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?

 

[wcaseyharman]

That would be a very high impedance fault, and I doubt it would remain that way long, especially in a plant environment.

There is always a minimum detectable fault current regardless of the configuration, but I think I can say with some certainty that it is rare a fault will remain as a high impedance, it seems once the fault current starts flowing the resistance of the fault path drops (unless it’s a tree out in the middle of nowhere in heavy rain, sometimes those just keep smoking)

 

[akhtarbiqadri1]

Thank you stevenal and wcaseyharman. Yeah, I do definitely need the training since this is my first time learning the protection, so I'm truly sorry if sometimes my question seems too dumb. I already do my own research on this topic but am still clueless in many ways.

Is there any way to calculate the current drawn by the station service from the grid based on this?

 

[wcaseyharman]

I’d just calculate it based on the transformer nameplate. I get 26A.

If you don’t want trips to the differential due to secondary faults you might calculate the primary fault current for a secondary fault. I get 650A primary. Depending on how big your machine is that might be low enough to ignore (by setting the 87 pickup above that value).

 

[akhtarbiqadri1]

I already calculate the primary fault current which is approx. 18 kA and that's really high from the "current" setting pickup value (0.23 Ie)

I think I'm doing something wrong

 

[stevenal]

Primary current from a secondary fault will be less (not more) by the inverse of the turns ratio (same as the voltage ratio). Remember energy is conserved, so when voltage goes up, current goes down. Convert the primary current to CT secondary current, then convert to per unit based on the relay tap setting.

Correction: Since you are concerned about current on the 150 kV side causing unbalance, you will need to step the current down one more time by 11/150 before applying the CT2 ratio and converting to per unit.

 

[akhtarbiqadri1]

Okay, thank you for the advice guys!

After doing my calculation, I got 26 A for the nominal current of Station Service Tr and the CT secondary current is 0.0874773

For the Main Tr, I got 0.69 and 0.94 (CT secondary current). Can I safely say that if the CT secondary current for SST is lower than the Main Tr, it means that it won't be any problem (red: fault-tripping) to install the 87T between CT2 and CT6? Or does it need more calculation? (can you give me a hint what to do)

 

[wcaseyharman]

Typical pickups I’ve seen are .3 (I think that’s usually in terms of the “relay tap “ but it depends on the relay. Check your relay manual)
What it means is load current won’t trip the differential if you have a typical pickup.

In an ideal world you’d set the pickup higher than a station service secondary fault to prevent the differential from tripping for an out of zone event. However, that does desensitize the differential (raise the trip point, so fault has to be larger) so there’s a trade off (art part of protection).

 

[akhtarbiqadri1]

We use 0.3 Ie for now. But, how to make sure that it won't trip if the generator stand-still and SST takes power from the grid? I still don't get it, sorry.

I already calculated the nominal current, rated current, and CT secondary current for MTR and SST. Can you tell me what I must calculate after that? Thanks

 

[wcaseyharman]

The “operate” current is the same whether the gen is running or not. The current from the gen is canceled out due to the proper polarity of the two sets of CTs. The only load current that will cause “operate” current is the loads from the smaller transformers without CTs as the transformer primary CTs will see the current if the gen is not running, and the gen CTs will see it if the gen is online. Makes no difference.

The only difference between the two scenarios is when there’s significant load the restraint current will help reduce the affects of steady state errors.

 

[akhtarbiqadri1]

So... is it impossible to do what I proposed in the beginning?

 

[wcaseyharman]

I’m saying you don’t need to. Differential will work fine as is.

 

[akhtarbiqadri1]

you don’t need to

Do you mean that I don't need to change the setting pickup value?

Here's what I calculate and my assumption (please CMIIW)

Currently, the setting pickup value (I_Pickup_Percentage Differential) is 0.1 Ie (Ie = 5 A)
The ratio turns of CT are 300:1 (HV side) and 3000:1 (LV side)

The primary side of MTR (150kV) CT2:
I_1 primary CT = 206.7 A
I_1 secondary CT = 0.6546 A

The secondary side of MTR (11kV) CT6:
I_2 primary CT = 2818.52 A
I_2 secondary CT = 0.8925 A

The slope setting is 10% and 65%

SST (on secondary side of CT):
I_3 primary CT = 26.24 A
I_3 secondary CT = 0.00875 A

Since the I_3 secondary CT is lower than 0.5 (0.1 Ie which is the setting pickup value), my conclusion is: It is safe to install the 87T between CT2 and CT6 when generator stand-still

Please tell me what's wrong with my calculation and obviously my conclusion.

Thanks.

(Here's the simulation that I tried)

 

[wcaseyharman]

All that seems fine.
Let’s check secondary faults on the SST. That could be up to 26/.04=650 A at 11kV, 47A at 150kV. That is only .15A secondary on the 300:1 CT, so that still should be less way than your pickup, so you’re good, your differential shouldn’t trip for out of zone faults. You could probably lower your pickup and still have plenty of margin.