8/11/2019 Differential Re

1/1

Electrical Project Installation Training Programme

Session VII 4

B. External protection provided to transformer at site :-1) Differential protection :- This is a unit type of protection provided for the equipments onHV/LV/MV side windings of the power transformer. This is connected to the protection cores ofthe C. Ts installed on each side of the transformer winding i.e. on HV/MV/LV side. The faultany where in these C. T.s is detected and the relay operates to isolate the transformer fromelectrical circuits. The ratio of the C. Ts connected on HV/MV/LV sides is matched according tothe voltage ratio of the power transformer. The differential relay is connected across the C.Tsecondary in such a way that the resultant current due to the fault shall be fed to the differentialrelay. The C.T secondary current is balanced at normal tap of the transformer provided withOLTC/off load tap changer. The differential relay is provided with a circuitry to meet with therequirement of distinguishing real fault and the charging inrush currents. Also the % bias isprovided to a relay operating ckts to meet the requirement of mismatch due to change intransformer ratio on account of tap change operation. Always the care is taken to match the C.Tsecondary currents through the differential relay such that under normal condition the operatingcircuit is stable for through faults /external faults. Some times use of auxiliary C. Ts is essentialfor matching the current balance on HV/MV/LV sides. While connecting the C.T. secondary tothe differential relays the care is taken to avoid mal operation due to zero sequence components(Vector group of transformer).

2) Restricted earth fault protection : - This protection helps easy and quick detection of fault inthe star connected winding of the power transformer. Generally this protection is connectedacross the C.T. pilots of the bushing C. Ts provided on star connected winding of the powertransformer. The relay operates whenever there is a fault in the tap changer or the star connectedwinding. Normally the balance three phase loads are feed through the transformer. There is nocurrent flow through the star point neutral to the earth in this normal situation one C.T. of similarratio and the protection class is provided in neutral side of the transformer which is used formatching/balancing the circulating current through the main C.Ts in case of external faults. TheREP relay is connected across this C.T matching ckt. In such a way that the relay shall operateonly in the event of fault in star winding & or OLTC of the power transformer.3) Overcurrent protection: - This protection is generally provided to monitor /restrict the loadingon the transformer to its rated capacity this protection also takes care for unclear faults on thedownstream of the power transformer. This type of protection is a backup type of protection withIDMTL characteristics. However the instantaneous relay can be supplemented to IDMTL relay toachieve quick isolation of the terminal faults/closer fault. Generally three over current withhighest element is provided on star connected HV side of the power transformer.4) Overcurrent and earth fault protection:- This protection is generally provided to isolate thetransformer in case of earth faults. This is IDMTL in nature and generally connected on LV sideof transformer. This type of protection is also connected on HV side of the transformer with deltaconnected primary winding for detecting faults in the transformer as well as in the event ofuncleared down stream faults as a backup protection.