Circulating Current Suppression in Modular Multilevel Converters Based on Hybrid Model Predictive Control

This paper proposed a control method for output and circulating currents of modular multilevel converter (MMC). The output and circulating current are controlled with the help of arm currents, which contain DC, fundamental frequency, and double frequency components. The arm current is transformed into a stationary reference frame (SRF) to isolate the DC and AC components. The AC component is controlled with a conventional proportional resonant (PR) controller, while the DC component is controlled by a proportional controller. The effective control of the upper arm and lower arm ultimately controls the output current so that it delivers the required power to the grid and circulating current in such a way that the second harmonic component is completely vanished leaving behind only the DC component. Comparative results of leg-level control based on PR controller are included in the paper to show the effectiveness of the proposed control scheme. A three-phase, five-level MMC is developed in MATLAB/Simulink to verify the effectiveness of the proposed control method.

Model predictive control (MPC) has emerged as a promising approach to control a modular multilevel converter (MMC). With the help of a cost function, the control objectives of an MMC are achieved easily by using an MPC approach. However, the MPC has several technical challenges and issues including the need of accurate system models, computational complexity, and variable switching frequency operation and weighting factor selection, when it comes to the control of an MMC. In the past few years, several research studies are conducted to address some of the challenges and issues in an MPC and developed several model predictive algorithms for an MMC. In this paper, the importance of each challenge and its impact on the system performance is discussed. Also, the MMC mathematical models used in the implementation of MPC are presented. Furthermore, some of the popular MPC algorithms are discussed briefly, and their features and performance are highlighted through case studies. Finally, summary and future trends of MPC for an MMC are presented.

THIS dissertation addresses the technical challenges associated with the operation and control of high-power modular multilevel converters. To improve the performance of modular multilevel converter (MMC), a generalized three-phase mathematical model with common-mode voltage (CMV) is proposed. By using the proposed mathematical model, the magnitude of circulating currents, capacitors voltage ripple, and the ripple in DC-link current during balanced and unbalanced operating conditions can be minimized. The modulation scheme and switching frequency are directly affected the output power quality and the performance of the converter and control method. In this dissertation, a novel sampled average and space vector modulation scheme is proposed. These modulation schemes are suitable to control the MMC with any number of submodules (without modifications), operates at low switching frequency, minimizes the ripple in output current and voltage harmonic distortion, and reduces the output filter size. For reliable operation of MMC, the voltage balancing among submodules is mandatory. This dissertation proposes a generalized single-stage balancing approach with reduced current sensors to control the MMC. The proposed balancing approach is suitable to implement with both phase-shifted and level-shifted pulse width modulation schemes. With the proposed approach, it is also possible to control the MMC with half-bridge and three level flying capacitor submodules. Also, an improved balancing approach often referred as the dual-stage balancing approach is proposed to minimize the voltage harmonic distortion and device power losses. This dissertation also proposes a direct model predictive control (D-MPC) approach to minimize the ripple in submodule capacitors voltage. To implement D-MPC approach, a discrete-time model of MMC with CMV is proposed. With the use of proposed model, the D-MPC approach does not require a cost function to minimize the circulating currents. The computational complexity is one of the major issues in the implementation of D-MPC approach for MMC. In this dissertation, a novel reduced computational MPC approaches named as dual-stage D-MPC and indirect model predictive control (I-MPC) approach are proposed. These approaches significantly minimize the computational complexity and, improve the voltage and current waveform quality while operating at the low switching frequency. Finally, the simulation and experimental studies are presented to validate the dynamic and steady-state performance of proposed methodologies. Index Terms • Modular Multilevel Converters. • Capacitors Voltage Balancing. • Pulse Width Modulation Schemes. • Circulating Currents. • Capacitors Voltage Ripple • Direct Model Predictive Control. • Dual-Stage Direct Model Predictive Control. • Indirect Model Predictive Control. • Total Harmonic Distortion.

In this paper, we consider an optimization problem solving for model predictive control (MPC) of a modular multilevel converter (MMC). An MMC consists of a large number of submodules. The objective of the MPC is to determine the best switching sequences for the submodules in the MMC to track the phase current references for T time horizons. The MPC is formulated as a nonlinear mixed-integer programming (MIP) problem with the on/off status of submodules as binary decision variables and MMC dynamic states such as phase currents, circulating currents and submodule capacitor voltages as continuous decision variables. With a large number of submodules and a large number of time horizons, the dimension of the nonlinear MIP problem becomes difficult to handle. Our contribution is to formulate this problem and solve this problem using Bender's decomposition. An example 5-level single-phase MMC is demonstrated for the proposed algorithm.

Reducing the power loss and handling short‐circuit faults in HVDC systems are crucial for economical and reliable operation of modular multilevel converters (MMC). This paper proposes a loss reduction method for AC voltage‐boosted FBSM‐MMC by controlling the second harmonic component in the circulating current. The relationship between converter loss and second harmonic circulating current (SHCC) is first derived for AC voltage‐boosted FBSM‐MMC. From the analysis, it can be seen that the converter loss can be reduced by injecting the optimal SHCC into the arm current. The topology comparison between traditional FBSM‐MMC and AC voltage‐boosted FBSM‐MMC is carried out to show the benefit of this method. Simulation results using PSCAD/EMTDC confirm the feasibility and validity of the proposed loss reduction method for AC voltage‐boosted FBSM‐MMC.

The requirement of a large number of sizeable capacitors in traditional modular multilevel converter (MMC) submodules is seen as a major barrier hampering its widespread use in medium-voltage applications. To reduce the value of capacitance, this paper presents a modified MMC (M-MMC) topology, which offers inherent alleviated capacitor voltage fluctuations compared with traditional MMC. Its operating principle, modulation, and capacitor voltage balancing strategies are described in detail. Moreover, a novel capacitor voltage fluctuation suppression scheme is also proposed to further reduce the second-order capacitor voltage ripples. Finally, both a three-phase prototype of M-MMC and traditional MMC have been built in the laboratory. The effectiveness of the proposed M-MMC topology and control methods is experimentally verified by comparing with the traditional MMC.

In this paper, a mathematical model and control methods of a modular multilevel converter are examined in per unit. Some control algorithms, like a sorting algorithm, circulating current control, proportional resonant controller and pulse width modulation techniques which are applicable on this converter are investigated. The average value model is used in per unit for the converter and it is combined with circulating current control method and propotional resonant current controller to suppress the second harmonic component in the circulating current. The effect of the circulating current harmonics on the resonance frequency is investigated. A Matlab/Simulink model is prepared for the simulation of converter modeled on the approach of the average value model and the results are generated from the simulation and they are discussed.

Modular Multilevel Converters (MMC) are known for their high modularity and good quality output power. The problems of circulating current and capacitor voltage balancing for the MMC have been widely studied in available literature but little work is done towards grid current control with fast dynamic response. Conventional current controllers use PI or PR controllers which are slow and thus degrade transient performance of the MMC. Recently reported Model Predictive Control (MPC) methods for MMC aim to control the grid current but MPC suffers from computational burden and difficulties in adjusting of weights. Space Vector based Hysteresis Current Controllers (SCHCC) have been well established for drive applications. In this work, an SVHCC has been developed for a grid connected MMC system. Simulation studies show fast current control for cases of sudden change of active power reference and current reference. The controller exhibits quick tracking of the current reference in all cases. The proposed method has been discussed for a MMC system with three submodules, but it may be extended to any number of submodules and can be easily implemented in a digital controller with less computational effort.

The Current control of voltage source inverters are realized by many techniques such as Proportional Integral (PI), Proportional Resonant (PR) and Hysteresis Current Control. The PR control works on an αβ frame and it involves less calculation process comparing to PI control. The αβ frame should be converted to dq frame in PI control. The PR control can easily implemented with digital control techniques due to the slow sampling frequency. PR control has very high proportional and integral gain at resonance frequency where it can mitigate low order harmonics. PR control can be implemented with other digital control techniques. In this study, a Seven-Level asymmetric cascade multilevel inverter simulation is realized with PR control. As a result of structure developed, the Total Harmonic Distortion (THD) is more lowered with respect to the structure without using PR control.

To improve the load frequency control (LFC) performance of power systems incorporating virtual power plants (VPPs) while reducing network resource consumption, a model predictive control (MPC) method based on a mixed time/event-triggered mechanism (MTETM) is proposed. This mechanism integrates an event-triggered mechanism (ETM) with a time-triggered mechanism (TTM), where ETM avoids unnecessary signal transmission and TTM ensures fundamental control performance. Subsequently, for the LFC system incorporating VPPs, a state hard constrained MPC problem is formulated and transformed into a “min-max” optimisation problem. Through linear matrix inequalities, the original optimisation problem is equivalently transformed into an auxiliary optimisation problem, with the optimal control law solved via rolling optimisation. Theoretical analysis demonstrates that the proposed auxiliary optimisation problem possesses recursive feasibility, whilst the closed-loop system satisfies input-to-state stability. Finally, validation through case studies of two regional power systems demonstrates that the MPC approach based on MTETM outperforms the ETM-based MPC approach in terms of control performance while maintaining a triggering rate of 33.3%. Compared with the TTM-based MPC algorithm, the MTETM-based MPC method reduces the triggering rate by 66.7%, while maintaining nearly equivalent control performance. Consequently, the results validate the effectiveness of the MTETM-based MPC approach in conserving network resources while maintaining control performance.

The matrix converter has emerged as a direct AC/AC converter and has attracted research attention. This work proposes a Proportional Resonant (PR) controller based on the Space Vector Modulation (SVM) method for the three-phase direct matrix converter and its application systems. The PR controller, compared with the widely used PI controller, has better steady-state error performance, specific harmonics compensation capability, and the ability to handle sinusoidal quantities. These enhance the tracking performance of the converters and benefit the current quality improvement and selective harmonics suppression. The PR controller can be implemented in the natural frame in a straightforward manner, which removes the frame transformations involved in the stationary (αβ) and synchronous (dq) reference frame based control strategies. This alleviates the computation burden. Simulation results verify the effectiveness of the proposed PR controller for the matrix converter applications.

Existing control strategies of modular multilevel converter (MMC) have many drawbacks, such as need of several parameter controllers, complex parameter tuning and huge computational burden. This paper proposes a novel control method based on the hybrid architecture of proportional integral (PI) control and model predictive control (MPC). The PI controller is used as external controllers to control the ac-side power and current, while the MPC controller is used to control the internal features of the MMC, i.e. suppressing the circulating current and balancing the capacitor voltages. The proposed hybrid method takes both advantages of PI and MPC strategies, which lightens the computational burden and improves dynamic performance and steady-state accuracy. The validity of the proposed method is verified by the simulation results of a 11-level MMC system using Matlab/Simulink.

This paper presents the hardware in the loop of control the position of electro hydraulic servo system based on Data Acquisition Card (NI-6014) and hybrid Model Predictive Controller (MPC). The realization of hybrid MPC depends on using Data Card and Matlab/ Simulink as the core of the control system. In addition, this paper discusses the using of Cuckoo Search Algorithm (CSA) for optimal tuning of the traditional controller parameters as a part of hybrid MPC controller. The experimental implementation of the studied system has been investigated using REXROTH components. The Electro Hydraulic Servo System mechanism (EHSS) plays an important role in industrial applications, especially in flight simulators, aircraft landing gear system, material testing machine and hydraulic press machines. The key reason of applying hydraulic systems on these applications is that it can give a high torque and high force. The theoretical and experimental results demonstrate that the values of performance criteria (settling, rise times and system overshoots) are the minimum in case of realization hybrid MPC position control over using traditional and MPC controllers. It also introduced that the integral time absolute error for various types of test signals (Multistep, sinusoidal and square signals) are a minimum values in case of using the proposed strategy in compared with the traditional and MPC controllers. In addition, hybrid MPC achieved the required physical limits of the actuator. As a general conclusion, one can conclude that the hybrid model predictive control has the priority of applying it in the field of electro hydraulic servo system due to the mentioned advantages.

Active-neutral-point-clamped (ANPC) converters can be decomposed into a low-frequency cell (LFC) and a high-frequency cell (HFC), and fundamental-frequency operation of the LFC is becoming popular. However, balancing the dc-link voltage and three flying capacitor voltages while maintaining very fast control of the output current is always a challenging task, especially when the flying capacitor is small. This article proposes a hybrid model predictive control (MPC) to guarantee the fundamental-frequency operation of the LFC and achieve the fixed-switching frequency operation of the HFC. In the hybrid MPC, the classical MPC regulates the LFC, whereas the duty-cycle-optimized MPC regulates the HFC. The capacitor voltage balance is achieved by adjusting the duty cycles to avoid the weighting factor tuning process. The proposed MPC features a fixed switching frequency and improves the steady-state performance while guaranteeing the fundamental-frequency operation of the LFC. Simulation and experimental results on a five-level ANPC are presented to validate the advantages of the proposed hybrid MPC method over the classical MPC.

Due to the ability to control several parameters by the cost function and the high dynamic response as well as direct modeling, the model predictive control (MPC) is among the best methods for controlling a modular multilevel converter (MMC). In conventional MPC, the optimal value of the cost function is obtained after calculating all switching states. If the number of sub-modules (SMs) increases, the computational volume also increases exponentially, which makes this method impossible to implement. In this paper, an improved indirect model predictive control (IIMPC) is presented to control the parameters of MMC, such as circulating and load currents and capacitor voltage. Moreover, this method can make the computational volume independent from the number of SMs. On the other hand, to control the voltages of the capacitors, a sorting algorithm has been used. The superiority and privilege of the proposed IIMPC are verified by simulation results of a five-level single-phase MMC with four SMs in each arm.