When the conventional half-bridge modular multilevel converter based HVDC (MMC-HVDC) transmission technology is applied to overhead UHVDC transmission lines, it confronts challenges such as difficult to isolate DC faults, unable to operate under reduced DC voltage, and unable to realize the on-line bypassing/insertion of valve group. To overcome the above challenges, this paper presents a non-blocking overhead MMC-HVDC transmission technology. The proportion design theory of full-bridge power sub-module in the overhead MMC-HVDC transmission system is described in detail, and the control architecture of the MMC-HVDC transmission system is redesigned. On the basis of the traditional dq axis inner loop current control, the DC current control inner loop is added. The requirements of the new control architecture and the inadaptability of the conventional half-bridge control for the overhead MMC-HVDC transmission system are discussed. The new control architecture is described, and the validity of the no-locking control is verified by experiments.

To improve the uninterrupted operation capability of hybrid modular multilevel converter (MMC), the control strategy considering unbalanced grid fault is proposed in this paper. A control loop including ANF-PLL, quasi resonant part, low voltage current limiting and zero-sequence circulating current suppressing is added based on AC/DC decoupling control. The negative-sequence current is suppressed, the overcurrent of the arm is limited and the zero-sequence double frequency current is eliminated. Firstly, the converter model under unbalanced condition is introduced. The impact is analyzed based on the expression of power and arm current. Then the negative-sequence current suppression strategy, active power control strategy and circulating current suppression strategy are designed to ensure the safe and stable operation of hybrid MMC. Finally, a two-terminal MMC-HVDC model is built with PSCAD/EMTDC. The proposed control strategy is verified by simulations under an unbalanced grid fault. The uninterrupted operation of hybrid MMC is achieved.

When MMC convertor operates in island model, if the power is supplied to the unbalanced load, especially single-phase load, negative sequence current will be generated. However, the existing island control strategy of MMC converter does not support the power supply demand of unbalanced load. To solve this problem, a novel island control strategy for MMC converter is proposed in this paper, which is suitable for the demand of unbalanced island load power supply. The control strategy includes two parts: positive sequence control and negative sequence control. The output of positive sequence control is the positive sequence component of modulation wave, which control the voltage amplitude of the PCC point. The output of negative sequence control is the negative sequence component of modulation wave, which balance the voltage drop on the impedance generated by negative sequence current, to make the negative sequence component of PCC point voltage zero. In this way, the PCC voltage will meet the requirements of national standard. Finally, a two-terminal MMC-HVDC system was set up in the platform of MATLAB/Simulink, and the effectiveness of the control strategy is verified.

Submodule voltage balancing algorithm of modular multilevel converter based high voltage direct current(MMC-HVDC) is important for the operation of modular multilevel converter (MMC). With the increase of the voltage level of MMC-HVDC, the number of submodules per single bridge arm increases correspondingly, and the great challenges have been posed to the effective execution of the voltage balancing algorithm. In this paper, a fast sorting algorithm based on dynamic range segmentation is proposed. Accor-ding to the finite and changing rules for the distribution range of the sorted elements, this algorithm achieves the fast dynamic range segmentation sorting and balances the field programmable gate array (FPGA) resources consumption by introducing the dynamic range segmentation method and using the rich random access memory RAM resources in the FPGA. The characteristics of the proposed met-hod and other voltage balancing algorithms in sorting accuracy, time consumption and resources consumed by FPGA are compared and analyzed. Finally, the real-time simulation is performed on the RT-Lab, and the simulation results verify the validity and correctness of the proposed dynamic range segmented sorting algorithm.

Due to the small system inertia and fast fault growing speed of DC transmission system, and there is no zero cross point for DC current, demands from DC transmission system on the breaker’s ability are rigorous. Based on the upcoming DC circuit breaker installation project for the Nan’ao three-terminal VSC-HVDC system, the world’s first mechanical DC circuit breaker for engineering application is researched. New control and protection strategies for the VSC system with DC circuit breaker are analyzed, some functions such as online opening/closing operation and fault isolation are added. Real-time simulation test is conducted by connecting the physical controller and protection devices to the simulation testing platform, and a series of tests on system characteristics and control-protection strategies are carried out. Typical faults in Qing’ao station are analyzed. Results show that by installing the DC circuit breaker, faults on Qing’ao station and Qinghui line (from Qing’ao station to Jinniu station) can be effectively isolated and tripped, while the other two stations are not affected. The paper provides a theoretical and experimental basis for drafting on-site project commissioning scheme. Futuremore,artificial negative pole to ground fault is also tested on site during commissioning, which fully verifies the reliability of DC circuit breaker in tripping a DC fault.

When a monopolar grounding fault occurs on DC line in Nan’ao multi-terminal VSC-HVDC project, it is necessary to ensure that the mechanical DC circuit breakers (DCCBs) can isolate the fault effectively and the non-fault stations can keep normal operation. Firstly the basic principle of the mechanical DCCBs recently installed in Nan’ao is introduced. Secondly, the line fault characteristics, the existing control and protection strategies, and the tripping characteristics of DCCBs are analyzed. Subsequently, a DC line differential protection with voltage criterion is put forward to locate the fault line quickly, and a method of switching setting values of protections in non-fault stations is presented to keep the normal operation of non-fault stations. Finally, the results of simulation in PSCAD/EMTDC and artificial short circuit test in Nan’ao project verify the feasibility of the strategy.

Based on Wudongde multi-terminal UHVDC demonstration project, the control strategy of multi-terminal hybrid UHVDC transmission system is researched to adapt to the characteristics of complex operation and high reliability requirements. The control mode and starting-up/shutting-down approach applied to multi-terminal hybrid UHVDC system are proposed, which could realize the matching of external characteristics of hybrid DC, two/three terminal stations starting-up/shutting-down, and the third terminal station online inputting/quitting. Also, the steady state control strategy of each terminal and multi-terminal coordinated control strategy are also studied, which could adapt to the complex operation of multi-terminal hybrid HVDC transmission system. Finally, the simulation results based on PSCAD/EMTDC show the validity of proposed control strategy, verifying the reliable and stable operation of multi-terminal hybrid UHVDC transmission system.

With the increase of the number and capacity of MMC-HVDC projects, the multiple MMC-HVDC feeding into weak AC system will be a very common scene in the future reginal power grid. This paper studies the control strategy for such scenes. In order to make the MMC station have the same characteristics as the synchronous generator, the synchronous generator emulation control (SGEC) is proposed. The control method consists of three loops, namely active power and frequency loop (PF loop), reactive power and voltage loop (QU loop), and inner current loop. The PF loop is used to simulate the swing process of the synchronous generator (SG). The QU loop is used to simulate the exciter system of the SG. The inner current loop serves to limit fault currents and increase MMC response speed. For the multi-infeed MMC-HVDC system, a novel control of dynamic power allocation is proposed. This control scheme can not only allocate the power among MMC stations precisely, but also achieve the secondary frequency regulation. Finally, the effectiveness of the proposed control strategy is verified through the PSCAD/EMTDC simulation of improved 3 machine 9 node system.

With the development of DC transmission and renewable energy technology, it has become a trend to construct a larger scale DC transmission network. At present, with the rapid development of multi-terminal VSC-HVDC transmission technology, the characteristics of DC circuit breaker, the core equipment, directly determine the reliability of DC power grid. Based on the above situation, a hybrid DC circuit breaker based on inductance coupling circuit is proposed, its basic principle is introduced, the mathematical model of inductance coupling circuit and the design method of power electronic components are given. Finally, aiming at the topology of hybrid DC circuit breaker designed in this paper, a simulation model is built in PSCAD/EMTDC. The feasibility of the technical scheme of DC circuit breaker based on inductance coupling circuit and the correctness and validity of the test circuit are verified by the prototype test. The switching-off current of the prototype is more than 9 kA.

Taking the new hybrid high voltage DC circuit breaker based on the forced commutation of the coupled negative pressure circuit as a research object, the circuit topology, working principle of the new DC circuit breaker are introduced. A modular control protection system based on high-speed fiber-optic communication and current, voltage, and time control protection strategies with high-speed fault detection are designed. The time sequence of opening and closing control of circuit breaker and its corresponding fast fault protection strategy are described in detail. Based on the PSCAD/EMTDC simulation platform, a circuit breaker simulation model is built. The simulation results show that the control protection strategy can effectively break the DC current, the fault clearing speed is fast, the dynamic characteristics are good during the breaking process and the all parts of the power electronic devices are reliably protected. The correctness and feasibility of the control strategy and fault protection strategy of the proposed hybrid DC circuit breaker are verified.

The most basic and important function of DC circuit breaker is to break the short circuit current in time under the condition of short circuit fault. In order to effectively evaluate the breaking capacity of DC circuit breaker, it is necessary to simulate the harshest short-circuit current breaking condition in practical project. A topology structure of an equivalent breaking test device is presented based on the characteristics of hybrid DC circuit breaker. The topology composition, operation principle, stress analysis of key components, and calculation method of electrical parameters are also discussed. The research results can be used as the theoretical basis for the development of test equipment platform and the related work of DC circuit breaker breaking test.

A hybrid HVDC system based on the modular multilevel converter integrated with DC circuit breaker (IDCB-MMC) and line commutated converter (LCC) is proposed. When DC short-circuit fault occurs, by coordinating the operation of the converter and the DC circuit breaker, the DC fault current can be forced to transfer to the energy absorbing branches which are parallel connected across the DC bus. Therefore, the bulky and costly solid power electronics switch branch in conventinal hybrid HVDC breaker can be discarded. The LCC can clear the DC fault current by switching into the inverter mode. Simulations are carried out on one pole of a two-terminal 1 000 MW/±320 kV LCC-MMC hybrid HVDC system. The simulation results show that the DC fault current can be cleared by the IDCB-MMC and the power transimission can be automatically recovered. Simulation results also verify the feasiblity of the hybrid HVDC system composed of LCC and IDCB-MMC.

Temporary short-circuit accidents of transmission lines will increase greatly in ±500 kV MMC-HVDC overhead transmission system, which need the DC circuit breakers (DCCBs) reclose to ensure the reliability of power supply. However, the transient operation characteristics for reclosing of DCCB are not yet clear. A ±500 kV pseudo bipolar MMC-HVDC overhead transmission system model is established, which includes the key equipment of converter station and the hybrid DCCB. The impact of the electromagnetic transient characteristics and key parameters of hybrid DCCB on the breaking and reclosing characteristics during the breaking and reclosing process is studied when a single pole occurs short circuit fault to ground. The simulation results show that the hybrid DCCBs could cut off the fault in a short time (less than 5 ms). Because the converter valve is not locked, the current of valve side of converter transformer would not reduce to 0 after the breaking of DCCB. After 150 ms of fault deionization, the current of valve side of converter transformer and the pole voltage would rise to the rated operating condition in 200 ms with oscillating condition. In addition, the breaking performance of DCCB reduces with the increasing of the capacitance of RCD branch, while the current transfer rate of solid branch and mechanical branch is not related to the capacitance of RCD branch. The results of this paper can provide the corresponding data reference for the performance check of circuit breaker of ±500 kV MMC-HVDC overhead transmission lines.

The energy of the AC grid is mainly stored in synchronous generators in the form of kinetic energy with a character of big inertia. In the study of AC grid stability, AC network is usually modelled as algebraic equations. While in DC grid, the energy is primarily stored in converter’s capacitor, as well as the DC network’s capacitor and inductor. The storage capacity of the DC network and the converter is even superior to the generator’s storage capacity in AC grid. Therefore, in the study of the DC grid stability, it is not proper to simplify the DC network as algebraic equation, but to take the dynamic differential equations of the DC network into consideration. In order to model any complex DC network precisely, a method using extended differential algebraic equations (DAE) to deduce the dynamic modelling of DC network is proposed. This method can eliminate the redundant state variables in the dynamic modelling of the DC network. Moreover, it can obtain the dynamic modelling of the DC network automatically through the submatrix junction and multiplication, thus save a lot of time in comparison to deduction of the state space system matrix manually. The accuracy of the method proposed is verified on a four-terminal DC network by comparing with the electromagnetic transient simulation and the small-signal simulation.

In view of the stability problem of voltage source converter based high voltage direct current(VSC-HVDC) transmission system, the state space model of two-terminal VSC-HVDC is established under different control strategies of voltage source converter(VSC), and the oscillation modes of VSC-HVDC are comprehensively analyzed. The eigenvalue method is adopted to compute eigenvalue and damping ratio of 23 ranks state matrix of system without including phase-locked loop (PLL), and the main oscillation modes are screened out through mode frequency and participation factors. The results show that the system mainly contains the subsynchronous oscillation(SSO) modes of DC lines, SSO modes of q-axis current and supersynchronous oscillation modes of d-axis current in the AC side of converter controlled with a constant active power. When the active power controller of converter is controlled with a constant active power, the constant alternating voltage controller is better than the reactive power controller considering least oscillation modes and maximum damping ratio; when VSC is controlled with a constant direct voltage, the effects of the constant alternating voltage controller and the constant reactive power controller on the oscillation mode are basically the same; most oscillation modes of VSC-HVDC are mainly related to the constant direct voltage controller and the state variables of VSC controlled with a constant active power, which are affected by the control loop of the converter.

Running hybrid HVDC transmission system in the case of communication failure will face a high security risk. An internal station fault at MMC side without communication may cause the capacitance voltage of the submodule to exceed the safety margin. A new solution is proposed in this paper, in which a bypass thyristor is connected in parallel at the sub-module port. The bypass thyristor will be triggered and a designed DC short-circuit fault will be constructed artificially. The DC short circuit fault will be identified by the DC line protection system at the LCC side, and the GS control strategy will be triggered to suppress the over-voltage of the submodule. Finally, a hybrid DC transmission model based on PSCAD is built, and the effectiveness of the scheme is verified by simulation, which ensures that the hybrid HVDC system can be run safely in the case of communication failure.