Mario Rizo

A Generalized Delayed Signal Cancellation Method for Detecting Fundamental-Frequency Positive-Sequence Three-Phase Signals

A novel scheme for obtaining the fundamental-frequency positive-sequence grid voltage vector based on a generalization of the delayed signal cancellation method is proposed in this paper. The technique is implemented by sampling and storing the instantaneous αβ voltage vector. A mathematical transformation is then proposed through which the current and delayed voltage vectors are combined. It is shown that the proposed transformation has unity gain for the fundamental-frequency positive-sequence voltage vector, while its gain is equal to zero for some chosen components. Cascaded transformations can then be used for eliminating the fundamental-frequency negative-sequence vector, as well as chosen positive- and negative-sequence harmonic vector components and, thus, for accurately obtaining the fundamental-frequency positive-sequence voltage vector. The output of the last transformation block is input to a synchronous reference frame phase-locked loop for detecting frequency and position of the positive-sequence vector. A proposal for making the scheme frequency adaptive is also presented. The good performance of the proposed method is verified with simulations and experiments by using distorted and unbalanced signals, containing fundamental-frequency as well as positive- and negative-sequence harmonic components. The proposed method frequency adaptation capability is also verified.

A Space-Vector Discrete Fourier Transform for Unbalanced and Distorted Three-Phase Signals

In this paper, a space-vector discrete-time Fourier transform is proposed for fast and precise detection of the fundamental-frequency and harmonic positive- and negative-sequence vector components of three-phase input signals. The discrete Fourier transform is applied to the three-phase signals represented by Clarkes αβ vector. It is shown that the complex numbers output from the Fourier transform are the instantaneous values of the positive- and negative-sequence harmonic component vectors of the input three-phase signals. The method allows the computation of any desired positive- or negative-sequence fundamental-frequency or harmonic vector component of the input signal. A recursive algorithm for low-effort online implementation is also presented. The detection performance for variable-frequency and interharmonic input signals is discussed. The proposed and other usual method performances are compared through simulations and experiments.

FPGA implementation of grid synchronization algorithms based on DSC, DSOGI_QSG and PLL for distributed power generation systems

Power electronic converters used in distributed power generation systems need grid synchronization systems. These synchronization methods can be divided into two fundamental parts. Whereas the first part proposes detecting grid signal sequences and harmonics, the second one is based on a PLL (Phase Locked Loop). DSC (Delay Signal Cancellation) y DSOGI-QSG (Dual Second Order Generalized Integrator-Quadrature Signal Generator) techniques are proposed in the first block. In the most of the cases, these types of algorithms are implemented by means of software in microprocessors or DSPs (Digital Signal Processors). In this paper, they are implemented on an FPGA. Thus, important advantages are achieved, like the increment of the input signals and control algorithm sample rate, parallel execution of different algorithms in order to get some information about magnitude, frequency and phase of different harmonics, etc.

Low voltage ride-through of wind turbine based on interior Permanent Magnet Synchronous Generators sensorless vector controlled

The importance of Wind Turbines based on Permanent Magnet Synchronous Generators (PMSG) has been increasing over the last years due to, fundamentally, its high efficiency. Fulfilling low voltage ride-through (LVRT) requirements is mandatory to guarantee the stability of the power systems under voltage dips, especially, in scenarios with high penetration of wind energy like Spain, Denmark, Germany, etc. This paper is focused on satisfying LVRT restrictions controlling both active and reactive powers delivered to the grid according to the more relevant grid codes. To achieve these requirements, this work proposes a method where kinetic energy storage in the turbine blades is carried out. DC active crowbar (braking chopper) is added in the converter, and, lastly, pitch angle control is used under long voltage dips. A control scheme, which emphasizes the regulation of the DC-link voltage, is designed and simulated in a Wind Turbine (WT) based on PMSG of 1MW with sensorless control.

Comparison of current controllers based on repetitive-based control and second order generalized integrators for active power filters

In this paper, two current controller schemes, based on repetitive controllers and proportional+resonant controllers using second order generalized integrators connected in parallel, are proposed and compared for shunt active power filters. Both controllers are well suited due to their optimum tracking and harmonic rejection capability. The objectives of this paper are: to analyze both controllers in the continuous and discrete time-domains; to study how to tune the parameters of the controllers and other auxiliary elements like PI controller in the case of repetitive-based controller; to show questions related to real-time implementation for the two cases; and, finally, to present some experimental and comparative results.

A space-vector discrete Fourier transform for detecting harmonic sequence components of three-phase signals

In this paper, a discrete-time algorithm for separately detecting the positive- and negative-sequence harmonic space-vector components of a three-phase signal is presented. The discrete Fourier transform is applied to the three-phase signals represented by the Clarkes aß vector. The proposed transform outputs the instantaneous values of the fundamental frequency and harmonic component vectors of the input three-phase signals. A recursive algorithm for low-effort online implementation is also presented. The detection performance for variable frequency and inter-harmonic input signals is discussed. The proposed method performance is verified through experiments.

Voltage Control Architectures for the Universal Operation of DPGS

The future concept of universal operation embraces those distributed power generation systems (DPGS) capable of getting disconnected from the main grid to start operating within an island. Once the cause of disconnection is solved, the DPGS is able to resynchronize and reconnect with the grid. In these conditions, the universal operation can be successfully implemented through a voltage control. Voltage control is manly adopted in uninterruptible power supply (UPS) and island applications. However, a study that solely focused on the universal operation is missing. The goal of this work is to help in the selection of the most suitable control structure as function of: 1) power dynamics, power quality, and robust stability in grid-connected; 2) voltage regulation and performance under local load steps in island mode; and 3) behavior during the reconnection with the grid. For that purpose, five architectures have been reviewed within a general mathematical framework and with experimental results.

Robustness analysis of Wind Turbines based on PMSG with sensorless vector control

Wind Turbine (WT) systems based on Permanent Magnet Synchronous Generators (PMSG) have become important mainly due to their high power factor and high efficiency. In order to reduce costs, maintenance and complexity, speed and position estimators can be used to replace the corresponding sensors in low and high power WT. This paper studies a PMSG flux observer, which is used along with a PLL to estimate both rotor speed and position. The robustness of the sensorless technique is analyzed against the PMSG parameters variation at low speeds, which it is usually the most problematic condition.

Frequency adaptive phase-sequence separation method based on a generalized delayed signal cancelation method

A new frequency adaptive method for detecting the Fundamental Frequency Positive Sequence (FFPS) voltage vector is proposed in this paper. The analytical developments demonstrate that the proposed method allows obtaining the FFPS αβ voltage vector canceling the effects of fundamental-frequency negative-sequence components, as well as the influences of all harmonic voltage vectors up to 24th order and also dc offsets. A high bandwidth output synchronous reference frame phase locked loop (SRF-PLL) is then used for obtaining the amplitude and position of the fundamental frequency vector. A scheme for making the proposed method adaptive to input frequency variations is also presented. The proposed method performance is verified through simulations and experiments.

Distortion-Free Saturators for Power Converters Under Unbalanced Conditions

Voltage controlled voltage source converters (VCVSC) and current controlled voltage source converters (CCVSC) have gained importance within distributed power generation systems and equipment for power quality improvement. The control action is susceptible to fall in saturation under disturbance transients or in case of poor control tuning. The use of existing saturation techniques for vector variables, such as three-phase voltage and current, distorts the reference waveforms under unbalanced conditions regardless the used reference frames--stationary (αβ-axis) or synchronous (dq-axis). These harmonics are quite detrimental for the electric grid and reduce the efficiency. This paper presents a distortion-free saturation methodology with two versions and analyzes the performance in function of the control structure (VCVSC and CCVSC) and the saturation point within the control loop. The experimental results prove the advantages of using the proposed saturators.