Reactive Power of Transformer: Loads, Losses, and kVA

Nameplate kVA, current present when the secondary is open, and reactive-power readings during operation are easy to mistake for the same kind of “transformer consumption.” Understanding reactive power of transformer starts with distinguishing reactive power, active equipment losses, and rated apparent power, then considering what no-load excitation and load power factor each indicate.

Distinguishing Power from Losses

Under sinusoidal steady-state conditions, apparent power S, active power P, and reactive power Q follow the power-triangle relationship. Q affects current, capacity use, voltage, and system power factor, but it is not an active loss measured in watts. A transformer’s rated capacity, stated in VA, kVA, or MVA, is the apparent power it can transmit continuously under specified cooling, temperature-rise, ambient, and frequency conditions. Winding heating is mainly affected by current, while insulation is affected by voltage; load active power alone does not represent these conditions.

For an actual transformer, input active power equals output active power plus the transformer’s own losses. Efficiency is output active power divided by input active power. Adding magnetizing reactive power to losses measured in watts mixes two different power quantities. Efficiency comparisons also need the same loading, power factor, temperature, and test method; values from different operating conditions should not be compared directly.

Two Components in No-Load Current

After AC is applied, the transformer needs magnetizing current to establish the core’s main flux. Magnetizing current continues even with the secondary winding open, and the core still incurs no-load loss. No-load current contains a reactive component needed to establish the core’s main flux and an active component that covers core loss. Its total value alone cannot directly give no-load loss in watts.

No-load loss mainly comes from core hysteresis and eddy currents and remains relatively constant when rated voltage and frequency are stable. Load loss includes winding resistance I²R loss and additional leakage-flux loss, and varies with load current.

Power Factor Under Load

For a three-phase transformer, rated apparent power can be expressed as S=√3UI, where U and I are rated line voltage and rated line current. At the same kVA, different load power factors can yield different active kW. When estimating capacity from a known load in kW, use the apparent-power basis and account for power factor, efficiency, harmonics, starting or shock loads, and future growth; comparing kW directly with nameplate kVA is not enough.

An ordinary transformer does not actively correct power factor. Input-side power factor is affected by the load, magnetizing current, and harmonics. Load power factor and short-circuit impedance both affect voltage regulation. Voltage regulation describes the change in secondary terminal voltage from no-load to a specified load; a specific value should be associated with the load current, power factor, temperature, and tap position.

Some specialized designs have additional functions. The third winding of certain three-winding transformers can connect reactive-power equipment. A phase-shifting rectifier transformer can use multiple secondary windings with different phase angles to cancel some low-order grid-side harmonics and improve input power factor. These are specific configurations and do not mean ordinary transformers automatically compensate for reactive power.