Risk Mitigation through Transient Protection of Transformer Bushings when Using Online Monitoring

Transients and very fast transients (VFT) are high frequency electrical disturbances that occur in power systems and can lead to the degradation of power system components such as bushings. The transient voltages are created mostly from switching operations of breakers and disconnector operations in gas insulated switchgear (GIS) but also from harmonics generated by power electronics.The trend of digitalization has increased the number of online monitoring devices added to bushings. Condition assessments in the field can reduce the risk of failures if performed properly. The most common online monitoring systems measures parameters of the main insulation of the bushing and needs to be permanently connect to it via the test/voltage tap.A VFT pulse have a high frequency content and in combination with online monitoring, a high voltage in the test tap region of the bushing can be built up due to the impedance of the connected monitoring cable. This may cause flashovers inside the tap or internal breakdowns between the outmost conducting layer connected to the tap and ground (C2), or in the cable insulation with severe consequences. To mitigate this problem, effective protection strategies need to be implemented to safeguard the bushing if online monitoring is to be used.In this paper requirements for protection devices for the bushing tap are given together with test results. The primary objective of a protection device is to safeguard the bushing from overvoltages in the tap area without altering the measurement signal. Furthermore, it must be able to act on the same time scale as the rise times of the VFTs.Since VFT behavior is not captured in standard impulse testing, new tests have been developed. Results will be shown not only for small scale test on a bushing protection device but also full-scale test with the protection device mounted on 550 kV RIP bushing exposed to VFT.