Basler Electric BE1-87T Transformer Differential Relay

the sensitivity to compensate for transformer voltage taps or CT mismatches. This is accomplished by

providing a restraint factor proportional to the current flowing through the protected zone (through

current).

BE1-87T relays maintain sensitivity at a specified ratio of trip current to through current. This ratio,

generally referred to as slope, is front-panel adjustable in 5 % increments from 15 to 60 %.

Second-Harmonic Restraint

Magnetizing inrush current presents another problem unique to transformer differential relays. Relays

must be capable of detecting the small differences in current caused by the shorting of a limited number

of turns, yet remain secure against the occurrence of magnetizing currents many times the transformer

rating (as seen at one set of terminals).

Although magnetizing inrush is usually associated with the energizing of the transformer, any abrupt

change in the energizing voltage may produce this phenomenon. Common causes are the transients

generated during the onset, evolution and removal of external faults. Desensitizing the relay only during

energization is therefore insufficient.

Magnetizing inrush produces an offset sine wave rich in all harmonics. BE1-87T relays use the second

harmonic to restrain operation because it predominates and because it does not occur in significant

magnitude or duration at other times.

Three-phase BE1-87T relays use second-harmonic sharing. The second-harmonic content of all three

phases is summed together to derive the restraint for each phase. As a result, the second-harmonic

inhibit range and the associated factory setting, is higher than on single-phase relays.

Fifth-Harmonic Restraint

Power transformer overexcitation causes additional exciting current to flow into one set of terminals. This

presents an apparent differential (or operating) current not attributable to an internal fault. Although

potentially damaging, overexcitation is not an internal fault and, therefore, is not an appropriate condition

for transformer differential relay operation. One of the principal components in the complex waveform

produced during overexcitation is the fifth harmonic. BE1-87T relays use fifth-harmonic restraint to inhibit

the differential relay operation.

Unrestrained Trip

Severe internal transformer faults may cause CT saturation. Under such circumstances, harmonic

restraint transformer differential relays may fail to trip because of the extremely high harmonic content in

the waveform. Lack of operation can result in severe transformer damage.

BE1-87T relays provide an independent unrestrained tripping function. When set above the possible

inrush current magnitude, this function provides high-speed protection for the most severe internal faults.