Voltage sourced converter based high voltage direct current (VSC-HVDC) with ±800 kV/5 000 MW ultra-high voltage and large-capacity level is an important development direction of HVDC technology. Considering the development status of VSC-HVDC technology, the advantages and disadvantages of converter configurations with symmetrical monopole wiring and symmetrical bipolar wiring are compared. For the ±800 kV/5 000 MW VSC-HVDC technology, the device characteristics and operation performance of single converter scheme and double converter scheme under symmetrical bipolar wiring are compared and analyzed. The analysis results show that the double converter scheme is more suitable for the ultra-high voltage, large capacity and long distance VSC-HVDC technology because of its more flexible operation modes and higher energy availability.

Comparing with the traditional two-terminal HVDC, multi-terminal hybrid HVDC can realize multi-source supply and multi-receiving end, and the loss and cost are relatively low, which offers a more flexible transmission method. In this paper, the characteristics of multi-terminal hybrid HVDC are introduced. Different control modes and operation modes are design. Steady-state control strategy is also designed to coordinate LCC and VSCs at each sides. Moreover, considering the characteristic of series connection of high/low voltage valve groups of UHVDC, the valve group balancing control is designed. A simulation model is established in PSCAD/EMTDC, smooth start-up, shut-down and steady-state operation are achieved, and the validity of the proposed strategy is verified.

For the three-terminal line commutated converter-modular multilevel converter (LCC-MMC) hybrid DC transmission system, the DC transmission power reducing or interrupting problem caused by AC side fault at the rectifier station is studied. And an AC fault ride-through coordination control strategy for rectifier converter station is presented. Firstly, the mathematical models of different types of converters are established and the characteristics after AC system faults are analyzed. Secondly, according to the different operation modes and the fault characteristics of DC voltage decrease and DC line contain 100 Hz harmonic, the coordination control strategy between the minimum trigger angle control at rectifier side and the maximum modulation ratio control at converter side is proposed. Thirdly, the original 100 Hz protection values are changed then the control modes can be switched automatically without stations communication. Finally, a hybrid DC transmission system model is established in PSCAD/EMTDC, and the control characteristics of the system under different working conditions are simulated and analyzed. Results show that the proposed control strategy can improve the transmission power of the DC system and reduce the probability of power transmission interruption when short curcuit fault occurs at the rectifier AC side.

In view of strong nonlinearity and uncertainty of VSC-MTDC systems, a novel sliding-mode current control strategy based on exponential reaching law is proposed. The inner loop current proportional integral (PI) control is replaced by a sliding mode current control when the direct current control is used in HVDC converter stations, which reduces a PI controller compared to traditional double loop PI control and simplifies the parameter adjustment process of the system. A sliding-mode variable structure controller is established in which the errors of the d-axis current and q-axis current in inner loop are selected as a hyperplane in the rotating coordinate system, and the global stability of the control system is analyzed via Lyapunov function, the fast adjustment of power can be ensured when the converter station switches off because of fault, the stability of DC voltage can be maintained when the equivalent impedance of AC system changes. Finally, a multi-terminal HVDC transmission system connected to large offshore wind farms is built in PSCAD/EMTDC, the proposed control strategy is compared with traditional PI control strategy, and simulation results indicate that the proposed control strategy possesses good dynamic and steady state performances.

This paper builds equivalent circuit of multi-terminal DC network composed of different types of converters and multi-terminal DC lines, and proposes the control methods of different types of converters and the coordination mechanism of control modes among converter stations in multi-terminal HVDC transmission systems. A general electromechanical transient model of multi-terminal DC transmission system is established, and the model is realized based on the AC/DC power system calculation and analysis software (DSP). The results of the model is compared with PSCAD model through an example system. The comparison results show that the response characteristics of the established multi-terminal DC system model are in good agreement with that of the PSCAD model, and the accuracy of the model is verified.

As a new solution of HVDC, hybrid multi-terminal HVDC combines the merits of LCC-HVDC and VSC-HVDC. Firstly, an electromagnetic transient model is constructed for a parallel hybrid three-terminal HVDC transmisstion project using PSCAD/EMTDC program. Secondly, two different strategies defined as the inverter voltage control and rectifier voltage control are proposed for the hybrid multi-terminal HVDC and change of the system operation point is researched in detail. At last, the effectiveness of the strategies is verified in the simulation of the constructed model, and the AC fault responses under different strategies are comparatively analyzed.

Aiming at Wudongde multi-terminal UHVDC project, this paper studies the influence of line commutated converter HVDC(LCC-HVDC) configured with dynamic reactive power compensation scheme or voltage source converter HVDC(VSC-HVDC)scheme on the stability of inverter side of Guangdong Power Grid. Firstly, STATCOM is selected as the dynamic reactive power compensation devices after comparing technical performance, voltage stability and real performance. Secondly, the stability of receiving-end grid is analyzed when LCC, LCC+dynamic reactive power compensation devices and VSC are used seperately when the receiving-end is at eastern Guangdong and at western Guangdong. Research results show that STATCOM should be put into operation in different levels for LCC scheme if the N-1 constraints are satisfied and the ability of withstanding severe AC system faults increases to the level of VSC scheme.

The fault probability of sub-modules in modular multilevel converter (MMC) stays at a high level, fast and precise diagnosing and isolating the faulted sub-modules are critical for MMC to improve the operation conditions of converter and avoid unnecessary outages. Based on analysis of open-circuit fault characteristics of insulated gate bipolar transistor (IGBT) of sub-modules, a sliding mode observer is constructed to observe arm current and detect swiftly open-circuit fault in IGBT sub-module. By using the difference and changing rate between the measured value and the calculated value of arm current, fault type of the sub-module can be distinguished, relevant part of the sub-modules is selected according to the types of fault, then by calculating the ratio of the actual value against the theoretical value of the current flowing through sub-modules, the faulted sub-module can be located. Simulation on an MMC-HVDC system established on PSCAD/EMTDC shows that the proposed fault diagnosis method can reduce the computation of fault location effectively, detect and locate IGBT open-circuit fault accurately and swiftly.

Distribution network is seriously affected by harmonics and noise, the calculation of voltage phasor is more difficult, and the traditional phasor measurement accuracy of power transmission network can not satisfy the demand of distribution network. In order to improve the measurement accuracy of voltage amplitude and phase angle of distribution network, a method based on conditional maximum likelihood estimation (CML) is proposed for measuring the phasor of micro-synchronous phasor measurement unit (μPMU) in distribution network. The algorithm establishes the signal model of three-phase unbalanced system. When the measurement is up to two unknowns and the orthogonal value of the measured matrix and the unit matrix is not equal to zero, the feature which the three-phase matrix is orthogonal to the eigenvector of the sample covariance matrix is used to obtain the measurement. Furthermore, the amplitude and phase expression of the voltage phasor are deduced by the geometric characteristic, which reduces the computational complexity. Simulation test is carried out by Matlab, and the simulation results show that the proposed method can improve the measurement accuracy of the voltage amplitude and phase angle of distribution network to a certain extent.

Aiming at the problem of weak low voltage ride-through performance and unstable output power of doubly-fed induction generator (DFIG), the superconducting magnetic energy storage-fault current limiter system(SMES-FCL) suitable for grid-connected DFIG is constructed and analyzed, which achieves dual functions of energy storage and fault current limiting. Considering the nonlinear characteristics of the grid-connected DFIG with SMES-FCL, fuzzy-PI control strategy is used to establish a voltage and current double closed-loop control system of the SMES-FCL inverter and realize coordinated control with chopper, which overcomes the shortcomings of traditional PI control including difficult parameter tuning and poor robustness. Simulation model of DFIG with SMES-FCL is established by MATLAB/Simulink, simulation analysis and research are carried out under multi-conditions, such as normal operation and short circuit fault. Results show that the proposed method can not only improve the output power stability of DFIG, but also enhance the low voltage ride-through capability.