Abstract
This paper presents a comprehensive investigation on high frequency (HF) switching transients due to energization of vacuum circuit breakers (VCBs) in offshore wind farms (OWFs). This research not only concerns the modeling of main components in collector grids of an OWF for transient analysis (including VCBs, wind turbine transformers (WTTs), submarine cables), but also compares the effectiveness between several mainstream switching overvoltage (SOV) protection methods and a new mitigation method called smart choke. In order to accurately reproduce such HF switching transients considering the current chopping, dielectric strength (DS) recovery capability and HF quenching capability of VCBs, three models are developed, i.e., a user–defined VCB model, a HF transformer terminal model and a three-core (TC) frequency dependent model of submarine cables, which are validated through simulations and compared with measurements. Based on the above models and a real OWF configuration, a simulation model is built and several typical switching transient cases are investigated to analyze the switching transient process and phenomena. Subsequently, according to the characteristics of overvoltages, appropriate parameters of SOV mitigation methods are determined to improve their effectiveness. Simulation results indicate that the user–defined VCB model can satisfactorily simulate prestrikes and the proposed component models display HF characteristics, which are consistent with onsite measurement behaviors. Moreover, the employed protection methods can suppress induced SOVs, which have a steep front, a high oscillation frequency and a high amplitude, among which the smart choke presents a preferable HF damping effect.
Highlights
Large offshore wind farms (OWFs) consist of extensive cable systems with a number of feeders and medium voltage (MV) wind turbine step-up transformers, the surge impedance of cable is much lower than that of overhead lines, and the same switching surge may lead to higher voltage transients.wind energy is an intermittent source, which causes breakers to switch wind turbine generators (WTGs) frequently and randomly
When high frequency (HF) switching overvoltage (SOV) propagate to the ends of Wind turbine transformers (WTTs), the amplitude and du/dt may be higher, which pose a hazard to the connected equipment and cause local overstress of insulation systems
Only feeder A is considered and the user-defined detailed vacuum circuit breakers (VCBs) model is placed at the upstream of WTT1 on feeder A, while other VCBs are simulated by built-in ideal breaker models in PSCAD/EMTDC
Summary
Large offshore wind farms (OWFs) consist of extensive cable systems with a number of feeders and medium voltage (MV) wind turbine step-up transformers, the surge impedance of cable is much lower than that of overhead lines, and the same switching surge may lead to higher voltage transients. Wind energy is an intermittent source, which causes breakers to switch wind turbine generators (WTGs) frequently and randomly. In the past few years, insulation failures caused by SOVs have been reported to be one of prominent issues in wind farms operations [2]. Such SOVs have a steep front and only last for a short duration (few milliseconds or less).
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