A multi-pulse acceptance comparison may need more than harmonic magnitudes. The relative phases determine how bridge contributions combine. Those phases are usable only when every channel shares a defined clock and reference. IEC 61378-1 covers industrial converter-transformer specification, design and testing [1].
State the measured phase, polarity and amplitude convention. RMS and peak harmonic values differ, and a current arrow reversal changes its phasor sign. Resolve those definitions before comparing tables from different instruments.
Use a reproducible time reference
Let θh be the phase of harmonic order h in one channel and θ1 the phase of its defined fundamental reference. The relative angle ψh = θh − hθ1 is invariant to a common change of time origin, under the same Fourier sign convention.
This does not make independently acquired channels synchronized. Their clocks and reference signals still need a demonstrated relationship. A channel delay τ creates frequency-dependent phase error ωhτ.
| Acceptance record field | Required content |
|---|---|
| Clock and trigger | Synchronization method and reference signal |
| Phasor convention | Fourier sign, RMS or peak and current polarity |
| Measured location | Phase, terminal and bridge group |
| Operating state | Tap, DC currents, firing strategy and temperatures where relevant |
| Source waveform | Background voltage/current harmonic evidence |
| Record processing | Duration, steady-state selection and leakage treatment |
| Primary referral | Actual connection and sequence transformation |
Retain bridge loading with the spectrum
Unequal bridge currents change contribution magnitudes. Tap or control changes can alter their phase relationships. A harmonic table detached from those states cannot establish that two tests evaluated the same duty.
Report each bridge group where accessible and the combined primary response. Distinguish measured quantities from values referred through a model. The transformation must follow the actual special winding connection; a conventional clock shift may not describe every harmonic sequence.
Convert timing skew into harmonic angle error
Assume a 50 Hz fundamental and channel delay of 20 µs. Phase error is 360° × h × 50 × 20 × 10⁻⁶: 0.36° at the fundamental and 3.96° at the 11th harmonic. Thus a common-looking small delay can resemble a materially larger harmonic mismatch. For an assumed 1° timing-error budget at that order, delay must be below 1/(360 × 11 × 50) = 5.05 µs. Verify channel timing before interpreting a 4° residual as a winding phase error.
Separate source background
Record the supply’s harmonic condition during the test. A changed background can alter converter conduction and the measured primary spectrum. Do not subtract harmonic magnitudes arithmetically when phase and interaction matter.
Use repeat records to assess phase stability. If a component is near the noise floor, its angle may be poorly determined even when the instrument displays a precise number. Report that capability limit rather than treating all angles equally.
Compare the defined phasors against the project’s criteria and their measurement uncertainty. Keep unresolved or incoherent components labeled accordingly.
Another engineer should be able to combine the recorded bridge phasors without guessing their reference. Retain the clock, polarity, processing window and bridge state with the spectrum; otherwise an apparent phase discrepancy may be a measurement mismatch.
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.