A double-star rectifier has two secondary groups whose conducting paths vary through the cycle. An interphase reactor interacts with their instantaneous voltages and currents. Its duty should be derived from those states rather than assigned from total DC output alone. IEC 61378-1 covers industrial converter-transformer specification, design and testing [1].
Draw the complete rectifier and reactor connection. Identify the conducting devices, group outputs and common-load path for each interval. Different reactor connections can produce different voltage fractions, so a generic “half the difference” rule is unsafe without the diagram.
Separate common and differential currents
With branch currents i1 and i2 directed toward the load, total current is iD = i1 + i2. Define differential coordinate iΔ = (i1 − i2)/2. Then i1 = iD/2 + iΔ and i2 = iD/2 − iΔ.
These are current identities. The reactor and branch voltages determine how iΔ evolves. Equal sharing is one possible state, not a condition imposed by the total-current equation.
| Conduction record | What to calculate |
|---|---|
| Conducting devices in each half group | Active phase-voltage paths |
| Instantaneous group output voltages | Voltage difference presented to the reactor network |
| Reactor terminal relationship | Actual voltage on each section |
| Branch current and overlap | Secondary conductor duty |
| Load current condition | Continuous or discontinuous behavior |
Check periodic volt-seconds
For a reactor section with voltage vR = N dΦ/dt under the chosen convention, flux change is ΔΦ = (1/N)∫vR dt. A periodic steady flux requires zero net integral over the complete repeating period, apart from the separately modeled resistance and other effects.
A nonzero ideal volt-second balance can indicate asymmetric operation, wrong polarity or an incomplete state sequence. It is not repaired by choosing a larger core without understanding the source of the offset.
Check peak and cyclic flux excursion as well as the net integral. Zero net volt-seconds does not establish a small excursion or adequate material margin.
Check sharing and section volt-seconds independently
Assume branch currents 550 A and 450 A toward a 1000 A DC load: differential current is (550 − 450)/2 = 50 A. For a separate ideal 100-turn reactor section, +20 V for 2 ms followed by −20 V for 2 ms gives zero net volt-seconds and a flux excursion of 0.00040 Wb. If the positive interval becomes 2.2 ms while the negative remains 2 ms, net flux change is 0.000040 Wb per modeled cycle. Investigate that imbalance and the actual section connection before increasing core size or assuming periodic equal sharing.
Compare unequal and interrupted states
Evaluate unequal group voltage, unequal firing where relevant, and the actual load-current waveform. If one group stops conducting during part of a light-load cycle, the reactor sees a different sequence from continuous balanced operation.
Map the resulting group currents into winding RMS, peak and harmonic duties. A low total DC ripple does not prove equal group heating.
Specify the interphase reactor from the connection’s voltage waveform, flux excursion and branch-current duty. Include required transient and fault states with their durations and remaining current paths; total DC current alone cannot establish that duty.
Retain an explicit measurement model and input uncertainties when comparing measured and calculated duty [2].
References
- IEC, IEC 61378-1:2011, Converter transformers – Part 1: Transformers for industrial applications.
- JCGM, JCGM 100:2008, Evaluation of measurement data — Guide to the expression of uncertainty in measurement, 2008.