The modular multilevel converter (MMC) has been a subject of increasing importance for medium/high-power energy conversion systems. Over the past few years, significant research has been done to address the technical challenges associated with the operation and control of the MMC. In this paper, a general overview of the basics of operation of the MMC along with its control challenges are discussed, and a review of state-of-the-art control strategies and trends is presented. Finally, the applications of the MMC and their challenges are highlighted.

Operation, Control, and Applications of the Modular Multilevel Converter: A Review

The authors discuss high-voltage power conversion. Conventional series connection and three-level voltage source inverter techniques are reviewed and compared. A novel versatile multilevel commutation cell is introduced: it is shown that this topology is safer and more simple to control, and delivers purer output waveforms. The authors show how this technique can be applied to either choppers or voltage-source inverters and generalized to any number of switches.<<ETX>>

Multi-level conversion : high voltage choppers and voltage-source inverters

Modular multilevel converters have several attractive features such as a modular structure, the capability of transformer-less operation, easy scalability in terms of voltage and current, low expense for redundancy and fault tolerant operation, high availability, utilization of standard components, and excellent quality of the output waveforms. These features have increased the interest of industry and research in this topology, resulting in the development of new circuit configurations, converter models, control schemes, and modulation strategies. This paper presents a review of the latest achievements of modular multilevel converters regarding the mentioned research topics, new applications, and future trends.

Circuit topologies, modeling, control schemes, and applications of modular multilevel converters

In this paper, the principle of modularity is used to derive the different multilevel voltage and current source converter topologies. The paper is primarily focused on high-power applications and specifically on high-voltage dc systems. The derived converter cells are treated as building blocks and are contributing to the modularity of the system. By combining the different building blocks, i.e., the converter cells, a variety of voltage and current source modular multilevel converter topologies are derived and thoroughly discussed. Furthermore, by applying the modularity principle at the system level, various types of high-power converters are introduced. The modularity of the multilevel converters is studied in depth, and the challenges as well as the opportunities for high-power applications are illustrated.

Modular Multilevel Converters for HVDC Applications: Review on Converter Cells and Functionalities

Theory for the dynamics of modular multilevel converters is developed in this paper. It is shown that the sum capacitor voltage in each arm often can be considered instead of the individual capacitor voltages, thereby significantly reducing the complexity of the system model. Two selections of the so-called insertion indices, which both compensate for the sum-capacitor-voltage ripples, are considered. The dynamic systems which respectively result from these selections are analyzed. An effective dc-bus model, which takes into account the contribution from the submodule capacitors, is obtained. Finally, explicit formulas for the stationary sum-capacitor-voltage ripples are derived.

Dynamic Analysis of Modular Multilevel Converters

This paper describes a modified phase-shifted carrier-based pulsewidth-modulation (PSC-PWM) scheme for modular multilevel converters (MMC). In order to reduce the average device switching frequency, a reduced switching-frequency (RSF) voltage balancing algorithm is developed. This paper also proposes a circulating current suppressing controller (CCSC) to minimize the inner circulating current in an MMC. Based on the double line-frequency, negative-sequence rotational frame, the three-phase alternative circulating currents are decomposed into two dc components and are minimized by a pair of proportional integral controllers. Simulation results based on a detailed PSCAD/EMTDC model prove the effectiveness of the modified PSC-PWM method and the RSF voltage-balancing algorithm. The proposed CCSC not only eliminates the inner circulating current but also improves the quality of the converter ac output voltage. A simple loss evaluation demonstrates that the RSF voltage-balancing algorithm and the CCSC reduce the converter power losses.

Reduced Switching-Frequency Modulation and Circulating Current Suppression for Modular Multilevel Converters

This paper applies nearest level control (NLC) to the modular multilevel converter (MMC). Since there are a number of submodules (SMs) in high-voltage applications of MMC and all SM voltages are required to be measured and sorted, if the uniform sampling frequency is not high enough, the SM will not be triggered as quickly as possible. Thus, the converter-output voltage levels and voltage harmonics will be affected. A method to systematically analyze the voltage harmonics is presented, considering the uniform sampling frequency. In order to select a proper sampling frequency, two critical values, which significantly influence the output voltage levels and voltage total harmonic distortions, are calculated. Simulation results based on PSCAD/EMTDC proved the validity of the proposed modulation scheme and the importance of a properly selected sampling frequency.

Impact of Sampling Frequency on Harmonic Distortion for Modular Multilevel Converter

There are second-order harmonics in the dc voltage and current when the MMC-HVDC system is under unbalanced grid conditions, even if the negative-sequence current controller is employed. This paper presents a supplementary dc voltage ripple suppressing controller (DCVRSC) to eliminate the second-order harmonic in the dc voltage of the MMC-HVDC system. The instantaneous power of the converter arm and phase unit indicates that there are zero-sequence double-line frequency components in the three-phase unit voltages when the ac system is under an unbalanced fault. Since the zero-sequence components cannot be offset by each other, they lead to the second-order harmonic in the dc voltage and dc current. The DCVRSC is developed to compensate the zero-sequence components in three-phase unit voltages. Simulation results based on a detailed PSCAD/EMTDC model prove that the DCVRSC can eliminate the second-order harmonic in the dc voltage. Meanwhile, the ac currents are kept balanced under the unbalanced fault conditions.

Suppressing DC Voltage Ripples of MMC-HVDC Under Unbalanced Grid Conditions

The hybrid cascaded multilevel converter is a newly introduced voltage-source converter with dc blocking capability. It has two key parts, that is, the director switch (DS) made of insulated-gate bipolar transistors in series and the wave-shaping circuit (WSC) containing stacks of full-bridge submodules (FSMs). This paper focuses on the modulation schemes of the DSs and the WSCs. First, to reduce switching losses of the DSs, a novel asymmetrical square-wave modulation at a very low frequency is proposed, and the energy flow of the WSC under this modulation is analyzed. Second, modified phase-shifted carrier-based pulsewidth modulation (PSC-PWM) is presented to determine the number of FSMs to be inserted or bypassed; and the specified FSMs to be fired are selected according to the capacitor voltage-balancing strategy. Third, the capability to limit the short-circuit current during the dc fault is investigated, and fast ac overcurrent protection is designed. The effectiveness of the modulation and control strategies is verified by PSCAD/EMTDC simulations.

Modulation and Control for a New Hybrid Cascaded Multilevel Converter With DC Blocking Capability

This paper evaluates the converter power losses of the modular multilevel converter based HVDC systems (MMC-HVDC). A junction temperature feedback method is proposed in order to estimate the power losses more accurately. Based on the data provided by the manufacture, the characteristic of semiconductor device is acquired. Both the switching and conduction losses are calculated with the real-time voltage and current waveforms, which are derived from the detailed PSCAD/EMTDC model. Thermal circuit model is applied to estimate the average junction temperatures of the semiconductors. The converter valve losses with different heat sink temperatures are also investigated. Power losses evaluation results demonstrate that MMC topology has the lower valve losses than the 2-level or 3-level VSC topologies.

Qingrui Tu

Papers

Reduced Switching-Frequency Modulation and Circulating Current Suppression for Modular Multilevel Converters

Impact of Sampling Frequency on Harmonic Distortion for Modular Multilevel Converter

Suppressing DC Voltage Ripples of MMC-HVDC Under Unbalanced Grid Conditions

Modulation and Control for a New Hybrid Cascaded Multilevel Converter With DC Blocking Capability

Power losses evaluation for modular multilevel converter with junction temperature feedback