220 volt DC system
The 220V DC system supplies direct current as source of
operating power for control, signaling, relays, tripping and closing of
switchgears, emergency motors of most important auxiliary systems.
Under normal conditions of station generation, the storage battery
units are kept floating in DC bus bars by means of the trickle chargers (also
known as float chargers). The trickle chargers of each battery unit, which is a
rectifier with AC input, is normally made to take all DC requirements of the
power station without allowing the battery to discharge. This is achieved by
maintaining the DC output voltage of trickle charger a few volts higher than
the voltage of the battery.
A typical 500 MW Main DC Plant is described below. In a power
plant Number of DC system are running parallel in different BOP packages like
Air compressor ,CHP ,AHP etc. Low voltage DC system is also sunning as per
requirement of the system like FSSS,DCS,ATRS etc. However we are describing
only main plant. Switchgear DC system at switchyard also not considered.
Main Plant consists
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.
Battery Cells
- Data Sheet
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).
Above is standard specification of DC voltage system followed by BHEL in 500 MW EPC contract. Cell
voltage sometimes changed into 2 V +/- 5% instead of 1 volt .
Following installation checks to be carried out during erection of the DC
system
1. Room to be checked thoroughly
about level , acid resistance tiles etc
2. Check the battery about damage , condition of battery i.e dry or wait.
3. If it is dry then prepare the electrolyte for filling.
4.Check all the cable connection from battery bank to DCDB
5. Check the installation of Battery charger and cabling.
6. Follow all safety precaution listed in safety manual
7. Use calibrated instruments for measurements
of parameters.
8. As we will be handling acid solution extra precaution is required for
safe handling.
9. Check the ventilation system including exhaust fans are working properly.
Continued part –ii for
commissioning of battery charger
