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Marracuen (Electrical) (OP)
28 Mar 08 2:39
1. How can I protect the delta side of a delta star transformer from earth faults. The star side is directly grounded.
2. Could somebody explain the differences betwen open delta and broken delta and its applications? Please forward he drawings of the open and broken delta.
slavag (Electrical)
28 Mar 08 4:54
Hello Marracuen.
1. On the your first Q, you have several threads below.
David, 521AB, Jghrist, Bill, explain it as well as possible.
Punglu also added drw with CT connection.
E/F protection on the Delta side havent any connection to second side.( grounded/earthed it, resistive or ungrounded/unearthed).
Delta side High side see E/F GF fault on the star side only as phase to phase fault, as NPS, but not as E/F ( GF).
and only in case of solid grounded.
1.1. Delta side E/F protection depend on the your system. In case od solid grounded, possible operation of 87T ( diff, of course if it enough sensetive and you have enough fault current) or 50/51 function.
But goodest solution is Punglu drw. simple 51/50N finction with residual connection.
In case of ungrounded system, you need function like to 67N/59N or very sensetive 50/51N--SEF.
2. In IEC land we use term open delta, but on this Forum, we have some agremment, for ANSI land open delta it's something other ( we used term V connection). ANSI land use term broken delta, and we IECland in this Forum try use only
broken delta term ( from time to time we forgot smile )
Hope it's help.
Best Regards.
waross (Electrical)
28 Mar 08 11:00
Open delta uses two transformers. Draw a delta bank and then remove one of the transformers.
Broken delta, draw a three transformer delta bank and show the jumpers between the transformers. Then remove one of the jumpers. This is used to prevent circulating currents in delta connected PTs. I saw it used once on a large transformer (Several hundred KVA) for the same reason, to prevent circulating currents.

"Why not the best?"
Jimmy Carter

Marmite (Electrical)
28 Mar 08 11:45
The broken delta connection of VT's is used to detect earth faults. The output of VT secondary windings connected in broken delta is zero when balanced voltages are applied, but under earth fault coditions a residual voltage is developed. This is commonly used to drive neutral displacement protection to prevent the backfeeding of earth faults from transformers with delta windings.
The open delta arrangement as described by Waross is used on distribution systems to supply large single phase loads with small amounts of three phase loads. It is also used for emergency operation where one unit of a delta-delta bank is disabled.
Helpful Member!  davidbeach (Electrical)
28 Mar 08 12:34
If you are looking to detect ground faults on the wye side from the delta side, it cant be done and that dead horse has been sufficiently beaten.

If you are looking to detect ground faults on the delta side from the delta side you have to look at voltage rather than current.  The best way is a modern numeric relay with VTs connected grounded-wye grounded-wye and the relay making the 3V0 calculation.  You could also take those three-phase voltages to a 3-phase 27/59 relay which could tell you which phase has the ground fault.  Or you can go cheap, use the ground-wye broken-delta VT connection to a single 59N relay and know that you have a ground fault but have no information as to what circuit it is on.
Bronzeado (Electrical)
29 Mar 08 17:36
Hi folks,

Waross, I think if we make a connection with two transformers we get a "V" connection. To make a open or broken delta connection we need three transformers.

Regarding the residual voltage relays (3V0) as described by Marmite, we have used them as a back-up protection in 69kV delta connected system when the zig-zag grounding tranformer is in maintenance.

In this case, when a phase-to-ground fault occurs in any feeder derived from the 69kV busbar, the 3V0 protection operates opening all the circuit breaker connected to that busbar as we don't know in which fedeer the fault is occuring (there is no current as the ground transformer is not in operation.

I wonder if anybody has an ideia of how we can avoid disconneting the faultless feeders.


H. Bronzeado
davidbeach (Electrical)
29 Mar 08 18:30
Open delta is two transformers, broken delta is three transformers and they are not the same thing by any stretch of the imagination.  It is only sloppy usage that would allow confusion between open delta and broken delta.

Two transformers can be connected in an open wye (V) or an open delta.  You can even have V on one side and open delta on the other; a wye-delta connection minus one transformer.

If you are tripping for ground fault based on voltage, you have to trip everything, or nothing.  You could trip, and reclose if the problem doesn't go away, then trip the next breaker, but loads still get tripped unnecessarily.  You could alarm rather than trip and do some investigation to locate the ground fault if your system can withstand indefinite operation with a grounded phase.
Bronzeado (Electrical)
29 Mar 08 19:58

Thank you for your promply reply and also for your solution on my question. You deserve a star! Congratulation!

Based in your ideia, we may be choose to trip the feeders sequentially until finding the faulted feeder. Only after that, the health feeders will be reclosed. This may avoid to submit some more loads to overvoltage.

Regarding the open and broken delta, I think we are discussing semantics. As a Portuguese speaker, we normally get use to call "V" conection for any connection that is making with two transformer, doesn't matter if they are wye or delta connected.

Also, for us, there was no "open wye" (nor "closed wye").

Best regards,

H. Bronzeado
davidbeach (Electrical)
29 Mar 08 20:13
No, not semantics.  Broken delta, open delta, and 'V' connection all describe specific individual connections.  None of them are interchangeable and all have different uses.  Using one when a different one is needed will not provide the desired results.
Marracuen (Electrical) (OP)
31 Mar 08 2:04
Hi Slavag:
I could not find the drw. of Punglu as you mentione in your answer to my quention of Delta star traformer  protection.
slavag (Electrical)
31 Mar 08 11:11
Please see attached thread.
thread238-212882: Dlta /Star 33/11 kv transfomer
slavag (Electrical)
31 Mar 08 11:43
Hello Bronzeado.
Few Q's.
Are you have SS with single BB or double BB?
Are you zig-zag trafo installed into trafo zone or have dedicated CB?
How much lines on BB and waht lenght, are part of lines are cables?
According to our operation rules:
 in case of trafo w/o
grounding points protection system must send:
1. Trip to trafo w/o delay in case of some zig-zag fault.
2. trip to trafo after 15min in case of some non-protection reason of zig-zag CB open position ( human mistakes).
Bronzeado (Electrical)
3 Apr 08 19:28
Hi Slavag,

Thank you for your reply.

Our substations have the main and auxiliar busbars. The zig-zag transformers are connected straight to the main busbar through a disconnector (no circuit-breaker). When this disconnector is opened, the residual voltage relay (3V0) is automatically activated. The 3V0 operation time is within maximum 2s.

Normally, the 69kV feeders are overhead lines with different length and performance.

The number of feeders in each substation are between 6 to 10.


slavag (Electrical)
4 Apr 08 0:57
Hi Herivelto.
I understand your's scheme, our solutions isn't applicable here.
In this case maybe use other scheme: Not one trip to all feeders and after cascade close ( one by one)-- used cascade trips.
 for example: first line trip after about 1 sec and others with step 0.2-0.3 sec or trip of pairs.
Maybe also check option of 67N for the ungounded/unearthed systems, of course if possible install ZS CT ( minimum for cables).

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