Design and Analysis of Different Droop Controllers for Energy Storage Devices in Smart Grids

Open Access
Author:
Adeyemo, Oluwatosin V
Graduate Program:
Electrical Engineering
Degree:
Master of Science
Document Type:
Master Thesis
Date of Defense:
November 29, 2018
Committee Members:
  • Javad Khazaei, Thesis Advisor
  • Peter Idowu, Committee Member
  • Scott Van Tonningen, Committee Member
  • Seth Wolpert, Committee Member
Keywords:
  • Battery energy storage system
  • Modified power factor control
  • small-signal modeling and analysis
Abstract:
The increased penetration of renewable energy sources (RES) into the grid has led to increased usage of energy storage systems in the grid. In particular, battery energy storage systems (BESS) are commonly used to compensate for the uncertain power generation of RES. BESSs are connected to the grid and controlled via voltage source converters (VSC). When multiple BESSs are connected to the grid in parallel, active and reactive power sharing are commonly controlled by droop control strategy. However, droop control does not take the power factor of the BESSs into account when performing power sharing. High system losses and increased equipment and maintenance costs can occur because of uncontrolled BESSs power factors. A control strategy which controls reactive power sharing of BESSs connected in parallel based on their power factors is proposed in this work. The control strategy is based on the modified power factor strategy, and focuses on making all BESSs act around the same power factor, by balancing the distribution of the reactive power load. The controller is simulated, tested and validated on a micro-grid system with three BESSs using the MATLAB/Simscape power system toolbox. The stability analysis of a scaled-down test system is also presented in this work. The scaleddown test system is a micro-grid connected to a BESS via a VSC and a filter. The VSC is a grid-supporting converter enhanced with primary and secondary system frequency and AC voltage magnitude droop controllers. This enables the BESS to be able to operate in both grid-connected and islanded modes. The small-signal model of the system is presented and the stability analysis of the system is performed in order to determine the stability margins of the system.