Background
An N-2 outage is a multiple contingency event in which two elements of the grid fail, such as two transformers, generators, or transmission lines. Power flow studies have shown that a credible N-2 outage of both Shasta-Cottonwood 230-kV transmission lines would result in an overload of the Shasta-Keswick 230-kV transmission line.
WAPA originally operated the Shasta yard in a split-bus configuration. This mitigated overload situations but limited both line and power plant operations. WAPA now operates the Shasta yard in a double-bus, double-breaker configuration. However, during high power flow conditions, WAPA’s transmission system and Bureau’s Shasta power plant were not capable of operating at maximum potential without reconductoring.
Shasta and Keswick substations are both high-profile lattice steel arrangements using 795 ACSR Drake for all strain bus and jumpers. The ampacity of 795 ACSR Drake as applied at the substations is around 940 amps. To remain compliant with reliability standards and increase electrical capacity to meet the growing demand, both substations would need to double the ampacity. The confined space relevant to upgrading the stations is illustrated in the picture of the Keswick substation in Figure 9.
Figure 9. Confined substation location
The conventional solution to increase the substation ampacity is to increase the conductor bus size. However, because these substations were designed for 795 ACSR conductors, an increase in the conductor size would mean a total replacement of the major structures in the substations. Cost and outage time for this solution would be significant.
Options Evaluated
Three diameter-equivalent conductors were considered, as follows:
- 795 ACSS Drake 26/7
- 795 ACSS Drake 26/7 HS285
- 795-T16 ACCR Drake
Justification for Conductor Selection
The 795-T16 ACCR conductor was selected because it could meet the original sag of existing ACSR conductors without increasing tensions. Furthermore, this conductor satisfied the increased power flow requirement and potential emergency conditions.
Although the ACCR installation cost was the highest, the fact that there was no need for structure modifications and outage time savings justified its purchase.
Installation Review
The substations were reconductored in 2013 and 2014. ACCR conductors were used for jack-bus and drop-down jumpers. Two 1590 AAC conductors were required for connecting from high-temperature strain bus to terminals on disconnect switches and circuit breakers that were not rated for high-temperature operation.
A 3M installation specialist provided hands-on training for the contractor at the preconstruction meeting (Figure 10). Because the hardware is like that of a regular ACSR conductor, the installation went smoothly. Complications were due to poor as-built drawings, material count shortage, and outage constraints. The shorter bus runs had some minor bird caging due to the short distance between compression t-tap connectors and dead-ends. Bolted t-taps were substituted to correct this problem.
Figure 10. Crew training during a compression connector installation
The ACCR 795-T16 conductor provides over 2000 amps. Now, each bay can carry the full output of the Shasta power plant (710 MW).