Power calculation algorithm under nonlinear loads and Hopf oscillator-based synchronization controller for grid-forming inverters in a microgrid
(English) In the grid-forming control of microgrids, the droop control is a widely used for the parallel operation of power inverters, but has some peculiarities as slow dynamic and inaccuracy in the power sharing. In the droop control, the power calculation of the active and reactive powers determi...
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| Tipo de recurso: | tesis doctoral |
| Fecha de publicación: | 2023 |
| País: | España |
| Institución: | Universitat Politècnica de Catalunya (UPC) |
| Repositorio: | UPCommons. Portal del coneixement obert de la UPC |
| Idioma: | inglés |
| OAI Identifier: | oai:upcommons.upc.edu:2117/396557 |
| Acceso en línea: | https://hdl.handle.net/2117/396557 https://dx.doi.org/10.5821/dissertation-2117-396557 |
| Access Level: | acceso abierto |
| Palabra clave: | Àrees temàtiques de la UPC::Enginyeria elèctrica |
| Sumario: | (English) In the grid-forming control of microgrids, the droop control is a widely used for the parallel operation of power inverters, but has some peculiarities as slow dynamic and inaccuracy in the power sharing. In the droop control, the power calculation of the active and reactive powers determines the speed of the controller capability to share linear and nonlinear loads. Therefore, the thesis address first the design of advanced filters to improve the dynamic response for droop-based inverters. And, a fast and accurate power calculation technique is proposed for three-phase inverters by using a two-stage combined Second Order Generalized Integrator (SOGI) approach to overcome this obstacle. The two-stage combined SOGI filter structure achieves the average active and reactive powers, used as the kernel for the droop-method operation. A small-signal model of a paralleled three-phase inverters is developed for determining the dynamic response of the system, which shows that the proposal is faster than the conventional droop method when both methods are designed under the same conditions. The proposal can be used to improve the stability and transient response of droop-operated inverters. After, a novel synchronization method based on Andronov-Hopf oscillators is proposed for driving paralleled single and three-phase power inverters, which show to have lower harmonics, faster dynamic response, and higher synchronization speed than the achieved by previous nonlinear Van der Pol oscillators, used as virtual oscillator controllers (VOC). The coupling strength between Hopf oscillators relies on the local feedback of the output current, which is able to synchronize with the rest of parallel inverters, without using communications. The Hopf equations considering the current feedback generates the voltage reference for the power inverters, and has a dual-loop that is designed in a static reference frame. The Hopf has a pre-synchronization stage that allows a softer connection of additional inverters. A small-signal state-space model for two parallel VSI is built and compared with a standard droop controller. The root locus exhibits a larger stability margin and lower parameter sensitivity. Finally, a grid-forming controller based on the Hopf oscillator is proposed that is able to synchronize and achieve accurate power tracking of the references. The Hopf model is analyzed and the synchronization region is obtained by using the Arnold tongue method. In this controller, the active power can be regulated by phasor, and a PI regulator is introduced in the reactive loop to control the amplitude voltage based on the Q - V relationship achieved by the Hopf. Matlab/PLECS simulation studies and experiments on a single phase and three-phase inverters platform are conducted in a microgrid laboratory to verify the proposed control strategies. The simulations of the proposed power calculation method presents a faster and more accurate performance when sharing nonlinear loads compared with the LPF-droop. In the case of the Hopf oscillator, the simulation and experimental results demonstrate a faster transient response and better robustness compared with the standard droop controller, and a wider operating range compared with the Van der Pol VOC. The Thesis has been organized in five sections: Section 1 introduces the background, state of the art, objectives, methodology, contributions and limitations of the work; Section 2 describes the proposed power calculation method using the combined SOGI filters approach; Section 3 is devoted to the Hopf oscillator control operating in an islanded microgrid; Section 4 describes the desing of the Hopf oscillator to operate with grid forming inverters; Section 5 is devoted to conclusions and forecast future line works. |
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