Tan-delta test (European) is almost identical to the "power factor" test (American).
In both cases we are applying a high-voltage to the insulation (up to 10kv but no more than normal L-G voltage) and measuring the phase angle of the resulting current.
The insulation itself is primarily capacitive (power factor 0.0), but contamination introduces some resisitve component (can raise power factor above 0.0). The higher the power factor, the more likely contamination is present in the insulation. Also the magnitude of the current is also measured which enables calculation of insulation capacitance. All of these parameters can be trended over time for even better diagnostic value. There is a lot that goes into interpreting the data... Doble Engineering is a great resource in this area.
Back to the difference between Tan-delta test and "power factor". In both cases we are characterizing the angle delta between voltage and current. Tan delta is the tangent of this angle while power factor is the cos of this angle. For almost any insulation specimen, this angle will be less than 10 degrees and the cos and tan are virtually identicial. I prefer to talk in terms of power factor because it is a concept already familiar to most electrical types.
Tan-delta test is intended for determining the degree of degeneration of high voltage insulation. That insulation is part of the capacitance between the line, or windings, and earth, or between two windings. The loss factor of such capacitor gives a very reliable information about the degeneration of insulation, so that high loss factors correspond to high degeneration. That meassurement must be performed at the working voltage, and there is a well known an documented procedure based upon the so called "Schering Bridge".
I should offer a correction to my discussion on similarities between power factor and tan delta.
They ARE very nearly identical for most test specimens. But my reasoning was incorrect.... cos and tan are not equal for small angles, but sin and tan are equal. The tan delta looks at the tangent of the angle delta which is "complementary" to the power factor angle theta (ie delta = 90-theta). The trigonometry gives the same result, since cos(theta)=sin(delta) which is very near to tan(delta).
