PF Correction Capacitors A Double Edged Sword!

Published On: Feb 20, 2019

Non-linear loads such as VFDs, LED lights, Digital equipment etc. are known to create harmonics in the electrical network. The presence of such loads in the electrical network adds to inductive loads and lowers the PF of the network.
Capacitors are often deployed to improve the PF, but they can also introduce harmonic resonance if specified incorrectly.

INTRODUCTION

At times, the very measures taken to drive better efficiency of the facility can backfire and in turn affect the performance of the facility or even bring down the whole facility causing huge economic loss. PF Correction Capacitors installed at the facility to improve the Power Factor can create such situation. For instance, general use of capacitors to improve PF by adding a reactive load for a series of motors can result in a side effect. Due to inductance from the motors and the added capacitance from the PF correction Capacitors the entire electrical network can get tuned to a dominant harmonic frequency which may in turn lead to harmonic resonance.

HARMONIC RESONANCE - WHEN CAPACITORS BACKFIRE!

The frequency of power supplied to capacitor banks, power distribution equipment such as transformers and non-linear loads in the electrical network is dependent on the frequency of supply power. As the frequency increases the impedance of transformers, motors etc. also increases. Where as in Capacitors, with increase in frequency, the impedance decreases. In India, the power is supplied at 50 Hz. When harmonics are introduced, the electrical network has multiple frequencies acting at the same time. When the inductive (load) and capacitive reactance (from Capacitors) in the electrical network are equal (known as a crossover point), in other words positive and negative impedance summed up resulting in zero impedance, it can lead to partial or total resonance at one of the harmonic frequencies. The equipment in the electrical network and also the capacitors deployed can fail or collapse when the resonant point is equal to a harmonic frequency present in the network.

All the electrical networks with a capacitor have this point of resonance. The phenomenon of harmonic resonance alters the normal flow of currents, and increased levels of distortion can create problems within the facility or nearby facilities and even at the Utility sub-station.

KNOW THE SIDE EFFECTS - TRANSIENTS AND OVERVOLTAGE

Transient Voltage

Capacitors of utilities and end users operate at different voltage levels. The transients are observed at end user facility as the utility charges the capacitor. But the voltage across end-user capacitors does not change instantaneously. A similar effect can be observed as Capacitors are switched off. At times, the voltage of end user capacitor is more than the voltage of capacitors at the utility. High transient voltages are created at the low voltage capacitor bank when the frequency of the switching transient is close to the resonant frequency of the power system and the utility capacitor is typically ten or more times the low voltage capacitor. The phenomenon is also termed a voltage magnification and it leads to overheating and reduces the life of equipment.

Overvoltage

Traditionally, fixed capacitors have been used as these are more economical. However, fixed capacitors, designed for higher loads, when permanently connected to the load may lead to increase in voltage. The voltage may rise above 110% of the rated voltage leading to overheating of transformers, malfunctioning of sensitive equipment etc.

Both the issues discussed above deteriorate the PQ of the electrical network, further creating several indirect problems which are difficult to trace.

BEFORE ADDING CAPACITORS – PF AND PQ CONSIDERATIONS!

In most cases, Customer add PF Correction Capacitors based on standard table provided by OEMs, an external recommendation from consultant, vendor or other expert. The process that is followed is also of simple evaluation of costs, often considering only the utility incentives and penalties, and selection. A more comprehensive approach which consider the PQ parameters, and with specific focus on the harmonic resonance, can be adopted to derive maximum benefits from investments in the Capacitors.

As mentioned earlier, there are chances of overall PQ to deteriorate after adding of capacitors, especially due to the presence of non-linear loads. The first thing to consider is whether the facility is installing capacitors first time, adding a new pack for improving the PF or considering a change due to problems in current installation. The facility will have to be studied from different aspect to address the issue and arrive at the right configuration of Capacitors.

The following points may be considered in specific, before adding any new capacitors:

What to check?	Can help to decide…
Is your non-linear load above 20% of the total load?	Fixed capacitors alone may not help to improve PF
Do you observe malfunctions or abrupt behaviour of sensitive electronic equipment?	Check of harmonic resonance in case Capacitors are being used. Perform a PQ study before installing the capacitors.
What are the incentives and penalties for PF imposed by utilities?	The overall returns on investments for the Capacitor Installations. Use the tool secqr made available by APQI to arrive at the right investments.
Has there been a recent failure of protective devices such as fuses etc.?	Check for harmonic resonance (in case of use of capacitors) or presence of harmonics in the electrical network.
Do you plan to expand the facility in immediate future? Does it include equipment with higher levels of harmonics?	Plan for appropriate selection and accurate location of the PF Capacitors to serve improving the PF for additional capacity, without creating harmonic resonance
Do you use DG as an alternative power source? Does it trip due to unexplained reasons?	Check for presence of harmonic resonance in the electrical network, in case you already have PF correction capacitors.
Have any of the capacitors installed in the facility failed recently?	Check for harmonic resonance in the electrical network
Have you observed overvoltage, noises from the Capacitor or swelling in capacitors?	Check for harmonic resonance in the electrical network

Mitigation techniques for the above issues may include:

Detuning – to move the resonance point of the electrical network from dominant harmonic frequency is observed
Harmonic Filters – to reduce the harmonic currents in the electrical network and ensure a low impedance path is available to the over current
Utility service – Measures to prevent voltage transients due to switching at Utility end can be discussed to improve the overall PQ of the facility.

The right mitigation approach can be arrived at only after a detail study of the load patterns and electrical signatures of the equipment in the electrical network. A case study on HT Reactor failures published by APQI also demonstrates the need for such detail studies. A booster pumping station (BPS) in South India with high power HT pumping motors reported sudden failures due to flashover of the current limiting reactors of the PF correction capacitor circuit. Extensive re-look on design aspects and simulations studies pointed out no major design issues. To find out the specific reasons for such failures (which happened after three to four years’ of working), it was decided to undertake power quality studies. You can read the complete case study here.

CONCLUSION

Capacitors are often seen as a panacea to all the PF problems, and sometimes even the PQ issues in the electrical network. While Capacitors will solve only a very limited spectrum in PQ issues, their role in improving the PF must also be seen with adequate caution. It is important to note that the instances of Capacitors failures are on the rise, mostly attributed to the increase in non-linear loads. The instances are not just sudden but also severe in their impact, may it be downtime or the risks to safety of man and equipment in the facility.

From blown Capacitors to severe and hard to detect harmonics in the electrical networks, the presence of harmonic resonance, induced by use of Capacitors, can have multiple implications. It is therefore required to exercise caution in implementing Capacitors as a solution to improve PF in harmonic rich environment. A measured analysis, backed by study of the electrical network is highly recommended. Also, the internal team must make efforts to understand the solution, whether its fixed capacitors, Detuning devices or harmonic filter, in order to earn the best RoI on the investments. Before you encash the rewards of using Capacitors to improve PF problems, it is important to be aware of any potential and unfavourable aspects that can turn the rewards back into the risks.