Generally there are many factors to be consider for Specifying Transformer but the most important factors for specifying transformer is listed below,
- Kilo volt ampere (kVA) Rating
- Voltage Ratings, Ratio, and Method of Connection (Delta or Wye)
- Voltage Taps
- Typical Impedance Values for Power Transformers
- Insulation Temperature Ratings
- Insulation Classes
- Sound Levels
- Effects of Transformer Failures
- Harmonic Content of Load
- Paralleling transformers
Kilo volt ampere (kVA) Rating
Below example gives the preferred kVA ratings of both single-phase and three-phase transformers according to IEEE C57.12.00-2010, IEEE Standard General Requirements for Liquid-Immersed Distribution, Power, and Regulating Transformers (ANSI).
Voltage Ratings, Ratio, and Method of Connection (Delta or Wye)
Voltage ratings and ratios should be selected in accordance with available standard equipment that is indicated in manufacturers’ catalogs. This is recommended, if at all possible, both from the viewpoint of cost and time for initial procurement and for ready replacement, if necessary.
| Single-phase [kVA] | Three-phase [kVA] |
| 3 | 9 |
| 5 | 15 |
| 10 | 30 |
| 15 | 45 |
| 25 | 75 |
| 37.5 | 112.5 |
| 50 | 150 |
| 75 | 225 |
| 100 | 300 |
| 167 | 500 |
| 250 | 750 |
| 333 | 1000 |
| 500 | 1500 |
| 833 | 2000 |
| 1250 | 2500 |
Voltage Taps
Taps are used to change the ratio between the high- and low-voltage windings. Manual de-energized tap changing is usually used to compensate for differences between the transformer ratio and the system nominal voltage. The tap selected in the transformer should be based upon maximum no-load voltage conditions.
Typical Impedance Values for Power Transformers
Typical impedance values for power transformers are given in Table 2. These values are at the self-cooled transformer kVA ratings and are subject to a tolerance of ±7.5%, as set forth in IEEE C57.12.00-1987 (ANSI).
Insulation Temperature Ratings
Transformers are manufactured with various insulation material systems. Performance data with reference to conductor loss and impedances should be referenced to a temperature of 40°C over the rated average conductor temperature rise as measured by resistance.
Insulation Temperature Ratings in °C
| Average conductor temp. rise * °C | Maximum ambient temperature °C | Hot-spot temperature differential * °C | Total permissible ultimate temperature* °C | Class of insulation system*°C |
| 55 | 40 | 10 | 105 | 105 |
| 65 | 40 | 15 | 120 | 120 |
| 80 | 40 | 30 | 150 | 150 |
| 115 | 40 | 30 | 185 | 185 |
| 150 † | 40 | 30 | 220 | 220 |
Insulation Classes Voltage
insulation classes and BILs are listed in Table
Voltage Insulation Classes and Dielectric Tests
| Dry transformers | Oil immersed distribution transformers | Oil immersed power transformers | |||||
| Nominal system voltage (kV) | Insulation class | Basic impulse level (kV) | Low- frequency test (kV) | Basic impulse level (kV) | Low- frequency test (kV) | Basic impulse level (kV) | Low- frequency test (kV) |
| 1. | 1.2 | 10 | 4 | 30 | 10 | 45 | 10 |
| 2.4 | 2.5 | 20 | 10 | 45 | 15 | 60 | 15 |
| 4.8 | 5.0 | 30 | 12 | 60 | 19 | 75 | 19 |
| 8.32 | 8.7 | 45 | 19 | 75 | 26 | 95 | 26 |
| 14.4 | 15.0 | 60 | 31 | 95 | 34 | 110 | 34 |
| 23.0 | 25.0 | 110 | 37 | 125 | 40 | 150 | 50 |
| 34.5 | 34.5 | 150 | 50 | 150 | 50 | 200 | 70 |
Sound Levels
Permissible sound levels are listed in Tables 5 and 6. Transformer sound levels can be a problem in commercial building interiors, especially where relative quiet is required, such as in conference rooms and certain office areas.
Effects of Transformer Failures
Transformer failures are rare. However, in high-rise buildings and in other buildings where the conditions for evacuation are limited, the effects of the failure of larger transformers can be serious. Air from transformer vaults should be exhausted directly outdoors. Well-designed transformer protection can minimize the extent of damage to any type of transformer. Dry-type transformers, including the cast-coil-type, if subjected to faults for an extended period, can burn and generate smoke. Liquid filled transformers can burst, burn, and generate smoke. Provisions can be made for dealing with these rare but still possible failure modes for large transformers in critical areas.
Harmonic Content of Load
Very recent developments have indicated failures of certain types of transformers due to nonlinear loads, which cause third and higher harmonics to flow through the windings.
When these harmonics are present, due to loads like computers, variable speed drives, electronic ballasts, HID lighting, arc furnaces, rapid mode switching devices, and similar electrical loads, consideration should be given to specifying a special transformer that is designed to withstand these harmonic currents and the fluxes they produce in the cores.
Paralleling transformers
When a transformer is able to be paralleled with another transformer, specifying %IR, %IX, and %IZ is required. Read more about matching transformers for parallel operation.
