REF

It seems that it is working properly, since it is a very sensitive ground fault detector, and most overhead line faults tend to be ground faults. Maybe it is misapplied?

My guess, if you are sure it is not working properly (from my very first observation today of this method of ground protection system since I started in power system engineering in 1975 in the US), your line current transformers are saturating before your neutral current transformer.

Perhaps the drawback of this theoretically very sensitive method of detecting a ground fault is that the current transformers need such a high ratio that your realized sensitivity is much worse than the theoretical 15% over winding rated current.

In the US, if we can protect the transformer windings for ground faults down to 10% of the winding voltage rating, we're doing good. For the delta side, on a solidly grounded system, in practice, a ground fault relay in the residual connection (51N) will not actuate reliably for fault current below the Current Transformer primary rating, i.e. a ground relay in the neutral of 1500/5 ampere current transformers will not trip even on the 0.5 ampere setting unless you have a good 5 amperes of secondary current.

Relay schemes that need to be restrained for magnetizing current seem complex. A transformer of the size to warrant that sort of protection should be protected with differential protection, mature and common enough designs to include easy set up for different current transformer ratios and saturation characteristics. Then the layer of protection for your transmission lines can be separate, and tailored to the needs of that system.

The actual power system grounding and application of your restricted earth fault relay is important for diagnosis, solid wye ground, zig-zag grounding transformer, multiple grounds on the system, delta or wye supply to your distribution line, available fault current, these details will be helpful to further diagnose your problem. Maybe the relays are set to the wrong current threshold setting, for instance.

"any fault happened with over head line" - Do you mean 400kV OHL. If true, it is clearly a matter of stability of REF protection, considering that you already confirmed the CT secondary wiring to relay including polarity of phase and neutral CTs.

The way forward is to get the stabilising resistor calculation verified.

Thank you all for their answers

yes I mean 400KV OHL

So the line is feeding into a delta winding, and your power system ground is remote?

Your lighting arresters (with a ground path) might be inside of your protection, unless your relays are fed from bushing current transformers...

dear

thanks for your response

yes our protection feed from bushing current transformer

thanks

Asser, Once you find out the real problem, do post the details.

I am supposing the transformer is YNd connected and is acting like zero sequence current source for earth faults in 400kV lines feeding the faults (though there is no source on secondary side of transformer).

Common problems I have come across with HiZ REF protection schemes:

1) The neutral CT is left shorted near (or on top of) the transformer.

2) The polarity of neutral CT is not verified during stability check and is incorrect.

thank you for your interest

We check the neutral bushing CT it is connected to the relay and the polarity it is ok,since when

transformer in service the Bais HV REF>>>Diff HV REF ,but when the fault happen in the line

the Bais HV REF become less than Diff HV REF

thanks

Just curious, are you using the High Voltage bushing current transformers for the line current signals? It certainly seems that the polarity is reversed, assuming the protection is designed to detect ground fault current within the transformer case.

I suppose it might not be the first time someone was not paying attention to the primary polarity marks when they assembled the current transformers to the bushings. Certainly difficult to verify visually at this point...

The secondary winding for your transformer is connected in Delta?

Do you have generation sources on both sides of your transformer?

Do you have transformer differential protection installed?

Dear sir

the transformer have three winding (LV,TV) 18KV Delat connected and HV 400KnutV

Star connected. All CT (one in neutral side and three on phases) bushing type

the same CT uses in differential protectiont and restricted earth fault protection

Each delta side connected to generator and star side connected to the 400kv grid

thanks

So it seems unlikely that the bushing current transformers for the differential protection would have been installed correctly and the phase CTs incorrectly, so the polarity is likely OK, as you have verified.

if you do have a ground fault on your lines, your generator needs to disconnect at some point before you melt or seriously damage one of your transmission phase conductors. If the faults are on the main line, then opening up your high side or low side circuit breakers is the correct response. If the faults are beyond other transmission circuit breakers, then your primary REF protection should have some time delay to allow the downstream or remote circuit breakers to open, selective tripping or coordination. Other options include distance relays, where the transmission conductors are modeled and the relay is set to respond for a fault fed by your generator(s), through your transformer, for some line distance from your transformer, uses predicted X/R fault currents, without overlapping zones of protection beyond distant protection zones.

Your current REF protection is supposed to be quite sensitive, but I suspect that it gives you no more sensitive protection in practice than your transformer differential protective relay, due to current transformer saturation, as I explained earlier for residual (CT neutral) connections. As a backup for your differentials, adding some time delay up to the damage curve of your line conductors would seem worthy of consideration. A chat with your system protection designer MIT be helpful.