OLTC+AVR Problems

[kobenaw]

We have an OLTC with Alstom MVGC as the AVR. Recently, we have been going down a lot on out of step. This is because we have a parrallel connected transformers with master-follower configuration. The problem only occurs between taps 4 and 5. The master moves to 5 but the follower stays at 4. What are the things to look out for?

 

[bacon4life]

Has the transformer previously gone past step 4 or is this a new installation? Is there a tap range limiter?

 

[kobenaw]

The transformer has gone past step 4 until recently. There is tap range limiter for tap 16, the last tap.

 

[krisys]

If the tap changer master-follower unit is tested and commissioned properly, falling out of step during the operation is quire rare.
Would it be a wiring problem?

 

[ScottyUK]

Does the follower relay on the tap position indicator? If so then ensure that the TPI is operating correctly - many use a series of low resistance steps and have low excitation voltages which can easily be affected by oxidation and dirt on the contacts.

 

[bacon4life]

Is tap zero the neutral tap? If the transformer is +7.5%/-2.5%, tap 5 might indicated a problem with the neutral.

Which method of master/follower are you using? We had one location where the follower command was driven by the sealed in raise contact on the master. Once in a while, the follower would finish tapping a fraction of a second faster than the master, and would start a second tap change just before the master finished.