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dhanshyam
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Join Date: Feb 2011
Posts: 6

No-Load Charging of Transformer & Neutral Current as Sum of Phase Currents

02/23/2011 11:39 PM

In our plant, we have a 3-phase,180MVA,13.8/220KV, Delta-star Generating Transformer with 12.45 percentage impedance at normal tap. The other day, we backcharged the GT from the 220KV side, leaving the 13.8KV side unloaded. The no-load phase currents on the primary side(220KV) were as follows: 2.0A, 2.0A, 2.0A(converted using CT ratios) in R, Y & B phases respectively. But the neutral current was 6A. The result was the same with every core of CT. The voltages on both the sides were satisfactory.

The neutral is solidly grounded. We initially thought that the neutral grounding was not properly done, but when it was measured with 4-wire method, the two earth pits showed the resistances of 0.17ohm and 0.13 ohm.

Thus our main observation was that the neutral current was the algebraic sum of the phase currents on the star side in all the 5 cores of CT we have on the 220KV side. Why did this happen? Is this any phenomenon related to no-load charging? If yes, what is it? Where is the unbalance coming from? From my knowledge, under balanced condition, IR+IY+IB= IN = 0

Please respond with your expertise and experience.
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loadshare
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Engineering Fields - Electrical Engineering - New Member Engineering Fields - Mechanical Engineering - New Member

Join Date: Jan 2011
Location: Jakarta
Posts: 162
#1

Re: No-load charging of Transformer & neutral current as sum of phase currents

02/24/2011 12:25 AM

Hi,

Wondering problem on your grounding. What is your earthing system ? Did you combine the grounding between souce/GT , other GT and distribution ?

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dhanshyam
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Join Date: Feb 2011
Posts: 6
#2
In reply to #1

Re: No-load charging of Transformer & neutral current as sum of phase currents

02/24/2011 1:00 AM

Yes, our neutral grounding is done by 2 numbers of 3m GI earthing electrode, connected in parallel with each other, with alternate layers of salt and charcoal in the mentioned proportion. The other end of the electrodes is joined to the main earthing grid, formed by perpendicular MS rods of 40 mm dia, separated by 16 meters and laid 1m beneath the finished ground level. The earthing of the source, GT and the entire distribution network of the substation are joined at suitable places using test links.
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vijaymail_electrical
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Join Date: Jan 2011
Location: India
Posts: 98
Good Answers: 2
#3
In reply to #2

Re: No-load charging of Transformer & neutral current as sum of phase currents

02/24/2011 1:14 AM

Hi,

Are you measuring the current in the relay or by using tong tester. Kindly check the neutral wiring behind the relay. When i was commissioning one 220kV S/s, i received this problem. When I checked the neutral wire was looped twice with the phase current, the current was getting added in the neutral path and we were getting error values. Kindly conduct a stability test for the trafo and check with each phase ON while the other two phases OFF.

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sushildvc
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Join Date: Apr 2011
Posts: 1
#8
In reply to #2

Re: No-load charging of Transformer & neutral current as sum of phase currents

04/18/2011 7:57 AM

Hi,

I m also facing the same problem as you faced So i need your help in this matter whether u solved that problem and if yes then kindly send the procedures.
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loadshare
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#4

Re: No-load charging of Transformer & neutral current as sum of phase currents

02/24/2011 3:47 AM

Hi,

This use for 3 pole breaker from power source. If you use 4 pole breaker you could try to separated grounding.

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mountk2
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Join Date: Jun 2010
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#5

Re: No-Load Charging of Transformer & Neutral Current as Sum of Phase Currents

02/24/2011 3:56 PM

You are saying that earthing "it was measured with 4-wire method"; To my knowledge the 4 wire method is to check soil resistivity and for earth resistance test 2 spike method will be applicable, any way referring to your query: Please realize that there always is some leakage reactance, run distinctly separate earthing wires from transformer tank to the pit, conditions will improve.

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rajukailas
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Join Date: Dec 2009
Posts: 14
#6

Re: No-Load Charging of Transformer & Neutral Current as Sum of Phase Currents

03/01/2011 1:11 PM

Sir

Charging current of transformer my not be purely sinusoidal to give 0 current through the neutral.

Full load current of a 180 MVA transformer is 450 A approximately. You are measuring a current of 2 A. Your CT operating point is very low in OCC, may not be in straight line portion of OCC.

When you load the transformer CT operating point will come to linear portion and the proportion of magnetisation current reduces in primary current. then it give balanced load.

We even faced EF tripping during charging of transformer during commissioning.

Regards

Raju
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manindra
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Location: Kolkata, West Bengal, India
Posts: 172
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#7

Re: No-Load Charging of Transformer & Neutral Current as Sum of Phase Currents

03/02/2011 7:29 AM

Hi,

My apprehension is that you obviously back charged the trafo from grid power source. If power source contains harmonics, all tripplen harmonics ( zero sequence components of currents) are passing to ground via neutral( since neutral is grounded). In addition to the above in no load condition , the current drawn by the trafo is predominantly magnetising current and this may aggrevate the above effect.

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dhanshyam (1); loadshare (2); manindra (1); mountk2 (1); rajukailas (1); sushildvc (1); vijaymail_electrical (1)

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