Control Strategy For Modular Multilevel Converter Research Articles

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.

Power Equalization Control Strategy for MMCs in Hybrid-Cascaded UHVDC System

Based on the hybrid-cascaded topology of ultra-high-voltage direct current (UHVDC) engineering, this study clarified the mechanism of unbalanced power generation among modular multilevel converters (MMCs) at the inverter side following the fault of the AC system at the rectifying side, and then proposed the power equalization strategy for MMCs. By performing closed-loop control on the active power deviation between constant-voltage and constant-power MMCs, it was possible to achieve automatic power equalization among MMCs after the occurrence of a fault so as to avoid the detrimental effect of a single MMC’s power fluctuation on the connected AC system. Meanwhile, the control enabling logic was designed to ensure the reliable input and stable exit of the control strategy throughout the disturbance period. Finally, a PSCAD/EMTDC platform was used to simulate various types of faults in the AC system at the rectifier side in order to validate the effectiveness of the proposed power equalization strategy.

An MV-Connected Ultra-Fast Charging Station Based on MMC and Dual Active Bridge with Multiple dc Buses

The diffusion of electric vehicles will be strongly related to the capacity to charge them in short times. To do so, the necessity of widespread fast charging stations arises. However, their intermittent demand represents a challenging load for grid operators. In order to relieve their impact on the electrical grid operation, integrating storage systems in the charging stations represents a potential solution, although it complicates the overall system management. Moreover, standard converter architectures for the MV grid interface require the installation of bulky transformers and filters. In order to cope with the mentioned problems, this paper proposes an ultra-fast charging station topology based on a modular multilevel converter (MMC) structure and dual-active bridge (DAB) converters. Thanks to the multilevel converter properties, the proposed charging station can be directly interfaced with the MV grid without requiring transformers or filters. Additionally, exploiting the degree of freedom in the converter control system, such as circulating components, offers uneven power distribution among the converter submodules that can be managed. Along with the MMC control strategy, the article addresses a straightforward methodology to select the main parameters of the DAB converter as a function of the involved grid power and circulating power contributions, with the primary goal of obtaining a trade-off between internal balancing performances and a broad soft-switching region without incurring in converter oversizing. The effectiveness of the proposed charging station is finally discussed through numerical simulations, where its behavior during a power demand cycle is analyzed.

Fast and fault‐tolerant model predictive control of MMCs under selective harmonic elimination

Selective harmonic elimination (SHE) is a suitable pulse-width modulation (PWM) technique for some applications of modular multilevel converters (MMC) with lower number of semiconductor submodules (SM). However, SHE can make it difficult to control circulating currents in the MMC. Moreover, the MMC is vulnerable to SM switch failures that reduce its output power magnitude and quality. This paper proposes a fault-tolerant model predictive control strategy for MMCs operating with SHE (FT SHE-MPC). The effects of switch failure on phase voltage waveforms are studied first. Then, a method is proposed to design the FT SHE waveform to preserve its harmonic performance. Next, the proposed FT SHE-MPC is designed to increase the capacitor voltages during an SM failure to maintain the fundamental component of the faulty phase voltage and control the circulating currents, maintaining balance among line voltages and the output power of the MMC. Additionally, the proposed method offers as much as 60% faster FT transition time and as much as 36% lower capacitor voltage ripple. The advantages of the proposed method are verified through offline and real-time simulation studies on a three-phase nine-level MMC in PSCAD/EMTDC software and RTDS.

A Critical Review of Modular Multilevel Converter Configurations and Submodule Topologies from DC Fault Blocking and Ride-Through Capabilities Viewpoints for HVDC Applications

Modular multilevel converters (MMCs) based on half-bridge submodules (HBSMs) are unable to prevent the AC side contribution to DC side fault currents, thus necessitating circuit breakers (CBs) for protection. A solution to this problem is using submodules (SMs) that are capable of blocking the flow of current from the AC grid to feed the DC side fault. The full-bridge submodule (FBSM) is one type of fault blocking SM where the presence of two extra switches ensures that in the event of a DC fault, the reverse voltage from the FBSM capacitor is placed in the path of the AC side current feeding the DC side fault through the antiparallel diodes. However, the additional semiconductor switches in the FBSMs increase the converter cost, complexity, and losses. Several SM configurations have been proposed in recent years that provide DC fault blocking capability with lower losses and device counts than those of FBSMs. Besides, many of the proposed hybrid converter configurations that combine different topologies to optimize converter performance are also capable of providing DC fault blocking. Furthermore, certain SM topologies are capable of riding through DC faults by remaining deblocked and operating in static synchronous compensator (STATCOM) mode to provide reactive power support to the AC grid. In this paper, noteworthy SM and MMC configurations capable of DC fault blocking and ride-through are reviewed and compared in terms of component requirements, semiconductor losses, and DC fault handing capability. The review also includes a discussion on control strategies for MMC arm/leg energy balancing during STATCOM operation.

Dual Control Structure of Modular Multilevel Converters for Power System Support

This paper presents an analysis of two philosophies to control the modular multilevel converter (MMC). Most MMC control strategies are based on the philosophy used in the traditional voltage-source converter, in which the dc-bus voltage is controlled through the ac-side power; this approach is referred to here as the classical control approach. Because the MMC has an additional degree of freedom, an alternative control philosophy has recently been proposed, in which the dc-bus voltage is controlled through the dc-side power. In this paper, a comparison of these control approaches is presented and their main differences are described. Considering the increasing requirements imposed by system operators to converter-interfaced systems, guidelines are also given to provide supplementary services such as virtual inertia provision, short-term frequency support, virtual capacitor emulation, and oscillation damping. The provided insights are useful for selecting the appropriate MMC control strategy and for developing new supplementary controls for facilities based on the MMC topology.

A novel non-redundant submodules fault-tolerant control strategy for MMC in medium voltage application

Modular multilevel converters (MMCs) are widely used in medium-voltage application. It is necessary to design the control strategy under sub-modules (SMs) failure conditions. To increase the SMs fault ride-though capability of MMC without redundant SMs, a non-redundant fault-tolerant scheme is presented in this paper. Firstly, the operation reliability of MMC is analysed. From the perspective of improving the MMC control strategy, the operation reliability of MMC can be enhanced without redundancy investment. Secondly, the operating characteristics of MMC under SMs failure are derived. And the control objective of the resilient capacitor voltage control strategy is concluded, which is not only appropriate for the MMC in normal state but also appropriate for the MMC in AC system voltage imbalance and SMs fault state. Then, the non-redundant fault-tolerant scheme is proposed, which provides the possibility of reducing investment and enhancing economy while ensuring the operation reliability for MMCs in medium-voltage application. Finally, the control scheme is verified by simulations and experiments on a MMC prototype.

Power Decoupling Control of MMC and Small-Signal Stability Analysis of AC/DC Distribution Network With Renewable Energy

AC/DC hybrid distribution network can realize the high penetration utilization of renewable energy and modular multilevel converter (MMC) is the key equipment to connect AC grid and DC grid. In this paper, the direct modulation based on the virtual resistor and the reference value of dc-bus voltage is adopted, and instantaneous-value model of three-phase half-bridge MMC is derived by introducing differential-mode and common-mode component representation. Then, the decoupling control of AC active power and reactive power is designed based on the linear active disturbance rejection control theory (LADRC). The total disturbances including the coupling term and capacitor voltage fluctuation can be estimated by extended state observer (ESO), and then cancelled exactly. Based on the simplified average-value model of MMC, the small signal stability analysis of three-terminal AC/DC distribution network with 26-level MMCs corresponding to the Tangjiawan-Jishan1-Jishan2 demonstration project is carried out by root locus and bode diagram method, which gives the guidance for the choice of the main circuit component and controller parameters, and shows that the virtual resistor greatly reduces the resonant peak of AC/DC distribution network. Simulation and experiment results verify the effectiveness of the power decoupling control strategy of MMC and the AC/DC distribution network can realize complex multi-directional power flow.

High-Frequency Resonance Analysis and Stabilization Control Strategy of MMC Based on Eigenvalue Method

For the high-frequency resonant accident of modular multilevel converter (MMC) in real engineering, in this paper, the 10th order state-space model of MMC is established based on the dynamic phasor method. The state-space model of MMC controller for power control is established according to the practical engineering. Then the time delay of input and output is considered in the controller, and the Pade fitting in state-space model applicable to the eigenvalue method is introduced to the complete state space model of MMC. Based on the proposed model, the MMC high-frequency resonance phenomenon is analyzed by the eigenvalue method, and the accuracy of the model is verified by simulation results. Besides, it is demonstrated that high time delay causes high-frequency resonance in the MMC system based on the participation factor and the system eigenvalue trajectory. Then, the coupling relationship between the input and output time delay inside the controller and the d-q axis time delay is investigated, and a new time delay compensation method to suppress high-frequency resonance is proposed based on Pade fitting, which can be used jointly with the traditional method with good effect.

SCIG Based Wind Energy Integrated Multiterminal MMC-HVDC Transmission Network

Modular multilevel converter (MMC) based HVDC system for renewable energy integration has attracted the researcher’s interest nowadays. This paper proposes a control strategy for MMC based multiterminal HVDC system for grid integration of squirrel cage induction generator (SCIG) based wind energy systems. Unlike the average model, this work models the MMC using the aggregate model and develops multiterminal HVDC transmission network in MATLAB/Simulink. It further develops the MMC multiterminal HVDC transmission network in real time digital simulator (RTDS). Instead of simplified current source, the proposed network considers the complete dynamics of SCIG based wind source from generation to integration. It employs field-oriented control for optimum wind energy tracking and forms isolated AC grids using feed forward controller. The proposed MMC controller performance has been tested under severe balanced and unbalanced disturbances. The results from aggregate model based MMC network in MATLAB/Simulink and those of the experimental MMC network in RTDS are in full agreement. The results confirm optimum wind energy tracking, fulfill grid code requirements, and improve low voltage ride through capability.

DC Power Control Strategy of MMC for Commutation Failure Prevention in Hybrid Multi-Terminal HVDC System

This article presents a control strategy for a modular multilevel converter (MMC) to prevent commutation failure of a line-commutated converter (LCC), forming a three-terminal hybrid HVDC transmission system, where one LCC sending end is connected to the large generation and two receiving ends (LCC inverter and MMC) are located near the load center. This configuration, one of the potential options, has been proposed to strengthen Korea electric power transmission system through the optimized use of existing assets and rights-of-way, extremely challenging to secure. The MMC power control strategy has been developed to regulate the AC voltage and the extinction angle of the LCC inverter. This indirect yet effective active and reactive power control of the LCC inverter terminal helps prevent the commutation failure (CF) of the LCC in emergency and maximize the benefits of the costly planning option. By establishing a theoretical foundation for this power control problem and relationship among the control parameters, we quantify the active power reference for MMC to secure the desired LCC extinction angle. A coordinated strategy has been developed for the AC filter, the on-load tap changer of a transformer, along with the MMC control to lower the risk of CF and its catastrophic impact on the whole power system. The validity and performance of the proposed control methods are demonstrated for the real Korea electric power planning cases using a real-time power system simulator.

Adaptive AC autoreclosing scheme in MMC‐based hybrid AC/DC transmission

Modular multilevel converter (MMC) is widely used in hybrid AC/DC power transmission system. Strong control capability of MMC provides a new way of the adaptive autoreclosing scheme in such systems to enhance the power supply reliability and system stability. A novel fault identification method is proposed in this study. It is based on characteristic signal injection utilising the control strategy of MMC and line coupling features. A characteristic signal from MMC can be injected to healthy lines which in turn induce characteristic signal to the faulty phase/line. Induced signals have different performances under normal or fault conditions. The proposed method is independent of electrical characteristics of recovery voltage and secondary arc, and multiple independent injections could be implemented. Finally, the adaptive autoreclosing scheme based on injection method is proposed. Simulation studies are carried out under the single line to ground and symmetrical faults, which strongly verify the effectiveness of the proposed scheme.

Modular Multilevel Converter Control Strategy Based on Arm Current Control Under Unbalanced Grid Condition

The existing control strategies of modular multilevel converter (MMC) balance the capacitor voltage on the premise that the active power of ac side is balanced with that of dc bus. Thus, the symmetrical ac-side current references and the unevenly distributed dc current references in three legs are obtained by coordinate transformation, precise calculation, and numerous filters under unbalanced grid condition. However, by controlling capacitor voltages, the active powers between ac side and dc bus can self-regulate to balance, and this could simplify the obtainment of ac-side current references and dc-bus current references. Based on this idea, this paper proposed a control strategy, which combines the multi-hierarchy control with the arm current control for MMC under unbalanced grid condition. Within the multi-hierarchy control, the symmetrical ac-side current references and the unevenly distributed dc current references in three legs could be obtained easily by three voltage controllers in the abc coordinate, avoiding coordinate transformation, precise calculation, and numerous filters in the existing methods. Besides, the employment of the arm current control removes the need of the three-sequence ac-side current controllers and the three-sequence circulating current suppressing controllers. The proportional regulator with a feedforward steady-state duty cycle is designed for arm current regulator, which can perfectly track its reference and is easy to design, avoiding the complicated design of proportional resonant (PR) controllers. Both system-level simulation results and low-level experiment results verify the feasibility and effectiveness of proposed strategy.

Feedback Linearization-Based Current Control Strategy for Modular Multilevel Converters

Modular multilevel converters (MMCs) are multi-input multi-output (MIMO) nonlinear systems. The control systems for MMCs are required to simultaneously achieve multiple control objectives, e.g., output current regulation, submodule capacitor voltage control, and circulating ripple currents suppression. Existing cascaded control strategies for MMCs achieve those control objectives with relatively complex controllers, and the controller parameter design is normally difficult for such nonlinear systems with highly coupled states. In view of this, a feedback linearization-based current control strategy is proposed for an MMC system in this paper. The nonlinear state function model of the MMC is presented and transformed to a linearized and decoupled form with the help of the input-output feedback linearization technique. Based on the linearized system, simple linear controllers are employed to regulate the output and inner differential currents of the MMC, which significantly reduces the difficulty in controller design. The stability of the proposed control strategy is analyzed. The experimental verification results show that, compared to the conventional cascaded control strategies for MMCs, the proposed feedback linearization control strategy is able to achieve improved steady-state and dynamic performances. The robustness of the proposed control strategy against parametric uncertainties is experimentally investigated.

A Novel Flexible Capacitor Voltage Control Strategy for Variable-Speed Drives With Modular Multilevel Converters

Modular multilevel converter (MMC) is very popular in medium-voltage applications, such as high-voltage direct current transmission, static Var generator, and motor drives. The arm capacitor voltage of MMC is usually constant and the output voltage is adjusted by regulating the pulse width modulation (PWM) index for switches in previous studies. In this paper, a novel flexible capacitor voltage control strategy for MMC as motor drives was proposed, with which the arm capacitor voltage was controlled flexibly according to the motor speed, by regulating the dc component and ac component of the PWM reference signal, and unsymmetrical control was used for the control of the upper arm and the lower arm in MMC. With the proposed control strategy, the circuit efficiency and the precision of the output voltage of the converter could be improved when the motor speed was low, and d i /d t and d v /d t of the motor windings could be decreased. These proposals were implemented on a laboratory prototype, and experiments were carried out. The validity of the control strategy under both steady and dynamic states was verified.

An Improved MMC Control Strategy with Single-Phase to Ground Fault-Tolerance Capability

In this paper, an improved modular multilevel converter (MMC) control strategy based on one-cycle control (OCC) is proposed. With the introduction of OCC, the stability, rapidity and disturbance rejection of MMC is greatly improved. Based on the volt-second equilibrium, the control is distributed to every single submodule (SM). For further stability, an unbalanced OCC strategy is designed based on feed-forward control for negative sequence voltage with no need for measuring the positive and negative current sequence, considering single phase to ground fault and the second harmonic ripples of the DC voltage. As the switch frequency of every SM is different, the virtual loop mapping is used in this paper to balance the capacitor voltage with less hardware dependence. And a new simplified structure is also proposed with RS flip-flop abandoned. At last, simulation and experiment results prove the validity of the control theory.