25 Class 220 dry-type transformers

 Class 220 dry-type transformers

Class 220 dry-type transformers are those based on glass fibre reinforced boards, aromatic polyamide paper conductor insulation and similar materials capable of operating at temperatures up to around 220ºC. 

They have now been somewhat eclipsed by cast resin encapsulated types. 

However, they do have some advantages over cast resin; they are a little more compact and thus lighter, they generally have lower losses and are up to 20 per cent cheaper than cast resin, and, most signifi cantly, they have better overload and short-circuit withstand capability. 

Although they are not capable of withstanding the same extreme environmental conditions as cast resin,present day dry types are greatly superior in this respect to those of the 1960s

when they were initially introduced. 

At that time, the conductor insulation or

‘paper’ covering was largely asbestos based in order to be able to achieve the

required temperature withstand capability. Even when properly impregnated,

this material was inclined to absorb moisture, which greatly reduced its insulation

properties. It was therefore very important to ensure that transformer windings

were properly dried out before energising, and even whilst in service it was

important to ensure that transformers were given a good dry environment. The

availability of aromatic polyamide paper from the mid-1970s greatly improved

this situation.

The construction of class 220 dry-type transformers is very similar to oilfi lled units. They may have conventional helically wound LV windings or these

may be foil wound. For all but the lowest ratings the HV winding conductor

will be rectangular in section so that HV windings may generally be disc

wound. Disc windings are to be preferred to multilayer helical type, since the

former arrangement gives a uniform distribution of the phase voltage throughout the axial length of the winding thus ensuring that the electrical stresses are

minimised. As previously mentioned, air is a poorer cooling medium than oil

and in order to ensure adequate cooling air fl ow through the windings vertical

ducts should be a minimum of 10 mm wide and horizontal ducts a minimum of

6 mm. Figure 7.33 shows the core and windings of a typical class 220 dry-type

transformer.

The modern aromatic polyamide papers are far less inclined to absorb moisture

than the earlier asbestos based materials. They will absorb about 1 per cent moisture for each 10 per cent relative humidity, so that at 95 per cent RH they

will contain 8–10 per cent water. However, even at this level of moisture content their electrical properties will be very little impaired. Table 7.2 gives the

electrical properties of NOMEX1 paper type 410, 0.25 mm thick, at varying

levels of relative humidity.

Such a good resistance to humidity might create the impression that varnish

impregnation of a winding having aromatic polyamide insulation is unimportant.

This is far from the case and it is necessary to properly vacuum impregnate the

windings with a silicone varnish if partial discharges, which will ultimately

lead to breakdown, are to be avoided. The object of the impregnation should

be to ensure that the insulation structure is free from voids, particularly in the

areas of high electrical stress, for example at the ends of windings in the close

vicinity of conductors, or between the open ends of layers, if a layer-type

winding is used. The dielectric constant of the aromatic polyamide is between

1.5 and 3.5, depending on the density of the material. The fi gure for air is, of

course, near to unity. In any composite insulation structure, that is aromatic

polyamide with air-fi lled voids, the electrical stress in each component material

will divide in inverse proportion to their dielectric constants, so that the stress

in the voids may be between 1.5 and 3.5 times that in the solid insulation 

and the reason why these can become a source of partial discharge is thus

quite clear.

Installation of class 220 dry types

The method of installing class 220 dry-type transformers is very similar to that

used for cast resin transformers. The transformer core and windings will normally be mounted on rollers and housed in a sheet-steel ventilated enclosure

incorporated into the LV switchboard with its LV busbars connected directly

to the switchboard incoming circuit breaker. It is not so convenient to provide

moulded HV connections directly onto the winding as is the case with cast resin

and, in addition, the paper-insulated windings are more easily damaged than

those of a cast resin transformer so it is best to avoid carrying out any unnecessary work in the close vicinity. It is desirable, therefore, that the HV supply

cable is not terminated internally within the enclosure but connected into an

externally mounted cable box. Adequate access to the enclosure should be

provided, however, to enable the windings to be cleaned and inspected about

once per year. This should preferably be a vacuum cleaning rather than by

blowing out dust deposits – a procedure which may embed foreign material in

undesirable locations.