Electric Power Distribution -- Transformer construction / installation

Transformer construction:

Transformer construction comprises of windings and magnetic core kept in a closed tank and only windings brought out for external termination purpose. The closed tank construction is essential for safety as well as protection of the internal parts of the transformer. Further it’s necessary to keep the high-voltage windings insulated from the other phases and the external surface. The most common construction is oil-filled transformer where the windings are immersed in a mineral oil having certain basic electrical characteristics like dielectric break down voltage, moisture content, etc. The oil also acts as the cooling medium which by natural circulation through the transformer radiators transfers the heat to the ambient. This construction is termed as ONAN (oil natural, air natural). Different types can be adopted to achieve higher cooling by external forced fans (ONAF) and by pumping oil using external devices (OFAF). The other type of transformer is dry type and as the name implies this construction does not employ oil but use impregnated insulation to act as the insulating medium. The construction is air cooled by natural ventilation or with external fans.

Layout and installation practice:

In designing the layout and installations of transformers, the following need to be considered: the location of the transformer whether it’s to be installed outdoors or indoors, ventilation, fire hazard, transformer grounding and noise, etc.

Typically, most transformers that are oil filled present a potential fire hazard, especially if the transformer contains oil. Therefore, it’s absolutely essential to consider this factor when deciding whether a transformer is going to be installed outdoors or indoors. Invariably, most oil-filled transformers are therefore usually installed outdoors, or when essential to have the installation indoors, then adequate ventilation and physical isolation is required. This is to ensure, that in the unfortunate event of the transformer oil igniting for whatever reason, the damage caused shall be restricted to the transformer alone and its immediate ancillary equipment, and shall not interfere with any other unit assemblies in its vicinity.

There is a general misconception that all oil-filled transformers are fire hazards. This is usually not the case for most mineral oil-filled systems, which are less of a fire hazard than most. In these transformers, the closed flash point is not lower than 140 °C and hence it shall not be possible to accumulate sufficient vapor in an enclosed space to be ignited upon exposure to a flame or any other source of ignition, below this temperature.

In any case, in order to sustain a flame, these oils require a wick to produce sufficient vapor to enable it to burn freely.

In the past usually there had to be a fault occurring prior to a fire that caused the rapid loss of oil through a rupture in the tank. Invariably, there will exist a high temperature, and as the oil is at this point exposed to the atmosphere, ignition will occur and then the transformer insulation will now serve as a wick for the oil and sustain combustion. On the other hand, if on the occurrence of a fault that causes a slow leak or drip of the oil onto a heated surface, if the temperature is high enough to ignite the oil/vapor, this fire could be fed continuously by the slow drip or leak. In many instances, this may go unnoticed until it escalates to a serious level.

There are many methods of avoiding or at the very least minimizing the risk of such hazards. Conventionally, it has been a general practice in many substations that employ oil-filled transformers and switchgear, to provide surfaces of chippings and a drainage sump to transport away any oil spillage that could potentially fuel a fire. This method however is not foolproof. It has been found that over a period, these chippings collect dust and grime and this grime would provide the wick for sustained combustion.

Alternatively, one could explore the possibility of providing a firewater sprinkler system, which could be automatically triggered on the event of a fire around the transformer. Note that water displaces oil and any old oil spillage would be washed up from the sump.

Therefore, it’s imperative that this large quantity of excess oil and water be removed as quickly as possible to deluge water treatment centers before allowing the water to enter into the storm water drains, devoid of oil. Provision should be made to separate oil from water and for containment before this separation.

Additional precautions may also be considered if the above solutions are not feasible, such as separation or segregation. Separation involves locating the transformer well away from all other equipment, but this may not be convenient, as there may be space constraints. On the other hand, segregation provides for firewalls to be built around transformers such that fires, if any, would be contained within the walls. This firewall or barrier must be suitably reinforced to be capable of withstanding any explosions from the transformer. In order to install transformers within buildings, it’s a general practice that one uses dry-type resin-encapsulated units rather than using liquid-filled transformers. In selecting the dielectric type the following must be adhered to:

• The dielectric must be non-toxic, biodegradable and must not present a hazard to the environment.

• The dielectric must have a fire point above 300 °C to be classified as a fire resistant fluid.

• The dielectric must not contribute to or increase the spread of an external fire nor must the products of combustion be toxic.

• Normal operation, electrical discharges or severe arcing within the transformer must not generate fumes or other products that are toxic or corrosive.

This does not mean that liquid-filled transformers cannot be used within buildings.

They would meet all of the above criteria and in addition are cheaper and smaller than cast-resin or other dry type units. However there must be provision for a total spillage of the dielectric with suitable sumps and/or bunded catchment areas, such that in the event that spillage occurs the building drains would not be flooded with the dielectric liquids. If the transformers are installed on higher levels, then suitable precautions must be taken to prevent leakages on the lower floors.

On the other hand the building must be made totally weatherproof and care taken to ensure that there would be no deluges due to pipe leaks on the dry type of transformers after installation. Needless to say, every installation should have proper ventilation.

Tansformers Noise:

By definition, noise is described as an unpleasant or unwanted sound. It’s inevitable that a transformer in operation emits noise. Transformer noise is continuous and it usually falls into the mid ranges of human audio spectrum. Invariably the noise that tends to irritate humans emanates from distribution transformers that are required to be located near homes and offices. Methods to attenuate this type of noise comes from employing various noise absorbing barriers such as existing walls, buildings, natural geographic topology (pits, artificial or natural), or even installing the transformer downwind from dwellings.