Switch-mode power amplifiers with fast-dynamic characteristics for DC source emulation in microgrid testbeds
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Date
2021-09
Authors
Jayawardana, Isuru
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Abstract
Recent development in microgrids (MGs) has led to rigorous testing of power electronic interface converters, ensuring their seamless integration into the power grid or used as an off-grid alternative. In photovoltaic (PV)-based MG testbeds, DC source emulators are established as a replacement for their real counterparts as they avoid the hassle of setting up bulky, expensive and uncontrollable testing environments. Meanwhile, DC source emulators based on switch-mode power amplifiers (PAs) have attracted a great deal of attention for their low cost, high efficiency and high power density over linear PA-based emulators on complete hardware testbeds or power hardware-in-the-loop (PHIL) simulations. However, switch-mode PA has much slower dynamic response time compared to linear PAs. It is essential to have a high-bandwidth PA in a DC source emulator, like a PV emulator, to replicate the actual dynamic source characteristics and test fast load converters and also in PHIL simulations for better accuracy and stability.
This thesis develops fast-dynamic switch-mode PAs for DC source emulation through the application of non-linear boundary control (BC). Firstly, this research studies the shortcomings in second-order BC schemes for buck derived converters cascaded to capacitive loads and non-linear switching converters and proposes a BC with a corrected second-order switching surface for buck-derived PAs to overcome the loading capacitor effect. The proposed voltage-mode PA is applied to develop a fast-dynamic standalone PV emulator for testing boost-derived PV converters with fast MPPT algorithms. A novel instantaneous output impedance matching controller is introduced to generate a stable and fast convergent reference signal in PV emulators regardless of the operating region in the I-V curve. Secondly, this thesis proposes voltage-mode and current-mode switch-mode PA designs based on a synchronous buck converter with a two-stage LC filter to maintain the boundary-controlled PA performance under all types of non-linear switching loads. Results show that proposed PAs guarantee fast-dynamic characteristics without increasing switching frequency and maintain system performance under non-linear switching loads. The thesis also presents the comprehensive design and implementation of a PHIL-based PV emulator with the proposed current-mode PA.
Moreover, PHIL testbeds that incorporate two source emulators are introduced to evaluate DC-coupled PV-battery energy storage system (BESS) converters and grid-connected electric vehicle (EV) battery chargers. Mathematical frameworks to describe small-signal dynamics and analyze the stability and accuracy of PHIL setups are presented in this thesis. All DC source emulator designs are validated via experimental prototypes, and results agree well with the theoretical studies.
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Keywords
Microgrid Testbed, Power-hardware-in-the-loop Simulation, Switch-mode Power Amplifier, Boundary Control, DC Source Emulator, PV emulator