Due to the challenges of DC circuit breaker design and the coordination between DC terminals, the development of line commutated converter based multi-terminal DC system is very slow. The appearance of full control devices and new topologies for voltage source converter, especially the promotion and application of the modular multi-level converter, have brought opportunities and challenges for the multi-terminal DC system. This paper mainly discusses the bulk power transmission from energy base to different load centers. Different multi-terminal DC schemes for 3-terminal DC transmission system are analyzed and discussed. The advantages and disadvantages of these schemes are compared. Finally, the hybrid multi-terminal DC scheme based on line commutated converter and voltage sourced converter is proposed, which can solve the commutation failure problem in the inverter station, simplify the coordination between different DC terminal and has great economic significance.

Narrow transmission corridor and limited power consumption ability of a single market have become the key problems to power transmission and consumption of large energy bases in China. UHV multi-terminal DC (UHV-MTDC) technology can meet the requirements of long distance and large capacity power transmission for multiple sending ends and receiving ends. UHV-MTDC can save transmission corridor and match the multiple sending ends and receiving ends flexibly, therefore it will help to solve the power transmission and consumption problems of large energy bases. UHV-MTDC is an important direction of DC technology development in the future. Combining with the design of the demonstration project of UHV-MTDC transmission system from Wudongde hydro plant to Guangdong and Guangxi province, the advantages and utilization situation of UHV-MTDC technologies are introduced and their applications in the future are analyzed.

Isolation of the fault DC lines during the initial fault period after is a major challenge in the multi-terminal DC (MTDC) system based on voltage source converter (VSC). Existing protection schemes and fault analysis methods are mainly based on the numerical simulations, which are lack of the reliable theoretical analysis. In this paper, a high frequency equivalent model of VSC based MTDC system is proposed for the fault current calculation of the system in the initial stage of DC fault. In the proposed model, the parallel connected capacitors of VSCs are regarded as short-circuited, and the initial fault current calculation can be reduced as a reduced-order RL and RLC circuit. In addition, the errors between the transient fault current calculation based on the high frequency equivalent model and the fundamental RL model are also evaluated. Numerous simulation are studied based on PSCAD/EMTDC. The results demonstrate that the proposed high-frequency equivalent model can be utilized for initial DC fault analysis of VSC based MTDC system under different fault locations and the accuracy of results can meet the engineering requirements.

A power flow calculation method of multi-terminal hybrid LCC-VSC DC grid based on multiple control modes is proposed. Firstly, the power flow model of the DC network component is analyzed, and the steady-state model of the hybrid DC grid is established. Then the classical Newton Raphson method is used to solve the equation which is established according to the control mode, the control strategy of the DC grid and the power flow equations under various control mode according to the type of converter. Thirdly, the AC/DC hybrid network is decoupled by sequential method, and boundary information of DC network and the AC grid are processed, and the AC/DC hybrid grid is solved iteratively. Finally, an improved PSS/E example is given to verify the validity and accuracy of the proposed method which provides an accurate initial value for the electromechanical transient simulation of the multi-terminal hybrid DC grid.

To investigate the small signal stability of multi-infeed HVDC system, a multi-input and multi-output interconnected model with variable speed is established. The model is divided into two separate subsystems which are composed of LCC-HVDC and the rest of the power system. When two open-loop modes of two subsystems are close to each other, a special condition of near strong mode resonance (NESMOR) will take place, and strong dynamic interactions occur between subsystems. Under the NESMOR condition, one of the closed-loop modes is on the right of the corresponding open-loop mode and the damping of the closed-loop mode will be wicker, thus the small signal stability of the system will degrade. A method is proposed to predict the positions of the closed-loop modes based on the open-loop state space equations,also an index and analysis process of evaluating the stability of the system are given. In addition, the impact of the length of the tie-line between the infeeds is studied. Finally, simulation study of example power system is presented to verify the accuracy of the proposed method.

Switching model is one of the key factors that affect the accuracy and efficiency of electromagnetic transient (EMT) simulation of power electronic systems. For the numerical oscillation problem of classical LC binary equivalent switch model, an improved method has been developed by adding a pair of virtual resistors with the same resistance and opposite symbols to the equivalent circuit and applying the exponential difference method. The new method has the advantages of invariable system admittance matrix, easy selection of equivalent parameters, and good numerical stability. Based on the proposed method, a Simulink toobox is developed to automatically generate the circuits’ calculation matrix according to the Simulink block diagram. An application example of a power electronic transformer is completed. Compared with the Simulink/SimPowerSystem model, the steady-state error of the proposed method is less than ±2%, and the simulation speed can be increased by tens of times, which shows practical value in the EMT simulation of multi-switch power electronic systems.

The two-terminal hybrid HVDC transmission technology with line commuted converter on the rectifier side and modular multilevel converter on the inverter side can reduce the risk of commutation failure and possesses the advantages of low cost and small loss meanwhile, which has a broad application prospect. In HVDC transmission system, single-pole to ground fault of transmission line is one of the most common fault types. Single-pole to ground fault analysis is of vital importance. This paper introduces an analytical method of fault characteristics of two-terminal hybrid DC transmission system in the case of single-pole to ground fault. It aims at the fault detection stage when electromagnetic transient stress changes sharply, uses the differential and common mode transformation methods to solve coupling problem of transmission line, the asymmetry problem of DC fault and other nonlinear problems, establishes the model of each part of the system, and finally gives an analytical result of the system fault characteristics. Simulation verification is carried out in PSCAD/EMTDC, and the coincidence between analytical results and simulation results is more than 95%.

Multi-terminal hybrid UHVDC system can provide a larger capacity, more economical and more flexible transmission mode, taking into account the technical advantages of LCC and VSC. Compared with the two-terminal DC protection system, the DC line protection is the most different in multi-terminal UHVDC system. Hybrid modular multilevel converters (MMC) based VSC comprising of half-bridge (HBSM) and full-bridge sub-modules (FBSM) has the characteristics of fault self-clearance of HVDC lines. Therefore, the demands for DC line protection in this system are different from multi-terminal VSC-HVDC systems with HBSM topology. Based on the requirements for DC line fault identification and clearing of multi-terminal hybrid UHVDC transmission system, the key problems of DC line protection in the system are analyzed. DC line protection configuration and the coordination strategy of DC line protection are also proposed in this paper. Finally, based on PSCAD/EMTDC simulation, the feasibility and validity of the proposed scheme are verified.

The configuration and engineering calculation of DC transfer switch are analyzed by considering the characteristics of ‘multi-terminal’ and ‘hybrid’ of multi-terminal UHVDC system. Aiming at the problem of complex distributional relationship caused by “multi-terminal” characteristics, a new transfer current calculation method of metallic return transfer breaker (MRTB) and earth return transfer breaker (ERTB), which avoids complex mathematical calculations by combining the random number theory and simulation modeling, is mentioned. The obtained approximate conversion current maximum value can be applied to the configuration of engineering switch oscillating circuit and arrester parameters. Aiming at the "hybrid" characteristics, neutral bus switch (NBS) is not suggested installing into the MMC converter station, its original protection function can be replaced by blocking MMC converter or expanding the protection scope of neutral bus differential protection. The conclusion is verified through simulation models.

In order to meet the increasing cooling demand of high-power power electronic devices, this paper proposes the natural circulation boiling cooling system. This system utilizes phase change process for efficient cooling and uses natural circulation to drive the cooling medium. Based on the theoretical basis of two-phase flow and heat transfer, a homogeneous flow system simulation model is established. The typical representative converter valve of high-power power electronic device is taken as an example to design the natural circulation boiling cooling system of the converter valve. On this basis, the experimental platform of the natural circulation boiling cooling system of the converter valve is built. The simulation and experimental results show that the homogeneous flow model can accurately reflect the two-phase flow process of the system, and the temperature control effect of the natural circulation boiling cooling system is implementable.

With the implementation of energy strategy and the construction of a strong smart grid in our country, hybrid multi-terminal UHVDC transmission technology is rapidly developing and will be an important part of the China power grid in the future. Therefore, the accurate and quantitative evaluation of reliability of the hybrid multi-terminal UHVDC transmission system is of great significance to the safety and reliability of power grid. Based on this, the typical structure and operation modes of conventional HVDC systems and flexible HVDC systems are combed, and main wiring diagrams of hybrid multi-terminal UHVDC transmission system composed of aforementioned two systems are further studied. For each wiring diagram, according to the effect of main component failure and its position in primary connection, the HVDC system is divided into several subsystems such as AC filter subsystem and converter transformer subsystem, and then subsystems are modeled respectively. Finally, based on the state enumeration method, evaluation reliability model of hybrid multi-terminal UHVDC transmission system combined by the subsystem reliability evaluation models is established. Take Wudongde hybrid three-terminal HVDC transmission project as an example, the reliabilities under seven possible wiring diagrams in the project are evaluated and compared, the reliability sensitivity analysis of the failure rate of HVDC transmission line is also carried out.

In order to analyze the receiving-end AC side's harmonic interaction mechanism of the converter station in the hybrid multi-infeed HVDC transmission system, the harmonic impedance model of hybrid multi-infeed HVDC transmission system is established. Harmonic multi-infeed interaction factor (HMIIF) is proposed to measure harmonic interaction. The results of analysis based on the harmonic impedance matrix show that, the law of harmonic interaction between LCC-HVDC and MMC-HVDC is basically the same as that between different LCC-HVDC systems. The harmonic interaction between LCC-HVDC and MMC-HVDC changes with the harmonic impedance of MMC's converter in the AC side and it increases with the decreases of the coupling impedance between DC placements; the harmonic interaction of other DC systems to a certain DC decrease with the growth of the DC's equivalent harmonic impedance of AC subsystems; the harmonic interaction of this DC system to other DCs increases with the growth of the DC's equivalent harmonic impedance of AC subsystems. The index HMIIF and harmonic interaction between two DC systems are inversely proportional relationship. Finally, The simulation results of a hybrid three-infeed HVDC transmission test system built by PSCAD/EMTDC verify the correctness of the conclusions.