Frederico F. V. Matos

Regenerative PWM source for power transformer loading tests

Most of power transformers, before being commercialized, must undertake a series of tests with the objective of analyzing their operational behavior under real operating conditions. Such tests sometimes demand a high amount of power increasing significantly their costs. The paper presents a proposed system which allows testing in closed loop, reducing the energy consumption to the natural losses of the equipment, thus constituting a regenerative system. A VSI-based test environment for power transformers where controlled active and reactive power can be supplied is designed, simulated and realized as a 50KVA- prototype. Special care is taken concerning the ac-filters in order to keep the harmonics within the limits set by standards valid for testing of power transformers. The simulation and experimental results show that the prototype fulfills the given requirements.

Doubly-fed induction generator control during unbalanced grid conditions

In the last few years, the number of grid connected wind conversion systems (WECS) has increased rapidly. One of the most commercialized WECS technologies is the doubly-fed induction generator (DFIG) that offers operational and economical advantages over other ones. Nevertheless, this system has as the main drawback the high susceptibility to grid disturbances. During unbalanced grid voltages, negative sequence component of flux arises, inducing high distorted rotor voltages that influence the system control. The classical control topology behavior and the performance of two control strategies (dual PI and resonant controllers) for controlling the negative sequence component are analyzed. The analysis are carried out through simulation results in a 2MW WECS model, and validated with experimental results in a 4kW scaled test bench.

Low-voltage ride-through analysis of permanent magnet synchronous wind generator with harmonic distortion and frequency deviation using resonant controllers

The use of wind turbine as electricity source is growing globally and as consequence the grid codes are becoming more restrict. The restrictions depends on the country that the system is installed and basically limits the system disconnections during determined faults and demands the injection or absorption of active or reactive power during those faults to ensure a fast return of the system to its normal active power generation. This paper presents a study of a wind energy conversion system operating under unbalanced voltage sags using resonant controllers. The control design is presented and the system is simulated using Matlab/Simulink. The proposed control structure is capable of controlling the active or reactive power delivered to grid during the voltage sags with no oscillations. Simulation results show the control structure response to the voltage disturbances.

Low-voltage ride-through of PM synchronous wind generator under asymmetrical voltage sags with constant power

As the wind energy generation becomes more frequently employed worldwide, specific grid codes concerning power quality are required for these systems. Several countries have determined that the wind turbines cannot be disconnected from the grid during voltage sags, depending on the severity of the sags. This has pushed manufacturers to implement low-voltage-ride-through strategies in their turbines. Some grid codes not only demand that the turbine remain connected but also that it must provide active or reactive power in order to aid the recovery from the fault. This paper presents a strategy of low-voltage-ride-through that uses resonant controllers in the synchronous reference frame in order to withstand unbalanced voltage sags in a permanent magnet synchronous machine topology with full-scale power converter. It presents also the current references calculation so that the output active or reactive power can be controlled with oscillations. Simulation results are shown to demonstrate the strategy.

A multilevel wind power conversion system with an open winding squirrel cage induction generator

The growing use of wind energy conversion systems around the world is pushing the individual power of wind turbines to higher values. Especially in offshore farms, where installation and maintenance have higher costs, high power wind turbines and their reliability become critical issues to make investments economically viable. In this context, multilevel converters, widely used in high power drive systems, have been gaining attention also for variable frequency generation systems. Among the several topologies developed for multilevel converters, there is a structure that employs 2 conventional two-level converters connected to an open winding machine, resulting in a three-level behavior. This structure will be studied in this paper, applied to wind power conversion with a squirrel cage induction generator. The structure is interesting for its high number of redundant switching states, compared to the conventional system, and for the possibility to sustain operation if one of the converters fails. The squirrel cage induction machine is a cheap and robust solution, which is very important when dealing with high power and offshore exploration. The resulting switching states and an initial modulator design will be presented along with the machine modeling, leading to a dynamic simulation of the system.