Step Up AC Transformer: Ratio, Ratings and Real Output

To understand the step up ac transformer, first identify the two sides according to the actual power direction: the primary winding connected to the power supply and the secondary winding supplying power to the load. When the secondary rated voltage is higher than the primary rated voltage in the same operating direction, it is called a boost. The primary and secondary annotation input and output relationships are not fixed and are equal to the low-pressure side and the high-pressure side.

First use the ratio to determine the pressure increase ratio

The voltage ratio of a single-phase ideal transformer can be written as V2/V1=N2/N1; a ratio greater than 1 is a boost, and less than 1 is a step-down. Taking the increase from 400 V to 10 kV as an example, the ideal ratio is 25. Ignoring loss time, the output current is approximately 1/25 of the input current, and the current relationship I2/I1=N1/N2 is opposite to the direction of voltage change.

At the same transmission power, increasing the line voltage will reduce the line current; the line resistance loss changes with the square of the current.

The input and output of an ideal transformer are considered to have equal power, but the actual output active power of the device is lower than the input, and the efficiency is less than 100%. Boost is not equal to power gain, nor does it mean that the transformer’s own load loss will necessarily decrease; if the current on the high-voltage side decreases, the loss will still be affected by the number of winding turns, conductor resistance, magnetic leakage and insulation structure.

Rated ratio not equal to the voltage at the loaded terminal

Rated ratio is a design parameter under specified tap position and rated conditions. The no-load measured ratio, no-load terminal voltage, and loaded terminal voltage are not the same concept. Source voltage deviation, load voltage drop, transformer impedance, and tap position all affect the output; the on-load terminal voltage is also related to winding resistance, leakage resistance, load current, and power factor. When the inductive load increases, the secondary terminal voltage usually decreases; for a specific capacitive load, the voltage drop may be reduced or even the terminal voltage may be increased.

Three-phase devices should not confuse line voltage and phase voltage. Rated line current can be calculated as I=S/(√3U); at the same capacity, the rated current on the high-voltage side is less than that on the low-voltage side. The √3 relationship in star and triangle connections affects the actual winding voltage and rated current, while the connection group determines the phase relationship between the primary and secondary line voltages, affecting the parallel connection of equipment or the connection of existing systems. The insulation on the boost side must also be determined according to the maximum voltage and overvoltage conditions of the system. The impact resistance requirements of the high-voltage winding cannot be reduced because the input side voltage is low.

Therefore, device specifications cannot be written as just “step-up”. Capacity, rated voltage and current on both sides, frequency, number of phases, connection group, impedance, tap, insulation, cooling and environmental conditions are all necessary information to determine whether the equipment is matched.

Isolation and reverse use should be determined separately

Boost only indicates the relationship between input and output voltages, not whether the windings are isolated. Independent primary and secondary windings can provide electrical isolation; the primary and secondary windings of an autotransformed structure share some windings and do not have the same isolation capability.

Many two-winding transformers can transfer energy in reverse on electromagnetic principles, but the principle of reversibility does not mean that a finished product can still meet the rated output after being reversed. The original voltage reduction design is to compensate for the number of turns with load voltage reduction arrangement. After reverse connection, the high voltage output may be lower than the result calculated based on the nameplate ratio; the original tap purpose and gate closure flow may also change, and protection needs to be re-coordinated. Only when the product documentation explicitly allows it, or after the manufacturer confirms the specific model, connection method, capacity, frequency, and environment, can it be determined whether it can reverse boost the voltage.

The power frequency transformer does not change the frequency

Conventional industrial frequency transformers rely on alternating magnetic flux to transmit AC energy, with the same input and output frequencies. Stable DC cannot be directly connected to windings designed for AC to complete voltage boosting, which may cause overcurrent and overheating; DC voltage boosting requires power electronic devices such as DC/DC converters.