Stabilizing Resistor in REF Protection

http://www.rmspl.com.au/solutions/2v73.pdf

This might be helpful, OK it deals with one brand of relay but gives some good information. http://search.woodward.com/PDF/IC/DOK-TD-IRI1-ERE.pdf

I've installed several REF relays but they all came with "the idiots guide to setting up". The senior engineer would provide all the setting information to me.

Very kind of you, Mr. Sridhar! But, as you have rightly guessed, I have already gone thro this guide manual of Areva and it is not of much help in my query. Anyhow, thanks again for your initiative.

Please note that in REF a CT is provided at each line/winding to be protected and the CT secondary windings are connected in parallel to the relay.

Idealy the outputof the CT's is proportional to the sum of the zero sequence currents in the line and neutral earth connection (If neutral connection is within the protected zone).

For internal earth faults that sum is equal to "2 x total fault current" but for external faults zero sequence currents are either absent or sum to zero in the line and neutral-earth connection

Stabilzation is carried out since the system is not ideal and is not meant to de-sensitize the reay.

As stated before of internal and external faults, Normally we are designing relays for faults within the zone where the relay has to protect it, but what do you actually mean internal fault?

Sir

During through faults one of the CTs go temporarily to saturation level. In other words we purposefully saturate one of the CTs by adding high impedance in relay path and permitting it to develop voltage enough to saturate one of the CTs. A saturated CT secondary is a short circuit path in parallel circuit of all CTs and REF relay.

During through faults, Currents from all other unsaturated CTs will be circulated through the saturated CT secondary since this is very low impedance path compared to branch in which Relay is connected(due to stabilizing resistor in series with relay).

During in zone fault no CT is there in saturated condition. So there is no low impedance path to circulate currents from CTs. All the CTs drive current through Relay circuit to operate it.

With Regards

Raju.S

A siturated CT does not provides a short circuit path, In saturation conditions only waveform gets distorted and output from the CT is unpredictable behaviour of spikes to cause relay malfunction, further CT saturation can permanantly alter the magnetic characterstics of the core. Neither adding of high value impedence is desireable since that will cause the CT's to develop voltages above 2 kv during maximum internal faults.

The high impedence REF setting and value of series stabalizing resistor is kept low enough to operate at a votage slightly higher than developed by the CT's under maximum external fault condition.

Hi Everyone.

I have a question.

For a 3000/1A CT, the kneepoint voltage is 740V and I have calculated the stabilsing resistance to be in the range of 438Ohms to 617 ohms for a 60% setting. The voltage developed by the CT during a fault is 7.6kV and the minimum primary residual current detectable is approx. 2700A. The fault in the primary side is 63kA.

Can someone comment on this scheme and let me know these values are acceptable or if there's anything worong with it.

Thank you