Problem in the Power Transformer

More detail about the physical arrangement is required, including a single line diagram and information about the length of the transmission lines. When you say charging do you mean that the uncharged side is connected to the transmission line which is also charged, has load or no load, or is open at or near the transformer terminals? Also what is the short circuit contribution from each transmission line?

I suspect that the 66kV side is less robust than the high side and that the transmission line is also connected to it and is being charged as well. Energizing an inductance with capacitance attached can lead to resonance which in turn may cause abnormally high currents to flow momentarily. These can lead to saturation of the core which results in a high level of harmonics, all of which can affect your relaying.

Best way to see what's happening is to use a Transient Data Recorder or a Recording Oscillograph/Oscilloscope and and capture as much data as possible.

btw: Are you sure about the currents and the percentage multipliers? 55% of 218.5 does not equal 215, and 105% of 437 does not equal 420, better check all the settings again

the over current setting 55% is of CTs connection ie 400/1A on HV side and over current setting on LV side is 1.05 % of CTs connection 400/1A .Also the line length of the line is 38 km. and we did not know the short circuit contribution of the transmission line. first of all the transformer is charged from 132kv side , it stood ok but when we on the breaker of 66kv side of transformer it show earth fault and the earth fault relay trip from 132kv side. the 66kv side of transformer is connected to the 66kv bus bar.

problem in power transfer is very important topic and not everyone can describe about this without having a knowledge.many problems occur when power is transfer from one place to another place its a very tough process.electromagnet suppliers provide a product that help in power tranformer.

You have mentioned that the primary of the neutral CT on the secondary side of the trafo has multi tapping 800-400-200/1A. Please indicate the tapping point at which it is set at present. If it is set at lower tapping you may switch over to higher tappings and see the result.

no sir the neutral CTs has only one core and it is connected on 200/1A. where as the CTs connected on HV & LV side of transformer has multiple setting as 800-400-200/1A and is connected on 400/1A on both sides

sehdev;

Not knowing enough details and with limited information it is very confusing.

However, from your description of the issue it appears your method and/or settings for the transformer testing is flawed.

If I understand you correctly:

Energizing the primary side of the transformer does not result in a trip condition but energizing the secondary side results in a "ground/earth fault" trip initiation.

You do not clarify but it appears you are inducing test voltage on the secondary side with the primary side "open" and disconnected (de-energized). (Are you using test equipment to do this or are you back-feeding the transformer from another source?)

Your question appears to be: Whether to alter the "ground/earth fault" pickup magnitude setting or the time duration setting on the secondary side of the transformer or at the station where the primary power is being provided from.

My questions are:

Where is the relay trip function occurring? At the primary supply source (station) or in the transformer secondary protection relaying circuit? (Location of the trip event dictates where to look for and address the cause.)

Have you tried to energize the transformer "unloaded" and disconnected from all cables and busses other than the "Y" earth/ground connection?

Remember:

1. The cable/wiring and buss present/exhibit charging load during energization.

2. Where you induce the test voltage physically on the buss can and will affect results of the test due to the direction of current flow induced though the earth/ground CT. (Test lead connections and/or source attachment.)

In any case:

I would suggest:

1. You first examine the physical location of the ground buss CT and make sure it is correctly positioned and properly polarized on the buss.

2. Meggar the secondary windings in referrence to the "Y" connection and verify your system impedance is balanced.

3. Make sure the "Y" connection and earth/ground connection is properly torqued and verify the impedance "Y" to earth/ground is less then 1 ohm impedance. (If the "Y" connection and/or earth/ground is not at very low impedance, the "B" phase voltage will become unstable as well as too high in magnitude which in turn will cause false relay tripping due to the unbalance and "wild leg" induced condition.

4. If checking all of these items does not reveal the cause of the issue, I would suggest you review and verify the testing method(s) as well as the relay settings especially on the GF time and magnitude parameters.

Hope this helps,

Jim