A PLL-Free Robust Control Strategy With Application For Grid-Tied Fuel Cell DGs Under Asymmetrical Voltage Sags
The power grid of the future will see a heavy penetration of clean energy-based distributed generation (DG), such as solar photovoltaic (PV) power plants, wind turbines, and fuel cells. Such generation is often interfaced to the grid through power electronic converters (PECs). The interface commonly employs the dc-ac voltage source converter (VSC) which offers high efficiency and flexible operation through rapid control of active and reactive power.
Productive integration of clean energy resources is, for the most part, reliant on effectively controlling the interfacing converters. Over the past few years, fuel cell (FC) DGs (FCDGs) have become increasingly common as they possess many useful traits. They have a higher efficiency than diesel generators and can be readily incorporated with other types of renewable energy sources (RES). They can be used as primary energy sources or to supplement the intermittent nature of RES power.
Enormous benefits can be reaped from their integration into the power system, alongside a communications infrastructure, toward a reliable and stable grid operation, and two-way power flow. They can serve as key building blocks toward the rapidly emerging smart grid [2–5].
Literature Overview and Research Motivation
Control of VSCs with an application for FCDGs and hybrid FC-based DG has been broadly researched [1, 6–15]. An increase in DGs has led to new and stringent grid-connection regulations and standards  essential for maintaining stability and reliability. One such requirement is the ability of a DG source to maintain grid-connection the event of a voltage dip , also known as the low-voltage ride-through (LVRT) requirement. If the sag is not uniform across all the three phases (asymmetrical), the VSC must be carefully controlled to maintain grid connection and stability. In addition, there may be regulatory requirements on the DG source, such as injecting a prescribed amount of power into the grid. Under these conditions, it is common to decompose the measured asymmetrical voltages and currents into a set of symmetrical ones and control them separately [18, 19]. Such an operation is carried out using specialized and carefully synthesized phase locked loops (PLLs) e.g., the second-order generalized integrator-frequency-locked loop (SOGI-FLL) . The controller design and PLL tuning tasks become nontrivial and challenging and also increase the controller’s computational complexity.
Most of the existing VSC control techniques for the grid-tied FCDG address only normal operation; asymmetrical voltage dips are not addressed. And many of the current LVRT-capable strategies for grid-connected RES utilize special PLLs, leading to extra design efforts and computational overhead. The key motivation behind this research is to address some of these limitations.
This research proposes a robust, LVRT-enabled control solution for grid-tied FCDG converters having lower computational complexity and design requirements than some existing techniques. The key contributions of this work are:
- An efficient strategy utilizing uncertainty-and-disturbance estimation and repetitive control  is introduced. It has a fast transient response and offers good disturbance rejection properties.
- The proposed controller has low computational complexity and can be tuned conveniently.
- The active power remains constant, and phase currents assume a sinusoidal shape during asymmetrical voltage sags without requiring a special PLL, further lowering complexity.
- Controller design is free from the dynamics of the FC or the dc-dc converter dynamics. It makes the strategy more robust and immune to modeling inaccuracies.
- The generational capacity of the system can be increased without controller redesign. Thus, the scheme can effectively withstand network topology changes.
These findings are described in the article entitled A PLL-free robust control scheme with application to grid-connected fuel cell DGs under balanced and unbalanced conditions, recently published in the journal Sustainable Energy Technologies and Assessments.
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