Abstract : Based on the phasor balance matrix of Ynd5 provided by Siemens, after phasor matrix transformation, it is found that the phase of the Y-side phasor of the 7UT51 relay remains constant after phasor balance matrix transformation for transformers with different clock group connections. The key lies in the change of the Δ-side phasor matrix. Application comparisons were made for single-phase grounding faults outside the zone, single-phase grounding faults within the zone, and two-phase short-circuit faults within the zone. Keywords : 7UT51 relay; phasor balance matrix; differential protection. Due to the different wiring methods of the high and low voltage sides of the transformer, there is a phase difference between the high and low voltage sides. Therefore, differential protection requires phasor balance and amplitude balance for the input current on each side. Traditional BCH type differential relays rely on external wiring and setting calculations to balance the number of compensation coil turns; the 7UT51 type differential relay, however, achieves this through calculations within the protection system itself. While calculating amplitude balance is understandable, its current input circuits on each side are generally Y-connected. So how is phasor balance calculated, and what is the underlying principle? Based on the phasor balance matrix of Ynd5 published by Siemens, the phasor balance matrices of transformers with Yndl1 and Dynl1 connection configurations can be derived. [b][align=center]For more details, please click: A Brief Analysis of the Phase Balance of the 7UT51 Type Differential Relay[/align][/b]