Power Factor Correction (open leg on a floating wye)

[RBloomer]

We had a switch fail to close on the A phase of a floating wye connected capacitor bank. The C phase fuse was found to be bad and was replaced, however the B phase fuse then blew. The bad switch was then found and replaced. After that the system operated normally. The contractor said the open phase caused the overcurrent on the other two phases. I would have thought that the current on the remaining circuit would reduce to 86.6% of normal. What am I missing?

 

[berkeman]

We had a switch fail to close on the A phase of a floating wye connected capacitor bank. The C phase fuse was found to be bad and was replaced, however the B phase fuse then blew. The bad switch was then found and replaced. After that the system operated normally. The contractor said the open phase caused the overcurrent on the other two phases. I would have thought that the current on the remaining circuit would reduce to 86.6% of normal. What am I missing?

Welcome to the PF.

Others will be able to give you a better answer than I can, particularly @anorlunda but it would help to know more about the situation. What is the nature of the load? If the load is mostly a constant-power type of load (like switching power supplies), then the current will go up in the other phases to try to supply the same power.

 

[RBloomer]

The capacitor bank consists of 6 400 Kvar capacitors, 2 in parallel per phase, connected in a floating wye. Each leg has a series 11.5 mH inductor. It operates at 13.8 kV. It is doing power factor correction for typical industrial loads i.e. many motors, ac inverter drives etc.

 

[magoo]

With a floating wye connection, each phase of the capacitor bank sees 13.8/sqrt(3) kV or 57.7% of the phase to phase voltage under normal conditions.

If you energize the same capacitor bank with A phase open (i.e., switch fails to close on this phase), you still have phase to phase voltage across the other two phases. Under this condition, these two phases would be seeing 13.8/2 kV or 50% of the phase to phase voltage.

So my conclusion is that you are not missing anything at all. 50/57.7 is the 86.6% that you expected if you are dealing with the current to the capacitor bank and not the line current.