In recent years our company has successfully developed the numeric high impedance differential protection function, which provides protection solution for busbar, transformer, generator, reactor and motor applications where fast clearance of faults is required with high stability against external fault conditions and fast & reliable performance against internal faults.
This operating principle requires stabilizing resistors (RST) in each phase and a non-linear resistor (MOV) is generally required across the relay circuit.
| This protection can also be widely used at neutral grounded star transformers, where high internal phase-to-ground fault detection sensitivity can be reached. It is also called as Restricted Earth Fault protection (87N). |  |
During internal faults the current transformers are quickly saturated, so the differential current can be highly distorted. Because of the current distortion, a special differential current measurement is required. Protecta uses special hardware solutions for the measurement:
- for restricted earth fault protection, the 4th channel of CT+/5153 current module can detect the peaks of the differential current. Therefore, the first three channels remain for the overcurrent protections;
- for autotransformer/busbar/generator/reactor/motor applications, the first three (1st – 3rd) channels of CT+/5116 current module can detect the peaks of the differential currents, so the module can be only used for high impedance differential protection.
More technical details can be found on the related product page:
https://www.protecta.hu/protecta_products/smartline/s24/variant-7