Analysis of the influence of the hottest DC bias o

Analysis of the influence of DC bias on transformer operation

the operation mode of DC unipolar earth circuit of Tianguang transmission network has led to the intensification of vibration and noise of some transformers inside, and similar events have occurred again after the operation of Sanguang DC transmission line. Some of the affected transformers are near the DC grounding electrode of the converter station, while others are far away from the grounding electrode. At the beginning of the incident, it was suspected that it was mainly caused by the harmonic current of the converter station, and then gradually focused on the DC bias of the neutral grounding transformer. While detecting the vibration, noise and harmonics of the power and power plant transformer, it also detected the magnitude of the DC current of the neutral line, hoping to find clues. However, how does the earth current flow into the transformer neutral? What factors are related to its size? Does DC bias affect the safe operation of transformer? How to effectively reduce the DC current of transformer neutral? People are trying to find the answer. Since monopole is considered by many countries as one of the operation modes of DC transmission, and more DC lines will be put into operation in the construction and planning of southern power, it is urgent to study the interference of geodetic DC on AC system and put forward an engineering scheme to reduce the impact of geodetic DC on AC equipment safety on this basis

the author has done some research on DC bias and transformer vibration, the solution method of DC bias and the technical measures to reduce transformer vibration. In this paper, the principle simulation method is used to demonstrate the influence of DC bias on transformer operation, analyze the mechanism of the transformer itself to alleviate the external DC influence, and use simulation examples to illustrate the correlation between DC unipolar earth current and DC bias of transformer.

1 DC bias and transformer vibration

DC bias is the main reason for the intensification of transformer vibration. Although people have some understanding of the negative impact of the operation mode of DC unipolar earth circuit, the incident of intensified transformer vibration in Nanfang power shows that the industry will face differentiation, and this impact is greater than expected

according to the field monitoring data, the vibration noise and harmonic of the transformer increase with the increase of the DC current of the neutral line, and the magnitude of the DC current of the neutral line is roughly related to the power transmission of the DC line and the polarity of the DC line under the unipolar earth operation mode

this phenomenon can be explained by the familiar saturation magnetization characteristics of the transformer core: the DC current flowing through the winding becomes a part of the transformer excitation current. This DC current biases the transformer core, changes the working point of the transformer, and moves a part of the original magnetization curve working area to the core magnetic saturation area. As a result, the total excitation current becomes a spike wave, which eventually leads to the increase of transformer vibration

1.1 linearized simulation of magnetic saturation characteristics

in order to highlight the main contradiction, we ignore the core hysteresis loop and express the nonlinear saturation characteristics of the core with the broken line of the two sections of slope in Figure 1, in which the slope k  1 of line OA is the normal slope of the linear section of the magnetization curve, and the slope K2 of line AB is the slope of the saturation section of the magnetization curve. The excitation current waveform obtained by using the broken line magnetization characteristic preserves the main characteristics of the original excitation current waveform, so as to ensure the effectiveness of the theoretical simulation

total DC current in the excitation current

1.2 bias current and transformer operating point

from the data in Table 1, it can be seen that the larger the bias DC current is, the more serious the current distortion is, and the faster the DC component in the distorted excitation current increases. From this, we can draw an important conclusion: only part of the DC current forced into the transformer winding becomes the magnetic bias current, and the other part is converted into the DC component after the excitation current distortion (which can also be understood as being offset by the DC component after the excitation current distortion). This characteristic of transformer is beneficial to mitigate the influence of external DC current to a certain extent. Of course, this self-control ability of the transformer is limited, which will affect the service life of the device at the cost of increased vibration and noise

the selection of the normal excitation characteristic operating point of the transformer and the operating voltage of the transformer bus also affect its ability to withstand DC current. If the normal operating point is selected at the saturation point of excitation characteristics, or the transformer operating voltage is higher than the rated voltage, the magnetic circuit tends to be saturated earlier, so the ability to withstand DC bias decreases; On the contrary, when the normal working point is far away from the saturation point of excitation characteristics, or when the operating voltage of transformer bus is lower than its rated voltage, the magnetic circuit is saturated later, so the ability to withstand DC bias is correspondingly improved

1.3 example analysis

take a large capacity double winding single-phase transformer group with rated voltage of 525kv and 26kv, rated current of 1247a, and no-load excitation current of 0.4% per unit value as an example, Try to calculate the change of transformer excitation current when the neutral line flows into 30A DC current. Assuming that the saturation of the magnetization characteristics of the transformer is 20, and the peak value of the rated no-load excitation current is located at the turning point of the saturation curve, there is

, that is, the peak value of the excitation current increases by 7.5 times

it should be noted that the above is only a schematic analysis. In actual calculation, only relying on the nameplate parameters of the transformer can not obtain the saturation data of its magnetization characteristics, and it is difficult to master the position of its normal working point. To improve the accuracy of quantitative analysis, it is necessary to supplement other equipment parameters

1.4 DC bias and transformer vibration

the influence of excitation current distortion on transformer vibration involves winding electrodynamics, hysteresis expansion in iron core, component vibration, etc., which is very complex. The relevant quantitative analysis needs to be further studied with the participation of transformer design experts and transformer operation experts. Generally speaking, the following explanation can be given: excessive DC bias makes the saturation of transformer core tend to be serious, resulting in increased magnetic flux leakage, intensified vibration and increased noise

the ability of the transformer to withstand DC bias during operation is related to the magnetic flux density and mechanical strength of the structure selected in the design, as well as its position in the electricity and the load level of the transformer at that time. It should be pointed out that the structural design of the transformer has considered that it can withstand the impact of short-circuit current electrodynamic force. Therefore, the increase of vibration caused by excitation current distortion will not affect the safety of the equipment in a short time, but the long-term continuous excessive vibration and the overheating of local parts caused by the increase of magnetic leakage may have an adverse impact on the fasteners of the transformer. The research on the ability of transformer to withstand DC bias should be carried out with the participation of transformer design experts and transformer operation experts, but the urgent task is to take effective vibration reduction measures for the actual transformer whose vibration and noise exceed the design value to ensure the safety of power equipment

it should also be pointed out that the above analysis is carried out for single-phase (or three-phase five column) high-capacity transformers with the same DC magnetic circuit and fundamental frequency AC magnetic circuit. The three-phase three column transformer has a large DC magnetic resistance, so it has a strong ability to withstand DC bias.

2harmonics and DC bias

harmonic monitoring is part of the electrical safety measures after the DC transmission project is put into operation. Since the converter station is the harmonic source, it is absolutely necessary to strengthen harmonic monitoring

it must be pointed out that the harmonic elimination equipment and harmonic elimination measures of modern converter stations have been very perfect. Under different operation modes, various harmonics generated in the process of converter have been well filtered, and the harmonic component flowing out of converter stations is very small, which usually conforms to national standards. The same is true under the operation mode of unipolar earth circuit. In a backward step, it is assumed that the unipolar earth current contains a certain number of harmonic components, so the DC current flowing into the neutral line of the transformer also contains harmonics, but the amplitude of the harmonic component is small, and its energy is not enough to strengthen or induce the vibration of the transformer

from the above analysis, the unipolar earth current flows into the winding through the neutral point of the grounding transformer, causing serious distortion of the excitation current, including harmonics. Table 2 lists the main harmonic components contained in the distorted excitation current. It can be seen that the greater the DC bias, the greater the harmonic component of the excitation current, and its trend is consistent with the field measurement results. It can be seen that the harmonic increase under the DC unipolar earth operation mode is not caused by the harmonic leakage of the converter station, but the by-product of DC magnetic bias, which is the main cause of harmonic increase.

3 origin of neutral line DC current

Where does transformer neutral line DC current come from? What factors are related to the magnitude of DC bias? The following principled answers can be made to the question: since the increase of transformer noise is only produced in the unipolar earth operation mode, the DC bias that leads to abnormal transformer noise must be related to the earth current in the unipolar earth operation mode, the grounding electrode and the polarity of the direct current line; Since the earth current flows into the winding through the neutral point of the grounding transformer and enters the AC, this phenomenon must also be related to the DC path of the AC. However, using traditional methods to analyze the vibration cases of Nanfang electric transformer can not give a satisfactory explanation, and it is difficult to carry out quantitative analysis. In order to overcome the limitations of traditional methods, the author analyzes the causes of DC bias from another perspective, and proposes a mathematical model and algorithm for solving the DC current of transformer neutral line. Limited by space, the relevant content will be described in another article of the author. Here, only the theoretical simulation results are listed, from which some conclusions are drawn

in the simulation data, the DC current component of the neutral line of the main transformer is directly proportional to the transmission power of the DC line, and changes direction with the change of the polarity of DC transmission. The two DC transmission lines operate unipolar at the same time with the same polarity, and the DC current components of the neutral line of the main transformer are superimposed or offset each other; When the operating polarity is opposite, the DC current components of the neutral line of the main transformer offset or superimpose each other, and the trend is consistent with the measured results in the literature. It can be concluded that when several DC transmission lines adopt the monopole 100 (3) 00hz as the ground circuit of high-frequency fatigue testing machine at the same time, it may have a greater impact on the grounding transformer

4 conclusion

to sum up, it is concluded that:

a) DC bias is the main reason for the increased vibration of transformer under the operation mode of DC unipolar earth circuit. Excessive DC bias makes the transformer core saturation tend to be serious, and the magnetic leakage increases, resulting in increased vibration and noise

b) only part of the earth DC current flowing into the transformer winding becomes the magnetic bias current, and the other part is converted into the DC component of the excitation current distortion. This characteristic of the transformer is conducive to reducing the impact of external DC current

c) the ability of the transformer to withstand DC bias during operation is related to its designed magnetic flux density, mechanical strength and structure, as well as the connection mode of the transformer, its position in the electricity and the current load level. Transformer design experts and transformer operation experts should be invited to participate in the study of transformer's ability to withstand DC bias. However, when the actual transformer vibration and noise exceed the design value, it is urgent to reduce the vibration and noise

d) DC bias is related to earth current, line polarity, grounding electrode, and DC path of AC. When multiple DC lines operate unipolar at the same time, the influence on the transformer may be superimposed or offset for a common experimental machine manufacturer

e) the technical measures to reduce DC bias should be found from the factors affecting DC bias. Before the implementation of the engineering measures to effectively reduce the DC bias, the change of