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POWER FACTOR - 3

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Power Factor Correction.

Capacitive Power Factor correction 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.

  An induction motor draws current from the supply, that is made up of resistive components and inductive components. The resistive components are:
    1)  Load current.
    2)  Loss current.
and the inductive components are:
    3)  Leakage reactance.
    4)  Magnetising current.

The current due to the leakage reactance is dependant on the total current drawn by the motor, but the magnetising current is independent of the load on the motor. The magnetising current will typically be between 20% and 60% of the rated full load current of the motor. The magnetising current is the current that establishes the flux in the iron and is very necessary if the motor is going to operate. The magnetising current does not actually contribute to the actual work output of the motor. It is the catalyst that allows the motor to work properly. The magnetising current and the leakage reactance can be considered passenger components of current that will not affect the power drawn by the motor, but will contribute to the power dissipated in the supply and distribution system. Take for example a motor with a current draw of 100 Amps and a power factor of 0.75 The resistive component of the current is 75 Amps and this is what the KWh meter measures. The higher current will result in an increase in the distribution losses of (100 x 100) /(75 x 75) = 1.777  or a 78% increase in the supply losses.

  In the interest of reducing the losses in the distribution system, power factor correction is added to neutralise a portion of the magnetising 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 penalise 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 laging inductive current flowing from the supply.

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 Correction".

BULK CORRECTION:

The Power factor of the total current supplied to the distribution board is monitored by a controller which then switches capacitor banks In a fashion to maintain a power factor better than a preset limit. (Typically 0.95) Ideally, the power factor should be as close to unity as possible. There is no problem with bulk correction operating at unity.

STATIC CORRECTION:

As a large proportion of the inductive or lagging current on the supply is due to the magnetising current of induction motors, it is easy to correct each individual motor by connecting the correction capacitors to the motor starters. With static correction, it is important that the capacitive current is less than the inductive magnetising current of the induction motor. In many installations employing static power factor correction, the correction capacitors are connected directly in parallel with the motor windings. When the motor is Off Line, the capacitors are also Off Line. When the motor is connected to the supply, the capacitors are also connected providing correction at all times that the motor is connected to the supply. This removes the requirement for any expensive power factor monitoring and control equipment. In this situation, the capacitors remain connected to the motor terminals as the motor slows down. An induction motor, while connected to the supply, is driven by a rotating magnetic field in the stator which induces current into the rotor. When the motor is disconnected from the supply, there is for a period of time, a magnetic field associated with the rotor. As the motor decelerates, it generates voltage out its terminals at a frequency which is related to it's speed. The capacitors connected across the motor terminals, form a resonant circuit with the motor inductance. If the motor is critically corrected, (corrected to a power factor of 1.0) the inductive reactance equals the capacitive reactance at the line frequency and therefore the resonant frequency is equal to the line frequency. If the motor is over corrected, the resonant frequency will be below the line frequency. If the frequency of the voltage generated by the decelerating motor passes through the resonant frequency of the corrected motor, there will be high currents and voltages around the motor/capacitor circuit. This can result in sever damage to the capacitors and motor. It is imperative that motors are never over corrected or critically corrected when static correction is employed.

Static power factor correction should provide capacitive current equal to 80% of the magnetising current, which is essentially the open shaft current of the motor.

The magnetising current for induction motors can vary considerably. Typically, magnetising currents for large two pole machines can be as low as 20% of the rated current of the motor while smaller low speed motors can have a magnetising current as high as 60% of the rated full load current of the motor. It is not practical to use a "Standard table" for the correction of induction motors giving optimum correction on all motors. Tables result in undercorrection on most motors but can result in over correction in some cases. Where the open shaft current can not be measured, and the magnetising current is not quoted, an approximate level for the maximum correction that can be applied can be calculated from the half load characteristics of the motor. It is dangerous to base correction on the full load characteristics of the motor as in some cases, motors can exhibit a high leakage reactance and correction to 0.95 at full load will result in overcorrection under no load, or disconnected conditions.

Static correction is commonly applied by using on e contactor to control both the motor and the capacitors. It is better practice to use two contactors, one for the motor and one for the capacitors. Where one contactor is employed, it should be upsized for the capacitive load. The use of a second contactor eliminates the problems of resonance between the motor and the capacitors.

CAPACITOR SELECTION:

Static Power factor correction must neutralise no more than 80% of the magnetising 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 sever damage to the motor and capacitors. Unfortunately, the magnetising current of induction motors varies considerably between different motor designs. The magnetising 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 magnetising 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 penalised.
Power factor correction must be correctly selected based on the actual motor being corrected. The Busbar software provides two methods of calculating the correct value of KVAR correction to apply to a motor. The first method requires the magnetising current of the motor. Where this figure is available, then this is the preferred method. Where the magnetising 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.

Static Power factor correction can be calculated from known motor characteristics for any given motor, either the magnetising current and supply voltage (method 1) or half load efficiency and half load power factor(method 2), or, as a last resort, table values can be used. These will almost always result in under correction

Bulk power factor correction can be calculated from known existing power factor, required new powerfactor, line voltage and line current.

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 remove from the 'stimulous' 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 minimise 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) Minimise the amount of power factor correction, particularly when the load is light. The power factor correction minimises losses in the supply. When the supply is lightly loaded, this is not such a problem.
2) Minimise 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 capaciotors on to the supply after the load has been applied and switch off the supply before or with the load removal.


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