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Voltage-controlled three-phase three-leg inverters are widely applied in various occasions. The inverters are generally switched to the current-controlled mode to limit the fault currents when short-circuit faults happen. Under symmetrical fault, the inverter can be controlled as a symmetrical, three-phase current source. However, the voltage limiting will happen under asymmetrical fault. As a result, the fault phase currents will be distorted and cannot be completely controlled. Considering that the voltage limiting is mainly caused by large load impedance of healthy phase under asymmetrical fault, the voltage limiting is avoided by paralleling the virtual impedance with the output filter capacitor in this paper. As a result, the current limiting loop is not broken and the fault phase currents can be effectively controlled. The implementation of virtual impedance in control system and the influence of virtual impedance on the stability of system are also analyzed in detail in this paper. The theoretical results are validated by experiments. [ABSTRACT FROM AUTHOR]

A new single stage three-phase ac–dc converter with four switches and galvanic isolation is proposed in this paper. The new converter is simple and uses fewer switches than previously proposed ac–dc converters of the same type. It is a bridgeless converter that can operate with continuous input current and with any pulsewidth modulated method suitable for a standard three-phase six-switch voltage source rectifier. In this paper, the operation, control, analysis, and design of the proposed converter are explained and its features are discussed. Experimental results obtained from a prototype that confirm the feasibility of the converter are presented as well. [ABSTRACT FROM AUTHOR]

This paper introduces the fault current behavior of full-bridge modular multilevel converters (MMCs) and hybrid MMCs, with a consideration of a non-simultaneous blocking of the two half-bridges in each sub-module. In order to protect insulated gate bipolar transistor (IGBTs) in the full-bridge sub-module, both sides of a full-bridge sub-module are not blocked at the same time point. It is assumed that the first blocking of both sides is random in each sub-module, so there are many possible voltages in each arm after the first blocking. Because of this, there are also many possible fault current slopes after the first blocking. The influence of this sequential blocking on the maximum fault current is analyzed in the study. This paper also determines the threshold current, which has to be decreased due to this phenomenon. An optimization method of threshold current is also proposed. Other new MMC sub-modules can also use this new blocking method to reduce the stress of IGBTs, whose working principle is also introduced in this paper. [ABSTRACT FROM AUTHOR]

In this paper, an overload and short-circuit protection method is proposed for voltage source inverter-based uninterruptible power supply (UPS) system. In order to achieve high reliability and availability of the UPS, short circuit and overload protection scheme are necessary. When overload or short circuit happens, using the proposed control method, the amplitude of the output current can be limited to a constant value, which can be set by the customer to avoid the destruction of the power converter, and to obtain a faster recovery performance as well. The detailed principle of the proposed protection method is discussed in this paper. It mainly contains three parts in the control diagram for current limit, first is the anti-windup in the voltage and current controllers, then the feedforward of the capacitor voltage to the current control loop, the last is the fast reset of the resonant part of the current controller when overcurrent happens. The procedure of developing the control method is also presented in the paper. Experimental results on a commercial UPS system are presented to verify the effectiveness of the control method. [ABSTRACT FROM AUTHOR]

Symmetrical ac-side current control is widely used for grid-connected modular multilevel converters (MMCs) under grid fault conditions. However, it is revealed that this control method can lead to unbalanced arm currents and consequently asymmetrical electrical stresses and nonuniform temperature distributions. This paper aims to analyze the mechanism of the unbalanced arm currents and propose an effective method to solve this problem. First, the electrical quantities inside MMCs are analyzed under unbalanced grid fault conditions. The analytical expressions of the unbalanced arm currents are derived considering different fault severities and operating conditions to reveal the “side-effects” of the symmetrical ac-side current control. After that, a zero-sequence ac voltage injection method is proposed. The injected voltage is directly derived from negative-sequence ac current controller and ac-side current with pure mathematical manipulations, which can be easily obtained without extra controllers. The effect of the injected voltage on the original modulation index is very limited according to an accurate quantitative analyzation. Moreover, the proposed method does not affect the already achieved symmetrical ac-side current control and the circulating current suppression. The effectiveness and performance of the proposed analysis and control method are verified by both simulation and experimental results. [ABSTRACT FROM AUTHOR]

The modular multilevel converter (MMC) with nearest level modulation (NLM) is widely used in the high voltage applications for low switching frequency and easy implementation. Existing literature has not provided a complete submodule (SM) fault ride-through scheme for MMC with NLM. In this paper, a strategy including fault detection, localization, redundancy, and recovery is proposed to ensure continuous operation of MMC under IGBT open-circuit faults conditions. It only requires a few hardware and software resources. The features of MMC and SMs with three types of failures are studied, respectively. Based on these, the fault detection method is proposed by using a simple hardware circuit, thus high computation complexity is avoided. Since current fault localization schemes are limited to MMC with carrier phase shifted pulse width modulation, this paper further proposes a strategy for MMC with NLM to locate the faulty SM and identify the fault type. After this, the fault redundancy and the proposed fault recovery method are applied to eliminate the fault and then exit the failure state. Therefore, the ability of SM fault ride-through can be realized. Analysis of failure characteristics are verified in simulation. Experimental results based on a single-phase MMC prototype with 11 SMs per arm are presented to demonstrate the validity of the proposed fault ride-through strategy. [ABSTRACT FROM AUTHOR]

In addition to field-oriented control and direct torque control, the model predictive control (MPC) is another typical control method for permanent-magnet synchronous machine (PMSM) drives. In some critical applications, fault-tolerant controls are usually required. The faults of PMSM drives in this paper are limited as a single-phase open-circuit fault, which may be caused by open circuit or short circuit of one power switch. In the postfault operation, the neutral point of the PMSM is connected to a redundant leg. As is known, MPC is a model-based control method. Since the PMSM has different mathematical models in both normal and postfault operations, two different MPCs (MPC-I and MPC-II) should be, respectively, designed for both normal and postfault operations according to traditional thinking. However, it is first found in this paper that the faulty PMSM drive can be treated as an equivalent healthy PMSM drive, and MPC-I can be shared by both normal and postfault operations without any modification. The major contribution of this paper is to find this fact, and verify it with theoretical analysis and experimental validation. Obviously, this finding will simplify the whole control method of PMSM drives considering single-phase open-circuit fault. [ABSTRACT FROM AUTHOR]

This paper proposes an open-delta operation as the fault-tolerant control strategy for the delta-connected cascaded converter. Based on the detailed power analysis, the fault-tolerant control enables continued operation and maintains dc capacitor voltages balancing during the occurrence of bridge cells fail. This enhances the system's reliability and availability. The simulation and laboratory test-bench, combining a seven-level delta-connected cascaded converter apply in a STATCOM operation and a PV system is tested to verify the proposed control technique. [ABSTRACT FROM AUTHOR]

In this paper, a current-fed modular multilevel dual-active-bridge (CF-MDAB) dc–dc converter is proposed for medium-voltage dc (MVDC) application. The proposed converter inherits favorite characteristics of DAB circuits including soft switching and small passive components. Thereby, high efficiency and high power density can be achieved. Moreover, with direct input and output dc current control, the CF-MDAB is suitable for a breakerless MVDC system since it can realize dc fault ride-though operation. In addition, the dv/dt in the converter is mitigated with the quasi-three-level modulation. In this paper, the proposed converter is applied to integrate the battery energy storage to a MVDC grid as an example to illustrate its operation principles and fault current control capability. The operation principles are presented for both normal and dc fault conditions; the dynamic models are also derived not only under normal operation mode but under dc fault operation mode as well. The control systems under different operation modes are designed, respectively, based on the developed mathematical models. A downscaled 40-kHz 3-kW CF-MDAB prototype was built in the laboratory. The experimental results under both normal condition and dc fault condition verified the analysis as well as the control performance of the proposed converter. [ABSTRACT FROM PUBLISHER]

Supporting the grid and improving its reliability have recently become major requirements for large distributed generation units. Under most grid faults, the accuracy of the traditional voltage support schemes (VSSs) is dramatically affected due to the existence of the zero-sequence voltage. Also, the traditional VSSs have been used only in the STATCOM applications, where the active power is zero. This paper proposes an advanced VSS in the converter-interfaced units, called zero-sequence compensated voltage support (ZCVS), to accurately regulate the three-phase voltages of the connection point within the pre-set safety limits. The proposed scheme not only compensates the zero-sequence component but also considers the active power injection. Unlike the traditional methods, the proposed VSS is adapted even in resistive distribution systems. The contribution of this paper is, however, ternate. As the second contribution, the limited active power oscillation (LAPO) is proposed to be augmented to the ZCVS. This feature limits the oscillation to a specified value which provides an adjustable dc-link voltage oscillation setting while simultaneously supporting the ac host grid, even under severe unbalanced faults. Third, the maximum active power delivery (MAPD) to the ac grid is also formulated for the ZCVS. The successful results of the proposed support scheme and complementary strategies are verified using selected simulation and experimental test cases. [ABSTRACT FROM AUTHOR]

This paper proposes short circuit and overload gate-driver dual-protection scheme based on the parasitic inductance between the Kelvin- and power-source terminals of high-current SiC mosfet modules. The paper presents a comprehensive analysis of the two schemes in question, including worst-case analysis used to assess their parametric dependence due to manufacturing tolerances and temperature variations, as well as the in-depth design procedure that can be generally applied to any power module containing a Kelvin-source. For verification, a compact 1.2-kV, 400-A half-bridge module integrating the two protection circuits was developed. The results obtained demonstrate a response time within tens of nanoseconds, and effectively validate their functionality under short circuit and overload scenarios. Finally, a 100-kW, 400-V dc three-phase voltage-source inverter was used to demonstrate the gate-driver with integrated protection functions under 105°C ambient temperature conditions. [ABSTRACT FROM AUTHOR]

The five-level nested neutral-point-pilot (NPP) topology, as a new structure for converters, bears the advantages of a high power density, robustness, and flexibility and is therefore suitable for high-voltage and high-power applications. For a multilevel converter, as the number of power electronic switches increases, the risk of switch failure increases, together with the complexity of fault detection and tolerance control. The requirements for a higher operational stability and reliability continue to grow. However, studies on fault tolerance for multilevel converters are limited. In this paper, a fault diagnosis and tolerance solution for a five-level nested NPP converter is proposed. For the fault diagnosis, a deep learning method integrating the wavelet packet transform and long short-term memory is presented. Both open- and short-circuit switch failures can be precisely detected and located without the requirement of a large sample set. Two software-based control strategies for fault tolerance are adopted, and low-cost hardware reconfigurations are also implemented to prevent failure expansion and ensure continuous operation. Furthermore, the voltages of dc-link capacitors and flying capacitors are effectively balanced with the improved algorithm even when a failure occurs. Finally, the effectiveness of the fault-tolerant strategy are proven by simulations and experiments. [ABSTRACT FROM AUTHOR]

In this paper, diagnosis and tolerant control schemes have been studied for common electrical faults in T-type three-level inverter fed dual-three phase permanent magnet synchronous motor (PMSM) drives for safety-critical applications. Based on vector space decomposition and double three-phase space vector modulation, diagnostic and tolerant control methods have been developed comprehensively for open-phase faults, open-switch faults, and short-switch faults in T-type three-level inverter fed dual-three-phase PMSM drives, in such way that misdiagnosis of different faults sharing similar faulty features can be avoided. In particular, a two-step diagnostic scheme is proposed in order to simplify the diagnostic process. The first step is to identify the faulty phase and fault category by analyzing current trajectories on harmonic subspace, and the second step is to determine the specific fault type in a small fault-searching area obtained from the first step. In the aspect of fault-tolerant control, a novel current compensation method is proposed for open-phase fault without changing the machine model, space vector diagram, and control framework. Open-switch faults and short-switch faults are tolerated by making full use of the remaining vectors after faults. The validity of the proposed fault diagnosis and tolerance schemes has been verified by experiments. [ABSTRACT FROM AUTHOR]

In this paper, a non-invasive method based on electromagnetic radiation (EMR) signature is proposed for the health assessment of the converters. Converters use insulated gate bipolar transistor (IGBT) because of its robustness. However, it suffers from internal degradation due to the rapid power cycle and thermal stress. Degradation of IGBT primarily increases the turn-off time as a consequence of elevated junction temperature. The coupling of IGBT with the parasitics of the circuit results in EMR and depends on the turn-off time of IGBT. It has been found that the EMR signature reduces with the increase in turn-off time. This concept is taken forward for the health assessment of the converters. However, in a practical scenario, the EMR generated from multiple converters get mixed up. This is commonly known as a problem of near-field source localization. We use a uniform linear array to capture the EMR signals near the converters. Subsequently, ESPRIT and MUSIC algorithms are used to localize all the converters. An inverse transformation of the localization algorithm separates the EMR signature of all the individual converters. The proposed health assessment algorithm computes the degradation level of converters, and the experimental results validate the proposed approach. [ABSTRACT FROM AUTHOR]

DC circuit breakers (DCCBs) are vital for the safe and continuous operation of large high-voltage dc grids. Insulated-gate bipolar transistor-based hybrid DCCBs (IHCBs) can interrupt currents quickly and have good operational characteristics. Thyristor-based hybrid DCCBs (THCBs) are superior to IHCBs in terms of interruption capacity because of the large surge current capacity of thyristors. Nevertheless, existing THCBs have limitations regarding one or more of the following characteristics: pre-activation, fast reclosing, quick interruption of small current and no requirement for an additional pre-charge source. This paper proposes a thyristor full-bridge-based dc circuit breaker (TFCB) with all favorable characteristics mentioned above. The TFCB contains a main conductor, a main breaker (MB), an auxiliary branch, and an energy absorber. After an interruption, the capacitor in the TFCB can restore its initial voltage quickly to guarantee that the TFCB is available for the next interruption. Quick interruption of small currents is achieved using the additional charge current provided by the auxiliary branch. Pre-activation is achieved using the conduction path in the MB. The parameter designs are analyzed to ensure that the TFCB reliably interrupts currents without damaging itself. The proposed TFCB is validated using prototype experiments and PSCAD/EMTDC simulations. [ABSTRACT FROM AUTHOR]

An isolated bidirectional dc-dc converter, which combines multiple two-level converters (TLCs) in parallel on the medium-voltage side and a modular multilevel converter (MMC) on the high-voltage side, namely the TLC-MMC converter, is a promising candidate for the interconnection of MVdc and HVdc grids. In utility applications, the availability of power converters is of great importance, which makes the fault-tolerant operation (FTO) capability a required feature. In this paper, an FTO scheme of the TLC-MMC converter in case of a TLC failure is addressed. The FTO capability is enabled by employing only mechanical disconnectors. The faulty TLC can be bypassed and isolated for maintenance, and the TLC-MMC converter can continuously operate with a reduced power capacity. A dedicated mode-transition strategy is developed, whereby the TLC-MMC converter can transit to the fault-tolerant mode seamlessly without resulting in overvoltage or overcurrent despite the involvement of low-speed mechanical disconnectors. Thus, the proposed approach can effectively enhance the availability of the TLC-MMC converter. The validity of the proposed approach is verified by both simulation and experimental test with a down-scale prototype. [ABSTRACT FROM AUTHOR]

Numerous studies show that power semiconductors are significant contributors to the overall failure rate of pulsewidth modulation (PWM) voltage source rectifier (VSR) systems. Aimed at this problem, this paper presents an improved diagnosis system for open-circuit faults of a three-phase PWM VSR based on ac current distortion characteristics, which consists of a diagnosis module, a register module, and a decision module. The diagnostic module employs a hysteresis comparator to judge the change trend of the currents, and thereby diagnoses the single open-circuit (OC) faults. The register module is employed to achieve multi-switch OC fault diagnosis, and the decision module is used to output the final result. The proposed method extends the previous work to multi-switch OC fault conditions as well as improves the diagnosis accuracy. The experimental results show that the proposed method can correctly achieve fault diagnosis, including 6 kinds of single-switch faults and 15 kinds of different double-switch faults. Additionally, the proposed method also shows superior anti-interference performance and high robustness under abrupt load transient conditions, different power factor conditions, unbalanced and/or distorted grid voltage conditions, and different ac filter structure conditions. Furthermore, the average diagnostic time of this method is only 3.68 ms. [ABSTRACT FROM AUTHOR]

Fault tolerance in wind turbines is considered crucial to increase their reliability and availability levels. This paper presents a fault-tolerant direct-drive permanent magnet synchronous generator (PMSG) using new direct control techniques, with the ability to handle power semiconductor open-circuit faults in the full-scale back-to-back converter. The fault-tolerant topology consists of a five-leg converter, with a shared leg connected to a generator phase and to its corresponding grid phase, through a triode for alternating current (TRIAC). The main contribution of this paper consists of the development of an alternative direct torque control and direct power control schemes for both machine-side converter and grid-side converter, respectively. Moreover, a reliable fault diagnostics algorithm without requiring additional sensors is also integrated, providing the information required to instantaneously trigger fault-tolerant remedial strategies. Simulation and experimental results are presented to validate the effectiveness of the proposed fault-tolerant PMSG drive. [ABSTRACT FROM AUTHOR]

In electric vehicles, a battery management system highly relies on the measured current, voltage, and temperature to accurately estimate state of charge (SOC) and state of health. Thus, the normal operation of current, voltage, and temperature sensors is of great importance to protect batteries from running outside their safe operating area. In this paper, a simple and effective model-based sensor fault diagnosis scheme is developed to detect and isolate the fault of a current or voltage sensor for a series-connected lithium-ion battery pack. The difference between the true SOC and estimated SOC of each cell in the pack is defined as a residual to determine the occurrence of the fault. The true SOC is calculated by the coulomb counting method and the estimated SOC is obtained by the recursive least squares and unscented Kalman filter joint estimation method. In addition, the difference between the capacity used in SOC estimation and the estimated capacity based on the ratio of the accumulated charge to the SOC difference at two nonadjacent sampling times can also be defined as a residual for fault diagnosis. The temperature sensor which is assumed to be fault-free is used to distinguish the fault of a current or voltage sensor from the fault of a battery cell. Then, the faulty current or voltage sensor can be isolated by comparing the residual and the predefined threshold of each cell in the pack. The experimental and simulation results validate the effectiveness of the proposed sensor fault diagnosis scheme. [ABSTRACT FROM AUTHOR]

This paper presents a new ultrafast dc solid-state circuit breaker (SSCB) that uses a silicon carbide cascode as the main switching and limiting semiconductor and an isolated photovoltaic driver to control it. The proposed topology is self-powered and fully implemented with discrete parts. The SSCB's cascode can work in three different states—fully on during nominal operation, linear mode for current limitation, and fully off to disconnect the load. The time the SSCB operates in linear mode and the maximum current limit is easily set by discrete components. Control inputs have also been included to reset the SSCB after a fault has been removed or to remotely switch it on or off. This device can be used in dc distribution avoiding deterioration due to the problems associated with electric arcs and mechanical aging of moving parts, limiting inrush currents and also minimizing conduction losses respect other kind of circuit breakers. Functional, thermal, and efficiency tests have been carried out with three different 380 V prototypes. Experimental results show the excellent behavior of the SSCB, it is able to block a 380 V short circuit failure in 570 ns; the authors have not found any faster results in the literature. [ABSTRACT FROM AUTHOR]

This paper presents a transistor open-circuit fault diagnosis method based on an average model and post-fault reconfiguration algorithms for a grid-tied single-phase inverter that transfers power bi-directionally. The transistor open-circuit fault occurrence is detected by comparing the average bridge arm pole-to-pole voltage deviation and threshold first. Then, the exact faulty transistor is identified by developed algorithms based on post-fault reconfiguration of control (PFRC) and of utilizing redundant leg (PFRUR). These two fault identification algorithms are suitable for different situations. Compared with the PFRUR, the PFRC can identify the faulty transistor without the help of redundant leg, but the identification time is longer. For inverters equipped with redundant leg already, the PFRUR can achieve faster identification speed so that inverters in reliability-oriented applications can return to normal operation after fault with short interruption. With average model, this method only requires existing signals sampled for control, thus it can be embedded in system easily without adding extra sensors and diagnosis circuits. The sampling frequency can be as low as switching frequency. Furthermore, this method is suitable for both inverters with L or LCL filters. Finally, experiments are carried out on a 550-W/10-kHz grid-tied single-phase inverter to verify the effectiveness. [ABSTRACT FROM AUTHOR]

The solid-state transformer (SST) has the ability to operate with only two input phases, which can be called single-phase open-circuit (SPO) operation. With this ability, the SST can maintain operation under permanent single-phase-to-ground faults, where the fault phase must be isolated, providing a better power supply sustainability than that of the conventional low-frequency transformer. In the absence of research on the SPO operation of an SST, this paper presents a study of it. The influence of SPO operation on the grid and the SST itself is analyzed. Under the SPO state, the SST will inject the negative-sequence current to the grid; meanwhile, the maximum load power will decline; thus, the optimized power factor under the SPO state is unity. The maximum power of △-SST is higher than that of the Y-SST under the SPO state owing to the △ connection. A control strategy for the SST medium-voltage cascaded H-bridge stage is proposed. SPO current control can be achieved by controlling the positive-sequence current. A proper negative-sequence voltage can be injected to synchronize the cluster voltages to obtain a lower current total harmonic distortion and smaller dc voltage ripple. In the △-SST, a proper zero-sequence current can be injected to fully use the power rating of the double-cluster branch. [ABSTRACT FROM AUTHOR]

This paper discusses the technical challenges and tradeoffs tied to design of medium-frequency transformers for medium-voltage high-power applications and emerging solid-state transformers. A dedicated design optimization algorithm is used to generate the set of all feasible transformer designs for the given electrical requirements, originating from a modular dc–dc solid-state transformer, and taking into account different geometry ratios, materials, and operating frequencies. All possible design choice variations are compared in terms of maximum achievable efficiency, gravimetric and volumetric power densities, thus identifying design limits for a given set of input parameters. The impact of modularity and choice of operating frequency directly related to selected semiconductors, reveal interesting trends that can aid overall solid-state transformer design optimization. [ABSTRACT FROM AUTHOR]

A novel fault diagnosis method and a fault tolerant scheme for open-circuit faults on a traction rectifier are proposed in this paper. When an open-circuit fault occurs in any leg of the rectifier, indication will be generated accurately to identify the faulty leg without utilization of any extra sensors. Furthermore, the faulty rectifier can be reconfigured to maintain its full output rate to prevent traction rectifier breakdown. The fault diagnosis process is neither related to control trigger signal nor the load fluctuation. The involved parameters are the input and output voltage of the rectifier, which are the most common parameters in rectifier control. Working under zero voltage and zero current, five additional switch sets are adopted for the fault-tolerant control process to reconfigure the topological structure between the traction rectifier and the traction transformer, meanwhile the original structure of the rectifier is retained. Analysis, design, and implementing consideration for both normal and abnormal operating situations of the traction rectifier are present in this paper. The experiment is processed to verify the effectiveness of the theoretical analysis. [ABSTRACT FROM PUBLISHER]

This paper extends the high-frequency transformer isolation concept to the Z-source (ZS) ac–ac converters and introduces a new family of high-frequency transformer isolated (HFTI) ZS ac–ac converters. The proposed HFTI-ZS converters retain all the benefits of their existing nonisolated counterparts, such as providing a larger range of output voltage with buck–boost functionality, reversing or maintaining the phase angle, reducing the in-rush and harmonic currents, and improving reliability. In addition to these benefits, the high-frequency (20 kHz) transformer (HFT) in the proposed ZS ac–ac converters provides the electrical isolation and safety with high power density as it eliminates the need for bulky and heavy line frequency (50 or 60 Hz) transformer for galvanic isolation, in applications such as dynamic voltage restorers, etc. The dc-blocking capacitor added in series with the HFT results in only ac voltage applied across its windings, which avoids its saturation. Various ZS-based HFTI ac–ac converters are proposed in this paper, and to verify their operation and advantages, example of quasi-ZS (qZS)-based isolated ac–ac converter is considered in detail. The commutation strategy is also developed to achieve the safe commutation, which avoids the current and voltage spikes without using any RC snubber. A 200-W laboratory prototype of HFTI-qZS ac–ac converter is fabricated and experiments are performed to validate the advantages of the proposed ac–ac converters. [ABSTRACT FROM PUBLISHER]

Operation and analysis of delta-connected ac motor-drive systems under fault-tolerant open-phase mode of operation is introduced in this paper for both scalar- and vector-controlled motor-drive systems. This technique enables the operation of the three-phase motor upon a failure in one of its phases without the need of a special fault-detection algorithm. It is mainly used to significantly mitigate torque pulsations, which are caused by an open-delta configuration in the stator windings. The performance of the fault-tolerant system was verified using a detailed time stepping finite element simulation as well experimental tests for a 5-hp 460-V induction motor-drive system and the results are presented in this paper This paper also compares the operation of this fault-tolerant mode of operation for the cases of scalar-controlled and closed-loop vector-controlled motor-drive systems. [ABSTRACT FROM AUTHOR]

In this paper, fault diagnosis and fault-tolerant control strategies have been studied comprehensively for dual three-phase permanent-magnet synchronous motor (PMSM) drives to improve the reliability. Based on direct torque control (DTC) with space vector modulation, a series of diagnostic and tolerant control methods have been proposed for five types of faults, namely, speed-sensor fault, dc-link voltage-sensor fault, current-sensor fault, open-phase fault, and open-switch fault. First, diagnosis and tolerant schemes are proposed for speed-sensor fault by estimating the rotor angle speed with the rotating speed of stator flux. Second, diagnosis and tolerant schemes are proposed for dc-link voltage-sensor fault by combining the current model based stator flux observer with the voltage model based stator flux observer. Third, a three-step method is designed to diagnose three types of faults related to current signals, namely, current-sensor fault, open-phase fault, and open-switch fault simultaneously. A vector space decomposition based current estimation method is proposed to achieve fault-tolerant control for the current-sensor fault, and the voltage compensation based fault-tolerant control is presented for both open-phase and open-switch faults. The experiments have been taken on a laboratory prototype to verify the effectiveness of the proposed fault diagnosis and fault-tolerant schemes. [ABSTRACT FROM AUTHOR]

In this paper, a nonisolated high step-up dc/dc converter, belonging to the family of reduced redundant power processing converters (R2P2), is investigated for high-power applications. The selection of this topology as the most suitable for such applications among those belonging to the R2P2 family is justified. Emphasis is given on the theoretical analysis of the real step-up voltage ratio of the converter, and voltage and current stresses under continuous and discontinuous conduction modes of operation, taking into account the nonideality of the topology components, namely the parasitic resistances of semiconductor devices and inductors and the voltage drop of diodes. The theoretical analysis and the behavior of the selected converter are experimentally validated through a 2 kW prototype. Moreover, the overall efficiency of the chosen converter is experimentally investigated for various step-up voltage ratios and for a high-power range, considering a waste heat recovery system for a maritime application. [ABSTRACT FROM AUTHOR]

The grounding of low-frequency distribution transformers is a critical concern that ensures the proper operation and protection of the distribution system. The power electronic transformer (PET), as a future alternative low-frequency distribution transformer, also needs grounding. This paper studies PET operation with arc-suppression-coil (ASC) grounding in the Y-connected medium-voltage (MV) stage, focusing on the following two aspects. [ABSTRACT FROM AUTHOR]

Cascaded H-bridge (CHB) inverter is the most widely used topology for a medium-voltage drive system due to the high degree of modularity, easier implementation of medium output voltage, and the ability to continuous operation under the cell fault condition. Because each power cell of CHB should have isolated dc source, multiwinding input transformer and three-phase active front end (AFE) are generally used for regenerative applications. The whole system can be simplified by replacing the three-phase AFE with single-phase AFE. However, if the control strategy of normal operation is adopted under the cell fault condition, input power imbalance among three phases inevitably occurs. In that situation, not only faulty cells, but also some unscathed cells should be excluded with giving up the maximum capability of the system, not to deteriorate grid current. This paper proposes a control scheme against the cell fault condition of the single-phase AFE CHB. By applying the proposed control scheme to the system, dc-link voltage of each cell and grid current are well regulated without imbalance even under the cell fault conditions. Finally, it can minimize the number of undamaged cells which should be turned off and maximize the capability of the system under the cell fault condition. Simulation and experimental results are provided to verify the effectiveness of the proposed scheme. [ABSTRACT FROM AUTHOR]

Fault-tolerance capability is a critical matter in power conversion systems that require continuous operation. Multiphase converters provide an inherent redundancy that gives them fault-tolerant capability. In addition, the interleaved operation of these converters provides them with some advantages regarding total current ripple, which otherwise would be lost under a fault condition. This issue can be solved by means of an identification and reconfiguration system. This paper proposes a method for open-circuit fault identification, based on the time-domain comparison of the residual of a state-space observer with signatures per phase. Signature online generation, together with a proper normalization and thresholding strategy, provides immunity with respect to the operation point. The use of the phase information associated to the interleaved operation allows to reduce the number of sensed variables, making it possible to identify the faulty leg by only sensing input and output signals, which are conventionally used for control purposes. Hence, the measurement of signals within every leg is avoided. Experimental results on a four-phase interleaved buck converter validate the method capacity to identify faults in two switching periods. Robustness regarding abrupt changes in the operational conditions was tested. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

This paper proposes a fault-tolerant control strategy of the open-winding inverter for dc-biased vernier reluctance machines (dc-biased VRMs), aiming to maintain the output capability after the open-circuit fault occurs in the inverter switches. DC-biased VRMs are with integrated winding current for both dc component for field excitation and ac component for torque generation. The torque generation is not associated with the current direction, making the full-bridge inverter be divided into two groups of devices. The proposed fault-tolerant control strategy is based on the regular full-bridge inverter for each open winding and can control the motor in the fault-tolerant mode with one group of devices. When an open-circuit fault of a device appears in the normal mode, the controller will switch to the fault-tolerant mode. After a short transient period, the motor will be able to work. Moreover, in the fault-tolerant mode, a pulsewidth modulation driving method is proposed to reduce the current ripple. The proposed method does not require any additional components, and the reliability of the drive control system with the open-winding inverter is improved. Experimental results verify the validity and feasibility of the proposed fault-tolerant control strategy. [ABSTRACT FROM AUTHOR]

For the purpose of increasing the reliability in a hostile environment, techniques of fault diagnosis have been reported for a three-phase voltage-source inverter (VSI). Based on the average current Park's vector method, this paper proposes a fuzzy-based fault diagnosis method for the VSI in the three-phase permanent-magnet synchronous motor drive. By utilizing the phase current information, the fault symptom variables are calculated by using the average current Park's vector method. The fuzzy logic approach is applied to process the fault symptom variables and obtain the faulty information of power switches. Compared with other fuzzy logic methods, the fuzzy logic design, fuzzy inputs, and fuzzy rules are different. The proposed fault diagnosis method can detect and locate not only the single or multiple open-circuit faults, but also the intermittent faults in power switches, which can improve the reliability of the motor drive system. The effectiveness of the proposed method is validated by both simulation and experiments. [ABSTRACT FROM AUTHOR]

In this paper, a high step-up quasi-Z Source (QZS) dc–dc converter is proposed. This converter uses a hybrid switched-capacitors switched-inductor method in order to achieve high voltage gains. The proposed converter have resolved the voltage gain limitation of the basic QZS dc–dc converter while keeping its main advantages, such as continuous input current and low voltage stress on capacitors. Compared to the basic converter, the duty cycle is not limited, and the voltage stress on the diodes and switch is not increased. In addition to these features, the proposed converter has a flexible structure, and extra stages could be added to it in order to achieve even higher voltage gains without increasing the voltage stress on devices or limiting the duty cycle. The operation principle of the converter and related relationships and waveforms are presented in the paper. Also, a comprehensive comparison between the proposed and other QZS based dc–dc converters is provided which confirms the superiority of the proposed converter. Simulations are done in power systems computer aided design (PSCAD) in order to investigate the maximum power point tracking (MPPT) capability of the converter. In addition, the valid performance and practicality of the converter are studied through the results obtained from the laboratory built prototype. [ABSTRACT FROM AUTHOR]

This paper presents a detailed analysis of short-circuit current behavior during interturn short-circuit faults in permanent magnet synchronous machines (PMSMs) by considering the short-circuit contact resistance. For this purpose, an finite element analysis (FEA)-based equivalent circuit model is developed to understand the circulating current behavior in the shorted turns. Various fault resistance and number of shorted turn combinations are examined at different torque and speed levels. To include saturation due to high fault currents, the inductance matrix of faulty machine is created in FEA environment and incorporated into the equivalent circuit model as four-dimensional lookup tables. In order to take loop responses into account, the model is controlled through field oriented control (FOC) with closed speed and current loops. An experimental setup is built to verify the simulation results using a PMSM with several winding taps. It is shown that the experimental results and the simulations are quite consistent with each other. The findings from this study are essential to predict fault severity, develop mitigation techniques and determine the safe operating area for faulty machines. [ABSTRACT FROM AUTHOR]

A transmission line emulator has been developed to flexibly represent interconnected ac lines under normal operating conditions in a voltage-source-converter-based power system emulation platform. As the most serious short-circuit fault condition, the three-phase short-circuit fault emulation is essential for power system studies. This paper proposes a model to realize a three-phase short-circuit fault emulation at different locations along a single transmission line or one of several parallel-connected transmission lines. At the same time, a combination method is proposed to eliminate the undesired transients caused by the current reference step changes while switching between the fault state and the normal state. Experiment results verify the developed transmission line three-phase short-circuit fault emulation capability. [ABSTRACT FROM AUTHOR]

This paper investigates the open-circuit faults of switching devices in the three-level T-Type inverter and proposes a new tolerant control method. The three-phase current and variation of the neutral-point voltage are detected to determine the fault switch. A carrier-based pulse width modulation is developed to tolerate switch fault by transiting from three-level to two-level switching. This makes the modulation scheme easier to be implemented compared to space vector modulation. For the neutral-point fault, the fault phase is operated by two-level modulation to produce the same rating voltage. On the other hand, the inverter is able to produce balanced voltage with reduced rating by keeping the phase of half-bridge fault to the neutral-point voltage. The neutral-point voltage is also able to be balanced by using two-level modulation since it does not contribute the neutral-point current. The performance and effectiveness of the proposed tolerant control are verified by a laboratory build. [ABSTRACT FROM AUTHOR]

For the purpose of reducing cost and volume, as well as to increase reliability in hostile environments, techniques of reconstructing three-phase current through a single current sensor have been reported for a three-phase alternating current motor vector control system. Based on the phase current reconstruction method, this paper proposes a novel open-circuit fault diagnosis method for the low-power voltage source inverter in the three-phase permanent-magnet synchronous motor drive. Other than sampling the single dc-link current sensor, the zero voltage vector sampling method (ZVVSM) is adopted to implement the fault diagnosis. By placing the single current sensor at a special position, ZVVSM is able to sample current during the two zero voltage vectors and reconstruct the three-phase currents. The reconstructed three-phase currents are used to generate the diagnostic variables that can detect and locate the faulty power switches. The PWM modulation strategy remains unchanged in this method and the reconstructed phase currents are with less waveform distortion and less harmonic contents, which are suitable for the fault diagnosis. The effectiveness of the proposed method is verified by experiments. [ABSTRACT FROM AUTHOR]

In an islanded inverter-interfaced microgrid (IIM), fault current is generally controlled at the same level of the normal operation current due to the limited capacities of inverters, rendering the conventional overcurrent protection methods impractical. To solve this issue, this paper presents a novel protection strategy, which incorporates flexible control of the inverter and setting of microprocessor-based relay (MBR). Once a short-circuit fault occurs at distribution line, a current limiting strategy combining with hysteresis control and maximum current root-mean square (RMS) control (MCRC) is applied for the inverter to limit the fault current within permissible range. In the MCRC mode, a percentage of fifth harmonic is injected to the fault current, facilitating the MBRs to identify the fault according to the harmonics components. Additionally, the fault current versus fault impedance ( $I_{{\rm{fault}}}- Z_{{\rm{fault}}}$ ) droop control is synthesized to make the inverters output larger fault currents when the faults are closer to the inverters, and hence, the MBRs can be coordinated with the inverse-time principle. The main advantages of this method are that it neither relies on high magnitude of the fault current to identify the fault nor needs communications among protective devices to ensure the coordination of protections. Experiments have been conducted to demonstrate the feasibility of this method. [ABSTRACT FROM AUTHOR]

The paper presents a stationary frame control strategy to achieve optimal current control for star-connected polyphase permanent-magnet machine under asymmetric phase faults, namely, phase open circuit (OC) and phase short-circuit (SC) condition. Current regulation under these faults is particularly challenging because optimal torque control strategy generates nonsinusoidal current references with unbalance in both fundamental and higher order working harmonics, to achieve minimal copper losses and torque ripple under fault condition. Under field-weakening operation, voltage limit introduces additional control problems. The paper describes a solution for these control issues by employing a novel controller in stationary frame. This control strategy allows minimal reconfiguration of the control structure from healthy to postfault operation. Extensive simulation and experimental results are presented as validation for the proposed strategy. [ABSTRACT FROM AUTHOR]

A comprehensive literature review shows that transformer-based solutions are superior for the mitigation of inrush currents than external (to the transformer) solutions. The use of air gaps and low-permeability (iron) materials are known techniques for this purpose. This paper investigates the effectiveness of these approaches for reducing inrush and phase-hop currents. Studies are carried out on toroidal transformers, due to their broad application in power electronics devices. Contrary to common belief, this paper demonstrates that air gaps do not reduce the inrush currents when a transformer is fully demagnetized. However, inrush currents can be mitigated by the use of low-permeability iron materials. It is also demonstrated that air-gaps significantly reduce inrush currents when transformers have residual flux, e.g., for phase-hop conditions. Analytical expressions are derived to compute the mitigation factor for a specific gap length. The results and formulae presented in this paper are verified with laboratory experiments, transient simulations with validated circuit models, and 2-D finite element simulations. [ABSTRACT FROM AUTHOR]

In this paper, a modulation and control method for the new transformerless unified power flow controller (UPFC) is presented. As is well known, the conventional UPFC that consists of two back-to-back inverters requires bulky and often complicated zigzag transformers for isolation and reaching high power rating with desired voltage waveforms. To overcome this problem, a completely transformerless UPFC based on an innovative configuration of two cascade multilevel inverters has been proposed. The new UPFC offers several advantages over the traditional technology, such as transformerless, light weight, high efficiency, low cost and fast dynamic response. This paper focuses on the modulation and control for this new transformerless UPFC, including optimized fundamental frequency modulation for low total harmonic distortion and high efficiency, independent active and reactive power control over the transmission line, dc-link voltage balance control, etc. The new UPFC with proposed control method is verified by experiments based on 4160-V test setup. Both the steady-state and dynamic-response results will be shown in this paper. [ABSTRACT FROM PUBLISHER]

Switch fault diagnosis and remedial actions are an important design aspect for isolated full-bridge dc-dc converters, and they can highly improve reliability of the whole system. In this paper, a fast switch short-circuit fault (SCF) diagnostic method is proposed for the phase-shifted full-bridge (PSFB) converter. The dc-link current and transformer primary voltage are treated as diagnosis criteria. Based on the combination of real-time criteria and switch gate-driver signals, the switch SCF can be identified rapidly, and thus further damage can be avoided. Besides, a remedial action for the faulty PSFB converter is introduced to keep the continuity of the system. The converter under fault can be reconfigured into an asymmetrical half-bridge converter. Moreover, a boost unit, including a switch and a diode, is inserted into between the output rectifier and filter capacitor to compensate the output voltage. The operational principle, design considerations, and implementation are discussed in this paper. Experimental results are shown to verify the validity of theoretical analysis. [ABSTRACT FROM AUTHOR]

This paper proposes a new thermography-based maximum power point tracking (MPPT) scheme to address photovoltaic (PV) partial shading faults. Solar power generation utilizes a large number of PV cells connected in series and in parallel in an array, and that are physically distributed across a large field. When a PV module is faulted or partial shading occurs, the PV system sees a nonuniform distribution of generated electrical power and thermal profile, and the generation of multiple maximum power points (MPPs). If left untreated, this reduces the overall power generation and severe faults may propagate, resulting in damage to the system. In this paper, a thermal camera is employed for fault detection and a new MPPT scheme is developed to alter the operating point to match an optimized MPP. Extensive data mining is conducted on the images from the thermal camera in order to locate global MPPs. Based on this, a virtual MPPT is set out to find the global MPP. This can reduce MPPT time and be used to calculate the MPP reference voltage. Finally, the proposed methodology is experimentally implemented and validated by tests on a 600-W PV array. [ABSTRACT FROM AUTHOR]

Switch fault diagnosis is an important design aspect for pulse width modulation (PWM) dc-dc power converters. It can prevent power converters from further damage, and also make preparations for remedial actions. In this paper, a fast switch fault diagnostic method is proposed for PWM dc-dc converters operating in continuous conduction mode. The proposed method utilizes the magnetic component (inductor or transformer) voltage for fault diagnosis. Based on the real-time voltage measurement and switch gate-driver signals, characteristics of switch open-circuit faults and short-circuit faults are rapidly extracted, and thus, switch faults can be quickly detected. The magnetic component voltage can be measured by an auxiliary winding in the magnetic core, and gate-driver signals can be easily got from the control circuit. Moreover, the fault detection can be implemented by a low-cost logical hardware circuit, and this circuit can be integrated into the control circuit. The fault diagnosis principle, design considerations, and implementation are discussed in this paper. Experiments are conducted to verify the theoretical analysis. [ABSTRACT FROM AUTHOR]

Modularity and high reliability from redundancy are the two attractive advantages of modular multilevel converters (MMCs). This paper elaborates a switch open-circuit fault diagnosis and a fault-tolerant operation scheme for MMCs with distributed control. The proposed fault diagnosis and fault-tolerant control method can significantly improve the reliability of the MMC while maintaining the modularity of its software implementation. By distributing fault diagnosis into submodules, its local controller is capable of identifying the switching devices in open-circuit fault without extra hardware circuitry. Based on the real-time measurements of submodule terminal voltage and arm current, single, or multiple faulty switches can be identified within 3.5 ms without triggering faulty alarms. Furthermore, a new fault-tolerant operation is proposed to maintain the output current, internal dynamics, and switching harmonics unchanged after the faulty submodule is bypassed. This is achieved by resetting the period and phase registers in the local controller according to the information of bypassed submodules. The control loops of the MMC are not influenced by the proposed fault diagnosis and fault-tolerant operation, making the operation transition seamless and reliable. Experimental results show that fault identification and system reconfiguration can be completed within 5 ms, and the MMC can seamlessly and smooth ride through the switch open-circuit faults without severe malfunction and catastrophic damages. [ABSTRACT FROM AUTHOR]

Open-circuit (OC) fault diagnosis of voltage source inverters on trains still has two problems to be improved. First, it is usually forbidden to change the system structures of trains or to add any extra sensor. Therefore, diagnostic methods which need pulse width modulation control signals or extra output voltage sensors are no longer suitable for trains. Second, loads on the train vary in a wide range and have significant influence on output currents. In order to solve these problems and increase the maintenance efficiency, an online diagnostic method for single switch OC (SSOC) faults and double switches OC (DSOC) faults is proposed. Supposing that three phases are balanced, it is proved that the trajectory of two output currents is an ellipse in the Cartesian coordination, and each OC fault has a unique trajectory. SSOC faults and DSOC faults can be isolated by detecting abnormal slopes and directions of several consecutive points on the trajectory. The fastest diagnostic process takes less than half a period. Load variation has impact on the size and shape of the trajectory but does not affect the diagnostic method. Experimental tests are carried out on dSPACE platform. Results show the accuracy of the mathematical model and the effectiveness of the online diagnostic method proposed in this paper. [ABSTRACT FROM PUBLISHER]

Taking into account extension of power systems, their interconnections, and increment of load demand, power capacity of networks is increased and the impedance of sources is decreased. Such variations in power systems resulted to occurring high ratings of fault currents. Fault current limiter (FCL) can be practically utilized in order to interrupt fault currents to a specified amount without requirement to upgrading protection devices. In this paper, a novel simple structure for nonsuperconducting FCL (NSFCL), which is called capacitor-based FCL (CBFCL), is proposed for limiting the high ratings of fault currents in power networks. The proposed NSFCL circuit is based on transferring electrical energy to a capacitor during fault occurrence, which can be used after fault removal. Simulation of the proposed NSFCL structure is done and the results are provided and analyzed. Additionally, the operation of the circuit is experimentally studied, and the similarity between the simulation results and the experimental results is validated. The provided simulation and experimental results ensure the practicability and possibility of employment of the proposed CBFCL. [ABSTRACT FROM PUBLISHER]

In order to improve the reliability and availability of the converters of wind turbines, condition monitoring and fault diagnosis are considered crucial means to achieve these goals. In this text, according to the current variation characteristics of the converter, this paper presents a novel approach for real-time diagnostics of open-switch faults in back-to-back converters of a doubly fed wind turbine. The average value of the normalized converter phase currents and the absolute normalized currents are used as principal quantities to formulate the diagnostic variables. The proposed fault-diagnostic variables prove to be carrying information about multiple open-switch faults. In addition to, by the combination of these variables with the average absolute values of the normalized converter phase currents, an adaptive fault-diagnostic threshold is proposed, which ensures the robustness of the diagnosis of single and double open-switch faults. Finally, through the diagnostic variables and the adaptive threshold, the dynamic fault-diagnostic method for open-switch faults in the back-to-back converter of a doubly fed wind turbine is formed. The simulation and experimental results also indicate that the fault-diagnostic method can not only diagnose the multiple open-circuit faults of a back-to-back converter, but also have a better robustness. [ABSTRACT FROM PUBLISHER]