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Comparison of zig zag and open delta transformers
4

Comparison of zig zag and open delta transformers

Comparison of zig zag and open delta transformers

(OP)
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?

RE: Comparison of zig zag and open delta transformers

Why don't you explain a whole lot more?  The two things you list in the thread title are entirely different, used for entirely different functions.  One would never "be used over the other."

RE: Comparison of zig zag and open delta transformers

(OP)
Okay, please explain why you think not. I have seen both used on the earthing of a MV distribution system. The open delta (with resistor) limited fault current with protection clearing a generator and in a separate system the zig zag allowed about 300A to blow down stream distribution fuses. Both were on isolated system supplied by generators. I also believe that both can be used on say the MV distribution system to provide providing an earth reference which is supplied from a delta winding of a step down transformer.
 

RE: Comparison of zig zag and open delta transformers

Do you perhaps mean broken delta?

A broken delta is created when a grounded wye - delta transformer has one corner of the delta "broken".  Across the break between the windings at the corner is a voltage proportional to 3V0.  The insertion of a resistor in this broken delta would then limit the amount of 3I0 that can flow when driven by the presence of the 3V0.

An open delta is formed when one of the three windings of a delta - delta transformer is omitted.  Open delta can be used for small three-phase services and was commonly used with electromechanical relays and metering when phase-to-phase voltages were sufficient.  The open delta blocks all zero-sequence current and has no application what so ever for system grounding.

Zig-zag transformers are apparently semi-common in Europe and quite rare in North America.

RE: Comparison of zig zag and open delta transformers

(OP)
Yes, in your terminolgy I am talking about broken delta with a resistor.  Any light shed on comparison with zig zag?

RE: Comparison of zig zag and open delta transformers

4
It's interesting that IEC 60076 part 1 defines Open Delta as:-
3.10.3
open-delta connection
the winding connection in which the phase windings of a three-phase transformer, or the windings for the
same rated voltage of single-phase transformers associated in a three-phase bank, are connected in series
without closing one corner of the delta

Whereas the IEEE 100
The Authoritative Dictionary of
IEEE Standards Terms
open-delta connection (power and distribution transformers)
A connection similar to a delta-delta connection utilizing
three single-phase transformer, but with one single-phase
transformer removed.

Even the learned bodies can't agree.

In the UK interconnected star/star earthing transformers predominate over the star/delta. The star winding provides substation auxiliaries. I have looked at all my transformer text books and can't find any texts suggesting one method is technically better than the other.
Maybe one of the transformer engineers on the forum will comment?
Regards
Marmite

RE: Comparison of zig zag and open delta transformers

Quote:

Yes, in your terminolgy I am talking about broken delta with a resistor.  Any light shed on comparison with zig zag?
Still completely different purposes.  The broken delta is used to measure zero-sequence voltage and can be used to trip a generator if a distribution system on the delta side of a GSU is isolate and becomes an ungrounded system.

The zig-zag transformer is used to establish a ground source on an otherwise ungrounded system.

A grounded wye - delta transformer can be used for the same purpose as a zig-zag transformer.  The choice between zig-zag and grd wye-delta is economic.

 

RE: Comparison of zig zag and open delta transformers

So, what term does the IEC use to describe an open delta connection if they use open delta to describe a broken delta?  Egads, why reinvent the wheel?

RE: Comparison of zig zag and open delta transformers

It's not mentioned or defined in that particular IEC Standard, but neither is the term broken delta in the IEEE list! I have previously seen the term Vee connection used to describe open delta, though.
In using a star/delta earthing transformer you have three potential alternative positions to place the resistor(s). 1. Break the delta and insert a resistor. 2. Install a resistor in the neutral. 3. Install a resistor in each line.
Each position has pro's and cons.

Regards
Marmite

RE: Comparison of zig zag and open delta transformers

Quote:

In using a star/delta earthing transformer you have three potential alternative positions to place the resistor(s). 1. Break the delta and insert a resistor. 2. Install a resistor in the neutral.
A zigzag connection is used to provide an earth reference.
A wye delta is used to provide an earth reference.
A broken delta is used to indicate a phase to ground voltage unbalance.
The fault current in the event of a grounded phase may be limited by the insertion of a grounding resistor with either a zig-zag scheme or a wye delta scheme. The resistor would be connected from the wye point to ground.
The broken delta may be used to limit the fault current by inserting an appropriately sized resistor across the break. This approach may have merit where it is desired to make a direct connection from the wye point to ground, but still limit the magnitude of a fault current.
 

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: Comparison of zig zag and open delta transformers

Marmite and David,

We have discussed this terminology in this forum before.

Fortunately, the IEC and IEEE are now trying to make compatible all the terms in the coming standards.

In Brazil, "open" and "broken" delta (three single-phase transformers connected in series without closing one corner) is called "delta aberto" ("aberto" is translated as "open"). With two transformer is "V" configuration.

Best regards,

Herivelto bronzeado  

RE: Comparison of zig zag and open delta transformers

(OP)
Waross et al

I agree that fundamentally you can use zig zag and y broken delta with a resistor to both provide a ground reference and limit current. The question is why one or the other. Is it to do with higher currents, I note that the zig zag applications are often high current (>300A) where the Yd is low currents, ie 6A for a high impedance earth. Does it come down to cost?
 

RE: Comparison of zig zag and open delta transformers

I guess I was mistaken, you can use a wye - broken delta transformer as a ground source.  

See Optimum Use of Oil-Filled Distribution Transformers to Design Resistance Ground Sources for Medium-Voltage Delta-Connected Power Sources, RICHARD L. DOUGHTY, SENIOR MEMBER, IEEE, AND JOHN VOSICKY, MEMBER, IEEE, IEEE TRANSACTIONS ON INDUSTRY APPLICATIONS, VOL. IA-321 NO. 2.. MARCH/APRIL 1985.

Abstract-A procedure is developed to design a resistance ground source for medium-voltage power systems with delta-connected source transformers. The optimum connection is shown to be three distribution transformers in wye/broken delta configuration with a 600-V resistor in the secondary circuit. The assumptions are that the transformer banks are used solely as a source of ground fault current, that the power system has no loads at line-to-neutral voltage, and that the ratio of source transformer to distribution transformer three-phase kVA is greater than 25.

Basically, you can use standard MV-LV single-phase distribution transformers in a bank for either a wye-delta with neutral resistor or wye-broken delta with a resistor in the break.  It is less expensive to use the wye-broken delta connection because a 600 volt resistor can be used.
 

RE: Comparison of zig zag and open delta transformers

A zig-zag transformer is probably a special order. I was looking for one about 12 years ago and the issue became a challenge to find a sales rep who knew what a zig-zag was.
Standard distribution transformers may be used for a broken delta scheme at distribution levels. Replacements are readily available.

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: Comparison of zig zag and open delta transformers

I hate to ask, but could someone post a diagram showing the two different types so we can understand this discussion. I understand open Delta but not Zig Zag.
Thanks
Roy

RE: Comparison of zig zag and open delta transformers

Thanks Marmite, another star for you, I haven't read it fully yet but looks like a good simple explanation on system grounding.
Roy

RE: Comparison of zig zag and open delta transformers

(OP)
Marmite
Thanks for the reference to I Gard. Could you have a look at figure 6 the answer seems to be wrong for sizing the resistor. Isn't the voltage 3 x Volts of the delta winding?
 

RE: Comparison of zig zag and open delta transformers

(OP)
Just to let all readers know I have been in contact with a good authority on zig zag and broken delta. His answer is that usually with high impedance broken delta is used, and low impedance zig zag. The latter is better suited to the 400 to 500A typically used Generally the above arrangements are the most economical which drives the decision, not their abilty to do the job.
 

RE: Comparison of zig zag and open delta transformers

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.
 

RE: Comparison of zig zag and open delta transformers

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.
 

RE: Comparison of zig zag and open delta transformers

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.

Quote:

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.

Quote:

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.

Quote:

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
  1. The ground fault current is controlled at 10 A or less; and
  2. A visual and/or audible alarm is provided to indicate clearly the presence of a ground fault.
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

---
Cory Anderson
http://startco.ca/

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