Supervisor:
Xiongfei Wang

Co-Supervisor:
Heng Wu

Assessment Committee:
Yajuan Guan (Chair)
Qiteng Hong, Professor, University of Strathclyde, UK
Marjan Popov, Professor, Technical University of Delft; Netherlands

Moderator:
Shuai Zhao

Abstract:

The increasing integration of inverter-based resources (IBRs) has introduced fault characteristics that differ significantly from those of synchronous generator (SG)-based systems, challenging the reliability of transmission line protection originally designed based on the behavior of SG. Among various schemes, distance protection—comprising distance, directional, and phase selection elements—is widely used and relies on the characteristics of impedance trajectories, sequence quantities, and incremental quantities.

Reliable operation of impedance trajectory-based elements requires similar source- and grid-side currents. Negative-sequence elements depend on highly inductive source impedances, while incremental quantity-based elements require 1) similar source dynamics before and during the fault, and 2) highly inductive source impedance. Kirchhoff’s Law shows that the preconditon for impedance-based elements still hold in both grid-following (GFL) and grid-forming (GFM) IBR systems under symmetrical faults with negligible fault resistance. However, GFL and GFM systems affect the remaining preconditions differently.

In GFL systems, mature grid codes, such as IEEE Std. 2800, mandate a highly inductive output impedance for negative-sequence components, but not for positive-sequence ones. Furthermore, the source dynamics precondition is often violated, causing potential malfunctions in incremental quantity-based elements. In SG-based systems, the impact of load conditions on total quantities is difficult to quantify, leading to the widespread use of incremental quantity-based elements. In contrast, this impact can be quantified in IEEE Std. 2800-compliant systems, enabling the modification of settings of the protective relays for improved performance.

In GFM systems, evolving standards and control strategies further complicate protection. The current limiting control (CLC) shapes output impedance, while outer-loop control defines the internal source. It is shown that the current reference saturation-based CLC is unsuitable for protection—a gap unaddressed in prior art. Moreover, incremental quantity-based elements remain unreliable due to unmet preconditions. To address this, a protection-interoperable fault-ride through (FRT) control strategy is proposed, incorporating modified power control and a highly inductive virtual impedance/admittance-based CLC.