ABB diff relays DIX109a/110a/111a

[veritas]

Has anyone experiences with these 1980's static diff relays from ABB? I am investigating an incident where a DIX110a maloperated for an out of zone fault most likely due to CT saturation. Relay was applied to a 420MVA gen trfr. I am particularly curious about the way that Idiff and Istab are calculated.

Id = abs(I1 - I2)
Istab = abs(I1 + I2)

The above means that for an inzone fault the Istab actually decreases. I am wondering whether this does not result in the relay operating for out of zone faults with CT saturation due to the phase shift in the current produced by the saturated CT? I have not seen this kind of diff algorithm in modern relays and am wonder as to how secure it is?

Thanks.

 

[veritas]

Relays were originally Asea Brown Boveri.

 

[ScottyUK]

Does the attachment help? I'm pretty sure this was a Brown-Boveri relay from Baden: if the document doesn't contain sufficient information then that's the bit of the company to approach.

 

[veritas]

Yes, it is a Baden relay. Thanks for the attachment. Will have a read. Spoke to the site engineers today. Apparently the fault was about 200km away on the 500kV network. Relay tripped due to a reflected 375Hz travelling - at least that's what they believe to be the cause. The Main 2 Protection (Siemens) never operated for the same fault.