Proportional Load Sharing and Stability of DC Microgrid
AC electrical distribution system is presently dominating whose engineering foundations were planned above hundred years ago. However, the debate between ac and dc distribution system has started again due to the evolution of dc loads and increasing use of renewable energy sources (RESs). Currently, depleting threat of conventional fuels, growing energy demand and prices, and ecological changes necessitate that considerable power to be produced through RESs. Microgrids are modern form of distribution system which can function autonomously or in combination with main supply grid. Microgrids can operate in low or medium voltage range which have their own power generation with energy storage and loads. The unique property of the microgrids is that they can work in islanded mode under faulty grid conditions which increases the reliability of power supply. This inspires that microgrid is an effective way of power generation and consumption. In the near future, the distribution system may consist of some interconnected microgrids with local generation, storage and consumption of power. Solar, wind and fuel cell technologies are playing an important role in electric power generation among various renewable sources. Most of these sources are inherently designed for dc or they are dc friendly. The growing use of these sources and fast evolution of domestic appliances from ac to dc attracting dc microgrids in the distribution system. DC microgrid system may be more efficient compared to the ac system because the integration of RESs in dc requires less conversion stages compared to ac. Additionally, the reactive power compensation and frequency synchronization circuits are not required in dc microgrids.
DC microgrids are not exempted from the stability concerns. In the first part of this thesis, voltage stability of dc microgrid based on decentralized control architecture is presented. Droop controllers are being used for voltage stability of dc microgrids. But droop control is not effective due to the error in steady state voltages and load power variations. Further, the voltage deviation increases with the increase in droop values which are not acceptable to the loads. Additionally, proportional integral (PI) controllers are being used to realize droop control for the stability of dc microgrid. The main reason to use these control techniques is due to easy implementation of their tuning method in industrial applications. However, PI controllers cannot ensure global stability. They exhibit slower transient response and control parameters cannot be optimized with load power variations. To address the aforementioned limitation, sliding mode control (SMC) is proposed for voltage stability of dc microgrid in this thesis. Main advantages of SMC are high robustness, fast dynamic response and good stability for large load variations. To analyze the stability and dynamic performance, mathematical model of a dc microgrid is derived. Controllability and stability of the modeled system are verified. Hitting, existence and stability conditions are verified through SM. Modeled dynamics of the system are graphically plotted which shows that system trajectories converge to the equilibrium point. Detailed simulations are carried out to show the effectiveness of SM controller and results are compared with droop controller. SMC showed good voltage regulation performance in steady state condition. The effect of transient on a step load is also investigated which confirms the good performance of the proposed controller. Further, a small scale practical setup is developed, and results are presented.
In the second part of this thesis, distributed architecture using SM controller is proposed for proportional load sharing in dc microgrid. The key objectives of the dc microgrid include proportional load sharing and precise voltage regulation. Droop controllers are based on decentralized control architecture which are not effective to achieve these objectives simultaneously due to the voltage error and load power variations. Centralized controller can achieve these objectives using high bandwidth communication link. However, it loses reliability due to the single point failure. To address limitations, a distributed architecture using SM controller utilizing low bandwidth communication is proposed in this thesis. Main advantages are high reliability, load power sharing and precise voltage regulation. To analyze the stability and dynamic performance, system model is developed and its transversality, reachability and equivalent control condition are verified. Furthermore, the dynamic behavior of the modeled system is investigated for underdamped and critically damped response. Detailed simulation results are carried out to show the effectiveness of the proposed controller.