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Transformer Fundamentals
- 2. Transformers • Transfer energy from one circuit to another by means of magnetic coupling • Used to transform voltage levels - Minimize transmission losses • S = VI; If V is high, I is low • Losses = I2Z, lower I = lower losses • Used to act as sinks for harmonics - Delta windings absorb triplins (3rd, 9th, 15th, etc.) • Applied in generation, transmission, distribution and utilization areas of the power system Transformer Fundamentals
- 3. Transformers are used throughout the bulk electrical system: Generation Transmission Distribution Utilization Transformer Fundamentals
- 4. Flux in Core Steel Core Primary Winding Secondary Winding Basic Transformer Transformer Fundamentals
- 5. R = resistance; X = reactance (inductive); N = No of turns; E = voltage Basic Equivalent Circuit Winding Losses (≈1.5% at full load) Magnetizing Losses (≈0.5%) √ √ Transformer Fundamentals
- 6. • V1I1 = V2I2 • N1V2 = N2V1 • N1I1 = N2I2 AA I = 5 A I = 10 A V = 100 V V = 50 V N = 100 N = 50 Ideal Transformer – No Losses Transformer Formulas Transformer Fundamentals
- 7. Transformer Formulas Primary Winding Secondary Winding Tertiary Winding E1 = 1000 N1 = 100 E/N = 10 N2 = 50 E2 = 50 X 10 = 500 N3 = 20 E3 = 20 X 10 = 200 Transformer Fundamentals
- 8. • Found in generation, transmission, and distribution areas of the power system - Used to transfer large amounts of bulk power to different voltage levels • Step Up, Step Down - Used to regulate transmission and sub-transmission voltages • Autotransformer • Typically iron core • Typically liquid insulation (wet vs. dry) • Two or Three Winding • With or without Taps • With or without Load Tap changers (LTC) Power Transformers Transformer Fundamentals
- 9. Bushing Cooler LTC LTC Control Cabinet Cooler Main Tank Power Transformers Transformer Fundamentals
- 10. 87T Ig • Two winding transformer, with REF Typical Applications Transformer Fundamentals
- 11. 87T Ig • Substation Differential Wrap, with REF Typical Applications Transformer Fundamentals
- 12. • Dual generator unit differential wrap Typical Applications Transformer Fundamentals
- 13. REF REF 87T 87T High Speed Trip for Bus Faults Main-Tie-Main Substation Typical Applications Transformer Fundamentals
- 14. From IEEE Press Book • Small 500 to 10,000 kVA • Medium 10,000 kVA to 100 MVA • Large 100 MVA and above • Less than 500 kVA not considered a power transformer Ratings and Classifications Transformer Fundamentals
- 15. • Core Form - Single path for the magnetic circuit - Less $$$ • Shell Form - Multiple paths for the magnetic circuit - Better through-fault withstand Windings Core Core Core Core Core Types Transformer Fundamentals
- 16. • Dry - Used where liquid spill cannot be tolerated - Small ratings, lower voltage distribution • Wet - Offer smaller size, lower cost and greater overload capacity - Liquids have greater coefficient of heat than dry insulation - Vast majority of power transformers use wet (liquid) insulation Insulation Materials Transformer Fundamentals
- 17. • Single Phase - Typical for lower voltage load-serving distribution - May be applied in higher capacities where a spare is desired - 4 transformers on site, 3 connected for three phase duty, 1 as a spare • Three Phase - Typical for T&D - Less expensive than 3 single phase transformers of the same rating - Vast majority of power transformers Single vs. Three Phase Transformer Fundamentals
- 18. • No load taps - Taps are adjusted under no-load conditions to bring secondary voltage to desired level - Cheaper than on-load tapchanger - Cannot dynamically adjust to voltage to load and line drop conditions • On-load tapchanger (LTC) - Taps are adjusted under load - Can respond dynamically to adjust voltage to load and line drop conditions Ratio Adjustment Transformer Fundamentals
- 20. • H1, H2, H3 - Primary Bushings • X1, X2, X3 - Secondary Bushings Transformer H1 H2 H3 X1 X2 X3 Wye-Wye H1 and X1 at zero degrees Delta-Delta H1 and X1 at zero degrees Delta-Wye H1 lead X1 by 30 degrees Wye-Delta H1 lead X1 by 30 degrees ANSI Standard Bushing Nomenclature Wye-Wye H1 and X1 at zero degrees Delta-Delta H1 and X1 at zero degrees Delta-Wye H1 lead X1 by 30 degrees or X1 Lags H1 by 30 degrees Wye-Delta H1 lead X1 by 30 degrees or X1 Lags H1 by 30 degrees Transformer Fundamentals
- 21. • Polarity – used to describe the phase relationship of single phase transformers - ANSI Standard • Additive if voltage is 8660 or below and the kVA is 200 or less (voltage across any two bushings can be rated) • Subtractive otherwise (voltage across any two bushings less than rated) • Angular Displacement – used to describe the voltage phasing on three phase transformers - ANSI Standard • Wye-wye and delta-delta; 0 degrees displacement • Wye-delta and delta-wye; X1 lags H1 by 30 degrees or “High leads low by 30” ANSI C57.12 & C57.105 Polarity & Angular Displacement Transformer Fundamentals
- 22. • Wye-Wye – Cheaper than 2 winding if auto bank – Conducts zero-sequence between circuits – Provides ground source for secondary circuit • Delta-Delta – Blocks zero-sequence between circuits – Does not provide a ground source • Delta-Wye – Blocks zero-sequence between circuits – Provides ground source for secondary circuit • Wye-Delta – Blocks zero-sequence between circuits – Does not provide a ground source for secondary circuit Winding Arrangements Transformer Fundamentals
- 23. • ANSI Y-Y & Δ-Δ @ 0° • ANSI Y-Δ & Δ-Y @ H1 lead X1 by 30° or X1 lag H1 by 30° Angular Displacement Transformer Fundamentals
- 24. • ANSI Y-Y & Δ-Δ @ 0° • ANSI Y-Δ & Δ-Y @ X1 lags H1 by 30° - ANSI makes our life easy • Euro-designations use 30° CW increments from the H1 bushing to the X1 bushings - Dy1=X1 lags H1 by (1*30°) 30° • or, H1 leads X1 by 30° - Think of a clock – each hour is 30 degrees 0 6 39 8 7 10 11 1 2 5 4 H1 X1 • Dy1 = X1 lags H1 by 1*30 = 30, or H1 leads X1 by 30 (ANSI std.) • Dy1 equivalent to ANSI DabY Transformer Fundamentals Polarity & Angular Displacement
- 25. *1 *1 *2 *2 *1 = ANSI std. @ 0° *2 = ANSI std. @ X1 lag H1 by 30°, or “high lead low by 30°” • IEC (Euro) practice does not have a standard like ANSI • Most common GSU connection is Yd1 (High lead low by 30°) • Obviously observation of angular displacement is extremely important when paralleling transformers! Angular Displacement Transformer Fundamentals
- 26. HV LV H1 H2 H3 X1 X3 X2 A B C a b c a b c A B C Assume 1:1 transformer • H1 (A) leads X1 (a) by 30 • Currents on “H” bushings are delta quantities Angular Displacement - Development Transformer Fundamentals
- 27. HV LV H1 H2 H3 X1 X3 X2 a b c A B C IA-IC IB-IA IC-IB A B C ab c Assume 1:1 transformer • H1 (A) leads X1 (a) by 30 • Currents on “X” bushings are delta quantities Angular Displacement - Development Transformer Fundamentals ©2008 Beckwith Electric Co., Inc.