Adaptability analysis on differential protection of converter transformers in a VSC-HVDC transmission system connected to offshore wind farms

Abstract

The fault characteristics of voltage source converter-based high voltage direct current （VSC-HVDC） transmission systems connected to offshore wind farms differ from those of conventional power systems. Consequently， there exists a potential risk of inadequate adaptability when conventional relay protection schemes are applied to the VSC-HVDC transmission system. This paper analyzes the impact of harmonic components induced by asymmetrical electrical quantities， negative sequence suppression， and current-limiting measures on the performance of differential protection during fault ride-through （FRT）. Sensitivity calculation formulas for differential protection considering sequence components are derived， and adaptability analysis is conducted. It is noted that differential protection is susceptible to odd-order harmonics during fault conditions， while negative sequence suppression and current-limiting measures may also diminish its sensitivity. Extensive simulation calculations are carried out using a simulated system of a practical demonstration project. The findings indicate a decrease in the sensitivity and reliability of conventional differential protection schemes.

Keywords

• vsc-hvdc transmission
• converter transformer
• differential protection
• harmonic blocking
• sensitivity