68 MOTORS AND DRIVES
Low harmonic drives ease stress on the power supply
Gregers Geilager explains how low harmonic drives continuously regulate the network and load conditions without affecting the connected motor.
T
he mains voltage supplied by electricity utilities to homes, businesses and industry should be a uniform sinusoidal voltage with a constant amplitude and frequency.
This situation is no longer found in mains grids. This is mainly because consumers take non-sinusoidal current from the network or have a non-linear characteristic, eg frequency converters, switching network parts or energy-saving bulbs. Because of the ever increasing use of non-linear
loads deviations becomes increasingly serious. Irregular power supply reduce the lifetime of motors and other equipment, and cables and transformers must be oversized to maintain proper operation. The compatibility levels of the standards EN50160/
EN61000 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 set functions without disruption in 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’. Experts refer to the relative harmonic content of a network on the basis of Fourier analysis and calculate
it to 0.5 kHz, corresponding to the 50th harmonic oscillation. The commonly used B6 bridge of an input rectifier generates a typical harmonic load of this type on the mains network, as does the use of energy-saving bulbs, TV sets, computers, phase-control systems and many other devices and systems. Excessive distortion causes malfunctions and even the destruction of electronic control systems, computers and control devices. High harmonic contents cause an overall lower
network efficiency, load idle compensating systems and may even cause their destruction.
Reducing feedback
Feedback from electronic power control systems can be reduced. In Danfoss frequency converters (except VLT Micro Drive), it is as standard limited by built in filter elements. If it is necessary to reduce the harmonic content in the mains network further, for example in case of weak networks or emergency power operation, a network analysis can indicate appropriate measures. Active filters and active front ends are very effective
in reducing harmonic oscillations up to 2kHz, and are used in addition to passive harmonic filters. For active filtering considerations must be given
to the effects above 2kHz, generated by these filters themselves. They make further measures necessary to keep the mains supply clean. Standard limits in this higher frequency range are still at the planning stage. The switching frequencies of active filters will cause
a peak at the switching frequency of the filter itself. This is above the range of current norms, but higher orders disturbance is of equal importance. Users should ask manufacturers about emission values and counter- measures to secure the operational safety of their plant. To avoid overloading and secure mains voltage
quality, a number of reductions, avoidance or compensation methods can be used with systems and devices producing harmonic currents. You can use the VLT Harmonic Calculation Tool to include specific counter-measures in the planning stage and therefore ensure the quality of your system. The network feedback of electronic devices can be calculated to within 0.5kHz, depending on the system configuration and standard limits. The operation of a plant supplied with a generator
Fig. 1. The upper red curve indicates the odd-shaped current curve produced by a frequency converter. Adding current in the shape of the green curve will re-establish the ideal sinusoidal curve.
can also be reliably simulated. Conversion of the network to generator operation is possible and takes the emergency power supply situation in account. Current standards (EN50160) are included in the analysis. At
www.hcs-software.danfoss.de you can get quick
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