Seismic Performance of Electric Transmission Systems
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SEISMIC PERFORMANCE OF ELECTRIC TRANSMISSION SYSTEMS
Eric Fujisaki, P.E. Pacific Gas and Electric Company, San Francisco, CA
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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
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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
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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
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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
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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)
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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
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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
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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
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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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