Microprocessor-based current differential relay with overcurrent, earth fault and thermal elements


The differential current relays type SKC is intended for the protection of generators, large motors, two winding power transformers and short lines.
The relay SKC feature the following characteristics:

  • seven current inputs, where six are provided for current phase measurements and the last one dedicated for residual current measurement;
  • internal compensation of phase shift and ratio matching for transformers differential function (87T); conventional external compensation can also be selected with external CT's;
  • stabilizing circuit with dual slope with variable bias makes the differential functions of SKC relay insensitive to external fault (87T, 87G);
  • second and fifth harmonic restraint stabilization with strong magnetizing inrush currents and overexitation condition (87T) based on numerical processing of differential current (DFT algorithm);
  • alernative set thresholds for 49-50-51-50N-51N/87N protection functions which are automatically enabled upon detection of high second harmonic above a programmable set-point to prevent tripping for transformer energization (inrush);
  • earth fault protection insensitive to thirth and higher harmonic components;
  • selectable operation curves with independent or dependent time (first thresholds) for overcurrent and earth fault functions;
  • trip counting performed for each function;
  • shorting values of input quantities corresponding to the four last operations (START or TRIP) and trace memory with pre and post trigger;
  • four final relays, independently programmable as energized/de-energized and latched/no-latched;
  • three signalling LEDs (ON,TRIP,ALARM);
  • two digital inputs circuits;
  • double set-point groups (bank A and bank B)
  • blocking circuits to perform accelerated protection schemes either with programmable logic from internal funtions with block circuits of external protection relays;
  • trip circuit supervision (TCS)
  • breaker failure protection (BF)
  • serial communication circuit RS485 with MODBUS RTU protocol;
  • real time clock with memory backup;
  • continuous monitoring of internal circuits ;(SELF TEST).
SKC - Serie S ()


Key:   standard standard optional optional alternative alternative


  • Transformer

Protection functions

  • Thermal image (49)
  • Phase overcurrent (50/51)
  • Residual overcurrent (50N/51N)
  • Differential protection for generators and motors (87G/87M)
  • High impedance restricted earth fault (87NHIZ)
  • Biased differential for transformer (87T)
  • Breaker failure (BF)