Submodule Failure Research Articles

Review of Fault Diagnosis and Fault-Tolerant Control Methods of the Modular Multilevel Converter Under Submodule Failure

Modular multilevel converters (MMCs) have attracted extensive research interests in various ac and dc conversion applications due to their modular structure and excellent harmonic performance. However, the large number of power switches increases the potential risk of submodule (SM) failure, which greatly challenges the safe and reliable operation of the MMC. This paper presents a detailed review of fault diagnosis and fault-tolerant control methods of the MMC under SM failures. On this basis, comprehensive comparisons are conducted among different fault diagnosis methods, and verification results are provided to analyze the advantages and disadvantages of the popular fault-tolerant control methods. Finally, the review is concluded, and future trends and research opportunities are discussed.

Fault detection and localization method for modular multilevel converters in offshore DC wind turbines

The offshore DC wind turbine, consisting of direct-drive permanent magnet synchronous generator (PMSG) and modular multilevel converter (MMC), has become a promising candidate for the large-capacity wind energy conversion system. Submodule (SM) failure detection and localization are crucial for reliable operation of the MMC. However, existing techniques mainly focus on the rated operation condition, which may not be suitable for the MMC in wind turbine due to varying wind speed. Moreover, proper injection of second-order harmonic circulating current (SHCC) is desirable to reduce capacitor voltage ripple and power loss, which may also affect the accuracy of fault detection. This paper proposes a fault-tolerant strategy immune to variable operation conditions, as well as SHCC injection, for the MMC with phase-shifted carrier modulation. Firstly, the inherent switching harmonics of circulating current with SM failure are analyzed, based on which the fault detection method is developed. Then, the localization approach is presented by measuring the difference of SM capacitor voltages. Compared with existing techniques, the proposed strategy requires neither estimator nor additional sensor, which is robust, simple, and economical. Finally, both simulation and experimental results demonstrate the effectiveness of the proposed method, which is not influenced by SHCC and varying wind speed.

A Sliding-Mode Observer for MMC-HVDC Systems: Fault-Tolerant Scheme With Reduced Number of Sensors

The modular multilevel converter (MMC) has been widely adopted in different applications, such as high-voltage direct current (HVDC), electric drives and static synchronous compensators. Despite the application, MMC realization requires a high number of power electronic components, sensors, and communication cables. This paper proposes a sliding-mode observer for capacitor voltages of the MMC, using the measurements of arm currents and arm voltages. A linear corrector within the hysteresis band is proposed to reduce the chattering in the observer dynamics. Moreover, guidelines for tuning the observer gain are presented. The SM capacitance value is included as a parameter uncertainty of the observer. The proposal reduces the requirements of voltage sensors and optical links in MMC and allows fault-tolerant operation. The proposed observer performance is evaluated through a simulation of 100 MVA three-phase MMC-HVDC during initialization, steady-state, and submodule failures. The effect of the switching and sampling frequencies on the observer performance is also evaluated. Finally, experimental results validate the proposal in a downscale 3 kVA single-phase MMC prototype.

Submodule Fault-Tolerant Strategy for Modular Multilevel Converter with Scalable Control Structure

Modular Multilevel Converter (MMC) topology is considered a good candidate for high-voltage applications. One of the reasons is that an MMC can quickly generate a higher voltage with an excellent sine wave with the series connection of many power blocks, called Sub-Modules (SMs). In such applications, the control system of an MMC can be challenging, and the possibility of an SM failure increases. As a result, the reliability and availability of the application reduce over time. To reduce the effects of SM failure, an MMC is usually equipped with Redundant SMs (RSMs). The RSMs are added into MMC arms as regular SMs to increase the application’s reliability and reduce downtime. This paper proposes a unique decentralized SM fault-tolerant control model for RSMs to participate in any SM sets. In an MMC arm, a dedicated controller is assigned to RSMs, while the group of SMs has their local controllers. The controller of the RSMs continually monitors the voltage of all the SM sets in the arm. If there is any failure, the controller of the RSMs activates a requested number of SMs to help local controllers to generate the desired voltage level. The proposed control system significantly reduces local controllers’ computational and communication requirements compared to conventional redundant controllers. The proposed control system is based on a distributed structure, so it does not limit hardware flexibility, such as the scalability and modularity of an MMC system. Besides, the separate controller for the RSMs significantly helps increase the reliability of an MMC application.

Overvoltage Suppression for Modular Multilevel Converter With Switch Open-Circuit Fault

To enhance reliability and exploit advantages, modular multilevel converters (MMCs) are mostly desired with uninterrupted operational capability during submodule (SM) open-circuit fault. Once an SM fault is located, the closing signal will be delivered to the corresponding bypass switch. However, due to the response delay of mechanical switches, capacitor voltage of the SM with lower switch fault will continue increasing, thus resulting in severe overvoltage and threatening the safety of MMC. In view of this, this article proposes an arm current reshaping-based method to suppress capacitor overvoltage under SM failure. During the period of overvoltage suppression, all the arm currents are reshaped to decrease capacitor charging of the faulty SM and maintain the terminal currents of MMC. Besides, since the capacitor voltage variations are also reshaped by the reshaped arm current, common-mode voltage is injected to satisfy the requirements of arm voltages. Consequently, the degree of SM overvoltage can be alleviated by over 50% without extra hardware investment. Simulation and experimental results confirm the feasibility and validity of the proposed method.

A Novel Fault-Tolerant Operation Approach for the Modular Multilevel Converter-Based STATCOM With the Enhanced Operation Capability

The operation capability of the modular multilevel converter (MMC)-based static synchronous compensator (STATCOM) is limited under submodule (SM) failures. When running in the STATCOM mode, the floating dc-link voltage of the MMC is adjustable. Therefore, this article proposes a novel fault-tolerant operation approach to enhance the operation capability of the MMC under severe SM failures. First, the dc-link voltage is dynamically regulated under SM failures, where the capacitor voltages of all SMs are slightly increased. The modulation ranges of the faulty and healthy phases can be enlarged, and the even voltage stress distribution can be realized. Then, a low-magnitude fundamental frequency zero-sequence voltage (FF-ZSV) is injected to balance the line-to-line voltages. Finally, the virtual energy idea is proposed, where the capacitor voltages of faulty SMs are considered to be clamped at the reference value. As a result, the arm energy reference will be the same in all six arms, and the arm energy balancing control is simplified. Detailed comparisons show that the proposed approach has an enhanced operation capability with lower capacitor voltage increment. Simulation and experimental results verify that even under severe SM failures, the proposed approach can still guarantee the fault-tolerant operation within the specified security region.

Unified Strategy for Fault-Tolerant Operation of MMC with Multiple SMs Failure Based on SMs Grouping Management

Modular multilevel converter (MMC) is distinguished by its modularity. In order to improve its reliability and avoid unscheduled maintenance, it requires the MMC to continue operating even though some of its submodules (SMs) have failed. In this paper, a grouping management method of SMs in MMC is first presented where every six SMs are conceptually grouped into a virtual subunit (SU), and on this basis, a simplified space vector modulation (SVM) implementation is developed. Then, the fault-tolerant solutions are proposed by investigating the post-fault operation of the MMC based on the virtual SU concept. It is analyzed that the SU failures can be categorized into three basic types, according to the phase where the failed SMs are located, and also multiple SM failures can be decomposed into one or multiple basic fault types. In this way, the fault-tolerant solutions are unified regardless of the number of faulty SMs. Further, the sorting capacitor voltage control is combined into the proposed methods to balance all of the SM capacitor voltages. With the proposed fault-tolerant method, the amplitude and THD of the line voltages are almost unchanged under fault conditions when compared with normal operations. The simulation and experiment verify the propositions.

Cascaded H-bridge Based STATCOM With Improved Ride Through Capability of Submodule Failures

This article investigates the operation of cascaded H-bridge (CHB) based static synchronous compensator (STATCOM) after failure in its submodules (SMs). Once a switch fails, the corresponding faulty SM is bypassed. In the post-fault operation, a zero-sequence voltage (ZSV) is injected to the converter phases to generate the maximum possible line-to-line voltage. Using a clamping technique, the fundamental component of injected ZSV (FZSV) is minimized according to the operating point of CHB STATCOM. But, FZSV leads to unequal flow of active power into the converter phases. In the proposed method, negative-sequence current is controlled to manage the active power flow within CHB phases. By doing this, the effect of FZSV on active power of CHB phases is compensated and the voltages of capacitors are kept balanced. This method ensures the continuous operation of CHB STATCOM with the minimum over-design and guarantees the voltage balance of capacitors after a failure. The presented simulation and experimental results validate the feasibility and effectiveness of the proposed method.

A novel non-redundant submodules fault-tolerant control strategy for MMC in medium voltage application

Modular multilevel converters (MMCs) are widely used in medium-voltage application. It is necessary to design the control strategy under sub-modules (SMs) failure conditions. To increase the SMs fault ride-though capability of MMC without redundant SMs, a non-redundant fault-tolerant scheme is presented in this paper. Firstly, the operation reliability of MMC is analysed. From the perspective of improving the MMC control strategy, the operation reliability of MMC can be enhanced without redundancy investment. Secondly, the operating characteristics of MMC under SMs failure are derived. And the control objective of the resilient capacitor voltage control strategy is concluded, which is not only appropriate for the MMC in normal state but also appropriate for the MMC in AC system voltage imbalance and SMs fault state. Then, the non-redundant fault-tolerant scheme is proposed, which provides the possibility of reducing investment and enhancing economy while ensuring the operation reliability for MMCs in medium-voltage application. Finally, the control scheme is verified by simulations and experiments on a MMC prototype.

A Novel Submodule Level Fault-Tolerant Approach for MMC With Integrated Scale-Up Architecture

A submodule (SM) level fault-tolerant method for modular multilevel converters (MMCs) with a scalable control design for medium- and high-voltage applications is proposed in this article. A large number of SMs, which is typically required in high-power applications, is divided into sets with a smaller number of SMs. A local controller is assigned to each set for the capacitor voltage balancing (CVB) and switching signal generations, and the SM level protection. A CVB control method is proposed to maintain SM voltages among sets within a prespecified band under SM failures, in which an MMC arm can operate with unequal numbers of SMs among sets. The computational and communication burdens are reduced in each local controller. Most importantly, changing the number of SMs is achieved without significant modifications in the control system, which improves the expandability and flexibility of the MMC system to easily scale-up to higher voltage levels. A ±320 kV/1000 MW MMC system has been studied in a real-time digital simulator (RTDS) and field-programmable gate arrays boards to validate the proposed fault-tolerant control method under SM failures.

A fault-tolerant strategy of hybrid modular multilevel converter based on level adjusting method

With the advantage of DC fault ride-through and the reduced cost, a hybrid modular multilevel converter (HMMC) has become a promising topology for HVDC applications. However, the complicated configuration of HMMCs with a large number and different types of submodules (SM) increases the risk of submodule failure. This paper proposes a novel HMMC fault-tolerant strategy by adjusting the inserted SM number residing in the fault phases, analyzes voltage fluctuation of the SM capacitor, and designed an SM capacitor voltage controller in coordination with fault-tolerant strategy. Meanwhile, by utilizing the negative voltage output characteristic of FBSMs, this strategy can significantly improve the SM fault tolerance capability of the HMMC. Moreover, as compared to conventional fault-tolerant strategies, the proposed strategy requires fewer components and can be easily implemented. Finally, the effectiveness of the proposed fault-tolerant methods is demonstrated by MATLAB/Simulink under different fault conditions and MMC systems.

On Converter Fault Tolerance in MMC-HVDC Systems: A Comprehensive Survey

Since they were first proposed, modular multilevel converters have been strongly studied in the literature. Due to their high component count, some concerns regarding their reliability arise, and several fault-tolerance schemes have been proposed in recent years. This article presents a comprehensive survey on the converter fault-tolerance strategies for modular multilevel converter (MMC) high-voltage direct current (HVdc) systems based on the available technical literature. Some MMC-HVdc solutions adopted in industry are reviewed. This work proposes the analytical expressions to determine the maximum number of submodule failures in the MMC-HVdc system. Thereafter, the limitations of the main converter fault-tolerance schemes under submodule failures are described and critically compared. A case study is conducted based on 1000-MVA Xiamen MMC-HVdc project with ±320-kV dc-link voltage, aiming to evaluate the effect of the faulty submodules on the reliability and power losses of the converter. Finally, the remaining challenges and opportunities in the converter fault tolerance in MMC-HVdc systems are stated as well as the future directions of this field.

Impact of Submodule Faults on the Performance of Modular Multilevel Converters

Modular multilevel converter (MMC) is well suited for high-power and medium-voltage applications. However, its performance is adversely affected by asymmetry that might be introduced by the failure of a limited number of submodules (SMs) or even by severe deviations in the values of SM capacitors and arm inductors, particularly when the number of SMs per arm is relatively low. Although a safe-failed operation is easily achieved through the incorporation of redundant SMs, the SMs’ faults make MMC arms present unequal impedances, which leads to undesirable internal dynamics because of unequal power distribution between the arms. The severity of these undesirable dynamics varies with the implementation of auxiliary controllers that regulate the MMC internal dynamics. This paper studied the impact of SMs failure on the MMC internal dynamics performance, considering two implementations of internal dynamics control, including a direct control method for suppressing the fundamental component that may arise in the dc-link current. Performances of the presented and widely-appreciated conventional methods for regulating MMC internal dynamics were assessed under normal and SM fault conditions, using detailed time-domain simulations and considering both active and reactive power applications. The effectiveness of control methods is also verified by the experiment. Related trade-offs of the control methods are presented, whereas it is found that the adverse impact of SMs failure on MMC ac and dc side performances could be minimized with appropriate control countermeasures.

New Submodule Improving Fault-Tolerant Capability of Modular Multilevel Converters

This article presents a new submodule (SM) circuit that facilitates the fault-tolerant capability for modular multilevel converters (MMCs) under various internal and external faults. The proposed SM circuit ensures continued operation of the MMC subsequent to multiple SM failures and symmetrical and asymmetrical grid faults without the need for additional hardware implementation. The proposed SM circuit can be simply integrated into conventional half bridge-based MMCs by using the switching signal adapter block. The switching signal adapter unit duly assigns a switching pattern to the SMs in each arm to obtain the most efficient current path and inherent capacitor voltage balancing features. A fully modular structure, enhanced internal fault management, external fault-handling capability, and ease of expandability are the key features of the proposed SM circuit. The configuration, modeling, and gating signal pattern of an MMC that adopts the proposed SM are discussed and analyzed in detail. Time-domain simulation studies using detailed nonlinear models validate the effectiveness of the proposed configuration and its principle of operation in an MMC-HVDC system under various fault conditions. In addition, hardware-in-the-loop test results confirm the feasibility of hardware implementation of the proposed SM circuit.

Detection and Localization Strategy for Switch Open-Circuit Fault in Modular Multilevel Converters

The modular multilevel converter (MMC) is one of the most attractive topologies for high-voltage and high-power applications. Reliability is an important challenge for MMCs due to large amounts of power switches in the phase leg of the MMC. To enhance the reliable performance of MMCs, an effective and accurate fault detection and localization strategy for MMCs with the nearest level modulation is proposed in this paper. According to the general characteristics of the MMC, the capacitor voltage of the faulty submodule (SM) is higher than the healthy ones’. Based on the sorting algorithm, the faulty SM will have the maximum voltage and it will be identified by the proposed fault detection method. The auxiliary monitoring is supplemented to the detection algorithm for the cases of faults occurring in multiple SMs. The faulty SM localization strategy is based on the differences of capacitor voltages. The faulty switches are located by the distinctions of capacitor voltage in different current intervals. The proposed strategy could be applied under the cases of single or multiple SMs failures and identifies three kinds of the power switches faults in the faulty SM. The validity of the proposed strategy is verified by the simulation results in MATLAB/Simulink.

Detection and Localization of Submodule Open-Circuit Failures for Modular Multilevel Converters With Single Ring Theorem

Submodule (SM) failure detection and localization is crucial to reliability improvement of modular multilevel converters (MMCs), which consist of numerous identical SMs. Upon SM failures, the capacitor voltages of the faulty SMs deviate from those of the healthy SMs. Hence, SM voltage consistency disruption can be utilized as an indicator of SM failures. To enable effective consistency evaluation of numerous SM voltages, the single ring theorem, which is an ideal analyzing tool for large size matrices, is applied to SM failure detection in this paper. The proposed SM failure detection method eliminates the need for ideal-state estimators and thus ensures sufficient robustness in terms of parameter uncertainties. Additional voltage/current sensors are not required, which is beneficial for cost efficiency. A statistical-analysis-based SM failure localization method enabling fast identification of the faulty SMs upon failure detection is also proposed. Both single and multiple SM failure detection and localization can be handled with the proposed method even if the faulty SMs are in different arms. The effectiveness of the proposed SM failure diagnosis method is verified by both simulation in MATLAB/Simulink and experimentation on a 13-level MMC prototype.

A Novel Fault-Tolerant Control of Modular Multilevel Converter under Sub-Module Faults Based on Phase Disposition PWM

Each arm of modular multilevel converter (MMC) consists of a large number of sub-module (SM) units. However, it also increases the probability of SM failure during the long-term system operation. Focusing on the fault-tolerant operation issue for the MMC under SM faults, the traditional zero-sequence voltage injection fault-tolerant control algorithm is analyzed detailed and its disadvantages are concluded. Based on this, a novel fault-tolerant control strategy based on phase disposition pulse-width modulation (PD-PWM) is proposed in this paper, which has three main benefits: (i) it has carrier and modulation wave dual correction mechanism, which control ability is more higher and flexible; (ii) it only needs to inject zero-sequence voltage in half a cycle of the modulation wave, which simplifies the complexity of traditional zero-sequence voltage injection control algorithms and much easier for implement; (iii) furthermore, the zero-sequence voltage can even be avoided injecting under the symmetrical fault conditions. Finally, the effectiveness of the proposed control strategy is verified with the simulation and experiment studies under different fault conditions.

Fast Diagnosis Method for Submodule Failures in MMCs Based on Improved Incremental Predictive Model of Arm Current

Fast Diagnosis Method for Submodule Failures in MMCs Based on Improved Incremental Predictive Model of Arm Current

Design and Implementation of a Modular Multilevel Converter With Hierarchical Redundancy Ability for Electric Ship MVDC System

Modular multilevel converter (MMC) can be considered as a promising candidate for modern electric ship medium-voltage direct current (MVDC) power system due to its high-voltage quality, low switching loss, good expansibility, and good redundancy. As the voltage level in MVDC is much lower than that in high-voltage direct current case, the number of submodules (SMs) of MMC in MVDC system is also much less. Therefore, the SM failure will have more significant effect on the safety operation. Since uninterrupted operation is one of the most important requirements in electric ship application, the special redundancy design for MMC is required. This paper thus proposes an MMC system with hierarchical redundancy ability, aiming to guarantee the ride-through operation under SM failure. The “hot-reserved” and “cold-reserved” SMs are prepared in the MMC by compromising the faulty SM number, voltage and current stress of SMs, SM power loss, and so on, and the hierarchical redundancy strategy is divided into several progressive layers to ride through the SM failure in different cases. The pretreatment units are designed to realize the proposed hierarchical redundant strategy smoothly, and without changing the existent modulation and sort-and-selection strategies designed for normal condition. The proposed design is explained in this paper, and verified by a three-phase MMC simulation and a single-phase MMC prototype.

Modular Multilevel High-Frequency-Link DC Transformer Based on Dual Active Phase-Shift Principle for Medium-Voltage DC Power Distribution Application

This paper proposes a multilevel high-frequency-link dc transformer (MDCT) based on dual active phase-shift principle for medium-voltage dc (MVDC) power distribution application. The proposed MDCT employs multilevel and multiplex conversion principle, which brings many advantages and makes the operation of MDCT quite different with the traditional dc transformer (TDCT) and modular multilevel converter. Compared to the TDCT scheme, the proposed MDCT has smaller circulating current and higher power factor; it can operate as a dc breaker to cutoff the connection with the MVDC distribution grid absolutely when a short fault occurs in the distribution grid; the redundant design can be achieved when some submodules failure to improve the reliability. In the paper, the topology, operation principle, modulation method, switching characterization, voltage, and power characterization, and control strategy of MDCT are presented and analyzed comprehensively. At last, a MDCT prototype is built and the experimental results verify the correctness and effectively of the proposed solution.