Line Differential Pickup Setting - SEL 411L

[aliz]

Hi,

I am wondering if any one has experience of line differential relay setting calculation. Its a 230kV three terminal line - two generating stations are feeding to utility. I want to calculate the 87L pickup value.
What are the factors should I consider?
Do I need to calculate IAR/IAL ratio? or calculating the charging current and CT saturation are sufficient to get the pick up value?

Appreciate your inputs.

Thanks.

 

[HamburgerHelper]

Set the 87 best you can and supervise it with overreaching 21 elements to prevent tripping from the taps.

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If you can't explain it to a six year old, you don't understand it yourself.

 

[davidbeach]

I find it helpful to set the 87L pickup points such that a test switch can be opened at maximum load without tripping the differential. Yes, that does remove a certain amount of sensitivity to high impedance faults but they are inherently also faults that cause at most minor impact to other loads.

 

[HamburgerHelper]

I suppose you could try setting negative sequence and zero sequence differential if your tapped high sides are deltas. You would have diff the was immune to your positive sequence generation. That won't help you for a 3 phase fault tough.

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If you can't explain it to a six year old, you don't understand it yourself.

 

[aliz]

Thanks for your comments.
I just see in the 311L manual, they mention 87L pickup value should be at least 20% of Load current and Charging current, and for 87L2 and 87LG they put half of the 87L pick up.

 

[davidbeach]

The 411L can compensate for line charging current. Takes some getting used to, looking at the 87L currents in the relay and seeing much less than expected; in that case almost all of the current was charging current and the relay showed close to zero amps in the 87L when the metering showed quite a bit more.