Testing Of Transformer
Testing of Transformers are
covered by IS:2026, 1977
(Re-affirmed 2001).
Tests :
Type Test :-
(a) Measurement of winding resistance
(b) Measurement of voltage ratio and check of vector group
(c) Measurement of impedance voltage / short circuit impedance
(nominal tap) and load loss
(d) Measurement of magnetising current and no load loss
(e) Measurement of Insulation resistance
(f) High voltage tests for Di-electric
(g) Temperature rise and
(h) Tests on On-load tap changers where appropriate
Routine Tests :
(a) Measurement of winding resistance
(b) Measurement of Voltage ratio and check of vector
group
(c) Measurement of impedance voltage/short circuit
impedance (nominal tap)
(d) Measurement of magnetising current and no load loss
(e) Measurement of Insulation resistance
(f) High voltage tests for Dielectric and
(g) Tests on tap-changers, where appropriate
CESC System of Testing
(1) Routine & Type Tests : CESC accepts certificates from
Manufacturers
(2) Acceptance Tests : Three types of Transformers
are tested for acceptance :-
(i) Transformers (new) received
from Manufacturers
(ii) Transformers repaired by
contractors
(iii) Transformers ready after in-
house repairing
Acceptance Tests :
(i) Measurement of winding resistance
(ii) Measurement of ratio and magnetising current ( by 3-
phase injection)
(iii) Check of vector group (by 3-phase injection)
(iv) Magnetic balance (by 1-phase injection)
(v) Measurement of Insulation resistance
(vi) Measurement of Polarisation Index
(vii) High Voltage test
Test Procedure & Philosophy of Testing
A. Measurement of Insulation Resistance (IR)
IR of each winding is measured by 1 kV Megger
Line terminal is connected to the winding. Other winding & tank is
connected together to earth
IR value is measured to check health of the insulation before
applying voltage to the transformer
B. Measurement of Ratio & Magnetising Current
/
Voltages VRY,VYB,VBR is measured. Also Vry,Vyb,Vbr,Vrn,Vyn,
Vbn is measured to calculate ratio.
Calculated ratio is checked with the ratio specified by the
manufacturer.
Magnetising current on each phase is measured across R’-R or
Y’-Y or B’-B keeping the respective fuses out.
For Dyn11 transformer IR > IY & IB and IY=IB.
Ratio & Magnetising current is measured to check
(i) the mentioned ratio
(ii) whether there is any inter turn short in the winding &
(iii) also to check the winding connection.
C.Vector Group Test
R-r is shorted
Vector Group is tested to check winding connection & Polarity.
Example : To check vector group Dyn11
VBY,VBb, VYy,VYb,VRn,VYn,VRY is measured
R,r
Check (i) VRY = VRn+VYn n
(ii) VBy>VBb y
(iii) VYy = VYb b
B Y
D. Magnetic Balance Test
Magnetic balance is checked by applying 1-phase A.C. Voltage (say
100 V) across each phase to Neutral on LV side (Star side).
1-Ph voltage is applied across rn and Voltages across Vyn & Vbn
are measured & studied. Value of Vyn should be more than Vbn.
Similarly 1-Ph voltage across yn and bn are applied in turn and the
voltages Vrn,Vbn & Vrn & Vyn are measured respectively &
studied.
y phase being the middle limb Vrn & Vbn should be more or less
equal and it’s value should be 0.5 Vyn when voltage is applied
across yn.
Magnetic balance is done mainly to check core healthiness &
distribution of flux. Leakage flux should be low.
E.Winding Resistance Measurement
Resistance is measured by Ducter Ohmmeter or any low resistance
(inductive) measuring instrument.
Resistances of RY , YB , BR on HV side & rn , yn , bn, ry , yb , br
on LV side are measured.
Winding resistance is measured to check any high resistance
in any winding.
F. Measurement of Polarisation Index (PI)
PI is measured by 5 KV Motorised Megger.
Readings of IR is taken after 1 min. & after 10 min.
IR value after 10 min.
PI =
IR value after 1 min.
PI value indicates dielectric Polarisation property & hence health of
the insulation.
PI value should be > 1.5 for new Transformer.
G. High Voltage Test
High voltage DC is applied for 1 min. Leakage current is noted.
Test voltage :
Rating Test voltage(DC) for 1 min.
New Transformer 6 KV 13.5 KV
11 KV 23.0 KV
Repaired Trans. 6 KV 9 KV
11 KV 15 KV
HVDC test is done to check health of the insulation.
3. Testing at site subsequent to
breakdown & necessary repair
Sequence of test :
(a) Measure IR of both the windings
(b) Measure winding resistance
(c) Measure ratio & magnetising current
(d) Check magnetic balance
(e) Carry out HVDC test
(f) Vector group test is not required at site.
Test equipment required :
1. Multimeter (suitable for measuring Voltage & Current)
2. 3-Phase test board with colour leads
3. Motorised Megger ( 5kV )
4. Hand driven 1 kV Megger
5. Ducter Ohmmeter / Low resistance (inductive) measuring Inst.
6. DC High Voltage Test Set
7. 1-Phase Variac
* 3-Phase supply with neutral is required..
Reference books :
1. The Performance & Design of Alternating Current
Machines : M.G.Say
CBS Publishers & Distributor
485 , Jain Bhawan
Bholanath Nagar
Shahdara , Delhi-32
2. J & P Transformer Handbook
Have A Nice Day……
RATIO TEST & MAGNETISING CURRENT MEASUREMENTS
A R r
n
400V y
400
400V Y
B
b
Ratio 6000/420v
AT NORMAL TAP
INPUT VOLTAGE MAG. CURRENT MEASURED VOLTS IN LV SIDE
TO HV (mA) (VOLTS)
WND(VOLTS)
R-Y Y -B B-R R Y B r-y y -b b - r r- n y -n b- n
400 400 400 14.5 10 10 28 28 28 16 16 16
MAGNETIC BALANCE TEST
R r
n
y
30V
B Y
b
Ratio 6000/420v
AT NORMAL TAP
SINGLE PHASE MEASURED VOLTS (VOLTS)
INPUT VOLTAGE IN
LV SIDE
r-n y -n b-n r-n y-n b-n R-Y Y-B B-R
30 - - 30 22 8 770 540 195
- 30 - 15 30 15 390 770 380
- - 30 8 22 30 220 540 770
VECTOR GROUP TEST
rR
n
400V y
400 TEST DONE AT NORMAL TAP
b
400V R – r kept shorted
B Y
Vector group Dyn 11
INPUT VOLTAGE MEASURED VOLTAGE (VOLTS)
(VOLTS)
R-Y Y -B B-R R-y R-b Y-y Y-b B-y B-b R-n Y-n
400 400 400 28 28 380 380 400 378 16 384
CONCLUTION Ry = Rb Yy = Yb By >Bb Rn+Yn =RY
TYPICAL WNDG. RESISTANCE FIGURES FOR
DISTRIBUTION TRANSFORMERS USED IN CESC
Transformer Rating HV resistance LV resistance
details between between phase
terminals of and neutral of
transformer in transformer in
Delta side at Star side
normal tap (milliohms)
(ohms)
6000/420 V. 200 kVA 2.5 - 2.7 5.3 – 5.7
Oil immersed 315 kVA 1.1 – 1.4 2.1 – 2.9
Dy 11 transformer 400 kVA 0.85 – 1.0 1.2 –2.0
600 kVA 0.4 - 0.6 1.1 –1.2
6000/420 V. 315 kVA 0.74 – 0.78 1.8 – 1.85
Dry type
Dy 11 transformer
11000/420 V. 315 kVA 4.8 – 5.0 2.7 – 2.8
Oil immersed
Dy 11 transformer
Trf.details 1 min-IR 10 min Ir PI value Leakage Leakage
Value value current of current of
(Meg.ohms) (Meg.ohms) HV side at LV side at
13.5 kV dc.(mA) 2 kV dc.(mA)
GEC make 315 10000 14500 1.45 0.01 0.01
kVA
6600/420 V Oil
immersed 3500 6500 1.86 0.01 0.01
Transformer
340 370 1.09 0.07 0.03
600 660 1.10 0.04 0.05
Kirloskar make 6000 9000 1.5 0.02 0.01
6600/420 V Oil
immersed
Transformer 3700 6000 1.62 0.01 0.01
1400 1450 1.04 0.04 0.03
