Xinmin Jin

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.

The PWM strategies of grid-connected distributed generation active NPC inverters

The Neutral Point Clamped topology due to high efficiency, low leakage current and EMI, its integration is widely used in the distributed generation (DG) systems. However the main disadvantage of the NPC inverter is given by an unequal distribution of the losses in the semiconductor devices, which leads to an unequal distribution of temperature. By using the Active NPC topology, the power losses distribution problem is alleviated. The modulation strategy is a key issue for losses distribution in this topology. In this paper two known strategies are discussed and a new proposed PWM strategy, namely the Adjustable Losses Distribution (ALD) PWM strategy is proposed for better losses distribution in the Active NPC (ANPC) topology. Simulations using Simulink and the PLECS toolbox have been done for evaluating efficiency of different NPC topologies and some experimental results are presented in this paper to validate the operation of the different strategies.

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.

An integrated multifunction DC/DC converter for PV generation systems

Due to the stochastic characteristic of photovoltaic (PV) generation, energy storage devices are indispensable to smooth the power flow and provide the electric energy with high quality. In this paper, an integrated DC/DC converter is proposed to realize high performance interfaces for PV array, battery and load. This converter consists of a boost converter for the PV array and a buck/boost converter for the battery, which can realize multifunction including power generation, charge and discharge. The grid-connected and stand-alone operation modes of this converter are analyzed. From this analysis, the control strategies are developed for both modes. A coordination scheme is proposed for the power balance at the stand-alone mode. Simulation and experiments verify the practical feasibility and effectiveness of this multi-function converter.

A voltage and frequency droop control method for microsources

This paper focuses on a control method for microsources which can work as AC voltage sources, instead of AC current sources. The microsources can not only provide AC power in grid-connected mode, but also control the frequency and voltage for the microgrid load in island mode. By using the voltage and frequency droop control method, the microsources can realize the “plug-and-play” concept: share the load automatically by using only the local information, i.e. the voltage, frequency, instead of the communications with the other microsources or the microgrid central control system. The microsources can work in parallel to the grid or as an island. When the microgrid system disconnects from the grid, caused by the grids short current or power failure, the microsources can keep on working for the microgrid loads, without restart process. Detailed analysis is shown in this paper, and experimentations on a 25kVA three phase inverter have been done to prove the performance of this method.

A New PWM Strategy for Grid-Connected Half-Bridge Active NPC Converters With Losses Distribution Balancing Mechanism

Photovoltaic systems technological development is driven by the request for higher efficiency and safety. These concerns influence also the choice of the power converter stage. Several topologies have been proposed and many of them are available commercially. Among them, the neutral point clamped (NPC) and derived topologies offers high efficiency, low leakage current, and low EMI. However, one main disadvantage of the NPC inverter is given by an unequal distribution of the losses in the semiconductor devices, which leads to an unequal distribution of temperature that can affect lifetime. By using the active NPC (ANPC) topology, where the clamping diodes are replaced by bidirectional switches, the power losses distribution problem is alleviated. The modulation strategy is a key issue for losses distribution in this topology. In this paper, two known strategies are discussed and a new PWM strategy, namely the adjustable losses distribution is proposed for better losses distribution in the ANPC topology. Simulations and experimental results help in evaluating the modulation strategies.

An improved control scheme for Doubly Fed Induction Generator during grid fault ride-through

The increasing wind power penetration brings new challenge for wind turbine to provide Low Voltage Ride-Through (LVRT) capability. Doubly Fed Induction Generator (DFIG) has been widely used in wind turbines; due to its cost-effective partially power rated of the converter and Variable-Speed Constant-Frequency (VSCF) operation. However, the DFIG system has two poorly damped poles and is sensitive to the grid disturbance. For the stator is directly connected to the grid, the symmetrical voltage dip results in a dc component in the stator-flux which reflects a serious oscillation in the synchronous reference frame. This transient stator-flux induces overvoltage and overcurrent in the rotor circuit. The paper analyzes the characteristic of the transient stator-flux under voltage dip. Based on the eigenvalues of the system matrix, the damping rates of the transient stator-flux are investigated. An improved control scheme of the Rotor Side Converter (RSC) is presented to accelerate the transient stator-flux damping rate. The proposed control improves the rotor current control performance and relieves the fluctuation of electromagnetic torque, dc-link voltage, stator current and rotor current. It is also beneficial to the fast reactive power support to the grid during voltage dip. The simulation results of a 1.5-MW DFIG system and experimental tests on a 90-kW DFIG rig validate the theoretical analysis and the feasibility of the proposed control scheme.

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.

A novel LVRT strategy for direct-drive wind turbines based on permanent magnet synchronous generator

Increasing penetration level of wind energy into the power system over the last decades has brought new issues and challenges. One of these concerns is the issue of low voltage ride through (LVRT). In this paper, given the characteristics of MW-level direct-drive wind turbines based on permanent magnet synchronous generator, a novel LVRT strategy is proposed, which is the combination of coordinate control of generator-side and grid-side converters and small un-loading branch. The implementation principle as well as calculation criteria are explained in depth. Finally, the feasibility of the proposed method is demonstrated by simulation results under balanced and unbalanced sag conditions.