PHOTOVOLTAIC BATTERY BASED POWER MANAGEMENT SCHEME FOR ISLANDED MICROGRID

Abstract

Energy plays a pivotal role in the development of all nations. Due to the limited quan tity of fossil fuel sources and the environment pollution created by these sources, the demand for pollution-free energy sources, such as renewable sources, has been increas ing rapidly. Among the available renewable sources, the solar energy based photovoltaic (PV) sources are gaining importance due to their free availability, ease of installation and flexibility in installed capacity. Apart from these advantages, the PV source is a promising option for the electrification of a remotely located community. However, due to the stochastic nature of this source and uncertainty in energy demand, uninterrupt ible supply can only be ensured by integrating other types of power sources such as battery storage. With this motivation, an efficient energy solution for the remote areas which do not have access to electricity yet is proposed in this thesis. To supply the energy demand of a remote area, a standalone Photovoltaic Battery (PVB) based hybrid unit is proposed. The droop concept is implemented to regulate the output voltage of an inverter proportional to the dc-link voltage (VDC). Further, the conservation voltage reduction (CVR) principles, together with dc bus signaling concept, are implemented to coordinate the operation of DC side converter (PV and battery) and inverter. Further, according to the state-of-charge condition and power limit of the battery charging controller, the PV source can either extract maximum power or operate in power regulation mode to prevent the battery from over charging. Similarly, to prevent the deep discharging or violation of the power limit of the discharging controller, automatic load shedding via low bandwidth communication line is carried out to operate the system stably. The proposed method also aims to min imize the size of the overall PVB system to reduce the total investment and operating cost considering energy saving achieved by CVR, while satisfying system operational constraints. This method also reduces the number of charging/discharging operations of the battery which, in turn, enhances the battery life. The performance of the pro posed technique has been investigated using PSCAD and RSCAD simulation tools on the low voltage IEEE European distribution network (EUDN) having a high R/X ratio. i Further, to increase the reliability and reduce the probability of loss of supply, multiple PVB units have been considered to be installed in the microgrid. To coordi nate the operation of multiple PVB units, a centralized-decentralized secondary control (CDSC) algorithm has been proposed. In the proposed CDSC control algorithm, the centralized secondary controller (CDS) evaluates the different coefficients based on the received information from each local controller (LC) of individual PVB unit. Subse quently, the decentralized secondary controller (DSC) calculates the required reference for the LC based on the coefficient received from the CSC. The CDSC algorithm shares the active and reactive power demand among the PVB units based on the actual gen eration considering the maximum charging/discharging limits of the battery converter and also ensures the stable operation of the microgrid. Further, the proposed algorithm also considers the maximum/minimum SOC limits of the battery. Furthermore, it also accounts for the absence of the battery within the PVB unit which may not be always available in service due to the absence/maintenance of it. Additionally, the proposed CDSC algorithm is further modified to ensure the stable operation of the microgrid even during the failure of communication network or during the failure/maintenance of the microgrid components. The robustness of the CDSC algorithm has been val idated using the PSCAD simulation tool considering separate phases of low voltage IEEE EUDN network having different sizes of PVB units and different characteristic of the loads. The simulation results confirm ability of the proposed algorithm to main tain the stable operation of multiple parts of the microgrid and also for sharing the load demand accurately among the available PVB units within a particular part of the microgrid. Furthermore, this algorithm also facilitates easy plug and play operation of the PVB units. Also, it enables the easy expansion and instalment of new PVB unit in the microgrid. Besides these, due to the inherently negligible inertia of the PVB units, the performance of the microgrid is also affected even due to a small delay in communication. Hence, the performance of the microgrid has also been investigated considering the uniform as well as non-uniform communication delays. The power flow analysis of the microgrid is necessary for the planning and op erational management of the microgird. To solve the power flow problem of a highly ii resistive distribution line, the conventional Newton Rapson power flow algorithm has been modified. Subsequently, the steady state model of the droop based PVB unit is derived and incorporated in the formulation of modified Newton Raphson (MNR) algorithm. The proposed model of the PVB unit accounts for various scenarios such as overcharging, deep discharging and non-availability of the battery. The accuracy of the proposed MNR algorithm has been established by comparing its results with those obtained by time-domain PSCAD simulation studies considering different phases of the low voltage IEEE EUDN. Furthermore, the effectiveness of the proposed algorithm and the steady state model of PVB unit is also confirmed by considering the ZIP load.