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Consequences of higher or lower SCR

Short answer

Higher SCR (low Xd) means larger air gap, higher cost, improved stability and voltage regulation, higher short-circuit current, and good line-charging capability. Lower SCR gives a smaller, cheaper machine but poorer stability, regulation, and fault levels. Typical values: hydro 1.0–1.5, turbo generators 0.5–0.7.

What could be the consequences of using higher or lower short circuit ratio (SCR) for a synchronous generator?

Short circuit ratio SCR≈1/XdSCR \approx 1/X_d (p.u.). Its value changes the size, cost and behaviour of a synchronous generator.

AspectHigher SCR (low XdX_d)Lower SCR (high XdX_d)
Air gapLargerSmaller
Size, weight, costLarger, heavier, costlierSmaller, cheaper
Field current and lossesHigherLower
Steady-state stability limit EV/XdEV/X_dHigher, more stableLower, less stable
Synchronizing powerHigherLower
Voltage regulationBetter (less drop)Poorer
Short-circuit currentHigher; heavier CBsLower
Line-charging capabilityGood, resists self-excitationPoor
EfficiencySlightly lower (more field loss)Slightly higher

Consequences in brief

  • Too high SCR: an over-sized, expensive machine with higher fault levels and more field losses; benefits may not justify the cost.
  • Too low SCR: cheaper machine but poor stability margin and voltage regulation; needs fast excitation (static, with PSS) and careful operation; risk of losing synchronism on a weak grid.
  • Typical values: hydro (salient pole) 1.0–1.5, turbo generators 0.5–0.7.

✓ Answer checked by Claude Opus 5.5 (high effort)

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