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High-voltage string design: a 3 kV DC circuit uses SCRs rated at 800 V each. With 25% derating, how many SCRs must be connected in series?

Difficulty: Easy

Correct Answer: 5

Explanation:

Introduction / Context:
Series connecting SCRs requires voltage derating to accommodate tolerances, temperature, and transient imbalance. This calculation finds the minimum number of devices for a specified DC bus.

Given Data / Assumptions:

Bus voltage V_total = 3000 V.
Device rating V_Rated = 800 V per SCR.
Derating = 25%, so usable per device is 0.75 * 800 V.

Concept / Approach:
With derating, allowable steady blocking voltage per SCR becomes V_use = 0.75 * V_Rated. The required count is N ≥ V_total / V_use, rounded up to the next integer.

Step-by-Step Solution:

Compute usable per SCR: V_use = 0.75 * 800 = 600 V.Compute count: N = 3000 / 600 = 5 exactly.Therefore, 5 SCRs in series meet the derated requirement.

Verification / Alternative check:

Total derated withstand = 5 * 600 = 3000 V → matches bus voltage (ignoring transients; equalizing networks still recommended).

Why Other Options Are Wrong:

4 devices give only 2400 V derated; 6 or more are conservative but not minimal; 8/7 oversize unnecessarily.

Common Pitfalls:

Forgetting to derate; confusing AC peak vs DC; not rounding up for non-integer results.

High-voltage string design: a 3 kV DC circuit uses SCRs rated at 800 V each. With 25% derating, how many SCRs must be connected in series?

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