Ingeniero

POWER TRANSFORMERS
CONNECTION DRAWINGS FOR ELECTRICAL TESTING
1. INSULATION RESISTANCE TEST
An insulation measurement is carried out between HV/LV, HV/GROUND and LV/GROUND
applying 5kV DC during10 minutes. Verifying the polarization index and individual values obtained,
the acceptance is made by means of comparison against experimental values tendency.

2. RATIO, POLARITY AND PHASERELATION TEST
Ratio verification for each tap. Obtained measurements must fulfil a ± 0.5% tolerance regarding
rated values. Siemens has a three-phase equipment in which different phasor combinations canbe

selected depending on the phase relation, besides, a logic circuit is incorporated to show a normal
or reverse polarity condition.

3. WINDINGS RESISTANCE MEASUREMENT
Windings resistance iscarried out between phases for each tap in HV and LV. Results are
confronted with calculation to accept or reject the test.

4. NO LOAD LOSSES AND MAGNETIZING CURRENT TEST
Connecting LV winding ata rated frequency and voltage using an appropriate measuring system a
current and losses register is filled to be compared with guaranteed values taking into account
tolerances established by ANSISTANDARD, Po: ±10%, Io: ±30%.
No load losses reduce with temperature, therefore a correction must be applied according to ANSI
C57.12.90
Our measuring system set includes CTs and PTs class 0.1 withnegligible angle displacement and
a power analyzer designed to transformers testing.

5. LOAD LOSSES AND SHORT CIRCUIT IMPEDANCE TEST

Load losses are composed by I2R and stray losses. I2Rcomponent increases with temperature
and stray component decreases.
Depending on the tap position HV winding with LV short-circuited is energized, increasing voltage
until getting rated current flows.Measured power losses and impedance voltage are corrected to a
reference temperature and compared with guarantee values.

6. LIGHTNING IMPULSE TEST
Test sequence for each phase is: one reduced...