23 DISTRIBUTION TRANSFORMERS

 DISTRIBUTION TRANSFORMERS

Distribution transformers are normally considered to be those transformers
which provide the transformation from 11 kV and lower voltages down to the
level of the fi nal distribution network. 

In the UK this was, until January 1995,
415 V three phase and 240 V phase to neutral. Now it is nominally 400 V three
phase and 230 V between phase and neutral. 

Of course, these are nominal voltages to be applied at consumers’ terminals and there are tolerances to take account of light loading conditions and regulation at times of peak load. 

Prior to January 1995, most distribution transformers were designed for a secondary open-circuit voltage on principal tapping of 433 V and it remains to be seen whether this situation will change in the long term. 

At the present time,  however, transformer voltage ratios have not changed, although it is possible that some adjustment of transformer off-circuit tappings might have been made at some points of the distribution network. 

Throughout the following section, therefore, in making reference to distribution transformer LV windings and systems, these will be termed 415 V or 0.415 kV. 

Except where specifi cally indicated to the contrary this should be taken as a nominal description of the winding or system voltage class and not necessarily the rated voltage of the winding or system in question.

Distribution transformers are by far the most numerous and varied types of
transformers used on the electricity supply network. 

There are around 500 000 distribution transformers on the UK public electricity supply system operated by
the Distribution Network Operators (DNOs) and a similar number installed in industrial installations. 

They range in size from about 15 kVA 3.3/0.415 kV to 12.5 MVA 11/3.3 kV, although most are less than 2000 kVA, the average rating being around 800 kVA. 

The vast majority are free breathing fi lled with oil to BS 148, but they may be hermetically sealed oil fi lled, dry type, or, occasionally,where there is a potential fi re hazard, fi re-resistant fl uids notably silicone fl uid, natural or synthetic ester or high molecular weight hydrocarbons which have a
fi re point in excess of 300ºC may be specifi ed. 

Environmentally friendly fluids such as natural esters are also becoming increasingly utilised in the USA in distribution transformers, particularly 

where there might be a perceived environmental risk in rural areas and by companies who wish to market themselves as ‘green.’

This section will fi rst discuss oil-filled units in some detail and later highlight those aspects which are different for dry-type transformers. 

As far as the constructional features of transformers using these are concerned, there are no signifi cant differences compared with oil-filled units apart from the need to ensure that all the materials used are compatible with the dielectric fluid. 

Most insulating materials, including
kraft paper and pressboard, are satisfactory on this score; if there are problems it is usually with gaskets and other similar synthetic materials.

Design considerations Distribution transformers are very likely to be made in a different factory from larger transformers. 

Being smaller and lighter they do not require the same specialised handling and lifting equipment as larger transformers. 

Impregnation under very high vacuum and vapour-phase drying equipment is not generally required. 

At the very small end of the range, manufacturing methods are closer
to those used in mass production industries. 

There are many more manufacturers who make small transformers than those at the larger end of the scale. 

The industry is very competitive, margins are small and turnround times are rapid.

As a result the main consideration in the design of the active part is to achieve the best use of materials and to minimise costs, and a 1000 or 2000 kVA transformer built in 2006 would, on reasonably close examination, appear quite different from one made as recently as, say, 20 years earlier.

Cores Simplicity of design and construction is the keynote throughout in relation to distribution transformers. 

Simplified cation has been brought about in the methods of cutting and building cores, notably by the reduction in the number of individual plates required per lay by the use of single plates for the yokes (notched yokes)
rather than the two half-yoke plates as would generally be used for a larger transformer. Nonetheless all joints are still mitred and low-loss high-permeability materials are widely used. 

Cores are built without the top yoke in place and, when the yoke is fi tted, this is done in a single operation rather than by laboriously slotting in individual packets of plates. 

Core frames have been greatly simplifi ed so that these have become little more than plain mild steel ‘U’ section
channels drilled in the appropriate places, and occasionally some manufacturers may use timber for the core frames. 

These have the advantage that there are no problems with clearances from leads, for example, to be considered in the design of the unit but they are not so convenient in other respects, for example it is not so easy to make fi xings to them for lead supports or to support an off-circuit tapchanger. 

Timber frames are now generally considered by most manufacturers
to be less cost effective than steel channels and are now generally tending to be phased out. 

It is, of course, hardly necessary to state that distribution transformer cores are invariably of a totally boltless construction. 

Wound cores, in which the core material is threaded in short lengths through the windings to form a coil
are common for smaller ratings up to several tens of kVA.

One occasion on which more sophisticated designs are widely used in distribution transformers is in relation to the use of the step-lapped core construction. 

This form of construction is to be regarded as the norm for most distribution transformer cores and was adopted for distribution transformers more rapidly than for larger units. 

There are a number of reasons

for this:

Joints form a greater proportion of the total iron circuit in the case of a small
distribution transformer core compared to that of a large power transformer and so measures to reduce losses at the joints will show a greater benefit.

Building a small core is so much easier than it is for a large core, so that
the more sophisticated construction does not present such an obstacle in
manufacture.

Distribution transformers tend to operate at poor load factors. Although this means that the magnitude of the load loss is not too important, iron loss is present all the time and it is therefore desirable to minimise its impact.

The competitive nature of the industry, discussed above, gives an incentive
to provide low losses and noise levels, both of which are improved by using
the step-lap construction.

Distribution transformer cores also represent the only occasion for which the use of amorphous steel has been seriously considered in the UK (and quite widely  adopted in other countries, notably the USA). 

The dimensions of the material currently available is one factor which prevents its use in larger transformers, but nevertheless some of the reasons discussed above for the adoption of the step-lap form of construction, namely the relative ease of building small cores and the importance of minimising iron losses .