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Insulated Gate Bipolar Transistor (IGBT) module is one of the most prone-to-fail components in the wind power converter. The reduction of the junction temperature swing by thermal management can improve the reliability of a wind power converter. Due to the lack of an applicable selection criterion, not many studies on the control target of junction temperature swing were reported. This paper proposes a thermal management strategy to find the control target of junction temperature swing. The proposed strategy considers the return on investment (ROI) of thermal management as a quantitative indicator to guide the control target selection. The selected control target enables optimal benefit of the thermal management. Results show that a 1%-increase in the total harmonic distortion would extend the lifetime of IGBT module by 0.144 years. Success of the proposed scheme lies in the efficient regulation of the junction temperature swing. Case studies with different mission profiles and experimental tests on a three-phase two-level inverter are presented to validate the proposed method.

Multi-terminal modular multilevel converter (MMC) power transmission is the current development trend in DC transmission. However, owing to its fast control and dynamic characteristics, it may interact with the other inductive devices or the grid and cause stability issues. This paper describes the use of the impedance analysis method in the frequency domain to analyze system stability. First, the multi-harmonic linearization method is used to model the impedance of a three-terminal MMC system. To make the model more accurate, the modeling considers the effect of frequency coupling caused by an asymmetric controller and the AC sampling site position. This paper then proposes an equivalent method of the grid impedance for the three-terminal system that can accurately judge the stability of the grid-connected system when the AC side is coupled. This work provides a theoretical basis for the prediction and suppression of the oscillation of MMC grid-connected system with coupling on the AC side. Finally, a three-terminal MMC model is built in Matlab/Simulink and the Nyquist stability criterion is used to verify the correctness of the analysis.

In order to further improve the efficiency of dual-active-bridge (DAB) converter, a hybrid five-variable modulation scheme with minimal root mean square (rms) current is proposed in this article, which combines the traditional phase-shift and pulsewidth modulation schemes. By adopting the proposed optimal scheme, a wider zero-voltage switching (ZVS) operation range and lower rms current level are achieved simultaneously when compared with other modulation schemes, which significantly improves the efficiency of the DAB converter. Moreover, for the complexity of the proposed modulation scheme with five variables, the Fourier decomposition based analytical expressions are derived for the converter and a generalized performance analysis results are presented. Besides, in order to avoid the analysis error, especially at light-load condition, the optimization strategy takes the rms current as the optimal objective, which includes the eight harmonic component of the inductor current rather than just considers the fundamental component. And an offline particle swarm optimization algorithm is introduced to acquire the optimal operating points for the whole power range. To substantiate the effectiveness of the proposed modulation scheme, a comprehensive comparison among related works in terms of the rms value and ZVS range were carried out under various operation condition. Finally, a 1.2 kW experimental platform was built and the highest experimental efficiency under the proposed modulation scheme reaches 96.8%.

This paper presents an in‐depth analysis for the two‐variable impedance model of a grid‐connected system. The two‐variable model describes the impedance using the frequency as well as the operating point variable. The impedance‐based stability analysis method cannot be directly applied because the conventional models are derived for a specific steady‐state operating point and the operating point varies during operation. Thus, this paper develops a new analysis method to shed light on the role the two‐variable model plays instability analysis. The proposed method not only analyzes the system stability at different operating points, but also enables the controller design to ensure system stability. Experimental results are presented to demonstrate the effectiveness of the proposed method.

Asymmetric duty modulation (ADM), as a recently proposed modulation scheme, provides new chance to improve the performance of dual active bridge (DAB) converter. On this basis, an optimal ADM (OADM) scheme is proposed in this article to minimize the inductor peak-to-peak current over the whole voltage and power range. First, the DAB converter adopting ADM scheme is analyzed in frequency-domain and time-domain, and four operating modes are established. Then, the minimum peak-to-peak inductor current for each operating mode is obtained under Karush–Kuhn–Tucker condition, and the corresponding optimal closed-loop control with low calculation complexity is derived. Then, the key values and features of the OADM scheme are compared with recent state of the art, mainly including the inductor root mean square current and soft-switching performance. Finally, an experimental prototype was built to validate the efficiency enhancement of the converter by adopting the proposed OADM scheme.

Grid‐connected inverter have been extensively used in the renewable energy grid‐connect systems, such as solar and wind. Interaction between the grid and the inverter may generate harmonic resonances that lead to reduced power quality and even instability. To analyse the stability of the inverter‐grid system using impedance‐based stability criterion, inverter impedance has to be known. Impedance measurement in the dq‐domain requires the precise synchronous phase angle to obtain accurate measurement. For impedance measurement in the stationary frame, inaccuracy usually results from not considering the effects of grid impedance and the frequency coupling. A new impedance measurement method in the stationary frame is proposed to obtain accurate measurement. The proposed method takes into account the grid impedance and its interaction with the inverter. Thus, it overcomes the limitations of current impedance measurement methods. More important, the proposed method enables accurate stability analysis results, whereas current methods may fail. Simulation and experimental results are presented to demonstrate the effectiveness of the proposed measurement method.

Wide bandgap (WBG) devices offer the advantages of high frequency, high efficiency, and high power density to power converters due to their excellent performance. However, their low parasitic capacitance and fast switching speed also make them more susceptible to switching oscillations. The switching oscillations can cause voltage and current overshoots, shoot-through, electromagnetic interference, additional power loss, and even device damage, which can seriously affect the performance of power converters and systems. However, a comprehensive and in-depth overview is lacking on this topic. This article reviews the types, the causes and negative effects, the effects of parasitic parameters and suppression methods of these switching oscillations, which is helpful for practical engineering. First, the switching oscillations are divided into different types, and their causes and negative effects are reviewed. Then, the effects of different parasitic parameters on the switching oscillations are overviewed. It is found that due to the different physical structures of silicon carbide metal-oxide-semiconductor field-effect transistors, enhancement-mode gallium nitride high-electron mobility transistors (eGaN HEMTs), and cascode GaN HEMTs, the effects are also different. Finally, the main methods of suppressing the switching oscillations are summarized, and the advantages and disadvantages of these methods are presented. Furthermore, future research works on this topic and the conclusion of this paper are drawn, which will help readers deepen their understanding of the switching oscillations of WBG devices, and inspire readers to better use WBG devices for high-frequency and high-efficient power conversion.

Multiple output LED drivers are necessary to achieve better performance and higher reliability in street lighting, tunnel lighting and LCD background lighting applications. Based on LCL-T constant-current characteristics, a modular multiple output LED driver is proposed in this paper. The LCL-T rectifiers are connected to the same voltage bus and work like current sources. However, since there are much higher voltage harmonics in the AC square bus voltage, it is very important to quantitatively analyze how these harmonics influence the precision of the output currents. In addition, all of the switches can achieve Zero Voltage Switching (ZVS) by proper design of the LCL-T network. Finally, a 200-W prototype with five channels is established with an efficiency of 92.25%.

Grid-connected current quality is seriously affected by the low-order background harmonics in the gird-inverter dynamic system. The point of common coupling voltage feedforward impedance reshaping method of the grid-connected inverter is generally applied to solve this problem, however about 90° phase lag of the inverter output impedance is brought in, which would deteriorate the adaptability of the inverter to the weak grid. To solve this problem, this article proposes a novel frequency-division virtual impedance shaping method, which shapes the output impedance of the grid-connected inverter with different coefficients in different frequency bands, respectively. The basic principles, parameters design, and application analysis are studied in detail. With the proposed scheme, the grid-connected inverter can be equipped with strong adaptability to a wide range of grid impedance and suppression ability to the background voltage harmonics simultaneously. Theoretical analysis and experimental results confirm the effectiveness of the proposed method.

The interaction between wind farm and power grid may cause the stability issue in the wind farm integration system. Practically, the integration system contains many collector feeders, and one feeder connects a large number of wind turbine generators. Moreover, frequency coupling effect is introduced in all parts of the wind farm system due to the asymmetrical controller of the wind turbine converter. So, it is complicated to analyse the stability of the whole integration system. This study proposes the modular admittance model for wind farm, which is easy to expand for modelling and analysis of the whole integration system. In addition, the operation condition of wind turbine generators in wind farm varies with the prevailing wind speed. The system stability may alter constantly according to the wind speed. How to effectively evaluate the system stability in the entire wind speed operation space is also a challenge. Thus, for wind farm system, this study develops an approach with two-variable admittance model and criterion based on magnitude and phase contours plot to overcome this problem. The proposed admittance model and stability analysis approach are verified by simulation.

The LLC resonant converters commonly adopt frequency modulation (FM) or combination of FM with phase-shift modulation to regulate its output voltage. However, in these control schemes, a variable switching frequency range is required, which makes the magnetic components design complicated. Therefore, in this article, magnetic control (or variable inductor control) is adopted to make the converter operating at constant switching frequency and constant duty cycle. The fundamental harmonic analysis is commonly used because of its characteristic of simplicity. However, the accuracy of this method is reduced and considerable errors occur when the switching frequency or output power changes. Therefore, an optimal design methodology based on time-domain analysis of the LLC resonant converter with magnetic control is proposed in this article. The proposed methodology can assure that the converter will operate in PO or OPO modes within the whole operating range, and zero voltage switching operation for primary switches and zero current switching operation for secondary rectifier will be guaranteed. In addition, by limiting the resonant tank root-mean-square current, the system efficiency is improved. A 200-W experimental prototype is built and the effectiveness of the proposed optimal design methodology is verified.

This paper presents a new power control strategy for the grid-connected converter under grid faults. Positive- and negative-sequence reactive powers are controlled separately to effectively support the unbalanced grid voltages. Both maximum phase current limit and maximum active power oscillation limit are considered to make sure that the converter operates safely under grid faults. The spare capacity is fully used to generate active power. Simulation and experimental results are presented to demonstrate that the reactive power is generated immediately and a gradual generation of active power reduces the load of crowbar or other energy transfer devices.

Recently, wind power technology has been developed rapidly. The interaction between the wind turbine generator and the grid may cause the stability issue and threats the reliable operation of the power system. So, it is vital to analyze the stability of the wind power grid-connected system. Due to the fluctuation of wind speed, the operation characteristics of the system vary, and the system exhibits different stability characteristics at the varied operating points. Thus, it is important to analyze the system stability in the entire operation space. The impedance-based analysis is an effective tool for the stability analysis of the grid-connected system. However, it is difficult to apply the conventional impedance/admittance model to characterize the system performance due to the fact that the system operating point often varies continually. To overcome this challenge, this paper proposes a two-variable admittance model that incorporates the wind speed into the conventional admittance model. In addition, a new stability criterion based on the magnitude and phase contour plot is developed to enable the stability analysis. Simulation and experimental results are presented to demonstrate that the proposed two-variable admittance model and the stability criterion are effective tools for stability analysis of the wind power grid-connected system.

Thermal management based on the regulation of the switching frequency brings benefits to the lifetime extension of the insulated gate bipolar transistor (IGBT) module and drawbacks to the total harmonic distortion (THD). To make a tradeoff between the benefits and drawbacks of thermal management, an optimization strategy based on the benefit-cost ratio (BCR) is to provide a criterion for selecting the switching frequency of thermal management in photovoltaic systems. Each 1% increase in THD can extend the lifetime of the IGBT module as long as possible under the allowed conditions. Case studies and experimental tests are performed to verify the effectiveness of the proposed strategy.

Grid-connected inverters are widely used in the renewable energy such as PV and wind power grid-connected systems. However, actual application results show output power of inverters has an impact on the stability when the system is weak grid. To overcome the stability problem, some control strategies have been proposed. However, few control strategies take into account dynamic response of the system. Moreover, some may cause degradation of dynamic performance like the adaptive control that the PLL bandwidth is reduced to improve system stability. Hence, this paper proposes a new control strategy, which is based on the conventional PLL structure. PLL bandwidth is increased under the proposed strategy when the output power is increased in weak grids. Further, the proposed control strategy enables fast dynamic response of PLL. Moreover, the system can operate stably in the entire output power range. Experiment results are presented to demonstrate the effectiveness of the proposed method.

The impedance-based analysis method is an effective way to study the small-signal stability of the grid-connected converter system. A valid impedance model is the premise of the accurate stability analysis. In this study, the impedance model of the grid side sampling modular multilevel converter (MMC) is established both in AC and DC side. Compared with the existing work, the effect of the transformer impedance, ratio and group on the impedance characteristic is considered in modeling, which improves the accuracy of model. the effect of transformer parameters and types on MMC impedance characteristic is analyzed, after verifying the valid of the model in Matlab/Simulink. The stability analysis results show that the model can provide a theoretical reference for the impedance modeling and analysis of the grid side sampling MMC.

Aluminum electrolytic capacitor (AEC) is one of the weakest components in power electronic converters. As the degradation of AEC happens, its equivalent series resistance (ESR) increases and the capacitance ( $C)$ decreases. Therefore, the online monitoring of ESR and $C$ to predict AEC’s life has great significance for ensuring safe and reliable operation of converters. In this article, an online monitoring scheme is proposed for buck converters, aiming to estimate the ESR and $C$ of AEC at the output side. The proposed scheme utilizes large-signal load transient trajectories to estimate the AEC parameters and has a relatively low sampling frequency. By analyzing the relationship between the transient trajectory and capacitor parameters, the ESR is directly calculated using the voltage and current step values at the initial instant of the transient. Furthermore, $C$ is calculated utilizing a calculation model derived from the output-voltage load transient trajectory. Corresponding simulation analysis and an online monitoring system implementation are provided. Furthermore, a 48–12-V buck converter with a digital proportional-integral (PI) controller and an analog $V^{2}$ controller is built to verify the proposed online estimation method. The experimental results of the estimated ESR and $C$ are consistent with the results measured by an LCR meter, and the estimation error is less than 10%.

High step-up boost converters with coupled inductor have attracted much attention in the fuel cell or photovoltaic grid-connected generation system, however, there are few literatures elaborated on the construction ideas and derivation methods of them. Accordingly, in order to obtain a clear roadmap on the derivation and inner connection of these converters, a comprehensive review and analysis are presented in this paper. First, the basic boost converter with coupled inductor is regarded as the basic topology, and its merits and demerits are analyzed in detail. Then, in order to address these demerits, various step-up techniques are introduced, such as the rectifier circuit, the active-clamped circuit, the multi-winding coupled inductor, and the voltage doubler rectifier; and numerous new topologies are continuously proposed by combinations and equivalent simplifications. In addition to a detailed synthesis of each topology, a comparative and quantitative analysis among some important converters is presented, and the optimal one is chosen to build a 250 W prototype. Finally, based on comparisons and analysis, the main characteristics and inner connections of these high step-up boost converters with coupled inductor are identified and clarified.

This paper presents a multi-period frame transient switching control (MP-TSC) scheme for buck converter with a controlled coupled inductor, aiming at improving the load transient response. The proposed buck converter utilizes a controlled coupled inductor as the output inductor. During a load transient, the coupled inductor is controlled as a small equivalent inductance so as to achieve fast transient regulation. While in the steady state, the coupled inductor behaves as a large inductance to reduce the output current ripple. Furthermore, on the basis of the proposed variable inductance circuit, an MP-TSC control scheme is proposed and implemented in a digital form. With the proposed control scheme, the load transient event is divided into n + 1 sub-periods, and in each sub-period, the capacitor-charge balance principle is used to determine the switching time sequence. Moreover, the control principle design and transient response index analysis are provided. Furthermore, its feasibility is validated in simulation and experiment with a 12–3.3V low-voltage high-current synchronous buck converter. Experimental results demonstrate that the voltage deviation of the proposed MP-TSC scheme (coupled coefficient of the controlled coupled inductor is 0.52) has improved by more than 79% compared to that with the auxiliary circuit disabled.

Active damping (generally by the feedback of capacitor current) is an effective solution to the resonance peak of LCL -type grid-connected inverters. However, the damping performance is significantly affected by the computation and pulse-width modulation delays in the digital control system, which results in poor system robustness. Moreover, the phase lag would be introduced by these control delays, leading to the constrained bandwidth and low-frequency gain of control loop. In this study, a novel compensation method based on non-instantaneous loading and pulse-width equivalence is proposed to completely eliminate the computation delay without any reliability problem. Moreover, compared to the traditional strategies, the proposal can be extended to most two-level converters because of its simplicity. Finally, the validity of the proposed method is confirmed by the experimental results from a 3 kW single-phase grid-connected inverter with an LCL filter.

Grid-voltage feedforward control is widely used in grid-tied converters. The reported studies show that the grid-voltage feedforward may reduce the system stability margin, and may lead to oscillation problem in renewable power generation system. This study revisits the effect of voltage feedforward control on system stability and especially focuses on low frequency oscillation with impedance-based method. The low frequency oscillation usually happens in industry around 100 Hz and shows frequency coupling characteristics. A converter equivalent impedance which includes the effects of voltage feedforward and also covers the frequency coupling introduced by phase locked loop (PLL) is established. Based on the built impedance, the effect on oscillation mechanism with voltage feedforward and PLL is revealed. The results show that the voltage feedforward could increase the system stability margin and may be employed to improve the system stability. Then how to design an optimal proportional voltage feedforward control is demonstrated. Experimental results verify the theoretical analysis and the design of the proportional grid-voltage feedforward control.

Admittance matrix has been used to represent the characteristics of three-phase inverter for grid-inverter system stability evaluation in both stationary, sequence, and dq domain. For the dq domain admittance matrix, the PLL and Park transformation introduce additional dynamics during the admittance measurement that may influence the application of dq model. For the stationary and sequence domain admittance matrix, the model measurement is not affected by the above additional parts but may also encounter other difficulties in actual measurement. First, current frequency response analyser (FRA) is not able to perform two-frequency measurement required by the admittance matrix. Second, practical limitations exist in the course of solving complex-valued admittance matrix equations. Here, a single stationary domain equivalent admittance of three-phase inverter for grid-inverter system with considering the interaction between grid and inverter is defined so that the above two difficulties can be avoided. Then, an equivalent admittance model of prototype inverter is constructed for stability analysis of the grid-inverter system using Nyquist stability criterion, instead of the generalised Nyquist criterion (GNC). The effectiveness of the proposed equivalent inverter admittance model is validated through simulation and experiment verification.

The dc-link capacitors are important components that tend to be one of the weakest part of power converters. Thus, it is necessary to monitor the aging parameters of dc-link capacitors in valuable power electronic systems. This paper proposes a simple yet effective variable electrical network (VEN) condition monitoring method for dc-link capacitors in three-phase pulsewidth modulation (PWM) ac–dc–ac power converters. The capacitance ( C ) and equivalent series resistance of dc-link capacitors are estimated through the designed VEN unit during the discharging process. This VEN method does not need the current sensor or injecting signals into the control loop. The above property is favorable for the hardware and controller design. Besides, the designed monitoring circuit can be an external unit or a built-in unit which offers a flexible solution for electrical devices. When the designed VEN is used as an external unit, neither the hardware design nor the programs in the controller needs to change, which is practical and economic for electrical devices in use. Experiments are carried out in a 55 kW ac–dc–ac inverter with different capacitors. The experiment results prove the method to be effective and accurate.

As the penetration rate of distributed energy in the power system continues increasing, the stability of the power system is affected by more factors. However, the small signal modeling method used for system stability analysis lacks a consensus. In this paper, the voltage source converter (VSC) system is used as an example to discuss the pros and cons of two methods. Building the full-order state-space model in time-domain and in frequency-domain, the impedance model has also been built. The time-domain system simulation has developed to verify the accuracy of two models in MATLAB/simulink. On the basis of the existing models, the impact of the system parameters will be analyzed.

Selective harmonic elimination pulse width modulation (SHEPWM) is a mainstream choice for low switching frequency applications. To realize the entire speed range operation of driving systems, switching between different modes is inevitable. Three-level half-wave symmetric SHEPWM has more control degrees of freedom to deal with various problems, but the phase distribution and switching angle are quite different from quarter-wave symmetric SHEPWM. Thus, some conventional smooth switching methods can not be directly applied to three-level half-wave symmetric SHEPWM to avoid the current impact problem. In this paper, by analyzing the harmonic characteristics of half-wave symmetric SHEPWM and smooth switching conditions, the proper simultaneous switching point is derived, and then a multi-mode hybrid modulation strategy based on half-wave symmetric SHEPWM suitable for three-level converters is obtained. It can avoid the current shock at the switching time under various power factor and modulation ratio. Some numerical analysis and simulation are carried out to prove the correctness and feasibility of the proposed strategy.

Parallel operation of IGBT module is an attractive solution to achieve high power capacity of power converters due to the physically limited current rating of available IGBT modules. Due to the stable operation of parallel-connected IGBT modules, thermal coupling effects among parallel-connected IGBT modules exist. However, current thermal resistance monitoring under electro-thermal coupling operation condition considering the coupling effect are still not specific fully investigated, which may lack of confidence in thermal resistance monitoring results. This paper presents a study for determination the thermal resistance of parallel-connected IGBT modules under electro-thermal coupling operation condition. The experimental results reveal that thermal resistance monitoring of IGBT module in parallel connection under more practical coupling operation condition are influenced by multiple influencing factors including power loss distribution, external cooling condition, thermal path degradation et.al, which is challenging for current monitoring methods. It is suggested that multiple coupling factors should be taken into consideration for robust thermal resistance monitoring.

System stability issues occur when the VSC-HVDC system is connected to the weak grid. This phenomenon may be caused by the AC and DC dynamics characteristics between the converters in the system. Although it is convenient to use the DC transmission line and the large capacity DC filter to decouple the two AC systems from each other or to make the DC side of the system equivalent to a constant voltage source and so on to facilitate the model analysis and reduce the model order, the AC and DC dynamic characteristics between the converters will be neglected. The AC and DC dynamic characteristics can lead to inaccurate results when analyzing system stability, and cannot provide theoretical guidance for system control or stability prediction after short circuit ratio changes. Therefore, this paper establishes the equivalent DC impedance model with the AC side dynamic characteristics including frequency coupling and the equivalent AC side impedance model with the DC side dynamic characteristics. Through the analysis of the equivalent system impedance model, it is concluded that the system impedance model considering frequency coupling is more accurate to analyze the system stability. The conclusion is verified by Matlab / Simulink simulation. The conclusion is validated by Matlab/Simulink simulation.

Grid-connected inverters are widely used in renewable energy systems. Interaction between the grid and the inverter, however, may generate harmonic resonances that leads to reduced power quality and even instability. To analyze the stability of the inverter-grid system using impedance-based stability criterion, inverter impedance has to be known. This paper proposes an inverter impedance estimation method, which takes into account grid impedance in inverter-grid system and all entries in the impedance matrix. Further, the accurate inverter impedance is obtained. Simulation and experimental results are presented to demonstrate the effectiveness of the proposed method.

As a promising control method of permanent magnet synchronous motors (PMSM), the finite control set model predictive control (FCS-MPC) can overcome the integral saturation and bandwidth limitation of proportional-integral controllers. However, the weight design of the cost function is indeed a serious subject, which directly affects the control performance of FCS-MPC and should be carefully considered. Currently, the weight design lacks a theoretical basis and has a strong randomness. In this paper, a novel weight design of FCS-MPC based on frequency domain response characteristics is proposed for PMSM driving systems. Different from the conventional experience-based weight design, this study extracts the frequency domain response characteristics of FCS-MPC, analyzes the complex relationships between the weights in the cost function and frequency domain response characteristics, and finally obtains new weight design rules. Some numerical analysis and simulation are carried on to prove the correctness and feasibility of the proposed weight design method.

In this paper, we propose a thermal management strategy to smooth the mission profile thermal cycle (MPTC) of power device in the wind power converter (WPC). The key of this technique lies in the established relationship between the working range of thermal management and the predefined ripple of MPTC. Advantages of the proposed control strategy over the existing methods are as follow. (1) This strategy only applies to the specified, not the whole, working range of the converter. As a result, normal operation of the converter will not be interrupted often. (2) This strategy enables effective ripple reducing in the MPTCs which have large temperature swings and then the economic efficiency can be improved greatly. Experimental tests on an inverter are presented to validate the effectiveness of this technique.

Injection of both positive and negative sequence currents is desired for three-phase voltage source converters (VSC) under unbalanced grid fault condition. However, the second-order oscillation exists in the positive and negative sequence synchronous reference frame. Several approaches have been reported to cope with this second-order oscillation. Here, a simple direct proportional plus integral current control method in double synchronous reference frame is proposed. The proposed method does not require the additional filter or decoupling circuit, and does not require signal reconstruction for the current reference. Theoretical analysis and experimental study show that the proposed current control not only achieves zero steady-state error, but also exhibits good transient response.

In recent years, solar photovoltaic (PV) power technology has been applied widely. The interaction between the PV generator and the grid, however, may cause the stability issue and oscillation in practical applications. Hence it is very important to analyze the stability of grid-connected PV system. Due to the fluctuation of solar irradiance, the operating characteristics of the grid-connected system alter, and the grid-connected system shows different stability at the altered operating points. Therefore, it is vital to analyze the grid-connected PV system stability at all operating points in the entire operation range. The impedance-based analysis method is a useful tool for stability analysis of the system. However, since the operating point of the system often alters continuously, it is difficult to use conventional impedance/admittance model to characterize the system performance. To overcome this drawback, this paper attempts to incorporate the solar irradiance into the conventional PV generator admittance model and develops a multivariable small signal admittance model for PV generator. Moreover, the PV cell temperature influence is also considered in the model. Further, a new analysis method is proposed to enable the stability analysis. Simulation and experiment were conducted to demonstrate the effectiveness of the model and the stability analysis method.

The power converter plays an important role in the energy conversion and management of wind turbine system. It is one of the core components in the system, but also one of the most prone to fail components according to industry-wide survey. Condition monitoring of power converter can improve the reliable operation of wind turbine system and reduce its unscheduled downtime. In this paper, a novel quasi-online monitoring technique based on the time-constants of heat sink temperature is proposed to check the health status of wind power converter. Advantages of the proposed technique include: (1) This approach enables the monitoring of power module degradation and cooling system deterioration in the converter simultaneously. (2) This method does not need to measure the junction temperature and power loss which reduces the cost and complexity of operation. Experimental tests were conducted to verify the effectiveness of this technique. Results also indicate that the maximum coefficient of variation of time-constants is below 1.7% under different converter operating conditions and ambient temperatures. In addition, the maximum error of time-constants in the application of condition monitoring is less than 3.1%.

The interaction between grid-connected inverters and the grid may cause stability issues, and compromise the reliable operation of the inverters. This study investigates the stability of a three-level neutral point clamped (NPC) inverter connected to the grid using impedance-based methods. Because the impedance model of a three-phase three-level NPC inverter has not yet been reported, this study fills the literature gap by analyzing the influence of three-level DC-side neutral point control on the impedance characteristics. By fully considering the DC bus dynamics and DC voltage control loop, and the current loop and phase-locked loop (PLL), the admittance model of a three-phase three-level NPC inverter is established and verified by simulation. Additionally, in the stability analysis of a three-level NPC inverter grid-connected system, the frequency coupling introduced by the PLL and DC bus dynamics is included with the help of an established admittance model. The stability of the grid-connected system under different grid short circuit ratios (SCR) and operating power levels is analyzed according to the Nyquist stability criterion. The experimental results revealed that the established impedance model of the three-phase three-level NPC inverter can properly represent the stability of this system.

As a key method to realize a high-performance control of IPMSM in six-step mode, which is of benefit to reduce the switching frequency and increase the output voltage, the conventional voltage phase controller can adjust the output torque by amending the voltage phase through an integrator. However, the assumption that the relationship between the torque and voltage phase is monotonically increasing, is not always correct. In fact, motor parameters may change the relationship, which results in the failure of driving the motor for the conventional voltage phase controller. In this paper, we analyze the effect of motor parameters on the monotonicity. Then, point out that the assumption in the conventional way is a critical limit for the voltage phase control of some motors. It limits the operation region seriously. After achieving the condition that can judge which case the relationship between the torque and voltage phase is in, a model based voltage phase controller in six-step mode for IPMSM is proposed. The operation region, which is not suitable for some conventional ways, can be approached well for the proposed method. Finally, some simulation results are also given to prove the correctness and feasibility of the proposed method.

When a T-type three-level converter suffers from switch open-circuit faults, it is not only necessary to ensure the three-phase symmetrical output voltages, but also the neutral point potential (NPP) oscillation must be solved. Compared to non-fault conditions, the loss of some switching states under switch open-circuit faults makes the NPP problem more serious. In this paper, an in-depth characteristic analysis on the NPP oscillation is carried out. Through a carrier modulation based analytical tool, some key conclusions that are quite different from NPP oscillation characteristics under non-fault conditions are obtained. These conclusions are very valuable for studying the NPP problem and its control methods under switch open-circuit faults. Then, a search optimization method based NPP control that is suitable for all kinds of switch open-circuit fault, is proposed for simulation verification of the correctness of the analysis and related conclusions in this paper.

When the impedance criterion is used to analyze the stability of the three-phase inverter-grid system, the impedance acquisition of the grid-connected inverter is particularly important. In the stationary reference frame, the equivalent admittance accurately characterizes the impedance characteristics of the inverter. But it varies with the grid impedance and contains the frequency coupling component. This makes it difficult to measure the admittance accurately by the off-the-shelf frequency response analyzer (FRA). In this paper, a method is proposed to estimate the inverter impedance, which takes into account the influence of grid impedance and the frequency coupling effect. Consequently, correct stability analysis of the grid-inverter system is guaranteed. Simulation and experimental results are presented to demonstrate the effectiveness of the proposed method.

Three-level inverters are adaptively used in renewal energy generation equipment, especially in high and medium power levels. The stability of three-level inverters is worth studying. In this paper, a diode clamped three-level inverter is studied. Based on the influence of DC side neutral point control on its impedance characteristics, the impedance model of neutral-clamped three-level inverters is established. Then, the stability of the inverters under different grid impedance and operating power is analyzed by using Nyquist stability criterion. The simulation results show that the impedance characteristics of three-level inverters can accurately judge the stability of the grid-connected inverters.

As a very promising technology of improving the steady-state and transient response of the torque, the finite-control-set model predictive control (FCS-MPC) of permanent magnet synchronous motors (PMSM) can easily handle the nonlinear problems when the motor needs to be driven in an over-modulation region. However, in order to achieve a similar steady-state as the conventional proportional integral (PI) controller in the linear modulation region and a higher output and faster response of the torque in the over modulation region, a huge vector set is necessary. Thus, we must prepare the vector set in advance and use it based on a look-up table (LUT). Since the calculation amount is so enormous, the online MPC is impossible, which limits its wide application. In this paper, a novel fast search theory is proposed to realize a high-speed and high-precision selection of the optimal vector from a huge vector set. Therefore, the online MPC becomes achievable with a relatively low calculation amount that is tolerated for some high-performance digital signal processing (DSP). Some simulation results are given finally to prove the validity and feasibility of the proposed method.

DC/DC converter and DC distributed power system (DC-DPS) have been widely used in many industrial fields. This paper systematically studies the rule and method of deriving the basic topologies of them. Starting with the overall structure of basic non-isolated DC/DC converters, it can be divided into three cascaded units. Each unit can further be divided into voltage-type (V-type) and current-type (C-type). Then, according to the basic circuit theory, four categories of basic non-isolated DC/DC converter generalized structures and twelve specific non-isolated DC/DC converter topologies are all derived out. Based on the same topology construction principle above, the basic DC V-type and Ctype distributed power system generalized structures are both established, which can be named DCV-DPS and DCC-DPS. On this basis, two practical DC-DPS topology structures are also given out as an application example. Finally, combined with high-power LED driving applications, a 90W prototype with three channel outputs based on the DCC-DPS topology structure is built and the experimental results are presented. It is proved that the proposed topology construction rule and analysis method are valid.

Impedance based criterion is proved to be efficient for stability analysis of interconnected system, yet few existing impedance models of grid-tied modular multilevel converter (MMC) take outer control loop and internal dynamics into consideration simultaneously, which may lead to wrong analysis results. After analyzing the relations between inner and outer loop, this paper establishes the AC side impedance model with DC voltage/power control as outer loop through multi-harmonic linearization. The simulation results indicate that the outer loop has significant impacts on the impedance. The established impedance model can precisely reflect the characteristics of internal dynamics and outer loop, which provides basis for accurate analysis and effective improvement of the stability of MMC.

A set of accelerated aging and automatic test system for IGBT modules is designed in this paper, including platforms of accelerated aging and parameter automatic test. Aging test platform is controlled by IPC, using DC power cycling methods and water cooling to heat dissipation. Combined with LabVIEW and high-speed digitizers, single pulse switch testing is achieved in automatic test platform through hardware circuit. In different stages of the IGBT module aging tests, change trend of each electrical parameters were measured under different working point. Collector-emitter conduction voltage drop VCE(sat), gate-emitter threshold voltage VGE(th), turn-off time toff, the current and voltage waveform during turn-on and turn-off are measured. The relationship between these electrical parameters and aging degree of the IGBT module is researched. It would be able to characterize the device and seek out external indicators of fatigue aging, reliable evidences for state assessment. Compared with the traditional manual testing methods, the automatic test system has characters: high accuracy, test speed, short development cycle and so on.

We propose a practical scheme to implement universal superadiabatic geometric quantum gates in decoherence-free subspaces in the trapped-ions system. The logical qubit is only encoded by two neighboring physical qubits, which is the minimal resource for the decoherence-free subspace encoding. Different from the nonadiabatic control in decoherence-free subspace (Liang et al. in Phys Rev A 89:062312, 2014), a new Hamiltonian to implement universal effective interaction between logical qubits is proposed in the scheme. The proposed gates are numerically demonstrated to be robust against both systematic errors and collective dephasing noises, which combine the advantages of superadiabatic geometric quantum control and decoherence-free subspace. Since the Hamiltonian we use relies solely on two-body interactions, our scheme would be promising to be realized experimentally in trapped-ions systems.

For a single converter, AC and DC are coupled to each other; for a converter cascade system, the converters interact with each other in some cases, which in turn affects the stability of the system. In this paper, the HVDC system is taken as the research object, and the harmonic linearization method is adopted to consider the interaction between the DC side and the AC side to analyze the stability of the cascade system. Experimental tests are performed to verify the effectiveness of the proposed method. According to the results, relevant measures to improve the stability of the cascade system are proposed.

The Phase-Locked Loop (PLL) plays an important role in stability of three-phase grid-connected inverter system. However, the existing literature all neglect the influence of PLL when analyzing the stability of multiple parallel grid-connected inverters system. So this paper builds the model of the system and analyzes the system stability with considering the influence of PLL. Besides, this paper demonstrates that there exist admittance coupling among multiple parallel inverters. Thus, owing to the coupling effect, it can not obtain the multiple parallel grid-connected inverters admittance by accumulating each inverter admittance in parallel directly. Modeling the inverter mutual coupling is presented in this paper. The correctness of the modeling approach and stability analysis are verified by experimental results with RT-LAB.

This paper presents an optimized active power control method for two-stage photovoltaic (PV) grid-connected inverter under unbalanced grid faults. The purpose of the control method is to reduce the active power deficit of grid by injecting the maximum active power of the PV system under the premise of ensuring the safety of the inverter itself. Safe operation region (SOR), which considers the limitation of DC bus overvoltage, overcurrent and the second-order active power oscillation faced by the inverter under unbalanced grid faults, of inverter is proposed to quantify the maximum active power that can be injected into the grid under grid faults. SOR is used to optimize the control of the PV inverter. The optimal control is performed with the relationship between the safe maximum active power of the inverter governed by SOR and the maximum power point of the PV panel. In high power condition, the output active power should be curtailed to the maximum output capability of the PV inverter. In low power condition, maximum phase current could be minimized for device thermal protection, or active power oscillation could be minimized for power quality improvement. Experimental results are presented to validate the effectiveness of the optimized active power control strategy.

Injection of both positive and negative sequence currents is desirable for three-phase voltage source converters under unbalanced grid fault condition. However, the 2nd order oscillation exists in the positive and negative sequence synchronous reference frame. Several approaches have been reported to cope with this 2nd order oscillation. In this paper, a simple yet efficient control method in double synchronous reference frame (DSRF) is proposed. The proposed method does not need additional filter or decoupling circuit, and does not require signal reconstruction for the current reference. Theoretical analysis and simulation study show that the proposed control method not only achieve zero steady state error, but also exhibits faster transient response. Experimental results verify the performance of the proposed current control method in DSRF.

In three-phase grid-tied inverter, grid voltage feedforward is helpful to improve grid-tied power quality for strong grid condition. This paper found that the voltage feedforward had different effect on power quality under weak grid condition compared with that in ideal grid condition. And recent reported studies show that grid voltage feedforward has negative effect for grid-inverter system stability. This paper analyzes the voltage feedforward effects on power quality and system stability, and proposes a proportional voltage feedforward scheme to improve system stability and reserve ability to improve power quality under weak grid condition. Then how to design the proportional voltage feedforward scheme is presented. Theoretical and experimental results verify the effectiveness of the proposed control method for improving system stability and power quality under weak grid condition.

The asymmetric inverter controller structure or parameters result in frequency coupling in phase domain for the three-phase grid-connected inverter system. And it is difficult to measure the inverter admittance matrix in engineering practice due to the coupling term. What's more, applying the coupled admittance matrix to analyze the system stability, the generalized Nyquist criterion (GNC) has to be used. Therefore, considering the frequency coupling due to phase-locked loop (PLL), this paper builds the single equivalent inverter admittance model of three-phase grid-connected inverter system. This single equivalent inverter admittance model is convenient for measurement in engineering practice. Besides, by applying this model, the system stability can be analyzed with Nyquist criterion. Compared with the approaches which use GNC, the stability analysis is simpler and it is easy to obtain the stability margin of the system to guide the controller design. The proposed model is verified by simulated measuring results. Experiment results prove the correctness of the stability analysis results.

As a basic building block, the phase-locked loop (PLL) has been widely used in the grid-connected voltage source inverter (VSI). But it has been shown recently that PLL has adverse impact on stability of grid-inverter system. In a weak grid, the PLL may cause unexpected harmonic, or even instability. In order to overcome this problem, this paper proposes a q-axis voltage feedforward control method to mitigate the impact of PLL. Specifically, the detailed analysis and implementation of proposed method are depicted. Finally, experimental results are presented to verify the proposed method.