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Tuesday, March 22, 2011

lead-acid battery explosion and prevention

Battery bank of power substation is very critical since it serves as standby power supply for the high voltage equipment and its relays to be operated so that power can be restored in case of massive blackout or total loss of power. Lead-acid batteries are the most common type of batteries used in substation battery banks. Advantages include the ability to supply high surge current, large power to weight ratio, relatively cheap, easy maintenance and long useful life.Hence, maintenance of battery banks are conducted monthly to ensure its effective and reliable operation.
Since lead-acid batteries produce combustible gases like hydrogen and oxygen, explosion could take place if an ignitor is near the batteries. Conducted maintenance of battery banks include monthly testing of specific gravity per cell, voltage per cell, temperature per cell, and total voltage of the battery bank. For lead-acid batteries, specific gravity must be maintained not lower than 1.22, voltage per cell must not be lower than 2.0V, and temperature is maintained below 45 deg. Celcius. Eventhough these maintenance are frequently done, explosion of lead acid batteries can still occur. Here are the possible causes and its corresponding prevention, provided the batteries are on-load:
1. The electrolyte is not maintained on its minimum and maximum level. On a lead-acid battery, there is a minimum and maximum level of the electrolyte to be maintained. The level must not be lower than the minimum since this will reduce the charging and discharging capacity of the station battery. It can lead to overheating during charging process since the electrolyte maintains the operating temperature of the battery. This overheating could damage the separators thus short circuiting of the plates could take place. If short circuit occurs, there is a possibility that an arc will develop thus igniting the gases.
The level of the electrolyte must not go beyond the maximum so that there will be enough space for the gases produced to be dissipated naturally. If the space for the dissipated gas will not be enough, there would be build-up pressure inside the station battery. Also, this will lead to oversaturation of the electrolyte with gas that the battery could no longer charge and discarge normally.

2. Presence of foreign object. Foreign objects are particularly dangerous especially when these materials are conductive which can cause shorting of positive and negative plates. Therefore, during battery testing and maintenance, always observe good housekeeping and keep dirt and other materials away from the batteries.

3. Deterioration of separator. The separator acts as insulator between positive and negative plates. If the plates are deteriorated, short circuit could take place. Proper inspection of the separators per cell can be done so that any noticeable damage or deformity could be further investigated or replaced.

4. Snapping of terminals from the plates. The terminal of the lead-acid batteries is composed of the main terminal (positive or negative) connected to a number of plates. If any of these plates is snapped or separated from the terminal, small arcing could occur which could ignite the combustible gases. Inspection should be done on the terminals if they are already corroded so that the battery can be repaired or replaced.

5. Deliberately placing ignitor near the lead-acid battery. This could happen if an innocent user does not know about the explosive nature of the gases produced by the battery or a saboteur wants to damage the battery. Only Authorized personnels should be allowed to conduct testing and maintenance of station batteries.


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