Abstract
This dissertation proposes a time-domain-based protection scheme for radial and loop connected microgrid systems with grid following controlled inverter-based resources (IBRs), such as solar photovoltaic (PV) generation and type-4 wind turbines. The protection scheme is designed to function during grid-interconnected and grid-isolated modes without changing the settings of the protection devices or requiring islanding detection schemes. The proposed scheme provides an ultra-high-speed sub-cycle directional element aided with low bandwidth communication between relays, indicating the fault zone in as little as four milliseconds for fast tripping to retain stability for severe faults. Like directional comparison schemes, relays identify whether faults are in-zone or out-zone in feeders with bidirectional power flow.The proposed directional element is based on time-domain superimposed quantities and uses the Park's transformation. The superimposed, also known as incremental, quantities are fault generated instantaneous components of voltages and currents largely dependent on the electric network parameters. The directional element indicates fault direction based on relative polarities of the transient voltages and currents by calculating the transient energy quantities. It is essential to ensure that the element accurately indicates the true direction if the quantities are impacted by IBR nonlinear fault current responses.
Most IBRs generate low fault current magnitudes and high-frequency transients, that are largely dependent on IBRs fast control and independent of fault location. The fast control response and fault current characteristic of IBR impact current quantities by changing both magnitude and angle. Newer IBRs fault responses are often predominantly positive sequence fault currents with insignificant negative sequence currents. The dissertation shows the response of the superimposed-based element to IBR supplied fault current.
After that, this dissertation proposes a solution to eliminating the impact of IBR on time-domain based superimposed quantities elements. More specifically, the element calculates the superimposed positive-sequence direct axis component of transient energy during faults. Superimposed voltage and current quantities are calculated using delta filters and decoupled double synchronous reference frame (DDSRF) filters. The proposed filtering method improves the reliability of the superimposed directional element when IBRs are present. The protection scheme is evaluated on a modified IEEE 34-bus distribution system simulated using the electromagnetic transients program (EMTP-RV).