Yibin Tong

Leakage current analysis of a single-phase transformer-less PV inverter connected to the grid

Due to the large surface of the PV generator, its stray capacity with respect to the ground reaches values that can be quite high. When no transformer is used in a grid-connected PV system, common-mode current, which caused by the common mode voltage, can flow through the stray capacitance between the PV array and the ground. It is quite harmful to the body safety and PV system. In order to avoid leakage current, different inverter topologies that generate no varying common-mode voltages, such as bipolar pulse-width modulation (PWM) full-bridge topology, NPC topology have been proposed. From the safety and energy saving viewpoint, it is necessary to develop a higher efficiency topology. In this paper, the generation mechanism of common mode current is discussed. Then different methods used to eliminate the leakage current are compared. Finally, the full-bridge which generates no varying common-mode voltage and requires the same low-input voltage as the bipolar PWM full-bridge is given. The proposed topology has been verified in the simulation based on MATLAB with satisfactory results.

Parallel interleaved grid-connected converters in MW-level wind power generation

With the individual capacity of wind power generation system increasing, the demand for high power grid-connected converters is growing, which is usually limited by the available semiconductor technology. One solution is multiple lower power units connected in parallel. In this paper, direct parallel and interleaved parallel are analyzed and compared. Then it focuses on two parallel interleaved converters. The generating mechanism of high frequency circulating current and low frequency oscillation circulating current are both studied. Also, an interleaved SVPWM and the corresponding control strategy of circulating current are introduced. Finally, the described strategy has been implemented in a 1.5MW grid-connected converter for wind power generation, providing a satisfactory performance.

Design algorithm of grid-side LCL-filter for three-phase voltage source PWM rectifier

A design algorithm for grid-side LCL-filter of three-phase voltage source PWM rectifier is presented, which allows to use reduced values of inductance, improve system dynamic performance and reduce cost compared to traditional L-filter. These advantages are even more attractive in medium and high power applications. In this paper, the design criterion and calculation procedures are introduced in detail. A design example is reported, and the obtained LCL-filter has been realized and tested by simulation and experiments. Experimental results show that the obtained LCL-filter can provide sufficient attenuation of current harmonics and meanwhile ensure a high grid-side power factor. The goodness of this design method is demonstrated.

Harmonic impacts of inverter-based distributed generations in low voltage distribution network

With the high penetration of distributed generations (DG) in low voltage (LV) distribution network, some power quality problems, such as harmonic impacts due to power electronics converters, are unavoidable. This paper analyzes the harmonic influence caused by DG inverters to the connection points and to other nodes in the network. The LV distribution network model with DG inverters is established by MATLAB/Simulink. And simulation results verify the analysis. Power quality issues of connection point when multiple DG inverters connected to LV network are presented in field measurements and some related problems are discussed.

Leakage current analysis of single-phase transformer-less grid-connected PV inverters

Transformer-less string PV inverter is getting more and more widely utilized due to its higher efficiency, smaller volume and weight. However, without the galvanic isolation, the leakage current limitation and operation safety became the key issues of transformer-less inverters. This paper simplifies the leakage current generation circuit model and presents a leakage current estimation method both in real time and frequency domain. It shows that the leakage current is related to the circuit stray parameters, output filter and common mode voltage. Furthermore, with the proposed analysis method, the leakage current generation of H-bridge with different modulation methods and HERIC inverter are discussed individually. At last, the presented method has been verified via simulation.

A turn-on switching losses study in a ZCT soft-switching converter

This paper presents a mathematical approach of calculating the turn-on switching losses in a ZCT soft-switching converter. In theory, the ZCT scheme enables all main switches and auxiliary switches to be turned on and off under zero-current conditions. For the diode reverse-recovery problem can not be eliminated at the IGBT turn-on transition even though the diode current is resonated to zero, and the switching losses in IGBT and diode can not be ignored. Instantaneous voltage and current waveforms of the ZCT soft-switching converter are studied and linearized, and a diode charge-control model is employed to formulate the turn-on switching losses equations. The proposed method is verified by experimental results using a three phase ZCT soft-switching converter.

Error analysis and limit value design of a simple sensorless control for MW-level DDWT-PMSG

Given the characteristics of MW-level direct-drive wind turbines based on permanent magnet synchronous generator (DDWT-PMSG), a simple sensorless zero d-axis current control (ZDAC) method is introduced, which gets rotor flux linkage position using saturation feedback double integrator based on coordinate transform (SFDICT). Its implementation principle is introduced in detail. Furthermore, performance of SFDICT under different harmonic components and different amplitude limits is discussed in depth. Error expressions under different amplitude limits are proposed, which provide design principle for the selection of amplitude limits. Based on error analysis, small amplitude limit with angle compensation is proposed, which can get better accuracy and quality. Finally, experimental results indicate feasibility of the proposed method.

Stability analysis on parallel of LCL-filter-based grid-connected converters in MW-level direct-drive wind generation using complex vector

With the individual capacity of wind power generation system increasing, the demand for high power grid-connected converters is growing, which is usually limited by the available semiconductor technology. One solution is multiple lower power units connected in parallel. LCL filters can provide much better ripple and harmonic attenuation. Therefore, they are more suitable for high power conversion systems. However, LCL filters may result in resonance, and thus cause the closed-loop control system unstable, especially in the parallel of LCL-filter-based converters which are connected to the grid by one transformer. In this paper, complex vector model of parallel of two LCL-filter-based converters is first introduced in details. Then using the superposition theorem, the performance of LCL filter imposed by separate excited sources is analyzed. Furthermore, current reference step response and voltage feedforward control are discussed. Finally, the related results are verified by simulation and experiment.

Analysis of voltage distortion for parallel PWM inverters with L filter

When multi-parallel inverters with L filter are connected to the grid through a long cable, the voltage at the point of common coupling (PCC) will be distorted due to the superposition of the voltage harmonic which are generated by different inverters. In order to solve this problem, the causes of voltage distortion are analyzed in this paper primarily. The correlation of the harmonic between the voltage in the PCC and the pulse width modulation (PWM) wave the output of the inverters is deduced, including the peak voltage, voltage harmonic, and the total harmonic distortion (THD). And then the solution of adding capacitor in the PCC is presented in this paper. Combined with the practical requirement, the method on the design the capacitor is listed. Finally, for illustration, the problems and solution for paralleled inverters are tested by simulation and experiments and the results provide the support to aforementioned theory and analysis.

The design of IGCT Gate-Unit equipped in the three-level NPC converter

Due to the “Hard Drive” technique implemented by the integrated Gate-Unit, “Integrated Gate-Commutated thyristor” (IGCT) has lots of outstanding features and becomes the best choices of medium voltage applications. In three level NPC IGCT converter, because of the complex commutation transient process, the IGCT works in different conditions such as anti-parallel diode forward recovery, reverse recovery, current commutated from anti-parallel diode to GCT and so on, the Gate-Unit must judge the conditions and do the correct responses such as status feedback, decrease the back-porch current and inner retrigger. In this paper, the characteristics of the GCT works in three-level NPC converters are detailed analyzed. A Gate-Unit to drive 4000A/4500V asymmetric GCT is designed and tested in a three-level NPC PEBB, and the experiment results verify the good performance of the designed Gate-Unit.