GUIDELINES FOR HOUSE LOAD OPERATION 500MW SETS

 

I.          CONDITION FOR HOUSE LOAD OPERATION: -

 

Unit will come on House Load operation by disconnecting from the grid and feeding power to its own auxiliaries when a unit is required to be isolated from the grid due to some external fault, i.e. fault beyond the step-up transformer, or it encounters such operating conditions when the grid frequency drifts beyond acceptable operating limits. The signal for house load will be communicated to various controls of the set. The unit shall continue to operate under house load mode till such time it can be resynchronized back to the grid when the external conditions permit.

 

 

 

II.       ADDITIONAL REQUIREMENT OF VARIOUS EQUIPMENTS.

 

A.             TURBO GENERATOR PROTECTION

 

                  Review of TG tripping on any external fault beyond step up transformer: Presently, for any external fault beyond step up transformer, the step up transformer circuit breaker trips along with tripping turbo generator unit through unit protection relay (86). The trip circuit in generator protection panel therefore needs modification such that external fault would cause tripping of step up transformer breaker alone and not the turbine generator unit. For this modification, the external fault signals i.e. back up over current protection, bus bar differential protection and pole slippage protection shall be wired separately to step up transformer circuit breaker tripping circuit instead of the unit protection relay (86).

 

                    Review of generator over frequency / under frequency setting: Another modification required is for under frequency and over frequency tripping. The turbo generator unit trips through unit protection relay when unit is operated beyond permissible operating frequency limits. Presently, the frequency signal is taken from generator PT. This protection is needed even during house load protection. Therefore, it shall be retained as it is. However, for isolating the unit during grid frequency variation beyond set limits, a separate frequency signal from the bus bar PT is required to be wired directly to step up transformer tripping circuit. The time settings of the grid frequency tripping signals shall be set lower than the existing under frequency and over frequency trip settings for proper discrimination. It is also possible that the operator can hand trip the step up transformer to isolate it from the main grid when the grid frequency drifts beyond the acceptable limits. The hand tripping of step up transformer will not cause operation of unit protection relay (86).

                     Checking of the interlocks of 52GX contacts on house load.

                     Discussion to be held with electrical side expert as to what should be in generator, bus, etc. to have smooth operation.

 

B            BOILER CONTROL

 

                  As far as boiler is concerned, the house load operation results in sudden load throw off - say from 100% to HP by pass capacity (60% BMCR). Even though boiler is designed for the house load operation, the setting and control procedures are necessarily to be established to achieve successful transfer. It is expected that upon closing of turbine control valves and in spite of quick opening of HP by pass valves, the boiler outlet pressure might increase suddenly resulting in opening of emergency relief valve and spring loaded safety valve at super heater outlet header. Drum safety valves are not expected to blow. However, the drum level control on auto and also in manual mode will become difficult while safety valves are blowing. Therefore, proper experienced manual intervention may be sometimes required during that time because safety valve popping has to be prevented. Opening of LP by pass valves, simultaneously with opening of HP by pass valve, will prevent the reheater pressurization.

 

                 Opening of generators step up transformer circuit breaker will result in the following feed forward signals to respective controllers.

 

BA) Quick opening command to HP bypass controls.

 

BB) Boiler run back command to coordinated master control.

 

BA)    HP BYPASS CONTROL: -

     After quick opening of HP by pass control valves, the HP bypass control is left in the normal pressure control mode of operation with reduced pressure set point. That is during the pressure control mode, after the quick opening command vanishes, the HP bypass set pressure value may be set as below.

 

     HP bypass pressure is derived from turbine set pressure in coordinated master control and biased by a fixed valve above the pressure set point for turbine. During house load operation this bias could be removed so that HP Bypass can take faster action.

There are two possible modes of pressure setting of turbine, viz. Sliding pressure mode and fixed pressure mode. The unit will normally operate at constant pressure mode upto 80% of the load (72% BMCR) and thereafter it will work on sliding pressure mode upto 100% load. The change over from one mode to another is at present done manually through push button provided in coordinator Coordinated master control console. For unit load above 80% (72% BMCR), the mode of control will could be sliding pressure. As per the curves for the sliding pressure operation furnished by BHEL, at 80% load (72% BMCR) the pressure will be around 120 kg/cm2 at turbine inlet (129 kg/cm2 at super heater outlet). Thereafter, from 80% load to 100% load the pressure will slide from 129 kg/cm2 to 162 kg/cm2 at SH outlet. When the unit assumes sliding pressure mode of operation, the fixed pressure set value will remain at 129 kg/cm2. During the changeover to house load operation when the mode of pressure control is put on fixed pressure mode from sliding pressure mode, the pressure set point therefore assumes original fixed pressure value i.e. 129 kg/cm2. The only modification required in the existing set up in Coordination Master Control is to provide a binary signal for automatic changeover of pressure of pressure control mode from sliding pressure to fixed pressure during changeover to house load operation. The binary signal can be derived from generator circuit breaker opening signal. However, in case the unit was already running on fixed pressure mode when the requirement for house load operation comes, then the operator has to manually set the pressure to a reduced value. It may also be required that the pressure biasing of HP bypass pressure set point is not above turbine pressure set point normally at 5 8 kg/cm2 to be set not more than 5 kg/cm2. The HP/LP bypass will come into operation immediately on opening of step up transformer and continue to be in service dumping about 60% of the steam flow to condenser initially and thereafter to dump any excess steam when boiler is stabilized at minimum technical loading. The present interlock scheme at Chandrapur already has the facility to open HP bypass valve on opening of step up transformer circuit breaker and once the HP bypass valve is fully opened, the existing steam pressure controller will takeover and modulate the HP bypass valve to control the steam at fixed pressure.

 

     

      The HP bypass spray temperature set point has to be suitably decided based in site trails to prevent Reheater overheating. When BP valves open fully due to fast opening, it is necessary to adjust the feed forwards to the BPE so that these open at least 75% even before there is any rise in temperature.

 

     

 

      When BP valves open fully due to fast opening, it is necessary to adjust the feed forwards to the BPE so that these open at least 75% even before there is any rise in temperature.

 

 

 

BB) BOILER RUN BACK

 

      The boiler run back command reduces the fuel flow to boiler upto the minimum value allowed per elevation by reducing the PA flow through mills and the feeders would then reduce the speed to maintain the same DP in the mill. If the mill DP was not in auto but the Mill PA flow was in auto then the operator must manually reduce the feeder speeds to prevent mill choking when PA flows are getting drastically reduced. If some mill is running without the PA flows in auto then whenever the house load operation starts the operator must be alert and reduce those PA flows manually and quickly. The adjustments for boiler run back rate and runback target are provided in the existing Coordinated Master Control, which have to be set at site suitably. This itself may not be sufficient to restrict the target load to 60% in case more than 4 mills are in operation. Therefore, all the top rows of mills in excess of 4 may have to be tripped on initiation of house load operation. Suitable modification in FSSS could be necessary. The decision can be taken only after one live trial is done. There is logic available in the FSSS for tripping excess of four mills when one PA fan or one FD fan trips. This logic can be motivated for the requirement of house load operation also. Based on the field trials, suitable automatic selection logic for the mill to be tripped may be established.

 

C.            TURBINE CONTROL

 

              On receiving GCB open signal, due to maximum selection of the two, the speed controller assumes control of the turbine and brings the turbine to the set reference speed. House load operation may not be resorted to if the turbine is in the hydraulic governor control mode.

 

      As the control valve will attain the position corresponding to house load condition, the HP first stage pressure will drop. This will cause the variable set point in LP bypass controller to be reduced. Simultaneously, the reheater pressure will tend to rise, LP bypass controller will act to open the LP bypass valve in order to maintain reheater pressure to the reduced LPBP set point or fixed LPBP set point pressure whichever is high. Spray water valves will open to de-superheat the bypassed steam being dumped to condenser.

 

With the rise in the CRH pressure, HP exhaust temperature will rise and the trimming device will operate to increase the flow through HP turbine, which facilitates in restricting the rise in HP exhaust temperature.

 

      While reducing the boiler load, care shall be taken to ensure main steam temperature not below 490 C to avoid TSE margin reduction to zero.

 

Following protections / interlocks and control should be reliable and available for house load operation: -

 

-          Turbine should be on Electro hydraulic governing.

-          Availability of turbine protections.

-          Functioning of load shedding relay.

-          Functioning of trimming device.

-          Availability of TSE.

 

           

            At the time of conducting the trial following must be monitored and put on fast mode trending in addition to normal data acquisition of operating parameters.

 

-          HP exhausts temperature.

-          Generator voltage.

-          Load (MW).

-          Speed.

-          Control valves positions.

-          Steam pressure before HPT.

-          Steam pressure after HPT.

-          IP exhaust pressure.

-          HP secondary oil pressure.

-          IP secondary oil pressure.

-          Control oil pressure.

-          Trip oil pressure.

-          Primary oil pressure.

-          Opening of CRH NRV.

-          Generation of sequence of events during the trial.

 

D.          TURBINE FEED PUMP

 

As the availability of electrical feed pump is ruled out because of it being fed from station transformer, the unit is to depend only upon the turbo feed pumps. Since the turbine load is drastically reduced, control will switchover to auxiliary control valve taking steam from cold reheat line being fed from HP bypass and turbine.

 

E.            OIL HANDLING PLANT

 

Power supply to oil Handling Plant is at present derived from station board. During house load operation, in all probability, there will not be any station supply and therefore these pumps will not be available for oil support for the boiler. Since the boiler load is expected around 60%, due to availability of HP bypass, there is less possibility for flame failure. However, this risk factor is to be noted for the house load operation. Because of very fast unloading of the fuel, there is a possibility of Furnace pressure going low or even flame failure. In Chandrapur the furnace pressure control is by VFD by which we will probably able to take large changes in ID fan speed as required.

 

III            PRE-REQUISITES FOR HOUSE-LOAD OPERATION

 

1.   CMC is to be in service-GCB open contact is required for run back. Presently this is from master trip relay. Though the turbine will come out of CMC mode, in the CMC station the Boiler Master must be still in auto.

 

2.   AVR should be on auto

 

3.   Following auxiliaries should be available for their regular operation:

                                                              i.      CW PUMPS

                                                             ii.      VACUUM PUMPS

                                                           iii.      CONTROL FLUID PUMPS

                                                          iv.      VAPOUR EXTRACTOR CF

                                                            v.      VAPOUR EXTRACTOR OIL

                                                          vi.      1 AOP

                                                         vii.      SEAL OIL PUMPS INCLUDING VACUUM PUMPS

                                                       viii.      PRIMARY WATER PUMPS

                                                           ix.      CONDENSATE PUMPS

                                                            x.      TD BFPs

                                                           xi.      ACW PUMPS

                                                         xii.      SERVICE AIR COMPRESSOR

                                                        xiii.      INSTRUMENT AIR COMPRESSOR

                                                       xiv.      ID FANS

                                                        xv.      FD FANS

                                                       xvi.      PA FANS

                                                     xvii.      MILLS

                                                    xviii.      SEAL AIR FANS

                                                       xix.      OTHER RUNNING AUXILIARIES OF BOILER

 

IV.Resynchronisation with the grid:

On attending the faults resulting into House load operation, set may be synchronised to the grid in the normal way, manually.

 

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