Electromagnetic Transient Analysis on a 400 kV Overhead-underground System

Hkctor G. Sarrniento, Senior Member IEEE, and Carlos Tovar G.

Abstmct--As part of a feasibility study to install a 400 kV Table I. Network configurations studied. double circuit system, this paper reports relevant results from an Electric energy Supply I IO SE1 Su00lv 10 SE . . . I electromagnetic transient analysis to determine overvoltage supplied from: Lag0 vca Madero via protection in an overhead-underground system. All computer Texcopo OHTL +UGC OHTL simulations were done with the Electromagnetic Transients ‘r.3c0c0 UCK UGC Program (EMTP). OHTL = Overhead transmission line

Keywords--Modeling and simulation, electromagnetic transient UGC = Underground cable

analysis, lightning overvoltages, switching transients, lightning This paper describes the part of this project where

arresters. electromagnetic transient performance is assessed. Transient overvoltage results were obtained for the

I. INTRODUCTION following phenomena: -

U nderground cables are extensively used in high load m density zones, as well as residential areas, in order to

distribute electric energy at medium voltage. They are also : applied at high and extra high voltage to transfer blocks of electric power, when overhead transmission is not acceptable ’ for different reasons: technical, environmental and/or ’ aesthetic.

Lightning strokes (back flashover and shielding failure) Underground cable switching Single-phase fault Open phase Switching in the GIS.

II. OVERVOLTAGES DUE TO LIGHTNING

Expected load growth in the metropolitan area of Mexico City, in particular the northeast region, makes it necessary to build new infrastructure. Luz y Fuerza de1 Centro (LFC) has established the need for a new 4001230 kV substation, called SE Lago, and its associated electric transmission, to improve the availability and reliability of the electric energy in such region.

Electric energy to SE Lago will be fed from a 400 kV double circuit taken from a loop of such voltage level that surrounds Mexico City. Due to the construction of a nearby new international airport, SE Lago has been designed to be gas-insulated (GIS). The proximity of the new airport also forces the electrical transmission to be underground for at least a portion of its route. Hence, along this 400 kV path, different configurations were studied: the whole trajectory underground or a combination of overheard line and underground cable.

These 400 kV configurations studied are summarized in Table I.

Overvoltages due to lightning can be of two types [I]:

1) Back flashover. High current lightning that hits overhead ground wires or

towers, can generate overvoltages of suflicient magnitude as to flashover the insulator string in the transmission tower. Due to the fact that most of the stroke current will flow into ground, the quality of the transmission tower footing resistance is of utmost importance in this case.

2) Shielding Failure. Lightning can strike directly one of the phase conductors,

this is called shielding failure. The resulting surges will penetrate the underground cable thrn the phase conductors. The most severe condition will arise when these overvoltages are just below the breakdown level of the insulation string.

What follows is a description of computer simulations for back flashover and shielding failure.

A. Circuit analyzed.

Hector G Sarmiento (hsu@iie.org mx) and Carlos Tovar G (ctg@ie.org.mx) are with lnstituto de lnvestigaciones EICctricas, Temlxco, Mexico.

A 9 km 400 kV overhead transmission line and a 15 km underground 400 kV cable, which terminates in the GIS SE Lago, form the circuit.

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At the end of the substation, the 400/230 kV, 330 MVA transformer is represented with a capacitance. To establish a severe case, the transformer is unloaded.

CABLE

‘2 + SE SF8

_---_

Figure I Circuit used for back flashover studies (SE=substatlon, T=Transformer, AP=lightning

arrester).

Figure 1 shows the circuit used for the back flashover simulations. Circuit data is presented in Appendix A.

R. Overvoltages caused by hackjlashover’.

As mentioned before, back flashover will occur when the voltage (due to lightning) between the phase conductor and tower exceeds the breakdown voltage of the insulator string. This value is I,4 17 kV for the 400 kV OIITL studied.

Sensitivity to tower footing resistance (TFR).- During back flashover, most of the stroke current flows to

ground thru the tower and overhead ground wire. Lowering the TFR can reduce voltage surges appearing in the underground cable.

As the TFR was varied from 10 to 100 ohms, three cases were studied:

I A base case with no protective scheme. 0 One lightning arrester (L.A.) at the overhead-

underground transition point (refer to Appendix A for the L.A. characteristics).

P Two L.A. An additional arrester at the GIS transformer.

Figure 2 shows a graph of the voltage peak at the GIS transformer vs. the TFR for the cases described above.

It is readily observed that a scheme consisting of two lightning arresters provides the best protection. The peak voltage magnitude recorded with this scheme is 786 kV. Even considering a 20% margin (common criteria used in insulation coordination) [2], the peak voltage falls below the transformer RIL, which in this case is 1,425 kV. If only one L.A. is applied, TFR must be kept below 40 ohms.

‘$20 4 -.-T--I ---, ___ I 10 20 40 60 60 100

Rt (ohms)

--‘-- NoLk --- ILh - ZL A.

Figure 2. Overvoltages at the substation transformer

Figure 3 shows overvoltage waveforms of the surges reaching the power transformer at the GIS with a TFR of 100 ohms.

e@J --- :..

\

‘\ --._- --__

8.00 R.OZ 0 0, 0.06 R.&a it.18 8.12 0.14 8.16 0 1R 0.20

Time sca,c: 10-t-3, s. a, JWC?m..” n*

Figura 3. Overvoltages at the power transformer, wth TFR =: 100 ohms.

The three waveshapes are related to the three cases studied. The base case (no lightning arrester) obviously shows the highest voltage peaks. The lowest curve refers to the case of two LA. installed; one at the overhead-underground transmission point, the other at the GIS transformer.

C. Overvoltages caused by shielding failure.

A shielding failure occurs when a lightning stroke directly hits a phase conductor instead of the tower or overhead ground wire. For these studies, the stroke waveform has the following characteristics: 1.5/70 ps and a magnitude of 60 kA [2].

Table II shows the resultant overvoltages due to a shielding failure in the 400 kV line. Since TFR does not intervene in this phenomena, values presented in the table cover only the three cases already listed: base case without a LA., one L.A. at the OII-UG transition, and an additional LA. at the GE power transformer. Three values are presented for each of the three cases: overvoltage recorded at the beginning of the

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cable, overvoltage recorded at the end of the cable (entering the GIS), and overvoltage recorded at the GIS trail&former.

Table II. Overvoltages due to shielding failure in the ransmission line

1 Cable 1 Cable 1 GIS 1 Lightning arrester in: 1 ;;;, 1 end [kV] 1 ;‘,“ef. 1

No LA. I 1415

I 2610

I 2978

I

OIITL-UGC transition I 195

I 1548

I 1 783

I

OHTL-UGC transition and at 795 961

L-l

783 GIS Transformer

It is noted from observing the magnitudes in the previous table, that two lightning arresters provide an adequate protection against shielding failure. With only one L.A. installed, overvoltages recorded reach values of 1,548 and 1,783 kV at the end of the cable, and at the transformer terminals, respectively. Both of these values are greater than the BIL of such system, which equals 1,425 kV. For the case with two L.A., even considering a 20% margin, overvoltage magnitudes fall below the BIL of the system.

III. OVERVOLTAGE DLJE TO ILIGHTNING

A. Switching transients in tfze overflecrrf-unrlergrourzti 400 k V system. The circuit used for these simulations is shown in figure 4. In this figure, the electric circuit consists of 9 Km. of double

circuit 400 kV overhead line, initiating from substation Texcoco. What follows is 15 km of underground 400 kV cable up until Lago substation.

400 xv 290 Kv

L&GO AJEROPUERTO

Figure 4. Overhead-underground circuit fed from Texcoco substation.

Transmission between Lago and Madero substations consists of a double circuit overhead line, 230 kV for 5.5 km. Energy delivery to Aeropuerto substation is thru a 230 kV underground cable, with a length of 5.0 Km.

Similar to the analysis performed for lightning performance, a base case with no protection was used as reference. This base case is then compared to two others, conslstmg of one L.A. in the overhead-undergound transition point, in one hand; and a third case, consisting of an additional arrester at the GIS transformer terminals.

Also presented are results in which a trapped charge remains in the cable. This simulates a re-energization of the cable shortly after a switching operation is performed.

Table III illustrates results from previously described switching overvoltages.

Table III Overvoltages in P.U. when energizing the circuit - -

WI0 LA.

With one LA.

1.61 1.81 1.85

1.60 I.80 183 I I I

w/trapped charge, no [..A. 1 2.02 1 2.54 1 2.60

From the previous table, the overvoltage’ calculated when installing on or two lightning arTesters (LA.) and having trapped charge is 2.167 p.u. (707 kV). This value is less than the Basic Switching Impulse Level that LFC specifies of 1180 kV. Even considering a 20% security margin (848 kV), one 01 two lightning arresters provide adequate protection.

For illustrating purposes, Figure 5 shows the overvoltage waveshape at Texcoco Substation, when energizing the OHTL and cable with a trapped charge of -1.0 p.u. and without lightning arrester. In this case, and overvoltage of 660 kV is registered at Texcoco, and at the end of the UGC the value is 850 kV.

Figure No. 5 Overvoltage waveshapc when energizing 400 kV cable.

The next group of transient studies deal with the performance of the 230 kV portion of the OMTL-UGC system that originates in the Lago Substation and feeds Madero and Aeropuerto substations.

Table IV shows results from these simulations, with overvoltages being recorded at Lago, Madero (MAD) and Aeropuerto (AER) substations.

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Table IV Overvoltages in P.U

OHTL to Madero substation, no L.A. I

I .02

UGC to Madero substation, no L.A.

substation, w/trapped

UGC to Aeropuerto substation, w/trapped charge, no L.A.

I .08

I .08

:nergizi lg the 230 kV

MAD

1.96

I.89

From the previous table, the maximum value observed is Figure 6. Overvoltage waveform for a single-phase fault

when the 230 kV UGC to Aeropuerto substation is energized at the end of the UGC.

(2.279 p.u.). This magnitude, with a 20% margin results in 5 14 kV, below the basic impulse switching level specified of 745

C. Open Phase.

kV, and no additional protection is necessary. Last group of studies refer to having an open phase in the

400 kV transmission system. Results are shown in table VI.

B. Single-phase fault.

Single-phase faults were the next group of disturbances analyzed. Results are presented in Table V.

Table V. Single-phase faults (19-f) in the 400 & 230 kV transmission system I

I+fat : TEX ’ OHTL-

UGC MO MAD AER

End of 400 kV OHTL

I .67 I .78 I.81 I.14

End of 400 kV UGC

I.71 I .86 1.87 I .26

Start of 230 kV OHTL

I .32 I .39 T I .40 I .56

Start of 230 kV UGC

I .32 I .39 I .40 I S6

All resulting overvoltages from single line faults, both at 400 and 230 kV levels, are below the insulation level of the underground cables.

When the fault is cleared, overvoltage in the faulted phase reaches 592 kV, while in the other phases the overvoltage is 386 kV.

For illustrating purposes, Figure 6 shows the overvoltage waveshape from a single-phase fault at the OHTL-UGC transition

.

Tabla No. VI Open phase in the 400 kV system.

Open Phase TEX OHTL- UGC &to

No L.A. I .06 I.98 I.98 Two L.A. I .05 I .67 I .6?

Application of two lightning arresters limit overvoltage at Texcoco and Lago substations, as well as in the overhead- underground transmission, The resulting overvoltages do not harm the equipment insulation.

Figure 7 show the overvoltage present at the end of the UGC with an open phase in the overhead transmission system. Maximum overvoltage without a lightning arrester is 648 kV.

Figure 7. Overvoltage a! the end of the 400 kV UGC with an open phase in the OHTL.

Lightning arresters will limit overvoltages, but cannot completely damp out temporary overvoltages. These surges can have a long duration and their magnitude may be close to the L.A. rating [2]. These conditions influence the capacity and thermal stability ratings of the L.A. to be selected.

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Therefore, determination of magnitude and duration of temporary over-voltages is important in arrester selection.

Temperature rise due to temporary overvoltages will determine thermal stability in the unit. If temperature at the zinc oxide block is greater than the design temperature, the unit can be destroyed. Temporary overvoltage capacity in the L.A. must be greater than the expected magnitudes in the system.

IV. SWITCHING TRANSIENTS IN THE 400 KV SYTEM WITH

UNDERGROUND CABLE

The circuit analyzed in this case consists of a 400 kV underground cable for the whole length (24 Km.), from Texcoco to Lago substations (refer to figure 4). The 230 kV system remains the same.

Results from simulating different events are shown in Table VII.

TableVII. Overvoltages in p.u. with the circuit consisting of cable from Texcoco to Lago.

Event TEX MW MAD AER

1 Energizing UGC w/o L.A. 1 I .55 1 1.74 1 --- 1 --- I

Energizing UGC w/LA. I .58 1.73 --- ---

Energizing UGC, witrapped charge, w/o L.A. 1.99 2.41 ___ ___

Single-phase fault at the end of the cable I .68 I .84 I .24 I.31

Overvoltages appearing at Texcoco and Lago substation are practically the same for the first two events, since their magnitude is not enough to make the lightning arrester work.

V. SWITCHING TRANSIENTS IN THE GIS

Overvoltage transient simulations in SF6 substation [X] consisted of energizing buses at the 400 kV level, and transformer energizing with two configurations: Gas insulated ducts up to the transformer, and electric connection between SF6 and transformer by way of a short length of underground cable.

Results are presented in Table VIII. The length of the cable connecting the transformer is varied from 20 to 80 m.

Tabla VIII. Overvoltaee within GIS rkV1 Transf. energization 1 BI 1 B2 1 Ap. 1 T

Without L.A., SF6 1 812 1 367 1 679 1 654

With L.A., 40 m. 762 363 623 625 With L.A., 60 tn. 761 359 630 629 With I A 80 m 748 358 631 631

Results from the previous table show that none of the overvoltage magnitudes‘surpasses the GIS BIL, which is 1050 kV, even considering a 20% margin in insulation coordination. Nevertheless, it is noted that the overvoltage present at the transformer terminals increases its magnitude when the length of the connecting cable is also increased.

VI. CONCLUSIONS

1. The following table shows the maximum overvoltages recorded when the protection scheme with two lightning arres;ers described before is applied. These overvoltages are compared with the insulation level used in the equipment.

Table IX. Summary of maximum overvoltages

Event Overvoltage

Backflashover I 163

Insulation level

I 425

Both for back flashover and shielding failure in the lightning performance analysis, overvoltages are controlled with the installation of two ZnO lightning arresters: One at the overhead-underground transition, the other at the terminals of the GIS power transformer. Both units at a voltage level of 400 kV. With a protective scheme consisting of two lightning arresters, no damaging overvoltages were recorded when switching the 400 kV underground cable. In fact, results do not show a clear difference when using one or two L.A. An adequate lighming performance of the electric system under study requires the application of a second lightning arrester at the power transformer terminal in the GIS. It is recommended to install this L.A. as close as possible to the transformer terminals, since the protective margin will be reduced if the distance between the transformer and arrester is increased.

VII. APPENDIX A

This appendix lists the main data used in the studies described in this paper.

a) Short Circuit equivalents Table X. Three-phase and single-phase short circuit le

Substation Pee 39 Pee 19 (MW) (MW)

X/R

Texcoco I2 891 14 331 14.8 Teotihuacan Madero

1 16412 1 t1;vt-l I .“&., ,

l!?Lt 2 “_T

1 I3 I86 1 14666 i 16.7

Pee = short circuit power

vels.

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b). Overhead transmission line parameters Table XIII includes typical inductances and capac$ances of Table XI Line uarameters in units/km SF6 insulated components, used in this study.

I Voltage [kV] I Parameter I Sequence

,A\ I I.\ I

Rn 1 0.3565 1 0.02660 L mHy. 1 3.3810 1 0.86900 C pF. 0.0070 1 0.01344 R!Z2 0.3290 0.02690 L mHy. 3.2170 0.98500 C pF. 0.00713 0.01184

c). Underground cable parameters.

Table XII shows cable parameters used in the calculations. These parameters represent a 230 and 400 kV XLPE underground cable for three different conductor sizes. It is assumed that cables are transposed each 715 m.

Table XII. Cable parameters in units/m. Voltage

Area [mm’] Parameter Sequence Ik”l 0 I +

230 RpR 244 14.5

2000 WL ptn 67.0 215 C pF. -_- 270. R@ 103 19.7

400 1600 WL l.tR I 85 1 225 c pF. --- ! 190

I I ICpF. I _-- 1 216 1

d). SF6 Substation Data

For the studies concerning switching within the GIS, the following substation layout was used. This layout corresponds to the 400 kV bay of the Lago substation

Figure No. 8 Layout of SF6 Lago substation at 400 kV.

The GIS has a breaker and a half layout. Voltage transformation consists of three single-phase 330 MVA units. For the purpose of electromagnetic transient analysis, each unit is modeled by a 6,000 pf capacitance.

Table XIII. Inductances and capacitances for the different components of the GIS

400 kV Equipment Cap. I Id.

CTs 50 0.24 PTs 100 0.24

Bushings 200 0.24 Cable terminals 80 0.24

VIII. ACKNOWLEDGMENT

The authors gratefully acknowledge the contributions of C. Crowley, G. Ibarguengoitia and M. Mariscurrena from Luz y Fuerza de1 Centro (Mexico City) for their technical support during these studies.

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IX. REFERENCES

Ragaler K. ed Surges in hiah voltage networks, Plenum Press N.Y., 1980 Hileman, AR., lnsultation coordination for Power Svstems, (New York: Marcel Dekker), 1999 Electromagnetic Transients Program Reference Manual (EMTP Theory Power), prepared by H.W. Dommel for Bonneville Power Administration, P.O.Box 3621, Portland, OR, 97208, U.S.A., August 1986. Marti, L., “Simulation of Electromagnetic Transients in Underground Cables using EMTP”, IEE ZND International Conference on Advances in Power System Control, Operation and Management, Dec. 1993, Hong Kong, p. 147. Jun Ozawa, et al ., “Lightning surge analysis in a multiconductor system for substation insutation design”, IEEE Trans. , PAS-104 1985, p. 2244-2254. Bul-Van, Q., GBeaulie, HHuyn, R.Rosenqvist, “Overvoltage Studies for the StLawrence Rtver 500 kV DC Cable Crossing”, IEEE Trans. on Power Delivery, Vol.6., No.3, July de 1991, p.1205. Wedepohl, L.M., C.S. Indulkar, “Switching Overvoltages in Long Cross bonded Cable Systems using the Fourier Transform”, IEEE Trans. on PAS, Vol.PAS -98, No.4, July/August 1974. Simms, J.R., “Overvoltage Protection of Gas-insulated Substations”, GEC Review, Vol.4, No.2, 1988, p.100 Thue, William A. ed., Electrical Power Cable Engineering, (New York: Marcel Dekker) 1999.

X.BIOGRAPHIES

Hector G. Sarmiento (M’1975, SM’90) was born in Mexico City. He has a M. of E. from Rensselaer Polytechnique Institute (Troy N. Y.) and a Ph.D degree fron Concordia University (Montreal Can.). He works for the lnstrtuto de lnvestigaciones Electricas since 1978 in the areas of analysis of transmission & dtstrtbution systems.

Carlos Tovar G. was born in Mexico City. He has a M.S. from Instituto Politecnico National (Mexico City) .). He works for the Instituto de lnvestigaciones Electricas since 1987 in the areas of analysis of transmission & distribution systems.

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