Redundant Submodules Research Articles

Fault-Tolerant Method of Modular Railway Power Conditioner Based on DC Bus Voltage Regulation

With the development of high-speed electrified railway, many serious power quality problems have arisen and caused widespread concern. In order to achieve comprehensive management of power quality, modular railway power conditioner (MRPC) has been applied to traction systems. However, due to the presence of multiple sub-modules (SMs). The reliability of the equipment is threatened as the number of SMs increases. In medium-voltage applications, system reliability can be improved by adding redundant SMs. However, when the redundant SMs are run out, the reliability of the whole system cannot be guaranteed. Therefore, a fault-tolerant method based on DC bus voltage regulation is proposed to improve the reliability of MRPC by modulation signal reconstruction. The proposed method not only achieves fault ride-through to maintain normal operation and improves the SMs voltage stress on the opposite converter, but also effectively improves the over-modulation. The effectiveness of the proposed method is verified by down-scale experiments.

Shared Redundancy Strategy to Improve the Reliability and Fault-Tolerant Capability of Modular Multilevel Converter

Utilizing redundant sub-modules is a well-known strategy to improve the reliability and fault-tolerant capability of the modular multilevel converter (MMC). Redundancy-based fault-tolerant strategy makes it possible to restore the full performance of the converter in the post-fault condition. The main drawback of this strategy is the high cost. In this configuration, each redundant sub-module (RSM) can only be utilized in a specific arm and it cannot be used in other arms. This limitation leads to inefficient utilization of the RSMs in conventional MMC. This paper proposes a shared redundancy strategy which allows RSMs to be used either in upper or lower arms. The derived reliability models in various case studies illustrate that proposed strategy significantly improves the reliability and useful lifetime of MMC. To investigate the effectiveness of the proposed strategy, simulation and experimental results are provided.

Semi‐hot redundant control method for modular multilevel converter‐based high‐voltage direct current systems under submodules fault

AbstractA modular multilevel converter‐based high voltage direct current (MMC‐HVDC) system involves a large number of cascading submodules (SMs). The failure of SMs may critically degrade the system performance. Therefore, several redundant SMs can be incorporated and reasonably controlled to ensure the system reliable and continuous operation even if certain SMs fail. This paper proposes a semi‐hot redundant method for MMC‐HVDC systems. In the proposed strategy, when the system operates normally, the redundant SMs are controlled in a lower voltage range. These SMs do not require frequent charging and discharging, which decreases the switching loss. When ordinary SMs fail, the faulty SMs are bypassed and the corresponding number of redundant SMs can be promptly recharged to the rated voltage to replace the faulty SMs. The semi‐hot reserved SMs can shorten the transient process of faulty SMs replacement, and in this process, the absence of faulty SMs does not considerably influence the system. Thus, the original control and modulation strategies of the system do not need to be modified when several SMs fail. The performance of the proposed method is compared with those of the existing methods, and its validity is demonstrated through a small‐scale experimental 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.

An Advanced Power Flow Control in Small Scale DC Power Structure by Using Multilevel Converter

Because they combine outstanding harmonic performance with low switching frequencies, multilevel transforms are attractive options in Small-Scale DC Power Ne2rks. High dependability can also be obtained by including redundant submodules into the cascaded transform chain. DC microgrids are developing as the next generation of smallscale electric power structures, with very low line impedance. This phenomena creates high currents in microgrids even with little voltage changes; hence, a power flow controller must have quick transient reaction and accurate power flow management. Multi-level transforms are used as power flow controllers in this work to provide high speed and high accuracy power flow management in a dc microgrid. Because a multi-level transform is employed, the output filter can be tiny. The linear controller, such as PI or PID, is established and widely used in the power electronics sector, but its performance degrades as system parameters change. In this paper, a neural structure (NN) based voltage management technique for a DC-DC transform is developed. This project also shows how to construct the output LC filter of a multi-level transform to meet a current ripple requirement. In comparison to typical 2-level transforms, we can demonstrate that a multilevel transform with a smaller filter may provide high-speed and high-precision power flow management for low line impedance situations. MATLAB/Simulink Simulation results are used to evaluate the control performance of each output current in the step response while accounting for transient variations in the power flow.

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.

Control Design and Fault Handling Performance of MMC for MMC-Based DC Distribution System

Modular multilevel converters (MMCs) have emerged as a viable choice in future DC grid architectures due to their scalability to meet voltage level requirements. However, MMC-based DC distribution systems are at risk of short-term outages during the faults in either the DC or AC networks feeding the MMC, so it remains a challenge to accomplish AC and DC fault ride-through (FRT) capability in such applications. To ensure stable operations of the DC terminals, FRT strategies are required for the faults on both the AC and DC sides of the converter. This paper proposes a FRT strategy for the AC and DC side of the converter to ensure stable and economically viable operation of the DC distribution network. The asymmetrical faults in the upstream AC grids are managed by using the integrated energy of the MMC. Whereas, the DC FRT capability of the MMC is accomplished by changing the redundant submodules of the MMC to full-bridge submodules (FBSMs), thus allowing a DC FRT to be achieved by using DC circuit breakers that are low cost and reduced in size. Applying the proposed DC FRT strategy, which makes possible the use of low-cost and reduced in size DC circuit breakers in DC distribution, results in a reduction in the overall initial investments.

T-shaped hybrid alternate arm converter with arm energy balancing control for battery energy storage systems

Abstract Multilevel voltage source converters (VSCs), such as modular multilevel converter (MMC), cascaded H-Bridge (CHB) and alternate arm converter (AAC), are competent topologies for battery energy storage systems (BESSs) due to modularity, scalability and low harmonic distortion. However, there is a lack of studies about interfacing AAC with a BESS due to the arm energy balancing issue. Redundant sub-modules (SMs) are inserted passively into MMC, CHB and AAC to achieve high reliability; consequently, some of them are constantly idling, resulting in low SM utilization. We propose a novel topology -T-shaped hybrid alternate arm converter (TSHAAC) for BESS applications. In addition to the aforementioned features, the proposed TSHAAC requires lower number of SMs than MMC and AAC, along with lower number of switches than CHB. Moreover, an adapted arm energy balancing control is proposed to take advantage of the redundant SMs that are idling to achieve faster balancing than in conventional AAC configuration. The simulation results validate the integration of TSHAAC configuration in a BESS; the adapted arm energy balancing control is able to improve the balancing duration by 27 %.

An Advanced Power Flow Control in Small Scale DC Power Structure by Using Multilevel Converter

Because they combine outstanding harmonic performance with low switching frequencies, multilevel transforms are attractive options in Small-Scale DC Power Ne2rks. High dependability can also be obtained by including redundant submodules into the cascaded transform chain. DC microgrids are developing as the next generation of smallscale electric power structures, with very low line impedance. This phenomena creates high currents in microgrids even with little voltage changes; hence, a power flow controller must have quick transient reaction and accurate power flow management. Multi-level transforms are used as power flow controllers in this work to provide high speed and high accuracy power flow management in a dc microgrid. Because a multi-level transform is employed, the output filter can be tiny. The linear controller, such as PI or PID, is established and widely used in the power electronics sector, but its performance degrades as system parameters change. In this paper, a neural structure (NN) based voltage management technique for a DC-DC transform is developed. This project also shows how to construct the output LC filter of a multi-level transform to meet a current ripple requirement. In comparison to typical 2-level transforms, we can demonstrate that a multilevel transform with a smaller filter may provide high-speed and high-precision power flow management for low line impedance situations. MATLAB/Simulink Simulation results are used to evaluate the control performance of each output current in the step response while accounting for transient variations in the power flow.

Submodule fault-tolerant control based adaptive carrier-PDPWM for modular multilevel converters

The modular multilevel converter (MMC) is considered a very promising candidate for medium-voltage and/or high-power grids. One of the major concerns in half-bridge MMCs, especially with large number of levels, is the fault occurs at the converter submodule (SM). Hence, an appropriate control with an SM fault tolerance of the MMC is required to enhance its reliability. This paper addresses the issue and proposes a fault tolerant control-based adaptive carrier phase disposition pulse width modulation (AC-PDPWM) technique for MMCs with superior symmetrical and asymmetrical fault-tolerant capabilities. The proposed control-based AC-PDPWM acts to regulate the flow of energy between arms, rebalance the SM capacitors, and redistribute the pulses to the SM switches. In this study, a single-phase grid connected MMC having both symmetrical and asymmetrical fault conditions is investigated. Compared with other control methods, the proposed control can tolerate the SM fault conditions without requiring redundant SMs or additional computational burden, which decreases the cost and volume of the MMC design and simplifies its control. Simulation and experimental results are included to verify the fault-tolerant capability of the proposed control.

Fault-Tolerant Operation of the DC/DC Modular Multilevel Converter Under Submodule Failure

This article proposes a tailored fault-tolerant operation strategy for the dc/dc modular multilevel converter (MMC) in the case of submodules (SMs) faults. The proposed strategy utilizes the unique characteristics of the dc/dc MMC to make the post-fault operation possible without the need of adding redundant SMs. These characteristics separate the fault-tolerant operation of the dc/dc MMC from the renowned dc/ac MMC and give the converter flexibility to function at different operating points. This feature allows the converter to reduce the number of SMs needed in each arm at the cost of degraded performance or reduced power transmission. The proposed strategy is investigated through three different case studies that are simulated in the MATLAB\Simulink environment. The obtained results confirmed the effectiveness of the proposed fault-tolerant operation strategy in keeping the converter operational despite several SMs faults.

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.

Analysis and Experimental Validation of Current-Fed Switched Capacitor-Based Modular DC Transformer

Medium-voltage dc (MVdc) power distribution grid is a key link to build dc power networks. In this article, a novel input-series-output-parallel modular dc transformer (DCT) with the high-frequency-link (HFL) voltage buck-boost matching strategy is proposed for MVdc grids application. The proposed DCT is composed of a current-fed switched capacitor structure and corresponding dual-active-bridge converters, and they need to operate collaboratively to achieve the great property. Comparing with the traditional switched capacitor-based DCT (SCDCT), the most salient merits of the proposed DCT include the HFL voltage buck-boost adaptive regulation, the low conducting and switching current in switches and the capacitor voltages self-balance characteristic. These merits are all beneficial for the improvement of efficiency, operating reliability, switching performance, and cost of DCT systems. Meanwhile, the proposed DCT reserves the advantages of SCDCT including redundant submodule design and dc fault isolating capability. The topology, buck-boost modulation principle, capacitor voltage self-balance analysis, switching behaviors, and comprehensive comparison with the traditional SCDCT are completed in detail. Finally, the experimental results validate the effectiveness and correctness of the proposed solution.

Arm energy balancing control in AACs: a comparative analysis

The alternate arm converter (AAC) is an emerging state-of-the-art topology for voltage-source converter (VSC) by combining two-level converter and modular multilevel converter (MMC). The AAC performs better than the MMC in terms of the ability to block DC-side faults and ride through AC-side faults. Moreover, the number of sub-modules (SMs) per arm of the AAC is reduced by about 50% compared with the MMC. The key issue with AAC is the energy deviation during the alternate operation between the upper and lower arms. Presently, several AAC balancing techniques have been proposed, but no comparative analysis has been performed to provide insight on the effect of each balancing technique on AAC performance. In this study, five proposed balancing techniques are reviewed accordingly, followed by comparing them in terms of four parameters – the presence of DC filter, balancing capability, the presence of redundant SMs and zero-current switching control. Furthermore, a simulation work has been performed to study and compare the effects of each of the five balancing techniques. Both literature and simulation-based comparative analysis are significant to help select appropriate balancing technique as well as to provide helpful insights to improve the existing balancing techniques.

Cost and reliability optimization of modular multilevel converter with hybrid submodule for offshore DC wind turbine

The hybrid modular multilevel converter (HMMC) consisting of half- and full-bridge submodules (SMs), with DC fault ride-through capability, have become a promising candidate for offshore wind energy conversion system, especially all-DC output wind turbines. The topology of HMMC needs to be optimized for lower cost and higher reliability with active redundancy. This paper presents an optimization method for HMMC topology with cost and reliability objectives. The reliability is defined to be not only reliable under normal operation, but also able to ride through DC fault even with faulted SMs, which is different from the former model. Firstly, reliability and cost models with power loss, considering active redundancy configuration, are derived. Then, cost and reliability, with different numbers of original and redundant SMs, are quantified by taking a case study of 10-MW and 10-kV HMMC for offshore DC wind turbines. Finally, a multi-objective optimization approach that regards maximum reliability and minimum cost is proposed, and the optimal solution from Pareto-optimal set is selected according to fuzzy membership function, which determines the optimal topology for HMMC. The results show that reliability can be improve with redundant full-bridge SMs (FBSMs) but weakened with excessive half-bridge SMs (HBSMs). Furthermore, the incremental cost with a redundant FBSM is approximately 3 times that of HBSM. The optimal topology, 10 original SMs per arm (5 HBSMs and 5 FBSMs) with 1 redundant HBSM and 4 FBSMs, is validated by comparing with the results of former reliability model and single-objective optimization.

Non-Ideal Proportional Resonant Control for Modular Multilevel Converters Under Sub-Module Fault Conditions

The Modular Multilevel Converter (MMC) has become the most promising topology for high-voltage and high-power applications. Sub-Module (SM) fault ride-through capability is one of the most important features of the MMC. To study the approach of controlling SM faults, the differences between hot-backup and cold-backup configuration of redundant SMs are analyzed. After that, an internal asymmetrical dynamic model for the MMC is analyzed, which shows that unexpected fundamental and 3rd harmonic components in the circulating current result in fluctuation of DC current. Based on this model and by integrating the conventional circulating current suppression method and the SM-fault tolerant approach, a non-ideal proportional-resonant (PR) controller is proposed and analyzed. This control scheme can suppress the fluctuation of the DC current of the MMC under SM faults without the knowledge of the number of faulty SMs and extra communication system. Its effectiveness is verified by both PSCAD/EMTDC simulations and scaled-down prototype experiments.

The Optimization of Redundancy Design for Solid State Transformer

As the essential device in future integrated energy system, the reliability of solid state transformer is highly related to the security and reliability of the distribution power grid. With the redundant sub-modules, the system reliability as well as the system cost of solid state transformer will both increase. The reliability of various redundancy designs for solid state transformer are analyzed and an objective function is studied for optimized solution. The redundancy design of relatively high reliability and low additional cost for various configurations are discussed, and the optimized solution which are appropriate in both reliability and economy is obtained.

Dynamic preventive maintenance strategy for MMC considering multi-term thermal cycles

With the integration of large-scale renewable energy, preventive maintenance (PM) of modular multi-level converter (MMC) has become a critical technique for guaranteeing the secure operation of long-distance transmission systems. However, existing PM strategies result in errors and extra costs because of ignoring the time-varying operational conditions (e.g. wind speed, air temperature and power current alternating) and constant redundant sub-modules (SM). To reduce PM-related costs with an acceptable level of reliability considering different operational conditions, a dynamical PM strategy is proposed. Compared with the traditional PM strategy, adjustable redundant SMs in every operational stage and PM interval are added to dynamically coordinate reliability and PM-related costs of MMC. The reliability model of MMC proposed in this paper considers the impact of different operational conditions based on multi-term thermal cycles of components and operating characteristics of MMC. Monte Carlo method is used to estimate the reliability of MMC, and the genetic algorithm is applied to optimize the PM interval and the number of redundant SM in every operational stage. Case studies show that the PM results vary with different operational conditions. Besides, compared with existing strategies, the proposed dynamic PM strategy could reduce the PM-related costs with an acceptable level of reliability by dynamically adjusting the PM interval and redundancy on every stage.

Reliability evaluation of modular multilevel converter based on Markov model

The modular multilevel converter (MMC) is now the most attractive topology for medium and high voltage power conversion applications with several advantages over the traditional voltage source converter (VSC). However, due to a large number of sub-modules (SMs) in the MMC, system reliability is a big challenge in its practical application, where each SM may be considered as a potential point of failure. In this paper, a reliability evaluation based on the Markov model is proposed for the MMC. The failure rates of the power electronic devices and SMs are firstly analyzed. Then, the Markov model and the state transition equation of the system are built in detail. A general reliability evaluation function is established, in which the mean time to failure and reliability evaluation of the MMC with redundant SMs are also discussed. Finally, a practical direct current (DC) distribution example for reliability evaluation is analyzed, and the results verify that the reliability evaluation based on the Markov model could provide a useful reference for project design.

Reliability Modeling and Evaluation of MMCs Under Different Redundancy Schemes

Due to the demand for high reliability, modular multilevel converters (MMCs) are designed with redundant submodules. Redundant submodules can be integrated into the converter by employing different redundancy schemes: the conventional active scheme, the load-sharing active scheme, and the passive scheme. Different schemes have different impacts on the improvement of converter reliability. The contributions of this paper include that an analytical method is proposed to evaluate the reliability of MMCs under different redundancy schemes and the factors’ influence on the converter reliability is analyzed to determine the proper redundancy scheme. Reliability models of MMCs under different redundancy schemes are built using Markov chains and the iteration method. Based on the proposed models, the effects of redundant schemes are evaluated in terms of the converter reliability. A case study is conducted to validate the feasibility and robustness of proposed models and to specify the conditions in the favor of each redundancy scheme. The benefits of sharing redundancy among arms are also explored from the reliability point of view. If insulated-gate bipolar transistors (IGBTs) and capacitors are dominant components in a submodule in terms of failure rates, the load-sharing active scheme performs better; otherwise, setting the redundant submodules in an idle state is more effective. It is also found that the number of required redundant submodules is greatly reduced by sharing redundancy among arms.