Partial Differential BB protection 1

[Mohammed Ibrahim]

Hi,
I need explanation for the below.
In one of my project, partial Differential BB is available using Incomer and Buc coupler CTs. Also series resistor is available in the circuit like High impedance circuit but no metrosil. And the settings seems as a overcurrent settings. the protection getting block signal from all OG feeders to avoid operation for fault at OG feeders.
I have seen other region partial Differential protection scheme. there no series resistor but the setting is coordinated with OG feeders and upstream.
My doubt is why series resistor is used in the scheme (refer attached).
The setting is 0.85% of 3200A (CT ratio) and 200ms delay.

Please post your feedback.
Thanks.

 

[cranky108]

I don't open links, but from you description, I would guess the resistor is to force current through what might be a saturated CT.

Yes the setting can be a current setting of an over current relay, for a partial differential.

 

[sushilksk]

It is stabilization resistor used for avoiding relay operation during CT saturation during through faults. Its value is calculated on maximum through fault current.

 

[Mohammed Ibrahim]

Thanks Guys for your post.
Also i have noticed that the system/transformer is low impedance grounding using earthing transformer (max current is 2000A)
Here as per the settings, 2720A (0.85% of 3200A) is available. obviously the protection will not operate for earth fault currents since the max earth fault current is less than the pickup settings.
As per some international/general standards, the BB protection pickup should be between 10% to 30% of minimum fault current.
so my final understanding is we have to compare the pickup with minimum phase fault current only (not earth fault current due to the above point).

Your every feedback will be much helpful to correct my understanding. so please...

 

[RRaghunath]

Dear Mr. Mohammed Ibrahim,
For High impedance busbar protection, the effective primary operating current is recommended to be between 10 & 30% of available minimum fault current at the busbar.
Partial differential protection is different from standard busbar differential protection in that there are no CTs in outgoing feeders that are wired in the Partial differential protection scheme. Thus the protection sees the load current of the busbar at any given point of time. That is the reason why the pick up setting cannot be less than the maximum expected loading on the busbar.
The variable resistors are for providing stability to the protection against CT saturation during out-of-zone faults.
Protection against Earth faults in the busbar requires one more element that is connected in the residual circuit of partial differential scheme and set sensitive.