Zero Sequence Currents Externally Circulating Between the Back to Back Modular Multilevel Converters in Parallel AC-DC Distribution Links

Modular Multilevel Converters have been proposed to be a viable topology for medium voltage and high power motor drives due to the stackable structure of the topology. This allows for a transformer-less drive, better output current THD and reduces input and output filter sizes compared to traditional topologies. Replacing the traditional Si IGBT based submodules with SiC MOSFETs has been shown to improve the overall efficiency of the MMC. However, the lower switching losses of SiC MOSFETs have not been fully utilized to realize further system level improvements in the MMC. In this paper, the impact of the SiC MOSFETs on the efficiency, power density and output current THD of the drive is compared against Si IGBTs using analytical and simulation. A 7 kV dc bus, 4.16 kV ac, 1 MVA, 0-1,000 Hz medium voltage variable speed drive based on 1.7 kV devices based is used for the comparison.

The auxiliary submodule power supply is a vital component of a modular multilevel converter submodule or any multi-submodule converter. Considering the high isolation requirements and difficulties to provide power from the ground potential, the auxiliary submodule power supply must be simple, work reliably and not compromise the submodule's reliability. A flyback supply from the submodule's dc link with multiple sets of isolated secondary windings solves at once the low-voltage generation and the required voltage isolation for semiconductor gate circuits and protection without need for an externally supplied low-voltage input to the submodule (high-isolation connection). This paper presents an isolated, flyback-based auxiliary submodule power supply with planar magnetic, printed circuit board integrated windings and multiple isolated outputs for a medium voltage modular multilevel converter as well as detailed electrical and magnetic mathematical modelling, technological integration challenges description and proper experimental test considerations.

The modular multilevel converter (MMC) is one of the most promising topologies for high- and medium-voltage applications. However, the conventional MMC relies on large arm inductance and complicated feedback control to maintain submodule (SM) capacitor voltages balanced, which requires a large number of voltage sensors and complicated arm current control loops. In contrast to the conventional perspective, the MMC can be regarded as a derivative from the general multilevel topology based on switched-capacitor (SC) structure. The SC structure endows excellent properties to the MMC, such as minimum or no arm inductor and automatic SM capacitor voltage balancing capability. This MMC with minimum or no arm inductor is named as SC-MMC. Since the arm inductances are very small, the SM capacitor voltages can be balanced automatically. Small stray/parasitic inductance is enough to limit inrush current during SM's switching instants to the rated current level. To unleash the automatic SM capacitor voltage balancing capability of the SC-MMC, a Y-matrix modulation (YMM) has been proposed, which does not require closed loop control or any sensor, has no limitations on the number of SMs, and works for both half-bridge and full-bridge SMs. An unresolved issue of the emerging YMM is that, in the published literature, the existence of the Y matrix with full rank has not been proved theoretically beyond the observation for the SC-MMC when the number of SMs is large. In this paper, an analytical proof of the existence of the full rank Y matrix is provided, which implies the automatic voltage balance of SMs for the MMC with arbitrary number of SMs in each arm. Simulation results using the YMM on a 21-level SC-MMC with minimum arm inductance are provided to verify the voltage balance theory/observation of the SC-MMC using the YMM.

The arm inductors utilized in a modular multilevel converter (MMC) can suppress the MMC circulating current, fault current, and output current, and circulating current ripples. This paper presents a four-step design and optimization method for the MMC arm inductor. The first step is the define the requirements and specifications of the arm inductor based on the MMC ratings and detailed modeling, in order to define the core material and select the appropriate winding wire as a second step. The third step is to develop a multi-objective optimization for the core design of the arm inductor, while the considered constraints are area product, maximum magnetic flux density, and window area, which is here done based on a Non-dominated Sorted Genetic Algorithm (NSGA-II). The objective functions are inductor total mass, total volume, total power losses, total core length, and air gap length. Finally, using the average ranking (AR) method, the best optimal solution is selected from the Pareto solution provided by NSGA-II.

The control complexity of modular multilevel converter (MMC) increases with the number of submodules per arm. In particular, the implementation of a pulsewidth modulation (PWM) scheme and a voltage balancing approach for MMC is a major challenge. This paper proposes a generalized low-cost less computational decoupled sampled average PWM scheme for MMC. In this approach, the reference output voltage is generated by averaging the two nearest voltage levels from the top and bottom arms in each sampling interval. In addition, a simplified voltage balancing approach is proposed for MMC. The proposed approach does not require a sorting technique to select the submodules. Instead, the relative comparison logic is presented. The effectiveness of the proposed modulation scheme and voltage balancing approach is evaluated on a single-phase MMC system with three-level flying capacitor submodules. The proposed approach is extended to a three-phase system, which results in dynamic balancing of the zero-sequence voltage. Hence, the zero-sequence current is minimized. The performance of the proposed three-phase approach is compared with that of the conventional phase-shifted carrier PWM scheme. The simulation and experimental studies show successful voltage balancing among submodule capacitor voltage, low harmonic distortion, and less ripple in output current.

To further improve the power supply sustainability of the ac/dc hybrid distribution networks, this article proposes a control strategy for the modular multilevel converter (MMC) under grid-side open-circuit fault, which is based on independent arm current control. Firstly, the behaviors of the MMC after the faults are analyzed, including single-phase and two-phase open-circuit faults. For single-phase open-circuit fault, the proposed strategy can be achieved by reconfiguring the amplitude and phase of the arm currents, thereby eliminating the zero-sequence current. For two-phase open-circuit fault, both the arm currents and voltages need to be reconfigured. After that, a method for detecting and locating specific faults is provided, which can be implemented by detecting the amplitude of the zero-sequence current. Besides, the postfault characteristics of the MMC under single-phase open-circuit condition are given, including submodule (SM) capacitor voltage ripple, circulating current, arm rms current and arm peak current. According to it, long-term postfault operation of the MMC can be guaranteed with 50% of the rated power. Additionally, stable dc-bus voltage and balanced SM capacitor voltage are satisfied even though one phase is disconnected. Moreover, with the proposed control strategy, although the MMC is able to operate for a short period of time in emergency situations when two phases are disconnected, it is not supposed to transmit power due to the inevitable large current on neutral conductors. Finally, the feasibility and effectiveness of the proposed control strategy is validated by simulation and experimental results.

Arm inductor in a modular multilevel converter (MMC) is used to limit the circulating current and dc short circuit fault current. The circulating current in MMC is dominated by second-order harmonic, which can be largely reduced with circulating current suppressing control. By analyzing the mechanism of the circulating current suppressing control, it is found that the circulating current at switching frequency becomes the main harmonic when suppression control is implemented. Unlike the second-order harmonic that circulates only within the three phases, switching frequency harmonic also flows through the dc side and may further cause high-frequency dc voltage harmonic. This article develops the theoretical relationship between the arm inductance and switching frequency circulating current, which can be used to guide the arm inductance selection. The experimental results with a downscaled MMC prototype verify the existence of the switching frequency circulating current and its relationship with arm inductance.

Zero sequence voltage suppression control for a modular multilevel matrix converter is investigated, which is required for applications to power systems and high power motor drives. The schemes of power flow control and capacitor voltage balancing control with voltage space vector modulation have been proposed. However, these control schemes have the problem they cause zero sequence current that affects protection for power systems and motors. To reduce the zero sequence current, the control scheme that suppresses zero sequence voltage by compensating voltage difference between input and output systems is proposed in this paper. Numerical simulation of a 9-level modular matrix converter and experiments using a 2-kW 5-level modular matrix converter are conducted to verify the control scheme.

This paper presents deeply analysis and comparison of different modulation techniques for the modular multilevel converter without using arm inductor. In case of PWM, losses are more and Total Harmonic Distortion (THD) performance is very poor. With the introduction of delta modulation in MMC, the losses will reduce and THD will improve. It can be seen that the THD is 4.9% for delta modulation and 3.82% for adaptive delta modulation without using arm inductance but THD is 13.15% for delta modulation with arm inductance. It means THD has been improved without using arm inductance. It has been observed that the adaptive delta modulation has better THD response as compared to delta modulation due to its several advantages. The THD performance has been calculated and mentioned at different modulation indexes. It has been also observed that the THD performance is better with delta and adaptive delta modulation.

— The modular multilevel converter (MMC) is considered to be emerging multilevel topology for medium to high power applications due to its modularity and highly reliable structure. However, large submodule (SM) capacitor voltage ripple and higher circulating current during variable frequency operation is the major drawback of the MMC. In the literature, higher-order harmonic elimination technique is presented to reduce SM capacitor voltage ripple and circulating current. In this article, effect of lower-order harmonics on circulating current is investigated. The effect of second-order fundamental frequency current harmonic I2f on arm inductor is analyzed. Based on detailed analysis, an optimal design method for selection of arm inductor is proposed. Proposed optimal design is capable of reducing the circulating current under variable frequency range as well as SM capacitor voltage ripple remains under specified limit during entire variable frequency operation. A detailed MATLAB model of MMC fed high power medium voltage drive is developed. Experimental setup is also developed to check the effectiveness of proposed optimal design method. Reduction in circulating current and control of SM capacitor voltage ripple is observed for entire range of frequency.

The differential current in Modular Multilevel Converters (MMCs) is inherently subjected to even order harmonics, which are mainly caused by the voltage mismatch between phase legs and dc bus. This issue may exist in both single-phase and multi-phase MMC systems and can be eased by repetitive control based active harmonic suppression methods, which regulate the circulating current through legitimate adjustment of the voltage applied on the arm inductance. Nevertheless, conventional repetitive controllers have relatively slow dynamic response because of the presence of one fundamental period delay, and all the sampled errors in the past one cycle have to be stored for closed-loop control. This paper presents an improved even-harmonic repetitive control scheme that requires less data memory with faster dynamic response. The steady state performance can also be enhanced if system disturbances e.g. frequency deviation are considered. In order to verify its practicality and effectiveness, PLECS simulation and experimental results are presented to show the excellent dynamic response and disturbance rejection of the proposed even-harmonic elimination scheme.

The voltage unbalance between the upper and lower capacitors is an important problem in the three-level PWM converter system, and it is known that the voltage unbalance is directly related to the zero sequence. In this paper, we provide a zero sequence current path by connecting the neutral point of the source transformer to the midpoint of the DC link capacitors. Further, we control the unbalanced voltage with the zero sequence current. The zero sequence current is controlled by the zero sequence voltage made from sinusoidal PWM. The effectiveness of the proposed scheme is verified by computer simulations and experiment.

Low device switching frequency is preferred in high-power converters to achieve higher efficiency, higher device utilization, and feasible cooling system design. However, low device switching frequency operation leads to higher peak voltage in submodule (SM) capacitors of medium-voltage modular multilevel converters (MMC). The main reason is unequal distribution of energy among SMs in each fundamental period due to fewer switching commutations available for balancing. This results in increased converter cost due to higher voltage rating of SM capacitors and semiconductor devices. Therefore, the main objective of this paper is to propose a new SM selection algorithm to reduce the peak voltage of SM capacitors without significantly increasing the device switching frequency. The main idea is to avoid unnecessary switching commutations by ensuring balancing of SM capacitor voltages only at the end of charging period. The proposed algorithm has been verified with extensive simulation studies of a 5L-MMC and it has been observed that peak voltage is kept within 10 % of the nominal value while limiting the device switching frequency in the range of 350 to 400 Hz.

The research paper presents the control strategy to reduce THD (Total Harmonic Distortions) losses by the implementation of the Nearest Level Modulation control technique in a Modular Multilevel Converter. Modular Multilevel Converter is found one of the leading technologies in Power Electronics & Control, its applications are very common in HVDC systems, FACTS (Flexible Alternating-current Transmission system), Variable frequency drives and Electric vehicles as well. The power quality of MMC is better and has lesser THD in comparison to conventional converters like 2-level converters with carrier-based modulation techniques. The MMC has been designed with high scalability and has high voltage and power capacity. Sub-module is an integral part of MMC which is built up as an identical and controllable part of it. This converter is also called a controllable voltage source (VSC). Researchers aim to come up with a detailed review of control methods and necessary operations applied to MMC-based systems for HVDC, particularly focusing to control the total harmonic distortions. Power converters use many modulation techniques, but the existing techniques contribute to a great part in switching losses. MMC up to 49 levels, by implementing the Nearest Level Modulation (NLM) technique, is robust and has less complexity for the systems like MMC-HVDC, and the levels control the total harmonic distortions. In this research paper, the reduction of THD by increasing the voltage levels in MMC is comprehensively evaluated. The simulation results in MATLAB/Simulink are used to examine and confirm the proposed control strategy for stable operation of MMC for HVDC application.

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.