Impact of Low Inertia Systems on Transmission Line Protection Schemes

Abstract

As the energy mix continues to evolve, integrating distributed generation and renewable resources into electricity transmission systems presents both opportunities and challenges. The increasing reliance on power electronic inverters, particularly in connection with battery storage systems and flexible loads, is altering the traditional dynamics of power transmission. This shift necessitates robust protection mechanisms to ensure system reliability and stability. Effective protection of transmission systems is critical to safeguarding infrastructure against faults. Current best practices predominantly involve the use of conventional protection schemes, including overcurrent and distance relays. These systems are designed to operate under the assumption of significant fault current levels that exceed normal load currents. However, the reduced system inertia and fault levels associated with modern generation technologies complicate this task. The transition to a more decentralized energy landscape diminishes fault current levels and challenges the effectiveness of traditional protection schemes. With lower fault levels, distance relays may struggle to differentiate between overload conditions and fault events, leading to potential mis-operations in protection systems. Consequently, this presents a pressing need to investigate alternative protection strategies. Main and back-up protection for transmission lines is commonly based on distance relays, which operate by comparing the measured ‘relay to fault’ impedance on the protected line or adjacent line with three or four pre-set quadrilateral or mho operating zones. Any increase in the source impedance, due to a reduction in the short-circuit fault level, affects the source to line impedance ratio (SIR); which can increase the errors associated with the measured impedance, i.e., the distance relay may under- or over-reach. This research aims to evaluate the performance of distance protection relays in various artificial networks using DIgSILENT Power Factory simulations, particularly under conditions influenced by LCC HVDC interconnections. The LCC HVDC transmission link is utilized to interconnect two AC grids. The HVDC link is known as a better option when compared to HVAC to connect geographically remote power grids reliably and economically; for example, links between GB and continental EUROPE. This is due to power losses and stability issues that could be caused by using an HVAC transmission link. The mature technology of LCC HVDC, which is converting from AC to DC and vice versa via thyristors, would act as a current source for the AC grid. However, because it is low in inertia, its contribution to the fault current might be insufficient and this will affect the protection system. Additionally, the research seeks to identify the factors affecting relay performance, such as intermediate infeed sources and fault resistance, and explore the potential of Travelling-Wave (TW)-based protection schemes as a viable solution to the declining fault levels observed in the British transmission network. The methodology employed involves simulating short-circuit and Electro-Magnetic Transient (EMT) responses in a controlled environment, complemented by laboratory testing using time-domain protection relays and TW generators. The study focuses on evaluating the operational performance of various protection schemes, including TW differential protection and incremental distance elements, under diverse fault scenarios. Key results indicate that the performance of distance protection, particularly the incremental or superimposed distance element TD21, is influenced by fault type, location, and source strength. Conversely, the travelling wave differential scheme TW87 demonstrated resilience, effectively distinguishing between external and internal faults, particularly during three-phase fault conditions. This robustness underscores the TW87 scheme's potential as a superior protection solution in the context of reduced fault levels. However, the scheme, as expected, did not operate when a single phase-earth fault occurs near a point on the voltage wave corresponding to a zero-crossing. This apparent limitation is not considered a problem, since single phase faults are unlikely to occur at a zero crossing, due to a lack of voltage at the fault point, and if it did, other protection elements are included within the protection scheme, and these will operate correctly. The findings contribute to the existing body of knowledge by clarifying the challenges posed by the evolving energy landscape on traditional protection strategies and advocating for the adoption of Travelling-Wave technology in transmission line protection. This research not only highlights the technical achievements in protection performance but also aligns with the initial objectives of addressing the limitations inherent in current systems. In conclusion, the study affirms the necessity for innovative protection solutions in the face of changing grid dynamics and offers insights into the practical applications of TW-based schemes. Future work should focus on expanding the scope of testing across varied configurations and exploring the integration of artificial intelligence in enhancing the responsiveness and adaptability of protection systems.

Date of Award	6 Jan 2025
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Haiyu Li (Main Supervisor) & Zhirun Hu (Co Supervisor)