Anti-Islanding Detector based on a Robust PLL

As required by international standards, the distributed energy generators, connected to the grid by an inverter, have to detect an islanding condition within a suitable time interval. In this paper, a phase-locked loop (PLL), based on a third-order prediction-correction filter, is proposed to implement an islanding detector with reduced detection time. Such a feature is obtained using the estimation of the grid angular frequency and acceleration provided by the PLL with a negligible time delay. The proposed approach is implemented on an industrial grade DSP and validated through the experimental comparison among different detection methods, such as rate of change of frequency (ROCOF) and Slip Mode frequency Shift (SMS). The combined use of ROCOF and SMS is also illustrated and discussed

LC Filter Design for On-grid and Off-grid Distributed Generating Units

This paper deals with the design procedure of an LC-based output filter for three-phase inverters to be used in both off-grid and on-grid scenarios. The aim of this procedure is to provide guidelines and component selection criteria for reducing the inverter output switching ripple in order to limit the interaction with the control algorithm and to increase the filter stability. A suitable combination of resistive and reactive components is used to realize the complete filter structure. The proposed procedure is applied to the design of the output power filter for a 40-kVA three-phase inverter

Resonant–Repetitive combined control for stand-alone power supply unit

This paper investigates a combined resonant–repetitive (RR) control structure for a three-phase four-leg DC/AC converter power supply. The RR control configuration is com-posed by a resonant controller tuned at the system fundamental frequency working in conjunction with a plug-in-type repetitive controller. The resonant part of the control scheme is used to as-sure prompt tracking of the inverter output voltage and to achieve as fast as possible system response to load variations; to this pur-pose, it is tuned at the fundamental frequency. At the same time,the resonant controller is able to stabilize the system without the necessity of any further additional controller; the repetitive part of the scheme is implemented for the fine regulation at the system harmonic frequencies. The proposed control configuration is used to regulate the power supply output voltage, providing very good tracking of the output voltage reference even in the presence of anon linear load. Experimental validation from a 40-kVA converter prototype is presented to validate the operation of the proposed converter and control

Load-adaptive zero-phase-shift direct repetitive control for stand-alone four-leg VSI

This paper deals with a dedicated load adaptive phase compensation algorithm to be used in Repetitive Control based stand-alone 4-leg VSI. The plant model is achieved, its inherent modifications according to the operating point are highlighted and used to properly adapt the Repetitive Control structure. Modification of the repetitive control parameters is described to obtain the desired phase compensation capabilities achieving a Zero-Phase-Shift condition at each harmonic. This allows to increase the gain of the Repetitive Controller at high order harmonics thus yielding a better VSI output voltages with strongly reduced THD and faster dynamic response. As a consequence, the VSI output voltages are almost independent from the loads to be fed and the 4-leg VSI with the proposed Zero-Phase-Shift Direct Repetitive Control is an ideal candidate to supply sensitive loads in microgrid, in particular for stand-alone applications

Symmetrical three‐phase seven‐level E‐type inverter for PV systems: design and operation

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