The electric power system is currently undergoing a period of unprecedented changes. Environmental and sustainability concerns lead to replacement of a significant share of conventional fossil fuel-based power plants with renewable energy resources. This transition involves the major challenge of substituting synchronous machines and their well-known dynamics and controllers with power electronics-interfaced generation whose regulation and interaction with the rest of the system is yet to be fully understood. In this article, we review the challenges of such low-inertia power systems, and survey the solutions that have been put forward thus far. We strive to concisely summarize the laid-out scientific foundations as well as the practical experiences of industrial and academic demonstration projects. We touch upon the topics of power system stability, modeling, and control, and we particularly focus on the role of frequency, inertia, as well as control of power converters and from the demand-side.

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Foundations and Challenges of Low-Inertia Systems (Invited Paper)

The phase-locked loops (PLLs) are probably the most widely used synchronization technique in grid-connected applications. The main challenge that is associated with the PLLs is how to precisely and fast estimate the phase and frequency, when the grid voltage is unbalanced and/or distorted. To overcome this challenge, incorporating moving average filter(s) (MAF) into the PLL structure has been proposed in some recent literature. An MAF is a linear-phase finite-impulse-response filter, which can act as an ideal low-pass filter, if certain conditions hold. The main aim of this paper is to present the control design guidelines for a typical MAF-based PLL. The paper starts with the general description of MAFs. The main challenge associated with using the MAFs is then explained, and its possible solutions are discussed. The paper then proceeds with a brief overview of the different MAF-based PLLs. In each case, the PLL block diagram description is shown, the advantages and limitations are briefly discussed, and the tuning approach (if available) is evaluated. The paper then presents two systematic methods to design the control parameters of a typical MAF-based PLL: one for the case of using a proportional-integral (PI) type loop filter (LF) in the PLL, and the other for the case of using a proportional-integral-derivative (PID) type LF. Finally, the paper compares the performance of a well-tuned MAF-based PLL when using the PI-type LF with the results of using the PID-type LF, which provides useful insights into their capabilities and limitations.

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Moving Average Filter Based Phase-Locked Loops: Performance Analysis and Design Guidelines

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

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.

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

A new control approach of integrating a solar photovoltaic (PV) with a battery storage is presented to a single-phase grid for residential and electric vehicle application. The main purpose of the proposed work is to feed a continuous power to the grid, thereby enhancing the viability of the battery energy storage support connected to the system. The charging and discharging of the battery achieve power leveling and load leveling along with increased reliability of the system. The multifunctional voltage-source converter acts as an active power filter and performs the harmonics mitigation along with reactive power compensation. In the proposed system, a unique control is developed for resynchronization of the grid during reconnection of the grid after the mitigation of a failure. The overall control of the system is adaptable under various practically occurring situations such as disconnection of the PV array, the battery, and the grid from the system. The detailed design and control of the proposed system are presented. The validity of the proposed system is performed through a laboratory prototype developed for a power rating of 2.2 kW connected to the utility grid. The performance of the system is found satisfactory under various disturbance, and the recorded results have been demonstrated.

Implementation of a Grid-Integrated PV-Battery System for Residential and Electrical Vehicle Applications

The knowledge of the phase, amplitude and frequency of the utility voltage is a fundamental aspect for the design of the grid-connected inverter systems. In this paper are presented the basic features of the PLL technique. A particular attention is dedicated to the Synchronous Reference Frame-PLL scheme. About the generation of the orthogonal voltages system an evaluation of the most employed techniques is shown: Transport Delay, Inverse Park Transformation, Hilbert Transformation, Second Order Generalized Integrators (SOGI). Moreover some problems in filtering are treated. The internal filtering - due to the PD structure - and the external filtering - due to the harmonics presence in the grid voltage. In both the cases the most employed solutions are shown. For the internal filtering: Low Pass Filter, Resonant Filter, Moving Average Filter, Repetitive Controller. For the external filtering two alternative schemes are presented: the Dual SOGI-PLL and the Enanched-PLL (EPLL).

A. Nicastri

Papers

Comparison and evaluation of the PLL techniques for the design of the grid-connected inverter systems