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?

 

[davidbeach]

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."

 

[Rodmcm]

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.

 

[davidbeach]

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.

 

[Rodmcm]

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

 

[Marmite]

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

 

[jghrist]

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.

 

[davidbeach]

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?

 

[Marmite]

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

 

[waross]

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

 

[Bronzeado]

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

 

[Rodmcm]

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?

 

[jghrist]

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.

 

[waross]

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

 

[roydm]

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

 

[Marmite]

See pages 7-10 of the attached document.
Regards
Marmite

 

[Marmite]

That doesn't seem to work. Here's a link to the location I took the original document from.

http://www.i-gard.com/pdf/application guides/Neutral Resist Guide.pdf

Regards
Marmite

 

[roydm]

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

 

[Rodmcm]

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?

 

[Rodmcm]

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.