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DANFOSS LTD - Harmonic Distortion – An Ever Increasing Problem

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, e.g. strip lights, light dampers, energy-saving bulbs and frequency converters.

Because of the constantly increasing use of non-linear loads, deviations become increasingly serious.

Irregular power supply influence the performance and operation of electrical equipment, so motors, frequency converters and transformers must be more highly rated to maintain proper operation.

Statutory basis of assessment
The compatibility levels of the standards EN 50160/EN 61000 and, for industrial environments, EN 61000, 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 in electrical supply networks.

How feedback occurs
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 rectifier part, typically a 6-pulse diode rectifier.

Any periodic function can be described as the sum of a series of sine-wave functions (called a “Fourier transformation”) The figure shows the current drawn by a typical drive with dc coils. Underneath it is deconstructed into pure sinus curves. All but the blue curve represents distortion – unwanted and often harmful currents.

Consequences
Excessive distortion causes premature aging of electronic equipment. This will, over time, increase the system cost. It will also lower the efficiency, stress – or even destruct - idle compensating systems and overheat components such as the supply transformers and cables.

Reducing feedback
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 reassure compliance with product and system standards and trouble free operation.

AC-coils
The most common technique is the use of AC-coils in front of or build-in the drive.
Thereby, a significantly lower current distortion can be achieved compared to a basic frequency drive without coils.

DC coils
Built-in DC-chokes dramatically reduces network feedback and ensures the drives comply with the limits of EN 61000-3-12. Although AC and DC-coils have similar harmonic reduction, DC-coil drive has higher efficiency, does no reduction of the voltage to the motor and has a lower fluctuation on the intermediate circuit leading to prolonged life of the product.

Extensive harmonic reduction techniques
In some few cases it can be necessary reduce the harmonic content further, for example in the case of weak networks or emergency power operation. Further mitigation equipment is needed to assure compliance with system standard EN 61000-2-4 or recommendations such as IEEE 519-1992 or G5/4. This can either by 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 depending on load changes, voltage imbalance and/or voltage pre-distortion.

Passive filtration
Different combinations of coils and capacitors forms 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, -easy add-on, filter for power systems with a large concentration of non-linear loads connected to the same distribution transformer.

Especially at currents lower than 300A, passive harmonic filter offers a cost and performance efficient solution.

Multi-pulse drive
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 are cancelling out each other. 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 2 kHz and keeps harmonics disturbance low for the full load range. Consideration must however be given to the effects above 2 kHz, 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.

Active Filters
For installation where harmonics troubles are first detected after full installation or a result of continues extensive use of non-linear loads, active filters offers an easy retrofit possibility.

Active filter can be installed in front of several loads and offers a reliable and effective solution. Central harmonic mitigation reassured the highest possible system efficiency and so lowest running expenses.

Low harmonic drives
The most common low harmonic drive is an active front end. Besides the benefits of reducing harmonics AFE’s also offers regenerative braking needed in some applications. The disadvantage of the AFE is it relative low efficiency and the increases du/dt on the controlled motor leading to higher insulation and bearing stress.

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 dV/dt, which allows the use of standard IEC60034-17/ 25 motors in most cases.

Email: drivesuk@danfoss.com


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Arundel Jones Associates Ltd Registered in England and Wales No. 07334149
Hill Farm, Linton Hill, Maidstone, Kent ME 17 4AL
Tel : 01622 745333 email
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News Categories : Low harmonic drive
Variable speed drive
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