📘 Short Circuit Ratio (SCR) – Synchronous Generator
Short Circuit Ratio indicates strength and stability of synchronous generator. Very important conceptual topic.
🔹 1️⃣ Definition
SCR = Field current required for rated voltage (from OCC) --------------------------------------------------- Field current required for rated current (from SCC)
In simple terms: SCR = I_f (for rated V on OCC) / I_f (for rated I on SCC)🔹 2️⃣ Relation with Synchronous Reactance
Very important relation:SCR ≈ 1 / X_s (per unit)
High SCR → Low synchronous reactance Low SCR → High synchronous reactance🔹 3️⃣ Physical Meaning
High SCR means:- Better voltage regulation
- Better stability
- Larger machine size
- Lower reactance
- Poor voltage regulation
- Less stable
- Smaller machine size
- Higher reactance
🔹 4️⃣ Typical Values
Turbo alternator (cylindrical rotor): SCR ≈ 0.5 to 0.6 Hydro alternator (salient pole): SCR ≈ 1.0 to 1.5🔹 5️⃣ Example
From OCC: Field current for rated voltage = 5 A From SCC: Field current for rated current = 8 A Find SCR.SCR = 5 / 8 = 0.625
Moderate stability machine.🔹 6️⃣ Important Observations
- SCR inversely proportional to X_s
- Higher SCR → Smaller voltage drop
- High SCR improves steady state stability
- Low SCR machines cheaper
🎯 GATE Important Points
- Remember SCR ≈ 1 / X_s (pu)
- High SCR → Better regulation
- SCR affects stability limit
- Conceptual question favorite area
High SCR = Strong & Stable Generator
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