Removal of Inadvertent Radial Line Contingency

1. Removal of Inadvertent Radial Line Contingency Shams Siddiqi, Ph.D. Crescent Power, Inc. (512) 619-3532 shams@crescentpower.net CMWG Meeting August 5, 2019

2. ERCOT Policy on Radial Lines • Network Operations Modeling Expectations for TSPs, REs, and QSEs: "The typical modeling criteria for contingency definitions are straightforward; contingencies are modeled breaker-to-breaker between autotransformers, non-radial AC transmission lines, or between a generator and its high-side breaker.“ • Thus, programmatically generated single contingencies should not include radial lines • Request ERCOT to remove Contingency SWCSBOO8 – a single-circuit radial line (Solstice to Rose Rock) contingency that was created by Contingency Builder in contradiction to this Policy

3. Impact of Inadvertent Radial T-1 • Contingency SWCSBOO8 is a single-circuit radial line (Solstice to Rose Rock) contingency that was created by Contingency Builder in contradiction to Policy • This T-1 contingency takes out both Rose Rock units – equivalent to U-2 contingency • Riggins is curtailed pre-contingency to keep flows on Fort Stockton to Solstice within limits post-contingency • ERCOT operates under U-1 security constraint – not U-2 • If this inadvertent radial T-1 is not considered in accordance with ERCOT Policy, then Riggins would likely not be curtailed under U-1 of Rose Rock Fort Stockton Solstice Far West Load x Riggins Rose Rock U2 Rose Rock U1

4. Acceptable Risk • Even though radial T-1 may be a credible single contingency, ERCOT policy is based on reasonable tradeoff between cost and reliability and NERC guidance on “Acceptable Risks” as follows: “It is just as pointless to assume that every disturbance, event, or equipment failure will reduce the reliability of the Interconnection. If we know (not simply assume) the failure of a radial transmission line, while likely, has little or no effect on the integrity of the 230 kV network that feeds it (i.e., the reliability consequences of building this radial line are known and acceptable), then the risk is acceptable. Conventional wisdom does not justify the costs of constructing duplicate transmission facilities or extending the far end of every radial line to another point on the high-voltage network.” • Generator forced outage rates are magnitudes higher than transmission forced outage rates – thus, U-2 is more likely than T-1. Yet, ERCOT operates the system under U-1 and not U-2. This decision to operate under U-1 is a tradeoff between cost and reliability as is the practice on not including contingencies of radial lines.

5. Proposed Next Steps • Request ERCOT to follow its own Business Practice of not considering contingencies of radial lines and thus remove the contingency SWCSBOO8 as soon as possible • Request ERCOT to provide the logic in Contingency Builder on excluding radial lines and why Solstice-Rose Rock radial line contingency was not removed from consideration • Request ERCOT to provide information on which other radial lines are on the list of Contingencies and provide examples of other radial lines similar to Solstice-Rose Rock that are not on the list of Contingencies