WHAT IS HARMONICS DISTORTION IN ELECTRICAL SYSTEMS?

Harmonic waves are a general phenomenon in many physical systems in everyday occurrence. Harmonics as pure tones go to form common musical notes, which are responsible for music and songs etc., and are useful in that sense. However, unwanted frequencies cause disturbance and noise and have to be kept away. Harmonics are common occurrence with all periodic waves- sound, electrical, RF transmission, thermal, mechanical systems etc. Even physical structures like buildings, bridges etc. have their own natural frequency of vibration, which can cause major problems if presence of any of harmonic frequencies matches with this natural frequency.

In electrical and electronic systems, harmonics are sinusoidal waves with frequencies as integer multiple of fundamental frequency (or power frequency). When present, they distort the fundamental frequency waveform, often drastically, causing unwanted currents and voltages to appear in system. They are also a source of noise / disturbance in electronic and communication systems.

Sources of harmonics are non-linear loads such as thyristors, rectifiers, discharge lighting, or saturated transformers and machines connected in the circuits. In the presence of harmonics, current and voltage waveforms get distorted and deviate from pure sinusoidal waveforms. The waveforms appear far from sinusoidal, often very distorted or irregular in shape.

In the early 1800s, French mathematician, Jean Baptiste Fourier formulated that a periodic non-sinusoidal function may be expressed as the sum of sinusoidal functions of frequencies which are multiples of a fundamental frequency f. This tool, the Fourier Series, is very useful in analysis of waveforms with harmonic content of various frequency components.

Harmonics can cause major problems in power systems. Most dangerous is the resonance effect, when some harmonic frequency may match natural resonance frequency of components in system like transformers, capacitors, inductive loads etc. Capacitors are particularly susceptible in combination with reactive loads. At resonance frequency, impedance of system can go dangerously low, causing high damaging currents or voltages. Special precautions have to be taken to avoid such resonance. For example, detuned reactors are often used to shift the resonance frequency to safe level.

Causes of Harmonics

Harmonics are generated and appear in electrical systems due to several reasons.

• Power electronic devices like thyristors, rectifiers, UPS etc.
• Variable frequency drives
• Electronic devices, LEDs, switching power supplies etc.
• Industrial load fluctuations
• Transformer saturation
• Welding arcs, furnaces, elevators etc.

Types of harmonics

• Even harmonics – Frequencies in even multiples of fundamental frequency, i.e. 2^nd, 4^th, 6^th, 8^th harmonics etc.
• Odd harmonics – Frequencies in odd multiples (3,5,7,9,11 etc.) of fundamental frequency
• Subharmonics – Frequency in integral division of fundamental frequency (e.g. 30 Hz, 20 Hz, 10 Hz for 60 Hz fundamental)
• Positive sequence harmonics– Harmonic waveforms in three phase systems following same phase sequency as fundamental (rotating in same direction)
• Negative sequence harmonics – Harmonic waveforms in three phase system rotating in direction opposite to that of fundamental frequency, i.e., phase sequency opposite to that of fundamental.
• Zero sequence harmonics – Harmonics in all phases are in phase with each other. This happens in harmonics of 3^rd order or multiples thereof. These are particularly dangerous as currents in neutral can be very high and damaging.

Effects of harmonics on power systems

• Overheating
• Voltage and current waveforms distortion, causing malfunction of sensitive equipment
• Neutral overloading- Zero sequence harmonic currents add up in neutral current, causing high currents in neutral conductors.
• Resonance effects
• Reduced efficiency due to higher KVA and higher losses in system.
• Malfunction of equipment like protection relays, meters and sensors due to distorted waveform.

Analysis of harmonics in waveforms

Any complex periodic waveform can be broken down into number of simple sinusoidal waveforms with different frequencies and amplitudes using mathematical analysis by means of Fourier series. The frequencies so obtained consist of a fundamental frequency and multiples thereof, which form the harmonics.

In a power supply system and electrical / electronic circuits, harmonic frequencies up to 40^th or even higher multiples can be obtained to get nearest approximation of supply waveform. In most cases, frequencies up to 21^st harmonic are usually considered sufficient, while those beyond are negligible.  Most significant harmonics in supply systems are 3^rd and 5^th, while 7^th to 11^th also can have considerable bearing on power quality. Higher order harmonics can also have to be considered in many cases, particularly for sensitive circuits.

Analysis of harmonic components is essential in order to understand their contribution and mitigate them by taking suitable measures. Such analysis forms an essential feature of most power quality audits.

Mitigation of Harmonics

Distortion due to harmonics (current / voltage) cause the waveform to become highly out of shape and irregular in nature. These waveforms can be analysed mathematically and broken down into a combination of series of sinusoidal waveforms of different frequencies. Harmonic analysers are used to detect harmonic frequencies and their contribution up to 40^th harmonics.

Harmonics are always undesirable in electrical and electronic systems, and efforts are made to reduce them to minimum possible to keep equipment and system safe. High or frequency filters are often used to keep harmonics at safe level. Many times, they cannot be compensated fully but are reduced to a safe level where they do not appreciably interfere in functioning of system and equipment.