Abstract: As an important electrical equipment in the AC power system, the normal operation of the transformer is directly related to the safety of the system. Differential protection is the main protection of the transformer, and the inrush current is one of the key factors affecting its correct operation. This paper analyzes the transformer magnetizing inrush current and its characteristics. Taking the single-phase transformer as an example, the mechanism of magnetizing inrush current is analyzed, and the common suppression measures are given.

Key words: transformer magnetizing inrush current second harmonic discontinuity angle

1. Transformer magnetizing inrush current and characteristics

Transformer is a static component manufactured according to the principle of electromagnetic induction, and is an important electrical equipment for voltage conversion in AC transmission system. When closing the circuit breaker to charge the transformer, sometimes, it can be observed that the pointer of the transformer ammeter swings greatly, and then quickly returns to the normal no-load current value. This inrush current is usually called the magnetizing inrush current.

In general, the transformer magnetizing inrush current has the following characteristics: First, the waveform presents a peak shape, indicating that it contains a considerable amount of aperiodic components and higher harmonic components, of which the second and third harmonic components are dominant. , and, over time, the second harmonic content of a certain phase may exceed more than half of the fundamental component. Second, the magnitude of the magnetizing inrush current is directly related to the initial voltage phase angle of the transformer no-load input. For a single-phase transformer, when the voltage zero-crossing point is switched on, the magnitude of the inrush current is the largest. Since there is a 120-degree phase difference between each phase of the three-phase transformer, the inrush current is not the same. Third, in the first few waveforms, the inrush current will have a discontinuous angle. Fourth, the time constant of inrush current decay is related to transformer impedance, capacity and core material.

2. Mechanism of exciting inrush current

The transformer magnetizing inrush current is caused by the saturation of the transformer core. When the iron core is not saturated, the slope of the magnetization curve of the iron core is large, and the excitation current is approximately zero; once the iron core is saturated, the slope of the magnetization curve becomes smaller, the current increases linearly with the magnetic flux, and eventually evolves into an excitation inrush current.

The following takes the no-load closing of a single-phase transformer as an example to analyze the mechanism of excitation inrush current. Assuming that the transformer is closed at time t=0, the voltage applied to the transformer is:

  (1)

In addition, the relationship between the transformer voltage and the magnetic flux is: (2)

Therefore: (3)

In formula (3), the first formula is the steady-state magnetic flux, and the last two formulas are the transient magnetic flux, which is the remanence of the iron core, which is related to the voltage at the closing time.

Taking into account the cost and process, the saturation magnetic flux of the commonly used power transformers is generally set at 1.15 to 1.4, and the operating voltage of the transformer should generally not exceed 10% of the rated voltage. Therefore, when the transformer is in steady state and normal operation, the magnetic flux will not exceed the saturation flux, and the core will not be saturated. However, in the transient process, such as when the transformer is closed at no load, due to the effect of residual magnetism, the operating magnetic flux may be greater than the saturation magnetic flux, resulting in saturation of the transformer. For example, the most serious one is when the voltage is zero-crossing and the switch is closed. If the core is remanent at this time, the non-periodic magnetic flux is reached after half a cycle, which will be much larger than the saturation magnetic flux, resulting in serious saturation of the transformer. 3. Suppression measures

For three-phase power transformers commonly used in the field, the measures to prevent differential protection caused by transformer inrush current mainly include the following categories.

3.1 Adopt fast saturation intermediate converter

When the differential protection is set according to avoiding the maximum unbalanced current, the differential protection with the principle of speed saturation can reduce the protection misoperation caused by the aperiodic component. For example, the BCH-2 type is an enhanced speed saturation intermediate converter. Differential Protection. The core parts of this differential protection are saturated intermediate converters with short-circuit coils and differential current relays. The existence of the short-circuit coil greatly increases the operating current of the relay when there is a non-periodic component current, thereby improving the performance of avoiding the excitation inrush current and the transient unbalanced current during external short-circuit. When using BCH-2 differential protection, it should be noted that the number of turns of the short-circuit coil is determined. For small and medium-sized transformers, due to the large inrush current multiple, the aperiodic component attenuates quickly when the internal fault occurs, and the protection action requirements are low. Generally, a larger number of turns is selected, while for large transformers, the internal inrush current multiple is small, and the aperiodic component is attenuated. If it is slow, and the protection action is required to be fast, a smaller number of turns should be selected. Whether the tap finally selected is suitable should be determined by the transformer airdrop test. At the same time, the sensitivity test should be carried out according to the minimum short-circuit current during internal short-circuit. If the requirements are not met, differential protection with braking characteristics should be selected. The same principle as the BCH-2 type also has the differential protection formed by the DCD-2 type differential relay.

In general, the longitudinal differential protection with the speed saturation principle has been gradually eliminated due to its large operating current and low sensitivity.

3.2 Second harmonic braking

According to the characteristics of the second harmonic contained in the exciting inrush current, the method of the second harmonic braking is designed. Once the protection detects that the second harmonic contained in the differential current is greater than the protection setting value, the protection relay will be blocked to prevent the excitation caused by the inrush current. protective action. The action criterion of the second harmonic braking can be written as: (4)

where and are the amplitudes of the fundamental and second harmonic components in the differential current, respectively, and are the second harmonic braking ratio. In field application, according to operating experience and no-load closing test, it is generally set according to the minimum second harmonic content under various excitation inrush currents. In general, the second harmonic braking ratio can be set to (15%, 20%).

The differential protection principle of the second harmonic braking is simple, easy to debug and high sensitivity, and is widely used in the current longitudinal differential protection of transformers. However, in a system with a relatively large capacitance component such as a static reactive power compensation device, there is also a large second harmonic content in the fault transient current, which affects the operating speed of the differential protection. If there is a fault in the transformer before the no-load closing, the faulty phase after closing is the fault current, and the non-faulty phase is the excitation inrush current. When the three-phase or gate braking scheme is adopted, the differential protection will be blocked. Because the magnetizing inrush current decays very slowly, the protection action time can be hundreds of milliseconds. This is also the main disadvantage of the second harmonic braking method.

3.3 The method of discontinuous angle identification

As mentioned earlier, in the first few waveforms, the inrush current will have a discontinuous angle. The steady-state differential current flowing into the differential relay during the internal fault of the transformer is a sine wave, and there will be no discontinuous angle. The method of discontinuous angle identification is to use this feature to identify the excitation inrush current and the fault current, that is, by detecting whether there is a discontinuous angle in the differential current waveform, when the discontinuous angle is greater than the set value, the differential protection will be blocked. The protection setting value of discontinuous angle braking is generally set to 65°. For the three-phase transformer with Y/d connection, the discontinuity angle of the asymmetrical inrush current is relatively large, the discontinuous angle blocking element can operate reliably, and the margin is sufficient; while the discontinuity angle of the symmetrical inrush current will be less than 65°. It is not a good way to further reduce the setting value, because the setting value is too small will affect the sensitivity and speed of operation in case of internal fault. Since the wave width of the symmetrical inrush current is equal to 120°, and the wave width of the fault current (sine wave) is 180°, an auxiliary criterion for the response wave width is added on the basis of the discontinuity angle criterion. When the wave width is greater than 140° The differential protection is also blocked at ° (with a 20° margin). Due to the principle of discontinuous angle, it can act quickly when the transformer is closed due to an internal fault due to the method of phase-by-phase blocking. This is an advantage over the second harmonic braking (three-phase or gate braking) method. For large transformers, the longitudinal differential protection of the two principles can be used at the same time, which can complement each other's advantages and speed up the action speed of internal faults, which is a good configuration scheme.

  references

[1] Wang Weijian. Principle and operation of relay protection of electrical main equipment [M]. Beijing: China Electric Power Press, 1996.

[2] Wang Weijian, Hou Bingyun. Theoretical basis of relay protection for large generating units [M]. Beijing: China Electric Power Press, 1989.