Protection and coordination relay 50 / 51G 3

[Eng.Edward.M]

Hello, I'm having a doubt about the 50 / 51G protection settings. My system has a transformer (138kV - 6.6kV) with an RGB limited to 400A.
All connections are with CT involving the three phases.
Thinking about a protection between two relays what would be the best way to protect the medium voltage panels.
Is it correct to leave the adjustments according to the attached diagram?

 

[RRaghunath]

I see you have 40A & 0.01s for outgoing feeder and 40A & 0.4s for the incomer. There is ~0.4s coordination margin between the two which is good.

 

[Eng.Edward.M]

I understand, thank you.
But that way the circuit was not uncoordinated? because both are with 40A current.

 

[RRaghunath]

It is time based coordination here.
When the fault current exceeds 40A, both the relays (the one in outgoing feeder as well as that in the incomer) will pickup but the incomer relay will not give output and the outgoing feeder relay will trip before that.

 

[Eng.Edward.M]

Just one more question.
These are the relay settings as shown in the image.
51G pick-up = 0.4 (40A)
51G time delay = Set time 0.5s
50G pick-up = 0.4 (40A)
50G time delay = Set time 0.01

The 50G protection, should it follow the same thinking as the 51G protection?

 

[jghrist]

51G pick-up = 0.4 (40A)
51G time delay = Set time 0.5s

If used, the 51G should have an inverse time characteristic, not a set time. As a definite time element, it is no different than the 50G with a time delay. 51G will never operate if the pickup is the same as the 50G, which has a shorter time.

With an NGR, however, there will be little decrease in fault current with distance, so inverse time elements are not really very useful.

 

[RRaghunath]

Device no. 50 is for protection with no intentional time delay. When we have time delay provision, it should be termed as 50TD. Similarly, 51 is for inverse characteristic. This is from IEEE C37.2.
However, these ae not strictly being implemented with the advent of new devices (electronic/digital/numerical) that come with numerous options.

 

[Eng.Edward.M]

Please, would you let me know what is the best way to adjust the 50 / 51G protections?

 

[davidbeach]

There is no "the best way". There can be many acceptable ways and a vastly larger number of unacceptable ways. Many of the acceptable way may involve balancing different considerations and different protection engineers will find different balance points to be the most acceptable. Different organizations take different approaches, weighing various factors differently. No one right answer, just the universal protection answer of "it depends". But as the last two posters said, 51 is an inverse time element and doesn't have a fixed time delay.

I’ll see your silver lining and raise you two black clouds. - Protection Operations

 

[RRaghunath]

"...best way to adjust.." - that's what is called protection coordination study and it cannot be taught / learnt through a forum like this. Requires a lot of personal effort, unfortunately.

 

[Eng.Edward.M]

Thank you all for your help