Harmonic Restraint Differential Relay for Transformer Protection:
The operation of the relays because of magnetising inrush current can be avoided by using kick fuses across the relay coils or using relays with inverse and definite minimum time (IDMT) characteristics. However, for EHV transformers, the relay current and time ratings necessary to ensure stability on the magnetising inrush current caused by switching-in the transformer are not adequate for providing high speed protection.
A high speed biased differential relay incorporating a harmonic restraint feature is immune to the magnetising inrush current. The magnetising inrush currents have a high component of even and odd harmonics (about 63 % of second harmonics and 26.8 % of third harmonics) while harmonic component of short-circuit currents is negligible. The use of these facts is made for restraining the relay from operation during initial current inrush. The harmonic restraint differential relay is sensitive to fault currents but is immune to the magnetising currents. The operating coil of the relay carries only the fundamental component of current only while the restraining coil carries the sum of the fundamental and harmonic components.
Basic circuit of an harmonic restraint differential relay is illustrated in Fig. 12.8. The restraining coil is energized by a direct current proportional to bias winding current as well as the direct current due to harmonics. Harmonic restraint is had from the tuned circuit (XC – XL) that allows only the fundamental component of current to enter the operating circuit. The dc and higher harmonics (mostly second harmonics) are diverted into the rectifier bridge feeding the restraining coil. The relay is adjusted so that it will not operate when the harmonic current exceeds 15% of the fundamental current. Both the dc and higher harmonics are of large magnitude during magnetising inrush.
The relay may fail to operate due to harmonic restraint feature if an internal fault has considerable harmonics, that may be present in the fault current itself due to an arc, or due to saturation of CT. Also, if a fault exists at the instant of energization of transformer harmonics present in the magnetising current may prevent the operation of the relay. This problem can be overcome by providing instantaneous overcurrent relay in the differential circuit which is set above the maximum inrush current but will operate in less than one cycle on internal faults. Thus fast tripping is ensured for all internal faults.
The other method used is harmonic blocking. In this method the harmonic component of magnetising inrush current is used for blocking a separate relay, called the blocking relay, whose contacts are in series with the contacts of the differential relay. The blocking relay contains a 100 Hz blocking filter in operating coil and 50 Hz blocking filter in the restraining coil. During inrush currents the second harmonic component is predominant and the blocking relay is blocked. The blocking relay contacts remain open. During short circuits, fundamental component is predomiÂnant, so blocking relay operates and relay contact circuit is closed.