20171218PAC to Staff Attachment Utah_DPU 10.11f.docx

Remove the Three Peaks – West Cedar RAS
Existing RAS Summary:
The existing Three Peaks – West Cedar RAS is designed to sectionalize the 138 kV system between Three Peaks and West Cedar for the loss of the Sigurd – Three Peaks 345 kV line. Under heavy southbound flow conditions on the Sigurd – Three Peaks – Red Butte – Harry Allen 345 kV transmission line, the N-1 loss of the Sigurd – Three Peaks 345 kV line could cause thermal overload and/or low voltage issues in the Cedar City area. The existing RAS resolved these issues be opening the Three Peak – West Cedar system if the flow on the Three Peaks 345/138 kV transformer (138 kV to 345 kV) exceeded 45 MW.
Sigurd – Red Butte #2 Line Addition:
A new 345 kV line between Sigurd and Red Butte was energized in May 2015. Addition of this new circuit will impact the existing Three Peaks – West Cedar RAS. Due to the addition of the Sigurd – Red Butte #2 345 kV line, the thermal overload and/or voltage issues previously seen for the loss of Sigurd – Three Peaks 345 kV line are eliminated, refer to Figures 1, 2 and 3. Thus, the Three Peaks – West Cedar RAS for the N-1 outage of the 345 kV line is no longer required. Also, the existing RAS does not account for the status of the new Sigurd – Red Butte #2 345 kV line and thus it will not be feasible to use this RAS under N-1-1 conditions of the 345 kV lines between Sigurd and Red Butte.
New Solar Generation in Cedar City Area:
New solar generation up to 400 MW in Cedar City area is expected to come online by end of 2016. The exiting Three Peaks – West Cedar RAS does not consider the solar generation located in the area. The N-1 outage of the Sigurd – Three Peaks or Sigurd – Red Butte #2 345 kV lines can cause the flow through the Three Peaks Transformer to exceed 45 MW. This can trigger the Three Peaks – West Cedar RAS for loss of the Sigurd – Three Peak 345 kV line resultinginto non-reliable operation.
Conclusion:
Since the existing Three Peaks – West Cedar RAS does not consider the status of the Sigurd – Red Butte #2 line and the new internal generation in the Cedar City area this RAS can miss-operate and must be removed from operation.
Figure 1: Base Case – Heavy Southbound Flow, Heavy Load
Figure 2: N-1 Loss of Sigurd – Three Peaks 345 kV Line
Figure 3: N-1 Loss of Sigurd – Red Butte #2 345 kV Line