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XYLENE POWER LTD.

ELECTRICALLY ACCELERATED PIPELINE CORROSION

By Charles Rhodes, P.Eng., Ph.D.

ABSTRACT:
A new threat to buried steel pipelines is wind farms. Many buried pipelines were originally designed and installed in rural areas with no contemplation that they would eventually be in close proximity to major grid connected electrical equipment (wind turbine transformers).

The purpose of this web page is to identify the circumstances under which proximity of buried steel pipelines to wind turbine electricity generators and comparable major electrical equipment leads to significantly accelerated pipeline corrosion. Of particular concern are large diameter high pressure pipelines where electrically accelerated corrosion at a linear scratch in the pipe's dielectric coating can lead to a pipe rupture failure with ensuing major personal injury and/or property damage. Generally the future proximity of wind turbine electricity generators was not contemplated when existing buried steel pipelines were designed and installed.

The method of wind farm power distribution recommended by GE is to use a grounded wye primary - isolated delta secondary transformer at each wind turbine and grounded wye power collector transformers at the local substation. This configuration has the advantage that it suppresses third harmonics generated within the wind turbine equipment. However, it is shown herein that if there are nearby buried steel pipelines, this method of wind farm power distribution can still lead to ground currents and related electrically accelerated pipeline corrosion at pipeline dielectric coating defects that are in the proximity of a wind turbine generator.

This ground current problem can be mitigated by use of a heavy gauge copper common ground conductor and by substantial setback of wind turbines from the pipeline axis. However, assuming an induced ground voltage at a wind turbine of about 5 volts with respect to the substation ground the minimum setback of a wind turbine from a pipeline that is required to avoid this problem of electrically accelerated corrosion is in the range of 100 m to 1000 m, depending on local ground circumstances. The best case circumstance is uniform top soil extending to a great depth. The worst case circumstance is shallow top soil over bed rock.

Before choosing the power distribution transformers and before a capital commitment is made to construct the wind turbine generator the actual decline in induced ground voltage with distance should be measured at any contemplated wind turbine generator site that is in the proximity of a buried steel pipeline.

This problem of electrically accelerated corrosion can be mitigated via use of low capacitance wind turbine transformers and via use of substation transformers which are optimized to minimize harmonic generation, but these measures significantly increase the overall cost of the wind farm.
 

LEGAL PROBLEM:
Often buried steel pipelines have legally protected exclusive corridors. However, the half width of these exclusive pipeline corridors is generally much less than the potential damage radius to buried steel pipelines due to ground current radiating from a wind turbine transformer or other heavy duty grid connected power transformer. Thus if such transformer grounds are not sufficiently set back from existing buried steel pipelines, damage to the pipelines can result that is no fault of the pipeline owner.

However, much of current property law never contemplated this issue. This issue is known to power transmission engineers who specialize in AC/DC conversion for long distance under water power transmission. However, there are only a few locations in North America where it is necessary to transmit bulk power any significant distance under water. Hence, outside of Quebec, most of the electrical engineering community in North America has no hands on experience with this issue. Generally the courts and property lawyers have no comprehension that a wind turbine electricity generator on land parcel A can damage a pipeline on nearby land parcel B which in turn can damage persons and property on another nearby land parcel C.

The existing law generally provides that if the pipeline on land parcel B fails and damages persons or property on land parcel C then the pipeline owner is responsible, and ignores the fact that it was the wind turbine on land parcel A caused the pipeline failure on land parcel B.

The subject of this web page is quantification of the setback of wind turbines from buried steel pipelines that is necessary to prevent electrically accelerated corrosion of the buried steel pipelines.

To understand how this setback is calculated it is necessary to understand how electrical corrosion protection of pipelines works and how this corrosion protection can be defeated by proximity to a wind turbine electricity generator.
 

EXTERNAL CORROSION PREVENTION BY USE OF A DIELECTRIC COATING:
External corrosion minimization is achieved first by use of a dielectric coating on the outside of the steel pipe which excludes both water and oxygen from the outside surface of the pipe. To the extent that this dielectric coating is properly applied and remains defect free there is no external pipe corrosion.

However, in the field there are many ways that the external dielectric coating can acquire defects. Examples include:
Scratches while in transport;
Scratches during handling while in storage;
Too much exposure of the dielectric coating to sunlight;
Scratches during field assembly;
Inadequate cleaning after welding;
Inadequate coating and failure of coating bonding in the vicinity of welds;
Scratches while being lowered into the gravel bed;
Scratches while gravel is being poured over the pipe
Scratches due to thermal or stress related pipe expansion/contraction within the gravel bed;
Scratches by frost heaving within or adjacent to the gravel bed;
Scratches due to inadvertent application of external force, such as by the side leg of a back hoe or boom truck;
Scratches by augers that are used to install adjacent utility poles;
Scratches by back hoe buckets that are used to install adjacent buried services including:
potable water, sanitary sewer, storm sewer, local natural gas distribution, telephone, Cable TV, data communication and electricity;
Scratches by backhoes that are used to create drainage paths during flood emergencies.
 

ELECTRICAL CORROSION PREVENTION:
The steel pipeline industry recognizes that in spite of its best efforts, the external dielectric coating on buried steel pipes may have application and installation defects and will likely eventually get scratched by mechanisms beyond the control of the pipeline owner. Hence buried steel pipelines also use an electricity based mechanism for external corrosion protection. However, this electricity based corrosion protection mechanism can be defeated by proximity to major grounded electrical equipment such as wind turbine electricity generators. Thus the combination of almost inevitable pipeline dielectric coating defects plus proximity of a wind turbine electrical generator can lead to electrically accelerated corrosion of a buried steel pipeline. If that pipeline contains high pressure natural gas, oil and/or toxic hydrogen sulphide (H2S) gas, a major public safety/fire/property damage incident can result.

There have been recent developments in more scratch resistant pipeline coatings. However, these new coatings do not solve the problem of hundreds of thousands of km of existing buried steel pipe with inferior coatings.
 

CORROSION:
Most buried pipeline external corrosion is a result of chemical interaction between the iron in the pipe and negative ions in surrounding water or wet soil. Hence the first technique used by pipeline companies to minimize corrosion is to choose a pipeline route that is high and dry rather than low and wet. If the pipe is above the local water table, when water seeps into the gravel around the pipe gravity will cause the water to drain down towards the bottom of the gravel, and hence water will not sit against the pipe. Provided that the gravel bed is adequate, most external corrosion occurs at times when part or all of the pipe is below the local water table so that drainage through the gravel is does not prevent water being in direct contact with the outside of the dielectric coated pipe.

Hence the focus of this document is on the circumstance when there is water in contact with the outside of the coated pipe. The fraction of a year when there is water in contact with the buried pipe in part determines the net external corrosion rate.

Ground water contains a mixture of ions. The water itself produces H+ and OH- ions. Salt used for snow melting commonly produces Na+ and Cl- ions. Nitrogen fertilizers produce NH4+ and NO3- ions. Phosphorus fertilizers produce H+ and PO4- ions. Potassium fertilizers produce K+ ions. Carbon dioxide from the air produces (HCO3)- ions. Other commonly encountered ions are sulphate SO4-- ions. There are a host of less common water soluble ions. The issue is that if the steel pipe is in direct contact with ground water there is a natural tendency for the iron in the steel to chemically combine with the negative ions in the water. This chemical process is known as corrosion. The more acidic the soil the higher the negative ion concentration in the water and the faster that corrosion occurs.

Assume that the dielectric coated pipe is immersed in ground water. Assume that a source of direct current (DC) electricity is connected between the buried steel pipe and the surrounding water. Due to the dielectric coating around the pipe the only places where current can flow are at imperfections in the dielectric coating.

Assume that the source of DC is connected such that the pipe is electrically more negative than the surrounding water. Then at the dielectric coating defects the negative pipe surface tends to repel the negative ions in the ground water and the corrosion rate is reduced. However, if the polarity of the source of DC is reversed so that the pipe is more positive than the surrounding ground water then the pipe attracts the negative ions in the ground water and the corrosion rate rapidly increases. This is electrically accelerated corrosion.
 

CORROSION PROTECTION:
For practical reasons most buried steel natural gas distribution pipelines are corrosion protected using sacrificial magnesium electrodes. This arrangement is sometimes referred to as a Galvanic Anode System for cathodic protection. Absent proximity of major grid connected electrical equipment and assuming that the imperfections in the dielectric coating are relatively small the magnesium electrodes produce a differential voltage that makes the steel pipe about 1.9 volts negative with respect to the surrounding ground water. Practical experience has demonstrated that in locations that are remote from major grounded electrical equipment this methodology is adequate to prevent normal corrosion. In remote areas where there is no nearby major electrical equipment the magnesium electrodes may be quite far apart. Magnesium is an expensive metal and is typically used in 25 pound blocks because it slowly corrodes away over time. Hence pipeline companies try to minimize their use of these magnesium electrodes by spacing them as far apart as circumstances reasonably permit at the time of original pipeline installation.

Each pipeline carrying a fluid emerges from the ground at the fluid source, at pumping/compressor stations and at consumer premises. Generally at these locations distribution pipe is connected to furnaces, boilers, pumps, compressors, etc. that are electrically grounded. On distribution pipes there are a large number of such random electrical ground connections so the pipe can be considered to be at ideal electrical ground potential. Hence the magnesium electrodes bias the ground water surrounding the pipes at 1.9 volts positive with respect to ideal electrical ground potential.

If an external electrically induced ground current changes the ground water potential by less than 1.0 volt at the pipeline then 0.9 volts of negative bias remain on the pipe with respect to the surrounding ground water and the pipeline corrosion protection is not seriously compromised. The 1.0 volt may consist of 0.5 volt DC + 0.5 volt AC. However, if the ground water potential near the pipeline drops by more than 2.0 volts rapid electrically accelerated pipe corrosion will occur at pipe dielectric coating defects.
 

TYPICAL WIND TURBINE POWER COLLECTION CONFIGURATION:
The dominant manufacturer of wind turbine generators in North America is GE. The output of a typical 3000 kVA GE wind generator is at 6000 volts and is connected to the grounded wye primary winding of an adjacent three phase wind turbine transformer.

The wind turbine transformer secondary is isolated delta connected, and typically operates at 34.5 kV phase to phase, 19.9 kV phase to ground. This power is fed via either overhead wires or via buried cables to a local substation. At the substation there is a 34.5 kV transformer which is wye connected with the neutral grounded.

This arrangement has the advantage that it suppresses third harmonics originating in the wind generator. However, a problem with this arrangement is that there is a third harmonic current path from the substation transformer, through the power collector conductors, through the wind turbine transformer capacitance and back to the substation through ground. The current through the transformer capacitance to ground is typically about 0.5 amperes.
 

ELECTRICALLY ACCELERATED CORROSION:
Now assume that in the proximity of a buried steel pipeline someone introduces a wind turbine electricity generator. Even with an isolated delta transmission line connection to the wind turbine transformer there is typically a ground current of about 0.5 amps that flows through the wind turbine transformer capacitance to ground. One of the causes of this current is third harmonic voltage generation in the substation transformers. Another cause is fundamental frequency voltage imbalance. Another cause is transformer asymmetry. An aggravating factor is single phasing due to a blown fuse. For all kinds of practical reasons the three phases are not perfectly balanced. This imbalance leads to additional ground current at the power line fundamental frequency. If the ground resistance is 10 ohms then the induced ground voltage is typically:
0.5 amps X 10 ohms = 5 volts.
Hence, the voltage of the water in the soil around the wind turbine generator also oscillates 5 volts RMS positive and then 5 volts RMS negative with respect to the substation ground. This ground water voltage oscillation will gradually diminish with increasing distance from the wind turbine generator. However, if at the pipeline the electrically induced voltage exceeds about 1.0 volts negative with respect to the substation ground, the pipe corrosion rate at any nearby coating defect will rapidly increase and a pipeline rupture failure may eventually occur.
 

DC CORROSION:
Wind turbines frequently incorporate DC power supplies to allow adjustment of their magnetic fields. Wind turbines with grounded wye neutral power distribution potentially provide a path that allows inadvertent generation of DC ground current. Depending on its polarity the DC ground current will either make the corrosion situation much worse or much better. However, due to the variable wind generator output power this DC ground current may from time to time reverse in polarity, depending on the wind turbine generator power output. If DC ground current flows between two nearby wind generators one of the wind generators will cause electrically accelerated corrosion in a nearby pipeline. Hence DC ground current should be totally eliminated at any wind farm in the proximity of a buried steel pipeline by use of a wind turbine transformer with a floating delta secondary.
 

GROUNDING WIND TURBINES:
For transformer protection it is necessary to use heavy duty lightning arrestors between the phase conductors and ground at each wind turbine, and to install a large ground mesh at each wind turbine. It is also desirable to electrically bond together the grounds of the wind turbines and the substation using a very thick oxygen free high conductivity (OFHC) ground cable. The intent of this ground cable is to provide a ground current path that has a lower resistance than the earth ground. All of the aforementioned measures for reducing stray ground currents and hence electrically accelerated corrosion have lightning protection consequences and associated costs.

The induced AC ground current can be further reduced by use of low capacitance wind turbine transformers and highly linear substation transformers. There is a related further tradeoff in transformer efficiency and cost.
 

FURTHER CORROSION REDUCTION:
For major pipelines the effect of ground current on pipeline corrosion can be further reduced by using a DC power supply to bias the buried steel pipe more negative than 1.0 volts with respect to the surrounding ground water. This technique is sometimes referred to as an Impressed Current Anode System. However, this technique is only practical in applications such as main transmission pipelines where the pipeline company controls all the external connections to the pipeline. Every such connection needs a dielectric fitting. There are many ways that this anti-corrosion technique can be defeated by action of Murphy's Law.
 

MURPHY'S LAW:
Murphy's Law basically says that if there is a way for something to be done wrong that leads to equipment damage and/or personnel injury, sooner or later someone will find it. Usually Murphy strikes as a result of insufficient personnel training.

With respect to the corrosion issues raised herein some common Murphy issues are:
1. A backhoe operator scratches a pipeline and then covers up that scratch without reporting it, or if he/she does report it the report lacks sufficient detail for a repair crew to later find the scratch and fix the dielectric coating on the pipe. A large scratch can eventually lead to a pipe rupture failure.
2. A thief learns where the magnesium electrodes are and steals them for their scrap metal value. Then there is no bias voltage protection and electrically accelerated corrosion is much worse.
3. Major transmission pipelines use DC power supplies instead of magnesium electrodes to provide bias voltage protection. However, such protection is easily inadvertently compromised by trades persons who fail to use insulated pipe hangers or who fail to use appropriate dielectric fittings or dielectric isolation on flange bolts.
4. Such power supplies may also be accidentally compromised by grounded remote metering circuits.
5. Such power supplies, unless monitored via a computerized alarm system, are easily accidentally left off or inadvertently turned off by persons who simply do not understand the operation of the corrosion protection system.
 

LIGHTNING ISSUES:
Wind turbines are constructed using tall steel masts. In a thunder storm lightning discharges preferentially strike these tall steel masts or strike overhead wires used for wind farm power collection. The wind turbine mast is grounded. The ground connection typically consists of metal rods that are driven into the ground in a circle around the wind turbine mast. Then when lightning strikes the wind turbine tower the maximum potential rise of the tower is limited to the product of:
(the lightning current) X ( the ground resistance).
For example if the peak lightning current is 200 amps and the ground resistance is 10 ohms the potential rise of the grounded wind turbine equipment is:
200 amps X 10 ohms = 2000 volts.

If lightning strikes a radial wire then a lightning arrestor connected to ground is used to prevent the voltage on the radial wire exceeding the breakdown voltage of the wind turbine transformer.
 

GROUND RESISTANCE:
The ground resistance is the electrical resistance through the ground between the ground rod(s) of the wind turbines and the ground mesh of the substation. Absent the buried pipelines the typical ground resistance for a single ground rod is 5 ohms in damp soil conditions and as much as 500 ohms in very dry soil conditions. The ground resistance can be reduced by using a multiplicity of ground rods in a circle around the wind turbine. For the purpose of the mathematical analysis contained herein the radius of that circle is Zo.

An issue of importance is rate of drop of induced ground voltage with radial distance from a wind turbine generator. This voltage decrease is a function of the physical distribution of subsurface ground resistivity.

We will examine herein two common cases, first the two dimensional (2 D) case where there is a uniform layer of water bearing top soil on top of a flat insulating base such as clay or bedrock. The second three dimensional (3D) case is where there is uniform water bearing soil that extends to a great depth.

Define:
Rho = electrical resistivity of water bearing soil
T = thickness of layer of water bearing soil
Zo = radius of ground rod circle that surrounds a wind turbine
Z = distance from the wind turbine
Zp = distance from the wind turbine to a nearby pipeline
Zm = distance from the wind turbine to a point where pipeline is connected to an ideal electrical ground (typically the electrical substation ground)
R = electrical resistance between wind turbine ground rods at radius Zo and sense rods at radius Zm.
dR = element of resistance between Z and (Z + dZ)
Pi = 3.14159
Vo = induced ground voltage at the wind turbine with respect to the electrical substation ground
Vp = induced ground voltage in ground water at pipeline
Ln = natural logarithm
 

GROUND RESISTANCE (2D):
An element of ground resistance dR takes the form of a flat cylinder of surface area (2 Pi Z T).
dR = Rho dZ / (2 Pi Z T)
R = Integral from Z = Zo to Z = Zm of:
Rho dZ / (2 Pi Z T)
= [ Rho / (2 Pi T)] Ln[Zm / Zo]

Similarly the ground resistance between Zo and Zp is:
= [Rho / (2 Pi T)] {Ln[Zp / Zo]}
 

INDUCED VOLTAGE AT PIPELINE (2D):
Voltage division caused by these two ground resistances gives:
(Vp / Vo) = {(Ln[Zm / Zo] - Ln[Zp / Zo] ) / Ln[Zm / Zo]}

Under these circumstances the wind turbine induced ground potential declines only very slowly with distance. For example, if:
Zo = 10 m, Zp = 100 m, Zm = 2,000 m
then:
(Vp / Vo) = {Ln[200] Ln[10]} / Ln[200]
= (5.298 2.303) / 5.298
= .565

Assume a 5 volt induced ground voltage at the wind turbine. Hence the induced voltage at the pipeline is:
5 volts X .565 = 2.826 volts.
This voltage is much greater than the 1.9 volts of protection provided by the magnesium electrodes. Hence, under these circumstances, even with a 100 m set back between each wind turbine and the closest buried pipeline, a wind farm will cause electrically accelerated corrosion at any nearby pipeline dielectric defect.

Assume that the setback is increased to 1000 m. Then:
Zo = 10 m, Zp = 1000 m, Zm = 2,000 m
giving:
(Vp / Vo) = {Ln[200] Ln[100]} / Ln[200]
= (5.298 4.605) / 5.298
= .1308

Assume a 5 volt induced ground voltage at the wind turbine. Hence the induced voltage at the pipeline is:
5 volts X .1308 = .654 volts.
This voltage is sufficiently small by comparison to the 1.9 volts of protection provided by the magnesium electrodes to not cause a significant increase in corrosion. Hence under these circumstances a setback of about 1 km between a wind turbine and the nearest buried steel pipeline is required to avoid electrically accelerated corrosion at any nearby pipeline dielectric defect.
 

GROUND RESISTANCE (3D):
An element of ground resistance dR takes the form of a hemisphere of area 2 Pi Z^2.
dR = Rho dZ / (2 Pi Z^2)

The ground resistance R between Zo and Zm is given by:
R = integral from Z = Zo to Z = Zm of
Rho dZ / (2 Pi Z^2) = (Rho / 2 Pi) [(1 / Zo) (1 / Zm)]

Similarly the ground resistance between Zo and Zp is given by:
Integral from Z = Zo to Z = Zp of
Rho dZ / (2 Pi Z^2)
= (Rho / 2 Pi) [(1 / Zo) (1 / Zp)]

Voltage division gives:
(Vp / Vo) = {[(1 / Zo) (1 / Zm)] - [(1 / Zo) (1 / Zp)]} / [(1 / Zo) (1 / Zm)]
= [(1 / Zp) (1 / Zm)] / [(1 / Zo) (1 / Zm)]
= [(Zm / Zp) 1] / [(Zm / Zo) 1]

Usually Zm >> Zo and Zm >> Zp.

Hence this formula simplifies to:
(Vp / Vo) ~ (Zo / Zp)

Thus for Zo = 10 m and Zp = 100 m then:
(Vp / Vo) ~ 0.1

Under these circumstances a 100 meter setback between the wind turbine generator and the nearest pipeline is sufficient to reduce an induced 5V ground potential oscillation at the wind turbine to 0.5 volts at the pipeline, which makes electrically induced corrosion negligibly small. The exact setback that is required depends on the actual induced ground voltage at the wind generator.
 

SUMMARY:
It has been shown that if the wind turbine induced ground voltage with respect to the substation ground is less than 1.0 volts then there is no significant increase in nearby buried pipe corrosion independent of wind generator setback from the pipeline. If the induced ground voltage at the wind generator is 5 volts and if the soil under a wind farm is conductive to great depth then the required setback of the wind turbine from nearby buried steel pipelines is about 100 m. However, if the topsoil sits on a nonconducting layer such as bedrock, then the required setback between the wind turbine and the nearest buried steel pipeline is about 1000 m.

Clearly when construction of a wind farm in proximity of a buried steel pipeline is contemplated the first thing that should be done is to measure the actual decline in induced ground voltage with distance.
 

VOLTAGE IMBALANCE AND WIND TURBINE TRANSFORMER CAPACITANCE:
In the above analysis the assumption was made that the electrically induced ground voltage at a wind turbine is typically 5 volts. It is worth exploring this assumption.

The principal sources of this voltage are ground current due to fundamental frequency phase voltage imbalance and ground current due to harmonic voltage generation within the substation transformers.

On November 15, 2013 the capacitance between the delta connected 34.5 kV winding of a standard 2000 kVA GE Prolec wind turbine transformer and ground was measured and found to be .022 micro farads.

GE estimated that the capacitance between the delta connected 34.5 kV winding of a standard 3400 kVA GE Prolec wind turbine transformer and ground would be .0374 micro farads. GE further indicated that, based on its experience with lower capacitance transformers for major solar power systems, for a sufficient cost premium the wind turbine transformer capacitance could be reduced by about a factor of two.

In an ideal 3 phase electrical distribution system with no harmonics the three phases have exactly the same voltages and currents with 120 degree phase shifts. In reality there are harmonics and phase voltage and phase current imbalances. The harmonics are due to nonlinearity and the imbalances arise because of assymmetry issues in the generators and the load. At the third harmonic frequency the ground current is the sum of the third harmonic currents in each of the phase conductors.

A good estimate of the induced ground voltage is:
Vo = (induced ground voltage)
= (the ground current) X (the ground impedance)
.
The ground impedance is slightly larger than the neutral resistance.
 

ESTIMATE OF THE FUNDAMENTAL GROUND CURRENT:
Assume a 34.5 kV three phase power collector system. Then the phase to neutral voltage is given by:
34.5 KV / (3^0.5) = 19.9 kV

Assume a single phasing condition. Then the fundamental frequency voltage driving the wind turbine transformer capacitance is 19.9 kV.
The capacitive reactance at the fundamental frequency is:
Xc = 1 / (2 Pi F C)
= 1 / (2 X 3.14159 X 60 Hz X .0374 uF)
= .07092 X 10^6 ohms
= 70, 920 ohms

The corresponding ground current at the fundamental frequency is given by:
19.9 kV / 70.720 k ohms
= 0.281 amps
 

ESTIMATE OF THIRD HARMONIC GROUND CURRENT:
Assume a 34.5 kV three phase power collector system. Then the phase to neutral voltage is given by:
34.5 KV / (3^0.5) = 19.9 kV

Assume that the substation transformer operates at a point on its core saturation curve where it produces 4% third harmonic energy into a resistive load. That point corresponds to a third harmonic voltage which is 20% of the fundamental voltage. Hence the third harmonic voltage driving the wind transformer capacitance is given by:
(.2)(19.9 kV) = 3.98 kV

The capacitive reactance Xc if the wind turbine transformer at the third harmonic is:
Xc = 1 / (2 Pi F C)
= 1 / (2 X 3.14159 X 180 Hz X .0374 uF)
= 10^6 ohms / (42.298)
= 23641.78 ohms

Hence the third harmonic contribution to the ground current is:
3.98 X 10^3 volts / 23,641.78 ohms = 0.168 amps
 

ADDITIONAL GROUND CURRENT DUE TO OTHER HARMONICS:
The total ground current is the sum of the ground currents at the 1st, 3rd, 5th, 7th, 9th, 11th etc. harmonics. If the substation transformers have any non-linearity which leads to harmonic voltage production this total ground current can easily exceed 0.5 amps.
 

GROUND POTENTIAL OSCILLATIONS IN PROXIMITY TO OTHER GROUNDED ELECTRICAL SYSTEMS:
Single Family Homes:
A typical single family home is fed by a 120 V / 240 V split phase transformer. All the major loads such as stove ovens, dryers, etc are 240 volt connected so that from the transformer's perspective they are balanced. Random small 120 volt loads are approximately equally divided across the transformer. When multiple homes share the same pole mounted utility transformer there can be ground current between the homes. However, the transformer imbalance seldom exceeds 1.0 volts so the ground voltage swing is restricted to 1.0 volts and no buried steel pipeline corrosion results.

Major Buildings:
Major buildings are normally delta fed and use a dedicated transformer for that building. Hence the only source of ground current is capacitance across the transformer. Major buildings also have substantial electrical grounds to provide lightning protection. Since the buried natural gas distribution pipe line is normally connected to the building's electrical ground, at the building the pipe line potential and the ground water potential are the same. Usually the buried natural gas feed line is very well insulated in close proximity to the building because magnesium is ineffective at altering the ground water potential in that region. In a typical high rise building that insulation is achieved by routing the pipeline through the underground parkade.
 

GROUND FAULT DETECTION:
If there is a buried pipeline close to a wind turbine electricity generator it is important to detect if the ground current is excessive and if so to cut off the wind turbine until the source of the ground current is found and the problem is fixed. Hence it is important to have a Ground Fault Interrupter (GFI) at each wind turbine. It is also important for a third party to routinely test, monitor and report on the setting of this GFI because the equipment owner may have no financial incentive to replace defective isolation transformers as long as the wind turbine is generating energy.
 

RECOATING:
An alternative solution is to re-coat the pipeline with dielectric material so that there is certainty that there are no dielectric defects in the proximity of the wind turbine electricity generators. This solution has been used in the USA in places where the pipeline company could not control the location of nearby wind turbines. However, it involves digging up, cleaning, re coating and reburying the pipelines in the vicinity of the wind generators.
 

CONCLUSIONS:
If the subsurface is characterized by a layer of damp soil on top of a high electrical resistivity layer such as bedrock, a wind turbine installation may need to be set back over 1 km from a buried steel pipeline to prevent electrically accelerated corrosion of the pipeline.

If the subsurface is characterized by a uniform water bearing soil that goes very deep, then setting wind turbine generators back over 100 m from a pipeline should prevent the wind turbine system significantly affecting the pipeline corrosion. The exact setback requirement is a function of the induced ground voltage which in turn is proportional to the induced ground current. This induced ground current can be minimized by use of suitably chosen substation power transformers, low capacitance wind turbine transformers, a common low resistance wind turbine tower ground conductor and related lightning protection.

It is strongly recommended that the change in induced ground voltage with distance be measured before a major investment is made constructing a wind farm in the proximity of existing buried steel pipelines. It is critical that the induced AC ground voltage in ground water in the proximity of every buried steel pipeline that is protected by magnesium electrodes always be less than about 0.5 volt and that the induced DC ground voltage in ground water in the vicinity of the buried steel pipeline protected by magnesium electrodes also always be less than 0.5 volts. These specifications should be met at all locations and at all wind turbine power levels.

If an electrical power supply is used for impressing a negative voltage on the pipeline with respect to the surrounding ground water safety considerations limit the maximum value of that negative bias voltage to about 24 volts. Hence the problem of electrically accelerated corrosion can be mitigated but not eliminated by raising the pipeline bias voltage. However, this is a measure that must be taken by the pipeline owner, not the wind farm owner. Further, making this measure reliable requires on going alarm monitoring and maintenance dispatch, that have related costs. This measure may also trigger hydrogen related stress corrosion cracking of the pipeline, so it is a measure of last resort.
 

This web page last updated January 27, 2014.

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