Design of zig zag earthing transformer in an 11kV system 4

[SurendranNair]

Can anyone quickly advise which standards give design criteria for the use of zig zag earthing transformers for an 11kV ungrounded system (sizing of the transformer)?

Thanks for your advise in advance

Surendran

 

[RalphChristie]

Hi

Do you want to change an ungrounded system into a grounded system?

Regards


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[CoryA]

I agree with RalphChristie’s line of questioning. If you were to utilize the Zigzag transformer so as to high-resistance-ground the system, you’ll only need one large enough to handle the let-through current of the neutral-grounding resistor.

I’ll attach a document to this post that may be of extra value.

 

[SurendranNair]

Thanks for your queries.. Ralph and Cory, very sensible. The system scenario is like this... 4 generators are to be connected to a common 11kV double bus switchboard. We are planning to have three such systems like this for the proposed power station. This means there will be 12 generators and three 11kV double-bus switchboards which are in turn connected to three 80MVA 11/132kV power transformers in a 132kV switchyard. Because of the 11kV double bus arrangement there will be two feeders to one Tx from the switchboards (11kV currents will be split into two circuits). The 11kV side of the Tx is delta and 132kV side is star connected. 132kV will be connected to the transmission network.

Threre will be several 11kV load feeders to many buidings (This arrangement is for a Technology park - consumes high energy).

The generators are of 11kV and the 11kV system is normally grounded via the generator neutrals (solid gounding proposed). However, when the power station is totally down, power to the buildings will be drawn from the network in which case the 11kV network would be ungrounded. In order to protect the 11kV network from ground faults under this scenario I proposed to install 3 zig-zag earthing transformers (one each to each Tx 11kV side in the switchyard).

So, I need to design the sizing of these earthing Txs which I haven't done before and so unaware of any standards or guidelines how to proceed. Onething I know is the sizing depends on the 11kV fault level.

Could anyone please throw some light into this? Is it a good adea to install an earthing Tx in series with a grounding resistor for limiting the high 11kV ground fault currents?

Thanks in advance for your time and any suggestion is much appreciated.

Surendran

 

[dpc]

Any medium-voltage system with generators or motors should be resistance-grounded to limit equipment damage caused by ground faults. This is standard practice for any industrial facility and recommended in IEEE Red Book.

You will have to decide on low-resistance versus high-resistance grounding, then size the grounding transformer based on the expected maximum ground current and current duration.

Historically, multiple generators connected to a common bus would be low-resistance grounded to allow selective coordination for ground faults. But there are tradeoffs, so you'll need to do some research. IEEE Red Book is a good starting point. There is also a new IEEE recommended practice for generator grounding that utilizes a hybrid system combining low resistance and high resistance grounding.

 

[SurendranNair]

Hi dpc, thank you very much for your immediate reply. It helps me a lot...
Surendran

 

[RalphChristie]

I am not really familiar with generators or with generator-grounding, but I suppose it might use the same guidelines as applying a zig-zag transformer to a Power transformer.

The first step is to decide how much ground fault current you want to allow.

If you plan to solidy ground the transformer, you will need to specify the grounding transformer impedance (normally given in **ohms** for grounding transformers)as well as how long the transformer must be able to handle this (normally 10 seconds). For a solidly-grounded system, the grounding transformer impedance largely determines the ground fault current.

If you want to resistance-ground the system, then you will need to specify the maximum fault current along with the duration.

The primary determination of transformer size will be the short-circuit current it has to produce and the duration. You do not need to specify a kVA rating. Transformer manufacturer needs to know:

Operating voltage/BIL
Desired impedance in ohms
Maximum short circuit current
Short circuit current duration
Maximum continuous current

At the risk of running this thought into the ground, keep in mind that a conventional delta-wye transformer can also be used as a grounding transformer. Zig-zag transformers are sometimes somewhat hard to find, virtually no one builds these any longer. It will be a special "one of" order and you will have trouble getting quotes. One clarification — a conventional "grounded"-wye/delta transformer can be used but often rated for shorter-than-continuous duty if it can be reasonably assure that relaying will operate to isolate the faulted circuit. For resistance grounding, the MV resistor can be placed at the grounding-transformer wye point. The delta winding can be left floating, and one CT in the loop used as a polarizing quantity.




This information is taken form the Westinghouse "Electrical Transmission and Distribution Reference Book" published in 1964:

A zig-zag grounding bank is typically used to provide a source of fault current so a phase to ground fault on a delta system can be detected and interrupted. The bank is sized based on the neutral current and the phase to ground voltage multiplied by a constant "K" which is determined by the length of time required for the relay to open the breaker. The phase to neutral voltage on a 12 kV system is 7.2kV (12/1.732). Assuming the phase to ground fault current to be the 4000 amps the three phase kva rating of the zig-zag bank is determined as follows:

(7.2)(4000)(K) = kVA

K is determined from the following table:
time K
10 sec 0.064
1 min 0.104
2 min 0.139
3 min 0.170
4 min 0.196
5 min 0.220




Regards
Ralph

[red]Failure seldom stops us, it is the fear for failure that stops us - Jack Lemmon[/red]

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[SurendranNair]

Hi Ralph,

Thanks for your time. The information you and dpc shared are very helpful. Now I have a rough idea where to start and how to proceed with my design work where I puzzled. Also, I have gone through the website

http://www.expert.ecmweb.com

where Mr. Tony Locker had answered many such questions of many engineers and some of his replies I found are quite informative too.

Wish you all a merry Xmas, a happy and prosperous New Year.

Regards

Surendran
Australia.

 

[isquaredr]

If the 80MVA 132/11kv Yd transformer will be supplying customers when the generator sets are down is it not the supply / transmission company's responsibility to provide a means of earthing ?
Where on this system will the control boudary be ?

 

[RRaghunath]

RalphChristie,

Thanks for the post. It is quite informative.

I have a question with regard to 'K' factor. For 10sec, the factor as per Westinghouse book comes to 6.4%. This is at variance with the factor 3% recommended in IEEE32 and 9.5 % (Overload factor of 10.5) recommended in IEEE 62.92 Part-2.
The IEEE62.92 Part-4 (Appendix) considers Ph-Ph voltage / 3 (and not Square root 3) in sizing the Zigzag transformer.
Any insight.
Thanks.