Soft-starting measures for transformer no-load power transmission

乱世煮酒论天下

Soft-starting measures for transformer no-load power transmission [Copy link]

A 200kVA transformer is prone to tripping during the power-on process due to the small capacity of the circuit breaker in the distribution cabinet. One solution I have thought of is to perform a soft start like a motor start, with resistors in series on the input side. When the secondary voltage reaches a certain value, these starting resistors are bypassed. How should the voltage during the power-on process be analyzed?

Transformer, primary coil N1, secondary coil N2, ignore the coil resistance, primary coil connected in series with resistor R, secondary coil connected with nothing. In this case, the transformer should not be treated as a simple inductor. According to the ideal coupling condition and coupling coefficient , the three-phase transformer is treated as a delta connection. Let's look at one phase first. The actual circuit is an AC circuit with a resistor and an inductor in series. At that time, the average inductance between phases was measured by a bridge at 8mH. A 40Ω resistor was connected in series. According to the voltage drop divided by the circuit inductance , the number obtained was small, much smaller than the line voltage of the starting resistor after the actual measurement at that time. I remember that the line voltage of the starting resistor after the input end was about 300V one minute after power-on. What is the reason? Even if the mutual inductance is taken into account, it should not be so large?

maychang

When the transformer is powered on, the large inrush current may cause the circuit breaker to trip. The power department's practice is to wait for a while after the circuit breaker trips and then close the circuit breaker. If the circuit breaker trips again after the second closing, wait for a while and then close the circuit breaker for the third time. The power department does not have a practice of connecting a series resistor to allow the transformer to "soft start".

乱世煮酒论天下

maychang posted on 2024-7-9 08:05 When the transformer is powered on, the excitation inrush current is large, which may cause the circuit breaker to trip. The power department's practice is to pause for a while after tripping and then close the circuit breaker. If the circuit breaker trips again after the second closing...

It is not the power sector, but the industrial equipment manufacturer, transformer buffer, such as the transformer of the high-voltage inverter uses this method. It is not important whether or not to use this method. Analyze this process, after the transformer is unloaded and a resistor is connected in series on the input side, how should the voltage at the input end of the transformer be considered and calculated?

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"The short circuit breaker capacity of the power distribution cabinet is small, and it is easy to trip during the power-on process? If the rated current of the circuit breaker is less than the actual load current, or the setting value of the thermal relay is too small, etc., it may trip."

"I thought of a way to start the motor softly, with resistors in series on the input side, and bypass these starting resistors when the secondary voltage reaches a certain value,"

A series resistor on the primary side can limit the inrush current, but if the resistor value is not selected properly, it may not effectively reduce the current peak or may cause excessive voltage drop and power loss.

maychang

Luanshi Zhujiu Luntianxia published on 2024-7-9 08:38 It is not the power department, but the manufacturer of industrial equipment, transformer buffer, such as the transformer of high-voltage inverter uses this method. Is there any way to use this...

[Look at one phase first. The actual circuit is an AC circuit with a resistor and an inductor in series. The average inductance between phases was measured by a bridge and was 8mH. A 40Ω resistor was connected in series.]

This view is incorrect.

The three primary windings of a three-phase transformer are wound on a three-phase iron core (three-column iron core or five-column iron core), and there is magnetic coupling between them. Select any two of the three ends of the corner-connected winding and measure the inductance between these two points. This is not the inductance of a single winding, but also includes the inductance of the other two windings (the two inductors are connected in parallel with the inductance of the winding you think is being measured). This is one. You use a bridge to measure inductance. The voltage applied by the bridge to the winding is very small, and the current passing through the winding is also very small, so the magnetic flux density in the iron core is also very small. However, the iron core, especially the silicon steel sheet iron core, is not linear. Its magnetic permeability is relatively low when the magnetic flux density is very small (called initial magnetic permeability). The magnetic permeability increases with the increase of magnetic flux density (which can be several times or even more than ten times the initial magnetic permeability). When the magnetic flux density increases to a certain extent, the magnetic permeability begins to decrease rapidly (called magnetic saturation). Therefore, the inductance measured by the bridge may be quite different from the inductance of the transformer when it is working. The inductance during normal operation is larger than the inductance measured by the bridge, which may be several times or even more than ten times. This is the second. The frequency used in bridge measurement of inductance is higher than the power frequency of 50Hz. The higher the frequency, the greater the influence of the distributed capacitance of the winding, and the measured inductance value decreases. This is the third. Therefore, the view of "checking one phase first among three phases" is wrong.

maychang

Luan Shi Zhu Jiu Lun Tian Xia published on 2024-7-9 08:38 It is not the power sector, but the industrial equipment manufacturer, transformer buffer, such as the transformer of the high-voltage inverter uses this method. Is there any way to use this...

This is a small three-phase three-column transformer. As can be seen in the figure, the three windings are delta-connected and are mounted on a three-column iron core. When you measure the inductance of the red-green winding (the left winding), the red-yellow winding (the right winding) and the green-yellow winding (the middle winding) are connected in parallel to the red-green winding. Obviously, the measured inductance of the red-green winding is the inductance of the left winding in parallel with the inductance of the middle winding and the right winding.

Considering that the magnetic permeability of the silicon steel core is very small when the magnetic flux density is very small, it is not very meaningful to measure the inductance between two terminals using a bridge.

乱世煮酒论天下

maychang published on 2024-7-9 09:22 Luan Shi Zhu Jiu Lun Tian Xia published on 2024-7-9 08:38 Not the power sector, industrial equipment manufacturers, transformer buffers, such as high-voltage inverters...

I see, it's the transformer like the one you posted, it's a step-up transformer, there are multiple sets of taps on the secondary side, it should be corner-connected, and there is no neutral point lead out

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After discussing and brainstorming, I really learned a lot.