transformers neutral connection 4

[newelecteng]

Hi all,

We have 4 step up transformers(dYN) (0.4/11kv) they are connected to common utility busbar (11kv). Each transformer fed by 3 generators at (0.4kv) side.
The 11kv side of transforms is (YN). Does we need to connect (every) transformer neutral to the ground or we just need to connect one transformer neutral.

PS: the 11kv grid is unbalance.

 

[waross]

A possible issue with connecting only one transformer to ground is the loss of your ground connection if the grounded transformer is taken out of service.
If there is any remote possibility of line to neutral loads, then all transformers should be solidly grounded.
If the system will be directly connected to ground then I would ground each transformer.
If you wish to use impedance grounding, you may consider using a neutral bus. All of the transformer neutrals will be connected to the neutral bus and the neutral bus will be impedance grounded. That way the system will be grounded by the same impedance regardless of the number of transformers in service.

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

 

[newelecteng]

thanks waross for your response.

If the neutral path of the transformers or Neutral Earthing Resistors is different (I mean in the omic value) is that will cause any problem.

We face problem when we try to synch the transformers with 11kv bus (it fail to synch). But, when we isolate the transforms neutral from the earthing (3 out of 4) it synch normally.

thanks,

 

[waross]

"If the neutral path of the transformers or Neutral Earthing Resistors is different (I mean in the omic value) is that will cause any problem."
It may cause problems with ground fault protection.

"We face problem when we try to synch the transformers with 11kv bus (it fail to synch)."
I have seen some generators sync'ed under extreme conditions.
You may have a setting issue.
I can't comment more without knowing what you are using for synchronizing gear.

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

 

[newelecteng]

Hi waross,

when every one of the 4 transformer connected to their (NER). all transformer fail to synch with 11kv grid. And we notice some circulation current between neutrals.

But when we isolate 3 transformer form their NER (left their neutral unconnected). all 4 transformer synch normally.

 

[waross]

I'm guessing here, because I don't have enough information.
I became responsible for a small diesel plant that had a wiring mistake in the synchronizer circuit that hardwired a 30 degree error.
Even with a 30 degree error, and about a 200 Volt difference across a 460 Volt breaker, most times synchonization was succssesful.
I am guessing that your unbalanced system has both phase to phase voltage differences and phase angle errors.
I am guessing that your automatic synchronizer settings are to close to allow for the unbalances on the 11 kV line.

"But when we isolate 3 transformer form their NER (left their neutral unconnected). all 4 transformer synch normally."
This is another issue that MAY be related to the unbalanced 11 kV system.
This is an issue that may become worse as more NERs are added to the system. That is, more transformers online, each with an NER.
There are a number of conditions which may cause a circulating current on the neutral.
Some are serious, some are not.
Parallel generators with different pitches will cause neutral circulating currents.
Line to neutral loads may cause harmonic currents on the neutral. Unbalanced line to neutral loads may cause neutral currents.
Not all neutral current is circulating current.
BUT, this is all guessing based on how I visualize your system. I may be completely wrong.

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

 

[newelecteng]

Yes we have many single phase transformer (11/0.24kv) that are connected to the grid. So, 11kv grid is unbalance for sure.
But, how this related to "This is an issue that may become worse as more NERs are added to the system. That is, more transformers online, each with an NER"

 

[jghrist]

If the single phase transformers are connected Ø-Ø on the primary, then even if there is unbalance, there will be no neutral current.

 

[waross]

Some comments about your 11 kV system first. Consider a simple case of one line to neutral load on A phase.
The neutral will carry current and so will have an IR voltage drop. This voltage drop will move the neutral position away from the system neutral in the direction of A phase.
A phase will carry current and so will have an IR voltage drop. This voltage drop will move the A phase vector position at the load away from the system A phase in the direction of the neutral.
The result is three-fold.
1. Unbalanced phase to phase voltages.
2. Phase angle errors.
3. Displaced neutrals.
Note; The line to neutral voltages may have been corrected by voltage regulators, but these only correct line to neutral voltages and do not much affect the other issues,
That is a fact of life with unbalanced single phase loading.
The settings on your sync-check relay must be wide enough to accept the unbalances or you will have difficulty synchronizing.
The floating neutrals make your generators more able to adapt to the unbalances.
However, it is common practice and a requirement of some codes that the transformers be solidly grounded when serving line to neutral loads.
From what I understand of your situation I would solidly ground all neutrals.
I may be wrong. I have learned to be wary of unknown systems. There may be factors of which I am unaware.
In regards to multiple NERs:
For example if your NERs are 5 Amps, With one transformer online a ground fault will be limited to 5 Amps. With 4 transformers online the ground fault current will be limited to 20 Amps.
In the event of an arcing fault 20 Amps will do more damage than 5 Amps. In some systems the extra current may interfere with ground fault coordination somewhere else on the system.
I must stress again that I am unaware of the details of your system and I am unaware of local practices.
I have tried to explain some of the basic effects of an unbalanced system.
By the way, A delta-wye transformer works well, but a wye-delta transformer is a bad idea.
Your generators are feeding the system with a delta-wye. No problem.
However if the system back feeds through the transformers they become wye-delta transformers.
Under some conditions, depending on site specifics such as the capacity of your plant compared to the overall system capacity and the PU Impedance of your transformers, I may consider high amperage NERs.
In the event of a line to neutral fault somewhere on the 11 kV system, The your transformers will act as wye-delta transformers and back feed into the fault.
The current will be limited by three times the transformer impedance.
If you have protection in place to cope with this safely, no problem.
If your protection will not provide adequate protection you may consider limiting the neutral current.
The size of the NERs in this case may be based on 3 times the transformer impedance plus enough reactance to limit the current to the transformer's maximum allowable current. In this case the allowable current may be based on 150% or more of rated full load current.
As to the transformer impedance and an NER value.
The transformer impedance may have six time as much inductive reactance as resistance (X:R ratio). If you add resistance you have to add it to the small value of transformer resistance with a small contribution from the inductive reactance.
If you add reactance you will not need as high an ohmic value.
Again, subject to site conditions.
I would investigate wireless reactors. This is an old old trick for increasing the impedance of a transformer for parallel operation with a transformer of higher PU impedance.
These are no longer commercially available, BUT.
You can add reactance by passing the grounding conductor through an old window type CT core. Passing the conductor through more than once multiplies the effect.
Again subject to site conditions. It depends on how much impedance you need.
I hesitate to suggest major changes to a working system.
However, you may consider changing the settings on your sync-check relay. That may solve your most pressing issues.

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

 

[waross]

"If the single phase transformers are connected Ø-Ø on the primary, then even if there is unbalance, there will be no neutral current."
Not on the system neutral, but the unbalances may cause a circulating current in the delta winding and a resulting neutral current from the OP's transformers.

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

 

[newelecteng]

thanks waross

you clarify a lot of things and we will consider your feedback.

thank you very much.

 

[newelecteng]

can you clarify this little bit:
"However if the system back feeds through the transformers they become wye-delta transformers.
Under some conditions, depending on site specifics such as the capacity of your plant compared to the overall system capacity and the PU Impedance of your transformers, I may consider high amperage NERs.
In the event of a line to neutral fault somewhere on the 11 kV system, The your transformers will act as wye-delta transformers and back feed into the fault.
The current will be limited by three times the transformer impedance."

 

[waross]

Wye-delta transformers.
Specifically wye-delta transformers with the wye neutral connected to the system neutral.
This connection is not recommended for distribution circuits.
Example #1:
Consider an unbalanced load on a system. The voltage will be lower on the heaviest loaded phase due to line impedance.
The neutral will shift away from the system neutral due to the voltage drop on the neutral.
As I have stated this unbalance will cause unequal phase angles, unequal line to line voltages and unequal line to neutral voltages.
Draw a vector sketch of a balanced wye system. Three equal length vectors originating from a common point and displaced from each other by 120 degrees.
Now imagine a line to neutral load on one phase. That will cause a neutral current and a voltage drop on the neutral conductor.
Modify your sketch to show the load on one phase. You can do that by trimming a few percent off of each end of one vector.
Now the line to neutral voltages are no longer equal, line to line voltages are no longer equal and the phase angles may not be equal.

Now feed that into a wye-delta transformer bank.
The vectors from your sketch may be transposed and connected head to tail at the same angles and with the same magnitude as your sketch.
As you construct your delta diagram you will find that the delta no longer closes on itself.
Rather than the normal equilateral transformer, there will be a gap or an overlap in one corner of the delta.
That gap represents a potential difference that will drive a current around the delta. The current will be limited by three times the transformer impedance.
This effect happens whether or not the transformer is loaded.
This will happen even if the transformer is transferring power in the opposite direction.
In the event of a line to ground (neutral) fault on the 11 kV system the wye-delta bank will transfer power from the healthy phases back into the faulted phase.
In the event that the 11 kV system loses a phase, the wye-delta bank will attempt to back feed the missing phase.
I have seen cases where an unbalance on the primary caused enough overload on a wye-delta bank to burn out a transformer in a few days but not enough current to clear the transformer fuses.
This effect is in addition to the transformer's normal load.
But, it depends. Sometimes it is not a problem, sometimes it is a serious problem.
Floating the neutral on the wye-delta bank allows the delta angles to change and greatly reduces the circulating current.
But floating the neutral leaves you with an ungrounded system.
There is no universal solution. The compromises that are made will depend on site conditions.




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

 

[newelecteng]

thanks waross for your information

 

[jghrist]

Not on the system neutral, but the unbalances may cause a circulating current in the delta winding and a resulting neutral current from the OP's transformers.

If all loads are connected Ø-Ø, then there will be no zero-sequence current. With no zero-sequence current, there will be no circulating current in the delta.

 

[waross]

If the line drop to the single phase loads causes a voltage unbalance at the wye side of the transformer, the unbalanced voltages will cause the circulating current.
Anything which causes an unbalanced voltage at the wye side of a grounded wye:delta bank will indirectly cause circulating currents in the delta.


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

 

[waross]

Actually I am not just talking theory. I have been involved in replacing a customer transformer that overheated to the point of failure from circulating currents caused by primary voltage unbalances that were in turn caused by uneven loading.
Long rural circuits were particularly trouble prone.

One of the worst cases that I saw was an extensive rural area served with only two phases.
To make matters worse, there was a dairy with three phase refrigeration connected open delta at the end of the circuit.
We were at the end of the three phase service. Two phases continued for about 15 or 20 miles.
The voltage unbalances were causing us severe motor heating issues.

Induction motors and wye:delta transformers have a lot in common when fed unbalanced voltages. They both try to correct any unbalances.
The wye:delta does it with circulating currents in the delta.
Motors do it with excess rotor current and unequal line currents that may be dangerously high on one or two phases.
Both motors and wye:delta transformers will try to replace the missing phase in the event that one phase is lost.

However it is correct that single phase line to line loading will not of itself cause circulating currents.
When single phase currents cause unequal voltages, the unequal voltages cause the problems.

Another interesting issue on unbalanced rural lines is the action of remote voltage regulators.
Faced with unequal loading and a greater voltage drop on one phase, the remote voltage regulators (typically three single phase units connected in wye) will correct the line to neutral voltages.
However the line to line voltages may be unequal.
When that happens there will also be phase angle errors, the phases will no longer be displaced by exactly 120 degrees.

On a positive note, I encountered one wye:delta installation that was inadvertently acting to correct a lot of unbalances on a long rural line.

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