Design Principles for Pole-Mounted Distribution Transformers
Design Principles for Pole-Mounted Distribution Transformers
(1) Location and Layout Principles
Pole-mounted transformer platforms should be located near the load center or close to critical loads, following the principle of “small capacity, multiple locations” to facilitate equipment replacement and maintenance. For residential power supply, three-phase transformers may be installed nearby based on current demand and future growth projections.
(2) Capacity Selection for Three-Phase Pole-Mounted Transformers
Standard capacities are 100 kVA, 200 kVA, and 400 kVA. If load demands exceed a single unit’s capacity, additional transformers may be installed. However, the pole structure and secondary wiring must be designed and constructed to accommodate the final planned capacity from the outset.
400 kVA: Suitable for city centers, high-density urban development zones, economic development areas, and town centers.
200 kVA: Applicable to urban districts, towns, development zones, and rural areas with concentrated loads.
100 kVA: Recommended for rural regions with low load density.
(3) Special Case: 20 kV Dedicated Supply Areas
In 20 kV overhead distribution networks where load demand is high but adding new sites is difficult, a 630 kVA pole-mounted transformer may be used after technical justification. Due to the limited capacity of low-voltage overhead lines, a multi-circuit radial cable network is recommended for downstream distribution. Depending on site conditions, the transformer may be mounted on three poles or on a concrete pad, ensuring structural safety.
(4) Transformer Type Selection
Newly installed or replacement three-phase pole-mounted transformers shall use S11-type or higher oil-immersed, fully sealed transformers. In areas with low but stable load rates or highly fluctuating loads, SH15-type or higher amorphous alloy low-loss transformers are recommended.
(5) Overload and Voltage Drop Prevention
To avoid overloading and low output voltage, the maximum operating current of the transformer should not exceed 80% of its rated current. If this limit is exceeded, consider adding new transformer sites or capacity upgrades.
(6) Conductor and Cable Specifications
Medium-voltage (MV) drop conductors: Use JKLYJ-50 mm² cross-linked polyethylene (XLPE) insulated aerial cable or YJV22-3×70 mm² power cable.
Low-voltage (LV) outgoing cables: Use YJV22-0.6/1.0 kV, 4×240 mm² cable—single run for ≤200 kVA units, dual parallel runs for 400 kVA units.
All HV and LV terminals on the transformer platform must be fitted with insulating covers—no exposed live parts allowed.
Transformers in remote areas must incorporate anti-theft measures.
(7) Protection Devices
HV side: Protected by drop-out fuses.
LV side: Protected by low-voltage circuit breakers.
(8) Transformer Siting Requirements
The installation location must:
Be close to the load center to minimize LV supply radius;
Avoid explosive, flammable, heavily polluted, or flood-prone areas;
Allow convenient HV feed-in and LV feed-out routing;
Facilitate construction, operation, and maintenance.
(9) Prohibited Pole Types for Transformer Mounting
Do not install transformers on poles that are:
Corner or branch poles;
Poles with service drops or cable terminations;
Poles equipped with line switches or other devices;
Poles at road intersections;
Poles in easily accessible or densely populated areas;
Poles in severely polluted environments.
(10) Grounding Requirements
For 10 kV transformers, working, protective, and safety grounds may share one grounding system.
For 20 kV transformers, HV and LV working grounds should ideally be separate, though they may share one system if grounding resistance is ≤0.5 Ω.
Maximum grounding resistance for the transformer: ≤4 Ω.
Each repeated ground in the LV network: ≤10 Ω.
Grounding electrodes must be buried ≥0.7 m deep, and must not contact underground gas or water pipes.
Electrodes may be installed vertically or horizontally.
Grounding down-conductors: minimum Φ14 mm round steel or 50×5 mm flat steel.
(11) Lightning Protection
Install surge arresters as close as possible to the transformer, preferably on the secondary (LV) side.
For directly grounded neutral systems using LV insulated conductors, the neutral must be grounded at the source.
At the ends of main and branch LV lines, the neutral must be repeatedly grounded.
To prevent lightning surges from entering buildings via LV lines, the metal ferrules of service drop insulators should be grounded (R ≤ 30 Ω).
In three-phase four-wire LV systems, the neutral must be repeatedly grounded at the point of entry into each customer premises.
Grounding conductor size requirements are the same as in (10).
(12) Integrated Distribution Box (IDB)
Select IDB models based on transformer capacity: 200 kVA or 400 kVA, mounted on the pole.
The IDB must include reserved space for staged capacitor banks and be equipped with an integrated monitoring and control unit capable of energy data logging and automatic reactive power compensation.
