A comparative analysis of circulating current controllers for modular multilevel converters

Dual three-phase (asymmetrical six-phase) induction motors have received significant attention in high power medium voltage drive applications. Typically, two three-phase voltage source converters are needed to feed such a drive system. In medium voltage applications, multilevel inverters are preferred as they provide a stepped output voltage waveform that reduces dv/dt stresses and provides a better output current waveform, which reflects positively on the machine developed torque and overall drive performance. Among different multilevel inverters, the Modular Multilevel Converter (MMC) has recently shown promise in medium and high voltage applications. For a dual three-phase machine, two MMCs with 6 arms each, are needed. This paper proposes a nine-arm MMC (9A-MMC) to drive such a motor. The proposed approach reduces the number of involved arms by 25%, i.e. reduces the number of required dc capacitors, semiconductor devices and their gate drive circuits, which reduces system complexity and cost. A complete analysis of the proposed system is presented and a simulation model is used to verify the proposed concept.

An isolated modular multilevel dc-dc converter (IMMDCC) consisting of two modular multilevel converters (MMCs) coupled via a medium-frequency transformer is competitive for medium- and high-voltage applications because of its high modularity and high-voltage handling capability. Employing the quasi square-wave modulation (QSM) scheme for MMCs of the converter can increase the dc-link voltage utilization ratio and realize soft-switching operation, compared with sinusoidal wave modulation methods. However, the accurate reactive power control at the ac side of the IMMDCC can hardly be achieved under the QSM mode. The ac-side reactive power and current stress increase considerably when the conversion ratio deviates from the transformer turn ratio. In order to address this issue, this paper presents an arm voltage decoupling control scheme for the IMMDCC by introducing a virtual buck/boost conversion stage between the dc-side voltage and the transformer voltage. The transformer primary and secondary voltages are kept matched to suppress the reactive power when the dc-side voltage varies. The theoretical analysis shows that the proposed control enhances the power transfer capability and reduces the ac-side current stress of the IMMDCC. In addition, submodule voltages are maintained even when the dc voltages exceed the designed value under the conventional control scheme, which implies that the voltage range of the converter is expanded. The simulation and experimental results confirm the effectiveness of the proposed modulation and control strategy.

Fractional frequency transmission system (FFTS) is a promising solution to offshore wind power integration. The back-to-back modular multilevel converter (MMC) and modular multilevel matrix converter (M3C) are universally regarded as two promising ac/ac converter candidates of the next generation. This paper performs a detailed comparison between back-to-back MMC and M3C with special emphasis on their total capacitor and semiconductor ratings. The arm current and capacitor ripples are first analyzed, then comparison results of a 220 kV, 500 MW case is given. In terms of peak-current-based and RMS-current-based total semiconductor ratings the M3C is 42.56 % and 22.97 % larger respectively, while its total capacitor rating is merely 33.15% of the back-to-back MMC. The better candidate can be determined according to the quantitative comparison results and the latest prices of semiconductors and capacitors.

This paper presents a new converter topology for voltage source converter based high voltage dc application, named modular embedded multilevel converter (MEMC). MEMC is based on a three level structure and consists of series-connected IGBT bridge stacks, like the popular modular multilevel converter (MMC) and series-connected thyristor stacks. The IGBT bridge stacks are leveraged to generate the output the voltage waveforms and to commute the thyristors. The basic operation principle and the methods to commutate the thyristors are explained in detail. Due to the three level structure, the total number of IGBT stacks are reduced by half compared with MMC, leading to much lower energy storage, weight, volume and system complexity. Also by replacing part of the IGBT stack in MMC with thyristor stack, both of the system conduction loss and switching loss can be further reduced. The simulation results proved the analysis and verified the feasibility and benefits of the proposed converter topology.

Modular multilevel converter (MMC) is a recently emerged multilevel topology for high-voltage high-power applications. However, in wind power application the performance of the MMC has not been deeply investigated. In this paper the application of MMC in wind energy systems is studied. The converters used to connect the wind turbine to the grid having the rated active powers of 2MW and 10MW are designed and investigated. Electrical losses and thermal loading of the power devices in the proposed converter solutions are analyzed. The efficiency of the MMC converter under different P/Q boundaries defined by grid codes is investigated and compared with two-level and three-level NPC converters. It is concluded that it is possible to use the MMC in wind power application and the losses are distributed evenly between the sub-modules of the MMC converter. However, inside a sub-module the losses of the power devices are not equal that can lead to the de-rating of the converter.

this paper explains internal capacitor energy balancing and circulating current control of Modular Multi-level Converters (MMCs). MMCs are considered as one of the promising multilevel converter topologies especially in high voltage applications such as HVDC transmission system. Circulating current is a unique feature in MMC, it arises from capacitor voltage differences between the three phases. The idea behind MMC is to use floating capacitors as a voltage sources, they are inserted or bypassed using a switching circuitry to generate the required output voltage level. To ensure a stable operation, capacitor voltages should be kept in the same level and the total converter energy should be uniformly distributed between the three phases. This can be achieved by controlling the current that circulates between the three phases. In this paper a three phase grid connected MMC is considered. An analytical study has been done, operating principles, mathematical model and control system have been described. Also, the system performance has been verified using simulation with Matlab-Simulink, simulation results were satisfactory and in conformity with the analytical study.

The Modular Multilevel Converter (MMC) is an emerging topology in the line of multilevel converters for high and medium voltage applications. This paper proposes the control technique to tackle the pressing issue of ac component in circulating current. By an additional minor loop having Proportional Resonator controller for suppresing ac components in circulating current to zero. With PR control it is also now possible to introduce the output current control. This paper also presents application of antiwind up control with PR to avoid problems like windup or rollover arise in working under saturation or sustained error input. This control can be applied to single phase also as opposed to methods discussed previously. Simulation results are been provided for 3 phase grid connected system, using circulating current control and tracking of output current to be given as feedback signal.

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 direct chain-link modular multilevel dc converter has raised great interest recently in medium/high-voltage dc-dc conversion due to the high power device utilization and lower power losses than the front-to-front configurations of modular multilevel converter. This paper introduces a single-phase chain-link modular multilevel buck-boost converter for medium voltage dc applications and presents an analysis methodology on its internal circulating current frequency. Its connection and comparison with the classic dc-ac modular multilevel converter are given first, and its dc and ac components are analyzed in the respective equivalent circuits. Then, the derivation methodology for the proper circulating current frequency with lowest internal reactive power is provided, which could minimize the current stress and decrease the power losses. Also, this method can be directly applied in the derivative topologies and further configurations to satisfy various conversion requirements. The theoretical analysis is verified by a set of full-scaled simulations and further verified against experimental tests on a down-scaled prototype.

Modular Multilevel Converter (MMC) is an emerging converter topology for medium and high voltage applications. Nearest level Modulation (NLM) is the conventional control topology used to control the MMC that produces the N+1 AC output waveform. In previous research work, the Modified NLM has been already proposed, producing a 2N+1 and 4N+1 output waveform while utilizing a half-bridge (HB) submodule (SM) topology. However, half-bridge-based MMC has a similar behavior as two-level Voltage Source Converter (VSC) and cannot block DC fault current in case of DC-side short circuit fault. So, in recent years, full-bridge-based MMC topology is preferably used by manufacturers as it has DC fault blocking capabilities. This paper presents the Modified NLM for Full bridge (FB) SM topology to take the critical benefits of FB SM topology and improve power quality. The proposed method is simpler to implement and produces a 4N+1 AC output waveform. The THD of the output voltage and current reduces to half compared to the conventional NLM method. The proposed method is verified using LabVIEW Multisim co-simulation and as well as real-time simulation.

To implement the STATCOMs for transmission and distribution applications, high power converters are needed. A multilevel converter is a logical choice for such applications as it can meet the imposed line side current and voltage harmonic distortion standards without extending its device ratings. Several types of monolithic multilevel converters have been proposed based on different structures of a DC link voltage (or current) to generate staircase output voltage (or current) levels. The multilevel current source converters (CSC) are also a potential configuration for STATCOM. Multilevel CSCs using fully controllable switches have the advantages of low output harmonics and controllable active and reactive power. During the last decade, modular multilevel converters have shown a breakthrough and have made their way to commercial high power applications. Modularity, in general, refers to a technique to develop comparably large systems by combining smaller subsystems. For power converter topologies, this means, a cascaded connection of converter cells seems to be an interesting solution to reach high voltage and high quality waveforms with a minimum complexity. This chapter provides an overview on monolithic and modular multilevel converter topologies, targeting high-power applications and in particular STATCOM. Modular multilevel converters can be built either straightforward by applying the modular building block cells or by a combination of cells with monolithic multilevel topologies. The monolithic and modular CSCs can also be realized by employing the duality concept on each corresponding Voltage Source Converter (VSC). This chapter summarizes the most recent developments made for multilevel converter topologies, by covering new promising topologies and operational issues. In addition, emerging trends, challenges, and possible future directions of the multilevel converter technologies are outlined.

Voltage Sourced Converters (VSC) are being used in High Voltage Direct Current (HVDC) power transmission to convert electrical power from AC to DC and vice versa and has the significant proportion of future DC transmission schemes. The converters have the ability to provide the connected AC networks with “clean” real and reactive power without the need for large passive harmonic filters, makes them attractive for many applications where plant size and weight are important. PHMMC and MMC both are the types of voltage source converter. Both, PHMMC and MMC are considered as the promising multilevel converter topologies especially in high voltage applications such as HVDC transmission system. Compared with the MMC, the Parallel hybrid MMCs can potentially reduce the ratings of the converter components and power losses. This paper discusses the comparison of the performance of 9 level PHMMC and 4 level MMC, showing the number of device count reduced in the PHMMC along with the simulation model of the PHMMC and MMC.

Modular Multilevel Converter (MMC) topology is an interesting solution for number of medium and high voltage applications. Key advantages over another multilevel topologies include flexibility of scaling in voltage and simple implementation of redundancy. In this publication control of DC-DC MMC is discussed. Analyzed converter consists of six branches connected in double wye structure. It enables bidirectional power transfer and step-up or step-down voltage conversion. The scope of the paper is branch energy balancing of the converter. Balancing system is derived from the analysis of branch powers during steady state operation. Operation of the control system is verified in the hardware in-the-loop (HIL) simulator provided by OPAL-RT Technologies. Complete control loop realized on hierarchical system was implemented. The system is composed with a top-level controller (based on ABB AC 800PEC) and a number of cell controllers build with Texas Instruments DSP (C2000 F28035).

The modular multilevel converter (MMC) is emerged as the most favourable multilevel converter topology for medium to high-voltage applications. Reliability is one of the major concerns in an MMC due to high fragile component count. Due to low-cost and high energy density, electrolytic capacitors (ECs) are usually preferred as floating capacitors in the MMC. However, the ECs are gradually degraded over the time due to inherent chemical processes, which results in a decrease in capacitance value. This results in an increase in voltage ripple across the SM capacitors and would distort the output waveforms. Moreover, the prolonged use of these aged capacitors could disrupt the MMC operation. Therefore, monitoring and failure detection of submodule (SM) capacitors in an MMC are pivotal to enhance the reliability. This paper presents an SM capacitance estimation strategy for an MMC using nearest level modulation (NLM). A modified voltage balancing control structure with NLM is presented to reduce the unbalance in the capacitor voltages. The proposed approach uses available hardware for converter control with easy implementation in the same controller. Extensive studies are conducted on a three-phase MMC in PLECS simulation platform. Furthermore, experimental results are presented to substantiate the effectiveness of the proposed approach.

Modular multi-level converters (MMCs) are one of the promising topologies in recent years for medium or high voltage applications. They are considered as the next generation DC/AC converters for medium/high voltage (MV/HV) motor drive applications due to their transform-less structures, high efficiency and modularity. Reliability is one of the most important challenges in MMCs, since a large number of power switching devices are used and each of these devices can be considered as a potential failure point. It is significant to detect and locate the fault accurately and apply appropriate protections in a timely manner. This paper investigates the behavior of the failure submodules and healthy submodules and use artificial neural network (ANN) classification algorithms for single-submodule open-circuit fault diagnosis. The ANN algorithm is implemented in field programmable gate array (FPGA) and the ANN parameters training is completed offline in Matlab. Experimental results of the proposed fault diagnosis for a single-module open-circuit fault are presented in this paper.