In an extended-delta or polygon winding, a terminal output can combine several segment voltages. A ratio tester may report a cumulative terminal relationship rather than the turns ratio of one segment. Comparing it with the wrong theoretical quantity can create a false failure. IEC 61378-1 covers industrial converter-transformer specification, design and testing [1].
Obtain the actual winding diagram and the instrument’s test configuration. Identify the applied voltage, measured terminal pair and treatment of other phases. The test’s excitation state belongs in the theory.
Define three different ratios
| Ratio | Definition | Appropriate comparison |
|---|---|---|
| Segment turns ratio | Turns linking corresponding magnetic excitation | Individual winding-segment design |
| Balanced line-voltage ratio | Line-to-line phasors under defined three-phase supply | Rated terminal relationship |
| Instrument test ratio | Voltages produced and measured by its specific test | Expected reading for that procedure |
They can coincide for some connections and differ for others. Conventional star-delta conversion factors are useful only when the connection and measured quantities justify them.
For a segment sum Eout = a exp(jθa) + b exp(jθb), terminal magnitude is |Eout| and phase is arg(Eout). Neither is determined by a + b alone unless the vectors align. Use the signed connection path rather than adding segment turn counts without their phase relationships.
Reproduce the test excitation analytically
A single-phase test with other phases connected or constrained can create a different magnetic state from balanced three-phase operation. Derive the expected reading using the qualified method and topology. Do not use the nameplate line-voltage ratio automatically as the expected number for every excitation configuration.
Primary source testing work on special rectifier transformers compares several methods and reports cumulative ratio and phase behavior. Match the test method to the theoretical quantity before assessing an automatic reading.
Predict the terminal ratio from the segment vectors
Assume a 50 Hz balanced test construction with output segments of 100 V RMS at 0° and 50 V RMS at 60°, and a 1000 V RMS input reference. Output is 100 + 50 exp(j60°), with magnitude 132.288 V and angle 19.107°. The terminal magnitude ratio is 7.5593, whereas adding magnitudes gives an incorrect ratio of 1000/150 = 6.6667, 11.81% low. Use the vector result only for this excitation and terminal pair; derive a separate expected reading if the instrument applies a different test configuration.
Prepare an expected-reading schedule
For every relevant tap and terminal pair, record theoretical measured magnitude ratio, phase reference and the procedure used to derive them. Keep rounding separate from measurement capability. If the tap changes relative segment proportions, calculate phase as well as magnitude.
Compare the actual test with that schedule and its uncertainty. If it disagrees, inspect selected tap, phase sequence, polarity labels and instrument mode before assuming a winding fault.
Use an independent qualified method where a critical interpretation remains unresolved. Automatic vector-group recognition can assist setup but should not override a verified connection model.
For each tap, compare the reading with the ratio and phase predicted for that exact excitation and terminal pair. If the schedule and instrument disagree, resolve configuration and polarity first, then use a qualified independent method where the winding interpretation remains uncertain.
Check ratio and phase on the actual special connection and selected tap [2].
References
- IEC, IEC 61378-1:2011, Converter transformers – Part 1: Transformers for industrial applications.
- Gandhi Jeyaraj, S., Milne, R., and Mitchell, G., On-Site Testing of Special Transformers, Megger technical paper, Sections 2–6.