Download Insulation Testing of Transformer

WELCOME
A PRESENTATION
ON
INSULATION TESTING OF TRANSFORMER
Dhritiman Nath, ET
CONTENTS
 Transformer
 Insulation
 Insulation
Tester
 Insulator Model
 Significance of Records
TRANSFORMER
 Transformer
is an electromagnetic device that is used
to change the electrical parameters keeping the
frequency constant
 It is the most efficient equipment in the field of
electrical engineering having efficiency upto 99 %
 Availability of transformer makes the use of
alternating current more advantageous than direct
current
 We have total 104 nos of transformer in 103 nos of
sub-station
INSULATION
Insulator
prevents the flow of current in a
unwanted direction
Transformers have transformer oil and paper as
it’s insulation
 Causes of insulation failure are –
 Thermal Breakdown
 Chemical deterioration
 Effect of oxygen and humidity
 Electro-chemical deterioration
INSULATION TESTER
 We
use insulation tester of megger
 D.C. voltage is applied to the insulator under
measurement
 The voltage levels that are available are –
250V, 500 V, 1000V, 2500V, 5000V
 Measurement is taken between H.V. - Earth, L.V.Earth and H.V.-L.V.
PROCEDURE
 Shutdown
the transformer and isolate it
 Discharge the winding capacitance
 Clean the bushings
 Short Circuit the windings
 Guard the terminals to eliminate surface leakage
over terminal bushings
 Connect “LINE”, “EARTH” leads
 Transformer neutral bushing is to be
disconnected from earth during test
 Apply test voltage and note the readings
CONTINUED…..
Records are taken at an interval of –
1. After 30 secs
2. After 1 minute
3. After 10 minutes
o The total current is the sum of following –
1. Capacitive current (IC )
2. Conduction current ( IR )
3. Surface leakage current (IL )
4. Absorptive current ( IA )
o
+
+
+
+
-
DIPOLES
-
-
-
+
+
-
INSULATOR MODEL
Total Current
C
R0
= Leakage current
= Conduction current
= Capacitive current & absorptive current
R1
Relative Current
GRAPH
= Total Current
= Absorptive Current
= Capacitive current
= Leakage Current
= Conduction Current
Time of Voltage Application(minutes)
Continued….

Insulation resistance reading :
 After 30 secs the reading is of,
IT = IC+ IA + IL + IR
 After 1 minute, the reading is of,
IT = IA + IL + IR
 After 10 minutes, the reading is of,
IT = IL + IR
Reading of 10 Mins
 Polarization Index =
Reading of 1 Min
Reading of 1 Min
 Dielectric Absorption Ratio =
Reading of 30 sec
SIGNIFICANCE

Polarisation Index :
IL + IC + I A
IL + I C
One minute measurement is not reliable and scientifically
acceptable as IR is temperature dependent
 P.I. gives result which is temperature independent and this gives
more accurate result.
 P.I. gives better idea of conduction and leakage current


If IL and IC are low compared to IA, then PI will be more than 2

If IL and IC are high compared to IA, then PI will be around 1
CONTINUED…..
AS per IEEE,
Insulation Resistance
Polarisation Index
Dangerous
Less than 1
poor
1.0-1.1
Questionable
1.1-1.25
Fair
1.25-2.0
Good
Above 2
Source : grouper.ieee.org
CONTINUED…..

Dielectric Absorption Ratio :
IC +IA +IL + IR
I A + IL + IR
Insulation Resistance
Questionable
DAR value
Less than 1.25
Adequate
Less than equal to1.6
Good
1.6 and above
Source: www.openelectrical.org

The DAR test is performed only when P.I. is found to be less
than 2 even for new materials.Under such cases a minimum
DAR value of 1.25 is required.
RECALL
Transformers
 Procedure
 Significance of data
