2.1 scope of operating voltage

In general, the operating voltage should not be higher than 105% of the rated voltage of the operating tap. For special use, it is allowed to operate at no more than 110% of the rated voltage.

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), press U (%) = 110-5k

Limit the operating voltage U.

2.3 influence and harm of high voltage on transformer

When the power supply voltage increases, the magnetic flux Ф m increases, which makes the excitation current im increase. The excitation current is reactive current, so the reactive power increases and the active power allowed to pass through the transformer is reduced.

In addition, with the increase of voltage and magnetic flux, the iron core will be saturated and overexcitation will be produced, resulting in the distortion of voltage and magnetic flux waveform of transformer (forming peak wave), and the high-order harmonic component will increase, which will increase the additional loss of motor and line, generate resonance overvoltage of the system, and damage the insulation of electrical equipment. At the same time, the high-order harmonic will interfere with the nearby communication lines.

For the transformer itself, due to the voltage rise, the transformer will produce over excitation. The over excitation of the transformer will inevitably lead to the overheating of the transformer core, which will lead to the aging of the core insulation, reduce the service life of the transformer and even burn the transformer.

2.4 causes of transformer overvoltage

After the power system is disconnected due to accident, load rejection overvoltage, ferromagnetic resonance overvoltage of some systems, improper adjustment of transformer tap changer gear, no-load transformer at the end of long line or other operation, generator frequency less than rated value, premature increase of excitation current, self excitation of generator, etc.

2.5 voltage regulation of transformer

The operation voltage of transformer is adjusted by tap changer. The tap changer of transformer can be divided into two types: off excitation voltage regulating switch and on load voltage regulating switch.

The non excitation voltage regulating switch can be shunted and adjusted only when the power is cut off. When adjusting the single-phase switch, it is necessary to pay attention to the consistency of the three-phase gears. After the adjustment, the transformation ratio test must be conducted. Only when the three-phase gears are consistent can the power be transmitted.

The on load tap changer can be tapped and adjusted under live condition. The following regulations shall be observed when adjusting the on load tap changer: the change of tap position, voltage and current shall be monitored during voltage regulation step by step.

The three-phase synchronous operation is suitable for the switch installed in three-phase transformer or on load tap changer of single-phase transformer group.

When the on load tap changer operates in parallel, its voltage regulation should be carried out step by step in turn.

When the on load tap changer and the off load tap changer operate in parallel, the tap voltages of the two transformers should be as close as possible.

2.6 transformer capacity during voltage regulation

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

The transformer capacity remains unchanged when the tap changer is changed within ± 5% of the off load voltage regulation range; when the on load voltage regulation range is large, such as ± 7.5% and ± 10% tapping range, when the maximum negative tapping is - 7.5% and - 10%, the transformer capacity shall be reduced accordingly due to the limitation of conductor current. If the manufacturer does not specify, the transformer capacity will be reduced by 2.5% and 5%.