Victor F. Mendes

Modeling and Ride-Through Control of Doubly Fed Induction Generators During Symmetrical Voltage Sags

Modern grid codes determine that wind generation plants must not be disconnected from the grid during some levels of voltage sags and contribute to network stabilization. Wind energy conversion systems equipped with the doubly fed induction generator (DFIG) are one of the most frequently used topologies, but they are sensitive to grid disturbances due to the stator direct connection to the grid. Therefore, many efforts have been done in the last few years in order to improve their low-voltage ride-through capability. This paper analyzes the behavior of the DFIG during symmetrical voltage sags using models in the frequency domain. A new strategy, the machine magnetizing current control, is proposed in order to enhance the system response during balanced dips. The method is derived on a theoretical basis and numerically investigated by means of simulation. Experimental results are presented and validate the proposed strategy. Finally, the practical aspects of the use of this strategy are discussed.

Behavior of doubly-fed induction generator during symmetrical voltage dips — Experimental results

Wind energy conversion systems using the doubly-fed induction generator (DFIG) are the most commercialized topology worldwide, because this technology uses converters dimensioned for ±30% of the system rated power, thus reducing the equipment cost. The direct connection of the DFIG stator to the electrical system increases its sensitivity to grid disturbances. In this context, this paper analyzes the DFIG behavior during balanced voltage sags using the machine classical model for the theoretical development and analyzing experimental results of a 2.2 kW test bench equipped with a crowbar device. The obtained results are useful in the development of strategies to improve the DFIG supportability during grid disturbances.

Interconnection and damping assignment passivity-based control of a PMSG based wind turbine for maximum power tracking

The use permanent magnet synchronous generators (PMSG) in wind turbines has been increase considerably in offshore applications due to its efficiency, low maintenance costs and reliability. PMSGs are connected to the grid by means of frequency converters. Generally a back-to-back full converter structure is preferred. This work presents the application of interconnection and damping assignment passivity-based control (IDA-PBC) in order of a PMSG based wind turbine. This strategy is combined with a mechanical torque estimator in order to track the maximum power point of the system. Simulation results show the operation of the proposed strategy during wind speed variations. Proposed strategy extracts maximum power of wind turbine and injects generated power into the grid.

A novel adaptive current harmonic control strategy applied in multifunctional single-phase solar inverters

The inverter multifunctional operation is based on the harmonic current compensation, generated by nonlinear loads. The traditional harmonic detection methods tracks all harmonic contents of the load current and the control tuning tends to be complex with low flexibility. In many works, the proportional resonant (PR) controllers are used to control the inverter current reference. However, one PR controller needs to be designed for each harmonic frequency and this fact increases the control algorithm complexity. Therefore, this work proposes a novel adaptive current harmonic control strategy applied in multifunctional single-phase solar inverters. The strategy is based on a novel detection method of the harmonic load current. The harmonic current detection method is frequency adaptive and able to detect the load harmonic current with higher amplitude. This method consists in a cascade association of two phase-locked loop based on second order generalized integrator (SOGI-PLL). The detected frequency is used as feedback by the proportional resonant controller. Therefore, only two PR controller are required: one to track the fundamental component and another to track the harmonic component with higher magnitude. Simulation and experimental results show the performance of the proposed control strategy, improving significantly the grid current quality.

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.

Modeling and control of a flexible photovoltaic array simulator

Photovoltaic (PV) systems have gained prominence in the current context of distributed generation systems. The fundamental element of a PV grid-connected systems is the power converter. Power converters must be tested and certified according to the international standards, before being commercialized. Therefore, a flexible equipment to test the converters is necessary. This paper presents the modeling and control structure of a solar array simulator (SAS) of 10 kW for testing power converters. The limitations of conventional structures are pointed out and an improved structure based on two converter stages is proposed. The feasibility of the proposed SAS is shown through simulations using the Matlab/Simulink. The simulator is tested for the system connected to resistive loads and to a photovoltaic converter model. The dynamical behavior is evaluated through solar irradiance variations and partial shadowing tests. The results of the proposed structure show flexibility and good levels of power quality.

Modeling and design of a flexible solar array simulator topology

Photovoltaic (PV) systems have gained prominence in the context of distributed generation systems. The fundamental element of a PV grid-connected systems is the power converter. Before marketed power converters must be tested and certified according to the international standards. Therefore, a flexible equipment to test the converters is necessary. This paper presents the modeling and control structure of a solar array simulator (SAS) of 10 kW for testing inverters for photovoltaic applications. The limitations of conventional structures are pointed out and an improved structure based on two converter stages is proposed. The feasibility of the proposed SAS is shown through simulations using Matlab/Simulink package and experimental results. The simulator is tested for the system connected to resistive loads and to a photovoltaic converter model. The dynamical behavior is evaluated through solar irradiance variations and partial shadowing tests. Results showed that the proposed structures presents high capacity to simulate different shapes of solar irradiance, besides allow flexible configuration of the irradiance profile.

Adaptive saturation for a multifunctional three-phase photovoltaic inverter

This paper presents an inverter control scheme, based on proportional-resonant controllers, where the inverter works in multifunctional operation. In this case, the reactive power and harmonic compensation of the load are ancillary services. A resonant control is implemented in stationary reference frame (αβ), where is not necessary the use of a phase-locked loop (PLL). The instantaneous power theory (IPT) is used to detect the harmonic current and reactive power of the load. These components are used as references in the inverter control strategy. Most important fact and less related in literature is the inverter current limitation strategy. Thus, this work introduces a dynamic saturation scheme for a multifunctional three-phase inverter. This strategy possibilities partial or total reactive power and harmonic current compensation without injection of low order harmonics, ensuring that the inverter current does not exceeds the rated current.

Single-phase multifunctional inverter with dynamic saturation scheme for partial compensation of reactive power and harmonics

Single and three-phase photovoltaic inverters are essential components of the photovoltaic (PV) systems to extracting the PV power and injecting it into the grid. Thus, in order to extract the maximum power of the solar array for various solar irradiation tracks, it is used a maximum power point tracker (MPPT) algorithm. Due to variations in solar irradiance, inverters have a current margin, which is not explored during the day. Thereby, many works have proposed the multifunctional operation. This concept consists in aggregate to the inverter control strategy other functions, such as harmonics and reactive power compensation. However, most important fact and less related in literature is the necessity of techniques to compensate partially reactive power and harmonics of the load, ensuring that the inverter works below the rated current. Hence, the present work proposes a current dynamic saturation scheme in order to compensate partially reactive power and harmonics of the load during the multifunctional operation. Simulations show that the dynamic saturation prevents the inverter to inject low-order harmonics, while ensuring the operation below the system rated current. Furthermore, control performance is evaluated for five grid-connected PV system in parallel association, in order to show the effectiveness of proposed control strategy for various dispersed PV systems in the grid. To ensure that the proposed method is applied with the maximum efficiency of the PV system, this work compares, during inverter multifunctional operation, the instantaneous and dynamic efficiency between three MPPT algorithms proposed in literature: perturb and observe; dP - perturb and observe; modified perturb and observe.

Analysis of direct power control strategies applied to doubly fed induction generator

This paper presents the results of an investigation into several direct power control (DPC) strategies for the rotor side converter (RSC) of a doubly fed induction generator (DFIG) based wind energy generation system. The schemes investigated DPC of 2-level hysteresis, DPC of 3-level hysteresis and DPC using Discrete Space Vector Modulation (DSVM), with a 5-level hysteresis. The results of Matlab-Simulink simulations and a comparison between the control schemes are presented, regarding variations of active and reactive power, machine parameters, and wind speed.