ELECTRICAL SYSTEM ( HT ) PANEL 11KV FOR EQUIPMENTS AND SYSTEM (PART -I)- ABC OF THERMAL POWER

Till last dispatch basics electrical system discussed which are essential for the erection and commissioning of the Power system. Earthling rods and flats are laid at different places when civil works under progress. Riser are kept at different places to get it connected. Slowly it becomes like a grid. There value etc are precisely monitored time to time. After that main plant DC system are getting erected .As it was told no AC system will run without  the DC input.

Two more systems are coming after that one is HT ( 11kv/6.6 KV /3.3KV system ). It allows to distribute power in power plant as bulk and also the source running equipment like BFP , ID ,FD PA etc motors . Higher rating equipments are connected with HT ac voltage and this will reduce the cable size considerably and easy handle at site.

Here is brief description of panel in a 500 MW power station. There will be variation site to site but more or less these feeders will be there. This will give a good idea of number of HT panel to be commissioned. Viewers comments /value addition  will be appreciated.

11 KV Switchgear

Generally a few companies supplied these panels and they are BHEL . ABB , C&G ETC . these panels are installed in service bay near unit one side at an elevation of 8.5m .

1.         PLANT DETAILS :

       THIS SYSTEM CONSISTS OF  11 KV AND 3.3 KV HT SWITCHGEARS (UNIT SWITCH BOARD BA,BB,CA,CB FOR UNIT #1 AND UNIT #2 AND STATION SWITCH BOARD 0BA,OCA FOR 11 KV/3.3 KV  STANDBY SWITCHBOARD  SUPPLY IN CASE OF BOTH GENERATORS OUT OF SERVICE) .

MAKE: BHEL,BHOPAL

RATINGS VOLTAGE : 11 KV & 3.3 KV 3PHASE 50 HZ AC , CURRENT:2750 A

11 KV SWITCHGEARS

I.The 1BA Board consists of the following HT Panels :

1.Incomer Panel ( incomer from UT A )

2.PT PANELS ( line /feeder PT  and Bus PT )

3.         Tie breaker from standby switchboard (0BA)

4 .        OUTGOING FEEDERS    

            3240 KW Cooling Water Pumps 1 & 2

2450 KW Primary Air fan        A (Unit #1)

            16MVA Unit Aux. Transformer A

            16MVA Misc. service Transformer()

            2MVA  Unit service transformer A

            3MVA VFD transformer (ID Fan 1A Channel 1&2)

            1.25MVA Bunker area service transformer

            1.6 MVA ESP Service transformers

            1.25 MVA CHP transformer 1A

            0.75 MVA Track Hopper Service Transformer

            1.6MVA Fuel oil service transformer

            7.5 MVA Station  aux. Transformer.

            2 MVA Station Service Transformer

            6.3 MVA CHP Aux.

             5. Spare feeders

            a) 3240 KW Motor feeder

            b)2 MVA Transformer feeder

II.The 2BA Board consists of the following HT Panels :

1.         INCOMER PANEL

            Incomer Panel ( incomer from UT A ) (Unit #2)

2.         PT PANELS ( line /feeder PT  and Bus PT )

3.         Tie breaker from standby switchboard (0BA)

4.         OUTGOING FEEDERS    

            3240 KW Cooling Water Pump 4

            2450 KW Primary Air fan        A (Unit # 2)

16MVA Unit Aux. Transformer A

            16MVA Misc. service Transformer

            2MVA  Unit service transformer A

            3MVA VFD transformer (ID Fan 2A Channel 1&2)

            1.25MVA Bunker area service transformer

            1.6 MVA ESP Service transformers

            1.25 MVA CHP transformer 1B

            0.75 MVA Track Hopper Service Transformer

            1.6MVA Fuel oil service transformer

            7.5 MVA Station  aux. Transformer.

            2 MVA Station Service Transformer

            6.3 MVA CHP Aux.

            5.  Spare feeders

            3240 KW Motor feeder

            2 MVA Transformer feeder

III. The 1BB Board consists of the following HT Panels :

1.         INCOMER PANEL

            Incomer Panel ( incomer from UT B ) (Unit #1)

2.         PT PANELS ( line /feeder PT  and Bus PT )

3.        Tie breaker from standby switchboard (0BA)

4. OUTGOING FEEDERS    

            10000 KW MD BFP (Unit # 1)

            3240 KW Cooling Water Pump 3

            2450 KW Primary Air fanB (Unit # 1)

16MVA Unit Aux. Transformer B

            2MVA  Unit service transformer A

            3MVA VFD transformer (ID Fan 1B Channel 1&2)

            1.6 MVA ESP Service transformers

            1 MVA Switchyard Service Transformer

            2 MVA Ash Sys Compressor Plant Transformer

            1 MVA Ash Water Power House Service Transformer

            2 MVA Off Side Plant Service Transformer

            2 MVA WTP Area Service  Transformer

            2 MVA Station Service Transformer

            0.5 MVA Raw Water Pump House Service Transformer

            1.6 MVA Administration Building Service Transformer

5. Spare feeders

            2450 KW Motor feeder

            1 MVA Transformer feeder

IV.The 2BB Board consists of the following HT Panels :

1. INCOMER PANEL   

            Incomer Panel ( incomer from UT B )  (Unit #2)

2. PT PANELS ( line /feeder PT  and Bus PT )

3. Tie breaker from standby switchboard (0BA)

4. OUTGOING FEEDERS    

            10000 KW MD BFP (Unit # 2)

            3240 KW Cooling Water Pump 5

            2450 KW Primary Air fanB (Unit # 2)

16MVA Unit Aux. Transformer B

            2MVA  Unit service transformer A

            3MVA VFD transformer (ID Fan 2B Channel 1&2)

            1.6 MVA ESP Service transformer

            1 MVA Switchyard Service Transformer

            2 MVA Ash Sys Compressor Plant Transformer

            1 MVA Ash Water Power House Service Transformer

            2 MVA Off Side Plant Service Transformer

            2 MVA WTP Area Service  Transformer

            2 MVA Station Service Transformer

            0.5 MVA Raw Water Pump House Service Transformer

            1.6 MVA Administration Building Service Transformer

5. Spare feeders

            2450 KW Motor feeder

            1 MVA Transformer feeder

Above description is typical of coal based super thermal power station feeder list. There may be variation site to site and of uses. But this will given an idea of HT drives and transformer required for HT drives.

Recently i was visiting a super thermal power station and there HT panel erection is going on. Some other picture on this will be on next dispatch.

continued to pat -ii

220 VOLT DC SYSTEM-BATTERY CHARGER -COMMISSIONING PART -I I-ABC OF THERMAL POWER PLANT.

COMMISSIONING PROCEDURE:

     TESTING OF BATTERY CHARGERS:

The insulation value shall be checked with 500 V DC Megger. During this test, all low voltage circuits  & Relays etc, shall be  bypassed ( shorting)

The panel shall be switched on at no load. For this, first make input switch ON & check the voltages at all phases indicated by lamps  & voltmeter by selector switch . Now the voltage is connected to the primary of the Main transformer which steps the voltage down as required for the specific DC output Voltage & isolates AC Input line from Battery circuits. The transformer's secondary is connected to the Full wave rectifier assembly.

The bridge is standard  three phase full wave half controlled silicon bridge. The voltage control is accomplished by the application of a positive pulse to the gate terminals of the SCR at the desired time. Before the pulse is applied , the SCR is open  and no current flows in the circuit.  As soon as SCR is fired, it operates as a standard silicon rectifier until the forward current is reduced to almost zero.

Check the no load output voltage at the output terminals.

Ensure the output voltage is steady without any trouble for 15 minutes.

Check for annunciation interlocks  etc.

If load is available, the equipment shall be loaded progreesively upto rated value

Check for voltage regulation, free from oscillation, ripple etc.

Check for dropping characteristics at higher loads.

Run the equipment for 4 hours and observe of stability and check for local heating.  When unit is loaded the ripple voltage level (peak to peak) may be observed . Follow the advice of Service engineer.

II          FIRST CHARGING OF BATTERY:

 Thumb rule says DC  Voltage needed for battery charging will be 2 times the no of cells in battery bank. The initial charging will take approximately 16 to 24 hours since the batteries are already charged and sent as maintenance free ones.

k board is firmly coIt is of utmost importance that the terminal of the battery is connected to +ve load of charging source. So polarity should be ascertained.

Commence charging not later than 24 hours after connecting all battery cells.

Give the first portion of charge at a rate of 100 Amps for 10 Hours

Discharge it using Water resistance or halogen lamps as recommended by supplier

Give the second charging at the rate of approximately 20 Amps till the signs of completion  of charge observed.

Select any one cell (except the end cell or regulating cell) as pilot cell out of every rack as given in the layout of the battery bank.

Voltage and specific gravity reading from pilot cells will indicate the state of charging of the whole battery system. Follow supplier’s recommendation

All cell terminals & connections are to be applied with pertroleum jelly to reduce contact resistance. No adjustment should be made which will disturb charging.

When all the cells have been greasing freely and when voltage and specific gravity of pilot cell has been constant over 3 successive hour readings, the cells can be deemed to be fully charged and the charging may be terminated. Then wipe down outside of all cells and clean the floor.

Put the battery system on trickle charging when charging completes.

The trickle charge should be adjusted to give optimum battery voltage of 1.53 - 1.67 volts per cell.

III         ENERGISING OF DISTRIBUTION BOARD:

Take final I.R. value of the board. Check ea
ethi ected with earth through wire or strip as shown in the drawing.

Ensure that all outgoing circuits are locked in OFF position.

Ensure that all safety procedures have been observed.

Check that polarities of D.C switch board battery charger and battery are matching.

Close the incomer switch of board to charge it.

Measure D.C Voltage at the Input.

Observe D.C board for any abnormality.

If board condition is satisfactory, check that all remote end fuses of outgoing circuits are removed.

Switch ON outgoing feeders one by one. Record the following for each circuit.

i.)         Polarity  at both ends of circuit

ii.)        Voltage at both ends of circuit.

IV       EARTHFAULT RELAY ON LOAD TEST:

To carry  out on load test for the Battery E/F relay, connect variable resistance box of the range of 15 to 150 K Ohms between +ve  pole of the board and earth

Keep selectivity setting plug of E/F relay  at 3 ms setting.

Cut out resistance slowly to simulate an E/F  and observe the current in milli ammeter when the relay operates.

Record resistance and current in mA

Carry out this test on each available setting on relay.

While doing test observe and record the direction of deflection in the ammeter. Check whether it confirms the pole on which the fault is simulated.

Repeat  once a again on -ve pole of the board.

The test is satisfactory if the operation of the relay confirms with the setting.

Note : Electrical commissioning demands cool approach not rushed to wards the job. Any mistake may cause permanent damage and that was witnessed many a time.

All instrument need to calibrated so that during dispute you can rely. This is also a common problem.

All hand tool should be insulated one and value checked before using.

Electrical signboard /safety signboard should be displayed .

220 VOLT DC SYSTEM-BATTERY CHARGER PRE-COMMISSIONING PART -I -ABC OF THERMAL POWER PLANT.

220 volt dc system with battery charger commissioning

Till last dispatch I have discussed about the basic requirement of electrical system in apower plant . In continuation with that commissioning of DC 220v system will be described briefly.

The system consist of :-

i.          Battery Bank

ii.         Battery Charger - Main and Standby

iii.        DC Distribution board.

The battery is installed in the battery room in the HT switchgear room.  This provides a 220 V DC supply to the relay and control circuits in the Generator. Generator transformer and Unit transformer protection and control panels, 11KV  and 3.3 KV HT switchgear and 4.5 V LT switchgears Unit and station service boards and aux. Control panels. As told you earlier 220 volt DC system is backbone of the electrical system .To energise AC system you need DC system.

Battery Cells - Data Sheet ( Typical 500 MW power plant

            Guaranteed AH Capacity at 10 Hrs.discharge

            Rate 1 Volt / Cell at 27 C                                -           990 AH

            Designation                                                     -           KPH 990 P

            Recommended range of floating/trickle

            Charging voltage                                            -           1.4 -- 1.42 V/Cell

            Recommended range of boost charging

            Voltage                                                            -           1.53 -- 1.67 V/Cell

            Trickle charging current                                 -           1 to 2 MA /AH

            Boost charging current                                   -           198 A

             Open circuit voltage of cell when completely

            Discharged at 27 C at 5 hrs discharge rage   -            1.0 V

            Dimension (mm)                                           -            522 L x 192 W x 405 H

            Amount of specific gravity or electrolyte

            Per cell  required for first filling at 27 C         -          18 Lit / cell, 1.19 +- 0.01

            Specific gravity of cell when fully charged                1.19+- 0.01

            Max. electrolyte temp. that cell can withstand

            Continuously                                                  -           50 C

            Net dry cell weight                                        -           45 Kg

            With electrolyte                                             -           68 Kg

            Cell container material                                  -           Polypropylene

            No of cells / Bank                                         -           169

             Charger - I & II

            Input Voltage                                                  -           415 V+- 10%

            Output Voltage                                               -           220 V DC

            Float Voltage                                                   -           236 V DC

            Boost Voltage                                                  -           287.3 V  DC

            Rectifier current limit                                     -           400 A

            Battery current limit                                       -           198 A

            Auto charge over current                               -           40 A

            Steady state output voltage Regulation (For +- 10% I/P

            Voltage variation                                            -           +-1%

            0 - 100 % load variation

            at a time).

The above data was given from a data sheet of particular power station but this may vary marginally  power station to power station depending on system design and integration for which I am not expert.

Pre -commissioning  

- Battery,  battery charger and distribution boards are installed as per drawing.

- Cabling between the battery / battery charger and the distribution boards completed .

-     

      Basic testing .quality checks of components like insulation testing ,continuity testing , dimension checking etc are over. Charge the distribution board live at 220 V DC in  a safe and orderly manner without load and keep it charged for 24 hours.

- To carry tests and checks to ensure that all the components on the boards are operating satisfactorily including cables to all the remote points in the 400 KV substation and control room.

-  AC distribution board for input supply to be charged and commissioned.

-  Each module will be checked individually

-  Each outgoing supply will be tested up to the fuses at the remote end.

-  Battery E/F will be simulated for both the poles and E/F relay sensitivity will be

    checked.

-  Following  services will be  required during ,testing and pre –commissioning

 - 415 V,  3 Phases, 50 Hz supply ,Adequate running water, Suitable voltmeter, ammeter,

  

   thermometer and hydrometer,Variable resistance or water load for discharging the battery.

    Electrolyte for cells as per IS-266, Pure battery grade distilled water, Variable resistance box

    of 15 to 150 K Ohms, 1 No.  500 VDC megger, Multimeter , hand tools etc.

-Standard safety equipment for handling acid and battery .  Also fire prevention equipment like

extinguishsers , Exhaust fans to take out fumes if any , etc should be made available. Different

precautionary sign board in and around the panel battery room etc.

continued part -II battery charger commissioning.