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This paper describes a theoretical and experimental study on a seamless transfer control strategy for the parallel operation of three-phase voltage source inverters (VSIs) for energy storage systems (ESSs). Moreover, phase-locked loop, seamless transfer control, and parallel control algorithm are optimized for simplicity in order to implement in a unified-controller. Finally, the proposed algorithm...
This paper presents an active islanding detection method which has zero non-detection zone. This method is the advanced version of the conventional slip mode frequency shift islanding detection method (SMS IDM). In this method the parameters of SMS IDM change based on the local load impedance value. The parameters value are calibrated periodically by measuring the impedance of the local load. The...
To investigate the instability issues of three-phase voltage source converters (VSCs) connected to a weak grid, this paper develops a modeling method based on the harmonic-balance approach. The nonlinear coordinate transformation in the control system would generate two sideband components. If the grid is not strong while the bandwidth of phase-locked loop (PLL) is high enough, the sideband components...
AC power electronics system are prone to instability due to the interaction between the power converters, and terminal characteristics of individual power converter based criteria are very attractive for analyzing stability of whole system. However, more and more AC power electronics systems emerge with the feature of variable fundamental frequency, while existing approaches for stability analysis...
Digital isolators are widely used in isolated power converters for transmitting high-frequency gating pulses. Opto-isolators generally suffer from low reliability, while emerging RF based isolators are expensive and power-hungry. In this work, two Phase-Locked-Loop (PLL) based synchronization schemes are introduced and demonstrated to operate a bidirectional Dual-Active-Bridge (DAB) dc-dc converter...
This paper presents robust control of grid connected voltage source converters (VSC) with LCL line filter without grid voltage sensors, measuring only converter inductor currents and DC link voltage. As grid impedance and step-down transformer inductance values are usually not known, proposed control algorithm relies only on converter side inductor value for control parameter, making it robust against...
Voltage feed-forward control (VFFC) is widely used due to its good low-frequency-harmonics suppression and easy implementation, however, it will worsen system stability under the weak grid condition. In this paper, the stability of single-phase grid-connected inverter with VFFC is studied based on impedance analysis method. With this method, the instability mechanism and the unstable area caused by...
Second-order generalized integrator (SOGI) based phase-locked loops (PLLs) are commonly used for grid voltage synchronization in single-phase grid-connected power converters. SOGI-PLLs are attractive because of their simple structure that makes them suitable for implementation even in low-end digital controllers. In this paper, an SOGI based fixed-parameter PLL structure with full dc offset rejection...
Grid synchronization has always been an important challenge for three-phase grid-connected converters under unbalanced and distorted grid conditions. Moreover, how to quickly and accurately extract multiple harmonic sequence information is essential for control systems. In this paper, a three-phase multiple harmonic sequence detection method is proposed for estimating both the fundamental and harmonic...
To address the instability issue in renewable systems of a radial-line structure with multiple current-controlled interface inverters, this paper proposes a practical stability criterion to easily analyze the system stability and a controller parameter design method to guarantee stable system operation with good oscillation damping performance. The proposed impedance-based sufficient stability criterion...
In this paper, control of a grid-tied converter as a voltage controlled converter is investigated for distribution level grids. Small signal model of the converter is presented and a control structure is designed to deliver desired active and reactive power. This controller is using power flow equations to control the converter as a synchronous machine and without the need for a dedicated synchronization...
This paper proposes a novel automatic mode transition control strategy for multiple inverters to operate in grid-connected and islanded modes without communication. When grid is available, all the inverters operate in grid-tied current control mode and transfer available power to the grid. On grid failure, they automatically shift to conventional droop control mode and shift back to grid-tied current...
In this paper, a new DSC-PLL (Delayed Signal Cancellation Phase Locked Loop) with FLL (Frequency Locked Loop) based on RDFT(recursive discrete fourier transform) method is proposed for coping with frequency variation. This method shows significant performance improvement for detection of fundamental positive sequence component voltage when the grid voltage is polluted by grid harmonics and frequency...
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