PDC

[22574]

A core balance ground ct connection was used in O/L in a Motor,
while residual ground CT connection was used for O/C at the secondary side of Transfomer going to the switchgear where the motor connected.

My question is, can I coordinate the two relays?

What is more sensitive during ground fault?

Thank you.

 

[davidbeach]

Core balance is not subject to CT mismatch, and the CT doesn't need to be sized for load. Other than that, both measure 3I0 and coordination should be no problem.

 

[JMatt]

I agree with davidbeach.

One additional comment:
Your core-balance CT's are likely much more sensitive than your residually connected phase CT's. You may want to check the amount of ground fault current available to ensure that the residually connected phase CT's respond properly.

For example, 2000/5 Ct's connected residually may not allow your relaying to detect a 10A ground fault on a HRG system, but a 50/5 core balance CT would perform very well.

 

[trosepe]

You also need to be careful in choosing the ratio of the core balance CT to avoid saturation issues if the system is solidly grounded. I looked into a coordination issue recently similar to your situation but the root issue was the saturation of the core balance CT.

 

[slavag]

Hi.
No problem with coordination.
You can put with core-balance CT a low setting..it will about 1% of In of CT ( of course all depend on ) , but with residual connection not less 10-12% of CT rating. Core balance isn't sensetive to CT mismatch, to partial saturation..for example in case of motor start.
That menat, coordination by current setting and by time setting. Standart configuration.
Good Luck.
Slava