Today, conventional power systems are evolving to modern smart grids, where interconnected microgrids may dominate the distribution system with high penetration of renewable energy and energy storage systems. The hybrid ac/dc systems with dc and ac sources/loads are considered to be the most possible future distribution or even transmission structures. For such hybrid ac/dc microgrids, power management strategies are one of the most critical operation aspects. This paper presents an overview of power management strategies for a hybrid ac/dc microgrid system, which includes different system structures (ac-coupled, dc-coupled, and ac–dc-coupled hybrid microgrids), different operation modes, a thorough study of various power management and control schemes in both steady state and transient conditions, and examples of power management and control strategies. Finally, discussion and recommendations of power management strategies for the further research are presented.

Overview of Power Management Strategies of Hybrid AC/DC Microgrid

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.

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

Pushed by the booming installations of single-phase photovoltaic (PV) systems, the grid demands regarding the integration of PV systems are expected to be modified. Hence, the future PV systems should become more active with functionalities of low-voltage ride through and grid support capability. The control methods, together with grid synchronization techniques, are responsible for the generation of appropriate reference signals in order to handle ride-through grid faults. Thus, it is necessary to evaluate the behaviors of grid synchronization methods and control possibilities in single-phase systems under grid faults. The intent of this paper is to present a benchmarking of grid fault modes that might come in future single-phase PV systems. In order to map future challenges, the relevant synchronization and control strategies are discussed. Some faulty modes are studied experimentally and provided at the end of this paper. It is concluded that there are extensive control possibilities in single-phase PV systems under grid faults. The second-order-general-integral-based phase-locked-loop technique might be the most promising candidate for future single-phase PV systems because of its fast adaptive-filtering characteristics and it is able to fulfill future standards.

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Benchmarking of Grid Fault Modes in Single-Phase Grid-Connected Photovoltaic Systems

Transient stability of a power network requires that generators remain synchronized and return to normal power export once a fault is cleared. For inverter-interfaced generators, one must ensure that current and voltage limiters do not latch-up and that controller integrators do not wind-up. A comparison of current-limiting strategies during fault ride-through of inverters to prevent latch-up and wind-up is presented. A voltage-controlled inverter with an inner current controller is used in this paper. Instantaneous limiting (saturation) and latched limiting with a variety of reset strategies are tested to check for correct operation when a fault is applied and cleared. All the cases were tested on an experimental system using 10-kVA inverters and low-impedance three-phase faults. The experimental results showing the current and voltage waveforms of the inverter are presented to test whether each strategy correctly transitioned from current limiting to normal operation once the fault was cleared and to examine the extent to which controller wind-up was a problem. Conclusions are drawn as to which current-limiting strategies provide good performance in ride-through and recovery from faults.

/pdf/comparison-of-current-limiting-strategies-during-fault-ride-4d86ruz39o.pdf

Comparison of Current-Limiting Strategies During Fault Ride-Through of Inverters to Prevent Latch-Up and Wind-Up

The application of Photovoltaic (PV) in the distributed generation system is acquiring more consideration with the developments in power electronics technology and global environmental concerns. Solar PV is playing a key role in consuming the solar energy for the generation of electric power. The use of solar PV is growing exponentially due to its clean, pollution-free, abundant, and inexhaustible nature. In grid-connected PV systems, significant attention is required in the design and operation of the inverter to achieve high efficiency for diverse power structures. The requirements for the grid-connected inverter include; low total harmonic distortion of the currents injected into the grid, maximum power point tracking, high efficiency, and controlled power injected into the grid. The performance of the inverters connected to the grid depends mainly on the control scheme applied. In this review, the global status of the PV market, classification of the PV system, configurations of the grid-connected PV inverter, classification of various inverter types, and topologies are discussed, described and presented in a schematic manner. A concise summary of the control methods for single- and three-phase inverters has also been presented. In addition, various controllers applied to grid-tied inverter are thoroughly reviewed and compared. Finally, the criteria for the selection of inverters and the future trends are comprehensively presented.

A comprehensive review on inverter topologies and control strategies for grid connected photovoltaic system

Ancillary services for distributed generation (DG) systems become a challenging issue to smartly integrate renewable-energy sources into the grid. Voltage control is one of these ancillary services which can ride through and support the voltage under grid faults. Grid codes from the transmission system operators describe the behavior of the energy source, regulating voltage limits and reactive power injection to remain connected and support the grid under fault. On the basis that different kinds of voltage sags require different voltage support strategies, a flexible control scheme for three-phase grid-connected inverters is proposed. In three-phase balanced voltage sags, the inverter should inject reactive power in order to raise the voltage in all phases. In one- or two-phase faults, the main concern of the DG inverter is to equalize voltages by reducing the negative symmetric sequence and clear the phase jump. Due to system limitations, a balance between these two extreme policies is mandatory. Thus, over- and undervoltage can be avoided, and the proposed control scheme prevents disconnection while achieving the desired voltage support service. The main contribution of this work is the introduction of a control algorithm for reference current generation that provides flexible voltage support under grid faults. Two different voltage sags have been experimentally tested to illustrate the behavior of the proposed voltage support control scheme.

Flexible Voltage Support Control for Three-Phase Distributed Generation Inverters Under Grid Fault

This paper proposes a new structure for grid synchronization based on generalized integrators (GI) that lead to fast detection of positive and negative components of the grid voltage. The GI are used as a band pass filter (GI-BPF) mainly to provide in-phase and quadrature components of grid voltage. The proposed structure consists in the cascaded connection of multiple GI-BPF to achieve a faster time response than a single GI-BPF but with the same bandwidth characteristics. This structure is used with a frequency locked loop and a positive sequence detector to operate under unbalanced voltage sags. Simulations and experimental results of a photovoltaic power system injecting power to the grid in the stationary reference frame are shown to validate this proposal. Comparisons between this proposal and one that uses a single GI-BPF in the sequence detector are provided. The results show that the proposed cascaded GI-BPF achieves a considerably time response reduction in the sequence grid detection under a fault with the same quality-filter characteristics.

Fast grid synchronization technique based on a multiple cascaded general integrator scheme for distributed generation inverters

Unbalanced voltage sags strongly deteriorate the performance of power converters and electrical machines connected to the distribution network In particular, a reduction of the power quality is noticed in these equipments, which is caused by a ripple in the output power and an increase of the current harmonic distortion Several solutions based on reactive power compensation have been introduced to cope with this problem Among them, static synchronous compensators have confirmed the superior performance due to the possibility of using all rated power exclusively to voltage support This paper presents a simple reactive power control algorithm for static synchronous compensators operating under unbalanced voltage sags The main feature of the proposed algorithm is that it allows a fine adjustment of the amount of reactive power injected by the positive- and negative-sequence components Thus the control algorithm provides voltage support at the output of the static synchronous compensator Specifically the proposed control conveniently corrects both the positive- and negative-sequence voltage Selected simulation results are provided to validate the proposal

Coordinated reactive power control for static synchronous compensators under unbalanced voltage sags

Grid faults are one of the most severe perturbations to deal with in grid connected systems. To support the grid under voltage-sags (dips), different reactive power control strategies are needed depending on the type of grid fault. One of the possible objectives when supporting the grid voltage under type C voltage sags could be to avoid undervoltage in the faulted phases and over-voltage in the non-faulted phase. Therefore, the fault can ride-through the perturbation and the system disconnection can be avoided. This may be accomplished by injecting positive sequence reactive power to raise the voltages and negative sequence reactive power to equalize them.

https://esaii.upc.edu/people/pmarti/12IECON.pdf

Reactive power control for voltage support during type C voltage-sags

A Voltage Sourced Inverter (VSI) intended to inject active power to the grid is presented. The VSI is designed with an LCL output filter. This objective is done by means of a sliding mode control, which is a widely used technique to control systems where a fast dynamic response, robustness and accuracy are required. However, variable switching frequency appears when the sliding mode control is implemented with a fixed-band hysteretic modulator, and consequently, a ripple current is spread all over the bandwidth. This leads to a cumbersome output system filter design. In addition, sensitivity to switching interferences between phases appear in three-phase systems. A solution to these limitations is proposed by means of a current sliding mode control where feedforward variables to create a variable hysteresis envelope are used. The surface derivation and principle of decoupling between phased is presented in this paper, furthermore, an analysis of frequency stabilization method and simulation results are shown.

Eduardo Alarcon-Gallo

Papers

Flexible Voltage Support Control for Three-Phase Distributed Generation Inverters Under Grid Fault

Fast grid synchronization technique based on a multiple cascaded general integrator scheme for distributed generation inverters

Coordinated reactive power control for static synchronous compensators under unbalanced voltage sags

Reactive power control for voltage support during type C voltage-sags

Decoupled sliding mode control for three-phase LCL VSI operating at fixed switching frequency