Performance Evaluation of Percentage Differential Relays on Power Transformer and Reliability Assesment in HVDC Grid Protection Scheme

Percentage differential relays remain the most sensitive protection tool applied as backup protection on power transformers, busbar, and generators. Relays sometimes do mis-operate with the current transformer being affected by external fault leading to saturation, and the subsidence current present after clearing external faults. The cause of misoperation of percentage differential relays cannot be ignored that it entirely depends on magnitudes more than directionality for tripping decisions. This paper covers evaluating differential element performance, analysis of transformer inrush current, internal faults, external faults, and overexcitation conditions. The accurate computing of current transformers is also included. This protection only applies to 10MVA and above on transformers; however, it is not limited to transformers, but also transmission lines, busbars, and generators. The balance of the paper is on reliability assessment based on the HVDC grid protection scheme operation.