How to Choose a Transformer

When choosing a transformer, there are two primary concerns: the load and the application. Several factors must be evaluated carefully while making the choice, to ensure that the needs of both primary concerns are met.

To use a cliché, it is typically a ‘no-brainer’ to choose smaller transformers. A unit with a kVA rating that is larger from the anticipated load can quickly be picked up. But if you are selecting a large unit for an electrical utility system, to be part of a large distribution network, you are typically making a much larger investment; thus the evaluation process is much more detailed and elaborate. With over 90 years of experience in this industry, Pacific Crest Transformers has put together a quick checklist to help you make your choice judiciously.

There are three major questions that influence your choice:

  • Does the chosen unit have enough capacity to handle the expected load, as well as a certain amount of overload?
  • Can the capacity of the unit be augmented to keep up with possible increase in load?
  • What is the life expectancy of the unit? What are the initial, installation, operational, and maintenance costs?

Evaluation Factors

The cost and capacity of the transformer typically relate to a set of evaluation factors:

1.    Application of the Unit

Transformer requirements clearly change based on the application.

For example: in the steel industry, a large amount of uninterrupted power is required for the functioning of metallurgical and other processes. Thus, load losses should be minimized – which means a particular type of transformer construction that minimizes copper losses is better suited. In wind energy applications, output power varies a great extent at different instances; transformers used here should be able to withstand surges without failure. In smelting, power transformers that can supply constant, correct energy are vital; in the automotive industry, good short-term overload capacity is a necessary attribute. Textile industries, using motors of various voltage specifications, will need intermittent or tap-changing transformers; the horticulture industry requires high-performance units that suit variable loading applications with accurate voltage.

These examples serve to underline that type of load (amplitude, duration, and the extent of non-linear and linear loads) and placement are key considerations. If standard parameters do not serve your specific application, then working with a manufacturer that can customize the operating characteristics, size and other attributes to your needs will be necessary. Pacific Crest regularly builds custom transformers for unique applications.

2.    Insulation Type (Liquid-Filled or Dry Type)

While there is still debate on the relative advantages of the available types of transformers, there are some performance characteristics that have been accepted:

o    Liquid-filled transformers are more efficient, have greater overload capability and longer life expectancy.

o    Liquid-filled units are better at reducing hot-spot coil temperatures, but have higher risk of flammability than dry types.

o    Unlike dry type units, liquid-filled transformers sometimes require containment troughs to guard against fluid leaks.

Dry type units are usually used for lower ratings (the changeover point being 500kVA to 2.5MVA). Placement is also a crucial consideration here; will the unit be indoors serving an office building/apartment, or outdoors serving an industrial load? Higher-capacity transformers, used outdoors, are almost always liquid-filled; lower capacity, indoor units are typically dry types. Dry types typically come in enclosures with louvers, or sealed; varnish, vacuum pressure impregnated (VPI) varnish, epoxy resin or cast resin are the different types of insulation used.

Liquid-filled types: choice of filler material

The choice of filler material is usually based on factors that include temperature rating of the transformer, mechanical strength of the coils, dielectric strength of the insulation, expansion rate of the conductors under various loads, and resistance to thermal shock of the insulation system.

Liquid-filled types: temperature considerations

Using fluid both as an insulating and a cooling medium, liquid-filled transformers have rectangular or cylindrical forms when constructing the windings. Spacers are utilized between the layers of windings to allow the fluid to flow and cool the windings and core.

Within the sealed tank that holds both core and coils, the fluid flows through ducts and around coil ends, with the main heat exchange taking place in external elliptical tubes. For transformers rated over 5 MVA, radiators (headers on the top and bottom) are used for additional heat transfer. Modern paper insulation in liquid-filled units allows a 65ºC average winding temperature rise.

Dry type: temperature considerations

Dry type insulation provides dielectric strength and ability to withstand thermal limits. Temperature rise ratings are typically 150 ºC, 115 ºC, and 80 ºC, based on the class of insulation used (see box).

3.    Choice of Winding Material

Transformers use copper or aluminum for windings, with aluminum-wound units typically being more cost-effective. Copper-wound transformers, however, are smaller – copper is a better conductor – and copper contributes to greater mechanical strength of the coil. It is important to work with a manufacturer that has the capability and experience to work with either material to suit your specific requirement.

4.    Use of Low-Loss Core Material

Core choice is a crucial consideration, and core losses should be determined properly. Losses that occur in the core are due to hysteresis and eddy currents. High quality magnetic steel should be used so that hysteresis losses are reduced; laminated cores are chosen to minimize eddy current losses.

5.    Protection from Harsh Conditions

It is very important that transformer core, coils, leads and accessories are properly protected, especially when used in harsh environments. Liquid-filled transformers should be of sealed-type construction, automatically providing protection for the internal components. For highly corrosive conditions, stainless steel tanks can be employed. Some approaches to building dry-type transformers for harsh environments include cast coil units, cast resin units, and vacuum pressure encapsulated (VPE) units, sometimes using a silicone varnish. Unless the dry-type units are completely sealed, the core/coil and lead assemblies should be periodically cleaned, even in non-harsh environments, to prevent dust and other contaminant buildup over time.

6.    Insulators

Dry-type transformers normally use insulators made from fiberglass-reinforced polyester molding compounds. These insulators are available up to a rating of 15kV and are intended to be used indoors or within a moisture-proof enclosure. Liquid-filled transformers employ insulators made of porcelain. These are available in voltage ratings exceeding 500kV. Porcelain insulators are track resistant, suitable for outdoor use, and easy to clean.

High-voltage porcelain insulators contain oil impregnated paper insulation, which acts as capacitive voltage dividers to provide uniform voltage gradients. Power factor tests must be performed at specific intervals to verify the condition of these insulators.

7.    Regulation

The difference between the secondary’s no-load voltage and full-load voltage is a measure of the transformer’s regulation. Poor regulation means that as the load increases, the voltage at the secondary terminals drops substantially.

8.    Voltage Taps

Even with good regulation, the secondary voltage of a transformer can change if the incoming voltage changes. Transformers, when connected to a utility system, are dependent upon utility voltage; when utility operations change or new loads are connected to their lines, the incoming voltage to your facility may decrease, or even perhaps increase.

To compensate for such voltage changes, transformers are often built with load tap changers (LTCs), or sometimes, no-load tap changers (NLTCs). (LTCs operate with the load connected, whereas NLTCs must have the load disconnected.) These devices consist of taps or leads connected to either the primary or secondary coils at different locations to supply a constant voltage from the secondary coils to the load under varying conditions.

9.    Life Expectancy

It is commonly held that the useful life of a transformer is the useful life of the insulation system. Insulation life is directly proportional to the temperatures being experienced by the insulation across operation. Winding temperatures vary, and hot spots at a maximum of 30ËsC above average coil winding temperature are usually acceptable for dry-type transformers. Hot spot temperatures are estimated by calculating the sum of the maximum ambient temperature, the average winding temperature rise, and the winding gradient.

Transformers typically have a ‘nameplate’ kVA rating, and this represents the amount of kVA loading that will result in the rated temperature rise under standard operating conditions. When used in these ‘standard operating conditions’, including the accepted hot-spot temperature with the correct insulation class, a ‘normal’ transformer life expectancy can be estimated.

10.    Overloading

Operating conditions can sometimes necessitate overloading of a transformer; and what this overloading means to the unit, in terms of what it can withstand without developing problems or faults is an important consideration. A primary issue is heat and its dissipation.

For example, if a transformer is overloaded to a factor of 20% above its rated kVA for a certain period of time, any heat developed in the coils may be easily transferred to the outside of the transformer tank depending on the period of overload. If this heat transfer occurs, then the chances of a fault occurring are small; but there is clearly a time period beyond which the transformer cannot continue to be in the overloaded condition; heat can start to build up internally within the unit and cause serious problems, leading eventually to a fault and a possible power outage. Heat dissipation issues are often addressed with built-in fans, thus augmenting the load capability of the transformer as well.

11.    Insulation Level

The insulation level of a transformer is based on its basic impulse level (BIL). The BIL can vary for a given system voltage, depending upon the amount of exposure to system over voltages a transformer might be expected to encounter over its lifecycle. If the electrical system in question includes solid-state controls, the selection of BIL should be done very carefully. These controls when operating chop the current, and may cause voltage transients.

12.    Shielding

A transformer’s ability to attenuate electrical noise and transients is an important consideration, especially when dealing with particular types of load. The application of a shield between the primary and secondary coils of a distribution transformer is often accomplished when solid state equipment such as computers and peripherals are being served.

13.    Placing Transformers Near the Load

Minimizing the distance between the unit and the principal load is clearly beneficial in several ways – apart from reducing energy loss and voltage drops, it also brings down the cost of secondary cabling. The downside here is that any placement of high-voltage equipment requires very close scrutiny of electrical and fire safety issues. A suitable balance can be achieved by using units that are pre-approved or permitted by insurance companies.

14.    Accessories

An added cost, accessories are installed when required. Examples include stainless steel tanks and cabinets for extra corrosion protection, special paint/finishes for corrosive atmospheres and ultraviolet light, weather shields for outdoor units, protective provisions for humid environments; rodent guards, temperature monitors, space heaters to prevent condensation during prolonged shutdown, optional location of openings for primary and secondary leads, tap changing control apparatus, and more.