Guidelines for seismic design of flexible buswork between substation equipment.pdf

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EARTHQUAKE ENGINEERING AND STRUCTURAL DYNAMICSEarthquake Engng Struct. Dyn. 2007; 36:191208

Published online 30 August 2006 in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/eqe.619

Guidelines for seismic design of flexible busworkbetween substation equipment

Jean-Bernard Dastous,

Hydro-Quebec, Institut de recherche, Expertise M ecanique, M etallurgie et Civil, 1800 Lionel Boulet,

Varennes, Que., Canada J3X 1S1

SUMMARY

During an earthquake, flexible buswork between interconnected equipment is stretched and compresseddynamically. This causes additional forces to be transmitted to the equipment. Design guidelines forflexible buswork have been determined through non-linear finite element simulations on models of typicalinstallations. For a proper design, the required amount of slack in the buswork is established using anestimation of the maximum horizontal relative displacement between equipment in a pair, with an additionallength function of the buswork shape and its corresponding stiffness. To avoid multi-connected equipmenteffects, all pairs of equipment within a given electrical phase must be designed in such way. Of utmostimportance, equipment must be designed with an additional static load at its attachment point, to takeaccount of the unavoidable forces transmitted by the buswork. From the previous criteria, a methodologyfor the design of universal flexible buswork has been established for use within Hydro-Quebec and shownto be a simple way to cover most pairs of equipment within a given voltage level and for a given seismicdemand. This methodology resulted in design tables specifying the required conductor length of possibleshapes, for different distances between equipment. The guidelines described in this paper are proposed

for possible adoption by other utilities. Copyrightq

2006 John Wiley & Sons, Ltd.

Received 3 November 2005; Revised 25 May 2006; Accepted 5 July 2006

KEY WORDS: earthquakes; flexible buswork; interaction; interconnection; stranded conductor; substationdesign

INTRODUCTION

During an earthquake, interconnected substation equipment experiences relative displacement

which stretch and compress dynamically the flexible buswork between them. This buswork is

usually made of one or more stranded all aluminium conductors of relatively short span, usually

Correspondence to: Jean-Bernard Dastous, Hydro-Quebec, Institut de recherche, Expertise Mecanique, Metallurgieet Civil, 1800 Lionel Boulet, Varennes, Que., Canada J3X 1S1.

E-mail: [email protected]

Copyright q 2006 John Wiley & Sons, Ltd.


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192 J.-B. DASTOUS

between 2 and 7 m. During its motion, the buswork transmits dynamic forces to the equipment,

which adds to the loads generated by the equipment response itself during the earthquake. It has

been suspected that such additional forces have been responsible to contribute to the failure of

equipment during earthquakes [1].

It is only in the last 15 years or so that this subject started to receive more attention fromutilities and researchers. Therefore, the seismic design guidelines of flexible buswork have been

until recently limited to stating that enough slack should be provided to accommodate the relative

displacement between equipment without impact; the slack being defined as the difference between

the free conductor length and the straight line distance between its attachment points. In order to

establish the slack in such guidelines, equations based only on geometrical considerations have been

proposed. They use estimations of the maximum relative displacement, based on a combination of

the maximum expected individual displacements of equipment in a pair, without considering the

effect of the flexible buswork itself. Since the buswork generates additional forces, it is important

that such effect be considered. It is indeed fundamental that the level of expected forces transmitted

by properly designed buswork be known, in order to specify it in design of the equipment itself.

Up to now, this effect has not been quantified precisely.

In the last decade or so, we performed different research projects that eventually led to a

design methodology taking account of all important parameters. The purpose of this paper is to

present the corresponding seismic guidelines that have been established within Hydro-Quebec for

flexible buswork, along with some of our main findings. It is our opinion that such guidelines are

transferable to other utilities as well, and many of them have indeed been accepted in an IEEE

Recommended Practice on the seismic design of flexible buswork [2].

DESIGN APPROACH RETAINED

In a substation, many different types of equipment are interconnected by flexible buswork. Most

equipment is one of two types. The first is the candle-like type, with a column (the equipment)resting on either a tubular or a lattice-type of support. Examples are instrument transformers

(CT, CVT), live-tank circuit-breakers and lightning arresters. The second is the frame type, which

usually consists of one or more frames superposed. Examples are disconnects switches and capacitor

racks. Both types are relatively simple structures and share the common characteristic that the

displacement at the attachment point of the flexible buswork is mostly function of their fundamental

frequency and corresponding cantilever-type mode shape [3].

While it is possible to design a unique flexible buswork according to each possible pair of

interconnected equipment, it is not always desirable in practice, particularly in the context of a

utility where the costs and work related to such individual design might be prohibitive. Indeed in

practice, there are almost an infinite number of possible combinations of equipment to interconnect,

due to the different types, manufacturers and supports among others. Nevertheless, one common

characteristic is that only equipment of the same voltage level is interconnected. Since equipment ofthe same voltage level shares similar characteristics such as natural frequencies within a given range,

similar heights and similar required insulation distances, this makes it desirable to design universal

configurations that would apply to all possible pairs of equipment within a given voltage level,

under a given seismic demand. While this would involve sometimes large degree of conservatism

to cover most cases, such design is much welcome in the context of a utility where simplicity of

use is often an inherent design criterion. Therefore, the design approach we retained was to aim at

Copyright q 2006 John Wiley & Sons, Ltd. Earthquake Engng Struct. Dyn. 2007; 36:191208

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GUIDELINES FOR SEISMIC DESIGN OF FLEXIBLE BUSWORK 193

universal configurations that would be suitable for most pairs of equipment for each of our high

voltage levels: 145, 230, 330 and 735 kV.

For reasons of interchangeability of equipment after an earthquake, flexible buswork was de-

signed within Hydro-Quebec for two levels of zero period acceleration (ZPA): 0.5g and 1g. These

levels cover most of our sites in accordance with the seismic criteria in the new edition of theNational Building Code of Canada (referred next as NBCC 2005), as described in Reference [4].

Therefore universal flexible buswork configurations were designed for the four voltage levels above,

for these two levels of acceleration, resulting in a total of eight sets of universal configurations.

REQUIRED SLACK

Basic requirements

The primary question to answer in order to design a flexible buswork for a given pair of equipment

is the amount of slack needed. As a minimum, it is straightforward to propose that it should at

least permit the maximum relative displacement that can occur between interconnected equipment.Otherwise, impact forces may be transmitted between equipment due to all slack being used

up. Indeed, as demonstrated qualitatively by analytical and numerical studies on simple models of

equipment and conductors, damageable interaction effects can be generated when not enough slack

is provided, causing an amplification of the equipments response, especially for the equipment of

higher frequency in a pair [5, 6]. This need translates as a minimum length, Lmin, given by

Lmin = c0 + e0 (1)

where c0 is the straight line distance between the attachment points of both equipment within a

pair and e0 is the maximum expected elongation of the flexible buswork, function of the maximum

expected relative displacement between equipment and the geometry: span and difference of heights

between attachment points.Equation (1) does not however take into account that, depending on the rigidity of the conduc-

tor(s), there is a rapid increase of stiffness and therefore of transmitted force, when a configuration

is close to being stretched completely. Also, it does not take into account the fact that dynamic

forces are generated even when sufficient slack is present [7] and therefore, that additional slack

might be needed in order that the transmitted forces are within a specified level. Furthermore, the

effect of multiple-connected equipment as in real installation (more than one pair) is also not taken

into consideration.

Numerical studies performed to establish the required amount of slack

In order to establish the required amount of slack, we performed numerical simulations using

the non-linear finite element method with representative models of installations comprising multi-connected models of existing equipment on their supports. Four representative models of instal-

lations were used, each comprising from 5 to 7 multi-connected equipment; their properties are

detailed in Table I and include first standalone fundamental frequencies in the longitudinal (con-

ductor direction) and transverse directions. Typical fundamental standalone frequencies of existing

equipment from a survey are presented in Table II for comparison; it is observed that our models

are representative.


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Table I. Characteristics of models used in numerical study.

Models

Characteristic 145 kV 230 kV 330 kV 735 kV

Conductor (kcmil) 1 1796 2 1796 1 4000 2 4000

Equipment 1 Rigid bus Rigid bus Rigid bus Disconnect switchSpan to next (m) 3.8 3.0 4.1 3.6flongitudinal (Hz) 5.31 4.99 4.52 1.32

ftransversal (Hz) 4.61 4.37 3.76 0.97

Equipment 2 Disconnect switch Disconnect switch Disconnect switch Rigid busSpan to next (m) 4.3 3.0 3.4 6.2flongitudinal (Hz) 3.42 4.62 4.02 1.38

ftransversal (Hz) 5.37 2.18 1.89 1.18

Equipment 3 Circuit breaker Circuit breaker Rigid bus Circuit breakerSpan to next (m) 3.2 2.5 4.0 5.2f

longitudinal(Hz) 2.90 1.25 4.33 0.56

ftransversal (Hz) 2.90 1.25 3.49 0.62

Equipment 4 Current transformer Current transformer Circuit breaker Current transformerSpan to next (m) 3.0 3.0 4.3 4.1flongitudinal (Hz) 1.67 2.25 0.80 0.69

ftransversal (Hz) 1.67 2.25 0.80 0.69

Equipment 5 Rigid bus Rigid bus Current transformer Disconnect switchSpan to next (m) 3.8 3.0 3.6 flongitudinal (Hz) 5.07 4.83 0.96 1.32

ftransversal (Hz) 4.35 4.14 0.96 0.97

Equipment 6 Disconnect switch Disconnect switch Disconnect switch Span to next (m) 3.8 3.0 4.1 flongitudinal (Hz) 3.42 4.62 4.02

ftransversal (Hz) 5.37 2.18 1.89

Equipment 7 Rigid bus Rigid bus Rigid bus flongitudinal (Hz) 5.31 4.99 4.52

ftransversal (Hz) 4.61 4.37 3.76

Table II. Summary of survey on fundamental frequencies of equipment (cantilever mode shape).

Retained forVoltage (kV) Minimum (Hz) Maximum (Hz) Average (Hz) design (Hz)

145 1.24 10.0 4.6 1.25230 1.21 8.9 3.4 1.25

330 0.60 4.3 2.0 0.60735 0.40 2.8 1.1 0.40

An example of one model is presented in Figure 1 for the 735 kV installation. The models were

subjected to different synthetic and historical earthquakes time histories as detailed in Table III,

corresponding to eastern Canadian earthquakes, apart from synthetic inputs matching the IEEE-693


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Figure 1. Example of finite element model used in numerical simulations for the 735 kV installation.

response spectrum [8, 9]. The latter was used in order to check the application of our findings to

west coast earthquakes as well, since this spectrum was designed to cover earthquakes anywhere in

the U.S., Canada and Mexico. The eastern Canadian synthetic inputs [10] Montreal and La Malbaie

were longitudinal only and were applied in horizontal directions at 0, 45 and 90 respective to the

longitudinal direction of the installation (conductor direction).

A non-linear conductor model accounting for the dynamic variation of bending stiffness in the

conductor when layers of strands interact with each other was used. Its description and validation

are described in Reference [11]. This model was shown to reproduce experimental results with

good agreement, thus proving to be a reliable tool for representative simulations within the range

of forces expected for short spans between substation equipment during earthquakes; for higherlevel of forces, such as for long spans of transmission lines, the validity of such model remains to

be demonstrated. We summarize next the main findings supporting our design methodology.

Results from numerical study

The parameter denoted by [12] has been used to determine the required amount of slack.

It provides the ratio between the demand e0: the maximum expected elongation of the flexible

buswork and the availability d0: the slack present in a configuration. It is given by

=e0

d0(2)

with d0

given by

d0 = s0 c0 (3)

where s0 is the free conductor length.

The availability e0 was established using the displacement of equipment in its standalone

configuration (without the effect of buswork) since from a design perspective, this quantity is easy

to obtain from seismic qualification reports (standalone qualification) or to predict using simple


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Table III. Inputs used in our study.

Earthquake Epicenter Scaling PGAand site Magnitude distance R (km) factor Component (g)

Montreal 6.0 30 0.85Synthetic A Horizontal 0.37Synthetic B Horizontal 0.44Synthetic C Horizontal 0.40Synthetic D Horizontal 0.37

Montreal 7.0 70 0.90Synthetic E Horizontal 0.27Synthetic F Horizontal 0.26Synthetic G Horizontal 0.31Synthetic H Horizontal 0.26

La Malbaie 7.0 20 0.75Synthetic A Horizontal 1.28Synthetic B Horizontal 1.27

Synthetic C Horizontal 1.45Synthetic D Horizontal 1.22

Saguenay 1988 5.7 52 2.9 124 0.38Chicoutimi-North 214 0.31site 16 Vertical 0.30

Saguenay 1988 5.7 52 2.9 360 0.37Les Eboulements 270 0.30site 20 Vertical 0.68

Nahanni 1985 6.5 24 2.0 360 0.39Battlement creek 270 0.37site 3 Vertical 0.36

IEEE 693 0.5 x 0.50Synthetic y 0.49

Vertical 0.40

IEEE 693 1.0 x 1.00Synthetic y 0.99

Vertical 0.80

analytical methods [3]. In our study, simulations on non-connected equipment were performed

first in order to obtain e0 under each input. Equation (2) is interpreted as follows: a value smaller

than 1 indicates that more slack than required to account for the expected relative displacement

between standalone equipment is present and vice versa. However, dynamic and other effects

might influence the actual value of required in practice so that even values smaller than 1 donot ensure an adequate design and vice versa. In our simulations, the availability d0 was varied

significantly in order to obtain large variations of . To do so, we imposed the same amount of

slackness s to all pairs within a model, with s defined as

s =d0

c0(4)


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Figure 2. MAFi for the 145 kV model under synthetic inputs for Montreal and La Malbaie, for slacknessvalues from 2 to 10% (values for >4 not shown).

Slackness values from 2 to 10% were used which led to wide variations of as slackness is not

related to the expected elongation e0.

To establish an adequate value of for design, we need to correlate it to the level of forces

transmitted to equipment. By our previous dynamic experiments [7], it is indeed clear that those

are unavoidable, even with large amount of slack. This has also been demonstrated by other

investigations, experimentally as well as numerically [1115]. Therefore, it would not be realistic

to expect to design a flexible buswork such that corresponds to negligible transmitted forces.

The force transmitted by the flexible buswork can be interpreted as an additional moment at the

base of the equipment insulator, often the most stressed and critical point, which often governs the

design of the insulator itself and consequently, the equipments structural resistance to earthquakes.

For this reason, we compared first the moment transmitted at the base of equipment without the

buswork: Mi0(t), to the moment with it: Mi (t), in order to asses the buswork effect. To do so, we

used the parameter MAFi (moment amplification factor for equipment i ), given by

MAFi =max|Mi (t)|

max|Mi0(t)|(5)

A value of MAFi over 1 therefore indicates that the buswork amplifies the moment transmitted. In

our study, the values of max|Mi 0(t)| were obtained along with e0 by simulations on non-connected

models.

As first step, we analysed our results by studying the variation of MAF i versus , as well

as according to the frequency of the equipment. We present in Figures 2 and 3 representative

results for the 145 and 230 kV configurations. It is first observed that the amplification from the

buswork can sometimes be severe, with values up to 12 but in counterpart, it is also observed thatsometimes it causes de-amplification with MAFi values smaller than 1. It is also observed that

even for smaller than 1, there is significant amplification in many cases as well as on average

and therefore, the effect of buswork cannot be neglected. In Figure 2 for the 145 kV, a trend is

observed with the frequency of equipment as larger amplifications occur for the equipment at

higher frequencies and to some extent, with larger values of at the same time. Inversely, it is

also observed that de-amplification may happen on all range of (including over 1) and that this


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Figure 3. MAFi for the 230-kV model under synthetic inputs for Montrealand La Malbaie, for slackness values from 2%.

Table IV. MAFi statistics under the Montreal and La Malbaie synthetics inputs,for slackness values from 2 to 10%.

Statistic 145 kV 230 kV 330 kV 735 kV

Minimum 0.63 0.37 0.43 0.49Maximum 11.9 2.72 3.29 2.92Average 1.89 1.16 1.03 1.09

happens most often for the lower frequency equipment. This has also been pointed out in previous

studies where it was demonstrated that the equipment of higher frequency in a pair is the one

experiencing the most amplification of its response, with also higher amplification on average for values over 1, while the equipment of lower frequency usually experiences less amplification and

even de-amplification of its response [5,6,12]. The same trends are also observed from Figure 3

for the 230 kV but to a milder extent. The more severe amplifications for the 145 kV might be

explained as in this configuration, the differences between the frequencies of successive connected

equipment items are more pronounced that for the other configurations (Table I). Therefore, the

difference of frequency between equipment in a pair has a definite influence on their responses.

Also we suspect that since most equipment in our models are connected on both sides (that is

part of two pairs at the same time), this may complicate their response as opposed to independent

pairs of equipment so that the clearer trend observed in References [5,6,12] with may not

always apply.

We present in Table IV statistics on MAFi for the different configurations under the eastern

Canadian inputs applied in the longitudinal direction. It is observed that there are significantlyhigher amplifications for the lower voltage level (145 kV) which as explained earlier, is related

to the fact that there are larger differences between successive equipment frequencies. Proba-

bly to a lesser extent, another contributing factor to this observation is related to the equip-

ment weight that is lighter on average at that level, so that the effect of the forces transmitted

by the buswork is relatively more significant to the contribution from the equipment weight

only (Mi0(t)).


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Figure 4. MAFi,1000 for the 230-kV configuration under synthetic inputs for Montreal and La Malbaie,for slackness values from 2 to 10%.

In order to identify the level of force that would be suitable as design value, we started

our investigation with the value already specified in our own internal seismic qualification stan-

dard for substation equipment: 1000 N/by sub-conductor [16]. This value was established be-

fore we started to study the interconnection problem as a dead load in the case of very tight

(small sags) catenary configuration under icing conditions. Since we had previously observed

in our dynamic experiments that a value around or less than 1000 N was expected with suf-

ficient slack and since we do not consider the simultaneous occurrence of earthquakes with

extreme icing conditions, it made sense to use such value as a starting point in our

investigation.

The following amplification factor to asses the required value of was used, similar to

Equation (5) but taking into account that the equipment design would be done on a standalone

basis, with an additional moment from the force generated by the conductor(s) at the attachmentpoint:

MAFi,1000 =max|Mi (t)|

max|Mi0(t)| + Mi,1000(6)

where Mi,1000 is the static moment generated at the base of the insulator by the additional load of

1000 N/sub-conductor at the attachment point.

A value of MAFi,1000 smaller than 1 therefore indicates that the value of 1000 N/sub-conductor

is enough to cover the amplification effect of the buswork. In practice for most equipment,

even values over 1 would be suitable to some extent as max |Mi0(t)| is often below the equip-

ments resistance due to the use of margins of safety in design; we suppose here implicitly

that equipment is designed exactly for the earthquake input under consideration. We presentin Figure 4 the same results from Figure 3 analysed under MAFi,1000. It is observed that

even though many values of are below 1 as sought, there are still values over 1 including

when


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Figure 5. MAFi,1000 for the 230-kV configuration for synthetic inputs for Montreal and La Malbaie, with around or smaller than 1 for all pairs of equipment under a given input.

some pairs while limiting it in others. We therefore hypothesized that the effect of multi-connected

equipment could sometimes cause impacting forces to be transmitted from some pairs to others.

Our analysis was then refined by examining values of MAF i,1000 only when would be around or

smaller than 1 for all pairs under a given input. The corresponding subset of results from Figure 4

is presented in Figure 5, where values of slightly over 1 are also included. It is now observed

that all MAFi,1000 are below 1 and this result was common in most cases to all our other models,

which led to the following basic design criterion:

Flexible buswork in all pairs of multi-connected equipment within a given installation should

have values around or below 1.

This criterion was further validated by simulations with values exactly equal to 1 for all pairs ofequipment in a model under the different inputs used in our study, which confirmed the adequacy

of this criterion. Note that where the additional amount of slack needed may lead to clearances

violation, a user may consider to relax this requirement in cases where equipment of similar

frequency are connected, since as pointed in References [5,6,12], the most severe amplifications

occur for pairs of equipment with larger differences in frequency.

In addition to this criterion, we decided after investigation to specify the following levels of

design forces to account for the effect of buswork:

For ZPA0.5g, a force of 1000 N/by sub-conductor should be used.

For 0.5g


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AMOUNT OF SLACK FOR UNIVERSAL FLEXIBLE BUSWORK DESIGN

Based on the previous section, the required length for adequate flexible buswork design is

given by

s0 = c0 +e0

+ L (7)

where 1. It is easy to demonstrate by geometric considerations that for equipment with

attachment points at different heights, a conservative estimate of e0 is given by 0, the maxi-

mum horizontal expected relative displacement between standalone equipment:

0 = max(x0,1(t) x0,2(t)) (8)

where x0,1 and x0,2 are, respectively, the horizontal displacements of equipment 1 and 2 in a pair

in their standalone configuration.

It was also demonstrated through our simulations for between 0.2 and 1 that on average aswell as in most cases, 0 is a conservative estimate ofc, the maximum horizontal displacement

between connected equipment. Considering all our simulations using the synthetic inputs for

eastern Canada for slackness between 2 and 10%, the average ratio c/0 observed was 0.84

for values between 0.2 and 1. Therefore, in order to develop universal configuration between

equipment regardless of the height of their attachment points, Equation (7) was modified to obtain

the following design formula:

s0 = c0 + 0 + L (9)

where it is observed that we used equal to 1 since for universal design purposes, 0 will

be established using an upper bound of the expected relative displacement based on the lowest

frequency of equipment expected at a given voltage level, thus providing in practice valuessmaller than 1 for the great majority of pairs as sought.

It can be demonstrated by geometrical considerations that 0 is approximated very closely

by considering the expected relative displacement in the longitudinal direction only, that it is

in the direction along the conductor from one equipment to the other. Indeed, although relative

displacement can be as significant in the horizontal direction transverse to the conductor, its net

contribution to the total elongation e0 is in most cases negligible, as it is secondary for most spans

and expected displacements, as opposed to the longitudinal contribution which dominates. This

is especially true in the context were 0 is already on average a conservative estimate ofc and

also since maximum relative displacements will rarely happen simultaneously in the transverse

and longitudinal direction anyway.

In the case of response spectrum design we need to estimate 0 since Equation (8) is related to

the use of an earthquake input defined through a time history signal only. To do so, it is natural touse a combination of the maximum standalone expected displacements from each equipment, as

those can be evaluated from a response spectrum. Among the combination methods at hand, we

can use the methods proposed in IEEE-693-2005 [8]:

The absolute sum combination:

0,abs =xmax,01 + xmax,02 (10a)


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Table V. Statistics on absolute error: |0,method 0|.

Number Standard MaximumMethod of samples Mean deviation observed (m)

Pairs of equipment with frequencies within 20%Absolute sum 1068 0.0338 0.0554 0.917SRSS 1068 0.0146 0.0366 0.648CQC 1068 0.0107 0.0191 0.214

Pairs of equipment with frequencies outside 20%Absolute sum 2832 0.0284 0.0310 0.329SRSS 2832 0.0150 0.0213 0.194CQC 2832 0.0150 0.0213 0.194

The SRSS combination:

0,srss =

x2max,01 + x

2max,02 (10b)

An application of the CQC combination [5]:

0,cqc =

x2max,01 2 12xmax,01 xmax,02 + x

2max,02 (10c)

with 12 the correlation coefficient between equipments 1 and 2, function of the frequencies and

damping values of interconnected equipment.

In order to assess which of the above methods is the most adequate, we used the simulations

performed on the non-connected installation models under the 12 synthetic eastern Canada inputs.

We then considered all combinations of equipment between all our equipment models, regardless

of their voltage level, in order to obtain a large variation in the possible combinations. Using the 26

different units of equipment in our models (Table I), we obtained 325 pairs of units of equipment(including pairs with the same equipment as either the rigid bus and/or the disconnect switch is

usually repeated within a given configuration), leading to 3900 possible values of0 under the 12

synthetic inputs.

To compare the methods, we computed the absolute difference between the value predicted by

each method and the true value obtained by simulation: |0,method 0| (absolute error). To

do full justice to the CQC method that should theoretically perform best whenever frequencies

of equipment are within 20% of each other, we performed our analysis on two separate groups:

one for which frequencies of equipment in a pair are within 20% and the other for the remain-

ing combinations. The statistics on the corresponding results are presented in Table V where we

present average and standard deviation obtained for the two groups. Even though not entirely

statistically consistent since the number of corresponding samples might be small, we also re-

ported the maximum error observed in order to demonstrate the size of errors that can sometimeshappens.

We first observe that for equipment with closely spaced frequencies, the CQC method provides

the best performance overall with a significantly smaller standard deviation and also, much smaller

mean and maximum observed error. It is also observed that the absolute method as preferred in

IEEE-693-2005 is clearly the poorest choice and that it can also provide very large errors sometimes,

with a maximum observed here of almost 1 m! Table V demonstrates that when frequencies are


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not closely spaced, the SRSS and CQC methods provide the same results as expected since

the correlation coefficient 12 is then almost zero. This group demonstrates furthermore that the

absolute sum method still remains the poorest choice. Since in IEEE-693-2005 the use of the

absolute sum method is the preferred method, with in addition a 1.5 multiplying factor to account

for uncertainties, we recommend revising the use of such method (and the 1.5 factor) as it furtherleads to increasing difficulties in terms of electrical clearances to respect, especially for low voltage

equipment with large expected displacements.

Based on the previous results, we recommend using first the CQC method, especially when the

use of slack is limited due to clearance requirements and whenever accurate values of frequencies

and damping are available. In our general design methodology we retained the SRSS method to

estimate 0 in order to cover uncertainties on the available properties of equipment. However,

used as such it provides a statistical average only of0 and may thus underestimate sometimes the

true relative displacement. In order to avoid this situation, we used a multiplying factor of 1.25:

0,est|95% = 1.25

x 2max,01 + x

2max,02 (11)

where 0,est|95% is the estimator of0 retained in our design methodology.

The 1.25 factor was empirically calibrated to cover over 95% of the cases studied; it was also

validated using the results of simulations under the IEEE-693 synthetic inputs.

To use Equation (11) for a given voltage level, we need to estimate bound values of xmax,01and xmax,02. In a previous study [3] we established that a value covering 95% of typical substation

equipment (candle-like or frame types) is given by

xmax,0i |95% = 1.6 Sd( fi , i ) (12)

where Sd( fi , i ) is the spectral displacement of equipment i of frequency fi and damping i . It

was observed that the statistical distributions of xmax,0i is approximately Normal.

For design purpose, we introduce Equation (12) in Equation (11) which results in the following

empirical equation to establish the maximum amount of slack using the response spectrum method:

0,max = 2

(Sd,1)2 + (Sd,2)2 (13)

where Sd,i is the spectral displacement of equipment i . Note that we cannot readily predict the

amount of cases covered using Equation (13) as its inherent statistical distribution is not Normal,

resulting from the product of 2 approximately Normal distributions.

To establish the slack required to cover most pairs for a given voltage level, we decided to use

Equation (13) with a realistic value of the lowest frequency expected at that voltage level and 2%

damping, along with posing f1 = f2, which results in the following design equation for slack:

0,max design = 2.8 Sd max (14)

where Sd max corresponds to the lowest frequency expected for a given voltage level. Equation (14)

is in most cases conservative as:

It uses the 1.6 factor in Equation (12). In practice this normalized value of the first modal

participation factor varies between 1 to a maximum observed of 2 [3].

It supposes both equipment at the same frequency which would result theoretically in a null

relative displacement using the CQC method if both equipment would be SDOF with the


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same amount of damping. However, since the difference of frequencies of equipment can

be over 20% with expected displacement of the same magnitude (e.g. equipment at 1 and

1.2 Hz), it made sense to use this combination in the intent of aiming at a universal design.

It uses a 1.25 factor in front of the SRSS estimate for 0.

APPLICATION OF UNIVERSAL DESIGN WITHIN HYDRO-QUEBEC

Design response spectra used and corresponding values of slack

In the NBCC 2005, response spectra are specified by site for over 650 Canadian cities for a

uniform risk level and for a damping value of 5%. However, in the context of designing buswork

and qualifying substation equipment to be interchangeable, we retained as stated two levels of ZPA

for design: 0.5g and 1g, which cover most of our sites. For this reason, we used custom made

design spectra that were tailored to cover most of our sites, for 2% damping [17]. Furthermore,

we used over them site-amplification factors as specified in the NBCC 2005, using the averageamplification factor between soils of type D (firm) and E (soft), which also encompasses most

of our sites. We present in Figure 6 the corresponding spectral displacements for our 0.5g level,

Figure 6. Spectral displacements from the Hydro-Quebec 0.5g2% damp-ing spectrum with average soil amplification between sites type D & E and

from the IEEE-693 spectra for the moderate performance level 0.5g.

Table VI. Design values of slack.

ZPA = 0.5g ZPA = 1g

Sd max 0,max design Sd max 0,max designVoltage (kV) (cm) (cm) (cm) (cm)

145 9.0 25 15.8 45230 9.0 25 15.8 45330 11.4 32 24.0 68735 13.2 38 26.9 76


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with a comparison with the IEEE-693 spectra for the moderate performance level (0.5g), for a

damping ratio of 2%. It exemplifies the differences for eastern Canadian sites where significantly

less energy is expected in the low frequency range.

We present in Table VI the value of spectral displacement Sd max as well as the design value

of slack retained for each voltage level and ZPA using Equation (14) with our design responsespectra.

ADDITIONAL VALUE OF SLACK TO ACCOUNT FOR CONDUCTOR STIFFNESS

The value of L in Equation (9) need to be determined according to the shape of configuration used.

Four basic shapes were retained for design and are illustrated in Figure 7. Those were identified in

an earlier study as suitable through testing [18] and are also now recommended in Reference [2].

Depending on the amount of slack needed and the required electrical insulation clearances, one or

more of such shapes can be applied for a given span and height difference between the attachment

points.To determine L for each of these shapes, we used the non-linear static finite element method to

study the variation of forces during stretching, with the horizontal displacements from

Table VI. The required value of L was identified as the one needed so that the behaviour remains

approximately linear. This was established somehow qualitatively by studying the plots of force

versus stretching and retaining the range without abrupt change of stiffness which usually happens

sharply when a configuration is close to being stretched completely. In this determination, we used

the lowest and highest values of span for each voltage level, with also a variation of a height

difference between the attachment points up to 1 m. The main observations from this study on all

shapes were:

When an adequate value of L is used, the catenary, double and triple-curvature shapes have

similar stiffness and behaviours. The parabola shape does not need any additional L and is much more flexible than the three

other shapes.

Figure 7. Basic shapes of flexible buswork.


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206 J.-B. DASTOUS

Table VII. Recommended values of L and other requirements.

Attachment Number of

Shape L (cm) Span (m) angles at ends () conductors

Catenary 0 All 0 0 1 or 2Parabola 0 1.54.5 90 90 Always 2

without ice with a spacer1.53 with

45mm of iceDouble curvature 5 2 m and above 45 0 1 or 2Triple curvature 10 3.5 m and above 45 45 1 or 2

The additional values of L retained for each shape are presented in Table VII, along with the

angles at the attachment points and recommendations on the span range and number of conduc-

tors. The span range and number of conductors recommended were established from our testingexperience and calculations, such that the configurations can maintain their intended shape without

collapsing.

It is observed that L for the catenary shape was specified to 0 cm although a value of 1 cm

was identified through our simulations. The reason is that since this value is small, since there

is already a high degree of conservatism in the value of 0,max design and since this shapes

makes it sometimes difficult to respect electrical clearances, a null value would be acceptable in

most cases.

Universal flexible buswork tables

Our design guidelines for flexible buswork took the form of tables for each ZPA levels, specifyinga value ofs0 (Equation (9)) for different shapes, for tabulated distances between attachment points

(c0). The mandatory electrical clearance requirements were met for attachment points that are equal

or higher to normalized bus bar heights for each voltage level. Those clearances were checked using

the non-linear static finite element method which proved to be reliable for such verifications [11].

Note that in general we used whenever possible, more slack than required as permitted by the

clearances since from an economical standpoint, the net additional cost of added conductor length

is negligible while providing additional conservatism; slacker conductors are also easier to install.

A subset from our design tables for the 145 kV is presented in Table VIII. It is observed

that not all values of c0 can be covered for each shape, due to either the amount of required

slack that caused a violation of clearance requirements and/or the span allowed for a given

shape.

Linear interpolation is to be used for intermediate values of c0. The use of such tables hasproven to be simple enough within Hydro-Quebec as to be of practical use; many of our sub-

stations have now been constructed or upgraded with them. Nevertheless, several cases have

arisen that required a custom made design. Also, when equipment attachment points in a pair

are not at the same height, several cases have been covered by the catenary shape that then

could be used with enough slack to meet the elongation requirement while respecting the

clearances.


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Table VIII. Design table for the 145 kV level for 0.5g and 1g.

0.5g

c0 (m): 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.00

s0 (m): catenary n/a 2.75 3.25 3.75 4.25 4.75 5.25 5.75 6.25s0 (m): double curvature n/a 2.85 3.35 3.85 4.35 4.85 5.35 5.85 6.32s0 (m): triple curvature n/a 4.20 4.67 5.15 5.60 6.05 6.50s0 (m): parabola 2.90 3.40 3.90 4.60 5.30 6.00 6.70 n/a

1g

c0 (m): 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.00s0 (m): catenary n/as0 (m): double curvature n/as0 (m): triple curvature n/a 4.20 4.67 5.15 5.60 6.05 n/as0 (m): parabola 2.90 3.40 3.90 4.60 5.30 6.00 6.70 n/a

CONCLUSIONS

Design guidelines for flexible buswork within Hydro-Quebec have been determined through

numerical simulations on models of typical installations. For proper flexible buswork design,

the value of required slack corresponds to the expected horizontal elongation between a given pair

of equipment with the addition of a given supplemental length function of the buswork shape and

its corresponding stiffness. To avoid multi-connected equipment effects, all pairs of equipment

within a given electrical phase must be designed in such way. At the same time, equipment should

be designed to accommodate a static additional load at its attachment point, accounting for the

dynamic effects of conductors, as those are unavoidable. In the future, such requirement should

be mandatory in the standalone qualification (without buswork) of electrical equipment.

From the previous criteria, a methodology for the design of universal flexible buswork has been

presented to cover most pairs of equipment within a given voltage level under a given seismic

demand. The required slack has been established for different conductor shapes and spans in order

to meet the mandatory electrical clearances requirements. This methodology resulted in design

tables specifying the required conductor length for different distances between equipment. Such

design is limited since not all distances can be covered for all shapes. In such cases, individual

design using the methodology presented here can be performed. The guidelines and methodologies

in this paper are proposed for possible adoption by other utilities.

REFERENCES

1. Schiff A. Guide to improved earthquake performance of electrical power systems. Report NIST GCR 98-757,

National Institute for Standards and Testing, Washington, DC, 1998.

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3. Dastous J-B, Filiatrault A, Pierre J-R. Estimation of displacement at interconnection points of substation

equipment subjected to earthquakes. IEEE Transactions on Power Delivery 2003; 19:618628. DOI: 10.1109/

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4. Adams J, Halchuk S. Open File 4459. Fourth generation seismic hazard maps of Canada: values for over 650

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