Purpose of the measurement
The resistance between all pairs of phase terminals of each transformer winding are measured using direct current. Furthermore the corresponding winding temperature is measured.
The measured resistances are needed in connection with the load loss measurement when the load losses are corrected to correspond to the reference temperature. The resistance measurement will also show whether the winding joints are in order and the windings correctly connected.
Apparatus and basic measuring circuit
The measurement is performed by TETTEX 2285 transformer test system.
This device is an automatic winding analyzer, optimized for three phase power and distribution transformer measurements.
- T1 – transformer under test,
- A – Ammeter,
- U – Voltmeter
- B – DC supply,
- Th – Thermometer
The principle of the measurement is as follows:
The voltage drop Udc caused by the direct current Idc an by the resistance RAB, RAC and RBC is measured.The resistances are then calculated from Udc and Idc using correction for the error caused by the internal resistance of the voltage measuring equipment.
The temperature is measured from oil filled thermometer pockets situated in the transformer cover by means of an electronic thermometer connected to the computer.
The resistance values and the average temperature are calculated. In the report the terminals, between which the resistances are measured, the connection, the tapping position and the average temperature of the windings during the measurement are stated.
Reference: Testing of power transformers – ABB