TEST & INSPECTION PROCEDURE FOR POWER TRANSFORMERS (As per IEC 60076 & IS 2026) FAT (Factory PreType Routine Acceptance Test) Description of Test Commissioning Remarks Test Test or Inspection Test Test Measurement of Y Y Y Y Y - Yes Insulation Resistance Measurement of Y Y Y Y Winding Resistance Ratio Test Y Y Y Y Vector Group Test Y Y Y Y Magnetic Core C-Customer Y Y C Y Balance Test Choice Measurement of Noload loss & Exciting Y Y C Y Current Measurement of Impedance Voltage Y Y C Y & Load Loss Calculation of Y Y N N N - No
Efficiency & Voltage Regulation Temperature Rise Y Test Measurement of Insulation Power Y Factor Lightning Impulse Y Test on Bushing Separate Source Voltage Withstand Y Test Induced Overvolatge Y Withstand Test Test on On-Load Tap Changer (If Y applicable) Tests on Bushing CT Y (If Applicable) Tests on Protective Devices (OTI, WTI, Y Buch Holz Relay & MOG) General Inspection Vacuum Test & Oil Tightness Test Tests on Insulation Oil (If Applicable)
N
N
N
Y
N
N
N
N
N
Y
N
N
Y
N
N
Y
C
Y
Y
N
Y
Y
C
Y
Y
Y
Y
Y
Y
Y
N
N
Y
Y
C
Y
As per Approved Drawing
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
66 & 132 kV 22 & 33 V 6.6 & 11 V < 6.6 kV
At 30 oC 600 500 400 200
At 40 oC 300 250 200 100
At 50 oC 150 125 100 50
At 60 oC 75 62.5 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 Actual Measured Volts Ratio Calculated Tap Applied Volts Ratio No. 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 1U1V 1V1W 1W1U 1U1V 1V1W 1W1U Conditions) (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.
