# 1) Transmission grid failures

- a. Wires above ground ruined by wind, debris, snow, ice, flood, fire, or earth movement

- b. Wires below ground affected by flooding, liquefaction, landslide, but much less vulnerable overall than above ground

# 2) Generator failures

- a. Limited access to cooling water or water of low enough temperature

- b. Hazard events that exceed operational limits

- c. Hazard events that exceed structural limits

- d. Flood or earth movement

- 3) Fuel and maintenance supply chain failures

- a. Port closures for fuel delivery

- b. Pipeline damage for fuel delivery

- c. Road damages for fuel delivery / repair operations

- d. Backup generators requiring diesel stored on site

A detailed description of the findings which lead to the above vulnerabilities is given below by subsystem component: first for transmission grid and then by fuel type, which includes both the generation facility vulnerabilities and supply chain considerations. In each section vulnerabilities to infrastructure and supply chain are given as well as possible methods to increase resiliency.

# **3.1 – Transmission Grid**

_Vulnerabilities._ In nearly every case study, Table 1, transmission infrastructure represented the most vulnerable part of the power system. Particularly in geographies with less redundant systems, the time required to restore transmission lines constituted the majority (or sole) reason for the lag in recovery time. This was seen in Puerto Rico, where restoration of power across the island took nearly a year despite many solar and other generation facilities’ quick recovery times (U.S. Energy Information Administration 2018) (see Section 3.3).

Transmission lines are vulnerable to most meteorological impacts of a hazard event, including wildfires, high winds, freezing rain, snow, earth movement (liquefaction, earthquake, landslides) and debris. The most common vulnerability and damages occurred from wind resulting from hurricanes, storms, winter weather or debris picked up by wind. All infrastructure in earthquake areas where liquefaction occur saw damages (Watson 2013). Wind was found in many case studies to topple transmission lines and poles (Thompson 2011; Sinclair Broadcast Group 2013; Hoffman and Bryan 2013; Sinclair Broadcast Group 2012; Dispatch 2015).

Wildfires present an interesting and unique vulnerability to transmission and distribution infrastructure. Various case studies illustrate that during high risk conditions (drought, high temperatures, high wind), curtailments were used to reduce the risk of transmission infrastructure causing a wildfire. The potential risk was illustrated in California (USA) in 2017 when transmission lines were identified as a likely cause of fire start. Here San Diego Gas & Electric was found liable for causing three fires that led to three deaths and the destruction of 1,300 homes. The utility ultimately paid out $2 billion in settlements (Daniels 2017). Recent wildfires have put Pacific Gas and Electric, a large utility in California, under scrutiny due to $10 billion in liabilities from fires in 2017 and unknown amounts from ongoing fires in 2018 (McNeely 2018). A key parameter in overall grid vulnerability was the geographic distance between generation facilities and end-use. Summary vulnerabilities identified in the case studies are detailed in Table 3, with detailed case study data presented in Appendix B and C.

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