Sizing NGR for parallel running Generators
Sizing NGR for parallel running Generators
(OP)
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?
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?






RE: Sizing NGR for parallel running Generators
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
http://www.srengineersct.com
RE: Sizing NGR for parallel running Generators
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
www.cvoes.com
RE: Sizing NGR for parallel running Generators
RE: Sizing NGR for parallel running Generators
David Castor
www.cvoes.com
RE: Sizing NGR for parallel running Generators
If we adopt a HRG, do we have to limit the current to 5A too?
RE: Sizing NGR for parallel running Generators
Rafiq Bulsara
http://www.srengineersct.com
RE: Sizing NGR for parallel running Generators
I'd suggest getting a good reference book on the topic and reading it over.
David Castor
www.cvoes.com
RE: Sizing NGR for parallel running Generators
RE: Sizing NGR for parallel running Generators
Can anyone suggest any good reading material on this topic?
RE: Sizing NGR for parallel running Generators
David Castor
www.cvoes.com
RE: Sizing NGR for parallel running Generators
See aplication to large generator in 5.2.3 Core damage cause by ground fault
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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.