2025-07-31
Three-phase pole-type transformers are very common in power distribution networks around the world. Their purpose is to reduce the high distribution voltage to a lower voltage suitable for households, businesses and light industries. If you have such a need, please contact our company.
1. Core Function:
Voltage Transformation: Steps down 3-phase high voltage (HV) from the primary distribution lines to 3-phase low voltage (LV) for local customers.
Connection: The primary (HV) windings are typically connected in Delta (Δ), while the secondary (LV) windings are connected in Wye (Y). This Wye secondary provides a neutral point, allowing access to both:
Phase-to-Phase Voltage: e.g., 400V, 480V (for motors, larger loads).
Phase-to-Neutral Voltage: e.g., 230V, 277V (for lighting, outlets, single-phase loads).
2. Key Components:
Tank: A sealed, weatherproof enclosure (usually steel) filled with insulating/cooling oil.
Core & Windings: Laminated steel core with three sets of HV (primary) and LV (secondary) windings – one set for each phase.
Bushings: Insulated feedthroughs connecting the internal windings to the external lines.
HV Bushings (3): Connect to the primary distribution lines (overhead).
LV Bushings (4): Connect to the secondary service lines. Typically 3 phase bushings + 1 neutral bushing.
Insulating Oil: Provides electrical insulation and cooling. Modern units often use less flammable or biodegradable oils.
Radiators/Fins: Increase surface area for better heat dissipation from the oil.
Tap Changer (Usually Off-Circuit/De-Energized): Allows adjustment of the turns ratio (typically ± 2.5% or ±5%) to compensate for voltage variations on the primary side. Adjusted manually only when the transformer is de-energized.
Lifting Lugs: For installation and removal.
3. Essential Accessories & Protection:
Surge Arresters: Mounted on the primary side (and sometimes secondary) to protect against lightning strikes and voltage surges.
Primary Fuses/Cutouts: Mounted before the transformer on the pole. These are expulsion-type fuses designed to isolate the transformer in case of internal faults or severe overloads. They visibly drop open ("drop-out") when blown.
Secondary Breaker/Fuse (Optional): Sometimes included on the transformer itself for secondary protection.
Grounding: The tank, neutral point, and core are solidly connected to the system ground (grounding rod/wire running down the pole) for safety and fault protection.
Pressure Relief Device: Acts like a safety valve in case of internal pressure buildup (e.g., from a severe fault).
4. Typical Specifications & Ratings:
Primary Voltage: 11kV, 15kV, 22kV, 33kV (region-dependent).
Secondary Voltage: 400Y/230V, 480Y/277V (common standards).
kVA Rating: Ranges widely, commonly 25kVA, 50kVA, 75kVA, 100kVA, 150kVA, 250kVA. Larger sizes exist but become heavy for pole mounting.
Cooling: ONAN (Oil Natural, Air Natural) - the standard for pole mounts.
Impedance: Typically 4-6% (standard distribution impedance).
Losses: Defined by standards (e.g., No-Load Losses, Load Losses).
5. Installation & Mounting:
Mounted high on a sturdy utility pole using robust steel brackets.
Positioned below the primary distribution lines and above the secondary service lines.
Requires careful consideration of pole strength, height, and electrical clearances.
Ground wire runs down the pole to a grounding electrode.
6. Applications:
Residential Subdivisions: Supplying groups of houses.
Commercial Areas: Supplying shopping centers, offices, small businesses.
Light Industrial Areas: Supplying small factories, workshops.
Rural Electrification: Supplying farms and remote communities.
Street Lighting Circuits.
7. Advantages:
Cost-Effective: Lower installation cost compared to pad-mounted or substation transformers.
Space Saving: Utilizes air space above ground, no ground footprint needed.
Accessibility: Relatively easy access for maintenance (though requiring bucket truck).
Durability: Designed for harsh outdoor environments.
Safety: Mounted out of public reach; fuses provide primary fault isolation.
8. Disadvantages:
Aesthetics: Considered visually intrusive by some.
Vulnerability: Exposed to weather, lightning, vehicle pole impacts, animals.
Access for Maintenance: Requires specialized equipment (bucket truck) and trained linemen. Causes traffic disruption.
Limited Capacity: Size/weight constrained by pole strength.
Oil Spill Risk: Potential environmental hazard if the tank is ruptured.
The 3-phase pole-mounted transformer is the workhorse of the medium-to-low voltage electrical distribution system. It efficiently transforms high voltage 3-phase power into the usable lower voltage 3-phase and single-phase power required by end consumers. Its pole-mounted design offers a practical, economical, and space-saving solution for widespread power delivery, despite some aesthetic and accessibility drawbacks.
