A REPORT ON SUMMER TRAINING AT PGCIL
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POWER GRID CORPORATION OF INDIA LIMITED (COMPANY PROFILE):Introduction:
Govt.. of Indi Indiaa Ente Enterp rpri rise se,, was was esta establ blis ishe hed d in 1989 1989 unde underr the the Indi Indian an POWERGRID, a Govt Comp Compan anie iess Act Act 1956 1956,, with with an init initia iall auth author oriz ized ed share share capi capita tall of Rs. Rs. 50, 50, 000 000 mill millio ion. n. POWERGRID, the Central Transmission Utility (CTU) of the country, is one of the largest
and best-managed transmission utilities in the world with its huge transmission network spread over the entire length and breadth of the country. POWERGRID consistently maintaining the availa availabil bility ity of its transm transmiss ission ion system system above above 99.50% 99.50% level level throug through h deploy deploymen mentt of latest latest Operation and Maintenance techniques at par with global standards. Mission:-
“Establishment and operation of Regional and National Power Grids to facilitate transfer of pow power er with within in and and acro across ss the the Regi Region onss with with reli reliab abil ilit ity, y, secu securit rity y and and econ econom omy, y, on soun sound d commercial principles.” Objective:-
The Corporation has set following objectives in line with its Mission and its status as "Central Transmission Utility":
Undertake transmission of energy through Inter-State Transmission System.
Discharge all functions of planning and coordination relating to Inter-State Transmission System witho
State Transmission Utilities;
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Central Government;
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State Government;
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Generating Companies;
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Regional Electricity Boards; 2
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Authority;
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Licensees;
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Transmission Licensees;
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Any other person notified by the Central Government on this behalf.
Exercise supervision and control over the Inter-State Transmission T ransmission System.
Efficient Operation and Maintenance of Transmission Systems.
Establish/augment and operate all Regional Load Dispatch Centers and Communication facilities.
To facilitate private sector participation on Transmission system through Independent Private Transmission Company, Joint Ventures.
To assist various SEBs and other utilities in up gradation of skills & sharing of expertise by organizing organizing regular conference conferences, s, tailor-made tailor-made training training workshops workshops directed directed towards towards specific technological and O&M areas and extending laboratory facilities for testing purposes etc.
Restoring power in quickest possible time in the event of any natural disasters like supercyclone, flood etc. through deployment of Emergency Restoration Systems.
Developmental Developmental Stages:
The phased development of POWERGRID at the time of its formation was foreseen as follows:
Phase-I: Transfer of Transmission facilities along with related manpower from Central / Centre–State Joint Venture Organizations.
Phase-II: Transfer of existing Regional Electricity Boards and Regional Load Dispatch Centers together with associated communication facilities. f acilities.
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Phase-III: Establish Power Pool to facilitate exchange of power between States/Regions leading to formation of National Power Grid.
Establishment Establishment of Transmission Transmission System
Powe Powerr Grid Grid Corp Corpor orat atio ion n of Indi Indiaa Ltd. Ltd. acqu acquire ired d its its init initia iall netw networ ork k of asse assets ts in 1992 1992 and and subsequently through the Power Transmission Systems Ordinance the Government of India acquired and transferred the power transmission infrastructure of four of India’s largest power gene genera rati ting ng comp compan anie iess to Powe Powerg rgri rid. d. Ther Therea eafte fter, r, tran transm smis issi sion on asse assets ts from from othe otherr cent centra rall gene genera rati ting ng comp compan anie iess were were also also tran transf sferr erred ed to us and and POWE POWERG RGRI RID D has has subs subseq eque uent ntly ly expanded the transmission infrastructure further all over India. I ndia. Transmission of electricity is defined as bulk transfer of power over a long distance at a high voltage, generally of 132 kV and above. In India, transmission lines have grown from 3,708 km in 1950 to more than 265,000 km now. There are 5 regional grids:
Northern Region: Delh Delhi, i, Harya Haryana na,, Hima Himach chal al Prade Pradesh sh,, Jammu Jammu And And Kash Kashmi mir, r, Punj Punjab ab,, Raja Rajast stha han, n, Uttaranchal and Uttar Pradesh
Eastern Region: Bihar, Jharkhand, Orissa Sikkim and West Bengal.
Western Region: Dadra and Nagar Haveli, Daman and Diu, Chhattisgarh, Goa Gujarat, Madhya Pradesh and Maharashtra.
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Southern Region: Andhra Pradesh, Karnataka, Kerala, Pondicherry and Tamil Nadu.
North-Eastern Region: Arunachal Pradesh, Assam, Manipur, Meghalaya, Mizoram, Nagaland and Tripura
POWERGRID’s network, as at September, 2008, comprises of over 69,000 circuit km of high voltage transmission lines and 116 sub-stations spread across the country. The inter-regional power transfer capacity of National Grid has been enhanced to about 17,000 MW from 14,100 MW in FY 2007-08. POWERGRID has further plans to enhance the capacity to more than 37,000 MW by 2012. POWERGRID has envisaged an investment program of Rs.55000 crore towards investment in transmission projects during the Government of INDIA’s eleventh five year plan beginning April 1, 2007 and ending on March 31, 2012. In FY 2007-08, it commissioned transmission projects worth about Rs. 6000 crore, thereby adding transmission network of 7,350 circuit kms, 9 EHV AC sub-stations and transformation capacity of more than 13700MVA. POWER GRID: NETWORK CAPABILITIES
Particulars
2005
2006
2007
2008
Transmission network (ckt kms)
50,745
55,120
59,461
66,800
Substations (number)
85
93
104
112
Transformation Capacity (MVA)
49,442
54,377
59,417
73,100
99.74
99.64
99.20
99.52
System Availability
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High Voltage Direct Current(HVDC) Introduction A high-voltage direct current (HVDC) electric power transmission system uses direct current for the bulk transmission of electrical power, in contrast with the more common alternating current systems. For long-distance distribution, HVDC systems are less expensive and suffer lower electrical losses. For shorter distances, the higher cost of DC conversion equipment compared to an AC system may be warranted where other benefits of direct current links are useful. The modern form of HVDC transmission uses technology developed extensively in the 1930s in Sweden at ASEA. Early commercial installations included one in the Soviet Union in 1951 between Moscow and Kashira, and a 10-20 MW system in Gotland, Sweden in 1954.
High Voltage Transmission High voltage is used for transmission to reduce the energy lost in the resistance of the wires. For a given quantity of power transmitted, higher voltage reduces the transmission power loss. Power in a circuit is proportional to the current, but the power lost as heat in the wires is proportional to the square of the current. However, power is also proportional to voltage, so for a given power level, higher voltage can be traded off for lower current. Thus, the higher the voltage, the lower the power loss. Power loss can also be reduced by reducing resistance, commonly achieved by increasing the diameter of the conductor; but larger conductors are heavier and more expensive. High voltages cannot be easily used in lighting and motors, and so transmission-level voltage must be reduced to values compatible with end-use equipment. The transformer, which only works with alternating current, is an efficient way to change voltages. Practical manipulation of DC voltages only became possible with the development of high power electronic devices such 6
as mercury arc valves and later semiconductor devices, such as thyristors, insulated-gate bipolar transistors (IGBTs), high power capable MOSFETs (power metal–oxide–semiconductor fieldeffect transistors) and gate turn-off thyristors (GTOs)
Advantages and Limitations of alternating current transmission The advantage of HVDC is the ability to transmit large amounts of power over long distances with lower capital costs and with lower losses than AC. Depending on voltage level and construction details, losses are quoted as about 3% per 1,000 km. High-voltage direct current transmission allows efficient use of energy sources remote from load centers. In a number of applications HVDC is more effective than AC transmission. Examples include:
Undersea cables, where high capacitance causes additional AC losses.
Endpoint-to-endpoint long-haul bulk power transmission without intermediate 'taps', for example, in remote areas
Increasing the capacity of an existing power grid in situations where additional wires are difficult or expensive to install
Power transmission and stabilization between unsynchronised AC distribution systems
Connecting a remote generating plant to the distribution grid
Stabilizing a predominantly AC power-grid, without increasing prospective short circuit current
Reducing line cost. HVDC needs fewer conductors as there is no need to support multiple phases. Also, thinner conductors can be used since HVDC does not suffer from the skin effect
Facilitate power transmission between different countries that use AC at differing voltages and/or frequencies
Synchronize AC produced by renewable energy sources
Long undersea cables have a high capacitance. While this has minimal effect for DC transmission, the current required to charge and discharge the capacitance of the cable causes additional I 2R power losses when the cable is carrying AC. In addition, AC power is lost to dielectric losses. 7
HVDC can carry more power per conductor, because for a given power rating the constant voltage in a DC line is lower than the peak voltage in an AC line. In AC power, the root mean square (RMS) voltage measurement is considered the standard, but RMS is only about 71% of the peak voltage. The peak voltage of AC determines the actual insulation thickness and conductor spacing. Because DC operates at a constant maximum voltage without RMS, this allows existing transmission line corridors with equally sized conductors and insulation to carry 29% more power into an area of high power consumption than AC, which can lower costs. Because HVDC allows power transmission between unsynchronised AC distribution systems, it can help increase system stability, by preventing cascading failure from propagating from one part of a wider power transmission grid to another. Changes in load that would cause portions of an AC network to become unsynchronized and separate would not similarly affect a DC link, and the power flow through the DC link would tend to stabilize the AC network. The magnitude and direction of power flow through a DC link can be directly commanded, and changed as needed to support the AC networks at either end of the DC link. This has caused many power system operators to contemplate wider use of HVDC technology for its stability benefits alone
Disadvantages The disadvantages of HVDC are in conversion, switching and control. Further operating an HVDC scheme requires keeping many spare parts, which may be used exclusively in one system as HVDC systems are less standardized than AC systems and the used technology changes fast. The required static inverter are expensive and have limited overload capacity. At smaller transmission distances the losses in the static inverters may be bigger than in an AC transmission line. The cost of the inverters may not be offset by reductions in line construction cost and lower line loss. In contrast to AC systems, realizing multiterminal systems is complex, as is expanding existing schemes to multiterminal systems. Controlling power flow in a multiterminal DC system requires good communication between all the terminals; power flow must be actively regulated by the control system instead of by the inherent properties of the transmission line. High voltage DC circuit breakers are difficult to build because some mechanism must be included in the circuit breaker to force current to zero, otherwise arcing and contact wear would be too great to allow reliable switching. Multi-terminal lines are rare
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Costs of high voltage DC transmission
Normally manufacturers such as AREVA, Siemens and ABB do not state specific cost information of a particular project since this is a commercial matter between the manufacturer and the client. Costs vary widely depending on the specifics of the project such as power rating, circuit length, overhead vs. underwater route, land costs, and AC network improvements r equired at either terminal. A detailed evaluation of DC vs. AC cost may be required where there is no clear technical advantage to DC alone and only economics drives the selection.
Rectifying and inverting Components Early static systems used mercury arc rectifiers, which were unreliable. The thyristor valve was first used in HVDC systems in the 1960s. The thyristor is a solid-state semiconductor device similar to the diode, but with an extra control terminal that is used to switch the device on at a particular instant during the AC cycle. The insulated-gate bipolar transistor(IGBT) is now also used and offers simpler control and reduced valve cost. Because the voltages in HVDC systems, up to 800 kV in some cases, exceed the breakdown voltages of the semiconductor devices, HVDC converters are built using large numbers of semiconductors in series. Though 800kV systems are still to be realised in India. The low-voltage control circuits used to switch the thyristors on and off need to be isolated from the high voltages present on the transmission lines. This is usually done optically. In a hybrid control system, the low-voltage control electronics sends light pulses along optical fibres to the high-side control electronics. Another system, called direct light triggering, dispenses
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with the high-side electronics, instead using light pulses from the control electronics to switch light-triggered thyristors (LTTs).
Rectifying and inverting system Rectification and inversion use essentially the same machinery. Many substations are set up in such a way that they can act as both rectifiers and inverters. At the AC end a set of transformers, often three physically separate single-phase transformers, isolate the station from the AC supply, to provide a local earth, and to ensure the correct eventual DC voltage. The output of these transformers is then connected to a bridge rectifier formed by a number of valves. The basic configuration uses six valves, connecting each of the three phases to each of the two DC rails. However, with a phase change only every sixty degrees, considerable harmonics remain on the DC rails. An enhancement of this configuration uses 12 valves (often known as a twelve-pulse system). The AC is split into two separate three phase supplies before transformation. One of the sets of supplies is then configured to have a star (wye) secondary, the other a delta secondary, establishing a thirty degree phase difference between the two sets of three phases. With twelve valves connecting each of the two sets of three phases to the two DC rails, there is a phase change every 30 degrees, and harmonics are considerably reduced. In addition to the conversion transformers and valve-sets, various passive resistive and reactive components help filter harmonics out of the DC rails.
The theory of the HVDC converter The considerations are restricted on the line commutated converter which so far has dominantly been used for HVDC systems. Although forced commutated converters have occasionally been proposed for very special applications such as tapping of an HVDC line, the thyristor based converter is still the only economical and well proven solution for bulk power transmission.
The Three-pulse Commutation group
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Of the numerous converter configurations which have come into use for a wide variety of applications, HVDC technology uses exclusively the three phase bridge circuit. In many respects this is optimal converter connection. The three-phase bridge consists of two three-pulse commutation groups connected in series. The star point loading is of no consequence, since it disappears when the circuit is expanded into the six pulse bridge. The starpoint loading has been taken into account by means of delta connection on the primary side. A symmetrical ac network with no impedance and with no impedance and with sinusoidal voltage U L is assumed, as is generally customary in converter theory. In addition, a completely smooth direct current (I d) effected by a smoothing reactor with infinite inductance (l d) is also assumed.
The Commutation process In actuality, commutation of direct current requires a certain amount of time. This is due to leakage inductances of the converter transformer, which only permit a current change of limited steepness. Thus for a short period of time, the releasing and the receiving phases are carrying current simultaneously. This is referred to as commutation overlap and its duration is defined as the overlap angle U. The leakage inductances are considered lumped elements on the valve side of the transformer. Assume that valve 1 carries the direct current and that at an arbitrary point in time (after the voltage intersection) valve 3 receives a control impulse. A current loop will be created with U v as the driving voltage. The leakage inductances of phases 1 and 3 will be the reactances which determine the current. This is simply a phase to phase short circuit of the transformer. The short circuit current flows through valve 3 in the forward direction and through valve 1 counter to the forward direct current. As soon as the short circuit current has achieved the amplitude of the direct current (the composite current is zero), valve 1 extinguishes. At this point, the commutation process has ended and valve 3 is carrying the entire direct current. Curve of the direct voltage during commutation is along the average value of the voltages of valves 1 and 3.
Basic HVDC converter control concepts The converter valves, as precise and virtually delay-free control elements, are the most important actuators of the HVDC control system. Moreover, in most cases the converter groups have an additional actuator in the form of the transformer tap changer. Although it does not 11
operate on a continuous basis and there are relatively long periods of idle time, it nevertheless is responsible for important control function.
Current control Current control mainly determines •
Steady state transmission power
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Changes in transmission power according to size and rate of change
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The dynamic behavior of the system including temporary overload
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Limitation of transient overcurrents determined by amplitude and duration
The loading of all essential components of an HVDC system, with the exception of filter circuits, is determined by the direct current or an alternating current proportional to the direct current. Therefore current control is also a very essential protective function. In HVDC two point systems, the rectifier generally assumes the task of the current control. It is occasionally advantageous to assign the current control function to the inverter. However, since the current control of the rectifier is needed as a proactive function, it is advantageous to also use it for this purpose during normal operation. Then it is always active and monitors itself.
Configurations Monopole and earth return In a common configuration, called monopole, one of the terminals of the rectifier is connected to earth ground. The other terminal, at a potential high above, or below, ground, is connected to a transmission line. The earthed terminal may or may not be connected to the corresponding connection at the inverting station by means of a second conductor. If no metallic conductor is installed, current flows in the earth between the earth electrodes at the two stations. Therefore it is a type of single wire earth return. The issues surrounding earthreturn current include 12
Electrochemical corrosion of long buried metal objects such as pipelines
Underwater earth-return electrodes in seawater may produce chlorine or otherwise affect water chemistry. An unbalanced current path may result in a net magnetic field, which can affect magnetic navigational compasses for ships passing over an underwater cable.
These effects can be eliminated with installation of a metallic return conductor between the two ends of the monopolar transmission line. Since one terminal of the converters is connected to earth, the return conductor need not be insulated for the full transmission voltage which makes it less costly than the high-voltage conductor.
BIPOLAR In bipolar transmission a pair of conductors is used, each at a high potential with respect to ground, in opposite polarity. Since these conductors must be insulated for the full voltage, transmission line cost is higher than a monopole with a return conductor. However, there are a number of advantages to bipolar transmission which can make it the attractive option. Under normal load, negligible earth-current flows, as in the case of monopolar transmission with a metallic earth-return. This reduces earth return loss and environmental effects. When a fault develops in a line, with earth return electrodes installed at each end of the line, approximately half the rated power can continue to flow using the earth as a return path, operating in monopolar mode.
Since for a given total power rating each conductor of a bipolar line carries only half the current of monopolar lines, the cost of the second conductor is reduced compared to a monopolar line of the same rating.
In very adverse terrain, the second conductor may be carried on an independent set of transmission towers, so that some power may continue to be transmitted even if one line is damaged.
A bipolar system may also be installed with a metallic earth return conductor. Bipolar systems may carry as much as 3,200 MW at voltages of +/-600 kV. Submarine cable installations initially commissioned as a monopole may be upgraded with additional cables and operated as a Bipole. 13
Corona Discharge Corona discharge is the creation of ions in a fluid (such as air) by the presence of a strong electric field. Electrons are torn from neutral air, and either the positive ions or else the electrons are attracted to the conductor, while the charged particles drift. This effect can cause considerable power loss, create audible and radio-frequency interference, generate toxic compounds such as oxides of nitrogen and ozone, and bring forth arcing. Both AC and DC transmission lines can generate coronas, in the former case in the form of oscillating particles, in the latter a constant wind. Due to the space charge formed around the conductors, an HVDC system may have about half the loss per unit length of a high voltage AC system carrying the same amount of power. With monopolar transmission the choice of polarity of the energised conductor leads to a degree of control over the corona discharge. In particular, the polarity of the ions emitted can be controlled, which may have an environmental impact on particulate condensation. (Particles of different polarities have a different mean-free path.) Negative coronas generate considerably more ozone than positive coronas, and generate it further downwind of the power line, creating the potential for health effects. The use of a positive voltage will reduce the ozone impacts of monopole HVDC power lines.
Applications Overview The controllability of current-flow through HVDC rectifiers and inverters, their application in connecting unsynchronized networks, and their applications in efficient submarine cables mean that HVDC cables are often used at national boundaries for the exchange of power. Offshore windfarms also require undersea cables, and their turbines are unsynchronized.
AC network interconnections AC transmission lines can only interconnect synchronized AC networks that oscillate at the same frequency and in phase. Many areas that wish to share power have unsynchronized networks. However, HVDC systems make it possible to interconnect unsynchronized AC networks, and also add the possibility of controlling AC voltage and reactive power flow. A generator connected to a long AC transmission line may become unstable and fall out of synchronization with a distant AC power system. An HVDC transmission link may make it 14
economically feasible to use remote generation sites. Wind farms located off-shore may use HVDC systems to collect power from multiple unsynchronized generators for transmission to the shore by an underwater cable. In general, however, an HVDC power line will interconnect two AC r egions of the powerdistribution grid. Machinery to convert between AC and DC power adds a considerable cost in power transmission. The conversion from AC to DC is known as rectification and from DC to AC as inversion. Above a certain break-even distance (about 50 km for submarine cables, and perhaps 600–800 km for overhead cables), the lower cost of the HVDC electrical conductors outweighs the cost of the electronics.
Transmission Planning Criteria Introduction The objective of system planning is to evolve a power system with a level of performance characterised by an acceptable degree of adequacy and security based on a trade-off between costs and risks involved. Insofar as power transmission systems are concerned , there are no widely adopted uniform guidelines which determine the criteria for transmission planning vis-svis acceptable degree of adequacy and security. The criteria generally depends on the factors such as availability of generation vis-à-vis demand, voltage levels, and configuration of the system, control and communication facilities and resource constraints. Practices in this regard vary from country to country. The common theme in the various approaches is the “acceptable system performance”. Even though the factors affecting system performance are probabilistic in nature, deterministic approach has been used most commonly, being rather easy to apply. For adopting probabilistic approach, long operating experience and availability of reliable statistical data regarding performance of system components, namely equipment failure rate, outage duration, etc, are essential. Such data are presently being compiled by a few utilities; but these are still inadequate to go in for a totally probabilistic approach. Hence it is considered prudent to adopt a deterministic approach for the present with a committed thrust towards progressive adoption of probabilistic approach.
Planning Philosophy
The transmission system shall be planned on the basis of regional self-sufficiency with an ultimate objective of evolving a National Power Grid. The regional self-sufficiency 15
criteria based on load generation balance may still dictate to have inter-regional exchanges with adequate inter-connection capacity at appropriate points taking into account the topology of the two networks, the plant mix consideration, generation shortages due to forced outages, diversity in weather pattern and load forecasting errors in either regions. Such inter-regional power exchanges shall also be considered these studies.
The system shall be evolved based on detailed power system studies which shall include:1. Power Flow Studies 2. Short Circuit Studies 3. Stability Studies 4. EMTP Studies to determine switching/temporary overvoltages .
The adequacy of the transmission should be rested for different feasible load generation scenarios.
The following options may be considered for strengthening of the transmission network:1. Addition of new Transmission lines to avoid overloading of existing system. (Whenever three or more circuits of the same voltage class are envisaged between two sub-stations, the next transmission voltage should also be considered.) 2. Application of Series Capacitors in existing transmission line to increase power transfer capability. 3. Upgradation of the existing AC transmission lines. 4. Reconductoring of the existing AC transmission line with higher size conductors or with AAAC. 5. Adoption of multi-voltage level and multi-circuit transmission lines. The choice shall be based on cost, reliability, right-of-way requirements, energy losses, down time (in case of upgradation and reconductoring options)
In case of generating station close to a major load centre, sensitivity of its complete closure with loads to be met(to the extent possible)from other generating stations is also studied. 16
In case of transmission system associated with Nuclear Power Stations there shall be two independent sources of power supply for the purpose of providing start-up power facilities. Further the angle between start-up power source and the NPP switchyard should be, as far as possible, maintained within 10 degrees.
The evacuation system for sensitive power stations viz., Nuclear power stations shall generally be planned so as to terminate it at large load centres to facilitate islanding of the power station in case of contingency. Contingency is the temporary removal of one or more system elements from service. The cause or reason for such removal may be a fault , planned maintenance/repair etc.
1. Single Contingency – The contingency arising out of removal of one system element from service. 2. Double Contingency – The contingency arising out of removal of two system elements from service. It includes a D/C line, two S/C lines in same corridor or different corridors, a S/C line and a transformer etc. 3. Rare Contingency – Temporary removal of complete generating station or complete substation (including all the incoming & outgoing feeders and transformers ) from service, HVDC bipole and stuck breaker condition.
Where only two circuits are planned for evacuation of power from a generating station, these should be two single lines instead of a double circuit line.
Reactive power flow through ICTs shall be minimal. Normally it shall not exceed 10% of the rating of the ICTs. Whenever voltage on HV side of ICT is less than 0.975 pu, no reactive power shall flow through ICT.
Thermal/Nuclear Generating units shall normally not run at leading power factor. However, for the purpose of charging, generating unit may be allowed to operate at leading power factor as per the respective capability curve.
Inter-regional links shall, in the present context, be planned as asynchronous ties unless otherwise permitted from operational consideration. 17
Load Generation Scenarios The load-generation scenarios shall be worked out so as to reflect in a pragmatic manner the daily and seasonal variations in the load demand and generation availability.
Load demands The profile of annual and daily demands will be determined from past data. These data will usually give the demand at grid supply points and for the whole system identifying the annual and daily peak demand.
Active power The system peak demands shall be based on the latest reports of Electric Power Survey (EPS) Committee. In case these peak load figures are more than the peaking availability, the loads will be suitably adjusted substation wise to match with the availability. The load demands at other periods (seasonal variations and minimum loads)shall be derived based on the annual peak demand and past pattern of load variations. From practical considerations the load variations over the year shall be considered as under:1. Annual Peak Load – It is the simultaneous maximum demand of the system being studied. It is based on latest Electric Power Survey (EPS) or total peaking power availability, whichever is less. 2. Seasonal variation in Peak loads(corresponding to high thermal and high hydro generation) 3. Minimal load – It is the expected minimum system demand and is determined from average ration of annual peak load and minimum load observed in the system for the last 5 years. 4. Off-Peak Load relevant where Pumped Storage Plants are involved or inter-regional exchanges are envisaged.
Reactive Power (MVAR) 18
Reactive Power plays an important role in EHV transmission system planning and hence forecast of reactive power demand on an area-wise or substation-wise basis is as important as active power forecast. This forecast would obviously require adequate data on the reactive power demands at different substations as well as the projected plans for reactive power compensation. For developing an optimal power system, the utilities must clearly spell out the substationwise maximum and minimum demand in MWs and MVARs on seasonal basis. This will require compilation of past data in order to arrive at reasonably accurate load forecast. Recognising the fact that this data is presently not available, it is suggested that pending availability of such data, the load power factor at 120/132KV voltage levels shall be taken as 0.85 lag during peak load condition and 0.9 lag during light load condition expecting areas feeding predominantly agricultural loads where power factor can be taken as 0.75 and 0.85 for peak load and light load conditions respectively. In areas where power factor is less than the limit specified above, it shall be the responsibility of the respective utility to bring the load power factor to these limits by providing shunt capacitors at appropriate places in the system.
Generation Despatches Generation despatch of Hydro and Thermal/Nuclear units would be determined judiciously on the basis of hydrology as well as scheduled maintenance program of the generating stations. Various norms are used for working out the peaking availability of different types of generating units. In case of nuclear units the minimum level of output shall be taken as not less than 70% of the rated capacity. Generation dispatches corresponding to the following operating conditions shall be considered depending on the nature and characteristics of the system. •
Annual Peak Load
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Maximum Thermal generation – It is the condition when hydro generation is low(not necessarily minimum)and thermal generation is kept maximum to meet seasonal peak loads(not necessarily annual peak load).In other words it is the condition when the gap between monthly peak demand and hydro power availability is maximum.
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Maximum Hydro generation – It is the condition when hydro power availability is maximum during the year. It is also known as High Hydro condition.
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Annual Minimum Load
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Special area dispatches – It is the condition when power output from all the generating stations located in an area (in close proximity) is kept at the maximum feasible level. Maximum Feasible level of a generating station is the maximum power output when all the units in a power station are in service, assuming no planned or forced outages. However, in case of power station/complex where six or more units exist, for every six units one unit –second largest-is assumed to be under annual planned maintenance.
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Special dispatches corresponding to high agricultural load with low power factor, wherever applicable. Off peak conditions with maximum pumping load where Pumped S torage stations exist and also with the inter-regional exchanges, if envisaged. Complete closure of a generating station close to a major load centre.
The generation dispatch for purpose of carrying out sensitivity studies corresponding to complete closure of generating station close to a major load centre shall be worked out by increasing generation at other stations to the extent possible keeping in view the maximum likely availability at these stations, ownership pattern, shares etc.
Permissible Line Loading Limits •
Permissible line loading limit depend on many factors such as voltage regulation, stability and thermal capacity etc. Thermal capacity of a line refers to the amount of 20
current that can be carried by a line conductor without exceeding its design operating temperature. Surge Impedance Loading (SIL) means a unit power factor load over a resistance line such that series reactive loss (I^2*R) along the line is equal to shunt capacitive gain (V^2*Y). Under these conditions the sending end and receiving end voltages and current are equal in magnitude but different in phase position. While SIL gives a general idea of the loading capability of the line , it is usual to load the short lines above SIL and long lines lower than SIL (because of the stability limitations).line loading can also be shown (in terms of surge impedance loading of uncompensated line)as a function of line length assuming a voltage regulation of 5% and phase angular difference of 30 degrees between the two ends of the line. In case of shunt compensated lines, the SIL will get reduced by a factor k, where k = sqrt (1-degree of compensation)
For lines whose permissible line loading as determined from the curve is higher than the thermal loading limit, permissible loading limit shall be restricted to thermal loading limit. •
Thermal loading limits are generally decided by design practice on the basis of ambient temperature, maximum permissible conductor temperature, wind velocity, etc. In India, the ambient temperatures obtaining in the various parts of the country are different and vary considerably during the various seasons of the year. Designs of transmission line with ASCR conductors in EHV systems will normally be based on a conductor temperature limit of 75 deg Celsius. However, for some of the existing lines which have been designed for a conductor temperature of 65 deg Celsius the loading shall be correspondingly reduced. In the case of AAAC conductors, maximum conductor temperature limit will be taken as 85 deg Celsius.
Temporary Overvoltages 21
These are power frequency overvoltages produced in a power system due to sudden load rejection, single-phase-to-ground faults etc. 420 kV system 1.5 p.u. peak phase to neutral (343 kV = 1 p.u.) 800 kV system 1.4 p.u. peak phase to neutral (653 kV = 1 p.u.)
Switching Overvoltages These overvoltages generated during switching of lines, transformers and r eactors etc. having wave fronts 250/2500 micro sec. 420 kV system 2.5 p.u. peak phase to neutral (343 kV = 1 p.u.) 800 kV system 1.9 p.u. peak phase to neutral (653 kV = 1 p.u.)
Reactive Power Compensation
Shunt Capacitors Reactive Compensation should be provided as far as possible in the low voltage systems with a view to meeting the reactive power requirements of load close to the load points thereby avoiding the need for VAR transfer from the high voltage system to the low voltage system. In the cases where network below 132/220 kV voltage level is not represented in the system planning studies, the shunt capacitors required for meeting the reactive power requirements of load shall be provided at 132/220 kV buses.
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Shunt Reactors Switchable reactors shall be provided at EHV substations for controlling voltages within the limits defined without resorting to switching-off of lines. The size of reactors should be such that under steady state condition, switching on and off of the reactors shall not cause a voltage change exceeding 5%. The standard sizes(MVAR) of reactors are:-
400 kV (3-ph units)
50, 63 & 80 at 420 kV 22
765 kv (1-ph units)
50, 63 & 110 at 800 kV
•
Fixed line reactors may be provided to control Temporary Power Frequency overvoltage [after all voltage regulation has taken place] within the limits defined , under all probable operating conditions.
•
Line reactors (switchable/controlled/fixed) may be provided if it is not possible to charge EHV line without exceeding the voltage limits defined. The possibility of reducing precharging voltage of the charging end shall also be considered in the context of establishing the need for reactors.
Static VAR Compensation (SVC) Static VAR compensation shall be provided where found necessary to damp the power swings and provide the system stability under conditions defined. The dynamic range of static compensators shall not be utilised under steady state operating conditions as far as possible.
Sub-Station Planning Criteria •
•
The requirements in respect of EHV sub-stations in a system such as the total load to be catered by the sub-station of a particular voltage level, its MVA capacity, number of feeders permissible etc. are important to the planners so as to provide an idea to them about the time for going in for the adoption of next higher voltage level sub-station and also the number of substations required for meeting a particular quantum of load. Keeping these in view the following criteria have been laid down for planning an EHV substation: The maximum fault level on any new substation bus should not exceed 80% of the rated rupturing capacity of the circuit breaker. The 20% margin is intended to take care of the
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increase in short-circuit levels as the system grows. The rated breaking current capability of switchgear at different voltage levels may be taken as:-
•
•
132 kV
--
25/31 kA
220 kV
--
31.5/40 kA
400 kV
--
40 kA
765 kV
--
40 kA
Higher breaking current capability would require major design change in the terminal equipment and shall be avoided as far as possible. The capacity of any single sub-station at different voltage levels shall not normally exceed :-
765 kV
--
2500 MVA
400 kV
--
1000 MVA
220 kV
--
320 MVA
132 kV
--
150 MVA
•
Size and number of interconnecting transformers (ICTs) shall be planned in such a way that the outage of any single unit would not overload the remaining ICT(s) or the underlying system.
•
A stuck breaker condition shall not cause disruption of more than four feeders for 220 kV system and two feeders for 400 kV system and one feeder for 765 kV system.
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Transmission Engineering
Major Components of Transmission Lines •
Conductor
•
Tower Design and foundation
•
Earth wire
•
Insulators
•
Hardware Fittings
•
Accessories
CONDUCTORS 25
BUNDLE CONDUCTOR SELECTION AND OPTIMIZATION
Size, Type and Configuration of conductor influences:-
•
Tower and its geometry
•
Foundations
•
Optimum spans
•
Rating and configuration of Insulator string
•
Insulator Swings
•
Ground clearance
•
Line interferences like electric field at ground, corona, radio & TV interference, audible noise etc.
CONDUCTOR SELECTION SCENARIO
•
SCENARIO A
Selection of character for a transmission line of identified voltage level and specified minimum power flow but power flow capacity becomes ruling factor in selection of conductor size (low voltage lines). 26
•
SCENARIO B
Selection of conductor for a transmission line with identified voltage level and a specified minimum power flow but voltage level become ruling factor in selection of conductor/conductor bundle size (EHV/UHV lines).
•
SCENARIO C
Selection of conductor for high power capacity long distances transmission lines where selection of voltage level and conductor/conductor bundle size are to be done together to obtain most optimum solution (HVDC Bipole)
CONDUCTOR BUNDLE SELECTION METHODOLOGY
•
Primary set of conductor bundle/sizes identified to start optimization
•
Parameters like insulation requirement, limits for corona, RIV, AN, thermal ratings, line losses and statutory clearances identified
•
Detailed analysis of various alternatives in respect of following to be carried out to select the configuration -
Basic insulation design and insulator selection Tower configuration analysis
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Tower weight and foundation analysis
-
Capital line cost analysis and span optimization 27
-
Line loss calculation
-
Economic evaluations(PWRR) of alternatives
-
Comparison of interference performance
-
Cost sensitivity analysis
CONDUCTOR OPTMIZATION PROCEDURE
PRIMARY SELECTION •
Thermal rating of the conductor/conductors
•
Manufacturing facilities
•
Expense of other utilities
•
System voltage alternatives
•
Construction convenience
•
Line loss considerations
•
Terrain conditions and ground profile
•
Span length requirements
•
Right Of Way limitations
CONDUCTOR SELECTION – DESIGN CONSIDERATIONS
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BASIC CONSIDERATIONS (NON VARIABLE) 1) Loading condition and reliability level for the transmission line. 2) Insulator co-ordination 3) Limit load condition for structure, conductor, insulator, and hardware as well as limit conditions for swing of conductor and insulator strings. 4) Allowable limits for: •
Electric and magnetic fields
•
Radio and TV interference
•
Space charge density 5) Minimum ground clearance 6) Parameters for economic evaluation
CONDUCTOR SELECTION FOR SPECIAL TRANSMISSION SYSTEM
•
UPRATING OF LINES -
Sag of the selected conductor at maximum operating temperature should not exceed the sag of the original conductor
- No extra loadings on the structure at various design considerations •
UPGRADING OF LINES
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•
-
Line interference in respect of RIV, TVI, AN, EF, MF etc. Should be within acceptable limits
-
Conductor surface gradient within acceptable limits
-
Asymmetric bundle
COMPACT LINES -
Lowest possible sag and swing for required quantum of power
-
Considerations involved in upgrading/up rating
CONDUCTOR BUNDLE SELECTION: ESTIMATION OF TOWER WEIGHTS AND FOUNDATION VOLUMES
For each alternative of conductor and insulator configuration
TOWER WEIGHT ESTIMATION •
Preliminary tower design studies conducted
•
Estimation based on regression analysis and empirical formulae
FOUNDATION VOLUME ESTIMATION •
Preliminary foundation design studies conducted
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CONUCTOR BUNDLE OTIMIZATION: TECHNO-ECONOMIC ANALYSIS 1) Capital cost of line Cost of each item, construction cost 2) Cost of line losses Annual loss cost = Annual demand cost + Annual energy Loss Cost 3) Results of economic evaluation(PWRR or Annual Cost basis) 4) Cost sensitivity
Conductor Types: •
ACSR
•
AAAC
•
ACAR
•
New Technology Conductors
Trapezoidal
ACSS
INVAR Self Damping
ALL ALUMINIUM ALLOY CONDUCTOR AAAC
•
Good Conductivity
•
High Tensile Strength 31
•
Superior Corrosion resistance compared to ACSR
•
Improved strength to weight ratio resulting in lower sag
•
Lower electrical losses
Aluminium Conductor Steel Supported (ACSS Conductor) •
•
Similar to ACSR except Aluminium Steel Core (High Strength) carries most of the load and hence less sag compared to conventional ACSR conductor under emergency loadings.
•
Can be operated at 200 degree C without loss of strength
•
Improved Conductivity
•
Better self damping characteristics
Compact Conductors •
Aluminium wires/strands shaped trapezoidal
•
Increased Aluminium area and hence higher current carrying capacity
INVAR Conductors •
•
Core made of alloy of Iron. Nickel having low thermal coefficient of expansion(1/3 rd that of steel) After certain transition temperature all load transferred to the core and hence lower sag compared to ACSR after transition temperature •
Can be operated up to 200 0C
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DESIGN OF TOWERS SALIENT DESIGN CONDITIONS
•
The reliability of transmission line towers depends on the appropriate selection of design criteria/parameters.
•
Climatic conditions play an important role in determining the reliability of transmission line tower.
•
A significant number of transmission line failures can be the result of wind speed exceeding design limits due to deficiencies in selection of design parameters/criteria.
EARTHWIRE Functions of Earth wire •
To protect conductor against lighting flashovers
•
To provide a path for fault current
Lighting Flashovers •
Direct Flashover Occurs due to shielding failure with lighting on the conductor, flashover taking place across the insulator string from conductor to ground.
•
Back Flashover
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Occurs due to high towering resistance with a high voltage at the grounded tower cross arm compared to conductor, resulting in a flashover across the insulator string from ground to conductor.
Maximum allowable fault current (I) through earthwire mainly depends upon •
Area of Earth wire(A)
•
Maximum permissible temperature
•
Time of short circuit(t)
I varies proportional to A and inversely proportional to sqrt (t)
HARDWARE FITTINGS AND ACCESSORIES FOR CONDUCTOR & EARTHWIRE
HARDWARE FITTINGS •
For attachment of insulator string to tower D-shackles, Ball clevis, Yoke Plate, Chain link
•
For attachment of insulator string to the conductor Suspension & tension assembly Fittings like D-Shackles, socket clevis, chain link
•
For protection of insulator string from power follow current Arcing horn
•
For making electric field uniform and to limit the electric field at live end Corona control ring/Grading ring 34
•
For fine adjustment of conductor sag -Sag Adjustment plate, Turn Buckle
HARDWARE FITTINGS- Design •
Arcing Horn The air gap is maintained for satisfactory performance under actual field conditions. For power follow current
•
Yoke Plate To withstand mechanical loads-Thickness & shear edge maintained To maintain sub conductor spacing
•
Corona Control Ring/Grading Horn To cover at least one live and insulator disc To cover hardware fittings susceptible for Corona/RIV
•
Suspension Assembly - Shaped to prevent hammering between clamp & conductor - To minimize static & dynamic stress in conductor under various loading conditions - Minimum level of corona/RIV performance - For slipping of conductor under prescribed unbalanced conditions between adjacent conductor spans
•
Tension Assembly -
To withstand loads of atleast 95% of conductor UTS
-
To have conductivity more than that of conductor 35
•
Sag Adjustment Plate/Turn Buckle -
To adjust sag upto 150mm in steps of 6mm.
Cost Engineering What is cost engineering? Cost Engineering is the science of cost finding by analysis of the processes and expenses of production, and charging of particularized expense factors, through process units rates, in the exact ratio of utilization.
Overhead transmission Overhead conductors are not covered by insulation. The conductor material is nearly always an aluminum alloy, made into several strands and possibly reinforced with steel strands. Copper was sometimes used for overhead transmission but aluminum is lower in weight for equivalent performance, and much lower in cost .
Underground transmission Undergrounding is more expensive, since the cost of burying cables at transmission voltages is several times greater than overhead power lines, and the life-cycle cost of an underground power cable is two to four times the cost of an overhead power line
Abstract cost estimate
1) Preliminary Survey & Soil Investigation 2) Land Acquisition for Substation and R & R Compensation 36
3) Cost of compensation for transmission lines •
Compensation towards trees, crops & PTCC
•
Compensation towards forest
4) Civil Works •
•
•
Infrastructure for substation Non Residential Buildings Colony for transmission lines and substation
5) Equipment (Supply & Erection Cost) •
Transmission Lines
•
Sub Stations
6) Misc. Tools & Plants 7) Maintenance during construction 8) Engineering & administration 9) Losses on stock 10) Contingencies 11) Custom Duty 12) Interest During Construction (IDC)
Substation Equipments 1) GIS Substation Equipment. 2) Circuit Breakers 3) Isolators 37
4) Current Transformers 5) Capacitor Voltage Transformers / Voltage Transformers 6) Surge Arresters: They should be provided near line entrances, transformers so as to achieve proper insulation coordination. These shall be fitted with pressure relief devices and diverting ports suitable for shattering of porcelain housing providing path for the flow of rated currents in the event of arresters failure.
Substation Support facilities 1) AC & DC Power supplies 2) Fire Fighting System: Fire fighting system in general conforms to fire insurance regulations of India. The fire fighting system is proposed with both AC motor & diesel engine driven pumps. Automatic heat actuated emulsifying system is proposed for transformers and reactors. 3) Oil evacuating, filtering, testing, & filling apparatus: To monitor the quality of oil for the satisfactory performance of transformers, shunt reactors and for periodical maintenance necessary oil evacuating, filtering, testing and filling apparatus would be provided at new substations. 4) Lighting & Communication: Adequate normal and emergency AC & DC lighting shall be provided in the control room of the substation. A telephone exchange of 24 lines shall be provided at new substation as a means of effective communication between various buildings of the substation. 5) Control Room
Line Accessories 1) Mid Span compression joint for conductor / earthwire 38
2) Repair sleeve for conductor 3) Flexible copper bond for earthwire: Flexible copper bonds shall be used for good electrical continuity between the earthwire and the tower. Two bonds per suspension tower and four bonds per tension tower shall be used. 4) Vibration Dampers for conductor / earthwire: Stockbridge vibration dampers shall be used to reduce the maximum dynamic train caused be Aeolian vibrations to a value of 150 micro-strain. 5) Spacer Dampers
Some Cost Reduction Techniques
By allowing multiple generating plants to be interconnected over a wide area, electricity production cost was reduced. The most efficient available plants could be used to supply the varying loads during the day. Reliability was improved and capital investment cost was reduced, since stand-by generating capacity could be shared over many more customers and a wider geographic area. Remote and low-cost sources of energy, such as hydroelectric power or mine-mouth coal, could be exploited to lower energy production cost. The capital cost of electric power stations is so high, and electric demand is so variable, that it is often cheaper to import some portion of the needed power than to generate it locally. Because nearby loads are often correlated, electricity must often come from distant sources. Because of the economics of load balancing, wide area transmission grids now span across countries and even large portions of continents. The web of interconnections between power producers and consumers ensures that power can flow, even if a few links are inoperative. The unvarying (or slowly varying over many hours) portion of the electric demand is known as the "base load", and is generally served best by large facilities (and therefore efficient due to economies of scale) with low variable costs for fuel and operations, i.e. nuclear, coal, hydro. Renewable sources such as solar, wind, ocean/tidal, etc. are not considered "base load" but can
39
still add power to the grid. Smaller and higher cost sources, such as combined cycle or combustion turbine plants fueled by natural gas are then added as needed. Long distance transmission allows remote renewable energy resources to be used to displace fossil fuel consumption. Hydro and wind sources can't be moved closer to populous cities, and solar costs are lowest in remote areas where local power needs are minimal. Connection costs alone can determine whether any particular renewable alternative is economically sensible. Costs can be prohibitive for transmission lines, but various proposals for massive infrastructure investment in high capacity, very long distance super grid transmission networks could be recovered with modest usage fees. High voltage direct current (HVDC) is used to transmit large amounts of power over long distances or for interconnections between asynchronous grids. When electrical energy is required to be transmitted over very long distances, it is more economical to transmit using direct current instead of alternating current. For a long transmission line, the lower losses and reduced construction cost of a DC line can offset the additional cost of converter stations at each end. Also, at high AC voltages, significant (although economically acceptable) amounts of energy are lost due to corona discharge, the capacitance between phases or, in the case of buried cables, between phases and the soil or water in which the cable is buried.
Environmental and Social Aspects
Forest Involvement / Clearance
As per the practice, preliminary route selection is done by POWERGRID based on such documents as Forest Atlas and the survey of India maps using “bee” line method, followed by field verification through walk over survey. All possible steps are taken to avoid the route alignment through the forests. In case where it becomes unavoidable due the geography of
40
the terrain, the alignment is made in such a way that the route through forests is barest minimum.
For the selection of optimum route, following points are to be taken into consideration: •
The route of the proposed transmission line does not involve any human rehabilitation.
•
Any monument of cultural or historical importance is not generally affected.
•
The route does not create any threat to the survival of the community.
•
It does not affect any public utility services like playground , school, other establishments, etc
•
It does not pass through any sanctuaries, national park, etc.
•
It does not infringe with the areas of natural resources.
Social Issues / R&R measures As per the prevailing law, land below transmission line is not required to be acquired and only land for substation is acquired. POWERGRID is following the practice of land management to minimize the land requirement to the barest minimum. Generally 20 to 30 hectares of land is required for constructing a substation depending upon the type of the voltage level. Even for this 20 to 30 hectare land, POWERGRID try to locate the substation on government land as far as possible and in the absence of govt. land private land is acquired. In order to insure the indigenous (Tribal) people do not suffer adverse affects, utmost care is taken to avoid acquisition of land belonging to tribal community. In spite of that, POWERGRID has developed an indigenous people (Tribal) Development Plan (IPDP) 41
which ensure that they receive culturally compatible social and economic benefits for any adverse affects.
Risk Analysis Revenue Risk The capital cost of the transmission system comprises of i) An equity component ii) A loan component This is recovered through the annual transmission charges consisting of return required for the equity, an interest for the loan component together with the depreciation charges, the O & M charges and interest on working capital from the beneficiaries as per Notification in proportion to the benefits derived by them. These are recovered in monthly fixed charges from the beneficiaries. In addition to annual charges Income Tax, FERV and incentives, etc. as per notification would also be payable. The Bulk Power Transmission Agreement (BPTA) which cover the payments for transmission charges for all the existing projects as well as those that may be included in future after approval by CEA already exists.
Regulatory Risk BPTAs have the provision that the transmission tariff for new / existing transmission assets commissioned as well as the additional tariff payable due to additional capitalization from year to year, etc. shall be computed by POWERGRID based on norms / methodology followed in the GOI notification dated 16.12.97 in accordance with norms to be specified by the Central Electricity Regulatory Commission (CERC) as amended from time to time.
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