1️⃣ Delta–Delta (Δ–Δ) Connection
| Parameter | Details |
|---|---|
| Basic Theory | Both primary & secondary windings are connected in delta. No phase shift between primary & secondary voltages. Good for low-voltage, high-current applications. |
| Advantages | • Handles unbalanced loads well • 3rd harmonic currents circulate inside delta → no distortion • No neutral required• If one transformer fails, system can run in Open-Delta (V–V) at 58% capacity • No phase shift (0°) — so parallel operation with Δ–Δ is possible |
| Disadvantages | • Requires more copper (higher phase current) • No neutral → cannot supply 1-phase loads • Not suitable for very high voltages (due to insulation needs) |
| Applications | • Low-voltage distribution • Industrial loads • For continuity of supply (Open-Δ option)• Rectifier loads, heavy machinery |
2️⃣ Delta–Star (Δ–Y) / Delta–Wye Connection
| Parameter | Details |
|---|---|
| Basic Theory | Primary is delta, secondary is star. Secondary has neutral → gives 3-phase 4-wire output. Introduces +30° phase shift (line voltages). |
| Advantages | • Ideal for step-up transformers (generation → transmission) • Secondary neutral available → 1-phase loads served • Delta on primary provides path for 3rd harmonic currents → voltage is sinusoidal • No floating-neutral or distortion issues |
| Disadvantages | • 30° phase shift → cannot be paralleled with Δ–Δ or Y–Y banks • Unbalanced load on secondary creates unbalanced currents on primary |
| Applications | • Generating stations (step-up) • HV transmission start point • Mixed loads (3-phase + 1-phase consumers) |
3️⃣ Star–Delta (Y–Δ) Connection
| Parameter | Details |
|---|---|
| Basic Theory | Primary is star (neutral grounded), secondary is delta. Phase shift of –30° (depending on vector group). Used mainly for step-down. |
| Advantages | • Primary sees only phase voltage = V_L / √3, so insulation requirement low → good for HV side • Stable secondary because Δ provides path for triple harmonics • Good for unbalanced loads compared to Y–Y |
| Disadvantages | • 30° phase shift → cannot parallel with Y-Y or Δ-Δ • Secondary has no neutral → cannot feed single-phase loads • If load badly unbalanced, secondary currents circulate in delta |
| Applications | • Substations (step-down HV → LV)• Industrial distribution • Where primary neutral grounding is required |
4️⃣ Star–Star (Y–Y) Connection
| Parameter | Details |
|---|---|
| Basic Theory | Both windings connected in star. Simple structure. Phase shift 0° between primary & secondary line voltages, but phase voltages shift 30°. |
| Advantages | • Cheapest for small, high-voltage transformers • Minimum insulation required (phase voltage = V_L / √3) • Simple construction• Neutral available on both sides |
| Disadvantages | • Serious problem of neutral shifting under unbalanced load → phase voltages become unequal • 3rd harmonic distortion unless: → neutral is grounded, OR → a tertiary delta winding is added • Cannot handle unbalanced loads well |
| Applications | • High-voltage small power transformers • When load is perfectly balanced • Systems where tertiary delta is added for stabilization |
🔥 Super-Summary for SSC JE (One-Look Revision)
| Connection | Phase Shift | Neutral? | Unbalanced Loads | Harmonics | Typical Use |
|---|---|---|---|---|---|
| Δ–Δ | 0° | ❌ No | ⭐ Excellent | 3rd harmonic circulates in Δ | Industries, continuity supply |
| Y–Y | 0° (line), 30° (phase) | ✔ Yes | ❌ Very poor | Needs grounded neutral / tertiary Δ | Small HV transformers |
| Y–Δ | –30° | ❌ | ⭐ Good | Δ handles 3rd harmonics | Step-down substations |
| Δ–Y | +30° | ✔ Yes | ⭐ Good | Δ handles harmonics | Step-up (generation) |
⭐ Exam Tip: Which connection where?
Generation → Transmission: Δ–Y (step-up, neutral on secondary)
Transmission → Distribution: Y–Δ (primary HV star)
Industries: Δ–Δ
Small HV units: Y–Y
Continuity (if one unit fails): Δ–Δ → V–V