Analysis and Control of Single Phase Grid Connected Modular Multilevel Converter in Per Unit

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.

The modular multilevel converter (MMC) has unique topological characteristics and has gained significant popularity in medium-voltage applications. However, during MMC operation, circulating currents inevitably arise, exacerbating arm current distortion, causing additional losses in the converter system, and thereby increasing system costs. This paper primarily addresses the circulating current issue in traditional half-bridge MMCs by introducing a control strategy combining model predictive control and proportional resonance controllers. First, a value function is established using a discrete prediction model of the output current, followed by a rolling optimization that combines a capacitor voltage sorting strategy to determine the duty cycles of each submodule in the arm. Secondly, a proportional resonance controller (PR) is designed to eliminate the second- and fourth-order harmonic components in the circulating current. Finally, the output of the resonance controller is used to correct the duty cycles, which are then compared with the PWM triangular carrier to generate more switching actions that suppress the circulating current. The effectiveness of the strategy is experimentally verified. The results show that the proposed method yields better output characteristics, smaller capacitor voltage fluctuations, and significantly suppresses harmonic components in the arm currents.

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.

One of the advantages of multiphase machine is the capability of fault tolerance. Variety of post-fault control methods based on hysteresis, proportional-integral (PI), proportional resonant (PR) and model predictive controller (MPC) have gradually been developed. Due to simple structure and decoupling of reduced-order matrix, it is used as an alternative to the Clarke matrix in the healthy mode when the fault happens. Although a lot of control methods adopted a reduced-order matrix as the basic derived technology, the performance of controllers is different, and they need to be compared in term of specific applications in fault tolerance. For this propose, the paper deals with the steady-state and dynamic performance of four current control methods, hysteresis control, PI control, PR control and model predictive control in open-circuit fault. The results shows that the PR control method has low torque ripple, copper loss and derating in healthy and post-fault stage, and the MPC control method has low torque ripple in fault stage and fast response capability after applying fault-tolerant methods.

Integration of photovoltaic (PV) power to the grid is achieved using three-phase inverters with high-quality current waveforms. The new grid codes impose a limit on the total harmonic distortion (THD) value of the inverter’s current waveforms. Due to simple structure and ease of design, proportional integral (PI) controllers are the most widely adopted methods to control three-phase grid-connected inverters; however under abnormal grid conditions, PI controllers may suffer from instability problems. To address the aforementioned problem, this article proposes an expert knowledge-based proportional resonant (PR) control for the integration of PV power to the grid under abnormal grid conditions. The proposed control method adjusts the control parameters based on rules generated from expert knowledge. Initially, parameters of the proposed control methods are optimized using particle swarm optimization (PSO) method. The proposed control is tested with a 100 kW grid-tied three-phase inverter unit under normal and abnormal grid conditions and its performance are compared with PI and fixed gain PR control methods. Moreover, the proposed control is tested experimentally with a scaled down 300 W grid-tied inverter lab prototype module. From the simulation and experimental results, it is verified that the proposed adaptive PR control technique is capable of providing low THD in the injected current under abnormal grid conditions compared with fixed gained PR controllers.

The large amount of power electronic submodules and semiconductor switching events in the Modular Multilevel Converter (MMC) introduces several challenges for efficient and accurate Electro-Magnetic Transient (EMT) simulation. Research efforts have focused on developing Average Value Models (AVMs) of MMC that enable fast and accurate dynamic simulation of the converter. This paper proposes a universal blocking-module-based AVM, which can simulate the MMC of different submodule types and provide accurate results for the MMC operating in both blocking and de-blocking modes. An analytical approach is included in the model to calculate the semiconductor losses of different submodule types in the MMC. The proposed model is validated against a detailed switching-based model and the state-of-the-art AVMs in a 41-level two-terminal MMC-HVDC system.

This paper proposes an enhanced control method for a high-voltage direct-current (HVDC) modular multilevel converter (MMC). To control an MMC-HVDC system properly, the ac current, circulating current, and submodule (SM) capacitor voltage are considered. The ac-side current is a fundamental frequency component, and the circulating current is a double-line frequency component. Existing control methods control the ac current and circulating current by separating each component. However, the existing methods have a disadvantage in that the ac-side current must be separated into the positive and negative sequences for control under an unbalanced voltage condition. The circulating current consists of not only negative-sequence components but also positive- and zero-sequence components under an unbalanced voltage condition. Therefore, an additional control method is necessary to consider the positive- and zero-sequence components of the circulating current. The proposed control method has the advantage of controlling not only the ac-side current of the MMC but also the circulating current without separating each of the current components to control each arm current of the MMC. In addition, it can stably control the positive- and zero-sequence components of the circulating current under the unbalanced voltage condition.

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.

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 proportional–integral (PI) control is usually applied in synchronous reference frame to regulate the output voltage of three-phase voltage source inverter (VSI). However, the PI controller is constrained by the capability of suppressing harmonic and negative sequence components; besides, its direct application in single-phase VSI is limited. While the proportional–resonant (PR) control can track ac reference and achieve harmonic suppression in stationary frame for both single-phase and three-phase VSI. Due to the high order of the resonant controller, lots of efforts are needed during the parameter design to get a satisfactory performance, such as trial-and-error. In this article, the way to approximately reduce the order of the resonant controller in the discrete time domain is presented, and then a method of selective pole placement and cancellation for the PR control design used in VSI is proposed. With the proposed method, the closed-loop pole-zero distribution of the PR control structure used in the VSI is well handled, and the parameters of the feedback controller and the reference feedforward controller are directly calculated by placing the selected poles and zeros in desired positions. The procedure of the parameter design is facilitated, and the performance of the VSI with PR control is improved.

Although the control of modular multi-level converters (MMCs) in high-voltage direct-current (HVDC) networks has become a mature subject these days, the potential for adverse interactions between different converter controls remains an under-researched challenge attracting the attention from both academia and industry. Even for point-to-point HVDC links (i.e., simple HVDC systems), converter control interactions may result in the shifting of system operating voltages, increased power losses, and unintended power imbalances at converter stations. To bridge this research gap, the risk of multiple cross-over of control characteristics of MMCs is assessed in this paper through mathematical analysis, computational simulation, and experimental validation. Specifically, the following point-to-point HVDC link configurations are examined: (1) one MMC station equipped with a current versus voltage droop control and the other station equipped with a constant power control; and (2) one MMC station equipped with a power versus voltage droop control and the other station equipped with a constant current control. Design guidelines for droop coefficients are provided to prevent adverse control interactions. A 60-kW MMC test-rig is used to experimentally verify the impact of multiple crossing of control characteristics of the DC system configurations, with results verified through software simulation in MATLAB/Simulink using an open access toolbox. Results show that in operating conditions of 650 V and 50 A (DC voltage and DC current), drifts of 7.7% in the DC voltage and of 10% in the DC current occur due to adverse control interactions under the current versus voltage droop and power control scheme. Similarly, drifts of 7.7% both in the DC voltage and power occur under the power versus voltage droop and current control scheme.

Presently the global wind energy capacity is increasing rapidly and it has become the fastest developing renewable energy technology. The early technology used in wind turbines was based on squirrel cage induction generators directly connected to the grid through normal converters. In this paper, a new modular multilevel converter based integrated wind energy system for reducing harmonics with different pulse width modulation techniques is proposed. Modular multilevel converter is the state of art technology and hence this converter is integrated with wind energy system. In this paper, harmonics, voltage and current responses at different loads at different wind conditions by using new proposed integrated wind energy system connected to a modular multilevel converter through multi winding transformer are compared. The proposed model of modular multilevel converter integrated to the wind energy system is developed in MATLAB/SIMULINK.

Modular multilevel converter (MMC) has been widely used in high-voltage direct current (HVDC) transmission system due to its advantages of modularity, low switching frequency and excellent output voltage waveforms. Proper operation of MMC is based on capacitor voltage balancing. Conventional MMC capacitor voltage balancing control needs a large number of voltage sensors and sorting algorithm which increases computing burden on the processor. In this paper, a capacitor voltage self-balancing MMC with DC fault ride-through capability is proposed. Capacitor voltage balancing control of this proposed MMC is significantly simple due to its self-balancing auxiliary circuit. Sorting algorithm is not needed, each phase leg only needs one voltage sensor for top SM. So control complexity and computing burden are reduced greatly. Furthermore, DC fault ride-through capability is also reserved. Simulation results validates capacitor voltage of this new MMC topology can be well balanced and DC fault ride-through capability is effective.

The large number of switching elements in modular multilevel converters (MMCs) contribute a tremendous computational burden for electromagnetic transient (EMT) simulation programs. Detailed equivalent models (DEMs) and average value model (AVMs) are currently two major types of accurate and efficient model. However, the DEMs are still computationally inefficient for the simulation scenarios in large-scale MMC based high-voltage direct current (MMC-HVDC) grids, as the models represent all submodule (SMs) switching events, and memorize all individual capacitor voltages. Though the AVMs provide a faster simulation speed by using a single equivalent capacitor on the DC side, they have a relatively low simulation accuracy compared to DEMs, especially under blocked mode. This paper proposes an enhanced computationally efficient model based on arm equivalence (AEM), which can accurately represent the dynamic behaviors in both de-blocked and blocked modes. Compared to the DEMs, the proposed AEM is more efficient, with no loss of accuracy, and the simulation speed is irrespective of the SM number. The accuracy and computational efficiency of the proposed model were validated against the DEM and AVM through several simulation scenarios in a two-terminal MMC-HVDC system on the power systems computer aided design/ electromagnetic transient in DC system (PSCAD/EMTDC) program.

The multilevel cascaded converters are extensively applied for high-voltage and high-power applications, which are considered as the most favorable topologies for the high power ac-drives and the high-voltage DC transmission (HVDC) systems using the voltage source converters (VSCs). This paper presents the dc capacitor balancing and effective current regulation methodologies for the multilevel cascaded converters. The dc-link voltage balancing and capacitor energy evolving mechanism of the MMCC is analyzed, and an effective control scheme is devised by utilizing two dc capacitor voltage control loops, including the total capacitor voltage controller (TCVC), the capacitor voltage balancing controller (CVBC). The predictive current controller is proposed to enhance the tracking accuracy of current control loop. The proportional-resonant controller (PRC) is used as circulating current damping controller of the modular multilevel converter, which is found to be highly effective to minimize the circulating current. The Electromagnetic Transient Program (EMTP) is utilized for digital simulation of the single-phase five-level modular multilevel converter. The effectiveness of the control scheme is validated by the simulation results. DOI: http://dx.doi.org/10.11591/telkomnika.v11i11.2994