To maintain power factor near to unity in the new kVAh based billing calculation, it is necessary to compensate the inductive reactive power of load in real time. To achieve this, we need to switch capacitors from APFC bank as per the requirement of the load. It requires a thorough study of load pattern at different parts of the working day, this will help us understand the load variation with respect to time and in turn help us select the right switching technology.
There are various types of switching technologies used to compensate the inductive reactive power in the system, namely,
a) Contactor
b) Discharge resistors in combination with contactor
c) Thyristor
d) Static VAR generator
(There are some more components used to achieve the real time compensation which will be overviewed at the end of this blog.)
a) Contactor:
This is the most common type of switching device used to turn on/off capacitors in the APFC bank. It is informed by manufacturers that a capacitor requires 40-60 sec to discharge itself completely, in addition to this, the life of contactors is defined by the switching instances.
Thus a total of around 60-100 sec are required to discharge the capacitor and also give time to contactor to maintain its life.
b) Discharge resistors in combination with contactor
The discharge time of a capacitor depends on the discharge resistors connected across the terminals of capacitors. Special discharge resistors are available which can reduce the said discharge of capacitor (40-60 sec) down to 10-15 sec, which is a major reduction in time.
Considering the time required to switch contactor and discharging capacitor, it would take around 30-50 sec to switch capacitors on/off as per load variation.
c) Thyristor
This an electronic switching device that uses diodes to let the current pass through it or stop the flow of current. As it uses diodes as a switching medium, there are no mechanical or moving parts here that would get deteriorated. The switching time of thyristor is about 5msec, which is a major reduction in switching time as compared to contactor.
To support this low switching time of thyristor, we also need to reduce the discharge time of capacitors to make the combination switch at high speed or in narrow time intervals, this is achieved again by discharge resistors.
d) Static VAR generator (SVG) :
There are devices that can compensate the inductive reactive power close to real time, one of such device is Static VAR generator. The working principal of this device is, it generates capacitive reactive power as per the inductive reactive power present in the system at that time.
As there no mechanical switching devices used to achieve the goal, there is no deterioration of any parts. The ambient temperature and the cleanliness plays a part to determine the deviation from the rated capacity.
Above mentioned devices are used to vary the capacitive reactive power as per the need to compensate the magnitude of inductive reactive power. The data received from energy analyser regarding the load fluctuations and inductive reactive power fluctuations, the right switching device can be decided.
Bonus section:
There are technically advanced devices that are used to maintain the power factor near unity available in the market,
a) Static VAR compensator:
This device is used to vary the inductive reactive power using 1-phase shunt reactors for each phase. It is used in combination with capacitor bank. Whenever capacitive reactive power is in excess in the system this SVC comes into play, which varies inductive reactive power to match the capacitive reactive power required to keep the power factor near unity.
b) Active harmonic Filter (AHF) :
This device works like SVG with additional feature to reduce the harmonics content in the system. As harmonics has an impact on true power factor, it is necessary to keep the harmonics within limits as specified by IEEE standards.
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