Comparison of zig zag and open delta transformers 4

[Rodmcm]

I'm trying to understand why one would be used over the other. They both seem to provide the same function, what is the difference (other than the obvious). Is it a function only of cost?

 

[NAZ55]

I-Guard Published a very interesting Q&A booklet and here's an abstract from this manual used to answer a similar question.


1. How do you select a zigzag transformer grounding or
Y/broken delta for delta system grounding?
2. How do you calculate kVA rating and voltage ratio of
the zigzag transformer and Y/broken delta (with voltage
system rating of 6.6kV and NGR rating of 125A)?
3. Which type of the zigzag transformer and Y/broken
delta do you recommend?

Answers:
1. A zigzag creates a neutral point. The advantage
is that it is physically and electrically smaller
than the Y/broken delta, so should be less expensive.
The disadvantage is that there are only
a couple of manufacturers, and UL/CSA is not
always available. Also, zigzags only create neutral,
so for a 4160V system, the neutral point would be
2400V. You could not add a 59 relay to this resistor
or pulse to locate ground faults. With a Y/broken
delta, the secondary can be any voltage you
choose, so the resistor will be < 240V, you can use
a relay and/or low-voltage CT, and you can pulse
the low-voltage resistor to locate ground faults.

2. In your 6.6kV system and 125A, pulsing is
not recommended. I suggest a zigzag transformer
with a rating of 125A. The line-to-neutral voltage (6.6kV/ 1.73=3815V) equals 476.9kVA. Since
it is 125A, I am assuming that it is rated for only
10 seconds; you can de-rate the transformer by a
factor of 10. The new rating is 476.9kVA / 10 =
50kVA.

3. It depends on your environment, but the
oil type typically has much longer lifespan than
the dry type and is better for harsh environments.
However, it is more expensive and requires some
maintenance.

 

[SudionoSu]

Just to side track a bit. What will be the protection scheme for Y/Broken Delta scheme? Currently, we have an 11KV system with two generators infeed (Wye connection with neutral floating). The 11KV bus is earthed through the Y/Broken delta transformer. The 11KV wye connection is earthed and the secondary resistor is put across the broken delta to limit the fault current.

Basically, for Earth fault protection, we have provided ZCT (Zero sequence/Core balance CT) in Alternator and Outgoing feeders which will operate relay function 67N (for Alternators) and 51N (for Outgoings).

For feeder going to NGT, we have not provided any ZCT however there is 3 Phase CTs connected to relay (Rngt) with protection function 50/51 and 50N/51N.

Please advise whether the above scheme is adequate?

Thanks in advance.

 

[CoryA]

I know that this post is fairly old, but it caught my eye when it popped up in a related Google search. I’d like to comment on zazmat’s post.

The advantage is that it is physically and electrically smaller than the Y/broken delta, so should be less expensive.

I am not sure that one thing can be electrically smaller than another.

The disadvantage is that there are only a couple of manufacturers, and UL/CSA is not always available.

Resistance grounding is more and more common today than in the past; zigzag-transformer manufacturers have responded in kind. In my experience, and my colleague’s experience, zigzag transformers have been made readily available to us with or without CSA certification.

Also, zigzags only create neutral, so for a 4160V system, the neutral point would be 2400V. You could not add a 59 relay to this resistor or pulse to locate ground faults.

This is untrue. The CEC specifically allows for an alarming system of up to 5 kV.

Canadian Electrical Code, Part One, 2006 - 10-1102 (2)
“Where line-to-neutral loads are not served, provision shall be made to automatically de-energize the system on the detection of a ground fault unless the electrical system is operating at 5 kV or less, in which case it shall be permitted to remain energized on the detection of a ground fault provided that
[ol a][li]The ground fault current is controlled at 10 A or less; and[/li]
[li]A visual and/or audible alarm is provided to indicate clearly the presence of a ground fault.[/li][/ol]

Article 250.36 of the NEC does not address systems above 1000 V and Article 250.186 does not require that an impedance-grounded system be a tripping system. What code dictates that the 4160-V system referenced above cannot be an alarming one? The use of an appropriate neutral-grounding-resistor monitor provides both 51N and 59N protection.

With regard to your question SudionoSu, it is worth mentioning that should the resistor across the broken-delta transformer fail, all 67N and 51N relays will fail to function. It is with this point that I address Rodmcm’s original question that began this thread.

The resistor’s continuity cannot be monitored when installed in a broken-delta configuration. If the resistor fails, current-sensing protection can no longer function; an important consideration. Choosing a zigzag transformer for resistance grounding allows for the use of a neutral-grounding-resistor monitor. Below is a link to a paper with additional information on the subject.

Monitoring Neutral-Grounding Resistors - An Update

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Cory Anderson

http://startco.ca/