What is high resistance grounding?
In a High Resistance Grounding (HRG), the neutral point or star point of transformer or generator is connected to one end of high resistance and other end of the resistance is connected to ground. High resistance grounding is used in applications where total interruption of the supply can not be afforded.
In a solid grounding system the neutral point is directly connected to earth and thus very high fault current flow through the equipment to ground. Contrary to this, in HRG system, the fault current is limited to 10 Ampere or less by the neutral grounding resistor. limiting fault current not cause escalating point of fault damage. Moreover, it limits the touch potential. The fault current flowing through the neutral can easily be detected with the protection system to locate ground faults.
In a solid grounding, the phase voltage of the sound phases does not rise because the neutral point is clamped to the earth potential. However, in the case of HRG, the phase voltages of the unfaulted phases will increase during ground fault. The increasing of phase voltage during ground fault may cause increased possibility of second ground fault.
To avoid this, line- to- ground and line- to- line insulation needs to be improved to take care of increased phase voltages during fault. High resistance grounded system reduce the possibility of arc flash between line-to-ground to make the system safer, however it does not limit line-to-line arc flash energy. one of the advantages of HRG system is that it control the transient overvoltage phenomenon which happens in the case of ungrounded system.
In an HRG system, the fault current is limited by high resistance in the neutral and earth circuit. Thus, the fault current gets limited and the protection can not be ensured with fuses or circuit breakers as a protective devices. The fault current is sensed by the current transformer by measuring the current in the neutral circuit.
The output of the relay can be used to trip the circuit breaker or can be used as an alarm. The system may be kept operating even after earth fault occurs in the system. Thus, the uninterrupted supply can be ensured. The detected ground fault can be repaired later on in planned manner
High resistance grounding and protective relays
In a solid grounding system, the system impedance only limits the fault current. The system impedance is generally low and the high magnitude of earth fault current similar to short circuit current flow through the neutral circuit and, it cause overcurrent protective devices to trip the faulty section.
IN an HRG system, current sensing protective relay is used to detect the earth fault. The relay can be used for either tripping the system or for alarm only. In an HRG system, overcurrent devices like circuit breaker and fuses can not isolate the faulty circuit as current flowing through the equipment gets limited by high resistance connected in the grounding circuit.
Typical applications for HRG
The HRG is adopted where high availability of the power supply is desired and power outage is not desired due to safety viewpoint. The electrical system of mining industry operates in a harsh environment like dust,moisture and vibration. The insulation of electrical system is more vulnerable in mining industry and on the other hand uninterrupted supply is must for safety point of view. The HRG system is a good choice for mining industry.
One installation can be rely on HRG to allow continued operation with one ground fault. The other electrical installation may be provided with ground fault relay with selective coordination of tripping.
In case of the fault, the upstream feeder must trip first to isolate the supply system and the main supply system having HRG must not trip,it should output alarm only. Read More: