How do you size and select power transformers effectively?

Paper 'Preparation of Transformer Specification' in internet could help. Utility engineers need to write specifications based on their need.

It is strongly recommend to consider the applicable Standards in the country and area the transformer will be installed in prior to anything else. In countries that applyed international standards selecting transformer working on voltages of American standards will be not reliable and the vice versa. Example, if the system voltages are 0.4, 11, 33...KV it will be not reliable if selecting transformers of 0.22, 6, 9,...KV due to below reasons: - Availability in local markets - Availability of spare parts - compatibility with public grid - local staff competencies - Compatibility with installed appliances

Transformer effeciancy is considered high (90- 95%), but it's considered as best practice to derates according to altitude and ambeint temperature. In addition to leaving safety service factor of 15-20% as a safe margin. Transformer duty cycle should be considered and inserted to the selection criteria.

Energy demand with tolerance factor can be used in selecting a power transformer by referencing existing distribution transformers... For example, a state has: ?1000 nos of 750kVA distribution transformers ?4000 nos of 500kVA distribution transformers ?3000 nos of 300kVA distribution transformers ?2000 nos of 200kVA distribution transformers ?500 nos of 50kVA distribution transformers Using MW conversion method to select an upstream power transformer (generation step up power transformers) for the above: ?750kA= 0.66MW*1000=660MW ?500kVA=0.44MW*4000= 1760MW ?300kVA=0.26MW*3000=780MW ?200kVA=0.18MW*2000=360MW ?50kVA=0.044MW*500=22MW Conclusively, 660+1760+780+360+22= 3582MW Hence, 9 nos of 500MVA(437.4MW)power transformers,will be selected

Consider reliability of your system. Too many facilities do not have backup for their power system. If we are talking about a transformer that is your primary source consider adding a twin transformer and splitting the load. Each transformer should be capable of suppling the entire load with switching capability for maintenance and failures. The percentage of cost of your overall build to provide redundancy to your power system is well worth it. Consider an automatic transfer scheme on the secondary equipment to speed restoration. It is no small feat to source and install a new transformer when there is a failure.

Consider scheduling impacts and lead times for the transformer you are specifying. Working directly with manufacturer representatives and evaluating various equipment configurations that are more readily available may help limit scheduling delays.

Transformer impedance directly affects let through fault current. Consideration must be given to downstream fault interrupting ratings for protective devices based on the transformer chosen.

1.voltage, current, frequency, application to the transformer 2. Determine the type of load,load diversity factor 3. Determine the inrush conditions, multiplier of 10.5times(inrush condition last for 6 cycles(0.10s) 4. Type of the transformer for required application, we have different types of transformers ester oil based, mineral oil, dry type and cast resin 5. Is projected future load growth. Usually about 20-30% factor is added 6. The performance evaluation of the transformer which can be efficiency and voltage regulation 6. Required winding and oil temperature rise based on ambient temp application. 7. Type of maintenance strategy to be implemented to the transformer 8. Criticality of the transformer 9. Asset model strategy(Life cycle)