Luis Garcia de Vicuna

Decentralized Control for Parallel Operation of Distributed Generation Inverters Using Resistive Output Impedance

In this paper, a novel wireless load-sharing controller for islanding parallel inverters in an ac distributed system is proposed. The paper explorers the resistive output impedance of the parallel-connected inverters in an island microgrid

Output impedance design of parallel-connected UPS inverters with wireless load-sharing control

This paper deals with the design of the output impedance of uninterruptible power system (UPS) inverters with parallel-connection capability. In order to avoid the need for any communication among modules, the power-sharing control loops are based on the P/Q droop method. Since in these systems the power-sharing accuracy is highly sensitive to the inverters output impedance, novel control loops to achieve both stable output impedance and proper power balance are proposed. In this sense, a novel wireless controller is designed by using three nested loops: 1) the inner loop is performed by using feedback linearization control techniques, providing a good quality output voltage waveform; 2) the intermediate loop enforces the output impedance of the inverter, achieving good harmonic power sharing while maintaining low output voltage total harmonic distortion; and 3) the outer loop calculates the output active and reactive powers and adjusts the output impedance value and the output voltage frequency during the load transients, obtaining excellent power sharing without deviations in either the frequency or the amplitude of the output voltage. Simulation and experimental results are reported from a parallel-connected UPS system sharing linear and nonlinear loads.

Hierarchical Control of Intelligent Microgrids

Worldwide, electrical grids are expected to become smarter in the near future. In this sense, there is an increasing interest in intelligent and flexible microgrids, i.e., able to operate in island or in grid-connected modes. Black start operation, frequency and voltage stability, active and reactive power flow control, active power filter capabilities, and storage energy management are the functionalities expected for these small grids. This way, the energy can be generated and stored near the consumption points, thus increasing the reliability and reducing the losses produced by the large power lines. In this article, the main concepts related to the configuration, control, and energy management of intelligent microgrids are reviewed.

Virtual Impedance Loop for Droop-Controlled Single-Phase Parallel Inverters Using a Second-Order General-Integrator Scheme

This paper explores the impact of the output impedance on the active and reactive power flows between parallelized inverters operating with the droop method. In these systems, a virtual output impedance is usually added to the control loop of each inverter to improve the reactive power sharing, regardless of line-impedance unbalances and the sharing of nonlinear loads. The virtual impedance is usually implemented as the time derivative of the inverter output current, which makes the system highly sensitive to the output current noise and to nonlinear loads with high slew rate. To solve this, a second-order general-integrator (SOGI) scheme is proposed to implement the virtual impedance, which is less sensitive to the output current noise, avoids to perform the time derivative function, achieves better output-voltage total harmonic distortion, and enhances the sharing of nonlinear loads. Experimental results with two 2-kVA inverter systems under linear and nonlinear loads are provided to validate this approach.

Control Scheme for Photovoltaic Three-Phase Inverters to Minimize Peak Currents During Unbalanced Grid-Voltage Sags

Nowadays, the majority of the photovoltaic (PV) power sources are connected to the public grid. One of the main connection problems occurs when voltage sags appear in the grid due to short circuits, lightning, etc. International standards regulate the grid connection of PV systems, forcing the source to remain connected during short-time grid-voltage faults. As a consequence, during the voltage sag, the source should operate with increasing converter currents to maintain the injection of the generated power. This abnormal operation may result in nondesired system disconnections due to overcurrents. This paper proposes a controller for a PV three-phase inverter that ensures minimum peak values in the grid-injected currents, as compared with conventional controllers. From the system analysis, a design method is presented in order to set the parameters of the control scheme. Selected experimental results are reported in order to validate the effectiveness of the proposed control.

Grid-Fault Control Scheme for Three-Phase Photovoltaic Inverters With Adjustable Power Quality Characteristics

The power quality of a three-phase photovoltaic (PV) inverter drastically deteriorates in the presence of grid faults with unbalanced voltages. A ripple in the injected power and an increase in the current harmonic distortion are the main noticeable adverse effects produced by this abnormal grid situation. Several grid-fault control schemes are nowadays available for operating under unbalanced grid voltage. These control schemes usually have extreme power quality characteristics. Some of them have been conceived to completely avoid power ripple during unbalanced voltage sags, but at an expense of high current harmonic distortion. With other schemes, the harmonic distortion is totally eliminated but at an expense of high ripple in the injected power. This paper further explores the performance of PV inverters under unbalanced voltage sags. It has three theoretical contributions: 1) a generalized control scheme, which includes the aforementioned grid-fault controllers as particular cases; 2) a control strategy based on the use of continuous values for the control parameters. This original approach gives adjustable power quality characteristics that cannot be achieved with the previous control schemes; 3) three different control algorithms for calculating the continuous values of the control parameters. These contributions are experimentally validated with a digital signal processor-based laboratory prototype.

Active and Reactive Power Strategies With Peak Current Limitation for Distributed Generation Inverters During Unbalanced Grid Faults

Distributed generation inverters have become a key element to improve grid efficiency and reliability, particularly during grid faults. Under these severe perturbations, inverter-based power sources should accomplish low-voltage ride-through requirements in order to keep feeding the grid and support the grid voltage. Also, rated current can be required to better utilize reactive power provisions. This paper presents a reference generator capable to accomplish these two objectives: to keep the injected currents within safety values and to compute the power references for a better utilization of the inverter power capacity. The reference generator is fully flexible since positive and negative active and reactive powers can be simultaneously injected to improve ride-through services. Selected experimental results are reported to evaluate the performance of the proposed reference generator under different control strategies.

Control Scheme With Voltage Support Capability for Distributed Generation Inverters Under Voltage Sags

Voltage sags are one of the main problems in transmission and distribution grids with high penetration of distributed generation. This paper proposes a voltage support control scheme for grid-connected power sources under voltage sags. The control is based on the injection of reactive current with a variable ratio between positive and negative sequences. The controller determines, also, the amount of reactive power needed to restore the dropped voltage magnitudes to new reference values confined within the continuous operation limits required in grid codes. These reference values are chosen in order to guarantee low current injection when fulfilling the voltage support objective. Selected experimental results are reported in order to validate the effectiveness of the proposed control.

An Adaptive Prefiltering Method to Improve the Speed/Accuracy Tradeoff of Voltage Sequence Detection Methods Under Adverse Grid Conditions

This paper deals with the improvement of the transient response and harmonic, subharmonic, and dc-offset voltage rejection capability of a grid voltage sequence detection scheme based on a second-order generalized integrator (SOGI). To perform that, the SOGI structure is first analyzed in deep, emphasizing both its tradeoff limits between settling time and harmonic attenuation and the sensitivity to grid subharmonics and dc-offset voltage. Then, a study of the effect of grid voltage harmonics and subharmonics in SOGI and in the SOGI-FLL and MSOGI-FLL structures is introduced. Hence, to overcome these problems, a new structure based on the use of the SOGI filter as prefilter for the previous structures is proposed to achieve a faster time response and higher harmonic rejection. This structure is used in a sequence detection scheme for the detection of the grid voltage components in the αβ-frame and it is applied in a three-phase PV system. Experimental and comparative results are shown to validate this proposal.

Voltage Support Control Strategies for Static Synchronous Compensators Under Unbalanced Voltage Sags

Static synchronous compensators have been broadly employed for the provision of electrical ac network services, which include voltage regulation, network balance, and stability improvement. Several studies of such compensators have also been conducted to improve the ac network operation during unbalanced voltage sags. This paper presents a complete control scheme intended for synchronous compensators operating under these abnormal network conditions. In particular, this control scheme introduces two contributions: a novel reactive current reference generator and a new voltage support control loop. The current reference generator has as a main feature the capacity to supply the required reactive current even when the voltage drops in amplitude during the voltage sag. Thus, a safe system operation is easily guaranteed by fixing the limit required current to the maximum rated current. The voltage control loop is able to implement several control strategies by setting two voltage set points. In this paper, three voltage support control strategies are proposed, and their advantages and limitations are discussed in detail. The two theoretical contributions of this paper have been validated by experimental results. Certainly, the topic of voltage support is open for further research, and the control scheme proposed in this paper can be viewed as an interesting configuration to devise other control strategies in future works.