Power Factor Correction (open leg on a floating wye)

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Discussion Overview

The discussion revolves around the effects of an open phase in a floating wye connected capacitor bank on the current in the remaining phases. Participants explore the implications of a switch failure and the subsequent behavior of the system under different load conditions, focusing on power factor correction in an industrial setting.

Discussion Character

  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant describes a scenario where a switch failed to close on the A phase of a floating wye connected capacitor bank, leading to issues with the C and B phase fuses.
  • Another participant questions the contractor's assertion that the open phase caused overcurrent in the other two phases, suggesting that the current should reduce to 86.6% of normal.
  • A participant provides details about the capacitor bank configuration, noting it consists of 6 400 Kvar capacitors in parallel per phase, with inductors, and operates at 13.8 kV for power factor correction of industrial loads.
  • Another participant explains that with an open A phase, the remaining phases would experience a phase-to-phase voltage of 50% of the total voltage, which aligns with the expected current reduction.

Areas of Agreement / Disagreement

Participants express differing views on the impact of the open phase on current levels in the remaining phases. While one participant supports the contractor's explanation, another challenges it, leading to an unresolved discussion regarding the actual behavior of the system.

Contextual Notes

The discussion includes assumptions about load types and their influence on current behavior, as well as the specific electrical characteristics of the capacitor bank setup. The implications of these factors remain unresolved.

Who May Find This Useful

Electrical engineers, technicians working with power factor correction systems, and those interested in the behavior of floating wye connected capacitor banks in industrial applications may find this discussion relevant.

RBloomer
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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?
 
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RBloomer said:
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.
 
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.
 
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.
 

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