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jtxln (Electrical)
14 Jun 06 0:12
I'm in the process of expanding the electrical distribution system of an existing Healthcare facility.  The expansion will consist of a new building that will house Medium Voltage (12.47 kV) switchgear, parallel connected generators (2 new and 1 future), and utility paralleling switchgear.  The generators will be rated 12.47 kV, 2250 kW.  My experience with 12.47 kV, paralleled generators is limited although the research I have done tells me that I should employ neutral grounding resistors (NGRs) or some other ground fault limiting devices on the generators.  Would someone be so kind as to answer the following questions?

1. Is it best to employ one resistor for the system or individual resistors for each generator?  The future generator will not be installed initially and the size of the third generator is not yet known due to unforeseen future load conditions.

2. What impact does utility paralleling have on sizing the resistors?

3. What other methods of limiting fault current (if any) might be more suitable or cost effiective?

Your assistance is much appreciated!
 
Helpful Member!  waross (Electrical)
14 Jun 06 1:15
Generally, neutral grounding resistors are connected to the system neutral.
If you ground the generator neutrals, you will have multiple, interconnected, ground and neutral paths.
Also, grounding the generators through resistors will result in different resistance values to ground when the number of connected generators changes, and will result in no resistance when the generators are off line. This will raise difficult issues with your protective relay systems, particularly the ground fault protection.

Suggestion.
Connect the generator neutrals solidly to the system neutral. There is no advantage to switching the neutral connection and a few serious disadvantages.

Make one connection from the system neutral bus to the ground grid.
This connection may be solid, low impedance, or high impedance as you wish.
Don't forget to run an adequate jumper from the system ground bus to the "Quiet ground bus" as per code and good practice.
respectfully
jtxln (Electrical)
14 Jun 06 1:47
If I understand you correctly, you stated that providing NGRs between the star point and ground of my 12.47 kV generators is not such a good idea.  I'm confused?  I understand your logic but this contradicts (at least I think it does) just about everything I've read about NGRs.  I was under the impression that NGRs are a good idea for MV applications in order to limit fault currents for the following reasons, 1. to protect the generators, equipment, and cables and 2. to protect personnel from high energy arcing faults.  Please advise!
davidbeach (Electrical)
14 Jun 06 2:19
Sorry, waross, but I disagree in this case.  Individual resistors is the most common condition and does not cause any significant problems for the protection system.  There are a variety of ways of getting from here to there, but individual resistors would be the most common based on what I've seen.

Resistance of the resistors is based on voltage and current desired.  A 10A resistor for a 12.47kV system would have a resistance of 7200V/10A or 720Ω.  Then you need to pick a time rating; 10 seconds is common - the protection system must clear the fault faster than this time.  Maximum ground fault from the generators to a fault on the system (or in another generator) will depend on the number of generators on line.  This can be easily handled using numeric relays and settings groups.  For those relays that will see a variable ground fault current, use different settings groups that correspond to the number of generators on line so that you have a setting in use that is appropriate for the number of generators on line.  You might want to check how the relay changes between settings groups; one popular brand goes off-line for a few seconds to change settings groups while at least one other brand does not have any off-line time between settings groups.

Utility paralleling will not impact the sizing of your resistors, although without the utility present, your MV system will be high impedance grounded and subject to overvoltages should you have the misfortune of a ground fault.  If all your loads are phase-to-phase or 3-phase, you should be OK if your insulation is all rated for phase-to-phase rather than phase-to-neutral, and that includes any arresters that could be energized only from your generators.

Grounding resistors are probably the most cost effective means of limiting ground fault current.  There is no particular means of limiting phase-to-phase or 3-phase fault currents other than inserting impedance in the line, such as a transformer.  That probably isn't worth the expense or losses.

While paralleled with the utility, it is highly likely that you will have a solidly grounded MV system, and then it will be high resistance grounded will operating on just your generators.  With the utility present, the generator grounding resistors will only protect the generator against internal ground faults after the generator breaker has opened.  Without the utility present, your high resistance grounded system will be better protected against phase-to-ground faults, but no different for the other faults.  Arcing faults will be less likely to occur while operating as a high resistance grounded system, but should one occur it is highly likely that it will involve at least one more phase, at which point the grounding resistance becomes moot.

My recommendations:

Individual grounding resistors.

10A for 10s is a common size.

Use numeric relays and settings groups to match protection to the number of generators on line.

Everything on the system rated for phase-to-phase voltages (cable insulation, transformer insulation, arresters).

No phase-to-neutral loads.

Wye connected VTs (ground faults easier to detect through voltage shift than current in some cases).
waross (Electrical)
14 Jun 06 2:25
The NGR's only limit the current to ground and so only protect against ground faults.
NGRs have no effect on line to line faults or line to neutral faults.
If they are used, they are applied in the connection between the system neutral and the system ground. There should only be one such connection for a system.
If you have a 600 volt generator feeding a 600/1200 volt transformer, and the connection to the facility system is on the 12,000 volt side of the transformer, the generator is a seperate system, seperated by the transformer and and the generator may have the neutral grounded or a NGR may be applied applied.
If your generators are generating 12,000 volts, when they are connected to the system they are part of the system and depend on the system connection to ground.
For the purpose of grounding, a "System" is an area surrounded by transformers and/or a service connection.
Your 12.7 kv system is limited on the one side by the service connection or the secondary winding of the supply transformer, if you have a supply transformer.
The other side of the 12.7 kv system is the primary windings of the stepdown transformers throughout your facility. Any 12.7 kv motors, other 12.7 kv equipment and the 12.7 kv generators are part of the 12.7 kv system for grounding purposes.
One connection from the neutral bus to the ground grid.
respectfully


jtxln (Electrical)
14 Jun 06 3:26
The generators are configured 12.47 kV WYE with no phase to neutral loads.  The neutrals will be tied to ground therefore the resistors should have an impact on a line to neutral faults only at the generator which technically would be a line to ground fault.  With that said, wouldn't the NGRs protect the generators under phase to ground conditions out in the system as well as a phase to neutral fault at the generator?

So what's the verdict, multiple resistors or simply one?  Also, wouldn't low resistance grounding resistor(s) be appropriate?  I'm not sure high resistance grounding is the route to take due to cost?
ScottyUK (Electrical)
14 Jun 06 8:34
High resistance grounding is used because it limits the prospective earth fault to a level at which serious stator core damage is unlikely in the event of a stator ground fault. The core of a large generator is a hugely expensive asset; the cost of a replacement justifies the extra cost of protecting it. 10A is at the upper end of what is considered acceptable - modern relays should be able allow the grounding resistor to be of higher ohmic value and reduce the fault current to a lower level.

I have limited knowledge of protecting embedded generation, but large utility generators usually employ a DYN* transformer between the generator and the system to allow the system to use whatever grounding method is required, while allowing the generator to operate with a high resistance grounding connection.

----------------------------------
  I don't suffer from insanity. I enjoy it...

jghrist (Electrical)
14 Jun 06 9:27
Connecting the neutral bus to ground through a single resistor will be ineffective if the 12 kV service is from a utility with a multigrounded neutral (most common situation in USA) and no isolating transformer.  In this case, the "system" as defined by waross will include the whole utility distribution system.

dpc (Electrical)
14 Jun 06 12:03
For a medium-voltage generator, some type of resistance grounding is strongly preferred.  But as jghrist points out, if you take resistance-grounded generators and tie them to a solidly grounded utility distribution circuit, you have not accomplished nearly as much as you might think.  If operated in parallel with the utility, a ground fault in the generator will get a big contribution from the utility source, regardless of the generator neutral grounding method.  This is why it is almost always better to isolate a local generator from the utility through a delta-wye transformer.

One resistor per generator is the safest and most common approach.  For low resistance grounding you do have to consider the third harmonic heating effects in the resistor, but this is normally not a major problem.  I would not recommend a switched neutral resistor arrangement unless the facility has the technical expertise to understand and maintain.
 
AusPowEng (Electrical)
14 Jun 06 17:38
This is always the problem with private parallel generation. When you have loss of supply from the network you would be expecting that your generators will look after critical loads at your facility. Whilst doing so your HV feeder to the supply transformer would remain disconnected until your utility restors power and let's you reconnect. Under these circumstances, it is preferable to establish your earthing scheme at each generators.

Conversely, when you are energising your system (let's say after a major shutdown) if you have a problem (earth fault) with your supply transformer you must be able to detect this in accordance with your local regulations and make sure you don't have a touch potential hazard.

Perhaps the MV switchboard is the best place to 'meet in the middle'. Have you considered a zig-zag (interconnected star) transformer located there?
waross (Electrical)
14 Jun 06 18:52
davidbeach,
We cross posted, I was not challenging your post.
I defer to those with more experience in medium voltage generation.
respectfully.

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