Active filters for wind generators

By Subodh Bhatia, managing director, Westek Electronics

Remotely located wind farms are likely to have a negative effect on power quality by virtue of the voltage distortion effect their harmonic current output has on soft transmission lines. Active filters can be an excellent harmonics mitigation solution by virtue of their rapid response to changing harmonics conditions due to, for example, wind variability.

Wind generators now account for a significant proportion of the total power generated in Australia. It does not yet compare with the impact in European countries, principally Denmark, Germany and Spain.

However as wind power expands in Australia there will be increased pressure on power quality from wind generators. In Germany, wind generation farms have to comply with that country’s FGW TR3/2, and generally with IEC61400-21/4. The spread of harmonics in wind generators is broad to thousands of hertz, though individual harmonic amplitudes can be low.

In the event of wind farms connected to the main grids by long transmission lines, the control of both power factor and harmonics is important because of the voltage regulation and distortion that can occur with soft transmission circuits. Active filters are a solution for harmonics, power factor control, and for the start-up phase – during which there can be a harmonic and reactive components that equal the rating of the generator. For a wind farm with ‘N’ wind generators, the total harmonic distortion effect is equal to N at their point of common coupling. The rationale for active power filters is a strong one in that invariably the harmonic patterns of wind generators are highly dynamic, cover a broad spectrum and include interharmonics.

Larger wind generators, certainly those in excess of one megawatt, are likely to be of the double-fed induction type (DFIG). Their popularity is due to their efficient operation under variable wind conditions. They utilise a double conversion stage so as to cope with varying rotational speed, Basically the rotor is supplied with variable current and frequency so as to allow operation in below-synchronous speed conditions. The DFIG system allows power output from both the stator and rotor, or depending on lower wind speeds, power draw by the rotor and export of power from the stator. Harmonics output from the larger systems is significantly lower than for similarly constructed generators below one megawatt. The DFIG systems may well require harmonics mitigation but are able to operate at unity displacement power. Directly connected induction generators are not ideal for very variable wind conditions, require reactive vars from the grid, and have a very large spread of harmonics and interharmonics. Synchronous generators, whether field or permanent magnet excited, have a large harmonics output by virtue of the converter-inverters on their outputs.

Active – rather than static harmonic filters – are the requisite mitigation method, principally because of the highly dynamic nature of the harmonic spectra. Broadband correction (sometimes referred to as global correction) provides for the rapid response characteristic and this is achieved by corrective non-sinusoidal current mimicking the distorted current from the generator minus the fundamental, which has been remove by a notch filter. Although individual broadband filters differ, the notch filter is more or less a standard feature and allows response within some tens of microseconds. The further correction of which a filter should be capable is to act as a sink for additional specific harmonics allowing for a selection harmonic order with specified amplitude and phase. This permits the setting up of a series of control loops that will allow tailoring of the mitigation system. Once all control loops are locked to their respective frequencies, they only react to changes in phase angle and amplitude and these parameters are calculated in real time during sampling.

Harmonics mitigation for Australian wind farms is a relatively more important issue than for many European countries because of the longer and therefore softer transmission lines connecting to stiff base load grids. Active filters can be employed both on individual generators or/and for bulk mitigation at the point of common coupling.