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
RE: Comparison of zig zag and open delta transformers
RE: Comparison of zig zag and open delta transformers
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
RE: Comparison of zig zag and open delta transformers
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
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
RE: Comparison of zig zag and open delta transformers
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
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
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
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
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
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
Thanks
Roy
RE: Comparison of zig zag and open delta transformers
Regards
Marmite
RE: Comparison of zig zag and open delta transformers
http:
Regards
Marmite
RE: Comparison of zig zag and open delta transformers
Roy
RE: Comparison of zig zag and open delta transformers
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
RE: Comparison of zig zag and open delta transformers
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
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 am not sure that one thing can be electrically smaller than another.
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.
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
- The ground fault current is controlled at 10 A or less; and
- 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
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Cory Anderson
http://startco.ca/