Date of Award


Document Type

Open Access Dissertation

Degree Name

Doctor of Philosophy in Electrical Engineering (PhD)

Administrative Home Department

Department of Electrical and Computer Engineering

Advisor 1

Bruce A. Mork

Committee Member 1

Leonard J. Bohmann

Committee Member 2

Jeffrey B. Burl

Committee Member 3

R. Andrew Swartz


Renewable energy sources are interfaced with the electrical grid using power electronic inverters. These inverter-interfaced resources have been deployed for nearly 20 years. Still, NERC only recently highlighted the vast gap between the actual behavior of these inverters during power system transients and those observed in simulations. Simulation models need significant improvements, mainly for developing accurate inverter current controls, phase-locked loops, and fault response during different power priority modes. Additionally, only time-domain electromagnetic transient simulation tools can fully represent the fault response of the inverter-interfaced resources.

The developed simulation model of the inverter-interfaced resource is based on the recommendations made in the IEEE-2800 standard, which aims to improve the performance of protective relays during faults. Several challenges with protective relays' inability to detect faults are attributed to such energy sources. The EMTP-ATP simulation package is used to develop the simulation model with the fault response benchmarked from the laboratory results presented in the literature. The source code for the developed model has been given to enable future research.

Ferroresonance is an overvoltage caused by the series excitation of a nonlinear inductor through a capacitor. It is known to occur only with lightly loaded distribution transformers. The residential solar generation is connected to the secondary of a single-phase distribution transformer. The transformer's loading is thereby hypothesized to be influenced by the power generated by the solar panel and the power consumed by the connected load. The hypothesis is confirmed by performing transient simulations using data from an ungrounded distribution network with unbalanced loading, which was thought to have experienced ferroresonance. A single-phase inverter-interfaced model modified from the benchmarked model is connected to the distribution transformer low voltage side. The ferroresonance occurrence is influenced by the net loading of the transformer instead of the actual load connected. Balancing the loads on all the phases and voltage-based control of the capacitor bank effectively avoided ferroresonance overvoltage.

Single-phase open conditions occur in transmission systems during automatic reclosing on temporary faults. These open-phase conditions may lead to the nonlinear transformer core being excited through a capacitor, leading to ferroresonance. The benchmarked IEEE-2800-based model is scaled up and connected to a modified IEEE 9-bus system to test this. In addition to the overvoltage observed during open-phase conditions, the transformer differential relay operates incorrectly depending on the control strategy applied and the priority mode chosen. A logic comparing the winding RMS currents to 1% of the current transformer ratio detects open-phase conditions and avoids incorrect relay operation. This has been implemented and tested to enhance the security of the relay without compromising the sensitivity.

The impacts of single-phase open conditions with distribution and transmission systems have been demonstrated. With distribution networks, ferroresonance can be challenging to predict due to varying net loading throughout the day caused by changes in the load or the output of the inverter, a little-known problem until now. Improvements in the design of distribution systems by balancing loads on all phases prevent ferroresonance scenarios. In addition, improving the operation by implementing voltage-based control of capacitor banks prevents scenarios causing ferroresonance.

In transmission systems during single-phase open conditions with inverter-interfaced resources, transformer differential protection can operate incorrectly. This incorrect operation could cause these sources to become wholly disconnected from the grid, leading to a severe shortage of generation. Such scenarios have been analyzed with time-domain electromagnetic transient simulation tools using benchmarked models of the inverter controls to understand their behavior during power system transients. The proposed transformer differential relay operation improvement can avoid such incorrect operations.

Available for download on Sunday, December 01, 2024