High Voltage ? Selectivity

[Lars]

Sometimes we experience that a primary High Voltage relay is cutting out the HV feeder even with smaller electrical faults secondary on a 4160/480 transformer.

The (old) electromechanical Westinghouse relay CO-6 50/51N has a Time Dial 1 - Amp.Taps 0.5 - CT 600/5

As I see it, the Amp Taps will cause a cut out with a current larger than 60A primary in more than 0.85 sec. (or is it 90A ? ? here I have my doubts)

To prevent those unintended cut out, I want to Increase the amp. range on my CO-6 relay.

To create an acceptable selectivity to the CO-8 51 relays (Time Dial 3 ? Amp.Taps 5 ? CT 600/5) I have in plans to increase the CO-6 amps to about 2000-5000A and to lower the Time Dial to 0.5 in order to spare the feeder if a cable fault should happen.

An amp. increase like this seems a little drastic to me, but it will meet all European regulations. Will it meet the US standards as well, referring to normal settings of a CO-6?

The HV feeder is an AWG 4/0 and the 1000 KVA transformer has a 200A load interrupter before and 1600A over current protection after the transformer.

Thanks,
Lars

 

[George Stolz]

Lars, welcome back.

Is this the same deal you were dealing with before?

 

[jcormack]

Relay OK

Relay OK

Have you had the relay checked/calibrated?
Is it an Instantaneus Trip? Time? Phase? Ground?
CT ratio's correct - CT's been tested?

I would adjust time dial first.............

 

[jcormack]

Why so low

Why so low

The tap setting for the CO-6 at 1 is too low - why would you not have that set for full rating of the 1000 KVA trans (138 amps) - I would begin at Tap 2 You could probably go higher (I don't have curves in front of me for that model)

 

[wanderer20001us]

Test the relays and set the relay per the coordination study values. If there is no coordination study, do one (get one done). This is the best way to get proper operation without risking increase fault damage.

 

[Lars]

Hi George.

Thank you for the welcome. Yes I am working the same thing with which you were so helpful last year. You and this great Forum got me convinced that my problem was a too low current setting, now I only need to convince my boss )

I see no reason to let the rating on the transformer decide the current settings on my CO-6, the transformer is protected by the 1600A breaker placed secondary on the transformer.
Only the HV 4/0 feeder cable need protection by relays and is over current protected by the CO-8, now I only need the short circuit protection provided by the CO-6

The relay was installed 1965 and the setting hasn?t been changed since then, maybe that is why there is doubt about my suggestion to increase the settings to maybe 5000A. I admit that it is a drastic increase from 60A to 5000A, but it will make the release curve look like a normal breaker curve compared to the CO-8 relays.

The above mentioned is why I want to be sure that my proposal to the settings are normal and not violating any NEC recommendations.

Wanderer20001us, jcormack
The CT and HV relay is being calibrated every 2 years, last time Nov. 2006 and no faults have shown.
The size of the CT is adequate for the CO-8 ? Maybe I have to change the CO-6 relay.
The instantaneous part of the CO-6 relay is for ground fault, but it hasn?t been activated.
We have a coordination study for all interrupters, but for some unknown reason the CO-6 is not a part of this study, which I am trying to change.

Thanks to all.
Lars

 

[jcormack]

Sorry missed the fact that you were using the CO-6 as 50/51N - I was thinking Phase OL - not Phase to ground - so ignore my transformer capacity comment. If the CO-6 50 (inst.) is not in play then examination of the tap settings and time-dial of the two curves is all you need to coordinate the two relays. The tap settings of 60 amp on the primary side and 600 on the secondary are similar (although the primary will begin a little sooner) - the time-dial is where the two diverge. With a time-dial of 1 the primary 51/N is always going to trip before the downsteam device. Your two options are lower the time-dial of the secondary or raise the time-dial of the primary. A quick glance at generic CO-6 & CO-8 curves shows me that a time-dial of 4 on the CO-6 would overlay the CO-8 at TD 3.

And I hope when you are saying you want to set the CO-6 to 5000 amps you are meaning to activate the 50 (Inst.) portion of the relay and not to set the tap & TD of the 51 to 5000 - that is NOT the proper way to use the rotating disk ! ! ! ! ! That portion is for time overcurrent not inst, fault!

 

[Rockyd]

Lars,

Might want to go here. We used there gear for Defense at Greely on ABB's equipment to meet the Seismic 4 requirements. Sounds like DEW line upgrades for white Alice sites (1965 ref)?