Important definitions Total System Black Out The system is said to be under total black out when all Regional utility generation has ceased to operate and there is no supply from external interconnections to the Western grid and it is not possible for the total system to function again without black start. Partial System Black out Blackout of a particular sonstituent system either partially or fully or of more than one constituents’ systems. System islands In case, part of the system is separated from the grid due to system splitting or through defence schemes and operates independently, it is called System Islanding. Normal System Conditions System will be declared to be normal when all islands are re-connected, all emergency and essential loads restored and adequate transmission restored enabling States to draw their scheduled requirements from ISGS. 1
The black start procedures should contain the following minimum details. 1. Generation Securing 2. Generation build-up · Survival and Start-up Power of Power Plant(s). · Power Station(s) with Black-start facility. 3. Build-up of islands 4. List of black start facilities, interstate/regional ties, synchronizing points and essential loads to be restored on priority 5. Detailed State/Utility-wise Restoration plans 2
SYSTEM RESTORATION APPROACHES 1. In systems with a fair dispersal of Generating stations with black start facilities, those generating stations where black start facilities are available, should be started up and islands formed around these generating stations by connecting essential loads. These islands are then interconnected at predefined locations where synchronizing facilities are available. The speed of restoration enhances with increase in number of black start facilities and their dispersal. Gujarat, Chhattisgarh and M. P have considerable number of black start facilities while Maharashtra suffers from lack of adequate number of black start generators. It is required to carefully monitor the operation of the island (reintegration with rest of the grid) due to small stiffness in islands. One engineer shall be specifically assigned to monitor one island. 2. While adding loads, care should be taken to ensure step by step addition keeping in view load characteristics, ie. , variation of load with respect to voltage and frequency and stiffness of island. During cold load pick up sudden starting of motor loads or power plant auxiliaries like BFP, PA fan etc. , can cause voltage dips due to drawal of high starting currents. Essential loads can be restarted in steps smaller than 5 MW. It is preferable to restore rotating type loads which co~tributes to inertia of the island. In any case, load pick up should not cause frequency excursions greater than 0. 5 Hz in the island. 3. The second approach could be followed in case self start facilities are available at only a few power stations or the stqrt up power has to be imported from neighbouring regions at one 0 or two points. In this approach, the start up power required to be extended to all the generating stations on priority basis while restoring few loads and transformers for voltage control. The start up power available from neighbouring regions at various interconnections have to be seriously explored since considerable assistance can be availed and the restored system is connected to stable external systems. The procedures have to be laid down for quickly harnessing these facilities. The restoration through this approach could be delayed due to problems in charging the lines, high voltage, lack of synchronising equipment at certain substations etc. and may involve system disturbances during restoration. 3
OPERATIONAL GUIDELINES. 1. Each SLDC will coordinate synchronisation of units and transmission lines in its system in consultation with WRLDC. For ISGS power stations , WRLDC will be coordinating for their synchronisation. WRLDC will coordinate interstate/ inter-regional startup power availability and also inter-state/inter-regionql synchronisation of the system. 2. Minimum 25 MW power flow from West to North will be maintained on HVDC back to back link at Vindhyachal during normal operating condition. 3. Black start generation usage priorities: a) provide startup power to hot units b) provide startup power to units that are cool; but capable of rapid restart c) restore stations auxiliary service to generating stations 0 and sub-stations. d) Pick up essential loads. 4. Transmission corridors used for startup power should be isolated from any damaged/faulty equipment and are of minimum length and minimum voltage level to reduce line charging. 5. The constituent receiving assistance during restoration process should restrict to the agreed quantum only since this may have an adverse effect on healthy system 0 rendering the assistance. 4
GENERAL GUIDELINES While each disturbance would be different and may require a different plan, nevertheless it would be useful to formulate general guidelines for the benefit of the load despatchers. These may be described as 0 below: i. The operators at generating stations and substations should have the knowledge of pre-planned synchronising locations and synchronising procedures. Synchronising should be done preferably at generating stations. ii. Switching procedures should be clearly laid down and periodically reviewed. iii. The part systems should be reintegrated only after adequate stabilization. iv. The transformer taps should be checked for desired settings to minimise voltage difference. v. The substation operators and load despatchers sxould make a check of the capacitor banks and reactors in service and accordingly to carry out the switching operations for voltage control. vi. Energising long high voltage lines and cables should be avoided until enough generating capacity is available. vii. Provision of islanding schemes area-wise, power station-wise and unitwise would enhance the ability to restore faster. These should be fully exploited. viii. Some of the generating units might have been saved due to successful islanding. Stabilization of such online generation is of top most priority. ix. Provision of start up power to nuclear plants should be given priority as poisoning of the reactors would delay restoration of nuclear units. x. In case of failure of main communication channels, guidelines for decision making should be given to all the major substations and generating stations. xi. Devising islands for power stations wherever small units could help in faster restoration of the grid are available. One small unit at these power stations can be islanded with radial loads and/or house load. 5
REQUIREMENT OF SURVIVAL POWER / AUXILIARY POWER Survival power can be defined as the power needed for avoiding the damage to the equipment in case of supply failure. This power is required for · Turbine emergency oil pump · Jacking oil pumps · Barring gear of the turbines · Lubricating oil pumps · Compressors for ABCB operation · Emergency lighting · Battery chargers of units, station, and communication and telemetry system The survival power required by 120 MW units is of the order of 250 -350 k. W while the requirement of 210 MW units is of the order of 350 -500 KW. As a general rule, the survival power requirement would be around 0. 25 -0. 30% of the unit capacity. Nuclear power plants should be supplied with survival power on priority basis. 6
REQUIREMENT OF START UP POWER The start-up power is the power required for the auxiliaries while the generating unit is restored. The requirement of start-up power by various units is as follows: Nuclear & : 7 to 8% of the unit capacity Thermal Hydro : 0. 5 to 1% of the unit capacity Gas : 1. 5 to 2% of the unit capacity. 7
GUJARAT Ukai (H) 4 x 75 Hydro Diesel 500 KVA One 75 MW gen. house set Mini Hydro 2 x 2. 5 Hydro Diesel 50 KW Kadana 4 x 60 Hydro Diesel 500 KVA Dhuvaran 1 x 27 + 4 x 63. 5 + 2 x 140 Gas Thermal Any one unit out of the units 1, 2, 3 & 4 1 x 63. 5 MW Islanding of any one units out of units 1, 2, 3, & 4 on to house load Diesel 1600 KVA x 2 700 KVA GIPCL 3 x 32 + 1 x 45 + 1 x 104 + 1 x 5 Gas Steam All units Diesel 141 MW 500 KVA Islanding of all units A. E. Co. Stn. C Stn. D, E, F 4 x 15 2 x 30 3 x 110 Thermal Stn-C islands Diesel 30 MW 500 KVA Islanding of 0 ne 30 MW unit GPEC 3 x 138 Thermal Diesel 3000 KVA Kawas 4 x 106 2 x 116 Gas Diesel 2700 KW Gandhar 3 x 144 + 1 x 225 Gas Diesel 3120 KW CCPP, Vatwa 3 x 39 + 1 x 45 Gas Essar 8
ower Station Installed Cap. (MW) Survival power Auxiliary Power Diesel Capacity Synch. Facl. GEB Ukai (Hy) 4 x 75 100 KW 250 KW 500 KVA Yes Ukai LBCH 2 x 2. 5 5 KW 20 KW 50 KW Yes Kadana (Hy) 4 x 60 7 KW 200 / 250 KW 500 KVA Yes Ukai (Th) 2 x 120 + 1 x 200 + 2 x 210 1. 447 MW 4. 5/ 8 MW - Yes 2 x 120 500 KW 12 MW 500 KW Yes Dhuvaran (Th) 1 x 27 + 4 x 63. 5 + 2 x 140 500 KW 5 MW 500 KW Yes Gandhinagar 2 x 120 + 3 x 210 175/350 KW P. U 7. 2/10. 7 MW P. U 500 KVA D. G. set under erection Yes Wanakbori 7 x 210 0. 8 MW 13 MW P. U 3 x 400 KW Yes 2 x 70+1 x 75 360 KW 4. 5 MW 1280 KW & 500 KW Yes h. Stn. C 2 x 30 250 KVA 9 MW 1 x 500 KVA Yes h. D, E, F, + 3 x 110 2 x 125 KVA GT + 3 x 33 Gas 3 x 32 Nil 350 KVA 1 x 500 KVA Yes ikka (Th) anandro A. E. Co 9
1. REACTIVE POWER BALANCE Objective To keep system voltage within allowable range Strategies Ø Energising fewer high voltage lines Ø Operating generators at minimum voltage levels (logging p. f) Ø Deactivate switchable capacity Ø Connect shunt reactors and tertiary reactors Ø Adjustment of transformer taps Ø Pick up loads with lagging p. f Ø Charge more transformers Ø Charge shorter lines Ø Operating synchronous condensers / SVCs where available Ø Avoid charging lines with series capacitors Concerns Ø Self excitation of generators and run away voltage rise. 10
2 LOAD AND GENERATION BALANCE Objective To maintain system frequency within allowable limits Strategies Ø Restore loads in small increments (minimum & essential) Ø Smaller and radial loads to be restored prior to larger and network loads Ø Feeders with U/F relays are restored later (bypass until frequency stabilizes) Ø Load restoration based on load characteristics Ø To get adequate inductive loading to compensate capacitive effect while charging high voltage long line, a concentrated load of large town/city should be released along with that of Railways. Ø Maintain frequency close to 50 Hz paying special attention to traction and other fluctuating loads Concerns Ø Size of load pick up depends upon the rate of response of prime movers Ø Load pick up in large increments led to collapse of the restored systems Twice in Gujarat on 9. 12. 95 Twice in Maharashtra on 9. 12. 95 11 Once in MP on 9. 12. 95
3 LOAD AND GENERATION COORDINATION PRIORITIES Restart Stage Ø Priority to restore power supply to generating stations and load dispatch centers. Ø Priority to supply start up power to hydro and gas units Ø Priority for providing backup/survival/startup power to nuclear power stations Ø Several load and generation islands formed Ø Black starting of small hydro or gas units Ø In each island, the objective is to supply station auxiliary power and start up power Ø The number of islands limited by sources of black start units Ø Each island should preferably be monitored by one load despatcher till reintegration. Re-integration stage Load restoration stage v In small steps v Observe frequency charges (< 0. 5 Hz) with load addition v Preferable to restore rotating type loads if possible v Cold load pick up is the main concern 12
4 MONITORING & CONTROL Ø Location of the fault and extent of collapse of the system should be ascertained before restoration Ø It is dangerous to restore a faulty line or faulty equipment Ø Ensure communication links between control centers, power plants and substations Ø SCADA system performance Inadequate displays Excessive alarms Protection tele-metering EMS 13
5 Communications ¨ Establishing communication between LDCs, Generating Stations and major substations. ¨ All important substations only to be kept in touch with and links with unimportant substations to 0 be cut off to avoid draining of batteries. ¨ All communication channels required for restoration process shall be used for operatyonal communication only till grid normalcy is restored (IEGC 6. 8. e) 14
6 PROTECTIVE SYSTEMS / SYNCHRONISATION Ø Check all interlocks Ø Sort out problems in closing of breakers due to low gas/air pressure Ø Avoid paralleling islands through weak ties. Ø Synchronizing facilities at sub-stations Ø Standing phase angles to be checked and difference reduced by generation control. Synchronisation only through synchro-check relays Ø Try to synchronise islands or part systems, near the generating stations requires less coordination and easier to control Ø Operate generators on lagging p. f Ø Generators supplying start up power should not be loaded beyond 80% Ø The capacity of the island to sustain the starting current of BFPs should be checked Ø If sub-systems are to be synchronized away from generating stations, be extremely cautious of standing phase angle differences (system occurrence on 28. 4. 93 in Maharashtra) 15
7 ENERGY STORAGE Loss of back up power supplies like: Batteries Battery chargers UPS Diesel sets Could affect · Operation of circuit breakers · Motor operated isolators · Communication · SCADA (one of the important causes of delays in restoration) 16
8 SWITCHING OVER VOLTAGES · Energise small sections of lines · Energise lower voltage lines · In case of parallel circuits, energise one ckt. · Control high voltages during restoration to avoid damage of Las/CVTs et. 9 Survival Power Ensuring availability of back up power supplies such as batteries, battery chargers, D. G sets to avoid effect on non operation of circuit breakers, communication systems etc. , which can cause delay in restoration. 10 Awareness of Restoration Plans Training and necessary documentation may be provided to Load Despatchers by respective LDCs. 11 Exchange of Information Exchange of information among SLDCs and between RLDC and SLDCs is essential for proper co-ordination. 17
12 SYSTEM STUDIES FOR MAKING RESTORATION PLANS 1 Power flow § Multiple islands § Study voltage problems § Generator excitation limits § Transformer taps 2 Dynamic stability § To study load generation co-ordination § While load pick up, generation response could be studied 3 EMTP § To determine what lines to be charged § Transient over voltages in switching 18
13. v RESTORATION PLAN Identification of collapsed power system components and equipment. v Restart and supply start up power first to hydro and gas stations v Startup power to thermal stations, auxiliary power to sub-stations v Co-ordination of power plant start up with load pick up to bring generators to their stable minimum generation levels v Restore in sub-systems if multiple sources of startup power available v Energising transmission lines with acceptable transient and sustained over voltages v While load pick up, check frequency decline v Reintegration of sub-systems v Ensure discipline and avoid over drawals until proper stabilization 19
14. ROLE OF LOAD DESPATCH CENTRES v Determine severity of collapse v Identify and initiate black start facilities v Import start up power from neighbouring states or regions v Import more power to meet essential loads from neighbouring states or regions v Decision making and guidance v Determine priority loads v Check unbalanced loading due to traction v Getting start up power from captive power plants, if possible v Ensure communication links, SCADA facilities v Guidance to sub-station/generating station operators from the results of EMS v Identify points of reintegration and synchronization v Reporting 20
15. TRAINING AND ROUTINE EXERCISES v Short time appreciation courses v Review of targets for restoration as soon as a black start facility or inter-state / inter-regional connection is stabilized. v Review of restoration plants after every occurrence v Updating of restoration manually and other documentation v The strategies of restoration should have alternatives to enable flexibility v Training by experts v Interactive training and case studies v All constituents should participate in training programmes v Formation of a command group v Mock exercises v Preparation of manuals on important telephone numbers etc. 21
16. (A) (B) OVER VOLTAGE CONTROL DURING RESTORATION Sustained power frequency over voltages v Due to lightly loaded lines v May cause under excitation of generators v May lead to self excitation of generators v Over fluxing of transformers (generate harmonic distortions and cause transformer over heating) Transient voltage or switching surges v caused by energisation and deenergisation of lines or v switching of capacitive elements v In conjunction with sustained o/v may cause arrestor failures 22
Harmonic Resonance Voltages v Oscillatory un damped or weakly damped v Of long duration v Originate from equipment non linearities and switching natural frequency of series resonance circuit formed by source inductance and line charging capacitance. v Magnetizing in rush due to transformer energisation v Lightly damped due to light loading of lines v Over fluxing of transformers (beyond 1. 1 pu) 2. Equipment limitations Transformers and Arrestors 1. 2 p. u for one minute 1. 4 p. u for 10 seconds Circuit breakers v Will have reduced interrupting capability v Can interrupt line charging currents upto 1. 2 p. u 23
3. Control of sustained over voltages v Sufficient under excitation capability on the generators v Connect lagging p. f loads and shunt reactors v Remove all sources of reactive power and switch off capacitor banks v Run generators at maximum possible reactive power output to allow margin to adjust for large charging reactive power during line switching v Tap staggering of transformers v Avoid extra parallel lines v Maintain low voltage profile on the lines to reduce line charging 24
4. Control of switching transients v Switching o/v may cause flash over and damage to equipment v Switching transients on fast transient caused by ill timed closure of breakers v To be controlled to 2. 5 p. u for 400 k. V and 1. 9 p. u for 800 k. V and 2. 3 p. u for others v Usually of fast front, low energy or slow front, high energy transients. v Keep steady state voltage below 1. 2 p. u. Keep generator terminal voltage around 0. 8 p. u 25
5. Harmonic Resonance v Transformers may get over excited and generate harmonics v Combination of system inductance and line capacitance forms a series resonance circuit which is excited by harmonic distortions produced by transformer saturation v Harmonics generated by magnetic current in rush can also lead to harmonic resonance v Sufficient load to be connected to the underlying system at both ends to damp oscillations v Lower order resonances produce higher over voltages (3 rd, 4 th, 5 th, 6 th harmonics) v To control over voltage due to transformer over excitation, user lower taps (system studies) v Harmonic resonance can be damped by connecting loads at both ends v Connect dead load on the transformer to be energized v Reduce number of highly loaded lines (in parallel paths) 26
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