Power Quality Basics in AC Power System

What is Power Quality?

Power generated and used is with a sinusoidal wave characteristic at 50 Hz at a power factor close to unity. If it is free from other waveforms of voltage and current, then Power Quality is good. If it is mixed with other waveforms of voltage and current, then Power Quality is poor.

Basic Definitions for Components of Power Quality

Active Power (kW) – Active Power is the component of total power that is used to perform the designated task by electrical equipment.

Reactive Power (kVAr) – Reactive Power is the component of total power developed by reactive components like inductors or capacitors in AC circuits. This is unused power that flows back to the source. Therefore, our endeavour should be to minimise or eliminate this component.

Apparent Power (kVA) – Apparent Power is the vector sum of active and reactive power. It is always advisable to have this power as close as possible to “Active Power.”

Power Factor – Power Factor is the ratio of active power (kW) to the apparent power (kVA). Power Quality is good if it is closer to unity. A power factor less than unity is a low power factor.

Power Traingle - Power Quality
Figure 1 Power Triangle

Fundamental Frequency – 50 Hz power frequency is Fundamental Frequency.

Harmonics – If voltage and current waveform deviate from the sinusoidal characteristics of the power then it contains harmonics. The frequency of those harmonic components is in integral multiples of the fundamental frequency. So, the frequency of these voltage and current components are 2x50Hz, 3x50Hz, 5x50Hz……….

Harmonic Waveforms - Power Quality
Figure 2 Harmonic Waveforms

Basic Formula’s

What is good or clean Power?

Ideally, if the Power Factor is unity and it is free from harmonics then Power Quality is said to be good otherwise it is bad. Low power factor and harmonics are also called power polluters.

Reasons for Poor Power Quality

We get the power free of harmonics at business establishments or even at our homes, but the nature of the loads will decide whether it will remain so or get polluted. If the load inside the premises is inductive like AC motors, water pumps, transformers, induction furnaces or loads with inductive chokes will make the power factor low because it will draw reactive power along with active power.

Whereas if the load is non-linear like rectifiers, variable speed drives, electronic devices, switched mode power supplies (SMPS), broadcasting equipments, banking machines and telecom systems. These devices have high-frequency switching while in operation. Because of this high frequency switching current breaks down to a level where it does not follow sinusoidal voltage waveform.

Impacts of Poor Power Quality

Low power factor means a considerable amount of reactive power requirement of load which will not convert into useful work done by the electrical equipment. That means power equivalent to reactive power is getting wasted.  

Now coming to harmonics. Non-linear load is characterized by changing impedance with applied voltage where other loads have a constant impedance. Once a voltage is applied across a non-linear load, the current drawn is non-sinusoidal even if the applied voltage is sinusoidal. So what? Should we get worried about it? What is the problem if it is so? Yes, we need to be worried because these harmonic currents will interact with the impedance of the distribution system to create voltage distortion that can harm distribution system equipments and the load connected to it. You can refer to Figure 2 for different waveforms. The major impact of harmonics on the systems are:

  • Nuisance tripping of switchgear devices like ACB and MCCB
  • Blown up of fuses.
  • Excessive heating and bursting of capacitors.
  • Damage to motors, transformers, and fluorescent lighting with choke due to excessive heating.
  • Damage to sensitive electronic devices.

So, I hope by now you strongly feel that power quality needs to be improved. You may be wondering what the ways are to improve the power factor and mitigate the harmonic distortion. Let’s discuss these aspects one by one.

Power Factor Improvement

Power Factor improvement means the reduction or elimination of reactive power consumption by the load. This is only possible if we feed the reactive power requirement of the load from other devices of just opposite reactive power requirements. Capacitor banks are the devices that feed the reactive power requirement of an inductive load if connected in parallel with the load. These are called shunt capacitors.

Refer calculation below for the selection of capacitor banks. Go back to the basic formula section.  The reactive requirement of the load is:

Reactive Power (Q) = VI Sin Φ kVar

So, we need to connect a capacitor bank of equivalent kVar. Let us understand this by an example. Assume we have a load of 100kW at 0.8 power factor and we want to make the power factor 1.

Before power factor compensation:

To achieve a unity power factor, we need to add a 75kVar capacitor bank. Once the Power factor is 1 then the source size requirement will be as follows:

Capacitor Size Selection Table

Capacitor Size in kVar = Active Power or Load Power (kW) x (tan Φ1 – tan Φ2)

(tan Φ1 – tan Φ2) values are listed in the table below. Pick up the value from the crossing point of the starting PF and desired PF and simply multiply it with the load kW to arrive at the required value of the capacitor for power factor improvement.

Table 1

Ways to Mitigate Harmonics

By using shunt capacitors, we can improve the power factor. Similarly, by using detuned rectors in series with capacitors we can reduce the impact of harmonics in the system. So, the next point is how does it work?

The impedance offered by reactors (inductors) is directly proportional to the frequency of the power supply. Reactors offering higher impedance to 3rd, 5th, and 7th harmonics. Therefore, these rectors stop harmonic amplification. This also safeguards capacitors and associated switchgear against switching inrush.

Since these reactors are connected in series with a capacitor then it forms an LC circuit. Reactors are selected in such a way that the resonance frequency is less than 90% of the dominant harmonic in the system.

Figure 3 Typical Capacitor-Reactor Circuit

The most common values for relative impedances (% impedance) are 5.7%, 7% and 14% respectively.


Power quality primarily revolves around two factors, one is the power factor and the second is the level of harmonics. A better power factor will save energy while low harmonics power will safeguard electrical equipments from damage.

We have not covered here how to design an LC circuit for the improvement of power quality. We will take this up in a separate article.



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