David Campos-Gaona

Fault Ride-Through Improvement of DFIG-WT by Integrating a Two-Degrees-of-Freedom Internal Model Control

A novel two-degree-of-freedom internal model control (IMC) controller that improves the fault ride-through (FRT) capabilities and crowbar dynamics of doubly fed induction generator (DFIG) wind turbines is presented. As opposed to other control strategies available in the open literature, the proposed IMC controller takes into account the power limit characteristic of the DFIG back-to-back converters and their dc-link voltage response in the event of a fault and consequent crowbar operation. Results from a digital model implemented in Matlab/Simulink and verified by a laboratory scale-down prototype demonstrate the improved DFIG FRT performance with the proposed controller.

A Novel Compensation Scheme Based on a Virtual Air Gap Variable Reactor for AC Voltage Control

Voltage control based on reactive power compensation is a fundamental aspect of the operation of ac electric power systems. This paper presents a novel shunt compensation scheme based on a virtual air gap variable reactor. The scheme is fully developed, from the adaptation of the virtual air gap principle to high-voltage applications and the determination of its expected performance, to the proposal of a digital cascade control using internal model and proportional-integral controllers. The suitability and flexibility of the device, and the voltage control and reactive power compensation scheme are verified by means of laboratory tests performed in a small-scale prototype. Measured results show that the proposed device and its control provide a robust load compensation scheme for ac systems.

Control of flywheel energy storage systems as virtual synchronous machines for microgrids

Microgrids are an attractive option in remote areas with elevated renewable resources. However, with or without grid connection, microgrids often results in weak grids. Hence, microgrids are much affected by the power variations and require energy storage systems to smooth them out. Flywheel based energy storage systems (FESSs) are gaining momentum in microgrids as, despite the limited amount of stored energy, they allow to interchange high power and have long useful lifetime. In addition, the state-of-charge is very simple to estimate as it only consists on measuring the spinning speed. In FESSs, the flywheel is attached to an electrical machine, which is connected to the grid through a back-to-back converter. Controlling the three phase converter as a virtual synchronous machine allows to overcome instability issues of the PLL-based control in a weak grids. For his reason, his paper proposes to control the grid-side converter of the FESS as a virtual synchronous machine. Small signal analysis is applied to the equations of the virual synchronous machine. The DC-link voltage is regulated by the machine-side converter. Simulation results are provided to illustrate the proposed concepts.

DC-Link Control Filtering Options for Torque Ripple Reduction in Low-Power Wind Turbines

Small wind energy conversion systems (WECSs) are becoming an attractive option for distributed energy generation. WECSs use permanent-magnet synchronous generators (PMSGs) directly coupled to the wind turbine and connected to the grid through a single-phase grid-tie converter. The loading produced on the dc link is characterized by large ripple currents at twice the grid frequency. These ripple currents are reflected through the dc bus into the PMSG, causing increased heating and ripple torque. In this paper, the PMSG inverter is used to control the dc-link voltage. In order to avoid reflecting the ripple currents into the PMSG, the feedback dc-link voltage is passed through a filter. The Butterworth filters, notch filters, antiresonant filter (ARF) and moving average filter (MAF) are considered. For a fair comparison, formulas are provided to tune the filter parameters so that dc-link voltage control will achieve the selected bandwidth. The different filtering options produce different levels of torque ripple reduction. The notch filter, ARF, and MAF obtain the best results and there is a tradeoff between the filter implementation complexity, bandwidth, overshoot, and the torque ripple reduction. Simulations and experiments using a 2.5-kW PMSG turbine generator validate the proposals.

Modeling and control design of a Vienna rectifier based electrolyzer

Hydrogen production is an interesting alternative of storing energy. Electrolyzers produce hydrogen through water electrolysis; the resulting hydrogen is later used to generate electricity by using fuel cells, that reverse the process. Electrolyzers use rectifiers to convert the grid ac voltage into dc voltage for supplying the electrolyzer cells. Previous research used a rectification process based on conventional rectifiers (diode- or thyristor-based) which draw non-sinusoidal current from the main grid. This requires increased filtering to prevent power quality problems and equipment malfunctioning/failure. In addition, previous literature assumed simplified models for the power electronics converters and lacked a detailed control system. The Vienna rectifier is a non-regenerative converter that produces sinusoidal currents with low losses due to the reduced number of active switches. This manuscript proposes using the Vienna rectifier as an interface to connect electrolyzers to the ac grid. The dc voltage applied to the electrolyzer is regulated by using another dc-dc converter, which is selected to be a synchronous buck converter for simplicity and maximum efficiency. In this paper, the models of the Vienna rectifier, synchronous buck converter, and the electrolyzer are developed along with their respective controls. The control system has the ability to function in two operation modes for the overall reference: hydrogen production and power demand. The first one is adequate for grid-connected operation and the later for off-grid operation. Simulation results are given to show the validity of the proposed procedures.

Dynamic mitigation of grid current harmonics using the power sphere concept in voltage source inverters

Voltage source inverters (VSI) connected to intermittent renewable sources may have periods of low active power production. When the power generated at the source is below the power rating of the inverter, the remaining unused rating of the inverter can be used to compensate the harmonic distortion in the grid. This paper presents the realization of a VSI with Active Power Filter capabilities, which is able to control the compensation of individual load harmonics up to the 17th as a function of the variation of the unused rating of a VSI. In order to select the number and magnitude of harmonic to mitigate, the paper analyses the utilization rating of the VSI in light of the power theory used in the latest IEEE standards. This analysis results in the derivation of a novel “power sphere” concept that is helpful to establish the magnitude of harmonic compensation needed to fully utilize the unused rating of a VSI at a given time. The performance of the selective compensation of individual harmonics in a VSI is validated in simulations and in a 3-phase 1kW DSC-controlled experimental platform.

Robust Active Damping in LCL -Filter-Based Medium-Voltage Parallel Grid Inverters for Wind Turbines

LCL-filter-based grid-tie inverters require damping for the current-loop stability. There are only software modifications in active damping, whereas resistors are added in passive damping. Although passive damping incurs in additional losses, it is widely used because of its simplicity. This paper considers the active damping in medium-voltage parallel inverters for wind turbines. Due to cost reasons, only minimal software changes are allowed and no extra sensors can be used. The procedure must be robust against line-inductance variations in weak grids. Double-update mode is needed so the resonance frequency is under the Nyquist limit. The bandwidth reduction when using active damping is also required to be known beforehand. Moreover, the design procedure should be simple without requiring numerous trial-and-error iterations. In spite of the abundant literature, the options are limited under these circumstances. Filter-based solutions are appropriate and a new procedure for tuning the notch filter is proposed. However, this procedure requires that the resistance of the inductors is known and a novel filter-based solution is proposed that uses lag filters. The lag filters displace the phase angle at the resonance frequency so that the Nyquist stability criterion is fulfilled. Simulations and experiments with a 100-kVA prototype validate the analysis.

MPPT and control design of a Vienna rectifier-based low power wind turbine with reduced number of sensors

Low power wind turbines (LPWT) are an interesting option for distributed generation. Maximum power point tracking (MPPT) procedures for LPWT are either based on look-up tables or use an algorithm. This paper proposes an MPPT algorithm for wind energy analogous to that of the incremental inductance for solar energy. The Vienna rectifier is an efficient non-regenerative power converter with only three switching devices and its use for LPWT can be advantageous. The permanent magnet synchronous generator (PMSG) is a common option for LPWT because of its efficiency. This manuscript explains the sensorless vector control of PMSG using the Vienna rectifier, which results to be analogous to that of the traditional full bridge converter for unity power factor. In addition, the unbalanced voltage in the Vienna rectifier capacitors is derived from the modulation indexes without requiring an additional sensor. Hence, the resulting LPWT has low losses by virtue of the Vienna rectifier, and lower cost and higher reliability by virtue of the sensorless vector control and unbalance voltage estimation. Simulation results validate the proposals of the paper.

Control design of a neutral point clamped converter based active power filter for the selective harmonic compensation

This paper presents the control of an active power filter (APF) based on a 3-phase, 3-level neutral point clamped (NPC) converter with selective harmonic compensation. To achieve the selective harmonic compensation, the APF use several synchronous rotatory frames, which are rotating at the angular frequency and sequence of their respective harmonics, to detect and control the magnitude and angle of each individual harmonic using d and q variables. A three dimensional space vector modulator (3D-SVPWM) is used to generate the compensation currents. Due to its multilevel topology, the proposed active power filter can be used in high voltage power quality applications, such as sub-transmission and distribution levels. Simulation results are shown to validate the proposed solution and corroborate the proper function of the multilevel active power filter.