Control Of Modular Multilevel Converter Research Articles

A voltage-balancing algorithm based on the gating signal rotation and Separated Circulating Current Controller applied on a Modular Multilevel Converter

The Modular Multilevel Converter (MMC) is being used for many medium/high-power energy conversion applications due to its superior advantages such as modularity, scalability, and low harmonic generation. However, the MMC control can be challenging. If the submodule (SM) capacitors are not properly balanced, a circulating current is induced due to the voltage mismatch between the capacitor submodules. This circulating current adds to the arm current, which increases its RMS and peak value, and thus increases system losses. Therefore, controlling the submodule capacitor voltage and the circulating current is an important step in any MMC control. In this paper, a Gating Pattern Rotation algorithm is used for SM capacitor voltage balancing, and an independent circulating current controller in the double reference frame is used to eliminate the circulating current. Furthermore, a voltage-oriented control is employed to eliminate harmonics and regulate the grid power factor. MATLAB Simulink simulates the control technique, and various simulation results are presented. With results showing their effectiveness in managing and improving MMC behavior.

Theoretical and simulation analysis of DC bus current fluctuation control strategies of modular multilevel converter under AC side fault conditions

Modular Multilevel Converters (MMCs) play a crucial role in high-voltage DC (HVDC) systems due to their adaptable control and swift response capabilities. However, AC side faults could introduce asymmetric double-line frequency components into the MMC's circulating current, jeopardizing DC bus stability. Although strategies to mitigate DC side fluctuations are well-established, a detailed comparison of their operational impacts remains unexplored. The operational effects of MMCs under AC side faults are firstly examined in this study, including issues related to grid-connected current asymmetry and DC bus current fluctuations. Subsequently, two targeted control schemes are devised based on the goals of suppressing the double-line frequency components and only the zero-sequence components of circulating current. Through simulation, these approaches are contrasted, delineating their advantages and disadvantages in terms of circulating current suppression, submodules(SMs) capacitance voltage fluctuation, and other performance metrics, which could provide a foundational reference for designing and selecting MMC control strategies under AC side faults.

A Data-Driven Control for Modular Multilevel Converters Based on Model-Free Adaptive Control with an Event-Triggered Scheme

Modular multilevel converters (MMCs) have gained widespread adoption in high-voltage direct current (HVDC) transmission due to their high voltage levels, low harmonic content, and high scalability. However, conventional control methods such as finite control set model predictive control (FCS-MPC) suffer from a heavy computational burden and sensitivity to system parameter variations, limiting the performance of MMCs. This paper proposes a data-driven approach based on model-free adaptive control with an event-triggered mechanism that demonstrates superior robustness against parameter mismatches and enhanced dynamic performance in response to sudden output changes. Moreover, the introduction of the event-triggered mechanism effectively reduces redundant operations, decreasing the computational burden and switching losses. Finally, the proposed strategy is validated through a MATLAB/Simulink simulation model.

Dependability Analysis of the Negative-Sequence Turn-to-Turn Fault Protection Schemes for MMC-HVDC Converter Transformers

This paper addresses two complex areas of research: the detection of turn-to-turn faults (TTF) in power transformers and the impact of inverter-based resources on the TTF protection scheme operation. Detecting turn-to-turn faults in power transformers by protection algorithms poses a challenge due to the minimal fault currents observed at transformer terminals. Yet, the demand for dependable TTF protection is very high because of the high fault currents inside the shorted turns and the resulting damage consequences. On the other hand, for such sensitive protection, adverse conditions such as transformer inrush currents or CT errors may lead to protection maloperation. Moreover, the fault current characteristic of the inverter-based source infeed is very different compared to the synchronous machine infeed, particularly concerning the negative-sequence current used in the TTF protection schemes, which calls for thorough research analysis. A simulation model of the converter transformer capable of simulating TTFs, and the Modular Multilevel Converter (MMC) for a High Voltage Direct Current (HVDC) transmission link, has been developed. The test results of turn-to-turn fault protection schemes in inverter-based generation-dominated power systems compared to the synchronous generator infeed are presented. The negative-sequence current protection quantities are analysed in more detail for TTFs with small and large number of shorted turns, i.e. without and with reactive negative-sequence current injection by the MMC control. Finally, the paper assesses the dependability of the transformer differential protection and sensitive TTF protection schemes in detecting faults with different numbers of shorted turns and fault resistance for TTFs occurring in the star and delta winding of the converter transformer.

Energy Control of Modular Multilevel Converters for Drive Applications at Low Frequency Using General Averaging

Energy Control of Modular Multilevel Converters for Drive Applications at Low Frequency Using General Averaging

Three-stage model predictive control for modular multilevel converters with comprehensive performance optimization

Three-stage model predictive control for modular multilevel converters with comprehensive performance optimization

Parallel multi‐rate simulation scheme for modular multilevel converter‐based high‐voltage direct current with accurate simulation of high‐frequency characteristics and field programmable gate array‐based implementation

AbstractThe real‐time simulation of the modular multilevel converter‐based high‐voltage direct current (MMC‐HVDC) transmission system has become a popular research topic. However, in order to meet the real‐time performance, the real‐time simulation technology will cause additional simulation errors for MMC‐HVDC, especially on its frequency characteristics. Therefore, a parallel multi‐rate simulation scheme for MMC‐HVDC is developed in this work to ensure accurate simulation of high‐frequency characteristics. Firstly, a non‐error method based on converter transformer decoupling is proposed to decouple the converter and alternating current system; direct current transmission line decoupling and arm decoupling methods are used to achieve decoupling among and within converters. A multi‐rate data synchronous mechanism is established by considering the differences among high‐frequency characteristics caused by delayed data interaction. Secondly, the computing architectures of the primary system solver and modular multilevel converter controller are designed based on a field programmable gate array (FPGA). The real‐time simulation platform for a four‐terminal true bipolar MMC‐HVDC is constructed based on the FPGA array. Thirdly, the factors in multi‐rate simulation affecting the simulation accuracy of high‐frequency characteristics are analysed. The simulator is shown to be accurate in steady and dynamic states. The authors also verify its applicability for further research on high‐frequency resonance based on control experiments.

Research on modular multi-level converter control based on passivity theory

All In view of the poor dynamic performance of the conventional PI control method in modular multilevel converter (MMC) systems, an up-to-date control strategy using passive theory is proposed. First, the topology of the system and its mathematical model are presented, then its Euler-Lagrange (EL) model is developed and its passivity is proved. Next, the dq-axis component of the current is chosen as the state variable to derive the passivity-based control (PBC) law and from this, the current passivity controller is designed. Simulation shows that compared with conventional PI control, the PBC in this paper can respond quickly to the change of current or power under different operating conditions, with good harmonic characteristics and better dynamic characteristics.

Optimized Control for MMCs with Reduced Power Loss and Extended Lifetime

Modular multilevel converters (MMCs) are widely applied to medium and high voltage occasions. The total power consumption of the submodule (SM) and the maximum power consumption of power devices in the SM are related to the operating costs and lifetime of the MMC. Existing literature only considers total power loss optimization or maximal power consumption optimization in MMC’s SM. In this article, a reduced loss and extended lifetime power loss optimum control (RLEL-PLOC) is introduced to inject the first-best second harmonic circulation into the MMC’s arm current. Compared with the conventional power loss optimization control, the proposed control could decrease the maximal power consumption of the semiconductor devices without increasing the total loss of the SM. According to study results of the MMC, compared with the circulating current suppression control (CCSC) method, the total power consumption of the SM could be reduced by 4.2% and the maximal power dissipation in the SM could be reduced by 5.4% with RLEL-PLOC. PSCAD simulation and MMC prototype experiment are also carried out, and the research results verified the availability of the put forward RLEL-PLOC for MMCs.

Arm‐current sensorless model predictive control for grid‐interfacing modular multilevel converter with reduced switching frequency

AbstractModel predictive control (MPC) emerges as an attractive alternative for the control of modular multilevel converters (MMCs) owing to its superior dynamic performance and ease of implementation. Nevertheless, conventional MPC's performance could be further improved by reducing dependence on weighting factors, switching frequency, and computational burden. To achieve these objectives, an arm‐current sensorless MPC is developed for a grid‐interfacing MMC in this article. The MPC is integrated with an arm‐current sensorless reduced switching frequency voltage balance algorithm that requires only switching vectors (where number of submodules per arm) to optimize the cost function. This method does not require knowledge of arm‐current direction to decide the charging and discharging state of the submodule capacitors. The developed control not only reduces the switching frequency of devices but also eliminates the need to sense the arm currents. As a result, it reduces the system's cost, complexity, computational burden, switching losses and provides a more reliable control method for MMCs with superior dynamic performance. Further, the effect of parameter mismatch on the developed MPC's dynamic performance and stability is studied. The developed MPC's effectiveness is validated through both simulation and experimental results and also compared with existing methods under different conditions.

Network based impedance analysis of grid forming based MMC-HVDC with wind farm integration

Grid forming control for modular multilevel converter based high voltage direct current (MMC-HVDC) transmission system is favorable to support the AC systems. This paper presents the hybrid AC-DC impedance model for grid forming based MMC-HVDC with wind farm integration. Comparisons of the hybrid AC-DC impedance with the traditional AC/DC impedance are presented, and the hybrid AC-DC impedance does not contain right half-plane poles caused by DC/AC coupling. Then, the network based impedance analysis is carried out to analyze the stability of grid-forming based MMC-HVDC with wind farm integration. It is found that grid forming based MMC-HVDC may lack damping with two modes of oscillations. Large inertia, DC/AC virtual impedance and wind farm integration can dampen the first mode oscillation, while only AC virtual impedance is effective for the second mode oscillation. The stability comparison of grid forming and grid following based wind farm integration is also presented. Moreover, MMC-HVDC with grid forming control cannot improve the stability of grid following based wind farm integration in weak grid where grid forming based wind farm performs better. Therefore, both wind farm and MMC-HVDC with grid forming control are better in weak grid. Simulation results in PSCAD/EMTDC validate the corresponding analysis.

Improved power loss balance control for modular multilevel converters based on variable capacitor voltage deviation predefined value under SM malfunction

When it concerns the reliable working of the modular multilevel converter (MMC), the power loss balance control cannot be ignored. Under the submodule (SM) malfunction condition, each arm of the MMC operates in an asymmetrical status, resulting in unbalanced power loss in every arm of MMCs, which affects the converter operation’s reliability. This essay proposes a variable capacitor voltage deviation predefined value-based power loss balance control (VCVDPV-PLBC), which could optimize faulty arms’ power losses by regulating capacitor voltage deviation predefined value to change power devices’ switching loss in MMCs under SM malfunction. On this basis, the coupling relationship between SM’s switching frequency and the capacitor voltage deviation predefined value is revealed. The raised VCVDPV-PLBC could effectually prolong the service life of power devices in faulty arms and strengthen the reliability of MMC system under the malfunction of the SM. The validity of the raised VCVDPV-PLBC was proved by simulation results with specialized software PSCAD/EMTDC and experimental studies with small-scale MMC setup.

A enhanced control strategy for capacitance voltage equalization in modular multilevel converter

In searching for the optimal control strategy of sub-module capacitance voltage equalization of Modular Multilevel Converter (MMC), the most important issue is how to address the significant increase in computing time of the controller, switching frequency of the sub-module, device switching loss and MMC fault rate led by the increase in the number of sub-modules of the upper/lower bridge arm of the MMC. Therefore, this paper proposes a control strategy based on the retaining factor method + improved quick sort, namely, an improved quick sort algorithm is used to sort the sub-module capacitance voltages to reduce the amount of computation of the MMC controller; at the same time, a retaining factor is introduced to participate in the improved quick sort to significantly reduce the switching frequency of the sub-module. MMC of large ship grid-connected photovoltaic power system is taken as a research object in this paper, a 31-level MMC model is built in Matlab/Simulink. Simulation results verify that the improved quick sort has a good rapidity compared to the algorithms of bubble sort, quick sort, bidirectional bubble sort, and improved merge sort, that the retention factor method has a good frequency reduction effect, and that the proposed retention factor + improved quick sort can provide a good performance compared to improved quick sort and retention factor + improved merge sort under conditions of the normal operation and introducing disturbance.

Modulation-Compensated Power Synchronization Control for Modular Multilevel Converters With Enhanced Performance Under Varying SCR

Modulation-Compensated Power Synchronization Control for Modular Multilevel Converters With Enhanced Performance Under Varying SCR

An MMC control strategy without bridge arm current sensors

Each bridge arm of a modular multilevel converter (MMC) is connected in series with multiple submodules. This structure increases the amount of current detection, making it necessary to have a number of current sensors in operation, which increases the system cost. For this reason, this paper proposes an MMC control strategy that does not require bridge arm current sensors. Firstly, the nearest level modulation (NLM) strategy requires the bridge arm current to be used to determine the submodule entry direction. In this case, this article proposes a method for determining the input direction of the submodule based on the capacitive voltage of the submodule, which calculates the input method of the submodule by analyzing the state quantities in the bridge arm voltage switching function, omitting the bridge arm current sensor and reducing the cost. Secondly, the traditional MMC circulation suppression method needs to detect the bridge arm current to calculate the circulation. However, this paper proposes an indirect circulation suppression method by suppressing the unbalanced voltage based on the analysis of the circulation generation mechanism. Finally, simulations and experiments validate the effectiveness of this method and lay the foundation for the next engineering application.

Virtual synchronous generator‐based control of modular multilevel converter for integration into weak grid

AbstractIntegrating Power Electronics Converters (PECs)‐based renewable energy sources can establish a weak grid due to the substantial inertia reduction leading to severe frequency fluctuations. In this paper, a control strategy based on the Virtual Synchronous Generator (VSG) concept is designed for a three‐phase Modular Multilevel Converter (MMC) connected to a weak grid. The proposed VSG control approach improves the frequency response during transient mode, resulting in a 66.7% enhancement at the Point of Common Coupling (PCC). The proposed strategy is shaped within four control loops wherein the VSG‐dependent output current control possesses the duty of enriching the MMC output current tracking capability. Focusing on the MMC output current‐based transfer function, the effectiveness of this capability is guaranteed using the frequency domain analysis including the Nyquist and Bode diagrams. Moreover, a comprehensive discussion was carried out regarding the impact of VSG on the voltage of submodule capacitors and the circulating current. Simulation results in MATLAB/Simulink environment validate the accuracy and weak grid‐connected MMC performance under the proposed VSG‐based control technique. To further verify the superiority of the proposed strategy, two control techniques including Droop and SEBIR are compared with the VSG proposed controller.

Predictive Control of Modular Multilevel Converters: Adaptive Hybrid Framework for Circulating Current and Capacitor Voltage Fluctuation Suppression

Modular multilevel converters (MMCs) are widely used in voltage-sourced, converter-based high-voltage DC systems due to their modular design, scalability, and fault tolerance capabilities. In MMCs, multi-variable control objectives can be employed by using model predictive control (MPC) due to its fast dynamic response and ease of implementation. Nonetheless, conventional MPC techniques for MMCs have shortcomings, including high computational requirements, poor circulating current, and capacitor voltage fluctuation suppression. First, this study proposes an adaptive MPC technique that adapts the number of candidate combinations to the steady and transient states, significantly reducing the computational burden. Second, an improved hybrid combination of an MPC with a proportional-resonance (PR) controller enhances the circulating current and capacitor voltage fluctuation suppression performance. According to the phase difference between the circulating current and the capacitor voltage, the circulating current and capacitor voltage can be suppressed at different times by changing the circulating current reference of the PR controller. The switching frequency can be reduced by using the PR controller’s output to adjust the input submodule number instead of changing the duty cycle. The proposed techniques were validated by simulations and experimental case studies with a three-phase grid-connected MMC.

Variable Sampling Frequency-Based SM Power Loss Balancing Control for MMCs With Bypassed Faulty SMs

The modular multilevel converter (MMC) is one of the most appropriate topologies for high power applications due to its high modularity and low harmonic distortion. As faulty submodules (SMs) are bypassed, the capacitor voltage and switching frequency of healthy SMs in faulty arm will rise, which causes unbalanced power losses distribution among the arms of MMCs and affects the reliability of MMCs. This paper proposes a variable sampling frequency based power losses balancing control (VSF-PLBC), which can balance power losses distribution among the arms of MMCs through adjusting sampling frequency for arm inserted SM number to modify switching losses in power devices in the MMC with bypassed SMs. The proposed VSF-PLBC can effectively improve the reliability of the MMC with the balanced power losses distribution among the arms in the MMC with bypassed SMs. The proposed VSF-PLBC is simulated with the professional tool PSCAD/EMTDC and is validated on a down-scale MMC prototype. The simulation and experimental results confirm the effectiveness of the proposed VSF-PLBC.

Equivalent model of nearest level modulation for fast electromagnetic transient simulation based on DC voltage control loops of sub‐modules in modular multilevel converter

AbstractThe nearest level modulation (NLM) is used to automatically generate ac side voltages of sub‐modules in modular multilevel converter (MMC). Existing literature seldom presents models that describe complex NLM processes. This paper proposes an equivalent model of the NLM to simply describe and simulate the complex NLM process. The main contribution of this study is to build the model of NLM with fast simulation speed for the application focuses on the simulation accuracy at the millisecond level. The duty cycle of the average model generated by the NLM in sub‐modules of the MMC is divided into two components, which are the stable component for generating the bridge arm voltage and the fluctuation component for balancing the dc side voltage of sub‐modules, respectively. The stable component is generated by the modulation wave of the MMC control system. By adding a voltage control closed loop to the sub‐module, the fluctuation component for balancing the dc side voltages of sub‐modules can automatically adjust the ac side voltages of sub‐modules. The proposed equivalent model of NLM improves the simulation speed and has the same effect with the traditional NLM within the low‐frequency range or with the time step of millisecond level.

High-Frequency Oscillation Suppression Strategy for Flexible DC Transmission Systems Based on Additional Joint Damping

Flexible DC transmission systems experience negative damping characteristics in the high-frequency range due to the delay in the system control link. This can lead to interactions between the impedance of modular multilevel converters (MMCs) and AC transmission lines, resulting in high-frequency oscillation (HFO) issues. To resolve this problem, a simplified MMC control model was developed, taking into account the current inner loop control link. The Sobol method was employed to analyze the factors influencing the impedance characteristics of MMC. Additionally, a combined approach involving a virtual passive filtering method and an equivalent current sampling method was proposed to provide additional damping. The study compared and analyzed the impact of additional damping control on the impedance characteristics and system stability of MMCs. Lastly, a flexible direct current grid connection simulation model was constructed using PSCAD to validate the effectiveness of the proposed additional damping control strategy.