Leading PFs and Equipment Damage

What's a PF?

"Guest"

If you can't deduce the meaning of PF in the context of "Electrical Engineering" and the word "Leading" in the title, you have no business perusing this forum. Please go elsewhere to waste bandwidth.

Please don't make us guess, it wastes a lot of time. Indicate what your confusion is and more clearly state the kind of examples you desire.

I know it may seem obvious to you, but we can't read your mind. For instance, do you want to hear of cases where leading power factor has caused equipment damage? Or do you want to know how a facility ends up with a leading PF and what it can lead to? Or do you not understand Power Factor at all?

can someone give me an example of an incident regarding Leading PF's

Incorrect power factor correction capacitor bank setup or operation springs to mind.

Data Centres or Large Server rooms will generate a Leading PF due to all the Dual Corded, Dual Switch Mode Power Supplies installed in servers (High Capacitance load).

The main point where this is a problem is when the site is is transitioning between Mains Power and Generator (or back), where the Power Factor correction and Mechanical services are offline and only the IT load is online, or if the UPS is in bypass and not "presenting" a laggin PF to the supply side.

Generally the Mechanical Services (Lagging PF) will balance the site as a whole out and with a good APFC unit should run day-to-day in the 0.90 to 0.95 lagging range.

At my last site the APFC was set up to drop a step if PF=0.95 was exceeded as the threat of Harmonics as we approach unity was high and the impact of an outage higher.

Regards,
Sapper

Leading power factor is caused by having too many capacitors on for the load, or in some cases by synchronous motors, but usually because of capacitors. This can be harmful to equipment, particularly electronic controls, because it will have a tendancy to raise the voltage. It might also affect your electric bill if your utility bills you in kVA and not kW or penalizes you for out of range power factor.

This is a great answer Keshi and excellent comment. It dosn't matter the degree of or class that we are in, there is something out there that we still don't know. As long as the question is genuine never count it to be silly, it could be the final stepping stone to someone's success.

Regards

Granville (bloke)

Dear Keshi, there was no discouragement about your question, It was about just no question asked.

Now you are clear.

The leading Pf is quite a bit (this is an understatement) damaging to the generator,

As you know the generator usually loves the scondary a bit trailing the primary, being induced. However if the secondary current is leading, so will be its flux. And there are a few papers i have seen, you may search the net and have a look at them, on the damage due to it.

Personally i have never encountered it, since we always see the fellows lagging behind .

In fact that is one of the reasons, you don't go for unity, since controlling it there will be a bit problem, and try to restrict it in nineties, below (I have seen a bit of posts with over unity , technically wrong, so it is nineties below or above)

And there may be a few other problems too, got to think about, but as far as I know, your electronic circuits should not bother about them.

Keshii123, I am more familiar with leading power factor being the situation where current leads the voltage, rather than voltage leading the current.

Was your definition just a typo, or are there different definitions in play depending on the application?

CJM

If you have a manual voltage regulator on a generator connected to the grid, during the weekdays when the grid is loaded usually no adjustment is required, but on the weekend where there is no load, the grid voltage drifts higher and the generator unity power factor or zero KVAR's, now becomes KVARs without having adjusted the generator voltage regulator. If the grid was a constant voltage and power factor no adjustment of generator voltage / excitation is required.

An Automatic Power Factor Controller adjusts the generator voltage regulator to maintain an operator set power factor.

If the Power Factor controller or generator exciter fails in unattended operation, protective relays (loss of excitation) are used to "bracket" the acceptable generator operation (lead or lag KVAR's / PF ). Normal best is Unity PF, some utilities need to "support grid voltage and will operate at 0.9 or .8 PF, but independent power producers want to keep generator winding heat lowest for longevity so operate at 1.0 or Unity PF.

This relates to "leading power factor" and generator damage, if the excitation is removed from a synchronous generator when the generator is online, the generator will start to "jump poles", usually the lights in the building will cycle bright and dim, the generator is now trying to suck in all of the KVARs from the grid, you will get a telephone call from the grid operator.

Do not apply excitation back to the generator, immediatly open the generator breaker to get it offline. If you reapply excitation, the generator rotor and stator are out of sync electricly and mechanicly and tremendous forces will bend shafts causing rotor and stator rub and considerable damage.

During power plant start-ups, stuff happens, you usually only need to experience it once or be told the story by someone who has done it, then you need time to reflect on the lessons learned. I have been present when a exciter fuse blew and I have been told the re-excite story by a friend and I saw the damage and I was there when a generator breaker was closed on a non-rotating generator and caused rotating exciter damage and ? This was over 10 years and more then 50 plants I was in.