Safety Aspects in Substation Voltage Uprating

7/27/2019 Safety Aspects in Substation Voltage Uprating

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I 1250 IEEE Transactions on Power Delivery, vol. 7, NO. 3, JULY 1992SAFETY ASPECTSinSUBSTATION VOLTAGE UPRATINGby

IEEE Substation Committee - W.G. El :Recommended Minimum Clearances in Substations*Abstract - Present arrester technology enables the use ofsmaller clearances and insulation levels than are currentlystandardized for HV substation. This permits voltage uprating orcompacting of substations such that substations can meet thesafety standards. For the uprated substation th e statement canbe made that if it met the safety standards before uprating, itwould automatically meet the standards after uprating. However,the relevant standards could be improved to clarify theinterpretation, and promote flexibi lity of design. The W.G. onrecommended clearances in substations has discussed the issuesand set forth, in this paper, their views, which are intended tohelp the engineer involved with upratingkompacting safety. Briefcomment on insulation coordination is given since this sets thebases for clearances. The needed choice of BIL values for thesame voltage rating in the HV range is supported, making thesafety clearance issue straightforward.Key words - Substations, safety, minimum clearances, voltageuprating.

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at a higher nominal voltage with the same clearances andchanges of only some equipment , for example, transformers orcircuit breakers.Voltage uprating of older substations or building newsubstations with smaller dimensions makes an economicimprovement possible, but it also raises several concerns,mainly with personal safety, insulation coordination, insulatorperformance under polluted conditions, environmental (fields, RI,corona) effects, short circuit withstand, and maintenance.Personal safety concerns are brought up by the decrease inclearances from those presently specified. Personal safety, asthe most important issue of substation uprating is the objective ofthe discussion in this paper. Since basic dimensions within thesubstation are determined by the insulation coordination aspects ,it is helpful to start with a brief comment on design proceduresfrom the viewpoint of insulation coordination.

INSULATION COORDINATIONINTRODUCTION Choice of Insulation Level

Insulation levels of HV substations per present practices [l]appear relatively much higher than those of EHV substations.Expressed in per units of rated voltage (peak value, line toground), the 550 kV BIL of a 115 kV** substation translates to5.6p.u. while the 2050 kV BIL of a 765 kV substation is only3.13 p.u. The reasons for this situation stem more from historicaldevelopment than from existing design requirements . Thepractices with HV substations were developed some decadesago when limitations of available knowledge were compensatedby higher insulation margins and when economic pressure onsubstation cost was not as high as it is today. To buy bettermargins by larger clearances was cheap with 115 kV. Theeconomy is quite different with 765 kV substations nowadays.Even though the comparison of relative insulation levels doesnot cover all the aspects, (for example, the clearances with EHVare dictated by the switching surge rather than the lightningsurge), it serves here to illustrate the possibilities to decreasethe insulation in the HV range. Indeed, the application of modemmetal oxide arresters could enable a decrease of insulationlevels by up to two steps for the substations up to 230 kV whileretaining the same or better margins than with 765 kV, as shown,for example, in [2]. A decrease in the insulation requirementsenables the substation engineer uprate substations for operation*Edited by - J. Panek, GE, W.G.El Chairman);M. Rockwell, B.C. Hydro; A.M.Sahazizian, Ontario Hydro.**For the sake of consistency, normal 60Hz voltages are used throughout thispaper except where a voltage is referenced from another document.91 SM 511-6 PWRD A paper recommended and approvedby the IEEE Substations Committee of the IEEE PowerEngineering Society for presentation at the IEEE/PES1991 Summer Meeting, San Diego, California, July 28 -August 1, 1991. Manuscript submitted January 25,1991; made available for printing June 25 , 1991.

Design of HV substations is usually done according toindustry or internal utility standards which directly specify theclearances and BIL's. This bypasses the need for detailedinsulation coordination studies to determine these values. Whilethe NEMA and ANSI standards generally [for example, 1, 3, 41offer a choice of insulation levels for EHV equipment (higher than242 kV), they do not provide such a choice for the equipmentbelow 242 kV. Some of the standards go even further and tie theBIL and clearances together and associate them directly to therated voltage. A good example is Table 1, reprinted from NEMASG6 [l] . The result is just one value of BIL and minimumclearance for a given system voltage below 242 kV.Even though the power frequency voltage stress has to betaken into consideration for insulation design, the equipment BILand the required substation clearances, are in general, given bythe maximum overvoltage, not by the system voltage. Within thesurge arrester's zone of protection, the maximum overvoltage islimited by the surge arrester protective effects. Thus,operationally both the required equipment BIL and the substationclearances are determined by the applied arrester. Since it ispossible to use various arresters for the same system voltage,various insulation levels should be possible as well.International standards [5] extend the choice down through theHV range. This supports the above reasoning and indicatespossible changes in our practices.Indeed, if modern surge arresters are applied and thesubstation design is based on an insulation coordination studyrather than on standardized practices, the acceptable BIL andminimum clearances resulting are substantially smaller thanthose given in the present standards (see Table 1). This is notonly a result of a theoretical study; it is supported by practicalexperience. Some utility substations have been indeed upratedand successfully operated with BIL's and clearances well belowthose standardized. For example, by the judicious application ofsurge arrestors and with the change out of some majorequipment , several 115 kV substat ions were uprated to 230 kVand operated with insulators and disconnects rated 550 kV BILinstead of the usual 900 kV and with clearances to groundcorresponding to 115 kV [5 , 61. Similarly a 69 kV substation hasbeen operated at 115 kV with 350 kV BIL insulators and

0885-8977/92/$3.0001992 IEEE


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1251Table 1Outdoor Substations - Basic Parameters

Recommended Phase Spacing RecommendedMinimum Center-to-Center- Inches (Meters) Minimu mMetal-to-Metal Bus Supports. ClearanceRated Withstand Volta ee Distance Between Vertical Brk Between Over-Impulse 60 Hz kV rms Rigidly Supported Horn Ga p Horiz onta l Disc. Switches head ConductorRated 1 . 2 ~ 0 p Wet Energized Ground Clearance Switch & Break Powe r Fuse s and Ground forLine Max. Volt Wave 10 Conductors Inches (Meters) Exp ulsio n Disc. Non-Expu lsion Type Personal SafetykV rms kV Crest SecMXls Inches (Meters) Recommended Minimum T v ~ euses Swit ches Riaid Conductors Feet (Meters)6 72.5 350 145 31( .79) 29 ( .74) 25( .64) 84(2.13) 72(1.83) 60(1.52) l l(3. 35)7 121 550 230 53(1.35) 47 (1.19) 42(1.07) 120(3.05 ) 108(2.74 ) 84(2.13) 12(3.66)8 145 650 215 63( 1.60) 52- 1/2( 1.33) 50( 1.27) 144(3.66) 132(3.35) 96(2.44) 13(3.96)9 169 750 315 72( 1.83) 61-1/2(1.56) 58( 1.47) 168(4.27) 156(3.96) 108(2.74) 14(4.27)10 242 900 385 89(2.26) 76 (1.93) 71(1.80) 192(4.88 ) 192(4.88) 132(3.35 ) 15(4.57)

1 1 24 2 1050 455 105(2.67) 90-l n(2. 30) 83(2.11) 216(5.49) 216(5.49) 156(3.96) 16(4.88)12 362 1050 455 119(3.02) 106 (2.69) 84(2.13)* 240(6.10) - 192(4.88) 18(5.49)13 362 1300 525 - - 104(2.64)* - - - -14 550 1550 62 0 - - 124(3.15)* - - - -15 550 1800 710 - - 144(3.66)* - - -16 800 2050 830 - - 166(4.22)* - - - -300(7.62)

WithstandS.S. CrestkV-

-65 075 980889898 2

Note: For insulator data, see the NEM A Standards Publication for High Voltage Insulators, Pub. No . HV 1-1973,* Ground clearance for voltages 362 k V and above are selected on the premise that at this level, selection of the insulation depends on switching surge levelsof the system. The values were selected from Table 1 of IEEE Transaction paper T-72-131-6 (Vol. No . 5, page 1924) which is a rep of the T ransmissionSubstation Subcommittee. For additional switching surge v alues refer to the above noted paperNote : Table reprinted from NEMA Publication SG6, 1977, App A. p. 2.disconnects instead of 550 kV. M ~ ~ ~ ~ ~ ~ ,ubstations arein operation under very adverse conditions: one of the uprated230 kV substations is at 6000 feet elevation and the 115 kVsubstation is in an industrial environment with heavy pollution.

similar to Table 1 should not be interpreted as restrictive. It is tobe noted that the ANSI insulation coordination standard [3]defines just a series of preferred values of BIL for medium andhigh voltage class (paragraph 5) and does not associate thevalues with the rated voltages. Such an association is left up tothe user.Relation of BILs to Clearances

Table 2Preferred BIL SeriesRated Maximum Preferred BIL'sVoltage kV rm s kV Crest72.5 350-250121 550-450-350145 650-550-450-350169 750-650-550-450242 1050-900-750-650-550

Table 3BIL and Minimum Clearance

BILkV Crest2503505506507509001050

MinimumPhase-to-GroundClearance in (m)17 (0.43)25 (0.64)42 (1.07)50 (1.27)58 (1.47)71 (1.80)83 (2.11)

Note:1. The clearances are based on NEMA SG6.2. These are operational or electrical clearances, not safetyclearances.3. The phase to phase clearances make no allowances fordevices designed to produce and interrupt an arc in free airsuch as horn gap switches or expulsion fuses.The standards should reflect a preferred BIL series or rangefor each nominal voltage as illustrated in Table 2. Each BIL,rather than each nominal voltage, should have an associated setof minimum standard clearances such as those shown in Table 3.Thus, the possibility of choice of various insulation levels for agiven system voltage should be open to the substation engineerand the codes and standards which gives values in a manner

Both the equipment insulation level and the clearance aregiven by the same maximum overvoltage. However, they do nothave to be tied together. Quite often they really should not: BILhas meaning only for equipment which is tested, clearancesshould be prescribed for assemblies which are not tested. Toimpose both represents overspecification. They should becalculated by procedure schematically pictured in Figure la. Sucha procedure gives the engineer the freedom to choose differentmargins for BIL and fo r clearances.Contrary to the above, the clearances used in currentpractices are derived by a procedure pictured in Figure lb, asdescribed in [113. The minimum clearances are calculated fromBIL's. The numerical values of clearances calculated in [111 havebeen embodied directly in the various standards and are stillused, as shown, for example, in Table 1. This was a pioneeringan d very useful work. However, the pyramiding of margins isbrought to attention: First, there is the margin between themaximum overvoltage (protec tive level) and the BIL. Thismargin is usually larger than the needed 15 or 20 % since the BILvalues have to be selected from a series of values which increasein somewhat large steps. Furthermore, there is another marginbetween the BIL and the clearance, consisting of a 10% correctioni n length of rod- rod gap on CFO over BIL and an additionalcorrection of 12% on structure. The numerical values, the varioussafety margins or the procedure itself may be questioned.Nevertheless, association of clearances with the BILs, howeverquestionable from the insulation coordination viewpoint, can beused with advantage for easier interpretation of safety rules, asdiscussed later. Here it suffices to say, that the clearancesextracted from Table 1 as shown i n Table 3, are coordinated withthe shown BIL.

The previous paragraph was concerned with the insulationcoordination aspects. In that respect the clearances are meant aselectrical or operational clearances. That is, clearances whichwill withstand the imposed dielectri c stresses. By theseclearances we mean the minimum distance between, for example,conductor and vertical or horizontal or other structure, or betweenthe conductor and ground or a structure which supports an


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6 C l e a r a n c e sL,5 1 M a rg i n I

tI n su l a t i o nC l e a r a n c e s

I M a rg i n I 3 I M a rg i n Iti g h e s tO v e r v o l t a g e

V o l t a g e

la )Figure I . Procedures to derive clearances.

H i g h e s tO v e r v o l t a g e

1b)

insulator, forms a base for an apparatus, etc. On a line to linebasis the minimum clearance represents the minimum distancebetween energized metallic parts of the equipments orconductors, etc. What these clearances do not include is someallowance for objects which could appear in between the twoelectrodes or allowances for personal safety. These aspects areconsidered apart.Beyond the solid objects which could be thrown in betweenthe electrodes by perhaps accidents, explosions, very strongwinds, etc., animals which can move into the space are ofconcern.Although animals and solid objects could move into thecritical space by coincidence, this coincidence would represent asecond contingency. In general, the insulation systems are notdesigned for double contingencies, because of the extremelysmall probabilities that both events (critical overvoltage andanimal or solid object ) would occur at the same time. It shouldbe pointed out, that this consideration does not represent anychange from previous practices: The presently used minimumclearances are based on the procedure given in [l 11 and there isno mention about animal or solid object clearances being acriterion. One of the discussions to that paper suggestsadditional margins to allow for animal presence. This point, eventhough recognized as having merit, is dismissed in the closure asa double contingency case: It is not likely that a bird would flyclose by at the same instant as lightning occurs.At lower voltages animals which bridge the clearance gap andcause a 60 Hz flashover may be of concern. However, suchevents are properly dealt with as reliability not insulationcoordination issues.

CLEARANCES FOR PERSONAL SAFETYConsideration for human beings are, of course, substantiallydifferent. Personal safety becomes the critical criterion asopposed to the mere sparkover within the substation. Moreover,it is appropriate to consider the double contingency that a personwould be in proximity of the energized parts at the same timewhen the lightning overvoltage occurs.

Vertical ClearancesFirstly, let us draw a clear distinction between the minimumelectrical clearances (line to ground - for example, conductor tostructure, and line to line) and the minimum safe clearances. Bysafe clearances we mean those defined (per National ElectricSafety Code [lo]) as clearances from live parts to anypermanent supporting surface for workers.. The distinction isapparent from Figure 2. While the clearance c represents theelectrical clearance designed not to sparkover, the clearance b

is the safe clearance designed for people to move safely in theproximity of energized electrodes. Figure 2 is complemented byTable 4a. which shows the appropriate numerical values asspecified by the Safety code (10 Table 124-1) for various nominalvoltages. (There are two specified minimum ground clearanceswith the 230 kV since two values of BIL are recognized with thatvoltage.)

Measure to Outs ideof To p of EnergizedInsu la to r Ski r t

I / b

b = 86+cbl = 36 + Cb = Vertical distance from a grounded support surfaced = Vertical distance from the ground to the porcelainbl = Horizontal distance from a grounded support

Where c = Electrical clearance.for workers to a live part.base on an insulator.surface for workers to a live part.

Note: Limits of approach distances of less than b or bl maybe applied under restrictive conditions.Figure 2. Safety clearances.

As pointed out before, insulation coordination procedure mayresult in minimum electrical clearances smaller than thosepresently standardized (Table 4a, Column c). This in turn couldresult in safe clearances less than those specified by the Safetycode, (see Table 4a, Column b). However, the NationalElectrical Safety Code does provide for this possibility. Quotingfrom that code (10, page 102):


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1253"Where surge protective devices are applied toprotect the live parts, the vertical clearances,Colwnn 2 of Table 124-1 may be red uced, providedthe clearance is not less than eight fee t and sixinches plus the electric clearance betweenenergized parts and ground as limited by the surgeprotective devices."

Table 4aVertical Clearances

NominalakV69115138161230230

Voltaee

Nat. El.Safety CodeANSI C2Section 12VerticalClearancebft in10 511 712 212 1014 1014 10

NEMA SG6ANSIC37Table 1GroundClearance(Minimum]ft in2 13 64 24 105 116 11

C

VerticalSafetyClearancedft in8 48 18 08 08 117 11

Ib) - IC)

Ground here means a structure supporting an insulator, notthe "supporting surface for workers". Thus, if, for example, a115 kV station was designed with a safety clearance 8-112 feet +electrical minimum clearance and by application of the arrester wekeep the maximum voltage stress such, that the same electricalclearance could hold for 230 kV nominal, then we would be leftwith the original 8-1/2 feet of safety for workers. From the otherviewpoint, we can say that addition of 8-1/2 feet to the minimumelectrical clearance, as limited by the arrester application, is infact more conservative than the present practices. This isapparent from Column " d n Table 4a. The safety clearances,taken as the difference between "b" and "c" values, are generallysmaller than 8-1/2 feet.It is concluded that with the vertical clearances the voltageuprating can be done safely in compliance with the NationalElectric Safety Code.Horizontal Clearances

Where appropriate, the horizontal clearance requirementsshould also be taken into account. The NESC does specify thehorizontal safe clearances but does not indicate how they can bemodified if surge protective devices are applied. It is logical todeduce that the same would apply as with vertical clearances,only the distance would be smaller than 8-1/2 feet. It would begiven by the reach, rather than the height, of a person. Table 4bshows in Column "b" the horizontal clearances as specified byNESC and the minimum ground clearances in Column "c" perexisting standards. The difference between the two is given inColumn "d". From these values and by comparison to Table 4a itappears that the safety clearance based on a reach distance of 3-1/2 feet would be a reasonable choice. Thus, the followingprocedure, analogous to the one for vertical clearance from apermanent supporting surface for people, as quoted above issuggested:

"Where surge protective de vices are app lied toprotect the live parts, the HORIZONTAL clearances,Column 3 of Table 124-1 may be reduced, providedthe clearanc e is not less than three fee t and sixinches plus the electric clearance between energizedparts and ground as limited by the surge protectivedevices."

(CAUTION: The above is not included in the NESC, it is only asuggestion by the authors of this report to provide guidance tothe engineer involved with substation uprating.)

NominalVoltaeeakV69115138161230230

Table 4bHorizontal ClearancesNat. El.Safety CodeANSI C2Section 12Horizon alClearadceblft in4 116 16 87 49 49 4

NEMA SG6ANSIC37Table 1GroundClearanceWinimum)ft in2 13 64 24 105 116 11

C

HorizontalSafetyClearanced1ft in2 102 72 62 63 52 5

(b) - IC)

This is similar to but not identical with limits of approachwhich are measured from the person as they move about butwhich may not involve permanent supporting surfaces.In those cases where the criteria cannot be met, considerationcould be given to other precautions (for example, railings) whichcould be applied to assure safety and prevent accidental contactsby personnel.Even though the above interpretations are straightforward,difficulties have been experienced in the industry. The fact is thatthe safety rules are meant in the first place to protect personnelwho do not have, neither are required to have, a deeper educationin electrical engineering. Such personnel have the right to checkthe safety rules and to understand them. In this context there aresome difficulties with the above quoted paragraph on vertical orhorizontal clearances. Namely, the end of the paragraph, quote"plus the electric clearance between energized parts and groundas limited by the surge protective devices" defines the safetyclearances, somewhat vaguely. The substation maintenancepersonnel would be much more receptive to a numerically givendistance, which can be easily checked by them, than tocalculations of clearance from arrester characteristics. Theminimum electrical clearance is here given by the BIL, not by thenominal voltage. Thus, for example, if the application of surgearresters enables the use of insulators rated at 350 kV BIL in a115 kV substation instead of the usual 550 kV BIL, then theminimum electrical clearance is 25 inches instead of the 42inches. The necessary clearance for personal safety is 8 feet 6inches +25 inches = 10 feet 7 inches instead of 11 feet 7 inchesshown by NESC for 115 kV, see Table 4a. This example coversa practical case for a 69 kV substation uprated to 115 kV.Clearances of Open Air Switches

Within this context the Table 3 can provide help.

Another aspect of clearances for personal safety is raised bymaintenance considerations. Specifically, clearances across theopen contacts of disconnect switches are of concern for personnelworking on a disconnected (and properly grounded) part of thesubstation. These clearances are specified in Table 3, page 9 of[4]. First part of that table is reproduced here, see Table 4c.Here again the length of break is associated with the BIL and,moreover, with the rated voltage. This would not permit uprating,however, interpretation of the text could:The key to clearance is given in [4], paragraph 2.4.3: Lengthof Break, quoted as follows:"The length of break of outdoor break switches,when in full open position, shall be at least 10% i nexcess of the dry arcing distance over the insulatorsand shall be such that the open ga p( s) will withstanda test voltage which is 10% in excess of the lowfrequency dry and impulse withstand test voltagegiven in Table I ."


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1254This sentence is followed by a reference to Table 3 in [4].This reference is superfluous and could be disregarded. Theimportant fact from the safety viewpoint is that the eventualsparkover is by the design of the switch forced to occur acrossthe insulator rather than across the open break distance. Thus,with the disconnected part grounded, the maintenance personnelare safe independently of the BIL or the rated voltage of theswitch. This would become apparent from the applicablestandard, if the length of break would be associated only with theBIL and not the rated maximum voltage. There should be a

choice of BILs for various rated voltages, as discussed inprevious paragraphs and shown in Table 2.Table 4cOutdoor Air Switches

LineXL67891011

RatedMaximumVoltage(kV rm&72.512114569242242

Rated Length ofBreakImpulse MinimumWithstand Metal-&MetalVoltage Single DoubleWave Disrance Distance1.2 x 50 ps Break BreakkV Crest (inches) (inch)350 32 (0.813 m) 22 (0.559 m)550 50 (1.27 m) 32 (0.813 m)650 60 1.52 mj 38 (0.965 mj750 68 1.73 m) 44 (1.12 m)900 84 (2.13 m) 50(1.27 m)1050 104 (2.64 m) 57 (1.45 m)

CONCLUSIONIn conclusion, a simple summary can be made: If a substationmet the safety criteria before uprating, it would also meet the criteriaafter uprating. Uprating by application of surge protective deviceswould reduce the electrical clearances, line to ground and line to line,but will leave the safety clearances unchanged and thereforesatisfactory. The above, of course, assumes that the same criteria isused and that it remains validThis wording really does not cover newly constructedsubstations. However, the newly const ructed substations can bedesigned to smaller BILs and clearances than those presentlystandardized and still provide personal safety corresponding topresent requirements. These requirements, embodied in the NationalElectrical Safety Code and some other standards, have to beinterpreted with care, but also with due flexibili ty, as discussed in theabove paragraphs. These interpretations should be made easier,particularly for personnel without deeper knowledge, by appropriate

changes in s t a n d a r d s .The most basic change would provide some freedomof choice ofBIL for various rated voltage in the HV range. This is embodied inthe insulation coordination standard [l], but not in some other,generally older standards. Updating of such standards s desirable sothat they do not limit the applications of new technologies andcorresponding economies. Changes in standards generally requireconsiderable length of time. Until it is accomplished, this paper canprovide support for the engineer striving for economy with safedesign in mind.REFERENCES

NEMA SG6-1977: Power Switching Equipment.Panek, J, Elahi, H.: Substation Voltage Upgrading, IEEETransactions on Power Delivery, July 1989, Vol. 4,No. 3,ANSI C 92.1: Insulation Coordination.ANSI C 37.32-1972: Schedules for Preferred Ratings, andApplication Guide for High-Voltage Air Switches, BusSupports and Switch Accessories.IEC Publication 7 1: Insulation Coordination.W.J. Lannes, K.W. Priest, et al., 230 kV Operation of aSubstation Designed for 115 kV by Controll ing VoltageTransients, IEEE Transactions, Vol. PAS-90, pp. 1698-1718.

pp. 1715 - 1724.

[71 R. Saavedra, Experience with Uprated Substations,IEEE/PES 1989 Summer Meeting, Panel Session Paper,Long Beach, CA, July 11 ,1989.[81 J.R. Stewart, Ongoing Work with Substation VoltageUprating .IEEE/PES Summer Meeting, Panel Session,Long Beach, CA, July 11, 1989.[9 ] H. Elahi, J. Panek, J.R. Stewart, H.R. Puente, SubstationVoltage Uprating: Design and Experience, Paper 496-0,IEEE/PES Summer Meeting, Minneapolis, Minn., July 15-19, 1990.[101 ANSI C2: National Electrical Safety Code, 1990.[1 13 AIEE Committee Report: A Guide for Minimum ElectricalClearances for Standard Basic Insulation Levels, AIEETransactions, Part I11 (PAS) Vol. 73, June 1954, pp. 636 -641.


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Discussion1255

ReferencesA. M. Mousa, (British Columbia Hydro, Vancouver, B.C., Canada): Iwish to congratulate Dr. Panek and his coauthors for an interesting paper.A study of the safety aspects Of uprating is havepreceded the building of uprated substations. The authors response to thefollowing comments and questions would be appreciated:

[I21 E. J. Yasuda and F. B. Dewey, BpAs New Generation of500 kvLines, IEEE Trans., Vol. PAS-99, No. 2, pp. 616-624, 1980.[13] H. Elahi et al., Closure of Substation Voltage Uprating: Designand Experience, IEEE Trans. on Power Delivery, Vol. 6, No.an d

3, pp. 1049-1057, July 1991.The last sentence of the first paragraph of the CONCLUSION of thepaper needs to be emphasized: safety requirements are not automati-cally met in the uprated substation unless the voltage withstand criteriahave not been changed. In this connection, the following is noted:(a) Same compact designs involve a reduction in voltage withstandcriteria. For example, the 500 kV compact line designs reported inRef. 12 are based on allowing one flashover per 10 switching surge(SS) operations while the typical industry standard is oneflashover/lOO S S operations.(b) Some of the uprated substations have experienced flashovers whichremain unexplained [131.In view of the above, there may be locations in some upratedsubstations where safety clearance requirements are not met andaccess needs to be restricted by use of barriers.As pointed out in the first sentence of the second paragraph of theCONCLUSION, the safety criterion given in the paper does not covernewly constructed substations. To cover this latter important case,there is a need for providing two things:(a) An equation for determining the limit of approach as a function ofthe overvoltage limits guaranteed by the applied arresters. Presum-ably those overvoltage limits can be correlated to the ratings of thearresters. The methods for calculating safety clearances reported in[14] and [I51 may be useful in that respect.(b) A guide regarding the amount of room which should be providedfor the mans body in the different situations. A basis for such aguide is already available in the material given in [16]. As anexample, [16] specifies the vertical reach of the worker as 2.25m ascompared to the 8.5 ft (2.59m) given in the paper.In a traditional substation design, three different insulation levelsoften exist: A BIL for the transformer, a somewhat higher BIL for thepost insulators, disconnect switches and circuit breakers, and a thirdvoltage withstand level higher than the above two applying to the airgaps to grounded structures. The above practice (reflected in Fig. Ib)may be justified by the fact that the overvoltage level appearing at a buspoint increases with increase in separation between that point and thenearest arrester. In the traditional layout, surge arresters were usuallyused only at the terminals of the transformers. Thus the existence of 3insulation levels may have been justified. The new compact designsinvolve use of more sets of arresters. Nevertheless, some of the pointsof the strain bus may be far enough from the arresters to justify havingtwo insulation levels.In fairness to the pioneers of traditional substation designs, it shouldbe mentioned that they used higher insulation levels not because oflimitations of their knowledge about insulation coordination but ratherbecause of the limitations of the equipment available them. Insulationlevels have dropped in 3 distinct steps when better equipment becameavailable:(a) The traditional gapped arresters of today represent a huge improve-ment over the arresters used 40 years ago. Insulation coordinationlevels in ANSI Standards have undergone at least one majoroverhaul to reflect that fact.(b) With introduction of EHV systems, switching surges became thegoverning parameter. Use of breaker pre-insertion resistors con-trolled the overvoltages at their generation point thus keepinginsulation levels from rising in proportion 60 Hz levels.(c) The introduction metal oxide arresters opened a new era by con-trolling both lighting and switching surge levels.

[141 IEEE Working Group; Live-Line Maintenance Methods, IEEETrans., Vol. PAS-92, pp. 1642-1648, 1973.[I51 P.C.V. Esmeraldo et al., Calculation of Minimum Safety Dis-tances for Live-Line Maintenance-A Statistical Method Applied to765 kV AC Itaipu Lines, IEEE Trans., Vol. PWRD-1, No. 2,[I61 CIGRE Working Group 06 of Substations Committee, The Effectof Safety Regulations on the Design of Substations, Electra, No .19, pp. 79-102, November 1971.

pp. 264-271, 1986.

Manuscript received August 22, 1991.

Working Group El: RECOMMENDED MINIMUM CLEARANCES INSUBSTATIONS: The authors wish to thank Mr. Mousa for his interest in thepaper and for his comments. Below is our reply:1.a: Flashover criteria for station and line insulation ought to be different.Thii is because the line insulation, having a self restoring insulation canaccept a higher number of flashover occurrence than the insulation ofstation equipment.Also, for substation faults,no high speed reclosures areallowed; while for lines they are often implemented. Also reliabilityrequirements influence the criteria, loss of a line and loss of the wholesubstation have different impact on the system. - The voltage withstandcriteria for the insulation of the substation equipment have not beenchanged.The failure of 2 cap and pin insulators remains unexplained. It is notclear if the failure had an electrical or mechanical cause. As described in[13), both insulators were 20 to 30 years old, i.e, they were exposed totransportation and handling during the new installation as well as totemperature variations during the years of service. A number of failures ofthe same insulators has also occured in other,- not upratedsubstations.The utilities concerned did not change their safety criteria, they replacedthe insulators with other units, often substation post.We agree that substation uprating necessitates careful examination ofsafety clearance requirementsat all locations.2. The principles of voltage upgradings are the same as for newsubstations except for the latitude of providing additional margin in newconstruction. Reduction of overvoltage stressescanalwaysbe accomplishedby additional arresters as in the case of uprating. - Thus the paper is fullyapplicable also to new, compacted, substations.Different levels of insulation within substation is an approach adoptedby many in the past. With the present surge arresters, protective levels havebeen significantly reduced. The range of protective levels allows one tochoose the most economical solution. - Prescribing the number ofinsulation levels within the substation isnot necessary.This paper recognizes the technological changes permitting upratiag.These changes include protective equipment, switching surge andtemporary overvoltage suppressing schemes (i.e.,closing resistors/reactors)and analytical ools (i.e.,EMTP).

1.b

3.

4.

In conclusion the safety aspects raised in this paper show the need of updatingand coordination between the provisions of some standards, related to thechoice of the BIL for a given rated voltage. There is a need of providing aguide for the designers regarding the minimum safety clearances inrelationship with the BIL and the available protective level, rather than onlytothe rated voltage, as it is now.Other exciting developments are destined to happen in the future andmetal oxide arresters will probably become obsolete. Hopefully, theengineers of the future will look back with kindness at the less knowl-edgable engineers who pioneered metal oxide arresters!

Manuscript received February 6, 1992.

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Safety Aspects in Substation Voltage Uprating