Abstract:In order to study switching transients in an offshore wind farm (OWF) collector system, we employ modeling methods of the main components in OWFs, including vacuum circuit breakers (VCBs), submarine cables, and wind turbine transformers (WTTs). In particular, a high frequency (HF) VCB model that reflects the prestrike characteristics of VCBs was developed. Moreover, a simplified experimental system of an OWF electric collection system was set up to verify the developed models, and a typical OWF medium voltage (MV) cable collection system was built in PSCAD/EMTDC based on the developed models. Finally, we investigated the influences of both the initial closing phase angle of VCBs and typical system operation scenarios on the amplitude and steepness of transient overvoltages (TOVs) at the high-voltage side of WTTs.
For the study of transient overvoltage (TOV) in an offshore wind farm (OWF) collector system caused by switching off vacuum circuit breakers (VCBs), a simplified experimental platform of OWF medium-voltage (MV) cable collector system was established in this paper to conduct switching operation tests of VCB and obtain the characteristic parameters for VCB, especially dielectric strength parameters; also, the effectiveness of the VCB reignition model was verified. Then, PSCAD/EMTDC was used to construct the MV collector system of the OWF, and the effects of normal switching and fault switching on TOV amplitude, steepness, and the total number of reignition of the VCB were studied, respectively, with the experimental parameters and traditional parameters of dielectric strength of the VCB. The simulation results show that when the VCB is at the tower bottom, the overvoltage amplitude generated by the normal switching is the largest, which is 1.83 p.u., and the overvoltage steepness of the fault switching is the largest, up to 142 kV/μs. The overvoltage amplitude and steepness caused by switching off VCB at the tower bottom faultily with traditional parameters are about 2 and 1.5 times of the experimental parameters under the same operating condition.
Line‐commutated converter‐based high‐voltage direct current (LCC‐HVDC) systems suffer from commutation failure (CF) when receiving‐end AC grid faults occur, which is a major drawback and threatens the security and stability of power systems. This paper presents a fast and accurate evaluation method of CF for the LCC‐HVDC systems, considering the CF probability characteristics. Firstly, based on the analysis of the CF probability characteristics from the view of short‐circuit impedance, the CF tolerance index (CFTI) and CF vulnerability index (CFVI) are proposed to quantitatively evaluate the LCC‐HVDC systems' risk of the possible‐ and inevitable‐occurrence CF, respectively. Secondly, four factors, that is, commutation voltage, fault occurrence time, DC current and firing angle command, are considered to calculate CF evaluation indices (CFEIs) and identify the CF risk area boundaries based on the theoretical analysis. Moreover, the relationships among short‐circuit impedance, fault position, and extinction angle are established based on the AC/DC decoupling method, which helps realize the decoupling of multi‐factors affecting CF during fault. Compared to earlier methods, this evaluation method combines the advantages of the accuracy of the electromagnetic transient simulation method and the rapidity of theoretical derivation. Finally, the validity and accuracy of the evaluation method are verified in the CIGRE HVDC benchmark model and IEEE 39‐bus system.
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