Technical Articles
Typically, power factor correction capacitors are fitted with discharge resistors connected directly across the terminals (two resistors between three phases). These resistors are mandatory safety requirement recommended by the capacitor standards, to discharge the residual voltage that remains in the capacitor, once it is switched off.
As per IEC 60831 (MPP capacitors) and IEC 60931 (APP capacitors) the capacitor should discharge to 75 V or below within 3 min as soon as the capacitor is switched off. This is to protect human beings from the risk of electric shock and also for safe re- switching of the same capacitor in APFC panel. As a general practice, discharge resistors bring the voltage to below 50 V in 1 min, which is safer.
If the capacitor is re-switched without discharging sufficiently, there is a possibility that the voltage difference across the contactor may shoot up to 1000 V. More over, this results in very high peak inrush current as the rate of change of voltage is very high (IC = C dV/dt). Such frequent switchings reduce the life of the capacitor and/or the contactor, and may cause premature failures. Hence while re-switching the capacitor (either in manual mode or auto mode) it is mandatory to discharge the capacitor to its 10% of the rated voltage.
In order to ensure the sufficient discharge, it should be ensured that the capacitor is re-switched only after 45 to 60 seconds. This time delay (for re-switching the same capacitor bank) can be set in APFC relay. In case, any application demands frequent switching of capacitors by contactors, time delay can be reduced by faster discharge of the residual voltage. Faster discharging can be achieved by replacing the existing resistors with new resistors of lesser resistance.
Formula to calculate the resistance is:
Where,
R = Discharge resistance value in Ω
t = Time for discharge from √peak voltage) to Vr (50 V), in seconds C = Rated capacitance per phase, in µF
Vn = Rated voltage of capacitor unit, in V
Vr = Permissible residual voltage, in V
The above formula is valid for resistors assembly in the delta connected capacitor (two resistors between three terminals), as shown in the figure.
However, opting for lower resistance for faster discharge will increase the power loss. At the same time, appropriate power rating (wattage) of the resistor should be chosen to ensure sufficient current carrying capacity of the resistor.
Power rating of the resistor (in watts) = 1.3*Vn*I = 1.3*Vn2/R
Rajesh R Shirodkar,
DGM-Corporate CommunicationRajesh Shirodkar is a seasoned marketing and business leader with over 20 years of experience in the automation and electrical industry, spanning marketing communications, sales, business development, and software solution selling. He has led successful brand transformation initiatives and high-impact marketing programs, including branding and account-based marketing campaigns, as well as industry events that drive visibility, engagement, and growth. With expertise in brand building, lead generation, solution selling, and sales enablement, Rajesh is known for translating technology and business offerings into compelling value propositions and delivering sustainable growth through strong stakeholder collaboration.
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