On overcurrent protection: short-circuit current, protection coordination and fuse sizing in IBRs dominated-distribution feeders

Abstract

The rapid integration of inverter-based distributed power generators are posing challenges for the protection of the electrical power system, especially due to the uncertainty and change in the level of short-circuit current (SCC) of distribution feeders. This thesis proposes three approaches. The first is a methodology to estimate the value of the SCC level at the installation points of overcurrent protection devices in distribution networks with high penetration of inverter-based resources (IBRs), for three-phase faults (3LG) and single-phase ground faults (LG). The second proposes a methodology for adjusting the characteristic curve of inverse-time overcurrent relays. And the third, two methodologies for sizing fuses to avoid solidarity tripping or to protect the conductor. The methodologies are applied in scenarios with high penetration of IBRs and do not require communication links, nor measurement of parameters in real time: minimal need to replace equipment already installed. Considering the estimation of SCC, the methodology presents a good assertiveness when compared with a computational simulation in MATLAB/Simulink. Errors are smaller for 3LG faults (< 2.1%). For LG faults, the errors are smaller when the IBRs are installed in the main fault trunk (< 10.3%) than in the lateral ones (< -13.0%). Despite the high errors, it is possible to estimate an extreme value of SCC to be used in the protection adjustment. In a scenario of 100% penetration of IBRs, there was a miscoordination between the protection devices: relay-relay and relay-fuse. Coordination was re-established using the proposed methodology for extreme IBR installation situations. In this coordination of the inverse time-overcurrent protection, the time dial of the overcurrent relays was modified. This modification accommodates a high penetration range of IBRs and, in the case of 100% penetration, it delayed the protection actuation by a few tens of ms. For fuses, it was found that depending on the level of penetration of IBRs on the lateral branch that the fuse protects, there may be an incompatibility of the limits used for its sizing. This incompatibility happens for high penetration levels when the maximum current through the fuse is higher than its nominal value. Methodologies to avoid sympathetic tripping and conductor protection were applied to calculate the appropriate fuse to protect the lateral branch and the maximum level of penetration required. The maximum penetration level decreases as the IBR can inject a greater SCC (e.g., 1.2 pu or 2.0 pu). The applied methodologies re-established the coordination of the distribution feeder, both for the main trunk and for the side branches. These proposals can be applied to other feeders and the penetration range to be met is defined by the network manager.