overcurrent doubt

[bejoyeleengineer]

Hello,
In the figure that i have attched ? as you can see inside the 6T2B1(transformer back up protection panel in substations) panel ? there are two overcurrent relays in the same panel(50/51-1 and 50/51-2 and the other one is 50N/51N-1 and 50N/51N-2 ). Not sure why two relays of the same function is put and that too into a single panel. Please explain.

 

[bejoyeleengineer]

overcurrent - doubt

overcurrent - doubt

I had one more doubt when i saw the figure. In the transformers neutral ground we can see two CT's - one connected to the 51G and the other to the 87REF. Both seems to be doing the same function as both these relays senses the ground fault current and gives the trip command to the lockout. so why not use the same relay for tripping both the lockouts or is there a difference in function for both...... Thanks in advance

 

[SG-1]

The figure is too small for me to make out all the numbers on each device, but I do see that the 51/50s are energizing Trip Coil 2 & the 87 is energizing Trip Coil 1 on the same breaker. Looks like it could just be backup protection.

 

[jim dungar]

In the days of electromechanical relays it was not uncommon to install multiple 50/51 relays. For example, if you wanted to have an alarm output with different than the trip point, you would have needed to relays.

Do you know what the relay settings are?

 

[bejoyeleengineer]

overcurrent - doubt

overcurrent - doubt

But i doubt if it's for extra output contacts because in this relay there are quite a lot of contacts still available for one relay. Back up protection could be the reason behind putting two relays in one panel. anyway kindly answer the next question about the difference between the 87REF and 51G relay.

 

[SG-1]

But i doubt if it's for extra output contacts because in this relay there are quite a lot of contacts still available for one relay. Back up protection could be the reason behind putting two relays in one panel. anyway kindly answer the next question about the difference between the 87REF and 51G relay.

The 51G is just basic overcurrent protection. It will trip the associated 86 if the current through it's CT exceeds the setting. My guess is that it is the backup protection.

The 87 is a differential relay & more complex. There is always more than one set of CTs connected. By the drawing the other CTs are in the 13.8KV switchgear. As long as both sets or both single CTs sense the same current, no matter how much, the relay does nothing because the CTs oppose each other and the currents cancel. Let just one CT see enough current & the relay will trip the associated 86. The 87's zone of protection is the transformer itself. If it sees current on one CT and not the other it knows the fault is in the transformer & goes for a trip.

 

[SG-1]

I have been doing a little more thinking about the 51G. If a ground fault occurs the the 87 may not detect it because it could be on the far side of the switchgear CT. In that case the 51G would act to clear the fault.

It still provides backup for the 87.

 

[bejoyeleengineer]

overcurrent doubts

overcurrent doubts

Thanks for the answers.I am happy for the answer that you have provided admist my silly questions. I know i have asked a very silly question but i really appreciate your patience on answering these questions. I will keep asking more questions as such. Thanks once again

 

[RoberteFuhr]

In many modern solid state multifunction relays you can have group settings. These settings are used for different operating conditions. One such use is to use group 1 settings for normal protection. Group 2 settings would be for a Maintenance Mode. The maintenance mode would have much lower settings to reduce the AF energy. This is sometimes done on LV substations to reduce the hazard for racking in breakers.

In your example, there also could be different facility operating modes that require distinctive settings to coordinate or protect equipment. I have seen situations where closing the bus tie breaker will cause feeder breakers tobe underrated for the available fault current. If the fault current gets too high, then control logic tells the upstream breakers to operate before the underrated feeder breakers operate. This is accomplished by using a group setting with lower pickup and time delays.

In short, you have to look at the big picture (the entire drawing) to try to determine the function of the two OC relays.