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Large-scale offshore wind farms have become the development trend in wind power generation. Long submarine cables are used to collect electrical energy in the collection networks of offshore wind farms. However, the distributed capacitance of cables cannot be ignored, which makes the impedance of the collection network complex and changeable. It is common to encounter high-frequency harmonic resonance (HFHR) problems when cables interact with wind turbine generators (WTGs). The HFHR may threaten the safe and stable operation of wind turbines. In order to solve this problem, firstly, the impedance of a collection network was constructed. Furthermore, the wind farm collection network was divided into the equivalent wind turbine subsystem (EWTS) and the remaining equivalent wind farm subsystem (REWFS). Then, the mechanism of HFHR was revealed based on the impedance stability analysis method. The effects of cable parameters, the number of connected WTG, and the grid impedance on the HFHR of the interconnected system were also analyzed. Finally, the hybrid damping control method, combining active damping control (ADC) and passive damping control (PDC), was proposed to suppress the resonance. A simulation and experiment were performed to verify the effectiveness of the analysis results and proposed control method.

To reduce the common-mode voltage (CMV) as well as the current total harmonic distortions (THDs) of 2-level voltage source inverters (2L-VSIs), a hybrid single and double-voltage vector (VV)-based model predictive control (MPC) method is proposed. Firstly, a two non-zero VV-based MPC strategy is proposed to reduce the CMV as well as the current THDs. Secondly, the influences of the dead time on this method are analysed in detail, and an improved VV preselection method is further proposed to remove the dead time effects. Thirdly, considering the impacts of the control delay and the current ripples, a new current sector division method is presented, based on which a hybrid single and double-VV-based MPC strategy is finally proposed. Furthermore, the current variation per control period under the action of different VVs is also analysed in-depth in this study to lay a solid theoretical foundation for dividing the current sector more properly. Both the simulation and experimental studies are carried out to verify the effectiveness of the proposed method.

To enhance generating capacity and reliability of multi-machine parallel power generation, implement the efficiency optimization, a current distribution control method based on differential evolution (DE) algorithm is proposed for the switched reluctance generator (SRG) in parallel. The load current is stabilized by the maximum value current sharing control (CSC) firstly. Then, the current distribution control method is adopted, which can adjust the reference current of each parallel generator in the current distribution part, so as to change the working point of each generator and implement the optimization of parallel system efficiency. The performances of the proposed control method are evaluated in detail by the simulation, and comparison with traditional current sharing control is carried out as well.

Submarine cables are used to collect electrical energy in offshore wind farms, of which the grounding capacitance cannot be ignored. The impedance of wind turbine generators (WTGs) interacts with the impedance of the network, leading to a risk of wideband harmonic resonance (WHR). In order to study the characteristics of the internal WHR in the collector system, firstly, the impedance models of a typical WTG and cable line are established, and the correctness of the established mathematical models are verified. Then, the wind farm system can be divided into the equivalent wind turbine subsystem (EWTS) and remaining equivalent wind farm subsystem (REWFS) based on the impedance stability theory. And the effects of the length of cable line, the number of trunk lines in the collector system, the number of connected WTGs and the equivalent grid-side impedance on the WHR are analyzed. Finally, the validity of the theoretical analysis is verified by MATLAB-based simulation results.

For the purpose of acquiring the accurate grid impedance, a complex coefficient filter (CCF) based grid impedance identification method is studied in this paper. High frequency non-characteristic harmonic voltages and currents are injected into the power grid by the power converter firstly. Next, a CCF is used to extract these high frequency voltages and currents from the sampled voltages and currents at the point of common coupling. Then, the grid impedance is calculated based on the extracted voltages and currents. Considering that the impacts of the DC offset on the grid impedance identification cannot be removed by the conventional CCF, an improved CCF is proposed in this paper by adding a high-pass filter to the forward channel of the conventional CCF. The filter characteristic of the proposed CCF is analyzed, which shows its DC offset suppression capability. Simulation results show the effectiveness of the proposed grid impedance detection method.

Bidirectional AC/DC converter can be reconstructed as a three-phase four-switch (TPFS) fault-tolerant converter when leg fault occurs to enhance its fault tolerant operation capability. In this paper, to further improve the control performance of the TPFS fault-tolerant converter, a new model predictive control (MPC) method is proposed. Considering that there are only four voltage vectors for the TPFS fault-tolerant converter, to reduce the current ripples, double voltage vectors are selected and applied per control period in the proposed method. To reduce the calculation burden, the duration time of each voltage vector is calculated based on the principle of the modulated MPC. Furthermore, the effectiveness of the proposed double-voltage vector based MPC strategy is analyzed in theory in detail for the first time and then it is further verified by comparative simulation studies.

To identify the grid impedance accurately, high frequency non-characteristic harmonic voltages and currents are often injected into the grid by the power converter firstly. Then, digital signal filters are used to extract the high frequency voltages and currents from the sampled voltages and currents at the point of common coupling. Finally, the grid impedance can be calculated. Due to effects of the grid background harmonics, such as 5th and 7th harmonic, the grid impedance identification precision is usually poor. In this paper, to improve the grid impedance identification precision, a new complex coefficient filter-based grid impedance identification method is proposed considering the effects of the grid background harmonics. To reduce the influences of the grid background harmonics, such as 5th and 7th harmonic, an improved complex coefficient filter is proposed by adding a 5th and 7th harmonic suppression module to the conventional complex coefficient filter. The filter characteristic of the proposed complex coefficient filter is analyzed in detail to show its properties. Additionally, a simplified implementation method of the proposed complex coefficient filter is also proposed to reduce the calculation burden. Finally, simulation studies are carried out to further show the effectiveness of the proposed grid impedance identification method.

Sliding mode observers are often utilized to achieve the speed sensorless control of permanent magnet synchronous motors. However, due to the nonlinearity of the sampling channel, DC offset is often contained in the sampled voltages and currents of the permanent magnet synchronous motor, which may further reduce the speed sensorless control accuracy. Besides, the speed sensorless control accuracy is usually very poor due to the low signal-to-noise ratio in the low speed region. In this paper, to improve the speed sensorless control accuracy in the low speed region and remove the effects of the DC offset, a new super-twisting sliding mode observer based speed sensorless control method is proposed for permanent magnet synchronous motors. First, based on the concept of the equivalent feedback, a new super-twisting sliding mode observer is proposed. Second, the effects of the DC offset on the speed sensorless control are analyzed in detail, and a second-order generalized integrator is used to remove the DC offset contained in the observed equivalent feedback signal. Then, a phase-locked loop is used to estimate the speed and the rotor position to achieve speed sensorless control. Finally, the effectiveness of the proposed control strategy is verified by a 6.6 kW permanent magnet synchronous motor speed sensorless control system.