2025-07-22
The following provides the typical specifications of a 2500 kVA floor-mounted transformer as well as key suggestions for selection. Users can view the transformer and obtain specific parameters by contacting Dayuan Power.
1. Primary (High) Voltage:
Common Ratings: 13.8kV, 14.4kV, 24.94kV, 34.5kV (L-L).
Configuration: 3-Wire Delta or 4-Wire Wye (e.g., 12470Y/7200V). CRITICAL: Match your utility's distribution voltage exactly.
2. Secondary (Low) Voltage:
Most Common: 480Y/277V (3-Phase, 4-Wire Wye).
Other Options: 208Y/120V, 240/120V (3-Phase, 4-Wire Delta), 600Y/347V. Match your facility's utilization voltage.
3. Vector Group (Connection Symbol):
Standard for Pad-Mounts (Utility): Delta-Wye (e.g., Dyn1 or Dyn11). Dyn11 is very common in North America.
Other: Wye-Wye (Yyn0), Delta-Delta (Dd0), etc. Confirm with utility/facility requirements.
4. Basic Impulse Level (BIL):
Primary: Standard depends on Primary Voltage (e.g., 95kV for 15kV class, 125kV/150kV for 25kV/35kV class). Higher BIL = better surge withstand.
Secondary: Typically 30kV BIL.
5. Impedance (%Z):
Standard Range: ~4.5% to 6.0%. 5.75% is a very common standard value.
Considerations: Lower %Z = lower voltage drop but higher fault current. Higher %Z = limits fault current but increases voltage drop. Consult utility/system studies.
6. Temperature Rise:
Standard: 65°C (Average Winding Rise by Resistance).
Alternative (Higher Capacity/Extended Life): 55°C (lower losses, cooler operation).
7. Insulation System / Thermal Class:
Standard: 220°C (Based on Aramid/Non-Cellulosic materials like Nomex, thermally upgraded paper). Older designs might be 150°C or 180°C.
8. Cooling Class:
Standard: ONAN (Oil Natural, Air Natural) - Radiators/fins only.
Optional: ONAF (Oil Natural, Air Forced) - Radiators with fans for higher capacity or hotter climates.
9. Sound Level:
Typical Range: 60 dB to 70 dB (Measured @ 1m under no-load).
Lower Sound Options: Available (e.g., 55 dB) using special core clamping, sound-deadening materials, lower flux density - important for noise-sensitive areas.
10. Winding Material:
Copper: Higher cost, lower losses, higher efficiency, better short-circuit withstand, smaller size/weight. Recommended for efficiency focus.
Aluminum: Lower cost, higher losses, larger size/weight. Common where initial cost is paramount.
11. Taps:
Type: Usually De-Energized Tap Changer (DETC) +/- 2x2.5% or +/- 4x2.5% (4 above, 4 below nominal = 9 positions total).
Location: Typically accessible without oil drain (compartment door).
Optional: On-Load Tap Changer (OLTC) - Significantly higher cost, used where constant voltage regulation is critical.
12. Efficiency & Losses:
Governed by DOE 2016 regulations (USA) or similar internationally (IEC 60076-20).
Tiers: Tier 1 (Basic), Tier 2 (High), Tier 3 (Premium). Recommend Tier 2 or Tier 3 for significant lifetime cost savings.
Typical Losses (Copper Windings, Tier 2):
No-Load Loss: ~1800 - 2500 Watts
Load Loss (@ 75°C): ~15,000 - 20,000 Watts
Exact values vary significantly by manufacturer and design. Obtain quotes with specific loss values.
13. Enclosure:
Material: Galvanized Steel (G90 min), Stainless Steel (higher cost, better corrosion resistance).
Rating: NEMA 3R (Standard - Rain, sleet, external ice), NEMA 4 (Watertight - hosing/driven rain).
Ventilation: Louvered openings with insect/rodent screens.
Security: Lockable, tamper-resistant doors/bolts. Lift-off or bolt-on cover.
14. Compartments:
Primary (High Voltage): Separately locked compartment with visible break device (loadbreak elbows or deadbreak bushings), surge arresters, grounding switch (optional but recommended).
Secondary (Low Voltage): Separately locked compartment with bushings/terminals, often includes metering (CTs/PTs).
Control: May house tap changer, gauges, protective relays (optional).
15. Bushings:
Primary: Deadfront, 200A or 600A loadbreak or deadbreak style (e.g., Elbow type - Cooper, ABB, T&B). Specify type to match utility standards.
Secondary: Typically bus-bar or cable lugs (e.g., 4x 600kcmil or 4x 750kcmil per phase/neutral).
16. Surge Protection:
Primary: Distribution-class surge arresters (MOV) standard on each phase. Specify kV rating (e.g., 10kV, 15kV, 27kV).
17. Liquid Level & Temperature:
Liquid Level Gauge (visible sight glass or magnetic type).
Liquid Temperature Gauge (dial thermometer).
Winding Temperature Indicator (optional, but recommended).
18. Protective Devices (Common Options):
Pressure Relief Device: Standard.
Buchholz Relay: Optional (detects internal gas generation from faults).
Sudden Pressure Relay: Optional (alternative/complement to Buchholz).
Over-Temperature Alarms/Trips: Optional (via WTI/LTI contacts).
19. Grounding:
Robust external grounding pads (multiple).
Internal core/coil ground brought to external pad.
20. Metering (Optional but Common):
Dry-type Current Transformers (CTs) in secondary compartment.
Potential Transformers (PTs) if needed for revenue metering.
Test switches/terminal blocks.
1. Prioritize Efficiency (Losses): Strongly recommend Tier 2 or Tier 3 efficiency (DOE 2016). The slightly higher initial cost is quickly recovered through drastically lower operating costs (energy losses) over the 25-40 year lifespan. Calculate the Total Owning Cost (TOC).
2. Specify Copper Windings: For superior efficiency, overload capability, short-circuit withstand, and longevity. The long-term benefits outweigh the initial cost premium over aluminum.
3. Confirm Utility Requirements FIRST: This is paramount. Utilities have very specific standards for:
Primary Voltage & Configuration
Primary Bushing Type (Loadbreak vs. Deadbreak, Amp rating, Manufacturer style)
Impedance (%Z)
BIL Ratings
Protection (Surge Arrester ratings, Grounding Switch)
Metering Compartment (CT/PT specs)
Enclosure Color/Painting
Do not proceed without this list.
4. Choose Reputable Manufacturers: Stick with established players known for quality and reliability in pad-mounted transformers (e.g., Eaton, Siemens, ABB, GE, Howard, ERMCO, Pacific Crest, WEG). Check references and service support.
5. Consider the Environment:
Sound Level: Specify lower dB if near residences, offices, or hospitals.
Corrosion: Specify stainless steel enclosure or enhanced paint systems (e.g., epoxy) for coastal, industrial, or high-pollution areas. NEMA 4 if exposed to frequent washing.
Temperature: Confirm ambient temperature range matches design. Consider ONAF cooling if consistently high ambient.
6. Specify Key Accessories: Don't assume they are included. Explicitly state:
Primary Surge Arresters (kV rating)
Grounding Switch (if required)
Winding Temperature Indicator (Recommended)
Specific CT/PT ratios if metering is needed
Desired number/range of DETC taps
7. Understand Testing: Ensure routine tests per IEEE C57.12.00/90 or IEC 60076 are performed and certified. Consider requesting witness testing for large orders.
8. Factor in Installation & Maintenance:
Ensure the pad is designed for the transformer's weight and dimensions (obtain from manufacturer).
Confirm clearances per NEC/IEEE standards and utility requirements.
Plan for future oil sampling/testing (DGA).
Ensure adequate ventilation around enclosure.
9. Get Detailed Quotes: Require quotes that list all specifications explicitly (voltages, %Z, BIL, losses/Tier, materials, accessories, sound level, warranty). Compare apples-to-apples.
