Stator ground fault relay ANSI 59N 1

[charlierod]

Dear Sirs,

I am dealing with the replacement of a generator protection system which includes an electomechanical relay type CUH90C from Brown Boveri for detecting stator grounds. The fact is that two voltage signals are wired to the relay namely one from the grounding transformer and one from VTs installed at generator terminals. I am more used to relays which only use voltage signal from the grounding transformer. Can anyone explain me how this relay works? Forces from induced currents on a disc?
I have not been able to find any info in the web.
Just to let you know I am a protection engineer from the numerical devices era so my knowledge about electromechanical devices is just bare.

Regards,

Charlie

 

[rasevskii]

This was most likely a 100% stator E-fault protection in that a generator voltage signal was used to de-inhibit the 180HZ element that had to be picked up in normal service. If the 180Hz was not present in service across the grounding trafo secondary, that indicated a ground fault close to the neutral end of the stator.

180Hz is the third harmonic component (60Hz system) 150Hz if 50Hz system.

The 95% element worked in the normal way across the grounding trafo secondary.

A similar method was used by ASEA in their 100% E/F protection type RAGEA.

rasevskii

 

[charlierod]

Hi Rasevskii,

Thanks for your reply. I think this might not be a 100% stator ground relay as you say based on the following reasons:

1. Third harmonic was not developed until the late 1970s and this relay seems much older.

2. It does not have a setting for third harmonic level and it is well known that this shows a high variation from machine to machine so as to be standardized as fundamental frequency neutral overvoltage. In fact this relay just have a time dial setting.

3. When applying nominal voltage only to coils sensing terminal voltage while maintaining neutral voltage signal at zero the relay does not trip.

4.Relay case shows a 90 degree connection of the two coils. In fact when applying voltages in phase to both coils, the relay does not trip. In order to get a trip the voltages there must have some phase displacement among them. Minimum pick up for nuetral voltage is about 5V secondary when the two signals are displaced by 90 degrees.

I think the terminal voltage signal is requiered to maintain a high degree of sensitivity and still have enough magnetic field to move the induction disk or whatever may be.

Your comments are highly appreciated.

Regards,

Charlie

 

[slavag]

Maybe terminal voltage is used as safety factor for 95% Stator EF, that means, two factor needs for 95% correct operation, voltage across grounding transformer and terminal voltage.
someting like to this used in the Siemens terminals today.

 

[rasevskii]

for charlierod:

Yes you are probably correct. I could not find any info on the CUH90 either. Generally BBC was very secretive about technical info on their relays, and only the buying client got the detailed description and wiring diagrams.

ASEA on the other hand was very generous with information, and published very good documentation on all their equipment. When the two companies merged forming ABB, a lot of policies got changed as well as a lot of older people being put out to pasture permanently.

The same happened later on, elsewhere in the electrical industry...

rasevskii

 

[charlierod]

Thanks you for posting. I will try to get documentation from customer and let you know