Modular Multilevel Converter Inverter Research Articles

Cascaded Smart Gate Drivers for Modular Multilevel Converters Control: A Decentralized Voltage Balancing Algorithm

Recent Modular Multilevel Converter (MMC) topology allows for drastic improvements in power electronic conversion such as higher energy quality, lower power semiconductors electrical stress, decreased Electro-Magnetic Interferences (EMI), and reduced switching losses. MMC is widely used in High Voltage Direct-Current (HVDC) transmissions as it offers, theoretically, no voltage limit. However, its control electronic structure is not modular itself. Especially, the insulation voltage between the submodule gate drivers’ primaries and secondaries depends on the number of submodules. The converter voltage levels cannot be increased without designing all gate driver isolations again. To solve that issue, the novel concept of distributed galvanic insulation is introduced for multilevel converters. The submodule’s gate drivers are daisy-chained, which naturally reduces the insulation voltage to the submodule capacitor voltage, regardless of the number of submodules. The MMC becomes truly modular as the number of submodules can be increased without impacting on the previous control electronic circuit. Such an innovative control structure weakens the link between the main control unit and the gate drivers. This inherent structural problem can be solved through the use of Smart-Gate Drivers (SGD), as they are often equipped with fast and bidirectional communication channels, while highly increasing the converter reliability. The innovation proposed in that work is the involvement of smart gate drivers in the distributed galvanic insulation-based MMC control and monitoring. First, the numerous benefits of smart gate drivers are discussed. Then, an innovative Voltage Balancing Algorithm directly integrated on the chained gate drivers is proposed and detailed. It features a tunable parameter, offering a trade-off between accurate voltage balancing and execution time. The proposed embedded algorithm features a low execution time due to simultaneous voltage comparisons. Such an algorithm is executed by the gate drivers themselves, relieving the main control unit in an original decentralized control scheme. A simulation model of a multi-megawatts three-phase grid-tied MMC inverter is realized, allowing validation of the proposed algorithm. Matlab/Simulink logic blocs allow us to simulate a typical CPLD/FPGA component, often embedded on smart gate drivers. The converter with the proposed embedded algorithm is simulated in steady-state and during load impact. The controlled delay and slew rate inferred by the algorithm do not disturb the converter behavior, allowing its conceptual validation.

Travelling wave directional pilot protection for hybrid LCC-MMC-HVDC transmission line

Currently, line commutated converter (LCC) and modular multilevel converter (MMC) based hybrid high voltage direct current (HVDC) is applied in actual power transmission system. Generally, LCC-MMC-HVDC transmission line adopts travelling wave (TW) protection as primary protection which includes voltage change criterion, voltage change rate criterion and current change rate criterion. In fact, it has threshold issue and no fault direction discrimination ability which could lead to wrong operation to external fault. Besides, it has not considered different structures of LCC rectifier station and MMC inverter station. In order to overcome these disadvantages, the paper proposes a novel TW directional pilot protection for hybrid LCC-MMC-HVDC transmission line. Firstly, special TW propagation process characteristic in hybrid LCC-MMC-HVDC transmission system is analyzed. Secondly, a novel fault direction discrimination algorithm and complete implementation scheme are proposed. Thirdly, using PSCAD/EMTDC, a ±400 kV hybrid LCC-MMC-HVDC transmission system simulation model is established. Eventually, a series of simulations prove that the proposed TW protection actions correctly with different fault locations, different fault resistances and different fault types.

Design and Control of Unidirectional DC–DC Modular Multilevel Converter for Offshore DC Collection Point: Theoretical Analysis and Experimental Validation

This paper presents the design and control of an advanced unidirectional dc–dc modular multilevel converter (MMC), which enables the integration of off-shore windfarms with the high-voltage direct current (HVdc) transmission system. The proposed converter consists of a single-phase MMC inverter, coupled with series-connected rectifier modules through a medium frequency transformer of multiple secondary windings. The modularity feature of the proposed converter enables scalability for different voltage levels. In addition to the galvanic isolation, the transformer also provides stepping gain to the output voltage. The proposed converter shows superior performance in terms of efficiency, losses, and devices utilization, when compared with the most competitive unidirectional cascaded dc–dc converters such as input series output series and input parallel output series. Furthermore, unlike the conventional d-q control method, which involves multiple transformations, this paper employs a simple proportional resonant control strategy that directly acts on the ac output of the MMC, under the stationary reference frame. The analytical design along with the simulation and experimentally validated results, confirmed the excellent performance of the proposed converter.

Comparison study of small-signal stability of MMC-HVDC system in different control modes

The control mode adopted by a modular multilevel converter (MMC) in an HVDC transmission system directly affects both dynamic and steady-state characteristics of the system. This paper, benefiting from a comprehensive dynamic-phasor based state-space model of MMC, presents a comparative analysis on operational stability of the MMC-HVDC station in PV-control mode (i.e., in active-power and AC-voltage-control mode) and PQ-control mode (i.e., in active- and reactive-power-control mode). Firstly, corresponding to the MMC rectifier /inverter in PV- and PQ-control modes, four linearized small-signal models are developed in MATLAB, and validated by electromagnetic transient (EMT) simulations in PSCAD/EMTDC. Then, based on eigen-analysis, the impact of AC-system strength, quantified by the short-circuit-ratio (SCR) and impedance-angle, on the small-signal stability for a MMC rectifier /inverter under PV- and PQ-control are fully compared. Finally, based on the participation factor analysis and parametric sensitivity analysis, this paper digs more deeply into the essential state-variables and critical control-parameters that significantly contribute to the instability of the MMC system under a very weak AC-grid condition. The results show that the stability-margin of the MMC system, either rectifier or inverter, is essentially different when under PV- and PQ-control. This paper provides an effective approach, through adjusting the most sensitive control-parameters, to mitigate the MMC system instability at weak AC-grid conditions, which is a valuable guidance for parametric selection and adjustment in practical engineering.

Unidirectional DC/DC modular multilevel converter for offshore windfarm with the control strategy based on stationary frame

This paper presents the design and control of an advanced unidirectional DC/DC modular multilevel converter (MMC) that facilitate the integration of off-shore windfarms with the high-voltage direct current (HVDC) transmission system. The proposed converter deploys a single-phase MMC inverter coupled with series-connected rectifier modules through multiple medium frequency transformers. Unlike the conventional dq control method, which involves multiple transformations, this paper also proposes a simple control strategy that directly acts on the AC output of the MMC, under the stationary reference frame.

Time/frequency domain modelling for grid‐connected MMC sub‐synchronous/super‐synchronous oscillation in PV MVDC power collection and integration system

This study proposes a comprehensive PWM-based modular multilevel converter (MMC) model in both time and frequency domain at the same time. The time-domain model not only accelerates simulation speed but also describe the characteristics of sub-modules, such as the dynamic voltage-balancing process of sub-module capacitors. The frequency-domain model provides the output impedance of the grid-connected MMC inverter in positive-sequence and negative-sequence coordinates. The proposed model, validated by results of different scenarios listed, at last, predicts and eliminates sub-synchronous/super-synchronous oscillations with impedance shaping method.

On DC-Side Impedance Frequency Characteristics Analysis and DC Voltage Ripple Prediction under Unbalanced Conditions for MMC-HVDC System Based on Maximum Modulation Index

In this study, we first briefly introduce the effect of circulating current control on the modulation signal of a modular multilevel converter (MMC). The maximum modulation index is also theoretically derived. According to the optimal modulation index analysis and the model in the continuous domain, different DC-side output impedance equivalent models of MMC with/without compensating component are derived. The DC-side impedance of MMC inverter station can be regarded as a series xR + yL + zC branch in both cases. The compensating component of the maximum modulation index is also related to the DC equivalent impedance with circulating current control. The frequency characteristic of impedance for MMC, which is observed from its DC side, is analyzed. Finally, this study investigates the prediction of the DC voltage ripple transfer between two-terminal MMC high-voltage direct current systems under unbalanced conditions. The rationality and accuracy of the impedance model are verified through MATLAB/Simulink simulations and experimental results.

Modeling, Control Design, and Analysis of a Startup Scheme for Modular Multilevel Converters

Featuring modularity and high efficiency, a modular multilevel converter (MMC) has become a promising topology in high-voltage direct-current transmission systems. However, its distributed capacitors lead to a more complicated startup process than that of a two-level converter. To fully understand this issue, the charging loops of an MMC rectifier and an MMC inverter during an uncontrolled precharge period are analyzed in this paper, with special focus on the necessity of additional capacitor charging schemes. Moreover, a small-signal model of a capacitor charging loop is first derived according to the internal dynamics of the MMC inverter. Based on this model, a novel startup strategy incorporating an averaging capacitor voltage loop and a feedforward control is proposed, which is capable of an enhanced dynamic response and system stability without sacrificing voltage control precision. The design considerations of the control strategy are also given in detail. Simulation results from a back-to-back MMC system supplying passive loads and experimental results from a scaled-down MMC prototype are provided to support the theoretical analysis and the proposed control scheme.

A Repetitive Control Scheme for Harmonic Suppression of Circulating Current in Modular Multilevel Converters

In a modular multilevel converter (MMC), the interaction between switching actions and fluctuating capacitor voltages of the submodules results in second- and other even-order harmonics in the circulating currents. These harmonic currents will introduce extra power loss, increase current stress of power devices, and even cause instability during transients. Traditional methods for circulating current harmonic suppression have problems such as limited harmonic rejection capability, limited application area, and complex implementation. This paper presents a plug-in repetitive control scheme to solve the problem. It combines the high dynamics of PI controller and good steady-state harmonic suppression of the repetitive controller, and minimizes the interference between the two controllers. It is suitable for multiple harmonic suppression, easy to implement, and applicable for both single-phase and three-phase MMCs. Simulation and experimental results on a single-phase MMC inverter proved the validity of the proposed control method.