Testing microprocessor-based numerical transformer differential protection

Transformer differential protection application has always been a challenge to protection engineers and commissioning personnel, even with older electromechanical and solid state technologies. The emergence of microprocessor-based protection, where the intelligence is implemented numerically in software algorithms, and the fact that different relay models, even from the same manufacturer, apply different algorithms and compensation techniques and use different terminology and notation, has made it increasingly more complicated to apply the protection and even more challenging to correctly test them during commissioning. This paper reviews how phase differential protection principles are implemented in numerical technology by different relay models, provide guidance in their application, and guide test personnel in performing their work. Test methods are proposed for use with modern test systems that can be applied to test all transformer phase differential protection functions easily and effectively.