Origin of harmonics
The presence of harmonics indicates a distorted current or voltage wave. The distortion of the current or voltage wave means that the distribution of electrical energy is disturbed and power quality is not optimum.
Harmonic currents are caused by non-linear loads connected to the distribution network. The flow of harmonic currents causes harmonic voltages via distribution-network impedances and consequently distortion of the supply voltage.
Origin of harmonics
Devices and systems that cause harmonics are present in all sectors, i.e. industrial, commercial and residential. Harmonics are caused by non-linear loads (i.e. loads that draw current with a waveform that is not the same as that of the supply voltage).
Examples of non-linear loads are:
- Industrial equipment (welding machines, arc furnaces, induction furnaces, rectifiers)
- Variable-speed drives for asynchronous or DC motors
- UPSs
- Office equipment (computers, photocopy machines, fax machines, etc.)
- Home appliances (television sets, micro-wave ovens, fluorescent lighting)
- Certain devices involving magnetic saturation (transformers)
Disturbances caused by non-linear loads: harmonic current and voltage
Non-linear loads draw harmonic currents that flow in the distribution network. Harmonic voltages are caused by the flow of harmonic currents through the impedances of the supply circuits (transformer and distribution network for situations similar to that shown in Figure M2).
Fig. M2:Single-line diagram showing the impedance of the supply circuit for a harmonic of order h
The reactance of a conductor increases as a function of the frequency of the current flowing through the conductor. For each harmonic current (order h), there is therefore an impedance Zh in the supply circuit.
When the harmonic current of order h flows through impedance Zh, it creates a harmonic voltage Uh, where Uh = Zh x Ih (Ohm law). The voltage at point B is therefore distorted. All devices supplied via point B receive a distorted voltage.
For a given harmonic current, the distortion is proportional to the impedance in the distribution network.
Flow of harmonic currents in distribution networks
The non-linear loads can be considered to reinject the harmonic currents upstream into the distribution network, toward the source.
Figures M3 and M4 show an installation disturbed by harmonics. Figure M3 shows the flow of the current at 50 Hz in the installation and Figure M4 shows the harmonic current (order h).
Fig. M3:Installation supplying a non-linear load, where only the phenomena concerning the 50 Hz frequency (fundamental frequency) are shown
Fig. M4: Same installation, where only the phenomena concerning the frequency of harmonic order h are shown
Supply of the non-linear load creates the flow of a current I50Hz (shown infigure M3), to which is added each of the harmonic currents Ih (shown in figure M4), corresponding to each harmonic order h.
Still considering that the loads reinject harmonic current upstream into the distribution network, it is possible to create a diagram showing the harmonic currents in the network (see Fig. M5).
Fig. M5: Flow of harmonic currents in a distribution network
Harmonics have major economic effects in installations:
- Increases in energy costs
- Premature ageing of equipment
- Production losses
