Beyond the nameplate - Selecting transformer compensation settings for secure differential protection

Microprocessor-based transformer differential relays offer fast and dependable protection for transformer faults while remaining secure for through faults. Differential elements in microprocessor-based relays compensate for the transformer winding connections internally based on the compensation settings provided by the user. Transformer differential settings are often misunderstood, and incorrect settings can lead to undesired operations. Relays use compensation settings to compensate for transformer and current transformer (CT) connections. Understanding transformer winding construction, phase-to-bushing terminal connections, and CT connections is crucial to the correct selection of transformer compensation matrices. This paper presents simple rules, based on fundamentals, for the correct selection of compensation settings and their application to several real-world installations. It discusses how different compensation settings (matrices) affect the differential elements and uses a real-world event to show how incorrect compensation settings can result in misoperation, even under load conditions. By analyzing two events from differential relay operations, the paper further shows that some compensation pairs, although valid in terms of producing a balanced differential current, could compromise the security of the differential element due to insufficient restraining current for certain external fault types with CT saturation. For both events, following the rules presented in the paper improved the security of the differential element and avoided the misoperations.