There are two basic mechanisms for the generation of interharmonics.
The first is the generation of components in the side-bands of the supply voltage frequency and its harmonics as a result of changes in their magnitudes and/or phase angles. These are caused by rapid changes of current in equipment and installations, which can also be a source of voltage fluctuations. Disturbances are generated by loads operating in a transient state, either continuously or temporarily, or, in many more cases, when an amplitude modulation of currents and voltages occurs. These disturbances are of largely random nature,
depending on the load changes inherent in the processes and equipment in use.
The second mechanism is the asynchronous switching (i.e. not synchronized with the power system frequency) of semiconductor devices in static converters. Typical examples are cyclo-converters and pulse width modulation (PWM) converters. Interharmonics generated by them may be located anywhere in the spectrum with respect to the power supply voltage harmonics.
In many kinds of equipment both mechanisms take place at the same time.
Interharmonics may be generated at any voltage level and are transferred between levels, i.e. interharmonics generated in HV and MV systems are injected into the LV system and vice versa. Their magnitude seldom exceeds 0.5% of the voltage fundamental harmonic although higher levels can occur under resonance conditions
Basic sources of this disturbance include:
- arcing loads
- variable-load electric drives
- static converters, in particular direct and indirect frequency converters
- ripple controls
Interharmonics can also be caused by oscillations occurring in the systems comprising series or parallel capacitors and transformers subject to saturation and during switching processes.
The power system voltage contains a background Gaussian noise with a continuous spectrum. Typical levels of this disturbance are in the range
(IEC 61000-2-1)
ARCING LOADS
This group includes arc furnaces and welding machines. Arc furnaces do not normally produce significant interharmonics, except where amplification occurs due to resonance conditions. Transient operation, being a source of interharmonics, occurs most intensively during the initial phase of melting (Figure 1).
Welding machines generate a continuous spectrum associated with a particular process. The duration of individual welding operations ranges from one to over ten seconds, depending on the type of welding machine.
ELECTRIC MOTORS
Induction motors can be sources of interharmonics because of the slots in the stator and rotor iron,particularly in association with saturation of the magnetic circuit (so-called „”slot harmonics“„). At the steady speed of the motor, the frequencies of the disturbing components are usually in the range of 500 Hz to
2000 Hz but, during the startup period, this range may expand significantly. Natural asymmetry of the motor (rotor misalignment, etc.) can also be a source of interharmonics – see Figure 2.
Motors with variable-torque loading, i.e. forge drives, forging hammers, stamping machines, saws, compressors, reciprocating pumps, etc., can also be sources of subharmonics. The effect of variable load is also seen in adjustable-speed drives powered by static converters.
In wind power plants the effect of the variation in turbine driving torque, resulting, for example, from the ”shadow effect“ of the pylon, can modulate the fundamental voltage component, thus becoming the source of undesirable, low-frequency components.
STATIC FREQUENCY CONVERTERS -INDIRECT FREQUENCY CONVERTERS
Indirect frequency converters contain a dc-link circuit with an input converter on the supply network side and an output converter (usually operating as an inverter) on the load side. In either current or voltage configurations the dc-link contains a filter which decouples the current or the voltage of the supply and load systems. For that reason the two fundamental (the supply and the load) frequencies are mutually decoupled. But ideal filtering does not exist, and there is always a certain degree of coupling. As a result, current components associated with the load are present in the dc-link, and components of these are present on the supply side. These components are subharmonic and interharmonic with respect to the power system frequency.
CURRENT-SOURCE LOAD COMMUTATED INVERTERS
Due to the semiconductor devices switching technique, these are classified as line commutated indirect frequency converters. A frequency converter (Figure 3) consists of two three-phase bridges P1 and P2 and a dc-link with reactor of inductance L . One of the bridges operates in the rectifier mode and the other in the inverter mode, although their functions could be interchangeable.
The presence of two rectifier bridges supplied from two systems of different frequencies results in the dc-link current being modulated by two frequencies–f1 & f2. Each of the converters will impose non-characteristic components on the dc link, which will appear as non-characteristic harmonics on the ac side, both in the load and in power supply system
We will See the "EFFECTS OF THE PRESENCE OF INTERHARMONICS" in next article soon,
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