Power Factor Correction Capacitors and their basic types

Most of the load on supply system is always lagging, and power factor of the system is low because of the nature of load (transformers, motors, electromagnets etc.). It becomes necessary to improve the power factor of system beyond 0.9, and often above 0.98, to get maximum utilization of generation and transmission system. Capacitors providing leading power factor (opposite in phase to inductive load component) are the most common natural solution to be added to a system to counter the lagging load and improve the power factor. Capacitors are specially designed for such duty, termed ‘power factor correction capacitors’, or KVAR capacitors (since they provide leading reactive power).

Power factor correction capacitors are used by all utilities and industries for improving load power factor to acceptable level. They may be used at low voltage side at consumer / substations, medium voltages or high voltage sides on transmission systems. Low voltage applications at distribution points or consumer premises are the most common. They are most commonly used in parallel with load, when they care called shunt capacitors, or may be connected in series with transmission lines, where they are called series capacitors.

Power capacitors are among the most stressed capacitors, and have to be designed meticulously. The capacitors may remain circuit for days or years without being switched off. They may be in open area or indoors, subject to vagaries of weather, as also fluctuations and transients in system throughout their life. They are expected to serve for decades without failures. Any failure on grid system may are problematic for the system, as it may entail disruption of services, and tripping of safety switchgear. Even partial failure (few of inner capacitor elements) can cause degradation of power factor level. Till 1970s, KVAR capacitors were made exclusively from paper and aluminium foil, with oil impregnation under vacuum. Most common oils were synthetic oils of polychlorinated biphenyl class (PCB). Polypropylene films made appearance in 1980s, and with the ban on PCBs, they quickly replaced paper capacitors. PP film / Aluminium foil / synthetic chemicals (as impregnating media). Most KVAR capacitors today are made with PP film, either with aluminium foil with oil impregnation or as Metallized Film capacitors.

PP / Foil / Oil (APP) capacitors

They have advantage of very low loss factor compared with paper. Impregnation of PP was very difficult, and films with rough surface (on one side or both sides) were developed to enable ease in impregnation. Paper + PP film mixed dielectric capacitors also were introduced. The latter are not common now. The windings are made on hollow cores and flattened with not-too-tight pressure, and then impregnated.

PP / Foil windings elements are made and then assembled in groups phase-wise, arranged in box with lid open, and then impregnated under vacuum. The top / lid is then sealed. Often, particularly for large and high voltage capacitors, fuses or pressure sensitive protection is incorporated in assembly to keep capacitor safe from explosion from untoward short-circuit failures and thermal runaway. 

APP capacitors have lower loss factor compared with MPP capacitors, and extended foil construction, where used, reduces the losses further. Oil helps remove all traces of air and moisture, thereby improving dielectric properties, and also acts as cooling medium, allowing for better design stresses and keeping temperatures uniform throughout the capacitor volume.

The capacitor is built from multiple number of elements per phase, such that failure of one element isolates it from others, and resultant capacitance reduction remains within permissible limits. It must be noted that in the absence of such isolation mechanism for faulty elements, the capacitor can explode like a bomb, causing severe damage to surroundings (even fire) and personnel nearby. The protection may be provided as individual fuses on elements, or a group of elements. Capacitors may sometimes also include discharge resistors inside across groups of capacitors. These capacitors are highly stable in capacitance (and hence KVAR) values, as also losses, throughout life, barring any chance isolation of faulty elements.

High voltage capacitors are usually made as single phase units, and then connected in star connection to get desired service voltage. They have number of elements in series parallel connections to reach desired voltage and KVAR rating. For example, three units of 6.6 KV capacitors connected in star give overall 11 KV working voltage. Common star point may be used for neutral connection or earth point. APP construction may be used for shunt capacitors in power systems, as also for series capacitors on transmission lines.

MPP Capacitors

Late 1990s saw the advent of Metallized Polypropylene Film (MPP) capacitors, mainly for economic reasons. Though MPP has the advantage of self-healing, it also means capacitance goes down and loss factor goes up slightly with each self-healing. Construction and manufacturing process of MPP capacitor for power factor correction therefore becomes very critical to ensure a reasonably long life. Often the sprayed ends of MPP capacitor elements are problematic because of air trapped in space under sprayed metal layer and stagger of film. To overcome this, some manufacturers impregnate the elements or capacitors under vacuum to remove the air and replace with dielectric oil. The oil could be a well refined vegetable oil or a synthetic chemical. Most common vegetable oils are castor oil and rapeseed oil.

Most MPP shunt capacitors are made using cylindrical windings, while sometimes even three phase cylindrical integral windings are made on machines. Good practices include a protection device for explosion proof construction, by way of expansion joint in can (which breaks the terminal wire connection) or a special terminal discs which isolate the capacitor in case of thermal runaway. MPP capacitors are mostly limited for use in power factor correction for low voltage applications up to 440 V AC.

MPP capacitors reduce the size of capacitors drastically compared with APP counterpart, because of elimination of foil layer and thinner dielectric film. Their self-healing property means any short-term transients or over-voltages will cause isolation of failure spot, thereby the capacitor remains in circuit without appreciable damage. Each such event of self-healing reduces the capacitance value, and also increases its loss factor by an infinitesimal value. However, over time, the capacitance value may drop to unacceptable level and losses increase beyond limits, and unless capacitor is removed from service in time, it may cause thermal runaway at some time. Capacitor manufacturers have to take this factor in design consideration and observe extreme precautions to get reasonably long life in service.

Modern manufacturing practices, as also improvements in materials and processes have enabled MPP capacitors to become most preferred choice for shunt capacitors up to 440 V AC systems, and or little higher.