Changzhou Yu

An LCL - LC Filter for Grid-Connected Converter: Topology, Parameter, and Analysis

In order to further cut down the cost of filter for grid-connected pulsewidth modulation (PWM) converter under the more and more stringent grid code, a new kind of high-order filter, named LCL-LC filter, is presented in this paper. The resonant frequency characteristics of the filter are analyzed, and a parameter design method on the base of the characteristics is also proposed in the paper. The proposed parameter design method can easily make full use of the existing research results about the traditional LCL filter parameter design. And then a parameter robustness analysis method based on four-dimensional graphics is proposed to analyze parameter robustness of the presented filter. Compared with the traditional one, the proposed analysis method can analyze the filter performance under variations of several parameters at a time without any iteration. The comparative analysis and discussion considering the LCL filter, the trap filter, and the LCL- LC filter, are presented and verified through the experiments on a 5 kW grid-connected converter prototype. Experiment results demonstrate the accuracy of theoretical analysis and prove that the presented filter has a better performance than two others.

Hysteresis Model Predictive Control for High-Power Grid-Connected Inverters With Output LCL Filter

Hysteresis model predictive control (HMPC) is a variant of finite-control-set model predictive control, which is mainly developed for high-power applications. In this paper, the HMPC scheme is proposed to control a high-power grid-connected two-level voltage source inverter with an inductive-capacitive-inductive (LCL) filter. To ensure the stable operation of the control system, the resonance characteristics of the system with an LCL filter based on HMPC are analyzed, and an active damping scheme is then given. On the other hand, the impacts of the modeling error on system stability and average switching frequency are analyzed in detail. Then, a new switching frequency-adaptive control scheme that uses the average switching frequency to adjust the parameter value of the predictive model online is presented. The new control scheme can keep the average switching frequency within an acceptable operational margin, while improving the stability of the control system. For demonstration, the proposed control scheme has been implemented on a small-scale grid-connected inverter system, and the results show that the theoretical analysis is correct and that the proposed control scheme is effective.

Modeling and Resonance Analysis of Multiparallel Inverters System Under Asynchronous Carriers Conditions

With increasing penetration of renewable energy sources into modern power systems, multiparallel inverters are commonly employed in the interface to the utility grid, giving rise to potential resonance problems. In order to better understand the issue, resonance of multiparallel inverters is analyzed under an asynchronous carrier. First, multiparallel inverter equivalent circuits are modeled within a controlled bandwidth. In order to investigate resonance characteristics under an asynchronous carrier, a crossover filter function is used to separate output currents of different frequencies for a two-parallel inverter to analyze the output current frequency response. With this approach, resonance characteristics are focused under asynchronous conditions, and the impact of shifting LCL parameters on resonance is examined. The influence of paralleled inverter number on output currents is examined, and the analysis is expanded to multiparallel inverter systems to determine the resonance characteristics. Experimental results validate the developed models and the proposed resonance characteristics.

Study of a Current Control Strategy Based on Multisampling for High-Power Grid-Connected Inverters With an LCL filter

The high-power grid-connected inverters with LCL filters have been widely used. Current control plays a key role in the grid-connected inverter control system. To cope with the inherent resonance of the LCL filter, active damping (AD) methods are usually employed. However, the AD performance is impaired by control delays which are introduced in the digital control implementation process. Besides, the lag phase due to control delays limits the system bandwidth and stability margin. The effects of control delays are more noteworthy in the high-power grid-connected inverter due to its low switching frequency. This paper investigates the current control strategy based on multisampling for the high-power grid-connected inverter with the LCL filter. First, the multisampled AD scheme is studied, which can reduce control delays effectively and improve AD performance. Besides, the multisampled control without additional damping whether passive or active is researched. Through the inverter-side current feedback, the system can realize single-current-loop control based on multisampling. Thus, the control system is simplified and it can be stable, and achieve better dynamic performance. Finally, experimental results show that the proposed control schemes are effective.

A reactive power sharing method based on virtual capacitor in islanding microgrid

For the problem that paralleled inverters cannot share reactive power proportional to their power capacity in islanding microgrid. Originally, the effect of inverter output impedance and line impedance to reactive power sharing with conventional droop control is analyzed. Following, a method adjusting the output reactive power by paralleling capacitor at the inverter output is proposed, and the relationship between paralleled capacitors, line impedance and reactive power is analyzed. On the basis, virtual capacitor method is realized to improve reactive power balance. Finally, the proposed method and analysis are verified through simulation.

Stability improvement of grid-connect inverter using combination of passive and active damping

the gird-connected converters with regulation of power factor to unity become to be more popular. These inverters are connected to the grid through L or LCL filters. An LCL filter is more effective and reduces harmonic distortion at lower switch frequency than an L filter. However, an LCL filter can bring stability problem. Even more, in practical engineering, there are some additional filters behind the LCL filter, which cause serious stability problem, this paper models a two-level-filter as an example. To improve the stability, one solution is to introduce a resistor in series/parallel with the filter capacitor or inductance which is called passive damping and another is to modify control algorithm to stabilize the system which is called active damping. This paper analyzes the reason why the effect of passive and active damping method is limited. To enhance the stability of the system, a method that combines the passive and active damping is proposed, this method possesses both the advantage of passive and active damping. Simulation and experimental results verify this combination.

A Reactive Power Sharing Strategy of VSG Based on Virtual Capacitor Algorithm

In the islanded microgrid, distributed generators are controlled with virtual synchronous generator (VSG) strategy to simulate rotor inertia and droop characteristics of synchronous generators, in order to enhance the voltage and frequency support capabilities. Since the capacity and location distribution of each VSG is random, the VSG output impedance, line impedance and its capacity are mismatched, resulting in inaccurate sharing of reactive power. Based on the study of reactive power sharing schemes without communication and system parameters detection, and aiming at the contradiction between reactive power sharing error and voltage control accuracy of existing schemes, a reactive power sharing strategy based on virtual capacitor is proposed. The strategy simulates characteristics of paralleling capacitor at the VSG output terminal, and compensates the output voltage according to adaptive control of VSG output reactive power, thus to reduce reactive power sharing error, and improve the voltage control accuracy meanwhile. The design of virtual capacitor parameters and a two VSG parallel system stability with proposed strategy are analyzed in this paper. The correctness and effectiveness of the proposed strategy is verified by experiments.

Resonance characteristics analysis of paralleled inverters based on Resonance Stability Margin

With increasing penetration of renewable energy sources into modern power systems, multi-parallel inverters are commonly employed in the interface to the utility grid, giving rise to potential resonance problems. Firstly, 2-parallel inverters equivalent circuit model is established based on its control. In order to decouple inverter output current, the components characteristics of inverter output are analyzed in different frequencies, and one of 2-parallel inverter output current under condition of synchronized and asynchronized carriers are worked out. An idea “Resonance Stability Margin” is raised to investigate and evaluate resonance characteristics, variables that represent the stability of the system are found in control system of single inverter, and are used to record when the system achieves the same closed-loop dominant pole position or open-loop phase margin under different conditions. Values of those variables recorded indirectly reflect the degree of resonance in the paralleled inverters system. Experimental results validate the developed resonance characteristics and the proposed resonance stability margin.

Overview on resonance characteristics and resonance suppression strategy of multi-parallel photovoltaic inverters

Photovoltaic(PV) inverters have been widely used in large-scale PV power generation systems to reduce the system mismatch and increase the output power. With the increasing installed capacity of PV power stations, the number of string PV inverters increases, which brings about risk of inverter resonance and influences the quality and stability of multi-parallel inverters grid-connected system. Thus, the problems of multi-parallel inverters resonance have become a hot research topic. This paper is focused on the case of grid-connected string PV inverter systems, and the grid-connected PV inverter resonance where resonance suppression strategy will be analyzed and discussed. Firstly, the structure of string PV inverter and multi-parallel inverters system are presented. Secondly, discussions are made about the model of multi-parallel inverters system and its resonance characteristics based on the multi-input-multi-output(MIMO) system, Norton equivalent circuit and carrier synchronization. Thirdly, two strategies of “inverter level” and “system level” are provided to summarize the multi-parallel inverters resonance suppression strategy. Finally, the development tendency of resonance suppression strategy and conclusions are prospected.

Harmonic reduction through resistive and inductive harmonic impedance

In order to improve the power quality, a kind of resistive harmonic impedance (RHL) has been reported. It can provide a reduction of the harmonic distortion, and be implemented using active power electronic devices with a resistance behavior in harmonic frequency. This paper proposes new type harmonic impedance: resistive and inductive harmonic impedance (RLHI). The expression for optimal inductance value in proposed RLHI is developed. And a simulation study based on Matlab/Simulink is carried out and the results verify that the proposed RLHI can provide more reduction performance of harmonic distortion than the one of RHI.