I am doing NGR sizing for 220KV system. I am wondering if anybody can direct me to size the size of NGR. Is there a rule of thumb for this calculation?
A neutral grounding resistor needs to have a generous wattage rating for several reasons:
1. The NGR has to tolerate a ground fault long enough for protective relays to operate. In the case of small systems where manual fault tracing is used, which is acceptable for some 480 volt and 600 volt industrial systems, the resistor needs to have a continuous rating.
2. The resistor should have a high enough voltage rating to tolerate a single point power cross with the next lower voltage. A pair of 240 volt baseboard heaters in series is reasonable for a 600 volt resistance grounded system NGR.
3. Ferroresonant phenomenon will tax the resistor, or at least it has to have enough damping to be like a water bucket with a large hole in the bottom. Different electrical engineers have different opinions as to how big the hole has to be.
4. The NGR has to supply enough current for automatic tracing of a ground fault, usually at least twice what system capacitance supplies.
5. The neutral grounding resistors also has to tolerate silent and invisible lightning such as Saint Elmo's Fire. During a rainstorm the is a ore of less steady rain of static electricity which is what Benjamin Franklin picked up with his kite experiment. If he had picked anything more powerful he would have been toasted.
By the way, 480 volt ungrounded systems have more than their fair share of lightning damage to motors than does 277Y480 solidly grounded or resistance grounded. There is hust no comparison. 480 volt ungrounded systems pick up very destructive amounts of static electricity during rainstorms. Similarly, a 200KV system with wires way high up in the air MUST have a path for static electricty.
For the rating that a 220KV system would require you would need wire wound resistors which do have some amount of inductance as they are both a resistor and an air core inductance.
You definitely do not want to skimp on resistor rating. You do not want this thing to blow up.
You do need lighting arrestors across the NGR on a 220KV system but these will have a certain clamping voltage and this has to coordinate with the insulation level and short time rating of resistor. If lets say that I want to connect 1 megohm resistors phase to ground on a 480 volt system strictly to bleed off static electricity, the best bet would be series strings of 240,000 Ohm 3 watt metal film resistors. These have a voltage rating of 750 volts each which coordinates with available surge arrestors. A resistor for 220KV has a few more problems such as voltage distribution during a lightning or switching surge so the problem is slightly more complicated.
Also, a resistor or inductor for a 220KV system is usually an open design that can withstand rain.
Any grounding resistor on a 220kV system should be tied to the secondary of a 220kV - 240V single phase transformer. That lets the transformer deal with the high voltages and leaves the resistor needing to deal with more manageable voltages. Check with the folks at
You are right about using a transformer to step down the voltage to a lower level ( 240 volts to 14,400 volts ) for the neutral grounding resistor. This is a common arrangement for over 600 volts. However, it needs to be able to handle excess voltage from ferroresonance, switching surges, and so forth without saturating. Therfore, the primary voltage rating of the grounding tranformer needs to be equal to the phase-to-phase voltage rather than phase-to-neutral.
I believe that the original post mentions a 220kV system, and I believe that I stated that the grounding transformer should be a 220kV - 240V transformer, which seems to me to be the phase-to-phase voltage rather than the phase-to-neutral voltage.
i am glad that my questuons sparkled a lot of discussions which has gien an idea what I should do for NGR sizing. We cann't rely on rule of thumbs toomuch about technical questions. Instead, we should do a detailed analysis for coodination and fault level.
