|Advanced control of power converters for microgrids with renewable energy resources and variable loads
|Chan, Ka Wing (EE)
Hu, Jiefeng (EE)
Electric current converters
Hong Kong Polytechnic University -- Dissertations
|Department of Electrical Engineering
|xxii, 153 pages : color illustrations
|For the last several decades, cascaded linear control (CLC) has been widely used in the field of power electronics. However, it presents several major problems. First, for the control structure, it suffers from "more loops, more complicated". Second, in order to drive the switching unit, pulse-width modulation (PWM) is necessary to be adopted, which results in a slow dynamic response. Third, proportional-integral-differential (PID) controllers are normally embedded in these CLC control loops, which results in a tedious work of repeatedly tuning the parameters and thus the implementation is restricted. Early in this century, the concept of microgrid (MG) was proposed to meet the proliferated green energies. And now it is suggested to be one of the future dominating forms of power supply. An MG can form an independent controllable unit with an integration of renewable energy sources (RESs), storages, loads and control systems. As a key component to interface RESs into the MG, the power electronic converter needs to be well regulated for the MG. The droop control method is typically used for these converters to deal with parallel-connected RESs by proportionally sharing their powers. Traditionally, voltage and current double loops are attached to the droop control method to produce driving signals for the switching units. However, subject to the limitations of these CLC double loops, the advantages of droop function may be compromised. In addition, for existing research about MGs, ideal and steady DC power sources are usually adopted to simulate a variety of RESs. However, in practice, this is deficient and incomplete since the intermittent nature of such energy resources is overlooked. Consequently, traditional CLC control may no longer be the best choice for MG control, and this results in the urgent requirement to develop advanced control methods to ensure a better control performance rather than using the CLC method. In these years, model predictive control (MPC), as a distinctly different technique from CLC control, has been introduced to effectively control these converters. MPC is based on the knowledge of system topology and the prediction of system behavior. Finite control set model predictive control (FCS-MPC) is an important branch of the MPC family. For FCSMPC, the optimal switching state of power converters is determined according to a prespecified cost function. By taking into account current system states, cost function can be solved over some future certain intervals. As reported in existing literature, compared with CLC, MPC can be utilized to achieve a better performance such as a faster dynamic response, an easier way to include nonlinear constraints and a better robustness, etc. Although MPC is advantageous, it is seldom reported about its application in the coordination control of multiple converters in MGs.
In this thesis, various power converters controlled by MPC have been investigated and developed, which involves bidirectional DC-DC buck-boost converters and AC-DC interlinking converters. They are controlled through analyzing the systemic dynamic behaviors (especially for inductors and capacitors used in the circuit). Then predictive model is built, cost function is defined, and optimization process is computed repeatedly. In this way, traditional CLC control loops have been replaced, and further, the coordinated control of these converters applied in various MG types has also been researched. Comprehensive cases related to the MG operation have also been studied such as islanded operation, grid-connected operation, islanding process, and grid synchronization. Besides, in order to address the aforementioned issues about simulating a more realistic DC power source, photovoltaic (PV) systems and/or wind turbines (WTs) as well as energy storage systems (ESSs) are combined to provide a DC common bus serving as a practical DC source. In this context, the intermittent nature of RESs influenced by natural environmental factors (like solar irradiation, ambient temperature, wind speed, etc) is considered. For the standard operation of an MG, the hierarchical control structure with three levels is universally accepted. In the first and basic level, the well-known droop control method serves as the primary control. Since droop control method will cause frequency and voltage deviations, secondary control is necessary to be adopted. In general, centralized secondary control is often used, which works via central PI controllers making adjustment signals based on the measurements from common buses. However, this centralized way suffers from a single point of failure and the potential breakdown of the whole system. Therefore, a washout filter based method which is a distributed secondary control using only localized measurements with higher flexibility and reliability is developed here, meanwhile, the power sharing belonging to the primary control domain is also achieved. In this thesis, a series of control schemes based on MPC principles has been developed for different MGs with various properties. Specifically, in an MG system, for DC-side control, MPC-based schemes are applied on bidirectional DC-DC converters to form a stable and robust DC bus; while for AC-side control, MPC-based schemes are applied on VSC-based interlinking converters to ensure a stable and high-quality AC voltage supply. Besides, the fluctuating power resulted from intermittent RESs has been respected and investigated. Moreover, a system-level energy management scheme (EMS), an auxiliary voltage support functionality, and a washout filter based power sharing method with voltage compensation are also developed for various MG applications. The proposed various MPC-based schemes have been validated by Matlab/Simulink case studies and controller-hardware-in-the-loop (CHIL) tests.
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