Capacitor Banks and Harmonic Issues 2

[Cotton]

I have an induction furnace which uses capaciators for power factor correction. I also have several large DC drives (1,000 HP and 1,250 HP) which produce enough harmonics to drive the power factor on the line side of the pf correction capacitors from +0.96 to +0.55, which causes the line current to increase. I have 2 questions: 1) How do I calculate which harmonic or frequency the capaciator bank is tuned to and 2) how do I calculate the required inductance to be placed in series with the capaciator bank to shift the tuned ferquency away from the generated harmonics?

I'm assuming its cheaper to filter the harmonics at the capacitor bank than placing chokes at each drive.

Any help will be apprreciated.

 

[busbar]

One reference is IEEE Std 1036-1992 …Application of Shunt Power Capacitors
Description at

http://ieeexplore.ieee.org/xpl/abs_...rAuthor=&arNumber=211735&a_id0=211735&count=1

 

[rezaa]

Try to contact Dr. Jewell at
ward.jewell@wichita.edu

-------------------------
Ward Jewell
Professor of Electrical Engineering
Wichita State University
300 Wallace Hall
Wichita, Kansas 67260-0044
316-978-6340
316-978-5408 (fax)

 

[rbulsara]

Capacitor manufacurer's engineering suppport should be of great help as they do this all the time.

I beleive the rule of thumb is rating of the series inductior is about 1% of the rating of capacitors and rated for 100% thru current. This is from a distant memory.

 

[jghrist]

I assume that by "...which harmonic or frequency the capaciator bank is tuned to..." you mean the resonant points. For a system that includes several capacitors, there are several resonant points. For a very simple system that can be represented by a single capacitor and a single source impedance, the resonant point can be determined by:

h=sqrt(X[sub]c[/sub]/X[sub]sc[/sub])

where X[sub]c[/sub] is the capacitive reactance of the capacitor bank and X[sub]sc[/sub] is the short-circuit reactance of the source.

Design of filters is more complex. The best filter may not be a simple series capacitor and reactor, but may be a compound filter. See:

J. Arrillage, D.A. Bradley, and P.S. Bodger, Power System Harmonics, John Wiley & Sons Ltd., Chichester, 1985.

 

[jbartos]

Suggestions to Cotton (Electrical) Mar 22, 2004 marked ///\\I have an induction furnace which uses capaciators for power factor correction. I also have several large DC drives (1,000 HP and 1,250 HP) which produce enough harmonics to drive the power factor on the line side of the pf correction capacitors from +0.96 to +0.55, which causes the line current to increase. I have 2 questions: 1) How do I calculate which harmonic or frequency the capaciator bank is tuned to
///The first, create the filter network transfer function, either for the filter attenuation, gain or impedance. Then, analyze its magnitude for its maximum, minimum or bands in terms of frequency. There are various filter topologies, or networks, depending on a type of the filter characteristics (attenuation, gain, etc. shape) required. See Reference:
Donald G. Fink, H. Wayne Beaty "Standard Handbook for Electrical Engineers," 13th (or 14th, current) Edition, McGraw-Hill, Inc. 1993, Section: Filters on page 2-57
\\\

and 2) how do I calculate the required inductance to be placed in series with the capaciator bank to shift the tuned ferquency away from the generated harmonics?
///Reference:
Steve Ashmore, Manager, Harmonic Filter Systems, High Voltage Capacitor Division, "Capacitor Banks and Harmonics," Subject VII-3, 1997 ABB Electric Utility Conference.\\

 

[electricuwe]

I would recommend to contact the supplier of your induction furnance on that topic.

Power supplies for induction furnances may look like a simple combination of some capacitors, inductors and transformers but it is not as easy as it looks like.

Post make, type and nameplate data of the induction furnace you are using. Maybe I can provide some useful hints.

 

[pyppyp]

The resonance frequency calculation as given by jghrist is applicable.

Before placing the capacitors this frequency has to be calculated.

If the system works on a "infinite" bus, where harmonics from other sources outside this installation are likely to be present, it will not be possible to install a "tuned" filter trap for a particular frequency. This will short-circuit itself, on charging itself.

The solution is to provide "detuned" filters. A combination of reactor and capacitor tuned just below the first predominant harmonic (5th in case of 6 pulse drive, 11th in case of 12 pulse drive).

We have commercially installed such systems.

It has to be noted that, the present capacitors cannot be convereted to any filters, due to increase in capacitor terminal voltage with insertion of series reactors.

 

[electricuwe]

pyppyp,jghrist,

you should also warn cotton that putting in a series inductor will increase the voltage at the capacitors.

 

[jbartos]

Suggestion: Visit

http://www.schneider-electric.ca/

http://www/en/techpaper/html/reseli.htm

http://www.ema.uni-wuppertal.de/paper/compens.pdf

http://www.mgeups.com/techinfo/techpap/articles/0246-e.pdf

(for smoothing choke design)
etc. for more info