Theory of Power Factor @ Unity

At unity power factor, the inductive and capacitive components of the load are equal. This can be a recipe for resonance effects which can cause large "circulating" currents to flow - large enough to trip circuit breakers. I have dealt with several cases now where a mis-adjusted power factor correction controller has created this problem, leading to 630A MCCBs nuisance-tripping with loads of less than 100A connected.

Paul, unity PF is all the energy supplied by the source, is used by the load this is

good your controller no good

Have considered this challenge for some time. We were taught that unit power factor is "possible" in industrial situation if there is a stable base load that can be run from a synchronous motor with the phase excitation suitabel adjusted (with feedback) from the "Q" sensor in your metering equipment.

The synchronous motor can then produce lead or lag power factor to minimise the error.

Yes, I agree with this. It is possible, but not practical using capacitors because the capacitance comes and goes in stages, which can be thought of as almost digital (at a macro level) whereas the inductance comes and goes in an analog fashion as loading changes slightly with induction motors. So unless you have an infinite number of small capacitors and a perfect control system, running at unity pf is virtually impossible. We say to strive for a 0.95 pf in order to allow for a "cushion" that prevents overshoot and resonance issues. That, and the fact that the difference in losses between 0.95 and 1.0 is so little that it is not worth risking attainment of 1.0 pf.

But by using a large sysnchronous motor as a synchronous condenser, you can come a lot closer to matching the changes in inductance in that analog fashion. Still, 0.99 is more realistically attainable than 1.0 even with that method, and most people who use it still leave them set to maintain 0.95 leading anyway because induction (motors) are often added and subtracted in large step changes, so even a synchronous condenser excitation controller may not be able to react fast enough.

During my engineering work experience (Vacation times), in a previous existence (job) there was some discussion with the local electrical authority about using synchronous devices to actually correct the district power factor.

Yes we had LARGE machines that could run synchronous and could be trimmed to be reactive in lead or lag. (Medium to high voltage running 24/7, 3 phase 6,6oo Volt so no transformer issues back to the local street mains.) We were running them for our own correction at the time.

There was even the beginnings of discussion about trading in "Q" value to everyone's benefit.

I moved on (back to final year studies) before that was finalised and do not know the outcome.