Candidate: Xiaoyou Zhang
Title: The Protection of Transmission Lines Connected to DFIG-Based WTGs
Date: August 6, 2021
Time: 11:00 AM
Supervisor(s): Pirooz Azad, Sahar
Recently, many countries have proposed various plans to address the issue of climate change, and increasing the capacity of renewables is one of the major common components of such plans. The uncertainty and variability of generation, introduced by renewable energy sources (RESs), pose significant protection challenges to the power systems. Type III wind turbine generators (WTGs), which are based on doubly-fed induction generators (DFIGs) and connect to the power systems via reduced-size converters, raises additional protection challenges such as the maloperation of distance relays due to the frequency deviation of the current measurement caused by the short-circuit characteristics of the DFIGs, and the impact of the fault resistance on the calculated impedance.
Although many studies have identified the challenges associated with the protection of power systems with RESs and have proposed various algorithms to address these challenges, only a few of them comprehensively discuss all the protection challenges within one system. In this thesis, a single test system is developed and used to illustrate the protection challenges and to provide a review of the existing protection schemes, which have been proposed in the literature to tackle the protection challenges associated with power systems with RESs. After introducing the protection challenges associated with the integration of RESs in the power system, this thesis focuses on the protection of transmission lines connected to DFIG-based WTGs. The protection challenge associated with the frequency deviation caused by the short-circuit characteristics of DFIG is further discussed in detail, and a modified permissive underreaching transfer trip (PUTT) scheme is presented to address the challenge. With the addition of a frequency tracking element, the modified scheme correctly prevents the maloperation of the distance elements during external faults and enables the trip of the relay during internal faults. Besides, the protection challenges associated with conventional distance relays at the terminal of DFIG-based WTGs that are caused by the fault resistance and the frequency deviation associated with the short-circuit characteristics of the DFIG, are addressed and investigated. A modified distance protection scheme is presented to address these protection challenges by using an averaging filter to correct the current phasors and removing the error term caused by the fault resistance in the measured impedance. Pure-fault circuits are used to calculate the pure-impedance of the WTG and pure-fault sequence networks are used to estimate the fault current flowing through the fault resistance. Simulation results show that, for various fault scenarios with different fault resistances, the developed modified distance protection scheme is able to accurately estimate the positive-sequence impedance between the fault and relay location, with low memory requirement.