SEMINAR ON ISLANDING BY PAWAN

What is islanding? Normally all Power generators are interconnected to form a network called ‘GRID’. Some times it so happens for a generator to come out of grid, either due to disturbance in grid or due to some other internal reasons. When generator gets disconnected from the grid it is ‘ISLANDED’. The advantages of Islanding in our network: Unit tripping can be avoided Tripping of major critical loads in steel plant & JPOCL can be avoided Generation loss can be avoided Oil consumption can be reduced or avoided If other unit got tripped, it can be restored fast Islanded unit can be connected back to grid much faster, once grid disturbance is cleared.`

How islanding happens? In our network detection of islanding at 220KV will be done under the following conditions: oSensing of open condition of Bus Coupler oSensing under voltage in Bus-2 (islanding bus). Settings are at 75% with 1.5 sec time delay & 73% instantaneous oUnder frequency setting is at 47.6 Hz with 2 sec time delay & 47.5 Hz instantaneous oOver frequency setting at 51Hz with dF/dt of 0.5 Hz/sec oSensing of no power flow <2% in both lines with dF/dt 2 Hz/sec oSensing of Line-1, Line-2 and either of ICT-1 220KV or 400KV breakers

Scenario generation & loads before Islanding: GT-1, CPP-2, ST-1, ST-2, T-1, T-2,T-3,T-4, T-5, T-7, T-8 & T-9 will be connected to BUS-2 (Islanding Bus) GT-2, T-6, T-10, L-1, L-2, ICT-1, ST-3 & ST-4 are connected to BUS-1 CPP-1 will be selected either in mode-1 or mode-2 depending upon their gas availability. Mode-1 will trip CPP-1 in case of islanding and Mode-2 will bring it to house load. CPP-2 has been chosen to put in parallel islanding along with one of our unit. Depending upon the CPP-2 generation and BF-3 status, following modes has been incorporated In case CPP-2 generation is less or it nor required in islanding scheme a face plate is provided in DCS to enable or disable CPP-2 tripping in line loading logic.

Consequences Islanding: In case of islanding & bus coupler open, a hard wired logic provided to trip both the 220KV lines. In this condition if ICT is healthy, then U#2 will survive else it will trip. L1 & L2 breakers open to non-islanding unit trip is removed Islanding detected signal will enable U#1 EHTC: To change over from LOAD to SPEED control A speed reference set point will be generated from EHTC based on below formula NR = NRTD = One hour average actual speed + ((Actual load / 150 )*0.04) rpm NRTD value is limited to rpm on upper side and rpm on lower side. NRTD will be fast tracking NR in speed control mode. Islanding detected signal will annunciated in control room, MSDS control room & JPOCL.

Operator Actions Post Islanding: Immediately after islanding, ensure fuel input is marginally more to boiler than required for islanding load. Depending upon the islanding load & Throttle pressure, HP Bypass should be kept open close to 15%. Match the HP Bypass fixed pressure set point to actual pressure & put HP Bypass in manual. Reduce the HP Bypass SP slowly as per requirement, to keep open the HP Bypass. If unit is running on full coal, as a measure of precaution oil support should be taken. Download Speed Ref SP in Speed ref D/L block with that of NRTD Further regulation in speed can be effected through the increase /decrease arrow in speed ref D/L station. It is to be noted that during the increase in frequency load will increase and accordingly fuel has to be increased.

ISLANDING CHECK LIST:  Islanded unit is running in speed mode  Bus coupler, L-1 & L-2 breakers are in open condition  Drum level in normal limits, if required take S/B BFP in service  Observe for actual speed, if it dropping trend continuously, inform to cut out portion of complex load  For any drop in network frequency, EHTC will try to maintain the frequency SP by injecting more steam flow through CV. At that point of time fuel should be sufficient in the boiler to cater for demand. Hence fuel input, frequency & pressure should be continuously monitored by operator  Islanding bus frequency fluctuation should be less 0.5 Hz / min  Generator PF and MVAR are normal  All equipments are in service / healthy

Normalization:  Ensure the grid conditions are stable & clearance obtained for synchronizing the bus coupler  Reset islanding master trip relay (in islanding panel at MSDS) & ensure islanding detected alarm gets reset in ECP in CR. Islanding cut out should not be done without resetting the relay at MSDS.  Match the speed reference D/L set point with that of NRTD & after matching synchronize the machine by closing the Bus Coupler.  Check the actual load of machine after synchronization.  If the load is less than 45 MW, Increase the Load with Speed Ref D/L Block, Load the machine more than 50 MW.  Set the load reference using Load reference D/L block such that the sum of load reference matches the Actual load.  Reset ISOLATED GRID at EHTC 2 graphic (now it will not reset until Master trip relay is reset, hard wiring done in both units).  Change over Turbine Control from SPEED to LOAD AT EHTC 2 graphic (now change over will not happen until ISOLATED GRID is reset).  Keep HP Bypass in AUTO & load the machine to full  Cut-in Islanding as per requirement

Learning from previous islanding incidents:  On 22/04/08 Islanded unit (U#2) tripped due tripping of feeder on phase failure. Later over load relay settings of feeders are reviewed. As preventive action sufficient oil support should be taken irrespective of available fuel.  On 01/07/08 Unit#1 got islanded while cutting in islanding, as Bus coupler open feedback is persisting. As precautionary measure, one additional alarm K35 relay operated hooked up in DCS to annunciate the operator. If K35 relay acted alarm is persisting, islanding should not be taken in service.

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