The SIGMA3-C is a combination of a ground fault protection and Resistor Path Fault (RPF) monitoring relay that protects systems from ground faults, open or short circuits within the Neutral Grounding Resistor (NGR) as well as in the conductor between the NGR and the source, typically a transformer or generator.
The SIGMA3-C is designed for both High Resistance Grounded systems (5kV or below with fault current limited to 10 amps or less) as well as Low Resistance Grounded systems (above 5kV with fault current above 10 amps and typically time-rated).
There are 4 components in a typical system, the Neutral Grounding Resistor (HRG or LRG), the SIGMA3-C relay, a Zero Sequence Current Sensor (ZSCS) (TXA or RX-YA), and an NGRXS-## sensing resistor.
The SIGMA3-C uses both current and voltage sensors to monitor the health and integrity of the ground path from the X0 source bushing to the ground. The NGRXS-## sensing resistor is used to measure the voltage across the NGR, and ZSCS is used to measure the current through the NGR.