Multi-terminal High Voltage DC Transmission Research Articles

A New Hybrid Dual Active Bridge Modular Multilevel Based DC–DC Converter for HVDC Networks

Multi-terminal high voltage DC transmission currently represents a leading technology in long-distance power transmission systems. Among the main technical challenges facing such technology, DC fault isolation, permitting different grounding schemes, providing interoperability, and high DC voltage stepping between different HVDC networks, and allowing high-speed power reversal without power interruption especially when connecting the pre-existing voltage source converters (VSC) and line commutated converters (LCC)-based HVDC networks. This paper introduces a new modular multilevel converter (MMC) based front-to-front DC-DC converter to interconnect two different types (LCC/VSC) of HVDC networks. The proposed topology comprises a voltage source MMC (VS-MMC) and a current source MMC (CS-MMC), while both are coupled via an AC link including the isolating transformer. The proposed topology can successfully provide an uninterruptible bi-directional power flow, high DC voltage stepping with a DC fault blocking capability, and low number of semiconductors due to the usage of only half-bridge SMs. The system design is provided with a detailed mathematical analysis. Furthermore, two active power control methodologies are proposed and compared. The first control technique is simpler and entails lower passive elements, while the second technique ensures a zero reactive power over the full range of active power flow. Furthermore, Losses analysis and comparison are provided between the two proposed control techniques. Finally, Control-Hardware-in-the-Loop (CHiL) test validation is employed to confirm the validity of the proposed system under healthy as well as different fault scenarios.

Optimal power flow in multi‐terminal HVDC networks with embedded high‐power DC–DC converters for voltage matching and flexible DC transmission

High-power DC–DC converters are integrated into multi-terminal high-voltage DC (MTDC) networks, typically for voltage matching or flexible DC transmission. This study addresses droop-controlled radial and mesh MTDC networks with transmission loss minimisation considering the impact of integrating a high-power DC–DC converter. DC voltage droop control can achieve minimum power loss in radial MTDC transmission network by controlling the power-sharing among the receiving nodes inversely proportional to their transmission line resistances. Updating the droop characteristics by an optimisation algorithm guarantees optimal power flow (OPF) during renewable source power generation fluctuation. Meanwhile, the mesh network DC voltage references can be updated by the OPF for transmission loss minimisation. This study presents the impact of incorporating a high-power DC–DC converter at different system access lines of a radial MTDC network while targeting efficient power transfer and DC transmission loss minimisation. The effect of introducing a high-power DC–DC converter on the droop characteristic settings during steady-state operation in a radial MTDC network is investigated. Moreover, the impact of a high-power DC–DC converter in mesh MTDC network with OPF operation is explored. To validate the presented concepts, the CIGRE B4 MTDC network is used as a benchmark system with a metaheuristic optimisation technique.

Power flow approximation for DC networks with constant power loads via logarithmic transform of voltage magnitudes

This paper proposes a logarithmic transformation of voltages (LTVM) for the power flow in DC grids. This problem is non-linear due to the presence of constant power loads (CPLs), which also introduce a negative resistance effect that can create numerical instability for conventional algorithms. The proposed methodology is applied to dc-microgrids, dc-distribution and multi-terminal high voltage DC transmission (MT-HVDC). Two main approximations are presented and compared in terms of computational performance and the accuracy of the solution. Simulation results performed in Matlab/Octave demonstrate the advantages of the proposed methodology using a complete set of test systems, from low to high voltage applications. The proposed methodology does not require any consideration about the topology of the grid (radial or meshed) or the number of constant power loads.

Optimisation of commutation circuit parameters for DC vacuum circuit breakers

As the environmental problems caused by the consumption of the non-renewable fossil energy are getting increasingly serious, the new energy power generation has received wide attentions. Multi-terminal high voltage DC (HVDC) transmission is one of the key technologies for improving the reliability and stability of new energy power generations. Besides, HVDC circuit breakers (CBs) become the bottleneck of the development and application for multi-terminal HVDC transmission power systems. Here, the active DC vacuum circuit breaker (VCB) is taken as the research object. Meanwhile, the optimisation design of the commutation circuit for the DC VCB is realised. According to Kirchhoff's law, the genetic algorithm is used to calculate the commutation parameters under different optimisation objectives, to obtain the lowest cost and the shortest sheath development time. Based on the software package MATLAB, the simulations and analyses of different optimisation objectives and parameters are carried out. Through the comparative analyses of the two optimisation objectives, the relationships between the development time of the sheath gap and the cost have been obtained.

Hybrid HVDC (H2VDC) System Using Current and Voltage Source Converters

This paper presents an analysis of a new high voltage DC (HVDC) transmission system, which is based on current and voltage source converters (CSC and VSC) in the same circuit. This proposed topology is composed of one CSC (rectifier) and one or more VSCs (inverters) connected through an overhead transmission line in a multiterminal configuration. The main purpose of this Hybrid HVDC (H2VDC), as it was designed, is putting together the best benefits of both types of converters in the same circuit: no commutation failure and system’s black start capability in the VSC side, high power converter capability and low cost at the rectifier side, etc. A monopole of the H2VDC system with one CSC and two VSCs—here, the VSC is the Modular Multilevel Converter (MMC) considered with full-bridge submodules—in multiterminal configuration is studied. The study includes theoretical analyses, development of the CSC and VSCs control philosophies and simulations. The H2VDC system’s behavior is analyzed by computational simulations considering steady-state operation and short-circuit conditions at the AC and DC side. The obtained results and conclusions show a promising system for very high-power multiterminal HVDC transmission.

Power flow and transmission loss analysis of modular multi‐level converter based multi‐terminal high‐voltage DC systems

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.

Coordinated Operation and Control of VSC Based Multiterminal High Voltage DC Transmission Systems

This paper deals with a multiterminal voltage source converter (VSC)-based high voltage dc transmission system (M-HVDC) connecting offshore wind farms to onshore ac grids. Three M-HVDC configurations studied, each with different control strategies. The voltage-current characteristics of VSCs are presented and VSC converter operation with different output powers from offshore wind farms is assessed. A generalized droop control strategy is mainly used to realize autonomous coordination among converters without the need of communication. Operation of the three configurations with respect to their control system is analyzed through PSCAD/EMTDC simulations and experimentation. Results show control performance during wind power change, eventual permanent VSC disconnection, and change in power demand from the ac grid side converter.