EPE Association: EPE 2021 - Modular Multilevel Converters

EPE 2021 - Modular Multilevel Converters
You are here: EPE Documents > 01 - EPE & EPE ECCE Conference Proceedings > EPE 2021 ECCE Europe - Conference > EPE 2021 - Topic 02: Power Converter Topologies and Design > EPE 2021 - Modular Multilevel Converters

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   A Supervised Capacitor Voltage Balancing Method of Modular Multilevel DC-DC Converters for Medium Voltage DC Application
By Jing SHENG [View]
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Abstract: In this paper, the submodule capacitor voltage fluctuation of high-frequency modular multilevel dc-dc (MMDC) converters is investigated firstly. Considering the high switching frequency and the control delay in practice, the issue of large capacitor voltage ripple in MMDC converters is revealed. Then a supervised capacitor voltage balancing method is proposed in this paper. This balancing method can avoid the continuous charging of submodule capacitors, which can reduce the voltage ripple by 61\%. Finally, the theoretical analysis and proposed method is validated on a 16kV MMDC prototype.

   Discussions on fault-tolerant operation of modular multilevel converters using mechanical bypass switches
By Laxman MAHARJAN [View]
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Abstract: Reliability and availability of modular multilevel converters such as SSBC (single-star bridge-cell) andSDBC (single-delta bridge-cell) converters can be significantly improved by achieving continuous operation when a bridge cell fails. In order to achieve this, each bridge cell is equipped with a bypass switch at its ac terminals. When a bridge cell in a phase or cluster fails, the faulty bridge cell, and two healthy bridge cells in the other two phases or clusters in case of symmetrical operation, are switched OFF and their ac terminals are short-circuited by turning ON their bypass switches. The converter is then operated with the remaining healthy bridge cells. The converter thus transitions from normal operation to fault-tolerant operation without requiring to turn OFF the whole converter. However, when a mechanical switch is used as a bypass switch, the transition period, i.e. the interval between the fault occurrence and the closing of the bypass switch, can range from tens of milliseconds to over a hundred milliseconds. During this period, the concerned bridge cells including the faulty one may work as diode rectifiers, thus increasing their dc capacitor voltages and leading to dc overvoltage. Moreover, the ac output voltages of the concerned bridge cells are not zero and act as a disturbance to the current control of the converter leading to degradation in output performance. This paper presents a simple solution to avoid the possible dc overvoltage and proposes a current control method to improve the converter output performance, both during the transition period. The former is based on the attenuation of the current flowing through the converter, while the latter is based on the cancellation or minimization of the effect of voltages appearing across the switched-off bridge cells including the faulty one. Experimental and simulation results validate the effectiveness of the proposed ideas.

   Single-phase Modular-Multilevel Converter Based on Concept of Universal Smart Power Module (USPM)
By Hitoshi HAGA [View]
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Abstract: This paper proposes control method for a power conversion system constructed by combining power conversion modules based on a new concept called Universal Smart Power Module (USPM). The USPM consists of a H-bridge including the controller, and a power converter is realized by using multiple modules. The proposed method avoids interference between the controllers installed in each module due to series or parallel connection of modules. In addition, the main controller sends the signals to suppress the deviation occurred in the operation of the module controller. These signals are updated through low-speed wireless communication. Therefore, it is necessary to verify low frequency communication and communication blackout. The main controller generates effective command values instead of instantaneous command values to overcome the communication problems. The module controller converts the effective value into an instantaneous value to realize normal operation. With this control method, the power conversion system can be operated even in the event of communication delay, infrequent communication, or communication blackout. In this paper, a single-phase Modular Multilevel Converters (MMC) with four modules was used in experiment for the purpose of effectiveness of the proposed method. As a result, the proposed control method was experimentally confirmed achieving an input current THD 2.63\% and an input power factor of 0.99 on steady-state and operating stable even in the event of communication delay, infrequent communication, or communication blackout.

EPE 2021 - Modular Multilevel Converters
You are here: EPE Documents > 01 - EPE & EPE ECCE Conference Proceedings > EPE 2021 ECCE Europe - Conference > EPE 2021 - Topic 02: Power Converter Topologies and Design > EPE 2021 - Modular Multilevel Converters
	[return to parent folder]

	A Supervised Capacitor Voltage Balancing Method of Modular Multilevel DC-DC Converters for Medium Voltage DC Application By Jing SHENG	[View] [Download]
Abstract: In this paper, the submodule capacitor voltage fluctuation of high-frequency modular multilevel dc-dc (MMDC) converters is investigated firstly. Considering the high switching frequency and the control delay in practice, the issue of large capacitor voltage ripple in MMDC converters is revealed. Then a supervised capacitor voltage balancing method is proposed in this paper. This balancing method can avoid the continuous charging of submodule capacitors, which can reduce the voltage ripple by 61\%. Finally, the theoretical analysis and proposed method is validated on a 16kV MMDC prototype.

	Discussions on fault-tolerant operation of modular multilevel converters using mechanical bypass switches By Laxman MAHARJAN	[View] [Download]
Abstract: Reliability and availability of modular multilevel converters such as SSBC (single-star bridge-cell) andSDBC (single-delta bridge-cell) converters can be significantly improved by achieving continuous operation when a bridge cell fails. In order to achieve this, each bridge cell is equipped with a bypass switch at its ac terminals. When a bridge cell in a phase or cluster fails, the faulty bridge cell, and two healthy bridge cells in the other two phases or clusters in case of symmetrical operation, are switched OFF and their ac terminals are short-circuited by turning ON their bypass switches. The converter is then operated with the remaining healthy bridge cells. The converter thus transitions from normal operation to fault-tolerant operation without requiring to turn OFF the whole converter. However, when a mechanical switch is used as a bypass switch, the transition period, i.e. the interval between the fault occurrence and the closing of the bypass switch, can range from tens of milliseconds to over a hundred milliseconds. During this period, the concerned bridge cells including the faulty one may work as diode rectifiers, thus increasing their dc capacitor voltages and leading to dc overvoltage. Moreover, the ac output voltages of the concerned bridge cells are not zero and act as a disturbance to the current control of the converter leading to degradation in output performance. This paper presents a simple solution to avoid the possible dc overvoltage and proposes a current control method to improve the converter output performance, both during the transition period. The former is based on the attenuation of the current flowing through the converter, while the latter is based on the cancellation or minimization of the effect of voltages appearing across the switched-off bridge cells including the faulty one. Experimental and simulation results validate the effectiveness of the proposed ideas.

	Single-phase Modular-Multilevel Converter Based on Concept of Universal Smart Power Module (USPM) By Hitoshi HAGA	[View] [Download]
Abstract: This paper proposes control method for a power conversion system constructed by combining power conversion modules based on a new concept called Universal Smart Power Module (USPM). The USPM consists of a H-bridge including the controller, and a power converter is realized by using multiple modules. The proposed method avoids interference between the controllers installed in each module due to series or parallel connection of modules. In addition, the main controller sends the signals to suppress the deviation occurred in the operation of the module controller. These signals are updated through low-speed wireless communication. Therefore, it is necessary to verify low frequency communication and communication blackout. The main controller generates effective command values instead of instantaneous command values to overcome the communication problems. The module controller converts the effective value into an instantaneous value to realize normal operation. With this control method, the power conversion system can be operated even in the event of communication delay, infrequent communication, or communication blackout. In this paper, a single-phase Modular Multilevel Converters (MMC) with four modules was used in experiment for the purpose of effectiveness of the proposed method. As a result, the proposed control method was experimentally confirmed achieving an input current THD 2.63\% and an input power factor of 0.99 on steady-state and operating stable even in the event of communication delay, infrequent communication, or communication blackout.