LOADFLOW STUDIES OF A GRID INTERFACED WINDFARM USING PSS®E
Submitted by
WAQAS ALI MEMON (Group Leader)
11EL87
NEELESH KUMAR
11EL78
UMAR MEMON
11EL140
JITENDER KUMAR
11EL132
KHAMISO KHAN
11EL147
ALEEM-UL-HAQUE
11-09EL02
Supervised By
ENGR. MOKHI MAAN CHANG
Co-Super vis vi sed By
ENGR. MAHESH KUMAR RATHI
DEPARTMENT OF ELECTRICAL ENGINEERING MEHRAN UNIVERSITY OF ENGINEERING & TECHNOLOGY, JAMSHORO Submi tted tted in parti al f ul fi ll ment ment of th e r equi r ement ment f or th e degree degree of the Bache Bachelor lor of E lectri lectri cal cal Engin eeri ng
JANUARY 2015
1
CERTIFICATE This is to certify that the work presented in this project report/thesis report on
“LOADFLOW ANALYSIS OF A GRID INTERFACED IN TERFACED WINDFARM USING PSS®E” is entirely written by the following following
student/s, themselves under the
supervision of Engr. Mokhi Maan Chang, .
Submitted by
WAQAS ALI MEMON (Group Leader)
11EL87
NEELESH KUMAR
11EL78
UMAR MEMON
11EL140
JITENDER KUMAR
11EL132
KHAMISO KHAN
11EL147
ALEEM-UL-HAQUE
11-09EL02
Project / Thesis Supervisor
External / Examiner
_____________________ ________________________________ ___________ Chairman Department of Electrical Engineering Mehran UET, Jamshoro
Date:___________________
2
ACKNOWLEDGEMENT At the outset, I pay homage and guidance to ALLAH ALMIGHTY, the most merciful, compassionate, gracious and beneficial whose help enabled to complete this thesis. We extend our profound sense of gratitude to respected supervisor Engr. Mokhi Maan Chang, Department of Electrical Engineering MUET Jamshoro under whose
abe guidance this thesis has been completed. We are indeed extremely grateful for her inspiring guidance and kind sympathetic attitude. Without that thesis would not have been the light of the day. We are highly thankful to Dr. Abdul Sattar Larik, Chairman Department of Electrical Engineering MUET Jamshoro, who always encouraged and advised us to complete the thesis well on the time. He extended all the possible co-operation in this regard. We are also thankful to all our teachers of our department who helped us to fulfill this thesis. And finally we are thankful to Engr. Mushtaq Ahmed Kerrio, Plant Manager and Engr. Shahid Ali, Sub-engineer at Zorlu Wind Power Farm .
3
ABSTRACT Wind power has evolved as a significant renewable energy source for the generation of electrical energy due to the growth of environmental concerns. Large wind farms with several hundred megawatts of rated power have been connected to grid, fulfilling the energy demands of a region or a country. The injection of new produced power into the existing power system resulting challenges regarding voltage levels, thermal limits, stability and constancy of frequency. In the same way when wind farms are connected to the existing power system introducing the new power to the system it also offers some challenges like reactive power compensation, fluctuations in active and reactive power and control strategies. It is therefore necessary to go through steady state and dynamic analysis to map the impacts of newly injected power of wind farms on the existi ng system. Thus the objective of this thesis is to carry out the load flow analysis of the wind farm integrated to the power grid, in order to study their overall impacts on the power system. For this purpose, a wind farm of 110MW having two aggregates of 24MW and three aggregates of 21MW connected offshore to the power grid by means of a power cable. Initially the steady state load flow analysis is carried out and then dynamic simulation is carried out to determine the fault ride through capability of the wind farm. All this simulation and load flow is performed using a software i.e. PSS/E by Siemens PTI. The results of the simulation are analyzed to study the Impacts of grid interfaced wind farm on the power system.
4
TABLE OF CONTENTS Title ................................................................................................................................ i Certificate ...................................................................................................................... ii Acknowledgement ....................................................................................................... iii Abstract ........................................................................................................................ iv Table of Contents ...........................................................................................................v List of Figures ............................................................................................................. vii List of Tables ............................................................................................................. viii CHAPTER 1
INTRODUCTION 1.1
Introduction ........................................................................................................1
1.2
Problem Statement ............................................................................................1
1.3
Objectives ..........................................................................................................2
1.4
Thesis Outline ....................................................................................................2
CHAPTER 2
LITERATURE REVIEW 2.1
Offshore Wind Farms ........................................................................................3
2.2
Transmission Strategies .....................................................................................4
2.3
LVRT .................................................................................................................6
2.4
Grid Integration..................................................................................................7
2.5
HVAC Transmission ..........................................................................................9
2.6
Reactive Power Compensation ..........................................................................9
2.7
FACTs Devices ................................................................................................10
2.8
Power Flow Analysis .......................................................................................11
CHAPTER 3
INTRODUCTION TO PSS®E 3.1
Introduction .....................................................................................................19
3.2
What is PSS®E? ..............................................................................................19
3.3
File types used in PSS®E ................................................................................21
3.4
Explanation of Tabs .........................................................................................22
3.5
Major system security tools offered in PSS®E ...............................................27
5
CHAPTER 4
PSS®E MODEL CONFIGURATION 4.1
Wind Farm Modeling Approach ......................................................................30
4.2
PSS®E Modeling .............................................................................................31
4.3
Grid Structure...................................................................................................32
4.4
Wind Farm Model ............................................................................................37
4.5
Wind Turbines .................................................................................................38
4.6
Layout of the Offshore Wind Farm .................................................................40
4.7
HVAC Transmission System ...........................................................................41
4.8
STATCOM for Steady State Simulation .........................................................42
4.9
Diagram of the Grid .........................................................................................44
4.10
Modeling for Dynamic Analysis ......................................................................45
CHAPTER 5
RESULTS & OBSERVATION 5.1
Introduction ......................................................................................................51
5.2
Power Flow Simulation Results .......................................................................51
Case1 Wind Farm without Reactive Power Compensation ........................................54 Case2 Wind Farm with Shunt Reactor .......................................................................60 Case3 Wind Farm with STATCOM ..........................................................................65 5.3
Dynamic Results ..............................................................................................70
CHAPTER 6
CONCLUSION 6.1
Conclusion .......................................................................................................80
6.2
Recommendations ............................................................................................81
REFERENCES...............................................................................................82
6
LIST OF FIGURES Fig1:Example of the results of a voltage drop test ....................................................................... 7 Fig2: Schematic Diagram of an SVC...........................................................................11 Fig3 – (a) Schematic Diagram of a STATCOM; (b) a STATCOM installation .........11 Fig4: Use of PSS®E in different fields ........................................................................20 Fig5: PV Analysis Curve .............................................................................................28 Fig6: Layout of wind turbines in Wind Farm .............................................................37 Fig7: PSS®E model of the offshore wind farm ...........................................................40 Fig8: HVAC transmission layout in PSS®E ...............................................................41 Fig9: Graphic representation of a STATCOM in PSS®E ...........................................42 Fig10: Single line diagram of the wind Farm connected to the Grid...........................44 Fig11: DFIG Model .....................................................................................................46 Fig12: Power Flow Data File .......................................................................................47 Fig13: PSS®E DFIG Generic Model ..........................................................................48 Fig14: Legend of the values presented in the single-line diagrams .............................51 Fig 15: Power Flow results of Wind Farm without Compensation .............................54 Fig16: color display of voltages at buses .....................................................................57 Fig 17: power Flow results of wind farm with Shunt reactor ......................................60 Fig 18: Color display of Voltages at different buses ...................................................60 Fig 19: Power Flow results of Wind Farm with STATCOM ......................................65 Fig 20: Color display of voltages at different buses ....................................................65 Fig 21: Single line diagram of the grid used for the dynamic simulations. Note: Bus 3005, where the fault occurs is marked in the orange rectangle ......................71
7
LIST OF TABLES Table1: 33KV and 220KV Line data ..........................................................................33 Table2: 400KV Line dat .............................................................................................34 Table3: Grid Transformer Data ..................................................................................35 Table4: Load Data........................................................................................................36 Table5: Shunt Compensator Data ................................................................................36 Table6: Wind Turbine Data .........................................................................................39 Table7: Wind Farm Transformer Data ........................................................................39 Table8: 33KV Cable Data ............................................................................................40 Table9: 150KV Cable parameters ................................................................................41 Table10: Onshore and offshore transformer data ........................................................42 Table11: STATCOM Parameters ................................................................................43 Table12: Color Representation of Voltages .................................................................45 Table13: Turbine Model WT3T1.................................................................................49 Table14: Generator Model WT3G1 .............................................................................49 Table15: Pitch Model WT3P1 .....................................................................................49 Table16: Electric Part Model WT3E1 .......................................................................50 Table17: Voltages at different buses............................................................................56 Table18: Data of Active and Reactive power generated .............................................56 Table19: Voltages at different buses including wind farms ........................................62 Table20: Active and Reactive power generated by different generators. ....................62 Table21: Voltages at different buses using STATCOM ..............................................67 Table22: Active and Reactive power generated using STATCOM.............................67
8
INTRODUCTION 1.1
INTRODUCTION
The utilization of wind turbine to produce electricity is increasing rapidly in different parts of the world. It has become one of the main alternatives for non pollutant and environmentally friendly type for power generation all over the world and in Pakistan also its benefits are being recognized and a large amount of wind power is planned to be added to the national grid in coming years. Not until recently, the contribution of wind power generation on the system stability was considered to be small. However with increasing in the wind farm capacity it is clear that disconnecting a large wind farm will result in loss of a big part of power generation in grid, which can aggravate instability problems. Due to increasing portion of wind power, wind turbines have to contribute in reactive power support during transient conditions. The grid connection procedure is changing and adaption to a large scale wind power expansion is continually made. This adaption will most likely lead to requirements of information regarding the wind power unit‟s electrical behavior and then to determine the impact of adding wind generation, and establish how the system can be upgraded. Therefore a simulator tool (PSS/E) is used to perform the load flow and to study the dynamic behavior of the grid interfaced wind farm. 1.2
PROBLEM STATEMENT
Wind power industry is developing rapidly, more and more wind farms are being connected into power systems. Integration of large scale wind farms into power systems presents some challenges that must be addressed, such as
System operation and control
System stability and power quality
This thesis describes modern wind power systems, presents requirements of wind turbine connection and analyzes the impacts on grid integrated wind-farm.
1.3
OBJECTIVES
9
The overall objective for this thesis is to illustrate the dynamic impacts from the wind farms on the existing power system. The following elements are included in the studies:
Establish a steady-state and dynamic model of wind-farm integrated power system
Carryout load flow and transient analysis to study the impacts on grid interconnected wind-farm
1.4
Introduction to PSSE and Simulation using PSSE software THESIS OUTLINE
This thesis consists of six chapters including this chapter. The content of each chapter are outlined as follows: Chapter 2 includes Literature Review in which offshore wind farms, LVRT, steadystate and dynamic analysis, grid codes and compensation devices are discussed and it mainly focuses on the challenges which are faced to integrate the wind farms to power grid. Chapter 3 includes the Introduction to PSS/E software introducing to the files, windows and components used for simulation and explaining different security analysis functions offered by it. Chapter 4 includes PSS/E modeling approach, discussing the Grid and wind farm data for the analysis and establishing a model in order to perform the d esired analysis. Chapter 5 includes the steady-state and dynamic analysis results of a power system connected with 110MW wind farm. Analysis is performed and results are discussed. Chapter 6 contains conclusion and recommendations regarding the future work on PSS/E.
10
CHAPTER 2
LITERATURE REVIEW 2.1
OFFSHORE WIND FARMS
Among the available types of renewable energy, wind power generation offers the advantages of mat u re technological systems and a rich track record, as well as lower generation costs. For these reasons, the introduction and popularization of wind power is advancing. The challenges involved in offshore wind power development may be divided into three key categories. The first is that of cost. Because offshore wind turbines are installed within ocean environments, the cost is said to run roughly twice that of onshore facilities. This includes the wind turbines themselves, the foundations (bases submerged in the waters), submarine cable installation work and other project aspects. In addition, the operation and maintenance (O&M; referring to parts replacement and other upkeep) work also differs from onshore wind turbines insofar as the demand for heavy expenditures [12,13]. Costs likewise vary by distances from the shore, water depth and other elements. Because recent offshore wind farms in Europe are steadily moving further away from continental areas a nd into deeper waters, installation costs are also on the rise. The second challenge category is technology. With early offshore wind turbines suffering frequent breakdowns in their step-up gears, generators, development of technology was advanced for means of raising reliability involving salt damage countermeasures and monitoring of wind turbine conditions. In addition, when moving installation locations from shallow to deeper waters, there is a need to increase per-turbine power generation in order to lower cost. This makes increased size and improved reliability a major theme in developing the technology for offshore wind turbines. The third challenge concerns social acceptance. Clearly, offshore wind power generation will never be realized without the understanding of fisheries
11
operators and other marine users. To earn their supports , environmental assessments are a must. The extension of wind power can have severe impacts on the transmission system because of the remote sitting and the possible problems for system security. Due to the fact that electrical energy cannot be stored in a substantial way, the need for short and long term power balancing can require an adjustment of the operational strategy of power systems with a high wind power penetration level. Besides, in case of windstorms or system disturbances (such as voltage drops), there exists the increased risk of a sudden and uncontrolled shutdown of the wind farms, which can severely affect the security of the system. Wind generation also has an influence on th e network‟s Voltage control capability. On one hand, wind turbine generators (WTG) can demand a large amount of reactive power (depending on the technology) and on the other hand they replace c onventional thermal power plants that have excellent voltage control capabilities. In order to reduce the impact of wind generation on the transmission system, the connection through HVDC based on voltage source converters (VSC) seems a promising solution. The major benefit of this technology is its ability to vary the reactive power supplied and, as a consequence, to help in supporting the voltage at the point of common coupling with the transmission network [12,13]. 2.2
TRANSMISSION STRATEGIES
The transmission link to the shore can be HVAC; line- commutated thyristor-based HVDC or VSC-HVDC. The AC connection is the solution adopted mostly by existing wind farms and has the following features:
The submarine AC cable generates a considerable reactive current due to its high capacitance (typically in the range of 100-150 KVAR/km for 33 kV XLPE (cross- linked polyethylene) cables, 1000 KVAR/km for 132 kV XLPE cables, and 6 – 8 MVAR/km for 400 kV XLPE cables). This reduces the active current carrying capacity of the cable and, for large distances, requires compensation devices;
12
because of the high capacitance of the cable, resonances between the onshore and the offshore grid can occur, leading to distortion of the shape of the voltage;
The AC local wind turbine grid and the main grid are synchronously coupled and all faults in either grid are noticed in the other;
The major advantage is the low costs for substations when compared to DC solutions. On the other hand, costs for cables are higher than for DC alternatives.
The main advantages of the DC link with respect to the AC link are the following:
The losses and the voltage drop in the DC link are very low and there is no charging current in the DC cable. There is virtually no limitation of the connection distance, only practical restraints of cable manufacturing and laying put a maximum to this distance;
There is no resonance between the cables and other AC equipment
Since the collection system and the main grid are not synchronously coupled,
the WTG‟s do not contribute significantly to short-circuit currents in the main grid;
The DC link provides faster control of active and reactive power than the AC link. Voltage source converters are able to control reactive power over the
complete operation range, for „classical‟ thyristor-based HVDC this is somewhat limited. This control capability makes it easier to comply with connection requirements. The thyristor-based HVDC solution is a technology that has proven itself on land but seems not particularly well suited for offshore applications. Converter stations and auxiliary equipments have demanding space requirements, which will lead to enormous offshore converter platforms. Moreover this technology is highly susceptible to AC network disturbances (resulting in commutation failures in the inverter station), which can cause a temporary shutdown of the HVDC system; for these reasons this technology has not been considered further in this paper. On the contrary HVDC technology based on VSC‟s seems to be very promising for offshore applications because it requires less auxiliary equipment and the converters themselves take less space than the thyristor-based version. The VSC‟s are able to
13
independently control both the active and reactive power exchanged with the AC grid and therefore they can take part in voltage regulation. The major drawback of this technology is the high converter losses, caused mainly by switching losses that depend on the switching frequency of the semiconductor devices [9]. 2.3
LOW VOLTAGE RIDE THROUGH CAPABILITY (LVRT)
Grid stability and security of supply are two important aspects for energy supply. In order to avoid power outages it is necessary that power generating plants should have control capabilities and protection mechanisms. In the past, these requirements were mainly fulfilled by conventional power plants. In the meantime, however, the share of renewable energy sources in the total electricity generation has become so significant that these sources too must con tribute to the grid stability. Therefore the transmission sys tem operators have established so called grid codes with certain critical values and control characteristics that the generating plants have to fulfill. An important part of these requirements is the so-called LVRT capability of generating plants. LVRT is short for Low Voltage Ride Through and describes the requirement that generating plants must continue to operate through short periods of low grid voltage and not disconnect from the grid. Short term voltage dips may occur, for example, when large loads are connected to the grid or as a result of grid faults like lightning strikes or short circuits. In the past, renew able generating plants such as wind turbines were allowed to disconnect from the grid during such a fault and try to reconnect after a certain period of time. Today, because of the significant share of renewable, such a procedure would be fatal. If too many generating plants disconnect at the same time the complete network could break down, a scenario which is also called a “blackout ”. For this reason the LVRT requirement has been established which is meant to guarantee that the generating plants stay connected to the grid. Additionally many grid codes demand that the grid should be supported during voltage drops. Generating plants can support the grid by feeding reactive current into the network and so raise the voltage. Immediately after fault clearance, the active power output must be increased again to the value prior to the occurrence of the fault with in a specified period of time.
14
These requirements which at the beginning only applied to wind turbines, now also have to be fulfilled by photo voltaic systems (PV) and most recently, by combined heat and power plants (CHP).
Fig 1: Example of the results of a voltage drop test. Figure 1 shows the result of a voltage drop test at a PV system. In this diagram the voltage drops to about 20% of the nominal voltage for a time of approx. 550ms. The PV inverter recognizes the voltage drop and feeds a reactive current of approx. 100% of the nominal voltage into the system for the duration of the fault in order to support the grid. After fault clearance the active power output is increased to the value prior to the occurrence of the fault within 160ms [10]. 2.4
GRID INTEGRATION
Integrating a wind farm in an electrical network poses a significant challenge to the grid. The impact varies with the strength of the grid and the size of the wind farm. As the wind farm capacity grows, grid integration issues may arise, as increasingly large amounts of electricity are fed into networks, either in distribution or transmission systems.
15
2.4.1
Grid Connection Requirements
Until some years ago wind farms were allowed to disconnect from the grid during a disturbance in the grid. This has changed significantly, due to the addition of large amounts of installed wind power capacity. The disconnection of a large wind farm would result in a significant loss of generation that could cause some stability problems to the network. Transmission system operators require nowadays for wind farms to stay connected under certain disturbances in the grid. These requirements are known as the fault ride through capability of the wind farm and are generally regulated in grid codes. As established in most grid codes, only under certain circumstances shall wind farms be disconnected from the grid following a grid fault, remaining otherwise connected in order to assist in the stabilization of the grid frequency or the voltage during fault, providing voltage back-up [7]. Apart from the fault ride through capability, other technical requirements must be fulfilled by the wind farm, since the increasing size of wind farms means that the rating of such installations will be comparable to that of traditional generating plants on the grid. These requirements include:
Control of active and reactive power (operation under a specified range for power factor);
Frequency range (with time durations for extreme conditions, permissible reduction at frequency extremes)
Contribution to network stability;
AC voltage control capability.
As the proliferation of wind power increases, wind farms will be bound to meet these demands, which may prove difficult depending, to greater extent, on the transmission system used between the wind farm and grid. The charging currents affecting AC cables represent a limitation for the HVAC cables and so some form of compensation for the surplus reactive power generated by the cable is necessary to met grid requirements [7].
16
2.5
HVAC TRANSMISSION
2.5.1
General Aspects
Connecting the wind farm to the grid by an AC cable is the most straightforward technical solution, as both the power generated by the wind farm and the onshore transmission grid are AC. The HVAC transmission offers some advantages:
Proven and low-cost technology;
Easy to integrate in existing power systems
Low losses over small distances
On the other hand there are some limitations of the HVAC system
There is an excessive amount of reactive power produced in the AC transmission cables
Increase in the cable length means increase in its capacitance which results in a reactive power increase, resulting in a transmission distance limit for AC systems
Necessary use of compensation systems (shunt reactors, STATCOMS, SVC, etc) at the ends of the cable [7]
2.5.2
Main Components of HVAC Transmission
A transmission system based on HVAC technology includes the following main components
AC based collector system within the wind farm known as point of common coupling.
2.6
Three core HVAC transmission cable
Offshore transformer
Reactive power compensation (onshore and/or offshore)
Onshore transformer REACTIVE POWER COMPENSATION
The solution for the large amounts of reactive power at the cable is to compensate the reactive power produced by absorbing reactive power, thus reducing the additional losses and increasing the maximum transmitting distance. The compensation is usually done by fixed or electronically controlled shunt reactors. The fixed shunt
17
reactor is the simplest device but the progress in FACTS (Flexible AC transmission system) devices, such as SVC (Static VAR compensator) or STATCOM (Static Synchronous Compensator), considerably extends the reactive power and voltage control possibilities offered by the switched shunt reactors. The voltage on a transmission network is determined by the reactive power flows. DFIG wind turbines have the capability of controlling reactive power flow through the connection network and supporting the voltage network which they are connected. However, in a large wind farm controlling individual DFIG wind turbine to regulate reactive power flow is not feasible. It may not be able to control the voltage in the grid. On many occasion, the reactive power and voltage control at the grid is achieved by using reactive power compensation [7]. 2.7
FACTS DEVICES
The SVC and the STATCOM are part of the FACTS device family, used for voltage regulation and power system stabilization, based on power electronics. These devices are capable of both generating and absorbing reactive power. The flexibility of use is the main advantage of these equipments, since they allow the continuous variable reactive power absorption (or supply). The reactive power is not proportional to the voltage at the bus is another advantage of FACTS devices. The FACTS devices also contribute in the improvement of the voltage stability and the recovery from network faults. The similarity of the SVC and STATCOM devices led to them being sometimes
referred generally as “Static VAR Compensators”. These are, however, different equipments. The SVC is based on conventional capacitor banks together with parallel thyristor controlled inductive branches. These inductive branches can either be TCR (Thyristor Controlled Reactor), used for linear injection of reactive power or TSC (Thyristor Switched Capacitor), used for stepwise injection of reactive power. A SVC device is represented in Figure where a linear diagram and an SVC installation (in an offshore wind farm) are represented [7].
18
Fig 2: Schematic Diagram of an SVC The STATCOM device uses a power electronic voltage source (VSC). The converter uses semiconductors with turn-off capability, such as Insulated Gate Bipolar Transistors (IGBTs). The benefits of the STATCOM (commercially known as “SVC
Light” by ABB or “SVC Plus” by Siemens), compared with the SVC, are the fact that the capacitor banks used are smaller and also there is no need for big air-cored inductors. Further advantages of the STATCOM are also found in the dynamic behavior (such as faster transient response). A simplified schematic diagram of a STATCOM is shown in Figure 3(a & b).
Fig 3 – (a) Schematic Diagram of a STATCOM; (b) a STATCOM installation 2.8
POWER FLOW ANALYSIS
In power engineering, the power flow analysis (also known as load-flow study) is an importance tool involving numerical analysis applied to a power system. Unlike traditional circuit analysis, a power flow study usually uses simplified notation such as a one-line diagram and per-unit system, and focuses on various form of AC power (ie: reactive, real and apparent) rather than voltage and current. The advantage in studying power flow analysis is in planning the future expansion of power systems as well as in determining the best operation of existing systems. Power flow analysis is being used for solving power flow problem. There are three methods can be used to
19
solve power flow analysis. The methods are Newton-Raphson method, FastDecoupled method and Gauss-Seidel method. This sub-chapter will discuss all three methods generally on formula or mathematical step in order to solve power flow problem [15]. 2.8.1
Newton-Raphson Method
Newton-Raphson method is commonly use and introduce in most text book. This method widely used for solving simultaneous nonlinear algebraic equations. A Newton-Raphson method is a successive approximation procedure based on an initial estimate of the one-dimensional equation given by series expansion. The Newton-Raphson method using the bus admittance matrix in either first or second
– order expansion of Taylor series has been evaluate as a best solution for the reliability and the rapid convergence. f(x)=c
(1)
If x(0) is an initial estimate of the solution, and ∆ x(0) is a small deviation from the correct solution, we must have f(x + ∆ x )=c
(2)
Expanding the left-hand side of the above equation in Taylor‟s series about x f(x ) + (df/dx)
∆x
+1/2! (d f/dx )
yields
(∆ x ) +...=c
(3)
Assuming the error ∆ x(0) is very small, the higher-order terms can be neglected, which result in
∆ c ≈ (df/dx)
∆x
where
∆ c(0)=c - f(x(0)) Adding ∆ x to the initial estimate will result in the second approximation x(1)= x(0) + ∆ c(0)/ (df/dx)(0)
(5)
20
Successive use of this procedure yields the Newton-Raphson algorithm
∆ c(k)=c - f(x(k))
(6)
∆ x(k)= ∆ c(k)/ (df/dx)(k
(7)
x(k+1)= x(k )+ ∆ x(k)
(8)
(7) can be rearranged as
∆ c(k)= j(k) ∆ x(k)
where j(k) = (df/dx)(k)
(9)
In power system analysis, J (k) is called the Jacobian matrix. Element of this matrix are the partial derivatives evaluated at X(k). It is assumed that J (k) has an inverse during
each iteration. Newton‟s method, as applied to a set of nonlinear equations reduces the problem to solving a set of linear equations in order to determine the values that improve the accuracy of the estimates [15]. 2.8.2
Gauss-Seidel Method
Gauss-Seidel method is also known as the method of successive displacements. To illustrate the technique, consider the solution of the nonlinear equation given by F(x)=0
(10)
Above function is rearrange and writes as x=g(x)
(11)
If x=(k) is an initial estimate of the variable x, the following iterative sequence is formed X(k+1)= g(x(k))
(12)
A solution is obtained when the difference between the absolute value of the successive iteration is less than a specified accuracy, i.e., | x(+k1)- x(k)|≤ ε
(13)
Where ε is the desire accuracy
21
The process is repeated until the change in variable is within the desired accuracy. So the Gauss-Seidel method needs much iteration to achieve the desired accuracy, and there is no guarantee for the convergence [15]. 2.8.3
Fast Decoupled Method
When solving large scale power transmission systems, an alternative strategy for improving computational efficiency and reducing computer storage requirements is the decoupled power flow method, which makes use of an approximate version of the Newton-Raphson procedure. The Fast decoupled power flow solution requires more iterations than the NewtonRaphson method, but requires considerably less time per iteration and a power flow solution is obtained very rapidly. This technique is very useful in contingency analysis where numerous outages are to be simulated or a power flow solution is required for on-line control [15]. For large scale power system, usually the transmission lines have a very high X/R
ratio. For such a system, real power changes ∆ P are less sensitive to changes in voltage magnitude and are most sensitive to changes in phase angle ∆δ. Similarly, reactive power is less sensitive to changes in angle and most sensitive on changes in voltage magnitude. Incorporate of these approximations into the Jacobian matrix in Newton-Raphson power flow solution makes the elements of the sub-matrices J 12 and J21 zero. We are then left with two separated systems of equations,
22
In well-designed and properly operated power transmission system: i) Angular differences between typical buses of the system are usually so small.
δ
ij
= (δi −δ j ) very small that results,
cosδij ≈1 sin δij ≈ 0.0 ii) The line susceptances Bij are many times larger than the line conductances Gij so that Gij sin δij << Bij . iii) The reactive power Qi injected into any bus i of the system during normal operation is much less than the reactive power which would flow if all lines from that bus were short circuited to reference. That is Qi << Vi 2 Bii .
∂Pi
Y =− V ij i
V
sin (θ +δ −δ ) i i
(16)
∂δ j
∂Qi
Y =− V V
V
∂P i sin(θ
i
ij i
+δ −δ )=
(17)
i
∂V j
∂δ j
In Eq.(16) and Eq.(17), the off diagonal elements of J 11 and J 22 are given by
∂Qi
Y
=− V V
V ∂V j
∂Pi
ij i
+δ −δ )=
sin(θ Ij
i
(18) ∂δ j
23
Using the identity sin(α + β)= sinα cos β + cosα sin β in Eq.(18) gives us
∂Q i
∂Pi
V =− V
= V
B cos(δ
−δ ) + G sin(δ −δ )
i
(19)
i
i
i
i
V
∂
∂δ j
The approximation listed above then yield the off diagonal elements
∂Q i
∂Pi
= V
= − VV B
(20)
i i V
∂
∂δ j
∂Pi n
=∑
Y VV
ij i j sin(θ ij +δ j −δi )
(21)
∂δi =1
≠i
∂Q i
V
i
− 2 V B = Q − V 2 B
=−
∂V
2
∂Pi
∂δ i
i
ii
i
i
(22)
ii
24
The diagonal elements of J 11 and J22 Applying the inequality Q i << Vi 2 Bii
∂Qi
∂Pi ≅
V
≅
i ii
i ∂δ
∂δ
i
− V 2 B
i
are shown in Eq. (8) and Eq. (9) respectively. to those expressions yields
(23)
Substitute Eq. (19) and Eq. (20) into Eq. (14) and Eq. (15), we obtain
We can also modify Eq. (24) and Eq. (25) to two decoupled systems of equations for n-bus network.
25
And
Bij are the imaginary parts of the corresponding Y bus elements. These were the basic introduction to all the related literature to understand the further work of our thesis in chapter 4 and 5.
26
CHAPTER 3
INTRODUCTION TO PSS®E 3.1
INTRODUCTION
The software tool PSS®E (Power Systems Simulation for Engineering) made by Siemens is heavily utilized to perform system studies. This software is used by many power companies. The reason that so many companies rely on PSS®E is because of the many features and abilities that it has to offer. The functionality and performance
of PSS®E doesn‟t come at a cheap price. The software costs roughly $90,000 per computer. Luckily for our thesis studies we have got university version of this software limited up to 50 buses. The hardest part of our thesis was actually learning how to use PSS®E. We spent numerous hours outside of PSS®E just reading the help files that in our opinion are somewhat difficult to apply to create a power-flow study. We learned how to use PSS®E to perform power-flow study. Our study involves adding a wind farm to the existing power grid. We were able to draw several conclusions about the feasibility of this addition [6]. 3.2
WHAT IS PSS®E?
Power System Simulation for Engineering (PSS®E) is composed of a comprehensive set of programs for studies of power system transmission network and generation performance in both steady-state and dynamic conditions. Currently two primary simulations are used, one for steady-state analysis and one for dynamic simulations. PSS®E can be utilized to facilitate calculations for a variety of analyses, including:
Power flow and related network functions
Optimal power flow
Balanced and unbalanced faults
Network equivalent construction Dynamic simulation
In this chapter our focus will primarily be on power flow, steady-state and dynamic simulations. PSS®E uses a graphical user interface that is comprised of all the functionality of state analysis; including load flow, fault analysis, optimal power f low, equivalency, and switching studies.
27
In addition, to the steady-state and dynamic analyses, PSS®E also provides the user with a wide range of auxiliary programs for installation, data input, output, manipulation and preparation [6].
Fig 4: Use of PSS®E in different fields. 3.2.1
Power Flow
A power flow study (also known as load-flow study) is a steady-state analysis whose target is to determine the voltages, currents, and real and reactive power flows in a system under a given load conditions. It is an important tool involving numerical analysis applied to a power system. Unlike traditional circuit analysis, a power flow study usually uses simplified notation such as a one-line diagram and per-unit system, and focuses on various forms of AC power ( i-e: reactive, real, and apparent). Power flow studies are important because they allow for planning and future expansion of existing as well as non-existing power systems. A power flow study also can be used to determine the best and most effective design of power systems [6]. 3.2.2
Dynamic Simulation
28
The dynamic simulation program includes all the functionality for transient, dynamic and long term stability analysis. The purpose of the dynamics is to facilitate operation of all dynamic stability analytical functions. The dynamic modeling simulation is used to ensure the reliability of electricity supply and to predict the performance of the system under a wide range of conditions and to identify any problems and scope measures needed for reliability [6]. 3.3
FILE TYPES USED IN PSS®E
PSS®E uses many types of files. Here is a brief description of important file types that may be used by PSS®E: *.sav – Saved case file
The saved case file is a binary image of the load flow working case. To conserve disk space and minimize the time required for storage and retrieval, saved cases (*.sav) are compressed in the sense that unoccupied parts of the data structure are not stored when the system model is smaller than the capacity limits of the program. *.raw – Power flow raw data file (input data file)
A raw file is a collection of unprocessed data. This means the file has not been altered, Compressed, or manipulated in any way by the computer. Raw files are often used as data files by software programs that load and process the data. These files contain power flow system specification data for the establishment of an initial working case. *.sld – Slider file (Single Line Diagram)
This file allows for performing network analysis studies on the grid. Sliders are visual displays of the grid. It includes buses, branches, lines, loads, Generators, transformers etc... All components should be color coded based on voltage flow. The slider file can also show the operational ratings (power flowing across the component relative to the capacity) of the listed components. *.txt – Text file
A text file (or plain text file) is a computer file which contains only ordinary textual characters with essentially no formatting.
29
*.dat – Input data file
PSS®E accepts large volumes of data from external sources time to time. Such large volumes of data could be typed directly into the PSS®E working case using the Spreadsheet View but this could be an onerous task. Voluminous data is best assembled in an input data file independent of PSS®E before PSS®E is started up. This file may then be used as the input source for PSS®E to feed the data through the appropriate input activity into the PSS®E working case [6]. 2.4
EXPLANATION OF TABS
After opening the *.sav file, there are 19 tabs to choose from at the bottom of the data file (shown below). Each tab can be accessed by clicking on it. Few tabs will be focused over here:
2.4.1
Buses
Followind are few important parameters of Buses used in PSS®E:
Displays the number assigned to a specific bus. Alphanumeric identifier assigned to bus "#". The name may be up to twelve characters. Bus base voltage; entered in kV.
Bus type code: 1 - Load bus (no generator boundary condition)
30
2 - Generator or plant bus (either voltage regulati ng or fixed Mvar) 3 - Swing bus 4 - Disconnected (isolated) bus 5 – Same as type 1, but located on the boundary of an area in which an equivalent is to be constructed.
Bus voltage magnitude; entered in per unit, V = 1.0 by default.
3.4.2
Branches: Following are few important parameters of Branches
Branch "from bus" number outside brackets with bus name and bus kV enclosed in brackets. Branch "to bus" number outside brackets with bus name and bus kV enclosed in brackets. Line R (pu): Branch resistance; entered in per unit. A value of R must be entered for each branch. Line X (pu): Branch reactance; entered in per unit. A nonzero value of X must be entered for each branch. Charging (pu): Total branch charging susceptance (imaginary part of admittance); entered in per unit. B = 0.0 by default.
Length: Length of line entered in user-selected units. 3.4.3
Load
Following are few important parameters of the Load:
31
This displays the Bus Number (where the load resides) outside of the brackets and displays the bus name as well as the bus voltage in kV inside the brackets. This is a alphanumeric load identifier. It is used to distinguish among multiple loads at the same "Bus Number/Name".
At buses in which there is a single load present, the ID = 1. A check mark indicates that a certain load at a "Bus Number/Name" is fully operational. If for any reason a certain load at a "Bus Number/Name" needs to be taken out of service, simply un-check that particular one. Active power component of constant MVA load; entered in MW. Reactive power component of constant MVA load; entered in MVAR. 3.4.4
Machines
The machines tab can be used to: 1. Add machines at an existing generator bus (i.e., at a plant). 2. Enter the specifications of machines into the working case. 3. To divide and distribute the total plant output power limits proportionally among the machines at the plant. The important parameters for the machines tab are described below:
32
This displays the Bus Number (where the machine is located) outside of the brackets and displays the bus name as well as the bus voltage in kV inside the brackets. This is a alphanumeric machine identifier. It is used to distinguish among multiple machines at a plant (i.e., at a generator bus). bus). At buses in which there there is a single machine present, ID = 1. A check mark indicates that a certain machine at a "Bus Number/Name" is fully operational or out of service. This shows the active power that the generator is putting out; entered in MW. This shows the minimum active power that the generator can output; entered in MW. This shows the maximum active power that the generator can output; entered in MW. This shows the reactive power that the generator is putting out; entered in MVAR. This shows the minimum reactive power that the generator can output entered in MVAR. This shows the maximum reactive power that the generator can output; entered in MVAR. 3.4.5
Two Winding Transformers Transformers
Each transformer to be represented in PSS®E is introduced by reading a transformer data record block.
33
The transformer data record block can be accessed by clicking on the two Winding transformer tab. The important parameters for this tab are explained below:
This states the first bus number outside of the brackets with the bus name and bus kV enclosed in brackets. It is connected to winding one of the transformers included in the system. The
transformer‟s magnetizing admittance is modeled on winding one. No default is allowed. This states the second bus number outside of the brackets with the bus name and bus kV enclosed in brackets. It is connected to winding two of the transformers included in the system. No default is allowed. A check mark indicates that a certain two winding transformer between two buses is fully operational. If for any reason a transformer needs to be taken out of service, simply un-check that particular one. The default is in service [6].
34
3.5
MAJOR POWER SYSTEM SECURITY TOOLS OFFERED BY PSS®E
This section briefly describes three power system topics that are usually covered at the undergraduate level which are offered b PSS®E. The three topics are: load flow analysis, transient stability analysis, and short circ uit analysis. 3.5.1
Load Flow Analysis
The main purpose of load flow analysis is to calculate bus voltages and transmission line MW and MVAR flows for a power grid. There are two types of power flow analysis:
AC power flow and
DC power flow calculation.
AC Power Flow: By solving the AC power flow problem, effectiveness of various
voltage control strategies could be identified. After performing the load flow concepts, the maximum power transfer capability of any system could be determined. DC Power Flow:
PSSE offers DC power flow options which could could be used to
perform fast contingency analysis to identify potential thermal overloads on the system. They can also compare the accuracy between the AC and DC load flow calculations [14]. 3.5.2
Voltage Stability Analysis
In addition to solving for bus voltages and line flows, PSSE can also be used to derive PV and VQ curves for different contingency scenarios, a process commonly used to assess power grid voltage stability. Through PV and VQ analysis, maximum power transfer capability of a transmission path could be determined while preserving voltage stability of the system. PSSE produces PV curves for each monitored bus so that the weakest node of the system can be identified. Figure presents a typical output of a PV analysis from PSSE for different contingency scenarios [14].
35
Fig 5: PV Analysis Curve 3.5.3
Transient Stability Analysis
Transient stability, also known as large signal stability, is a measure of the ability of synchronous generators to remain in synchronism when the power system suffers from a disturbance. The disturbance could be a fault successfully cleared by protective relays, tripping tri pping of a major generator, or loss of a large la rge load. These t ypes of disturbances can cause rotors to accelerate, resulting in an increase in internal angle and the potential loss of transient stability. For a single-machine-infinite-bus system (SMIB), the electrical power output of the generator is significantly reduced during the fault-on period, so that the mechanical input of the generator exceeds its electrical output. As a result, the transient kinetic energy of the generator increases rapidly as the rotor accelerates. When the fault is finally cleared, the generator needs to dissipate all the excess transient kinetic energy into the power system. If proper protection schemes are not in place to isolate the fault fast enough, the generator can lose synchronization (i.e. falling out of step) and suffer severe damages. Transient stability is affected by many factors such as the mechanical inertia of the generator, pre-contingency generator output level, and the excitation control of the generator. The Equal Area Criterion can be used to determine the transient stability of a SMIB if the synchronous machine is represented as a classical generator, i.e., a constant voltage source behind a transient reactance. The Equal Area Criterion states that for a
36
given contingency, the area corresponding to the destabilizing effect of the fault must not exceed the area corresponding to the restorative strength of the post-fault system. The critical clearing time is defined when the fault is cleared so that the generator is just marginally stable. The generator angle at the critical cle aring time is known as the critical clearing angle. The Equal Area Criterion is, however, only applicable for a single-machine-infinite bus system. When there are multiple generators, the time domain simulation technique must be used. This is the method which is used by PSSE. The interactive structure of PSSE guides through a step-by-step process which provides a greater understanding of the major factors which influence the transient stability of a system. PSSE exposes to analysis that cannot practically be done by hand calculations. Furthermore, the interactive nature of PSSE is conducive to sensitivity analysis, that is, different elements or variables of the system can be changed and the observer can immediately see how these changes affect system stability (e.g. varying the length or location of a fault) [14]. 3.5.4
Short Circuit Calculation
For the purpose of power system protection, it is important to introduce fault calculation which is also known as short circuit calculation. For the short circuit calculation, introduction to the concept of symmetrical components and the formation of positive, negative, and zero sequence networks is necessary. Then interconnection of the sequence networks depending on the type of fault under consideration is performed, in order to calculate the fault current levels. This short circuit calculation facility facilit y is offered by PSSE [14].
37
CHAPTER 4
PSS®E MODEL CONFIGURATION 4.1
WIND FARM MODELLING APPROACH:
The modeling approach of wind farms is based on Wind Grid Code Requirements. The aim of Wind Grid Code is to provide a description of the technical and operational requirements that are to be met by the developers and operators of WindPowered Generating Stations (WPGS) that wish to connect to the national grid. The Wind Code applies to all WPGSs that are or have applied to be connected to the transmission grid. Before the interconnection of wind farm to transmission grid the impact and design study is performed which covers:
Impact of the Wind Farm connection on the power system security and reliability of supply.
Design of the appropriate direct assets from the commercial boundary.
Design of the necessary Infrastructure Reinforcement of the Transmission System.
Based on grid code and wind grid code requirements a new connection (a generator or load) could be connected into the transmission network if the following conditions are fulfilled:
System operation security after the connection shall not be negatively affected.
The quality and reliability of demand supply shall comply with grid code requirements
The steady state and transient stability of power system should have sufficient security margin.
All relevant planning data will be used for detailed modeling of Wind Farms using PSS®E simulation platform. In this case two common calculations are essential for assessing the impact of Wind Farm on security and reliability of power system:
Power flow calculation
Transient stability
38
4.1.1
Power Flow Calculations
A power flow calculation is to determine the power flows on transmission lines and transformers and the voltage profile of system bus bars. This calculation is fundamentally important for the planning and design of the connection of wind farms to the transmission grid. N-1 Security criterion (system should be able to withstand the loss of any single components like lines, transformers, cables or generators) is essential for the proper design of transmission networks to ensure the security and reliability of power supply. System performance is compared to operating limits and criteria. Short Circuit calculations also play a very significant role for the proper selection of high voltage equipment and the setting of protection relays [8]. 4.1.2
Transient Stability Studies
The objective of transient stability studies is to examine whether wind farms will have a negative effect on the transient stability of power system. The power system response for a defined set of disturbances, typically three phase and single phase faults cleared by tripping of transmission elements such as lines, transformers, generators or bus bars. The response of conventional or wind turbine generators is checked to see that all machines have an adequate stability margin, damping of power system oscillations is acceptable and that the voltage recovery following fault clearing is adequate [8]. 4.2
PSS®E MODELING
In order to simulate the behavior of a wind farm, appropriate models of the wind farm, the transmission system and the electrical grid have to be constructed. Only then is it possible to analyze the steady-state and the transient behavior of the power system. The software used to model the system and perform the simulations in the work of this
thesis
is
the
PSS®E
software.
PSS®E
stands
for
Power
System
Simulator/Engineering and it is a software tool provided by Siemens Power Technologies International (PTI). It is used by most utilities in the world to perform power system simulations, as it allows the performance of power flow analysis, dynamic simulations and stability studies, among other features. PSS®E is composed of a comprehensive set of programs for studies of power system transmission network
39
performance in both steady-state and dynamic conditions, which is an obvious reason for its widespread use by transmission and distribution systems operators [1]. 4.3
GRID STRUCTURE
The electrical grid that will be used for steady-state analysis in which the wind farm will be integrated is the “savnw” network, provided by PSS®E as an example of a relatively large grid, as it has 23 buses, 6 generators and 7 loads. In order to properly use this grid, a few changes were made, namely to the operating frequency and voltage levels. These changes were made taking into account values used in the Asian electrical grid [1].
Additional changes to the original “savnw” network were performed since the wind farm rated at 110MW will be placed in a bus where the existing generation is of 750 MW. Therefore, adjustments in active and reactive power of the network are required, which can be made by adapting the power of the loads and the shunt compensators. As so, the changes made to the original network can be summarized:
The frequency was changedto 50 Hz (the frequency in use in most parts of the world, including Pakistan) since the frequency of original example grid is 60 Hz (typical of American grids)
The voltage levels were changed from 500 kV, 230 kV and 22 kV to, respectively, 400 kV, 220 kV and 33 kV. These voltage levels are common in most Asian Grids, including Pakistan
The 110 MW offshore wind farm was added, replacing an existing power plant of 750 MW
Most of the parameters of the grid are presented in detail: 4.3.1
Lines
As a result of changing the grid frequency to 50 Hz, the reactance and the susceptance of the branches have to be converted, since these parameters depend on the grid frequency. So new parameters (except those of 400kv lines) are given in table 1 using following equations [1]
40
X 50Hz = X 60HZ × (ω50Hz / ω60Hz ) B50Hz = B60HZ × (ω50Hz / ω60Hz )
Line From
To
Bus
Bus
X 50H z
B 50H z
[p.u.]
[p.u. ]
153
154
0.0375
0.08333
153
154
0.045
0.125
153
3006
0.01
0.025
154
203
0.0333
0.08333
154
205
0.00278
0.075
154
3008
0.01833
0.25
203
205
0.0375
0.006667
203
3003
0.0375
0.06667
3001
3004
0.00667
0
3002
3005
0.045
0.075
3003
3005
0.045
0.075
3003
3005
0.045
0.075
3005
3006
0.025
0.05833
3005
3007
0.02083
0.05
3005
3008
0.04167
0.1
3007
3008
0.02083
0.05
Table1: 33KV and 220KV Line data
41
4.3.2
400KV Line Parameters
The values for the 400 kV line parameters calculated are shown in Table 2
Line From bus
R [p.u]
X [p.u]
B [p.u]
To bus
151
152
0.00183
0.0201
0.5623
151
152
0.00183
0.0201
0.5623
151
201
0.00091
0.01005
0.2811
152
202
0.00146
0.01608
0.4498
152
3004
0.00128
0.01407
0.39361
201
202
0.00091
0.01005
0.28115
201
204
0.00091
0.01005
0.28115
Table2: 400KV Line data
42
4.3.3
Transformers
For the two- winding transformers in the “savnw” grid, the transformer reactance is frequency dependent, so their values also have to be adapted to the 50 Hz frequency, according to Equation X 50Hz = X 60HZ × (ω50Hz / ω60Hz ) B50Hz = B60HZ × (ω50Hz / ω60Hz ) The values of the results of the calculations for the transformers in the grid are shown in Table 3
Buses
X50Hz [p.u.]
From
To
Bus
Bus
101
151
0.01133
152
153
0.00417
201
211
0.01771
202
203
0.01354
204
205
0.0125
205
206
0.01111
3001
3002
0.0125
3001
3011
0.00833
3004
3005
0.01354
3008
3018
0.07083
Table3: Grid Transformer Data
43
4.3.4
Loads and Shunt Compensators
Addition of the wind farm rated at 110 MW to bus 102 of the grid in the place of the existing 750 MW conventional power plant leads to an imbalance of active and reactive powers. This imbalance can be compensated by reducing the consumed power in the loads. As so, the values of the load powers of the grid are presented in Table 4. The shunt compensators are also regulated in order to maintain the voltage in the buses at values within reasonable values (between 0.95p.u and 1.05p.u). Table 5 outlines the values of the shunt compensators of the grid [1]. Pload [MW]
Bus
Qload [Mvar]
153
200
100
154
500
450
154
400
350
203
200
100
205
1000
700
3005
100
50
3007
200
75
3008
100
50
Table 4: Load Data
Bus
Bshunt [Mvar]
151
0
153
150
154
300
201
300
203
100
205
300
Table 5: Shunt Compensator Data
44
4.4
WIND FARM MODEL
The model for the offshore wind farm is based in an existing installation: the Lillgrund Wind Farm. This wind farm is located off the coast of Sweden, at a distance of 9 km from the Point of Connection in the onshore grid and the transmission is achieved by a combination of an AC sea cable (7 km long) and an AC land cable (2 km long). With 48 wind turbines, rated at 2, 3 MW each, the total capacity of the wind farm is 110 MW. Figure 6 depicts the layout of the wind turbines in the Lillgrund wind farm, used as a reference here.
Fig 6: Layout of wind turbines in Wind Farm As can be seen in Figure 6, the internal grid of the Lillgrund wind farm consists of 33 kV sea cables divided in five feeders and each of these feeders connects 9 or 10 wind turbines to the offshore substation. The total is 48 wind turbines rated at 2, 3 MW. The modeled wind farm is based on this layout. The turbines are joined in aggregates and then connected to the offshore substation bus via 33 kV AC cables [1].
45
4.5
WIND TURBINES
The wind turbines used are the GE 1.5 MW model, available in the PSS®E Wind package. This model is of a DFIG (Doubly Fed Induction Generator) wind turbine developed by General Electric and released for PSS®E simulation and testing.
In order to match the 110 MW of the reference wind farm, a total of 74 1.5 MW wind turbines were used in the model. These are joined in five aggregates: two aggregates of 24 MW (16 wind turbines for each aggregate) and three of 21 MW (14 wind turbines per aggregate). This actually adds up to 111 MW, which is the value assumed hereby for the wind farm power. For PSS®E simulation of the wind farm, two distinct models are designed for the wind turbines: the Steady-state model (which allows the power flow simulations) and the transient model (used for Dynamic simulations) [1]. 4.6
STEADY-STATE MODEL
The load flow provides initial conditions for dynamic simulations. In the Load Flow parameters of the PSS®E, the wind turbines were modeled as five conventional generators, rated at 24 MW and 21 MW. The values specified on the existing generator record are outlined in Table 6. Note that the values in Table were calculated considering the values in the Individual WTG Power Flow Data of the PSS®E Wind User Guide and multiplying by the number of lumped elements, as recommended by the PSS®E guide. Since the model used is of a DFIG wind turbine, both the active and reactive power can be controlled. As so, for the wind turbine aggregates, for the HVAC transmission system, the wind turbines were regulated for a unit power factor, which means no reactive power is generated by the wind turbines. The reason is that the AC cable already produces a significant amount of reactive power, so an additional quantity of reactive power generated by wind farm would deteriorate the system behavior [1].
46
24MW
21MW
Aggregates
Aggregates
Pgen{MW}
24
21
Pmax{MW}
24
21
Pmin{MW}
1.1
1
Qgen{Mvar}
0
0
Qmax{Mvar}
0
0
Qmin{Mvar}
0
0
Mbase{MVA}
26
23
Xsource{p.u}
08
08
Table 6: Wind Turbine Data 4.6.1
Step-Up Transformers
The 0.69/33 kV transformers that adapt the voltage at the generation buses (690 V) to the voltage of the internal grid of the wind farm (33 kV) have the parameters presented in Table 7. As each aggregate contains 14 or 16 wind turbines, the rated power of the transformers is respectively, 24.5 MVA and 28 MVA [1]. Transformer Parameters
Value
Unit Rating {MVA}
175
Unit Rated Voltage {kV/kv}
0.69/33
Unit Impedance{%}
5.75
Unit X/R
7.5
Table 7: Wind Farm Transformer Data
47
4.6.2
33 kV Cables
The cables used to link the five buses where the wind turbines are connected to the offshore bus are 33 kV AC cables. The cable parameters are presented in Table 8 Cable
Length
Rp.u
Xp.u
Bp.u
1
0.003857
0.010101
0.000957
102
0.8
0.003085
0.008081
0.000766
3
102
0.6
0.002314
0.006061
0.000574
4
102
0.4
0.001543
0.00404
0.000383
5
102
0.2
0.000771
0.00202
0.000191
From Bus
To Bus
[km]
1
102
2
Table 8: 33KV Cable Data 4.7
LAYOUT OF THE OFFSHORE WIND FARM
Figure illustrates the wind farm designed in PSS®E, where the wind turbines, the step-up transformers and the 33 kV cables mentioned above are represented [1].
Fig 7: PSS®E model of the offshore wind farm
48
4.8
HVAC TRANSMISSION
The HVAC transmission is the simplest alternative for the transmission of the electrical power of the offshore wind farm to the onshore grid. The main components are the AC transmission cable and the two transformers: onshore and offshore. Figure 8 illustrates the HVAC transmission scheme, with the AC cable and both transformers. For proper compensation of the reactive power generated in the cable, shunt reactors (or a STATCOM device) were applied to one or both ends of the cable. They are not however represented here [1].
Fig 8: HVAC transmission layout in PSS®E 4.8.1
150KV Power Cable
The power cable chosen is a 100Km, 150 kV XLPE cable. 150 kV is a typical option for offshore wind farms, used in, for example, the Horns Rev wind farm, in Denmark. Hence using the parameters of the power cable in Horns Rev Cable From Bus
20
To Bus
21
Rkm
Xkm
Ckm
Length
[Ω/km]
[Ω/km]
[µF/km]
[km]
0.039
0.12
0.19
100
Rp.u
0.0173
Xp.u
0.053
Bp.u
1.34235
Table 9: 150KV Cable parameters
49
4.8.2
Onshore and Offshore Transformers
The offshore transformer adapts the voltage from the 33 kV of the internal grid to the 150 kV of the power cable. As for the onshore transformer, it is used to increase the voltage from the 150 kV of the cable to the 400 kV of the onshore grid. Table10 shows the offshore and onshore transformers data [1]. Buses
Rated Voltage Impedance {%}
Rating {MVA}
X{p.u.} 100MVA Based
From Bus
To Bus
Location
Rated Voltage {KV}
20
102
Offshore
33/150
13.8
160
0.08625
21
151
Onshore
150/400
15
200
0.075
Table 10: Onshore and offshore transformer data 4.9
STATCOM FOR STEADY-STATE SIMULATION
For power flow analysis, the STATCOM is modeled as a FACTS device with the parameters adjusted as to simulate the behavior of this device. In both cases of the use of the STATCOM (in HVAC transmission) it was modeled with the same parameter s.
Fig 9: Graphic representation of a Statcom in PSS®E The parameters presented in Table 11 are the most important values used in power flow analysis of the STATCOM and they are obtained using PSS®E Manual for FACTS devices
50
Parameter
Rectifier
Device Number
1
Terminal Bus
0
Control Mode
Normal
P Setpoint {MW}
0.00
Q Setpoint {MVAR}
0.00
V Send Setpoint
1.02
Shunt Max {MVA}
64
RMPCT {%}
100
Bridge Max
0
V term max {pu}
1.1
V term min {pu}
0.9
V Series Max {pu}
1
I Series Max {MVA}
64
Dummy Series X{pu}
0.05
V Series Reference
Sending end voltage
Table 11: STATCOM Parameters
51
4.10
DIAGRAM OF THE GRID
The resulting grid, after the changes performed, is presented in the single-line diagram shown in Figure, in which the offshore wind farm is already included. Bus 102 referred above, is marked in the figure 10. The legend of the colors used in the figure for each voltage value is presented in Table 12
Fig 10: Single line diagram of the wind Farm connected to the Grid
52
Color
Rated Voltage {KV}
RED
440
BLACK
220
PURPLE
150
DARK GREEN
33
BLUE
20
ORANGE
13.8
BORDEAUX RED
0.7
Table 12: Color Representation of Voltages 4.11
MODELING FOR DYNAMIC ANALYSIS
The dynamic data file so called (.dyr) file in PSS®E consist of dynamic parameter data for all conventional synchronous generators, turbines, exciters governors and other devices. The first step in dynamic simulation using initial dynamic file is to enter the detailed dynamic model data for Wind Farm, which is saved in a file. This file contains a group of records, each of which defines the location of a dynamic WTG model in the grid along with the constant parameters of the model. The PSS®E university version 33 provides a dynamic model for a DFIG wind turbine. The model includes generator, electrical control, wind turbine and pitch control. Dynamic simulation is performed based on the load flow data that provide the transmission grid, load, and generator data. In this study, dynamic analysis is performed to investigate the WTGs model response subjected to grid disturbances. The most relevant disturbance for the study case is a three-phase symmetrical short-circuit fault on an onshore bus. Additional fault events can be simulated on the different buses of the Wind Turbine Generation Station (WTGS) to see the behavior of the wind farm but here our focus will only be on the simulation of fault on the 400KV onshore bus named as 3005 [8].
53
4.11.1 PSS®E Dynamic model of DFIG
The variable speed wind turbine using DFIG are more popular technology which is used word-wide due to advantages such as high energy efficiency and controllability. DFIG is basically a standard, wound rotor induction generator with a voltage source converter connected to the slip-rings of the rotor. The stator winding are coupled directly to the grid and the rotor winding is connected to power converter as shown in figure 11. The steady-state and dynamic characteristics of DFIG are dominated by the power converter. The converters allow the machine to operate over a wider range of speed, and control active and reactive power independently. This means that DFIG have the capability to participate in steady-state and dynamic VOLT/VAR control. In some DFIG designs, a crow-bar or DC chopper circuit may be used to short the rotor-side converter during a close-in transmission fault to avoid excessively high DC link voltage and keep the machine running. If the rotor-side converter is shorted, the dynamic behavior is similar to an induction generator. During a low voltage event, t he converter tries to retain full in control of active and reactive currents. DFIG can be designed to meet low voltage ride-through requirements without external reactive power support. Converters are current-limited devices, and this plays a major role in the dynamic response of DFIGs to grid disturbances. DFIGs also have a pitch control to optimize energy capture [8].
Fig 11: DFIG Model The PSS®E wind turbine model WT3 was used for the dynamic simulation study with the objective of simulating the dynamic performance of a wind Farm employing DFIG technology. The generic WT3 model is included as a standard model in the Dynamic Model Library of PSS®E version 33. The WT3 model can only be used 54
when the generator is specified as a wind generator and not as conventional generator in the power flow data file. As shown in the fig 12
Fig 12: Power Flow Data File
0 if this is not a wind machine (this is the default value).
1 if this is a wind machine which participates in voltage control, with the
values of QT and QB on the data record specifying the machine‟s reactive power limits.
2 if this is a wind machine which participates in voltage control, with the
specified power factor and the machine‟s active power setting (PG on the data record) used to set the machine‟s reactive power limits.
3 if this is a wind machine which operates at a fixed power, with the
machine‟s reactive power output and reactive power upper and lower limits all equal, and set based on the specified power factor and the machine‟s active power setting [8]. 4.11.2 Power Factor
ignored if the wind control mode is 0
is used in setting the machine‟s reactive power limits when the wind control mode is 2 or 3
Negative value may be specified when the wind control mode is 3, and is interpreted as a leading power factor (i.e., the wind machine produces active power and absorbs reactive power).
The WT3 generic wind turbine model consists of the following main modules:
WT3G generator/converter module
WT3E electrical control module
WT3T turbine module and
WT3P pitch control module.
55
Figure 13 shows the interaction between these modules.
Fig 13: PSS®E DFIG Generic Model The DFIG includes 4 modules which are responsible for:
WT3G1, doubly-fed induction generator which is mostly an algebraic model to calculate the current injection to the grid based on commands from controls, with or without the PLL control.
WT3E1, electrical control including the torque control and a voltage control.
WT3T1, the turbine model including a two-mass shaft mechanical system and
a simplified method of aerodynamic conversion, namely ΔP=Kaero*θ*Δθ where P is mechanical power, θ is a pitch angle; this method was validated against results obtained when using the Cp matrix
WT3P1, the pitch control.
Tables 13, 14, 15, 16, show the dynamic data for Double Fed Inductive Generator based on WT3 Generic Wind Model. Values for the parameters of these models were based on typical values given in the PSS®E Wind manual. The number of aggregated wind turbines was changed in the models to 14 for the 21 MW aggregates and 16 for the 24 MW aggregates [8].
56
Symbol
Value
Unit
Vw
1.25
p.u
H
4.95
MW*sec/MVA
DAMP
0
p.u P/pu
Kaero
0.007
Const.
Theta 2
21.98
Deg.
Htfac
0.875
Hturb.h
Freq
1.8
HZ
DSHAFT
1.5
P.U P/pu
Table 13: Turbine Model WT3T1
Symbol
Value
Unit
Xeq
0.8
p.u
Pll gain
30
Con
Pll integrator gain
1
Con
Pll maximum
0.1
Cons
Turbine MW rating
0.5
MW
No.of lumped WT-s
number of turbines for each feeder
Integer
Table 14: Generator Model WT3G1
Symbol
Value
Unit
Tp
0.3
Sec
Kpp
150
p.u
Kip
25
p.u
Kpc
3
p.u
Kik
30
p.u
Tetamin
0
Deg
Tetamax
27
Deg
RTetamax
10
Deg/sec
PMX
1
p.u on Mbase
Table 15: Pitch Model WT3P1
57
Symbol
Value
Unit
Symbol
Value
Unit
Tfv
0.15
Sec
T_Power
5
sec
Kpv
18
p.u
Koi
0.05
Con
Kiv
5
p.u
VMNCL
0.9
Con
Xc
0
p.u
VMAXCL
1.2
Con
Tfp
0.05
Sec
Kqv
40
Con
Kpp
3
p.u
X1Qmin
-0.5
Con
Kip
0.6
p.u
X1QMax
0.4
Con
PMX
1.12
p.u
Tv
0.05
Con
PMN
0.1
p.u
Fn
1
con
QMX
0.309
p.u
Wpmin
0.69
p.u
QMN
-0.309
p.u
Wp20
0.78
p.u
IPMAX
1.1
p.u
Wp40
0.98
p.u
TRV
0.05
Sec
Wp60
1.12
p.u
RPMX
0.45
p.u
pwp
0.74
p.u
RPMN
-0.45
p.u
Wp100
1.2
p.u
Table16: Electric Part Model WT3E1
58
CHAPTER 5
RESULTS & OBSERVATIONS This chapter presents the results of the steady-state simulations performed on the grid with the offshore wind farm, as described in Chapter 4. Results are analyzed and discussion of the results is also performed. 5.1
INTRODUCTION
The power flow analysis carried out comprehends numerical calculations of active and reactive power flows and node voltages. PSS/E software is used, in the scope of this thesis, for the power flow analysis of power grid with an offshore wind farm. Special attention is given to the Point of Common Coupling (hereby designated as PCC), i.e., the point of connection of the wind farm with the remaining grid. The voltage at the PCC, the active power injected and the reactive power injected/absorbed in the PCC is analyzed as a part of the power flow study. Power losses for each transmission system are also assessed. The power flow results are presented by the single-line diagrams of the network (from the PSS/E load flow software). The active and reactive flows at each end of the branches and the voltage magnitude and angle at each bus are depicted in each figure of this chapter. Figure depicts how each of the values from the power flow calculations are represented in the power flow result figures [1].
Fig 14: Legend of the values presented in the single-line diagrams. 5.2
POWER FLOW SIMULATION RESULTS
For the HVAC transmission system, the power flow analysis focuses on the voltage level and reactive power flow in the wind farm and the power cable connecting the wind farm to the grid. Therefore, some reactive power compensation options are
59
studied with the objective to compensate the reactive power flow at different buses of Wind farm so as to keep the voltage in to the desires range of 0.95pu to 1.05pu. The main criteria for the choice of the value of the compensation device is the power factor at the PCC, which is chosen to be approximately of 0.9, a typical value in grid integration of wind farms. As so, the shunt reactor chosen for offshore compensation only absorbs certain amount of reactive power, as the 0.9 power factor at the PCC is guaranteed with this value. An estimate of the reactive power produced by the 150 kV AC cable can be made taking into account Equation. The approximate amount of reactive power produced by the 100 km cable is given by equation below
Note that, for HVAC power flow, the wind turbine generators supply no reactive power, since there is already an excess of reactive power, as a consequence of the shunt capacitance of the AC cable. Therefore, the capability of the DFIG machines of providing voltage support to the grid, by supplying reactive power, is not considered for the present study. Different strategies are made in order to compensate the reactive power produced due to the power cable that is interconnecting the wind farm to the grid. This reactive power needs to be compensated in order to keep the voltage profile of all the buses and generators at the wind farm into desired range of 0.95pu to 1.05pu. It must be noted that in this study of wind farm, doubly fed induction generators are used which have the capability to control the reactive power flow but for the wind farms of larger capacity like 110MW wind farm this approach of compensating the reactive power seems less effective. Due to which different compensation devices are connected to keep the control of reactive power flow in order to keep the voltages at all the buses of the power system into desired limits. Here in this study three cases are discussed which are as follows:
Wind farm without compensation devices
Wind farm with shunt reactor
Wind farm with STATCOM
60
These cases are created to analyze the importance of the external compensation devices into the wind farms. Each case simulation describes the reactive, active power flows at different buses and
voltages at different branches and buses. It also uses „„Enable contour‟‟ facility of PSSE to show the voltage level of different buses.
61
CASE1 WIND FARM WITHOUT REACTIVE POWER COMPENSATION After developing the steady-state model of the power system having wind farm connected to it, the load flow analysis is performed in order to carry out the impact studies. Full Newton-Raphson method is used to perform the power flow studies. In Case 1 the wind farm can be seen connected to the power system. In this case no compensation device is connected to the wind farm and the results are obtained as shown in fig15.
Fig 15: Power Flow results of Wind Farm without Compensation In table given 17 the result of voltages at different buses can be observed. It can be seen that at all the buses of the wind farm the voltages are extremely high as compared to the desired value of the voltages. The voltage is as high as 1.19pu whereas the desired voltage limit is 1.05pu. The voltage reaches to the extreme value due to high capacitive reactive power produced by the power cable, which needs to be compensated in order to bring the voltage under normal ranges.
62
Bus Number
Bus Name
Base KV
Code
Voltage(p.u)
Angle (deg)
1
PARK 1
33.0
1
1.1952
8.35
2
PARK 2
33.0
1
1.1950
8.34
3
PARK 3
33.0
1
1.1948
8.31
4
PARK 4
33.0
1
1.1947
8.29
5
PARK 5
33.0
1
1.1945
8.27
20
BUS OFFSHORE
150.0
1
1.1973
4.42
21
BUS ONSHORE
150.0
1
1.1426
2.60
101
NUC-A
33.0
2
1.0221
3.28
102
NUC-B
33.0
1
1.1944
8.26
151
NUCPANT
400.0
1
1.0343
-1.35
152
MD500
400.0
1
1.0172
-4.46
153
MD230
220.0
1
0.9992
-5.68
154
DOWNTN
220.0
1
0.9705
-9.23
201
HYDRO
400.0
1
1.0400
-2.87
202
EAST500
400.0
1
1.0208
-4.74
203
SUB500
220.0
1
0.9981
-7.65
204
SUB230
400.0
1
1.0142
-4.98
205
URBGEN
220.0
1
0.9800
-8.61
206
HYDRO_G
18.0
2
1.0807
-3.61
211
MINE
20.0
2
1.0307
2.47
3001
E-MINE
220.0
1
1.0279
-1.62
3002
S-MNE
400.0
1
1.0257
-2.31
3003
WEST
220.0
1
1.0121
-2.58
3004
WEST
400.0
1
1.0015
-4.74
3005
UPTOWN
220.0
1
1.0001
-5.78
63
3006
RURAL
220.0
1
0.9816
-5.71
3007
CATDOG
220.0
1
0.9835
-8.13
3008
MINE_G
220.0
3
1.0400
-8.17
3011
CATDOG_G
220.0
2
1.0360
0.00
3018
GEN BUS1
0.7
2
1.1952
-4.19
90001
GEN BUS2
0.7
2
1.1950
8.64
90002
GEN BUS3
0.7
2
1.1948
8.62
90003
GEN BUS4
0.7
2
1.1947
8.56
90004
GEN BUS5
0.7
2
1.1945
8.54
Table 17: Voltages at different buses Table 18 shows the active and reactive power generated by different generating units. It can be seen that power factor of the wind farm is 1, due to which no reactive power is supplied by the WTGs to the Grid. Bus Number
Bus Name
Code
PGen(MW)
QGen(MW)
Voltage (p.u)
101
NUC-A
2
750.0
100.0
1.0221
206
URBGEN
2
800.0
458.8
0.9800
211
HYDRO_GEN
2
600.0
252.2
1.0400
3011
MINE_G
3
367.4
112..3
1.0400
3018
CATDOG_G
2
100.0
80.0
0.9835
90001
GEN BUS1
2
24.0
0.0
1.1952
90002
GEN BUS2
2
24.0
0.0
1.1950
90003
GEN BUS3
2
21.0
0.0
1.1948
90004
GEN BUS4
2
21.0
0.0
1.1947
90005
GEN BUS4
2
21.0
0.0
1.1945
Table 18: Data of Active and Reactive power generated
64
Figure 16 color display of the voltages at the different parts of the power system. Here it can be seen that color at all the buses of the wind farm is red which indicates that voltages at these buses are ver y high as compared to the normal voltage range.
Fig 16: Color display of voltages at buses This is the power flow program which shows the convergence of the solution. The method used was full Newton-Raphson power flow method.
65
SIEMENS POWER TECHNOLOGIES INTERNATIONAL 50 BUS POWER SYSTEM SIMULATOR--PSS(R)E University-33.4.0 INITIATED ON THU, JAN 08 2015 2:26 PSS(R)E PROGRAM APPLICATION GUIDE EXAMPLE BASE CASE INCLUDING SEQUENCE DATA Diagonals = 34 Off-diagonals = 52 Maximum size = 82
ITER DELTA/V/ 0 0.33741( 1 0.14515( 2 0.02800( 3 0.00815( 4 0.15212( 5 0.02969( 6 0.00332( 7 0.00032( 8
DELTAP BUS
BUS DELTAANG
23.1846( 1
)
2.6871( 3018
)
0.2356( 3018
)
0.0015( 3018
)
0.0001( 3018
)
0.0281( 3018
)
0.0028( 206
)
0.0001( 206
)
0.0000(
151
)
0.28003( 151
)
0.11360( 201
)
0.01425( 20
)
0.00050( 3008
)
0.01307( 3018
)
0.00413( 205
)
0.00043( 205
)
0.00004( 205
)
DELTAQ
BUS
BUS 11.6593( 90001
)
3018
)
206
)
206
)
206
)
206
)
)
0.0317( 206
3018
)
0.3256( 206
)
)
1.5351( 3018
206
)
3.5016( 3018
)
)
0.2126( 3018
211
)
1.0717( 90001
)
)
13.6337( 90001
211
)
0.0025(
66
0.00003( 9
206
)
0.0000(
0.00000( 205
206
)
)
0.0002(
206
)
Reached tolerance in 9 iterations
Largest mismatch: -0.00 MW MVA at bus 206 [URBGEN 18.000]
-0.02 Mvar
System total absolute mismatch: MVA
0.02 0.04
SWING BUS SUMMARY: BUS# X-- NAME --X BASKV QGEN QMAX QMIN 3011 MINE_G 112.3 600.0
13.800 -100.0
PGEN
PMAX
PMIN
367.4
900.0
0.0
67
CASE 2 WIND FARM WITH SHUNT REACTOR In case a shunt reactor is connected to the offshore bus to compensate the reactive power. As discussed earlier the value of the shunt reactor is chosen so to obtain the power factor of 0.9 lag at Bus 102 (point of common coupling, PCC). Same Full Newton-Raphson method is used to perform the load flow and the results are obtained. The Active and reactive power flows and voltage at all the buses can be observed.
Fig 17: power Flow results of wind farm with Shunt reactor Figure 18 shows the color display of the voltages at the different parts of the power system. Here it can be seen that color at all the buses of the wind farm changed from dark red to sky blue by the use of the static VAR compensator of proper rating at the offshore bus, which indicates that the voltages are under the normal ranges.
Fig 18: Color display of Voltages at different buses
68
The table 19 shows the voltages at different buses of the power system including that of the wind farm buses. It can be observed that by the use of the shunt reactor at the offshore bus, the voltages are brought below the max voltage limit of 1.05pu. Bus Number
Bus Name
Base KV
Code
Voltage(p.u)
Angle (deg)
1
PARK 1
33.0
1
0.9864
12.99
2
PARK 2
33.0
1
0.9862
12.97
3
PARK 3
33.0
1
0.9860
12.93
4
PARK 4
33.0
1
0.9850
12.90
5
PARK 5
33.0
1
0.9856
12.88
20
BUS OFFSHORE
150.0
1
0.9908
7.23
21
BUS ONSHORE
150.0
1
1.0132
3.14
101
NUC-A
33.0
2
1.0200
3.35
102
NUC-B
33.0
1
0.9855
12.85
151
NUCPANT
400.0
1
1.0267
1.31
152
MD500
400.0
1
1.0134
4.47
153
MD230
220.0
1
0.9961
5.69
154
DOWNTN
220.0
1
0.9700
9.26
201
HYDRO
400.0
1
1.0400
2.89
202
EAST500
400.0
1
1.0196
4.76
203
SUB500
220.0
1
0.9974
7.68
204
SUB230
400.0
1
1.0142
5.01
205
URBGEN
220.0
1
0.9800
8.64
206
HYDRO_G
18.0
2
1.0303
3.66
211
MINE
20.0
2
1.0597
2.60
3001
E-MINE
220.0
1
1.0301
1.63
3002
S-MNE
400.0
1
1.0268
2.31
3003
WEST
220.0
1
1.0248
1.63
69
3004
WEST
400.0
1
1.0095
2.31
3005
UPTOWN
220.0
1
0.9995
2.59
3006
RURAL
220.0
1
0.9973
4.75
3007
CATDOG
220.0
1
0.9799
5.80
3008
MINE_G
220.0
3
0.9522
5.72
3011
CATDOG_G
220.0
2
1.0420
8.15
3018
GEN BUS1
0.7
2
1.0340
8.20
90001
GEN BUS2
0.7
2
0.0040
0.00
90002
GEN BUS3
0.7
2
0.0070
4.21
90003
GEN BUS4
0.7
2
0.0056
13.42
90004
GEN BUS5
0.7
2
0.0040
13.39
Table19: Voltages at different buses including wind farms
The table 20 gives details of the active and reactive power generated by different generators. It is observed that no reactive power is generated by the WTGs and none of the generator is overloaded. Bus Number
Bus Name
Code
PGen(MW)
QGen(MW)
Voltage (p.u)
101
NUC-A
2
750.0
49.3
1.0200
206
URBGEN
2
800.0
481.7
0.9800
211
HYDRO_GEN
2
600.0
122.9
1.0400
3011
MINE_G
3
368
120.6
0.9822
3018
CATDOG_G
2
100.0
80.0
0.9864
90001
GEN BUS1
2
24.0
0.0
0.9864
90002
GEN BUS2
2
24.0
0.0
0.9862
90003
GEN BUS3
2
21.0
0.0
0.9852
90004
GEN BUS4
2
21.0
0.0
0.9859
90005
GEN BUS4
2
21.0
0.0
0.9858
Table 20: Active and Reactive power generated by different generators.
70
This is the power flow program which shows the convergence of the solution. The method used was full Newton-Raphson power flow method.
SIEMENS POWER TECHNOLOGIES INTERNATIONAL 50 BUS POWER SYSTEM SIMULATOR--PSS(R)E University-33.4.0 INITIATED ON THU, JAN 08 2015
2:26
PSS(R)E PROGRAM APPLICATION GUIDE EXAMPLE BASE CASE INCLUDING SEQUENCE DATA
Diagonals = 34
ITER DELTA/V/ 0 0.15868( 1 0.15219( 2 0.02687( 3 0.00775( 4 0.15558( 5 0.03122( 6 0.00347( 7
Off-diagonals = 52
DELTAP BUS 23.1846( 211
)
1.5258( 3018
)
0.2099( 3018
)
0.0011( 3018
)
0.0001( 3018
)
0.0295( 3018
)
0.0030( 206
)
0.0001(
BUS DELTAANG 151
)
0.27989( 151
)
0.04278( 201
)
0.00490( 205
)
0.00047( 3008
)
0.01331( 3018
)
0.00434( 205
)
0.00045( 205
)
Maximum size = 82
DELTAQ
BUS
BUS 9.4961( 90001
3018
)
3018
)
206
)
206
)
206
)
)
0.3419( 206
)
)
1.6009( 3018
206
)
3.6260( 3018
)
)
0.2050( 3018
211
)
1.0471( 21
)
)
12.4269( 20
211
)
0.0349(
71
0.00035( 8 0.00003( 9
206
)
0.0000( 206
)
0.0000(
0.00004( 205
206
)
0.0029(
0.00000( 205
)
206
)
206
)
206
)
)
0.0002(
Reached tolerance in 9 iterations
Largest mismatch: MVA at bus 205 [SUB230
0.00 MW 220.00]
0.02 Mvar
System total absolute mismatch: MVA
0.02 0.05
SWING BUS SUMMARY: BUS# X-- NAME --X BASKV QGEN QMAX QMIN 3011 MINE_G 120.6 600.0
13.800 -100.0
PGEN
PMAX
PMIN
368.0
900.0
0.0
72
CASE3 WIND FARM WITH STATCOM In the third case the strategy which is used is the replacement of the shunt reactor with the STATCOM at the offshore bus for the compensation of the reactive power to keep the voltages closer to the nominal values. The power flows and voltages can be seen in the figure 19.
Fig 19: Power Flow results of Wind Farm with STATCOM Figure 20 shows the color display of the voltages at the different parts of the power system. Here it can be seen that color at all the buses of the wind farm changed from dark red to light green by the use of the static synchronous compensator (STSTCOM) of proper rating at the offshore bus, which indicates that the voltages are under the normal ranges.
Fig 20: Color display of voltages at different buses
73
Table 21 shows the voltages at all the buses of the power system after the use of the STATCOM at the offshore bus for the purpose of reactive power compensation. It can be observed that voltage at all the buses of the wind farm is in the desired range of 0.95pu to 1.05pu. Bus Number
Bus Name
Base KV
Code
Voltage(p.u)
Angle (deg)
1
PARK 1
33.0
1
1.0160
12.25
2
PARK 2
33.0
1
1.0159
12.22
3
PARK 3
33.0
1
0.0156
12..18
4
PARK 4
33.0
1
1.0155
12.16
5
PARK 5
33.0
1
1.0153
12.13
20
BUS OFFSHORE
150.0
1
1.0200
6.81
21
BUS ONSHORE
150.0
1
1.0315
3.09
101
NUC-A
33.0
2
1.0200
3.35
102
NUC-B
33.0
1
1.0151
12.11
151
NUCPANT
400.0
1
1.0276
1.31
152
MD500
400.0
1
1.0139
4.46
153
MD230
220.0
1
0.9964
5.68
154
DOWNTN
220.0
1
0.9701
9.24
201
HYDRO
400.0
1
1.0400
2.87
202
EAST500
400.0
1
1.0197
4.74
203
SUB500
220.0
1
0.9975
7.76
204
SUB230
400.0
1
1.0142
4.99
205
URBGEN
220.0
1
0.9800
8.63
206
HYDRO_G
18.0
2
1.0300
3.64
211
MINE
20.0
2
1.0395
2.77
3001
E-MINE
220.0
1
1.0301
1.62
3002
S-MNE
400.0
1
1.0269
2.31
3003
WEST
220.0
1
1.0249
2.59
3004
WEST
400.0
1
1.0098
4.74
3005
UPTOWN
220.0
1
0.9998
5.76
74
3006
RURAL
220.0
1
0.9977
5.71
3007
CATDOG
220.0
1
0.9801
8.14
3008
MINE_G
220.0
3
0.9824
8.18
3011
CATDOG_G
220.0
-2
1.0400
0.00
3018
GEN BUS1
0.7
-2
1.0350
4.20
90001
GEN BUS2
0.7
-2
1.0160
12.65
90002
GEN BUS3
0.7
-2
1.0158
12.62
90003
GEN BUS4
0.7
-2
1.0156
12.53
90004
GEN BUS5
0.7
-2
1.0153
12.48
Table 21: Voltages at different buses using STATCOM The table 22 gives details of the active and reactive power generated by different generators. It is observed that no reactive power is generated by the WTGs and none of the generator is overloaded. Bus Number
Bus Name
Code
PGen(MW)
Voltage (p.u)
QGen(MW)
101
NUC-A
2
750.0
58.1
1.0200
206
URBGEN
2
800.0
478.8
0.9800
211
HYDRO_GEN
2
600.0
2.9
1.0400
3011
MINE_G
3
368
119.5
1.0200
3018
CATDOG_G
2
100.0
80.0
1.0000
90001
GEN BUS1
2
24.0
0.0
1.0000
90002
GEN BUS2
2
24.0
0.0
1.0000
90003
GEN BUS3
2
21.0
0.0
1.0000
90004
GEN BUS4
2
21.0
0.0
1.0000
90005
GEN BUS4
2
21.0
0.0
1.0000
Table22: Active and Reactive power generated using STATCOM.
75
This is the power flow program which shows the convergence of the solution. The method used was full Newton-Raphson power flow method.
SIEMENS POWER TECHNOLOGIES INTERNATIONAL
50 BUS 33.4.0
POWER
SYSTEM
SIMULATOR--PSS(R)E
INITIATED ON THU, JAN 08 2015
University-
2:21
PSS(R)E PROGRAM APPLICATION GUIDE EXAMPLE BASE CASE INCLUDING SEQUENCE DATA
Diagonals = 34
ITER DELTA/V/ 0 0.14079( 1 0.15348( 2 0.02622( 3 0.02811( 4 0.15556( 5 0.03124( 6
Off-diagonals = 52
DELTAP BUS 23.1846( 211
)
1.3339( 3018
)
0.2089( 3018
)
0.0010( 102
)
0.0030( 3018
)
0.0294( 3018
)
0.0029(
BUS DELTAANG 151
)
0.27984( 151
)
0.04093( 201
)
0.00786( 205
)
0.01292( 21
)
0.01327( 3018
)
0.00436( 205
)
Maximum size = 82
DELTAQ
BUS
BUS 8.4145( 90001
)
3018
)
3018
)
206
)
206
)
)
1.5914( 3018
206
)
3.6299( 3018
)
)
0.1963( 90001
211
)
1.0247( 90001
)
)
12.1903( 20
211
)
0.3405(
76
0.00346( 7 0.00035( 8 0.00003( 9
206
)
0.0001( 206
)
0.0000( 206
)
0.0000(
0.00045( 205
206
)
0.0346(
0.00004( 205
206
)
206
)
206
)
206
)
206
)
)
0.0029(
0.00000( 205
)
)
0.0002(
Reached tolerance in 9 iterations
Largest mismatch: -0.00 MW MVA at bus 206 [URBGEN 18.000]
-0.02 Mvar
System total absolute mismatch: MVA
0.02 0.05
SWING BUS SUMMARY: BUS# X-- NAME --X BASKV QGEN QMAX QMIN 3011 MINE_G 119.5 600.0
13.800 -100.0
PGEN
PMAX
PMIN
367.4
900.0
0.0
77
5.3
DYNAMIC RESULTS
After undergoing the Steady-state simulation and observing the results of that study, dynamic simulations are performed on the grid under analysis. The response of the wind farm and that of the grid to a symmetrical short circuit fault on a grid is analyzed and results are discussed. 5.3.1
Introduction
The analysis of the dynamic behavior of the offshore wind farm comprehends the response of the wind farm to voltage and frequency disturbances in the grid. The study of this work is focused on the response of both the offshore wind farm and the onshore grid to a three-phase fault in an onshore bus, bus 3005, as marked in Figure 21
78
Fig 21: Single line diagram of the grid used for the dynamic simulations. Note: Bus 3005, where the fault occurs is marked in the orange rectangle Nature of Disturbance and clearance time:
For all simulations, the following conditions were applied:
The fault applied is a bolted symmetrical three-phase applied one second after the start of the simulation.
The fault clearing time is 1 second (45 cycles) and the simulation time is 20 seconds.
79
Behavior of the following parameters of the grid and wind farm is analyzed here during and immediately following the disturbance:
Voltage response at different buses and WTGs.
WTG speed response
WTG active power response
WTG reactive power response
Pitch angle response
The analysis is carried out with the objective of assessing the fault ride through capability of the offshore wind farm, i.e. the requirement for the wind farm to stay connected to the grid during the disturbance, thus contributing to the reestablishment of the normal operation. The fault ride through capability of the wind turbines in the offshore wind farm is guaranteed by the under/over voltage disconnection relays of the wind turbine generators. These devices allow the operation of the wind turbines
even when the terminal voltage decreases. This capacity of “riding through” a fault is limited to defined voltage dips and fault durations. So, and according to the implemented fault ride through characteristic (as defined in Figure), the wind turbine will only trip if the fault that occurs across the terminals of the machine are outside the defined limits. Following are the results of different parameters during and after the occurrence of fault: 5.3.2
Response of voltage during the fault at the faulty bus, Bus 9005
After the occurrence of the three-phase short circuit at the Bus 9005, the voltage drop occurs almost in the entire transmission system. More precisely, the closer the location of the fault, the greater will the voltage drop be. This transient voltage drop lasts as long as the time of activity of protective relays in order to isolate the fault. In graph 1, is shown the impact of short circuit fault at the faulted bus. Response of voltage could be see during and after the occurrence of Fault. After 1 sec when the fault occurs, the voltage of bus 9005 drops down to (0 p.u) and after the clearance of fault the voltage restores to its normal value.
80
Graph1: Response of voltage during the fault at the faulty bus, Bus 9005 5.3.3
Response of voltage at offshore bus, Bus 20
Graph 2 shows that during the fault on bus 3005, the 150Kv offshore bus voltage drops to 0.45pu. After fault clearance, the voltage gradually recovers to around 0.98pu.
Graph2: Response of voltage at offshore bus, Bus 20
81
5.3.4
Response of voltage at 400 KV onshore bus, Bus 21:
Graph 3 shows that during the fault on bus 3005, the 150KV offshore bus voltage drops to 0.63pu. After fault clearance, the voltage gradually recovers to around 0.98pu
Graph 3: Response of voltage at 400 KV onshore bus, Bus 21
82
5.3.5
Response of voltage at terminals of WTG1 during the fault:
Graph 4 shows that during the fault at bus 3005, the WTG terminal voltage shows a similar pattern, except that during the fault the minimum terminal voltage is 0.18pu approximated to 0.2pu. The range of terminal voltage drop to other WTGs is from 0.2pu to 0.23pu, depending on location of the installed units. The simulation results indicate that all DFIGs have the ability to ride through the fault, which is in compliance with Wind Grid Code requirements.
Graph4: Response of voltage at terminals of WTG1 during the fault
83
5.3.6
WTG1 active power response during the fault:
Graph 5 shows that during the fault the WTG electrical power output suddenly decreases to a very low value (0.001pu). The difference between the mechanical input power and electrical output power causes an increase in the rotor speed and therefore the rotor starts to accelerate. The torsion oscillation in the drive-train model is reflected in the output power of the wind turbine. Oscillation of power output after the fault is cleared will cause mechanical stress in the drive train system. Approximately ten seconds after the fault is cleared, the power output recovers to the pre-fault value of 16MW.
Graph 5: WTG1 active power response during the fault
84
5.3.7
WTG1 reactive power response during the fault:
During the fault, the rotor speed increases, giving a larger negative slip. This is because the electric power has decreased to almost zero whereas the mechanical power is assumed to be the same. As a result, WT3P module responds by altering the blade pitch to decrease mechanical power. The reactive power output from selected wind turbine generator WTG1 is shown in graph 6. Before the fault occurs, the generated reactive power is near zero and the wind farm operates at unity power factor. It can be seen that each WTG units during the fault provide reactive power support to the grid, as is required by the Wind Grid Code.
Graph 6: WTG1 reactive power response during the fault
85
5.3.8
WTG1 speed response during the fault
Graph 7 shows the generator rotor speed response after the fault occurs. The generator speed is oscillating for about 5 seconds after the fault. 16 seconds after fault is cleared, the rotor speed recovers to the pre-fault value after some 16 seconds.
Graph 7: WTG1 speed response during the fault
86
5.3.9
Pitch angle response after fault event
As can be seen in graph 8, the turbine blade pitch angle is incre ased during the fault in order to reduce the power input from the wind turbine. The pitch angle oscillates in response to the oscillation of the speed of the turbine. For a few seconds after fault clearance, the turbine shaft speed decreases while the pitch angle increases due to the effect of pitch compensation controller trying to reduce the input power to the turbine by increasing the pitch angle.
Graph 8: Pitch angle response after fault event
87
CHAPTER 6
CONCLUSION & RECOMMENDATIONS 6.1
CONCLUSION
Wind power has evolved as a significant renewable energy source for the generation of electrical energy due to the growth of environmental concerns. Large wind farms with several hundred megawatts of rated power have been connected to grid. When wind farms are connected to the existing power system introducing the new power to the system, it offers some challenges like reactive power compensation, fluctuations in active and reactive power and Voltage control strategies. It is therefore necessary to go through steady state and dynamic analysis to map the impacts of newly injected power of wind farms on the existi ng system. Thus the objective of this thesis is to carry out the load flow analysis of the wind farm integrated to the power grid, in order to study their overall impacts on the power system. For this purpose, a wind farm of 110MW having two aggregates of 24MW and three aggregates of 21MW connected offshore to the power grid by means of a power cable. Initially the steady state load flow analysis is carried out considering three cases which are
Wind Farm without compensation
Wind farm with Shunt Reactor
Wind Farm with STATCOM
On the basis of these three Cases the voltages and power Flows at different buses were determined and it was ensured that Reactive power and voltages are in the normal ranges. Then dynamic simulation is carried out by creating a fault at a grid bus for a second in order to determine the fault ride through capability of the wind farm. For all this Steady-State and Dynamic simulation Siemens PTI software Known as PSS®E is used.
88
The results of the simulation are analyzed to study the Impacts of grid interfaced wind farm on the power system. 6.2
RECOMMENDATIONS
In our thesis HVAC transmission system was used to interconnect the offshore wind farm to the power grid and results were studied. For the future work we would recommend that DC link converter could be taken in PSS/E and HVDC transmission model could be developed to interconnect the offshore wind farm to the Grid, and then the results of both HVAC and HVDC transmission systems could be compared to see the impacts of each on the power system.
89
