Ferroresonance in isolated 6 kV network

[Anonymous user]

Dear collegues,

Our client has a problem with ferroresonance in isolated 6 kV network. In some cases, when earth-fault occurs, after isolation of faulty line, in voltage transformers voltage of 25 Hz is induced and after some time explosion of voltage transformers happen. They tried to solve this problem by installation of resistor in the open delta winding of voltage transformers, but problem continue to happen even after that. Now they asked us to try to solve this problem by using relay protection.

Does anyone have an idea how to solve this problem using Schneider Electric protection relays? Also any other idea will be helpful.

I will be very thankful if anyone can help with this problem.

[Ian_Young · Accepted solution]

Hi Marko,

Cahier technique n° 190 is a very good document on the topic of ferroresonance. 

https://www.se.com/us/en/download/document/ECT190/ 

As this document states to have classical ferroresonance you need

• simultaneous presence of capacitances with non-linear inductances,
• existence in the system of at least one point whose potential is not fixed (isolated neutral, single fuse blowing, single phase switching....),
• lightly loaded system components (unloaded power or instrument voltage transformers...) or low short-circuit power sources (generators).

So it is best to understand where these are coming from. It sounds like when the faulty line is tripped they are disconnecting from the system earth and the VT becomes the earth for an isolated system. If the system is isolated even briefly then the VT needs to be rated accordingly.

We find similar situations occurring at data centres during transition to local supply. What is the application in this case?

[Anonymous user]

The application in this case is mine of coal.

Thank you very much for answer.