Danfoss Drives - Page 3

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 supplies influence the performance and operation of electrical equipment, so motors, frequency converters and transformers must be more highly rated to maintain proper operation. Source of interference e.g.: ■ Strip lights ■ Power converters ■ Frequency converters ■ Light dampers ■ Computers ■ Switch mode power supplies Interference sink e.g.: ■ Control systems ■ Power converters ■ Frequency converters ■ General radio receiving systems ■ Data transmission wires Interference 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”. Experts refer to the relative harmonic content of a network on the basis of Fourier analysis and calculate it to 2.5 kHz for 50 Hz, corresponding to the 50th harmonic oscillation. Relationship between compatibility, immunity and emission levels System disturbance level Equipment immunity level A satisfactory operating production system is obtained when each installed equipment has an immunity that’s higher than the total allowed grid distortion. For EN 61000-2-4 the immunity level is a minimum of 10% where as the planned highest distortion level is 5 or 8% depending on the installation. This leaves very few parts (red scattered) of the installation with temporary problems. Normal distribution of disturbance and immunity level 3

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