Modular Multilevel Converter Structure Research Articles

Current stress optimization of nonagonal MMC for FFTS‐based offshore wind power application

SummaryNonagonal Modular Multilevel Converter (MMC) is a new type of three‐port AC‐AC converter suited for applications in FFTS‐based offshore wind power. It can accommodate three AC ports with only nine branches, greatly reducing the cost and size of energy conversion. However, the multiplexing structure of nonagonal MMC inevitably increases the peak branch current resulting in high semiconductor ratings, which weakens its advantage in cost and size. Since the control degrees of freedom are reduced, traditional current attenuation methods are not applicable. To address this issue, in this paper, a parameter adjustment method is proposed for nonagonal MMC to reduce the current stress. By establishing the mathematical model of nonagonal MMC, phase angle difference between ports of nonagonal MMC is proved to be a key influencing factor of peak branch current. By adjusting phase angle difference, the average current stress of nonagonal MMC can be reduced by as great as 66.31%. Finally, the correctness of the conclusions of this paper is verified by RT‐LAB results.

Analysis, design and control of a hybrid railway power conditioner considering power rating reduction

In order to effectively compensate negative sequence current (NSC) of the V/v traction system and reduce the capacity and cost of railway power conditioner (RPC), a low-cost hybrid railway power conditioner (HRPC) technology scheme considering regenerative braking energy is proposed in this paper. The proposed HRPC consists of an RPC with a modular multilevel converter structure (MMC-RPC) and a railway static var compensator (RSVC). First, the circuit topology of the proposed scheme and the principle of NSC compensation considering regenerative braking energy are given. Then, the capacity configurations of MMC-RPC and RSVC are presented to reduce the capacity of active equipment and further lower the investment cost of HRPC. On this basis, the HRPC with energy storage units (ESU-HRPC) is studied to recycle the remaining regenerative braking energy, and the difference from the proposed HRPC is further analyzed. Subsequently, the capacities of HRPC and ESU-HRPC are designed based on the measured power data from a heavy-haul railway in China as an example. Besides, the system control strategy is given. Finally, the proposed technical schemes are verified by the hardware in loop (HIL) experiments.

Impedance Modeling and Stability Analysis of Energy Controlled Modular Multilevel Converter

In recent years, the modular multilevel converter (MMC) has been widely used in high-voltage dc (HVDC) transmission, which, however, brings about various stability issues, such as sub- -synchronous oscillation (SSO). To deal with these stability issues, many active and passive damping methods have been proposed. In fact, the MMC itself has more control degrees of freedom compared to the two-level voltage-source converter (VSC), such as internal energy, circulating current, etc., which can be exploited to improve the system stability. Based on the idea, the internal energy of MMC is utilized to suppress the SSOs in this paper. The frequency-coupling impedance model of MMC with energy controller is first developed based on the harmonic state-space (HSS) method by considering the internal dynamics and all control links, which is verified by frequency scanning result in simulation. The stability analysis of the energy controlled MMC-HVDC for wind farm integration is conducted and a modified energy control structure of MMC is also proposed to further improve the stability of the interconnected system.

A Step-Down Resonant Modular Multilevel DC/DC Converter With Extendable ZVS Range Based on Asymmetric Triangular Current

In this article, a new step-down resonant modular multilevel dc/dc converter is proposed for medium-voltage applications. The proposed converter is derived from the combination of the modular multilevel converter (MMC) and resonant converter with a passive auxiliary circuit. It can achieve zero voltage switching (ZVS) over a wide input voltage range for all switches compared with the conventional dc/dc MMC. Specifically, the asymmetric triangular compensating current provided by the passive auxiliary circuit is first introduced to flow into the bridge arms. Then, the current bias caused by the MMC structure is eliminated in the lower side switches of the submodules to achieve ZVS. Moreover, asymmetrical pulsewidth modulation is applied to double the equivalent switching frequency and obtain a high step-down ratio. In this way, the resonant current is reduced, and the conversion efficiency is improved. Besides, the simulation and experimental results of the prototype are presented to demonstrate the viability of the proposed converter. The results convincingly verify the ability of the proposed converter to offer ZVS for all the switches over a wide input voltagerange.

Analysis of typical fault characteristics of modular multilevel converter for flexible HVDC transmission

Modular multilevel converter (MMC) has been widely used in VSC-HVDC system for its smooth output waveform, low harmonic content and low switching loss. But compared with the conventional LCC-HVDC system, modular multilevel converter has a short development history, few engineering applications, high failure rate and lacks of very effective protection strategies, which limits its further development. Therefore, this paper firstly introduces topological structure of MMC and its working principle. Then the commonly used control policy of dual-terminal MMC-HVDC project is elaborated. Finally, the transient electrical characteristics of the typical faults are detailedly analyzed through the combination of theoretical analysis and simulation, such as three phase short circuit of converter valve, single phase grounding of converter valve, and the bridge arm short circuit. The conclusion provides an essential theory for the research of the converter protection strategy in the MMC-HVDC project.

Virtual control method applied to Modular Multilevel Converter

The DC transformer based on the Modular Multilevel Converter (MMC) structure has the advantages of robust scalability, low output voltage harmonic content, easy realization of back-to-back connection, and high power transmission efficiency. It is widely used in DC distribution networks. However, traditional control methods hardly provide inertia for the DC distribution network, thereby reducing the stability of the distribution network when new energy output fluctuations, DC distribution network failures, or other disturbances occur. Aiming at the problem that the DC bus voltage is easily affected by power fluctuations, the paper proposes a virtual inertia control method applied to the DC transformer with MMC structure. Similar to the introduction of virtual synchronous generators in the AC distribution network to enhance the system inertia, the paper designs a control strategy for dynamically adjusting the virtual inertia with a virtual capacitor added, which is based on the traditional two-phase shift control. The paper also presents the calculation method of the virtual capacitance. When the power in the DC microgrid fluctuates significantly, the strategy can effectively enhance the inertia of the DC distribution network and suppress the voltage fluctuation of the DC bus. The paper finally establishes a DC distribution network model with distributed power sources in the PLECS simulation software, and verifies the effectiveness of the control strategy compared with the traditional strategy.

Circulating current control of modular multilevel converter by wild spider foraging optimization based fractional order proportional integral derivative controller

Modular Multilevel Converter (MMC) plays a vital role in high voltage industries because of its high rating power conversion. Due to its usage in high voltage rating power conversion and switched capacitor usage in MMC structure, there arises a problem of unbalanced capacitor voltage, which causes circulating current and disturbance in output current regulation. To manage these problematic parameters, a FOPID (Fractional Order Proportional Integral Derivative) controller has been utilized, due to its dynamic tracking and fast response. Secondly, the gain values of FOPID are not efficient, and they are optimized for each control group at all times of MMC working conditions. To provide a dynamic gain value by considering the dynamic change of error tracking parameters, Wild Spider Foraging Optimization (WSFO) algorithm has been developed based on the foraging behaviour of wild spider searching food (gain values) in view of changing the error of tracking parameters. The proposed algorithm has been evaluated in MATLAB Simulink by modeling the MMC structure with FOPID controller. The parameters of FOPID are optimized by bio-inspired algorithms like WSFO, Artificial Bee Colony (ABC) and Particle Swarm Optimization (PSO). The outcomes of the proposed WSFO-FOPID provide minimum circulating current and effectively balance the capacitor voltage in MMC. When the effectiveness of the results has been verified with the existing ABC and PSO optimization approaches, the proposed algorithm outperforms.

Research on the Structure and Control Strategy of a Novel Power Electronic Transformer for AC/DC Hybrid Distribution Network

Power electronic transformers (PETs), as the core devices of the energy internet, are the key to achieve both effective consumption for renewable energy and the safe and coordinated operation for AC/DC hybrid system. In order to overcome the shortcomings of the existing PETs, a novel PET with an improved structure that applicable for multi-voltage level AC/DC hybrid distribution network is proposed. The topology of the proposed PET is analyzed, and the corresponding control methods are suggested for different parts. The input stage utilizes the modular multilevel converter structure and applies the virtual synchronous machine control strategy to enhance the inertia and damping of the system. The power of the output stage is adjusted flexibly and that enables the PET to provide certain power support to the upper grid and participate in its primary frequency regulation. A combined connection of input-series output-series and input-series output-parallel is applied for the dual-active-bridge modules of the isolation stage to enable network interconnection and electrical isolation of AC/DC grids with significantly different voltage levels. A power coordinated control method is then proposed to meet the power demand of the distribution networks connected to the output stage and ensure stable operations of PET simultaneously. The reliability and efficiency of the proposed PET topology and control strategy for AC/DC hybrid distribution network are finally verified via PSCAD/EMTDC simulation.

Virtual Submodule Predictive Control method for Power Electronic Transformer based on MMC Structure

In order to achieve flexible and efficient operation of intelligent power distribution, solving the problems of traditional distribution transformer such as large volume and weight, easy to generate harmonics when overload, and need supporting protection equipment to protect it when failure, etc. We propose a power electronic transformer structure based on modular multilevel converter (MMC). Firstly, we consider the multi-dimensional control target of MMC converter to establish a mathematical model. Then a virtual submodule predictive control method is proposed. The method introduces the concept of virtual submodule to realize the optimal switching state rapid mapping and reduce the switching loss of MMC. Finally, the experimental results show that the mmc-based power electronic transformer has excellent dynamic steady-state performance and can effectively overcome the high loss of traditional predictive control.

A unified MMC reliability evaluation based on physics-of-failure and SM lifetime correlation

The reliability of a modular multilevel converter (MMC) is determined both by the component lifetime distribution and the MMC structure. In this paper, a unified submodule (SM) component lifetime is estimated based on the physics- of- failure, and the overall reliability of the MMC system is evaluated considering the SM lifetime correlation. First, the component lifetime estimation is performed in three sub-steps. (a) The current stress of each component, i.e., the insulated gate bipolar transistor modules (IGBTs) and capacitors, is computed according to the working principle of the MMC. (b) The core temperatures of the components are calculated using thermal equivalent networks. For the IGBTs, the thermal cycles are extracted to compute the cumulative damage. (c) The lifetime of each device is estimated in a static state using the lifetime model and damage model. Second, the lifetime Weibull distribution of each component is obtained using the Monte Carlo method. This distribution links the component lifetime estimation and MMC reliability analysis. Finally, considering the SM lifetime correlation, the overall reliability of an MMC system with redundancy is evaluated using a Copula function. The reasons for the impact of SM lifetime correlation on MMC reliability are revealed in a case study.

Non‐linear behavioural modelling of device‐level transients for complex power electronic converter circuit hardware realisation on FPGA

Detailed device-level models of the insulated-gate-bipolar-transistor (IGBT) and diode are essential for power converter design evaluation for providing insight into circuit and device behaviours, as well as to shorten the design cycle and reduce costs. In this study, the non-linear behavioural models of IGBT and power diode are utilised for emulating the modular multilevel converter (MMC) on the field programmable gate array. For digital hardware-in-the-loop (HIL) emulation, these time-domain continuous models are discretised and linearised prior to being designed into the corresponding hardware modules using the hardware description language VHDL that features a fully paralleled and pipelined implementation. A circuit partitioning approach is adopted according to the MMC structure to enhance computation efficiency and then, detailed information from the system-level performance to the specific features of individual switches is available. HIL emulation and the subsequent comparison with results from the commercial off-line simulation tools prove that the complex IGBT and diode models can be involved in the efficient simulation of large-scale power converters.

The diode-clamped half-bridge MMC structure with internal spontaneous capacitor voltage parallel-balancing behaviors

In order to significantly reduce the number of voltage sensors and computation boards in the modular multilevel converter (MMC), this paper proposes a diode-clamped half-bridge MMC with full capability of capacitor voltage parallel-balancing. The newer generation of MMC structure can be re-constructed from the traditional half-bridge MMC by integrating one clamped diode and damping resistor into each submodule (SM), and four auxiliary voltage balancing circuits across all three phases. The proposed MMC topology can naturally balance its SM capacitor voltages regardless of modulation and triggering techniques. This is due to the fact that all the SMs in the MMC belong to 6 Spontaneous Capacitor Parallel Behaviors (SCPBs), through which the voltages of SM capacitors are automatically balanced. The inserted SM capacitor is now parallel with the adjacent bypassed SM capacitor. By designing proper RC constant, the adjacent SMs in the proposed MMC will have equal capacitor voltage ripples. Although voltage balancing is naturally guaranteed, in order to average the switching losses, open loop nearest level modulation (NLM) and carrier phase shifted sinusoidal PWM (CPS-SPWM) techniques are used in the proposed MMC. The electromagnetic transient (EMT) simulations on PSCAD/EMTDC and the down-scaled prototyping results will validate the new topology.

Design, implementation and testing of a Modular Multilevel Converter

The Modular Multilevel Converter (MMC) is a power electronic structure used for high voltage adjustable speed drives applications as well as power transmission applications and high-voltage direct current. MMC structure presents many advantages such as modularity, the absence of a high voltage DC bus and very low switching frequency. It presents also some disadvantages such as modeling complexity and control due to the large number of semiconductors to control. The objectives of this paper are to present the methodology to design a laboratory MMC converter and its control system. This methodology is based on an intensive used of real-time simulation, to develop and test the control algorithm is proposed. This MMC prototype must be as realistic as possible to a full scale MMC, with a large number of sub-modules (i.e. 640 kV on the DC side, a rated power of 1 GW and 400 sub-modules). A control hardware integrating distributed processors (one for each arm) and a master control is presented. The protocols to validate sub-modules, arms and the converter are explained.

A Novel STATCOM Based on Diode-Clamped Modular Multilevel Converters

A new static synchronous compensator (STATCOM) based on the diode-clamped modular multilevel converter (DCM2 C) is proposed in this paper. In this converter topology, the capacitor voltage is clamped by using a low power rating diode in each submodule. The quantity of voltage sensors is significantly reduced and is free from the number of voltage levels. Furthermore, the voltage balancing control method becomes very simple and the capacitor voltage balance speed is fast. Based on the structure of modular multilevel converter, the DCM2C-STATCOM has the capability of Var compensation and negative-sequence current compensation. The topology characteristics and compensation control method of DCM2C-STATCOM are investigated in this paper. Experimental results obtained from a laboratory prototype validate that the capacitor voltage of the proposed DCM2C-STATCOM can be well balanced and the Var and negative-sequence current compensations are effective.

A New Topology of Multilevel VSC Converter for a Hybrid HVDC Transmission System

This paper introduces a new multilevel converter topology for a hybrid HVDC system comprising line-commutated converter (LCC) and voltage source converter (VSC). Among the existing modular multilevel converter (MMC) topologies for the hybrid HVDC, a mixed MMC structure with half-bridge submodules (HBSMs) and full-bridge submodules (FBSMs) has characteristics of reduced system cost, low operation loss, but still keeping capability to cope with dc short-circuit fault. However, it is very difficult for the conventional hybrid MMC structure to balance the submodule capacitor voltages under dc-bus voltage sliding since each MMC arm is a mixture of HBSMs and FBSMs. To solve the defect of the conventional hybrid MMC structure, an asymmetric mixed MMC, in which one arm is made of series-connected HBSMs and other arm is made of FBSMs, is devised. The proposed asymmetric MMC can regulate the dc-bus voltage freely without uncontrollable submodule capacitor voltages. The problems of the conventional MMC structure and the validity of asymmetric MMC are verified by both computer simulation and experimental results.

Novel Control of a Modular Multilevel Converter for Photovoltaic Applications

The number of applications of solar photovoltaic (PV) systems in power generation grids has increased in the last decade because of their ability to generate efficient and reliable power in a variety of low installation in domestic applications. Various PV converter topologies have therefore emerged, among which the modular multilevel converter (MMC) is very attractive due to its modularity and transformerless features. The modeling and control of the MMC has become an interesting issue due to the extremely large expansion of PV power plants at the residential scale and due to the power quality requirement of this application. This paper proposes a novel control method of MMC which is used to directly integrate the photovoltaic arrays with the power grid. Traditionally, a closed loop control has been used, although circulating current control and capacitors voltage balancing in each individual leg have remained unsolved problem. In this paper, the integration of model predictive control (MPC) and traditional closed loop control is proposed to control the MMC structure in a PV grid tied mode. Simulation results demonstrate the efficiency and effectiveness of the proposed control model.

A Three-Phase Modular Multilevel DC–DC Converter for Power Electronic Transformer Applications

A three-phase modular multilevel dc-dc converter is proposed and analyzed for power electronic transformer (PET) applications. Similar to a dual active bridge converter, the proposed converter comprises two three-phase inverters/rectifiers, coupled via a medium frequency (MF) transformer. The modular multilevel converter structure is used in the medium-voltage side to meet the high-voltage requirements and to reduce the dv/dt stress on the MF transformer. The frequency of the voltage through MF transformer (isolation frequency) is the same as the switching frequency of the power semiconductor devices, and zero-voltage switching-ON can be achieved for all the devices. With only one three-phase MF transformer, the proposed topology can greatly simplify the transformer design compared with the existed input-series output-parallel dc-dc converter structures for PET applications. A dual-phase-shift method is presented to control the output power and to balance the submodule capacitor voltages. Simulations and experimental results are provided to validate the theoretical analysis.

Research on Control Strategy of AC-DC-AC Substation Based on Modular Multilevel Converter

Significant disadvantages in power quality especially the unbalance problem and neutral sections restrict the evolution of conventional traction power supply system. A new traction power supply system based on three-phase to single-phase converter is studied, which can transfer active power from three-phase grid to single-phase catenary. One catenary section could be utilized in the new traction power supply system instead of the multiple split sections in conventional system. Three-phase to single-phase converter is the core equipment of new traction power system. MMC (modular multilevel converter) structure of AC-DC-AC substation is proposed in this paper. To solve the problem of the capacitor voltage balancing in MMC, a parallel sorting algorithm based on field programmable gate array (FPGA) is studied. And the correctness and effectiveness of the algorithm are verified by experiments. In addition, it is inevitable that the AC grid voltage will be unbalanced caused by the fault in the new system. Therefore, this paper focuses on the analysis of the effect of the unbalanced grid voltage on the operating characteristics of the MMC system. Finally, the correctness of the theoretical analysis is verified by simulation.

Internal Power Flow of a Modular Multilevel Converter With Distributed Energy Resources

This paper examines the internal power flow mechanisms that exist within a generalized modular multilevel converter (MMC) and examines alternatives for integration of distributed energy resources (DERs) within the MMC structure. Each phase leg of the MMC consists of two series connected strings of submodules, where each string of submodules is referred to as a phase arm. Based on analytically developed inter-arm power flow relations, a control methodology is proposed, which provides fully independent control of each arm’s real power flow, facilitating extreme levels of inter-arm power transfer. This eliminates the need for uniform integration of DER units across all submodules of the MMC. Among others, viable operating configurations are shown to include DERs only integrated in the upper (or lower) phase arms and DERs integrated only in the upper and lower phase arms of a single phase leg. Power balance is maintained internal to the MMC via dc and ac currents circulating within the converter without distorting input dc or output ac currents. To maximize conversion efficiency, a mechanism for minimizing the necessary circulating ac currents under any inter-arm power flow condition is also identified. Comprehensive simulation results validate both the developed model and controls.

The Control Strategy of Establishing the Voltage of DC Side in MMC

This paper discusses the establishing method of DC voltage of modular multilevel converter (MMC). Firstly the structure of MMC and the charging principle of sub-modules (SMs) are introduced in order to control the DC voltage of MMC and reduce the switching loss. It is pointed out that the DC voltage control in MMC includes two parts, the DC voltage establishment of system and the voltages control of SMs. For the DC voltage control in system, the control theory is analyzed, and the corresponding control strategy is proposed. Meanwhile an improved voltage balance control strategy for SM is proposed by the improvements of the traditional voltage sorting method to reduce the high switching frequency caused by the blind action of switching devices in the SM control. Finally, the simulation results based on PSCAD/EMTDC show that DC voltage in MMC can run in the rated value steadily, and the switching frequency is reduced significantly.