risk analysis ii, c.a. brebbia (editor) © 2000 wit press ...€¦ · starting from the station...

Seismic risk mitigation of high voltage power

stations by means of anti seismic devices

S. Bellorini & F. Bettinali

Enel.Hydro — B.U. Hydraulic and Structural Centre, Milan, Italy

Abstract

The paper resumes the main results of the seismic risk and reliability analysis ofan Italian high voltage open air power station, which suffered a significantearthquake in 1976 [1]. The probabilistic estimation of the costs of restoration ofthe electrical components damaged by the earthquake has been compared withthe ones relevant to the installation of anti seismic devices.The main results of an EC partially funded research project [2] dealing with theseismic mitigation of a Gas Insulated Station by means of a Rolling Ball typeisolation system are described in the following.

1. Introduction

Many recent seismic events have clearly indicated that, without appropriateprecautions, a major earthquake may damage important parts of the electrictransmission and distribution network. Such inadequate seismic behaviourinfluences negatively all other lifeline utilities in the emergency situationfollowing an earthquake, when the availability of electrical power is veryimportant for rescue operations. Thus seismic reliability analyses of HV stationsare becoming more and more important also for the owners which need to know,in addition to the earthquake induced damages, the relevant restoring costs. Theexpected benefits related to possible structural retrofitting of electricalequipment, aimed at their seismic risk reduction, can be quantified. Theevaluation of the seismic risk of electrical equipment and of the whole substationhas to be performed assuming the real site seismicity and theexperimental/numerical fragility of equipment and civil structures.

Risk Analysis II, C.A. Brebbia (Editor) © 2000 WIT Press, www.witpress.com, ISBN 1-85312-830-9

434 Risk Analvsis II

2. Resume of the methodology of seismic reliability analysis ofa high voltage open air power station

Starting from the station electrical scheme, the probability of having loss ofelectrical power on the 380 or 132 kV bus bar has been evaluated [3, 4]. Themethodology of analysis consisted in the following steps:

• subdivision of the station into its major sub-units (macro-components like380 kV line module, 132 kV line module, 132 kV bar parallel module,auto transformer module and 380 kV bar parallel module) which groupedsome significant electrical equipment;

* evaluation of the fragility of each macro-components (380 kV bar module inservice, 380 kV bar module out of service, 132 kV bar module in service and132 kV bar module out of service, electric feeding system) starting from theknowledge of components fragility.

The fragility of every macro-component has been defined as the capability toprovide specific functions, namely the connectivity and the isolation of bus bars,line module and autotransformer module, the electrical feeding for the electricalfeeding system. Within the macro-component functional scheme the probabilityof protection call has been considered too, because the protection system maystrongly influence the substation configuration after the seismic event. Theperformances of the common substation electrical feeding and of the stationbuildings, hosting the control and protection instrumentation, can heavilyinfluence the station behaviour.• evaluation of the substation fragility starting from the macro-components

fragility knowledge.

0.00 0.10 0.20 0.30

Acceleration [g]

0.40 0.50

Fig. 1: Main electrical components fragility curves


Risk Analysis II 435

To evaluate the component fragility (represented by a curve, which relates theprobability of structural damage, and the consequent isolation loss, with aparameter that characterises the earthquake intensity, e.g. maximum groundacceleration) the following activities have been carried out:• realistic seismic input definition (from seismicity historical data and on site

recorded data);• experimental tests (static to estimate the mechanical parameters - Young

modules, yield and failure strength of ceramic and steel - and dynamic, aimedat the identification of electrical equipment dynamic parameters - naturalfrequencies, mode shapes and damping coefficients -) [5]

• numerical analysis of the components (by means of a realistic finite elementmodel set up with experimental data, able to correctly reproduce theinternal/external connections). Some typical results, in terms of fragilitycurves for the most critical electrical equipment are shown in fig.l. It is worthnoting that the most sensitive component (highest probability of structuralfailure during earthquake) is the Current Transformer (TA).Finally the substation fragility has been carried out taking into account the

consequences of all the different possible failure combination. In [1] the riskanalysis gave the probability of occurrence of the above mentionedconfigurations with reference to 30 years station life.

3. A probabilistic estimation of the cost of HV componentsrestoration and of their retrofitting by means of anti seismicdevice

A numerical code (ARB/Station [6]), home developed at Enel.Hydro, hasprovided the mathematical support in performing reliability analysis. It relies ona fragilities database while the operational and the functional schemes of riskanalysis are based on a dynamic event tree structure .

The code can also quantify the consequences of an earthquake by means ofthe expected restoration costs of the damaged electrical equipment. The costevaluation is probabilistic, in the sense that it s computed as the product of thecost of the components and the probability of occurrence of a seismic event witha defined ZPA. The probable medium cost is defined as the cost that has the50% probability of being exceeded, while the maximum expected cost isevaluated as the cost that has the 99% probability of not being exceeded.

It s worth mentioning that, in the present work, the evaluation of the costscovers only the restoration costs of the components and not the ones of theunsuccessful supplying of electric power during an earthquake. These costs arevery high because they depend on the out of service time, hours or days.By means of the ARB code, the probable Medium restoration cost for atypical HV open air station was estimated in 260 kEURO, as the sum of therestoration costs related to specific ZPA ranges (Tab. I, 2^ row). In the sameway the Maximum expected restoration cost is evaluated (Tab. I, 3"* row).The retrofitting of the electrical equipment, through anti-seismic devices, wasextensively investigated as a mean for reducing the probability of the station outof service and, consequently, of the high costs of component restoration. In [4]it s shown the not negligible influence of the current transformer (TA) on the


436 RiskAnalvsis II

whole station functionality (132kV and 380kV line in service). In fact astructural improvement of the same component, by means of the seismic baseisolation, produces benefit to the whole station, reducing the HV line out ofservice from 8.5% to 6% in 30 years of plant useful life, (fig.2).Considering that the improvement cost of the current transformer is relativelysmall if compared with its restoration (about 4kEURO versus 25kEURO) theseismic base isolation solutions appears to be very interesting.

4. Seismic risk mitigation of a Gas Insulated Station (GIS)

4.1. The importance of seismic risk and reliability analysis

Due to their strategic positions in the electric power network, substations and inparticular the GIS ones are essential for the correct functioning of the system. Ifa fault occurs, the consequences can be serious or even disastrous for the finalconsumers, like hospitals, industries and administrations.Several degrees of fault can be taken into account:• Uncontrolled operation (apparatus operates without having received an

order) can lead to faults on the network and damage to the substation itself.• Malfunctioning, breaking of mechanical parts, operating mechanisms,

connection. This does not cause major faults but makes revision andreconditioning operations necessary. The energy supply is guaranteed.

• Displacement of internal parts, leakage and SF6 gas losses: this can lead tointernal arcs if live parts are at a significant amount distance. This will makea substation compartment unusable.

• Breaking-up of structures or enclosures: substation becomes unusable,people nearby can be injured and internal faults can cause damage tosubstation compartments

Table I: Estimated costs of substation equipment restoration for each ZPA range

Acceleration [g]

Probable medium cost[kEURO]

Max expected cost [kEURO]

0-0.1

18

120

0.1-0.2

18

80

0.2-0.3

80

205

0.3-0.4

113

155

0.4-0.5

32

41



0.0%0.1 0.2 0.3 0.4

Ground acceleration [g]0.5

Fig. 2: Reduction of station fragility (380kV bar out of service) due toTA seismic base isolation

As a consequence, seismic risk and reliability analysis is becoming more andmore important for Open Air HV Stations and it s essential for GIS because ofthe characteristic of the closeness of the electrical components.

Enel.Hydro and other European companies, in the framework of REEDS, anEC funded Brite Euram project [2] focussing on innovative energy-dissipationand isolation devices, have evaluated the feasibility of the installation of a newanti seismic system, composed of Rolling Ball devices, under a portion, a phase,of a Gas Insulated Substation (GIS). In the project it has been demonstrated howthe developed anti-seismic devices can efficiently mitigate the seismic risk.Numerical analyses and laboratory (shaking table) experimental tests have beencarried out on the component traditional fixed and on the same with a baseisolation system.

4.2. The anti seismic devices

Because for light structures, like GIS, traditional devices as for example the onebased laminated rubber-steel, are difficult to achieve, a Rolling-Ball RubberLayer (RBRL) device, produced by TARRC [7] has been designed. In the devicethe functions of supporting the structure and providing a horizontal restoringforce are carried out by two separate elements. The rubber layer providesdamping to the system in order to reduce the magnitude of the horizontaldisplacement. Prototype devices, in which the horizontal restoring force is


438

Fig. 3: A 3D finite element mesh of the GIS

provided by cylindrical elastomeric bearings, have been developed in the project,manufactured and tested. The device stiffness is relatively high at very smallamplitudes because the balls have to roll out of the 'pits' produced bydeformation of the rubber under the static vertical load. This increased stiffnessprovides an intrinsic restraint. The stiffness plateau (a constant value) is reachedat large displacement. The damping is fairly constant with displacementamplitude; for the devices used in the tests it is about 10%.

4.3. The GIS station

The electrical system, analysed in the project, is the tallest phase of one bay of a420 kV GIS substation. It s produced by ALSTOM (a partner of the project); sixmeters high, made of Aluminium alloy, and of about 5000kg weight. Theelectrical insulation is ensured by a pressurised SF6 gas mixture, [8].

A 3D finite element numerical model (fig.3) was set up and the results ofmodal and spectrum numerical analysis were compared with the observeddynamic behaviour of the mock-up. This was tested in two configurations, fixedbase and isolated base taking into account, in the latter, the influence of adjacentstations by adding extra masses.

The interface with the soil, a rigid steel base frame, assured a rigid bodymotion. For the numerical analyses standard time-histories (IEC AF2 and AF5[9]) and recorded ones (Toimezzo and Calitri Italian earthquakes) have beenadopted as seismic input.

Comparison of the horizontal accelerations at the top of the mock-up and ofthe stresses obtained in both the configurations showed how the anti seismicsystem is able to reduce the responses by 60% giving rise to no breakage in the



most stressed Aluminium alloy enclosure, (fig.4). The ability of RBRL devicesto improve the seismic resistance of an existing G1S is thus confirmed, [10, 11]

4.4. Solutions and advantages of using the base isolation technique

An anti seismic design can rely on:a) Structure strengthening (i.e. by modifying the structural design so as toprevent any damage),b) Base isolation and/or energy dissipation inclusion (i.e. by limiting theaccelerations transmitted to the structure, so reducing inertial forces).

In the current practice, the following approaches are usually foreseen:• If the ground acceleration is lower than 0.2g, it is assumed that the standard

GIS is able to face seismic excitation without any problems;• If the ground acceleration is between 0.2g and 0.3g additional strengthening

of the steel constructions is necessary, thus giving rise to an increase of 2 or 3% of the global cost;

• If the ground acceleration is greater than 0.4g or 0.5g, substantialmodifications, depending on the figure, are necessary: a) strengthening ofsteel construction, b) decreasing of busbar length and c) design modificationof GIS topology (increasing of enclosure thickness).

Those last modifications imply an extra-cost of 10 to 15% if compared to thestandard design; costs of extra calculations have also to be added. On the otherhand and according to the project results, base isolation seems to be veryinteresting because of the decrease in the stress level the standard GIS canwithstand an earthquake having a Peak Ground Acceleration of 0.5g.

4.5. Cost benefits analysis

The economical benefits for a station base isolated with RB devices has beenevaluated and the cost estimation carried out by referring to a typical complete420kV GIS station in both the fixed base and isolated base configuration.According to the experience of ALSTOM and to Enel.Hydro information comingfrom previous cost-benefit analysis, an economical comparison including bothdirect and indirect costs between the two solutions has been considered.


440 Risk Analysis II

The direct costs considered have been the following:• GIS equipment (including low voltage and control devices)• Assembly, mounting, gas filling, supervision and tests• Extra costs for anti seismic design (3-15% of GIS)• Supporting steel frame" Rolling Ball Isolation System• Extra costs for additional computationThe indirect costs considered are those associated with the station being out ofservice . This cost depends on a fixed MW/day flowing through the station, aload reduction factor of 2 (after the earthquake the energy request usuallydecreases) and a fixed EURO/kWh cost of the unsold energy. Depending on thelevel of damages caused by the earthquake on the substation, it is possible tohave a long period of interruption in energy delivery (10 days for a partial repairand even much more if the equipment destruction is significant).For a high seismicity level (greater than 0.3g and up to 0.5g) the base isolatintecnique seems to be very attractive, because its direct costs are lower than thefixed base solution cost, which requires strengthening and other not negligiblestructural modification.For a low-medium seismicity level (from 0.2g to 0.3g) the direct costs are rathersimilar, but the base isolated system covers the risk that a seismic event stronger

TEST IEC AF2 triaxial

Time [sec]

Fig. 4: Comparison between fixed base and isolated base GIS

than the design earthquake occurs during the lifetime of the substation (30

years).Multiplying the costs (direct and indirect) by the risk, the potential saving is

around 30 kEURO, which balances the small extra cost of Rolling-Ball systemshowing the economical advantages of base isolation solution in a very largerange of seismic situations. It has to be noted that costs related to the base


Risk Analysis 11 441

isolated solution use prototype and laboratory system device costs that can beobviously reduced by industrial production.

5. Conclusions

The numerical-experimental methodology adopted has allowed the evaluation ofthe substation fragilities, i.e. the probability of occurrence of the main substationfunctions in the case of a seismic event. For the examined open air substation theexpected frequency of out of service of 380 kV and 132 kV bar module in 30years have been evaluated in 6% and 13% respectively.

Furthermore the current transformer has been shown to be the most criticalcomponent of the station and the effect of its retrofitting on the main stationfunctions (reduction of the 380kV bar out of service from 8.5% to 6%) has beenestimated. A probabilistic estimation of the costs of electrical componentrestoration, after an earthquake, has also been carried out.

The stress reduction induced by a seismic base isolation technique in a GIShas been demonstrated, for both high and medium level earthquakes.

Thanks to the general methodology for the reliability analysis developed itwill be possible to efficiently examine other stations located in different seismicareas.

Acknowledgements

A special thank to Dr. Keith Fuller and to Dr. Alan Muhr of TARRC, to Dr. EricSerres and Alexandre Diehl of ALSTOM for their active role in the REEDSproject and in particular to the electric equipment activities and to Dr. GiuliaBergamo of Enel.Hydro-ISMES for her co-ordination of all the experimentalactivities on GIS prototype.

6. Bibliography

[1] S. Bell orini, F.Bettinali, S.Clementel, F.Gatti, V.Carrettin, G.Zafferani,Seismic Risk Evaluation For High Voltage Power Stations: A CaseStudy , 37 * Cigr Session, 30 August- 5 Sept 1998, Paris.

[2] Synthesis Report Contract n. BRPR-CT96-0141 - REEDS, Optimisationof energy dissipation devices, rolling systems and hydraulic couplers forreducing seismic risk to structures and industrial facilities , projectfunded by EU under Brite Euram programme.

[3] Internal Report Enel.Hydro, Analisi di affidabilit strutturale di stazionielettriche a fronte di eventi sismici: applicazione del codice di calcoloARB/Stazioni alia stazione ENEL AT di Udine Ovest, ENEL PIS n.5524,Gennaio 1998.


442

[4] Internal Report Enel.Hydro Analisi di affidabilit strutturale della

stazione 380KV di Udine Ovest: analisi dei macrocomponenti costituentila stazione , ISMES Doc. n. RAT-DMM-893/95, Prog. ASP-7618.

[5] Internal Report Enel.Hydro Prove statiche di rottura sui sostegni isolanti380KV del progetto unificato ENEL, ISMES Doc. n. RAT-DIS -1661/93,Prog. ASP-7149.

[6] Internal Report Enel.Hydro Realizzazione del codice ARB/Stazioni per1 analisi dell affidabilit strutturale delle stazioni: manuale utente Vers.

2.0 , ISMES Doc. n. RAT-STR-2976/96, Prog. ASP-7149.

[7] K.N.G. Fuller et al., "Optimisation of visco-elastic, elasto-plastic andviscous dampers, shock transmitters and rolling-ball systems in theframework of the EC-funded reeds project", Proceedings of Post-SMiRTConference Seminar on Seismic Isolation, Passive Energy Dissipation and

Active Control of Vibrations of Structures, Cheju, Korea, 1999

[8] Confidential Report ALSTOM REEDS- Evaluation of technical andeconomical benefits of the innovative devices —Six monthly report ,

B3GA1102, November 1998.

[9] IEC 1166, Guide for Seismic Qualification of High Voltage AlternatingCircuit Breakers , first edition 1993/04.

[10] Internal Report Enel.Hydro, The benefits of the seismic base isolation fora 420kV GIS Station , ENEL PIS n. 5856, Ottobre 1999.

[11] Confidential Report Enel.Hydro-PIS REEDS- Evaluation of technicaland economical benefits of the innovative devices —GIS fragility ,B3EN1102, January 1999.


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