S11 200 kVA 10 kV Oil-Immersed Distribution Transformer: Parameters and Construction

The S11 200 kVA 10 kV oil-immersed distribution transformer is a three-phase, two-winding unit used to step a 10 kV or 11 kV primary supply down to a 400 V secondary for a commercial building, a plant sub-distribution board or a municipal distribution point. It uses a corrugated tank instead of a tube-and-plate radiator tank, a cold-rolled grain-oriented silicon steel core produced on an automatic lamination line, and a coil wound from high-strength enamelled or paper-covered conductor. This article summarises the construction, the catalogue parameters and the selection rules for the rating.

Corrugated Tank and Cooling Arrangement

The corrugated tank performs three functions at the same time. It provides the heat-transfer surface that replaces the tube or plate radiators of a conventional tank, it flexes with the thermal expansion and contraction of the oil so that no separate expansion volume is needed, and it damps the pressure variations produced inside the tank under load cycling. The result is a lighter transformer that holds less oil and does not breathe, so the oil stays out of contact with air and moisture and the unit operates without routine oil treatment.

The tank is manufactured on an automatic production line. The steel is degreased, acid cleaned, phosphated and given an electrophoretic pretreatment before powder spraying, and the coating is then cured at high temperature. The finish resists corrosion, which allows the transformer to be installed outdoors, in humid climates and in industrial atmospheres where airborne contaminants would attack an untreated steel surface.

Core and Coil Construction

The core is made from cold-rolled grain-oriented silicon steel laminations cut and stacked by step-lap technology on an automatic line. Step-lap joints stagger the joint positions between adjacent laminations so that the reluctance of the magnetic path at the corner is kept low, which directly reduces no-load loss and no-load current. The core is clamped as an integral unit, and the clamping frame also holds the coils, so the winding behaves as a rigid body under short-circuit forces.

The coil is wound from high-strength enamelled wire or paper-wrapped wire, with even ampere-turn distribution and a graded insulation structure between the high-voltage and low-voltage windings. The transformer body is built without a hanging-core arrangement, which simplifies assembly and keeps the insulation distance between the windings and the tank uniform. Sealing components are made from acrylic rubber compounds selected for resistance to both photo-ageing and thermal ageing, because a degraded gasket is the usual cause of oil seepage on a sealed unit.

Catalogue Electrical and Mechanical Parameters

Adjacent ratings on the same platform are 250 kVA at 400 W no-load loss and 3200 W load loss, and 315 kVA at 480 W no-load loss and 3830 W load loss, both with a 4.0 percent impedance in the 10 kV class. Efficiency limits for these ratings are checked against GB 20052-2020, and technical parameters against GB/T 6451-2015.

Why Low No-Load Loss Matters at This Rating

No-load loss is produced whenever the transformer is energised, whatever the load, so on a lightly loaded distribution transformer it is the dominant part of the annual energy consumption. The no-load loss of 340 W at 200 kVA is achieved by using a low-loss grain-oriented core, a step-lap joint design and a flux density below saturation, and the same design choices reduce the no-load current to about 1.5 percent of rated current. Because the magnetising component drawn from the network falls, the reactive power requirement of the distribution feeder also falls, which releases capacity on the upstream network.

The economic benefit of a low-loss core depends on the load profile. A transformer that carries a steady base load benefits mainly from a low load loss, while a transformer that is energised for long periods with little connected load benefits mainly from a low no-load loss. The two cases lead to different core and conductor specifications, so the load profile should be part of the technical schedule.

Installation, Protection and Testing

The transformer is suitable for indoor or outdoor installation on a plinth or a pole platform, with the bushings and cable boxes arranged to suit the incoming and outgoing cables. Protection on the high-voltage side is usually a fuse or a circuit breaker rated to clear a secondary fault while tolerating magnetising inrush. The tank, the core clamp and the neutral of the low-voltage winding must be earthed so that earth-fault protection operates, and the earthing conductor must be sized for the prospective fault current.

Before dispatch the unit is subjected to the routine tests specified for liquid-immersed power transformers: winding resistance measurement, ratio and vector-group verification, no-load loss and no-load current measurement, load loss and short-circuit impedance measurement, applied-voltage and induced-voltage dielectric tests, and an oil dielectric strength test. Where the project requires it, temperature-rise and lightning impulse tests are performed as type tests, and the test certificates are issued with the equipment.

Design and performance are referenced to IEC 60076-1 for general requirements, IEC 60076-2 for temperature rise, IEC 60076-3 for insulation levels and dielectric tests, and IEC 60296 for the mineral insulating oil.

Frequently Asked Questions

Q: What are the losses of an S11 200 kVA 10 kV transformer?
A: Catalogue data for this series gives 340 W no-load loss and 2730 W load loss in the sealed tank version, with a no-load current of about 1.5 percent and a short-circuit impedance of 4.0 percent.

Q: Why is a corrugated tank used instead of a tube radiator?
A: The corrugated wall provides the cooling surface and the expansion volume at the same time, so the transformer holds less oil, weighs less and does not breathe air into the tank.

Q: Is the low-voltage voltage fixed at 400 V?
A: No. Secondary voltages of 380/220 V, 400/230 V and 480/277 V are all produced, and the value is fixed at the order stage because it determines the number of turns in the low-voltage winding.

Q: How is the core made in this series?
A: The core is stacked from cold-rolled grain-oriented silicon steel laminations using step-lap technology on an automatic lamination line, which keeps the joint reluctance and the resulting no-load loss low.

Q: What protection devices are normally fitted?
A: A gas-actuated relay where the rating requires it, a pressure-relief device and oil-level indication on larger tanks, plus a temperature indicator; the low-voltage and high-voltage protection is provided by the switchgear on each side.

Q: How should the unit be transported and stored?
A: The transformer is moved by the lifting lugs provided and kept upright, and the oil level, the seals and the bushings are inspected before energisation; the tank is designed to resist the mechanical loads of normal road transport without core displacement.