Yuanbin He

A New Design Method for the Passive Damped LCL and LLCL Filter-Based Single-Phase Grid-Tied Inverter

A higher order passive power filter (LLCL filter) for the grid-tied inverter is becoming attractive for industrial applications due to the possibility to reduce the cost of the copper and the magnetic material. However, similar to the conventional LCL filter, the grid-tied inverter is facing control challenges. An active or a passive damping measure can be adopted to suppress the possible resonances between the grid and the inverter. For an application with a stiff grid, a passive damping method is often preferred for its simpleness and low cost. This paper introduces a new passive damping scheme with low power loss for the LLCL filter. Also, a simple engineering design criterion is proposed to find the optimized damping resistor value, which is both effective for the LCL filter and the LLCL filter. The control analysis and the power loss comparison for different filter cases are given. All these are verified through the experiments on a 2-kW single-phase grid-tied inverter prototype using proportional resonant controllers. It is concluded that, compared with the LCL filter, the proposed passive damped LLCL filter can not only save the total filter inductance and reduce the volume of the filter but also reduce the damping power losses for a stiff grid application.

An LLCL Power Filter for Single-Phase Grid-Tied Inverter

This paper presents a new topology of higher order power filter for grid-tied voltage-source inverters, named the LLCL filter, which inserts a small inductor in the branch loop of the capacitor in the traditional LCL filter to compose a series resonant circuit at the switching frequency. Particularly, it can attenuate the switching-frequency current ripple components much better than an LCL filter, leading to a decrease in the total inductance and volume. Furthermore, by decreasing the inductance of a grid-side inductor, it raises the characteristic resonance frequency, which is beneficial to the inverter system control. The parameter design criteria of the proposed LLCL filter is also introduced. The comparative analysis and discussions regarding the traditional LCL filter and the proposed LLCL filter have been presented and evaluated through experiment on a 1.8-kW-single-phase grid-tied inverter prototype.

A Modified LLCL Filter With the Reduced Conducted EMI Noise

For a transformerless grid-tied converter using pulse width modulation, the harmonics of grid-injected current, the leakage current, and the electromagnetic interference (EMI) noise are three important issues during designing of the output filter. In this paper, the common mode and the differential mode EMI noises are investigated for the LCL- and LLCL-filter-based single-phase full-bridge grid-tied inverters. Based on this, a modified LLCL-filter topology is proposed to provide enough attenuation on the conducted EMI noise as well as to reduce the dc-side leakage current. The parameter design method of the filter is also developed. The comparative analysis and discussion on four filter cases (the conventional LCL filter, the conventional LLCL filter, the modified LCL filter, and the modified LLCL filter) are carried out and verified through simulations and experiments on a 0.5-kW, 110 V/50 Hz single-phase full-bridge grid-tied inverter prototype.

An Efficient and Robust Hybrid Damper for

A high-order (LCL or LLCL) power filter with a small grid-side inductor is becoming more preferred for a grid-tied inverter due to less total inductance and reduced costs. In a microgrid, the background harmonic voltage (BHV) may distort the injected currents of the grid-tied inverters. In order to resist the effect of the BHV, a feedforward voltage compensator and a proportional resonant regulator with harmonic compensation are often adopted. However, they still have their own limitations, particularly when there are higher order BHVs at the point of common coupling and when the equivalent grid impedance widely varies due to the different numbers of grid-tied inverters in parallel. Thus, an extra damper should be inserted to keep the system stable. In this paper, the control bandwidth limitation of a multiloop control active damping (AD) method is analyzed and illustrated by the capacitor-current-feedback AD. Based on this, a single-loop current control with a hybrid damper is proposed for a single-phase LCLor LLCL-filter-based grid-tied inverter. A step-by-step design of the controller method is also introduced in detail. Experiments on a 2-kW prototype fully demonstrate the strong robustness of the stability and the high harmonic rejection ability of the inverter using the proposed control method.

LCL

The single-phase power converter topologies evolving of photovoltaic applications are still including passive filters, like the LCLor LLCL-filter. Compared with the LCL-filter, the total inductance of the LLCL-filter can be reduced a lot. However, due to the resonant inductor in series with the bypass capacitor, the differential mode (DM) electromagnetic interference (EMI) noise attenuation of an LLCL-filter-based grid-tied inverter declines. Conventionally, a capacitor was inserted in parallel with the LC resonant circuit branch of the LLCL-filter to suppress the DM EMI noise. In order to achieve a small value of capacitor as well as to minimize the additional reactive power, a novel simple DM EMI suppressor for the LLCL-filter-based system is proposed. The characters of two kinds of DM EMI suppressor are analyzed and compared in detail. Simulations and experiments on a 0.5-kW 110-V/50-Hz single-phase grid-tied inverter prototype are accomplished to confirm the analysis.

- or

Deadbeat control is commonly used in grid-connected inverter with L filter, but it faces the challenge of having filter resonance in inverter with LCL filter. Although many active damping techniques have been proposed to tackle such phenomenon, their digital implementation would introduce nonminimum phase characteristics. Furthermore, the plant viewed by the deadbeat controller is of high order, making the system performance be susceptible to the drift of the filter parameters. This paper introduces a new perspective of using boundary control with second-order switching surface to reduce the order of the plant viewed by the deadbeat controller. The structure hybridizes the merits of the deadbeat control in its simplicity and the boundary control in achieving wide control bandwidth. Small-signal dynamic modeling of the boundary control is formulated. The performance sensitivities of the overall system to filter parameters and grid inductance variations are studied with the derived models. An online grid inductance estimation algorithm is proposed to assure sufficient phase margin under an extremely weak-grid condition. A 2-kW, 220-V, 50-Hz prototype with the switching frequency of 8 kHz has been built and evaluated. Its steady-state and transient behavior, and harmonic rejection capability under stiff- and weak-grid conditions are discussed.

LLCL

Grid-tied voltage source inverters using LCL filter have been widely adopted in distributed power generation systems (DPGSs). As high-order LCL filters contain multiple resonant frequencies, switching harmonics generated by the inverter and current harmonics generated by the active/passive loads would cause the system resonance, and thus the output current distortion and oscillation. Such phenomenon is particularly critical when the power grid is weak with the unknown grid impedance. In order to stabilize the operation of the DPGS and improve the waveform of the injected currents, many innovative damping methods have been proposed. A comprehensive overview on those contributions and their classification on the inverter- and grid-side damping measures are presented. Based on the concept of the impedance-based stability analysis, all damping methods can ensure the system stability by modifying the effective output impedance of the inverter or the effective grid impedance. Classical damping methods for industrial applications will be analyzed and compared. Finally, the future trends of the impedance-based stability analysis, as well as some promising damping methods, will be discussed.

-Based Grid-Tied Inverter With Strong Grid-Side Harmonic Voltage Effect Rejection

Cascaded boundary-deadbeat controller has been proven to be effective in controlling single-phase grid-connected inverter with LCL output filter. Such architecture mitigates filter resonance and offers good stability under stiff- and weak-grid conditions. However, its merits are offset by requiring many sensors, dedicated control loop to regulate the operating frequency, and high-precision intracycle information of the circuit variables to dictate the states of the switches. A modified cascaded boundary-deadbeat control law with reduced number of current sensors, the use of current band to regulate the operating frequency, and intracycle information recovery mechanism of the filter capacitor voltage for a virtually-grounded three-phase grid-connected inverter with LCL filter is presented. It inherits the merits of allowing the inverter to exhibit fast dynamic response and mitigating filter resonance. The contaminated intracycle information of the filter capacitor voltage is recovered so as to estimate and predict state trajectories accurately. Furthermore, a dc bus voltage feedforward injection scheme with reduced number of voltage sensor is proposed. It utilizes the duty cycle information of the gate signals to compensate the effect of the unbalanced dc bus capacitor voltages on causing modulation saturation and current distortion. The system characteristics under parametric variations will be studied. A 3-kW prototype has been built and evaluated under stiff- and weak-grid conditions.

A Simple Differential Mode EMI Suppressor for the

High-frequency pulse width modulation (PWM) in inverter-fed induction motor drive systems are widely used in industrial applications because of their flexible speed control and energy efficiency. However, high-frequency pulses induce overvoltage spikes to the motor via long cable. Such phenomenon would cause serious deterioration of the motor and cable. A passive overvoltage suppression techniques of low-loss “RL-plus-C” filter has been proposed recently. It has not only some merits of simple structure, low cost, and good robustness, but also a significant merit of low power dissipation. Based on the design method of “RL-plus-C” filter, this paper presents a new passive filter design method which can both eliminate voltage surges and greatly reduce inverter-induced bearing currents. The theoretical analysis and the design method are introduced in detail. Simulation results are good agreement with the theoretical analysis.

LLCL

Many active damping (AD) techniques have been proposed to deal with current oscillation in grid-tied voltage source inverter with an LCL filter. Among them, the proportional-capacitor-voltage-feedforward (PCVFF) has been found to be an effective way. However, there is a lack of detailed modeling, design, and analysis of the PCVFF. This paper will elucidate the characteristics of the PCVFF AD scheme by using the virtual impedance model, analyze the dynamic characteristics under wide grid impedance variation, and give a step-by-step controller design procedure for the proportional-resonant-plus-harmonic-compensation (PR+HC)-based grid current controller with the PCVFF AD. The control method has been applied to a 3kW, 220V, 50Hz prototype. The steady-state and transient performances of the inverter under a wide variation of the grid inductance from 0.0025 pu to 0.15 pu will be studied and evaluated.