Choice of MV/LV transformer

Characteristic parameters of a transformer

A transformer is characterized in part by its electrical parameters, but also by its technology and its conditions of use.

Electrical characteristics

• Rated power (Pn): the conventional apparent-power in kVA on which other design-parameter values and the construction of the transformer are based. Manufacturing tests and guarantees are referred to this rating
• Frequency: for power distribution systems of the kind discussed in this guide, the frequency will be 50 Hz or 60 Hz
• Rated primary and secondary voltages: For a primary winding capable of operating at more than one voltage level, a kVA rating corresponding to each level must be given.

The secondary rated voltage is its open circuit value

• Rated insulation levels are given by overvoltage-withstand test values at power frequency, and by high voltage impulse tests values which simulate lightning discharges. At the voltage levels discussed in this guide, overvoltages caused by MV switching operations are generally less severe than those due to lightning, so that no separate tests for switching-surge withstand capability are made
• Off-circuit tap-selector switch generally allows a choice of up to ± 2.5% and ± 5% level about the rated voltage of the highest voltage winding. The transformer must be de-energized before this switch is operated
• Winding configurations are indicated in diagrammatic form by standard symbols for star, delta and inter-connected-star windings; (and combinations of these for special duty, e.g. six-or twelve-phase rectifier transformers, etc.) and in an IEC-recommended alphanumeric code. This code is read from left-to-right, the first letter refers to the highest voltage winding, the second letter to the next highest, and so on:

  - Capital letters refer to the highest voltage winding
  D = delta
  Y = star
  Z = interconnected-star (or zigzag)
  N = neutral connection brought out to a terminal
  - Lower-case letters are used for tertiary and secondary windings
  d = delta
  y = star
  z = interconnected-star (or zigzag)
  n = neutral connection brought out to a terminal
  - A number from 0 to 11, corresponding to those, on a clock dial (“0” is used instead of “12”) follows any pair of letters to indicate the
    phase change (if any) which occurs during the transformation.
A very common winding configuration used for distribution transformers is that of a Dyn 11 transformer, which has a delta MV winding with a star-connected secondary winding the neutral point of which is brought out to a terminal. The phase change through the transformer is +30 degrees, i.e. phase 1 secondary voltage is at “11 o’clock” when phase 1 of the primary voltage is at “12 o’clock”, as shown Figure B21 which can be seen in the section Common winding arrangements. All combinations of delta, star and zigzag windings produce a phase change which (if not zero) is either 30 degrees or a multiple of 30 degrees.
IEC 60076-4 describes the “clock code” in detail.

Characteristics related to the technology and utilization of the transformer
This list is not exhaustive:

• Choice of technology

The insulating medium is:
  - Liquid (mineral oil) or
  - Solid (epoxy resin and air)

• For indoor or outdoor installation
• Altitude (<= 1,000 m is standard)
• Temperature (IEC 60076-2)

  - Maximum ambient air: 40 °C
  - Daily maximum average ambient air: 30 °C
  - Annual maximum average ambient air: 20 °C
For non-standard operating conditions, refer to “Influence of the Ambient temperature and altitude on the rated current”.

Description of insulation techniques

There are two basic classes of distribution transformer presently available:

• Dry type (cast in resin)
• Liquid filled (oil-immersed)

Dry type transformers
The windings of these transformers are insulated by resin between turns and by resin and air to other windings and to frame. The resin is usually cast under vacuum process (which is patented by major manufacturers).
It is recommended that the transformer be chosen according to the IEC 60076-11, as follows:

• Environment class E2 (frequent condensation and/or high level of pollution)
• Climatic conditions class B2 (utilization, transport and stockage down to -25 °C)
• Fire resistance (transformers exposed to fire risk with low flammability and self extinguishing in a given time)

The following description refers to the process developed by a leading European manufacturer in this field.
The encapsulation of a winding uses three components:

• Epoxy-resin based on biphenol A with a viscosity that ensures complete impregnation of the windings
• Anhydride hardener modified to introduce a degree of resilience in the moulding, essential to avoid the development of cracks during the temperature cycles occurring in normal operation
• Pulverulent additive composed of trihydrated alumina Al (OH)3 and silica which enhances its mechanical and thermal properties, as well as giving exceptional intrinsic qualities to the insulation in the presence of heat.

This three-component system of encapsulation gives Class F insulation (Δθ = 100 K) with excellent fire-resisting qualities and immediate self-extinction. These transformers are therefore classified as nonflammable.
The mouldings of the windings contain no halogen compounds (chlorine, bromine, etc.) or other compounds capable of producing corrosive or toxic pollutants, thereby guaranteeing a high degree of safety to personnel in emergency situations, notably in the event of a fire.
It also performs exceptionally well in hostile industrial atmospheres of dust, humidity, etc. (see Fig.12).

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Fig. B12: Dry-type transformer

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Liquid-filled transformers
The most common insulating/cooling liquid used in transformers is mineral oil. Mineral oils are specified in IEC 60296. Being flammable, safety measures are obligatory in many countries, especially for indoor substations. The DGPT unit (Detection of Gas, Pressure and Temperature) ensures the protection of oil-filled transformers. In the event of an anomaly, the DGPT causes the MV supply to the transformer to be cut off very rapidly, before the situation becomes dangerous.
Mineral oil is bio-degradable and does not contain PCB (polychlorinated biphenyl), which was the reason for banning askerel, i.e. Pyralène, Pyrolio, Pyroline...
On request, mineral oil can be replaced by an alternative insulating liquid, by adapting the transformer, as required, and taking appropriate additional precautions if necessary.
The insulating fluid also acts as a cooling medium; it expands as the load and/or the ambient temperature increases, so that all liquid-filled transformers must be designed to accommodate the extra volume of liquid without the pressure in the tank becoming excessive.
There are two ways in which this pressure limitation is commonly achieved:

• Hermetically-sealed totally-filled tank (up to 10 MVA at the present time)

Developed by a leading French manufacturer in 1963, this method was adopted by the national utility in 1972, and is now in world-wide service (see Fig. B13).

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Fig. B13: Hermetically-sealed totally-filled tank

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Expansion of the liquid is compensated by the elastic deformation of the oil-cooling passages attached to the tank.
The “total-fill” technique has many important advantages over other methods:
  - Oxydation of the dielectric liquid (with atmospheric oxygen) is entirely precluded
  - No need for an air-drying device, and so no consequent maintenance (inspection and changing of saturated dessicant)
  - No need for dielectric-strength test of the liquid for at least 10 years
  - Simplified protection against internal faults by means of a DGPT device is possible
  - Simplicity of installation: lighter and lower profile (than tanks with a conservator) and access to the MV and LV terminals is unobstructed
  - Immediate detection of (even small) oil leaks; water cannot enter the tank

• Air-breathing conservator-type tank at atmospheric pressure

Expansion of the insulating liquid is taken up by a change in the level of liquid in an expansion (conservator) tank, mounted above the transformer main tank, as shown in Figure B14. The space above the liquid in the conservator may be filled with air which is drawn in when the level of liquid falls, and is partially expelled when the level rises. When the air is drawn in from the surrounding atmosphere it is admitted through an oil seal, before passing through a dessicating device (generally containing silica-gel crystals) before entering the conservator. In some designs of larger transformers the space above the oil is occupied by an impermeable air bag so that the insulation liquid is never in contact with the atmosphere. The air enters and exits from the deformable bag through an oil seal and dessicator, as previously described. A conservator expansion tank is obligatory for transformers rated above 10 MVA (which is presently the upper limit for “total-fill” type transformers).

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Fig. B14: Air-breathing conservator-type tank at atmosphere pressure

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Choice of technology

As discussed above, the choice of transformer is between liquid-filled or dry type.
For ratings up to 10 MVA, totally-filled units are available as an alternative to conservator-type transformers.
A choice depends on a number of considerations, including:

• Safety of persons in proximity to the transformer. Local regulations and official recommendations may have to be respected
• Economic considerations, taking account of the relative advantages of each technique

The regulations affecting the choice are:

• Dry-type transformer:

  - In some countries a dry-type transformer is obligatory in high apartment blocks
  - Dry-type transformers impose no constraints in other situations

• Transformers with liquid insulation:

  - This type of transformer is generally forbidden in high apartment blocks
  - For different kinds of insulation liquids, installation restrictions, or minimum protection against fire risk, vary according to the class of 
    insulation used
  - Some countries in which the use of liquid dielectrics is highly developed, classify the several categories of liquid according to their fire
    performance. This latter is assessed according to two criteria: the flash-point temperature, and the minimum calorific power. The
    principal categories are shown in Figure B15 in which a classification code is used for convenience.    

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Code	Dielectric fluid	Flash-point (°C)	Minimum calorific power (MJ/kg)
O1	Mineral oil	< 300	-
K1	High-density hydrocarbons	> 300	48
K2	Esters	> 300	34 - 37
K3	Silicones	> 300	27 - 28
L3	Insulating halogen liquids	-	12

Fig. B15: Categories of dielectric fluids

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The determination of optimal power

Oversizing a transformer
It results in:

• Excessive investment and unecessarily high no-load losses, but
• Lower on-load losses

Undersizing a transformer
It causes:

• A reduced efficiency when fully loaded, (the highest efficiency is attained in the range 50% - 70% full load) so that the optimum loading is not achieved
• On long-term overload, serious consequences for

  - The transformer, owing to the premature ageing of the windings insulation, and in extreme cases, resulting in failure of insulation and
     loss of the transformer  
  - The installation, if overheating of the transformer causes protective relays to trip the controlling circuit-breaker.

Definition of optimal power
In order to select an optimal power (kVA) rating for a transformer, the following factors must be taken into account:

• List the power of installed power-consuming equipment as described in Chapter A
• Decide the utilization (or demand) factor for each individual item of load
• Determine the load cycle of the installation, noting the duration of loads and overloads
• Arrange for power-factor correction, if justified, in order to:

  - Reduce cost penalties in tariffs based, in part, on maximum kVA demand
  - Reduce the value of declared load (P(kVA) = P (kW)/cos φ)

• Select, among the range of standard transformer ratings available, taking into account all possible future extensions to the installation.

It is important to ensure that cooling arrangements for the transformer are adequate.

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