Harmonics in electrical systems

What are harmonics?

Harmonics are unwanted currents that are integer multiples of the fundamental line frequency (50 or 60 Hz). For a 50Hz supply (fundamental frequency), 2^nd harmonic is 100Hz, 3rd is 150Hz and so on.

Causes of harmonics

Nonlinear loads cause harmonics to flow in the power lines. Some loads cause the current to vary disproportionately with the voltage during each half cycle.

A large portion of non-linear loads is constituted by SMPS-based power supplies. These supplies draw current in high-amplitude, mostly non-sinusoidal, short pulses which create distortion that travels back into the power supply and can affect other loads connected to the same source. Their current and voltage have waveforms that are non-sinusoidal, containing distortions, whereby the fundamental frequency (50 or 60Hz) has numerous additional frequencies superimposed upon it, creating multiple frequencies within the normal sine wave. The multiple frequencies are harmonics of the fundamental frequency.

Linear loads - loads where the voltage and current follow one another without any distortion to their pure sine waves). Examples of linear loads are resistive heaters, incandescent lamps, and constant speed induction and synchronous motors.

Examples of non-linear loads

• Switch-mode power supplies;
• Ballast-based fluorescent lighting;
• Variable speed motors and drives;
• Photocopiers;
• Personal computers;
• Laser printers;
• Fax machines;
• Battery chargers;
• Some UPSs

Some effects of harmonics

• Harmonic currents can overload cables and protection circuit breakers due to increased current, thereby causing false tripping of breakers.
• Increased current flowing in the neutral conductor of a circuit - especially due to the 3^rd, 9^th and 15^th harmonics (triple harmonics) which can cause neutral current to rise to as much as 1.73 times the phase current, even under balanced load conditions.
• Equipment and generator malfunctions and/or failures due to excessive voltage distortion.
• Overheating of electrical distribution equipment, cables, transformers, standby generators, motors etc. which in extreme cases can cause fire.

(In transformers and motors, hysteresis loss and eddy current increases with frequency and since harmonics increase the frequency seen by the equipment, these losses lead to increased heating in induction equipment).

In cables, the effective impedance of the cable increases at high frequencies thereby limiting the current carrying capacity of the cable leading to overloading and overheating.

• Equipment malfunctions due to excessive voltage distortion
• Reduction in the speed of electric motors - at the 5th harmonic, counter electromotive force (CEMF) which acts in opposition to the direction of rotation may eventually cause motor speed to decrease.
• Increase cost of utility power to the end user due to metering error and lower system power factor problems.
• Increased internal energy losses in connected equipment, causing component failure and shortened life span
• Poor crest factor and its related problems.

Classes of harmonics

Harmonics can broadly be categorized as:

Positive Harmonics:

These are harmonics with the numbers 1, 4, 7, 10, 13, etc. (note a common difference of 3 starting from the fundamental frequency harmonic 1). They produce magnetic fields and currents rotating in the same direction as the fundamental frequency harmonic.

Negative Harmonics:

These are harmonics with the numbers 2, 5, 8, 11, 14, etc. (also note the common difference of 3 starting from the second harmonics). They develop magnetic fields and currents that rotate in a direction opposite to that positive frequency set.

Zero Sequence Harmonics:

Harmonic numbers 3, 9, 15, 21, etc.(note a common difference of 3 starting from the third harmonic). They do not develop usable torque, but produce additional losses in the machine.

Mitigating/eliminating effects of harmonics

Over-sizing the neutral conductor:

Over-sizing the neutral conductor does not eliminate harmonics but can help mitigate its effect. This is because the neutral conductor impedance is lowered and so it can carry more current which may result from harmonics but with lesser risk of overloading and overheating which may lead to fire.

Multiple Neutral conductors:

This effectively produces the same effect as above only that each phase has its own neutral conductor rather than a common one.

Over-sizing transformers:

Loading transformer to not more than 60% of its capacity helps the transformer to deal with harmonic related problems such as eddy current, hysteresis losses and copper losses.

Eddy current losses: Power dissipated due to current circulating in metallic material (core, windings, case, and associated hardware in motors, etc.) as a result of electromotive forces induced by variation of magnetic flux.

Hysteresis: The energy loss in magnet material that results from an alternating magnetic field as the elementary magnets within the material seek to align themselves with the reversing magnetic field.

Using K-rated transformers:

K-rated transformers are transformers designed with high K-factor so that they can handle the high heat resulting from harmonics.

Using equipment which generates less harmonics:

Using linear loads and equipment with linear power supply which generate little or no harmonics

Harmonic Filters:

Harmonic filters are design to pass harmonic contents to ground. They are usually designed to handle specific frequencies. With good design, they can be used to eliminate harmonics.