Irregular power supplies influence the performance and operation of electrical equipment, so motors, frequency converters and transformers must be more highly rated to maintain proper operation. Gregers Geilager reports.
The mains voltage supplied by electricity utilities to homes, businesses and industry should be a uniform sinusoidal voltage with a constant amplitude and frequency.
This ideal situation is no longer found in any power grid. This is mainly because consumers take non-sinusoidal current from the grid or have a nonlinear characteristic, for example, strip lights, light dampers, energy-saving bulbs and frequency converters. Because of the constantly increasing use of non-linear loads, deviations become increasingly serious.
The compatibility levels of the standards EN50160/EN61000 and, for industrial environments, EN61000, apply within the scope of the EMC directives. It is assumed, in principle, that when these levels are observed, all devices and systems fulfil their specified functions without disruption on or to electrical supply networks.
The distortion of the sinusoidal curve shape of the supply network, as a result of the intermittent use of electricity by the consumers connected to it, is called 'network feedback' or 'harmonic distortion'. In the case of variable speed drives, harmonic current distortion is caused by the input rectifier component, typically a six-pulse diode rectifier. Excessive distortion causes premature ageing of electronic equipment. This will, over time, increase the system cost. It will also lower the efficiency and stress - or even destroy - idle compensating systems and overheat components such as the supply transformers and cables.
In state-of-the-art frequency converters, harmonic feedback is, as standard, limited by built-in filter elements such as ac or dc-coils. These are often sufficient to ensure compliance with product and system standards and trouble-free operation. The most common technique is the use of ac-coils in front of or built-in to the drive. Thereby, a significantly lower current distortion can be achieved compared to a basic frequency drive without coils. Built-in dc-chokes dramatically reduce network feedback and ensure the drives comply with the limits of EN61000-3-12. Although ac and dc-coils offer similar harmonic reduction, a DC-coil drive has higher efficiency, does not reduce the voltage to the motor and has a lower fluctuation on the intermediate circuit, leading to prolonged life of the product.
Harmonic reduction
In some few cases it can be necessary to reduce the harmonic content further, for example in the case of weak networks or in operation on emergency power supplies. Further mitigation equipment is needed to ensure compliance with system standard EN61000-2-4 or recommendations such as IEEE519-1992 or G5/4. This can either be done by passive or active means.
Harmonic reduction via passive means is a well known approach, where technologies like multi-pulse drives or passive filters offer a robust and inexpensive mitigation form. The mitigation performance of passive solutions is unfortunately highly dependent on load changes, voltage imbalance and/or voltage pre-distortion.
Different combinations of coils and capacitors form different passive filters tuned to the individual drive. They are either tuned for individual harmonic cancellation or for reduction of a range of frequencies. They offer a practical and easy add-on filter for power systems with large concentrations of non-linear loads connected to the same distribution transformer. Especially at currents lower than 300A, passive harmonic filters offer a cost effective and performance efficient solution.
Multi-pulse drives, also called transformer solutions, are drives where the current is supplied via two or three parallel- connected 6-pulse rectifiers over a phase-shifting transformer. In theory, the 5th and 7th harmonic currents (for 18-pulse also the 11th and 13th) of the individual phase-shifted rectifiers cancel each other out. Since some voltage imbalance or harmonic background distortion is always present, a complete cancellation of the 5th and 7th (11th and 13th) is never achieved.
Active harmonic mitigation equipment is still considered a high cost solution. Active mitigation is very effective in reducing harmonic oscillations up to 2kHz and keeps harmonics disturbance low for the full load range.
Consideration must, however, be given to the effects above 2kHz, generated by these units themselves. In some cases they make further measures necessary to keep the mains supply clean. Users should ask manufacturers specifically about emission values and counter-measures to secure the operational safety of their plant.
For installation where harmonics problems are first detected after full installation or as a result of continued extensive use of non-linear loads, active filters offers an easy retrofit possibility. Active filters can be installed in front of multiple loads and offer a reliable and effective solution. Central harmonic mitigation ensures the highest possible system efficiency and so the lowest running costs.
The new Danfoss Low Harmonic Drive is a combination of an active filter and a standard ac-drive. It does not offer regenerative braking but is designed with the fewest possible components in the main current path, optimising the energy efficiency and has low output peak voltage and du/dt, which allows the use of standard IEC60034-17/25 motors in most cases.
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Gregers Geilager is with Danfoss Drives A/S, Graasten, Denmark. www.danfoss.com or www.driving-perfection.com
