Friday, March 06, 2015

Plant DC System

How DC System Works?

The DC system draws AC power from the plant electrical distribution system, converts it to DC, and supplies the plant critical DC loads as well as the UPS system. The DC system also includes the batteries which provide backup power supply to the DC loads during plant emergency (black out).DC supply system includes batteries, battery chargers, battery switch boxes, DC distribution and sub-distribution boards and DC motor control centers. 

When DC System Can Be Utilized?

During normal operating conditions, battery chargers, together with DC distribution boards, supply main power to the emergency lighting and UPS inverters, and auxiliary control power to the switchgears, DCS, communication equipment and multiple protection and control panels of the plant.

 During plant blackout, the batteries supply power to emergency drives such as emergency oil pump, vacuum breaker valves, etc. to shut down the plant safely. In addition, the batteries maintain essential control power (e.g. circuit breaker tripping, alarm annunciation etc.).

Plant DC system schematic diagram

Power Flow

The DC system draws incoming 480V AC power from the 480V switchgears.The 480 V AC is then rectified to 120V DC in the thyristor bridge rectifier circuit. The rectified 120V DC is supplied to the DC distribution boards, MCCs and UPS system for further distribution. A part of the rectified DC charges the batteries.
  
Operating Mode

1.  FLOAT MODE

This is the normal mode, in which the charger supplies the DC load and keeps the battery in full charge by supplying float charge current. Because it is a constant voltage mode, the battery charger current will tend to taper off to near zero as the battery becomes fully charged.The voltage chosen is that value at which the internal losses are compensated by the charge current into a charged battery.The battery is maintained fully charged yet remains below gassing voltage so that the rate of loss of electrolyte is negligible.

2. AC SUPPLY FAIL MODE



When the AC supply to both of the charger fails, the charger is switched off and the battery starts supplying the DC load without any interruption.

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