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Small signal stability of inverter based droop controlled microgrid is a major problem from past decade. In this work, an optimal state feedback controller is designed to improve the stability of the two inverter based test microgrid. The detailed full order small signal model of the test microgrid is developed in state space considering network and load dynamics. Eigen value and sensitivity analysis...
In three-phase grid-tied inverter, grid voltage feedforward is helpful to improve grid-tied power quality for strong grid condition. This paper found that the voltage feedforward had different effect on power quality under weak grid condition compared with that in ideal grid condition. And recent reported studies show that grid voltage feedforward has negative effect for grid-inverter system stability...
Almost all existing relevant literatures are focused on the multi-inverter system that only contains one type of inverter. Till now, the interactions in multi-inverter system consisting of polytype inverters have been not investigated systematically. To fill this gap, the paper is aimed at the stability study of the generic system containing multiple types of inverters. The mathematic model of interactions...
In cascaded-type microgrid, the synchronization and power sharing of distributed generators (DGs) become two new issues that needs to be addressed urgently. In this paper, a unified droop control is proposed and its stability is analyzed in the islanded mode. The proposed scheme can ensure the system stability and accurate power sharing without communication in four-quadrant operations. Finally, simulation...
Grid-connected inverter is an important part of the grid-connected system. Compared with the traditional L or LC filter, LCL filter has a better high-frequency harmonic attenuation performance. However, LCL filter has resonant peak, which has a great influence on the stability of the system. This paper first analyzes the effect of passive damping method on the resonance peak; then a double closed-loop...
Digital-physical hybrid real-time simulation is considered to be an advanced method for future power system simulation. However, the physical interface will inevitably exist DC component, which causes negative effects on practical applications, such as degradation of simulation performance, damage to physical equipment, etc. A novel DC component elimination control strategy for the digital-physical...
With the improvement in the power electronic technologies, medium voltage dc (MVDC) electrical distribution systems are being considered for on-shore and off-shore applications. These future MVDC electrical distribution systems are expected to provide the possibility of easy interfacing of the renewable energy sources, improving the dynamics of the system and also help in reducing the carbon footprint...
Photovoltaic units that are connected to the grid through dc/dc converters and inverters in distributed generation, are controlled via a synchronous-generator-like voltage control mechanism. While other control schemes such Synchronverters are used to control the inverter frequency and power at a fixed dc link voltage, the proposed approach considers both the dc-link voltage dynamics and the inverter...
The stability of grid-connected voltage-source inverters (VSI) is deteriorated in a weak grid due to the interaction between the increasing grid impedance and the inherent inverter impedance. To improve the stability of LCL-filtered VSI, an impedance-phase reshaping method is proposed in paper. With this output current feedback compensator, the phase margin at the impedance intersection can be improved...
Full Feed-forward (FF) approach has been proved to be an effective method to suppress the output current distortion of the LCL type grid-connected inverter (GCI) due to grid voltage harmonics. Since most of the existing FF approachs are realized by digital control, a futher study of the digital control based (DCB) FF approach has been carried out in this paper firstly. It is shown that, there are...
In this paper, the stability study of a multiple offshore parallel long-cable-connected inverters (MOPLI) systemhs has been presented. It is shown that, since this MOPLI system is consisted of a large number of different inverters and long cables, its stability assessment actually faces big challengs. In addition, though there are lots of existing impedance-based stability criteria about prallel inverters,...
Grid-connected Voltage Source Inverters (VSIs) equipped with LCL filters are attractive in utility applications for their better power quality features, while the inherent LCL resonance may limit the current control loop bandwidth or even threaten system stability. Simple but effective single-loop current control strategies have been studied for obtaining a stable system, where either the inverter-side...
Electrical energy generation is evolving from centralized towards distributed energy resources (DER) like wind and solar power plants. In such power systems, new challenges arise for the design of voltage source converter (VSC) control strategies. In the last few years synchronverters have gained interest from the research community due to their capability of emulating synchronous machines (SMs) and...
The resonant stability problems of LCL filter have become big challenges for grid-connected inverter in weak grid. This paper takes the effect of non-minimum phase time delay on the ratio between resonance frequency and sampling frequency into account and use it to design the LCL filter. Both LCL resonance frequency and inverter-side inductor and filter capacitor related L1C resonance frequency are...
The increasing use of grid-connected inverters to connect renewable energy sources to a power grid will have globally important effect on grid performance. A mismatch between the grid and inverter impedance may cause harmonic resonances, which can lead to instability of the grid and disruption of inverter operation. The grid dynamics vary over time, so the inverter should adapt to the varying conditions...
In view of LCL-type grid-connected inverter system, traditional methods can hardly realize multigoal analysis and optimization, thus, a research is carried out on the visualization method based on D-partition method. In consideration of the discrete characteristic of digital control, this paper infers the establishment conditions of D-partition method in z domain, and compares it with D-partition...
A low voltage ride-through solution for an indirect three-level matrix converter based open-end winding drive is demonstrated. The proposed method uses the input filter capacitors to exchange the inertial energy from the motor and thereby maintain the flux in the motor during grid fault conditions. The input filter capacitors act as the voltage source for inverters during the fault. Ride-through is...
Penetration levels of solar photovoltaic (PV) generation on the electric grid have increased in recent years. In the past, most PV installations have not included grid-support functionalities. But today, standards such as the upcoming revisions to IEEE 1547 recommend grid support and anti-islanding functions-including volt-var, frequency-watt, volt-watt, frequency/voltage ride-through, and other inverter...
Capacitor-current-feedback active damping method is commonly used in LCL-filtered grid-connected inverters, which attenuates the resonance peak by emulating a virtual resistor in parallel with the filter capacitor. Taking the digital control delay into account, the emulation component becomes frequency-dependent, and it will turn negative when the filter resonance frequency becomes higher than one-sixth...
To avoid the stability problem of grid connected inverter caused by the conventional phase locked loop (PLL), the direct power control of voltage source inverter in virtual synchronous reference frame without PLL is investigated considering grid voltage disturbance in the paper. Based on the small signal analysis methods, comparison between conventional PLL and no PLL is made. It can be found that...
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