CURRENT RELAY OPERATION TO GFCI IN STARTER

[freddycor]

hello to all
Before than other thing,please excuse my english, i hope somebody can help to me in this cuestion.
Some time ago i see in a old starter for winding rotor motor ( by resistances ), a system that i think is to provide protection to the motor for unbalanced current and/or ground fault. This consist in the 3 main cables of primary ( 4,160 V )in starter, pass together trough CT of ratio 9/0.5 amp, conected to a current relay in series with the coil ( secundary of CT ).
This is a Westinghouse design in 1968 or around.
Im interesting in the theory of operation of this system, and i'm thinking about to make a similar system to protect small transformers ( 45 to 500 kva ), it is right???
I hope anybody can ilustrate me.
Thanks a lot .

Alfredo Corona
Mexico

 

[DanDel]

This appears to be a typical ground fault detection system, measuring zero sequence current. Normally, the vector sum of the current in the three phase conductors would equal zero. If there is a ground fault, the vector sum of the current in the three phase conductors would then be equal in magnitude to the ground fault current.

 

[busbar]

To add to DanDel’s comments freddycor, no. Zero-sequence overcurrent tripping cannot protect a transformer by itself. Per-phase overcurrent relaying is also needed for adequate protection of transformers.

 

[freddycor]

thanks for sus responses. Probably Im not explain very well the option to protect with this system to transformers, Im refer to protect THE LOAD in secundary ( connected in Y ) of transformers, against GF ( lost of phase will work good this system ??? ).
Just now Im thinking about it, because here in this area is in the coast and very wet conditions that many substations in 34.5 KV have trouble with contamination of insulators and is some common that they having loose of one phase, and burn some threephasic motors/loads. ( Subestations that no receive apropiate maintenance periodicially ).
Im talking about TRs from 45 to 500 KVA with no protection more than the monopolar fuse in primary, and main breaker in secundary.
I hope somebody have an idea if is possible to apply this system or something simmilar, to provide aditional protection considering the actual conditions of operation descrited above.
Thanks a lot for your time and energy !!!!

Alfredo Corona
Mexico

 

[DanDel]

freddycor, what you describe is a standard GF protection system. Installed and setup correctly, it will protect the downstream system from excessive ground fault damage.

 

[buzzp]

If the secondary is a Y and you want to protect the motors from GF or single phase conditions, then what you describe will work and as was pointed out, is one common method of measuring ground fault current.
If you want to protect the transformers you will need more than GF protection. If you want to protect the motors against single phasing as a result of fuses blowing then what you propose will work. You have to be careful that you do not try to interupt the circuit (contactor or starter for motor) in the event of a short circuit. That task belongs to the circuit breaker. Measuring voltage (vectors) is more common for detecting single phase conditions than measuring the GF current, with attention given to so called 'regenerated voltages'.
It will be interesting to see what amplitudes of current are flowing to ground as a result of moisture.

You could use this info to find the trend and make decisions on rising GF current.

 

[freddycor]

hi guys!!!

thanh you very much for your responses. I hope more people have some experience in this case ( if somebody have a system simmilar in operation please let me know yours results/recomendations ).

Thanks in advance.

Alfredo Corona
Mexico

 

[jbartos]

Suggestion: The transformer of larger size may also be protected by the transformer differential relay 87T. The transformer secondary side power distribution is often protected by the system grounding applied to the transformer secondary side, e.g. transformer neutral.