Ana Valdeolmillos Rodríguez

LQG Servo Controller for the Current Control of

This paper presents a current control scheme for a voltage-source converter connected to the grid through an LCL filter. The proposed current control is based on the linear quadratic (LQ) Gaussian (LQG) servo controller technique, which combines the LQ regulator and the Kalman filter (KF). The use of the KF instead of a Luenberger estimator improves the quality of the estimation of state variables in noisy environments, increasing the noise immunity of the control loop. This paper focuses on giving some criteria and practical hints to choose both the KF and LQ design parameters to achieve null steady-state tracking error, channel decoupling, and noise immunity. Furthermore, this paper analyzes the steady-state KF approximation that allows a significant decrement in the computational complexity of the algorithm at nearly no cost in accuracy. Simulation and experimental results prove the correct operation of the proposed LQG current control and the possible advantages of using KF to estimate the state vector.

LCL

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.

Grid-Connected Voltage-Source Converters

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.

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

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.

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

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.

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

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.

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

Starting from previous works [1], this paper analyzes the robustness and stability of two current controller schemes for shunt active power filters, based on repetitive controllers and proportional+resonant controllers using second order generalized integrators connected in parallel. Both controllers are well suited due to their optimum tracking and harmonic rejection capability. The objectives of this paper are: to present a brief description of both controllers, study their behavior under drifts in the value of the plant parameters and in the frequency, and also analyze the suitability of these controllers to work in a current control scheme when a nonlinear load is demanding asymmetric currents. Finally, some comparative results will be shown.

Distortion-Free Saturators for Power Converters Under Unbalanced Conditions

This paper realizes an exhaustive analysis and comparative study about the different existing techniques up to now in order to develop repetitive-based controllers applied to selective harmonic compensation in shunt active power filters. Starting from the generic repetitive controller, the advantages and drawbacks of the different versions that have arisen from the original model are compared. The paper also explains the importance of having a correct and accurate model of the plant in order to ensure stability and good performance of the entire system, justifying the method used to carry out the design of such model. The response of different controllers is compared in presence of disturbances, variations in the nominal value of the plant parameters and the computational load.

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

This paper proposes a new MPPT algorithm (‘Maximum Power Point Tracker’) for photovoltaic panels (‘PV’) with NPC converters (‘Neutral Point Clamped’). One of the characteristic of this proposal is that it is not necessary to modify the voltage references in order to know the state of the MPP (‘Maximum Power Point’). The fundamentals are based on the use of the existing ripple in the NP (‘Neutral Point’) in the NPC converters. Given that the form of the ripple is known, it is possible to determine the power that is supplied by each maximum and minimum of the voltage ripple. The execution period of the algorithm does not depend on the time constant of the DC-bus, rather the frequency of the voltage ripple, which in the NPC converter is three-time the frequency of the grid.

Analysis of repetitive-based controllers for selective harmonic compensation in active power filters

The suitability of Superconducting Magnetic Energy Storage (SMES)-based energy systems has been widely tested in the field of wind energy, being able to supply power in cases such as low wind speeds or voltage dips and store energy when there are surpluses. The aim of this paper is to analyze, improve and compare the performance of two SMES-based systems that differ in the type of power converter they use. These are a Voltage Source Converter (VSC) and a Current Source Converter (CSC). Their characteristics, control system and losses are analyzed and compared in order to improve load-leveling. In order to obtain the higher reliability and accuracy possible, a co-simulation between MATLAB/Simulink® (running the control system) and PSIM® (running the power system with commercial IGBTs and diodes) has been executed. Simulation results are shown under normal operating conditions.