500mw Guidelines for House Load Operation 500mw Sets

8/10/2019 500mw Guidelines for House Load Operation 500mw Sets

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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 andfeeding power to its own auxiliaries when a unit is required to be isolatedfrom the grid due to some external fault, i.e. fault beyond the step-up

transformer, or it encounters such operating conditions when the gridfrequency drifts beyond acceptable operating limits. The signal for houseload will be communicated to various controls of the set. The unit shallcontinue 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 withtripping turbo generator unit through unit protection relay (86). The

trip circuit in generator protection panel therefore needsmodification such that external fault would cause tripping of step uptransformer breaker alone and not the turbine generator unit. Forthis modification, the external fault signals i.e. back up over currentprotection, bus bar differential protection and pole slippageprotection shall be wired separately to step up transformer circuitbreaker 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 unitprotection relay when unit is operated beyond permissible operating

frequency limits. Presently, the frequency signal is taken fromgenerator PT. This protection is needed even during house loadprotection. Therefore, it shall be retained as it is. However, for

isolating the unit during grid frequency variation beyond set limits, aseparate frequency signal from the bus bar PT is required to bewired directly to step up transformer tripping circuit. The timesettings of the grid frequency tripping signals shall be set lowerthan the existing under frequency and over frequency trip settingsfor proper discrimination. It is also possible that the operator canhand trip the step up transformer to isolate it from the main grid

when the grid frequency drifts beyond the acceptable limits. The


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hand tripping of step up transformer will not cause operation of unitprotection 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 loadoperation, the setting and control procedures are necessarily to beestablished to achieve successful transfer. It is expected that uponclosing of turbine control valves and in spite of quick opening of HP

by pass valves, the boiler outlet pressure might increase suddenlyresulting in opening of emergency relief valve and spring loaded

safety valve at super heater outlet header. Drum safety valves arenot expected to blow. However, the drum level control on auto and

also in manual mode will become difficult while safety valves areblowing. Therefore, proper experienced manual intervention may besometimes required during that time because safety valve popping

has to be prevented. Opening of LP by pass valves, simultaneouslywith opening of HP by pass valve, will prevent the reheaterpressurization.

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 bypasscontrol is left in the normal pressure control mode of operation withreduced pressure set point. That is during the pressure control

mode, after the quick opening command vanishes, the HP bypass setpressure 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 thepressure set point for turbine. During house load operation this biascould 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 willnormally 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 atpresent done manually through push button provided in coordinatorCoordinated master control console. For unit load above 80% (72%BMCR), the mode of control will could be sliding pressure. As perthe curves for the sliding pressure operation furnished by BHEL, at

80% load (72% BMCR) the pressure will be around 120 kg/cm

2

atturbine inlet (129 kg/cm2at super heater outlet). Thereafter, from

80% load to 100% load the pressure will slide from 129 kg/cm2to


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162 kg/cm2at SH outlet. When the unit assumes sliding pressuremode of operation, the fixed pressure set value will remain at 129kg/cm2. During the changeover to house load operation when themode of pressure control is put on fixed pressure mode from slidingpressure mode, the pressure set point therefore assumes originalfixed pressure value i.e. 129 kg/cm2. The only modification required

in the existing set up in Coordination Master Control is to provide abinary signal for automatic changeover of pressure of pressurecontrol mode from sliding pressure to fixed pressure duringchangeover to house load operation. The binary signal can be

derived from generator circuit breaker opening signal. However, incase the unit was already running on fixed pressure mode when therequirement for house load operation comes, then the operator has

to manually set the pressure to a reduced value. It may also berequired that the pressure biasing of HP bypass pressure set pointis not above turbine pressure set point normally at 5 8 kg/cm2tobe set not more than 5 kg/cm2. The HP/LP bypass will come intooperation immediately on opening of step up transformer and

continue to be in service dumping about 60% of the steam flow tocondenser initially and thereafter to dump any excess steam when

boiler is stabilized at minimum technical loading. The presentinterlock scheme at Chandrapur already has the facility to open HPbypass valve on opening of step up transformer circuit breaker andonce the HP bypass valve is fully opened, the existing steam

pressure controller will takeover and modulate the HP bypass valveto control the steam at fixed pressure.

The HP bypass spray temperature set point has to be suitablydecided based in site trails to prevent Reheater overheating. When

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

When BP valves open fully due to fast opening, it is necessary toadjust the feed forwards to the BPE so that these open at least75% even before there is any rise in temperature.

BB) BOILER RUN BACK

The boiler run back command reduces the fuel flow to boiler uptothe minimum value allowed per elevation by reducing the PA flowthrough mills and the feeders would then reduce the speed tomaintain 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 manuallyreduce the feeder speeds to prevent mill choking when PA flows aregetting drastically reduced. If some mill is running without the PAflows in auto then whenever the house load operation starts the

operator must be alert and reduce those PA flows manually andquickly. The adjustments for boiler run back rate and runbacktarget are provided in the existing Coordinated Master Control,


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which have to be set at site suitably. This itself may not besufficient to restrict the target load to 60% in case more than 4mills are in operation. Therefore, all the top rows of mills in excessof 4 may have to be tripped on initiation of house load operation.Suitable modification in FSSS could be necessary. The decision canbe 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 FDfan trips. This logic can be motivated for the requirement of houseload operation also. Based on the field trials, suitable automaticselection 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 bringsthe turbine to the set reference speed. House load operation may

not be resorted to if the turbine is in the hydraulic governor controlmode.

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 setpoint in LP bypass controller to be reduced. Simultaneously, the reheaterpressure will tend to rise, LP bypass controller will act to open the LP bypassvalve in order to maintain reheater pressure to the reduced LPBP set point orfixed LPBP set point pressure whichever is high. Spray water valves will opento de-superheat the bypassed steam being dumped to condenser.

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

While reducing the boiler load, care shall be taken to ensure mainsteam 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 normaldata acquisition of operating parameters.

-

HP exhausts temperature.- Generator voltage.


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- 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 feedpumps. Since the turbine load is drastically reduced, control will switchover toauxiliary control valve taking steam from cold reheat line being fed from HPbypass 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 stationsupply 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 HPbypass, there is less possibility for flame failure. However, this risk factor is tobe noted for the house load operation. Because of very fast unloading of thefuel, 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 willprobably 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 willcome out of CMC mode, in the CMC station the Boiler Master mustbe still in auto.

2. AVR should be on auto


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3. Following auxiliaries should be available for their regular

operation:i.

CW PUMPSii. VACUUM PUMPS

iii. CONTROL FLUID PUMPSiv. VAPOUR EXTRACTOR CF

v.

VAPOUR EXTRACTOR OILvi. 1 AOPvii. SEAL OIL PUMPS INCLUDING VACUUM PUMPS

viii. PRIMARY WATER PUMPSix.

CONDENSATE PUMPSx. TD BFPsxi. ACW PUMPS

xii. SERVICE AIR COMPRESSORxiii.

INSTRUMENT AIR COMPRESSORxiv. ID FANSxv. FD FANSxvi. PA FANS

xvii.

MILLSxviii.

SEAL AIR FANSxix. OTHER RUNNING AUXILIARIES OF BOILER

IV.Resynchronisation with the grid:

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

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500mw Guidelines for House Load Operation 500mw Sets

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