We have a 40 MVA auto transformer of 115 kv /69 kv with high side portion of the winding connected with an ABB tap changer (Type UZ. It has a strange behavior. When the 69 kv voltage is for example, becomes low, it will reduce the turn in the primary portion and will increase the 69 kv side voltage. Similarly, if the 69 kv voltage increases, it will increase the primary turns and the secondary voltage will reduce. This operation is normal. But during this process, it is observed that the primary voltage also changes. If the 69 kv increases, the 115 kv decreases and vise versa. The primary voltage does not stay steady. In the case of two winding transformer, primary voltage always stay steady and only secondary voltage changes during tap changer operation. Why auto transformer behave differently? Both side of the auto transformer is connected with strong source and loads. In fact it is in between transmission lines. Is this causes the problem?

 

Azizam... this seem to be a copy of what was posted on thread # http://control.com/thread/1445226180.

Following are several questions asked on the first thread:

1) Presuming the Xfmr's are in parallel, can you send a simple SLD or hand-drawn sketch with pertinent data?

2) Also include the number of taps provided in each Xfmr.

3) Nameplate should hold key. Please post NP data on this forum.

Regards,
Phil Corso