Supervisor:
Zhe Chen

Co-Supervisor:
Claus Leth Bak

Assessment Committee:
Nathaniel Taylor, Department of Electrical Engineering. KTH Royal Institute of Technology

Erik Cornelius Wytze de Jong, Head of Flex Power Grid Lab and part-time associate professor at Eindhoven University of Technology

Moderator:
Zhenyu Yang

Abstract:

Numerous renewable energy sources (RESs) have been coupled with the modern power grid for environmental protection, so the grid is gradually changing from synchronous generator (SGs) dominated to power electronics dominated. The RES power plant is an important way to utilize renewable energy, especially for wind-rich or solar-rich areas, and the generated electricity is often transmitted to a far end by sub-transmission or transmission lines. These transitions will bring new challenges for transmission network protection because of their very different fault signatures such as the limited current, frequency offset, and variable sequence impedance angles.

The transmission network mainly installs directional elements, distance relays, differential relays, and auto reclosers. The most commonly used in the transmission line are fault component-based directional elements including positive-sequence fault component-based directional elements, negative-sequence directional elements, and zero-sequence directional elements. Since the sequence equivalent impedance angles of converter-interfaced renewable energy sources (CIRESs) depend on the executed fault ride-through (FRT) strategy and the fault conditions, and they may present a capacitive feature in some cases, directional elements may fail to operate. To manage this problem, a novel fault coordinative control scheme has been developed to guarantee the compatibility of CIRESs with line directional elements while the current limiting condition of CIRESs is satisfied.

Distance relays on the RES side also face severe challenges due to the remote infeed for the line integrated with CIRESs. The remote infeed current can amplify the influence of fault resistance and make the apparent impedance out of the protected range. Inspired by this challenge, two new methods based on active control are advised. One is to design a novel fault control strategy to adjust the fundamental frequency current from CIRESs for CIRESs compatible with traditional distance relays. The other one is to inject a second harmonic from the inverter of CIRESs and detect the apparent harmonic impedance to determine inside or outside-of-zone faults.

Moreover, the regulated current angle of CIRESs may result in a big current angle disparity between two terminals of the line under the traditional FRT, which will result in the sensitivity decline and even misoperation of differential protection, so three schemes are suggested to manage this issue. For the first method, a novel fault control strategy is suggested to regulate the phase angle difference of less than 60°, so the operations of differential protection could be enhanced. The latter two methods are based on the protection algorithm design. A pilot protection scheme based on the comprehensive current magnitude ratio is developed because the fault current from CIRESs is far lower than that of the grid. In addition, the improved Euclidean distance is applied to evaluate the similarity of fault currents between two terminals to detect internal faults.

Finally, a control-based scheme is suggested to detect the fault nature to reduce the secondary impact due to auto reclosing. For this method, CIRESs are controlled to be a voltage source, and the fault property could be distinguished by detecting the overcurrent after the current circuit breaker on the RES terminal is reclosed. Due to the low injected current and the short injection time, the secondary impact is reduced a lot for permanent faults. The suggested method behaves well for different fault types and remote high-resistive faults.

In this thesis, the protection adaptive problems and the corresponding solutions are verified by offline simulation in PSCAD. In addition, the real-time digital simulator (RTDS)-based experiment testing is also done to validate the suggested protection schemes.