In a multi-terminal high-voltage direct current (HVDC) system, DC circuit breakers (DCCBs) are conventionally connected in a star configuration to enable isolation of a DC fault from the healthy system parts. However, a star connection of DCCBs has disadvantages in terms of loss, capacity, reliability and so on. By rearranging the star connection, a novel delta configuration of DCCBs is proposed. The total loss of the proposed delta configuration is only 33.3% of that of star configuration, yielding a high efficiency. Moreover, any DC fault current is shared between two DCCBs instead of one DCCB in the faulty branch suffering the fault current. Thus, DCCB capacities in the proposed delta configuration are only half of those in a star arrangement. Additionally, in the case of one or two DCCBs out of order, the power can still be transferred among three or two terminals, thereby affording high supply security of all HVDC links. On the basis of the DCCB delta configuration, two novel DC fault protection structures with external and internal DC inductances are proposed. Their characteristics are discussed and it is shown a DC fault can be isolated using slow DCCBs. The results demonstrate DC fault tolerant operation is achieved by using the proposed DC fault protection structures with delta configuration.

The high-voltage multi-terminal dc (MTDC) systems are foreseen to experience an important development in the next years. Currently, they have appeared to be a prevailing technical and economical solution for harvesting offshore wind energy. In this study, inertia mimicry capability is added to a voltage-source converter-HVDC grid-side station in an MTDC grid connected to a weak ac grid, which can have low inertia or even operate as an islanded grid. The presented inertia mimicry control is integrated in the generalised voltage droop strategy implemented at the primary level of a two-layer hierarchical control structure of the MTDC grid to provide higher flexibility, and thus controllability to the network. Besides, complete control framework from the operational point of view is developed to integrate the low-level control of the converter stations in the supervisory control centre of the MTDC grid. A scaled laboratory test results considering the international council on large electric systems (CIGRE) B4 MTDC grid demonstrate the good performance of the converter station when it is connected to a weak islanded ac grid.

A novel analytical approach to reliability evaluation of voltage-source converter-based multi-terminal direct current (VSC-MTDC) integrated offshore wind plants is presented in this study. To treat the multi-terminal grid precisely and efficiently, the approach combines the state evaluation method and network flow method. A DC line equivalent component is also proposed to consider the operation modes, failure clean measures as well as interactions between different DC lines of VSC-MTDC. Employing this approach, the authors can obtain both frequency and probability indexes of reliability. The usefulness and effectiveness of the proposed method are verified by case studies. The comparison between three popular schemes is also carried out.

Interconnected voltage-source converter based multi-terminal high-voltage DC (HVDC) systems provide better system redundancy, higher flexibility and capability of exchanging power between multiple areas. Recent developments in modular multi-level converter technology makes multi-terminal HVDC (MTDC) transmission more promising than before. This study provides an in-depth analysis of the power flow and transmission loss for MTDC systems with different grid topologies. A voltage–power droop control based algorithm is applied to solve the power flow problems. The main contribution of this study is the novel way of determining and calculating the transmission line losses for different MTDC network topology configurations. Three MTDC system topologies are investigated. Simulated case studies are used to observe the system power flow and transmission losses for all three topologies.

This study presents a ‘seek’ and ‘check’ approach to determine the operating area of the modular multilevel converter based high-voltage direct current (MMC-HVDC) transmission systems. First, the equivalent circuit for analysing the operating area of MMC-HVDC systems is derived and the operating area is represented by discrete operating points. Then, three operating constraints are proposed according to the MMC operation principles, i.e. the voltage, current and voltage stability constraints. Therefore, the determination of the operating area is transformed into checking whether the operating point meets the proposed constraints. The case studies are performed on a one-terminal MMC-HVDC system under six typical short circuit ratios. The results show that the top boundary of the operating area is limited by the voltage constraint, and the bottom boundary of the operating area is limited by the current constraint. The influence of the third harmonic injection and the ac impedance characteristic on the operating area is also studied. Lastly, the validity of the ‘seek’ and ‘check’ approach is proved by time-domain simulation results.

The modular multilevel converter (MMC) will be used in power systems more and more widely. The main circuit parameter design of the MMC impacts the initial investment and the operating performance of the system. In this study the parameter design problem of several important elements in the main circuit of the MMC, such as the link transformer, the arm reactor, the sub-module (SM) capacitor and the SM power electronic devices are analysed. Based on the fundamental equivalent circuit of the MMC, the design method for the rated valve side no-load voltage of the link transformer is proposed; and the variation range of the rated valve side no-load voltage is determined as (1.00–1.05)/2 times of the DC side voltage. By introducing the concept of the equivalent capacity discharging time constant H, the nearly constant relationship between the fluctuation ratio of the SM capacitor voltage and the H-value for different projects is discovered, which means the H-value is independent of particular projects. In this study, the corresponding parameters of several practical projects are analysed and the recommended H-value is given as 40 ms. The voltage and current stress of the SM power electronic devices is investigated for the four extreme operating conditions by the analytical formulae of the MMC; and it is discovered that the current stress of the SM power electronic devices changes greatly with the operating condition. By introducing the concepts of the phase unit series resonance frequency and the circulating current resonance frequency, the principle for designing the parameter of the arm inductor is established. Through analysing the corresponding parameters of several real projects, the recommended value of the phase unit series resonance frequency is given as the rated frequency of the connected AC system.

This study focuses on the possibility of modular multilevel converter (MMC) loss optimisation by modulation without any additional circuit, where a common-mode voltage is injected into a nearest level modulator. Owing to the common-mode voltage injection, the proposed modulation strategy increases the fundamental voltage. Therefore, for a given voltage, operation is possible with the submodule (SM) voltage of at least one SM inserted each arm. This reduces the number of SMs required, resulting in reduction of conduction losses and leading to significant cost savings and/or returns on investment for facilities. Furthermore, the injection of common-mode voltage also decreases the total SM transitions, resulting in reduction of switching losses. The proposed modulation strategy is valid under both normal and unbalanced ac fault conditions for MMCs. The common-mode voltage extraction method is provided and its harmonic components are extracted. Then, the SM saving capability of the injection-based nearest level modulation is discussed and the SM saving limit is presented in relation with modulation indexes. The optimisation of both conduction losses and switching losses is analysed. The effectiveness of the proposed modulation strategy under both normal and unbalanced ac fault condition is verified by simulations of a 101-level three-phase MMC in MATLAB/Simulink.

DC droop control strategy is usually used to improve load sharing in increasingly popular DC distribution networks. However, the conventional droop control strategy suffers from considerable terminal voltage drop and influences by line impedance. In this study, the authors propose a new robust droop control strategy to overcome these drawbacks. They first develop a mathematical model of the proposed robust controller and use the load terminal voltage as a feedback signal. They further treat the line impedance as part of the equivalent output impedance of individual power converters and minimise the inaccuracy of load sharing by regulating robust coefficients. They analyse the influence of the robust coefficients on the system stability. They have verified the robust control strategy with both power system computer-aided design/electromagnetic transients including DC simulation and control hardware in the loop semi-experimental method. It is shown that the robust concept improves the load sharing accuracy and suppressing the load voltage fluctuation.

The generation of offshore wind power is less predictable. This can cause the overload of offshore DC transmission system and thus requires the curtailment of wind power. To reduce the amount of wind power curtailment, a method of optimising DC power flow using DC power flow controller (DC-PFC) is proposed. The analytical expression of coordinating DC-PFCs and converters in controlling the power flow of the DC system has been created. Method has been developed to optimise the power flow of DC grids within which control setting changes automatically in different wind conditions to reduce both the power curtailment and power losses. The proposed method has been demonstrated and validated on a 9-port DC system. It is concluded that both the curtailment of wind power and power losses are effectively reduced by inserting DC-PFCs into DC grids.

Traditional low voltage high power wind energy conversion system (WECS) requires heavy cables and large step-up grid interfacing transformers. In recent years, power rating of offshore wind turbines already exceeds 10 MW, and medium voltage (MV) converters have become strongly demanded. Modular multilevel converter (MMC) is suitable for MV high power applications. However, there are two technical difficulties for MMC when applied in MV permanent magnet synchronous generator (PMSG) WECS. The first one is the large sub module (SM) voltage fluctuation caused by low frequency and high amplitude PMSG phase current. The other one is the low voltage ride through (LVRT) problem. Overall control strategy based on carrier-phase-shifted -sinusoidal pulse width modulation technique of the MMC WECS is proposed. For the first difficulty mentioned, a second-order circulating current injection method based on closed-loop control is proposed. For the LVRT problem, a distributed braking chopper solution is provided. The effectiveness of the injection method and the distributed braking chopper solution are validated by simulation of a 5 MW/3.3 kV MMC WECS. Influences of the current injection amplitude on SM voltages and arm currents are discussed and a trade-off method is given to optimise system design, which is also verified by simulation.

A high-power double frequency bidirectional symmetrical DC–DC resonant converter is proposed to reduce the sizes of the passive components and increase the cost-effectiveness. Inspired by the bipolar and unipolar modulations for the single-phase inverter, an asymmetrical duty cycle double frequency modulation is originated for high-power bidirectional resonant converters whose switching frequency and power density are significantly limited by the high-power devices. The equivalent operating frequency of the resonant tank becomes twice of the switching frequency, which effectively reduces the volumes and costs of the passive components, especially for high-power high-voltage applications. Moreover, the proposed converter has the same output characteristics for the two power flow directions, which not only symmetrises the bidirectional frequency control, but also simplifies the design of the resonant components. The converter operation principle is illustrated and the converter performances are analysed including the principle of the double frequency modulation, output characteristics and soft switching condition. Finally, a 5 kW prototype is built to verify the analysis, and the resonant components for the conventional bidirectional resonant converter are also designed and compared to illustrate the advantages of the proposed converter.

DC wind farm (DCWF) with series-connected DC wind turbines (DCWTs) is proved to be a potential solution of offshore wind power collection. The coupling behaviour of series-connected DCWTs is described in detail. Possible wind energy curtailment during the period of wind turbine voltage limitation and its key impact factors are firstly quantitatively derived. A novel variable speed control strategy is proposed under voltage limiting condition of the DCWT to improve its wind energy capture. This control algorithm can be implemented in the local DCWT controller without the communication need. The specific variable speed control strategy of the DCWT is realised in a generalised averaged model. Dynamic simulation cases under different operation conditions have been conducted to evaluate the effectiveness of the proposed control strategy. It is found that the proposed control strategy might be a solution for wind energy curtailment, which will significantly improve the performance of the series-connected DCWTs between the time scale above seconds and below minutes.

This study analyses a surge capacitor failure on the neutral bus in a ±500 kV high voltage direct current (HVDC) converter station. Both overvoltage and insulation of the surge capacitor are investigated through simulation, practical record data analysis, and experimentation. A highly accurate HVDC model is established on the basis of the topological structure and parameters of the HVDC project. Using this model, the failure process is simulated by combining neutral bus overvoltage record analysis and voltage divider performance test. Results indicate that the 246 kV uncharacteristically high overvoltage on the neutral bus is a measurement error caused by voltage divider fault. From the field test, difference equation of the voltage divider is derived to reconstruct the actual overvoltage, the amplitude of which is 73 kV. The amplitude is significantly lower than that of the switching impulse withstand level of the surge capacitor, which should not cause capacitor failure. Therefore, the primary cause of capacitor failure is insulation damage rather than overvoltage. Finally, maintenance measures are proposed and applied in the station.

This study introduces an advanced decentralised control method for DC microgrids which can be adopted both in the grid-connected mode and the islanded mode with a small change. Compared with the conventional droop control methods, there are three improvements in this proposed control method. First, in the grid-connected mode, the proposed control method can output the specified current, which is useful for the economic dispatch of the microgrid. Second, in the islanded mode, the proposed control method can realise accurate current sharing among distributed generations (DGs). Third, in the islanded mode, the proposed control method can directly restore the average voltage of the bus without the extra secondary control, which can ensure the power quality and reduce the complexity of the control system. The high reliability of the proposed control method is illustrated by the analysis of the stability. Also, the time-delay effect caused by the finite bandwidth of the communication network is analysed and the limited bandwidth of the communication network can be calculated out. All the conclusions are verified by the corresponding simulation tests based on MATLAB/Simulink, also the real-time hardware-in-loop tests are conducted to evaluate the performance of the control method when applied practically.