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Power transformer differential protection 87T has been used for decades. With static or electromechanical 87T relays, external interposing current transformers are used in order to compensate for phase-angle shift and current magnitude variation between the two sides of the protected power transformer. With numerical differential relays, external interposing current transformers are not required. Necessary magnitude and phase-angle compensation are provided internally within the relay by software. However, special power transformers with nonstandard or variable phase-angle shift, such as converter transformers or phase shifting transformers, cannot be easily protected with classical 87T differential relays. By using modern numerical technology, it is now possible to make differential protection relay 87T, which can properly calculate differential currents for any power transformer, regardless of its fixed or variable phase-angle shift and current magnitude variations.