Alternative schemes for eliminating harmonic voltage amplification in a distribution network

The medium voltage sub-networks of the Swiss Unterengadin region are fed from substations along the 380-kV transmission lines that tie Switzerland to Austria and Italy. High harmonic voltages have been observed in the region after suffering severe disturbances that led to some low voltage equipment damage. Measurements have shown that during certain network configurations the 7th harmonic voltage contents far exceed the IEC harmonic limit level, whereas the 5^th harmonic voltage levels are close to such limits. This paper summarizes a comprehensive study carried out, using frequency and time-domain simulations, to determine the exact cause of the problem, analyses the influencing factors and devises alternative solutions. The following alternative countermeasure schemes are presented: changing network topology; passive shunt filters; blocking series filters; and active filters