CHAPTER 1 OVERVIEW OF THE PROJECT
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1.1 INTRODUCTION TO PROJECT
Power quality is certainly a major concern in the present era; it become especially important with the introduction of sophisticated devices, whose performance is very sensitive to the quality of power supply. Modern industrial processes are based a large amount of electronic devices such as programmable logic controllers and adjustable speed drives. The electronic devices are very sensitive to disturbances and thus industrial loads become less tolerant to power quality problems such as voltage dips, voltage swells, and harmonics. Voltage dips are considered one of the most severe disturbances to the industrial equipment. A paper machine can be affected by disturbances of 10% voltage drop lasting for 100ms. A voltage dip of 75% (of the nominal voltage) with duration shorter than 100ms can result in material loss in the range of thousands of US dollars for the semiconductors industry. Swells and over voltages can cause over heating tripping or even destruction of industrial equipment such as motor drives. Electronic equipments are very sensitive loads against harmonics because their control depends on either the peak value or the zero crossing of the supplied voltage, which are all influenced by the harmonic distortion. This project analyzes the key issues in the Power Quality problems, specially keeping in mind the present trend towards more localized generations (also termed as distributed and dispersed generation) and consequent restructuring of power transmission and distribution networks. As one of the prominent power quality problems, the origin, consequences and mitigation techniques of voltage sag problem has been discussed in detail. The study describes the technique of correcting the supply voltage sag in a distribution system by power electronics based device called Distribution STATCOM (D-STATCOM). D-STATCOM injects a current into the system to correct the voltage sag. The steady state performance of DSTATCOM is studied for various levels of voltage sag levels.
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CHAPTER 2 HARMONICS IN POWER SYSTEM
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2.1 INTRODUCTION TO HARMONICS
One of the biggest problems in power quality aspects is the harmonic content in the electrical system. Generally, Generally, harmonics may be be divided into two types: types: 1) voltage harmonics, and and 2) current harmonics. Current harmonics is usually generated by harmonics contained in voltage supply and depends on the type of load such as resistive load, capacitive load, and inductive load. Both harmonics can be generated by either the source or the load side. Harmonics generated by load are caused by nonlinear operation of devices, including power converters, arc-furnaces, gas discharge lighting devices, etc. Load harmonics can cause the overheating of the magnetic cores of transformer and motors. On the other hand, source harmonics are mainly generated by power supply with non-sinusoidal voltage waveform. Voltage and current source harmonics imply power losses, Electromagnetic Interference (EMI) and pulsating torque in AC motor drives. Any periodic waveform can be shown to be the superposition of a fundamental and a set of harmonic components. By applying Fourier transformation, these components can be extracted. The frequency of each harmonic component is an integral multiple of its fundamental. There are several methods to indicate of the quantity of harmonics contents. The most widely used measure in North America is the total harmonics distortion (THD), which is defined in terms of the amplitudes of the harmonics, Hn, at frequency nw0, 2 where w0 is frequency of the fundamental component H1 and n is integer. The THD is mathematically given by whose amplitude of H1
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2.2 ADVANTAGES OF MULTI LEVEL POWER INVERTER:
1. The multi level power inverter is a strong candidate topology for the future naval ship propulsion systems considering advantages over traditional inverters 2. Lower switching losses 3. Higher voltage capability , 4. Higher power quality ,and 5. They are suitable for medium to high power applications. 6. They are an ideal inter phase between a utility and renewable energy sources such as photovoltaic or fuel cells. 7. Their efficiency is very high (>98%) because of the minimum switching frequency 8. They can improve the power quality and dynamic stability for utility systems. 9. Switching stress and EMI are low. 10. Because of their modular and simple structure, they can be stacked up to an
almost unlimited number of levels .
2.3 AN INTRODUCTION TO POWER SYSTEM HARMONICS
The objective of the electric utility is to deliver sinusoidal voltage at fairly constant magnitude throughout their system. This objective is complicated by the fact that there are loads on the system that produce harmonic currents. These currents result in distorted voltages and currents that can adversely impact the system performance in different ways. As the number of harmonic producing loads has increased over the years, it has become increasingly necessary to address their influence when making any additions or changes to an Installation. To fully appreciate the impact of this phenomenon, there are two important concepts to bear in mind with regard to power system harmonics. The first is the nature of harmonic-current producing loads (non-linear loads) and the second is the way in which harmonic currents flow and how the resulting harmonic voltages develop.
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2.4 Linear and non-linear loads
A linear element in a power system is a component in which the current is proportional to the voltage. In general, this means that the current wave shape will be the same as the voltage (See Figure2.1). Typical examples of linear loads include motors, heaters and incandescent lamps. – Voltageandcurrentwaveformsforlinear
Figure2.1
On the other hand, the current wave shape on a non-linear load is not the same 16 as the voltage (See Figure2.2). Typical examples of non-linear loads include rectifiers (power supplies, UPS units, discharge lighting), adjustable speed motor drives, ferromagnetic devices, DC motor drives and arcing equipment
Figure 2.2 – Voltage and current waveforms for non-linear loads The current drawn by non-linear loads is not sinusoidal but it is periodic, meaning that the current wave looks the same from cycle to cycle. Periodic waveforms can be described mathematically as a series of sinusoidal waveforms that have been summed together (See Figure 2.3). The sinusoidal components are integer multiples of the fundamental where the fundamental, in the United States, is 60 Hz. The only way to measure a voltage or current that
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contains harmonics is to use a true-RMS reading meter. If an averaging meter is used, which is the most common type, the error can be Significant.
Figure 2.3 Waveform with symmetrical harmonic component Each term in the series is referred to as a harmonic of the fundamental. The third harmonic would have a frequency of three times 60 Hz or 180 Hz. Symmetrical waves contain only odd harmonics and un-symmetrical waves contain even and odd harmonics. A symmetrical wave is one in which the positive portion of the wave is identical to the negative portion of the wave. An un-symmetrical wave contains a DC component (or offset) or the load is such that the positive portion of the wave is different than the negative portion. An example of unsymmetrical wave would be a half wave rectifier. Most power system elements are symmetrical. They produce only odd harmonics and have no DC offset. There are exceptions, of course, and normally symmetrical devices may produce even harmonics due to component mismatches or failures. Arc furnaces are another common source of even harmonics but they are notorious for producing both even and odd harmonics at different stages of the process.
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2.5 Harmonic current flow
When a non-linear load draws current, that current passes through all of the impedance that is between the load and the system source (See Figure2.4). As a result of the current flow, harmonic voltages are produced by impedance in the system for each harmonic.
Figure 2.4 – Distorted-current induced voltage distortion
These voltages sum and when added to the nominal voltage produce voltage distortion. The magnitude of the voltage distortion depends on the source impedance and the harmonic voltages produced. If the source impedance is low then the voltage distortion will be low. If a significant portion of the load becomes non-linear (harmonic currents increase) and/or when a resonant condition prevails (system impedance increases), the voltage can increase dramatically. Power systems are able to absorb a considerable amount of current distortion without problems and the distortion produced by a facility may be below levels recommended in IEEE 519. However, the collective effect of many industrial customers, taken together, may impact a distribution system. When problems arise, they are usually associated with resonant conditions.
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2.6 Harmonic currents can produce a number of problems, namely:
o Equipment heating o Equipment malfunction o Equipment failure o Communications interference o Fuse and breaker mis-operation o Process problems o Conductor heating
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CHAPTER 3 VOLTAGE DIPS
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3.1 INTRODUCTION TO VOLATAGE DIPS A voltage dip is a short-term reduction in, or complete loss of, RMS voltage. It is specified in terms of duration and retained voltage, usually expressed as the percentage of nominal RMS voltage remaining at the lowest point during the dip. A voltage dip means that the required energy is not being delivered to the load and this can have serious consequences depending on the type of load involved. Voltage sags – longerterm reductions in voltage – are usually caused by a deliberate reduction of voltage by the supplier to reduce the load at times of maximum demand or by an unusually weak supply in relation to the load. Motor drives, including variable speed drives, are particularly susceptible because the load still requires energy that is no longer available except from the inertia of the drive. In processes where several drives are involved individual motor control units may sense the loss of voltage and shut down the drive at a different voltage level from its peers and at a different rate of deceleration resulting in complete loss of process control. Data processing and control equipment is also very sensitive to voltage dips and can suffer from data loss and extended downtime. The cost implications are very serious and are discussed in Section 2. There are two main causes of voltage dips; starting of large loads either on the affected site or by a consumer on the same circuit and faults on other branches of the network.
3.2 Dips caused by large loads
When heavy loads are started, such as large drives, the starting current can be many times t he normal running current. Since the supply and the cabling of the installation are dimensioned for normal running current the high initial current causes a voltage drop in both the supply network and the installation. The magnitude of the effect depends on how „strong‟ the network is, that is, how low the impedance is at the 22 point of common coupling (PCC) and on the impedance of the installation cabling. Dips caused by starting currents are characterized by being less deep and much longer than those caused by network faults – typically from one to several seconds or tens of seconds, rather than less than one second. On-site problems, caused by too high resistance in the internal cabling, are easily dealt with. Large loads should be wired directly back to the origin of the appropriate voltage level – either the PCC or the secondary of the supply transformer. If the problem is caused by the impedance of the PCC – i.e. the supply is too „weak‟– then further 11
action is required. One solution, if applicable to the equipment in question, is to fit a soft starter so that the starting current is limited to a lower value but is required for rather longer. Another solution is to negotiate with the supply company for a lower impedance connection – but this may be expensive depending on the geography of the network in the area.
Fig. 3.1 The cause of voltage dips If the cause of the voltage reduction cannot be controlled, then other equipment will be needed to compensate for it. Suitable equipment ranges from the traditional servo controlled mechanical stabilizers to electronically controlled tap changers and dynamic voltage restorers
3.3 Dips network originating from faults
The supply network is very complex. The extent of a voltage dip at one site due to a fault in another part of the network depends on the topology of the network and the relative source impedances of the fault, load and generators at their common point of coupling. Figure 1 shows an example. A fault at position F3 results in a dip to 0 % at Load 3, a dip to 64 % at Load 2 and to 98 % at Load 1. A fault at F1 will affect all users with a dip to 0 % at Load 1 and to 50 % for all other loads. Notice that a fault at Level 1 affects many more consumers more severely than a fault at Level 3. Loads connected at Level 3 are likely to experience 12
many more dips than a load connected at Level 1 because there are more potential fault sites – they are affected by Level 1 and level 2 faults. Loads at Level 2 and 1 are progressively less sensitive to faults at Level 3. The„closer‟ the load is to the source, the f ewer and the less severe the dips will be. The duration of the dip depends on the time taken for the protective circuits to detect and isolate the fault and is usually of the order of a few hundred milliseconds. Since faults can be transitory, for example when caused by a tree branch falling onto a line, the fault can be cleared very soon after it has occurred. If the circuit were to be permanently disconnected by the protection equipment then all consumers on the circuit would experience a blackout until the line could be checked and reconnected. Auto reclosers can help to ease the situation, but also cause an increase in the number of dips. An autorecloser attempts to reconnect the circuit a short time (less than 1 second) after the protection equipment has operated. If the fault has cleared, the autoreclose will succeed and power is restored. Loads on that circuit experience a 100 % dip between disconnection and autoreclose while other loads see a smaller, shorter dip between the fault occurring and being isolated, as discussed above. If the fault has not cleared when 24 the autorecloser reconnects, the protective equipment will operate again; the process can be repeated according to the program set for the particular autorecloser. Each time the autorecloser reconnects the faulty line another dip results, so that other consumers can experience several dips in series. Utility performance in deregulated markets is partly - in some countries, such as UK, solely - judged on the average „customer minutes lost‟, taking into account interruptions exceeding, typically, one minute. Minimizing this statistic has resulted in the widespread application of autoreclosers and an increase in the probability of dips. In other words, long term availability has been maximized but at the expense of quality.
3.4 Equipment sensitivity
Computers are now essential to all businesses, whether as workstations, network servers or as process controllers. They are vital to all data processing transactions and many communications functions, such as email and voice box systems. It was the introduction of computer equipment that first highlighted the problem of voltage dips (in fact, most power quality problems) and early installations were plagued with seemingly random failures that resulted in considerable support effort being required. The learning process resulted in the production of the Computer and Business Equipment Manufacturers Association (CBEMA) curve (Figure 2). This curve has since been modified and is now known as the Information Technology Industry Council (ITIC) curve (Figure 3) 13
and a version of it has been standardized by ANSI as IEEE 446 (Figure 4). Duration of an event is plotted against voltage with respect to the nominal supply voltage and the curves define the envelope within which equipment should continue to function without interruption or data loss. As far as dips are concerned it is the lower limit line that is of interest. This line represents the boundary between survivable and non-survivable dips. In an ideal world there would be just one curve that represented real-world supply network performance and to which all equipment would comply. In fact, while quite a lot of equipment meets the requirement of one or other of the standard curves, the performance of supply networks falls far short
Fig.3.2 CBEMA curve
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Fig.3.3 ITIC curve
3.4
ANSI
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curve
3.5 Characteristics of equipment sensitivity
Electronic equipment power supplies, such as those used in personal computers (PC) and programmable logic controllers (PLC) employ a reservoir capacitor to smooth out the peaks of the full wave rectified waveform, so they should be inherently resilient to short duration dips. The larger the capacitor, and the greater the difference between the stored capacitor voltage and the minimum required for the internal voltage converters to operate, the better t he resilience will be. Designers will always try to reduce the size of the capacitor to a minimum to reduce size, weight and cost while ensuring that the charge stored is just sufficient at minimum voltage and maximum load. For good dip resilience a much larger capacitor is required, at least twice as large to enable the equipment to ride through one cycle, and 100 times as large if a one-second 27 ride through was required. An alternative design strategy is to keep the minimum input voltage as low as possible to maximize the hold up time of the system.
This is the approach taken, by default, in equipment designed to work over a wide range of voltage. The hold up time will be much greater with a 230 V supply than it will be with a 110 V supply. There is no technical problem in making a dip resistant power supply but it is not done because it is not an issue that users raise with manufacturers and there are cost implications. Nevertheless, the cost of making a PC or PLC resilient to 1 second dips is very small compared to the cost of improving the network assets to prevent such a dip occurring. Variable speed drives can be damaged by voltage dips and are usually fitted with under voltage detectors that trip at 15 % to 30 % below nominal voltage. Variable speed drives with enhanced ride through capability are the subject of a later section of this Guide.
Induction motors have inertia so they may help to support the load during a short dip, regenerating energy as they slow down. This energy has to be replaced as the motor reaccelerates and, if the speed has reduced to less than 95 %, it will draw nearly the full start-up current. Since all the motors are „starting‟ together, this may be the cause of further problems. Relays and contactors are also sensitive to voltage dips and can often be the weakest link in the system. It has been established that a device may drop out during a dip even when the retained voltage is higher than the minimum steady state hold-in voltage. The resilience of a contactor to dips depends not only on the retained voltage and duration but also on the point on the waveform where the dip occurs, the effect being less at the peak. Sodium discharge
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lamps have a much higher striking voltage when hot than cold, so that a hot lamp may not restart after a dip.
The magnitude of dip that will cause a lamp to extinguish may be as little as 2 % at the end of life or as high as 45 % when new. Most appliances and systems incorporate one or more of the above elements, and so will exhibit problems when subject to dips. Figure 5, below, suggests that it is cheaper and more reliable to design equipment to be resilient to dips, rather than to try to make the whole process, whole plant, or the whole electricity distribution system resilient. As shown here, the cost of solution increases rapidly as the point of cure is moved from equipment through plant to infrastructure.
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CHAPTER 4 SOLUTIONS TO POWER QUALITY PROBLEMS
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4. SOLUTIONS TO POWER QUALITY PROBLEMS
4.1 INTRODUCTION
There are two approaches to the mitigation of power quality problems. The solution to the power quality can be done from customer side or from utility side . First approach is called load conditioning, which ensures that the equipment is less sensitive to power disturbances, allowing the operation even under significant voltage distortion. The other solution is to install line conditioning systems that suppress or counteracts the power system disturbances. A flexible and versatile solution to voltage quality problems is offered by active power filters. Currently they are based on PWM converters and connect to low and medium voltage distribution system in shunt or in series. Series active power filters must operate in conjunction with shunt passive filters in order to compensate load current harmonics. Shunt active power filters operate as a controllable current source and series active power filters operates as a controllable voltage source. Both schemes are implemented preferable with voltage source PWM inverter s [5], with a dc bus having a reactive element such as a capacitor. Active power filters can perform one or more of the functions required to compensate power systems and improving power quality. Their performance also depends on the power rating and the speed of response. However, with the restructuring of power sector and with shifting trend towards distributed and dispersed generation, the line conditioning systems or utility side solutions will play a major role in improving the inherent supply quality; some of the effective and economic measures can be identified as following.
4.1.1 Thyristor Based Static Switches:
The static switch is a versatile device for switching a new element into the circuit when the voltage support is needed. It has a dynamic response time of about one cycle.To correct quickly for voltage spikes, sags or interruptions, the static switch can used to switch one or more of devices such as capacitor, filter, alternate power line, energy storage systems etc. The static switch can be used in the alternate power line applications. This scheme requires two
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independent power lines from the utility or
could be from utility and localized power
generation like those in case of distributed generating systems . Such a scheme can protect up to about 85 % of interruptions and voltage sags.
4.1.2 Energy Storage Systems :
Storage systems can be used to protect sensitive production equipments from shutdowns caused by voltage sags or momentary interruptions. These are usually DC storage systems such as UPS, batteries, superconducting magnet energy storage (SMES), storage capacitors or even fly wheels driving DC generators . The output of these devices can be supplied to the system through an inverter on a momentary basis by a fast acting electronic switch. Enough energy is fed to the system to compensate or the energy that would be lost by the voltage sag or interruption. In case of utility supply backed by a localized generation this can be even better accomplished.
4.1.3 Electronic tap changing transformer:
A voltage-regulating transformer with an electronic load tap changer can be used with a single line from the utility. It can regulate the voltage drops up to 50% and requires a stiff system (short circuit power to load ratio of 10:1 or better). It can have the provision of coarse or smooth steps intended for occasional voltage variations. 4.1.4 Harmonic Filters
Filters are used in some instances to effectively reduce or eliminate certain harmonics. If possible, it is always preferable to use a 12-pluse or higher transformer connection, rather than a filter. Tuned harmonic filters should be used with caution and avoided when possible. Usually, multiple filters are needed, each tuned to a separate harmonic. Each filter causes a parallel resonance as well as a series resonance, and each filter slightly changes the resonances of other filters.
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4.1.5 Constant-Voltage Transformers:
For many power quality studies, it is possible to greatly improve the sag and momentary interruption tolerance of a facility by protecting control circuits. Constant voltage transformer (CVTs) can be used [6] on control circuits to provide constant voltage with three cycle ride through, or relays and ac contactors can be provided with electronic coil hold-in devices to prevent disoperation from either low or interrupted voltage.
4.1.6 Digital-Electronic and Intelligent Controllers for Load-Frequency Control:
Frequency of the supply power is one of the major determinants of power quality, which affects the equipment performance very drastically. Even the major system components such as Turbine life and interconnected-grid control are directly affected by power frequency. Load frequency controller used specifically for governing power frequency under varying loads must be fast enough to make adjustments against any deviation. In countries like India and other countries of developing world, still use the controllers which are based either or mechanical or electrical devices with inherent dead time and delays and at times also suffer from ageing and associated effects. In future perspective, such cont rollers can be replaced by their Digital – electronic counterparts. ] 4.2 USE OF CUSTOM POWER DEVICES TO IMPROVE POWER QUALITY
In order to overcome the problems such as the ones mentioned above, the concept of custom power devices is introduced recently; custom power is a strategy, which is designed primarily to meet the requirements of industrial and commercial customer. The concept of custom power is to use power electronic or static controllers in the medium voltage distribution system aiming to supply reliable and high quality power to sensitive users. Power electronic valves are the basis of those custom power devices such as the static transfer switch, active filters and converter-based devices. Converter based power electronics devices can be divided in to two groups: shunt connected and series-connected devices. The shunt connected devices is known as the DSTATCOM and the series device is known as the Thyristor Controlled Swiched Capacitor(TCSC). It has also been reported in literature that both the TCSC and DSTATCOM have been used to mitigate 21
the majority the power system disturbances such as voltage dips, sags, flicker unbalance and harmonics,stability problems. For lower voltage sags, the load voltage magnitude can be corrected by injecting only reactive power into the system. However, for higher voltage sags, injection of active power, in addition to reactive power, is essential to correct the voltage magnitude. Both TCSC and DSTATCOM are capable of generating or absorbing reactive power but the active power injection of the device must be provided by an external energy source or energy storage system. The response time of both TCSC and DSTATCOM is very short and is limited by the power electronics devices. The expected response time is about 25 ms, and which is much less than some of the traditional methods of voltage correction such as tap - changing transformers.
4.3 MODELING OF CUSTOM POWER DEVICE AND SIMULATION RESULTS
As mentioned in the previous section that custom power devices could be the effective means to overcome some of the major power quality problems by the way of injecting active and/or reactive power(s) into the system. This section deals with the modeling of DSTATCOM . Consequently some case studies will be taken up for analysis and performance comparison of these devices. The modeling approach adopted in the paper is graphical in nature, as opposed to mathematical models embedded in code using a high-level computer language. The well-developed graphic facilities available in an industry standard power system package, namely, MATLAB (/Simulink) [12], is used to conduct all aspects of model implementation and to carry out extensive simulation studies. The control scheme for these devices is shown in Fig.1. The controller input is an error signal obtained from the reference voltage and the value rms of the terminal voltage measured. Such error is processed by a PI controller and the output is the angle δ, which is provided to the PWM signal generator. The PWM generator then generates the pulse signals to the IGBT gates of voltage source converter
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Fig 4.1 The PI Controller
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CHAPTER 5 DISTRIBUTION STATCOM
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5.1 INTRODUCTION TO DSTATCOM This chapter presents the operating principles of DSTATCOM. The DSTATCOM is basically one of the custom power devices. It is nothing but a STATCOM but used at the Distribution level. The key component of the DSTATCOM is a power VSC that is based on high power electronics technologies. The Distribution STATCOM is a versatile device for providing reactive compensation in ac networks. The control of reactive power is achieved via the regulation of a controlled voltage source behind the leakage impedance of a transformer, in much the same way as a conventional synchronous compensator. However, unlike the conventional synchronous compensator, which is essentially a synchronous generator where the field current is used to adjust the regulated voltage, the DSTATCOM uses an electronic voltage sourced converter (VSC), to achieve the same regulation task. The fast control of the VSC permits the STATCOM to have a rapid rate of response. The DSTATCOM is the solid – state based power converter version of the SVC. Operating as a shunt – connected SVC, its capacitive or inductive output currents can be controlled independently from its connected AC bus voltage. Because of the fastswitching characteristic of power converters, the DSTATCOM provides much faster response as compare to SVC. DSTATCOM is a shunt connected, reactive compensation equipment, which is capable of generating and or absorbing reactive power whose output can be varied so as to maintain control of specific parameters of the electric power system. DSTATCOM provides operating characteristics similar to a rotating synchronous compensator without mechanical inertia, due to the DSTATCOM employ solid state power switching devices it provides rapid controllability of the three phase voltages, both in magnitude and phase angle. In addition, in the event of a rapid change in system voltage, the capacitor voltage does not change instantaneously; therefore the DSTATCOM reacts for the desired responses. For example, if the system voltage drops for any reason, there is a tendency for the DSTATCOM inject capacitive power to support the dipped voltages.
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5.2 Operating Principle of the DSTATCOM
Basically, the DSTATCOM system is comprised of three main parts: a VSC, a set of coupling reactors and a controller. The basic principle of a DSTATCOM installed in a power system is the generation of a controllable ac voltage source by a voltage source inverter (VSI) connected to a dc capacitor (energy storage device). The ac voltage source in general, appears behind a transformer leakage reactance. The active and reactive power transfer between the power system and the DSTATCOM is caused by the voltage difference across this reactance. The DSTATCOM is connected to the power networks at a PCC, where the voltage-quality problem is a concern. All required voltages and currents are measured and are fed into the controller to be compared with the commands. The controller then performs feedback control and outputs a set of switching signals to drive the main semiconductor switches (IGBT‟s, which are used at the distribution level) of the power converter accordingly. The basic diagram of the DSTATCOM is illustrated in Fig 5.1.
Fig 5.1 Block Diagram of the voltage source converter based DSTATCOM The ac voltage control is achieved by firing angle control. Ideally the output voltage of the VSI is in phase with the bus (where the DSTATCOM is connected) voltage. In steady state, the dc side capacitance is maintained at a fixed voltage and there is no real power exchange, except for losses. The DSTATCOM differs from other reactive power generating devices (such as shunt Capacitors, Static Var Compensators 26
etc.) in the sense that the ability for energy storage is not a rigid necessity but is only required for system unbalance or harmonic absorption. There are two control objectives implemented in the DSTATCOM. One is the ac voltage regulation of the power system at the bus where the DSTATCOM is connected and the other is dc voltage control across the capacitor inside the DSTATCOM. It is widely known that shunt reactive power injection can be used to control the bus voltage. In conventional control scheme, there are two voltage regulators designed for these purposes: ac voltage regulator for bus voltage control and dc voltage regulator for capacitor voltage control. In the simplest strategy, both the regulators are proportional integral (PI) type controllers. Thus, the shunt current is split into d-axis and q-axis components. The reference values for these currents are obtained by separate PI regulators from dc voltage and ac-bus voltage errors, respectively. Then, subsequently, these reference currents are regulated by another set of PI regulators whose outputs are the d-axis and q-axis control voltages for the DSTATCOM.
5.3 Principle of Voltage Regulation 5.3.1 Voltage Regulation without Compensator:
Consider a simple circuit as shown in Fig 5.2. It consists of a source Voltage E, V is the voltage at a PCC and a load drawing the current Il. Without a voltage compensator[8], the PCC voltage drop caused by the load current Il, shown in fig as ΔV, s l ΔV = E −V = Z * I , S =VI * ,so S* =V * I
The voltage change has a component ΔVr in phase with V and component ΔVx, which are illustrated in Fig 5.2(a). It is clear that both magnitude and the phase of V, relative to the supply voltage E, are functions of the magnitude and phase of the load current namely the voltage drop depends on both the real and reactive power of the load. The component ΔV is rewritten as ΔV = I R + jI X
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Fig 5.2 A Simple Circuit for demonstrating the voltage regulation principle.
Fig 5.2 (a) Phasor diagram for uncompensated
5.3.2 Voltage regulation with DSTATCOM:
Now consider a compensator connected to the system. It is as shown in Fig 5.2(b) shows vector diagram with voltage compensation. By adding a compensator in parallel with the load, it is possible to make
E = V
by controlling the current of the compensator.
Is =Ir + Il Where Ir is the compensating current.
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Fig 5.2(b) Phasor diagram for voltage regulation with compensation
5.4 APPLICATIONS OF DSTATCOM Typical STATCOM applications: • Utilities with weak grid knots or fluctuating reactive loads • Unbalanced loads • Arc furnaces • Wind farms • Wood chippers • Welding operations • Car crushers & shredders • Industrial mills • Mining shovels & hoists • Harbor cranes
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CHAPTER 6 MODELING OF DSTATCOM
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6.1 DSTATCOM VOLTAGE REGULATION TECHNIQUE The DSTATCOM improves the voltage sags and swell conditions and the ac output voltage at the customer points is improved, thus improving the quality of power at the distribution side In this thesis the voltage controller technique (also called as decouple technique) is used as the control technique for DSTATCOM. The method is already discussed in the previous topic. This control strategy uses the dq0 rotating reference frame because it offers higher accuracy than stationary frame-based techniques. In this VABC are the three-phase terminal voltages, I abc are the three-phase currents injected by the DSTATCOM into the network, Vrms is the root-mean-square (rms) terminal voltage, V dc is the dc voltage measured in the capacitor, and the superscripts indicate reference values. Such a controller employs a phase-locked loop (PLL) to synchronize the three phase voltages at the converter output with the zero crossings of the fundamental component of the phase-A terminal voltage. The block diagram of a proposed control technique is shown in Fig 6.4. Therefore, the PLL provides the angle φ to the abc-to-dq0 (and dq0-to-abc) transformation. There are also four proportional-integral (PI) regulators.
The first one is responsible for controlling the terminal voltage through the reactive power exchange with the ac network. This PI regulator provides the reactive current reference Iq*, which is limited between +1pu capacitive and -1pu inductive. Another PI regulator is responsible for keeping the dc voltage constant through a small active power exchange with the ac network, compensating the active power losses in the transformer and inverter. This PI regulator provides the active current reference Id*. The other two PI regulators determine voltage reference V d*, and Vq*, which are sent to the PWM signal generator of the converter, after a dq0-to-abc transformation. Finally, Vabc* are the three-phase voltages desired at the converter output.
6.2 IMPEMENTATION OF D-STATCOM The test system employed to carry out the simulations concerning the DSTATCOM actuation for voltage sag compensation is shown in Fig.6.1 Such system is composed by a 230 kV, 50 Hz transmission system, represented by a Thevenin equivalent, feeding a distribution network through a 3-winding transformer connected in Y/Y/Y, 230/11/11 kV. To verify the working of a DSTATCOM, a variable load is connected at bus 2. During the simulation, in the period from 500 to 900 ms, the switch S1 is closed .The above test system is simulated under the environment of Matlab - Simulink and power system block set (PSB) the model used for this purpose is shown in the fig.6.2.
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Fig 6.1 block diagram DSTATCOM
Fig6.2. MATLAB Simulation model of test system with DSTATCOM
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A set of simulations was carried out for the test system shown in Fig.6.5. The simulations relate to three main operating conditions. 1) In the simulation period 500 – 900 ms, the load is increased by closing switch S1. In this case, the voltage drops by almost 27% with respect to the reference value. 2) At 900 ms, the switch S1 is opened and remains so throughout the rest of the simulation. The load voltage is very close to the reference value, i.e., 1 pu. 3) In order to gain insight into the influence that capacitor size has on D STATCOM performance, simulations were carried out with different size of capacitors. The total simulation period is 1.4 s. The rms voltage at the load point for the case when the system operates with no D-STATCOM. Similarly, a new set of simulations was carried out but now with t he D-STATCOM connected to the system. Where the very effective voltage regulation provided by the D-STATCOM can be clearly appreciated.When the Switch S1 closes, the D-STATCOM supplies reactive power to the system, shows the regulated rms voltage corresponding to a 750 F capacitor, where a rapid regulation response is obtained and transient overshoots are almost nonexistent. This contrasts with cases where the capacitor is undersized. For instance the rms voltage for the case when a 75 F capacitor is employed.
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CHAPTER 7 RESULTS AND ANALYSIS
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7.1 DSTATCOM RESULTS FOR TEST SYSTEM:7.1.1 Voltage response of the test system without DSTATCOM:Fig7.1 Voltage response of the test system without DSTATCOM
Fig.7.1. shows the rms voltage at the load point for the case when the system operates with no D-STATCOM. It has been seen observed that there is voltage dip when the load is increased by closing the circuit breaker of fig.6.6.
71.2 Voltage response of the Test system with DSTATCOM (With 750uf capacitor)
Fig 7.2(a) Voltage response of the test system with DSTATCOM (With 750uf capacitor) Fig. 7.2(a) shows voltage response of the te st system with D-STATCOM (With 750uf capacitor). The result shows that, the D-STATCOM supplies reactive power to the system and compensated the voltage dip to maintain constant voltage profile. It also shows that a rapid regulation response is obtained and transient overshoots are almost nonexistent with a 750uf capacitor.
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7.1.3 Voltage response of the test system with DSTATCOM (With 75uf capacitor)
Fig7.2(b) Voltage response of the test system with DSTATCOM (With 75uf capacitor) Fig. 7.2(b) shows voltage response of the t est system with D-STATCOM (With 75uf capacitor). The result shows that, the D-STATCOM supplies reactive power to the system and compensated the voltage dip to maintain constant voltage profile. It also shows that a voltage regulation response is obtained with little transient overshoots using a 75uf capacitor. From the above diagrams it is proved that DSTATCOM reduces voltage dips and so that it improves the power quality.
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37
LITERATURE SURVEY
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LITERATURE SURVEY
The last decade has seen a marked increase on the deployment of end user equipment that is highly sensitive to poor quality control electricity supply. Several large industrial users are reported to have experienced large financial losses as a result of even minor lapses in the quality of electricity supply . A great many efforts have been made to remedy the situation, where the solutions based on the use of latest power electronic technology figure prominently. Indeed custom power technology, the low voltage counterpart of the more widely known flexible as transmission system (FACTS) technology, aimed at high voltage power transmission applications, has emerged as a credible solution to solve many of the problems relating to continuity of supply at the end user level. The various power quality Problems at the Distribution level are voltage sag and swells, fluctuations, harmonics, flickering etc . Recently, various power electronic technology devices have been proposed especially to be applied to medium voltage networks, generally named custom power. Custom power concept introduced by N.G.Hingorani has been proposed to ensure high quality of power supply in distribution networks using power electronics devices. Additionally, various custom power devices are based on the voltage source converter technology introduced by N.G.Hingorani and L.Gyugyi . At present, wide range of very flexible controllers, which capitalize on newly available power electronics components, are emerging for custom power applications. Among these the Distribution static Compensator (DSTATCOM) and dynamic voltage restorer (DVR), both of them based on the VSC principle given by L.Gyugyi , and the SSTS are the controllers which have received the most attention. The modeling and analysis of these custom power devices has applied for the study of power quality by Olimpo Anaya-Lara and E Acha
presenting
comprehensive results to assess the performance of each device as a potential custom power application. The different control techniques of DSTATCOM are discussed in papers . SungMin Woo, Dae- wook kang, Woo-Chol Lee, Dong-Seok Hyun, have demonstrated a new control technique for 4 reducing effect of Voltage Sag and Swell with DSTATCOM . In this Thesis the DSTATCOM is simulated with Voltage Regulation Technique . The interest in distributed generation has considerably increased due to market deregulation, technological advances, governmental incentives, and environment impact concerns . At present, most distributed generation Installations employ induction and synchronous machines, which can
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be used in thermal, hydro, and wind generation plants . Although such technologies are well known, there is no consensus on what is the best choice under a wide technical perspective. In the paper by Prof.Mrs. P.R.Khatri, Prof.Mrs. V.S.Jape, Prof.Mrs. N.M.Lokhande, Prof.Mrs. B.S.Motling, they discussed the main problems associated with DG and also how to interface the DG to the utility systems. M. I. Marei, E. F. El-Saadany and M. M. A. Salama, in their work dealt with the Flexible Distributed Generation proposed a novel control scheme for the nonlinear link connecting DG to the distribution network using a current controlled Voltage Source Inverter (VSI).
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CONCLUSION Power quality measures can be applied both at the user end and also at the utility level. The work identifies some important measures that can be applied at the utility level without much system upset (or design changes).This project has presented models of custom power equipment, namely D -STATCOM, and applied them to mitigate voltage dip which is very prominent as per utilities are concerned. The highly developed graphic facilities available in MATLAB/ SIMULINK were used to conduct all aspects of model implementation and to carry out extensive simulation studies on test system. A new PWM-based control scheme has been implemented to control the electronic valves in the two – level VSC used in the D-STATCOM . As opposed to fundamental frequency switching schemes already available in the MATLAB/SIMULINK. This characteristic makes it ideally suitable for low-voltage custom power applications. It was observed that in case of DSTATCOM capacity for power compensation and voltage regulation depends mainly on the rating of the dc storage device. It can be concluded from the TCSC results that the impedance can be varied from capacitive mode to inductive mode so that the voltage can be raised during heavy inductive load periods and the voltage can be reduced to within limits so that voltage is controlled. By using TCSC no harmonics are introduced. Since the supply current is pure sinusoidal stability can also be improved by varying the reactance.
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REFERENCES
H. Hingorani “Introducing custom power” IEEE spectrum, vol.32 no.6
June 1995 p 41-48.
Ray Arnold “Solutions to Power Quality Problems” power engineering journal 2001 pages: 65-73.
John Stones and Alan Collinsion “Introduction to Power Quality” power engineering journal
2001 pages: 58 -64.
Gregory F. Reed, Masatoshi Takeda, "Improved power quality solutions using advanced solid-
state switching and static compensation technologies," PowerEngineering Society 1999 Winter Meeting, IEEE
G. Venkataramanan and B. Johnson, “A pulse width modulated power line conditioner for
sensitive load centers,” IEEE Trans. Power Delivery, vol. 12, pp. 844– 849, Apr. 1997.
N.G. Hingorani and L. Gyugyi, “Understanding FACTS: Concepts and Technology of Flexible
AC Transmission Systems”, 1st edit ion, The Institute of Electrical and Electronics Enginee rs,2000.
F. Z. Peng, H. Akagi, and A. Nabae, “Compensation characteristics of the combined system of
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M.H.Haque “Compensation of distribution system voltage sag by DVR and DSTATCOM”
Power Tech Proceedings, 2001 IEEE Porto , Volume: 1 , 10-13 Sept. 2001 Pages:5 pp. vol.1
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