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Fast Open-circuit Fault Detection Method for Defective Switches in Nested Neutral Point Clamped (NNPC) Converter
Abstract
Recently, the nested neutral point clamped (NNPC) converter has been developed for medium voltage applications. This converter has been considered for its desirable properties, such as application in a wide range of voltages, the low voltage stress on the switches, and excellent output waveform quality. This study presents a fast open-circuit fault detection method in semiconductor switches. The proposed method only requires the measurement of output voltage. The output voltage of the converter is investigated in different switching states. If there is a discrepancy in the converter output in normal mode and after the fault, the defective switch is identifiable. The proposed method has been investigated in the MATLAB Simulink environment under different scenarios. The simulation results confirm the desired performance of the proposed method.
... In [19], a method for detecting the open circuit fault in the NNPC converter is proposed by comparing the measured and expected values of the output voltage, output current, and voltage of capacitors. Also, in [20], by an output voltage sensor comparing the output voltage measured and expected, open-circuit faults are rapidly identified in different switches. However, in both methods presented, the converter is turned off after identifying the fault location, and no solution is provided to continue the operation after the fault detection. ...
... After fault detection, a converter with a fault-tolerance capability is recommended to raise availability. This paper uses the method presented in [20] to detect open circuit faults. ...
... 1. Identification of faulty switch and phase by the method presented in [20]. ...
The nested neutral point clamp (NNPC) converter is one of the most promising solutions for medium voltage motor drive applications. However, reliability due to the presence of semiconductor switches in the structure of this converter is one of the most critical concerns. The semiconductor switches are always one of the main components prone to failure. Switch failure can result in adverse effects, including system shutdown, poor system performance, and a reduction in overall system stability. Therefore, fault-tolerant converters are required to raise system availability in industrial applications. This research investigates and proposes the configuration of a fault-tolerant NNPC converter that uses a redundant phase to create a fault-tolerant converter. The redundant phase is in the bypass mode under normal operating conditions. After detecting the fault, the redundant phase will replace the faulty phase by utilizing a simple control scheme. Also, the proposed fault tolerance method is based on the cold reservation method, which will not increase the switching losses in the converter. The proposed fault-tolerant method’s feasibility and effectiveness are confirmed by simulating the NNPC converter under various scenarios in the MATLAB/Simulink environment.
... The demand for NNPC converters has gradually enhanced and become a critical part of Medium-Voltage and High-Power (MVHP) applications, particularly with the growing acceptance of renewable energy systems [9], [10]. The highlighted features of its structure make this type of converter particularly well-suited for applications like Medium-Voltage (MV) motor drives and distributed generation systems. ...
The Nested Neutral Point Clamped (NNPC) converter, functioning as a voltage source Converter (VSC), provides an effective solution for applications requiring Medium-Voltage and High-Power (MVHP). Earlier implementations of this converter typically required many sensors to maintain capacitor voltages equally in all series Half-Bridge Sub-Modules (HB-SMs). This research introduces two methodologies to minimize sensor requirements for computing capacitor voltages using novel algorithms based on estimation methods. These strategies simplify the converter control procedure by eliminating individual current and voltage sensors specified for HB-SMs. The proposed approaches ensure precise voltage balance with minimal estimation error by continuously adjusting capacitor voltages using estimated values from previous iterations and switching signals. Extensive MATLAB Simulink tests verify the effectiveness of these techniques across diverse practical scenarios. The results highlight significant simplification of sensor complexity while maintaining strong performance in NNPC Converter applications, emphasizing the importance of sensor implementation in the cost and operational effectiveness of VSCs.
... Recently, the modular multilevel converter (MMC) has been used frequently for its various advantages [1], [2], [3]. These include fully modular design, easy construction, excellent output waveform quality, and fault tolerance in high and medium power applications [4], [5]. ...
The modular multilevel converter (MMC) is a favored topology in the industry, but its reliability is at risk with an increase in the number of sub-modules (SMs) due to a rise in switching components. The essential need for maintaining capacitor voltage balance in each arm leads to increased complexity and cost, as numerous voltage sensors are required. This study introduces an innovative approach to minimize the number of voltage sensors by employing an enhanced algorithm for open-circuit fault detection in switches. The proposed scheme organizes each arm into groups, each containing two SMs and one voltage sensor, aiming to reduce the overall sensor count. A novel fault detection mechanism is presented, identifying open-circuit faults by comparing group output voltages in healthy and defective conditions. The capacitor voltage estimation algorithm in the sensor reduction scheme is noted for its simplicity compared to other methods. The effectiveness of these methods is validated through simulations and experimental implementations across diverse scenarios, affirming their reliability.
... In recent years, voltage source converters (VSCs) have gained wide acceptance in high-voltage direct current (HVDC) applications as a potential substitute [1], [2]. The HVDC systems based on VSCs have advantages over those based on thyristors, including the ability to supply low voltage distributed networks, independent control of active and reactive power, and easier controllability. ...
The alternate arm multilevel converter (AAMC) is a new type of voltage source converter topology introduced as an attractive option for medium voltage and high-power applications. In AAMC, many series full-bridge sub-modules (FB-SM) are used to achieve high-voltage applications. Despite this, a large number of voltage and current sensors are needed in this converter to balance the capacitor voltage of the SMs. This paper proposes a new method to estimate the SM capacitor voltage that needs only one voltage sensor in each arm and a current sensor in the converter’s output to balance capacitor voltages. By using a proposed method based on a Kirchhoff voltage law and circuit relations govern on AAMC, the capacitor voltages can be determined. Contrary to voltage balancing control schemes based on individual sensors, the number of voltage sensors in the proposed method has been substantially reduced, and arm current sensors are eliminated. In addition, there is low estimation error since the capacitor voltages are calculated with high accuracy and updated depending on the arm voltage sensor and switching signals. Analysis of the proposed method in MATLAB Simulink for different scenarios indicates its validity.
In multilevel converters (MCs), system components such as capacitors and semiconductor switches are very susceptible to damage. Furthermore, utilizing voltage sensors to balance capacitor voltages increases the system’s cost and complexity while decreasing reliability. This paper presents methods for voltage balancing of capacitors, capacitance monitoring, and open-circuit fault detection in a nested neutral point-clamped (NNPC) converter with a reduced number of voltage and current sensors. In the proposed method, converter capacitors’ voltage and current sensors are eliminated, and only the output sensors are employed to control the converter. In order to balance the voltage of capacitors, their voltages are estimated in three cases without using individual sensors. Comparing the capacitors’ measured and estimated voltage change ratios is used to assess their health in the monitoring method. Thus, the capacitors’ status can be checked by considering the system’s health indicator during each period. Furthermore, the open-circuit fault detection method can identify defective switches within one processing cycle by comparing the converter’s measured and standard output voltages in all switching states. The proposed methods have been evaluated in an experimental study of the system, and the results confirmed the effectiveness of the methods based on the new system configuration.
In this paper, a new hybrid configuration of nested neutral point clamped (NNPC) inverter is suggested which generates a 9-level phase-to-neutral ac side voltage. The circuit configuration and operation principle of the introduced converter are discussed first. In the suggested topology, the regulating of the voltages of capacitors is achieved by exploiting the redundancy in switching states. On the other hand, a higher number of generated levels allows us to use the low-frequency modulation with accepted near sinusoidal output waveform. In this study, a capacitor voltage balancing strategy based on the nearest level control (NLC) algorithm is introduced. In the introduced strategy, the switching power loss is significantly reduced in comparison to the conventional high-frequency PWM techniques while the voltages of the capacitors are kept balanced with an acceptable voltage ripple. The simulation results are provided to validate the feasibility of the proposed MLI and the performance of the presented balancing method.
In the present scenario, modular multilevel converters (MMCs) are considered to be one of the most promising and effective topologies in the family of high-power converters because of their modular design and good scalability; MMCs are extensively used in high-voltage and high-power applications. Based on their unique advantages, MMCs have attracted increasing attention from academic circles over the past years. Several studies have focused on different aspects of MMCs, including submodule topologies, modeling schemes, modulation strategies, control schemes for voltage balancing and circulating currents, fault diagnoses, and fault-tolerant control strategies. To summarize the current research status of MMCs, all the aforementioned research issues with representative research approaches, results and characteristics are systematically overviewed. In the final section, the current research status of MMCs and their future trends are emphasized.
Recently developed modular-multilevel converter (M-MC) has a major attention in the industry and research works which is moving into feasible technology for many medium and high-power applications. M-MCs have been improved by existing power conversion technology in several aspects, including efficiency, modular structure, power quality, transformerless operational capability, fault tolerant and redundancy operations having a low expense, standard components utilisation, high availability, excellent quality of output wave forms. However, as several challenges combined with M-MC topologies, include reduced voltage stress, compact size, and substantially lower semiconductor losses. These attributes the lower operating cost for high-power applications such as distribution systems and high-voltage direct current (HVDC) transmission systems. The main contribution of this study is to bring the scrutiny of M-MC applications and different circuit topologies. These M-MC configurations consist of converter structures with sub-module converters and M-MC family members. In such a way, the M-MC applications are generalised and it results the size and cost halved, reduced conduction and switching losses and increases system reliabilities. Moreover, in each application of M-MC, drawbacks, and advantages have been discussed in detail. This review presents the latest performance of M-MC with respect to the mentioned research areas and new applications.
This paper proposes a fault-tolerant strategy to recover the full performance of modular multilevel converter (MMC) under sub-module (SM) fault condition for regenerative medium-voltage drive application. The proposed method has the feature to retrieve line voltage amplitude during post-fault condition back to its nominal value. In this method, missing capacity of converter due to fault condition is compensated by all remaining healthy SMs. The main advantage of proposed method in comparison with previous works is that it imposes a lower voltage stress on switches in post-fault condition without severe circulating current. Remaining healthy SMs, also, operate at a similar condition which leads to the feature of homogeneous distribution of loss and temperature in all parts of the converter. The proposed method is applicable for regenerative MMC-based drives as well as back-to-back configuration of MMC without adding cost or complexity to whole system. A generalized approach is presented and its result compared with other methods. Simulation and experimental results are provided to validate performance and feasibility of the proposed method which can effectively enhance the fault-tolerant capability of this converter.
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 amount 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 MMCs, 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 SMs localization strategy is based on the difference 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 failure and identify 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.
Modular multilevel converter (MMC) has been one of the most popular candidates in high voltage applications. However, the reliability is a critical issue due to a large number of power switching devices and capacitors applied in the MMC. To improve the reliability of the MMC, a fast and reliable diagnosis strategy for the open-circuit fault is proposed in this paper, where submodule voltage sensors are relocated to the upper switching device. Furthermore, a voltage observer based on the submodule voltage sensor is established to monitor the capacitor and realize the power control of the MMC under normal operation. A Boolean logic operation based fault indicator is put forward based on the relationship of the operation state and binaried output of voltage sensors, which could detect and locate the open-circuit faults of the MMC very fast. The ratio of the increment of the observed and measured capacitor voltage during the period of positive arm current is applied to monitor the capacitor. The effectiveness of the proposed fault diagnosis strategy and the capacitor monitoring strategy is verified by the experiment results.
Finite control-set model predictive control (FCS-MPC) is an alternative strategy, in particular, for multi-level converters. However, the computational burden is rapidly increased with the number of voltage vectors to be predicted and objectives to be controlled. This will hinder the development of the FCS-MPC strategy. In this paper, an improved FCS-MPC is proposed for nested neutral point-clamped converters (NNPCs) under both balanced and unbalanced grid conditions. Specifically, to reduce the computational burden, an optimized voltage vector selection process based on deadbeat technique is embedded into the proposed design. Meanwhile, a simplified cascade-free control strategy is presented to directly govern the DC-link voltage and reactive power in NNPCs while the flying capacitor voltages are maintained balanced. Moreover, the inclusion of a power compensation approach is presented for the FCS-MPC method in an attempt to obtain desired grid-side currents under nonideal grid-side voltage conditions. The novelty of this work lies not only in a compatible reference design that directly allows to formulate the optimal control problem using a simplified cascade-free control strategy, but also in the utilization of combining the proposed FCS-MPC and the power compensation technique for suppressing the distorted grid-side currents under unbalanced grid conditions. The performance of the proposed strategy for NNPCs is evaluated by simulation studies in various operating conditions.
The high number of power semiconductors in multilevel converters makes them susceptible to failure. Therefore, one of the main concerns in utilizing multilevel inverters is their reliability. This paper proposes a new simple fault diagnosis and fault handling method to increase the robustness and reliability of a flying capacitor multilevel inverter (FCMLI) which is one of the most prominent multilevel inverters. The proposed method is capable of diagnosing failed switch(es) and reconfiguring the switching sequence such that the output voltage is maintained similar to a normal operation condition. The proposed scheme identifies failed switch(es) by using the information about the charging state of the capacitors and the applied switching sequence. After identifying the failed switch(es), the algorithm bypasses the failed switch(es) and converts the control signals of the faulty leg from an M-cell Llevel configuration to an (M-F)-cell L-level configuration (where F is the number of failed switches). The most attractive feature of the proposed control scheme is that any number of failed switches can be tolerated, as long as the number of functional switches is higher than the minimum number of cells required to build a full-binary L-level FCMLI. Simulation and experimental results are presented that verify the effectiveness of the proposed method.
A new space vector modulation method for common-mode voltage reduction in four-level nested neutral point clamped (NNPC) inverter is proposed in this paper. The proposed method employs switching states with minimal absolute common-mode voltages to synthesize given reference vectors. A variety of virtual vectors are constructed to facilitate the synthesis. The effect of dead time on common-mode voltage reduction is analyzed. Appropriate switching sequences are designed to avoid the effect. With the proposed method, the common-mode voltage of the inverter is reduced to 1/18 of dc-link voltage, 80% off compared with sinusoidal pulse width modulation (SPWM) and conventional space vector modulation (C-SVM), which contributes to the design of transformer-less medium voltage (MV) power conversion and drives. The proposed method is verified by simulation results.
A capacitor voltage balancing method for a nested neutral point clamped (NNPC) inverter is proposed in this paper. The NNPC inverter is a newly developed four-level voltage source inverter (VSI) for medium voltage (MV) applications with properties such as operating over a wide range of voltages (2.4-7.2KV) without the need for connecting power semiconductor in series and high quality output voltage. The NNPC topology has two flying capacitors in each leg. In order to ensure the inverter can operate normally and all switching devices share identical voltage stress, the voltage across each capacitor should be controlled and maintained at one third of dc bus voltage. The proposed capacitor voltage balancing method takes advantage of redundancy in phase switching states to control and balance flying capacitor voltages. Simple and effective logic tables are developed for the balancing control. The proposed method is easy to implement and needs very few computations. Moreover, the method is suitable for and easy to integrate with different pulse width modulation (PWM) schemes. The effectiveness and feasibility of the proposed method is verified by simulation and experiment.
In wind turbine generation (WTG) systems, a back-to-back converter with a neutral-point-clamped (NPC) topology is widely used because this topology has more advantages than a conventional two-level topology, particularly when operating at high power. There are 12 switches in the NPC topology. An open-switch fault in the NPC rectifier of the back-to-back converter leads to the distortion of the input current and torque vibration in the system. Additionally, an open-switch fault in the NPC inverter of the back-to-back converter causes the distortion of the output current. Furthermore, the WTG system can break down in the worst case scenario. To improve the reliability of WTG systems, an open-switch fault detection method for back-to-back converters using the NPC topology is required. This study analyzes effects of inner and outer open-switch faults of the NPC rectifier and inverter and describes a novel open-switch fault detection method for all possible open-switch faults in the back-to-back converter.
The nested neutral-point clamped (NNPC) converter is a four-level converter topology for medium-voltage applications with interesting properties such as operating over a wide range of voltages (2.4–7.2 KV) without the need for connecting the power semiconductor in series, high quality output voltage, and less number of components in compare to other classical four-level topologies. The control and balance of the flying capacitors (FCs) of the NNPC converter can be done by different control techniques taking advantage of the large number of redundant switching states. This paper presents a simple single-phase modulator for the NNPC converter, which can be applied to each phase of a three-phase NNPC converter. The proposed simple technique can control and balance the FCs to their desired values. Performance of the proposed technique under different operating conditions is investigated in the MATLAB/Simulink environment. The feasibility of the proposed converter is evaluated experimentally.
In this paper, a new voltage source converter for medium voltage applications is presented which can operate over a wide range of voltages (2.4–7.2 kV) without the need for connecting the power semiconductor in series. The operation of the proposed converter is studied and analyzed. In order to control the proposed converter, a space-vector modulation (SVM) strategy with redundant switching states has been proposed. SVM usually has redundant switching state anyways. What is the main point we are trying to get to? These redundant switching states help to control the output voltage and balance voltages of the flying capacitors in the proposed converter. The performance of the converter under different operating conditions is investigated in MATLAB/Simulink environment. The feasibility of the proposed converter is evaluated experimentally on a 5-kVA prototype.
This paper presents a fault detection and isolation (FDI) method for a Modular Multilevel Converter (MMC). This method can locate an open-circuit faulty device accurately and quickly. Based on a sliding mode observer (SMO) and a switching model of a half bridge, the approach is to assume the location of the fault, modify the observer equation and then compare the measured and observed states to verify, or otherwise, the assumption for possible fault locations. This technique requires no additional measurement elements and can easily be implemented in a microcontroller. The operation and robustness of the proposed method are confirmed using simulation results.
This review focuses on faults in Modular Multilevel Converter (MMC) for use in high voltage direct current (HVDC) systems by analyzing the vulnerable spots and failure mechanism from device to system and illustrating the control & protection methods under failure condition. At the beginning, several typical topologies of MMC-HVDC systems are presented. Then fault types such as capacitor voltage unbalance, unbalance between upper and lower arm voltage are analyzed and the corresponding fault detection and diagnosis approaches are explained. In addition, more attention is dedicated to control strategies, when running in MMC faults or grid faults. This paper ends up with a discussion of other opportunities for future development.