Transmission upgrades needed to west of Cascades north interface – study

The West of Cascades North (WOCN) interface, which comprises a group of 500-kV lines in the area west of the Cascades from the Canadian border to south of Seattle, will increasingly face conditions that push the interface beyond its voltage stability limit and should be upgraded, according to a ColumbiaGrid study.

Adverse effects can affect a broader area including Washington’s Puget Sound region and Oregon’s Willamette Valley, according to the study released April 3.

The 72-page Cross-Cascades Study Team report says the WOCN interface will be increasingly stressed during unusually cold winters which cause high peak winter loads, and when the region’s thermal generation is periodically replaced by remote east-side wind or hydro generation.

“Otherwise, the path tends to be loaded well below its transfer limit of about 10,200 MW,” the report states.

Increasing use of renewable and hydro power is another factor that lends toward voltage instability. The use of renewable generation is especially problematic when remotely located renewable or hydro energy displaces thermal generation located nearer the load centers, study participants said.

“As we displace generation north of Portland [Ore.], the lines become overloaded, even for a single-line outage contingency,” ColumbiaGrid senior planning engineer Gordon Comegys told TransmissionHub at the group’s March 15 planning meeting in Portland, Ore.

The study identifies 10 project options, which in varying degrees, would help alleviate system constraints that arise when either, or both, conditions are present.

The project identified as having the greatest benefit is a recommendation to build a new 120-mile, 500-kV transmission line from the Army Corps of Engineers’ Chief Joseph dam in Bridgeport, Wash., to the town of Monroe in western Washington, termed the Chief Joe-Monroe #2 line.

However, the study cautions that to fully realize the benefits of new cross-Cascades line alternatives, it will take additional mitigation to address effects that manifest outside the WOCN interface to yield increases in the WOCN’s available transfer capability.

The study also identified projects that are less expensive and have shorter implementation time lines than building new lines, though the benefits were less robust. Those included reconductoring existing lines, voltage upgrades, and adding series capacitors to existing lines to help alleviate overloading of the WOCN interface.

The study objective did not include addressing the conditions that would trigger the need for a project. As a next step, the report recommends performing a study to identify conditions – including load, generation, and interchange patterns – that would trigger the need to build the project. Such a study could be performed by ColumbiaGrid under certain conditions, or by another organization or utility, Comegys told TransmissionHub on April 6.