Assessment of over-current protection schemes for Superconducting Cables

Objectives and context: 

The objective of the studies presented in this paper aim to assess the sensitivity and stability margins of over-current relays for the protection of Superconducting Cables (SCs) with High Temperature Superconducting (HTS) tapes. 

The ultimate goal of this research is to identify the limitations and particularities of the conventional IDMT relays to provide sensitive, reliable and coordinated protection for the SC-based power grids.

Problem definition:

The deployment of multi-layer SCs within power grids has been identified as a promising solution, as they present a plethora of technically-attractive features such as high current-carrying capability with near-zero losses and can be utlised for developing bulk power corridors. Nevertheless, SCs pose many fault-related challenges accounting for power system protection, due to their unique electro-thermal properties and the quenching phenomenon.

The performance of the SCs is mainly determined by the electromagnetic properties of the HTS tapes and their behavior can change depending on system conditions. Specifically, under transient conditions, when the current exceeds a current limit (known as ‘critical current’), the HTS tapes automatically quench, and they start to present exponential increase in their resistance. Therefore, the transition to the resistive state imposes a dynamic change to the equivalent impedance of the system and affects the fault current magnitudes, introducing an adverse impact on the well-established protection schemes such as overcurrent relays. Furthermore, it has been observed that one factor that predominantly affects the quenching process is the presence of high values of fault resistance. 

Methods / approach: 

The assessment of the performance of over-current protection schemes has been conducted in an EMT environment, using a verified IDMT model and a SC, both developed in Matlab/Simulink. Specifically,  a SC with triad configuration and 5km length was integrated at 33kV of a transmission network which incorporates high penetration of converter-interfaced generation.  

A systematic iterative transient simulation analysis has been conducted, considering all types of internal faults with fault resistance up to 50 Ω, occurring at different locations over the SC’s length. The obtained fault current signatures were fed into the IDMT relays to perform the sensitivity analysis of the settings. Additionally, the impact of the quenching process on the protection coordination was assessed under different types of external faults and load switching events, occurring on the adjacent lines. 

Outcomes & conclusions.

The presented research identified that due to the inherent challenge of the dynamically changing resistance and the mutable operating stages of the SC, the conventional static short circuit level analysis for the protection design and coordination would not be effective. Therefore, RMS and EMT simulation studies should be considered when designing protection schemes for grids integrating SCs. 

The results would present the challenges of the IDMT relays to provide reliable protection, highlighting the decreased sensitivity under the quenching phenomenon in conjunction with highly resistive internal faults. Additionally, the diminished stability margins under external faults and load switching events (which cause the quenching of the HTS tapes) and the subsequent limitations for the protection coordination will be analysed extensively within the paper.