Showing papers by "Luiz A. C. Lopes published in 2004"

An improved perturbation and observation maximum power point tracking algorithm for PV arrays

Abstract: This paper discusses a new implementation of a perturbation and observation (P&O) maximum power point tracking (MPPT) algorithm that can mitigate/reduce the main drawbacks commonly related to the P&O method. This is achieved with peak current control, small perturbation values and sampling of instantaneous values, instead of averaged, to speed up the system response and reduce the oscillations around the maximum power point (MPP). The number of samples per switching cycle, type (synchronized/ unsynchronized) and ideal instant for sampling (maximum or minimum current) are investigated in order to obtain fast calculation of the direction of the next perturbation. Experimental results with the proposed control scheme implemented in a DSP are presented.

Analysis and comparison of islanding detection methods using a new load parameter space

Abstract: Anti-islanding protection is a mandatory feature for grid-connected inverters. The effectiveness of passive islanding detection methods (IDMs) is usually demonstrated by means of non-detection zones (NDZs) represented in a power mismatch space (/spl Delta/P /spl times/ /spl Delta/Q). A number of active type IDMs with improved performance have been presented but their effectiveness cannot be compared using the /spl Delta/P /spl times/ /spl Delta/Q space. This paper proposes a new load parameter space for comparing the effectiveness of IDMs where the influence of the load resistance can be easily assessed. Equations that represent NDZs for the /spl Delta/P /spl times/ /spl Delta/Q and proposed load parameter space are derived and verified by simulation.

Closed-loop state variable control of dynamic voltage restorers with fast compensation characteristics

Abstract: Voltage sags are the most prevalent type of power quality disturbances, and the dynamic voltage restorer (DVR) can provide load voltage support with a power rating only a fraction of that of the load. However, most reported control strategies are using open-loop feed forward control, because of the fast voltage compensation requirement, in order to reduce the impact of voltage sags to sensitive loads. It is shown that the open-loop control produces poorly damped response due to the presence of the switching harmonic LC filter. Closed-loop control permits active damping of the LC filter, and a closer tracking of the reference load voltage. Multiloop feedback PI control has attracted considerable attention, but it has difficulties meeting the fast dynamic response requirement because of the dynamic separation of two cascaded loops. This work presents a closed-loop state variable control strategy to handle the time varying control references in voltage sag compensation applications. It removes the inner current control loop that is included in the multiloop control structure, instead, the derivative of the output current is used to increase the dynamic response of the control system. Furthermore, a software based current observer is built to obtain the output current and its derivative, and to minimize the noise effects. The theoretical considerations are verified through simulation and experimental tests.

A multi-featured single-phase utility interface with reduced DC link capacitor for distributed power sources

Abstract: This paper discusses the operating principles and implementation of a multi-featured single-phase utility interface for distributed power sources. It controls the active power injected into the utility grid to keep the DC bus voltage regulated and compensates for the reactive power demanded and the current harmonics created by a local load. The DC bus voltage control loop includes a voltage ripple estimator so that the low order harmonics (2nd, 4th, 6th...) can be cancelled from the feedback signal. This way, one can use a small non-electrolytic capacitor in the dc bus without distorting the grid current despite of an increased dc bus voltage ripple. Besides, the loop bandwidth can be increased for a faster response. A three-level hysteresis current control scheme that presents reduced average switching frequency is used to impose a sinusoidal grid current in phase with the grid voltage. Experimental results in a scaled-down prototype demonstrate the feasibility of the proposed power electronic interface.