Multi-busbar sub-module modular multilevel converter

Modular multilevel converter (MMC) plays increasingly significant roles in large scale power electronics system including high voltage direct current (HVDC) system, static synchronous compensator (STATCOM), large scale energy storage, motor control, and so on, thanks to its advantages including modular configurations, reduced dv/dt, low total harmonic distortions, and low power losses. The classic sub-module (SM) topologies (e.g. half or full bridge types) all have in common their single connection arrangement between each SM in their series connection within a stack; i.e. a single busbar. This single busbar arrangement does come however with some drawbacks in terms of performance, reliability and flexibility. The lack of redundant switching states limits the potential optimization for the whole MMC. To solve the above mentioned issue, this thesis presents the control and performance of a new topology of SMs for MMCs, which uses multiple parallel connections between SMs and is referred to as multi-busbar sub-module (MBSM). Stacks made entirely of MBSMs can see improved functionalities such as pre-charging capability, capacitor paralleling, lower power losses, improved reliability, and a rational bypass mechanism in the event of SM failure. The soft-parallel mechanism is proposed to maintain voltage balancing without the requirement of additional spike current inductors. Despite the fact that the number of semiconductors in MBSM MMC has been doubled, semiconductor losses have been reduced to 80% of those in its counterpart. Simulation results have verified the characteristics of a FB MBSM MMC in an HVDC scenario. Several advanced control schemes for the control of the MBSM MMC are also investigated, including an algorithm to automatically generate independent variables state space models from linear electrical circuits, a model predictive control-based start-up controller to simplify the SM pre-charge procedure and at the same time improve the transient performance, and a reinforcement learningbased low-level controller to achieve low switching frequency operation of the MBSM MMC. The control schemes are validated by detailed theoretical analysis and simulation results. Besides, some MBSM applications in the operation scenarios of STATCOM are studied. Two topologies of delta-configured, partially rated energy storage (PRES) MBSM STATCOM and their corresponding low-level controllers are presented to improve the active power output capability. The soft parallel of MBSM is more effective in reactive power mode than active power mode due to the location of ES, which sees their current circulation limited to their own SM capacitor. The proposed controller for the MBSM STATCOM dynamically switches between two operation modes to reduce the converter losses over the extended range of active power. Simulation results confirm the earlier point, in that PRES-MBSMSTATCOM performs better at pure reactive power set-points and marginally better at high active power. This is explained by the fact that MBSM operates more frequently in soft-paralleling mode when the ES releases less power, i.e. reactive power set-points. Then the MBSM concept is further extended to a structure with more busbars, named multi-H-bridge SM, aiming at solving the current sharing issue of paralleled discrete SiC MOSFETs in large current applications. When compared to conventional FBSM constructed directly paralleled SiC MOSFETs, simulation results show that the current sharing performance against on-state resistance mismatch is improved and the switching loss is reduced. The same converter rating can be achieved with fewer MHSMs compared with Si IGBT SMs. Finally, the designing process of a benchtop-scale, low-voltage, open-source, and affordable hardware prototype of a MMC, the μMMC, is presented with a case study of a three-phase inverter-mode MMC. The proposed μMMC is configured as full bridge SMs type in the experiment, yet the flexible structure makes it capable to be configured as other SM types, including MBSMs. The cost for a single μMMC could be around 50 pounds. The control framework and concrete implementation are presented in detail. With the application of the μMMC, the STM32Cube Hardware Abstraction Layer, and the MATLAB/Simulink hardware support packages, it is possible to shorten the transition process from simulation to hardware realization to several hours. The experiment setup and results of a three-phase inverter mode MMC validate the proposed μMMC’s effectiveness, scalability, and convenience

A Class of Control Strategies for Energy Internet Considering System Robustness and Operation Cost Optimization

Aiming at restructuring the conventional energy delivery infrastructure, the concept of energy Internet (EI) has become popular in recent years. Outstanding benefits from an EI include openness, robustness and reliability. Most of the existing literatures focus on the conceptual design of EI and are lack of theoretical investigation on developing specific control strategies for the operation of EI. In this paper, a class of control strategies for EI considering system robustness and operation cost optimization is investigated. Focusing on the EI system robustness issue, system parameter uncertainty, external disturbance and tracking error are taken into consideration, and we formulate such robust control issue as a structure specified mixed H2/H∞ control problem. When formulating the operation cost optimization problem, three aspects are considered: realizing the bottom-up energy management principle, reducing the cost involved by power delivery from power grid (PG) to microgrid (MG), and avoiding the situation of over-control. We highlight that this is the very first time that the above targets are considered simultaneously in the field of EI. The integrated control issue is considered in frequency domain and is solved by a particle swarm optimization (PSO) algorithm. Simulation results show that our proposed method achieves the targets

Robust Control Method for DC Microgrids and Energy Routers to Improve Voltage Stability in Energy Internet

The energy internet (EI) is a wide area power network that efficiently combines new energy technology and information technology, resulting in bidirectional on-demand power transmission and rational utilization of distributed energy resources (DERs). Since the stability of local network is a prerequisite for the normal operation of the entire EI, the direct current (DC) bus voltage stabilization for each individual DC microgrid (MG) is a core issue. In this paper, the dynamics of the EI system is modeled with a continuous stochastic system, which simultaneously considers related time-varying delays and norm-bounded modeling uncertainty. Meanwhile, the voltage stabilization issue is converted into a robust H ∞ control problem solved via a linear matrix inequality approach. To avoid the situation of over-control, constraints are set in controllers. The problem of finding a balance between voltage regulation performance and constraints for the controllers was also extensively investigated. Finally, the efficacy of the proposed methods is evaluated with numerical simulations

Robust Mixed <em>H</em>₂/<em>H</em>_∞ Controller Design for Energy Routers in Energy Internet

In this paper, a class of mixed H2/H∞ controller is designed for an energy router (ER) within the scenario of an energy Internet (EI). The considered ER is assumed to have access with photovoltaic panels, wind turbine generators, micro-turbines, fuel cells, diesel engine generators, battery energy storage devices, flywheel energy storage devices, loads, and other ERs. Two types of control targets are considered. First, due to the access of large-scale renewable energy sources, the DC bus voltage deviation within the ER system shall be regulated. Second, an optimal energy management strategy shall be achieved, such that the autonomous power supply-demand balance within each ER is achieved with priority and the rational utilization of controllable power generation devices and energy storage devices are realized. When these objectives are considered simultaneously, the control issues with respect to ER is formulated as a mixed robust H2/H∞ control problem with analytical solutions provided. Several numerical examples are given, and the feasibility and effectiveness of the proposed method are demonstrated

A Comprehensive Review on the Power Supply System of Hydrogen Production Electrolyzers for Future Integrated Energy Systems

Hydrogen energy is regarded as an ideal solution for addressing climate change issues and an indispensable part of future integrated energy systems. The most environmentally friendly hydrogen production method remains water electrolysis, where the electrolyzer constructs the physical interface between electrical energy and hydrogen energy. However, few articles have reviewed the electrolyzer from the perspective of power supply topology and control. This review is the first to discuss the positioning of the electrolyzer power supply in the future integrated energy system. The electrolyzer is reviewed from the perspective of the electrolysis method, the market, and the electrical interface modelling, reflecting the requirement of the electrolyzer for power supply. Various electrolyzer power supply topologies are studied and reviewed. Although the most widely used topology in the current hydrogen production industry is still single-stage AC/DC, the interleaved parallel LLC topology constructed by wideband gap power semiconductors and controlled by the zero-voltage switching algorithm has broad application prospects because of its advantages of high power density, high efficiency, fault tolerance, and low current ripple. Taking into account the development trend of the EL power supply, a hierarchical control framework is proposed as it can manage the operation performance of the power supply itself, the electrolyzer, the hydrogen energy domain, and the entire integrated energy system