Assertion–Reason (HVDC thyristor triggering) Assertion (A): Light triggering is very suitable for HVDC transmission valves. Reason (R): Light triggering provides complete electrical isolation of the gate drive from high-voltage stacks.
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ABoth A and R are correct and R is correct explanation of A
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BBoth A and R correct but R is not correct explanation of A
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CA is correct but R is wrong
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DA is wrong but R is correct
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EBoth A and R are wrong
Answer
Correct Answer: Both A and R are correct and R is correct explanation of A
Explanation
Introduction / Context:Large HVDC converters use series strings of high-power thyristors. Triggering each device safely and reliably at very high potentials demands exceptional isolation and noise immunity. Light-triggered thyristors (LTTs) solve this by using fiber-optic light pulses instead of electrical gate leads.
Given Data / Assumptions:
- HVDC valve stacks operate at tens to hundreds of kilovolts.
- Fiber optics provide galvanic isolation and immunity to electromagnetic interference.
- Light-triggered device includes an integrated photosensitive gate structure.
Concept / Approach:Electrical gate-drive distribution up a high-voltage stack is complex, requires robust insulation, and can pick up noise. Fiber-optic links transmit light pulses to each device, eliminating conductive paths and thereby achieving complete electrical isolation between control and power circuits.
Step-by-Step Solution:Recognize HVDC valves require precise, isolated triggering.Fiber optics deliver light pulses directly to the thyristor gate region.Isolation is intrinsic; there is no galvanic connection to control ground.Thus, light triggering is well suited; the reason correctly explains the assertion.
Verification / Alternative check:Modern HVDC projects commonly employ fiber-optic gating and LTTs to reduce complexity, improve reliability, and enhance immunity to dv/dt and EMI.
Why Other Options Are Wrong:
- (b) denies the causal link—incorrect.
- (c) or (d) contradict established HVDC practice.
- (e) both statements are well-supported by industry practice.
Common Pitfalls:
- Confusing light triggering with opto-coupled electrical gates; LTTs integrate optical sensitivity.
- Ignoring need for per-device timing alignment in long strings.
Final Answer:Both A and R are correct and R is correct explanation of A