Abstract:Due to the capacitive effect of power cables, the phenomenon of harmonic resonance amplification tends to occur in the offshore wind power collection system via power cables, which leads to decline in power quality. The distributed power flow controller (DPFC) is a device based on the voltage source converter, which can regulate power flow and control harmonics at the same time. In this paper, the frequency-domain correlation model of the offshore wind power system is constructed, and the reason for harmonic resonance amplification is analyzed based on this model. Then, the harmonic mitigation method of connecting the DPFC to the offshore wind power system is proposed, and the harmonic characteristics of the offshore wind power system with the DPFC are derived. Based on the these characteristics, a control strategy is designed, which reduces the harmonic voltage content of the grid-connected point by controlling the DPFC to track the harmonic compensation voltage that makes the harmonic voltage amplitude at the grid-connected point zero in real time. The simulation results show that the proposed harmonic mitigation method and control strategy of the offshore wind power system based on DPFC can effectively reduce the harmonic voltage at the grid-connected point and improve the power quality.

Abstract:With the increase of installed capacity and proportion of wind power, the problem of extremely slow electromagnetic transient simulation speed of large-scale wind farms is becoming more and more prominent. In view of the complexity of the existing modeling methods, the inability to reflect internal characteristics of the wind farm, and the lack of flexibility, this paper proposes an electromagnetic transient controlled source based decoupling acceleration model (CS-DAM) for the large-scale offshore wind farm. This method realizes the decoupling between the internal components of the wind turbine (WT) and the ports between different WTs by transmitting the voltage and current information between the ports through the controlled source, so that the high-order matrix in the system solution process is decomposed into multiple small matrices to achieve simulation acceleration. The effectiveness of the method is proved based on the node analysis method and Kirchhoff’s law. The proposed modeling method of CS-DAM, which can build a model by dragging existing modules in the simulation software, is simple. The CS-DAM can accurately simulate various transient and steady-state conditions of the wind farm, and the acceleration effect is obvious. Meanwhile, the proposed CS-DAM can complement the methods proposed in the existing literature and expand their flexibility. Finally, wind farm models with different numbers of WTs are built in PSCAD/EMTDC to verify the simulation accuracy and acceleration effect of the CS-DAM. The results show that the proposed CS-DAM can achieve acceleration effects of 1~2 orders of magnitude, and has higher simulation accuracy compared to the detailed model.

Abstract:The medium-voltage DC (MVDC) collection and DC transmission systems of wind turbines based on distributed diode rectifiers do not require offshore booster stations and offshore converter stations, and have outstanding economic advantages for medium-to-long-distance and medium-scale offshore wind power. Aiming at its unique transmission current ripple problem, a multi-unit dynamic phase-shifting reference frame (DPSRF) is established and voltage vector control oriented to the d-axis of the DPSRF coordinate system is proposed to interleave the rectification current vectors and output current ripple of the distributed diode rectifiers. The staggered distribution of output current ripple effectively reduces DC transmission current and voltage ripple. Considering the influence of the dynamic phase shift angle in the ripple interleaving algorithm, a small-signal state-space model of the wind turbine grid-side converter is established, and the weak damping link of the system is identified based on the characteristic root analysis, and a stabilization control strategy is proposed. Taking advantage of the system’s potential to avoid DC circuit breakers, a fault location strategy based on the disturbance injection is formulated, and the relay protection logic and turbine operation sequence are used to realize fault localization and isolation of multiple fault locations in the DC internal network without a communication system. Finally, PSCAD simulation and RT-LAB-based hardware-in-the-loop experiments verify the effectiveness of the proposed control and protection strategy.

Abstract:After a busbar fault in the low-frequency transmission system of offshore wind power takes place, the equivalent potential and impedance of the converter-type power source are no longer considered constant like traditional power grids due to the switching and adjustment process of control. The paper theoretically analyzes the adaptability of busbar current differential protection and clarifies the impact modes of fault control strategies on the sensitivity of differential protection. A busbar protection is proposed based on time-domain full component model identification, utilizing the different equivalent fault models corresponding to internal and external busbar faults. This protection principle reflects the changes in the topology of the protected components themselves, and theoretically has the advantage of not being affected by power source characteristics. It is also applicable to other converter-type power grids. At the same time, the proposed protection is implemented in the time domain without the problem of phasor extraction, and the operation speed is fast. Finally, a simulation model is built in PSCAD/EMTDC to verify the reliability and speed of the proposed busbar protection.