sir,

i would like to know that how battery works in gas turbine generator once turbine shutdown? mean from where the battery getting power source to get charge after discharge?

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Ramesh,

The battery used for gas turbine-generator control and protection is being charged continuously by a battery charger. The charger is usually a single-phase, 440 VAC-to-125 VDC (nominal) charger. Some gas turbine-generators have two chargers, both connected to the battery in a redundant configuration (if one fails, the other immediately begins supplying the charging current).

Under normal operating conditions, the battery charger actually provides the current to any load connected to the battery. That's right; the charger supplies the current to the turbine control system, including discrete inputs and 125 VDC solenoid outputs, and generator/transformer protective relays requiring 125 VDC power, the fire protection/detection system, the Emergency L.O. Pump motor starter, 125 VDC Emergency Lighting Circuit, the generator exciter regulator (if powered by 125 VDC), etc. If you observe the ammeter on the charger, you will see that it is usually providing 5-10 Amperes to the loads connected to the battery. That current is not charging current; it's the current required by loads connected to the battery. So, the charger is actually supplying the current required by the loads connected to the battery, while keeping the battery "topped off."

The charger is usually only capable of providing approximately 30-50 Amperes, which is more than enough for continuous, steady-state operation of the loads powered by the battery. If the load connected to the battery exceeds the capacity of the charger (when there is AC available to power the charger) the battery will supply the additional current, such as for the Emergency L.O. Pump or for a 125 VDC starter motor of a diesel engine turbine starting means. Some generators use the 125 VDC battery for "flashing" the generator field to help build up generator terminal voltage during starting. Some times the flashing current required is more than the battery charger capacity, and in that case the battery will provide any additional current required during the flashing period.

When there is no AC available for the charger, then the battery will provide all of the current for any load connected to it. Of course, the capacity of the battery is limited--it can't provide current forever without being charged. But, the battery is sized to provide sufficient current to protect the turbine-generator during an emergency shutdown from rated speed to zero speed, capable of providing sufficient current to any DC pumps required to supply oil to bearings or seals during coastdown to zero speed.

Once at zero speed without charging current, the battery is sized to provide current for some period of time to cycle the Emergency L.O. Pump to provide lube oil to the bearings for cooling them (if the bearings and the shaft are hot and left un-cooled, the bearing material will soften and deform so the Emergency L.O. Pump is cycled periodically to maintain some cooling and to prevent running the battery down quickly (by cycling the pump motor on and off)).

So, the battery, under normal conditions, is continually being "charged" by the battery charger, and the battery charger is really supplying all of the current required for normal operation of the unit. Certain conditions may require slightly more current than the battery charger can provide and under those conditions the battery will supply the additional current. When there is no AC power supply to the battery charger, the battery will supply the power, but not indefinitely.

Some turbine-generators are capable of "black start," meaning they can be started without AC power for motors and control systems. In this case, the battery will supply current to power loads during starting, but as these units approach rated speed the generator actually provides power to critical loads, sometimes including the battery charger. Once the unit is at rated speed, the turbine-generator is capable of supplying the auxiliary loads, including the battery charger.

It is important to maintain the battery per the manufacturer's instructions in order to keep the capacity (amp-hours) as high as possible. This includes periodic load tests and/or "float charging". Note that Mark V, Mark VI and Mark VIe Speedtronic turbine control systems are NOT supposed to be subjected to more than 140 VDC and that the float charge settings of many battery chargers are perilously close to 140 VDC. That means that during float charging, the Speedtronic must be powered-down to prevent damage to the power supplies and components.

Battery maintenance includes periodic checks of electrolyte temperature and specific gravity, as well as visual inspections of cells and plates. If sediment in the bottoms of the cells builds up and begins to short the cell plates together the capacity of the cell can be severely reduced, and the cell can actually reverse polarity. If a lot of current is supplied by the battery with a cell which has reversed polarity the total battery voltage will be significantly reduced during the high-current condition and that can cause damage to come electronic components.

Again, battery maintenance is very important--critical--to turbine-generator protection, including the control system.

Lastly, it was noted above that the battery charger is supplying the current to the loads connected to the battery under normal conditions. This leads some people to believe that the turbine control system can be powered by the battery charger output when there is some problem with the battery. That is <b>FALSE.</b> The battery serves as a "filter" on the output of the charger, which has some significant waverform ripple which the turbine control system power supplies and components cannot withstand. Never power a Speedtronic turbine control system with battery charger output if the battery is not also connected to the charger output.

Hope this helps!