Summit Export Rating

Jay Campbell July 2005 Summit Export Rating

Introduction This study report recommends a new export rating for Path 24 (Sierra-to-Summit/PG&E). The study work was based upon the WECC summer 2005 basecase: 05-hs2a.sav. Hence this recommendation is only good for the WECC “summer” season (June 1 to November 1). The recommended path export rating is 10 MW. To achieve that rating, tripping the Silver Lake #141 CB for N. Valley Rd-to-Penny’s tap overloads is recommended.

Study Summary The Path 24 rating is limited by two load areas: North Tahoe and Reno/Sparks. The North Tahoe loads subtract directly from the Phase shifter capacity (150 MW – 40 MW) 110 MW. The path rating is also very dependant on the California/Northwest/Silver Lake/Stead loads. Following an n-1 contingency, no line should be above its emergency limit.

Any path 24 export will cross various 120kV lines in the Reno area (141/114, 106, 142, 125, 127). The single worst contingency is loss of the #142 line (N. Valley Rd.-to-Reno). With 10 MW export (measured at Summit), many line segments are overloaded.

Beginning with the WECC summer operating case, all loads in the Reno and North Tahoe areas were adjusted to match those seen at the peak load hour, 5 PM on Thursday July 14, 2005. Twenty six outages were examined. Those outages are:

1. Sierra's #101 Line 2. Sierra's #102 Line 3. Sierra's #133 Line 4. Sierra's #106 line 5. PG&E Drum PH #1, T1, G1&2 6. PG&E Drum PH #1, T2, G3&4 7. PG&E Drum PH #2, G5 8. PG&E Drum-Bell Line 9. PG&E Drum-Rio Oso #1 Line 10. PG&E Drum-Rio Oso #2 Line 11. PG&E Rio Oso 230/115kV Xfmr #1 12. PG&E Placer-Bell Line 13. PG&E Placer 115/60kV Xfmr 14. PG&E Placer-Goldhill #1 Line 15. PG&E Placer-Goldhill #2 Line 16. PG&E Gold Hill 230/115kV Xfmr 17. Sierra's #607 Line Outage 18. PG&E Drum-Spaulding Outage 19. Sierra's #114 Line 20. Sierra's #141 Line 21. Sierra's #142 Line 22. Humboldt-Midpoint (#3419) 345 kV line 23. Alturas 345 kV line 24. Sierra's #125 Line 25. Sierra's #127 Line 26. Sierra's #606 Line

Discussion Without any RAS scheme, the Path 24 will have to import to prevent cascading overloads. Figure #1 shows the system with all lines in service and 10 MW export at Summit. Figure #2 shows the system after the #142 line (outage #21 above). Close inspection shows the N. Valley Rd. #141 segment above its summertime maximum; 155 MVA. Please refer to Figure #3 for the Stead area voltage. The 60 kV and 24.9 kV busses area at 1.04 pu.

Figure #4 shows the system if the N. Valley Rd. #141 overcurrent relay, installed in May 2005 to prevent conductor damage, were to operate. The Mt. Rose-to-Washoe line segment is then overloaded (143 MVA on a 123 MVA summertime conductor rating).

Clearly, any Summit export above ~10 MW may lead to cascading outages and tripping the N. Valley Rd. #141 CB is inappropriate. As a temporary fix, the existing conductor overload relay (implemented in the #141 line SEL relay) can be re-programmed to send a trip to the Silver Lake #141 CB instead. That event will effectively move all Stead/Silver Lake load from the 120kV system onto the Valley Rd 606 line.

Figures #5 & #6 shows the system if the Silver Lake #141 CB is tripped instead of the N. Valley Rd. #141 CB. No cascading overloads exist and the Silver Lake voltages are acceptable. While the 60 kV is low, .93 pu, the regulated 24.9kV bus still has acceptable voltage.

Table 1 shows the Sierra system overloads after a #142 line trip, but without tripping the Silver Lake #141 CB. Figure #7 shows the system flows, after tripping the #142 line plus tripping the Silver Lake #141 CB. Table 2 shows the system for that same condition. Both Figure #7 and Table 2 include a 10 MW Summit export. Regarding the N. Valley Rd OC relay pick-up value, it should be set at the conductor rating of 745 amps. In the last three years, the actual current on the #142 CB at Valley Rd has only reached 745 amps once: May 27 concurrent with a 100 MW Path 24 export schedule.

Summary What’s clear is that substantial load has been added in the Reno area since the last Path 24 export studies were performed (summer 2001). Additionally, since then the Penny’s Tap-to-Silver Lake 120kV line was built, along with new transformers at Silver Lake, Northwest (shifting load to the north end of the Truckee Meadows) and Steamboat. These transformer improvements have been made with little corresponding transmission infrastructure additions. All these changes have driven down the Path 24 export capability.

Additional studies will be performed for Winter 2005-2006 to re-rate Path 24 for low temperature conditions.

As a final note, Table 1 & Table 2 both show an apparent overload, post contingency. Those lines actually have a thermal rating far in excess of 300 MW. The PSFL rating is driven by the Bordertown phase shifter. Existing operating procedures call for manually reducing the Alturas line flow upon phase shifter overload. Table 1---Equipment Overloads, #142 Line Trip & NO Silver Lake #141 CB Trip, 10 MW export, Summer 2004 Loads ======SUMMARY OF OVERLOADED LINES AND TRANSFORMERS ======------FROM bus------TO bus------ACTUAL FLOWS--- RATING flow RATING base cont Cnt # ---name--- -kv- ar # ---name--- -kv- ar -ID- --mw- -mvar --mva [MVA] amps [AMPS] load load # freq ------40537 "HIL TOP " 230 40 64058 "HIL TOP " 345 64 "1 " 339 41 341.5 300.0 <=== <=== 0.95 1.14 22 18 64400 "PENNY TP" 120 64 64125 "VAL RD N" 120 64 "1 " -176 18 176.9 161.0 858.6 775 0.84 1.11 21 3 64017 "BRDRTNPS" 345 64 64018 "BRDRTWN " 345 64 "1 " 330 -71 337.6 300.0 337.1 502 0.93 1.12 22 1 64037 "RUSTY SP" 120 64 64265 "RUSTY SP" 25 64 "1 " 43 15 45.5 46.7 <=== <=== 0.98 0.98 1 26

Table 2---Equipment Overloads, #142 Line Trip & Silver Lake #141 CB Trip, 10 MW export, Summer 2004 Loads ======SUMMARY OF OVERLOADED LINES AND TRANSFORMERS ======------FROM bus------TO bus------ACTUAL FLOWS--- RATING flow RATING base cont Cnt # ---name--- -kv- ar # ---name--- -kv- ar -ID- --mw- -mvar --mva [MVA] amps [AMPS] load load # freq ------40537 "HIL TOP " 230 40 64058 "HIL TOP " 345 64 "1 " 339 41 341.5 300.0 <=== <=== 0.95 1.14 22 17 64017 "BRDRTNPS" 345 64 64018 "BRDRTWN " 345 64 "1 " 325 -64 331.2 300.0 331.2 502 0.93 1.10 22 1 64400 "PENNY TP" 120 64 64125 "VAL RD N" 120 64 "1 " -157 20 158.3 161.0 765.8 775 0.84 0.99 24 2 64037 "RUSTY SP" 120 64 64265 "RUSTY SP" 25 64 "1 " 43 15 45.5 46.7 <=== <=== 0.98 0.98 1 26