Using a Step Down Transformer as a Step Up Transformer

Theoretically and in reality, this can be done. We have tried this setup in our sister utility company and it works.

The thing to watch out for when applying this scheme is to properly size the surge arresters so when there is reverse power flow, the arresters will not fail.

How did you size your arresters on the 11 kV side? Did you based it on the 11kV line configuration? Or did you based it on the primary delta configuration of the Dyn11 transformer? How did you connect it? Wye or delta?

In our case, our 69 kV line supplying power through the Dyn1 69kV/13.8kV power transformer is using 4-wire wye configuration. Normally, we use 3 units, 60 kV, 48kV MCOV arresters and connected it wye on the delta side of the power transformer and 3 units 12 kV 10.2 kV MCOV connected in wye on the 13.8 kV side.

Now, if we install the above transformer with the power flow from 13.8kV to 69kV step-up, the 60 kV arresters connected in wye will now be subjected to line-to-line voltage surges through the delta. The arresters will most likely fail since originally they were sized based on the line to ground configuration and not line-to-line.

[Our colleagues in our sister utility company, experimented on this and each time the power transformer (Dyn1) was energize at the wye side, the 60 kV arresters at the delta side all failed.]

In this case, the more appropriate size would now be 90 or a 96 rated kV arrester and connected in delta.

To complicate matters, let's say our Dyn1 transformer will be used to step up and step down power at anytime depending on the power flow direction. In this case we have 2 options:

1. On the delta side, we will install 3 units 90 or 96 kV connected in delta and inside this delta, we will also connect 3 units of 60 kV arresters arrange in wye configuration. In this arrangement, we will be assured that whichever direction is the power flowing our arresters will be able to perform effectively without failing.

2. On the delta side, we will use the 3 units 60 kV arresters and connect them in wye. Additionally, between the star point of the wye arrangement and the ground connection, we will install another surge arrester rated accordingly. This 4th arrester will be the one to dissipate the added voltage surge when there is a reverse power flow.

For option 2, you may use the ABB guide in sizing the 4th arrester. Or you may refer to the IEEE surge arrester standard.

Hope you find this useful.