Transformer differential relay with speed-up apparatus

A differential relay for protecting a transformer disposed in a power system against internal faults, said transformer having a plurality of windings, each winding having associated therewith a current measuring device which produces a signal representative of the current through said winding, said power system including a breaker operative to interrupt current through said transformer, said differential relay comprising;

• receiving means for connection with said current measuring devices for receiving said winding current signals from said current measuring devices;
• restraint signal deriving means interconnected with said receiving means for deriving a highest restraint signal from said winding current signals;
• operating signal deriving means interconnected with said receiving means for deriving a highest restraint signal from said winding current signals;
• operating signal deriving means interconnected with said receiving means for deriving an operating signal from said winding current signals;
• fault detecting means interconneced with said restraint signal deriving means for receiving said operating and highest restraing signals for detecting an internal fault condition in said transformer and for generating a fault signal representative of said latter detection;
• current inrush means interconnected with operating signal deriving means for detecting a current inrush condition in said transformer and for generating after a predetermined time interval a current inrush signal representative of said latter detection;
• delay means interconnected with said fault detecting means for delaying said fault signal by a delay time corresponding to said time interval;
• generating means interconnected with said delay means for generating a trigger signal for operating said breaker, said trigger signal when produced by said delayed fault signal being inhibited by the presence of said current inrush signal;
• ratio means governed by the ratio of said operating signal to said highest restraint signal and said fault signal for detecting a transformer internal fault condition and for generating, without substantial delay and independent of said generating signal, a quick response signal for tripping said breaker, whereby said quick response signal is operative to be produced regardless of said delay time;
• and
• recovery inrush current means governed by said highest restraint signal and said fault signal for detecting a recovery inrush current and for generating a recovery current inrush signal indicative of said latter detection and for inhibiting the generation of said quick response signal.

A differential relay for protecting a multiphase transformer disposed in a power system against internal faults, said transformer having a plurality of windings, each winding having associated therewith a current measuring device which produces a winding current signal representative of the current through said winding, said power system including a breaker operative to interrupt current through said transformer, said differential relay comprising:

• a differential relay unit for connection with said current measuring device for each phase of said multiphase transformer, each said differential relay unit comprising:
• receiving means for receiving said winding current signals from said current measuring devices;
• restraint signal deriving means interconnected with said receiving means for deriving a highest restraint signal from said winding current signals;
• operating signal deriving means interconnected with said receiving means for deriving an operating signal from said winding current signals;
• fault detecting means interconnected with said restraint signal deriving means for receiving said operating and highest restraint signals for detecting an internal fault condition in said transformer and for generating fault signal representative of said latter detection;
• current inrush means interconnected with said operating signal deriving means for detecting a current inrush condition in said transformer and for generating after a predetermined time period a current inrush signal representative of said latter detection;
• delay means interconnected with said fault detecting means for delaying said fault signal by a delay time corresponding to said time interval;
• logic means governed by said delayed fault signals of each phase of said multiphase transformer to generate a signal for operating said breaker, said trigger signal being inhibited by the presence of any one of said current inrush signals;
• and
• speed-up circuit means governed by said highest restraint signal, said operating signal and said fault signal of a corresponding differential relay unit for detecting an internal fault condition in the corresponding winding of said transformer and for generating in response to said latter detection, without substantial delay and independent of said trigger signal a quick response signal for operating said breaker, whereby each speed-up circuit operates to circumvent said delay time.