Development of control schemes to enhance stability and dynamic performance of islanded inverter-based microgrids

Abstract

Renewable Energy Sources (RESs) such as solar, wind/micro wind and hydro/micro hydro based power generations have gained considerable attention worldwide due to global warming, fast depletion of fossil fuels along with growing energy demand. Generally, power generation from these RESs is in the range of tens of kilowatts to fraction of megawatts and due to this, these energy sources are usually connected at distribution level in order to reduce power losses in long transmission. Therefore these sources are also called as Distributed Generations (DGs). RESs based DG units produce uctuating active power due to their intermittent nature. Further, the output of these DG units is either a DC or a variable frequency AC. Therefore, these DG units are interfaced to the distribution network or the local loads through a front-end inverter, named as Inverter-Interfaced Distributed Generation (IIDG). A recently evolved concept is to group a few of these IIDG units and a cluster of loads together to form a small local power system, called an inverter-based or AC microgrids (ACMGs). AC microgrids (ACMGs) can be operated either in an island mode or in a grid connected mode of operation. Stability of ACMGs is not a critical issue in grid connected mode of operation as the sti grid would be responsible for their stable operation. However, in the island mode of operation, it is an important concern due to the intermittent and low-inertial nature of IIDG units. The reason for this lies in the fact that in the islanded mode, IIDG units are responsible for maintaining the frequency and the voltage within their speci ed limits while sharing the load among the IIDG units in a stable manner. Therefore, stability of Islanded Inverter-based Microgrids (IIMGs) largely depends on the power sharing control algorithm. A widely accepted droop-based power sharing control approach has been used to share real and reactive powers among IIDG units. The low inertial nature of IIMGs makes them more vulnerable to instability even under small change in operating conditions. Apart from this, there are several other factors which may further degrade the stability of these IIMGs. These factors are interaction between generation and load dynamics, poor damping of low frequency modes associated with the droop controllers. Thus, stability and dynamic performance enhancement of the IIMG isan important concern for its satisfactory and reliable operation. The main focus of this thesis is to develop control schemes to enhance the sta- bility and dynamic performance of IIMGs feeding multiple types of passive loads, Recti er Interfaced Active Load (RIAL) and dynamic Induction Motor (IM) load, simultaneously. Various types of passive loads include resistive (R) load, inductive (RL) load and constant power load (CPL). In this thesis, a generalized model of IIMGs feeding R load, RL load, CPL, RIAL and dynamic IM load has been de- veloped. The developed model has been used to investigate the impact of load dynamics as well as load sharing among IIDG units on the stability and dynamic performance of IIMGs. Based on these investigations, decentralized, centralized and two-level hierarchical controllers have been developed. The proposed controllers are designed based on a robust extended Linear Quadratic Gaussian (LQG) control, which combines the Kalman estimator with the Linear Quadratic Regulator with Prescribed Degree of Stability (LQRPDS). In order to obtain the optimal values of the diagonal weighting matrices of Kalman estimator and LQRPDS, a bi-objective optimization problem has been formulated and solved using a fast and elitist multi- objective Non-dominated Sorting Genetic Algorithm-II (NSGA-II). The proposed controllers produce supplementary control signals to the local power sharing con- troller of each IIDG unit and DC voltage as well as AC current controllers of the RIAL. Eigenvalue analysis and time-domain simulations have been performed to validate the e ectiveness of the proposed controllers. The proposed controllers pro- vide robust control performance under various load con gurations as well as small load disturbances. Further, to enhance the dynamic performance of IIMGs under large load disturbances, a control algorithm for the Battery Energy Storage System (BESS) has been developed and evaluated for the load-leveling application.