A hybrid modular multilevel converter （MMC） with full-bridge and half-bridge submodules can clear the DC fault through either converter-blocking or active-clearing approaches. However，there is still a lack of comprehensive analysis and comparison on the difference of fault-clearing characteristics，parameter design considerations and applicable occasions between the two approaches. Accurate dynamic models for the fault-clearing process of the converter-blocking and active-clearing approaches are established in this paper，respectively. Based on the established models，the effects of the distance of overhead lines and the proportion of full-bridge submodules on the fault-clearing time and the capacitor over-voltages are analyzed. The control strategy of the DC-link voltage and the submodule capacitor voltage for the active-clearing approach during fault-clearing process is proposed. Taking a 1 500 km long-distance DC transmission system based on hybrid MMCs as a case-study，the DC fault clearing process and performance of two fault-clearing approaches are compared through theoretical analysis simulations for various conditions. The main limiting factors of the two fault-clearing approaches are revealed and the principle for selecting DC fault-clearing strategies and the methodology for designing parameters of hybrid MMC are proposed.

Multi-terminal line commuter converter-high voltage direct current （LCC-HVDC） transmission can realize power supply from multiple energy bases or power receiving in multiple load centers，thus it is a flexible and economical technical scheme. For multi-terminal HVDC，online entry and exit of the third converter station is the key strategy to realize flexible switching of operation modes. Based on the technical roadmap of high speed switch，the online entry and exit strategy of the third converter station in multi-terminal LCC-HVDC system is studied，the advantages and disadvantages of strategies for different strategies are compared and analyzed from the perspectives of the performance of high-speed parellel switch，the impacts on equipment，and the power loss of AC system. In order to reduce the operation overstress of the equipment and create good conditions for the opening and closing of high speed switch，this paper proposes a strategy for online entry and exit of the third converter station in multi-terminal LCC-HVDC. Finally，the feasibility of the proposed strategy is verified by the field test results.

The Tangjiawan three-terminal flexible DC distribution network project successfully applied the world's first ±10 kV three-port hybrid DC circuit breaker，which can realize the connection of multi-terminal converter stations and rapid fault isolation，thus ensuring the safe and reliable operation of the system. the RTDS simulation system is designed for the topology of the three-port hybrid DC circuit breaker in the Tangjiawan three-terminal flexible DC distribution network project in Zhuhai，combined with the control and protection FPT test requirements of the DC transmission project to design the external interface，and finally based on the hybrid DC circuit breaker breaking strategy，the Path2 hybrid DC circuit breaker breaking logic and commutation process in the three-port hybrid DC circuit breaker are simulated and verified. The simulation test results show that the RTDS simulation system can break the fault current of 2 kA within 3 ms. At present，the three-port hybrid DC circuit breaker RTDS simulation system is connected to the Tangjiawan three-terminal flexible DC distribution network project （physical layer） control and protection RTDS test system.

In order to make suggestions on the selection of starting resistance and the starting strategy of in the Kunliulong transmission project. This paper first introduces the start-up circuit of the flexible converter station. Then the startup loops under different topologies are analyzed，and the startup characteristics under different topologies are obtained. Finally，for the flexible converter station in Kunliulong transmission project，a PSCAD/EMTDC model is built to simulate the impact energy，and the results obtained are consistent with theoretical calculations. According to the simulation results，the requirements for the starting resistance shock absorption energy of the flexible converter station in Kunliulong transmission project and the system starting suggestions are proposed.

The hybrid multi-terminal DC transmission system realizes operation mode conversion，fault isolation and protection functions through the cooperation of various DC circuit breakers，and studying the action characteristics of DC circuit breakers after various faults is of great significance for discovering potential risks of the system. This paper constructs the Kunliulong （Kunbei-Liuzhou-Longmen） UHV three-terminal hybrid DC system and DC circuit breaker PSCAD/EMTDC simulation model，traverses the typical faults on the AC side，inner station and DC side of the system，and analyzes the operating characteristics of the circuit breaker in different fault recovery processes. The most serious fault state of each circuit breaker is obtained. The research results will help to find the weak parts of the system，and provide reference and basis for the system operation and maintenance and dispatching.

In Jiangsu power grid，the simultaneous commutation failure of multiple DCs will seriously threaten the stable operation of the power grid. A coordinated control strategy for commutation failure recovery of multi-infeed DC systems with hybrid cascade DC system is proposed. Firstly，by analyzing the limitations of multi-infeed effective short circuit ratio （MIESCR），the multi-infeed operating short circuit ratio （MIOSCR） is proposed，and the recovery sequence coordination principles that should be followed for the commutation failure recovery in multi-infeed DC systems is summarized. Then，based on MIOSCR，a two-stage coordination control strategy for the commutation failure recovery of multi-infeed DC systems with hybrid cascade DC system is proposed to reduce the number of commutation failure. Finally，taking Jiangsu power grid as an example，the simulation results in PSS/E show that the proposed strategy can effectively reduce the number of continuous commutation failure and shorten the time of commutation failure recovery in multi-infeed hybrid DC systems.

Large capacity ultra high voltage （UHV） multi-terminal hybrid DC involves many technical characteristics，such as multi-terminal，hybrid，double valve group and so on，the design is difficult and the operation characteristics are complex. Combined with the system characteristics and stability strategy of China Southern Power Grid，firstly，the AC/DC fault ride through strategy suitable for Kunliulong UHV multi-terminal DC project is proposed，and the fault-ride-through and recovery characteristics are clarified. Then the actual operation of the project is analyzed，the key operation control performances such as DC line fault ride through characteristics，AC fault ride through characteristics and on-line exit characteristics of the third station are analyzed. The actual operation characteristics have fully verified the effectiveness of the multi-terminal hybrid UHVDC key operation control strategy，and the DC system owns good recovery performance under various fault conditions，which effectively ensures the safe and stable operation of the multi-terminal hybrid UHVDC project under various fault conditions. The research can guide the actual operation of multi-terminal hybrid UHVDC and provide reference for the subsequent control function design of multi-terminal hybrid UHVDC.

High-resistance faults reduce the sensitivity and reliability of the main protection of flexible DC lines，and seriously affect the safety of the system. In order to solve this problem and further improve the reliability of the protection，this paper proposes a new principle of single-ended protection based on the DC reactor voltage Manhattan distance. Firstly，the reasons for high resistance fault to affect the protection selectivity are analyzed，and the shortcomings of the existing DC reactor voltage protection principle are revealed. Then，by analyzing the transient characteristics of the DC reactor voltage and the blocking effect of the DC reactor，the reasons for the difference in the voltage waveform of the DC reactor during the internal and external faults are explained，and the Manhattan distance is introduced to describe this difference. In this way，an overall protection scheme based on DC reactor voltage Manhattan distance is proposed. This scheme can detect faults only by measuring single-ended DC reactor voltage，which is easy to implement and has good economic efficiency. Finally，a DC transmission system model is built in PSCAD to verify the feasibility and superiority of the protection scheme.

UHVDC multi-terminal hybrid transmission system adopts a double valve group series connection，and the failure of a valve group will have a great impact on the system. After a single valve group fails，we hope to minimize the impact of the failure，ensure the sustainable operation of the system，and improve the overall operating time. The control system must react quickly to remove the failed valve group to avoid affecting the normal operating valve group. The valve group failure exit control strategy in converter high-voltage DC （LCC-HVDC） projects has been widely used in various projects，but voltage source converter ultra-high-voltage DC（VSC-UHVDC） systems and UHVDC multi-terminal hybrid systems have no previous engineering application in the industry. Therefore，it is necessary to research valve group failure exit control method for UHVDC multi-terminal hybrid system. This paper proposes a fault-valve group exit control method based on the hybrid bridge modular multilevel converter（MMC） valve，a hardware closed-loop test is performed on the RTDS test platform. The RTDS test results prove that the control method proposed in this paper can quickly isolate the failure valve group and adjust the operating state of the system after the failure of a single valve group，without causing the entire system to be shut down.

Due to the limitation of the breaking capacity，speed and cost of the DC circuit breaker，the DC power grid faces the problem of fault isolation. This paper proposes a hybrid fault current limiter （FCL） based on commutation capacitors. After fault，the current limiting branch will be put into the fault loop by commutation capacitors to suppress the fault current. The current limiting branch consists of thyristor，commutation capacitor，current limiting resistor and current limiting inductor. In this paper，the topology structure of the current limiter is given，and its current limiting principle in different stages of the fault is analyzed； Kirchhoff's law is used to list the loop equations of the commutation capacitor in different stages，and the current limiting effects under different parameters are theoretically analyzed so as to be the theoretical basis for parameter design. The four-terminal DC power grid model is built in PSCAD/EMTDC to verify its current limiting effect. The simulation results show that the proposed FCL can effectively slow down the discharge speed of the sub-module capacitor，so that the rising rate of short circuit current at 5 ms after the fault decreases from 2.254 kA/ms to 1.015 kA/ms，the increase of fault current is effectively suppressed and the breaking capacity of the DC circuit breaker is reduced.

Kunliulong UHVDC is the world's first hybrid multi-terminal UHVDC project in which the VSC station adopts the series structure of high and low converter. Before starting，it needs to charge the high and low converter in turn. As for the tripping event of Liuzhou converter station during charging process of high and low values /value group，this article first points out that the later charging converter jassively charged by DC side is a special phenomenon of VSC-UHVDC. Combined the mechanism analysis of this special phenomenon and the charging logic of valve control system，the reason of tripping event is found. Finally，based on the above analysis，the corresponding improvement measure is put forward and verified through RTDS simulation. The proposed scheme has been successfully applied to Kunliulong HVDC project. It has provided a guarantee for projects debugging and subsequent safe and stable operation，which also has an important reference value for future VSC-UHVDC projects.

The VSC-HVDC transmission system has been widely used in power system of China. The reliability of its core equipment，the converter valve，directly affects the safe and stable operation of the power grid. With the emergence of harmonic oscillations in VSC-HVDC projects around the world，in the type test，in addition to meeting the experimental items specified in the IEC standard，the harmonic tolerance capability of the converter valve also needs to be assessed. For this reason，a harmonic current injection control method is proposed based on the converter valve type test platform. An equivalent model of the current control circuit is established，which is compared with the conventional current control strategy based on a proportional-resonance （PR） regulator. By comparison，it proves that the proposed harmonic current control method based on the second-order low-pass filter can significantly improve the amplitude-frequency characteristics of the current control loop and eliminate the steady-state fluctuations of injected harmonics. Finally，the PSCAD software is used to simulate and verify the proposed current control method，and the control performance of the steady-state harmonic current based on the conventional PR regulator and the second-order low-pass filter is compared，which shows the effectiveness of the proposed current control method. Further，test verification is carried out on the converter valve type test platform. In the steady state，the harmonic current tracks the given value，which can meet the engineering verification needs of the converter valve harmonic current stress.

According to the system requirements of multi-terminal upgrading and reconstruction of LCC HVDC power transmission system，this paper studies the topology structure and power transmission mode of multi-terminal HVDC transmission system. A reconstruction method based on the application of DC high-speed switch （HSS） in the intermediate converter station is proposed，and the basic ideas and specific measures of DC field transformation project under extremely site constraints are discussed. In addition，this paper demonstrates the advantages of three-dimensional design in converter station engineering in multi-terminal reconstruction project. This research is based on the first three-terminal transformation HVDC transmission project in China to use the original LCC HVDC transmission channel，and its system design and engineering practice are of typical significance.

Analysis is carried on the risks of the DC system with common ground electrode in the process of monopolar metallic and ground return mode conversion，including the impact on equipment，such as converter transformers and switches，as well as control and protection system. A new wiring solution for solving the influence is proposed to install a current-limiting resistor and its bypass switch between the high-speed grounding switch （HSGS） and the grounding grid of the converter station. The resistance design method of the grounding current-limiting resistor is also given. The limited current and the neutral line voltage in the process of DC operation mode conversion are proposed based on theoretical analysis and verified by commissioning. The influence of the grounding method on the DC system during the fault transient conditions are analyzed by PSCAD/EMTDC simulation. The results show that the solution can effectively solve the mutual influence of the common grounding electrode on the converter station during the conversion process of the DC operation mode，and does not affect the steady-state and transient stress requirements of the polar line and neutral line equipment. For the DC control and protection design，only the sequence control needs to be modified for the built project，and the protection configuration does not need to be modified.

As the resources of transmission corridors become increasingly scarce，UHV hybrid multi-terminal DC transmission systems are expected to become the mainstream trend in the development of DC transmission systems. Reliability assessment of them has become a very important part of power system planning，design and operation. First，the reliability evaluation model of the UHV hybrid multi-terminal DC transmission system is established，and then the reliability evaluation algorithm based on the characteristic parameter matrix is proposed to realize the reliability evaluation of UHV hybrid multi-terminal DC transmission system. Finally，based on relevant calculation examples，the proposed method and conventional method are used to evaluate and analyze the reliability respectively. According to the comparison of the reliability evaluation calculation results，the efficiency and accuracy of the proposed method are verified. For complex UHV hybrid multi-terminal DC transmission，the system has good applicable value.

Flexible DC transmission technology can build a flexible and efficient power grid structure，with the advantages of flexible control and self-replacement. It represents the development and research direction of DC transmission field，and has been widely promoted and applied at home and abroad. This paper investigates the development status of equipment reliability standards in the field of flexible DC transmission in China，combs the reliability requirements of flexible DC engineering，and analyzes the current construction of reliability standards in this field. Based on the current status of reliability standards in non-flexible HVDC transmission，the preliminary assumptions for the construction of reliability standards for flexible HVDC transmission systems are proposed，which provides a reference for the construction of reliability standard systems for flexible DC transmission equipment in China.