Safety clearance between high voltage test systems and parts of air-insulated substations in operation

In the context of high voltage testing on-site within operating air insulated substations (AIS), the question of safe distances between high voltage test systems and live switchgear parts is an important safety issue. Until now no official guidelines or published state of the art exists. That question of safe distance arises particularly in testing of GIS, hybrid switchgear, AIS and high-voltage cable with air terminations. The risk of sparkover in general between the test system and a switchgear part under operating voltage is assessed. Generally, the occurrence of transient overvoltage cannot be excluded both in the tested system, as well as in the operating system. Considering the worst case, transient differential voltages in the range of 2 MV may occur during tests even at nominal voltage of 400 kV in AIS. Depending on the physical model, safety clearances between 8 and 11 metres in 420 kV AIS must be result in this worst case. A simple method for determining a safe distance based on basic principles of insulation coordination relevant to the question of maximum sparkover distance is presented. In order to cover all cases, the proposed method includes certain safety margins; in this form, it has already been applied by the authors in the past. Hence, the proposed method seems to provide the required safe distances and may be seen as a starting point for discussions to formulate a harmonized and commonly accepted state of the art.