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 .