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DC transmission is becoming more and more popular.
is currently a mature technology for long-distance and large-scale power transmission.
However, DC transmission will also have some by-products and side effects, which is worth paying attention to.
, which is the abnormal noise of power transformer caused by DC bias, is relatively common.
transformer noise
power transformer has noise. It mainly includes cooling system noise and body noise.
cooling system noise comes from the fans and oil pumps of the cooling system, so there is no need to discuss it here.
transformer body noise is the hysteresis stretching and contraction caused by the core silicon steel sheet under the action of an alternating magnetic field. The electromagnetic suction force generated by magnetic leakage between the joints of the silicon steel sheet and the laminate caused vibration of the core , and the magnetic leakage generated by the load current in the winding will cause vibration of the winding and oil tank.
DC bias
DC transmission system When running in the polar mode, since the earth is used as a loop, a DC current flows through it. Therefore, if the neutral point of the AC power transformer near the grounding pole is grounded, DC will flow into the transformer winding, as shown in the figure below.
Power transformer windings contain DC, which can easily lead to the iron core being saturated and working in a biased state, which will cause changes in the hysteresis characteristics of the silicon steel sheet.
The figure below is a typical hysteresis curve with the DC content. It can be seen that with the increase of DC content, the peak of the hysteresis curve characteristics of silicon steel sheets increases significantly.
In addition, DC bias will also increase magnetic leakage inside the transformer, and the electromagnetic force between the silicon steel sheets, windings and turns will increase, causing the transformer vibration to increase.
In summary, under the influence of DC bias, the noise of the transformer will increase.
Compared to the normal noise of the power transformer, I tend to call it anomaly noise.
Characteristics of noise caused by DC biased magnetism
The abnormal noise of power transformer caused by DC biased magnetism is summarized in several characteristics, and you can pay attention to it.
1. The noise size is not related to the transformer load;
2. The noise is discontinuous and has obvious intermittent characteristics, which is mainly related to the operating mode of the DC transmission system. For example, the noise increases for several hours or days, and then returns to normal on its own.
Third, the increase in noise may be accompanied by the increase in the transformer temperature and the reactive loss.
4 The oil chromatography and other test data of the transformer are normal.
Example of DC biased magnetization triggering noise
transformer has specifications for abnormal noise generated by DC biased magnetization.
We can measure the DC current of the neutral line, the normal transformer noise and abnormal noise, and approximately determine whether the increase in noise is related to DC bias.
The following figure is the curve relationship between the DC current content in the winding and the increase in the transformer sound level value given by the GB/T1094 transformer sound level measurement.
Assuming that an abnormal noise increase in a three-phase power transformer.
transformer high voltage side rated current 472.4A, no-load current 0.049%, converted to a famous value of 0.23A.
High voltage side rated current | No-load current percentage | No-load current is a value | ||
No-load current is a name value | ||||
472.4A | 0.049% | 0.23 |
measure the neutral linear DC current on the high voltage side of the transformer is 0.6A, which is converted to the single-phase winding DC current of 0.2A, and the unity value relative to the no-load current is 0.87.
View the graph given by GB above, the added value of transformer noise is about 10dB (A).
In this way, if the transformer noise abnormality and the normal data can basically meet the above calculation relationship, it can be used as a criterion for the abnormal increase in the power transformer noise caused by DC bias.
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