CT and PT failute temporary repair

[npower0073]

lately on plant we faced aproblem with the secondary of one CT and one VT. CT was used for protection 8000/1 5P20 toroid, VT (or PT for US) was used for exciation control 15750/V3 / 120 V Cl.0.2s.
In order to put the unit back in service i put in parallel the secoindar of the PT of another phase , same class, to have the voltage measurements.
Also for the current i put the secondary winding of another phase in series in order to have the same current.
This worked but i dont know if i should keep it this way or change the failed CT and PT with new ones.

 

[slavag]

Npower.
Maybe I didn't understand you:
Change phase of voltage for excitation it's maybe O.K., if AVR used only one phase meas.
But protection in series with another phase isn't possible , maybe you meant another core of same CT?
Could you please send scheme of changes.
Regards.
Slava

 

[davidbeach]

slavag, while I don't recommend doing it you can get all three phase currents out of two CTs but you will never get ground current. On high resistance grounded industrial systems it was not uncommon to find two CTs used on three phase systems. Put CTs on A and C (was the most common, any two would work) and wire them to the A and C inputs of the relay(s), then parallel them (not series) and run that combination the other direction through the B input. Assuming no ground current (an acceptable assumption when maximum ground current is 5A or 10A) we have A+B+C=0 or B=-(A+c).

npower0073, it never works to put the output of two CTs on different circuits in series. There are useful things than can be accomplished by putting them in parallel, but series will always result in problems.

 

[slavag]

David, is good for motor protection, where used only two CT's. I agree with you, it's common practic in old application, work many years w/o problems. 8000/1A it's some big size generator.
I assume was some misunderstanding wit Npower0073.
Regards.
Slava

 

[npower0073]

Thank you very much for your answers. What i told you is that i tried to put the output of one CT secondary in series with two inputs of protection relays in order to cover two phase current inputs with on CT secondary. For some reason it didi not work and thats why i ask you.

 

[slavag]

Hi Npower.
Please send drw's: before and after changes.
Very difficult recommend something without drw's. Maybe, you didn't connect correctly star-points of CT's secondary.
Regards.
Slava