Measures to reduce electromagnetic interferences on substation secondary circuits

This paper presents measuring results of transferred overvoltages in the secondary circuits of instrument transformers caused by switching operations of disconnector in air insulated substations 220 kV, in Bosnia and Herzegovina. Obtained results show that the maximum amplitude of these overvoltages can be up to 940 V_peak (220 kV hydroelectric power plant Grabovica). In order to ensure electromagnetic compatibility it is necessary to adequately damp high overvoltages by instrument transformer, or to use some of the measures for reducing transferred overvoltages in secondary circuits. The conducted measurements show that in case of using unshielded secondary cables on current transformer terminals, overvoltages are much higher than in case of using shielded cables (3.7 times higher in measuring core and 6.1 times higher in protective core). Paper gives suggestions for eliminating the causes of interference, reducing coupling on the path of signal transmission and increasing protection from interference in the secondary devices.