Transformer operating voltage regulations

2.1 Operating voltage range

The operating voltage should generally not be higher than 105% of the rated voltage of the operating tap. For special usage conditions, operation at no more than 110% of the rated voltage is allowed.

2.2 Relationship between current and voltage

The relationship between current and voltage is as follows:

When load current/rated current = K, (0≤K≤1), u(%) = 110-5K²

The operating voltage U is limited.

2.3 The impact and harm of excessive voltage on transformers

As the power supply voltage increases, the magnetic flux Фm increases, thereby increasing the excitation current Im. The excitation current is a reactive current, so the reactive power increases and the active power allowed to pass through the transformer decreases.

In addition, as the voltage increases, the magnetic flux increases, causing the core to saturate and produce over-excitation, causing the voltage and magnetic flux waveforms of the transformer to be distorted (forming peak waves), and the high-order harmonic components to increase, thereby increasing the additional losses of the motor and lines, generating resonant overvoltage in the system, and damaging the insulation of electrical equipment. At the same time, high-order harmonics will interfere with nearby communication lines.

For the transformer itself, the voltage increase will cause over-excitation of the transformer, which will inevitably cause the transformer core to overheat, causing the core insulation to age, reducing the life of the transformer or even burning the transformer.

2.4 Causes of overvoltage in transformer

After the power system is disconnected due to an accident, some systems may experience load shedding overvoltage, ferromagnetic resonance overvoltage, improper adjustment of the transformer tap changer gear, no-load transformers at the end of long lines or other operations, premature addition of excitation current when the generator frequency does not reach the rated value, and generator self-excitation.

2.5 Voltage adjustment of transformer

The operating voltage adjustment of transformer is realized through tap changer. The transformer tap changer is set up in two types: off-excitation voltage adjustment and on-load voltage adjustment.

Off-excitation voltage adjustment can only be performed under power outage conditions. When adjusting the gear, it is necessary to pay attention to the consistency of the three-phase adjustment gear for single-phase switches. After adjusting the gear, the transformation ratio test must be performed to confirm that the three-phase gears are consistent before power can be supplied. On-

load voltage adjustment can be performed under energized conditions. When adjusting the gear, the following regulations should be observed: the voltage should be adjusted step by step, and the changes in the tap position, voltage, and current should be monitored during the voltage adjustment.

The switches installed in phases of three-phase transformers, or the on-load tap changers of single-phase transformer groups, should be operated synchronously in three phases.

When on-load voltage adjustment transformers are operated in parallel, their voltage adjustment operations should be performed synchronously step by step in turn. When on-load

voltage adjustment transformers and off-excitation voltage adjustment transformers are operated in parallel, the tap voltages of the two transformers should be as close as possible.

2.6 Transformer capacity during voltage adjustment

When the voltage is adjusted, the capacity of the transformer is stipulated as follows:

when the tap is changed within the range of ±5% without excitation voltage regulation, the transformer capacity remains unchanged; when the on-load voltage regulation range is larger, such as the ±7.5% and ±10% tap range, at the maximum negative tap, that is, at -7.5% and -10% tap, due to the limitation of the conductor current, the transformer capacity should be reduced accordingly. If the manufacturer has no regulations, it is usually reduced by 2.5% and 5%.