Q1: Maintenance of capacitor banks
- A: Maintenance of a capacitor bank doesn’t require much time or energy. However it is important that regular inspections be carried out. This will help prevent early failure and pick up any fault. An inspection routine would include the following: 1.Check and clean filters 2.Ensure tightness of all electrical connections 3.Ensure all fuses are not damaged 4.Ensure all discharge resistors are operational 5.Ensure all contactors are operational 6.Remove all dust and deposit buildup 7.Tong test capacitor current
Q1: Power factor correction control relay
- A: When selecting a power factor correction relay the following main functions should be considered: 1.Measurement of the required reactive power and control the capacitor switching according to the power factor desired or pre-set value. 2.Indication of power factor, preset parameters and specified installation data. 3.Disconnect the capacitors when a system voltage drop occurs, this will prevent significant overvoltages in the installation and the subsequent damage to switchgear insulation. 4.Allow manual control. 5.Provision for a visual display of signal lamps for monitoring the number of capacitors steps switched into the system. 6.Possible implementation into a building management system
Q1: Voltage improvement
- A: Power factor capacitors decrease distribution system voltage drops and fluctuations during the start of large inductive loads.
Q1: Power loss reduction
- A: Distribution system losses are also reduced through power factor correction by reducing the total load current in the system.
Q1: Filter reactors
- A: In most cases it is necessary to reduce the effects of the harmonic currents. One way of reducing harmonic currents is to install an inductance (filter reactor) in series with the capacitor. The filter reactors protect the electrical installations and equipment but it does not eliminate the harmonics. The reactor value should be calculated and designed in order to reduce the resonant frequency of the circuit to a value lower than that of the lowest harmonic in the system. A capacitor equipped with a filter reactor is protected from harmonics regardless of the layout of the network to which it is connected. Except in some cases when switching in steps the inductance and capacitance values
Q1: Capacitor ratings
- A: Power factor correction capacitors are rated in electrical units called “VAr”. One VAr is equivalent to one volt-ampere of reactive power. VAr is the unit of measurement for indicating just how much reactive power the capacitor will supply.
Q1: Power factor correction capacitors
- A: A poor power factor can be improved by adding power factor correction capacitors to the plant’s distribution system. Correction capacitors provide needed reactive power (kVAr) to the load. Therefore, the Electricity Supply Company is freed from having to supply it. Power factor correction capacitors reduce the total current supplied by the Electricity Supply Company to the load and as a result the distribution system capacity is increased.
Q1: Power Factor Correction and Soft Starters
- A: Individual or ‘Static’ power factor correction capacitors can be used with soft starters provided they are installed on the input side of the soft starter and switched via a dedicated contactor only after the motor has reached full speed. The contactor should be AC6 rated for the motor full load current. Automatic or ‘Bulk’ power factor correction systems make use of a power factor controller to monitor changing power factor and automatically switch capacitors as needed. When used with a soft starter the automatic switching of capacitors should be inhibited until the motor is running at full speed. When a soft starter is installed in close proximity to a power factor correction capacitors (less than 50m) and used without a main contactor, the switching of capacitors whilst the soft starter is not passing motor current can also lead to premature starter failure.The use of a main contactor is therefore recommended when; • Multiple soft starters are installed along with static power factor correction capacitors. • A soft starter is installed along with a bulk power factor correction system. Connecting power factor correction capacitors to the output of a soft starter will cause equipment failure due to severe over voltage. This over voltage is created by resonance between the inductance of the motor and the power factor capacitance.
Q1: Power Factor Correction and Variable Speed Drives
- A: As a general rule, standard power factor correction systems should not be used when there are Variable Speed Drives (VSD’s) connected to the same point of connection unless some precautions are taken. Two situations arise when using power factor correction systems: 1. Installation of the power factor correction between the VSD and the motor, and 2. Installation of the power factor correction on the line side of the VSD In the first case, power factor correction should not in any case be connected between the output of the VSD and the motor. In typical DOL situations, some installations will have a fixed KVAR value of capacitors sized to counteract the motors inductive reactance hence increase the power factor on the supply line. Be cautious when replacing DOL components with a VSD – if there are PF correction capacitors connected to the motor remove them as premature damage to the inverter and motor will occur due to the high frequency switching voltage occurring on the output of the inverter. Most capacitors are not designed to withstand the high switching currents produced by VSD’s. In the second case, power factor correction can be installed on the line side of the VSD but only under certain conditions. In all cases, VSD’s produce a certain level of harmonic distortion back into the main supply. This harmonic distortion is in the form of both THID (total harmonic current distortion) and THVD (total harmonic voltage distortion) and the levels of this THD is dependant upon the size of the drive, the supply transformer impedances, short circuit levels, primary and secondary voltage levels plus cable lengths and cable size. Because of the inherent harmonic distortion produced by the VSD’s, the capacitors within power factor correction equipment will cause any THD to be amplified which results in higher voltage impulses applied to the input circuits of the inverter and the energy behind the impulses is much greater due to the energy storage of the capacitors. This will in turn prematurely damage the input rectifier of the VSD causing costly repairs. In addition, the increased current and voltage transients on the line side are passed back through the PFC capacitors causing increased operating voltage and current, which produces higher operating temperatures and may cause premature failure of the capacitors. By reducing the effects of THVD and THID through the use of input reactors, harmonic filters, active harmonic filtering on the line side of the VSD or using a 12-pulse rectifier (or even better an Active Front End solution), this reduces the effect of the transient impulses which can damage both the VSD and the PFC capacitors. Ensure that the capacitors used in the PFC system have a harmonic tolerance level greater than the harmonic distortion produced by the VSD installation.
