Heavy-Duty Industrial Grid Node

The 1250 kVA Oil Immersed Transformer is engineered to serve as the central energy hub for heavy industrial facilities and large commercial districts. With a rated low-voltage current output of approximately 1800 Amps, this unit provides the massive capacity required to drive large motor control centers (MCCs) and heavy machinery.

Acting as a critical step-down link, this 11kv transformer efficiently converts medium voltage grid power (11,000V) to a stable 400V or 415V utilization level. It is designed with enhanced insulation coordination to withstand grid switching surges, ensuring the safety of downstream low-voltage assets.

Cooling Scalability: ONAN to ONAF

A distinct feature at the 1250 kVA level is the adaptability of the cooling system. While the standard unit operates on ONAN (Oil Natural Air Natural), the tank design often includes provisions for cooling fans.

Overload Potential: By upgrading to ONAF (Oil Natural Air Forced) cooling (adding external fans to the radiators), the capacity of this unit can effectively be boosted by 15% to 20%. This allows facility managers to handle unexpected load growth without immediately replacing the entire transformer.
Radiator Configuration: Unlike smaller units that use fixed corrugated walls, the 1250 kVA model typically employs detachable panel radiators. This offers better heat dissipation efficiency and simplifies maintenance, as individual radiator banks can be isolated or replaced if damaged.

Cooling fans installed on 1250 kVA 11kV transformer radiators

for ONAF overload capacity.

High-Voltage Engineering

Reliability at the MV side is paramount. As a premier 11kv transformer, the high-voltage windings are wound using multi-layer insulation paper to meet specific Basic Impulse Level (BIL) standards (typically 75kV or 95kV peak). This ensures the unit remains resilient against lightning strikes or switching impulses common in 11kV overhead distribution networks.

Technical Data Sheet

Technical Parameter	Specification
Rated Power	1250 kVA
LV Rated Current	~1804 A (at 400V)
Primary Voltage	11 kV (Standard)
Impulse Withstand (BIL)	75 kV / 95 kV
Impedance Voltage	5.0% (IEC Standard)
Cooling Method	ONAN (with ONAF provision)
Radiator Type	Detachable Panel Radiators
Total Weight	Approx. 3500 - 4200 kg

Installation & Connection Context

Given the high current output and voltage class, the installation requirements are specific:

HV Termination: The 11kV side can be fitted with a cable box tailored for XLPE cables or open bushings for overhead line connection, depending on the site structure.
LV Termination: Due to the difficulty of terminating multiple heavy cables for 1800A, the LV side is usually fitted with a large busbar palm or a specialized flange for Busbar Trunking System (BBT) connection.
Foundation: This unit requires a reinforced concrete plinth capable of supporting a total weight exceeding 3.5 tons.