Limitations in Operation of High Voltage Equipment Resulting of Frequent Temporary Overvoltages

Introduction

The distribution and transmission networks integrate huge amounts of renewable generation sources and experience changes in the load behaviour of the consumers. These changes impact the voltages in the grids. In numerous countries, frequent power-frequency temporary (short and long duration) overvoltages (TOV) are observed. Although the high-voltage equipment is tested according to equipment-relevant standards (e.g. IEC 62271 series), with this type of overvoltage, it is not clear whether recurring TOVs or longer-duration TOVs are also covered by these tests. Moreover, today, the maximum operating voltage (U_m), which is mostly equal to rated voltage U_r, is used as a design value to which numerous tests (e.g. slow-front, fast-front overvoltages) are referred. Frequent TOVs with amplitudes above U_m increase the probability of switching actions and short circuits at the time of an instance of TOV. Consequently, tested overvoltages referred to Um are lower as at the time of instance of TOV (U_TOV > U_m). For this reason, there is a need to consider in more detail the different limitations in applications of various equipment that may be exposed to TOVs.

Analysis of the network conditions and TOV characterisation

The network conditions that can lead to power-frequency TOVs have been listed and categorised in a novel way. “Long-duration power-frequency TOV” are longer than 2 s and typically require manual or automated action of the system operator. The resulting amplitudes can lead to 5 % - 10 % above Um. The “short-duration power-frequency TOV”, typically in a time frame of 0.2 s to 2 s, depends on the regulation and automatic control of the voltage level. The last category is TOVs originating from some resonance phenomena in the grid. The comparison of different network connection and operation codes shows that there is no unified approach for the specification of the TOVs. They are rather dependent on the network type, country strategy and voltage levels. The survey conducted among TSO's and DSO's shows that the long-duration power-frequency TOVs are a reality phenomenon today. For the purpose of this work, the average or typical voltage surge equal to U_TOV=1.05×U_m has been defined.

The lifetime reduction of the equipment insulation subjected to continuous U_TOV can be very significant for most of the equipment considered. Hence, the increase of the continuous voltage and introduction of new voltage levels is not supported. The lifetime of the equipment insulation exposed to recurrent TOV should be considered. As long as the expected reduction in lifetime is minimal and acceptable from an asset management perspective it can be considered manageable.

Apart from insulation ageing, additional equipment-specific applications should be considered because TOV can restrict the application area of considered equipment.

Main Findings for considered equipment

Current and voltage instrument transformers

The TOV will lead to an increase in the short-circuit current. It should be verified by the user if the resulting current level remains below the tested values. Moreover, the TOV increases the risk of triggering ferro-resonance oscillations with inductive VTs. The application of designs with air gaps or with additional damping elements needs to be verified or considered. In particular, for GIS applications where the ratio of capacitance relation is unfavourable concerning triggering ferro-resonance oscillation, the resistive damping ability of VT is crucial. Finally, inductive VTs are frequently used for the discharging of non-grounded disconnected network parts. TOV will increase the amount of energy that needs to be discharged....