125VDC in MV SWGR

[weressl]

Hello guys,

I am currently reviewing bids on MV Swgr. I am new at this and I wanted to know where is the 125VDC power source in the SWGR come from? Also, what does the 125VDC service inside the SWGR? I know this might sound vague but does 125VDC source usually provided by the vendor (who provides the SWGR) or is it coming from some place else outside the SWGR, then fed into the SWGR?

Also, what does the 125VDC power? Circuit breakers, CT's,etc?

Thanks

DC is provided for the operation of essential equipment when AC power is lost or not available. DC is normally provided by lead/acid batetry banks. 48V used if the distances are short and all equipment is in the same switchgear building and one would use 125V if there is an outside switchyard and the distribution concerns itself with voltage drop. Remember lower voltage, lower number of DC cells, higher reliability.

MV switchgears operate differently than regular contactors. They had a motor that charges the opening/closing springs and have DC solenoids that mechanically trip said springs. After loss of power some breakers are required to trip and the DC is needed. If the control voltage is derived from the bus, then you would not be able to close the main incioming breaker, since there is no power on the bus:roll:. So the chargins motors and the breaker operating circuits are usually powered by the DC, but there is a trend to power all protective relaying and metering from the same source for simplicity and also that you can retrieve the stored data in case of power failure and analyze it before restoring power.

PT's are usually mean Potential Transformers used for metering and CPT's are used for Control Power Transformers, where DC voltage is not used.