THE EFFECT OF EARTHING OF CABLE SHEATHS ON FAULT CURRENT DISTRIBUTION

Neil McDonaghESB International, Ireland

ABSTRACT

In urban areas, high voltage underground cables are commonly used for the transmission and distribution of electricity. Many such high voltage cables have metallic sheaths or screens surrounding the conductors, and/or armour and metallic pipes surrounding the cables. During earth faults applied to directly earthed systems, these metallic paths are expected to carry a substantial proportion of the total fault current, which would otherwise flow through the general mass of earth, while returning to system neutrals. These alternative return paths must be considered when determining the extent of the grid potential rise at an electrical plant due to earth faults. This paper examines fault current distribution following a single phase to earth fault at a high voltage urban substation. Sub A 110kV substation is fed by two 110kV connections to Sub B and Sub C substations. Both feeders are pipe type cables. A network model incorporating the substation earth grid and cable sheathsis built using proprietary software and the results presented. Current injection tests are carried out to verify modelled results. Finally the implications of alternative return paths provided by cables are discussed and conclusions are drawn.

Keywords: Earthing, Grounding, cable sheaths, Substation, Fault current distribution, Cable sheaths, current injection test, computer modelling.

INTRODUCTION

During a phase to earth fault on a directly earthed system, fault current will return to source transformer neutrals by whatever means are available, primarily through the general mass of earth and via metallic connections. These metallic connections may be provided by shield wire or counterpoise in the case of overhead lines and by cable sheath, armour, pipes and counterpoise in the case of underground cables. Fault currents may also return along unintentional paths such as gas pipelines, railway lines, or any metallic connection.

Fault current that returns via the earth must pass through the substation earth grid to the earthed transformer neutral. This will produce a grid potential rise with respect to remote earth (GPR). It is desirable to reduce the GPR at substations during earth faults for a number of reasons; principally to reduce touch and step voltage hazards that may exist at the substation, but also to minimise transfer voltage hazards along other utilities such as telecommunications circuits, railway lines, gas pipe lines, etc.

GPR (volts) = Ig * EGR (Equation 1)

GPR = potential rise with respect to remote earth (volts)Ig = earth fault current in amperesEGR = earth grid resistance in ohms

It is clear to see from Eq 1 that the only two ways to decrease the GPR are to decrease the earth fault currentIg or to decrease the earth grid resistance EGR.

To reduce the earth grid resistance a number of methodsmay be used. The substation earth grid in question may be extended or reinforced, perhaps through the use ofvertical earth rods, satellite earth grids, etc.

Fig 1 Grid Potential Rise Plot

If = Im + Ig (Equation 2)If = Total fault currentIm = current travelling along metallic return paths

Any continuous metallic connections away from the substation that are connected to earth at the substation and at some other remote location will provide an alternative path for fault current thus, reducing the amount of current flowing through the earth at the site of interest (Eq 2).

However it must be noted that voltages transferred along these metallic connections may be hazardous if a sufficient earthing system is not present where these connections are earthed.

During a phase to earth fault the faulted phase will carry fault current. This fault current induces a current in parallel metallic return paths, such as cable sheaths, so that some of the current which could have travelled through the mass of earth instead travels along these return paths, thereby reducing the current component contributing to GPR (Eq2). [1]

Fig.2 Phase to Earth Fault

This principle is illustrated in Fig 2 where a typical phase to earth fault is displayed. (It must be noted that this diagram is merely illustrative and the direction of current flow indicated by the arrows is a simplistic illustration.).

If the earth fault current is reduced then the GPR is also reduced Eq 1. Therefore it can be seen that a continuousmetallic connection, such as cable sheath, armour, pipe, counterpoise or shield wire, along a faulted phase can reduce the GPR at substations at both its ends by virtue of these two mechanisms, namely conductive and inductive paths.

Case study

Sub A is located in a large urban centre and fed by two 110kV connections to Sub B and Sub C. Each of the 110kV feeders is a 110kV pipe type cable. Cable sheaths surround each of the cores, armour surrounds the three cables. The pipe, armour and sheaths all provide a return path for fault current to various earthed neutrals on the system. The mechanism by which these metallic return paths carry fault current is two-fold. Firstly they provide a series impedance connection to Sub A thus reducing the proportion of current that is forced to flow through the impedance to remote earth of the earth grid at Sub A thus reducing the overall grid potential rise or GPR.

Secondly sheaths in the vicinity of the faulted phase carry induced current away from the fault location. A large proportion of current will therefore circulate in the core of that faulted phase and the metallic return paths. This paper examines this phenomenon using industry standard software CDEGS [2]. Modelled results are also backed up current injection test results

CABLE MODEL

Fig. 3 shows the diagram on which the cable model was based. Each phase and sheath were modelled, the pipe is also modelled. Due to software constraints, it was not possible to accurately model the armouring that covers all three cables as shown in Fig. 3. Therefore three models were constructed in an attempt to accurately model self impedances and mutual impedances between cores, sheaths and pipes.

Fig. 3 City Type Cable

1. copper core 6. aluminium sheath (APL) sheath2. conductor screen 7. bedding 3. XLPE insulation 8. armouring4. Insulation screen 9. steel pipe with insulation 5. semi-conducting tape

Model A: the cable was modelled as shown in Fig. 3 but without armouring.

Model B. The cable was modelled as in model A but with individual armouring around each cable. The three individual armours have the same cross-sectional area as the actual armour shown in Fig 3.

Model C. The pipe was modelled as having the same cross section as the pipe and armour combined and was modelled as close as possible to the cable cores to account for the mutual impedance path provided by the armour

Faulted phase If

Metallic return paths Im

Current path through earth Ig

Fault

Substation earth grids

Cable model analysis

A number of different scenarios were created in order to analysis the difference in performance of each cable model. All calculations are based on the description of the network provided in the introduction. For allcalculations it is assumed that Sub B and Sub C have 1Ωearth grid resistances. The earth grid resistance at Sub A and the distance between Sub A and Subs B and C are altered to analyse the effect on earth fault current and therefore GPR. The distance between Sub A and Sub B is equal to the distance between Sub A and Sub C for all calculations. During all calculations current is injected along the feeder to Sub C.

For calculations 4, 5 and 6 the feeder to Sub B will be disconnected. All six calculations are carried out for 3 cable lengths: 2km, 5km and 10km. The following EGR’s are used at Sub A: for calculations 1 and 4: 1Ω, for calculations 2 and 5: 5Ω and for calculations 3 and 6:20Ω. It can be seen from these results (Table 1) that for each calculation the earth fault current calculated usingModel B cable is between those calculated for Model A and Model C. This is understandable for the following reasons: Model A under-estimates the series impedance of the cable by neglecting the armour. Model C over-estimates the mutual coupling between the faulted phaseand the pipe. Model B may slightly over-estimate the mutual coupling between the faulted conductor and the armour but Model B is seen as the most accurate and will be used in all further analysis.

Table 1 Earth Fault Current and GPR per kA of Injected Current

2km 5km 10kmcalc Ig GPR Ig GPR Ig GPR

A 27.9 27.9 48.4 48.4 59.5 59.5

B 21.7 21.7 37.5 37.5 46.2 46.21

C 17.4 17.4 29.4 29.4 35.7 35.7

A 8.8 44.2 20.4 102.2 34.8 174.0

B 6.9 34.4 15.9 79.4 27.0 135.22

C 5.6 28.0 12.8 64.1 21.5 107.5

A 2.4 48.9 6.0 120.2 11.7 233.2

B 1.9 38.0 4.7 93.6 9.1 181.73

C 1.6 31.2 3.8 76.6 7.4 147.9

A 37.0 37.0 55.7 55.7 63.3 63.3

B 28.7 28.7 43.2 43.2 49.1 49.14C 22.8 22.8 33.5 33.5 37.9 37.9

A 15.6 77.8 32.8 164.2 48.5 242.4

B 12.1 60.5 25.5 127.6 37.6 188.15C 9.8 49.1 20.3 101.7 29.5 147.3

A 4.7 94.5 11.4 228.9 21.4 427.6

B 3.7 73.5 8.9 178.2 16.6 332.66

C 3.0 60.4 7.3 145.2 13.4 268.8

CALCULATIONS

Fig. 4 Fault Circuit Diagram

Zsc= Series impedance of metallic return path to Sub CZsb= Series impedance of metallic return path to Sub BZmc= Mutual impedance between faulted phase and

metallic return paths on feeder to Sub CZmb= Mutual impedance between faulted phase and

metallic return paths on feeder to Sub B

Fault current will take all available paths back to the source, but the majority of current will flow along the lowest impedance path. For a fault at Sub A fed from Sub C current will return along the following paths to return to the source at Sub C: through the earth at Sub A and returning through the earth at Sub C, through the metallic return paths (both Zsc and Zmc) from Sub A to Sub C and through the metallic return paths to Sub B (Zsb) then through the earth at Sub B and returning through the earth at Sub C

The proportion of current that flows through each path will depend on the relative impedances of each path. If the system was completely isolated from all other electrical and metallic systems there would be only three return paths. In reality the situation may be quite different as all three substations may have metallic connections to medium voltage substations or Sub A and Sub C may be indirectly connected via other 110kV cable sheaths, shield wires or a combination of both.

On each feeder there are 10 metallic components that must be considered (Fig. 5); three cores, three sheaths, three armours and the pipe. There is a mutual impedance between each of these components and the faulted phase. Each of these paths will also have a series impedance between the two substation earth grids, although the two non-faulted phases will not act as return paths. Therefore there are ten series impedance paths to be considered and forty-five mutual impedance paths that must be considered to created an accurate mathematical model of the cable.

Imputing accurate positional and material property information CDEGS will calculate self impedance of each component and the mutual impedance of each component to every other component. These results are

used in conjunction with line lengths and earth grid resistances in order simulate complete circuit model.

Fig.5 Cable Model B

In order to verify this model results shall be compared to formulas [3].

Equation 3

++

++++

−=

++++

++++++

Bmp,3A

B)mp,2A

Bmp,1A

BcnABnm1,A

Bm1,2A

Bnm1,ABc1A

Rlz(R

Rlz(R

)Rlz(R

)Rlz(R-----)Rlz(R

||

|------------)Rlz(R

)Rlz(R-------)Rlz(R

f

n

2

1

I

I

I

I

Ig = -If - I1 - I2 - .............-In (Equation 4)RA = Earth grid resistance at Sub ARB = Earth grid resistance at Sub Bl = length of feederZc1 = series impedance along return path 1Zm1,2 = Mutual impedance between path 1 and 2Zmp,1 = mutual impedance between faulted phase and

return path 1I1 = Current flowing along return path 1

(a) Fig. 6 (b)

7

6

5

4

3

2

1

o

o

o

o

o

o

o

167.27@66.63

167.27@66.63

144.84-@147.87

143.87@570.16

@160.147114.48

160.147@114.48

104.93-@410.134

o

o

o

o

o

o

o

.12757.461@175

.12757.461@175

136.974-127.51@

735491.7@151.

.03598.734@168

.03598.734@168

129.093-300.13@

Fig 6 (a) resultant current matrix from equation 3.Fig 6 (b) resultant current matrix give by CDEGS.

A circuit with 2 substations was created to compare results. Both earth grid resistances are 1Ω. The

connection between the substations was 5km in length. The injected current was 1000 + 0j A. The resultant current matrices can be seen in Fig 6 (a). Each metallic return path is labelled with a number: 1, 2 and 3 are cable sheath, -1 is the sheath on the faulted phase. 5, 6 and 7are armouring surrounding each phase, -5 is the armouring around the faulted phase. 4 is the steel pipe.

It can be seen in both Fig 6 (a) and (b) that the metallic return path carrying the most current is number 4, the steel pipe, followed by number 1, the sheath on the faulted phase. The differences between (a) and (b) may be due to a number of factors. Primarily equation 3ignores the effect of the two non faulted phases and current leakage along the length of the feeder while CDEGS takes this into account

The earth fault current calculated from equation 4 is 50.1 A @ 230o. The earth fault current calculated by CDEGS is 43.2 A @ 237o.

Variation in GPR at Sub A

A number of different circuits, similar to those examinedin section 2.2, were created in order to analyse a phase to earth fault. The cable lengths to Subs B and C were altered. The EGR at Subs B and C was 1Ω for all tests and while the EGR at Sub A was varied. The earth fault current and GPR was calculated in each case. These results are shown in Table 2 and Fig. 7.

Table 2 Earth Fault Current per kA of injectedcurrent

lengths (km) 2 5 10 20EGR

1 21.66 37.54 46.17 50.572 14.23 28.72 40.29 47.765 6.87 15.89 27.03 38.9710 3.67 8.87 16.53 27.9220 1.9 4.68 9.08 16.91

Variation in GPR due to change in cable length

0

50

100

150

200

250

300

350

400

2 5 10 20Cable lengths (km)

GP

R (v

olts

) per

kA

of

inje

cted

cu

rren

t

20 ohm

10 ohm

5 ohm

2 ohm

1 ohm

Fig. 7 Plot of results from Table 2

It can be clearly seen from Table 2 and Fig. 7 that an increase in Cable length or EGR causes a rise in the GPR at Sub A.

Steel pipeCable core

Armour

Cable sheath

MEASURED RESULTS

A current injection test was carried out at an urban substation in the spring of 2005. The circuit used for the current injection was similar to the description of the network provided in the introduction. The faulted phase was on the feeder to Sub C. The lengths of the feeders to Sub B and Sub C were 4.7km and 7.2km respectively. The EGR at Sub A was modelled as 40 Ω, while the EGRs at Subs B and C were assumed to be 1Ω. The results shown are averages of measurements taken for a number of different injected currents. The magnitudes displayed are percentages of the fault current.

It must be noted that a number of issues were noted during measurement. At times there were current surges of up to 10A being measured on the pipe of feeder C when no current was being injected. Injected currents varied between 10A and 137A; therefore a 10A surge could have a serious effect on measurements. When the current on this pipe was measured, erratic variations were noted. It must also be noted that medium voltage connections were not considered and it is expected that a proportion of current may have taken this path. It was not possible to measure the currents on the armour. In reality the cable sheaths, armours and pipe are bonded at every joint.

Table 3 Measured ResultsSub B Sub C

Injected current mag angle mag angleR phase 0 0 100 0R sheath 2.42 180 26.69 -141.4S sheath 2.58 180 11.29 -162.8T sheath 2.48 180 11.23 -164Pipe 8 180 *85.5 -170

Table 4 Results Calculated by CDEGSSub B Sub C

Injected current mag angle mag angleR phase 0 0 100 0R sheath 0.139 -30.1 30 -129S sheath 0.131 -33.8 9.81 168T sheath 0.131 -33.8 9.81 168R Armour 0.081 -23.0 12.7 -137S Armour 0.077 -28.5 5.71 175T Armour 0.077 -28.5 5.71 175Pipe 2.81 53.7 50 150

Table 5 Results Calculated from Equation 3Sub B Sub C

Injected current mag angle mag angleR phase 0 0 100 0R sheath 0.21 180 40.9 -104.9S sheath 0.175 120 11.36 159.9T sheath 0.175 120 11.36 159.9R Armour 0.125 134 14.72 -144.8S Armour 0.102 127 6.613 167T Armour 0.102 127 6.614 167Pipe 3.46 -146 58.63 142.7

** Error due largely to interference on feeder CTable 6 Earth Fault Current and GPR per 100A of

injected currentI g GPR (V)

Measured **56.7 ***Equation 3 3.13 11.16CDEGS 2.79 12.55

***GPR cannot be estimated from fault current as medium voltage circuits were connected to the substation earth grid. Also it was not possible to disconnect a the feeders in order to record a voltage measurement andthus calculate the GPR and resistance to remote earth of substation earth grid

Calculated results are also displayed from CDEGS (table 4) and an alteration was made to equation 3 (Table 5) in order to incorporate a second substation into the model.

Variations in results (Table 6) may be due to a number of factors. Sections of the pipes are very old and insulation may not still be intact. In reality the cables, armours and pipes are bonded at every cable joint, this is not considered in either of the calculated models. Medium voltage connections were not considered at Sub A which may lower the effective EGR seen by the fault at Sub A.This would raise Ig but lower the GPR significantly.

CONCLUSIONS

It can been seen from measured and computed results that metallic return paths provided by cable sheaths,armours and pipe have a very positive effect in reducing the GPR, and consequently the hazards caused by large GPRs, by carrying large proportions of the fault current away from substations through metallic return paths as opposed to through the earth. Current flows due to conductive and inductive effects through these return paths. Replacing non-shield wire overhead lines with shield wire overhead lines or underground cables can significantly reduce hazards associated with high voltageearth faults. While this may not always be economically desirable or possible, the presence of metallic return paths should always be considered when calculating earth fault currents.

REFERENCES

1. Electricity Association Technical Specification 41-24 Issue 1, page 23, 1992.

2. CDEGS version 11.3.107. SES Technology Ltd, 20043. Electricity Networks Association, Engineering

Recommendation S34, page 25 1986.

AUTHOR’S ADDRESSThe author may be contacted at

Power Systems StudiesESB International Stephen Court18-21 St Stephen’s Green Dublin 2. neil.mcdonagh@esbi.ie