Tan Delta (TD) is a measurement of the dielectric losses in the machines stator winding insulation when AC voltage is applied to it. Ideal insulation behaves like a capacitor (purely reactive current) yet real insulation has a resistive leakage current. Tan delta is a measurement of the overall stator winding insulation and if it's stable under voltage stress, through measuring the resistance current divided by the capacitive current, where higher tan delta readings mean increased dielectric losses, insulation deterioration and possible thermal instability/moisture ingress/ contamination/delamination/voids. The test is performed as per the IEC 60034-27-3 and/or IEEE 286 standard(s), in voltage steps with the 'tip-up' measured i.e. the TD change with voltage.

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