This paper proposes a novel control for voltage-source inverters with the capability to flexibly operate in grid-connected and islanded modes. The control scheme is based on the droop method, which uses some estimated grid parameters such as the voltage and frequency and the magnitude and angle of the grid impedance. Hence, the inverter is able to inject independently active and reactive power to the grid. The controller provides a proper dynamics decoupled from the grid-impedance magnitude and phase. The system is also able to control active and reactive power flows independently for a large range of impedance grid values. Simulation and experimental results are provided in order to show the feasibility of the control proposed.

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Adaptive Droop Control Applied to Voltage-Source Inverters Operating in Grid-Connected and Islanded Modes

The steady increase in photovoltaic (PV) installations calls for new and better control methods in respect to the utility grid connection Limiting the harmonic distortion is essential to the power quality, but other requirements also contribute to a more safe grid-operation, especially in dispersed power generation networks For instance, the knowledge of the utility impedance at the fundamental frequency can be used to detect a utility failure A PV-inverter with this feature can anticipate a possible network problem and decouple it in time This paper describes the digital implementation of a PV-inverter with different advanced, robust control strategies and an embedded online technique to determine the utility grid impedance By injecting an interharmonic current and measuring the voltage response it is possible to estimate the grid impedance at the fundamental frequency The presented technique, which is implemented with the existing sensors and the CPU of the PV-inverter, provides a fast and low cost approach for online impedance measurement, which may be used for detection of islanding operation Practical tests on an existing PV-inverter validate the control methods, the impedance measurement, and the islanding detection

A digital controlled PV-inverter with grid impedance estimation for ENS detection

This paper introduces a new intelligent-based approach for detecting islanding in distributed generation (DG). This approach utilizes and combines various system parameter indices in order to secure the detection of islanding for any possible network topology, penetration level and operating condition of the DG under study. Hence, every parameter index displays characteristics for a given set of events. The proposed technique uses the data-mining technology to extract information from the large data sets of these indices after they are screened off-line via massive event analyses using network simulations. The technique is tested on a typical DG with multiple distributed resources and the results indicate that this technique can successfully detect islanding operations. In addition, this technique can also overcome the problem of setting the detection thresholds inherent in the existing techniques by optimizing their settings

Intelligent-Based Approach to Islanding Detection in Distributed Generation

Naval ships as well as aerospace power systems are incorporating a greater degree of power electronic switching sources and loads. Although these components provide exceptional performance, they are prone to instability due to their high efficiency and constant power characteristics that can exhibit negative impedance nature at certain frequencies. When designing these systems, integrators must consider the impedance versus frequency at an interface (which designates source and load). Stability criteria have been developed in terms of source and load impedances for both dc and ac systems, and it is often helpful to have techniques for impedance measurement. For dc systems, the measurement techniques have been well established. This paper introduces a new method of impedance measurement for three-phase ac systems. By injecting an unbalanced line-to-line current between two lines of the ac system, all impedance information in the traditional synchronous reference frame d-q model can be determined. For medium-voltage systems, the proposed technique is simpler and less costly than having an injection circuit for each phase. Since the current injection is between only two phase lines, the proposed measurement device can be used for both ac and dc interfaces. Simulation and laboratory measurements demonstrate the effectiveness of this new technique.

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Small-Signal Impedance Measurement of Power-Electronics-Based AC Power Systems Using Line-to-Line Current Injection

An active power filter implemented with a four-leg voltage-source inverter using a predictive control scheme is presented. The use of a four-leg voltage-source inverter allows the compensation of current harmonic components, as well as unbalanced current generated by single-phase nonlinear loads. A detailed yet simple mathematical model of the active power filter, including the effect of the equivalent power system impedance, is derived and used to design the predictive control algorithm. The compensation performance of the proposed active power filter and the associated control scheme under steady state and transient operating conditions is demonstrated through simulations and experimental results.

Improved Active Power Filter Performance for Renewable Power Generation Systems

A method for power system impedance estimation is presented. The method employs a power converter to inject a voltage transient onto the supply system. As the technique employs controlled power electronic devices it may be used as a stand alone piece of a portable measurement equipment, or it may be embedded into the functions of an active shunt filter for improved harmonic control. The impedance is estimated through correlation of the measured voltage and current transients. Simulations and experimental results demonstrate the measurement technique is highly accurate and effective.

A technique for power supply harmonic impedance estimation using a controlled voltage disturbance

This paper introduces an improved power system impedance measurement technique. The technique injects a short duration current spike into the supply, and derives the impedance from the measured current and voltage. The method is shown to work extremely well under most conditions, by experimental evaluation using a 45-kVA laboratory prototype. Degradation is observed when a voltage-source-type power-electronic load is connected directly to the measurement equipment. An alternative signal processing strategy, based upon the Prony method is proposed to improve the estimation under these conditions. This method also reduces the overall transient data capture time, thus allowing a faster, online calculation of the impedance.

Impedance measurement for improved power quality-Part 1: the measurement technique

This work describes the development of a new "stand-alone" active shunt filter. The control employs online power system impedance measurement to optimize the current control algorithm and to provide the filter's reference currents. The current control employs additional rotating frames of reference at fifth and seventh harmonic frequencies and is specifically for high kilovolt-ampere applications where the switching frequency is constrained to 4 kHz. This results in an improved immunity to disturbances such as supply distortion, pulse-width modulation (PWM) imperfections and processing delays, and provides excellent steady-state current control. Experimental results confirm the operation of the new controller.

Impedance measurement for improved power quality-Part 2: a new technique for stand-alone active shunt filter control

This paper describes a method for on-line measurement of power system impedance to source. The method employs a power electronic circuit to inject a small current disturbance onto the energised power network, and the measurements of the disturbance current and resultant voltage transient are used to identify impedance. Simulation results demonstrate the effectiveness of the technique for identification of the impedance of transmission lines and linear loads. An alternative data processing technique is introduced in order to address the measurement problems associated with nonlinear loads. This technique is also illustrated using simulation.

Power system impedance measurement using a power electronic converter

This paper describes a method for on-line measurement of power system impedance to source. The method employs a power electronic circuit to inject a small current disturbance onto the energised power network, and the measurements of the disturbance current and resultant voltage transient are used to identity impedance. Simulation results demonstrate the effectiveness of the technique for identification of the impedance of transmission lines and linear loads. An alternative data processing technique is introduced in order to address the measurement problems associated with nonlinear loads. This technique is also illustrated using simulation.

B. Palethorpe

Papers

A technique for power supply harmonic impedance estimation using a controlled voltage disturbance

Impedance measurement for improved power quality-Part 1: the measurement technique

Impedance measurement for improved power quality-Part 2: a new technique for stand-alone active shunt filter control

Power system impedance measurement using a power electronic converter

System impedance measurement for use with active filter control