1. What is the use of TAN(δ) value - TAN(δ) value is used to determine the deterointation of the insulating material in the insuators and thus determining their life.
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2. Theory of TAN(δ) test -
a) When an insulator is connected across line & earth it behaves like a capacitor.
b) In ideal condition, only capacitive current Ic will flow through it. This current Ic leads voltage by 90 degree .
c) But in real world, since we cannot make 100% pure capacitance there exist an effective series resistance (ESR) with capacitance which creates a path for IR current to flow. This causes the resultant current IL to lead voltage V by (90-θ)degree.
from phasor diagram (fig-D),
from phasor diagram (fig-E),
d) From TAN(δ) test we get the value of capacitance C & TAN(δ), using this values we can calculate R.
Testing Two-winding transformers
Test connections for Delta - Star type Transformer-
1] The transformer must be taken out of service and isolated from the power system.
2] Ensure the transformer is properly grounded to the system ground.
3] Take a Armoured wire from a spare conductor long enough to short all 3 phases of HV bushing.
4] Take another armoured wire and short all 3 phases of LV bushing with neutral.
5] Connect HT lead to the shorted HV bushing, LT lead to shorted LV bushing and connect test kit guard cable to transformer earth.
6] Use following table
* In test no. 1 the capacitance and Tan Delta between HV & LV bushing is measured CHL.
* In test no. 2 the LV bushing gets grounded and Capacitance between LV to HV to ground is measured CHG + CHL.
* In test no. 3 LV bushing is guarded, so capacitance between HV bushing and ground is measured CHG.
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7] Now reverse the connections, connect HT lead to LV shorted Bushing and LT lead to HV shorted Bushing.
* In test no. 5 the capacitance and Tan Delta between LV & HV bushing is measured CLH. (test no.1 = test no.5)
* In test no. 6 the HV bushing gets grounded and Capacitance between HV to LV to ground is measured CHL + CLG.
* In test no. 7 HV bushing is guarded, so capacitance between LV bushing and ground is measured CLG.
To Crosscheck results capacitance from,
(Test no.2 - Test no.3) = Test no.1
similarly,
(Test no.6 - Test no.7) = Test no.5
8] Individual tests should be performed on each HV and LV bushing. Bushings equipped with a potential/ test tap should have the UST test performed and the GST on those without test taps.
9] For transformers that have wye-wye configuration, and the neutrals internally cannot be separated, Test No. 1 & 2 and Test No. 5 & 6 cannot be measured. In this case short the high voltage bushings and the low voltage bushings together and perform a GST test. Test voltage should be suitable for the rating of the low voltage winding.
10] Enter all the nameplate information of the transformer. Note any special or unusual test connections or conditions. Enter ambient temperature and relative humidity and a general indication of weather conditions at the time of the test.
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