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This paper proposes an adaptive saturation module to enhance the transient stability of grid‐following inverters after voltage‐dip inception and fault‐clearance moment. The equal‐area criterion reveals that a large acceleration area will be obtained due to the step change of current reference according to grid codes, which may lead to the loss of synchronism with the grid. It is found that the saturation module used with the phase‐locked loop can enhance the transient stability performance by reducing the acceleration area. To simultaneously adapt to the two cases (the inception and clearance of the voltage‐dip fault), an adaptive saturation module that can automatically adjust the clamping mode is proposed, which is the main contribution of the paper. The selection of the threshold value for the saturation module is also presented. A comprehensive comparison is made between the proposed solution and state‐of‐art solutions. Finally, the effectiveness of the proposed control strategy is confirmed by the experimental tests. [ABSTRACT FROM AUTHOR]

Power quality is a critical issue in power systems that can affect the performance and reliability of sensitive equipment. The dynamic voltage restorer (DVR) is a widely used technology for improving power quality, but it mainly focuses on mitigating voltage sags and often neglects the issue of harmonic distortion. This paper proposes a DVR voltage compensation strategy based on the discrete Fourier transform (DFT). This strategy utilizes the DFT to analyze the voltage waveform, separating the fundamental and desired harmonic components. With this strategy, the DVR can effectively compensate for voltage sag and achieve selective harmonic compensation, thereby improving the power quality of the power system. Additionally, a method utilizing hardware circuitry and an LED as a timestamp is proposed to dynamically estimate digital siginal processor (DSP) resource occupancy rate in real‐time, ensuring effective compensation while minimizing resource occupancy. A 3 kW DVR experimental prototype was constructed and tested to evaluate the practicality of the proposed strategy. The simulation and experimental results show that the method proposed in this paper successfully realizes the compensation of voltage sag and harmonics in the power system, and improves the stability and reliability of the power system. [ABSTRACT FROM AUTHOR]

Abstract This paper proposes an adaptive saturation module to enhance the transient stability of grid‐following inverters after voltage‐dip inception and fault‐clearance moment. The equal‐area criterion reveals that a large acceleration area will be obtained due to the step change of current reference according to grid codes, which may lead to the loss of synchronism with the grid. It is found that the saturation module used with the phase‐locked loop can enhance the transient stability performance by reducing the acceleration area. To simultaneously adapt to the two cases (the inception and clearance of the voltage‐dip fault), an adaptive saturation module that can automatically adjust the clamping mode is proposed, which is the main contribution of the paper. The selection of the threshold value for the saturation module is also presented. A comprehensive comparison is made between the proposed solution and state‐of‐art solutions. Finally, the effectiveness of the proposed control strategy is confirmed by the experimental tests.

Abstract Power quality is a critical issue in power systems that can affect the performance and reliability of sensitive equipment. The dynamic voltage restorer (DVR) is a widely used technology for improving power quality, but it mainly focuses on mitigating voltage sags and often neglects the issue of harmonic distortion. This paper proposes a DVR voltage compensation strategy based on the discrete Fourier transform (DFT). This strategy utilizes the DFT to analyze the voltage waveform, separating the fundamental and desired harmonic components. With this strategy, the DVR can effectively compensate for voltage sag and achieve selective harmonic compensation, thereby improving the power quality of the power system. Additionally, a method utilizing hardware circuitry and an LED as a timestamp is proposed to dynamically estimate digital siginal processor (DSP) resource occupancy rate in real‐time, ensuring effective compensation while minimizing resource occupancy. A 3 kW DVR experimental prototype was constructed and tested to evaluate the practicality of the proposed strategy. The simulation and experimental results show that the method proposed in this paper successfully realizes the compensation of voltage sag and harmonics in the power system, and improves the stability and reliability of the power system.

Abstract A multi‐inverter‐fed power system is susceptible to small‐signal instability owing to weak grid influence. This study proposes a black‐box method for small‐signal stability analysis of a multi‐inverter‐fed power system based on Bode diagrams. Not only did it consider the technical confidentiality on the engineering site, but it also took into account the presence of right‐half‐plane (RHP) pole in the aggregated impedance, achieving Nyquist circle acquisition through Bode diagrams. The stability analysis process is thus more accurate, intuitive, and convenient. First, black‐box impedance fitting for a single grid‐connected inverter and impedance aggregation for a multi‐inverter‐fed power system are discussed. Second, the principle of the proposed method is elaborated. Using the Bode diagram, the number of RHP poles of the aggregated impedance and actual number of circles for Nyquist curves are identified. Third, a multi‐inverter‐fed power system is constructed, and the effectiveness of the proposed method is verified using case analysis and hardware‐in‐the‐loop real‐time experiments based on RT‐LAB. Finally, the advantages of the proposed method are revealed by comparing it to the IF method.

Abstract A three‐level neutral point clamped (3L‐NPC) voltage source inverter (VSI) topology can be advantageous in electric vehicles with a high DC‐link voltage and a high switching frequency. A bulky DC‐link capacitor is not an option; thus, the DC‐link capacitor's sizing considering the traction system characteristics is an important design step. This paper investigates how the DC‐link capacitor size of a 3L‐NPC VSI gets affected by the combination of the interdependent characteristics of an electric drive, such as its power factor, modulation index, current, and fundamental frequency, with the effects of the modulation methods. Five pulse width modulation (PWM) methods, of which three of them have an active neutral point potential control, are compared in terms of their neutral point potential (NPP) oscillations. Then, the size of the DC‐link capacitor is determined for each PWM method so that the NPP ripple is kept under desired limits at all operating conditions. It is shown that both the modulation technique and the electric machine characteristics influence the capacitor size. For example, electric machine design modifications can introduce more than a 30% reduction in capacitor size. Finally, DC‐link and NPP oscillations with different PWM methods are experimentally validated in a scaled‐down 3L‐NPC inverter.

Abstract Interconnecting multiple microgrids (MGs) to form a MG cluster (MGC) is significant to improve the MG power supply reliability and increase the new energy accommodation capacity. However, for the traditional MGC in complex operational conditions, the following technologies need to be further researched, such as mutual power support among MGs, voltage support and power quality improvement and so on. Focusing on the above problems, a novel virtual synchronous port control (VSPC) strategy for MGC is proposed. Firstly, a multi‐port soft open point (SOP) is used to interconnect each sub‐MG in the MGC, to achieve a better flexible interconnection among sub‐MGs. Secondly, on this basis, the VSPC strategy is proposed. By adopting the DC voltage control of multi‐port SOP and the virtual synchronisation control of interconnecting ports, multiple voltage source converters (VSCs) are cluster controlled to provide the sub‐MGs with superordinate ports with virtual synchronisation characteristics, which ensures the dynamic balance of the power inside the MGC and achieves better voltage and frequency support capability. Finally, the example results verify the feasibility and effectiveness of the proposed VSPC strategy.

Abstract Aiming at the problem of single‐phase switch tube open circuit fault in inverters, a current residual vector three‐phase four‐switch fault‐tolerant control strategy based on the mixed logic dynamic (MLD) model of permanent magnet synchronous motor (PMSM) drive system is proposed. Firstly, the MLD and three‐phase four‐switch mathematical models of the motor drive system are established and a calculation method of DC bus capacitance is proposed. Secondly, to improve the dynamic performance of the motor, an improved vector control based on the MLD model is proposed. Finally, by setting the threshold and dividing the area of the current residual vector, the accuracy of inverter fault diagnosis is improved, and the fast and smooth switching of the drive system after the fault is realized. Simulation and experimental results show that this strategy can effectively reduce the influence of calculation error and system noise, and has strong robustness for the change of motor speed and load.

A three‐level neutral point clamped (3L‐NPC) voltage source inverter (VSI) topology can be advantageous in electric vehicles with a high DC‐link voltage and a high switching frequency. A bulky DC‐link capacitor is not an option; thus, the DC‐link capacitor's sizing considering the traction system characteristics is an important design step. This paper investigates how the DC‐link capacitor size of a 3L‐NPC VSI gets affected by the combination of the interdependent characteristics of an electric drive, such as its power factor, modulation index, current, and fundamental frequency, with the effects of the modulation methods. Five pulse width modulation (PWM) methods, of which three of them have an active neutral point potential control, are compared in terms of their neutral point potential (NPP) oscillations. Then, the size of the DC‐link capacitor is determined for each PWM method so that the NPP ripple is kept under desired limits at all operating conditions. It is shown that both the modulation technique and the electric machine characteristics influence the capacitor size. For example, electric machine design modifications can introduce more than a 30% reduction in capacitor size. Finally, DC‐link and NPP oscillations with different PWM methods are experimentally validated in a scaled‐down 3L‐NPC inverter. [ABSTRACT FROM AUTHOR]

A multi‐inverter‐fed power system is susceptible to small‐signal instability owing to weak grid influence. This study proposes a black‐box method for small‐signal stability analysis of a multi‐inverter‐fed power system based on Bode diagrams. Not only did it consider the technical confidentiality on the engineering site, but it also took into account the presence of right‐half‐plane (RHP) pole in the aggregated impedance, achieving Nyquist circle acquisition through Bode diagrams. The stability analysis process is thus more accurate, intuitive, and convenient. First, black‐box impedance fitting for a single grid‐connected inverter and impedance aggregation for a multi‐inverter‐fed power system are discussed. Second, the principle of the proposed method is elaborated. Using the Bode diagram, the number of RHP poles of the aggregated impedance and actual number of circles for Nyquist curves are identified. Third, a multi‐inverter‐fed power system is constructed, and the effectiveness of the proposed method is verified using case analysis and hardware‐in‐the‐loop real‐time experiments based on RT‐LAB. Finally, the advantages of the proposed method are revealed by comparing it to the IF method. [ABSTRACT FROM AUTHOR]

Interconnecting multiple microgrids (MGs) to form a MG cluster (MGC) is significant to improve the MG power supply reliability and increase the new energy accommodation capacity. However, for the traditional MGC in complex operational conditions, the following technologies need to be further researched, such as mutual power support among MGs, voltage support and power quality improvement and so on. Focusing on the above problems, a novel virtual synchronous port control (VSPC) strategy for MGC is proposed. Firstly, a multi‐port soft open point (SOP) is used to interconnect each sub‐MG in the MGC, to achieve a better flexible interconnection among sub‐MGs. Secondly, on this basis, the VSPC strategy is proposed. By adopting the DC voltage control of multi‐port SOP and the virtual synchronisation control of interconnecting ports, multiple voltage source converters (VSCs) are cluster controlled to provide the sub‐MGs with superordinate ports with virtual synchronisation characteristics, which ensures the dynamic balance of the power inside the MGC and achieves better voltage and frequency support capability. Finally, the example results verify the feasibility and effectiveness of the proposed VSPC strategy. [ABSTRACT FROM AUTHOR]

Aiming at the problem of single‐phase switch tube open circuit fault in inverters, a current residual vector three‐phase four‐switch fault‐tolerant control strategy based on the mixed logic dynamic (MLD) model of permanent magnet synchronous motor (PMSM) drive system is proposed. Firstly, the MLD and three‐phase four‐switch mathematical models of the motor drive system are established and a calculation method of DC bus capacitance is proposed. Secondly, to improve the dynamic performance of the motor, an improved vector control based on the MLD model is proposed. Finally, by setting the threshold and dividing the area of the current residual vector, the accuracy of inverter fault diagnosis is improved, and the fast and smooth switching of the drive system after the fault is realized. Simulation and experimental results show that this strategy can effectively reduce the influence of calculation error and system noise, and has strong robustness for the change of motor speed and load. [ABSTRACT FROM AUTHOR]

Abstract Inertia is the core index of the estimation of power system frequency stability; a simple estimation method of grid‐forming inverter inertia based on the rate of change of frequency (RoCoF) is proposed in this paper. The method records and fits the active power curve by applying RoCoF to obtain the value of the grid‐forming inverter inertia, and it is simpler and higher accuracy compared to existing methods. In addition, the inertia and damping are analysed to support the system, and the relationship equation between the grid frequency slope change and the output active power is derived. Considering the theoretical analyses and influencing factors, the inertia and damping are obtained by using the method of function fitting. The test steps show the simplicity of the proposed test method. Finally, the effectiveness of the proposed method is verified in MATLAB/Simulation and two 1.575‐MW parallel grid‐forming electrochemical energy storage power plants.

Abstract In this study, to solve total harmonic distortion (THD) and voltage stress on switches, challenges have been made by presenting a new multi‐level inverter (MLI). The proposed topology can operate with both symmetric and asymmetric sources. The proposed topology in symmetric and asymmetric mode with 18 switches can produce 17 and 49 voltage levels, respectively, with THD values of 4.14% and 1.45%, which passes the IEEE harmonic standard. The main advantage of the proposed topology is reducing the number of circuit devices, which reduces the price, volume, and complexity of the circuit by reducing the number of circuit devices. The proposed topology reduces the THD, the number of power electronics components, and voltage stress by increasing the output levels. The proposed topology is compared with other topologies and the advantage of the proposed topology is shown in terms of the number of switches and drivers. In addition, the proposed topology is compared in terms of the cost function. The power losses based on the thermal model of the proposed topology have been analysed and simulated. Simulation and experimental results are presented for different scenarios such as load type, load changes, and modulation index changes to validate the proposed topology.

Abstract Z‐source‐based half‐bridge inverters are a type of Z‐source inverters whose outer stage is a half‐bridge inverter. This paper presents a generalized structure from which 16 configurations can be derived. Once this topology has been introduced, one of the derived topologies, out of 16, is analyzed in detail based on the appropriate control method. According to these analysis results, this topology's boost‐factor and some equations to design passive and active devices are obtained. Next, it will be tried to derive the equations for each topology based on the obtained results for one of the configurations. In the next step, the topology that has been analyzed is considered to evaluate the efficiency of this topology using the power losses study. Hereafter, the generalized configuration's merits and downsides are studied by comparing it with other similar topologies. Finally, the experiment in the final section checks the operation and validity of the obtained results.

Inertia is the core index of the estimation of power system frequency stability; a simple estimation method of grid‐forming inverter inertia based on the rate of change of frequency (RoCoF) is proposed in this paper. The method records and fits the active power curve by applying RoCoF to obtain the value of the grid‐forming inverter inertia, and it is simpler and higher accuracy compared to existing methods. In addition, the inertia and damping are analysed to support the system, and the relationship equation between the grid frequency slope change and the output active power is derived. Considering the theoretical analyses and influencing factors, the inertia and damping are obtained by using the method of function fitting. The test steps show the simplicity of the proposed test method. Finally, the effectiveness of the proposed method is verified in MATLAB/Simulation and two 1.575‐MW parallel grid‐forming electrochemical energy storage power plants. [ABSTRACT FROM AUTHOR]

In this study, to solve total harmonic distortion (THD) and voltage stress on switches, challenges have been made by presenting a new multi‐level inverter (MLI). The proposed topology can operate with both symmetric and asymmetric sources. The proposed topology in symmetric and asymmetric mode with 18 switches can produce 17 and 49 voltage levels, respectively, with THD values of 4.14% and 1.45%, which passes the IEEE harmonic standard. The main advantage of the proposed topology is reducing the number of circuit devices, which reduces the price, volume, and complexity of the circuit by reducing the number of circuit devices. The proposed topology reduces the THD, the number of power electronics components, and voltage stress by increasing the output levels. The proposed topology is compared with other topologies and the advantage of the proposed topology is shown in terms of the number of switches and drivers. In addition, the proposed topology is compared in terms of the cost function. The power losses based on the thermal model of the proposed topology have been analysed and simulated. Simulation and experimental results are presented for different scenarios such as load type, load changes, and modulation index changes to validate the proposed topology. [ABSTRACT FROM AUTHOR]

The quasi‐Z‐source H‐bridge grid‐connected inverter (QHGCI) is well known for its advantages of the void of the shoot‐through problem and the high DC‐voltage utilization. But the existence of the common‐mode leakage current in the power frequency cycle, lower power density, and higher thermal stress make it hard applicable to the grid‐penetrating application. Thus with the purpose to conquer the problem relating to the QHGCI, an innovative transformerless Z‐source photovoltaic grid‐connected inverter with a coupled inductor coil (TZPGCI‐CIC) is proposed. The circuitry topology and an unipolar sine pulse width modulation strategy are first introduced in short. Thereafter, the common‐mode voltage in the whole working process is derived and evaluated through the detailed operating mode analysis, in which a constant value of it has been theoretically revealed. Lastly, a prototype platform of TZPGCI‐CIC is set up and its good performance on leakage current suppression, and lower thermal stress are validated with the experimental results. [ABSTRACT FROM AUTHOR]

Z‐source‐based half‐bridge inverters are a type of Z‐source inverters whose outer stage is a half‐bridge inverter. This paper presents a generalized structure from which 16 configurations can be derived. Once this topology has been introduced, one of the derived topologies, out of 16, is analyzed in detail based on the appropriate control method. According to these analysis results, this topology's boost‐factor and some equations to design passive and active devices are obtained. Next, it will be tried to derive the equations for each topology based on the obtained results for one of the configurations. In the next step, the topology that has been analyzed is considered to evaluate the efficiency of this topology using the power losses study. Hereafter, the generalized configuration's merits and downsides are studied by comparing it with other similar topologies. Finally, the experiment in the final section checks the operation and validity of the obtained results. [ABSTRACT FROM AUTHOR]

Abstract This paper proposes a switched Z‐source inverter with high voltage gain and low current stress on elements. Also, a new switching method corresponding to the proposed topology is introduced. This method is based on the usual Maximum Constant Boost (MCB) method. Hence, it has the advantages of the MCB method, such as low voltage stress on the inverter switches and high voltage gain. Also, eliminating part of the inverter's switching in this modulation method improves the inverter's efficiency. In addition to the Shoot‐Through (ST) duty cycle, another variable (N) affects the inverter's boost factor in this switching method. The proposed inverter's dynamic analysis shows that introducing N in the modulation method improved the dynamic characteristic of the inverter. Also, this variable as a control input can stabilize the dc side of the inverter in the closed‐loop control of the inverter. For the proposed topology, steady‐state analysis, dynamic analysis, elements design and efficiency calculation are performed. Also, the proposed topology is compared with the conventional topologies regarding boost factor, the number of elements, current stress on elements and efficiency to show its superiority. Finally, the experimental results are presented to verify theoretical discussions.

Abstract This paper presents an improved switched‐inductor quasi‐Z‐source inverter with a higher boost factor than similar inverters, which integrates a switched‐capacitor and a three‐winding switched‐coupled inductor into a conventional quasi Z‐source inverter (q‐ZSI). The proposed inverter is called a switched‐capacitor trans quasi Z‐source inverter (SCT‐q‐ZSI). In this topology, the input current is continuous, and a high voltage gain is reachable with lowduty cycles (D). In this structure, capacitors are used instead of the side diodes of the switched‐inductor quasi Z‐source inverter. The proposed converter has a low inrush current at the startup which might damage the converter components. The performance of the proposed inverter is investigated at different operating modes and the voltage and current equations of all elements are calculated. The analysis, derivation of boost factor, capacitor voltages, and components, of the proposed inverter, are carried out and are compared with similar inverters. Finally, analytical, simulation, and experimental results are also presented to validate its advantages.

Abstract Grid‐connected inverters are the core equipment in the renewable power system. There are multiple current sensors which may affect the driving module of the switch in inverter. In multiple current sensor fault diagnosis, the coupling between fault components makes fault diagnosis difficult. This paper presents an offset fault diagnosis method of multiple current sensors based on the average modulation voltage model. Based on the influence of current sensor offset fault in the system, the average modulation voltage model is established in three‐phase stationary coordinates. The difference between the measured value of the current and the actual value is estimated through the model when the offset fault occurs. And then the fault is located. Experimental results show that the fault can be located accurately and fault tolerant control can be performed by this method when there are offset faults in multiple sensors simultaneously.

Abstract A secondary control method is proposed for accurate reactive power sharing as well as frequency and voltage restoration in islanded AC microgrids (MGs). The proposed method consists of an MG secondary controller, local secondary controllers for distributed energy resources (DERs), and a low‐bandwidth communication link for broadcasting common data from the MG secondary controller to DERs. The broadcasted data includes the MG point of common coupling voltage magnitude and a common vertical shift for frequency and voltage restoration. Local secondary controllers calculate specific shifts for the Q‐V droop characteristic of each dispatchable DER and the V‐Q reverse droop characteristic of each photovoltaic (PV) system, aligning their operating points with the Q‐VPCC and VPCC‐Q droop characteristics, respectively. By employing VPCC as a common global variable, coordination of reactive power generation of all dispatchable DERs and PV systems is achieved, enabling accurate reactive power sharing. Importantly, in the proposed scheme, the required communication bandwidth and the communication burden are minor and are not increased with the number of DERs. Additionally, the DERs are relieved of the need for data transmission capability. The small signal stability of the proposed method is examined and its effectiveness is validated through Hardware‐in‐the‐Loop (HIL) experimental results.

Abstract Because of uncertain random noise and unknown disturbance, the observer‐based inverter fault diagnosis meets the problems of large deviation of current observation value and high false alarm rate. An inverter open‐circuit fault diagnosis method is proposed via residual performance evaluation. Firstly, the interval sliding mode observer is constructed by combining the interval observer with the sliding mode observer. Three phase current estimates are obtained and current residuals are calculated. Secondly, the residual performance indicators are calculated using the evaluation function. The fault detection is derived by comparing residual performance with the threshold. Thirdly, a residual information table is established based on the fault information contained in the residual for fault isolation. Finally, the proposed algorithm is validated on simulation and physical platforms. Compared with traditional observer‐based diagnostic methods, the introduction of residual performance evaluation improves the diagnostic speed by more than 50%. The interval sliding mode observer has higher estimation accuracy and anti‐interference ability in the presence of noise and external disturbances.

This paper presents an improved switched‐inductor quasi‐Z‐source inverter with a higher boost factor than similar inverters, which integrates a switched‐capacitor and a three‐winding switched‐coupled inductor into a conventional quasi Z‐source inverter (q‐ZSI). The proposed inverter is called a switched‐capacitor trans quasi Z‐source inverter (SCT‐q‐ZSI). In this topology, the input current is continuous, and a high voltage gain is reachable with lowduty cycles (D). In this structure, capacitors are used instead of the side diodes of the switched‐inductor quasi Z‐source inverter. The proposed converter has a low inrush current at the startup which might damage the converter components. The performance of the proposed inverter is investigated at different operating modes and the voltage and current equations of all elements are calculated. The analysis, derivation of boost factor, capacitor voltages, and components, of the proposed inverter, are carried out and are compared with similar inverters. Finally, analytical, simulation, and experimental results are also presented to validate its advantages. [ABSTRACT FROM AUTHOR]

This paper proposes a switched Z‐source inverter with high voltage gain and low current stress on elements. Also, a new switching method corresponding to the proposed topology is introduced. This method is based on the usual Maximum Constant Boost (MCB) method. Hence, it has the advantages of the MCB method, such as low voltage stress on the inverter switches and high voltage gain. Also, eliminating part of the inverter's switching in this modulation method improves the inverter's efficiency. In addition to the Shoot‐Through (ST) duty cycle, another variable (N) affects the inverter's boost factor in this switching method. The proposed inverter's dynamic analysis shows that introducing N in the modulation method improved the dynamic characteristic of the inverter. Also, this variable as a control input can stabilize the dc side of the inverter in the closed‐loop control of the inverter. For the proposed topology, steady‐state analysis, dynamic analysis, elements design and efficiency calculation are performed. Also, the proposed topology is compared with the conventional topologies regarding boost factor, the number of elements, current stress on elements and efficiency to show its superiority. Finally, the experimental results are presented to verify theoretical discussions. [ABSTRACT FROM AUTHOR]

Grid‐connected inverters are the core equipment in the renewable power system. There are multiple current sensors which may affect the driving module of the switch in inverter. In multiple current sensor fault diagnosis, the coupling between fault components makes fault diagnosis difficult. This paper presents an offset fault diagnosis method of multiple current sensors based on the average modulation voltage model. Based on the influence of current sensor offset fault in the system, the average modulation voltage model is established in three‐phase stationary coordinates. The difference between the measured value of the current and the actual value is estimated through the model when the offset fault occurs. And then the fault is located. Experimental results show that the fault can be located accurately and fault tolerant control can be performed by this method when there are offset faults in multiple sensors simultaneously. [ABSTRACT FROM AUTHOR]

Abstract The quasi‐Z‐source H‐bridge grid‐connected inverter (QHGCI) is well known for its advantages of the void of the shoot‐through problem and the high DC‐voltage utilization. But the existence of the common‐mode leakage current in the power frequency cycle, lower power density, and higher thermal stress make it hard applicable to the grid‐penetrating application. Thus with the purpose to conquer the problem relating to the QHGCI, an innovative transformerless Z‐source photovoltaic grid‐connected inverter with a coupled inductor coil (TZPGCI‐CIC) is proposed. The circuitry topology and an unipolar sine pulse width modulation strategy are first introduced in short. Thereafter, the common‐mode voltage in the whole working process is derived and evaluated through the detailed operating mode analysis, in which a constant value of it has been theoretically revealed. Lastly, a prototype platform of TZPGCI‐CIC is set up and its good performance on leakage current suppression, and lower thermal stress are validated with the experimental results.

Abstract Photovoltaic (PV) transformer‐less single‐phase inverters are widely used in the solar generation systems because of low cost, high power density, and high efficiency. However, there is a common mode leakage current in the transformer‐less inverters, which may endanger personal safety. In order to reduce the common mode leakage current, extra semiconductors should be added into the PV inverters to decouple the dc side from the ac side. However, the common mode leakage current is affected by the environmental condition and parameters of power devices. Since grounds of the PV array and the output terminal of the inverter are shorted, the common mode leakage current in the doubly grounded inverters can be eliminated thoroughly. Topologies of the doubly grounded inverters are reviewed in this paper, which can be divided into two categories, that is, hybrid topologies and topologies using energy storage elements. Derivation of the reviewed topologies is given. Different topologies of doubly grounded transformer‐less single‐phase inverters are compared.

Abstract This paper presents a simple three‐dimensional space vector modulation method in ghγ coordinates for a three‐level four‐leg neutral‐point‐clamped inverter. The idea uses redundant vectors as centre points to establish two‐level space vector diagrams simplifying the original space, the first sector of which is designated as subspaces, where a modulation process is implemented. The duty cycle and all possible switching sequences in a tetrahedron can be calculated automatically without manual storage and can avoid a full‐scale search process. Additionally, the neutral point potential can be balanced by three types of charge through cancellation. Such features enable the inverter to demonstrate good performance under closed‐loop control. Moreover, the proposed method can be applied to a multilevel inverter. Simulation and experimental results demonstrate the effectiveness of the proposed concept.

Traditional electric springs can only stabilize the voltage of critical loads (CL), while sacrificing the operational stability of non‐critical loads (NCL), the status of these two loads is obviously unequal, exposure to fluctuating voltages for extended periods of time can be hazardous to NCL. To solve this problem, a control method based on electric spring (ES) with switchable smart load (SSL) that does not distinguish between CL and NCL is proposed. This paper briefly introduces first the definition, principle and control strategy of the ES with SSL. Then, the relationship between the working state control angle in this control strategy and the working state of ES is analysed in detail. A method to obtain reasonable value ranges of working state control angle by setting boundary conditions is studied. Finally, an experimental platform is built based on DSP TMS320F28069 for experiments, experimental results show that ES can stabilize the voltage of different loads in different time by adopting this control strategy, the working state of ES can be adjusted and the four‐quadrant operation of electric spring can be realized by changing the working state control angle. [ABSTRACT FROM AUTHOR]

The current study proposes a topology for switched Z‐source inverter. In the switching method used for this structure, unlike the conventional, the active network switches' duty cycle is not equal to the shoot‐through duty cycle. So, in addition to the shoot‐through duty cycle, the duty cycle of impedance network switches is also involved in the boost factor. Besides, by extracting the small‐signal model, it is observed that the proposed topology has two control inputs on the dc‐side. The advantage of the additional control input is demonstrated by designing a closed‐loop control for the case where the inverter is used as an interface between the photovoltaics (PV) and the grid. In addition, the steady‐state analysis in different operating modes and the switching pattern are presented for the proposed topology. Comparing the number of elements and the boost factor of the proposed topology with several conventional topologies indicates that besides the control advantage, this topology has a higher voltage gain. Finally, simulation and experimental results are presented to verify theoretical discussions of the proposed inverter. [ABSTRACT FROM AUTHOR]

This paper presents a simple three‐dimensional space vector modulation method in ghγ coordinates for a three‐level four‐leg neutral‐point‐clamped inverter. The idea uses redundant vectors as centre points to establish two‐level space vector diagrams simplifying the original space, the first sector of which is designated as subspaces, where a modulation process is implemented. The duty cycle and all possible switching sequences in a tetrahedron can be calculated automatically without manual storage and can avoid a full‐scale search process. Additionally, the neutral point potential can be balanced by three types of charge through cancellation. Such features enable the inverter to demonstrate good performance under closed‐loop control. Moreover, the proposed method can be applied to a multilevel inverter. Simulation and experimental results demonstrate the effectiveness of the proposed concept. [ABSTRACT FROM AUTHOR]

Photovoltaic (PV) transformer‐less single‐phase inverters are widely used in the solar generation systems because of low cost, high power density, and high efficiency. However, there is a common mode leakage current in the transformer‐less inverters, which may endanger personal safety. In order to reduce the common mode leakage current, extra semiconductors should be added into the PV inverters to decouple the dc side from the ac side. However, the common mode leakage current is affected by the environmental condition and parameters of power devices. Since grounds of the PV array and the output terminal of the inverter are shorted, the common mode leakage current in the doubly grounded inverters can be eliminated thoroughly. Topologies of the doubly grounded inverters are reviewed in this paper, which can be divided into two categories, that is, hybrid topologies and topologies using energy storage elements. Derivation of the reviewed topologies is given. Different topologies of doubly grounded transformer‐less single‐phase inverters are compared. [ABSTRACT FROM AUTHOR]

Wind energy as the cleanest source of renewable energy requires a highly efficient lightweight generator that provides maximum power density while having the least vibration noise and maintenance. In this study, an axial air gap transverse flux machine is presented, and all excitation sources are located in the stator. This structure provides lower core loss, weight and cost due to the full utilisation of the permanent magnets, SMC‐free structure and short magnetic flux path. In fact, by combining the features of a flux‐switching machine into a transverse flux generator with an axial air gap, it is possible to improve the performance of a direct‐drive wind turbine generator by overcoming traditional structures' challenges. To analyse the axial transverse flux switching permanent magnet generator performance characteristics, 3D finite element simulations have been performed, which have been validated by comparing them to the practical results of a single‐phase prototype. The results are in agreement with an acceptable error that is caused by manufacturing uncertainties. [ABSTRACT FROM AUTHOR]

Abstract A three‐phase to single‐phase direct AC power generation system based on the open‐winding permanent magnet synchronous generator (PMSG) with the dual‐inverter is proposed in this study for the standalone power system. The negative DC terminals of the dual‐inverter are commonly connected, and the positive DC terminals are reconstructed as the output port of the single‐phase AC power; the three‐phase AC power generated by PMSG could be converted to be the high power quality single‐phase AC power directly without any LC filters. Different from the traditional OW‐PMSG system, zero‐sequence current is employed to achieve adjustable frequency and voltage of the single‐phase AC power, and the corresponding dual voltage‐loop control strategy with the separate positive‐sequence and zero‐sequence currents regulation of PMSG is given to achieve high output performance. The effectiveness of the proposed control strategy and the output characteristics have been verified by simulation and the experiment of the prototype, and the results show that the high‐quality single‐phase AC power can be obtained at a wide speed range and at different load operating conditions.

This paper presents a diagnostic method of inverter open‐switch and open‐phase faults in multiphase permanent magnet synchronous motor (PMSM) drives. First defined are adequate variables called virtual current vectors. The projection of the zero‐sequence current component on these variables was used to define two simple fault indices. High sensitivity to fault is thus induced but with a good robustness to transient states and variation of machine parameters. The mathematical development of the proposed method is provided and supported by experimental tests conducted on two prototypes of multiphase machines in the laboratory: sinusoidal and bi‐harmonic PMSMs. The experimental results confirm the effectiveness and the robustness of the proposed method and its capability to detect the single and multiple open‐switches and open‐phase faults in the electric drive. [ABSTRACT FROM AUTHOR]

Abstract Inter‐turn faults are most commonly the initial stator winding fault in power trains with Voltage Source Inverters. Without counteractions, these faults can propagate and cause catastrophic failures in safety critical applications like all‐electric aircraft. The propagation of an inter‐turn fault can be reduced, by limiting the local temperature at the fault. In this article, a fault tolerant control scheme is designed and validated, which reduces the heat of an inter‐turn fault by a controlled adaption of the operating point. The control strategy utilizes the hyperbolas of constant torque and the controller design is based on a linear time‐invariant system description. A machine learning regression is used for the online heat estimation and a 3D lumped‐parameter model to evaluate the thermal effectiveness. To test the control scheme, an inter‐turn fault is actively switched, which generates the local heat equal to 13% of the overall stator copper losses. The control scheme adopts the operating point within 30 electric periods, which reduces the generated heat of the inter‐turn fault by 70%. This causes a temperature reduction at the fault locally from 367°C to 175°C, which extends the approximated insulation life from 30 s to 5000 h.

Abstract The operation of parallel‐connected servo drives has the advantages of increasing the torque (hence the power) capacity, reliability, redundancy and modularity. However, smaller discrepancies of the system clocks of the independent microcontrollers, asynchronous pulse width modulation (PWM) carrier signals and hardware differences can make the output voltages of the parallel modules non‐identical, which results in circulating currents. These circulating currents limit the parallel operation, introduce additional power dissipation, cause unbalanced power distribution and degrade the control performance. To eliminate the circulating currents separated DC supplies, coupled inductors (CIs), PWM techniques, modified control techniques etc. are proposed in the literature. In this study, a fault‐tolerant parallel‐connected gallium nitride enhancement‐mode high electron mobility transistor voltage source inverter‐based servo drive scheme that eliminates the need for CIs and separate DC supplies and also enables the use of a standard proportional‐integral current control and a standard space vector PWM approach is proposed and tested. To evaluate the performances of the proposed scheme, up to six servo drives (limit‐6 × 7 Arms) are connected in parallel and tested under various conditions with a 24 V permanent magnet synchronous motor (nominal‐61.9 Arms). The results show that the parallel operation not only increases the torque capacity of the system but also increases modularity, flexibility, reliability and also redundancy.

Abstract In today's power grid, a great number of inverter‐based distributed energy resources (DERs) are connected and are mainly designed to supply power without considering the voltage and frequency deviations of the grid. Therefore, distribution system operators (DSOs) are challenged with an increase in grid events because of the random implementation of DERs. Voltage levels can vary beyond predefined limits at the point of connection and are currently not evaluated by DSOs. Summarized here is the development of a simulation model for evaluating the impact of support functions integrated in inverter‐based DERs. The model aims to help grid operators simulate voltage and frequency events and study the impact of DERs to the grid with respect to different settings of integrated support functions. A model is developed in MATLAB/Simulink conforming to European standards and regulations. Grid dynamics can be evaluated by imitating voltage and frequency deviations. Support functions can be either adjusted according to the situation or turned off. Together with adjustable settings according to DSO request, this model offers flexibility and insight in the capabilities of DERs to solve voltage and frequency issues. Case studies show that the model corresponds to expected behaviour and can be used for further development.

Abstract Large‐scale power systems exhibit more complex dynamics due to the increasing integration of inverter‐based resources (IBRs). Therefore, there is an urgent need to enhance the situational awareness capability for better monitoring and control of power grids dominated by IBRs. As a pioneering Wide‐Area Measurement System, FNET/GridEye has developed and implemented various advanced applications based on the collected synchrophasor measurements to enhance the situational awareness capability of large‐scale power grids. This study provides an overview of the latest progress of FNET/GridEye. The sensors, communication, and data servers are upgraded to handle ultra‐high density synchrophasor and point‐on‐wave data to monitor system dynamics with more details. More importantly, several artificial intelligence (AI)‐based advanced applications are introduced, including AI‐based inertia estimation, AI‐based disturbance size and location estimation, AI‐based system stability assessment, and AI‐based data authentication.

Abstract Grid‐forming inverter control is recently discussed for bulk power systems and is already in use for islanded microgrids. A common control type is the droop control. Numerous variants of the basic droop control have been proposed. However, there is lack of performance comparison of the droop variants in literature. Their superiority has only been demonstrated for some specific microgrid scenarios. This work composes benchmark scenarios to assess and compare the applicability of droop control variants and also their combination with virtual impedances under practical conditions. A number of microgrid topologies and the interaction with synchronous machines are considered to benchmark the performance. Static criteria, such as the steady‐state power sharing, as well as dynamic stability criteria, are taken into account for modal analysis. To guarantee a meaningful comparison, a genetic algorithm tailored to the problem is used to optimise controller parameters for each controller type. Results indicate that the combination with virtual impedance has a more decisive effect on stability than the droop variant. The outcome is relevant for microgrid stability analysis in numerous contexts, such as optimal placement of inverters or topology optimisation, where the choice of the most suitable controller type with optimised parameter sets is key.

Abstract Conventional flip‐flops are more vulnerable to particle strikes in a radiation environment. To overcome this disadvantage, in the literature, many radiation‐hardened flip‐flops (FFs) based on techniques like triple modular redundancy, dual interlocked cell, Quatro and guard‐gated Quatro cell, and so on, are discussed. The flip‐flop realized using radiation hardened by design Quatro cell is named as the improved version of Quatro flip‐flop (IVQFF). Single event upset (SEU) at inverter stages of master/slave and at output are the two drawbacks of IVQFF. This study proposes a guard‐gated Quatro FF (GQFF) using guard‐gated Quatro cell and Muller C‐element. To overcome the SEU at inverter stages of IVQFF, in GQFF, the inverter stages are realized in a parallel fashion. A dual‐input Muller C‐element is connected to the GQFF output stage to mask the SEU and thus maintain the correct output. The proposed GQFF tolerates both single node upset (SNU) and double node upset (DNU). It also achieves low power. To prove the efficacy, GQFF and the existing FFs are implemented in 45 nm Complementary Metal Oxide Semiconductor (CMOS) technology. From the simulation results, it may be noted that the GQFF is 100% immune to SNUs and 50% immune to DNUs.

Abstract A new Lyapunov‐based adaptive controller (LBAC) for a single stage three‐phase grid‐connected PV system (GCPVS) is proposed here. The uncertainties in solar irradiation affects the performance of the GCPVS. Further, ageing in the DC‐link capacitor affects the PV voltage tracking. To achieve an improved performance of the GCPVS under both high and low irradiation, and under an aged DC‐link capacitor, a new LBAC scheme has been proposed to control the PV voltage. In this LBAC, the gains of the controller are adapted online by a Lyapunov‐based adaptation mechanism to provide the nominal tracking response of PV voltage. Further, a maximum power point tracking algorithm and grid current controllers are designed to transfer the maximum PV power to the grid, ensuring good power quality. As a result, improved performances of the GCPVS are achieved in terms of low PV voltage ripple, enhancement of delivered grid energy, and low grid current total harmonics distortion (THDs). Computer simulation studies are carried out in MATLAB/Simulink environment to verify the effectiveness of the proposed LBAC. Further, experiments are performed in a 2.5 kW GCPVS prototype for real time implementation of the proposed controller. The total harmonics distortion (THD) of grid currents is found to be within IEEE‐1547 standards.

A three‐phase to single‐phase direct AC power generation system based on the open‐winding permanent magnet synchronous generator (PMSG) with the dual‐inverter is proposed in this study for the standalone power system. The negative DC terminals of the dual‐inverter are commonly connected, and the positive DC terminals are reconstructed as the output port of the single‐phase AC power; the three‐phase AC power generated by PMSG could be converted to be the high power quality single‐phase AC power directly without any LC filters. Different from the traditional OW‐PMSG system, zero‐sequence current is employed to achieve adjustable frequency and voltage of the single‐phase AC power, and the corresponding dual voltage‐loop control strategy with the separate positive‐sequence and zero‐sequence currents regulation of PMSG is given to achieve high output performance. The effectiveness of the proposed control strategy and the output characteristics have been verified by simulation and the experiment of the prototype, and the results show that the high‐quality single‐phase AC power can be obtained at a wide speed range and at different load operating conditions. [ABSTRACT FROM AUTHOR]

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Abstract This study proposes a generic mathematical modelling and decoupling fault‐tolerant vector control for dual three‐phase permanent magnet synchronous machine (PMSM) with one phase open based on the conventional dual three‐phase voltage source inverters, accounting for the mutual coupling between two sets of three‐phase windings and the second harmonic inductance. When the dual three‐phase PMSM has one phase open, the permanent flux‐linkages are asymmetric and there are second harmonic components in the conventional synchronous reference frame (dq‐frame). Based on the proposed mathematical modelling, both permanent magnet flux‐linkages and currents become DC values in the dq‐frame, and therefore, the conventional proportional integral (PI) controller can be used to regulate the dq‐axis currents. Then, a decoupling fault‐tolerant vector control with/without a dedicated feed‐forward compensation is proposed to validate the correctness of the proposed mathematical modelling. Experimental results on a prototype dual three‐phase PMSM with one phase open show that the second harmonic dq‐axis currents can be well suppressed simply by the conventional PI controller and dedicated feed‐forward compensation. It also shows that the decoupling fault‐tolerant control based on the proposed modelling and control method has excellent dynamic performance, which is equivalent to the vector space decomposition control for the healthy machine.

Abstract In normal operation mode, sensorless speed‐controlled motor drives should be more reliable, less expensive and less bulky than their counterparts with a speed sensor. The sensorless drive structure allows avoiding the electromagnetic and vibratory sensitivity of the mechanical sensor, the additional connecting circuits, additional space for the sensor, its price and maintenance requirements. However, in postfault operation mode, the robustness of speed sensorless control strategies has been poorly addressed in the study. Accordingly, this study deals with an experimental study of the robustness and the availability of speed‐controlled induction motor (IM) drives without speed sensor in the presence of inverter open‐switches faults. For that purpose, a super twisting algorithm‐based observer for motor speed estimation is proposed for experimental investigation. Experimental results are performed upon 3‐kW IM drive by using dSPACE digital signal processing controller board. Experimental investigations evaluate the performances of the method used for the control without speed sensor in prefault, postfault and regeneration operation modes.

Abstract Model predictive fault‐tolerant current control (MPFTCC) of permanent magnet synchronous generator (PMSG) drives can make a valuable contribution to improving the reliability and availability levels of wind turbines, because back‐to‐back (BTB) converters are prone to failure. However, MPFTCC suffers from excessive computational burden, because the BTB converter is treated as one system where all feasible voltage vectors (VVs) are used for prediction and evaluation. Accordingly, a computationally efficient MPFTCC algorithm for a PMSG drive is developed and proposed with the ability to handle insulated‐gate bipolar transistor open‐circuit faults. The candidate VVs of both machine‐ and grid‐side converters are separately predicted and evaluated, which significantly reduces calculation effort. The proposed reconfigurable converter is a five‐leg power converter with a common leg that connects the generator first phase to the grid three‐phase, ensuring proper postfault reconfiguration of the grid‐side inverter. Moreover, a three‐switch rectifier is adopted to achieve fault tolerance of the PMSG‐side rectifier. Performance of the considered MPFTCC strategies is evaluated by experimental means.