Calculation of Resonating Inductor for Tan Delta Test Kit?
What is Resonating Inductor?
A Resonating Inductor is nothing special but it is just an Inductor having a specified value of Inductance. Resonating word is added here just to give idea of its purpose and working principle. Now you can guess. Obviously this inductor will be used to create a resonant circuit for getting certain advantage.
Why Resonating Inductor is Required for Tan Delta Test?
Well, a Tan Delta Test Kit has a specified output current rating which varies from make to make. If you check the data sheet of different make Tan Delta Test Kit, you will notice this output current rating. You may also notice that Kit has two type of current rating, one is continuous and another short time current rating. Continuous current rating is less than short time current rating. Let us take an example for better understanding. Let us consider ISA make TDX5000 Tan Delta Test Kit. The current rating of this kit is tabulated below.
Output Voltage

Output Current

Duration (in second)

12 kV

300 mA

120

125 mA

>1 hour


100 mA

Continuous

Thus the maximum output current rating of this kit is 300 mA. Now suppose we need to perform Tan Delta Test of Turbo Generator Stator. The capacitance of Turbo Generator Stator is normally of the order of 400nF. The reactance offered by this capacitance
X_{C} = 1 / 2πfC
= 1 / (2×3.14x50x400 nF)
Frequency is assumed to be 50 Hz in above calculation.
X_{C} = 7961.8 Ω
Thus the current output of the kit at output voltage 12 kV,
I = 12kV / 7961.8
= 1.5 A
Thus the kit is required to have an output current of 1.5 A even at its highest output voltage of 12 kV. But the maximum current rating of Kit is only 300 mA. This means that we cannot perform Tan Delta Test of Generator with this kit.
To overcome the above problem of output current requirement, resonating inductor is used in parallel with the test specimen. Parallel connection of resonating inductor with test specimen reduces the impedance. Due to reduced impedance, the kit is able to deliver output current within its rating.
Rotating electrical machines have generally high insulation impedance. Due to high impedance, the kit becomes unable to deliver current for measurement of tan delta. Hence for such machines the resonating inductor is required.
How to Calculate the Value of Required Resonating Inductor?
Basis of Selection: The basis of selection of resonating inductor should be the highest capacitance of insulation of the rotating electrical machine used in the industry. For power generation utility, the capacitance of stator of Generator is highest. So, this should be considered for the choosing suitable value of resonating inductor.
Calculation of Resonating Inductor:
Let the highest value of insulation capacitance = 400 nF
As resonating inductor is connected in parallel with the test specimen, it forms a resonating circuit with the insulation capacitance. For resonance to take place, the inductive reactance must be equal to capacitive reactance. This means,
X_{C} = X_{L}
⇒ 1 / ωC = ωL
⇒ L = 1 / ω^{2}C
= 1 / 4π^{2}f^{2}C, since ω = 2πf
For our example, C = 400 nF and f = 50 Hz. Putting these values, we get
L = 25 Henry approx.
Therefore the required value of Resonating Inductor = 25 Henry. This resonating inductor can be used for tan delta and capacitance measurement of all rotating equipment as it is selected based on the maximum value of insulation capacitance of the utility.