TEST & INSPECTION PROCEDURE FOR POWER TRANSFORMERS (As per IEC 60076 & IS 2026)

Description of TestType Test

Routine Test

FAT (Factory Acceptance Test) or Inspection Test

Pre-Commissioning Test

Remarks

Measurement of Insulation Resistance

Y Y Y Y Y - Yes

Measurement of Winding Resistance

Y Y Y Y

Ratio Test Y Y Y Y

Vector Group Test Y Y Y Y

Magnetic Core Balance Test

Y Y C YC-Customer Choice

Measurement of No-load loss & Exciting Current

Y Y C Y

Measurement of Impedance Voltage & Load Loss

Y Y C Y

Calculation of Efficiency & Voltage Regulation

Y Y N N N - No

Temperature Rise Test

Y N N N

Measurement of Insulation Power Factor

Y Y N N

Lightning Impulse Test on Bushing

Y N N N

Separate Source Voltage Withstand Test

Y Y N N

Induced Overvolatge Withstand Test

Y Y N N

Test on On-Load Tap Changer (If applicable)

Y Y C Y

Tests on Bushing CT (If Applicable)

Y Y N Y

Tests on Protective Devices (OTI, WTI, Buch Holz Relay & MOG)

Y Y C Y

General Inspection Y Y Y YAs per Approved Drawing

Vacuum Test & Oil Tightness Test

Y Y N N

Tests on Insulation Oil (If Applicable)

Y Y C Y

1. MEASUREMENT OF INSULATION RESISTANCE:

Transformer Temperature : oC

Make of Megger :

Sl. No. of Megger :

Ø Recommended Test Voltage: 2.5kV or 5kV for all HV Windings & 500V or 1000V for all LV Windings

Description HV to Earth LV to Earth HV o LV

1 Minute Value

The Minimum values of IR Values shall be :

Voltage Class of Winding

Minimum IR Values in Mega Ω

At 30 oC At 40 oC At 50 oC At 60 oC

66 & 132 kV 600 300 150 75

22 & 33 V 500 250 125 62.5

6.6 & 11 V 400 200 100 50

< 6.6 kV 200 100 50 25

2. MEASUREMENT OF WINDING RESISTANCE:

Ø Use a good quality low-resistance meter or use Kelvin's Double Bridge

Ø Perform measurement on all Taps

Ø Check with the Transformer Routine Test Certificate

Transformer Temperature: oC

Make of Meter/Bridge

Sl. No. of Meter/Bridge

Tap No.Winding Resistance in Ohms

1U1V 1V1W 1W1U 2u2v 2v2w 2w2u

1

2

3. RATIO TEST:

Ø Perform the test on all taps

Ø Apply 3 Phase, 50Hz, Low Voltage, say, 415V to the HV Winding with the LV winding open circuited and measure the voltage in the secondary winding

Ø Use good quality voltmeters for measurement; use same meter for measuring HT & LT Volts

Ø Check with the ratio declared in the transformer name plate/routine test certificate

Ø Caution: Do not apply voltage to LV Terminals, as this would induce dangerously high voltages at the HV Terminals

Tap No.

Applied Volts Measured Volts Ratio CalculatedActual Ratio

1U1V 1V1W 1W1U 2u2v 2v2w 2w2u 1U1V/2u2v 1V1W/2v2w 1W1U/2w2u

1

2

4. VECTOR GROUP TEST:

Ø Short the terminals 1U & 2u using a wire of right size (say 2.5 sq.mm Cu.)

Ø Apply 3 Phase, 50Hz, Low Voltage, say, 415V to the HV Winding with the LV winding open circuited and measure the voltage in the HV/LV windings

Ø Use good quality voltmeters for measurement; use same meter for measuring HT & LT Volts

Ø Check if the equations for the relevant vector group are satisfied

Ø Caution: Do not apply voltage to LV Terminals, as this would induce dangerously high voltages at the HV Terminals

For example, for Dyn 11 Vector Group, the following equations must satisfy:

i) 1V2w = 1V2v

ii) 1W2w < 1W2v

iii) 1U2n +1V2n = 1U1V

Similarly, equations could be derived for any other vector group and test results can be crosschecked.

5. MAGNETIC CORE BALANCE TEST:

Ø Apply 2 Phase, 50Hz, Low Voltage, say, 415V to the HV Winding with the LV winding open circuited and measure the voltage in the HV windings

Ø Use good quality voltmeters for measurement; use same meter for measuring applied & measured Volts

Ø Check if the balance conditions are satisfied

Ø Caution: Do not apply voltage to LV Terminals, as this would induce dangerously high voltages at the HV Terminals

Applied Volts Measured Volts Remarks (Equations for Balance Conditions)

1U1V 1V1W 1W1U 1U1V 1V1W 1W1U

(415) --- --- --- (398) (17) 1V1W > 1W1U

--- (415) ---- (212) ---- (213) 1U1V =1W1U

---- ---- (415) (31) (384) ---- 1U1V <1V1W

6. MEASUREMENT OF NO-LOAD LOSS & EXCITING CURRENT:

Ø Perform the test on normal tap

Ø Apply 3 Phase, 50Hz, Low Voltage, say, 415V to the HV Winding with the LV winding open circuited and measure the voltage, current & power in the HV winding

Ø Use good quality meters for measurement

Ø Check with the exciting current/no-load loss declared in the transformer routine test certificate

Ø Caution: Do not apply voltage to LV Terminals, as this would induce dangerously high voltages at the HV Terminals

Applied Volts (V) Measured Current (mA)

Terminal Volts Winding Current

1U1V 1U

1V1W 1V

1W1U 1W

Note: From the above data, we can calculate that if the exciting current is 'x' milliamperes for the test voltage applied (say 415V), what would be the exciting current for the rated primary volts of the transformer.

Exciting current at rated primary volts = ("x"/415) x Rated primary voltage

of the transformer.

The current calculated above shall be approximately 2% to 4% of the rated transformer primary current.

7. MEASUREMENT OF IMPEDANCE VOLATGE & LOAD LOSS

Ø Perform the test on normal tap

Ø Apply 3 Phase, 50Hz, Low Voltage, say, 415V to the HV Winding with the LV winding short circuited and measure the voltage in the HV winding and current & power in the LV winding

Ø Use good quality meters for measurement

Ø Check with the % impedance/load loss declared in the transformer name plate/routine test certificate

Ø Caution: Do not apply voltage to LV Terminals, as this would induce dangerously high voltages at the HV Terminals

Applied Volts (V) Measured Current in LV Winding (A)

Terminal Volts

1U1V

1V1W

1W1U

Note: From the above, we can calculate that if for the test voltage (say 415V), the SC Current is 'y' amps, then what it would be for the rated primary voltage of the transformer.

Rated SC Current = ('y'/415) x Rated Primary Voltage

Then, we can calculate % impedance:

% Impedance = (Rated Secondary FLC/Rated SC Current) x 100

8. Test on On-Load Tap Changer: All operational checks (Manual &

Motorised Operations)

9. Tests on Bushing CT: All tests as per IS 2705 like Polarity, Winding Resistance, Accuracy, Ratio Test, Mag.Characteristics, etc.

10. Tests on Protective Devices: Check all protective devices for correct operation, settings, calibration, etc.

11. General Inspection: Check for Physical Dimensions, Clearances, Creepages, etc, as per approved drawings; Inspect for any visual damages to any components.

12. Tests on Insulating Oil: BDV shall be 60kV across a gap of 2.5mm with 12.5mm Dia. Spheres. Ask for test certificate.