Black Start Procedure
Introduction: Modern power system is
operated in grid mode. Various interconnected power systems operating in
synchronous. Grid mode operation is stable due to large system bias and high
inertia. Also seasonal and daily diversity in load and generation pattern is favorable
for optimum utilization of resources and cost of power. However operational
coordination is a task. Indian grid is monitor and control by SLDC, RLDC and
NLDC at appropriate levels. Grid system operating within limit of angular
stability, voltage stability and frequency is secure. Operation near or beyond
the limit is highly risky. System downfall starts with generator cascading or
network cascading and ends in total collapse having total power failure in
large area.
Electricity is used for various purposes like heating, cooling, pumping, lifting, moving, cutting, polishing, lighting and so on. But use for lighting is most common by mass. Hence practically word LIGHT becomes synonym of ELECTRICITY. Therefore conversation for electricity is लाइट बिल, लाइट गई, लाइट बंध, लाइट आई, लाइट कब आयेगी, etc.
Therefore non availability of electricity is regard as absence of light means darkness. Nothing is visible in darkness and all around is black. Therefore total power failure is called BLACKOUT.
Actually blackout refers to intentional switched off all the light sources during night hours as precautionary measure against air raid during war time. Whereas black out in this case is failure of electricity during day or night. The activity to restart the power system after black out is called BLACK START.
Preface: Black
out is a rare event. Therefore operating staff have limited or no practical
experience for operation or witness black start process. Handling of power
system during black start is all the way different from normal operation. Power
system at this stage is fragile due to infant state having very low inertia and
small system bias. A small disturbance in load and / or generation cause uncontrollable
wide and fast frequency variation. Also high voltage due to meager power flow
on lines forces generator operation under excited. Therefore very probability is
rotor falling out of step due to load surge because of poor synchronizing
torque with under excited operation. Regular system
operators are experts for matured power systems similar to physicians treating adults. But black start
operation is a specialized job like pediatrician for
child. Treatment
of power system like normal operation during restoration process may encounter unforeseen problems and
aggravate the condition and results in setback in restoration process. Primarily
non availability of power supply causes financial loss to supply company on
account of undelivered energy. But consequential unaccountable loss due to inconvenience
to public is larger. Passengers in train are stranded on the way and elevators,
homes and offices without light, fan and air conditioner, outage of industries and
so on. Therefore restart activities has to be fast but safe and sound.
Black Start Scheme is different than this black start procedure. Most of the power plants have station specific black start scheme. It is self start procedure using in house power source without waiting for external auxiliary power. Black start scheme is a listing of sequential steps for the purpose. Such self restart station is operated with internal or nearby external load.
Black Start: Black start is the activities to provide auxiliary power, restart the generators, synchronize, interconnect and load release managing frequency and voltage within safe operating range. Blackout can be total grid collapse due to cascading or partial confined to specific area due to network break up. Black out is sudden unforeseen event leading to panic for grid operators at LDC, power plants, transmission and distribution system including public. Generally they are unaware of what happened and what to do. So likely to make some blunders. But they may be calm and quit and ready for their role provided they are trained for such event and their role.
Power system is dead after blackout having no power in the affected area because of tripping of all generators and some lines except islanded station. Power system might have operated very erratically just before the collapse. Power system might have fragmented due to tripping of lines. Some segment might have survived wherein load and generation match by chance. But mostly segment collapse due to mismatch of load and generation. However chance of survival is better for planned segment known as island.
There is possibility that some of grid elements of power plant or transmission network have developed faulty condition due to abnormal operating condition before collapse. Those elements require inspection, testing and rectification before taking back in service. Restart activity has to be started considering non availability of such faulty and doubtful elements till they are set right and ready for operation. Hence it is utmost important to have this information before proceeding for restart. Otherwise restart activity stuck-up at midway. Time and efforts put on so far is wasted.
The black start procedure hereunder is generalized. However it can be useful for small, medium, large partial or total blackouts. Steps for restart with precautions and hints from the procedure according to respective system configuration and post occurrence status will be useful for fast and firm restoration.
There are three alternatives for start up. Each may be suitable according to post occurrence system status, field amenities, working condition and availability of experienced operators.
Synchronous Restoring: Grid linked restoring. This procedure is appropriate for partial blackout having live grid power feasible for auxiliary supply. Grid power from convenient substation is taken to waiting power plant by energizing lines from station to station in steps. Power from grid is most suitable because of stability without frequency excursion and over voltage problem. Additional advantage is a chance to feed some essential loads in addition to auxiliary power if grid can spare. However import has to be restricted in consultation with control authority. Blind unilateral import makes grid unsafe due to overloading in network or low frequency. Best part is all upcoming generators are connected to grid. So generators are stable and start picking without strict load generation matching. Under frequency load suppression schemes and planned island schemes kept inactive because these are planned for normal load condition.
However this restoring method is rather slow. Auxiliary power can be extended to waiting stations one after the other. Lines from source to destinations are taken in service step by steps via appropriate route through intermediate sub stations. These critical operations performed in proper sequence with due precautions and close coordination with all concern. Of course this method is preferred as it is the most stable method.
Integrated Restoring: Single cluster restoring. This is suitable when grid power is not available for restoration. There can be one or more islanded and / or self started power stations. Out of all these available live station one is selected as common source for auxiliary power supply for restart procedure. Selected source has sufficient level of generation for the purpose at convenient location to facilitate trouble-free quick transfer of power to most of the power plants. Power supply from selected station is extended to all waiting power plants for auxiliary as in synchronous restart method. Upcoming generators are synchronized with this single group. Generators in the group are operated with active governor control mode with higher droop for faster frequency response. The group is getting larger with addition of generators improving stability. Under frequency load suppression and island schemes are not activated at this stage. The system is synchronizes with the grid when permissive.
Isolated Restoring: Multi clusters restoring. This method is of restart is the option when grid power is readily not available but more scattered sources of limited power are available in affected area. Such situation is due to more successful islanding and self started stations. Limited power at isolated sources is used for auxiliary power to waiting stations close to respective source. This restoration method is faster because of concurrent activity for auxiliary supply in different cluster. Multiple sub systems are developed with only few power plants and associated loads. But these small systems are not stable due to small bias and low inertia. Generators are operated in active governor control mode with high droop. Under frequency load suppression and island schemes are not activated at this stage. Rigorous monitoring of each cluster is essential till they are interconnected. Cluster wise quick control for load generation matching is required for security of the clusters. Load adding or dropping have is in small steps to avoid impact on generators stability. Each cluster is sub system having different frequency and can be interconnected only after synchronizing checks. Load is added in the respective cluster when generator pickup. But care is required to avoid inter cluster line switching during expanding cluster to loads. Voltage rise is expected while charging lines for adding load, power transfer or synchronizing. Hence generators operated under excited to absorb reactive power. Than generators is likely to fall out of step due to surges of line charging, tripping, load switching etc. Strict monitoring and control is required for load generation matching within territory of respective cluster.
Critical part of this method is synchronization of clusters. Generally power from one cluster is taken up to other cluster where it is synchronized with the bus. Normally power plant operators are conversant with synchronization of generator with bus. But the task here is unusual to synchronize receiving line with the bus. Sometime it is not feasible to synchronize at power plant of either end. In that case synchronizing is done at intermediate substation where it is feasible. Power from one cluster is taken to the bus at intermediate substation. Power received from other cluster is synchronized with the bus.
Self start power plant may be gas turbine or hydro turbine operated stations. Because they require small auxiliary power that can be fed by starting local DG set provided for this purpose. However availability of auxiliary power from self start station depends on various factors. Battery condition of DG set, fuel stock, operator availability, water release from irrigation linked project, etc.
Procedure: This procedure consists of four level restoration functions. Procedure is prepared based on problems encountered in diverse situations during system rebuilding after blackouts. The prime objective is early recovery without hurdles and breaks. Prime task in black start is to provide auxiliary power to waiting power plants. First part is to find capable stable power source for auxiliary power at convenient location. Next part is to fix priority to supply auxiliary power where earliest generation is possible. Decision in the matter requires information about actually serviceable resources available in blackout condition. Post occurrence condition of various power system elements is received from all sources. Next task is rebuilding grid synchronizing generators and establishing interconnections. Load releasing as per availability of power along with network strengthening and maintaining safe frequency and voltage profile.
LDC level: As known this center is unique master controller for grid operation. Same is the role during black start also. Black start is a special unique task. Load dispatchers at LDC are key persons to streamline the recovery process without hurdles and breakdown. Experienced and conversant engineer for black start procedure is assigned overall control of operations. Other LDC engineers help to coordinate different tasks such as auxiliary power extension, cluster wise load generation matching, clusters synchronization and interconnection, power flow monitoring and control, getting updates from power plants and substations, liaison with field and authorities, etc. System operators at power plants, substations and distribution system may not be conversant with restart activity. LDC engineer handling particular task should guide in details step by step for correct sequence and precautions. They have to be specific in instruction and double sure even at the cost of repetition to avoid mal operation.
Post occurrence status: Start is with first hand information about the area affected. Message send to power plant, sub-stations and field offices in affected area about the black out. Massage has basic information like total supply failure in xxxxx area due to power system disturbance and resumption of power is likely to take about xx hours. This is required to avoid inquiry from field offices and consumers.
Power system could have erratic operation just before the blackout. Possible to have fast wide and abnormal variations in various parameters like voltage, current, frequency, speed, pressure, temperature, vibrations, etc. Erratic operation may develop problem in some part of power system. So it is not available for service till rectified or checked for healthiness. Such information is collected from power stations and substations. Information about grid status, nearest live station and feasibility of power import etc is collected from appropriate sources. Information is useful for course of action for restart.
Startup Power Planning: Auxiliary power is prime
requirement for startup of power plants. Therefore first task is to manage
auxiliary power supply for all waiting power plants. There may be more than one
source of power supply and numbers of power plants waiting for auxiliary power.
Restart is started with the pair of source and waiting station by which
earliest restart is possible.
Power Source: Consideration for source of power.
Grid Power: Power supply for auxiliary
is available from the live grid in case of partial blackout. Grid supply is stable and therefore most preferable
source for auxiliary power supply to waiting stations. Also possible to supply
auxiliary power to more power stations simultaneously and for essential load to
the extent grid can spare. Power can be taken from the most convenient station
of live grid. Generators returning to service are in synchronism with the grid
and hence stable and strict load generation matching is not essential. But grid
power import is controlled as per permissible limit only. Because excess power import
may result in critical network loading or frequency drop and jeopardize live
grid.
Islanded Station: Generally planned islanding is implemented in power system. Section of power system with generation and matching load gets isolated automatic by protective system at planned operating conditions during power system disturbance. So source of power for restart is readily available from such island after blackout. This source is not stable as it is standalone station of small capacity having small bias and low inertia. So requires careful handling for extending supply to waiting station and starting auxiliaries. However is the option when grid source is not feasible.
Self Start Stations: Hydro, Gas and Liquid fuel power plant have small size generators compared to thermal and nuclear power plants. Auxiliary power required is very less due to small size and low percent requirement. DG set of appropriate capacity is installed for quick in house source of auxiliary power for plant. Procedural steps to start DG set, auxiliary, power plant and in house and external loading for stability is Black Start Scheme for particular station. Though power is not available readily but is the quick source. Availability of power from such source depends on status of fuel, battery, starter, operator of DG set, dam level, allowable discharge etc. Requirement of such operation is very rare and therefore possibility of hitch due to above reasons. This source is also not stable as it is standalone station of low capacity having small bias and low inertia. However is useful when grid source and island power is not feasible.
Waiting Stations: Many power plants may be waiting for auxiliary power to proceed with restart activity. Process of taking power from source to waiting station is critical requiring careful operations in correct sequence. It depends on power source, location of waiting station, network route and experts available for parallel activity for more stations or one by one. Concurrent activity is preferred for fast restart when feasible but requires distinct instruction and operation without intermix. However avoid more than one switching at a time. Allow time gap for stabilization. Priority of station for auxiliary supply is fixed on following four criteria.
Power Requirement: General impression may be to prefer large power stations to have maximum
power at the earliest. But practical it is different with following
consideration. Auxiliary power requirement of steam turbine driven generators
is about 10% of its capacity and minimum requirement is about 6% with single
set of auxiliaries. So this is possible when available power source is capable to
deliver required power and sustain switching surge due to high capacity
auxiliary. Also this much supply has to continue till it starts generating only
after few hours. Whereas auxiliary power required for hydro, gas, liquid fuel power
plant is very less because of small capacity and low parentage requirement and
starts generating within few minutes only.
Location: Location of the waiting power plant with respect to source station is important. Waiting station far from the source has more intermediate stations on the route. More transits means more operations, more time, more switching surges, more chance of mal operation and possibility of high voltage. Therefore near is better.
Generator status: In blackout condition, all the generators are out except successful island. Some generator may not be readily available for operation due to some problem developed during erratic operation. Hence generator set status is important. Priority for auxiliary power should be for serviceable generators. Top priority is for set in hot standby condition.
Route status: Intermediate stations and lines between source and target power plant needs to be in serviceable condition. Information received from transmission status report is useful to find alternate shortest serviceable route for the purpose.
Networking: All the lines and transformers are dead in black out condition. Complete transmission network is taken back in service in three stages as under.
Providing auxiliary power to waiting power plants.
Interconnections of power plants or clusters.
Network strengthening with growth of system.
High voltage condition is very likely at initial stage because of meager load on lines. Therefore minimum required lines, substations and transformers are taken in service. Following precautionary measures are suggested.
Linking route should be of lower voltage of shorter length and minimum intermediate stations. Better to have common route for multiple destination where feasible. Bus and/or line reactors are kept in circuit where voltage shoot up is expected. Inter bus transformer charged from one side and other open is alternative where reactor not available. Capacitor banks are kept out of circuit at this stage. Some important precautionary measures are given in transmission level operations.
Power system restarts in synchronization with the grid in synchronous restoring method. Whereas power system restarts in group isolated from grid in case of integrated restoring method. So this has to be interconnected with grid at permissive condition. Synchronizing is required while connecting to the grid. But this is simple because no problem for instability, over voltage or surges because both the system are matured. Normally line is charged from the grid up to the key station in restored system where synchronization is done.
But task is rather difficult in clusters restoring
method. Because all cluster have to be interconnected to form single system
before linking to grid. All interconnection connections require synchronizing.
But clusters are small delicate sub system having limited power. Clusters with low
inertia and small bias are unstable. Therefore handling of cluster is risky due
to problems of high voltage, switching surges and load fluctuations. Repetitive
operation of synchronization and interconnection is tedious job having chance
of mal operation.
Slip frequency up to 0.1 Hz, phase angle up to 15 or 20 degree and voltage deviation up to 5 or 6 percent is permissive during normal synchronization. But these limits should be narrow for safe synchronization of these delicate systems. Slip frequency not more than 0.05Hz, phase angle not more than 5 degree and voltage deviation not more than 2 percent may be ideal for secured synchronization. This is labyrinth and time consuming operations requiring extra care.
Loading: Initially
live generation is available from grid or successful island or self start
stations within short time. These are limited sources and primarily used for
providing auxiliary power to waiting stations. Islanded or self started stations
are operating with house load and adjacent external load for minimum
requirement for stability. Once the supply is extended to waiting station,
these local loads can be curtailed as required to make room for load of auxiliaries.
After fulfilling the requirement of auxiliary power, spare power if any, is supplied preferably to specific users load only. Specific user load is quick contactable non fluctuating small motors load that is feasible to adjust as necessary for stability of source station. Motor load adds system inertia and bias for rigidity. Small motor has negligible impact while ON-OFF and also suitable for adjustment of load. Fluctuating load like rolling mill or railway etc is not suitable at this stage. There should be a list of such load center near each power plant as ready reference in emergency. Other essential load can be catered when system grow to sustain such loading.
System starts growing with generators back in service during restart process. Load is resumed as per priority (according to list of category wise priority) in accordance with available generation. Initially bare minimum lines are taken in service to avoid over voltage in very light load condition. Line loading increases with increase of generation and load. Therefore line parallel to loaded circuit is taken in service to share the load. Accordingly lines are taken in service in steps as required to strengthen the network with growth of system. Picking generation, adding load and network strengthening are concurrent activities to avoid over loading as well over voltage.
Power Plants
Level: One
experienced engineers at each station has
to coordinate
all local activity at power plant. He is single source link between his station
and LDC for information and operation. He is suppose to collects post
occurrence status details from concerns for generators, auxiliaries and switch
yard without waiting for call from LDC. He will provide all required
information regarding plant to LDC and coordinate locally for implement
instruction from LDC.
Status report: Abnormal operation during collapse have probably pose some problem like bursting of diaphragm, failure of lightening arrester/current transformer, buchholz operation of GT/UAT, sparking, burning of contact/jumper, leakages, mechanical fault, fire etc. Such element have to be checked for healthiness or require rectified before it can be back in service. Generator system status considering time required for inspection, testing, rectification, replacement, availability of spare and worker etc is estimated and reported to LDC for onward planning of restart. Reporting has to be systematic without haste and ambiguity to avoid confusion. Following details regarding status of generators is useful for planning at LDC.
Live on line - Successful islanding. Generation level, load catered, margin for pickup, etc
Quick Start - Feasibility of self start gen-set with associated
requirements.
Hot standby - Gen-set that can restart quickly on timely auxiliary
power.
Waiting - Gen-set ready for startup waiting for auxiliary
power.
Holdup - Gen-set under rectification with expected time for
ready.
Switchyard - Data similar to transmission system.
Above
vital information is very useful to LDC to proceed for restart activity. Progress
in the matter is also updated at each milestone.
Care for Auxiliary Power: LDC manages with transmission system for auxiliary power as per strategy. Activity and precautions at power plant depends upon the role as source or sought of auxiliary power.
Source Station: Governor and AVR systems have to be in active mode to take care for load and voltage changes. Waiting station is to start auxiliaries one by one in consultation of source station and LDC. Each time source station has to ascertain spare capacity required for auxiliary item to start. Curtail home or external load for margin as above and raise frequency above normal before switching.
Waiting Station: Clear bus to take power for auxiliary. Avoid diversion of power to other than auxiliaries. Auxiliary items are switched on one by one with time gap allowing source to reset for next step. Start small motors first when feasible to add stability to source before impact of high load. When possible start motor with no or partial load and gradually load afterward.
Synchronizing: Generators are synchronized
with the local bus that may be with grid, cluster or source only depending on restart
method adopted by LDC. Generators are operating with grid in synchronous
restart method. So they are stable and no need for individual load generation matching.
Generators can pick up just monitoring outlets loading.
Generator is not stable initially in integrated restart method but stability improves with growth of the system. Governor and AVR systems have to be in active mode. Load generation matching is required because of small system not connected to grid. Frequency, voltage and line loading have to be monitored and regulated. System has to be synchronized with the grid at permissive stage.
Generators are not stable in cluster restart method. Governor and AVR system have to be in active mode. Strict load generation matching for each cluster is very essential for stability of cluster. Due care is required for independent operations within territory avoiding inter cluster operation. Clusters have to be synchronized with each other and with the grid at appropriate stage.
Loading: Loading of generators depends
on method of restart. Generators back to service can pick up as per permissible
ramping in case of synchronous restart method. No need to match load
generation. Only network loading is watched. LDC manages with transmission and
distribution to release load according to priority. Picking generation in other
cases is in coordination with load released. Progress is slow because adding
load requires connecting lines and substations. Operations for network
extension is critical and performed with due care.
Transmission Level: Each substation particularly on backbone network has one experienced engineer as coordinator. Resumption of power supply takes some time. Therefore they have to cautious to avoid undue power usage. Available stored power is used sparingly to avoid no power condition. Switch off non important carrier channels, prefer manual operation instead of remote operation, operate isolator instead of breaker in dead system and so on. Avoid connecting unpredictable unstable RE sources.
Status Report: Station coordinator collects post occurrence abnormal condition of equipment at the station and ready to report to LDC on call. Non availability of any equipment may be due to burning of contact or jumper, failure of LA-CT-CVT, breaker low air/gas pressure, buchholz operation, fire, etc. Estimate expected time for checking healthiness and rectification considering availability of spare and worker. Correct ON/OFF status of breakers is significant while extending power to dead station for startup auxiliary or interconnection. Meters reading zero in dead system, visual remote indicator may have fused lamp or supply break. So have to double sure about ON/OFF status of breakers. Fault estimation and local inspection is required when line or transformer has tripped by protective system during system collapse. Information about workable condition of synchronizing arrangement with instruments and operator is also reported.
Operation for Startup Power: This is a series of sequential
operations to provide auxiliary power from source to waiting power plant. Sequence
of operation is important and not to ignore in haste. Operations performed as
per guidance from LDC. Terminology used during operational dialogue should have
explicit meaning for status and operation to avoid ambiguity and waste of time in
recall and clarification. Suggestive terminology is as under. This is useful
during black start as well normal operation also.
Bus Clearing: Switching off all power outlets from the concern bus. This means isolation of bus from incoming and outgoing lines, inter bus transformers and bus coupler but not bus PT. This arrangement is necessity to feed power only to target without diversion to elsewhere. Otherwise power supplied to bus also received by wide area in onward network. These uncontested stations start switching lines and load assuming resumption of supply. Ultimately unexpected load and lines cause abnormal low frequency and high voltage result in failure of source system.
(No need to operate breaker of incoming, outgoing lines, inter bus transformers and bus coupler etc. This may cause low air/gas pressure and breaker operation is held up when required. Concern isolators can be operated for this purpose when system is dead)
Charging: Switching ON line/transformer from one side while other side is open. In this condition power does not flow but only small charging current appears.
Test Charging: This is similar to above but when healthiness of line/transformer is not sure such as when line/transformer has tripped by protective devices. Generally test charging is done with lower setting of protective device to avoid backward reflection on healthy system.
Charged: Line/Transformer is ON from one end only whereas other end is open. Such status is for specific purpose. Charged line is for capacitive reactive power support at connected bus to boost voltage. Sometime it may be to safeguard against theft of conductor. Charged transformer may be for control of voltage rise at connected bus OR to keep warm to maintain dielectric strength of insulation system.
Taking in Service: Switching ON charged line/transformer from other end to put in service to enable power flow. (Actual power flow depends on system parameters)
In Service: Line/Transformer is ON from both sides allowing path for power flow. (However power may not flow when system is dead as in black out OR when voltage and phase angle are same at both ends.)
Tripped: Automatic breaker made OFF by protection devices.
Switch OFF: Intentionally breaker is made OFF for specific purpose. (Avoid words “Hand Trip’ or ‘Manual Trip’ to avoid confusion with automatic trip.
Standby or Available: Equipment is ready to push in service as no work permit pending or no doubt for its healthiness.
Operation is simple when waiting station is next door to source station. First confirm that bus at waiting station is clear. Charge line from source station and take in service at waiting station to energies the bus. Auxiliary load started one after other with short break to readjust at source station. Source station should manage and confirms the spare capacity required for auxiliary item to start and raise frequency above normal. Connected external or home load is reduced to make spare capacity as required. Procedure is almost similar when one or more substations between source station and waiting station. The bus of first intermediate substation is energies as above after isolating it from all power outlets. High voltage is expected in case of high voltage long line. In that case inter bus transformer is charged with other end open. Subsequent substations are energies in similar way up to waiting station. All operations perform in consultation of LDC to eliminate the scope of error.
Synchronization: As usual incoming generator is synchronized with the local bus. In case of synchronous restart method local bus is charged from grid supply to provide auxiliary power. So restarted system is operating with the grid and is most stable.
In case of integrated restart method generators operate in synchronism but not connected to the grid. So this forms independent subsystem which is synchronized with the grid at permissive stage. Power supply from grid is brought to any convenient substation in subsystem where incoming line from the grid is synchronized with bus of substation.
But in clusters restart method generator operates in synchronism with small cluster of 2 to 3 power plants. First all such clusters have to be synchronized with each other to form subsystem. This is a critical operation due to fast and wide frequency variation because of low inertia and small bias. Power from one cluster is taken up to convenient substation in other cluster where incoming line is synchronized with the bus. Procedure of bus clearing etc is followed at all intermediate substation as detailed in power extending for auxiliary power supply. Synchronizing line with bus is unusual operation differing from synchronizing generator with bus. Such event is exceptional and therefore operator may be unaware of such felicity, its condition and operating procedure. Therefore better to confirm before this operation about feasibility for synchronizing line with bus at receiving station. Sometimes synchronizing is not possible at any substation from both clusters. Find out one nearby dead substation where synchronizing is possible. Bus of this substation is cleared by isolating from of all outlets and charged taking power from one cluster. Line from other cluster to this substation is charged and synchronized with bus at this station. Direct short line between these clusters can be taken in service after synchronization of pair. Instead of dead substation, third cluster can also be intermediate provided synchronizing feasibility.
Frequency, phase and voltage matching is required in all synchronization. Very close matching is required for synchronizing delicate clusters to avoid untoward incident. Slip frequency not more than 0.05Hz, phase angle not more than 5 degree and voltage deviation not more than 2 percent may be ideal for secured synchronization. Both clusters operated close to normal frequency and voltage. Parameter matching is achieved by adjustment in concern cluster. All operations are executed in consultation of LDC.
Loading: During initial period of restart minimum lines are taken in service as safeguard against over voltage problem. With the time system starts growing with generators back to service. Load is resumed according to priority as per available generation. So over voltage problem subsides and over load condition arise in some section of network. Therefore network strengthening is required to avoid lines tripping on over load. Line on parallel path of critically loaded section is taken in service to share load and relieve the overload. However proceed with monitoring voltage profile and loading condition.
Field Level: Though limited but has key
role in initial period of black start. Sincere and efficient working at field
level contributes safe and fast recovery of power system.
Information: Distribution sub-stations and distribution field offices in close contact with consumers should reply to public with basic information received from LDC. Field offices should restrain from vague reply like ‘supply has failed from upstream’. Field offices should also avoid giving contact number and direct to LDC and upstream substations for further inquiry. This is very necessary to avoid undue calls from public keeping phone and operator engaged and upset operators with annoying arguments. Such interference from public hampers the progress of restart operation and ultimately delays in recovery.
Avoid: Do not switch ON all distribution feeders when supply is resumed at substation. Wait for loading instruction or inquire but avoid adding load without confirmation. Avoid switching long rural feeder having wide spread uncontrollable load. Also avoid charging of distribution substation in odd remote area because load control is impossible by local operators. Initially fluctuating and varying load like rolling mill and railway etc is avoided. Avoid self decision to supply power for strong representation and compelling for urgent need of power to particular group of loads. Strictly avoid taking link line to other substation in service without instruction/consent of LDC. Power at other station may be from other cluster having different frequency. Act of connecting two sources without matching parameters may result in havoc.
Loading: Load is to release as per directive of LDC. When advised for loading, prefer to supply local load that can be quickly controlled as required. Consistent motor load is most suitable for better stability. Station operator should be at phone to receive instructions of LDC and implement immediately without waiting for consent of other authority.
Normalization: During restart process all field operations were controlled as required for system rebuilt. Switching loops to other stations, distribution feeders, under frequency scheme and supply to peculiar load were restricted. Load was regulated by switch on and switch off as required in restart process. Industries were advised not to start their process in view of unstable supply.
Power system consider as normal when major generators are back to service operating with the grid and almost full transmission network is back in operation. LDC announces power system for normal operation. Grid operation is handed over to regular operating team at LDC, power plants and substations. All distribution feeders and inter linking loops allowed to switch on. Islanding schemes and under frequency automatic load shedding schemes are put back in service. Consumers inform about normal supply to use as usual.
Black start procedure is commenced by LDC and completed by distribution.
Conclusion: Rebuilding of power system from scratch after blackout to normal operating condition is tough task. Early recovery without any hitch may be the result of efficient coordinated efforts by all levels. Procedure has four level functions but knowledge about functions at other level is very useful for timely coordinated actions for early recovery. Knowledge of functions at all level is essential for LDC as navigator. Some important information is repeated with this purpose. This generalized procedure is followed according to system setup, blackout status, availability of resources and trained and accustomed man power.