Electromechanical Relay Settings

[psuzig]

Hi folks. I was hoping someone could help clarify the application of an old Westinghouse CO-8 protective relay and associated CT's. The CO-8 relay in question has a tap setting range of 1 - 12, 1200:5 CT's, and is protecting a 1,200 amp feeder. It's my understanding that the tap setting is directly related to the long time pickup current rating of the device. For example, if a tap of 5 is selected, the pickup current would be ( 1200 / 5 ) x 5 = 1,200 amps. With this example in mind, I have two questions:

1. Does the pickup current rating establish the overcurrent protective device rating for the circuit/feeder?
2. When would a tap setting above 5 be used with XXXX:5 CT's? Several engineers that I work with believe that if the CT's have a secondary rating of 5, any tap setting above 5 will potentially saturate the CT's. i.e. In the example above, if a tap of 12 was selected, the pickup current would be ( 1200 / 5 ) x 12 = 2,880 amps, well above the 1,200 amp CT primary rating and 1,200 amp feeder ampacity.

Thanks for your help.

 

[busbar]

http://www.abb.com/global/abbzh/abb...6311E&e=us&c=A91AA5E649C3ADCF412567A900467A57

 

[RalphChristie]

Re question 1: Overcurrent relays has a minimum operating current, known as the current setting of the relay. The current setting must be chosen so that the relay does not operate for max load current in the circuit been protected, but does operate for a current equal or greater to the minimum expected fault current. Although by using a current setting that is only just above the max load current in the circuit, a certain degree of protection agains overload as well as faults may be provided, the main function of overcurrent protection is to isolate primary system faults and not to provide overload protection

Tap range selection depends on the fault current as seen by the relay, which is determined by the ct-ratio and its operating characteristics under fault conditions. The magnitude of fault current available at a given location is usually determined by a fault study.

Re question 2: Protection equipment is intended to respond to fault conditions, and is for this reason required to function at current values above the normal rating. Protection class CT's must retain a reasonable accuracy up to the largest relevant current.

In the IEC-world the accuracy class and the accuracy limit factor of the CTs stands on the nameplate, like 1200/5 5P10 15VA.

15VA = Rated burden
5P = Accuracy class (5P or 10P)
10 = Accuracy limit factor (5,10,15,20 and 30)

For a current 10 times the normal current, (In this case 1200 x 10 = 12,000A) there will be a max error of 5% - this is well above your current of 2880A

Also, the plug setting of a electro-mechanical relay does have a effect on the burden. The burden is the load imposed on the CTs by the relay. The lower tap of the relay places a higher burden on the CT's and they must have adequate performance to meet such demand. This is often the case in low ratio CT's and the mistaken impression is created that the relay is at its most sensitive setting when on its lowest tap when in actual fact the CT's may saturate under these conditions, causing the relay to respond more slowly if at all.

I'm not sure how the ANSI notification for CT's work.



Hope it helps
Regards

Ralph

 

[jbartos]

Suggestions to psuzig (Electrical) May 26, 2004 marked ///\\Hi folks. I was hoping someone could help clarify the application of an old Westinghouse CO-8 protective relay and associated CT's.
///The relay is now under

http://www.abb.com\\

The CO-8 relay in question has a tap setting range of 1 - 12, 1200:5 CT's, and is protecting a 1,200 amp feeder. It's my understanding that the tap setting is directly related to the long time pickup current rating of the device. For example, if a tap of 5 is selected, the pickup current would be ( 1200 / 5 ) x 5 = 1,200 amps. With this example in mind, I have two questions:
///CO-8 is inverse time overcurrent relay often now replaced by MCO relay. Visit

http://www.abbus.com/abbrelays/pdf/100/41_122m.pdf

for MCO.
Normally:
1. Overcurrent relay ANSI No. 50 protects circuit against shorts and heavy overloads near short
2. Overload relay ANSI No. 51 protects circuit against overloads, e.g. 110%, 115%, etc.
Both relays have to be aligned with the sensor rating that is normally equal or lower than the circuit breaker rating.
\\1. Does the pickup current rating establish the overcurrent protective device rating for the circuit/feeder?
///Yes, ANSI Dev. No. 50.\\2. When would a tap setting above 5 be used with XXXX:5 CT's? Several engineers that I work with believe that if the CT's have a secondary rating of 5, any tap setting above 5 will potentially saturate the CT's. i.e. In the example above, if a tap of 12 was selected, the pickup current would be ( 1200 / 5 ) x 12 = 2,880 amps, well above the 1,200 amp CT primary rating and 1,200 amp feeder ampacity.
///The fault currents normally saturate CTs. The CT is designed to withstand 20xFLA rating overload. It depends on the relay-CT alignment how the fault is cleared. Often, CT are sized on the circuit breaker frame size current rating, e.g. 1200A circuit breaker frame, 1200:5A CT. Often, CO-8 relay is specified for a ground fault protection.\\

 

[jghrist]

I agree with RalphChristie. Generally, overcurrent relays are used for fault current protection, not overloads, on MV distribution feeders and HV transmission lines. Overloads are avoided by proper planning and reviewing loads regularly. I would normally set the pickup to be at least 200% of the expected peak load to avoid operation when there is no fault.

The pickup current rating does not establish the overcurrent protective device rating for the circuit/feeder. The inverse time characteristic allows a current higher than the pickup current for a significant time. If the pickup current were set at 1200A on a 1200A breaker, it would not prevent the breaker from being loaded higher than its rating. The pickup current settings because they do not limit the mamentary current.

The continuous current will not saturate the CTs. The 5A rating is based on thermal limits. Some CTs will have a rating factor over 1.0 to allow currents higher than nominal.