Q4Electrical Machine Design
Question
Q.4. What do you mean by Short Circuit Ratio (SCR) with reference to synchronous generator. Explain the factors being affected by SCR.
Answer
Short Circuit Ratio (SCR) is the ratio of field current for rated open-circuit voltage to field current for rated short-circuit current; it affects voltage regulation, stability limit, machine size/cost, and short-circuit current magnitude.
The Short Circuit Ratio (SCR) of a synchronous generator is defined as:
SCR is approximately the reciprocal of the per-unit unsaturated synchronous reactance (SCR ≈ 1/Xd, in per-unit), making it a direct, practically-measurable indicator of the machine's internal impedance relative to its rating.
Factors affected by SCR:
- Voltage regulation: a higher SCR (lower Xd) gives inherently better (smaller) voltage regulation, since less internal voltage drop occurs for a given load current, meaning terminal voltage varies less with changing load.
- Steady-state stability limit: a higher SCR increases the maximum power transfer limit (proportional to 1/Xd in the simplified power-angle relation P=EV sinδ/Xd), giving the machine a greater stability margin against falling out of synchronism during disturbances.
- Physical size and cost: achieving a higher SCR requires a larger air gap (to reduce armature reaction and hence Xd), which in turn requires more field MMF (larger field winding, larger pole structure) to establish the same air-gap flux, substantially increasing the machine's physical size, weight, and material cost for a given power rating.
- Short-circuit current magnitude: a higher SCR (lower Xd) machine draws/produces a larger short-circuit current for a given fault, since fault current is inversely proportional to Xd, requiring correspondingly higher-rated switchgear and protection equipment.
- Parallel operation and reactive power capability: a higher SCR machine can more readily share reactive power/load with other machines and the grid without excessive circulating currents, generally offering more flexible operation within an interconnected power system.
Because of this trade-off between performance (voltage regulation, stability) and economics (size, cost, fault current rating), SCR selection is a significant design decision: turbo-alternators (constrained to small air gaps by their high-speed cylindrical rotor construction) typically have lower SCR values (around 0.5-0.7), while salient-pole hydro-generators (which can more readily accommodate a larger effective air gap due to their construction) typically have higher SCR values (around 1.0-1.5), reflecting the different design constraints and priorities of these two major synchronous machine categories.
In practice, SCR is one of the key parameters specified by a purchaser when ordering a large synchronous generator, since it directly signals the expected voltage regulation and stability behavior the machine will exhibit in service, alongside its rated reactances.