Alternating current (AC) electrical systems use power in different ways. Some simple loads like heaters which are purely resistive allow current to flow synchronised with the alternating voltage. This is the most efficient way of transmitting power through cables and transformers. But some loads like motors and other devices which have windings cause the alternating current waveform to lag behind the voltage. This gives rise to lagging displacement power factor. This means that more current is needed to provide the same power because the voltage and current are not working in exact harmony. This is a less efficient way of transmitting power as it increases heat loss due to the increased current.
Systems which incorporate large numbers of induction motors such as on conveyor systems, have low power factor usually in the range of 0.4 to 0.7, A power factor of 0.5 means that twice as much current is required to supply the motors than if the power factor was 1.0, or perfect. This effectively increases the load on the distribution boards, switchboards and on the main incoming power supply. A poor power factor on the main supplies can incur penalty charges on top of the normal tariff, as well as additional heat losses which increase energy consumption.
To correct poor displacement lagging power factor, capacitors which have leading displacement power factor are used. Usually these are arranged in banks which are automatically switched on in stages, depending on the reactive power to be compensated.
How does it work?
The PF controller monitors the incoming supply voltage and current via a current transformer on one of the phases. By comparing the phase difference between the incoming supply current and the voltage it can switch in stages of capacitance as required to counteract the inductive load and thus improve the power factor.
Choice and configuration
Power factor correction capacitors have to be configured to provide the optimum combination of control and cost effectiveness. Parameters to be assessed include:
Total reactive load to be corrected
Number and sizes of capacitor stages
Speed of switching response required, which will affect:
Choice of switching device i.e. contactors or solid state switches
(Contactor control requires a time delay between switching to allow capacitors to discharge to avoid switching onto charged capacitors. Solid state switching can synchronise voltages or switch at zero points on the current waveform to avoid surges and thus switch more responsively)
The requirement for anti-resonance or detuning reactors
Cooling or ventilation requirements (capacitors age more quickly with heat)
PFC units require maintenance
Capacitors have a limited life and can deteriorate due to factors such as heat, and voltage transients. When capacitors fail, they can emit smoke, possibly activating smoke detection systems.
Replacement on a regular programme can reduce the risk of this happening. Also over a period of time, loads can change and alter the amount of PF correction required. Routine service visits to PFC units include the following checks:
Visual check of unit for signs of heat, damage or deterioration
Measurement all capacitor stages to determine that capacitors are healthy and not deteriorating.
Checking discharge resistors
Cleaning or replacement of air filters
Checking operation of fans and thermostat settings
Manually switching in each stage to verify correct operation
Testing insulation resistance to earth and earthing arrangements
Verification of controller settings.Measurement of harmonic content of supply voltage and/or current flow to capacitors. Excessive harmonics will cause increased currents to flow in capacitors.
PFC is not always the answer
Loads which draw current high in harmonic content such as switched mode power supplies, and invertors might also have low power factor but this will not be displacement power factor. This will be power factor influenced by the shape of the waveform and can only be corrected using active harmonic conditioners which detect the harmonic content and generate and inject the reverse harmonics to cancel them out and thus improve the true power factor as well as the displacement power factor.
For more information, and how we can help with PFC maintenance, contact us at firstname.lastname@example.org or +44 (0)1245 496281.