What is dissipation factor ? | What is tan delta test ? | What is loss of angle test ? | Testing of insulator
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The tan delta test or Dissipation factor or loss of angle test is use to check healthiness of insulation of the equipment like cable, winding, current transformer, potential transformer, transformer bushing.
The tan delta test or Dissipation factor or loss of angle test is use to check healthiness of insulation of the equipment like cable, winding, current transformer, potential transformer, transformer bushing.
👉 Content :
- Principle of Tan Delta Test.
- Basic Procedure of Tan Delta Testing.
- Purpose of tan delta testing.
- Why we use Very Low Frequency voltage for tan delta testing.
- Permissible limits of Tan delta test
👉 Principle of Tan Delta test:
When insulator is connected across line and earth or between two conducting plates ,it behaves as a capacitor and insulating material act as a dielectric material of capacitor.
A in above fig (a) shows the insulator material.
To understand how we can get dissipation factor first we take a pure insulator
In pure ideal insulator, insulating material acts as 100 % pure dielectrical material it make a 100% pure capacitor, the electric current passing through the insulator, only have capacitive component.
There is no resistive component of the current Ir = 0, as there is no impurity in material.as shown in fig (b)
In a pure capacitor, the capacitive electric current leads the applied voltage by 90°.
In a practical world, there is No 100% pure insulator.its dielectric property is decrease due to the aging of insulators, the impurities like dirt and moisture enter into it. and this impurity is represent as small resistance in equivalent circuit.
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| Equivalent circuit |
These impurities provide the conductive path to the current. due to this impurity an electric leakage current flowing from line to earth through the insulator has a resistive component.
Form above we can say that for a good insulator, the resistive component of electric leakage current is quite low.
The healthiness of an electrical insulator can be determined by the ratio of the resistive(Ir) component to the capacitive component ( Ic).
For good insulator, this ratio would be quite low. This ratio is commonly known as tanδ or tan delta. Sometimes it is also referred to as dissipation factor.
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| Vector diagram |
Capacitive component(Ic) of current lead voltage by 90° and it plot on Y axis.
vector sum of both capacitive and resistive current is total current (I), I = Ic+Ir
this total current I make δ ( delta ) angle with Y axis.
From above diagram we clearly say that ratio of Ir and Ic is Tanδ ( tan delta).
👉 Basic Procedure of Tan Delta Testing:
It is high voltage test. and safety is very important for this test as we use high voltage.
TAN DELTA KIT is use for this test .
Step 1: Isolate equipment form the system.
Step 2: Make connection of Tan delta kit with the equipment under test.
Step 3: First apply rated voltage , the value of tan delta is under limit.
Step 4: Now increase voltage 1.5 to 2 times of rated voltage.
Step 5: The tan delta kit have controller and tan delta measuring unit, measuring unit takes measurement of tan delta values. A loss angle analyzer is also connected with tan delta measuring unit to compare the tan delta values at normal voltage and higher voltages and analyze the results.
👉 Why we use Very Low Frequency voltage for tan delta testing:
If the frequency of the applied voltage is high, then capacitive reactance(XC=1/ 2πfc) of the insulator becomes low, hence the capacitive component of electric current is high.
The resistive component(R) is fixed; it depends upon applied voltage and conductivity of the insulator.
At high frequency as capacitive current, is large, the amplitude of the vector sum of capacitive and resistive components of electric current becomes large too.
Therefore, required apparent power for tan delta test would become high, So to keep the power requirement low for this dissipation factor test, very low-frequency test voltage is required.
Frequency range for tan delta test 0.1 to 0.01 Hz depending upon size and nature of insulation.
Second reason is ,
As we know ,
👉 Purpose of tan delta testing:
1) Detecting the incipient weaknesses in HV insulation.
2) Predicting the remaining life expectancy of the equipment under test.
3) To get the “Benchmark Reference Reading” of equipment when the equipment is new, clean, dry and free from impurities.
👉 How to analyse result of Tan delta test:
1) If equipment under test is new , the tan delta test result is benchmark for future. it is use to predict health of insulation in future.it become standard results for that equipment.
2) If equipment under test is old , than its current tan delta result is compare with old results and if it found that is violate the permissible limits than we can say there is weakness in insulation and Equipment is to be replace.This is due to the aging of insulators, the impurities like dirt and moisture enter into it.
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