why is transformer rating expressed in kVA instead of kW?

Answer 1

Because VA(apparent) = vector sum of W (active)+ VAr (reactive)
Transformers consume reactive power also.
Reactive power is consumed by inductance.
Reactive power is produced by capacitors.

Answer 2

Goes back to the concept of total power being composed of two parts, the real and imaginary power. For most applications, we only think about the amount of real power we need to get a job done. And, if the power factor is close to one (reactive loads primarily with very few motor loads), this is true. However, the imaginary power is still flowing and has to be generated and accounted for. If the power factor is above 95%, the effect is small enough that you could ignore it in most applications. However, tell that to a transformer outside a heavy industrial area where the power factor, if not corrected, could be 60%. In that case, the transformer may only be 100% loaded at, say 1000 kW but the total load would be 166% based on kVA. On a summer day, most transformers like that would overheat and fail. So, again, the transformer would fail from the total power in kva, not just the real.

Answer 3

Although both of the prior answers are correct, the real reason is somewhat more obvious.

It is not guaranteed that the input voltage will equal the output voltage; neither is it necessarily true that the input current equals the output current. However, input kVA DOES equal output kVA (apparent power includes both real 'kW' and imaginary 'kVAR' components).

Most transformers provide some sort of 'step-up' (output voltage higher than input voltage) or 'step-down' (output voltage lower than input voltage) function in addition to the isloation capability.

Answer 4

kVA takes into account that the load may be a reactive load where the voltage and currect can be out of phase. The product of the RMS voltage and the RMS current only give the true power for a purely resistive load.

Answer 5

The Rated power of the transformer is the sum of the VA (volts x amps) for all of the secondary windings
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Transformer losses are produced by the electrical current flowing in the coils and the magnetic field alternating in the core. The losses associated with the coils are called the load losses, while the losses produced in the core are called no-load losses.

What Are Load Losses?

Load losses vary according to the loading on the transformer. They include heat losses and eddy currents in the primary and secondary conductors of the transformer.

Heat losses, or I2R losses, in the winding materials contribute the largest part of the load losses. They are created by resistance of the conductor to the flow of current or electrons. The electron motion causes the conductor molecules to move and produce friction and heat. The energy generated by this motion can be calculated using the formula:

Watts = (volts)(amperes) or VI.

According to Ohm’s law, V=RI, or the voltage drop across a resistor equals the amount of resistance in the resistor, R, multiplied by the current, I, flowing in the resistor. Hence, heat losses equal (I)(RI) or I2R.

Transformer designers cannot change I, or the current portion of the I2R losses, which are determined by the load requirements. They can only change the resistance or R part of the I2R by using a material that has a low resistance per cross-sectional area without adding significantly to the cost of the transformer. Most transformer designers have found copper the best conductor considering the weight, size, cost and resistance of the conductor. Designers can also reduce the resistance of the conductor by increasing the cross-sectional area of the conductor.

What Are No-load Losses?

No-load losses are caused by the magnetizing current needed to energize the core of the transformer, and do not vary according to the loading on the transformer. They are constant and occur 24 hours a day, 365 days a year, regardless of the load, hence the term no-load losses. They can be categorized into five components: hysteresis losses in the core laminations, eddy current losses in the core laminations, I2R losses due to no-load current, stray eddy current losses in core clamps, bolts and other core components, and dielectric losses. Hysteresis losses and eddy current losses contribute over 99% of the no-load losses, while stray eddy current, dielectric losses, and I2R losses due to no-load current are small and consequently often neglected. Thinner lamination of the core steel reduces eddy current losses.

The biggest contributor to no-load losses is hysteresis losses. Hysteresis losses come from the molecules in the core laminations resisting being magnetized and demagnetized by the alternating magnetic field. This resistance by the molecules causes friction that results in heat. The Greek word, hysteresis, means "to lag" and refers to the fact that the magnetic flux lags behind the magnetic force. Choice of size and type of core material reduces hysteresis losses.

Answer 6

A transformer's iron losses depend on the magnitude of the flux which, in turn, is proportional to voltage, while its copper losses depend on the winding currents. As both iron and copper losses contribute to the maximum operating temperature of the transformer, it follows that a transformer must be rated in terms of voltage and current. In alternating current systems, the product of voltage and current is apparent power, expressed in volt amperes.