Search

In this article, the soft-switching technique is an important way to achieve high frequency and high conversion efficiency for transformerless grid-connected inverters (TLIs) with distributed photovoltaic generation. Meanwhile, new technical standards require TLIs to have a wider power factor operation range to support grid voltage, which means that there are positive power regions and negative power regions in a grid period. In order to fulfill soft-switching for high-frequency working switches during all power regions, a pair of zero-voltage-transition (ZVT) resonant cells with different capacitor connections are first derived in this article, and two equivalent two-quadrant resonance networks are originated by using the pair of ZVT resonant cells alternatively. Furthermore, defined connection points of the obtained two-quadrant resonance networks in TLIs are explored, and a guideline to invent new ZVT-TLIs is summarized. As a result, two families of ZVT-TLIs with full power factor range (ZVT-TLIs-FPF) are obtained. In order further to verify the correctness of the proposed guideline, the obtained ZVT-HERIC-FPF is taken as an example to analyze its operation principle. Finally, experimental results from a 1 kW prototype at 100 kHz are provided to verify the effectiveness of the proposed structuring method. [ABSTRACT FROM AUTHOR]

For current-controlled grid-connected inverters with LCL filter, the essence of grid voltage full feedback is to improve the output impedance of grid-connected inverters to infinity, which can eliminate the influences of grid voltage distortion and the non-negligible grid impedance. However, existing full feedforward strategies are complex and control delay cannot be compensated precisely, which leads to the instability of system when the full feedforward of the voltage at the point of common coupling under the weak grid is adopted. This article first proposes a simplified full feedforward strategy based on grid-side inductor volt-age differential feedback active damping (GIVD-AD). Second, repetitive prediction (RP) is used to compensate for the control delay. Then, a virtual impedance branch is added and an adaptive algorithm for modifying virtual impedance based on the difference between predicted grid voltages by Newton interpolation prediction (NIP) and RP is proposed. With the proposed method, grid-connected inverters can work stably when grid impedance changes suddenly and exhibit strong rejection ability against grid-voltage harmonics. Finally, simulative and experimental results from a 3-kW single-phase grid-connected inverter are provided to prove the effectiveness of the proposed strategy. [ABSTRACT FROM AUTHOR]

Soft-switching technique contributes to higher conversion efficiency and switching frequency for transformerless grid-connected inverters (TLIs), which is beneficial to improve power density and reduce cost. However, existing soft-switching TLIs can only operate with unity power factor; as a consequence, they could not supply reactive power required by newly technical standards. In order to solve this issue, a novel zero-voltage-transition (ZVT) scheme has been proposed in this article, and two resonant tanks with a compact structure are applied in H6-type topology. The derived ZVT inverter enables high-frequency switches of both positive and negative power regions to be turned on and off under zero-voltage conditions, as well as all auxiliary switches can be turned on with zero-current switching. Meanwhile, unipolar differential-mode voltage and constant common-mode voltage characteristics are also maintained. Finally, comprehensive experiment results from a 100-kHz/1-kW prototype are provided to verify the correctness of the proposed inverter. [ABSTRACT FROM AUTHOR]

Transformerless grid-connected inverters (TLI) feature high efficiency, low cost, low volume, and weight due to using neither line-frequency transformers nor high-frequency transformers. Therefore, TLIs have been extensively investigated in the academic community and popularly installed in distributed photovoltaic grid-connected systems during the past decade. This article analyzes and summarizes the state of the art of TLI techniques, three rules of maintaining constant common-mode voltage (CMV) of TLIs at switching frequency have been concluded from a generic CMV analysis model at the beginning. Second, suppression methods of leakage current (LC) and dc current injection (DCCI), so-called two key challenges for first-generation TLIs, have been classified and discussed in detail, respectively. Finally, future trends and some challenges of TLIs based on wide bandgap devices named second-generation TLIs have been presented in this article. [ABSTRACT FROM AUTHOR]

Soft-switching techniques of transformerless photovoltaic grid-connected inverters (TLIs) can significantly reduce switching losses, as well as soften switching processes. Conventional dc–ac soft-switching configurations proposed by Dr. Deepakraj M. Divan are invalid in TLIs because of the leakage current problem. In order to develop soft-switching techniques in TLIs, this paper proposes a new soft-switching configuration and a procedure to guide the invention of soft-switching TLIs. First, this paper proposes two basic resonance tanks related to dc bus polarities, and then, uses these basic tanks to elevate four popular full-bridge-type TLIs according to the proposed guideline. As a result, four soft-switching TLIs are gained. Second, this paper picks obtained soft-switching highly efficient and reliable inverter concept (HERIC) as example to analyze its soft-switching operation principle and performance. As a consequence, all active switches of the gained soft-switching HERIC circuit are switched under both of zero-current turn-on and turn-off conditions; the reverse recovery problem of freewheeling diodes is alleviated owing to the zero-current turn-off of diodes; meanwhile, the common-mode voltage at the switching frequency scale is still constant. Finally, some experimental results from a 3-kW universal prototype at 50-kHz switching frequency are provided to verify the effectiveness of the main contributions of this paper. [ABSTRACT FROM AUTHOR]

Unipolar sinusoidal pulsewidth modulation (SPWM) full-bridge inverter brings high-frequency common-mode voltage, which restricts its application in transformerless photovoltaic grid-connected inverters. In order to solve this problem, an optimized full-bridge structure with two additional switches and a capacitor divider is proposed in this paper, which guarantees that a freewheeling path is clamped to half input voltage in the freewheeling period. Sequentially, the high-frequency common-mode voltage has been avoided in the unipolar SPWM full-bridge inverter, and the output current flows through only three switches in the power processing period. In addition, a clamping branch makes the voltage stress of the added switches be equal to half input voltage. The operation and clamping modes are analyzed, and the total losses of power device of several existing topologies and proposed topology are fairly calculated. Finally, the common-mode performance of these topologies is compared by a universal prototype inverter rated at 1 kW. [ABSTRACT FROM AUTHOR]

The current-voltage-fed bidirectional dc-dc converter, which refers to a current-fed inverter at low voltage side and a voltage-fed inverter at high voltage side, can realize zero voltage switching (ZVS) for the switches with the use of phase-shift (PS) technology. However, the current-fed switches suffer from high voltage spike and high circulating conduction loss. In order to solve these problems, a novel phase-shift plus pulsewidth modulation (PSP) control ZVS bidirectional dc-dc converter is proposed in this paper. By adopting active clamping branch and PSP technology, the converter can realize ZVS for all switches in a wide range of load variation while input or output voltage varies. In addition, a novel control strategy with one port voltage regulation and another port current regulation is proposed to make energy bidirectional conversion freely. The operation principle is analyzed and verified by a 28V/270V conversion prototype rated at 1.5kW. [ABSTRACT FROM AUTHOR]

This letter proposes a fast current control scheme for ac drives by minimizing the latency from current sampling to the updation of a pulsewidth-modulation (PWM) command. By reconstructing the architecture of a linear controller, the minimal necessary steps to obtain the output voltage are deduced as the primary calculation, and the other computation work of the current loop is done in the postcalculation after updating the PWM command. With the proposed method, the computational delay is reduced with negligible loss in the available modulation index, and a higher stability margin or system bandwidth is realized. Also, the developed controller structure exhibits inherited antiwindup characteristics. The technique has been validated by experimental investigations on a permanent magnet synchronous motor. [ABSTRACT FROM AUTHOR]

The grid inverters play an important role in renewable energy systems, transferring the dc power to the ac grid. Galvanic isolation is usually needed between the dc source and the grid due to safety reasons and grounding leakage current mitigation. In this article, a soft-switching isolated grid inverter is proposed based on the current-fed matrix-type configuration. The topology of this converter is composed of a current-fed H-bridge on the side of dc source and a direct matrix converter on the grid side, which are linked by a high-frequency transformer. The current-fed H-bridge on the primary-side can smooth the dc-source current and control the dc-source current directly. The avoidance of dc-link energy-storage element in the single-stage matrix converter can prolong the system's life. A space-vector-modulation-based soft-switching scheme is proposed for the matrix converter on the grid side and a model-based commutation method is adopted in the current-fed H-bridge on the dc side. With the proposed modulation methods, the soft switching can be realized for all devices in the converter. Finally, a laboratory prototype of the proposed converter has been built to verify the effectiveness of the proposed method experimentally. [ABSTRACT FROM AUTHOR]

This article proposes an improved space vector modulation (SVM) for the current-source rectifier (CSR). Due to the overlap time during commutations of power switches, the pulsewidth-modulation (PWM) currents of CSR may suffer from a number of low-order (mainly fifth and seventh) harmonics, which seriously degrade the harmonic performance of currents and voltages on the grid side. Especially, the overlap-time effect becomes an important issue when the increased switching frequency is applied to CSR. To mitigate these low-order harmonics of the PWM converter currents resulted from the overlap-time effect, the relationship between the overlap time and the low-order harmonics in PWM currents and grid currents are analyzed at first. Then, the design process of the LC filter and the proposed SVM with optimal overlap-time distribution is presented in details. Finally, experimental results are conducted on a low power laboratory prototype to verify the effectiveness of the proposed method. [ABSTRACT FROM AUTHOR]

Brushless modular spoke type permanent magnet (MSTPM) machine has exhibited prior electromagnetic (EM) performance over conventional surface-mounted PM machines for in-wheel traction system in electric vehicles (EVs). However, analysis of thermal behavior of MSTPM machines is rather insufficient, even accurate thermal analysis is of significant importance due to poor heat dissipation condition inside the wheel hub. Conventionally, the losses produced in electrical machines by EM prediction is equivalent to the heat source in normal thermal analysis, and only the resultant thermal behavior is investigated. However, the reaction of temperature rising on the EM performance is neglected. In this article, a bi-directional coupled electromagnetic-thermal analysis method is proposed and carried out by a combination of lumped parameter thermal network (LPTN) model and finite element method (FEM), where both steady-state temperature distribution and transient-state temperature rise are investigated for MSTPM machines. By finite number of iterations between magnetic and thermal fields, the electromagnetic performance and thermal behavior can be predicted more accurately due to the coupling effect considered. The coupled model-predicted results are verified by 3D-FEM and experimental measurement, which shows that the proposed method has advantages in both computational efficiency and accuracy as well as can be applied to other electrical machines. [ABSTRACT FROM AUTHOR]

Thermal drift-caused current measurement offset error usually results in unfavorable torque ripples of fundamental frequency, and hence a compensation method for current offset error is proposed in this article, which is initially derived based on the nonsalient-pole machines and then extended to the salient-pole machines. By filtering the difference between the predicted currents from a simple nominal model and the measured currents, the offset error is directly estimated and then compensated with minimal effects on the current dynamics. It is proved that the estimation is accurate if the filter has unity gain at the frequency of the offset error. Therefore, the proposed method is free of the current regulator type and shows strong robustness against machine parameter deviations. It can also stably work even when the output voltage is saturated. Two kinds of filters are designed in the stationary reference frame, and the low-pass filter with low bandwidth is recommended for its easy implementation. Finally, the effectiveness of the proposed method is verified by simulations and experiments. [ABSTRACT FROM AUTHOR]