Other Sources of DC Stray Current

Summary of Equations

GROUP EXERCISE

2.2.2 Other Sources of DC Stray Current

Besides impressed current CP systems, there are other sources of DC stray current:

• DC transit systems

• DC welding equipment

• high-voltage DC transmission systems (HVDC)

• DC rail systems in mines.

Because these sources have a variable loading nature, the resulting stray current activity is dynamic (i.e., effects vary in magnitude and often location with time).

Another source of dynamic stray current, telluric currents, is discussed in Chapter 4.

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2.2.2(a) DC Transit Systems

The electrification of transit systems in the late 1800s throughout North America resulted in considerable interference corrosion on gray cast iron watermains. Much of the early attempts to mitigate this interference eventually led to the development of CP technology.⁵

Figure 2-31: Typical Stray Current Paths Around a DC Transit System

The load current (IL), after passing through the trolley motor, divides into a number of current paths depending on the resistance of each path.

therefore: IL = IR + Is + Ie [2-34]

Although the rails provide a relatively low-resistance path, the current leakage off the rails can be 5 to 10% of the load current. This may seem a small percentage, but the stray currents can be substantial because the start-up load current can be several hundred amperes for a single trolley and several thousand amperes for a subway train.

5 Kuhn, R.J., Cathodic Protection of Underground Pipelines from Soil Corrosion, API Proceedings, Nov.

1933, Vol. 14, p.164.

DC sub -station

ground

pick-up

I_s I_s

I_e IR

I_e I_s

discharge O/Hpowerconductor

metallicstructure (e.g.,watermain) running

rails

CP Interference

Not only will the magnitude of the stray current vary with time of day and whether the vehicle is accelerating or decelerating, but the location of stray current pick-up on the metallic structure will change as the trolley moves along the rail. Thus, a structure-to-soil potential recording will have a dynamic appearance (Figure 2-32).

-3500 -3000 -2500 -2000 -1500 -1000 -500 0

10:00 11:00 12:00 13:00 14:00 15:00 16:00 17:00 18:00 19:00 20:00 21:00 22:00 23:00 0:00 1:00 2:00 3:00 4:00 5:00 6:00 7:00 8:00 9:00 10:00

Time

Potential wrt CSE (mV)

Figure 2-32: Typical Structure-to-Soil Potential Recording with Time Caused by Interference from a DC Transit System

The potential-time recording of stray current effects from a DC transit system has a distinctive pattern. There are considerable potential fluctuations during the morning and evening rush-hour periods, light activity in the middle of the day and late evening, and virtually no changes during the early morning hours.

Although the stray current pick-up locations change with time, the discharge sites are predominantly in proximity to the substation ground. In urban areas, localized stray current can discharge from water piping around electrically discontinuous joints and from crossings with other utilities remote from the substation ground.

Determining the impact of transit-caused stray current on metallic facilities in urban areas requires considerable potential and current recording, starting in the vicinity of the substation grounds and along the transit system route.

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2.2.2(a)(i) Analysis of Transit System Stray Currents

A comprehensive method of analyzing dynamic stray current activity involves the construction of beta curves from current and potential measurements.

A line current survey can be conducted to determine the magnitude and direction of the currents flowing along the pipeline, providing a way to locate the source of the interference. However, this requires that some means be available to measure pipeline currents at a number of locations throughout the area of interest (which is not always possible). In cases where the pipe is exposed or rises above grade, this can be done using a pipeline current clamp (Figure 2-33). Otherwise, as Figure 2- 4 shows, IR-drop test stations must be used.

Figure 2-33: Current Clamp Used to Measure Pipeline Currents

CP Interference

-Substation

A B C

Load

Positive Rail

Negative Return Rail

E D

+ V - + V - + V - + V - + V

-IA IB IC ID IE

Figure 2-34: Line Current Survey to Locate Source of Interference Using IR-Drop Test Stations

A line current survey is conducted by taking a series of simultaneous pipeline current measurements at adjacent locations on the pipeline, and plotting these measurements with respect to one another. The measurements may be conducted simultaneously using either global positioning system (GPS) synchronized data loggers or manually using a two-person crew who communicate by radio. The best way to illustrate the procedure is through the following example.

In Figure 2-34, a set of pipeline current measurements are made at the IR-drop test station at location “A”; a second set of current measurements is simultaneously recorded at Location “B.” The measurements obtained at “B” are plotted against those obtained at “A” in Figure 2-35, and it is found that the relationship is linear and the data produce a line having a slope of greater than unity. Because the relationship is linear, the currents measured at “A” and “B” must emanate from the same source of interference. Also, because the slope is greater than 1, more line current exists at location “B” than at location “A”; the pipeline must therefore be picking up interference current in this area.

Simultaneous measurements are also taken at locations “B” and “C,” producing the second chart in Figure 2-35. Here, the plot is once again linear. However, the slope of the line is unity. This indicates that there is no net pick-up or discharge of interference current between these two locations.

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In the third chart of Figure 2-35, the slope of the line is less than unity; this indicates that interference current is discharging from the pipeline and that mitigative measures must be taken in this area. Note that in all three of these charts, the plots do not pass through the origin. This is an indicator that there are other currents also flowing along the pipeline that are unrelated to the interference currents, such as CP current.

In the fourth chart of Figure 2-35, there is no correlation between the currents measured at location “D” and those measured at location “E.” This indicates that the line currents at location “E” must emanate from some other source of interference. This other source could simply be another load somewhere else along the transit system, or it could be a source that is totally unrelated to the transit system.

ΔIB

ΔI_A ΔI_A

ΔIB

ΔIA > 1

I_B I_C

ΔIC

ΔIB

ΔIC

ΔI_B = 1

I_C ID

ΔID

ΔIC

ΔID

ΔIC < 1

I_D IE

Non-Linear

Figure 2-35: Line Current Plots for Example in Figure 2-34

A second type of survey that can be conducted is the exposure survey, where pipe-to-soil potential measurements are recorded simultaneously with pipeline currents (Figure 2-36). At each location, current is plotted versus potential to determine the point of maximum discharge.

CP Interference

-Substation

Load

Positive Rail

Negative Return Rail

A B C D

+ V -- V

E + V

-- V V

-- V

IA IB IC ID IE

Figure 2-36: Exposure Survey to Locate Point of Maximum Exposure

As an example, the first two charts in Figure 2-37 show that as the current flowing along the pipe increases, the potential of the earth becomes more positive with respect to the pipeline (i.e., pipe potential becomes more negative). This indicates areas of current pick-up. Because the slope of the plot is greater at “A” than at “B”

(i.e., potential variations are greater per unit of interference current), location “A”

is considered to be the point of maximum current pick-up.

At location “C,” the pipe potential is unaffected by the interference current;

consequently, there is neither pickup nor discharge in this area. This location should also correspond to the point of maximum current flow along the pipeline.

At location “D,” the earth potential becomes more electronegative (i.e., the pipe potential becomes more electropositive) as the current flowing along the pipeline increases; this indicates a point of current discharge. Although it has a positive slope, Location “E” is also a point of current discharge because the direction of current flow along the pipeline is in the opposite direction to that at locations “A”

through “D.” Because the absolute value of the slope of the plot at “D” is greater

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than that at “E,” Location “D” is the point of maximum exposure—where mitigative measures must be taken.

ΔI I

Figure 2-37: Exposure Survey Plots for Example in Figure 2-36

A third type of survey that can be conducted is the mutual survey, which does not involve the measurement of pipeline currents. Voltages are measured between the pipeline and the interfering system; simultaneously, pipe-to-soil potentials are measured at the point of maximum exposure (Figure 2-38). The pipe-to-soil potentials Ep/s are plotted versus the pipe-to-rail potentials Ep/r. If a correlation exists, as shown in Figure 2-39, then the source of interference has been positively identified.

-Figure 2-38: Mutual Survey to Confirm Source of Interference

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α V_p/s

ΔV_p/s

V_p/r ΔV_p/r

Figure 2-39: Pipe-to-Soil Potential Versus Pipe-to-Rail Potential for Example in Figure 2-38

Plots that relate pipe-to-soil potential to pipe-to-rail potential generally are called beta curves because the slope of the linear plot is called beta. The equation of the line is as follows:

r p r

p r p

s p s

p V V

V V _/ _/

/ /

/ =α+β

Δ + Δ α

= [2-35]

Because the to-soil potential in this example is a linear function of the pipe-to-rail potential (Figure 2-39), and because the pipe-to-soil potential is also a linear function of the pipeline current (Figure 2-37), it follows that pipeline current is a linear function of the pipe-to-rail potential. Therefore, it has been argued that an exposure survey may be conducted without measuring pipeline currents at all. This line of reasoning calls for simply recording pipe-to-soil potentials at various locations along the pipeline (Figure 2-40) and plotting these versus pipe-to-rail potential (Figure 2-41).

CP Interference

V +

- - V + - V +

-Substation

Load

Positive Rail

Negative Return Rail

+ -V

A B C

Figure 2-40: Exposure Survey Conducted Without the Measurement of Pipeline Currents

α V_p/s ΔV_p/s

Location A ΔV_p/r

V_p/r

α V_p/s

Location B V_p/s Δ

Δ = 0

V_p/r V_p/r

α V_p/s

ΔV_p/s Location C

ΔV_p/r V_p/r

Figure 2-41: Exposure Survey Plots for Example in Figure 2-40

In general, the steeper the slope of the beta curve, the greater the pick-up or discharge. However, the polarity of the slope will depend upon the point of connection for the voltmeter measuring the pipe-to-rail potential.

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2.2.2(a)(ii) Mitigation of Transit System Stray Currents

Mitigation methods for minimizing the deleterious effects of DC transit system stray currents are similar to those used for ameliorating CP stray currents. They include:

• electrical isolation of rails and substation

• electrical bonds

• reverse current switches

• forced drainage bonds

• CP.

On existing transit systems, stray current has been reduced significantly by improving the isolation between the rail and ballast. This is accomplished by installing insulating pads between the rail and ties, between the hold-down plates and the rail, and ensuring that the ballast is well drained. These measures, coupled with disconnecting the negative rails from electrical grounds, have proved relatively successful in many instances. Disconnecting the DC substation from electrical ground allows the rails and transit vehicles to electrically float in a manner that requires the installation of switching devices that connect the rails to earth if a specific rail voltage-to-ground potential is exceeded. The effectiveness of substation isolation in minimizing stray current activity is therefore lost during the time that the safety switches are activated.

For new transit systems, it has become common to electrically isolate the entire rail pocket if the rail is embedded in the road surface (Figure 2-42a) or isolate the rail from ties (Figure 2-42b).

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Figure 2-42b: Typical Direct-Fixation

Isolating Fastener

Source: Fitzgerald J.H. and Lauber, M.D., Stray Current Control for the St. Louis Metrolink Rail

System, MP, Vol. 34(1), Jan. 1995, p.22

Figure 2-42a: Typical Embedded Track Installation

Source: Sidoriak, W., Rail Isolation on the Baltimore Central Light Rail Line, MP, Vol. 32(7), July 1993, p.36

Some transit systems use a separate isolated rail (so-called fourth rail) as a current return path, which negates the need to isolate the running rails.

The earliest attempts to mitigate the corrosive effects of transit stray currents simply involved running bonds from the utilities to the negative bus at each substation. This provided an electronic path for the stray current to return, thus reducing the amount of stray current in the electrolytic path as shown in Figure 2-43.

Figure 2-43: Typical Utilities Drainage System at a Transit Substation

Facilities such as lead-sheathed power cables, steel gas piping, telephone grounds, and iron water piping would be connected in series with a switch and a shunt to the

shunt

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negative bus. The shunt provides a means of recording the stray current magnitude and direction.

One weakness of this drainage arrangement is that providing a direct low resistance path for the stray current causes the underground structures to pick up more current than they would otherwise. For structures with electrical discontinuities, such as iron watermains, this can result in more severe corrosion at the isolating joints.

A second disadvantage of the direct bond drainage system becomes evident where there are multiple substations and many trains. The utilities represent an alternative path to the rails between substations, and the stray currents can actually reverse. This situation is depicted in Figure 2-44.

3rd rail

Figure 2-44 Schematic Showing Circulating Current between Transit Substations Through Direct Bonds to Utilities

With the transit load located between substations “A” and “B,” it will draw some of the load current from each station. Hence, each substation’s load current has an alternative path through the utility bonds back to its respective source.

To prevent circulating currents, reverse current switches can be installed in each bond. These devices present a high resistance in one direction (the reverse direction) and a low resistance in the other (direction of intended drainage). There are several types of reverse current switches,^[6]as listed in Table 2-2, each with differing operational characteristics.

6Munro, J. I., Comparison and Optimization of Reverse Current Switches, NACE, Corrosion/80, Paper No. 142, March 1980.

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Table 2-2: Types of Reverse Current Switches

Type Characteristics

Electromagnetic (relay) Requires AC power to operate the relay, relay must conduct all current, may be slow to open

Diodes (germanium, silicon)

Requires a minimum of 0.4V to conduct, have resistance, subject to surge failures and reverse voltage breakdown Hybrid (relay in parallel

with diodes)

Smaller relay required because diodes carry most current and are subject to reverse voltage breakdown.

Potential Controlled

Rectifier (Figure 2-40) Can drain all the stray current but are relatively expensive.

Although CP is beneficial in mitigating transit system stray current, the stray currents are often so large that they preclude mitigation with galvanic anodes. Moreover, large-capacity impressed current systems in an urban area will likely create interference on other facilities. CP thus has limited effectiveness.

One the most successful measures is the use of a forced drainage bond. As shown in Figure 2-45, a forced drainage bond is a bond with a potential-controlled rectifier connected in series with the bond.

Potential Controlled

Rectifier

structure

buried reference electrode

I_s

Figure 2-45: Forced Drainage Bond Using a Potential Controlled Rectifier

The voltage output of the auto-potential rectifier varies depending on the potential measured between the structure and a buried reference electrode. If the measured potential is more positive than the potential set on the controller, the rectifier output

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voltage increases to force more current through the bond. With a DC voltage source in series with the bond, the bond resistance is negative. The negative resistance ensures that all the stray current is drained from the structure and there is no residual stray current in the soil path. Nevertheless, the controller must be adjusted so that there is no bond current during periods of no stray current activity. Otherwise, the transit system rails and grounding system will be corroded.

To be completely effective, the forced drainage bond must be located at the point of maximum discharge. Just as with a resistance bond, if the structure is not electrically continuous then a forced drainage system will aggravate corrosion at any isolating joints.

2.2.2(b) High Voltage Direct Current (HVDC) Electrical Transmission Systems

HVDC systems that transmit large blocks of electrical power over long distances have operating cost advantages over high voltage alternating current (HVAC) transmission. Unlike HVAC systems, there are no inductive or capacitive losses on HVDC. Moreover, for lengths greater than approximately 800 km, the power savings easily justify the extra capital costs to build the AC/DC converter stations and their extensive electrical grounding systems.

HVDC systems are built to operate in bi-polar mode; that is, there is both a positive and negative circuit with large grounding electrodes at each terminus as illustrated in Figure 2-46.

positive cables

I_dc

negative

AC / DC Converters

cables load

end

supply end

L > 800 km

I_dc

Figure 2-46: Electrical Schematic for a HVDC System

Under normal operating conditions, the DC line currents are typically in the 1000A range and imbalance currents are approximately 1 to 2% of the line currents. Such small currents do not pose a significant stray current risk on

CP Interference

underground metallic structures because the electrodes are intentionally located remotely from other utilities.

During emergency operating conditions, where either the positive or negative cable networks are faulted or de-energized for maintenance, the line current passes through the earth via the grounding electrodes. Under these circumstances, the system is operating in monopolar mode.

HVDC grounding electrodes are large compared to impressed current groundbeds, although CP anode materials such as high-silicon iron and coke are often used.

The electrode is typically in the shape of a ring having approximately 100m diameter and a depth of 1 to 2m. Despite the large size and relative remoteness, the voltage gradient around the electrode can be appreciable even a long distance away when the electrode is passing hundreds of amperes.

For example, the voltage rise in earth at some distance “x” from such an electrode can be estimated using Equation 2-36.

2 x

Hence a metallic structure located 1 km from the electrode would be exposed to 4V during monopolar operation under the foregoing conditions. It is claimed that the HVDC system will operate in monopolar mode a small percentage of time.

Nevertheless, the rather large voltage gradients can present a serious corrosion risk on some structures on a cumulative basis.

CP Interference

Also, the effect can be either a positive or negative potential shift on the structure (Figure 2-47) depending on which of the power circuits has the outage.

Time

-4.0

t₁ t₂ t₃

-3.0 -2.0 -1.0 0.0 +1.0

t₄ - E

+ E Es/s

(V_cse)

Figure 2-47: Potential-Time Plot for a Metallic Structure being Interfered-with by a HVDC System

Assume that the potential plot in this figure was from a structure located near the supply end groundbed. Under this scenario, the negative shift from t1 Æ t2 would result from a failure on the positive circuit and the positive shift from t3 Æ t4

would result from a failure on the negative circuit. Note that the potential shifts are not necessarily equal—even if the stray current is the same—because the cathodic and anodic polarization characteristic can be different.

Most structures would not extend the full 800km, nor be close enough to the electrode to make it economical to install a bond. Because of the large voltage shifts, galvanic anodes many not adequately compensate. The most practical mitigation method is to use an impressed current system powered by a potential controlled rectifier. Not only would the CP power supply be able to counteract the large positive potential shifts, but during the negative shift periods it would shut down—thus minimizing the stress on the coating if the structure was a coated steel pipeline.

2.2.2(c) DC Welding Operations

Welding operations on ships and barges have been known to create stray current interference, sometimes so severe that it has resulted in the sinking of the vessel.

In document NACE CP Interference January 2008 (Page 84-102)