Circulating Current Harmonics Suppression for Modular Multilevel Converters Based on Repetitive Control

Binbin Li,Dandan Xu,Dianguo Xu 전력전자학회 2014 JOURNAL OF POWER ELECTRONICS Vol.14 No.6

Modular multilevel converters (MMCs) have emerged as the most promising topology for high and medium voltage applications for the coming years. However, one particular negative characteristic of MMCs is the existence of circulating current, which contains a dc component and a series of low-frequency even-order ac harmonics. If not suppressed, these ac harmonics will distort the arm currents, increase the power loses, and cause higher current stresses on the semiconductor devices. Repetitive control (RC) is well known due to its distinctive capabilities in tracking periodic signals and eliminating periodic errors. In this paper, a novel circulating current control scheme base on RC is proposed to effectively track the dc component and to restrain the low-frequency ac harmonics. The integrating function is inherently embedded in the RC controller. Therefore, the proposed circulating current control only parallels the RC controller with a proportional controller. Thus, conflicts between the RC controller and the traditional proportional integral (PI) controller can be avoided. The design methodologies of the RC controller and a stability analysis are also introduced. The validity of the proposed circulating current control approach has been verified by simulation and experimental results based on a three-phase MMC downscaled prototype.

Reduction Method of Circulating Current for Parallel Connected Inverters Using Proportional Resonant Control

Hye-Won Choi,Kyo-Beum Lee 전력전자학회 2023 ICPE(ISPE)논문집 Vol.2023 No.-

This paper proposes a reduction method of circulating current for parallel connected inverters using proportional resonant (PR) control. Paralleled inverter has been increasingly applied in many power conversion systems such as renewable energy systems, motor drives, and energy storage systems. Modular inverters have several advantages of convenient maintenance and adjustable rating of power by the number of modules. However, a circulating current of paralleled inverter is generated due to the difference in the output parameters in each module. The circulating current distorts the quality of output current and deteriorates the performance of the parallel systems. To mitigate the problems, a reduction method is proposed using the PR controller of the paralleled connected inverter. The PR control method extracts the third harmonics of output current to reduce the circulating current of low-frequency component. The proposed method controls the low-frequency circulating current applying only controllers. The simulation results demonstrate the validity of the proposed control method.

Circulating Current Harmonics Suppression for Modular Multilevel Converters Based on Repetitive Control

Li, Binbin,Xu, Dandan,Xu, Dianguo The Korean Institute of Power Electronics 2014 JOURNAL OF POWER ELECTRONICS Vol.14 No.6

Modular multilevel converters (MMCs) have emerged as the most promising topology for high and medium voltage applications for the coming years. However, one particular negative characteristic of MMCs is the existence of circulating current, which contains a dc component and a series of low-frequency even-order ac harmonics. If not suppressed, these ac harmonics will distort the arm currents, increase the power loses, and cause higher current stresses on the semiconductor devices. Repetitive control (RC) is well known due to its distinctive capabilities in tracking periodic signals and eliminating periodic errors. In this paper, a novel circulating current control scheme base on RC is proposed to effectively track the dc component and to restrain the low-frequency ac harmonics. The integrating function is inherently embedded in the RC controller. Therefore, the proposed circulating current control only parallels the RC controller with a proportional controller. Thus, conflicts between the RC controller and the traditional proportional integral (PI) controller can be avoided. The design methodologies of the RC controller and a stability analysis are also introduced. The validity of the proposed circulating current control approach has been verified by simulation and experimental results based on a three-phase MMC downscaled prototype.

Master–slave modulated model predictive control to optimize current tracking for parallel cascaded H‑bridge power supplies

Bichen Yan,Haihong Huang,Haixin Wang 전력전자학회 2023 JOURNAL OF POWER ELECTRONICS Vol.23 No.1

Conventional distributed control has achieved a great deal of success in multi-parallel cascaded H-bridge (CHB) power supplies. However, conventional distributed controllers can have a seriously imbalanced output power due to a mismatch of the hardware or control parameters and form an unstable circulating current path between the CHB converters, which can lead to a couple of problems. (1) The stability and tracking performance of the branch current can become degraded. (2) The current-sharing reactor can be easily saturated. To optimize all the branch currents of multiple parallel CHB power supplies, a master–slave carrier-based model predictive control method is proposed in this paper. This control strategy can achieve an optimized current tracking performance and effective circulating current suppression. In addition, through carriers, H-bridge interleaving and branch synchronization can be easily achieved, which fixes the switching frequency. Thus, the advantages of model predictive control and interleaving can be combined. Based on the generalized branch current predictive model derived in this paper, when more than two paralleled CHB converters have different output powers, an optimized branch current response can be achieved and circulating current can be eliminated. Simulation and experimental results verify the effectiveness of the proposed method.

Three‑stage model predictive control for modular multilevel converters with comprehensive performance optimization

Ping Tuo,Zheng Gong,Xi Zheng,Chun Zhao,Xiaojie Wu 전력전자학회 2024 JOURNAL OF POWER ELECTRONICS Vol.24 No.7

Model predictive control (MPC) is recognized as an efficient control method for the modular multilevel converter (MMC), owing to its advantages, such as good robustness, rapid dynamic response, and multi-objective control. However, due to the coupling relationship between the ac-side current and the circulating current, the existing MPC has an impact on the ac-side current while suppressing the circulating current. In this paper, the relationship between the ac-side current performance and circulating current suppression is discussed in detail, and a three-stage MPC (TS-MPC) strategy is proposed to optimize the comprehensive performance. With the ac-side current control, circulating current control, and comprehensive optimization control, the optimum performance of both the ac-side current and circulating current suppression is realized while maintaining a low computational burden. Moreover, a grouping sorting algorithm is designed to reduce the calculation burden and to balance the capacitor voltages. The steady-state and transient performances of the proposed TS-MPC strategy have been verified by experimental results, which validates its correctness and effectiveness.

Novel Method for Circulating Current Suppression in MMCs Based on Multiple Quasi-PR Controller

Jian Qiu,Lijun Hang,Dongliang Liu,Shengbao Geng,Xiaonan Ma,Zhen Li 전력전자학회 2018 JOURNAL OF POWER ELECTRONICS Vol.18 No.6

An improved circulating current suppression control method is proposed in this paper. In the proposed controller, an outer loop of the average capacitor voltage control model is used to balance the sub-module capacitor voltage. Meanwhile, an individual voltage balance controller and an arm voltage balance controller are also used. The DC and harmonic components of the circulating current are separated using a low pass filter. Therefore, a multiple quasi-proportional-resonant (multi-quasi-PR) controller is introduced in the inner loop to eliminate the circulating harmonic current, which mainly contains second-order harmonic but also contains other high-order harmonics. In addition, the parameters of the multi-quasi-PR controller are designed in the discrete domain and an analysis of the stability characteristic is given in this paper. In addition, a simulation model of a three-phase MMC system is built in order to confirm the correctness and superiority of the proposed controller. Finally, experiment results are presented and compared. These results illustrate that the improved control method has good performance in suppressing circulating harmonic current and in balancing the capacitor voltage.

Circulating current suppression control for modular multilevel converters based on restricted self-redundant states prediction

Hu, Xing,Zhang, Jianzhong,Deng, Fujin,Chen, Gui,Chen, Qiang,Din, Zaki ud The Korean Institute of Power Electronics 2020 JOURNAL OF POWER ELECTRONICS Vol.20 No.5

Circulating current exists among phases or between the DC link and the three phases in a modular multilevel converter (MMC). Suppression control of the alternating components in circulating current is a critical issue for the stable and efficient operation of an MMC. Due to the redundancy and symmetry of MMCs, some of the self-redundant states of MMCs can be used to suppress the alternating components in circulating current without affecting the output performance of the converter. In this paper, a theoretical derivation of the boundary of redundant states is given. Then a suppression strategy for circulating current is proposed based on prediction control and the boundary of redundant states. The selection range of the redundant states is narrowed in this case, which reduces the computation burden when compared to the original method and ensures excellent performance in the suppression of circulating current. Simulations and experiments are carried out to verify the effectiveness of the proposed strategy.

Lifetime and reliability improvements in modular multilevel converters using controlled circulating current

Kadandani, Nasiru B.,Dahidah, Mohamed,Ethni, Salaheddine,Muhammad, Musbahu The Korean Institute of Power Electronics 2021 JOURNAL OF POWER ELECTRONICS Vol.21 No.10

Circulating current has been an inherent feature of modular multilevel converters (MMC), which results in second-order harmonics on the arms currents. If not properly controlled, the circulating current can affect the lifetime and reliability of a converter by increasing the current loading, loss distribution, and junction temperature of its semiconductor devices. This paper proposes controlled circulating current injection as a means of improving the lifetime and reliability of an MMC. The proposed method involves modifying the reference modulating signals of the converter arms to include the controlled differential voltage as an offset. The junction temperature of the semiconductor devices obtained from an electro-thermal simulation is processed to deduce the lifetime and reliability of the converter. The obtained results are benchmarked against a case where the control method is not incorporated. The incorporation of the proposed control method results in a 68.25% increase in the expected lifetime of the converter and a 3.06% increase on its reliability index. Experimental results of a scaled down laboratory prototype validate the effectiveness of the proposed control approach.

Novel Method for Circulating Current Suppression in MMCs Based on Multiple Quasi-PR Controller

Qiu, Jian,Hang, Lijun,Liu, Dongliang,Geng, Shengbao,Ma, Xiaonan,Li, Zhen The Korean Institute of Power Electronics 2018 JOURNAL OF POWER ELECTRONICS Vol.18 No.6

An improved circulating current suppression control method is proposed in this paper. In the proposed controller, an outer loop of the average capacitor voltage control model is used to balance the sub-module capacitor voltage. Meanwhile, an individual voltage balance controller and an arm voltage balance controller are also used. The DC and harmonic components of the circulating current are separated using a low pass filter. Therefore, a multiple quasi-proportional-resonant (multi-quasi-PR) controller is introduced in the inner loop to eliminate the circulating harmonic current, which mainly contains second-order harmonic but also contains other high-order harmonics. In addition, the parameters of the multi-quasi-PR controller are designed in the discrete domain and an analysis of the stability characteristic is given in this paper. In addition, a simulation model of a three-phase MMC system is built in order to confirm the correctness and superiority of the proposed controller. Finally, experiment results are presented and compared. These results illustrate that the improved control method has good performance in suppressing circulating harmonic current and in balancing the capacitor voltage.

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

Aditya Shekhar,Thiago Batista Soeiro,Laura Ramirez-Elizondo,Pavol Bauer 전력전자학회 2019 ICPE(ISPE)논문집 Vol.2019 No.5

During the operation of a three line ac link in parallel with a ac-dc-ac link system that employs Back-to-Back (B2B) Medium Voltage Modular Multilevel Converters (MMC), a path for the low frequency zero sequence currents is shown to exist. Apart from causing additional losses, these currents can result in limitations on power capacity. A detailed analysis on the influence of cell capacitance, arm inductance and link conductor length on the circulating zero sequence currents and the means to mitigate them through a simulated control strategy is investigated. The simulations show that the dc link employing B2B-MMC can successfully operate in parallel with ac link without transformer isolation by controlling the zero sequence currents.