Relay 87 (Differential Current) For Transformer 3

[akhtarbiqadri1]

Is 87T able to change the setting value automatically according to generator status so 87T won't activate for tripping while the generator is in stand-still status?

 

[cranky108]

Maybe. If you set the logic that way. But why would you do that? Why would you be seeing a fault when the generator is at standstill?
Or is this a people problem?

 

[stevenal]

When your generator is still, I would expect your 87T to sum to zero and not activate. Why would you need to block?

 

[dpc]

It's possible. You will need some logic and a contact wired to the relay to change settings.

 

[akhtarbiqadri1]

Thanks for the answers. I'll give the diagram here:

So, I want to add 87T between CT2 and CT6 so the 11 kV line will be protected by 87T. The status quo is: The 11kV network is only protected by 50/51G and 27 relay (between CT3 and CT4). Last time, there was an incident when a 3-phase short circuit happened. It takes more than 1.36s to tackle that incident. So, I want to add 87T by installing an additional CT (CT6) to extend the protection range of the 11kV network and to reduce the time needed to tackle the incident.

My question is: When the generator is in stand-still status, will "backfeeding" happen? And if back-feeding did happen, would that trigger 87T and cause the system tripping? (because the intake gate tr and station service tr take power from the grid and the generator is not always running)

 

[RRaghunath]

When the auxiliary power is being drawn from the Generator step-up transformer (GSUT) with Generator 11kV breaker open, there will be current in CT2 but no current in CT6. This current will be seen as differential current by 87T.
Also, if there is a fault on secondary side of either of the auxiliary transformers, that current also will be seen as differential current by 87T.
So, it is important to set the Differential pickup of 87T so that it doesn't maloperate for auxiliary load current or for faults on secondary side of the auxiliary transformers.
Once the above is ensured, the protection will work well as desired by you.
Just to mention - the arrangement proposed by you is quite common in Gas Turbine power stations and you may not find it difficult to locate appropriate guidance in setting the relay.

R Raghunath

 

[cranky108]

Or you can add CT's on the service transformers, or set the minimum trip higher. Other option is not use an 87 relay, use a 51 relay for an over current differential.

 

[stevenal]

Star for RRaghunath. Keep it simple, avoiding the need for extra operator steps or auxiliary switches and relay logic.

 

[akhtarbiqadri1]

Thanks to you all, especially Raghunath, for answering my question. However, I still have further questions.

As you said, I need to set the differential pickup. But, it doesn't change the fact that if the station service takes power from the grid, the 87T will always read it as a fault right? Since CT6 won't read any current but the CT2 will read the current that flows from the grid to the station service tr. (CMIIW)

 

[davidbeach]

Leave the connection at CT3. Then also program a directional overcurrent element using the CT3 current looking back toward the generator and aux transformers. Or simply install CTs on all three low-side branches, have them all as boundaries of the diff zone and include directional overcurrent for faults beyond the zone. Standard relay I'd consider for that application comes with five 3-phase current inputs for the diff zone and includes the ability to do directional overcurrent on each CT input.

When one this sentence into the German to translate wanted, would one the fact exploit, that the word order and the punctuation already with the German conventions agree.

-- Douglas Hofstadter, Jan 1982

 

[akhtarbiqadri1]

Would it make it to the same configuration with the same problem too? With CT3 - CT4 protected by 50/51 and 27, it needs more than 1.36 s to tackle the short circuit incident. Thus, the fire incident occurred. I want to reduce the time needed (by installing the 87T on CT2 and CT6).

 

[davidbeach]

Add CT7 and CT8. CT2 is winding S. Then CT6, CT7, and CT8 are windings U, W, X. Directional back into things can be much faster than a non-directional that looks out into the connected system.

When one this sentence into the German to translate wanted, would one the fact exploit, that the word order and the punctuation already with the German conventions agree.

-- Douglas Hofstadter, Jan 1982

 

[akhtarbiqadri1]

I'm sorry, David. I can't understand what winding s, u, w, and x refer to. Can you please enlighten me by telling me the references? Thanks

 

[RRaghunath]

"need to set the differential pickup. But, it doesn't change the fact that if the station service takes power from the grid, the 87T will always read it as a fault right?"
The current drawn by auxiliary transformers from grid will be below the differential current pickup threshold of 87T.
So, the relay will display it as differential current but will not cause any operation of the relay.
Read my earlier post again, for better understanding.

R Raghunath

 

[akhtarbiqadri1]

So, I need to set the differential pickup by considering 2 conditions, when the generator is running and stand-still. What if the current drawn by auxiliary tr from the grid (condition 2) is higher than the setting value from condition 1?

 

[davidbeach]

The relay I'd select for that application, for pretty much any transformer differential application, has five 3-phase current inputs, S, T, U, W, X.

When one this sentence into the German to translate wanted, would one the fact exploit, that the word order and the punctuation already with the German conventions agree.

-- Douglas Hofstadter, Jan 1982

 

[9wearSuave]

You’d probably need to make 2 groups of pickup settings that switch whether the gen is on or off.

 

[stevenal]

Thanks to you all, especially Raghunath, for answering my question. However, I still have further questions.

As you said, I need to set the differential pickup. But, it doesn't change the fact that if the station service takes power from the grid, the 87T will always read it as a fault right? Since CT6 won't read any current but the CT2 will read the current that flows from the grid to the station service tr. (CMIIW)

No, see the diagram. The horizontal line is the minimum. Looks like it was set to 0.3 pu in this example. Suggest setting this value in your relay so that station service load and fault current are in the no trip region below the line .

 

[akhtarbiqadri1]

Suggest setting this value in your relay so that station service load and fault current are in the no trip region below the line

What if the current drawn by station service tr is far higher than the fault current? Is it okay?

 

[stevenal]

Load current can't exceed fault current. You may need some training in this. SEL has some good classes.