The fast dc line protection is one of the key techniques for the multi-terminal flexible dc grid whose main technical difficulty lies in quick and reliable fault identification. The traditional protection principles for the ac system are not suitable for the flexible dc grid, and those for the conventional dc transmission system can also not be applied directly. Therefore, the frequency-domain traveling-wave boundary characteristics after faults are analyzed first in this paper to find the critical condition for distinguishing the internal and external faults as well as the forward and backward faults. And then, a novel single-ended transient-voltage-based protection strategy for flexible dc grid was proposed, which mainly consists of the fault identification criterion and the direction criterion. The protection scheme used wavelet transform to extract transient voltage energy. In order to eliminate the influences of the wavelet transform boundary effect and converter blocking, the principle to select the proper data window was proposed. Compared with the existing traveling-wave-based dc protection, the proposed method has a higher reliability and stronger ability to endure high transition resistance, which was verified by lots of simulation cases based on the PSCAD/EMTDC.

For droop control in voltage source converter based multi-terminal HVDC systems, the determination of droop coefficients is a key issue, which directly affects both power distribution and DC control performances. This paper proposes a novel design of droop coefficients considering the requirements of power distribution, DC voltage control and system stability. Considering the power margins of different converters, the ratio relationship among droop coefficients is established. Converters with larger power margins take bigger portion of power mismatch to avoid overload problem. Furthermore, the integral square error of converters DC voltage is adopted as the DC voltage control performance index, and optimization of droop coefficients to achieve coordinated DC voltage control of steady-state deviation and transient variation, is derived. Finally, the constraint of droop coefficients is established to guarantee the DC system stability after power disturbance. Case studies are conducted on the Nordic 32 system with an embedded 4-terminal DC grid to demonstrate the feasibility and effectiveness of the proposed droop control scheme.

Voltage source converter based multi-terminal HVDC (VSC-MTDC) technology has shown many advantages. However, following a large disturbance, the DC voltage and the output power of the VSC station with fixed droop control may hit their limits, thereby reducing the DC voltage control ability of the whole MTDC system. Here an adaptive droop control scheme is proposed on the basis of both the DC voltage deviation factor and the power sharing factor. It contributes to ensure the DC voltage and the power loading rate of each converter within their limits during large disturbances, and the power sharing capability of the whole MTDC system remain high. The working principle of the proposed adaptive droop control is discussed in details. Case study and simulations are performed based on the PSCAD/EMTDC model of a $\pm$320kV four-terminal VSC-HVDC system to validate the effectiveness of the proposed adaptive droop method.

The overhead line will be widely applied in the future flexible high voltage dc (HVDC) grid. Correspondingly, effective reclosing strategy which can identify the fault property must be configured for the dc circuit breaker (DCCB) to improve the system power supply reliability. However, if the DCCB is reclosed directly, large second overcurrent may occur to damage the vulnerable devices in the system. Therefore, the inherent voltage characteristic difference between different dc faults (nonpermanent or permanent) is analyzed in the paper. And then a novel DCCB reclosing strategy for the flexible HVDC grid is proposed. Compared with the method which recloses DCCB directly, the proposed strategy can avoid the second damage under permanent fault condition, because it can identify the fault property without completely reclosing the DCCB (only the residual dc current breaker is reclosed before fault property identification). Moreover, compared with the typical sequential auto-reclosing strategy, the proposed strategy significantly reduces the additional control and design requirements on the DCCB. Finally, simulation cases based on the PSCAD/EMTDC are carried out to verify the correctness of the theoretical analysis and the superiorities of the proposed reclosing strategy.

Medium voltage direct current (MVDC) distribution system has numerous advantages over medium voltage alternative current (MVAC) distribution system. However, two shortcomings of the existing hybrid direct current circuit breaker (DCCB) with load current switch (LCS), including high construction cost and high operating losses, have constrained the application of existing hybrid DCCBs in MVDC distribution system. To overcome these two shortcomings, a novel multi-port hybrid DCCB with negative voltage source (NVS) has been proposed in this paper. First, the general topology and working principle of the multi-port hybrid DCCB with NVS are presented. Then, focused on current commutation method and economic performance, comparisons between different hybrid DCCBs have been carried out. The analysis shows that by using multi-port hybrid DCCB with NVS, the construction cost could be reduced dramatically and the operating losses could be reduced to be comparable with alternative current circuit breaker (ACCB) which are negligible. In the end, to fill the blank of experimental research, based on the topology of multi-port hybrid DCCB with NVS, a prototype with rating voltage of 2kV has been established, and the test result have verified the multi-port hybrid DCCB with NVS.

With the rapid development of multi-terminal direct current (MTDC) grids, high voltage direct current circuit breakers (HV-DCCB) capable of interrupting currents within 5ms are in urgent demand. The mechanical DCCB has always been one of the main proposals for HV-DCCBs. The current commutation is the prerequisite for mechanical DCCBs to interrupt currents successfully. Focused on the current commutation difficulty, a novel mechanical DCCB has been investigated in this paper. A simulation model of the novel mechanical DCCB in MTDC system for fault current interruption was established in PSCAD. By analyzing the simulation results, the detailed transient varying patterns of current, voltage and energy in the novel mechanical DCCB during its interruption process are revealed. To verify the feasibility of the novel mechanical DCCB, a prototype was developed in Lab., and current interruption tests were carried out on the prototype. Experimental results have shown a current of 5.3kA can be interrupted by the prototype within 3ms. In the end, failures that occurred in initial experiments are discussed and relative protection measures have been proposed. By adopting these measures, failures are successfully avoided and the normal operation of the novel mechanical DCCB are ensured.