750 kVA 11 kV Oil-Immersed Distribution Transformer

The 750 kVA oil-immersed distribution transformer is a three-phase step-down unit that converts an 11 kV primary voltage to a 400 V secondary voltage for commercial, residential, and light industrial power supply. A typical configuration is the S11-750 KVA-11/0.4 KV model, which uses mineral oil for insulation and cooling and is designed for reliable long-term operation with low maintenance.

Product Overview

The transformer is designed and tested to IEC 60076, with a rated power of 750 kVA, a frequency of 50 Hz, and three-phase construction. Cooling is by the ONAN method, in which the insulating oil circulates naturally and the tank surface is cooled by natural air. The windings are made of copper for low losses and high reliability, and the vector group is Dyn11, which provides a neutral point on the low-voltage side and reduces harmonic influence. An off-circuit tap changer (NLTC) with a tapping range of plus or minus 2 x 2.5 percent allows the ratio to be adjusted when the unit is de-energized. Reference losses for the unit are 0.98 kW no-load loss and 7.5 kW load loss.

Technical Specification

Item	Specification
Model	S11-750 KVA-11/0.4 KV
Type	Oil-immersed distribution transformer
Standard	IEC 60076
Rated Power	750 kVA
Frequency	50 Hz
Phase	Three
Cooling Type	ONAN
Primary Voltage	11 kV
Secondary Voltage	0.4 kV
Winding Material	Copper
Vector Group	Dyn11
Impedance	4.5%
Tap Changer	NLTC
Tapping Range	Plus or minus 2 x 2.5%
No-load Loss	0.98 kW
Load Loss	7.5 kW
Accessories	Standard configuration

Typical Applications

Commercial and residential power supply: apartment buildings, hotels, shopping malls, and hospitals receive power at a usable voltage level.

Industrial facilities: factories and workshops use 750 kVA units to power production equipment.

Small-scale distribution substations: the transformer steps down voltage for local distribution networks.

Decentralized power sources: several 750 kVA units can be installed in parallel to serve a large load such as 7500 kVA when a single larger transformer is impractical.

Tap Changer Types

Two voltage regulation methods are used for distribution transformers. No-load voltage regulation (NLTC) means that the tap position is changed only after the transformer is de-energized. On-load voltage regulation (OLTC) allows the tap to be adjusted while the transformer remains energized, which is suitable for networks with frequent voltage fluctuation. The S11-750 model is supplied with an NLTC as standard, and an OLTC can be arranged for special projects.

Installation Requirements

The high-voltage down lead and high-voltage bus should use copper-core or aluminum-core rubber-insulated wire.

The phase-to-phase distance of the high-voltage bus, down lead, and drop-out fuse should be maintained according to the applicable safety distance requirements.

The distance between low-voltage lines and high-voltage down leads should not be less than 150 mm.

Low-voltage lines must not pass through high-voltage down leads, and the sag of the leads should be neat and consistent.

The transformer shell, circuit breaker, arrester ground wire, and low-voltage neutral should be grounded together first.

The transformer platform should be kept horizontal, with the diagonal height difference of the platform kept within 30 mm.

Advantages of Oil-Cooled Transformers

Oil-immersed transformers are effective thermal insulators and heat dissipators. The mineral oil provides excellent dielectric insulation between windings, transfers heat efficiently from the core and coils to the tank surface, and protects internal parts from moisture and dust. In addition, the mature manufacturing process keeps the cost competitive while delivering high efficiency and long service life, which makes the oil-cooled design the standard choice for outdoor medium-scale distribution.

Frequently Asked Questions

Q1. What does ONAN cooling mean?

ONAN stands for oil natural, air natural. The insulating oil circulates by natural convection and the heat is released to the surrounding air through the tank surface, without pumps or fans.

Q2. Why is the Dyn11 vector group used?

Dyn11 provides a neutral point on the low-voltage side, allows single-phase loads to be supplied, and reduces the influence of harmonics on the network, which improves power quality.

Q3. What is the difference between NLTC and OLTC?

NLTC, or off-circuit tap changer, is adjusted with the transformer de-energized. OLTC, or on-load tap changer, changes taps while the transformer is energized, which is needed where grid voltage varies frequently.

Q4. Can several 750 kVA transformers be installed in parallel?

Yes. Multiple units can share a large load, for example ten 750 kVA units for a 7500 kVA requirement, when a single large transformer is difficult to transport or install. Vector group, ratio and impedance must be matched for parallel operation.

Q5. What maintenance does an oil-immersed transformer require?

Routine maintenance includes checking the oil level and condition, inspecting gaskets and fittings, testing the protective devices, and periodic oil sampling for dielectric strength and moisture content.

Q6. What tests are performed before delivery?

Routine tests per IEC 60076 include winding resistance, voltage ratio and vector group, insulation resistance, no-load loss and current, load loss and impedance, and dielectric withstand tests.