Enhancement of fault current contribution from inverter-based resources

Abstract

The reduction in levels of fault current infeed as inverter-based resources (IBR) displace synchronous machines undermines the ability of a conventional protection system to identify and isolate faults in an effective manner and is therefore a concern for system operators (SOs). This observation provided the motivation to investigate the limitations of IBRs when injecting fault current and to explore how these limitations might be overcome. This thesis investigates techniques aimed at significantly increasing Fault Current Contribution (FCC) from an IBR system so that renewable energy resources can continue to be deployed without compromising the protection system. The techniques for enhancing FCC are at three different levels of an IBR system: at semiconductor or device level, circuit level and system level. The first study uses phase change materials (PCM) to provide a short-term overload rating to insulated-gate bipolar transistors (IGBTs) and found them to have very limited potential to provide FCC. A Finite Element Analysis (FEA) of heat-flow concluded that, although the PCM was useful for dealing with short over-load currents, it was unsuitable for facilitating large fault currents of several times normal load current. The view was that if the fault current cannot be created at device level through better thermal management, then a circuit level innovation would be required. The second study investigates series/parallel switching of submodules in modular converters. This takes advantage of the fact that during a fault, the line voltage is reduced, and if it falls below 0.5 pu then half of the sub-modules (SMs) can be put into parallel with the other half to double the FCC (2 pu) at half the voltage (0.5 pu). Similarly, if the voltage drops below 0.25 pu, parallel connection of four groups of SMs would enable 4 pu current capability. A model of a static synchronous compensator (STATCOM) was developed, inspired by the alternate arm converter (AAC), with the director switch of the AAC used as part of the reconfiguration circuit. The conclusion of this study was that the penalty paid in power losses in the additional semiconductor devices used for reconfiguration is reasonable for the 2 pu FCC case but not at the 4 pu FCC case. The third study was based on circuit reconfiguration but beyond the converter itself and in this case the windings of the coupling transformer of a STATCOM. Sections of winding were switched using thyristors to tap-change the transformer by a large factor. Using the proposed thyristor-based electronic tap-changer (eTC), the number of turns of the grid-side winding was reduced during a voltage dip, so that larger current can be delivered to the network for the same converter current. The STATCOM was controlled in the natural frame (abc frame) and this control is used to actively drive the currents in the tap-changer thyristors to zero when needed so that they can be commuted rapidly. The transformer was configured to give a normal ratio of 1:4 and be able to tap-down to 1:2 and 1:1 to increase FCC to 2 pu or 4 pu. Theoretical analysis of, and operating principles for, the proposed eTC, together with their associated control schemes, are verified by time-domain simulation at full-scale. The case-study circuit demonstrates delivery of substantial fault current contribution (FCC) of up to 4 pu at the point of common coupling (PCC) in less than half a cycle (10 ms) after detection of three- and single-phase faults. The results demonstrate that the proposed eTC is a good candidate for the enhancement of fault current from IBR systems that employ coupling transformers, allowing them thereby to make a contribution to future electricity networks dominated by IBR.Open Acces