Parallel Generator Earthing Transformer Disadvantages 4

[ScottPET]

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)?

 

[alehman]

If I understand the question, an earthing transformer isn't a replacement for a resistor.

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.

 

[ScottPET]

Thanks for the reply alehman. We are investigating replacing the earthing transformers (which have LV resistors across the secondary terminals), with HV resistors.

 

[rbulsara]

what size gens?

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...

 

[alehman]

Disadvantages commonly given for medium voltage resistors - they are made of thin wire and hence fragile. They are larger and require greater insulation. They are more expensive.

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.

 

[RRaghunath]

I agree with Alehman. The MV resistors connected directly to the generator neutral are fragile due to the requirement of high ohmic value and can break when a sudden fault is applied.

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.

 

[ScottyUK]

Transformer earthing is common for large generating plant. On these large machines the priority is to protect the stator core and this requires that earth fault current be limited to 10A or less. On an MV machine this would necessitate high ohmic value resistors, which are fragile as indicated by others above, and their use is not usually favoured. I'm interested to learn about the policy of ABB regarding direct resistor earthing. Is this worldwide?





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If we learn from our mistakes,
I'm getting a great education!

 

[busbar]

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

 

[rbulsara]

I am interested in cases of resistor failures...

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?

 

[DrSpark]

rbulsara
Did you mean withstand and dissipate the entire fault current except 10Amps??

 

[rbulsara]

how much fault current do u expect to go through a resistor rated for 8000V, 10A for 10 seconds during a dead L-G short...on a 13.8kV ssytem..Dr.spark?

 

[rbulsara]

or any other part of the circuit for that matter..

 

[alehman]

One major manufacturer seems to have both types of products. I notice that for load bank applications, their literature indicates that it may be more economical to provide a low voltage unit with a step-down transformer.

http://www.avtron.com/neutral_grounding_resistors.htm

http://avtron.com/permanent_load_banks.htm#MdLoadBanks

 

[QCE]

I would think the step down transformer equipment would be a lot cheaper!

Kinda neat, I have never seen this other MV resistor system in use.

 

[rbulsara]

QCE:

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..

 

[QCE]

My understanding is that they either want a transformer and a small resistor or a one large resistor. I have never seen a system installed with a large resistor.

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.

 

[rbulsara]

QCE:

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....

 

[rbulsara]

Generators of large utiltiy or prime duty generators may have different needs and view...

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...

 

[ScottPET]

Thanks everyone for your input.

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 ?

 

[alehman]

Most MV generators need some type of grounding impedance to prevent stator damage in the event of a ground fault. You must consult the manufacturer for these requirements. Large generators are normally installed with high impedance grounding. I don't see any inherent problem with a MV resistor for this.

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.