Suralaya was the biggest Power Plant in Indonesia consist of 4 units of 400 MW each and 3 units of 600 MW. They can deliver 3400 MW to the Jawa Bali grid with the capability to produce 22 to 24 TWh per annum. This figure mention that more than 25 % power and energy from Suralaya maintain Jawa Bali capability to deliver power and energy to the end consumer within acceptable high security and reliability.
When failure put Suralaya out of the grid severe condition established immediately and blackout will happen soon since no other plants could react to replace Suralaya at the moment.
This paper will describe several blackout in Jawa-Bali caused by the unwanted tripping of Suralaya Power plant since the beginning.
Suralaya Steam Power Plants.
Suralaya Steam Power Plants located in the western of Jawa in Banten Province consist of 7 units 4 times 400 MW plus 3 times 600 MW. The first unit of 400 MW entered to the grid for commercial operation in 1984 since then entered unit no. 2 in 1985, unit no 3 and 4 in 1989 and the last unit of 600 MW in 1997. All plants directly connected to 500 kV bus of Suralaya Substation without any generator circuit breaker installed at the terminal generators.
Those plants being operated by PT Indonesia Power which was the one of sister company of PT PLN state owned electricity company.
In order to generate electricity, Suralaya SPP used coal as main primary energy for all units. Except for Unit No.1 and No. 2 could use Marine fuel Oil instead of coal up to its nominal power capabilities, Unit no 3 and 4 could use MFO up-to 50 % of its nominal power then unit No 5 to 7 can only be used coal to generate electricity through out its capability.
It should also be understood that for starting and continuous operation Suralaya SPP needs electricity to power their auxiliaries, coal handling facilities and any accessories to assist Suralaya SPP keeps generating electricity. So when there were interruption from the grid which disconnected whether outgoing feeders from generators or any supply to auxiliaries, it could be suspend any generating activities then black out will happen immediately.
Those were phenomena which had been experienced to Suralaya SPP from the early day of its operation and had been given very bad impact to the Jawa-Bali power system.
Cranking Suralaya SPP
When any units of Suralaya SPP should be restarted-up say after outage for maintenance, first it should be prepared fresh water, pulverized coal and igniters ready to fire diesel oil for first firing and several fan and pump turned on to assist start up. Those apparatus can only be powered from the grid since there were not enough electric power provided inside plants. The amount of power required about 16 MVA for 400 MW and about 20 MVA for 600 MW. Bear in mind that due to high capacity motors which should start it needs power supply with high level of short circuits capability.
Depend on situation that start up should be initiated, start up process divided into three categories.
First category was Cold Start Up, it was defined that if units were out for such a long time and then the first metal temperature below 190 degrees C, start up called as Cold Start Up and indeed takes such a high energy to start and also take time before connecting to the grid almost more than 24 hours before it can deliver its first megawatt.
The second category was Warm Start Up, it was defined that if the first metal temperature more than 190 degrees C but below 370 degrees start up process called warm start-up. This process quite moderate for coal fired power plant since it takes time between 8 to 24 hours before it can deliver its first megawatt.
The third category was hot start up. It was defined that if first metal temperature above 370 degrees C. In this case it can be restarted less than 8 hours (normally about 4 hours) before it can deliver first megawatt to the grid.
Since the power and energy required to start was very hugh back feeding from the grid are necessary. Any abnormal condition that could suspend back feeding will prevent Suralaya to start.
Experiencing with blackout.
Since the beginning of planning to build Suralaya SPP there were so many dispute and peoples argue whether Jawa-Bali power system can withstand to the existence of such a big plants and connected directly to 500 kV system since the highest voltage at that time only 150 kV and the interconnections among subsystem only develop not more than 1000 MW during peak load. It means that instability due to reactive compensation could rise uncontrollable voltage since reactive power generated by 500 kV lines was very huge compare to the load and no other means could fully absorb that reactive power especially during low load condition..
As time goes by the first unit of Suralaya entered to the grid which consisted of one 500 kV line from Suralaya to Gandul and step down to 150 kV using 500 to 150 kV Inter-bus Transformer 500 MVA in Gandul Substation . From this substation 150 kV grid were interconnected to other plants and subsystem and then reconnected to Suralaya SPP through 150 kV lines. This 150 kV indeed has a role of main back feeding to feed auxiliaries and another purposes especially as a main power to assist construction of another plants in Suralaya.
The first failure had been occurred in 1985 when two circuits of 150 kV which had been feeding the auxiliaries were tripped out caused the only one units of Suralaya tripped as well and then Jawa-Bali in the blackout situation immediately. This occurrence were surprised to everybody who worked out in power system operation and then several precautions were taken out both from generator side and from grid operator side. Nevertheless since grid are opened to any possible failure due to the uncertainties of environments so failure could not be suppressed down to zero even apparatus were designed to do so.
As a result several failure followed by black out had been hit Jawa-Bali power system most of them were directly been struck Suralaya SPP. According to the fact that has been recorded there were at least 7 (seven) blackout which has been initiated by Suralaya SPP. Among them there were four accident that gave special attention to the stakeholder .
Accident on 24 October 1988 at 13.13 WIT. This was the unwanted incident during construction of Unit No.3 and No.4 when by accident two workers cut the main instrument air piping system which convey pressurized air to operate control and instrument system of Suralaya Unit No.1 and No.2. This incident happened though the engineer has been carefully plan check and instruct necessary action to cut and reconnect any intended pipe. At that time Unit No.1 and No.2 had been operating at full load and deliver almost 800 MW to the grid which had got load about 2500 MW. Since generation were cut more than 25 % of load then Jawa-Bali fell down in to blackout.
Accident on 17 August 1991 at 00.22. This unwanted occurrence really terrible since it has been happened right in the Independence Day of the Republic of Indonesia. The main cause of failure was flash over happened around the body of 500 kV circuit breaker No. 4 at Suralaya Substation. Following those phenomena while breaker try to isolate itself, the interrupting driving part was broken down and the ruins were touch the other parts of substation put all breaker in substation immediately off then cuts all circuits towards Suralaya substation. All Suralaya generators (at that moment Unit No. 2, 3 and 4 mean 1200 MW in operation) tripped out and then caused cascaded tripping to others generators. This failure put Jawa-Bali in blackout immediately since the overall loads only 3635 MW. Further inspection found that flash over around 500 kV circuit breaker had been caused by dust mixed with sea water which evaporated from the open sea water channel across the substation.
Accident on 13 April 1997 at 10.00. This Sunday nightmare had been happened due to the one 500 kV line from Suralaya to Gandul tripped while the other line put out for maintenance. Since then Jawa-Bali blackout due to the fact that at that moment Suralaya SPP had been loaded around 1200 MW almost more than 25% of Jawa-Bali loads. The situation became complicated since back feeding for Suralaya SPP had been too late to be sent and no other generators could immediately picked up the Jawa-Bali load. Recovery time became very long it took more than 12 hours to recover Jawa-Bali System back to normal.
Accident on 12 and 13 September 2002 at 17.52 This recent accident has been happened due to the tripping out of 500 kV lines from Cibinong-Gandul followed by tripping of 500 kV line from Cilegon to Cibinong caused cascaded tripping of all Susralaya generators which had been operating 6 Units ( Unit No. 2 out for maintenance and in the following day the near same incident had been happened while only one unit (Unit No.3) just picked up load and still ramp up to 350 MW. ). This accident had given worst impact almost to all Suralaya generators since in the previous accidents every blackout phenomena initiated by pulling down the power frequency but in the incident on 12 September 2002 power frequency rose up significantly (recorded about 53.5 Hz) meant that generators had been in over speed condition for several minutes before any protection react to protect generators from further damage. The result of this impact found that some bearing in Turbo Alternator No. 5 and 7 became overheated and this fact forced the engineer to make careful inspection prior restart-up. It means that recovery took such a long time waiting for clearance from Suralaya engineer. Finally restarting had been granted after one day check and observe any damage to all Suralaya turbo alternators.
Actions which should me made to avoid damages.
Heat sustained on turbines and condensers was the first priority to watch and protect otherwise heat stress and over pressure will destroy the apparatus. Further actions should immediately be done following those phenomena to :
a. Avoid any damage.
b. Keep any apparatus in such condition that later on ready to restart..
c. Maintain any remaining power within internal system to speed up recovery process.
Safety first action to open vacuum releasing valve will exhaust heat from turbine to the atmosphere. This action will put turbine while seed down to zero pumping out any remaining steam and while keeping vacuum it will also avoid turbine from pressurizing which prevent further damage on pressure protecting apparatus. Another advantage is that restart up can easily advanced soon after back feeding power arrive.
Maintaining ready to start position was the preferred condition while it will speed up restarting it can also maintain all vital apparatus which assist normal operation of the plants. The vital apparatus mean lubrication system, turning gear motor, air heater motor, hydrogen system, control, instruments and relay protection , burner management system and also lighting system to assist operator restart-up. Power source for those apparatus during blackout should be provided from internal emergency sources either from diesel or gas turbine generating set. Suralaya SPP has got Diesel Generating set for each plant and recently was added by 17 MW gas turbine generating set especially to cover Unit no.5 to 7.
Since all vital apparatus required such a big power to restart, it has been set up to avoid Suralaya really in the without power condition by applying several arrangement in system operations.
The first arrangement called island operation. The island operation means that local load can be fed by instantaneous remaining power generated after blackout condition. Selecting local load that should match to the remaining or minimum power of that island has been done by Under Frequency Relays dedicated to use only in island operation establishment. After the accident on 12 September 2002 instead of having under frequency relays, to establish island operation in Suralaya local network it has been also applied Over Frequency Relays and re-set Over Speed protection to maintain generators were matched to the local load.
The second arrangement was called House Load operation. This term means that if emergency condition occur and island operation by accident fail to establish some generators should capable for certain period can withstand with load dedicated to them. For Suralaya No. 5 to 7 since steam by-pass system can withstand in operation loaded up to 25 MW each for les than 30 minutes. Normally this situation could help operators to do any possible action to prevent all generators from further damage.
Those actions that bound to maintain safety were recorded and maintained in proper Standing Operating Procedures applied both to the normal operation and in emergency conditions.
• Even to restart only one plant, it is necessary for SURALAYA to have back feeding from the grid immediately after blackout occurred
• Maximum Back-Feeding Time needs to avoid Suralaya SPP from any damages should be less than 60 minutes. This will minimize damages to the main apparatus (i.e Turbine, Air Heater & Condenser etc )
• If back feeding recover more than 2 hours should Aux Boiler be started providing auxiliary steam to assist firing process.
• Due to the complexity of control activities HOUSE LOAD only be kept less than 2 hours. Any action to resynchronize to the grid should initiate immediately to avoid further tripping.
• From the very beginning of operation, Suralaya has been faced with various blackout conditions. Most of them caused by external failure.
• Except the blackout on 12 September 2002, blackout phenomena sensed and initiated by decreasing power frequency before any tripping taken place, so UFR might hold an island operation.
• On the last blackout power frequency course was unexpected. It first went up so not any UFR could hold island operation and then immediately went down following plants tripped out by over-speed protection apparatus
Author wish to thank to every body which help providing data and information. Author also wish to thank to PT Indonesia Power and Business Strategic Unit Jawa-Bali Control Center and Transmission System. Who also gave the opportunity to collect the information.