OVERVIEW

By definition a dry-type epoxy resin transformer is one that relies solely on the surrounding air for cooling, thus eliminating the need for liquid insulating mediums.

Additionally and by the proper selection and treatment of the epoxy resin, these transformers are usually regarded as fire proof transformers.

Keeping this in mind and adding it to our « Safety Concern and Principles » we have pushed the design edge even further as we have conceived Copper-Cast transformers to be the utmost in Power, Quality and Safety resulting in devices with the following added benefits:

  • An extra quiet operation that will eliminate the need for special sound proofing
  • Fire proof properties that will eliminate the need for special fire extinguishing properties
  • Extremely low losses resulting in fantastic operational gains and savings in no time
  • A tough and resilient design that will allow for operation at ambient temperatures exceeding 50 degrees Celsius
  • A thorough and comprehensive insulation design resulting in a safe operation up to 1200 m altitude
  • A simple and compact layout that will allow Copper-Cast to be placed anywhere, especially close to load centers thus eliminating the need for long cables and cable losses.
  • A production system that was heavily re-engineered and that allows us to provide you with a 5 years guarantee against manufacturing defects

With this in mind you can be sure that your capital investment in our transformers is the right choice as the revenues and benefits will come up in no time.

overview

INSULATION AND DISTANCES

Many technicians and trained engineers have tendency to overlook the actual operating conditions of a transformer when it fails, rushing into the presumption of a poor design or a bad manufacturing experience.

Whilst this might be a major cause of transformers failure, some other hidden causes need to be identified to avoid potential problems in the future.

Among these problems is the clearance issue. This becomes a major concern especially when we are entering the medium voltage, medium to high power transformers.

A list of various transformers configuration and clearances is available on this page and it is crucial to respect it when installing your transformers to avoid, flashovers, heat trapping and allow a free movement of technicians and individuals around the transformers.

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CONNECTIONS

Connections to and from transformers in general and medium voltage transformers in particular are very sensitive, as special care and precaution has to be taken to avoid a string of problems and difficulties.

When considering a medium voltage transformer connections can be made in any configuration (i.e. low voltage from top and medium voltage from top, low voltage from below and medium voltage from top…)

Nevertheless some major points have to be taken into consideration; these are:

  • The minimum distance between the cables (Both the HV and LV) to the live parts has to be 120 mm
  • If coming from top the cables have to be properly supported so they don’t lay on the connection points of the transformer and break them
  • The cables shouldn’t be very short, they will cause stress and strain on the connection interface
  • The cables shouldn’t be very long they will cause looseness on the connection interface and lead to sparking and degradation of the busbars
  • All local and international standards have to be carefully observed while connecting the transformers
connection

VENTILATION

Even with their utmost efficiency, Copper-Cast transformers are still electrical devices that generate losses.

This and the effect of other electrical devices and components that might be present in the substation or the electrical room, will add to the heat generation and dissipation.

Left to the natural convection effect, the room will never get rid of all this heat; therefore before installing a transformer the room ventilation has to be carefully studied and arranged according to these guidelines:

Air Inlet Section = 0.18 x P / √H
Air Outlet Section = 1.1 x Air Inlet Section

With P being the total loss of the transformer in kilo Watts at 120 degrees C

In situations where the room is badly ventilated the general codes of practice recommend installing a ventilation fan on the Air outlet with an air draft of:

Air Draft = 0.13 x P (applicable up to an ambient of 40 degrees Celsius)

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OVERLOADING

Due to their high and improved thermal constants, cast resin medium voltage transformers in general and Copper-Cast transformers in particular can withstand high cyclical overloads.

By this we mean that if operated at a certain loading factor L (explained herein) the transformer can be overloaded by a certain percentage, for a certain time without reducing its useful life.

The loading factor L is the ratio of actual constant load to the rated transformer power and is given by:

L = Actual Constant Load / Rated Power

For example and referring to the tables on the left of this page we can see the overloading capacity of various loading factors,
namely L = 0.75, L=0.50 and L=0.25
respectively in tables 1, 2 and 3.

On these graphs one can also appreciate the effect of ambient temperature on the overloading graphs and see that if we are to bring back our transformers to a certain operating regime for the remainder of the day a certain overloading of x% can be attainable depending on the factors stated above.

In any case as this is a design specific characteristic and it is recommended to come back to S.E.T.F technical department and study each case of overloading separately.

DE-RATING

Copper-Cast transformers are usually manufactured to operate in an average ambient temperature of 40 degrees Celsius.

Unlike other transformers, they will be fully rated at this ambient temperature and will require no de-rating.

It is also a very common practice to liaise with the customer and design transformers that will operate even at temperature reaching 55 degrees Celsius.

Nevertheless it will always be handy to keep in mind that a transformer deigned to give its full rated power at a certain ambient temperature, can be operated at a higher or lower ambient temperature by a proper factor.

If this factor is applied then the transformer can be expected to deliver properly during its full life, without quick degradation or deterioration.

As a quick example you will see the de-rating curve for a transformer designed to give it full rated output at 50 degree Celsius and the consequent power that can be drawn at other ambient temperatures.

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