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.

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