The lightning impulse test assesses transformer insulation's ability to withstand lightning overvoltages, involving the generation of an artificial lightning impulse through capacitors. The negative impulse voltage is applied to the transformer, and the response is analyzed to identify insulation failure. The test is performed on high-voltage windings, ensuring that other terminals and windings are grounded during the process.

1. LIGHTNING IMPULSE TEST

2. Purpose of test:
 The purpose of the test is to secure that the transformer
insulations withstand the lightning overvoltages which may
occur in service.
METHOD OF TESTING:
 The testing is conducted by artificial generation of
lightning impulse using IMPULSe GENERATORS.(FIG 1)
 Capacitor banks are charged in parallel from a d.c source and
is discharged in series to create impulse voltage of wave front
1.2micro seconds&tail of 50 micro seconds.(FIG 2)
 There are 20 capacitors of 150 kv each &together they
produce 3 million volts.(shown only 4 capacitors in fig)

3. FIG: impulse generator
C1 = impulse capacitor
Rh = charging resistor
Rf = low-ohmic discharging resistor
Rc = high-ohmic discharging resistor
F1 = spark-gaps
F2 = calibration resistor
C2 = capacitance of voltage divider
R = resistance of voltage divider

4.  The impulse wave applied is negative because positive wave
can cause distarge from line to earth since all coils are
earthed.(FIG 2)
 The impulse voltage is applied to the transformer under test,
if the insulation has failed then response wave will be
distorted or will be chopped.
 The response is obtained in an impulse analyser taken by
means of potential divider.

5. FIG-2

6.  The lightning impulse test is normally applied to the H.V.
winding. The impulse test-sequence is applied successively to
each of the line terminals of the tested winding. The other
line terminals and the other winding are earthed (FIG 3).

7. TRANSFORMER IMPULSE TESTING
CONNECTIONS
FIG-3

8. RATED WITHSTAND VOLTAGES FOR
TRANSFORMER WINDINGS
HIGHEST VOLTAGE RATED LIGHTNING WITHSTAND
VOLTAGE
KV r.m.s KV peak
15 95
121 350
245 650
420 1050
800 1800

9. Test circuit
CG-Impulse generator Pd-potential Divider
Rs-series resitor to adjust wave front C load- load capacitance(transformer)
Rp-parallel resitor to adjust wave tail

10. procedure
 The impulse voltage wave form is obtained at reduced level
from the potential divider by polishing its front & tail using Rs
and Rp.
 The sphere is calibrated to know the voltage to be applied or
can be checked using analyser.
 All winding except the test winding is earthed .
 The calibrated impulse wave is applied to the test windings.
 Its response is recorded using impulse analyser.

11. Presented by
JIBIN
JOY