Analysis of rail potential and stray current in MVDC railway electrification system | SpringerLink

Simulations are performed to analyze rail potential and stray current in an MVDC-RES environment. In the conventional high-speed AC-RES, the distance between the adjacent TSSs is usually 50 km. As in the MVDC-RES the losses are less, the distance between the TSSs can be increased. For simulation purpose, the analysis is performed on a 50 km track (similar to current AC corridors), and a 75 km track (similar to prospective MVDC-RES corridors) [3]. Moreover, for detailed analysis, simulations are performed for both grounded and floating electrification schemes in the single as well as multi-train scenario. The parameters used in the simulation model are listed in Table 2.

Table 2 Parameters used in simulation model

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4.1

Grounded scheme simulations for 50 km and 75 km track

An extended model of Fig. 5 is used in this section for simulating 50 km and 75 km tracks. The switching mechanism is intelligently controlled to simulate the moving train scenarios. The stray current and rail potential profiles are analyzed at specified locations, i.e., at the mid-section and at one-fourth of the section, for the complete journey of the train. If the maximum value of the stray current at any location for the complete journey of the train is less than the maximum allowable average of 2.5 A/km, then the average value must always be lower.

4.1.1

50 km grounded track

Figure 7 depicts the result of rail potential at specific locations, i.e., at the one-fourth and mid-section of the traction line, for the complete journey of the train. In both cases, rail potential is less than the maximum allowable limit of safe rail operation.

Fig. 7

Rail potential at the mid-section and one-fourth of the section

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The results of stray current at the one-fourth (12.5 km) of the section and mid-section (25 km), for the complete journey of the train on 50 km grounded track, are shown in Fig. 8. It reveals that the stray current reaches its maximum value when the train reaches the location of interest.

Fig. 8

Stray current monitored for 50 km grounded track

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4.1.2

75 km grounded track

The rail potential at the one-fourth of the section (19 km) and mid-section (37.5 km) for the complete journey of the train is shown in Fig. 9. Results reveal that the rail potential at the mid-section increases up to 77 V, approaching the maximum safe limits of 90 V. It can be concluded that increasing the distance further between the adjacent TSSs can cause risk of electric shock to humans and animals near the traction line.

Fig. 9

Rail potential monitored at specified locations on the 75 km grounded track

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The stray currents profile at a distance of the one-fourth of the track (19 km) and at the mid-section (37.5 km), for the complete journey of the train, is provided in Fig. 10. The results reveal that the maximum stray current at mid-section is still less than that of the average safe value of 2.5 A/km. Although the current is less than the maximum limit for the case of a single-train operation between adjacent TSSs, two or more trains can increase that value to hazardous levels.

Fig. 10

Stray current monitored at specified locations on the 75 km grounded track

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4.1.3

Verification of simulations

Simulation results for stray current and rail potential depicted for the grounded system can be verified through Eq. (2). The verification is performed for complete profile of stray current at the one-fourth of 50 km (12.5 km). Figure 11 shows a simplified version of Fig. 5.

Fig. 11

A simplified model of Fig. 5 for the verification of stray current (50 km grounded track)

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Given the train direction from left to right, initially when the train is in area A, the track current \(I_{2}\) is the only current that causes rail potential and stray current at the one-fourth distance or at the 12.5th km resistor. \(I_{2}\) can be monitored through an ammeter \(\text{A}_{\text{a}}\) which can be used in Eq. (2) for the estimation of stray current at the 12.5th km. As the train continues its journey and enters in area B, the track current \(I_{1}\) now causes rail potential and stray current at the 12.5th km. Current \(I_{1}\) can be monitored through ammeter \(\text{A}_{{\text{b}}}\). \(I_{1}\) can be used in Eq. (2) for the estimation of stray current at the 12.5th km of the track for the train in area B. The portions of track currents \(I_{2}\) and \(I_{1}\) which cause stray current at the 12.5th km are provided in Figs. 12 and 13, respectively.

Fig. 12

Track current \(I_{2}\) at the 12.5th km for train journey in area A

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Fig. 13

Track current \(I_{1}\) at the 12.5th km for train journey in area B

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Considering the values of track currents from the above figures, the stray current at the 12.5th km is estimated through Eq. (2), for the train positioned at 5, 12.5, 25, and 40 km. The results are compared with the simulated values of Fig. 8, as shown in Table 3. It can be observed from Table 3 that the estimated values are consistent with the simulation results.

Table 3 Validation of stray currents

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4.2

Floating scheme simulations for 50 km and 75 km tracks

As discussed earlier, the phenomena of stray current and rail potential are more complex in a floating scheme. For simulations, 50 km and 75 km tracks are considered, which represent the distances between adjacent TSSs for 25 kV AC-RES and 24 kV MVDC-RES, respectively.

4.2.1

50 km floating track

Simulations are performed to find the rail potential and stray current at the one-fourth (12.5 km) and at mid-section (25th km) for the complete journey of the train. The results of the rail potential and stray current are presented in Figs. 14 and 15, respectively. Unlike the grounded scheme, the rail potential and stray current now show negative values also. This is mainly because there is no intentional grounding at TSSs and the stray current follows the path of lower potential.

Fig. 14

Rail potential profile at specified locations for the 50 km floating track

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Fig. 15

Stray current profile at specified locations for the 50 km floating track

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4.2.2

75 km floating track

Simulations are also performed for the proposed lengths of the MVDC-RES. The results of rail potential and stray current at the one-fourth (19 km) of the section and at mid-section (37.5 km) are provided in Figs. 16 and 17. The results reveal that the values of stray current and rail potential for floating scheme are much lower as compared to the values obtained for grounded scheme track.

Fig. 16

Rail potential profile at specified locations for the 75 km floating track

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Fig. 17

Stray current profile at specified locations for the 75 km floating track

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4.2.3

Verification of simulations

The simulation results of the rail potential and stray current depicted for the floating track can be verified through Eqs. (3) and (4), respectively. The verification is only performed for the complete profile of stray current at the one-fourth (19th km) of the 75 km track. Figure 18 shows a simplified version of Fig. 5.

Fig. 18

A simplified model of Fig. 5 for verification of stray current for the 75 km floating track

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Given the train direction from left to right, initially when the train is in area A, the track current \(I_{2}\) is responsible for rail potential and stray current at the one-fourth of the distance or at the 19th km resistor. \(I_{2}\) can be monitored through an ammeter \(\text{A}_{{\text{a}}}\) and further used in Eq. (3) for the estimation of stray current at 19th km. As the train enters area B, the track current \(I_{1}\) will now be responsible for rail potential and stray current at the 19th km. \(I_{1}\) can be monitored through an ammeter \(\text{A}_{{\text{b}}}\) and used in Eq. (3) for the estimation of stray current at the 19th km of the track. The portions of track currents \(I_{2}\) and \(I_{1}\) that cause stray current at the 19th km are shown in Figs. 19 and 20.

Fig. 19

Track current \(I_{2}\) at the 19th km for train journey in area A

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Fig. 20

Track current \(I_{1}\) at the 19th km for train journey in area B

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Considering the values of current from the above figures, the stray current can be estimated at the 19th km through Eq. (3) for the train positioned at 10, 19, 30, and 60 km. In Table 4, the results are compared with the simulated values. It can be observed from Table 4 that estimated values agree with the simulation results.

Table 4 Validation of stray current for floating track simulation

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4.3

Simulations for 300 km multi-train multi-TSS scenario

The stray current in a grounded track increases to its maximum level when the train reaches at the mid-section between two adjacent TSSs. However, the behaviors of the stray current and rail potential must not be the same in the case of a multi-train multi-TSS scenario. For detailed analysis, simulations are performed for both grounded and floating schemes. To consider a more realistic simulation model, a 300 km track is considered, which resembles the newly build Chengdu—Chongqing long-distance high-speed line. Moreover, as per the timetable/schedule of the considered line, 10 active trains are considered in simulations.

4.3.1

300 km multi-train multi-TSS grounded track scheme

For simulation, a 300 km grounded track is considered, which has five TSSs each at a distance of 75 km from each other. Ten trains are equally distributed, each at a distance of 30 km from the other. The combination of TSSs and trains is provided in Fig. 21.

Fig. 21

Simulation scenario for a 300 km grounded track

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The obtained stray current results are provided in Fig. 22, which is observed from 60 different locations along the track. It is found that the maximum stray current along the track is not more than 2 A/km, which is less than the averaged allowable value of 2.5 A/km. It is also noteworthy that the measured current is the total current leaking through the considered grounding resistance that further has to divide and flow toward TSSs. This division will depend upon the distance between the TSS and concerned track location; the more the distance between the TSS and train location, the less the stray current in that direction will be.

Fig. 22

Stray current profile for the 300 km grounded track

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The simulated rail potential is provided in Fig. 23. Although the stray current is within the limits, the rail potential slightly increases from the maximum limit of 90 V/km. Practically, a combination of grounded and floating scheme can be achieved through diode or thyristor grounding, which can considerably decrease the rail potential.

Fig. 23

Rail potential profile for the 300 km grounded track

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4.3.2

300 km multi-train multi-TSS floating scheme

Simulation scenario for a floating scheme is given in Fig. 24. The model is same as discussed in previous section, where the only difference is that the ground terminals are removed.

Fig. 24

Simulation scenario for the 300 km floating track

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The simulation result of stray current is provided in Fig. 25. Locations of TSSs and trains are same as discussed in previous section. It can be observed that the stray current is now reduced to a maximum value of about 1.5 A/km.

Fig. 25

Stray current profile for 300 km floating track

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The profile of rail potential is shown in Fig. 26, revealing a considerable decrease in rail potential for floating scheme. The maximum value of rail potential is approximately −70 V which is less than that of safe limits of −90 V.

Fig. 26

Rail potential profile for the 300 km floating track

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4.4

Safe operation distances between adjacent TSSs in MVDC-RES

Considering the constraints of rail potential and stray current, analysis is performed to determine the maximum safe distance between adjacent TSSs. For simulations, two trains, each of 8 MW, are placed at equal distance from each other as well as from the TSS. Simulations are performed with different track lengths in the grounded and floating schemes. For the grounded scheme, the results of rail potential and stray current are provided in Figs. 27 and 28, respectively. The rail potential profile in grounded scheme depicts safe results for distances up to 70 km. For the track lengths of 80 km and 90 km, the rail potential reaches to hazardous levels. Unlike rail potential, stray current remains under safe limits even for the 90 km track.

Fig. 27

Rail potential with different track lengths in the grounded scheme

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Fig. 28

Stray current for different track lengths in the grounded scheme

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Simulations results of rail potential and stray current for different tracks under the floating scheme are illustrated in Figs. 29 and 30, respectively. Rail potential under the floating scheme depicts safe results for track lengths of up to 80–85 km. Similarly, the stray current remains under safe values for the track lengths of up to 90 km.

Fig. 29