Short time rating of Generator NGTR

[sakaran51]

Hi,
What is the recommended short time duty of Generator Neutral Grounding Transformer and its loading resistor? Is it for 10 seconds,30 sec,1 min or 5 minutes? Is there any standard?

 

[7anoter4]

Since this grounding system is connected with a ungrounded neutral grid the first fault-phase-to-ground-does not mean an actual short circuit. But it cannot work for an indefinite time. If the system may work for a long period with a phase grounded another way has to be chose.However,usually a phase-to-ground fault can be cleared in a few seconds.
The high resistance grounding equipment (zig-zag transformer and Grounding Resistor) should have a continuous-duty rating when the service continuity (alarm on first fault) is prime concern. Short-time rated devices (10 seconds, 1 minute or 10 minutes) are used on systems where the first fault is cleared automatically with a relay. With these devices, the fault must be removed within a time period of the short-time rating. Note–these devices should be ideally protected by a relay with inverse Time Current characteristics. The relay should be set to pick-up at or below the Maximum CONTINUOUS current rating of the Resistor. The time duration will be increased according to I2t=K (a constant). For example, at 50% rated current, a 10 second rated Resistor can only carry current for 40 seconds. In any case the relay characteristics must co-ordinate with the characteristic de-rating curve of the Grounding Resistor to prevent damage to the resistor.

 

[dpc]

There's an ANSI standard for grounding resistors - can't recall the number right now. You need to decide on how you want to ground your system and how you will respond to a ground fault. That will dictate the requirements for the grounding resistor.

For low resistance grounded system, in general, any ground fault must be removed from the system as soon as possible. Therefore, these are generally rated for 10 sec or 30 sec at their full current rating. It is your responsibility to make sure that the fault is removed within that time based on your protection settings. But low resistance grounding systems must also be able to carry any continuous neutral to ground current during normal operation - this is typically third harmonic current. That gets a bit trickier since the ANSI standard for 10 sec ratings does not specify any continuous rating.

For high-resistance grounded system, you may elect to trip on ground faults or continue to operate with a single ground fault. This is will dictate the resistor rating.

A good reference is "Industrial Power System Grounding Design Handbook" by Dunki-Jacobs, et al.

 

[sakaran51]

When high resistance grounding is adopted for generator grounding,the NGTR is designed generally for =/< than 10 Amperes rating on the primary side and the stator earth fault relay is set to trip the generator and clear the fault within a short duration,say 1 second.But one find some of the generators are provided with 5 min. rated NGTRs in spite of lower fault clearance time.Is it really required? Or in case the e/f relay fails to clear the fault for any reason,a higher short time duration is recommended,until the fault is cleared by some other back up protection?