The control of harmonic distortion on an EHV system by the use of capacitive damping networks

The privatisation of the United Kingdom electricity supply system has created conditions in which the pattern of reactive power flow can change rapidly. In order to respond quickly to such changes and maintain security and quality standards the need for additional switched compensation which can be easily re-located has been identified. This paper considers the requirements for large mechanically switched capacitor (MSC) banks on the UK EHV (400 and 275 kV) systems which will not produce any significant magnification of pre-existing harmonic distortion. Results of measurements on the EHV systems have shown an upward trend in distortion levels particularly at 3rd, 5th and 7th harmonics. The paper describes the need to design the MSC as a damped harmonic filter and the choice of a C-type configuration to limit losses. Details of the equipment ratings, in the presence of high levels of pre-existing distortion, and the equipment testing procedures are described