The Differences between Grounding Transformers and Distribution Transformers

Grounding transformers (GT) differ from "standard distribution transformers" (DT) because they are used to establish a return path for ground fault currents on a system which is otherwise isolated or effectively un-grounded. This differentiates the construction in a couple of ways.
Grounding transformers must be designed to meet two basic criteria:

1. They must be able to carry the continuous phase and neutral currents without exceeding their temperature ratings.
2. They must be able to carry the fault current without excessive heating that deteriorates the conductors or adjacent insulation.

It is in the second parameter which most widely separates grounding transformers from distribution transformers. DTs are designed to carry a fault current, which is limited by their impedance, for a maximum duration of 2 seconds per standards.  Whereas the GT must carry a fault current that is not limited by its impedance, for durations exceeding the 2 second limitation. Often this time is 10 seconds or more. The GT design must be such that at the end of this extended time period, the conductor temperature is below the critical thermal limit as identified in the standards.

DT: Main Concerns
The DT main concern is for heating caused by loading. Radiators are added to the transformer to help the insulating fluid control the steady state temperature rise, but these do not help during fault conditions. Heat generated during a fault happens in such a short period of time (usually seconds) that the calculation assumes "all heat is stored" in the conductor because heat dissipation does not occur fast enough to combat the rapidly heating conductors. The GT takes this into account and is designed such that the conductor can handle the fault heating without relying on insulating oil for heat transfer during the fault.

Many GT specifications recognize this and allow the steady state cooling to be calculated using the magnetizing current and HV I2R loss resulting from energizing the core only. This leads to some misconception that the DT is better cooled, but the opposite is during faults.

Another subtle difference is the way the two devices "see" faults. The DT typically sees a line to ground fault or maybe, a line to line fault, but since the GT is providing a return path to the network, it typically sees a zero sequence fault which impresses the fault current equally on all three legs simultaneously. To combat the forces generated, GT conductors are always copper for maximum strength to cross section ratio, and because copper has a higher thermal withstand capability. GT coils are always circular on cruciform cores to gain the maximum form stability. Distribution transformers often utilize rectangular coil construction which does not have the same form stability offered by the circular coil technology.

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.
Top Questions
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:

• Liquid-filled transformers are more efficient, have greater overload capability and longer life expectancy.
• Liquid-filled units are better at reducing hot-spot coil temperatures, but have higher risk of flammability than dry types.
• 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.
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.