Transformer with High Neutral-Ground Current

[lukmanhakim]

Hi all.
I have many transformer medium to low voltage (20kV/400V-230V) with wiring connection delta-delta, have neutral-ground value Current closely to value of phase-neutral current (Neutral-groung N-G= 80 Amp and phase-neutral R-N=105 Amp, S-N=100 Amp, T-N=103 Amp). value of grounding resistance arround = 1,4 to 3,5 Ohm.
could any one can help me give an detail explanation, what is wrong with that?
thanks for help.

 

[LarryFine]

Lukman, welcome to the forum!

It sounds like a corner-grounded Delta from your description, so the grounded line will carry current.

What's weird is that you're stating current between two points as one would usually state voltages.

 

[jim dungar]

Hi all.
I have many transformer medium to low voltage (20kV/400V-230V) with wiring connection delta-delta, have neutral-ground value Current closely to value of phase-neutral current (Neutral-groung N-G= 80 Amp and phase-neutral R-N=105 Amp, S-N=100 Amp, T-N=103 Amp). value of grounding resistance arround = 1,4 to 3,5 Ohm.
could any one can help me give an detail explanation, what is wrong with that?
thanks for help.

If you have a Delta connected secondary, it is unlikely that you have 400-230V. If you have a neutral point, it is unlikely you have a delta secondary. You probably have a 400Y/230 wye connected system although it is possible you have delta with a grounding transformer.

Is your neutral point solidly grounded or is there a grounding resistor?
Do you have any loads connected at 230V?

 

[robbietan]

If you have a Delta connected secondary, it is unlikely that you have 400-230V. If you have a neutral point, it is unlikely you have a delta secondary. You probably have a 400Y/230 wye connected system although it is possible you have delta with a grounding transformer.

Is your neutral point solidly grounded or is there a grounding resistor?
Do you have any loads connected at 230V?

I have a case here where the wye-wye poco transformer is supplying power to a wye-delta customer transformer. the zero sequence current is quite high, the customer has to readjust the trip settings of his zero sequence protection to avoid nuisance trippings

 

[jim dungar]

I have a case here where the wye-wye poco transformer is supplying power to a wye-delta customer transformer. the zero sequence current is quite high, the customer has to readjust the trip settings of his zero sequence protection to avoid nuisance trippings

Are any of the wye connections solidly grounded?

The delta connected load should not be passing zero-sequence current to the wye side of the transformer.

What type of zero sequence protection do you have?

 

[robbietan]

Are any of the wye connections solidly grounded?

The delta connected load should not be passing zero-sequence current to the wye side of the transformer.

What type of zero sequence protection do you have?

overcurrent Io with a timer. the poco transformer, as I have been told, are solid grounded wye on both sides. I imagine it is the unbalance from the poco supply that is magnifying the Io as seen from the customer secondary. When the customer is using his generators, his transformer becomes delta-wye and the Io figures drop dramatically

 

[Smart $]

...
Do you have any loads connected at 230V?

In asking this question, coupled with the OP, I can't tell whether you are 1) overlooking him saying phase-to-neutral current, or 2) looking "through" the likelihood of his phase-neutral current actually being Line current... :roll:

::::::::::

lukman':

Where are you taking these measurents, at the xfmr itself or elsewhere...???

What is the neutral grounded to, i.e. what electrodes? ...and where?

Are there any other electrical systems nearby... especially corner grounded?​

 

[jim dungar]

In asking this question, coupled with the OP, I can't tell whether you are 1) overlooking him saying phase-to-neutral current, or 2) looking "through" the likelihood of his phase-neutral current actually being Line current...

I asked the question about Line-Neutral connected loads, early in the discussion, because I was not clear on the actual system, when the OP said

... 400V-230V) with wiring connection delta-delta...

and then talked about a neutral.

 

[Smart $]

I asked the question about Line-Neutral connected loads, early in the discussion, because I was not clear on the actual system, when the OP said and then talked about a neutral.

Well, any way we look at it, the OP has a few "holes"

 

[jim dungar]

overcurrent Io with a timer. the poco transformer, as I have been told, are solid grounded wye on both sides. I imagine it is the unbalance from the poco supply that is magnifying the Io as seen from the customer secondary. When the customer is using his generators, his transformer becomes delta-wye and the Io figures drop dramatically

Normally the wye-delta customer transformer would block the utility zero sequence current from being reflected to the delta side.

How does connecting a generator make the transformer into delta-wye?

I can not create a mental picture of the one-line.

 

[robbietan]

Normally the wye-delta customer transformer would block the utility zero sequence current from being reflected to the delta side.

How does connecting a generator make the transformer into delta-wye?

I can not create a mental picture of the one-line.

here is a very simplified diagram of the customer. when the customer gets power from the poco, the transformer is not de-energized and acts as a wye-delta transformer to some loads inside the customer's power plant

 

[Cold Fusion]

Hi all.
I have many transformer medium to low voltage (20kV/400V-230V) with wiring connection delta-delta, have neutral-ground value Current closely to value of phase-neutral current (Neutral-groung N-G= 80 Amp and phase-neutral R-N=105 Amp, S-N=100 Amp, T-N=103 Amp). value of grounding resistance arround = 1,4 to 3,5 Ohm.
could any one can help me give an detail explanation, what is wrong with that?
thanks for help.

lukman-
Your description and numbers don't sound like a transformer with a delta secondary. "400-230" is a Wye connected voltage.

I'm going to assume the transformer has a Wye secondary

"Neutral-groung N-G= 80 Amp "
Here I'm not sure what current you are discussing. Is the the current on the neutral conductor leaving the transformer going to the switchboard?

Or is this the current on the bonding jumper between the transformer neutral and the ground (transformer steel)?

I'll need a little more information about the transformer is connected.

cf

 

[jim dungar]

here is a very simplified diagram of the customer. when the customer gets power from the poco, the transformer is not de-energized and acts as a wye-delta transformer to some loads inside the customer's power plant

So your facility is fed by a single transformer which is a wye-wye 440Y230 secondary. You have two co-generation sources, The first is a wye connected POCO, the second is a generator conneced through a delta-wye step up transformer. The generator transformer sometimes acts as a step-down wye-delta when the generator is not 'running'.

Am I correct so far?

Are you saying your problem is that you are experiencing zero sequence current problems on the generator side of the transformer when the generator is not running?

Is your generator a wye connected unit with a neutral grounding resistor?

 

[robbietan]

So your facility is fed by a single transformer which is a wye-wye 440Y230 secondary. You have two co-generation sources, The first is a wye connected POCO, the second is a generator conneced through a delta-wye step up transformer. The generator transformer sometimes acts as a step-down wye-delta when the generator is not 'running'.

Am I correct so far?

Are you saying your problem is that you are experiencing zero sequence current problems on the generator side of the transformer when the generator is not running?

Is your generator a wye connected unit with a neutral grounding resistor?

I have measured the zero sequence current jump to 50 times the "normal" value when he facility is on poco power (and the generator "off") and the zero sequence current return to almost zero when the facility is using the generators (poco supply is isolated).

I have no idea about the generator (the facility management wouldn't let me near it LOL) but I mean to ask them now that you have given me an idea.