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Sizing NGR for parallel running Generators

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sinhchoo

Electrical
Nov 21, 2009
23
We have a 14MVA, 6.6kV generator generating power to the 33kV network through a 14MVA 6.6kV/33kV Transformer. This 14MVA Generator is grounded through a 762ohm grounding resistor to limit the fault current to 5A. Now we are going to add one unit of 2MVA generator to the 6.6kV system to run parallel with the 14MVA Generator.

My first question is how should we ground the neutral of the 2MVA Generator? If Neutral Grounding Resistor is required how should I size it?

In previous thread, xxjohnh posted a reply as follow:
"Most low voltage generators are soldly earthed and small, up to 2500 kVA.Trip on earth fault.
Most medium voltage generators 2 TO 10 Mva are low resistance grounded, 100 to 400amps, 10 secs, to limit damage to expensive machinery.Trip on earth fault.
Larger units 10 to 1000 Mva tend to be unit connected so they are high restistance grounded, 10 amps or so continous through a distribution transfomer or resistor. Sometimes just alarm and don't trip on ground fault."

My second question is why should we limit the fault current to so low current for large generator?
 
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1. Refer to IEEE Green Book, Std 142 for grounding.

2. To limit the damage to the generator and/or to trip it before it sustains a damage (due to external ground faults) that requires a repair before putting it back in service.

Large fault currents can quickly damage a generator requiring expensive repair and prolonged outage. Also phase to ground faults are the most common type of faults, so limiting GF currents makes sense.

Rafiq Bulsara
 
Since your large machine is high-resistance grounded, I would stay with that approach for the new smaller generator. The less fault current, the less damage to the stator of the machine for an internal ground fault - that is the primary factor.

If the two machines are tied to the same bus, both should be high-resistance grounded, or else you risk losing a lot of the benefit of high-resistance grounding.

David Castor
 
The smaller generator is located 300m away from the larger generator and will be connected to a different bus. the two buses will be linked with a 150sq.mm cable.
 
If a LRG is adopted for the 2MVA Generator, does it mean any external faults will choose this 2MVA Generator neutral as a return path (as this is a lower impedance path) rather than choosing the 14MVA Generator neutral which is with HRG path?

If we adopt a HRG, do we have to limit the current to 5A too?
 
The problem with using LRG on the small generator comes when there is a fault inside the big generator. Rather than a few amps of fault current, you would now have a few hundred amps, assuming the small generator was on-line.

I'd suggest getting a good reference book on the topic and reading it over.



David Castor
 
Really, truly shouldn't be adding any new generation the the existing 6.6kV bus. The 2MVA machine should connect to the 33kV bus through its own transformer. Every generator should always be on its own transformer winding so that there is no zero sequence coupling between machines. Besides, how are you going to push 16 MVA of generation through a 14MVA transformer?
 
Thank you for all your inputs. To have a clearer picture, this is a biogas plant, at present, the plant is exporting about 9MW of power. The 2MVA new generator will only have about 1MW of energy to export as it will provide supply to the plant own usage.

Can anyone suggest any good reading material on this topic?
 
The IEEE Red Book is a good starting point. Blackburn's Protective Relaying Book has discussion on this. For in depth info, try Industrial Power Systems Grounding Handbook by Dunki-Jacobs, et al.

David Castor
 
1- Answer to second question-
See aplication to large generator in 5.2.3 Core damage cause by ground fault


2-Answer to first question
Your 6.6 kV system was defined as high impedance grounding, so you have to keep the same philosophy of grounding.
2MVA generator shall be sized of same manner as existing installation. With both generator in parallel you have o fault limited to 10A.(or less, it depends on capacitance to ground of new generator).
You shall analize the seletivity of protection and circulating current of both generators in parallel.
One solution would be grounding both resistors by single pole on load switches.
Keep only one generator grounding during operation , to reduce circulating current and guarantee seletivity of ground generator protection.
 
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