Control strategies for single-phase grid integration of small-scale renewable energy sources: A review☆
Introduction
Addressing the ever increasing global demand on reliable and sustainable electricity has become a main concern of electricity sector. At the moment, fossil fuel power plants are the backbone of world's electricity generation system. On the other hand, they are recognized as a major cause for the environmental pollutions [1]. Today, millions are willing to enjoy the benefits of improved lifestyle and environment with much more electricity generated from the renewable energy sources [2], [3]. In 2010, renewable energies accounted for 16.6% of the world's primary energy demand, while a share up to 50% worldwide is feasible by 2040, according to a report published by the European Renewable Energy Council (EREC) together with its member associations (EPIA, ESHA, ESTIF, EUBIA, EUREC Agency, EWEA, AEBIOM and EGEC) [4].
Along with ambitious plans to increase the share of renewable energies in developed countries, such as EU 2020 political renewable energy goal [5], and progresses in renewable energy utilization in developing and newly industrializing countries [6], [7], [8], [9], [10], [11], [12], [13], [14], small-scale electricity generation systems, such as small hydro turbines, roof-mounted photovoltaic and wind generation systems, and commercially available fuel cells are being widely utilized at the distribution level [15], [16]. The general structure of a small-scale renewable energy source, which is usually interfaced with the single-phase grid through a stage of power electronic devices, is depicted in Fig. 1. The source-side power conversion system extracts the maximum electricity from the available renewable input power and its topology and control strategy highly depends on the type of the renewable energy source. The focus of this paper is on the grid-side converter. The main objective of the grid-side converter is to control the power flow between the ac system and the dc link powered from the renewable resources. The single-phase voltage source converter (VSC) is the best solution for interfacing with a single-phase grid. It offers several advantages such as bidirectional power flow capability and low distortions at the ac side current and the dc side voltage. Another promising feature is that it can independently control the active and reactive power exchanged with the ac network; therefore, it can improve the voltage profile, and is able to operate in weak ac systems. These advantages make the single-phase VSC a successful solution for grid integration of small-scale renewable energy sources.
To regulate the power exchange with the single-phase grid, and at the same time, reduce the harmonic distortions in the ac current, different current control structures have already been proposed, such as current hysteresis control (CHC) [17], [18], [19], [20], [21], [22], [23], [24], [25], [26], voltage oriented control (VOC) [27], [28], [29], [30], [31], and proportional-resonant (PR) based control [32], [33], [34], [35], [36], [37], [38], [39], which all have their own pros and cons.
This paper gives an overview of the main characteristics of these control strategies. The paper also includes a discussion of some implementation aspects such as the fictitious signal generation and the single-phase grid synchronization techniques. Then, through extensive simulations a comparative study of the presented control strategies is presented and discussed. The simulations are supported by experiments.
Section snippets
Control strategies for single-phase grid-tie converters
Advanced power control strategies for single-phase converters imitate the concept of decoupled active and reactive power control of three-phase converters which is realized in the synchronous reference frame. In this way, the ac current is decoupled into active and reactive power components, Id and Iq, respectively. These current components are then regulated in order to eliminate the error between the reference and measured values of the active and reactive powers. In most cases, the active
An overwiew of fictitious phase generation techniques
The single-phase circuit has one phase conductor with a neutral return. In order to extend the stationary (αβ) or synchronous (dq) reference frame control strategies to single-phase systems, at least two orthogonal signals are necessary. So, a second fictitious phase should be properly generated to model a single-phase circuit as an equivalent virtual two-phase circuit.
In the literature, there are several successful works for generating the fictitious phase including the transport delay [54],
An overwiew of single-phase grid synchronization techniques
A lot of synchronization techniques, for both single-phase and three-phase applications, are available in the literature [61]. Thanks to their simplicity, robustness, and effectiveness, the phase locked loops (PLLs) are the most accepted ones. Generally speaking, a PLL is a closed loop feedback control system, which synchronizes its output signal in phase, as well as in frequency, with the fundamental component of the grid voltage. Among various techniques, currently, the synchronous reference
Performance comparison
Computer simulations in MATLAB/Simulink were conducted for all three current control structures in order to present a performance comparison under different situations. Also, experimental verifications have been performed based on a TMS320F28335 digital signal controller from Texas Instruments. The block diagram of the studied single-phase grid connected converter is shown in Fig. 1. In simulations, the dc link is replaced with a constant dc supply and in experimental, a Chroma 6260 DC power
Conclusions
From the above conducted studies, one can conclude that if wisely designed, both VOC and PR present similar steady-state and transient performances and can successfully achieve accurate regulation with fast dynamic response with minimum overshoot and harmonic distortions. The same performance can be achieved with the CHC only when the average switching frequency and as a consequence the switching losses are quite high. On the other hand, in spite of three-phase systems, where the instantaneous
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