gas insulated substation grounding system design using the

<@>-.. 2012 China International Conference on Electricity Distribution (CICED 2012) Shanghai, 5-6 Sep. 2012

Gas Insulated Substation Grounding System Design Using the

Electromagnetic Field Method

Jie LlU and Farid P. DA WALIBI Safe Engineering Services & technologies ltd. Canada

[email protected]

Abstract-This paper presents an accurate

electromagnetic field theory method to model and

analyze a realistic grounding system involving gas

insulated substation (GIS). The GIS structure,

ground conductors, massive reinforced steel rebar,

and aboveground bus bars were modeled

accurately in detail to evaluate GIS safety. The

study takes the effects of the inductive coupling

between GIS inner high-voltage buses and outer

enclosures into account. Soil resistivity test data

interpretation and fault current distribution

analysis were carried out as well. The study

demonstrates how advanced simulation

approaches can be used to analyze complex

grounding issues for a GIS substation and

produce accurate results.

Keywords-Grounding system, GIS substation, Electromagnetic field.

I INTRODUCTION A grounding system design of the gas-insulated

substation (GIS) is a complicated task to provide an adequate grounding system and meet specific criteria with regards to personnel safety and integrity of equipment during a fault condition. There are a few additional challenges unique to simulation approaches.

First, in a conventional substation grounding study, it is assumed that the entire grounding system being designed constitutes an equipotential structure and no induction involve in the analysis. However, in the case of GIS, the GIS equipment has the metal enclosing gas-insulated switchgear and the inner high-voltage buses that are completely contained within the outer pipe type enclosures. Under fault conditions, especially when the fault is inside the enclosure, induction between faulted inner buses and associated enclosures can result in significant induced currents in the enclosures that may generate sufficient voltage drops along the enclosures. The same thing may happen between the enclosures and the ground conductors as well. Therefore, an additional internal fault within the gas-insulated bus system as suggested in IEEE Std. 80-2000 has to be examined when there is a flashover between the bus conductor and the inner wall of the enclosure [1]. Secondly, the aboveground bus bars are parallel to the ground conductors and there is indeed inductive coupling between aboveground bus bars and ground conductors when a grounding system has a rather large size. Moreover,

Brian F. MAJEROWICZ Baltimore Gas and Electric Co. USA

[email protected]

when a large amount of circulating currents exists within the substation, they are also flowing along

the aboveground bus bars from local transformers to the fault location and then returning back to the sources through the ground conductors. This may cause different potential along different parts of the grounding grid. Finally, the GIS necessitates a fraction of the land area required for conventional substations. Because of this smaller area, it may be difficult to obtain adequate grounding since GIS is still subjected to the same magnitude of fault current as conventional substations. Frequent bonding and grounding of GIS enclosures will be needed to minimize hazardous touch and step voltages within the GIS area. Additional measures include the use of massive reinforced steel rebar in the concrete foundation of the GIS building that are connected to the GIS grounding system.

Therefore, how to accurately model the GIS structure, ground conductors, massive reinforced steel rebar and aboveground bus bars and correctly simulate fault currents, circulating currents and various fault locations become crucial. Previous study has already been carried out on this related subject [2]. This paper presents a grounding study for such a situation using an advanced grounding and EMF analysis software package [3] based on the safety criteria provided by IEEE Standard 80-2000 [1].

II COMPUTATION METHOD An electromagnetic field theory method, which is an

extension to low frequencies of the moment method used in antenna theory, is used for the computations. By solving Maxwell's electromagnetic field equations, the method allows the computation of the current distribution (as well as the charge or leakage current distribution) in a network consisting of both aboveground and buried conductors with arbitrary orientations and which are bare or coated. This approach takes induction effects fully into account. In other words, the computation results contain the combined effects of the inductive, conductive and capacitive interference.

This approach takes induction effects fully into account. It is an exact method that eliminates all of the assumptions mentioned in the conventional method and takes into account circulating currents and aboveground bus bars as well. It accounts for attenuation, phase-shift and propagation effects in the electromagnetic fields when moving away from the current sources. It models correctly the GIS phase conductors and enclosures that play a major role in discharging more realistically and accurately the fault current along the GIS structure ground bonding locations that are connected to the grounding grid through the steel rebar in concrete.

CTCED2012 Session 2 Paper No FP0224 /6

2012 China International Conference on Electricity Distribution (CICED 2012) Shanghai, 5-6 Sep. 2012

III SYSTEM DESCRIPTION The substation under study is situated upon

approximately 1100 feet by 650 feet land. The substation functions as both a medium voltage distribution substation and a high voltage switching station. Operating voltages are 13.8 kV, 34.5 kV, 99 kV, 115 kV, and 230 kV. The future 500 kV is also taken into consideration. Fig. 1 shows a plan view of the grounding systems. The top half part having denser conductors is the GIS area and the bottom wider part is the existing grounding grid. Fig. 2 shows a three-dimensional view of a portion of the GIS structure.

Fig. 1 The grounding system of substation.

Fig. 2 A thr.ep_,1>m,pm view of a portion of the GIS structure.

N SOTLMoDEL Soil resistivity measurements constitute the basis of any grounding study and are therefore of capital importance. Soil resistivity measurements were made at seven representative and accessible locations in the substation area. A detailed interpretation of the soil resistivity measurements was carried out and the short spacing and large spacing soil resistivity measurement data were combined to derive a worst case soil model which has been used in the grounding design as shown in Table I.

Table I Soil model

Resistivity (ohm-m) Thickness (feet)

Top Central Bottom

150 320 90

1.2 3 10

00

V FAULT CURRENTDISTRIBUTION ANALYSIS

The touch and step voltages associated with the grounding system are directly related to the amount of the fault current discharged into the soil by the grounding grid. The presence of numerous overhead shield wires and grounded transformer neutrals that are connected to the substation grounding system provide many locations where the fault current can enter or exit the grounding grid and circulate within the ground conductors. A circuit model of 115 kV and 230 kV existing overhead transmission lines and their related remote stations (terminals) that provide fault current contributions was built to determine the actual fault current discharged into the earth by the grounding grid shown in Fig. 3. The maximum length of the lines is about 17 miles. The currents are identified in Table II. Moreover, the fault currents for the 99 kV, 500 kV and other lines were determined accordingly and shown in Table III.

Terminal #1

Uno115#2

lIne115#1 Terminal #5

Terminal #3

Terminal #4 Fig. 3 The circuit model of 1 15 kV and 230 kV lines.

Table II Fault contributions of 1 15 kV and 230 kV lines

Line # Remote Fault Current (A) <

Station De::rees Line230# 1 Terminal # 1 562 < 96 Line230#2 Terminal # 1 3,227 < 103 Line230#3 Terminal #2 2,2 15 < 105 Line230#4 Terminal #3 5,722 < 103 Line230#5 Terminal #3 5,5 19 < 103 Line230#6 Terminal #4 1,929 < 103 Line230#7 Terminal #4 1,985 < 105 Line 1 15# 1 Terminal #5 6 1 1 < 105 Line 1 15#2 Terminal #5 646 < 108

Total Fault Current 22,406.96 < 103.4

Table III Fault contributions of 99 kV and 500 kV lines

Line # Fault Contribution (A) < Degrees Line99# 1 768 < 97 Line99#2 1,00 1 < 98 Line99#3 702 < 103

Line500# 1 18,242 < 9 1

CTCED2012 Session 2 Paper No FP0224 Page2/6


Line500#2 18,307 < 90 Total Fault Current 38,990 < 9 1

The total fault current at the substation is summarized in Table IV. The values shown in this table have been used in the grounding grid analysis. The grounding design is required to be satisfactory for a total fault current of about 62 kA. However, the earth current discharged by the grounding systems was computed to be 17,818 A. There are a large amount of circulating and returning currents. In this case, neglecting the circulating and returning currents and ignoring the aboveground bus bars in which these currents are flowing in the model can result inaccurate computations.

Table IV Summary of fault current distribution

Type of Current Current (A) < Degrees Total Current at the Fault Bus 6 1,527 < 96

Total Circulating and Returning Currents through Shield Wires

43,709 < 96 and Grounded Transformer

Neutrals Total Net Current Discharged

17,8 18 < 96 into the Grounding Grid

VI SAFETY EV ALVA nON

A. Computer model The entire network for the substation modeled is shown

in Fig. 4. It includes the aboveground GIS structure, ground conductors, massive reinforced steel rebar, aboveground bus bars and underground cables. The mesh size of the grounding grid at the GIS area is about 15' by 15'. The depth of the grounding grid is 2'. All ground conductors and ground rods are modeled in the computer model. Various types of conductors such as black solid coppers, 350 and 500 kcmil stranded coppers and 4/0 stranded coppers are modeled. The lengths of copper ground rods are 8' and 10'. The fence is 2.5' inside the edge of the grid and has a fence ground wire of #110 stranded copper. The fence posts are tied to the ground wire per 40 feet.

The reinforced steel rebar in the concrete foundation of the GIS building has been modeled. The overall dimension of the foundation is about 40' by 200'. The mesh size of the rebar is 4' by 4' which is a conservative assumption since it is much less dense than the real situation.

Fig. 5 shows the detailed configuration of the GIS structure. Fig. 6 illustrates the computer model created for the GIS structure. Each line in Fig. 6 represents a GIS bus and its enclosure. A phase conductor is modeled inside the enclosure as the GIS bus. The height of the GIS busses varies from 4' to 18.' The diameter of the GIS enclosure varies from 14" to 24". The wall thickness of the enclosure is about I". The enclosure is grounded at its two ends outside the GIS building and also grounded densely (maximum per 8') at various locations along their length.

veground Abo GIS Structure

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u

n �

, ,

1"

rgloun .... - - -

,n / , �

Grounding Conductors, Steel Rebar, and Fences

Aboveground Bus Bars

� 1i!!Il . 1 1 1. � l It

�-� JIf I 1m !=; I�

� ""'b �I I II t( === �

Fig. 4 Overall substation network modeled.

Fig. 5 Side views of the GIS structure.

Fig. 6 A computer model of the GIS structure.

Several representative fault locations were selected for the analysis as shown in Fig. 7. The phase to ground faults are simulated by shorting the phase conductor to the ground conductors and the phase to enclosure faults are simulated along various parts of the enclosure by shorting the phase conductor to the enclosure.



Fault L 0 Locations

- 10""-1.0 -I

T

I� :i /1--r �

• [ � = 1 "'"

Fig. 7 Representative fault locations.

B.Safety limits The maximum acceptable touch and step voltages applicable to areas without a crushed rock layer as listed in Table V have been calculated based on the IEEE Standard 80-2000. The X/R ratio is 20 and the fault duration is 0.4 second. The surface layer resistivity is 150 ohm-m.

Table V Maximum acceptable touch and step voltages

Surface Layer Maximum Acceptable Surface Resistivity Touch Voltage Step Voltage Material (ohm-m) (V) (V)

Native Soil 150 2 12.7 333.9 Crushed

3000 743.9 Rock

2,458.6

Note that no additional surface layer such as crushed rock could be considered inside the GIS building. The reference surface of the building was selected as being uncovered concrete. Concrete in intimate contact with local soil has the same resistivity as the surrounding soil. In practice, the building surface concrete is much drier than local soil. For example, the resistivity of dry concrete ranges from about 2,000 to 10,000 ohm-m. Consequently, the touch and step voltage design limits for the native soil used in the GIS building should be considered to be very conservative, unless water covers totally or partially the concrete.

C. Computer results Various cases have been analyzed and touch and step voltages all over the grounding grid have been examined. However, in this paper, only the touch voltages for two typical cases are reported to illustrate the computation results. Fig. 8 shows the touch voltages computed throughout the substation when the fault is outside the GIS building but near the GIS middle part. The maximum computed touch voltage is 1235 V and it occurs at the north-east corner which is not accessible to people. However, the touch voltages around the GIS area (the top part of the grid) are below 372 V which are below the touch voltage limit indicated in Table V (i.e., 743.9 V) for areas with a crushed rock layer. Fig. 9 shows the touch voltages within the GIS building only. The maximum touch voltage is 45.4 V. This maximum touch voltage is below the touch voltage limit indicated in Table V (i.e., 212.7 V) for areas without a crushed rock layer.

600

100

400 ��_ -625 -125 375

XA)<JS(�

Ta.d1 Vdtage rJegl (Vdts) [VIbrs]

MaxirnnnValue: 1234.908 MnirrumValue: 2.008

• ,;; 123491

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Fig. 8 Touch voltages throughout the GIS substation area.

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Fig. 9 Touch �1t;e7';rthr;'the GIS building (fault outside but near GIS middle part).

Moreover, Fig. 10 shows the touch voltages within the GIS building only when the fault is an internal fault inside the north GIS enclosure. The maximum touch voltage is then 39.4 V which is even less than in the case when the fault is outside the GIS structure but near the GIS middle part. It is interesting to note that when the fault is inside the enclosure, the induction between faulted inner buses and associated enclosures evenly discharges the fault current through the enclosure everywhere to the soil and actually reduce the maximum touch voltage.



XAXIS (FIET)

Tarl1\t:ftEg!fIAql.(\us)[IfItrn]

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Fig. 10 Touch voltages within the GIS building (fault inside the north GIS enclosure).

In this study, for comparison purposes, the analysis of the grounding system was also carried out without the modeling of GIS structure and the above ground bus bars. In this case, this approach accounts for the potential drop along conductors due to their self-impedance. However, it does not take the mutual impedance of conductors into account.

Two modeling scenarios have been analyzed. First, only the net earth current of 17,818 A has been applied to the grid. Another one is that the detailed currents have been simulated at various locations and the currents include the faulted phase currents, returning currents along the shield wires, and circulating currents.

As for each scenario, two typical fault cases that when the faults are inside the GIS building at the north part of GIS building and outside the GIS building but near the GIS middle part are modeled and reported in this paper to illustrate the computation results of touch voltages. In areas far from the location of the fault, both methods provide similar results in terms of the overall maximum touch and step voltages. However, these two scenarios give different computation results of touch voltages inside the GIS building.

In the case of only the net earth current of 17,818 A applied to the grid, Fig. 11 shows the maximum touch voltages computed throughout the GIS building when the fault is outside the GIS building but near the GIS middle part. The maximum computed touch voltage is 23.1 V. It occurs at the north end and the middle part of the GIS building. This maximum touch voltage is lower than in EMF computation where it is 45.4 V. The reason is that when only the net current discharged by the grid is considered, there is much less current discharged into the grid at the fault location than in the real situation. Thus the ground potential rise is lower and it results lower touch voltages as well. While, Fig. 12 shows the maximum touch voltages computed throughout the GIS building when the fault is at the north GIS enclosure location. The maximum computed touch voltage is 86.4 V. It occurs at the north

end and quite localized. This maximum touch voltage is much higher than in EMF computation where it is 39.4 V because no induction effect of the GIS enclosure is considered. The fault current is leaked out more locally from the ground conductors and massive reinforced steel rebar than in the case of EMF computation with the grounded enclosure that will distribute the fault current evenly within the GIS building .

�

XAXlS (FEET) Tcu::hWtcuerv'l<gn (Wts) [V'b:sj

.<

.<

.<

.<

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Fig. 11 Touch voltages (net current and fault near GIS middle part).

/

-

100

170

-

11 o -

" -330

.< L.

.<

.< -

.<

.< -c

.<

/ .<

I

Fig. 12 Touch voltages (net current and fault inside the north GIS enclosure).

In the case of the detailed currents applied to the grid, Fig. 13 shows the maximum touch voltages computed throughout the GIS building when the fault is near GIS middle part. The maximum computed touch voltage is 63.2 V. It occurs at the middle part of the GIS building. This maximum touch voltage is higher than in EMF computation where it is 45.4 V and also higher than in the case of only the net earth current applied to the grid. Fig. 14 shows the maximum touch voltages computed


2012 China International Conference on Electricity Distribution (CICED 2012) Shanghai, 5-6 Sep. 2012

throughout the GIS building when the fault is at the north GIS enclosure location. The maximum computed touch voltage is 256.2 V. It occurs at the north end and again quite localized. This maximum touch voltage is much higher than in other cases. The maximum touch voltage is increased dramatically when the detailed currents are modeled such as a large amount of circulating and returning currents.

If the aboveground GIS structure and bus bars are modeled and coupling between them and the ground conductors are accounted for, in most case, there would be a phase angle difference between the induced potential along the GIS enclosure and the GIS iuner bus and between the ground conductors and the aboveground bus bars as well. Accordingly, touch voltages near the fault location are relatively low. However, in this case, the computation was carried out without the modeling of GIS structure and the above ground bus bars. Moreover, when the detailed currents are applied to the grid, much more current (about 62 kA) is discharged at the fault location which results in a rather high ground potential rise. Due to these two reasons, the touch voltages in this case are quite higher than other cases.

290

270

=

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100

17 0

150

130 -

11 o -

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50

I

XAXlS (FEET) Tcu::hWbge MEgn. (\.tits) [Wrs]

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Fig. 13 Touch voltages (detailed currents and fault near GIS middle part).

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150

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=,Tcuh\JfugeMg1(\d1s)[VIlts]

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M;oOmmWL.e: 253219 Mtimmvalue: 5.3)1

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Fig. 14 Touch voltages (detailed currents and fault inside the north GIS enclosure).

VII CONCLUSIONS This study shows how advanced simulation approaches

can be used to analyze complex grounding systems involving gas-insulated substations. It is necessary to model accurately the GIS structure, ground conductors, massive reinforced steel rebar and aboveground bus bars and correctly simulate fault currents, circulating currents in order to determine touch voltages accurately and avoid overestimating or underestimating them. There is a wide range of practical grounding system scenarios which can be studied by using this electromagnetic field method.

Acknowledgments

The authors wish to thank Safe Engineering Services & technologies ltd. for the fmancial support and facilities provided during this research effort.

REFERENCES

[1] IEEE Guide for Safety in AC Substation Grounding, IEEE Std. 80-2000, The Institute of Electrical and Electronics Engineers, Inc., January 2000.

[2] .T. Ma, .T. Liu and F. P. Dawalibi, 2008, "Application of Advanced Simulation Methods and Design Techniques to Interconnected Grounding Systems ", Proceedings, The 17th Coriference of the

Electric Power Supply Industry (CEPSI), Macau, October 27 - 3 1, 2008.

[3] CDEGS Software Package, 2012, Safe Engineering Services & technologies ltd., Laval, Quebec, Canada, Version 14, June 2012.


gas insulated substation grounding system design using the

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