The increasing cost of energy and penalties set by many electric
utility companies are resulting in a widespread application of
power factor correction capacitors. However, their application
can lead to harmonic and transient problems which need to be addressed.
Benefits and guidelines for power factor correction are covered
in the companion paper on this system. This paper will attempt
to clarify the concerns resulting from their application.
HARMONIC CONCERNS OF SHUNT CAPACITORS
Power factor capacitors can drastically change the frequency response of an industrial power system. Figure 1 illustrates the system equivalent (or driving point) impedance as a function of frequency when shunt capacitors are applied. Note the shift of the parallel resonances, the points of high impedance, which occurs by varying the size of the capacitor bank. If these parallel resonances correspond to harmonic frequencies of nonlinear loads at the facility, they will cause significant voltage distortion. At industrial plants, almost all harmonic distortion occurs at the 5th and 7th, (and to a lesser extent 11th or 13th) harmonics, which are typical of adjustable speed drives (ASDs). In office buildings, distortion at the 3rd harmonic is possible due the electronic loads, such as computers.
Solutions to Harmonic Concerns
To reduce the above harmonic concerns, system operating restrictions
may be applied, or the power factor correction capacitors can
be configured as filters. The filters are tuned slightly below
the harmonic frequency of concern to allow for the manufacturing
tolerances in the filter components and system changes, and avoid
stressing filter components.
General Guidelines for Filter Applications
1. Start with a single filter, tuned slightly below the lowest harmonic frequency e.g.: 4.7th harmonic (282 Hz) filter to avoid resonances at the 5th (300 Hz). The filter kVAR (size of the capacitor) should be established by the amount of power factor correction needed.
2. Check the driving point impedance of the system with the filter in place allowing for tolerances in filter components.
3. Check the distortion levels.
4. If the distortion levels are still high, provide additional
filters for other harmonic frequencies. IEEE Std 519-1992 calls
for a 5% limit on the voltage distortion for low voltage systems.
Transient overvoltages are always of concern when capacitors are
switched. The concerns are twofold:
1. Overvoltages at the customer owned power factor capacitors
(and therefore the customer's system) occurring when utility capacitors
are switched.
2. Overvoltages within customer facilities associated with the
switching of their power capacitors.
The analysis for the two above conditions is similar, and is presented
in the following.
Overvoltages at The Switched Capacitors
When a capacitor is switched, a transient overvoltage oscillation
occurs between the capacitor and the system inductance. This overvoltage
can be as high as 2 per-unit (twice the normal voltage), but will
normally be less due to damping in the system. If the capacitor
switching device prestrikes (on closing) or restrikes (on opening),
voltage across the capacitor can be doubled or tripled, depending
on the number of prestrikes or restrikes.
Overvoltages at Locations Away From the Switched Capacitor
(Voltage Magnification)
Switching of capacitor banks can create overvoltages at remote
locations with smaller shunt capacitors. Figure 2 illustrates
how the magnitude of the overvoltages is a function of both capacitor
bank sizes. If larger capacitor is 10 times or more the smaller
unit, the two circuits have similar resonant frequencies, and
there is little damping in the system, the magnified transients
can be in the range of 3.0 to 5.0 per unit. Such overvoltages
can cause nuisance tripping of sensitive electronic equipment
and stress system insulation causing damage to components such
as motors and transformers.
Methods to Reduce Transient Overvoltages Due to Capacitor Switching
1. Use of switching devices that do not prestrike or restrike.
2. Synchronous switching of capacitors.
3. Installation of high energy metal oxide arresters.
4. Use of, properly designed, tuned filters in lieu of capacitors.
5. Use of isolation transformers or series reactors at the sensitive loads.
SELF EXCITATION OF INDUCTION MOTORS
When an induction motor and its power factor correction capacitor
have a common switching device, transient overvoltages may upon
opening that switching device. The capacitor bank, if improperly
sized, will lead to the self-excitation of the motor, which
will then act as a generator, resulting in some high overvoltages
which may damage the motor and the capacitor.
To avoid potentially dangerous overvoltages at induction motor
terminals, the kVAR rating of the motor-switchable power factor
capacitor should not exceed the kVAr requirement to correct
the no load power factor to unity.
HIGH INRUSH CURRENTS UPON BACK-TO-BACK SWITCHING OF CAPACITOR
BANKS
Energizing a capacitor bank with another bank in service on the same bus may lead to high inrush currents which may damage the capacitors, cause the CTs to saturate, and blow capacitor fuses. The switching devices should also be rated for capacitor switching.
The voltage transients associated with back-to-back switching
of capacitor banks are not considered to be problematic.
Proper application of multi-stage capacitor banks may require inrush-current-limiting reactors, or high induction type buses.
MEASURES TO REDUCE OVERVOLTAGES FROM CAPACITOR SWITCHING
Based on field experience, the following recommendations can be
made regarding the application of shunt capacitors:
To avoid dynamic overvoltages, do not energize capacitor banks and transformers simultaneously.
To limit switching overvoltages and interference with sensitive components, do not energize capacitor banks within five minutes after their deenergization.
To eliminate ferroresonance, avoid situations which may lead to
single-phasing a capacitor bank ( one leg of a Y or due to a fuse-blowing)
connected to a transformer.
This paper is not a complete analysis or applicable to all
systems. The application of capacitors for power factor correction
and voltage support must be carefully considered to prevent operating
difficulties and equipment damage. Each system is unique and solutions
should be modeled on a computer to determine the optimum type
of capacitor system, amounts of correction needed and where it
should be installed.
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