PROTECTION AND SECURITY ENHANCEMENT OF ACTIVE AC DISTRIBUTION SYSTEM USING DIFFERENTIAL TECHNIQUES

Abstract

The AC electric power distribution system is undergoing a profound transformation due to the integration of Distributed Energy Resources (DERs), giving rise to active distribution systems (ADSs). The synergy of advanced technologies and DERs is working cohesively to bolster grid resilience, addressing the pressing challenges posed by carbon emissions reduction, evolving consumer demands, and the need for a flexible and robust grid operation in the face of resource fluctuations, natural disasters, and escalating cyber and physical threats. The escalating frequency of these threats has led to significant power disruptions, resulting into the urgency of finding resilient solutions. An effective strategy in this pursuit has been the integration of DERs, giving rise to the concept of microgrids (MGs). This integration has not only improved system efficiency and reliability but has also given rise to the concept of multi-microgrids (MMGs). MMGs involve interconnecting self-sufficient microgrids in close proximity, presenting a promising solution for challenges in rural electrification. MGs and MMGs play a crucial role in ensuring uninterrupted power supply to loads using DERs situated in proximity to the demand side. Additionally, they contribute to reducing carbon emissions by utilizing clean energy sources. Despite these advantages, protection challenges have emerged due to the bidirectional power flow, intermediate current injection, and bidirectional current flow during faults in these evolving systems. Traditional protection relays designed for unidirectional power flow face efficacy challenges in these transformed systems. The MMG concept, while offering distinct advantages, presents unique protection challenges as each microgrid operates as an individual cluster with two-way power exchange capabilities. Further, the low impedance faults (LIFs) and high impedance faults (HIFs) usually occur in ADS. Moreover, the occurrence of HIFs in ADSs poses a specific challenge due to their random and non-linear fault currents. Conventional protection methods struggle to accurately identify HIFs, which can lead to arcing and sparking, posing risks of equipment damage and fire hazards. Addressing these challenges require the adoption of advanced protection techniques modified for the intricacies of distribution systems with integrated DERs. Ensuring the reliable protection of ADSs using a differential protection approach demands resilient communication channels, considering the vulnerability of the system to cyber-attacks with the evolving distribution system infrastructure. In summary, the ongoing evolution in AC electric power distribution systems, marked by the integration of DERs and the emergence of MGs and MMGs, necessitates a re-evaluation of traditional protection methods. As the grid transforms, the emphasis on resilience, cybersecurity, and advanced protection techniques becomes imperative for ensuring a reliable and secure power supply. Therefore, this thesis suggests methods considering the protection issues discussed above for ADSs. The primary objective of the thesis is to tackle substantial protection challenges and cyber threats by developing techniques that improve system reliability and resiliency. Thus, in this thesis, a method utilizing differential energy linked to positive sequence current components is introduced for detecting HIFs. Further, an innovative unified fault detection method using cumulative sum of the differential negative sequence impedance angle as a fault detection parameter, is proposed in this thesis to identify both LIFs and HIFs occurring in ADSs. Moreover, in real-time scenarios, the identification and classification of LIFs and HIFs faults hold equal significance. Thus, a fault detection index is designed in this thesis to differentiate between LIFs and HIFs. Additionally, a fault classification index is introduced to categorize these faults. The technique uses the energy linked to the differential value of the superimposed component within the positive sequence impedance for its implementation. Moreover, it is developed for the distribution systems integrated with a single DER unit, and MMGs. Furthermore, an alternative approach is put forth for distribution systems equipped with MMGs. This method introduces a fault detection index that relies on the positive sequence angle ratio between pre-fault and post-fault currents. This innovative fault detection index contributes to the precise identification and classification of both low and high impedance faults, enhancing overall system reliability. Additionally, the thesis focuses on developing a cyber-resilient protection method for ADSs. Moreover, the designed method also classifies these malicious data attacks with the actual physical faults occurring in the ADSs.