Measuring of Harmonics in the range 2-9 kHz

Measuring of Harmonics in the range 2-9 kHz

Background

Harmonics up to order 40 (2 kHz at 50Hz) are measured and evaluated according to the standards EN 61000-3-2 and -12. This frequency range includes the emission of typical power supplies or thyristor circuits. Starting from 9 kHz the usual RF EMC standards apply, which have to be regarded beside the low frequent effects.
 For the range from 2 to 9 kHz exist until now no standards which limit the emissions. But this range is actually used by devices like switched power supplies, electronic ballasts and frequency converters. The missing limits reduces the effort in development and production to meet them.
 In the last years there are more and more dispersed generation systems like photovoltaic or wind energy, which feed power into the net. Also here modern semiconductor components like IGBTs use the frequency gap between 2 and 9 kHz. The actual existing and in future growing net disturbances in this frequency range are the reasons, why measurements are necessary.

Standards

EN61000-4-7 describes in Annex B a method how to measure and group harmonics above order 40.
 The standard requires in principle following:

• DFT with 200ms rectangular window
• No gaps, no overlap
• Groups of width 200Hz, each consists of 40 lines in 5Hz steps:
   At 50Hz systems the first group starts at 2005Hz, group 35 ends at 9kHz.
   At 60Hz systems the first group starts at 2405Hz, group 33 ends at 9kHz.
   The groups are named according to their mid frequency. At a 50Hz system the first current group would be called I₂₁₀₀ and the last voltage group U₈₉₀₀.

Limits for the 2 to 9 kHz range are defined for example in the "Technischen Richtlinien für Erzeugungsanlagen am Mittelspannungsnetz", section 2.4.3, published by "BDEW (Bundesverband der Energie- und Wasserwirtschat e.V.)". The allowed harmonic currents are relative to the short circuit power at the point of coupling. For example in a 10kV net, the allowed current (relative to short circuit power) is for order n: 0,18 A/(MVA*n) for n > 40.

Measuring technique

Usually the current/voltage signals can not be measured directly with any instrument, but you need very high accurate voltage/current sensors.
 Following a list of usual ZES ZIMMER® current/voltage sensors, each with nominal value and error at 9kHz:

To improve the reproduce ability of measuring, the source side impedance must be specified accurately. A special net impedance with a well defined impedance in the range from 2 - 9 kHz is used between power source and EUT. The use of a power source is recommended but not necessary.
 The values of such a net impedance are defined in EN 61000-4-7:2002 + A1:2009 as well as the impedance |Z| over frequency. The impedance grows about linear from 3kHz to 10 Ohm at 9kHz.
 
 The measuring instruments Precision Power Analyzer LMG500 and Precision Power Analyzer LMG95 together with the testing software of systems CE-Test61k can perform such measuring according to the standard. Future limits can be implemented firmware and software updates (see Download Area).