Neutral current in transformer

[prc]

A 100 MVA 11/132 KV generator transformer with OLTC is connected to grid.At rated tap( 138 kV, there is no neutral current.
The grid voltage was 133 kv and transformer terminal voltage after regulation is also near to it.Line currents are also equal and balanced.
When tap is changed to 133 KV, grid voltage was 135 KV and after regulation the voltage difference between transformer terminal voltage and grid was 5-6 Kv and a neutral current of 23 A ( nearly 5% of full load current) is seen.When line currents which are not balanced,are vectorially added, the net current is coming near to 23 A.Customer says transformer is faulty and that is why neutral current is coming!

Waht exactly is happening when we a trf with terminal voltage say 129 Kv is connected to infinite grid at 135 KV?Will neutral current will be generated on that occasion? If terminal voltage is more than grid voltage what will happen?Then also neutral current is seen.

 

[waross]

When the voltages are different there will be a reactive current flowing in addition to the load current.
It is a fact of life and transformers that the neutral current will equal the vector sum of the line currents.
The question should be;
Why do the line currents become unequal when more reactive current is flowing?
You may have a faulty tap changer.
That would explain the change in current balance when the tap setting is changed.

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

 

[ScottyUK]

Possibilities...

[li]Tap winding ratio mismatch (unlikely but possible).[/li]

[li]Mis-connected tapchanger[/li]

[li]Defective tapchanger[/li]

[li]Core saturation giving rise to triplen harmonics, which are bound to run up in the neutral.[/li]

First thing I'd look at is whether the neutral current is fundamental frequency or harmonics. I say this because it is the simplest thing to verify: a winding ratio check on a big transformer takes a fair bit of setting up just because of access problems, and an OLTC inspection is a pretty major task.

You could probably use a standard LV current probe clipped around the neutral earthing conductor and get away with it almost every instance except during a major fault when you will end up having a very bad day, or clip over the neutral CT secondary with a more sensitive probe. If you have an neutral earthing resistor then the LV probe will be fine provided it is clipped on at at the solidly earthed end of the NER.


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If we learn from our mistakes I'm getting a great education!

 

[rockman7892]

If there are no loads directly connected to the nuetral should there even be nuetral current weather or not the loads are unbalanced?

 

[ScottyUK]

If more than one transformer on a common system has its neutral point connected to the mass of earth then the possibility of neutral current exists. Transmission system transformers almost always have a star winding on the HV side, and it will invariably be grounded either through a resistance or solidly. Ungrounded transmission systems are very unusual so in practice neutral current tends to flow to some extent, just normally it is very small.


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If we learn from our mistakes I'm getting a great education!

 

[slavag]

Hi. 5% of full load current is very bad.
It's GSU, I think it's solidly grounded trafo.
Xm.. in additional to Scotty's tests. Are possible meas residual current on the HV side and what is differntial and restrained current in the 87T protection?
Tap changer...problem with contact...
Best Regards.
Slava

 

[waross]

The grid voltage was 133 kv and transformer terminal voltage after regulation is also near to it.Line currents are also equal and balanced.

When tap is changed to 133 KV, grid voltage was 135 KV and after regulation the voltage difference between transformer terminal voltage and grid was 5-6 Kv and a neutral current of 23 A ( nearly 5% of full load current) is seen.When line currents which are not balanced,are vectorially added, the net current is coming near to 23 A.

As I understand the information, when the taps are set at 138 kV and the voltage regulated by the generator the currents are balanced.
As I understand the second information, when the tap changer is changed, the currents become unbalanced.
Case #1; If the change is abrupt when changes are made to the tap changer setting this points to a tap changer issue.
However, the problem may be developing at the 138 kV tap setting and only becoming noticeable at the 133 kV setting.
This could be the result of a voltage problem anywhere on the system.
I am going to take a leap of assumption here and suggest that the 135 kV line voltage on a 138 kV line coincides with heavy loading.
There may be a slight voltage unbalance on the 138 kV line under heavy loading.
Possible causes may be:
A faulty tap changer in a remote transformer.
An unbalanced transmission line. (Too few of improperly located transitions?)
Possibly an unbalanced line that only comes into use during heavy loading.
A an automatic capacitor bank somewhere that is missing a phase on a step that only comes into use during heavy loading.
There may be an issue with unbalanced voltages (One possibility is a faulty tap changer in either your transformer or another transformer.)
There may be an issue with unequal line voltage drops or impedances. (One possibility is poor transitions.)
Unequal phase current balance with increased load. (One possibility may be a switching capacitor bank with a missing phase in one or more of the higher steps.)
Can you give us any more information, prc?

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

 

[prc]

Gentlemen,the client is also raising the doubts expressed by you. But I disagree.I am not suspecting anything wrong with OLTC as it was commissioned barely one month back after checking every thing ie Resistance at all taps,ratio, and continuity during tap changing etc.

The neutral current is seen when tap changer is raised to a higher tap and also when lowered to a lower tap.So my conclusion neutral current is due to difference in the voltage on trf terminal (after regulation) to grid terminal voltage.How or why it happens?

The voltages are balanced always.See below:

Tap No12- 21.3 A neutral current, 68 MW load from generator, 264,270,268 A line currents, 135,135,135 KV, 0,245,125 degrees phase angle.

Tap No9- 0 neutral current, 80MW loading, 323.6,323.7,323.9A,132.6,133,132.9 kV, 0,240.7,120.7 degrees phase angle.

I came across such a problem in GTs some 35 years back.In a GT (140 MVA) utility noted a neutral current of 10 A.Out of curiosity I arranged to measure neutral current in many GTs and trfs around the country. I found always some current.Is it that we have never noticed this and this will be there always?

 

[waross]

You may wish to review:
thread238-228877

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

 

[prc]

Sorry Waross,that is not relevant here.Just before energising ratio had been checked and everything was fine.At 9th tap everything is fine.Then also tap winding and tap changer is in circuit.So anyhting abnormal with transformer is ruled out.Anyway taking DGA to confirm.