CT's and numerical relays 1

[ballenden]

Hi,

Need help. We have a 100MVA shunt reactor connected to 400kV bus. Additionally, this shunt reactor has a star connected tertiary that is loaded in emergency conditions.

It's differential protection is misbehaving. Diff protection trips the breaker whenever the breaker is switched (ie. closed). The protection is applied to only across the main winding and uses, thus, the 3 HV post type CT's (Class 10P) and 3 Neutral CT's (internal to SR) (Class X). The protection terminal is latest numeric relay.

Could it be the impact of DC offset on CT's?
Is the application desirable ito different CT classes used?

Thanks.


"Die Here is my Herder..."

 

[RalphChristie]

Hi ballenden

Another Afrikaans speaking person!

I am not really familiar with reactors, and not on that voltage level. But I suppose it would be oil-immersed reactors, either the coreless type or the gapped iron-core type. Could the trip possibly be due to an inrush current? The gapped iron-core design is subject to more severe energizing inrush than the coreless type. Do you use a harmonic restraint function in the relay?

Regarding different CT-classes:
Due to the low burden imposed on the CTs when using numerical relays, I do not think combining Class X and Class 10P would make that difference - although the aim should be to use the same class CTs from the same batch. You should incorporate any difference between the CTs (mismatch) in your settings.

Regards
Ralph

[red]Failure seldom stops us, it is the fear for failure that stops us - Jack Lemmon[/red]

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[ballenden]

Ralph,

The Reactor is oil-immersed, whether it is gapped iron or coreless I do not have this info yet. I imagine the inrush phenomena to be more prominent in the gapped iron design?

Yes we use 2nd harmonic restraint for energizing. I see how the design or type of reactor is important, this ultimately determines the 2nd harmonic restraint level.

Please comment.



"Die Here is my Herder..."

 

[SMB1]

Hi ballenden

Do you have a fault recorder there?

Check the wave form of the three phase.
I think the relay oprerated because switching of RL circuit.

I'll check and come back to you!!!

 

[RalphChristie]

Hi ballenden

That is a good idea - a fault recorder. Most new numerical relays do have an internal fault recorder that is been activated a few cycles before the fault and for a few cycles after opening the breaker. From the waveshapes you might get an idea what is going on.

The magnetic circuit of a gapped iron-core is constructed in a manner very similar to that used in a power transformer. So, yes, the inrush will be more prominent in the gapped iron design.

I've checked through:
IEEE Guide for the Protection of Shunt Reactors
Power System Relaying by Horowitz and Phadke

One of the principal difficulties with shunt reactor protection is false relay operation during energizing or deenergizing the iron core. During these periods, dc offset with long time-constants and low frequency components of the reactor energization current cause most of the problems. High impedance differential relays rather than low impedance relays are recommended if this problem occurs.

Also, according to these Guides, a Resonant Phenomena can also occur, but it normally cause misoperation of impedance relays.

Best Regards
Ralph


[red]Failure seldom stops us, it is the fear for failure that stops us - Jack Lemmon[/red]

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