Arm integrated double capacitor submodule for modular multilevel solid‑state transformers with DC short‑circuit fault ride‑through capability

Yinyu Yan,Yichao Sun,Wanxin Guo,Hao Jiang,Minqiang Hu 전력전자학회 2024 JOURNAL OF POWER ELECTRONICS Vol.24 No.6

Modular multilevel converter-based solid-state transformers (MMC-SST) usually use the input series output parallel structure or the dual active bridge (DAB) structure. These two structures suffer from low power supply reliability and high economic costs, respectively. Therefore, based on hybrid frequency modulation, this paper proposes an arm integrated double capacitor submodule (AIDCSM)-type MMC-SST topology. This topology effectively reduces the number of switching devices by integrating the submodule arms, while enabling the topology to possess DC short-circuit fault ride-through capability. When compared to the DAB-type MMC-SST with half-bridge submodules, the proposed AIDCSM-type MMC-SST saves 2/5 switching devices and 1/2 high-frequency transformers, and enables uninterrupted operation under DC short-circuit faults. At the same time, the control strategy of the proposed AIDCSM-type MMC-SST is thoroughly investigated under both normal operating and DC short-circuit conditions. Simulation and experimental results demonstrate the correctness and effectiveness of the proposed topology and control method.

A Hybrid Modular Multilevel Converter (MMC) for MVDC Application

C. Wang,Y.M. Yang,P.Y.Zhu 전력전자학회 2019 ICPE(ISPE)논문집 Vol.2019 No.5

This paper presents a hybrid modular multilevel converter (MMC) applied in medium voltage dc (MVDC) system that is able to generate more voltage levels on the arms when the same or even less number of sub-modules are utilized. Each arm of the presented MMC is comprised of the upper part (made of series-connected half-bridge submodules (HBSMs)) and the lower part (made of seriesconnected full-bridge sub-modules (FBSMs)). The voltage across series-connected FBSMs is only equal to half of the HBSM voltage. FBSMs are used to generate voltage levels with small voltage steps, which are added onto major voltage steps of series-connected HBSMs to smooth the voltage waveforms in order to improve the power quality. In this way, no extra filters and sub-modules are required for MMC with a small number of sub-modules, and therefore, the cost and footprint of the converter applied in MVDC system is reduced. The mathematical formulas and claims are validated by simulations using PSCAD/EMTDC.

A Hybrid Modular Multilevel Converter Topology with an Improved Nearest Level Modulation Method

Wang, Jun,Han, Xu,Ma, Hao,Bai, Zhihong The Korean Institute of Power Electronics 2017 JOURNAL OF POWER ELECTRONICS Vol.17 No.1

In this paper, a hybrid modular multilevel converter (MMC) topology with an improved nearest level modulation method is proposed for medium-voltage high-power applications. The arm of the proposed topology contains N series connected half-bridge submodules (HBSMs), one full-bridge submodule (FBSM) and an inductor. By exploiting the FBSM, half-level voltages are obtained in the arm voltages. Therefore, an output voltage with a 2N+1 level number can be generated. Moreover, the total level number of the inserted submodules (SMs) is a constant. Thus, there is no pulse voltage across the arm inductors, and the SM capacitor voltage is rated. With the proposed voltage balancing method, the capacitor voltage of the HBSM is twice the voltage of the FBSM, and each IGBT of the FBSM has a relatively low switching frequency and an equalized conduction loss. The capacitor voltage balancing methods of the two kinds of SMs are implemented independently. As a result, the switching frequency of the HBSM is not increased compared to the conventional MMC. In addition, according to a theoretical calculation of the total harmonic distortion of the electromotive force (EMF), the voltage quality with the presented method can be significantly enhanced when the SM number is relatively small. Simulation and experimental results obtained with a MMC-based inverter verify the validity of the developed method.

A Hybrid Modular Multilevel Converter Topology with an Improved Nearest Level Modulation Method

Jun Wang,Xu Han,Hao Ma,Zhihong Bai 전력전자학회 2017 JOURNAL OF POWER ELECTRONICS Vol.17 No.1

In this paper, a hybrid modular multilevel converter (MMC) topology with an improved nearest level modulation method is proposed for medium-voltage high-power applications. The arm of the proposed topology contains N series connected half-bridge submodules (HBSMs), one full-bridge submodule (FBSM) and an inductor. By exploiting the FBSM, half-level voltages are obtained in the arm voltages. Therefore, an output voltage with a 2N+1 level number can be generated. Moreover, the total level number of the inserted submodules (SMs) is a constant. Thus, there is no pulse voltage across the arm inductors, and the SM capacitor voltage is rated. With the proposed voltage balancing method, the capacitor voltage of the HBSM is twice the voltage of the FBSM, and each IGBT of the FBSM has a relatively low switching frequency and an equalized conduction loss. The capacitor voltage balancing methods of the two kinds of SMs are implemented independently. As a result, the switching frequency of the HBSM is not increased compared to the conventional MMC. In addition, according to a theoretical calculation of the total harmonic distortion of the electromotive force (EMF), the voltage quality with the presented method can be significantly enhanced when the SM number is relatively small. Simulation and experimental results obtained with a MMC-based inverter verify the validity of the developed method.

Analysis of submodule capacitor overvoltage during DC-side fault in hybrid MMC-based HVDC system

Yunbeom Gim,Zhengxuan Li,Jingwei Meng,Zexi Deng,Qiang Song 전력전자학회 2019 ICPE(ISPE)논문집 Vol.2019 No.5

The hybrid modular multilevel converter (MMC) consisting of half-bridge submodules (HBSMs) and full-bridge submodules (FBSMs) is one of the promising converter topologies for HVDC system, due to its fault blocking capability without applying circuit breakers and relatively lower cost compared with FBSM-MMC. However, the energy stored in the lines will cause an overvoltage in the capacitor voltages of the FBSMs. This overvoltage is significant especially when the HVDC system contains long distance transmission lines. The capacitor overvoltage can threat the safe operation of the system and the lifetime of the components, and therefore shall be appropriately regulated. This work analyzes the DC fault current blocking process of a hybrid MMC based HVDC system, determines the major factors which influence the capacitor overvoltage, and proposes corresponding mitigation measures. The simulation results of a two-terminal HVDC system on PSCAD/EMTDC with various test conditions are presented to validate the proposed analysis.

A Three-Terminal Hybrid HVDC System Based on LCC and Hybrid MMC with DC Fault Clearance Capability

Wei Han,Ruizhang Yang,Wang Xiang,Chao Liu,Weidong Ma,Jinyu Wen 전력전자학회 2019 ICPE(ISPE)논문집 Vol.2019 No.5

In this paper, a three-terminal hybrid HVDC system based on LCC and hybrid MMC is proposed, which has the ability of DC fault clearance. The system combines the merits of hybrid HVDC system and multi-terminal HVDC system, which is able to deliver bulk renewable power to multiple receiving power grids at a relatively lower cost. By utilizing hybrid MMC as the inverters, the system has the capability to ride through DC faults. The topology and the operating characteristics of the system are introduced in detail. The design of the hybrid MMC is analyzed. The control strategy and the DC fault ride through strategy are proposed. Finally, the model of three-terminal HVDC system is built in PSCAD / EMTDC. The simulation verifies the effectiveness of the control strategy and the DC fault ride through strategy. The results indicate that this kind of HVDC system has a good application prospect in bulk power transmission to the multiple remote receiving regional power grids and the DC fault ride through ability proposed for the system operates well.

A Fault Tolerant Control Technique for Hybrid Modular Multi-Level Converters with Fault Detection Capability

Mahmoud Abdelsalam,Mostafa Ibrahim Marei,Hatem Yassin Diab,Sarath B. Tennakoon 전력전자학회 2018 JOURNAL OF POWER ELECTRONICS Vol.18 No.2

In addition to its modular nature, a Hybrid Modular Multilevel Converter (HMMC) assembled from half-bridge and full-bridge sub-modules, is able to block DC faults with a minimum number of switching devices, which makes it attractive for high power applications. This paper introduces a control strategy based on the Root-Least Square (RLS) algorithm to estimate the capacitor voltages instead of using direct measurements. This action eliminates the need for voltage transducers in the HMMC sub-modules and the associated communication link with the central controller. In addition to capacitor voltage balancing and suppression of circulating currents, a fault tolerant control unit (FTCU) is integrated into the proposed strategy to modify the parameters of the HMMC controller. On advantage of the proposed FTCU is that it does not need extra components. Furthermore, a fault detection unit is adapted by utilizing a hybrid estimation scheme to detect sub-module faults. The behavior of the suggested technique is assessed using PSCAD offline simulations. In addition, it is validated using a real-time digital simulator connected to a real time controller under various normal and fault conditions. The proposed strategy shows robust performance in terms of accuracy and time response since it succeeds in stabilizing the HMMC under faults.

A Fault Tolerant Control Technique for Hybrid Modular Multi-Level Converters with Fault Detection Capability

Abdelsalam, Mahmoud,Marei, Mostafa Ibrahim,Diab, Hatem Yassin,Tennakoon, Sarath B. The Korean Institute of Power Electronics 2018 JOURNAL OF POWER ELECTRONICS Vol.18 No.2

In addition to its modular nature, a Hybrid Modular Multilevel Converter (HMMC) assembled from half-bridge and full-bridge sub-modules, is able to block DC faults with a minimum number of switching devices, which makes it attractive for high power applications. This paper introduces a control strategy based on the Root-Least Square (RLS) algorithm to estimate the capacitor voltages instead of using direct measurements. This action eliminates the need for voltage transducers in the HMMC sub-modules and the associated communication link with the central controller. In addition to capacitor voltage balancing and suppression of circulating currents, a fault tolerant control unit (FTCU) is integrated into the proposed strategy to modify the parameters of the HMMC controller. On advantage of the proposed FTCU is that it does not need extra components. Furthermore, a fault detection unit is adapted by utilizing a hybrid estimation scheme to detect sub-module faults. The behavior of the suggested technique is assessed using PSCAD offline simulations. In addition, it is validated using a real-time digital simulator connected to a real time controller under various normal and fault conditions. The proposed strategy shows robust performance in terms of accuracy and time response since it succeeds in stabilizing the HMMC under faults.

Capacitor voltage balancing method for hybrid modular multilevel converters based on second‑harmonic voltage injection

Hongxu Li,Qin Wang,Qunfang Wu,Lan Xiao,Jinbo Li 전력전자학회 2024 JOURNAL OF POWER ELECTRONICS Vol.24 No.4

Due to different output voltages, capacitor voltage imbalance occurs between half-bridge sub-modules (HBSM) and fullbridge sub-modules (FBSM) in hybrid modular multilevel converters (MMCs) under a boosted modulation index (m). To address this issue, a capacitor voltage balancing method based on second-harmonic voltage injection is proposed in this paper. The mechanism of the proposed method is to eliminate the DC component in the charging power of sub-modules under the restriction that the HBSM cannot output negative voltage. The proposed method is divided into two cases according to m. Case 1 is used when m ≤ 1.4, while Case 2 is implemented when m > 1.4. When compared with similar capacitor voltage balancing methods, the proposed SVI method has a simpler structure that can greatly reduce the number of computations. Simulation and experimental results verify the effectiveness of the proposed capacitor voltage balancing method.

Bipolar HVDC MMC 전송을 위한 비대칭 혼합 회로구성

정재정 대한전기학회 2020 전기학회논문지 Vol.69 No.3

Among the MMC topologies for bipolar HVDC systems, the asymmetric mixed MMC is considered as promising and attractive circuit due to the cost saving, less loss, DC fault ride-through capability, and free and quick DC bus voltage regulation. In this paper, the reasonable circuit configuration of an asymmetric mixed MMC in bipolar HVDC transmission system is introduced. In the application of asymmetric mixed MMC for the bipolar HVDC system, the induced DC bias voltage into AC transformer at normal operation according to the placement of submodules is investigated and discussed. Furthermore, the optimal submodule arrangement is proposed considering the busbar single line-to-ground fault at AC side of MMC.