For 12-pulse rectifier operation, which transformer connections are used to feed the two 6-pulse bridges to obtain the required 30° phase shift?

Introduction / Context:
To reduce ripple and harmonic content, high-power converters often use 12-pulse rectification, realized by paralleling or series-connecting two 6-pulse bridges fed from secondaries with a 30° phase displacement. The transformer vector group choice is crucial to create this displacement.

Given Data / Assumptions:

• Two 6-pulse bridges require a 30° phase shift between their AC inputs.
• Standard transformer connections provide known phase shifts (e.g., Y-Δ provides 30°).
• Equal voltage magnitudes are assumed for proper current sharing/harmonic cancellation.

Concept / Approach:

A Y-Y secondary provides a 0° reference, while a Y-Δ secondary provides a ±30° phase shift. Feeding separate 6-pulse bridges from these secondaries yields outputs that, when combined, form a 12-pulse system with reduced characteristic harmonics and improved DC quality.

Step-by-Step Reasoning:

Verification / Alternative check:

Harmonic cancellation rule: 12-pulse cancels 5th and 7th line currents when the secondaries are displaced by 30°. Y-Y and Y-Δ is a textbook solution to achieve this.

Why Other Options Are Wrong:

• Y-Y & Y-Y or Δ-Δ & Δ-Δ: zero relative displacement, no 12-pulse harmonic benefits.
• Y-Δ & Y-Δ: both displaced but equal; no 30° difference between the two feeds.
• Δ combinations without a complementary 0° feed do not provide the needed relative 30° shift.

Common Pitfalls:

• Confusing absolute phase shift with relative shift between the two bridges.
• Ignoring vector group standard phase relationships.

Final Answer:

Y - Y and Y - Δ transformers