VPI Dry-Type Transformer: Construction, Insulation Classes and Duty Limits

What Vacuum Pressure Impregnation Does to a Winding

A VPI dry-type transformer uses air for cooling and for external insulation, and replaces oil or cast resin with a resin-sealed winding. The winding is first dried under vacuum to remove moisture from the conductor film and the insulating paper or glass tape. Resin is then admitted while the vacuum is held, and pressure is applied so that the resin is driven into every gap between turns, between layers and between the conductor and its insulation. The coil is cured afterwards, so the finished winding behaves as a stiff, void-free solid rather than as a bundle of separate turns.

The point of the pressure stage is void removal. Any unfilled cavity inside the insulation is a place where partial discharge can start and where moisture can collect, and discharge in a cavity slowly erodes the insulation until the winding fails. Filling the cavities under pressure is what raises the discharge inception voltage and gives the type its long service record in damp and dirty locations.

Winding Build and Insulation Classes

VPI windings are wound from enameled rectangular or round copper conductor with insulating tape or paper between layers, and the whole assembly is impregnated after winding. Because the cured resin, not a cast mould, provides the mechanical keying, section dimensions can follow the electrical design closely. Insulation systems are quoted by thermal class, and the limiting temperature rise in service is set by IEC 60076-11 for dry-type power transformers.

Item	Value normally specified	Reference
Insulation class, common	Class F (155 degrees C) or class H (180 degrees C)	IEC 60076-11
Winding average temperature rise	100 K for class F, 125 K for class H	IEC 60076-11
Routine dielectric tests	Applied voltage and induced voltage tests, partial discharge where applicable	IEC 60076-11
Partial discharge limit	10 pC, a common specification figure for higher voltage classes	IEC 60076-11
Loading guidance	Emergency and cyclic loading curves for dry-type units	IEC 60076-12
Enclosure protection	IP20 to IP23 standard, up to IP54 for sealed outdoor use	IEC 60529

Note that the class letter fixes the maximum hot-spot temperature the insulation system will tolerate, while the temperature rise figure is the average winding rise permitted when the unit runs at rated load in the reference ambient. A class H unit does not automatically run cooler; it is allowed to run hotter before its insulation life is consumed.

Losses, Impedance and Loading Envelope

Because there is no oil circulation to carry heat away from the core and windings, a VPI unit reaches its temperature limit at a lower current density than an equivalent liquid-immersed transformer. The practical consequences are a slightly larger core and winding section for the same rating, and a shorter permissible overload duration. The loading curve matters more in service than the nameplate, so the design should be checked against IEC 60076-12 for the actual daily load profile rather than against the nominal rating alone.

Impedance is set by the winding geometry in the same way as for an oil-immersed unit, and it governs the downstream protection settings. Where a VPI transformer replaces an oil-immersed unit of the same rating in an existing installation, the impedance difference changes the fault level at the low-voltage terminals, so the protection grading should be reviewed rather than copied.

Enclosure, Cooling and Installation Practice

The standard ventilated enclosure protects against contact and against moderate dust and offers a well-defined chimney effect for cooling. Where the unit is exposed to rain, dust or wash-down, the enclosure is closed and the transformer must be de-rated or fitted with forced ventilation. Sealed outdoor enclosures are usually quoted to IP54 under IEC 60529, with the heat balance checked at the specified ambient because a sealed box traps air that a ventilated one would flush.

Installation is straightforward compared with an oil-filled unit: no bund, no fire separation and no oil containment, so the transformer can sit in a load centre, on a mezzanine or next to the switchboard it serves. A minimum clearance of about 0.8 m in front of the enclosure is a sensible working figure for maintenance access, and the floor loading must include the weight of the enclosure, not only of the transformer. Earthing should be bonded at two points and the winding temperature sensors wired to alarm and trip contacts before energisation.

Frequently Asked Questions

Q: Does VPI mean the same thing as cast resin?
A: No. Cast resin windings are moulded in a resin that sets in the mould and becomes the mechanical structure of the coil. VPI windings are wound conventionally and then impregnated and cured, so the coil keeps its own insulation build and the resin seals rather than shapes it.

Q: What temperature rise is allowed for a class H VPI transformer?
A: IEC 60076-11 permits a winding average temperature rise of 125 K for class H insulation, against 100 K for class F. The class reflects the thermal endurance of the insulation system and not a guaranteed operating temperature.

Q: Why is the overload capability lower than for an oil-immersed transformer?
A: Heat is removed by air rather than by circulating oil, so the winding has less thermal inertia and the hot spot rises faster under overload. The permissible duration should be read from the dry-type loading guide in IEC 60076-12 for the actual ambient and load cycle.

Q: Where should a VPI dry-type transformer be installed?
A: Inside buildings and in public-access locations, such as load centres, basements, hospitals, shopping centres and stations, where an oil-filled unit would need fire separation, bunding or a dedicated oil containment room.

Q: What partial discharge figure should be required?
A: For medium-voltage dry-type windings a limit of 10 pC is the figure normally written into the specification, measured during a routine partial discharge test. The test result, not the design intent, is what confirms that the impregnation is sound.

Q: Can a VPI transformer be installed outdoors without an enclosure?
A: Not in normal practice. The winding is sealed against moisture but the core and connections are exposed, so an enclosure rated to at least IP23 is used outdoors, and a sealed IP54 enclosure where dust, rain or wash-down is expected.