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Home Motor Control Power Factor Power Factor Correction

Capacitive Power Factor Correction

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Power Factor Correction.

Capacitive Power Factor correction (Power Factor Compensation) is applied to circuits which include induction motors as a means of reducing the inductive component of the current and thereby reduce the losses in the supply. There should be no effect on the operation of the motor itself.

In the interest of reducing the losses in the distribution system, power factor correction is added to neutralize a portion of the magnetizing current of the motor. Typically, the corrected power factor will be 0.92 - 0.95 Some power retailers offer incentives for operating with a power factor of better than 0.9, while others penalize consumers with a poor power factor. There are many ways that this is metered, but the net result is that in order to reduce wasted energy in the distribution system, the consumer will be encouraged to apply power factor correction.

Power factor correction is achieved by the addition of capacitors in parallel with the connected motor circuits and can be applied at the starter, or applied at the switchboard or distribution panel. The resulting capacitive current is leading current and is used to cancel the lagging inductive current flowing from the supply.

Corrected Current Vectors

Capacitors connected at each starter and controlled by each starter is known as "Static Power Factor Correction" while capacitors connected at a distribution board and controlled independently from the individual starters is known as "Bulk Power Factor Correction".


Capacitor selection.

Static Power factor correction must neutralize no more than 80% of the magnetizing current of the motor. If the correction is too high, there is a high probability of over correction which can result in equipment failure with severe damage to the motor and capacitors. Unfortunately, the magnetizing current of induction motors varies considerably between different motor designs. The magnetizing current is almost always higher than 20% of the rated full load current of the motor, but can be as high as 60% of the rated current of the motor. Most power factor correction is too light due to the selection based on tables which have been published by a number of sources. These tables assume the lowest magnetizing current and quote capacitors for this current. In practice, this can mean that the correction is often less than half the value that it should be, and the consumer is unnecessarily penalized.
Power factor correction must be correctly selected based on the actual motor being corrected. The Electrical Calculations software provides two methods of calculating the correct value of KVAR correction to apply to a motor. The first method requires the magnetizing current of the motor. Where this figure is available, then this is the preferred method. Where the magnetizing current is not available, the second method is employed and is based on the half load power factor and efficiency of that motor. These figures are available from the motor data sheets.

For example:
Motor A is a 200 KW 6 pole motor with a magnetizing current of 124A. From tables, the correction applied would be 37KVAR. From the calculations, this would require a correction of 68.7 KVAR
Motor B is a 375KW 2 pole motor with a half load efficiency of 93.9% and a half load power factor of 0.805, the correction recommended by the tables is 44 KVAR while the calculations reveal that the correction should be 81.3KVAR

Supply Harmonics.

Harmonics on the supply cause a higher current to flow in the capacitors. This is because the impedance of the capacitors goes down as the frequency goes up. This increase in current flow through the capacitor will result in additional heating of the capacitor and reduce it's life. The harmonics are caused by many non linear loads, the most common in the industrial market today, are the variable speed controllers and switchmode power supplies. Harmonic voltages can be reduced by the use of a harmonic compensator, which is essentially a large inverter that cancells out the harmonics. This is an expensive option. Passive harmonic filters comprising resistors, inductors and capacitors can also be used to reduce harmonic voltages. This is also an expensive exersize.
In order to reduce the damage caused to the capacitors by the harmonic currents, it is becomming common today to install detuning reactors in series with the power factor correction capacitors. These reactors are designed to make the correction circuit inductive to the higher frequency harmonics. Typically, a reactor would be designed to create a resonant circuit with the capacitors above the third harmonic, but sometimes it is below. (Never tuned to a harmonic frequency!!) Adding the inductance in series with the cpacitors will reduce their effective impedance at the supply frequency. Reducing the resonant or tuned frequency will reduce the the effective impedance further. The object is to make the circuit look as inductive as possible at the 5th harmonic and higher, but as capacitive as possible at the fundemental frequency. Detuning reactors will also reduce the chance of the tuned circuit formed by the capacitors and the inductive supply being resonant on a supply harmonic frequency, thereby reducing damage due to supply resonances amplifying harmonic voltages caused by non linear loads.

Detuning Reactors.

Detuning reactors are connected in series with power factor correction capacitors to reduce harmonic currents and to ensure that the series resonant frequency does not occur at a harmonic of the supply frequency.
The reactors are usually chosen and rated as either 5% or 7% reactors. This means that at the line frequency, the capacitive reactance is reduced by 5% or 7%.
Using detuning reactors results in a lower impedance, increasing the current, so the capacitance will need to be reduced for the same level of correction.
When detuning reactors are used in installations with high harmonic voltages, there can be a high resultant voltage across the capacitors. This necessitates the use of capacitors that are designed to operate at a high sustained voltage. Capacitors designed for use at line voltage only, should not be used with detuning reactors. Check the suitability of the capacitors for use with line reactors before installation.
The detuning reactors can dissipate a lot of heat. The enclosure must be well ventillated, typically forced air cooled.
The detuning reactor must be specified to match the KVAR of the capacitance selected. The reactor would typically be rated as 12.5KVAR 5% meaning that it is a 5% reactor to connect to a 12.5KVAR capacitor.

detuned capacitors

Supply Resonance.

Capacitive Power factor correction connected to a supply causes resonance between the supply and the capacitors. If the fault current of the supply is very high, the effect of the resonance will be minimal, however in a rural installation where the supply is very inductive and can be a high impedance, the resonances can be very severe resulting in major damage to plant and equipment. Voltage surges and transients of several times the supply voltage are not uncommon in rural areas with weak supplies, especially when the load on the supply is low. As with any resonant system, a transient or sudden change in current will result in the resonant circuit ringing, generating a high voltage. The magnitude of the voltage is dependant on the 'Q' of the circuit which in turn is a function of the circuit loading. One of the problems with supply resonance is that the 'reaction' is often well removed from the 'stimulus' unlike a pure voltage drop problem due to an overloaded supply. This makes fault finding very difficult and often damaging surges and transients on the supply are treated as 'just one of those things'.
To minimize supply resonance problems, there are a few steps that can be taken, but they do need to be taken by all on the particular supply.
1) Minimize the amount of power factor correction, particularly when the load is light. The power factor correction minimizes losses in the supply. When the supply is lightly loaded, this is not such a problem.
2) Minimize switching transients. Eliminate open transition switching - usually associated with generator plants and alternative supply switching, and with some electromechanical starters such as the star/delta starter.
3) Switch capacitors on to the supply in lots of small steps rather than a few large steps.
4) Switch capacitors on o the supply after the load has been applied and switch off the supply before or with the load removal.

Power Factor correction is not applied to circuits that draw either discontinuous or distorted current waveforms.

Most electronic equipment includes a means of creating a DC supply. This involves rectifying the AC voltage, causing harmonic currents. In some cases, these harmonic currents are insignificant relative to the total load current drawn, but in many installations, a large proportion of the current drawn is rich in harmonics. If the total harmonic current is large enough, there will be a resultant distortion of the supply waveform which can interfere with the correct operation of other equipment. The addition of harmonic currents results in increased losses in the supply. This results in distortion powerfactor

Power factor correction for distorted supplies can not be achieved by the addition of capacitors. The harmonics can be reduced by designing the equipment using active rectifiers, by the addition of passive filters (LCR) or by the addition of electronic power factor correction inverters which restore the waveform back to its undistorted state. This is a specialist area requiring either major design changes, or specialized equipment to be used.

Power Factor Introduction
Displacement Power Factor
Power Factor Correction
Bulk Correction
Static Correction
Power Factor Calculations
Distortion Power Factor

Last Updated ( Wednesday, 06 April 2011 22:48 )  

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