Protection

Battery Bank.

Rating of batteries: 80AH, 220V generally for 132/33KV Level Sub-stations, 200AH to 250AH with 220V DC and above for 220/132KV voltage levels and above.

                                                   Fig: 220V, 200AH  Battery set.

Importance of batteries in EHT sub-stations: The batteries are used for protection of the sub-station equipment from internal faults like Transformer internal faults [Electrical Internal faults, Thermal faults of the Transformers] and external faults like line faults [i.e earth fault, over load faults] and back up protection of the equipment. The batteries are simply can say the heart of the Sub-station, no electrical sub-station can be commissioned without batteries along with charger. The batteries are using along with 3ø Ac/DC FC/BC Charger for protection of  the sub-station total equipment from external and internal faults even in the condition of Station LT supply to charger failed as batteries can feed DC sufficiently for 4 to 6hrs with previous charging.

  

80AH to 250AH, EXIDE, AMARRAJA, STANDARD, electroflo and Ub-Mech make batteries are used for all EHT sub-stations..

Maintenance: The batteries should be clean to avoid self discharge on [+ve terminal to –ve terminal].Inspection of voltages, sp. Gravities, level of the float of battery and connection between the battery terminals to avoid failure of DC as the batteries are connected in series to obtain 220V DC.

Recently the VRLA Maintenance free batteries are used for EHT Sub-Stations rather than Lead acid cells. But the maintenance free battery set makes an open circuit, if one of the cells in battery bank opens. This leads to failing of the 220V DC supply to the protection system.[relays, breakers of the Sub-station].

[precaution: 1]The protection of the sub-station cannot be run with on 220V DC charger supply in the condition of battery DC fails as no reliability of station Transformer LT 440V LT supply.

[precaution: 2]Even though continues availability of the AC supply from station Transformer supply, the bus voltage of LV Bus [33KV] may decreased suddenly due heavy fault current flow due to any external fault simultaneously, the 3Ø LT AC voltage of station transformer out put falls to a great extent and further the charger DC Output also decreases fails the total protection of the sub-station.

Precaution: 3] No one battery or battery bank should not allow short circuit and monitoring the DC leakage current [in mA maximum of 50mA] of the battery bank. If dc leakage observed on either of +ve or –ve terminal, should check out the leakage and disconnect the leakage path in the load circuit.

The DC Leakage circuit;

   
Process of  commissioning of the batteries of different make and capacities;

Sl. .No.	Particulars	Standard		Emco		Ub-mek		Electro flo
		80AH	200AH	80AH	200AH	80AH	200AH	80AH		200AH
A	Initial filling of the batteries
1	Electro lyte sp. Gravity	1.19 0 @27oC		1.19 0 @27oC		1.19 0 @27oC		1.19 0 @27oC
2	Soaking time	12 to 16hrs.		12 to 16hrs.		12 to 16hrs.		12 to 16hrs.
B	First charge	But not >24Hrs.		But not >24Hrs.		But not >24Hrs.		But not >24Hrs.
1	Initial charging duration	100Hrs.		80Hrs.		50Hrs.		100Hrs.
2	Rating of charging	4A for 60hrs.	10A for 60hrs.	3.2A for 10hrs.	8.0A for 80hrs.	6.0A for 50hrs.	15.0A for 50hrs.	4A for 60hrs.	10A for 60hrs.
		Rest for 4Hrs.		--				Rest for 4Hrs.
3.		4A for 40hrs.	10A for 40hrs.					4A for 40hrs.	10A for 40hrs.
4.	Maximum permissible temperatures	50oC	50oC	50oC	50oC	50oC	50oC	50oC	50oC
5.	During charging readings [Voltage]	2.55V	2.55	2.40	2.40	2.40	2.40	2.5	2.6
6.	At the end of the charging [Sp.Gravity @27oC  ]	1200+0.05	1200+0.05	1200+0.05	1200+0.05	1200+0.05	1200+0.05	1200+0.05	1200+0.05
C]	Capacity /discharge  test
1	Discharge current	8A	20A	8A	20A	8A	20A	8A	20A
2	Test procedure :	After completion of the 1st charge, allow the battery to stand on open circuit for not less than 12hrs. butnot more than 24Hrs. discharge the battery through a variable resistor  or acetated water load at constant current equal to I=0.1C, the discharge shall be stopped when the closed circuit voltage of each battery falls to 1.85V, and specific gravity falls to 1.130 obtained in the duration of 10Hrs, maximum.
D]	Recharging
1.	Normal  recharge
2.	Current and end voltage of  the cell	8A up to 2.35V to 2.4V	20A up to 2.35V to 2.4V	11.2A up to  2.4V	28A up to  2.4V	8A up to  2.4V	20A up to  2.4V	8A up to  2.4V	20A up to  2.4V
3.	Finishing charge  Current and end voltage per cell	4A up to 2.55V to 2.65V	10A up to 2.55V to 2.65V	5.6A  up to 2.40 full Volts.	14A  up to 2.40 full Volts.	4A up to full charge	10A up to full charge	4A up to  2.65V	10A up to  2.6V
4.	End sp.gravity of the cell at 270C	1.200+0.05	1.200+0.05	1.200+0.05	1.200+0.05	1.200+0.05	1.200+0.05	1.200+0.05	1.200+0.05
5.	Maximum charging range	12A	30A	16A	40A	18A	45A	12A	30A

Note:Temperature correction: if cell temperature is different form 27⁰C, correction to specific gravity is to applied is 0.0007 per each Degree variation of the below or above the 27⁰C

Maintenance of the batteries:

a] Keep the battery room dry and well ventilated.

b] Keep the battery and its surrounding dry.

c] Keep the battery top surface clean and dry forever.

d] Ensure the tightness of the all battery connector erminals.

e] All battery terminals should be cleaned and covered with thin layer of the petroleum jelly.

f] Traces of corrosion to be removed properly and to be cleaned the terminals with distilled water.

g] Metal kettles should not be used for topping up of distilled water to maintain float level of the each battery.

h] care to be taken when work with metal tools on battery bank to avert short circuit of the battery terminals.

i] When handling electrolyte or concentrated acid, safety measures to be taken like wearing of apron and glows.

j] like bare lights, smoking of cigarette which creates sparks  should be avoided in the battery room.  

Connected Battery Charger:

k] very weak or discharged battery cells should be recharged separately to avert over charging of  remaining batteries in the battery bank..
l] at least once in a 15 days,battery bank should be discharged [i.e kept off the float charger in the load condition of the battery bank] for 1:00 Hrs. to 1Hrs.:30Mins.
m] Check the each battery cell voltage and specific gravity in the load condition to ensure the healthiness of the  each battery cell.
n] ensure the perfect continuity of the batteries by taking voltage of the each cell with voltmeter/multi meter from other adjacent cells terminals as below.

              The voltmeter connectors connected at +ve terminal of the 108^th cell and –ve terminal of the 100^th Cell i.e the volt meter measures the voltage of the 109^th cell including voltage drop due to contact resistances of  connecting bars ‘a’, ‘b’ and internal resistance of the cell. If any loose contact between cells from +ve of 108^th cell to –ve of 100^th cell, the voltmeter shows above 2+0.02 V or abnormal voltage. Sometimes Heat dissipation may be observed due to lose contact resistance I²R loss. The float charger is to be kept on immediately to reduce the load on the battery bank and tight the battery terminals with safety precautions. [i.e the float charger feeds the load in especially in the cases of continues load of EHT sub-stations protection and auxiliaries.

Distilled water or demineralized water  Topping up

Topping up of Distilled water or de mineralized water [as per IS 1069] is necessary up to sufficient level of float of each battery. Distilled water should not be filled above the float red mark which leads to lower the specific gravity of the battery cell.

Topping up of  Distilled water or demineralized water [as per IS 1069] is necessary up to sufficient level of float of each batter is mostly suggestible just before the battery bank boost charging.

Battery trouble	cause	Remedy
over  charging	Excessive gassing and buckling of plates, filling of active material on +ve plates, increasing of the temperature.	Reduce the float or boost charging voltage to a value which attains the specific gravity of electrolyte is 1.200.       Or add distilled water if level is near or below to red mark.        Ensure the accuracy of the voltmeter on FC/BC charger if necessary to avoid over or low charging of the batterys.
Under charging	Low specific gravity of electrolyte,         Reversal of voltage, light colour plates, buckling of the plates.	Put the battery bank in the boost condition which increases the voltage 2.16 V to 2.2/each cell attains specific gravity 1.200 of  electrolyte.       Carefully check leaking of the electrolyte from cell if found arrest the same. Ensure that no earth leakage in the battery circuit or load circuit in the field which lead unwanted discharging of the battery bank.
Corrosion of the  plates	Impure electrolyte	Disconnect the battery from the circuit, remove the electrolyte, flush with distilled water and refill with fresh electrolyte. Further separately charge   the cell up to 1.200+0.005 specific gravity and 2.00+5% V and reconnect the cell in the circuit.
Flaking of active      mterial	Charging done at high rate, material impurity applied on plates.	Charging and discharging limits should be limited between 2.4V to 1.85V
Lose of voltage,  battery voltage falls too on discharging condition	Excessive sulphation, loose connections,        Corroded terminals.	sulphation at initial stage can be rectified by low rate of repeated charging and discharging of the cells.
Continues lowering of the electrolyte level	Loss  of  water in the electrolyte due to evaporation by too high float voltage or over charging.        Leakage of electrolyte.	Corroded parts should be cleaned with warm distilled and cover with thin layer of patrollium gelly or vasaline.       Check  and arrest the leakage.
Over flowing of the electrolyte level  even without the addition of the distilled water	Due to humid atmosphere in the battery room causing condensation on the cell top surface.	Replace container incase of any leakage.        Improve ventilation in the battery room.

DC Chargers:

            Main Input AC supply is fed from AC Distribution Board-3h. 33KV/415V, 100KVA Station Transformer for 132/33KV sub-stations connected to 33KV bus side and feeds to AC Distribution Board. The Float cum/Boost Trickle Charger connected to 3Ø AC Supply on AC Distribution panel. The AC supply converted in to 220V to 245V DC and feeds battery bank, relays and auxiliaries. The Float cum boost 220V battery charger is very important to maintain the batteries healthy charge condition. If the discharging time is more during the line clears/interruptions on 33KV Bus or 33KV Station transformer, the boost charger is used to boost the batteries up to good level i.e specific gravity should be around 1.2+5% and voltage around 2.0 volts + 10% in next load condition. Example: FC/BC Charger Make: HEE 220V Dc, 200AH Float Current: 8 Amps, Boost Current: 16 Amps 

     

Float charging: the charger supplies 220V to 235V to load as well as batteries to light charging to hold full charge [i.e compensate to self discharge and backup during protection operated times]. The charging current is order of 0.2A to 0.5A only.

Boost Charger: the charger is utilized for boost the batteries by supplying 235V to 290V DC, current is maximum of 8A for 80AH batteries and 20A for 200AH batteries. During the boost charging, the entire battery set doesn’t connect across the load as the boost voltage more than system operating voltage i.e >220+10%. It causes failure of protective relays, contractors, and trip, close coils in the breakers. Hence the load is connected from 1to84^th cell of the battery bank to reduce the load voltage up to rated voltage. Entire battery bank connected to boost charger to obtain full charging with DC voltage from 235V to 290V.

Float cum boost charging; This type of battery charging isolates the load and feeds the load on float charging mode. The station batteries are fed through boost mode separately. This type of charging is used when the batteries discharged condition up to thresh hold limit [i.e Voltage <1.9/each cell]. It will improves charge of the station batteries quickly with 235V to 290V can be varied by course voltage tapes of the boost transformer [i.e connected in 3ph. AC supply to feed boost