Effects of Harmonics on Capacitors
The Effects of Harmonics on Capacitors include additional heating – and in severe cases overloading, increased dielectric or voltage stress, and unwanted losses. Also, the combination of harmonics and capacitors in a system could lead to a more severe power quality condition called harmonic resonance, which has the potential for extensive damage. Consequently, these negative effects will shorten capacitor life.
Capacitors are typically installed in the electrical power system – from commercial and industrial to distribution and transmission systems – as power factor correction devices. However, even though it is a basic component of a harmonic filter (aside from the reactor), it is not free from the damaging effects of harmonics. In a power system characterized by high harmonic distortion levels, capacitor banks are vulnerable to failures.
IEEE Limits
IEEE 18-2002 states that a capacitor is designed to operate at a maximum of 135% of its reactive power (kVAR) ratings. In addition, it must withstand a continuous RMS overvoltage of 110%, peak overvoltage of 120%, and an overcurrent of 180% of nameplate rating. Although the standard did not specify the limits for individual harmonics, the above percentages can be used as basis to determine the maximum allowable harmonic levels.
Harmonic Effects
The reactance of a capacitor bankis inversely proportional to the frequency, as can be noted in the formula,
Xc = 1/(2πfC)
where:
Xc = Capacitive reactance
C = Capacitance
f = Frequency
As a result, the capacitor bank acts like a sink, attracting unfiltered harmonic currents. This effect increases the thermal and dielectric stresses to the capacitor units (i.e. overload).
To illustrate, consider a harmonic-rich electrical system with 5thharmonic voltage of around 20% the fundamental. A4160 V, 300 kVAR capacitor bank has a reactance of 57.7 Ω at the fundamental frequency (e.g. 60 Hz) and shall draw a capacitive current of 41.6 A according to Ohm’s Law. On the other hand, the capacitor reactance is only 11.54 Ω at the 5thharmonic (5 x 60 = 300 Hz). Subsequently, this same capacitor bank energized with 5thorder harmonic voltage will also draw 41.6 A.
Fundamental Current:
I1= 4.16 kV/(√3)(57.7 Ω)
I1= 41.62 A
5thHarmonic Current:
I5= (20%)(4.16 kV)/(√3)(11.54 Ω)
I5= 41.62 A
Total RMS current:
Irms = √(I12+ IH2) = √(41.622+ 41.622)
Irms = 58.86 A or 141.4% of Fundamental Current (I1) – could blow capacitor fuses
Increase in Capacitor Current Due to Harmonics
In such cases, nuisance blowing is expected since most capacitor fuses are sized based on the 135% kVAR limit. Otherwise, capacitor unit shall suffer overloading and heating. This shows why nuisance capacitor fuse blowing and/or breaker tripping indicate very high harmonic distortion levels in the area.
Moreover, frequent switching of nonlinear magnetic components such as reactors and transformers can generate harmonic currents that will increase capacitor loading.
Harmonic Resonance
A serious concern arising from the use of capacitors in an electrical power system is the possibility of system resonance. This effect imposes voltages and currents that are higher than would be the case without resonance.
Harmonic resonance in a power system may be classified as parallel or series resonance, and both types are present in a harmonic-rich environment. Parallel resonance causes current multiplication, whereas series resonance produces voltage magnification. Substantial damage to capacitor banks would result if the amplitude of the offending frequency is large enough during resonant conditions. Also, there is a high probability that other electrical devices on the system would also be damaged.
For such reason, harmonic analysis must be performed before installation of a power factor improvement capacitor bank to ensure that resonance frequencies do not correspond with prominent harmonics contained in the currents and voltages.
References:
IEEE 519-1992. Recommended Practice and Requirements for Harmonic Control in Electrical Power Systems
Sankaran, C. (1999). Effects of Harmonics on Power Systems 1
