What is the reason for tripping the HRNG (High Resistance Neutral Grounded)in MV systems though they are intended for the purpose of service continuity?
Another reason for HRG is to limit equipment damage (generators, transformers, motors) in the event of a ground fault. Because of this, tripping on ground fault is common at MV, even with high resistance grounding. The unfaulted phases will see full line voltage to ground and this overvoltage is another reason to trip.
Failure of the resistors or cabling to the HRG. These systems sit forever and often are not maintained or tested over the years.
The HRNG protection systems I've worked with generally have a CT feeding the control box. There are settings for alarm and trip currents and often some delay available. Too much current (shorted resistors or cables) and the system isn't doing its job. Most of the systems I've worked with alarm only, but there may be certain cases where a trip of the source breaker may be warranted.
I've never seen a medium-voltage HRG system that did not trip on detected ground fault. These have been industrial and power generation systems. Low voltage is another story.
I would alarm until just before reaching the time the HRG was rated for, then trip, e.g., if the HRG was sized for 10 s, alarm from the moment the fault was detected until just before it hit 10 s, then trip, maybe at 9 s.
xnuke
"Live and act within the limit of your knowledge and keep expanding it to the limit of your life." Ayn Rand, Atlas Shrugged.
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.
It depends on the total capacitive current of the subject MV system. If it happens to be over 10A, it is not recommended to keep the system / faulty feeder in service.
The HRNG system in unit connected generators meets this requirement and the generator system is allowed to be in service awaiting a planned shutdown.
In case of MV distribution systems with cable feeders supplying various loads, the capacitive current is unlikely to be that low (10A) and hence the faulty feeder is wired for trip.
