YNd11 (earth fault at delta side) 6

[newelecteng]

Hi all

I have YNd11 transformer used as stepdown from 33kv(YN) to 11kv(d) to feed loads in 11 kv side

-The question is If we have earth fault in 11kv systems how we can detect it.

-is there will be reflection on star side of this earth fault.

-Is it possible to have earth fault in this case or there will be no earth fault current especially we don't have earthing transformer.

Thanks in advance

 

[waross]

If the delta is not grounded, the ground fault current will be the charging current of the ungrounded phases.
You may detect it with an artificial neutral with a current limiting resistor from the neutral to ground.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[newelecteng]

Waaross

Is there any chance that charging current will be reflected in the star ground side

 

[waross]

Charging current may be a few Amps.
Whatever it is, it will be less on the 33 kV side.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[che12345]

Mr. newelecteng (Electrical)(OP)22 May 23 22:02
"....I have YNd11 transformer used as stepdown from 33kv(YN) to 11kv(d) to feed loads in 11 kv side.....The question is If we have earth fault in 11kv systems how we can detect it....#2. is there will be reflection on star side of this earth fault....#3. Is it possible to have earth fault in this case or there will be no earth fault current especially we don't have earthing transformer".
I try to answer your questions for your consideration:
1. One of the methods is the use of 3 PTs with secondaries in open D; with a resistor across the open ends. Note: This resistor is NOT earthed. The working principle is by detecting the Voltage across the resistor.
2. Any EF current on the 11kV un-earthed d winding would be extremely low. Therefore it is impractical to detect it on the primary side.
3. It is possible that any of the 11kV lines to be accidently earthed. There will be very low EF current irrespective of whether you have earthing transformer or not.
Che Kuan Yau (Singapore)

 

[waross]

The PTs referenced by Mr. Che will be connected in wye on the 11 kV side, with the wye point grounded.
In the event that one phase is grounded, the voltage across that PT primary will drop to zero.
Th voltage across the broken delta will rise to the secondary voltage of the PTs.
(Open delta, broken delta;
In many places in the world the term "Open delta" refers to three transformers in delta with one corner open.
In North America this connection is called a "Broken delta" and the term "Open delta" is used to describe a three phase circuit fed by two transformers.
You want three transformers with the secondaries in delta with one corner connection "Open" or "Broken".

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[davidbeach]

And some where around that broken delta one phase shall be grounded, and it's almost always on one side of the resister or the other. In an open delta it is almost always the B-phase secondary that is grounded.

But why oh why that transformer? Should be a Dyn1. Any transformer that isn't a GSU should be wye on the load serving side of the transformer and in most cases a grounded wye. Then a delta on the source side means that low-side ground faults aren't seen as ground faults on the high-side, rather as a lower level phase-phase fault. Makes the protection of the transformer simpler as well since it doesn't become a ground source to the high-side.

I’ll see your silver lining and raise you two black clouds. - Protection Operations

 

[Padlock01]

I would rather make the transformer Dyn11 and install an 11kV NER which solve the earthing issue.

 

[RRaghunath]

You will be able to sense a 11kV system earth fault using Voltage displacement protection. But, discrimination as to which of the 11kV outgoing feeders has fault is not possible. Typically, the operator is expected to switch off one by one 11kV feeders to see whether the earth fault alarm vanishes to identify faulty feeder.
If a second fault were to happen in the mean time, it will most likely be a Phase to Phase fault with associated major damages.
In case the 11kV system has substantial (lengthwise as well as size wise) cabling, the cable leakage currents could be substantial and by installing Zero sequence CTs in each of the outgoing feeders it may be possible to identify individual faulty feeder using zero sequence current detection.
Another important point to address with YNd transformers is maloperation of YN side incomer earth fault protection for an earth fault elsewhere in the 33kV system. This means the earth fault protection on 33kV YN side incomer to be time delayed or there should not be any protection other than Restricted Earth fault protection (87N on YN side).

 

[Kiribanda]

You have to use BROKEN DELTA method connected to a voltage relay to detect any EF at the 11kV DELTA side
for alarm/ tripping.

 

[che12345]

Mr. newelecteng (Electrical)(OP)22 May 23 22:02
1. There seems to be confusion on the term "Open delta" and "Broken delt". It depends the location i.e. Europe or the US and is further confused by the local muddy practice etc....
2. I wish to clarify my earlier proposal is having THREE PTs on the 11kV and also THREE secondaries. The THREE secondaries are in DELTA formation with only ONE conner open with only ONE resistor across it. The resistor is NOT earthed.
3. With the setup clarified, you may call it whatever you like. Caveat emptor.
Che Kuan Yau (Singapore)

 

[davidbeach]

The resistor, per se, may not need to be grounded but it is essential that one point in the delta is grounded. That point is generally at one end or the other of the resistor but could be at one of the other corners. Traditionally, the winding on one side of the resistor was used as the synch check voltage and the grounding location was selected to the benefit of the synch scope. Today it's rather moot as one simply brings the secondary of a wye-wye VT to the relay and the relay calculates the value of 3V0.

I’ll see your silver lining and raise you two black clouds. - Protection Operations

 

[wcaseyharman]

If you use a YY PT for the relay to detect 3Vo to detect ground faults you’ll want a set of aux PTs configured in a broken delta anyway - the 11kV bus capacitance can go into resonance with the saturable inductance of the PTs and cause ferroresonance. (This happened to my company about 10 years ago on a 6.6kV bus when the designer did not include the broken delta) The broken delta resistor will damp out those oscillations. A 60 ohm resistor is commonly used on 13.8kV generator busses going back at least 75 years.
Ungrounded busses outside generating station MV busses seem to be going out of favor - high resistance grounded seems much more preferred for fault tolerant systems these days as they have most of the benefits and few of the drawback of ungrounded systems. Without knowing anything about the system I would consider a different transformer connection (delta wye or a wye-wye) or add a grounding bank, and install neutral resistance and associated relaying. That’s just my 0.02$

 

[waross]

However you call the secondary connection and however you detect a ground fault, you need a ground reference on the primary, 11kV system.
The most common way is to arrange the primaries of the PTs in grounded wye configuration.
During a ground fault, two of the PTs will experience line to line voltage and the PTs must be selected for line to line voltage, not line to neutral voltage.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[newelecteng]

Thanks all

Why we need to ground open delta PT at one corner?

 

[davidbeach]

If the secondary circuit isn’t grounded the voltage to ground can become quite high due to coupling with the primary. The ground is there for safety.

I’ll see your silver lining and raise you two black clouds. - Protection Operations

 

[slavag]

59N, residual overvoltage protection (broken delta, voltage displacement, etc.)
But, it is common detection for whole system.
You can use Varmetric, IosinF0 for feeder detection.