Effects of Harmonics for power factor correction Capacitor Banks

Do the harmonics in your distribution system is a potential threat for your capacitor bank?

Yes. When a capacitor bank is installed in a harmonic rich environment, they create a low impedance path and magnify the magnitude of current and voltage harmonics in a system resulting in parallel resonant effect. With this resonant phenomena, the elevated current in the system will drive to overheat the capacitors and this heating causes to degradation of the dielectric material of the capacitors, which may result in its eventual failure with the time.

What is Harmonic Resonance?

As mentioned, there are lot of nonlinear loads in any electrical distribution system and they create harmonic currents which flows throughout the system. With the increment of harmonic order (increment of frequency) the inductive reactance increases whereas the capacitive reactance decreases. When capacitor banks are installed in a system, there will be a crossover point where inductive and capacitive reactance is equal at a given harmonic frequency. This crossover point is called resonant point and every system with a capacitor has a parallel resonant or series resonant point. In harmonic frequencies at the point where parallel resonance occurs, that elevated harmonic current excites into the electric circuit and causes significant voltage distortions in the system. When an electrical networks are lightly loaded they are more sensitive to harmonic distortions and the installation of fixed capacitors to any distribution network should be carefully reviewed considering the harmonic existence of the network.

Symptoms and characteristics of harmonic resonance

Capacitor fuse blow: At resonant conditions, due to high capacitor currents fuses will be operated.

Capacitor failure: Insulating layers of the capacitors can be damage due to overtime overheating or excessive voltage stresses.

Voltage distortion: For resonant condition the distortion causes due to one or two closely spaced harmonic orders. Using a power quality analyzer, harmonic order that is causing the resonance can be identified.

Equipment failure: Low level resonance can go unnoticed for a long time. Usually the symptoms are unexplained failure of sensitive power supplies, electronic loads, transformer overheating etc.

 

Difference between Series Resonance and Parallel Resonance

Figure 1: Parallel Resonance Vs Series Resonance

As per the diagram it is evident that the parallel resonant point is always lower than the series resonant point in a practical power system installation where series resonance creates a low impedance path whereas parallel resonance creates a large impedance which even in the presence of small current can create large harmonic voltage drop and cause voltage stress related damages.

Installing a just selected harmonic filter is not the solution. A thorough investigation should be adhered in order to mitigate these negative impacts.

When installing a power factor correction capacitor bank the possible resonance frequency can be identified as per the following equation.

 

What is Harmonic Filter Resonance?

Even with the harmonic filters, potential problematic scenarios can be happened. Assume if a capacitor in a filter fails, the loss of capacitance results in movement of series and parallel resonant point which before failure was located in ‘safe’ spots but may have shifted to more problematic regions after capacitor failures. The resulting filter could draw excessive harmonic current (due to low impedance path series resonance creates higher currents) and fail or create high voltage distortion (due to higher impedance parallel resonance create more voltage distortions).

Can circuit resistance involve in mitigating the resonance?

Resistance in the power system provides a damping which is helpful in reducing the catastrophic effects of power system resonance. It should be noted that the circuit resistance does not remove harmonics, it only reduces the damaging results caused by resonance currents by reducing the peak currents. Resistive loading in the system provides a relative beneficial impact on peak impedance. Electricity utilities have the advantage of this since they have the feasibility of changing the physical location of harmonic filters, capacitor banks to an optimum location that offers extra series line resistance. But when it comes to Industrial installations, they do not have this feasibility on altering the locations and usually they have limited options while installing capacitor banks or filters.

A tip for identifying the abnormal Harmonic affects in your system with a Power Quality Analyzer

When you measure the system condition with a power quality analyzer there may be significant amount of harmonic currents for different harmonic orders, but usually characteristic harmonic decrease as frequency increases. As an example, 5th order harmonic content should be higher than the 7th, the 7th order harmonic content should be higher than the 11th, and so on.

More Tips for your concern

  • High order harmonics resonance such as the 23rd harmonic and beyond that can bring catastrophic outcomes even at low magnitude since it causes to generate significant heat and interferences
  • Converting 480V capacitors to 480V filters shouldn’t be done due to application of continuous overvoltage may damage the capacitors.
  • When using higher rated capacitors in lower rated system calculate de-rate the KVAR accordingly to the existing system.
  • If you keep blowing fuses or tripping a breaker on a capacitor bank, measure the capacitor current and look for high harmonic currents.

In order to reduce the adverse effect of harmonics and to ensure proper operation and maintenance of plant machinery, it is required to do a power quality study and install power factor improvement or harmonic mitigating filters based on expert suggestion. To ensure system compatibility with international standards as IEEE STD 519-1992 harmonic study is required. According to IEEE STD 399, when the harmonic loads are about 30 % or more with respect to total plant loads, the harmonic impacts needs to be examined.

 

Based on the nature of installed equipment in the plant, proper measures and expert suggestions need to be taken before installation of power factor correction equipment in the plant. IEEE STD 1531 2003 is an IEEE technical guide that defines the application and specification which should be adhered for the designing, controlling and protection of harmonic filters.

References

[1] D. J. Carnovale and E. /. Cutler-Hammer, “Power Factor Correction and Harmonic Resonance: A Volatile Mix Capacitors may be meant to correct problems, but they can also introduce them if incorrectly specified,” Eaton.com. [Online]. Available: https://www.eaton.com/content/dam/eaton/products/low-voltage-power-distribution-controls-systems/power-factor-corrections/power-factor-correction-harmonic-resonance-white-paper-IA02607001E.PDF.
[2] “Harmonic resonance in power systems – voltage disturbance,” Voltage-disturbance.com. [Online]. Available: https://voltage-disturbance.com/power-quality/harmonic-resonance-in-power-systems

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