Introduction

Super fast and selective protection is required to protect power transformers in case of faults like internal or external short circuits. Faults with high fault currents that are not cleared instantaneously may lead to long term loss of a critical asset and very high repair costs – if a repair is still possible. Therefore it is state of the art to protect power transformers with differential protection systems.

Differential protection systems are very selective and can act very fast. However, since a power transformer transforms the electricity to a different voltage level some complication is introduced when it comes to applying differential protection. While differential protection of generators or motors is a quite straight forward technique, protection of power transformers with differential protection needs to overcome some challenges. Those challenges are the transformation ratio of the currents, phase shift depending on the vector group of the transformer, elimination of the zero sequence component if networks with different grounding systems are linked together by the transformer, inrush current, etc. (Fig. 1, Fig. 2)

Electromechanical relays have managed these challenges by applying interposing transformers and solutions like delta connected CTs. (Fig. 3)

Numerical relays apply a software transformer model to the currents received from the CTs making the use of interposing transformers obsolete. The differential characteristics of transformer differential relays are in principle in line with other differential protections. A biased characteristic is in place to desensitize the protection for:

• Measurement errors of the CTs
• Measurement errors of the relay
• Influence of the transformer tap changer
• CT saturation effects
• Magnetization current of the transformer (Fig. 4)