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#46133 - 12/13/04 09:38 AM Ground Fault Protection for Mobile Generators
winnie Offline
Member

Registered: 09/15/03
Posts: 652
Loc: boston, ma
Hi,

I am working on a project where I need 'electrician's practical advice', rather than detailed code analysis.

As I've stated in the past, I am doing research on electric motor systems, and have been studying code in order to better know how these motor systems will be applied once they reach production. We are now working on a project where we will be mounting a diesel gen-set on a vehicle on a temporary basis, and using the output of the generator to run an experimental electric motor/drive system.

The system will be operating at 440V three phase with approximately 80KW peak loading. Because the electronic drives are using a simple (dumb) six step rectifier without power factor correction, we are planning on using a gen-set with at least 130KVA capability. These are the only loads on the generator, so we don't care about harmonic voltage distortion on the generator output; we simply care about damage to the generator itself because of excessive harmonic currents. We will be operating two motors through two electronic drives, and may split the gen-set to two smaller systems.

NEC does not apply to this application, although OSHA rules almost certainly do apply. Customer requirements for protection of their vehicle, and personal requirements for the protection of my own skin set the standards for this installation.

All cabling between equipment (generator to electronic drives, drives to motors) will be SO cable. The SO will be sleeved in copper braid to provide EMI shielding, and then further overwrapped with sleeving to protect the copper from abrasion. In addition to any ground bonding done using conductors inside the SO, the copper braid will be bonded at both ends.

Each electronic control will include a circuit breaker on the input, at their rated current level, as well as a lock-out compatible on-off switch. The electronic control will provide overload protection for the motors. The motors will have direct embedded temperature monitoring. The generator will have its standard overcurrent protection.

First question: is there anything in the above (code or not) which makes you jump up and say 'you are asking for trouble'?

What I am looking to do is add ground fault detection to this mix, shutting down the generator if there is insulation failure anywhere between the generator and the motor. I'd like the ground fault pickup to be adjustable between 1A and 10A. Extremely sensitive ground detection will result in nuisance tripping, since there will be some amount of capacitive coupling between the motor winding and frame, especially at the electronic control switching frequency.

The Cummins generators that we have been looking at have Ground Fault Detection available as an option, however the minimum pickup level is 100A.

Second Question: Where should I be looking to find a cost effective and reasonable way to add ground fault detection to this system? I am currently searching for 'ground fault relay', and going down the list of suppliers to call, but I'm hoping that someone here has already installed sensitive ground fault detection on a generator, and can point me in the right direction.

Best Regards,
Jonathan Edelson

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#46134 - 12/13/04 12:25 PM Re: Ground Fault Protection for Mobile Generators
Bjarney Offline
Moderator

Registered: 04/10/02
Posts: 2561
Loc: West-Southern Inner-Northeast ...
 
Jonathan, you are effectively describing high-resistance system grounding, that although not “mainstream,” is frequently used in industrial settings. There is a paper that discusses the application, although not specifically for a generator as sole power source. See www.neiengineering.com/papers/paper1JN.pdf.

The idea is to use a resistor connected from the generator-neutral point of stator windings to equipment ground. At 480V and under 1000kVA, the resistor is sized to limit ground-fault current to roughly one ampere. That is about a 280-ohm, 300-watt device, with a 480:120 potential transformer with primary connected across the grounding resistor with secondary connected to the relay.

Typically used is a two-setpoint protective (ANSI device 59N or 59G) voltage relay—one setting is configured to alarm/annunciate for initial manual intervention. A higher setting that can be applied to shunt-trip associated circuit breakers for ground faults. One example of the relay is a Basler BE1-59N at www.basler.com/html/pcs59-87.htm#BE1-59N The relay has adjustable- voltage pickups of a few volts with time curves set for the application, to continuously withstand phase-to-ground voltage {scaled by the PT} without damage. The relay also has third-harmonic restraint to improve security.




[This message has been edited by Bjarney (edited 12-13-2004).]

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#46135 - 12/13/04 03:25 PM Re: Ground Fault Protection for Mobile Generators
winnie Offline
Member

Registered: 09/15/03
Posts: 652
Loc: boston, ma
Bjarney,

Thanks for the pointer to the relay and the link to the paper. I am aware of the general concepts of using resistance grounded systems; but for some reason I was having tunnel vision in this case, and only thinking in terms of a solidly grounded system and then using some sort of current transformer based ground fault detection. I will be pointing the engineer who is selecting the generator to this; this would seem to be a perfect application for an impedance grounded system

-Jon

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#46136 - 12/15/04 06:57 AM Re: Ground Fault Protection for Mobile Generators
winnie Offline
Member

Registered: 09/15/03
Posts: 652
Loc: boston, ma
As a follow on question, how does the use of 'line to neutral' loads affect the installation?

I understand that NEC only permits high resistance grounding when line to line loads are served. I'd like to understand the reason for this restriction.

The text that Bjarney pointed out notes that an 'ungrounded' system is really a mis-named; instead it should be called a capacitively grounded system. A balanced, ungrounded 'wye' system will pretty much have its neutral near ground potential, until a fault occurs. Once a fault occurs, the system will 're-reference' and the neutral will be at elevated potential. The same thing happens in a resistance grounded system.

I can understand that any equipment which is designed with the expectation that neutral will be near ground potential might have a problem with this. In the event of a phase to ground fault in a resistance ungrounded 480V wye system, the neutral will suddenly go from approximately 0V to approximately 277V. At the same time, I could see that this only depends upon the quality of the insulation system; a system designed to operate with 277V between its supply terminals, with good enough insulation to operate with 480V to ground at any of its supply terminals, should have no problem operating on a resistance grounded system, even with a fault.

Am I missing other aspects of the interaction between line-neutral loads and resistance grounded systems?

The reason that this comes up is that it turns out that the control power supplies in the inverter systems are supplied line to neutral. This means that the bulk of the load is line to line, but there will be a couple of amps of line-neutral loading.

-Jon

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#46137 - 12/15/04 02:39 PM Re: Ground Fault Protection for Mobile Generators
Bjarney Offline
Moderator

Registered: 04/10/02
Posts: 2561
Loc: West-Southern Inner-Northeast ...
(Re)read, until you thoroughly understand, the Nelson paper and Soares' book on Grounding.

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