Reactive Power Compensation Medium voltage networks, which have a very low harmonic load and in which in the future no important power electronic converters will be operated and no data transmission by line traps will be installed are typical applications for non-reactor based reactive power compensation systems. Investigations in the form of long-term measurements of the harmonic ratios of public and industrial medium-voltage networks show that a tendency towards increasing voltage distortion caused in particular by the 5th harmonics takes place. In consideration of this circumstances and while operating or planning powerful electronic converter systems reactor based reactive tuned power compensation have to be applied. Reactive power increasingly is stressing the networks. For the user created significant costs, caused by losses of the apparent current on the supply lines and electrical machines (I ² x R). Furthermore, increasingly reactive energy is billed by the energy provider. This phenomenon applies to both capacitive and inductive reactive load. 21kV/14MVAr-Reactive Power Compensation in a wind farm to fulfill the grid code