Theses and Dissertations Collection

Historically, power systems have been dominated by large synchronous generators. Synchronousmachines provide benefits, including stabilizing the power system during disturbances since they have large inertia and damping. With the presence of machine governors and automatic voltage regulators, the regulation of voltage and frequency can be attained. Nowadays, power system generation has shifted to more environmentally friendly generation, especially in distribution systems with much research exploring the impacts of having a high percentage of renewable energy-based distributed generator penetration and microgrids. coal-fired generators are being displaced in response to low capital and operating costs for natural gas generation and the rapid growth of renewable generation due to technical advances combined with government policies to reduce CO2 and other greenhouse gas emissions. Wind and photovoltaic (PV) generation are becoming the predominant type of renewable generation for new installations, with the addition of the deployment of battery energy storage systems becoming a common research topic that is moving into industry practice. These technologies are connected to the power grid through power electronics-based voltage source converters (VSC).

Wind and PV based distributed energy resources are typically considered intermittentand operated in a non-dispatchable fashion, meaning their electrical output cannot be varied at any given time to meet the electric demand. More importantly, the VSCs in many wind and PV applications use grid-following control schemes, which means they have fast tracking of changes in angle or frequency to support peak power transfer and to limit over current conditions, effectively resulting in low inertia. In microgrid applications, where the grid is often weak, the lack of inertia would affect the frequency and jeopardize the system stability in terms of voltage and frequency and reduce the dependability when using renewable generation. In large interconnected power grids, the reduction of system inertia causes a faster rate of change of frequency that impacts electrical grid stability following large disturbances, resulting in possible DG tripping and frequency collapse before the under-frequency load shedding relays have time to trip. Converter controls based on grid-following schemes are especially susceptible to problems in vulnerable grid situations when the effective short circuit ratio of the inverter-based resources is low.

To support a large increase in the deployment of inverter based resources (IBR), thecontrols for IBR need to include functions to mitigate the negative impact of present IBR controls on system stability. One solution to this problem is using grid-forming inverter controls that incorporate virtual inertia (VI) emulation to mimic the behavior of a synchronous generator. Virtual inertia can be defined as the ability to control the system frequency by varying the active power output of an inverter based distributed generator or energy storage element to improve the dynamic frequency stability. VI can be achieved by programming the controls for a battery energy storage system to create virtual inertia with equation-based characteristics tailored to system configurations.

This work focuses on applying different methods to create virtual inertia supportcombined with other inverter control functions to maintain system frequency to avoid instability conditions. These methods include droop control schemes and swing equation based control schemes. The virtual inertia approach will be compared to system responses with the emerging industry practice of adding synchronous condensers with added flywheels to create inertia and retain some of the behavior of a synchronous generator. A detailed synchronous generator model is built to compare the VI methods’ effectiveness in this study.

The simulations are performed using a power system electromagnetic transient program to simulate the power system. The study system includes a renewable energy source, in this case, multiple photovoltaic sources combined with battery energy storage systems. These components are connected to the IEEE 9-bus transmission test system. The abilityof the proposed methods to emulate inertia will be evaluated under multiple system conditions, along with assessing what would be needed to apply these techniques in practice.