Assessment and mitigation of zero missing phenomenon for compensated cables and harmonic filters

摘要

This paper describes the assessment of a ‘zero missing’ phenomenon (ZMP) observed in circuit breakers, where the current does not cross zero through the breaker contacts for a period of time. Consequently, the breaker phases cannot be opened during faults on the system. The assessment is performed for a planned offshore wind farm (>200MW) which will connect to a National Grid 400kV substation in Great Britain. The duration of ZMP depends on the type of fault, location of fault, amount of compensation (high risk if >50% compensated), rating of the equipment and fault impedance. All these factors are considered for the assessment of this project. There are two scenarios considered for the analysis; one with a harmonic filter compensated with a shunt reactor on the 400kV side, and the other consisting of the export cable and filter compensated with a shunt reactor on the 220kV side. Different mitigation methods are analysed in this paper but each has its own advantages and disadvantages, where selection of the specific method depends on the system conditions. The complete system has been modelled and analysed in PSCAD (EMTDC) software. The system is analysed with the shunt reactor operating at different taps, different fault levels of the grid, and various unbalanced faults. Mitigation methods are then analysed and proposed for the system. These include point on wave switching, tapping the shunt reactor, energisation of capacitive and inductive equipment at different times, and sequential switching. This paper will show that for the 220kV scenario, sequential switching with dedicated circuit breakers for both export cable and reactor, is a technically feasible solution to overcome the issue of ZMP. For the 400kV scenario, it was determined that separate energisation of the filter, cable and reactor is the best option, as this can be achieved without breaching reactive power compliance under the steady-state conditions.