Neutral Grounding Resistor (NGR)
NGR or NER is an equipment which is used to protect transformers or generators by installing on the null-earth connection path. The main purpose of installing this equipment is to limit the maximum earth fault current, so that the power system equipment do not suffer damage and also within a specified time limit, protection relays can operate. The NGR is a backup device for the main protection system, which reduces the risks of probable fault for the equipment and their operators. Paarsun NGR resistors are designed according to IEEE-32 standard.
Advantages of using NGR:
- To reduce burning and melting effects in faulted electrical equipment like switchgear, transformers, cables, and rotating machines.
- To reduce mechanical stresses in circuits/equipment carrying fault currents.
- To reduce electrical-shock hazards to personnel caused by stray ground fault.
- To reduce the arc blast or flash hazard.
- To reduce the momentary line-voltage dip.
- Secure controlling of transient over-voltages and prevention of blackouts in earth fault circuits.
- To improve the detection of earth fault in a power system.
Main specifications of NGR
Rated Voltage is the effective voltage value (Vrms) at the rated frequency that applies between the equipment terminals, under standard operating conditions, and over a rate time period. The rated voltage of NGR is usually equal to the rated voltage of the line to the neutral (phase voltage) of the power system.
Rated Current is the effective current value of the null point that passes through the equipment under standard conditions within the rated time, taking into account the thermal limits in accordance with the standard. The choice of the rated current values depends on the characteristics of the power system, the protected equipment, as well as the types of protective systems used.
The resistance value is equal to the ratio of the rated voltage to the rated current, and this ratio is the design criterion at 20 ° C. The measured DC resistance at ambient temperature is corrected for temperature of 20 ° C and the maximum tolerance allowed is ±10%. Depending on customer requirements, it is possible to produce resistors with lower tolerance.
Rated Time is the time during which the rated current passes through the equipment under the specified conditions, taking into account the thermal limits in accordance with the standard. According to the standard, the recommended rated times are 10 seconds, 1 minute, 10 minutes and extended time. (Rated times outside of the mentioned values can also be used). In practice, due to the adjustment of protective equipment, the duration of the fault passing through the NGR is much less than its rated time. In determining the rated time, possible occurrence of consecutive short-circuit faults should also be considered.
Rated Continuous Current
Rated Continuous Current is the amount of current that can be continuously passed through the equipment, taking into account the thermal limits in accordance with the standard. As a result of this current flow, the temperature of the element will reach steady state temperature.
Rated Time Temperature Rise
Rated Time Temperature Rise is the maximum temperature increase over the steady state temperature caused by the rated current flow over the rated time. For information on temperature rise refer to the IEEE-32 standard.
Design and Manufacturing Standards of NGR:
ANSI / IEC 60529-2004
ANSI / NETA-ATS-2009
DIN ISO 2768