Transmission Service Request Study

TSR-08-1124A TSR-08-1124B

January, 2009

Background Tri-State received two transmission service requests and subsequent applications for long-term firm transmission service in separate letters each dated December 2, 2008. The Transmission Customer requested 50 MW of Long-Term Firm Point-to-Point Transmission Service over Tri-State’s system from a new 115kV interconnection point located between the existing Bravo Dome and Clapham substations (Point of Receipt) to the Ojo 345kV bus (Point of Delivery) (TSR-08-1124A). The same Transmission Customer requested an additional 32 MW of Long-Term Firm Point-to-Point Transmission Service over the same path (TSR-08-1124B). A System Impact Study Agreement (TS-08-0076) already in effect for this same Customer’s earlier 82 MW Transmission Service Request (TSR-08-0715) now withdrawn, was utilized to perform this System Impact Study. A previously conducted LGIP System Impact Study (TI-04-1214) investigated the injection of 80 MW and 120 MW of generation at the same location on the Bravo Dome to Clapham 115kV line. That study included power flow, transient stability and short circuit analysis and should be referenced for a discussion of the performance of the transmission system with generation additions on the requested path. This study evaluates the ability of the transmission system to provide the requested Long-Term Firm Point-to-Point Transmission Service taking into consideration Tri-State’s native load requirements over the next ten years and the projected operating conditions of the transmission system. Actual facility ratings have been verified so that maximum transmission capacity can be offered.

Scope This study consists of the following:

• Evaluation of the Available Transfer Capability (ATC) of the requested path which begins at a new interconnection on the 115kV line between the existing Bravo Dome and Clapham substations and continues to Gladstone, Springer, Black Lake, through the Taos 345kV/115kV transformer(s) and to the Ojo 345kV bus.

• Identification of thermal overloads or violations of voltage criteria resulting from

providing Transmission Service over the requested path.

• Determination if voltage stability analysis is required to identify inadequately damped response to system disturbances resulting from providing the requested Transmission Service.

• Impacts of Tri-State native load growth through the ten year planning horizon as it will

affect the committed uses on the path and the ability of Tri-State to provide the requested Transmission Service.

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Study Assumptions The study was performed utilizing the WECC Approved 2013 Heavy Summer (2013HS) base case with Tri-State’s 2010, 2013 and 2018 loads inserted. A light load base case was also considered, however it was determined, as explained in the Methodology section of this report, that the highest loading across the requested path depended on native load-serving needs. A light load scenario reduces the native load demand such that it does not result in the maximum flow across the requested path. Therefore, the heavy summer case was utilized to determine the Available Transfer Capability. The following modifications were made to the heavy summer case.

• 82 MW of generation was injected at a new 115kV bus located halfway between the existing Bravo Dome and Clapham substations (1.03 per unit voltage modeled in the System Normal and N-1 cases)

• 10 MW at 0.95 power factor of motor load was added at Bravo Dome West • Approximately 50 MW of motor load was modeled at Rosebud • The Springer – Black Lake – Taos 115kV lines were rated at 230 MVA • The Gladstone to Rosebud 115kV line was rated 169 MVA • The Gladstone to Springer 115kV line was rated 169 MVA • A 15 MVAR capacitor bank was added at York Canyon

The calculated ATC values for each year were determined based on assumed northeast New Mexico load, TOT transfer schedules and resource dispatch assumptions. The following TOT transfer schedules were selected for the power flow cases and were judged to be acceptable for determining ATC values.

• TOT2A ~250 MW, Path 48 ~1600 MW during System Normal cases • TOT2A ~240 MW, Path 48 ~1600 MW during N-1 scenarios

Methodology The following definitions were used to determine Available Transfer Capability (ATC):

Equation 1: ATC Equation ATC = Total Transfer Capability (TTC) – Existing Transmission Commitments (ETC) –

Transmission Reliability Margin (TRM)

• TTC was determined in accordance with Tri-State’s Engineering Standards Bulletin – Reliability Criteria for System Planning and Service Standards which states the maximum loading criteria for transmission lines as a percent of the continuous rating. Based on that criteria, TTC in this study was defined as 100 percent of the conductor

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thermal rating of the lowest rated line section in the requested path. Therefore, TTC was determined to be 169 MVA which is the thermal rating of the Rosebud to Clapham to Gladstone to Springer 115kV lines.

• ETC is defined as Tri-State native load-serving needs, existing commitments for transmission service and existing commitments for purchase/exchange/delivery/reserves.

• TRM is defined as loop flow across the requested path. WECC operating practice requires transmission providers to accommodate some through-flow which may decrease ATC.

It was determined through the power flow analysis that the Gladstone to Springer 115kV line was the most heavily loaded line section in the requested path. Therefore, ETC for purposes of this study was defined as the flow on the Gladstone to Springer 115kV line. The flow from Gladstone to Springer defines ETC since the flow reflects the amount of power that is required for local load-serving needs, including Tri-State’s Taos and Black Lake loads and includes transmission line losses. Additionally, it should be noted that the flow on the Gladstone to Springer 115kV line includes through-flow north to south from Walsenburg towards York Canyon. This inclusion of through-flow power meets the definition of TRM for this study. Therefore, it was reasoned that the highest flow on the Gladstone to Springer 115kV line defines ETC and also includes the accommodation of through-flow power, or TRM. The maximum flow from the Gladstone 115kV bus to the Springer 115kV bus was determined with and without the injection of 82 MW into the Bravo Dome to Clapham line. Power flow analysis was performed using the National Electric Reliability Corporation (NERC)/Western Electricity Coordinating Council (WECC) planning standards. Power flow analysis was used to identify any line loading exceeding 100% of its continuous thermal rating and to monitor the voltage performance of the transmission system. Normal voltage violations were limited to the conditions where per unit voltages were between 0.95 and 1.05. Emergency voltage violations were limited to the conditions where per unit voltages were not less than 0.90 or greater than 1.10. TOT2A and Path 48 flows were kept at levels judged to be typical of heavy summer conditions. Phase shifters were not adjusted from the starting WECC base case settings. The following N-1 scenarios were simulated:

• Walsenburg-Gladstone 230kV line outage • Norton – Hernandez 115kV line outage • San Juan – Ojo 345kV line outage • Colinas – Rowe Tap 115kV line outage

From the above, it was determined that the San Juan – Ojo contingency resulted in the maximum flow across the requested path. There are thirteen power flow diagrams attached to this report. The first set of three power flow diagrams reflects the System Normal cases in order from 2010, 2013 to 2018 without the injection of 82 MW of generation at the requested Point of Receipt. The next diagram models the

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worst N-1 contingency without the 82 MW (present system only). The next set of three diagrams adds the generation at the Point of Receipt. The third set reflects the worst N-1 contingency. And the final set models a sensitivity discussed later in this report. Results To determine ATC for the requested path, the worst local N-1 scenario was simulated to maximize the flow of ETC and TRM across the Gladstone to Springer 115kV line. As stated in the Methodology section of this report, the maximum flow on the Gladstone to Springer 115kV line occurs during an outage of the San Juan – Ojo 345kV line. During an outage of the San Juan – Ojo 345kV line, the flow on the Gladstone to Springer line was determined to be 102 MVA (year 2010 loads) for the system prior to the 82 MW injection. This value (102 MVA) defines ETC and includes TRM. Based on the above analysis, the ATC prior to the 82 MW injection is as follows: ATC = TTC – ETC – TRM ATC = 169 MVA – 102 MVA = 67 MVA Normal power flow on the Springer to Ojo portion of the requested path is west to east due to the generation at San Juan and Four Corners and the fact that much of the Northeast New Mexico system is radial with a high concentration of motor load. Therefore, injecting 82 MW of generation at the Point of Receipt demonstrated the expected result of displacing local load and reducing west to east flow on the Springer to Ojo portion of the requested path. Under System Normal conditions for example, and with the injection of the 82 MW, the flow from Ojo to Taos was reduced by approximately 18 MVA and was still in the west to east direction. The injection of 82 MW at the Point of Receipt increases the flow on the Gladstone to Springer portion of the requested path by approximately 40 MVA (year 2018). However, acceptable steady state performance was achieved during System Normal conditions and during the most severe N-1 contingency (a loss of the San Juan – Ojo 345kV line). No violations of the NERC/WECC/Tri-State system planning standards were observed and the rating of the lowest rated transmission line conductor was not exceeded for the worst N-1 scenario. Since the power flow simulations demonstrated that the 82 MW injection displaces local load and provides voltage support to the transmission system, it was reasoned that the loss of the 82 MW was an appropriate N-1 contingency to determine ETC. That is, the 82 MW injection contributes to meeting Tri-State’s native load-serving needs. Therefore, the maximum flow on the Gladstone to Springer portion of the requested path with this N-1 contingency was determined to be 68 MVA (year 2018 loads). Again, this value (68 MVA) defines ETC and includes TRM. Based on the above analysis, the ATC with the 82 MW injection is as follows: ATC = TTC – ETC – TRM ATC = 169 MVA – 68 MVA = 101 MVA

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The following table summarizes the calculated ATC results. As discussed above, ETC for this study includes TRM.

Table 1: Results of N-1 Power Flow Analysis (MVA)

Year Contingency TTC (MVA)ETC

(MVA)ATC

(MVA) Pre-project N-1 169.0 102.1 66.9

2013 N-1 169.0 66.9 102.1 2018 N-1 169.0 67.8 101.2

A sensitivity power flow was run removing the approximately 60 MW of motor load modeled at Rosebud and Bravo Dome during an outage of the San Juan – Ojo 345kV line to determine the affect of this loss of load on the Gladstone to Springer line flow. As expected, the east to west power flow across the requested path from Gladstone to Springer increases. However, the thermal rating of the transmission line conductor was not exceeded for the study period. As stated in the Background section of this report, the TI-04-1214 System Impact Study should be referenced for a discussion of potential voltage problems associated with operation of the Bravo Dome compressor motors. No further voltage stability analysis, other than that performed for the TI-04-1214 study, is considered necessary. As stated in the Methodology section of this report, TTC was determined to be 169 MVA which is the thermal rating of the conductor of the Gladstone to Springer to Clapham to Rosebud 115kV line sections. The study confirmed that there are existing 115kV metering CT’s located at Taos, Springer and Clapham (2 sets) that are only rated/operated at 600 amp. Therefore, in order to realize the 169 MVA thermal rating of the line conductors, these CT’s will need to be replaced.

Conclusion The Transmission Customer requested a sum of 82 MW of Long-Term Firm Point-to-Point Transmission Service over Tri-State’s system from the Bravo Dome-Clapham 115kV line to the Ojo 345kV bus (east to west direction only). Based on the results of this study, the requested 82 MVA is available for the customer for the period covered by this study. The Transmission Provider should monitor the 115kV metering CT’s located at Taos, Springer and Clapham for upgrade/replacement as required. Reassessment of the path ATC upon termination of this TSR will be necessary as loads, generation dispatch and operating conditions change.

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Appendix 1

Steady State Power Flow Diagrams in MVA

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BackgroundScopeConclusion Appendix 1