Many harmonics are generated in the HVDC converters during operation. To avoid the harmonics to

reach the AC network, AC filters are installed at the converter station. A filter bank does usually

consist of band pass filters for the major harmonics and high pass filters for the higher order harmonics.

The band pass filters are designed to work as a short circuit for one specific harmonic current.

If the filter parameters such as fundamental frequency or capacitance in the capacitor banks start to

alter, the filter performance will decrease.

By being able to compensate the tuning frequency of the filter it is possible to always keep a good filter

performance. Variable filters can be designed by either controlling the capacitance or the inductance.

The capacitance can be controlled by connecting or disconnecting tuning capacitors. The inductance

can be altered by using adjustable reactors with tap changers.

Variable AC filters can be designed with a narrower bandwidth and higher Q-factor (quality factor),

compared to a passive filter. A higher Q-factor will provide lower the active power losses.

A smart design of the variable filter may decrease the footprint of the filter. The decrease will lead to

smaller filter yards and lower costs for prepared surface.

A smart control system must be used to calculate the appropriate filter characteristics. The control

system must be applied with special features to make the filter run as smooth as possible. Some

features are synchronized connection, hysteresis and detection of filter failure.

Variable AC filters are a requirement for HVDC stations including CCC (Capacitor Commutated

Converter) compensation. In connection with CCC the band pass filter will earn a very narrow

bandwidth. If the AC filters is not variable, the performance will decrease a lot during frequency

deviations.

The conclusion is that a variable filter will increase the filter performance when the frequency is

deviating and the ambient temperature is altering the filter capacitance. Installation of variable filters

offers smaller footprints, lower losses, better performance and lower costs.