What Is a Stray Current?
Stray current refers to the current flowing outside the designed or specified circuit, and is also called "stray current". It flows through the soil and has nothing to do with the protected piping system. This current enters the pipeline from a part B of the pipeline. This part is called the cathode. After the current flows along the pipeline for a distance, it flows out of the pipeline C. This part is called the anode. Here, the pipeline generates anodization, that is, stray current corrosion. Stray current corrosion is electrochemical corrosion in nature, and anode corrosion is more serious. Take the DC traction system as an example.
Stray current
Stray current refers to the current flowing outside the designed or specified circuit, and is also called "stray current". It flows through the soil and has nothing to do with the protected piping system. This current enters the pipeline from a part B of the pipeline. This part is called the cathode. After the current flows along the pipeline for a distance, it flows out of the pipeline C. This part is called the anode. Here, the pipeline generates anodization, that is, stray current. corrosion. Stray current corrosion is electrochemical corrosion in nature, and anode corrosion is more serious. Take the DC traction system as an example.
- Chinese name
- Stray current
- Foreign name
- Stray Current
- Meaning
- Current flowing outside the designed or specified circuit
- nickname
- Stray
The stray current mainly refers to the current that does not move according to the prescribed path. It exists in the soil and is not related to the equipment system that needs to be protected. This stray current in the soil will enter the pipeline through a part of the pipeline, and after moving a certain distance in the pipeline, it will leave the pipeline and return to the soil, and these currents will corrode where they leave the pipeline.
Corrosion of running rails and their accessories
Traction current flows back to the traction substation through the running rail. As shown in Figure 2-1, because the running rail cannot be completely insulated from the ground, part of the current will leak from the running rail to the ground. At this time, the running rail is in a corrosive battery. The anode is very susceptible to corrosion. The data show that the stray current corrosion of the track is more obvious in tunnels and forks, and some places need to be changed again in 2 to 3 years.
Corrosion of the running rail and its accessories generally occurs on the contact surface with other objects. These corrosions are difficult to find from the outside, and the rails need to be replaced when they are discovered. Therefore, it is very harmful.
Damage to reinforced concrete structures
Stray currents can corrode the steel bars in reinforced concrete structures, but they do not affect the concrete itself. When stray current flows into the reinforced concrete structure, the steel bar is the cathode, and hydrogen evolution corrosion will occur, and the generated hydrogen gas cannot escape, so isostatic pressure will be formed to disengage the steel bar from the concrete. When stray current flows out of the rebar, the rebar is the anode, which will cause corrosion and produce products such as rust, which will increase the volume of the rebar itself and cause a large pressure inside the concrete to cause it to crack. It is very difficult to repair and replace damaged reinforced concrete structures.
Corrosion of buried pipelines
At present, buried pipelines are mainly natural gas pipelines, water pipes, heating pipelines, petroleum pipelines, cables, etc. It is easy to collect stray currents and suffer corrosion. If the pipeline is close to the subway system or transmission line, it is easily affected by stray currents, so it should be paid attention to in the design and construction process.
Threat to personal safety
When the buried pipeline approaches or crosses the high-voltage AC transmission line, the current generated by the AC transmission line generates magnetically induced voltage on the pipeline, so that the potential of the pipeline to ground is not zero. If the pipeline voltage is too high, it may pose a threat to the personal safety of operation and maintenance personnel.
In the subway system, when the traction current does not return smoothly and a large amount of stray current flows into the ground, it will cause the voltage between the rail and the structural steel bar to rise, posing a threat to the personal safety of platform passengers.
Affects the normal operation of electrical equipment
In the area where the stray current is serious, it may cause the cathodic protection potentiometer to alarm, interrupt the work, and may cause some electrical equipment to malfunction, which will affect the normal operation of the electrical equipment. If the insulation between the track and the soft pillow is damaged, a large stray current will be generated and the drain cabinet may be burned.
Impact on communication
The electrical bursts and surges generated by pantographs (boots) are an important part of urban clutter and can cause interference to surrounding communication equipment. In addition, the contact wires in the vehicle are high-harmonic transmitting antennas, and the laying of the wires will pollute the electromagnetic environment in a short distance.
Abnormal corrosion
When the line is introduced into a building such as an operation warehouse, a repair warehouse, and a delivery inspection warehouse, if the insulation construction is not good, a certain degree of electrical connection will occur between the rails and the building, which will cause the leakage current to increase and produce abnormally severe noise. Stray current corrosion.
There are two methods for distinguishing the DC stray current interference: the appearance discrimination method and the electrical discrimination method.
(1) Appearance judgment method
For buried pipelines, if it is corroded by DC stray current, its appearance is: the tendency of pitting corrosion is great, the wound surface is smooth, the edges are relatively neat, and sometimes there is metallic luster. When water is present and severely corroded, the electrolysis process can be clearly observed. However, in the case of soil resistivity greater than 10,000, stray current corrosion is generally difficult to occur.
In comparison, the appearance characteristics of natural corrosion are: the corrosion product is black or yellow, the rust layer is relatively loose, the tendency of pitting is small, the wound surface is not smooth, the edges are uneven, and the surface is rough after removing the corrosion product.
(2) Electrical judgment method
Because stray current is difficult to measure directly, whether the pipeline is affected by stray current is usually judged by the positive offset value of the pipe ground potential compared to the natural potential. At this time, the soil potential gradient can be used to determine the strength of the stray current. According to China's petroleum industry standard "Technical Standard for DC Drainage Protection of Buried Steel Pipelines" (SY / T 0017-2006), when the pipe ground potential at any point of the pipeline is positively offset by 20mV from the natural potential or the soil near the pipeline When the potential gradient is greater than 0.5mV / m, DC stray current interference is considered; when the pipe ground potential at any point of the pipeline is positively offset by 100mV from the natural potential or the soil potential gradient near the pipeline is greater than 2.5mV / m, Take protective measures in a timely manner. The European standard EN50162 stipulates that four methods can be used to judge whether there is stray current interference: tube ground potential offset from natural potential, tube ground potential fluctuation, soil potential gradient near the pipeline, and current value in the pipeline.
Table 1 Standards for judging the level of DC interference in China
Tube ground potential forward offset (mV) | DC interference level |
<20 | weak |
20 200 | in |
200 | Strong |
Table 2 Judgment criteria for the strength of stray current in China
Soil potential gradient (MV / m) | Strength of stray current |
0.5 | weak |
0.5 5 | in |
5 | Strong |
Judgment of AC stray current interference: In the "Technical Standard for AC Drainage Protection of Buried Steel Pipelines" SY / T 0032-2000, determine the index of drainage effect based on soil acidity and alkalinity: AC interference under weak alkaline The voltage should be 10V; under neutral conditions, the AC interference voltage 8V; in acidic conditions, the AC interference voltage 6V. In foreign countries, the evaluation is from the perspective of personal safety and pipeline corrosion. The European standard CEN / TS 15280 uses AC current density and AC / DC current density ratios as evaluation standards.
Stray current drain protection
the way | Direct drainage | Polar drainage | Forced drainage | Ground drain |
Scope of application | Determined anode zone in disturbed pipe | Positive and negative alternation of ground potential on the disturbed pipe | Small potential difference between track and pipeline | Interrupted pipes that cannot drain directly to the source of interference |
advantage | Simple economy Works well | Easy installation Wide range of applications | Large protection range Special occasions where other drainage methods cannot be applied | Wide range of applications Less interference to other facilities |
Disadvantage | Limited application range | Poor protection when the pipeline is far from the rails | Requires power Aggravated rail corrosion | Poor effect Need auxiliary ground bed |
Stray current cathodic protection
method | Forced current | Sacrificial anode |
advantage | Large protection range Not limited by soil resistivity The greater the amount of work, the more economical Long protection device life | No external power required Less interference to adjacent metal structures Small administrative workload Economical hours |
Disadvantage | Requires external power Interference with nearby metal structures Heavy workload of management and maintenance | High resistivity environment is uneconomical Not applicable when the coating is poor Limited output current |
Other protective measures for stray current
When there are power plants near the port where the ship is moored, DC electrified railways and river crossing cables should be monitored for the presence of stray current, and the direction and location of the stray current source should be determined by measurement.
It is not allowed to use seawater or river water for discharge or voltage reduction. Do not use broken cables to avoid stray current.
For long-term electric welding operations on large ships, the complete set of electric welding machines should be moved to the hull.
The ship's power supply system and electrical equipment must be well-insulated and regularly checked to prevent leakage. [1]
