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.

In DC distribution networks, the uncertainties of renewable energies and loads cause frequent voltage fluctuations. In this paper, a voltage regulation strategy based on the combined coordination of centralized optimal control (COC) and adaptive droop control (ADC) is proposed for droop-controlled DC distribution networks. In the COC strategy, the operation reference power of voltage source converters (VSCs) is optimized using the bus injection power measurements to improve the voltage profile and balance the power loading rates of different VSCs. The VSC capacity margin constraint is established to ensure that the capacity margin of each VSC is sufficient to cope with the unexpected fluctuation of loads and renewable energies. Then, an ADC strategy is adopted to handle voltage fluctuation between adjacent two COCs. In the ADC strategy, the operation reference power of the VSCs is further adjusted to recover the voltages. The voltage regulation is achieved based on the relationship among the VSC voltages and the minimum/maximum voltages of the whole network, which does not depend on the information of each nodal voltage and thus real-time network communication is avoided. The simulation results show that the proposed voltage regulation strategies can effectively improve the voltage profile under various uncertain environments.

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.

Modular multilevel converter (MMC) based medium voltage DC (MVDC) distribution system has become a promising option in the future smart grid architecture due to many distinguishing features. However, MMC-MVDC distribution system is prone to face the short-term outage problem during bipolar DC faults, so how to implement DC fault ride-through (FRT) capability for MMC-MVDC is still a challenge. This paper intends to change the redundant half-bridge submodules (HBSMs) of the HB-MMC into the redundant full-bridge submodules (FBSMs), thus, a practical DC FRT method is proposed by using the combination of the hybrid MMC and the hybrid DC circuit breakers (CBs) for the MMC-MVDC system. Then, the DC fault current characteristics of the MMC with the proposed FRT method are discussed. Lastly, data analysis and simulations of a three-terminal MMC-MVDC system have been performed in PSCAD/EMTDC to validate the effectiveness of the proposed DC FRT method. The proposed DC FRT method is not only easy to implement, but also reduces the requirements and the investment of the DC CB.

The isolated bidirectional DC/DC converter (IBDC) plays an important role in DC buses connection, voltage conversion, electrical isolation and bidirectional power transmission for DC distribution networks. In this paper, a multiple modular IBDC scheme based on hybrid- dual-active-bridge (HDAB) for medium-voltage (MV) and high-voltage DC (HVDC) distribution network is proposed. In the proposed HDAB, the switches in diagonal position of full-bridge are replaced by half-bridge sub-modules (SMs), the proposed HDAB maintains advantages of conventional DAB and achieves bidirectional fault handling capability, avoiding the application of large DC-link capacitors and DC circuit breakers, reducing the volume and construction cost. Besides, high-frequency-link (HFL) voltage regulation strategy is also proposed in this paper, adjusting the HFL voltages in HDAB to match the HFL transformer voltage adaptively, thus improving the efficiency and applicability of IBDC under the fluctuation of power flow and DC bus voltage. The topology, operation principle, control structure and corresponding bidirectional fault handling and HFL voltage regulation strategy of proposed IBDC scheme are investigated. The experimental results verify correctness and effectiveness of proposed scheme and control strategy, promoting the adaptability and practicality for both IBDC and DC distribution network.