What are Power Transformer Voltage Ratings

A power transformeris an electric device that is used to step up or step down the voltage level of its supply source. The stepping up or down depends upon the number of turns of primary and secondary winding- Transformers have two windings, being the primary winding and the secondary winding. The primary winding is the coil that draws power from the source. The secondary winding is the coil that delivers the energy at the transformed or changed voltage to the load. If the number of turns on both the windings is the same, and the losses of the transformer are negligible, we may conclude that the voltage across each of the windings is the same. In this case, the transformer is just utilized in isolating two electrical circuits. Generally, a power transformer is used in stepping up the voltage of the supply to decrease the transmission losses, and then stepping down is done for the distribution purpose at the load centers. Power Transformers are larger size devices that transfer the energy to the public electricity supply or the substation.

Power Transformers are used in transmission networks so they do not directly connect to the consumers, thereby reducing load fluctuations. They are used in transmission networks of higher voltages for step-up and step-down applications (400 kV, 200 kV, 110 kV, 66 kV, 33kV) and are generally rated above 200MVA. They are used for transmission purposes at heavy load, a high voltage greater than 33 kV & 100% efficiency. It also has a big size as compared to a distribution transformer. It is used in generating stations and transmission substations with high insulation levels. Average loads are about only 75 percent of full load and these are designed in such a way that max efficiency occurs at 75 percent of full load. A power transformer is, in fact, a bridge between the power generator and the primary distribution grid. Based on specifications and ratings, a power transformer can be divided into three categories –

1) Small Power Transformer

2) Medium Power Transformer

3) Large Power Transformer

The main use of this device is to convert the low voltage high current to a high voltage low current.

In Power Transformer Voltage Ratings, Engineers rate power transformers according to the maximum output voltage and current they deliver. For a given unit, we'll often read or hear about the volt-ampere (VA) capacity, which equals the product of the nominal output voltage and maximum deliverable current. A transformer with 12 V output, capable of providing up to 10 A of current, has a VA capacity of 12 V x 10 A, or 120 VA. The nature of power-supply filtering makes it necessary for the power-transformer VA rating to significantly exceed the actual power in watts that the load consumes.

A high-quality, rugged power transformer, capable of providing the necessary currents and/or voltages, constitutes an integral and critical part of a well-engineered power supply. The transformer is usually the most expensive power-supply component to replace if it burns out, so engineers always choose the appropriate transformer ratings when designing and building a power supply.

When a transformer is to be used in a circuit, the voltage, current, and power-handling capabilities of the primary and secondary windings must be taken into consideration. When nominal values of voltage, current, and power are specified, they represent the middle point of the respective maximum and minimum rated values. The maximum voltage that can safely be applied to any winding is determined by the type and thickness of the insulation used. When a better (and thicker) insulation is used between the windings, a higher maximum voltage can be applied to the windings.

Transformers that are used at the generating station to step up generated voltage are normally referred to as power transformers. These transformers are usually rated above 500kVA and are present between the generator and the distribution circuits. These transformers are also known as step-up transformers. Their construction varies with rating and installation locations. For outdoor use, they are usually oil-immersed whereas power transformers intended for indoor use are primarily dry type.

Kilovolt-Ampere kVA is the rating normally used to rate a transformer. The size of a transformer is determined by the kVA of the load. In many circumstances, the power required by the load is equivalent to the rating of the transformer expressed in either VA or kVA. For example, a 1KW (1000 Watts) load would require a 1kVA transformer at unity power factor. Depending on the kVA rating, power transformers are classified into;

• Small power transformers: 500 to 7500kVA
• Medium power transformers: 7500kVA to 100MVA
• Large power transformers: above 100MVA

Medium and large power transformers are equipped with added arrangements for cooling, tap changing arrangements, and Buchholz relay for internal fault protection. In addition to that, an oil conservator tank also presents all power transformers.

Power transformer at the end of the secondary transmission, makes 132 kV voltage level steps down to 33 kV or 11 kV as per requirement. From this point, the primary distribution of power starts to distribute power to different distribution stations. At the end of the primary distribution, the distribution stations receive this power and step down this voltage level of 11 kV or 33 kV to 415 V (Line Voltage). From these distribution stations to consumer ends, 415 V is kept to sustain for utilization purposes.

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