Transformer inrush current generation mechanism and suppression measures of

Abstract: transformer as an AC power system electrical equipment, its normal running directly related to the security of the system. Differential protection as the main protection of the transformer magnetizing inrush current is one of the key factors affecting the correct action. Article analyzes the transformer magnetizing inrush current and its characteristics, for example, a single-phase transformer magnetizing inrush mechanism, given the common measures of suppression.

Keywords: transformer inrush current, the second harmonic dead angle a transformer inrush current and characteristics of stationary elements of the transformer is made based on the principle of electromagnetic induction, AC transmission system for voltage conversion electrical equipment when the aggregate on the circuit breaker to the transformer charging, and sometimes can be observed to the transformer ammeter pointer swing Subsequently, quickly returns to the normal no-load current value, the inrush current is typically known as the magnetizing inrush.

In general, the transformer magnetizing inrush current has the following characteristics: First, the waveform presents spire shape, that which contains a considerable component of the non-periodic component and the higher harmonic component, wherein the higher harmonic mainly to the secondary and tertiary and, over time, a second harmonic content may exceed the fundamental component of more than half a second, the voltage of the magnitude of the magnetizing inrush current Load Transformer directly related to the initial phase angle for a single-phase transformer to that, when voltage Zero invested, the the magnetizing inrush maximum amplitude. 120 degree phase difference between the phases of the three-phase transformer inrush current are not the same as the third, in the first few waveform, the inrush current will occur intermittently angle. Fourth, the inrush current decay time constants and transformer impedance, capacity, and the core material and so related.

2, the magnetizing inrush current generation mechanism transformer magnetizing inrush current is saturated by the transformer core caused by the core slope of the magnetization curve of the core is unsaturated, the magnetizing current is approximately zero, once the core saturation, the slope of the magnetization curve becomes small, the current vs. significant flux linear growth, eventually evolved into the magnetizing inrush.

Case magnetizing inrush current generation mechanism following a single-phase transformer load closing set transformer closing at time t = 0, is applied to the transformer voltage:

Further, the relationship between the transformer voltage and magnetic flux as: (2

Therefore: (3

The formula (3 type for the steady-state flux formula for transient flux core remanence, and the closing moments of the voltage.
Count and costs, and processes commonly used in modern power transformer saturation magnetic flux is generally set to 1.15 to 1.4, while the operating voltage of the transformer should generally not exceed 10% of the rated voltage, therefore, the transformer steady state during normal operation, the magnetic flux does not exceed the saturation flux core will not saturate. transient process, such as the transformer load closing, due to the remanence, run the flux, there may be greater than the saturation magnetic flux, resulting in saturation of the transformer, for example, most serious voltage at time zero, closing, if the core remanence, non-periodic flux after half cycle flux reached, will be greater than the saturation flux caused by the transformer is heavily saturated. posted on Free papers Download Center
3, suppression measures to prevent transformer inrush current caused by the differential protection measures for the three-phase power transformers used in the scene, mainly the following categories.

3.1 speed saturated intermediate current transformer differential protection malfunction caused in accordance with the to escape maximum unbalanced current tuning, the the belt speed saturated principle of differential protection to reduce non-periodic component protection, such as BCH-2 is an enhanced type speed saturated the intermediate converter differential protection. central part of this differential protection with short circuit coil saturation middle of the converter and the differential current relay. the shorted turn presence of aperiodic component current the operating current of the relay is greatly increased, thereby improving the the transient unbalanced current performance escape when the inrush current and the external short-circuit. using a BCH-2 type differential protection more to determine the turns of the short circuit of coil turns, to pay attention to escape inrush better the performance, but the long delay internal short circuit relay action, small and medium-sized transformers multiples of magnetizing inrush and internal fault aperiodic component decay faster, lower protection action requirements, the general election than large number of turns, and the large transformers, internal inrush current multiples small, non-periodic the component attenuation slow, and require protection action is fast, you should choose a smaller number of turns. suitability final selection of tap transformer airdrop test to determine the same time, the minimum short-circuit current sensitivity test should be an internal short circuit to if does not meet the requirements, should be selected with the braking characteristics of the differential protection the same principle as the BCH-2 DCD-2 differential Following protection electrical constitute the differential.

Overall, the longitudinal differential protection with speed saturation principle action current, low sensitivity, and transformer internal fault, due to the presence of non-periodic component delay action, has gradually be eliminated.

Containing the second harmonic characteristics the 3.2 second harmonic restraint in accordance with the magnetizing inrush current, the design of the second harmonic restraint, once protection is detected the poor stream containing the second harmonic is greater than the protection setting value on lockout protection relay, preventing caused by magnetizing inrush current protection action. braking action second harmonic criterion can be written as: (4
Which were poor flow in the fundamental and second harmonic component amplitude of the second harmonic restraint than field applications, based on operating experience and load switching test, generally in accordance with escape various The magnetizing inrush current, the smallest content of the second harmonic tuning Generally, secondary harmonics suppression ratio can be set to (15%, 20%.

Second harmonic restraint differential protection principle is simple, easy to debug, high sensitivity, and is widely used in the current transformer differential protection., Static var compensator device capacitance component system fault transient current second harmonic content, resulting in differential protection operation speed is affected load closing before the transformer there is a failure, closing after the fault phase fault current, non-fault phase magnetizing inrush current, using three phase or the door brake the program when the differential protection is bound to be atresia. magnetizing inrush current decay is very slow, the protected action time may be as long as hundreds of milliseconds which is the second harmonic restraint method main drawback.

3.3 intermittent angular discrimination method mentioned earlier, the first few waveform, the inrush current will appear intermittent angle. Steady-state differential current transformer internal fault into the differential relay is a sine wave, not interruptions angle. Intermittent the angular discrimination method is the use of this feature to identify inrush current and fault current, and whether there is intermittent angle by detecting the difference between the current waveform, when intermittent angle is greater than the setting value differential protection atresia intermittent brake protection setting angle value is generally located 65 °. three-phase transformer for the Y / d Wiring asymmetric inrush discontinuity angle is relatively large, intermittent angle closure element is capable of reliable operation, and an adequate margin, while the symmetry inrush discontinuity angle will be less than 65 ° further reduce the setting value is not a good method, since the sensitivity and movement speed of the setting value is too small to affect the internal fault. As symmetry inrush pulse width equal to 120 °, and the fault current (sine wave width 180 °, intermittent angle criterion based on the additional criterion of a secondary reaction wave wide large wave in 140 ° (20 ° margin differential protection principle atresia. intermittent angle due according to phase atresia, internal fault in the transformer closing fast action it is superior than the second harmonic restraint (three-phase or door brake for large transformers, both using the two principles longitudinal differential protection, can play a complementary advantages, speed up internal fault movement speed, after all, a good configuration program.

References [1] Wang Weijian main electrical relay protection principle running [M] Beijing: China Electric Power Press, 1996.
[2] Wang Weijian, Hou Ping-Yun large units relay theoretical foundation [M] Beijing: China Electric Power Press, 1989. Posted in the free papers Download Center

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