Harmonics basics

Introduction

The presence of harmonics in electrical systems means that current and voltage are distorted and deviate from sinusoidal waveforms.Harmonic currents are caused by non-linear loads connected to the distribution system. A load is said to be non-linear when the current it draws does not have the same waveform as the supply voltage. The flow of harmonic currents through system impedances in turn creates voltage harmonics, which distort the supply voltage.

What is Harmonics?

We all know that the electricity generated and distributed takes a sinusoidal wave form. This wave form is generated at a frequency of 50 cycles / second (or 50 Hz). It is also known as main/fundamental frequency waveform. This is the ideal waveform that is suitable for smooth operations of various electrical equipments, appliances connected to the grid.

However, due to various reasons that will be discussed in next section, this pure sinusoidal waveform gets distorted. Additional waveforms (unwanted) get added on top of the fundamental sinusoidal waveform. A Fourier Analysis of these waveforms reveal that these waveforms are in multiple of the fundamental sinusoidal waveform frequency of 50 Hz. These frequency components that have higher frequencies than fundamental frequency are called “Harmonics”.

Source of harmonics

Harmonics are generated by various electrical equipments connected to the electrical grid. These equipments include:

  • Arc Furnaces;
  • Electric welding machines;
  • Thyristor controlled devices like Variable Speed Drives, Soft Starters, Rectifiers;
  • In some cases transformers, rotating machines;
  • Elevators, Inverters, UPS, Battery Chargers.

Grouping of harmoncs

The harmonics are grouped into positive (+), negative (-) and zero (0) sequence components.

  • Positive sequence harmonics (harmonic numbers 1, 4, 7, 10, 13, etc.) produce magnetic fields and currents rotating in the same direction as the fundamental frequency harmonic.
  • Negative sequence harmonics (harmonic numbers 2, 5, 8, 11, 14, etc.) develop magnetic fields and currents that rotate in a direction opposite to the positive frequency set.
  • Zero sequence harmonics (harmonic numbers 3, 9, 15, 21, etc.) do not develop usable torque, but produce additional losses in the machine. The interaction between the positive and negative sequence magnetic fields and currents produces torsional oscillations of the motor shaft. These oscillations result in shaft vibrations.

Adverse effects of harmonics

  • Increased losses due to distorted waveform. Equipments may get overheated. E.g. transformer overheating, increased losses due to harmonics;
  • Some of the precision electrical equipments like relays may fail to perform accurately as they are calibrated to work with fundamental waveform at rated values. Harmonics cause false conditions and may lead to equipment malfunction;
  • Capacitors may get overloaded and may lead to failure;
  • Undesirable resonance effect seen between inductive and capacitive components;
  • Electromagnetic interference with communication systems;
  • Due to introduction of harmonics it may generate irregular voltage and current values in the grid and may unnecessarily increase overall demand in utility grid.
  • Neutral overload in 3-phase systems.

Effect on transformer

  • Increase of iron losses / hysterisis loss
  • Increase of eddy current losses

Transformers that are required to supply power to nonlinear loads must be derated based on the percentages of harmonic components in the load current and the rated winding eddy current loss.

Effect on capacitor banks

  • Most capacitors are designed to operate at a maximum of 110% of rated voltage and at 135% of their kVAR ratings. In a power system characterized by large voltage or current harmonics, these limitations are frequently exceeded, resulting in capacitor bank failures.
  • Since capacitive reactance is inversely proportional to frequency, unfiltered harmonic currents in the power system find their way into capacitor banks, These banks act like a sink, attracting harmonic currents, thereby becoming overloaded.

Effect on cables

The primary effect of harmonics on power cables is the additional heating due to increase in the I2R losses. This can be attributed to the two phenomena known as skin effect and proximity effect, both of which vary as a function of frequency as well as conductor size and spacing. Also, cables involved in system resonance, may be subjected to voltage stress and corona, which can lead to dielectric (insulation) failure.

Effect on motors

Electric motors experience losses due to hysteresis and eddy currents set up in the iron core of the motor. These are proportional to the frequency of the current. Since the harmonics are at higher frequencies, they produce higher core losses in a motor than the power frequency would. This results in increased heating of the motor core, which (if excessive) can shorten the life of the motor. The 5th harmonic causes a CEMF (counter electromotive force) in large motors which acts in the opposite direction of rotation. The CEMF is not large enough to counteract the rotation; however it does play a small role in the resulting rotating speed of the motor.

Various harmonic indicators

THDu is an indicator of the distortion of the voltage wave. Below are given indicative
values of THDu and the corresponding consequences in an installation:

  • ≤ 5%: normal situation, no risk of malfunctions,
  • 5 to 8%: significant harmonic distortion, some malfunctions are possible,
  • ≥ 8%: major harmonic distortion, malfunctions are probable. In-depth analysis and
    the installation of mitigation devices are required.

THDi is an indicator of the distortion of the current wave. The current distortion can be different in the different parts of an installation. The origin of possible disturbances can be detected by measuring the THDi of different circuits. Below are given indicative values of THDi and the corresponding phenomena for a whole installation:

  • ≤ 10%: normal situation, no risk of malfunctions,
  • 10 to 50%: significant harmonic distortion with a risk of temperature rise and the resulting need to oversize cables and sources,
  • ≥ 50%: major harmonic distortion, malfunctions are probable. In-depth analysis and the installation of mitigation devices are required.

Which harmonic orders must be monitored and mitigated ?

The most significant harmonic orders in three-phase distribution networks are the odd orders (3, 5, 7, 9, 11, 13 ….). Triplen harmonics (order multiple of 3) are present only in three-phase, four-wire systems, when single phase loads are connected between phase and neutral. Utilities are mainly focusing on low harmonic orders (5, 7, 11, and 13).

Generally speaking, harmonic conditioning of the lowest orders (up to 13) is sufficient. More comprehensive conditioning takes into account harmonic orders up to 25.

Harmonic amplitudes normally decrease as the frequency increases. Sufficiently accurate measurements are obtained by measuring harmonics up to order 30.

Controlling harmonics

Presence of harmonics is not good for both utility operators as well as individual customers. Various adverse effects of Harmonics are as follows:

  1. Limiting non linear load devices (e.g. inverters, UPS) to below 30% of total load on a transformer.
  2. Use of Harmonic Filters: In cases where it is not possible to limit the non linear load, use of Harmonic filters is recommended.

A Harmonic filter helps to trap the harmonic frequency thereby freeing up the fundamental frequency. In this, the filters are tuned to specific harmonic such as 3rd, 5th, 7th , 11th.

A Harmonic filter provides low impedance to tuned harmonic frequency and bypasses it to earth and thus eliminates harmonics from power supply.

Oversizing of equipment

  • Derating of power sources (generators, transformers and UPSs) means they must
    be oversized;
  • Conductors must be sized taking into account the flow of harmonic currents. In addition, due the skin effect, the resistance of these conductors increases with frequency. To avoid excessive losses due to the Joule effect, it is necessary to oversize conductors;
  • Flow of harmonics in the neutral conductor means that it must be oversized as well.

Reduced service life of equipment

When the level of distortion THDu of the supply voltage reaches 10%, the duration of service life of equipment is significantly reduced. The reduction has been estimated at:

  • 32.5% for single-phase machines
  • 18% for three-phase machines
  • 5% for transformers

 

 

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