Protection of Transmission Lines Connected to IG-Based Wind Farms

Abstract

Over the last few decades, renewable energy sources have been attracting great attention due to the increased cost, limited reserves, and the adverse environmental impact of fossil fuels. Among them, wind energy is one of the fastest-growing renewable energy sources worldwide. Wind farms (WFs) comprise a considerable share of the installed capacity of renewable sources in the power grids. With the large integration of WFs in the power grid, the fault ride-through (FRT) requirement has become an essential part of the modern grid codes to increase grid reliability and stability. WFs with FRT capability are required to remain connected to the power grid during fault conditions for a specific period. This will result in WFs contributing to the fault current and changing the system fault current characteristics. Such changes in the fault current characteristics significantly affect the operation of the protection systems. This dissertation will mainly focus on doubly fed induction generator (DFIG)-based WFs and will study their negative impacts on the operation of conventional protection relays, particularly the ones that protect the transmission lines connected to DFIG-based WFs. Considering different negative impacts of DFIG-based WFs on protection systems due to their large slip range, the short-circuit behaviour of a DFIG is evaluated in two different aspects: 1) close-to-zero slip operation and 2) large slip operation. During close-to-zero slip operation of a DFIG-based WF, the short-circuit behaviour of the DFIG is similar to that of a fixed-speed squirrel cage induction generator (SCIG); therefore, fixed-speed SCIG-based WFs are also evaluated in this dissertation. In this situation, a conventional distance relay located at the fixed-speed SCIG or DFIG terminal fails to operate correctly and loses its coordination with the downstream relays for a balanced fault in its backup zone due to the negligible magnitude of the fundamental component of the fault current after several hundred milliseconds. Regarding DFIG-based WFs with FRT capability during large slip operation, the fault current frequency fed by DFIG-based WFs deviates from the nominal frequency during a fault, which affects the operation of conventional protection relays with distance or frequency elements. In this dissertation, two new relaying schemes based on distance elements for the protection of transmission lines connected to the fixed-speed SCIG- and DFIG-based WFs are presented to overcome the aforementioned challenges. To overcome the protection problem associated with the operation of distance relays at the terminal of fixed-speed SCIG- or DFIG-based WFs in case of a balanced fault in the backup zone, a new relaying algorithm requiring only local measurements called modified distance element type I is presented. To detect a fault, the modified distance element type I uses the impedance measured at the relay location together with the fault current waveform injected by the SCIG or DFIG. The reliable performance of the modified distance element type I under different types of faults is verified on a 4-bus test system. The obtained results demonstrate the robustness of the modified distance element type I against fault impedances and system disturbances such as power swing and overload conditions. To overcome the protection challenges associated with the operation of distance relays at the terminal of DFIG-based WFs during large slip operation of DFIGs, a new pilot protection scheme with minimum bandwidth requirements called modified distance element type II is also presented. The developed algorithm relies on the frequency tracking of the fault current injected by the DFIG-based WF. By implementing the modified distance element type II in a 4-bus test system, it is verified that the new relaying algorithm provides reliable protection over the entire length of the transmission line connected to the DFIG-based WF. Moreover, the modified distance element type II accompanied by the modified distance element type I provides proper backup protection for the adjacent lines.