This article deals with the vector control in dq axes of a three-phase grid-connected photovoltaic system with single-stage topology and low-voltage-ride-through capability. The photovoltaic ...generator is built using an array of several series-parallel Suntech PV modules and is modeled as a Lookup Table (two-dimensional; 2-D). The requirements adopted when grid voltage sags occur are based in both the IEC 61400-21 European normative and the allowed amount of reactive power to be delivered according to the Spanish grid code, which avoids the disconnection of the inverter under grid faults by a limitation in the magnitude of the three-phase output inverter currents. For this, the calculation of the positive- and negative-sequences of the grid voltages is made and a conventional three-phase Phase-Locked Loop is used for the inverter-grid synchronization, allowing the control of the active and reactive powers solely with the dq components of the inverter currents. A detailed enhanced flowchart of the control algorithm with low-voltage-ride-through capability is presented and several simulations and experiments using Matlab/SIMULINK and the Controller Hardware-in-the-Loop simulation technique, respectively, are run for several types of one- and three-phase voltage sags in order to validate its behavior.
The quality of power and current control are the greatest challenges of grid-connected wind farms during abnormal conditions. The negative- and positive-sequence components of the grid currents may ...be injected into a wind generation system during grid faults, which can affect the power stability and damage the wind system. The proposed work assures a low-voltage ride through capability of doubly-fed induction generator- based wind turbines under the grid voltage sag. A new technique to protect the wind system and to recompense the reactive power during failures of the utility grid according to the Spanish grid code is proposed. The control design is implemented to the power converters, and the grid current regulation is developed by using proportional-resonant regulators in a stationary two-phase (αβ) reference frame. The control performance is significantly validated by applying the real-time simulation for the rotor-side converter and the hardware in the loop simulation technique for the experiment of the generator’s grid-side converter control.
This review article is mainly oriented to the control and applications of modular multilevel converters (MMC). The main topologies of the switching modules are presented, for normal operation and for ...the elimination of DC faults. Methods to keep the capacitor voltage balanced are included. The voltage and current modulators, that are the most internal loops of control, are detailed. Voltage control and current control schemes are included which regulate DC link voltage and reactive power. The cases of unbalanced and distorted networks are analyzed, and schemes are proposed so that MMC contribute to improve the quality of the grid in these situations. The main applications in high voltage direct current (HVDC) transmission along with other medium voltage (MV) and low voltage (LV) applications are included. Finally, the application to offshore wind farms is specifically analyzed.
•This paper gives a performance study of the Positive Sequence Detector plus a dqPLL.•A real-time digital simulation platform is used to validate the results.•With low-level harmonic pollution it is ...not a necessity to use a complicated synchronization algorithm.•A significant attenuation of the harmonic contamination is attained when the PSD block is used.•A good power factor can be obtained even though variations in frequency occur.
In a distributed generation (DG) system, several renewable agents are connected to the low-voltage 3-phase utility grid through an inverter which is used as power condition and must guarantee the higher efficiency of the renewable agent. To attain this level of efficiency, a unitary power factor (FP) between the inverter currents and the utility grid voltages is necessary, and a synchronization algorithm is needed for the perfect synchronization between the renewable agent and the 3-phase utility grid. Within this context, this paper gives a performance study of the Positive Sequence Detector plus a Synchronous Reference Frame Phase-Locked Loop (PSD+dqPLL) as the synchronization algorithm, evaluating its accuracy under different conditions and studying their advantages and drawbacks. A grid-connected photovoltaic system with a nominal power of 6kW is used so as to evaluate the behavior of the synchronization algorithm when the 3-phase utility grid is affected by some disturbances such as voltage unbalances, frequency variations and harmonic distortions. Firstly, several simulations with a disturbed 3-phase utility grid using MATLAB/SIMULINK from The MathWorks, Inc. are shown, and secondly, the previous tests are run in a Real-Time Digital Simulation (RTDS) platform in order to validate the obtained results with simulations.
The uninterruptible operation of grid-connected renewables under the occurrence of grid voltage sags is addressed in this work. This is achieved due to the incorporation of an enhanced control ...algorithm of a renewable source. The low-voltage ride-through algorithm was developed in accordance to the voltage profile introduced by IEC 61400-21 regarding grid voltage sags. To guarantee continuous operation of the renewable agent during voltage sags, not only instantaneous reactive power but also instantaneous active power under moderate voltage sags was injected to the utility grid fulfilling grid code requirements. A dual second-order generalized integrator frequency-locked loop synchronization algorithm was used to estimate the system’s frequency, together with the positive and the negative sequences of the three-phase utility grid voltages when unbalanced sags occurred. The current control was made in a stationary reference frame by using proportional-resonant regulators, and a DC voltage source was used to emulate the primary energy from any type of renewable system. The validation of the proposed control algorithm was conducted for a three-phase grid-connected renewable system with an apparent power of 500 kVA. The results from several experimental tests demonstrated the proper behavior of the enhanced algorithm.
•Modular multilevel converter (MMC) that uses the near level control (NLC).•Influence of the number of switching modules and the switching period.•Comparison of the harmonics of voltage and current ...with grid codes.•Connecting the MMC to the electrical grid without using any coupling inductor.
This paper is focused on the modular multilevel converter (MMC) topology that uses the near level control (NLC) method. Specifically, it addresses the relationship between the number of levels or switching modules, the switching frequency and the harmonics superimposed on the generated voltages and currents, making a comparison with the high and medium voltage AC codes. Furthermore, it also assesses the possibility of connecting the MMC to the electrical grid without using any coupling inductor, either using a transformer or simply directly. Finally, it shows how to automate the simulations necessary to select the number of levels and the switching frequency.
•Control of the modular multilevel converter (MMC).•Reduction of the effect of grid harmonics.•Proportional resonant (PR) controllers and a harmonic compensator (HC) scheme.•Compliance with the low ...voltage ride through (LVRT) regulation.
This paper is about the control of Modular multilevel converters, an innovative technology in the design of converters, which is beginning to be included in real installations. Papers about this topic include simulation models, circulating current reduction, voltage modulators, capacitor voltage balancing and control issues. The scheme for current source regulation used in this article includes all control loops, which are, from the outermost to innermost, DC bus voltage regulator, current regulator, voltage modulator, capacitor voltage balancing, and a PLL for the synchronization to the grid. Disposition-sinusoidal pulse width modulation is used as the voltage modulator, and an enhanced control strategy in the stationary reference frame for 3-phase MMCs is used for the inner current control loops. Very detailed simulations of the complete control system have been performed for both the enhanced control strategy in the stationary reference frame, and the well-known control in the synchronous reference frame, as well as some experiments using the hardware-in-the-loop simulation technique. The validation of these control strategies is made by a comparison of the capability of each one to compensate the harmonic distortions of the utility grid according to the grid code. The correct operation has been tested in the case of a strong/weak grid, unbalances and grid failures.
•Frequency-adaptive vector control of a grid-connected PV system.•MSOGI-FLL synchronization algorithm.•Low-Voltage Ride-Through (LVRT) capability with improved limitation of the amplitude of the ...three-phase inverter currents.•Controller Hardware-in-the-Loop (CHIL) simulation technique.
The low-voltage ride-through service is carried out in this paper according to the voltage profile described by the IEC 61400-21 European normative when short-duration voltage sags happen, and some instantaneous reactive power is delivered to the grid in accordance with the Spanish grid code; the mandatory limitation of the amplitude of the three-phase inverter currents to its nominal value is carried out with a novel control strategy, in which a certain amount of instantaneous constant active power can also be delivered to the grid when small or moderate voltage sags happen. A Multiple second order generalized integrator frequency-locked loop synchronization algorithm is employed in order to estimate the system frequency without harmonic distortions, as well as to output the positive- and the negative- sequence of the αβ quantities of the three-phase grid voltages when balanced and unbalanced voltage sags happen in a frequency- adaptive scheme. The current control is carried out in the stationary reference frame, which guarantees the cancellation of the harmonic distortions in the utility grid currents using a Harmonic compensation structure, and the implementation of a constant active power control in order to protect the DC link capacitor from thermal stresses avoiding the appearance of large harmonic distortions at twice the fundamental frequency in the DC link voltage. A case study of a three-phase single-stage grid-connected PV system with a maximum apparent power about 500 kVA is tested with several simulations using MATLAB/SIMULINK firstly, and secondly, with some experiments using the Controller hardware-in-the-loop (CHIL) simulation technique for several types of voltage sags in order to do the final validation of the control algorithms.
This paper presents five synchronization algorithms that are commonly used in grid-connected renewable systems. The algorithm structures are studied and their corresponding block diagrams are shown ...and explained in detail. The behaviors of the algorithms are studied according of their response under voltage unbalances, frequency variations and low-order voltage harmonics. The advantages and disadvantages of the algorithms are discussed by analyzing some simulations done with MATLAB/SIMULINK tool from The MathWorks, Inc. Later on, the final validation is done by doing real-time digital tests of the synchronization algorithms with a Real-Time Digital Simulator (RTDS) platform. A grid-connected photovoltaic system with a nominal power of 10kW is used to evaluate the responses of the synchronization algorithms when the utility grid is affected by disturbances.
In this paper, an implementation of the control and the synchronization algorithms for a Voltage Source Inverter used as the power conditioner for Photovoltaic renewable energy in a grid-connected ...structure is carried out. Its main purpose is to show, in a simple manner, the design and combined operation of the control and synchronization algorithms for attaining the proper behaviour of the Grid Inverter when the 3-phase utility grid is disturbed by voltage unbalances, frequency variations and harmonic distortions, according to power quality standards.
In order to obtain a high efficiency of the system during perturbations, a Proportional Resonant controller with a Harmonic Compensator structure is designed for the control algorithm, whereas a Dual Second Order Generalized Integrator Frequency-Locked Loop (DSOGI-FLL) is used as the synchronization algorithm.
In order to validate both the control and the synchronization algorithms, some simulations using MATLAB/SIMULINK from The MathWorks, Inc. are shown firstly, and secondly, some real-time digital simulations are carried out.
•A DSOGI-FLL is used to detect the frequency.•Low order harmonics are introduced in the utility grid.•High rejection of the harmonic components is attained.•Unitary power factor operation is attained.•Combined operation of the DSOGI-FLL and the PR regulator.
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