Test done on Power transformers. Insulation Resistance test, Winding Resistance test, Ratio Measurements, Magnetic balance test, Tan delta test, DIssolved gas analysis for transformer, Sweep frequency response analysis.

1. Testing of Transformer
BY – GEETESH VERMA
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2. Why Testing of Transformer ?
•To validate design specifications
•To verify quality of manufacturing process
•To establish present condition and condition trend
•To determine if damage has occurred
•To detect winding deformation
•To detect displacements between high and low voltage windings
•To detect partial winding collapse
•To detect shorted or open turns
•To detect core movement
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3. Important Tests done in site Periodically –
• Insulation Resistance Test or Megger Test
• Winding Resistance Test
• Ratio Measurements
• Magnetic Balance Test
• Tan(δ) test
• Dissolved Gas Analysis for Transformers
• Sweep Frequency Response Analysis Test | SFRA Test
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4. Transformer Winding Resistance Test
•A DC test current is injected across the measured winding (-s), the voltage drop as well as the
test current is measured and the resistance is calculated.
•For purely resistive objects the measurement is trivial and very easy to perform.
•When the measurement circuit is highly inductive, measurement may become rather complex.
•Always try to saturate the core. This happens typically when the test current is about 1% of rated
current.
•The time required for readings to stabilize will vary based on the rating of the transformer, the
winding configuration, and the current output selected.
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5. The mathematical expressions, where U is Voltage, R is resistance, I is current, L is inductance and t is
the time,
Winding resistance R
times the current I, i.e.
what we want to
measure.
“error” term
(inductance) x (change of current)
The solution to this issue is to keep
the test current constant over
time.
“error” term
(current) x (inductance change)
It is difficult to reduce. In practice
the only way is to wait for the
inductance change to stabilize.
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6. 6
TAP POS.
HV WINDING RESISTANCE (m-ohm)
UV UW VW
3 25.02 25.70 25.03
LV WINDING (m-ohm)
R-N Y-N B-N RY YB BR
4.130 4.111 4.153 7.955 7.92 7.755
Winding Resistance Test readings of a Transformer.

7. Transformer Ratio Measurements
1. The tap changer of transformer is kept in the
lowest position and LV terminals are kept open.
2. Apply 3-phase 415 V supply on HV terminals.
Measure the voltages applied on each phase
(Phase-phase) on HV and induced voltages at
LV terminals simultaneously.
3. After measuring the voltages at HV and LV
terminals, the tap changer of transformer
should be raised by one position and repeat
test.
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8. 8
TAP
POS.
Calculated
Ratio
Measure ratio
R Y B
3 1.59420 1.5925 1.5928 1.5926
Ratio Measurement test readings of a Transformer.

9. Magnetic Balance Test
It is performed to detect faults / defects in the magnetic core structure, shifting and/or de-
shaping of windings, failure in inter-turn insulation or fault in Tap Changer
These faults change the effective reluctance of the magnetic circuit which then affects the
magnetizing current required to establish flux in the core.
Test Procedure -
1. Connect single phase 230 Volts supply between first and second phase of primary side of
transformer (between 1U & 1V)
2. Measure voltages between 1U-1V, 1V-1W, and between 1W-1U,
3. Similarly measure voltages between 2U-2N, 2V-2N & 2W-2N
4. Then connect single-phase supply between second and third phase on primary side
(between 1V-1W)
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10. 5. Again Measure voltages between 1U-1V, 1V-1W, and between 1W-1U
6. Repeat Similar Procedure for LV side.
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2U-2N 2V-2N 2W-2N
239.0 207.9 31.20
138.9 239.5 99.80
36.5 201.6 239.1
1U-1V 1V-1W 1W-1U
229.7 112.15 117.5
195.2 229.75 33.6
198.1 31.4 229.7
Magnetic Balance test readings of a Transformer.
*all readings in Volts

11. Dissipation factor measurement/ tan(δ) test
•Tan(δ) performed to investigate the condition of the insulation of power transformers and bushings.
•The dissipation factor/tan(δ), is the ratio of Resistive component of current to Capacitive component
of current flowing through a insulator.
•A pure insulator when is connected across line and earth, it behaves as a capacitor. There is no
resistive component of the current, as there is zero percent impurity.
•For good insulator, resistive component of leakage electric current is quite low.
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12. • In the vector diagram, the system voltage is drawn along x-axis.
• Conductive electric current i.e. resistive component of leakage current, IR
will also be along x-axis.
• As the capacitive component of leakage electric current IC leads system
voltage by 90°, it will be drawn along y-axis.
• Total leakage electric current (IR+ IC) makes an angle δ (say) with y-axis.
• Now, from the diagram above, it is clear, the ratio, IR to IC is nothing but tanδ or tan delta.
• NB: This δ angle is known as loss angle.
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13. Interpretation of tan(δ) Measurement
1. Comparing the results of previous tests to determine, the
deterioration of the condition of insulation due to ageing effect.
2. Determining the condition of insulation from the value of tanδ,
directly.
If the insulation is perfect
The loss factor will be
approximately same for all
range of test voltages.
If the insulation is not sufficient
The value of tan delta increases
in the higher range of test
voltage.
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14. 14
Meas.
Combination
Test Voltage Mode Capacitance(nF) Current(mA) Tan δ
HV-E
2 KV
GST-GA
2.89418 1.820 0.1988
5 KV 2.9004 4.56 0.3295
HV-LV
2 KV
UST-A
3.7693 2.3687 0.3349
5 KV 3.7681 5.9252 0.3099
LV-E 2 KV GST-GA 7.8876 4.9632 0.3205
Tan Delta test readings of a Transformer.

15. Dissolved Gas Analysis for Transformers
•Oil sample analysis is a useful, predictive, maintenance tool for determining transformer health.
•The breakdown of electrical insulating materials and related components inside a transformer
generates gases within the transformer.
•The identity of the gases being generated can be very useful information in any preventive
maintenance program.
•This method involves sampling the oil and testing the sample to measure the concentration of
the dissolved gases.
•It is recommended that DGA of the transformer oil be performed at least on an annual basis
with results compared from year to year.
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16. Insulating mineral oils for transformers are mixtures of many different hydrocarbon molecules
The fundamental chemical reactions involve the breaking of carbon hydrogen and carbon-carbon
bonds.
Energy is needed for the breaking of the chemical bonds and this comes from the energy contained
in the fault of the transformer.
Gases Formed
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18. Sweep Frequency Response Analysis Test
| SFRA Test
•A transformer can be considered as a complicated distributed network of resistance, inductance,
and capacitance or in other words a transformer is a complicated RLC circuit.
•Because of that each winding of a transformer exhibits a particular frequency response.
•In SFRA a sinusoidal voltage Vi is applied to one end of a winding and output voltage Vo is
measured at the other end of the winding. Other windings are kept open.
•As the winding is itself a distributed RLC circuit it will behave like RLC filter and gives different
output voltages at different frequencies.
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19. After transportation, heavy short circuit faults, transient switching impulses and lightening impulses etc., if
we do same Sweep Frequency Response Analysis test and superimpose the present signature with the
earlier patterns and observe some deviation between these two graphs, we can assess that there is
mechanical displacement and deformation occurred in the winding.
Plot of output voltages against the corresponding frequencies.
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20. Three methods are commonly used to assess the measured traces:
 Time-based – current SFRA results will be compared to previous results of the same unit.
 Type-based – SFRA of one transformer will be compared to an equal type of transformer.
 Phase comparison – SFRA results of one phase will be compared to the results of the other phases of
the same transformer.
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21. THANK YOU
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