Parallel Generator Earthing Transformer Disadvantages
Parallel Generator Earthing Transformer Disadvantages
(OP)
We have a client looking to replace his earthing transformers with new transformers. These transformers are to be connected to 3 parallel 11kV generators (one per generator), with resistors on the secondaries.
Our normal preference (w.r.t. power transformers) is for earthing resistors, as they are linear, i.e. start limiting current immediately, whereas reactors and transformers may not limit current for up to 8-10 cycles, and they can saturate.
On the other hand, we know that resistors have been known to "collapse" when subjected to a fault, or a series of faults, whereas reactors and transformers are more robust in this regard.
Anyone know of other disadvantages with earthing transformers on parallel generators that would necessitate the installation of resistors instead (or alternatively reasons to keep transformers over resistors)?
Our normal preference (w.r.t. power transformers) is for earthing resistors, as they are linear, i.e. start limiting current immediately, whereas reactors and transformers may not limit current for up to 8-10 cycles, and they can saturate.
On the other hand, we know that resistors have been known to "collapse" when subjected to a fault, or a series of faults, whereas reactors and transformers are more robust in this regard.
Anyone know of other disadvantages with earthing transformers on parallel generators that would necessitate the installation of resistors instead (or alternatively reasons to keep transformers over resistors)?






RE: Parallel Generator Earthing Transformer Disadvantages
If by "earthing transformer" you mean a single-phase transformer connected between neutral and ground, you still need a resistor or reactor connected to the secondary of the earthing transformer. The advantage being that the grounding device is rated for low voltage.
RE: Parallel Generator Earthing Transformer Disadvantages
RE: Parallel Generator Earthing Transformer Disadvantages
I have seen resistor only being more common mode of 11kV-15kV generators grounding....up to several units in parallel consisting of up to 2MW each...
RE: Parallel Generator Earthing Transformer Disadvantages
You may wish to consider your ground protection requirements as well as well. Some sense voltage across the grounding resistor. You may have to modify your system to accomodate the change.
RE: Parallel Generator Earthing Transformer Disadvantages
The distribution transformer (with secondary connected low ohmic value resistor) type earthing arrangement is robust more popular (Ref. CIGRE survey reports) world wide.
The transformer primary is always rated for phase-phase voltage to preclude saturation and resultant problems.
Having said that, I learnt that ABB (one of the majors in power plant supplies) always provides direct connected MV resistors for generator neutral earthing and claim that their resistors are proven for reliability.
Trust the above is helpful.
RE: Parallel Generator Earthing Transformer Disadvantages
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If we learn from our mistakes,
I'm getting a great education!
RE: Parallel Generator Earthing Transformer Disadvantages
Consider that application of MV resistors in place of transformers/LV resistors may require a PT per stator for 59G{/27G?} sensing.
ANSI-region reference IEEE C62.92.2-1989
RE: Parallel Generator Earthing Transformer Disadvantages
If some is trying to tell me that modern industry can not manufacture a reliable resistor to withstand 10 amps for 10 seconds..I would not buy any electrical machinery or device...
So how simple a device have to be to make it reliable?
RE: Parallel Generator Earthing Transformer Disadvantages
Did you mean withstand and dissipate the entire fault current except 10Amps??
RE: Parallel Generator Earthing Transformer Disadvantages
RE: Parallel Generator Earthing Transformer Disadvantages
RE: Parallel Generator Earthing Transformer Disadvantages
http://www.avtron.com/neutral_grounding_resistors.htm
http://avtron.com/permanent_load_banks.htm#MdLoadBanks
RE: Parallel Generator Earthing Transformer Disadvantages
Kinda neat, I have never seen this other MV resistor system in use.
RE: Parallel Generator Earthing Transformer Disadvantages
The original question is related to the grounding resistor and transformer for MV generators...not load bank..
alehman's last post may have mislead you, inadverently..
RE: Parallel Generator Earthing Transformer Disadvantages
The cons stated above for not using the large resistor are that they are unreliable and you can not run your protection off them easily.
RE: Parallel Generator Earthing Transformer Disadvantages
You need to walk around little more...I am not sure where you are..I am only speaking of commercial/institutional installations in the USA...where most common mode of MV generators (standby and emergency usage)is resitor grounded....
and I am speaking of installation backing up data centers for large institutions....
RE: Parallel Generator Earthing Transformer Disadvantages
On generators at users end for standby or emergency duty....resitors are very adequate..it is not that these R are carrying or seeing any continuous voltage all the time or there are ground fault occuring on regular basis...
Applications has to be viewed in proper light and not just comparing theoratical behaviour of a transformer and resitor irrespective of other factors that need to be considered for making a decision..
I am not against transformers for grounding or not saying that one is superior to other..but only commenting on views expressed for R being unreliable...that is a very difficult pill to swallow...when someone says a finding reliable resitor is a major obstacle...
RE: Parallel Generator Earthing Transformer Disadvantages
In Australia, MV resistors are often used on power transformers to limit EF current to 10A (11kV txf secondary voltage), for which normally only 50/51 protection is used. Apart from the difficulty of employing 27/59 protection, is there a specific reason why MV resistors should not be used on generators ?
Any opinions on earthing transformers not limiting the earth fault current initially, e.g. first 8-10 cycles ? Would this be enough to cause stator damage ?
RE: Parallel Generator Earthing Transformer Disadvantages
Another mfg-
http://powerohm.com/catalog/NGCAT00.pdf
Low impedance grounding (just enough impedance to prevent damage from ground faults) is sometimes used on smaller generators.