RECOMMENDED PRACTICE 2.1 Type

2.1.1 Static type Battery Chargers shall be used.

2.1.2 Silicon Controlled Rectifiers (SCR) type battery chargers are provided for both float and boost chargers.

2.2 Scheme

2.2.1 The charging equipment shall be suitable for charging each battery in float as well as in boost mode. The float charger should be provided with Automatic Voltage Regulator to maintain the output voltage within +1% of the set value under all load conditions and for an input voltage variation of +10% and frequency variation of +5%. Boost charging mode should have current control and limiting facility. The boost charger shall be provided with automatic constant current regulator to maintain the current output within +2% of the set value for AC input voltage and frequency variation of +10% and +5%

respectively. The load limiting feature shall be provided for the chargers.

2.3 Type and quantity of chargers

2.3.1 Wherever 2x100% capacity batteries are provided for each service, 2x100% combined float cum boost charger shall be provided (For ex.

Typical scheme of battery and charger connection is shown in sketch no. TCE.M2- EL-CW-S-2633 R0.

(b) For unit loads of each gas turbine, coal handling plant, switchyard etc. one battery, 2x100% combined float cum boost charger shalls be provided. Typical scheme of battery and charger connection is shown in sketch no. TCE.M2-EL-CW-S-2632 R0.

2.3.4 Type and Quantity of chargers

The requirement of type and standby chargers based on different types of plants and unit sizes are presented in Table below:

Sl.No .

Type of Plant Quantity of Batteries

1.1.1 Unit Load 1X100% for each unit

Station Load 1x100% for station load 1.1.3 I & C System 2x100% One per

battery

Floot cum Boost

NIL. I&C system loads 1.1.4 Switchyard 1x100% One per

battery

1.2.1 Unit loads 2X50% for power &

1.2.2 Control loads 1X50% for control loads 1.2.3 Station loads 1X100% One per

battery 1.2.5 I & C System 2x100% One per Floot cum NIL. I&C system

battery Boost loads 1.2.6 Switchyard 1x100% One per

battery

2.1 For all unit sizes

2.1.1 Unit loads 1x100% One per battery 2.1.2 STG loads &

Station loads 2.1.3 I & C System 2x100% One per

battery

3.1 Unit size upto 20 MW

3.1.1 Unit & Station loads 3.2.1 Unit loads in one

block

Set loads in one block

3.2.2 Station loads 1x100% One per battery

4.1 All Unit Sizes 2X100% for complete

5.1 All Types of Plants

(a) Float charger current of the associated battery.

(b) Continuous load of the DC system :

Continuous load of the DC system is obtained from the battery sizing calculations made as per Design Guide for Battery, Part – A and Part – B. It shall be recognised that 10 hour rating indicated in battery sizing does not consider the continuous load of the excitation system, boiler DC control panels, etc. As such the DC loads which exist during the normal running of the plant shall also be considered over and above the 10 hour loads indicated for the battery.

(c) Largest DC motor current.

(d) 25% margin shall be considered over and above loads.

2.4.3 The float charger for I&C system battery shall be capable of meeting the highest of the I&C system loads which will occur during the starting or tripping/shut down or running of the unit/plant.

2.5 Boost charger

The rating of the Boost charger should be adequate to charge the battery after the battery is completely discharged within a period of 10 hours for lead acid and 8 hours for Nickel-Cadmium batteries. The boost charging voltage for lead acid battery will be between 2.40 to 2.75 V/cell and for nickel cadmium it will be between 1.53 V to 1.67 V for high discharge type (H), 1.54V to 1.69V for medium discharge type (M) and 1.55V to 1.7V for low discharge type (L). During boost charging of battery, the load is fed by the float charger. During boost charging the battery, the battery shall be disconnected from the load to avoid uneven charging of the battery bank).

3.0 DISCUSSION

3.1 The spare charger unit wherever provided should have facilities for connection to the DC distribution board. Drawing no. TCE.M2-EL-CW-S-2633 R0 shows typical spare charger connection. The design of the leads from the spare charger should be such that chance of wrong connection (reversal of polarity) is totally eliminated.

3.2 The leads interconnecting the Battery, Charger and DC distribution board should be single core, armoured cables. The cable size should

be rated for the emergency loads (2 hour or 1 hour of battery). The armouring of the cable should be of aluminium.

3.3 The cables shall be sized considering the voltage drop within acceptable limits of 5% from the battery to load terminal.

3.4 The Blocker diodes connected between the end cell Tap and the DC bus should be rated to carry the 1 min. output of the battery. Two diodes in series are preferred to safeguard the D.C system against short circuiting of one diode.

3.5 The ripple content of the DC output nominal voltage of the Float and Boost charger shall be within +1% of the nominal output voltage when battery is disconnected.

3.6 The transformer rating of the float & boost charger offered by the Vendor shall be checked for it's suitability by asking the sizing calculation from the charger supplier.

4.0 ENCLOSURES

i. Float cum boost charger TCE.M2-EL-CW-S-2631 R0 with 2x100% batteries

ii. Float cum boost charger TCE.M2-EL-CW-S-2632 R0 with 1x100% battery

iii. Float and boost charger TCE.M2-EL-CW-S-2633 R0

with 1x100% battery