Power Factor

[dnkldorf]

Hi folks..

 

I have contacted 2 different suppliers of PF caps and filters. Eaton and Ge.

 

First Eaton's guy, is a sub and is too busy it seems with co-generation on other projects to get back to me, and now I contacted GE's guy. Anyway to make a long story short, he wants me to go back and do different measurements on both incoming services, basically start all over again....

 

So, getting to my question: Is there a step by step procedure out there anywhere, starting from the line diagram, through the harmonic measurements, that one should follow and chart? Or is this all up to the person you are contacting?

 

I've been working on this project for over 6 months and can't get any definate answers from suppliers.

 

Any help?

 

Thanks..

[marke]

Hello dnkldorf

 

Welcome to the forum.

 

I am not quite sure what you are looking for here. It appears that you wish to now how to apply power factor correction and I assume that you are concerned about the effect of harmonics.

Perhaps you can give a clearer description of what you are trying to achieve so that we can help you better.

 

Best regards,

[dnkldorf]

Thanks for the welcome marke,

 

Here is what I got...

 

A machine shop, buying voltage at the 33KV side, metered at the 4160V (secondary)...

 

They own and maintain their own substation..

 

From the 4160V, they feed (2) services...(1) 4160X480 on the back side of the building via a 600KVA single dry transformer...(1) 4160X208/120Y utilizing (3) pot transformers on the side of the building. Each pot is rated 150KVA for a 450KVa bank...

 

Power factor flucuates from .69-.75, from what I see on their power bills. They are currrently paying and average of $1200 a month penalty based on 2 years bills I have....

 

My first thought was to make a line diagram of the plant, starting at the 480V service. The 600KVA transformer has no impedeance markings on it.

I drew this service out, from the disconnects, to every panel, subpanel, and then to to every machine including every transformer I could find. Noted on the plans sizes of transformers, impedeance voltage KVa size ect..

 

I contacted Eaton after I was done the 480V side, this is where most of the machine loads are connected. The 208/120 side has mostly office loads, office lighting, ect..

 

I gave the info to eaton, and they looked at it and told me this was easy, install a 150KVAR bank on the 480V side and we are done..not so easy I thought.

 

I brought up some research I have done, about harmonics and filters, and they tell me to go back and redo things again...ugh..I took some harmonic spapshots, but these were on (1) of the parallel sets of 500MCM that are on the 480V side, this I don't think should matter..Now they tell me I need to install a line reactor, and now we'll be done...

 

Still to me, not that easy....since all AC loads come of the 208 side..Now they don't get back to me, they are busy on co-genertaion with other people....ugh..

 

So I call GE, they tell me to start all over, with thier site survey forms, and electric bills that I have..still can't be that easy...

 

I am reading about overvoltages when loads get lowered. The plant runs about 16 hours a day, with loads constantly changing, and they shut down every Saturday night and sunday. Shoudn't I be looking for or trending info daily to find the lowest load and PF first, not just going off a power bill that shows the lowest PF for the month?

 

There should be some step by step plan for doing this, so I ask..can you help me?

 

Or am I outhinking the whole process?

 

 

Dnk.

[marke]

Hello dnkldorf

 

This is not an unusual situation and there is no real issue with applying power factor correction.

Some basic things to consider though are:

1) There are a number of ways in which the penalty is measured and applied, but the major reason for applying power factor correction is to reduce the peak load. Correction under light load conditions is not an issue interms of achieveing tangible results, but in some regions may be an issue for reducing penalties. Many of the penalties are based on the power factor at maximum load, or purely on the KVA maximum demand.

2) There are two components of power factor, i) displacement power factor and ii) distortion power factor. Displacement power factor can be improved by the addition of power factor correction capacitors. Distortion power factor requires comples and expensive filters.

3) Harmonics are present on the supply due to the rectifiers on VSDs and other equipment. Thes harmonics will increase the current through a power factor correction capacitor and cause premature failure depending on the magnitude and frequency of the harmonics. Detunig reactors can be fitted in series with the power factor correction capacitors to reduce the effect of the harmonics on the capacitors. - these reactors can produce a lot of heat so they need to be well ventillated.

4) There are tow major forms of power factor correction installation, bulk correction applied at one point, (often at the point of supply) and static correction (applied at each motor).

5) Power factor correction can cause a resonant circuit between the capacitor and the reactance of the supply and transformers. - if floating or permanently connnected capacitors are used, there can/will be resonance issues at light load.

 

Provided that the poor power factor is primarily due to displacemnt power factor (motors etc) then it can be improved by the addition of power factor correction capacitors. If static correction is used, be careful not to correct to greater than the magnetising current of each motor being corrected. Additional correction can be applied as bulk if required.

Design the correction to provide correction to high loads and consider no correction at light load. i.e. when the load is less than say 20% of the capacity, shut the correction down to minimise resonance problems. - ensure that the penalty is based on the pf at high load and that the pf at light load is not relevent!!

If there are voltage harmonics present, use a detuning reactor in series with each capacitor bank. If you use detuning reactors, ensure that you use high voltage capacitors that are designed for use with detuning reactors. The voltage across the capacitors can be above line voltage if the harmonics are high.

 

One of the problems that can occur with bulk correction systems is where there is no ligh load lock out and the controller switches a bank in and out on an unloaded transformer. This will produce high voltage transients and can damage other equipment. This does not occur with static correction as the capacitor is only applied when the load is connected.

 

Best regards,