Experimental study of surge protective devices to apply in dc power supply system at power substation

Impulse interferences appear in DC power supply system when switching, short circuits or lightning strokes to substations occur. Interferences under consideration result in malfunctions of relay protection and emergency situations. To mitigate these interferences, at present, avalanche diodes or surge protective devices (SPDs) mainly based on metal-oxide varistors with gaps are used. Protective devices are installed on positive and negative pole of DC power supply main switchboard using common mode scheme. Specific high-frequency diodes are not applied because they allow low currents only. Propagation of interferences along DC power supply system was investigated experimentally at operating power substations. Interferences was injected in DC circuit at open air switchgear by means of special generator and then measured at DC switchgears and at relay protection panels. Different types of protection devices were examined at laboratory to define their characteristics. They are avalanche diodes, SPDs and their combinations. This study showed that diodes provide lower protection level than SPDs but they are damaged by voltage impulses having short rise time. SPDs have less response time but in some situations are unable to dissipate energy produced by current flowing. Conclusions were made concerning combined usage of diode and SPD with intermediate decoupling inductor.