Transformers
Transformers change voltage levels between different parts of your electrical system. They step voltage up for transmission efficiency or step down for distribution and utilization.
Transformer types
Power transformers:
- Large units for utility substations
- Ratings: 5 MVA to 500 MVA
- Voltages: 115kV to 13.8kV, or similar ranges
- Oil-filled with external cooling
Distribution transformers:
- Medium-duty for building distribution
- Ratings: 75 kVA to 5 MVA
- Voltages: 13.8kV to 480V typical
- Liquid or dry type
Dry-type transformers:
- Indoor building installation
- Ratings: 15 kVA to 2500 kVA
- Voltages: 480V to 208V, or 600V to 480V
- Air-cooled, no oil
Creating a transformer
- Drag Transformer from the component toolbar.
- Drop on canvas.
- Connect primary side (top handle) to upstream bus.
- Connect secondary side (bottom handle) to downstream bus.
- Configure ratings in edit panel.
Rated kVA
Transformer capacity in kilovolt-amperes (kVA):
Common ratings:
- Small: 15, 30, 45, 75, 112.5, 150 kVA
- Medium: 225, 300, 500, 750, 1000, 1500, 2000, 2500 kVA
- Large: 3000, 5000, 7500, 10000 MVA and up
Select kVA based on total connected load plus growth margin (typically 125-150% of calculated load).
Primary and secondary voltages
Primary voltage - Input (high voltage side):
- Where power enters the transformer
- Examples: 13.8kV, 4.16kV, 480V
Secondary voltage - Output (low voltage side):
- Where power exits the transformer
- Examples: 480V, 208V, 120V
Common voltage pairs:
| Primary | Secondary | Application |
|---|---|---|
| 13.8kV | 480V | Utility to building distribution |
| 4.16kV | 480V | Campus distribution |
| 480V | 208V | Step-down for lighting/receptacles |
| 208V | 120V | Small transformers for controls |
Must match system voltages on both sides.
Impedance percentage
Transformer impedance (Z%) determines short-circuit current and voltage regulation:
Typical impedance by size:
| kVA Rating | Typical Z% |
|---|---|
| 15-150 | 2-3% |
| 225-500 | 3-4% |
| 750-1500 | 4-5.5% |
| 2000-2500 | 5-5.75% |
| 5000+ | 5.75-7% |
Higher impedance = lower short-circuit current but higher voltage drop under load.
Vector groups
Vector group indicates winding connections and phase shift:
Common vector groups:
- Dyn11: Delta primary, wye secondary, 30° phase shift - most common for distribution
- Yy0: Wye-wye, no phase shift - utility transmission
- Yyy0: Three-winding transformer
- Dz0: Delta-zigzag for grounding
Most building transformers use Dyn11 configuration.
Tap changer settings
Tap changers adjust secondary voltage by changing transformer turns ratio:
No-load tap changer (NLTC):
- Adjusted when transformer is de-energized
- Fixed positions: typically ±2.5% or ±5%
- Must be changed manually by technicians
On-load tap changer (OLTC):
- Adjusts while transformer is energized
- Automatic voltage regulation
- Common on utility transformers
- Rare on building transformers
Most building transformers have fixed taps set at installation.
Cooling types
Dry-type (air-cooled):
- Forced air (fans) or natural convection
- Indoor installation
- No oil = no fire/environmental hazard
- Limited to ~2500 kVA
Liquid-filled:
- Mineral oil or silicone fluid
- Better cooling than air
- Outdoor or vault installation
- All sizes available
- Requires spill containment
Cooling designation:
- ONAN: Oil natural, air natural (no fans/pumps)
- ONAF: Oil natural, air forced (fans on radiators)
- OFAF: Oil forced, air forced (pumps + fans)
Connection configurations
Single-phase transformers:
- Two bushings (H1-H2 primary, X1-X2 secondary)
- Used for residential and small commercial
Three-phase transformers:
- Six bushings minimum (three-phase each side)
- One tank with three phase windings
- More economical than three single-phase units
Configure phase count to match your system.
Autosizing impedance
ekx can auto-populate impedance percentage based on kVA rating:
- Enter the rated kVA.
- System suggests typical Z% for that size.
- Accept default or override with nameplate value.
Use actual nameplate impedance when available for accurate short-circuit calculations.
Transformer annotations
After power flow analysis, transformers show:
- Loading percentage (kVA / rated kVA × 100%)
- Primary (HV) side current
- Secondary (LV) side current
- Color-coded by loading (green ≤90%, yellow >90% to 100%, red >100%)
See Power flow results.
Related topics
- Buses - Connecting transformers to buses
- Power flow analysis - Transformer loading calculations
- Short-circuit analysis - Transformer impedance effects