Seismic Performance of Electric Transmission Systems

SEISMIC PERFORMANCE OF ELECTRIC TRANSMISSION SYSTEMS

Eric Fujisaki, P.E. Pacific Gas and Electric Company, San Francisco, CA

Overview

  Introduction   Historic performance of electric transmission

systems   Substation buildings   Substation equipment/ components   Underground transmission cables   Recent R&D activities   Research needs

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PG&E’s Electric Transmission System

  Mix of old and new equipment, structures, buildings, nearly 1,000 substations

  Transmission lines mostly overhead, some underground in Oakland and SF Peninsula

  Older Buildings—Indoor substations mostly in urban areas  URM or Lightly-reinforced masonry  Non-ductile reinforced concrete shear wall  Most have steel gravity frames

  Seismic capability varies

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Seismic Hazard

  Bay Area  Several active fault

systems  High concentration of

substations  High concentration of

customers

4 Mapsource:USGS,2008

Recent Historic Performance

 One large earthquake—Loma Prieta  Several moderate earthquakes, e.g.,

Morgan Hill, Coalinga, San Simeon  Substation damage

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Loma Prieta (6.9Mw), 1989

  19 phases of live tank breakers damaged, 500kV, several 230kV

  25 poles of disconnect switches damaged, 500kV

  Several radiator leaks on transformers, 500 and 230kV

  15 current transformers, 500kV   7 CCVTs damaged, 500kV   Numerous 230kV switches damaged   Rigid bus work damage 500, 230kV

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Coalinga (6.4Mw), 1983

  Transformer bushing leaks, 500/230kV

  2 transformers sheared anchors, shifted, uplifted

  2 support frames of OCBs damaged

  2 PTs shifted   1 OCB with friction clips

shifted, 60kV   1 battery rack shifted 7

  12 phases of live tank circuit breakers damaged, 500kV

Morgan Hill (6.2Mw), 1984

Coalinga & Morgan Hill Earthquakes

Historic Performance— Live Tank Breakers

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Historic Performance— Instrument Transformers

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Historic Performance—Buses, Switches

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Historic Performance—Radiators

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Historic Performance— Connectors at Elevated Switches

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Historic Performance— Elevated Switches

Underground Transmission Cables

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Type Vintage Typical Installation

High pressure gas filled (HPGF)

1950s-1960s 6”-8” steel pipe casing, pressurized with N2 gas

High pressure oil (fluid) filled (HPFF)

1970s-1980s 6”-8” steel pipe casing, pressurized with oil

Solid dielectric (XLPE)

Late 1990s-Present

Cables in PVC conduits, encased in concrete duct bank

Underground Transmission Cables— Likely Failure Modes

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Cable Type Failure Modes

HPGF HPFF

  Loss of pressure boundary   Excessive curvature (short bend radius)

XLPE   Cable crushing/ pinching   Excessive curvature (short bend radius)

Liquefaction Zones

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  Segments of underground transmission lines pass thru high risk liquefaction zones

  Best addressed by contingency planning and system redundancy

Mitigation Efforts

  Substation building retrofit   Seismic qualification standards

implementation   Targeted equipment replacement   Equipment anchorage retrofit   Capital improvements—new equipment

procurement, system redundancy   Contingency planning

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Building Retrofit

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Seismic Qualification Standards Implementation

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New and Replacement Equipment

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Equipment Anchorage

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Current R&D Activities

  Application guide for connected equipment   Network studies of seismic performance   Transformer bushing test protocols   Station post insulator studies

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Application Guide for Connected Equipment

  Interaction between connected equipment recognized as a likely cause of failure in past earthquakes

  IEEE 1527, “Recommended Practice for Design of Flexible Buswork …” approved in 2006

  Complex methodology, limited acceptance   Application guide to supplement IEEE 1527 has

been drafted (Dastous and Der Kiureghian)

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Recommended Shapes

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Triple curvature

Catenary Inverse parabola

Double curvature

  Conventional wisdom—Sufficient cable slack leads to Seismic Terminal Force ≈ 0

  Experiments and nonlinear analyses—Non-trivial Seismic Terminal Force may occur even if slack is sufficient

  Further work needed

Terminal Force

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Network Seismic Risk Assessment— SERA*

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  Simulate/ assess post-earthquake damage state of a network

  Motivation:   Identify potential weaknesses in the system   Asset management, benefit/ cost analysis   Emergency resource planning   Loss estimation for insurance

* System Earthquake Risk Assessment

SERA Analysis—Modeling

  System topology (limited model, includes 46 important substations in SF Bay Area)  System geographic information  Equipment connectivity  Substation connectivity (transmission lines/ towers)

  Component fragilities  Different failure modes for each equipment  Based on historic data, tests, judgment

  Seismic hazard  Scenario earthquakes  Geotechnical hazards (e.g. liquefaction, landslide)

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Research Needs

  Interaction effects (particularly terminal loading issues)

  Deformation capacities of buried transmission cables

  Base isolation/ supplemental damping technologies for substation equipment

  Insulator post-shaking test damage detection, porcelain and composites

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