The past few weeks have been quite busy with transformer inspections, where I’ve been participating as a witness. It’s been a valuable learning experience, especially since this is all new to me. I got to see firsthand how testing is conducted in practice, which gave me a much deeper understanding of the procedures.
I observed three major tests: No-Load Loss Test, Load Loss Test, and Temperature Rise Test. Each test serves a different purpose in ensuring the efficiency, reliability, and quality of transformers.
1. No-Load Loss Test
Objective:
To measure the iron losses in the transformer core. These include:
-
Eddy Current Loss: Caused by circulating currents in the core; minimized by using laminated core sheets.
-
Hysteresis Loss: Caused by repeated magnetization and demagnetization of the core due to alternating current; minimized by using high-grade CRGO (Cold Rolled Grain Oriented) steel.
Importance:
Distribution transformers operate 24 hours a day, often at no-load. Reducing iron loss is therefore critical for energy efficiency and cost-effectiveness.
Procedure:
-
The low-voltage (LV) side (400 V) is supplied with rated voltage.
-
The high-voltage (HV) side is left open.
-
Current, power loss, and frequency are measured using a Yokogawa digital power analyzer (with CT/PT ratio set manually or in auto mode).
Yokogawa Power Analyzer No Load Loss Measurement Data
Acceptance Criteria:
According to Nepal Electricity Authority (NEA) standards, maximum permissible no-load losses are:
| kVA Rating | No-Load Loss (W) |
|---|---|
| 300 | 550 |
| 200 | 365 |
| 100 | 220 |
2. Load Loss Test
Objective:
To measure copper losses in transformer windings under load.
Importance:
This test has financial significance, since load losses represent wasted energy when the transformer is supplying load. They must be corrected to a reference temperature of 75 °C for standard comparison.
Procedure:
-
The transformer is supplied with rated current on the HV side.
-
The LV side is short-circuited.
-
Copper losses are measured with a power analyzer and corrected to 75 °C using:
where Pt is the measured loss at temperature t.
Rated Current Calculation:
Acceptance Criteria (NEA standard):
| kVA Rating | Load Loss (W) |
|---|---|
| 300 | 3000 |
| 200 | 2100 |
| 100 | 1210 |
3. Temperature Rise Test
Objective:
To determine the maximum temperature rise of transformer windings and oil under rated load conditions.
Procedure:
-
Initial Step:
-
Measure winding resistance (HV and LV) at ambient temperature.
Winding Resistance Measurement -
Record no-load and load losses.
-
-
Loading Phase:
-
Total equivalent losses (sum of no-load and load losses) are supplied continuously for 6–7 hours.
-
The breather must remain connected to ensure proper oil expansion and pressure balance.
-
-
Temperature Measurement:
-
Oil temperatures are measured using thermometers inserted into oil pockets (top, bottom, and opposite sides of cooling fins).
-
Ambient temperature is measured using three thermometers.
-
For distribution transformers, winding temperatures are estimated indirectly through resistance measurement. For power transformers, built-in probes provide direct oil and winding temperature readings.
-
-
Limits:
-
Maximum temperature rise allowed: approximately 50 °C above ambient (e.g., 80 °C when ambient is 30 °C).
-
-
Winding Resistance Method:
-
After the loading phase, HV and LV windings are shorted together.
-
A current of 1 A is injected, and resistance is measured at ~30-second intervals.
-
Readings are plotted to determine winding resistance immediately after shutdown.
-
From this, winding hot-spot temperature is calculated using:
where R1 = cold resistance, R2 = hot resistance, and t = test temperature.
-
Conclusion
The three transformer tests; No-Load Loss, Load Loss, and Temperature Rise, provide a complete assessment of transformer efficiency, copper and core quality, and thermal endurance. Witnessing these tests gave me practical insights into transformer performance evaluation and a much deeper understanding of how theoretical standards are applied in real-world conditions.