How to Choose a Substation Transformer


Tom Steeber, Pacific Crest Transformer

Transformer requirements change dramatically based on the application and the load. Several factors must be evaluated carefully during a selection process to ensure the unit selected meets some primary needs.

The load type, the primary voltage, and the secondary voltage that might be required by load equipment are key factors. Other important factors include the frequency in hertz (Hz) and phase (whether single or three-phase), and the kilovolt ampere (kVA) load, factoring in possible increases. Finally, the choice also depends upon whether the transformer will be used indoors or outdoors, and if it will be floor or wall-mounted.

Making the right choice involves asking the right questions. Following is a list of questions that will help you clarify your needs, as well as provide an overview of evaluation factors to consider as you make your selection.


Questions to Guide You


The answers to these four questions will guide you on your quest for the perfect transformer. For each unit you consider, be sure to ask:

1. Does the unit have enough capacity to handle the expected load, as well as a certain amount of overload?
2. Can the unit’s capacity be augmented to keep up with possible future load increases?
3. What is the unit’s life expectancy?
4. What are the initial, installation, operational and maintenance costs?


Transformer requirements clearly change depending upon the application. Take the example of wind energy, where output varies greatly at different times. Transformers used here must be able to withstand surges without failure. Another example might be a utility substation, where transformer reliability makes the difference between a neighborhood that has power and one in the dark. In the automotive industry, good short-term overload capacity is a key attribute. A steel industry transformer requires a large amount of uninterrupted power for the intensive metallurgical and other processes, so this application might require a particular type of transformer construction that minimizes copper losses.

The following questions will help you select the right transformer for a particular application:

  • What is the primary voltage?
  • What secondary voltage is required by load equipment?
  • What is the frequency (in Hz) and phase (single or three-phase) for both the primary and secondary voltage?
  • What is the existing kVA load? How might the load increase in the future?
  • Is the transformer to be used indoors or outdoors?
  • Is the transformer to be floor or wall-mounted?
  • Is an auto transformer or a double-wound transformer required?



Liquid-filled or Dry-type Transformer


Two main types of transformers are available—liquid-filled and dry-type insulation. Debate about which is better is ongoing. Generally accepted performance characteristics indicate that liquid-filled transformers are more efficient, have greater overload capacity and longer life expectancy. They are better at reducing hot-spot coil temperatures but have a greater risk of flammability than dry-type transformers. Unlike dry-type units, liquid-filled units sometimes require containment troughs to guard against fluid leaks. Dry-type units are usually used for lower ratings (the changeover point is considered to be 500kVA to 2.5MVA).

Another consideration in selecting between the two types is whether the unit will 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.

The choice of the filler in liquid-filled transformers is usually based on the transformer’s temperature rating, the coils’ mechanical strength, dielectric strength of the insulation, expansion rate of the conductors under various loads and the insulation system’s resistance to thermal shock. Using fluid both as an insulating and a cooling medium, liquid-filled transformers have rectangular or cylindrical forms when constructing the windings. Spacers are used 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 65 C average winding temperature rise.

Dry-types typically come in enclosures with louvers, or sealed. They are typically insulated with varnish, vacuum pressure impregnated (VPI) varnish, epoxy resin or cast resin. Dry-type insulation provides dielectric strength and ability to withstand thermal limits. Temperature rise ratings are typically 150 C, 115 C, and 80 C, based on the class of insulation used.


Other Considerations


Choosing the proper winding material: Transformers use copper or aluminum for windings. Aluminum-wound units are typically less expensive and usually the most cost-effective. Copper-wound transformers, however, are smaller, because copper is a better conductor, and copper also contributes to the coil’s greater mechanical strength. In making a decision on which to choose, it is especially important to work with a manufacturer who can help you evaluate which would be best for your application, and has the capability and experience to work with either material to suit your specific requirement.

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.

Protecting the transformer from harsh conditions: It is important to ensure that transformer core, coils, leads and accessories are properly protected, especially when used in harsh environments. Liquid-filled transformers should be sealed, automatically providing protection for the internal components. For highly corrosive conditions, consider stainless steel tanks. To protect dry-type transformers from harsh environments, cast coil, cast resin and vacuum pressure encapsulated (VPE) units can be used, sometimes with 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.

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 generally use porcelain insulators, which 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. Operators must perform power factor tests at specific intervals to verify that these insulators are still in good condition.

Transformer regulation: The difference between the secondary’s no-load voltage and full-load voltage is a measure of the transformer’s regulation. This can be determined by using the following equation:

Poor regulation means that as the load increases, the voltage at the secondary terminals drops substantially.

Voltage taps: Even with good regulation, a transformer’s secondary voltage 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 the lines, the incoming voltage to the facility may either decrease or 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. It is important to discuss whether your application is likely to require voltage taps during your selection process.

Transformer life expectancy: A transformer’s useful life is generally considered to be the same as that of its insulation system. Insulation life, in turn, is directly proportional to the temperatures to which the insulation is exposed. Winding temperatures vary, and hot spots at a maximum of 30 C 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, which represents the amount of kVA loading that will result in the rated temperature rise under standard operating conditions. You can estimate a normal transformer life expectancy by using these standard operating conditions, including the accepted hot-spot temperature with the correct insulation class.

Overloading: Operating conditions can sometimes necessitate overloading a transformer. It is crucial to determine how much overloading a unit can withstand without developing problems or faults. Heat dissipation is a primary issue. For example, if a transformer is overloaded 20 percent above its rated kVA for a 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, the chances of a fault occurring are small. However, there is clearly a period at which the transformer can no longer remain overloaded. Heat will build up inside 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, which augment the transformer’s load capability.

Insulation level requirements: The transformer’s insulation level is based on its basic impulse level (BIL), defined as the maximum peak voltage that a piece of equipment can withstand before insulation breaks down and the equipment shorts out. The BIL can vary for a given system voltage, depending upon the amount of exposure to system over-voltages a transformer encounters during its lifecycle. The BIL must be carefully selected if the electrical system includes solid-state controls, because such controls chop the current when operating and may cause voltage transients.

Shielding: A transformer’s ability to attenuate electrical noise and transients is important, especially when dealing with particular load types. A shield is often placed between a distribution transformer’s primary and secondary coils when the transformer services solid state equipment such as computers and peripherals.

Placing transformers near the load: Minimizing the distance between the unit and the principal load reduces energy loss and voltage drops and brings down secondary cabling costs. Placement of high-voltage equipment, however requires close scrutiny of electrical and fire safety issues. Selecting units that are pre-approved or permitted by insurance companies helps provide a balance.

Accessories: Necessary accessories increase costs and, therefore, should be choosen wisely. 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, and tap changing control apparatus.

Work with a reliable manufacturer: One final factor exists that will greatly impact the selection process, given the number of issues to be considered and the need for them to work in concert. It is important to work with a manufacturer that can match the transformer operating characteristics, size and other attributes with the needs you identify during your evaluation process.

Tom Steeber is vice president of marketing and sales with Pacific Crest Transformer. More information at

Two main types of transformers exist: liquid-filled and dry-type insulation.


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