Wang Lei

(China Railway 16th Bureau Group Electrification Engineering Co.,Ltd.,Beijing 100018)

Abstract:The construction of city subway connects the whole city.People can reach every place in the city by taking subway, which brings great convenience to the travel of city residents.However, the abnormal rise of subway track potential during subway operation has brought some negative impacts on the normal operation of the subway and the personal safety of passengers.Therefore,the research on this issue is of great significance to the future development of subway.

Keywords:subway track;abnormal potential;research

With the rapid development of urban subway construction in our country, the phenomenon of abnormal rise of subway track potential has received more and more attention.Although the problem of subway track potential rising has existed for a long time, there is little research on track potential anomalies in our country,and there is no concrete and feasible theoretical guidance and successful experience.

I.Hazards Caused by Abnormal Elevated Potential in Subway Track

At present,the traction power supply mode of urban subway rail transit is rail return DC power supply, and this kind of power supply saves construction investment to a great extent.However,due to the resistance of the track itself,the voltage drops when current flows through the track,and the pressure difference between the track and the ground generates the track potential.It is impossible to completely isolate the track from the ground,which will lead to current flowing from the track to the ground,current leakage and serious corrosion of the buried metal steel structure.Nowadays, many subways have the problem of abnormal rise of rail potential[1].For example,the track potential of Nanjing Metro Line 1 is abnormally high.When the rail potential limiting equipment fails to contact the ground, once the cooperation between the rail potential limiting equipment and the frame protection is deviated,the protection frame is abnormal;which leads to the power failure of the catenary and causes great damage to the running of the subway.Such problems have already occurred in the subway operation in major cities,and even worse,casualties are more likely to occur.

II.Reasons for Abnormal Rise of Metro Track Potential

Among them,the rail potential itself will be affected by many factors,such as feed section,load sharing, load current, return line resistance,distribution and rectifier position,etc.However,there is a positive correlation between the length of the feed section itself and the rail potential.The longer the feed section is,the higher the rail potential will be.The load current will have an opposite effect on the rail potential.The lower the load current is,the higher the rail potential.Under the condition of setting the working condition,it can be found that there is also a positive proportional relationship between the track resistance and the return line resistance,and the larger the return line resistance,the greater the track resistance will be[2].

(i)Metro Rail Potential's Distribution

When the current flows through the subway track,a certain amount of current will leak into the earth,and there is excessive resistance between the rail and the earth,resulting in a certain potential difference between the rail and the earth.It is very difficult to calculate the rail potential existing between substations because the transitional resistance has a great relationship with the type of subway track,the conductivity of the environment in which the subway is located,the weather conditions and subway accessory equipment.Let's make a brief analysis of the potential distribution of subway tracks[3].

During the operation of the subway,the train gets current from the traction network,flowing to the negative pole through the track and the earth,and finally returns to the substation.When current flows through the track, some stray current will be generated,causing the track to change in the positive and negative range near the load and the substation,thus causing the track to deform.Two polar regions:an anode region and a cathode region.In the anode region,the current flows to the earth via the track;in the cathode region,current flows from the earth back to the track.There is no clear position in the range of anode and cathode areas,but varies with the position of the train[4].Specific analysis is shown in Figure 1.

Fig.1 Qualitative Analysis Diagram of Rail Voltage

At point A,the train receives current from the contact network,and the current flows to A via the train via the contact network.At this time,the current flowing through the track is the largest,then the current flows to points B and C and then returns to the substation.In this process,the current flowing through the track slowly leaks to the ground,forming an anode region,and the current on the track changes and gradually decreases.When the current flows through points E and F,the current in the ground flows back to the track one by one,thus generating a cathode.At points B and C,the grounding current and the track current meet and the current returns to the substation,then the track current at points B and C will become larger.If the difference in transition resistance between the train and the substation is insufficient,the rail potential from the train to the substation will show a high-low-,low-and high change trend.Therefore,there will be a large track potential around the train and the return point.On the subway platform,when passengers are ready to get on or off the subway,once the passengers touch the train and the ground at the same time, the passengers'bodies will bear the voltage,and the passengers'bodies will be damaged once the rail potential rises abnormally.

(ii) Poor Shunting of Railway Signal Track Circuits

First,it is related to rust on the rail surface.Rail is very important in track circuit.It is through wheels and rails that the circuit is shunted.

Due to external environmental reasons,rails are easily rusted in the open air,eroded by wind,rain and sediment or under various natural conditions,and the surface of the rail is easy to generate an oxide layer.When the rail circuit is shunted,the oxide layer separates the surface of the rail from other parts,which increases the resistance.At the same time,the voltage value received by the resistor cannot make it operate normally,resulting in poor shunting.

Secondly,it is related to the shunt resistance of the wheel set.When the train is in contact with the rail,the pressure is exerted on the rail by the wheel,and the resistance on the two rails is the shunt resistance.In the railway track circuit,the value of shunt resistance represents whether the railway track circuit is in a normal state.When the shunt resistance is less than the standard shunt resistance,the shunt state is within the normal range;however,if the shunt resistance is greater than the standard shunt resistance,poor shunt will occur.

III.Solutions for Abnormal Elevated Metro Track Potential

Generally speaking, people usually use rail potential limiter to limit the rise of rail potential.However,in order to ensure the safety of people's lives and property,it is necessary to set rail potential to ground in the setting process of rail potential limiter and the human body withstand voltage-time characteristic curve to ensure that it is within a reasonable range and will not cause damage to human body.After the setting of the device is completed,the subway track potential can be monitored in real time during daily operation.Once it exceeds the normal value, the rail potential can be reduced by temporarily connecting the rail and the grounding grid to protect personal safety.

(i)Detection Steps to Abnormal Rise Subway of Track Potential

Generally speaking,the abnormal rise of the track potential will cause the track potential control equipment to operate,so the research method has great applicability,and the track potential control equipment will only operate when the track potential abnormally rises.If the equipment fails,it is likely to be a wrong response.Therefore,the key point to be solved is to test whether the track potential is too high.When the rail potential is normal,the operation of the rail potential control equipment is likely to be a failure of the equipment,and the equipment should be overhauled.When it is detected that the rail potential is indeed abnormally elevated,it must be analyzed and handled according to actual reasons[5].The general operation flow is as follows: (1)Starting from the rail potential control equipment,checking whether the performance of the rail potential control equipment is abnormal and whether it can operate normally;(2)Checking the resistance in the vertical direction of the rail and the resistance in the vertical direction of the rail joint,and analyze whether the resistance is too large or too small;(3)Detecting the layout of the track potential and current to determine the area where the potential abnormally rises,so as to do a good job in the basic work for determining the triggering factors in the next step;(4)Detecting the correlation and change trend between the subway power support system and the subway operation;(5)Detecting the change trend between the negative bus at the substation and the running status of the subway,and comprehensively understand the reasons for the abnormal rise of subway track potential.

(ii)Measures Taken for Various Reasons

First,if the rail potential control equipment moves incorrectly, it should be repaired and reformed.The solutions to specific problems are different:if the detection is not accurate enough,the detection circuit can be reset[6];if the problem is caused by software quality,certain equipment should be replaced to strengthen the ability of equipment to interfere.

Secondly, when the subway rail potential detection finds that there is no abnormal rise,but temporary fluctuation is caused by the interference of external factors on the test, some parameters or adjustment methods of working principle can be made to the rail potential limiter to ensure the accuracy of the test and the normal operation of the equipment.

Thirdly, when the subway track potential detection finds that there is indeed an abnormal rise,the possible influencing factors include subway current consumption,line length,loop resistance,etc.The rectifier position, power system and track equipment of the existing subway operation line cannot be changed.Therefore,when the rail potential increases due to factors such as the length of the line,measures must be carefully selected and comprehensive control measures can be adopted to control the rail potential.The resistance will be affected to some extent by the resistance of the return line;especially the rail joint resistance will seriously affect the resistance.The rail joint resistance can be ensured by selecting a more reliable way to connect the rail joints.

(iii)The Control Strategy of Bad Shunt

For the problem of poor shunting,the following methods should be adopted to improve it.For the first point mentioned above,new technologies should be actively introduced.At present,high voltage pulse track circuit technology is mainly introduced to detect whether the train occupies the track circuit section.The application of high voltage pulse track circuit technology can break down the corrosion problem of the track surface,so as to carry out accurate detection.The second point mentioned above can be classified and managed according to different road sections so as to determine different treatment methods under different objective conditions.The third point is to determine what will happen to the insulation layer,thus focusing on it.

IV.Conclusion

With the development of society,major cities are constantly building and improving urban subway lines,making it easier and faster to travel in cities.However,the occurrence of abnormal rise in subway rail potential brings certain risks to the safe operation of the subway.Therefore,the study of abnormal rise in subway rail potential can better serve the development of the subway.