Technical Articles
A capacitor is a passive device which has two conductors separated by a dielectric of infinite resistance, ideally. Hence, it should offer only capacitive reactance, with zero resistance and zero inductance values. But practically, the dielectric of the capacitor will offer a finite resistance along with the capacitance. This finite resistance is called as Equivalent Series Resistance (ESR), and its equivalent circuit can be represented as below:
The ESR in the capacitor causes the leakage current to flow through the dielectric of the capacitor. This results in real power loss (I2 *ESR) called as dielectric loss. Higher the ESR, higher is the power loss, and hence the heat generated by the capacitor is also more. The heat generated should be dissipated properly; otherwise it may result in significant temperature rise. A good quality capacitor has very low ESR value.
In a lossless (ideal) capacitor, the current leads the voltage exactly by 90°. But there is always a small shortfall in the lead angle from 90°, because of the dielectric loss. The difference in angle is called loss angle (δ). The following diagram represents loss angle (δ) in the impedance plane.
The tangent of the loss angle (loss tangent) is defined as the ratio of the capacitor's equivalent series resistance (ESR) to the capacitive reactance (XC).
Power Loss = I2 * ESR
= (ωCV) x (ωCV) x ESR where, ω= 2πf
= (ωC) x (ωCV2) x ESR
= (ωC) x Q x ESR
= Q x ESR / XC
Power loss = Q tan δ[ from Eq 1]
Where, Q is the reactive power rating of the capacitor (in Var).
Typically, for good quality power capacitors, tan δvalue is less than 0.0002. Power loss of any capacitor can be computed if tan δvalue is known as explained in the example below:
For a 10 kVAr capacitor, consider the value of tan δas 0.0002.
Total Watt Loss = 10 × 1000 × 0.0002 = 2 W
Per kVAr Power Loss = (Total Watt Loss)/(kVAr) = 2/10 = 0.2 W/kVAr
Normally, the tan d value is available in the routine test certificate of the power capacitor.
Sourav Dasmodak,
Product Management & Marketing (Powergear - ACB)Product Owner of Air Circuit Breaker (ACB) of Lauritz Knudsen for Domestic & International Market. I can talk to you about Electrical Products' Sales, Business Development, Market Expansion, Cracking Critical Strategic Account, handling Key Account & of course how to develop & motivate Channels along with the organizational growth. Having near about one and a half decade of experience across the country with major electrical manufacturers (Top 4).
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