Dielectric dissipation


If an alternating voltage V of frequency f is applied across an insulation system comprising capacitance C and equivalent series loss resistance RS, then the voltage VR across RS and the voltage VC across C due to the resulting current I are:
V = (VR2 + VC2)½

The dielectric dissipation factor of the insulation system is the tangent of the dielectric loss angle d between VC and V:
d = VR / VC = RS / XC = 2pfCRS
= XCtand = tand / 2pfC
Note that an increase in the dielectric losses of a insulation system (from an increase in the series loss resistance RS) results in an increase in tand. Note also that tand increases with frequency.

The dielectric power loss P is related to the capacitive reactive power QC by:
P = I2RS = I2XCtan
d = QCtand

The power factor of the insulation system is the cosine of the phase angle f between VR and V:
f = VR / V
so that
d and f are related by:
d + f = 90°

tand and cosf are related by:
d = 1 / tanf = cosf / sinf = cosf / (1 - cos2f)½
so that when cosf is close to zero, tand » cosf

Note that the series loss resistance RS is not related to the shunt leakage resistance of the insulation system (which is measured using direct current).