ELECTRICAL SYSTEM 220 V DC SYSTEM FOR MAIN PLANT- ABC OF THERMAL POWER STATION

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