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An Improved Method to Reduce the Number of Sensors in the Modular Multilevel Converters (MMCs)
Abstract
In this article, the combination of a new capacitor voltage sorting method and an open-loop control approach based on the energy estimation of the arms is used in the modular multilevel converters (MMCs). The presence of circulating current, the use of multiple voltage and current sensors, and the voltage imbalance of the capacitors increase the cost and uncertainty of using such converters. Therefore, the proposed sensor reduction method provides the ability to control the converter by using a total of two voltage sensors in the arms and a current sensor in the output, and other required parameters in each phase are calculated only by using circuit relationships and relying on the concepts of multilevel converters. The open-loop control method of arms energy estimation, which converts the circulating current of the converter to a constant value at any moment in time, makes it possible to determine the size and direction of the arm currents of the converter only by using an output current sensor. The proposed method continuously maintains stability in the capacitor voltages, and multiple voltage sensors are removed from each sub-module (SM).
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The submodule (SM) capacitance value of a modular multilevel converter (MMC) is so large that the reliability and cost of the MMC are affected. By existing methods of second‐harmonic circulating current injection with a closed‐loop control strategy, the SM capacitance value can be greatly reduced. However, the design of the SM capacitance value is not given, and extra loss is brought in. This study gives the design of the SM capacitance value when the second‐harmonic circulating current is injected with a closed‐loop control strategy. Then, an adaptive control method based on SM capacitor voltage fluctuation feedback is proposed to reduce the extra loss caused by the second‐harmonic circulating current injection. In addition, for the case without SM capacitor voltage sensors, a method based on real‐time calculation of the injected second‐harmonic circulating current is proposed to reduce the extra loss without SM capacitor voltage sensors. Finally, the accuracy of the design and the validity of the proposed methods are verified by simulation and experimental results.
This paper presents a new voltage estimation method for the submodule (SM) capacitor in a modular multilevel converter (MMC). The proposed method employs a Kalman filter (KF) algorithm to estimate the SM voltages of the converter. Compared with sensor-based methods, this scheme requires only one voltage sensor to achieve the voltage-balancing of the converter. This sensor is connected to the total arm voltage; the proposed algorithm requires also the switching patterns of each upper SM switch which are provided from the processor used without the need for extra sensors. This impressive reduction in the number of voltage sensors improves system reliability and decreases its cost and complexity. Extensive simulation and experimental analyses carried out to validate the proposed estimation scheme under different conditions include steady-state analyses and investigation of the effect of variations in capacitance and inductance and the effect of low carrier and effective switching frequency on the accuracy of the estimation, step changes in load conditions, faults and a range changes in DC voltage. The results obtained are validated based on a single-phase MMC.
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 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.
The internal control of a modular multilevel con- verter aims to equalize and stabilize the submodule capacitor voltages independent of the loading conditions. It has been shown that a submodule selection mechanism, included in the modulator, can provide voltage sharing inside the converter arm. Several procedures for controlling the total stored energy in each converter arm exist. A new approach is described in this paper. It is based on estimation of the stored energy in the arms by combining the converter electromotive force reference, the measured alternat- ing output current, and the known direct voltage. No feedback controllers are used. Experimental verification on a three-phase 10 kVA prototype is presented along with the description of the new procedure. Index Terms—Energy balance, modular multilevel converter (M2C), modulation, open-loop control, prototype.
This paper presents a new method to measure the voltage across the submodule (SM) capacitors in a modular multilevel converter (MMC). The proposed technique requires only one voltage sensor per arm. This reduces the number of sensors required compared to conventional sensor-based methods. Therefore, the cost and complexity of the system are reduced, which in turn improves the converter’s overall reliability. The proposed method employs an exponentially weighted recursive least square (ERLS) algorithm to estimate the SM capacitor voltages through the measured total arm voltage and the switching patterns of each SM. There is thus no need for extra sensors to measure these control signals as they are directly provided from the controller. The robustness of the proposed method is confirmed via introducing deviations for the capacitance values, dynamic load changes, DC voltage change and start-up transient condition. Simulation and experimentally validated results based on a single-phase MMC show the effectiveness of the proposed method in both, steady-state and dynamic operations.
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.
Modularmultilevel converter topologies and applications-a review
- M Priya
- P Ponnambalam
- K Muralikumar
M. Priya, P. Ponnambalam, and K. Muralikumar, "Modularmultilevel converter topologies and applications-a review," IET
Power Electronics, vol. 12, no. 2, pp. 170-183, 2018.