GENERAL DIRECTION SOUGHT AND SPECIFIC QUESTIONS TO BE ANSWERED

DATE: June 28, 2011 STAFF: Brian Janonis Brian Moeck, PRPA Pre-taped staff presentation: none WORK SESSION ITEM FORT COLLINS CITY COUNCIL SUBJECT FOR DISCUSSION Electric Transmission Update. EXECUTIVE SUMMARY Brian Moeck, General Manager of Platte River Power Authority, will provide an update on the reconstruction of the Dixon Creek to Horseshoe 230 kv transmission line, along with a general discussion on undergrounding transmission facilities. Additionally, Mr. Moeck will provide information on the proposed construction of walls around the Dixon Creek Substation at Overland Trail and Drake and the Timberline Substation, just south of Prospect on Timberline GENERAL DIRECTION SOUGHT AND SPECIFIC QUESTIONS TO BE ANSWERED This work session is for information purposes upon request of City Council. No specific direction from Council is being sought at this time. BACKGROUND / DISCUSSION Platte River is currently in the process of reconstructing the Dixon Creek to Horseshoe line. Dixon Creek Substation is located at the west end of Drake Road and Horseshoe Substation is just north of 57th Avenue on Shields/North Taft Avenue in Loveland. The project is being completed in three segments. The first segment built for the project was the new 2.5 mile-long underground section that runs from Trilby and Shields in Fort Collins to the Horseshoe Substation in Loveland. That segment of the project was completed in late summer 2010. The second segment of the project is the rebuilding of an existing 115kV wood pole overhead transmission line owned by Tri-State that connects a small switching station at the south end of Horsetooth Reservoir to the Trilby Substation at Shields and Trilby. Tri-State Generation and Transmission Association owns the 115kV transmission line which was installed in the early 1970s. Platte River and Tri-State agreed on a contract which allows Platte River to rebuild the four-mile overhead transmission line and convert it to a double circuit 230kV design. Platte River will own the 230kV circuit for the connection to the underground transmission segment to Loveland and pay for the rebuilding. Tri-State will own the 115kV circuit connecting the Horsetooth Tap Switching Station to the Trilby Substation. No new right-of-way was needed for this segment of the project, which was completed three months ago.

June 28, 2011 Page 2 The final transmission segment to be installed for the project is the rebuild of the existing Western Area Power Transmission 115kV transmission line from Dixon Creek Substation to the south end of Horsetooth Reservoir. Platte River plans to rebuild this almost four mile long transmission segment within the existing right-of-way using overhead poles that are identical to the poles installed several years ago along Overland Trail when Platte River rebuilt that portion of Western s transmission line. The rebuilt transmission line will support two transmission circuits: one will be the replacement for an existing Western transmission line using a slightly larger conductor size; the second circuit will be the Platte River 230kV circuit. As noted in the attached Platte River ten-year transmission study (Attachment 1), the next transmission line project Platte River has planned in the Fort Collins area will be construction of a new line to connect a new substation in the Northeast portion of the City. This is not anticipated to occur until after 2015 when the City advises Platte River that transmission support is needed for a new NE Fort Collins distribution substation. Undergrounding Transmission Lines Questions have been raised about the feasibility of undergrounding transmission lines in Fort Collins. Utilities staff, along with PRPA, have developed a very rough non-engineered estimate of the cost of undergrounding the transmission lines within the city. A map detailing the area transmission lines along with ownership information is attached (Attachment 2). The roughly estimated cost to underground the lines within the city is approximately $350 million. PRPA estimates that it would take approximately 10 years to underground all of the lines within the city. NOTE: Western Area Power Authority and Tri-State share many of the transmission poles in the City. They would have to be consulted and their approval gained to underground their lines. Staff has calculated the approximate rate impact if the project were financed over a 30-year period at 5%. The table below reflects the approximate rate increases required. As noted, costs are a very rough estimate. Engineered plans will be required to determine actual pricing. Rate Class Rate Increase R Residential 18.3% RD Residential Demand 20.2% GS General Service 21.7% GS50 Small Commercial 24.5% GS750 Large Commercial Industrial 30.2% CONTRACT Contract Customer 36.1% FLOOD LIGHTS Flood Lights 7.1% TRAFFIC Traffic signals 27.7% Since no customer benefits more than any other from undergrounding the transmission line then the cost could also be allocated as a fixed cost. The increase per customer would be $29 per month per account.

June 28, 2011 Page 3 ATTACHMENTS 1. PRPA Ten-Year Transmission Plan 2011-2020 Study Report 2. Foothills Area Map 3. 2011-2020 Transmission Plan

Platte River Power Authority Ten-Year Transmission Plan (2011-2020) Prepared by PRPA System Planning January 27, 2011

Table of Contents I. Executive Summary II. III. IV. Scope Assumptions Criteria V. Procedure VI. VII. VIII. Results o Operating Horizon o Near-Term Planning Horizon o Longer-Term Planning Horizon o Transient Stability Analysis o Prior Outage Initial Conditions o PRPA Sub-Area Reactive Power Assessment o Short-Circuit Analysis Conclusions Additional Reports Exhibit 1 Exhibit 2 Exhibit 3 Exhibit 4 Exhibit 5 Exhibit 6 Exhibit 7 Exhibit 8 Exhibit 9 Exhibit 10 Exhibit 11 2011-2020 Transmission System Map and 2011-2020 Plan of Service Diagrams Foothills Study Area PRPA 10-year Load Forecast by Substation PRPA Load and Resource Allocations Study Procedure Forced Outage Contingencies Transient Stability Fault Descriptions PRPA Sub-Area Reactive Power Assessment Matrix Power Flow Study Results Transient Stability Study Tabular Results Transient Stability Study Plots Page 2 of 881

I. Executive Summary The Platte River Power Authority (PRPA) Ten-Year Transmission Plan (2011-2020) is developed to ensure the reliable delivery of electricity to its municipal owners in Estes Park, Fort Collins, Longmont, and Loveland, Colorado and to other PRPA transmission customers. The planning studies and reliability assessments for the near-term and longer-term planning horizons demonstrate that the PRPA transmission system meets the performance requirements of the Western Electricity Coordinating Council (WECC) and of the North American Electric Reliability Corporation (NERC) Standards TPL-001 through -004. PRPA transmission projects planned for the next ten years are listed in the following Table 1 in order of in-service date, and are illustrated on the 2011-2020 Transmission System Map and 2011-2020 Plan of Service Diagrams in Exhibit 1. Table 1: PRPA Planned Transmission Projects In-Service Project Name Description Purpose March 2011 Richard Lake 115kV Substation Addition of Richard Lake-Waverly 115kV Line. New delivery point to serve growing load for TSGT. Interconnection April 2011 Loveland East 115kV Substation Expansion Add 115/12.47kV transformer T3 and complete ring bus configuration. New delivery point to serve growing load. May 2011 July 2011 December 2011 May 2012 October 2011 May 2012 College Lake 230kV Substation Fordham-Fort St. Vrain 230kV Line Harmony 230kV Substation Terminals Upgrade Meadow 115kV Ring Breaker Timberline 230/115kV Substation Expansion Dixon-Horseshoe 230kV Line Sectionalize Dixon-Laporte Tap 230kV Line section with new substation and 0.5 mile of double-circuit 230 kv line. Approximately 21.4 miles of underground/overhead sections. Expansion of Fordham to 230/115kV Substation with two 230/115kV transformers. Rebuild Del Camino Tap-Meadow-LongmontNW- Fordham 115kV Lines to double-circuit 230kV capability. Create LongmontNW- Rogers-Terry 115kV 3-Terminal Line. Upgrade Longs Peak-Fort St.Vrain 230kV Line. Modify CT tap and transformer relaying. Add one breaker to complete the 115 kv ring bus. Add 115/13.8kV transformers T3 & T4. Approximately 9.4 miles. Rebuild WAPA s Dixon-Horsetooth Tap 115kV Line to doublecircuit 230kV capability. Rebuild TSGT s Horsetooth Tap-Trilby 115kV line to doublecircuit 230kV capability. New Trilby- Horseshoe 230 kv underground line. Expansion of Horseshoe to 230/115kV Substation with two 230/115kV transformers and two 35-MVAR 115kV Capacitor Banks. New delivery point to serve growing load for PSCo. Necessary to meet WECC and NERC performance requirements. Provide a second 230kV source to the Longmont area. Remove conditional line ratings on the Boyd and Timberline lines. Improve reliability to Meadow Substation. Meet PRPA design criteria. New delivery point to serve growing load. Necessary to meet WECC and NERC performance requirements. Provide a second 230kV source to the Loveland area. Page 3 of 881

Table 1 (Continued) In-Service Project Name Description Purpose May 2012 Fordham 115kV Substation Expansion Add 115/12.47kV transformer T3. New delivery point to serve growing load. November 2013 Crossroads 115kV Substation Expansion Add 115/12.47kV transformer T2 and a Ring Breaker. New delivery point to serve growing load. November 2013 May 2013 May 2015 August 2015 May 2016 Rebuild LongmontNW- Harvard 115kV Line Timberline 230/115kV Substation Expansion Boyd 230/115kV Substation Expansion Fort Collins Northeast 115/13.8kV Substation Timberline 230/115kV T1 Replacement Connect Harvard 115/12.47 kv transformers T1 & T2 to different bays at LongmontNW Substation. Create two 115kV underground lines to cross Harvard Street. Add 230/115kV transformer T2. Add 230/115kV transformer T2. Rebuild TSGT s Timnath-Boxelder 115 kv Line double-circuit. Create Richard Lake- Boxelder/FortNE 115 kv Line. (Alternative site near Cobb Lake 115 kv Substation.) Replace 230/115kV transformer T1 with new transformer. Improve reliability to each transformer. Meet PRPA design criteria. Improve system reliability in the Fort Collins area. Improve system reliability in the Loveland area. New delivery point to serve growing load. Improve system reliability in the Fort Collins area. Existing transformer installed 1976. II. Scope The study area is the Foothills Area Transmission System (Foothills System) located in northern Colorado as shown in Exhibit 2. The PRPA transmission system is situated in the Foothills System. The near-term (years one through five) and longer-term (years six through ten) planning horizons were studied and the results documented herein over a range of forecasted system demands and subject to the various contingency conditions defined in the NERC Standards TPL-001 through -004 for Categories A, B, C, and D. III. Assumptions 1. Loads are represented at the high-voltage busses. 2. PRPA detailed representation with substation transformers and low-voltage bus loads are not used in this study. However, power factors have been adjusted for high-voltage bus representation. 3. Voltage criteria violations on the transmission system are of more concern at load busses than at non-load busses. Page 4 of 881

IV. Criteria PRPA adheres to NERC Transmission Planning Standards and WECC Reliability Criteria, as well as internal company criteria for planning studies. PRPA s power flow simulation criteria: Category A System Normal N-0 System Performance Under Normal (No Contingency) Conditions (Category A) NERC Standard TPL-001-0 Voltage: 0.95 to 1.05 per unit Line Loading: 100 percent of continuous rating Transformer Loading: 100% of highest 65 C rating Category B Loss of generator, line, or transformer (Forced Outage) N-1 System Performance Following Loss of a Single Element (Category B) NERC Standard TPL-002-0 Voltage: 0.92 to 1.07 per unit (PRPA) 0.90 to 1.10 per unit (all others) Line Loading: 100 percent of continuous rating or emergency rating if applicable Transformer Loading: 100% of highest 65 C rating Category C Loss of Bus or a Breaker Failure (Forced Outage) N-2 or More System Performance Following Loss of Two or More Elements (Category C) NERC Standard TPL-003-0 Voltage and Thermal: Allowable emergency limits will be considered as determined by the affected parties and the available emergency mitigation plan. Curtailment of firm transfers, generation redispatch, and load shedding will be considered if necessary. Category D Extreme Events (Forced Outages) N-2 or More System Performance Following Extreme Events (Category D) NERC Standard TPL-004-0 Voltage and Thermal: Evaluate for risks and consequences. If applicable, use allowable emergency limits as determined by available emergency mitigation plan. Curtailment of firm transfers, generation redispatch, and load shedding will be considered if necessary. Transient stability criteria require that all generating machines remain in synchronism and all power swings should be well damped. Also, transient voltage performance should meet the following criteria: Following fault clearing for Category B contingencies, voltage may not dip more than 25% of the pre-fault voltage at load buses, more than 30% at non-load buses, or more than 20% for more than 20 cycles at load buses. Following fault clearing for Category C contingencies, voltage may not dip more than 30% of the pre-fault voltage at any bus or more than 20% for more than 40 cycles at load buses. Page 5 of 881

In addition, transient frequency performance should meet the following criteria: Following fault clearing for Category B contingencies, frequency should not dip below 59.6 Hz for 6 cycles or more at a load bus. Following fault clearing for Category C contingencies, frequency should not dip below 59.0 Hz for 6 cycles or more at a load bus. Note that load buses include generating unit auxiliary loads. NERC Standards require that the system remain stable and no Cascading occurs for Category A, B, and C disturbances. Cascading is defined in the NERC Glossary as The uncontrolled successive loss of system elements triggered by an incident at any location. Cascading cannot be restrained from sequentially spreading beyond an area predetermined by studies. A potential triggering event for Cascading will be investigated upon one of the following results: A generator pulls out of synchronism in transient stability simulations. Loss of synchronism occurs when a rotor angle swing is greater than 180 degrees. Rotor angle swings greater than 180 degrees may also be the result of a generator becoming disconnected from the BES; or A transmission element experiences thermal overload and its transmission relay loadability is exceeded. (PRPA sets its transmission relays so they do not operate below 150% of the continuous rating of a circuit.) V. Procedure The studies were performed by PRPA System Planning using the Siemens-PTI PSS/E computer simulation software versions 30.3.2 and 32.0.1. The transmission system models were developed from models prepared by WECC. Previous planning studies by PRPA, the Foothills Planning Group, and the Colorado Coordinated Planning Group (CCPG) have concluded the heavy summer loading scenarios cover the most critical system conditions over the range of forecasted system demand levels. Both heavy and light load scenarios were studied for each the near-term and longer-term planning horizons to conduct a thorough assessment for all seasons. Transmission topology and system demand were modified according to which season and year are studied. Light load scenarios apply to Spring and Fall system conditions and heavy load scenarios apply to Summer and Winter system conditions. WECC Approved base cases were selected accordingly and load, generation, and transmission topologies were updated as necessary with the most recent modeling representations of the planned PRPA and Foothills systems. The study cases include both existing and planned facilities, the expected system conditions, and the effects of any Bulk Electric System (BES) equipment planned to be out-of-service during the critical demand levels. 1 All normal operating procedures and the effects of all control devices and protection systems are modeled. 1 PRPA makes every effort to avoid removing a BES facility or equipment including protection systems from service for planned maintenance or construction during the summer peak demand levels or during other high-risk system conditions when PRPA may implement No Touch procedures. PRPA performs system studies when a BES facility is scheduled to be removed from service. Page 6 of 881

Reactive power resources are included in the model to ensure adequate reactive resources are available to meet system performance. The PRPA 10-year Load Forecast by Substation is listed in Exhibit 3. PRPA uses its high load forecast for reliability margin to reflect uncertainties in projected BES conditions. The PRPA Load and Resource Allocations for each base case studied are provided in Exhibit 4. These exhibits represent the projected PRPA customer demands, firm transfers, and generation dispatch modeled in the bases cases. All projected firm transfers are modeled according to the data for loads, resources, obligations, and interchanges described in the Associated Material document provided with each approved WECC base case. The generation dispatch in each base case was modified to fully stress the PRPA system by setting Rawhide to its maximum output. See Exhibit 5 for the study procedure where the modified generation dispatch values are documented. All Category A and B contingencies and certain Category C and D contingencies were simulated using the Matrix routine written for contingency analysis on the PSS/E computer simulation software. The Category C and D multiple contingencies studied are those that would produce more severe system results or impacts based on the Transmission Planners knowledge of the system and engineering judgment. The rationale for selection considers facilities at significant substations, large generation stations, and lines involved with large bulk transfer paths, common rights-of-way, common structures, and shared circuit breakers. Computer simulation software solution methods are as follows: Pre-contingency Post-Contingency Area Interchange Control Off Off Phase-Shifter Lock Lock TFMR LTC Adjust Adjust Switched Shunt Reactor/Capacitor Adjust Lock DC Taps Adjust Adjust All busses and branches in Zones 706 and 754 of the WECC base cases are monitored for criteria violations. A list of simulated forced outage contingencies is provided in Exhibit 6. The PRPA transmission system is fully contained within Zones 706 and 754 and completely studied by the list of contingencies. Study results were reviewed and assessed for compliance with the WECC and NERC standards. Planned upgrades, additions, or corrective actions needed to meet the performance requirements are identified and included in the transmission plan for Category A, B, and C contingency conditions which cause a criteria violation. System performance problems associated with Category D extreme events are evaluated for possible actions to reduce the likelihood or mitigate the consequences of the extreme event. Page 7 of 881

VI. Results Operating Horizon (2010) Powerflow and transient stability studies were performed for the operating horizon. The results and mitigating actions are documented in the Foothills Area 2010 Summer Assessment report dated July 12, 2010, and in several planned outage study reports conducted throughout 2010 for the Foothills Area. The summer season has system performance problems that may occur for contingencies during higher load levels and lower CBT generation levels. The winter season has fewer problems than the summer season for contingencies during higher load levels and lower CBT levels. The difference between these summer and winter study results is typical for the PRPA transmission system, the Foothills System, and the CCPG footprint, and also demonstrates the historical pattern of why the summer season representation is the most critical system condition studied over a range of forecasted system demand levels in these areas. With the Rawhide Plant generating at its maximum capacity, mitigating actions are necessary to reduce Rawhide generation for two NERC Category C contingencies and for one NERC Category D extreme event involving two or more of the four 230kV transmission lines connected to the generation facility. The findings are documented in the Rawhide Operating Limitations Study Report dated July 12, 2010. These symptoms continue into the near and longer term planning horizons. In past Ten-Year Transmission Plan study reports these conditions were mitigated by the Laporte 230 kv Substation Expansion Project. However, for financial reasons in 2010 this project was removed from PRPA s ten-year capital budget in favor of allowable mitigating actions. Transient stability studies were performed for the operating horizon in a 2010 Heavy Summer scenario and the results documented in the TOT 7 Transfer Path Transient Stability Study dated November 8, 2010 (TOT 7 Study). The TOT 7 Study showed the TOT 7 transfer path and the surrounding Foothills transmission system remains stable with satisfactory damping characteristics. Also the transient voltage dip and frequency results from the study show the system responds adequately to the simulated disturbances. Near-Term Planning Horizon (2011-2015) PRPA has transmission plans to achieve the required system performance throughout the planning horizon. In 2004 the 2004-2014 Ten-Year Transmission Plan included two significant 230 kv projects necessary to meet system performance requirements in the near-term planning horizon. These two projects are the Fordham-Fort St.Vrain 230 kv Line in the Longmont area originally expected to be in service by Spring 2007, and the Dixon-Horseshoe 230 kv Line in the Fort Collins/Loveland area originally expected to be in service by Spring 2008. PRPA has been (and still is) working diligently to complete these projects as soon as possible but has experienced a number of delays. Reasons for the delay have been the County land use 1041 Regulation processes, right-of-way acquisitions, negotiations for rebuilds of transmission facilities owned by others, changes in line routing, modifications to transmission line design, a Page 8 of 881

delay in the issuance of long-term bonds due to uncertain financial markets, and occasional construction delays. At this time the expected completion dates for the Fordham-Fort St.Vrain 230 kv Line and the Dixon-Horseshoe 230 kv Line are July 2011 and May 2012 respectively. The schedule for implementation of these and all other PRPA transmission projects is given in Table 1 at the beginning of this report. The lead times for the Fordham-Fort St.Vrain 230 kv Line and the Dixon-Horseshoe 230 kv Line projects are designed for completion to occur as soon as possible by coordinating multiple contractors for overhead and underground line construction activities to work around each other simultaneously on both projects, while at the same time trying to avoid activities that require line outages during the summer months. Detailed transmission construction schedules for these and other near-term projects and a Ten-Year Capital Transmission Budget for all PRPA transmission projects were developed by PRPA System Engineering. Each year since 2004 PRPA has addressed the criteria violations with mitigation actions in its annual Operating Assessments provided to the WECC Reliability Coordinator, the Transmission Operators, and the Balancing Authorities in the area so all affected parties are prepared to respond to system problems that might occur. In the meantime until these transmission projects can be completed, PRPA will continue to perform annual operating assessments and provide the results and mitigating actions to affected parties. The majority of performance problems appearing on the PRPA system in the 2010 Summer and 2010-2011 Winter construction operating assessments are due to the delay of these two significant 230 kv line projects. As project in-service dates change PRPA makes the associated changes to system topologies in the WECC base case models. Power flow and transient stability studies were performed for the near-term planning horizon and the results are documented in this report for the 2015 Heavy Summer and the 2014 Light Autumn scenarios. The same seasons were studied in the CCPG NERC/WECC Compliance Report and Reactive Margin Analysis dated December 28, 2010 (CCPG Study). All but one of the criteria violations are associated with the Rawhide Plant generating at its maximum capacity and can be mitigated by a reduction of generation levels. There were no transient stability criteria violations for the PRPA transmission system. (See the Transient Stability Analysis section for details.) Voltage stability studies were performed for the near-term planning horizon and the results documented in the CCPG Study for the 2015 Heavy Summer and 2014 Light Autumn scenarios. There were no criteria violations for the PRPA transmission system. Power flow results of the near-term planning horizon studies for the PRPA transmission system are summarized in the following Table 2. The primary reasons for the more favorable nearterm results, as compared to the 2010 Summer operating horizon results, are the additions of the Fordham-Fort St.Vrain 230 kv Line and the Dixon-Horseshoe 230 kv Line. Page 9 of 881

Table 2: Near-Term Planning Horizon (2011-2015) Case Criteria Violations Mitigation Plan 2014 Light NERC Category C forced outage of Rawhide-Ault & Rawhide- Autumn 2 Timberline 230kV lines overloads Dixon-Rawhide-Timberline 2015 Heavy Summer 3 3-terminal 230kV line by 103% of 472MVA rating. NERC Category D extreme event for loss of Ault-Timberline & Rawhide-Timberline & Ault-Rawhide 230kV lines overloads Dixon-Rawhide-Timberline 3-terminal 230kV line by 103% of 472MVA rating. NERC Category C forced outage of Rawhide Timberline & Dixon Rawhide Timberline 3-terminal 230kV lines (Timberline BKRFAIL 1186) overloads Ault Rawhide 230kV line by 123% of 378 MVA rating. NERC Category C forced outage of Rawhide-Ault & Rawhide- Timberline 230kV lines overloads Dixon-Rawhide-Timberline 3-terminal 230kV line by 102% of 472MVA rating. NERC Category D extreme event for loss of Ault-Timberline & Rawhide-Timberline & Ault-Rawhide 230kV lines overloads Dixon-Rawhide-Timberline 3-terminal 230kV line by 103% of 472MVA rating. NERC Category C forced outage of Rawhide Timberline & Dixon Rawhide Timberline 3-terminal 230kV lines (Timberline BKRFAIL 1186) overloads Ault Rawhide 230kV line by 116% of its 378 MVA rating and the Laporte 230/115kV Transformer by 115% of 184MVA rating. NERC Category C forced outage of Longs Peak County Line & Slater-Longs Peak-Meadow 3-terminal 115kV line causes Longs Peak bus voltage to 1.07pu. Reduce Rawhide Generation to 640MW (net) in 15 minutes from time of forced outage to avoid conductor sag limit. Evaluate the Ault-Rawhide terminal equipment for rating increase to 472 MVA conductor sag limit. In the meantime, reduce Rawhide Generation to 560MW (net) in 6 minutes from time of forced outage to avoid conductor sag limit. Reduce Rawhide Generation to 640MW (net) in 15 minutes from time of forced outage to avoid conductor sag limit. Evaluate the Ault-Rawhide terminal equipment for rating increase to 472 MVA conductor sag limit, allowing more time for generation reduction to unload the transformer. In the meantime, reduce Rawhide Generation to 575MW (net) in 7 minutes from time of forced outage to avoid conductor sag limit. De-energize capacitor bank at Longs Peak Substation to reduce bus voltage. 2 Study case used w as derived by CCPG and its members and originated from the WECC 14LA 1 approved case 14la1sa1p.sav that w as posted to http:/ / w w w.w ecc.biz on 3-10-2010. 3 Study case used w as derived by CCPG and its members and originated from the WECC 15H S2 approved case 15hs2a1p.sav that w as posted to http:/ / www.wecc.biz on 5-3-2010. Page 10 of 881

Longer-Term Planning Horizon (2016-2020) Power flow and transient stability studies were performed for the longer-term planning horizon and the results documented in this report for the 2018 Light Autumn and the 2020 Heavy Summer scenarios, and in the CCPG Study for the 2020 Heavy Summer scenario. All criteria violations are associated with the Rawhide Plant generating at its maximum capacity and can be mitigated by a reduction of generation levels. There were no transient stability criteria violations for the PRPA transmission system. (See the Transient Stability Analysis section for details.) In the 2020 Heavy Summer scenario it was determined there was one criteria violation associated with TSGT facilities. For a NERC Category B forced outage of Slater-Longs Peak- Meadow 3-terminal 115kV line there is an overload on the Dacono-Erie 115kV line of 111% of 109MVA rating. After conferring with TSGT, this criteria violation can be mitigated by adjusting or replacing metering equipment at the Erie Substation thus increasing the facility rating of the Dacono-Erie 115kV line to 166MVA. The Weld-Promontory 230 kv Project was removed from the study cases due to lack of support for the project. A similar form of this project remains in the TSGT planning stages and PRPA will monitor the progress of this 230 kv project through the Foothills Planning Group where coordinated transmission planning occurs with three other interconnected transmission owners. Voltage stability studies were performed for the longer-term planning horizon and the results documented in the CCPG Study for the 2020 Heavy Summer scenario. There were no criteria violations for the PRPA transmission system. Results of the longer-term planning horizon studies for the PRPA transmission system are summarized in the following Table 3 and indicate favorable system improvements from planned transmission projects added to the system. Page 11 of 881

Table 3: Longer-Term Planning Horizon (2016-2020) Case Criteria Violations Mitigation Plan 2018 Light NERC Category C forced outage of Rawhide-Ault & Rawhide- Autumn 4 Timberline 230kV lines overloads Dixon-Rawhide-Timberline 2020 Heavy Summer 5 3-terminal 230kV line by 103% of 472MVA rating. NERC Category D extreme event for loss of Ault-Timberline & Rawhide-Timberline & Ault-Rawhide 230kV lines overloads Dixon-Rawhide-Timberline 3-terminal 230kV line by 103% of 472MVA rating. NERC Category C forced outage of Rawhide Timberline & Dixon Rawhide Timberline 3-terminal 230kV lines (Timberline BKRFAIL 1186) overloads Ault Rawhide 230kV line by 123% of 378 MVA rating. NERC Category C forced outage of Rawhide-Ault & Rawhide- Timberline 230kV lines overloads Dixon-Rawhide-Timberline 3-terminal 230kV line by 102% of 472MVA rating. NERC Category D extreme event for loss of Ault-Timberline & Rawhide-Timberline & Ault-Rawhide 230kV lines overloads Dixon-Rawhide-Timberline 3-terminal 230kV line by 103% of 472MVA rating. NERC Category C forced outage of Rawhide Timberline & Dixon Rawhide Timberline 3-terminal 230kV lines (Timberline BKRFAIL 1186) overloads Ault Rawhide 230kV line by 116% of its 378 MVA rating and the Laporte 230/115kV Transformer by 119% of 184MVA rating. Reduce Rawhide Generation to 640MW (net) in 15 minutes from time of forced outage to avoid conductor sag limit. Evaluate the Ault-Rawhide terminal equipment for rating increase to 472 MVA conductor sag limit. In the meantime, reduce Rawhide Generation to 560MW (net) in 6 minutes from time of forced outage to avoid conductor sag limit. Reduce Rawhide Generation to 640MW (net) in 15 minutes from time of forced outage to avoid conductor sag limit. Evaluate the Ault-Rawhide terminal equipment for rating increase to 472 MVA conductor sag limit, allowing more time for generation reduction to unload the transformer. In the meantime, reduce Rawhide Generation to 575MW (net) in 7 minutes from time of forced outage to avoid conductor sag limit. 4 Study case used w as C:\ Study_Program_2010\ Projects\ Transplan\ 2018LA _PRPA \ 18LA_PRPA.sav w hich w as created from the CCPG derived case w hich originated from the WECC 14LA1 approved case 14la1sa1p.sav that was posted to http:/ / www.wecc.biz on 3-10-2010. 5 Study case used w as derived by CCPG and its members and originated from the WECC 2020H S approved case 20hs1a1p.sav that w as posted to http:/ / w w w.w ecc.biz on 6-9-2010. Page 12 of 881

Transient Stability Analysis The purpose of the Transient Stability analysis is to evaluate the stability performance of the PRPA transmission system and of generators at the Rawhide Power Plant and surrounding area. This analysis was performed for the near-term and longer-term planning horizons to evaluate how the system responds to SLG and 3 faults with various clearing times and forced outages. The rationale for the selection of contingencies and extreme events to be evaluated considers facilities at significant substations and power plants, lines involved with large bulk transfer paths, common rights-of-way, common structures, and shared circuit breakers to satisfy all contingency conditions and types of faults defined in NERC Table 1 of the TPL standards. A list and description of the disturbances run in the transient stability simulations are provided in Exhibit 7. There were no transient stability criteria violations for the PRPA transmission system which remains stable with satisfactory damping characteristics for all NERC Category A, B, and C events. The following two NERC Category D extreme events resulted in Rawhide generator instability for all cases studied: 1. Category D2 a 3 fault at the Ault end of the Ault-Rawhide 230 kv line with failure of a pilot protection system and operation of a backup Zone 2 relay with a delayed clearing 6 time of 30 cycles. The Rawhide units pulled out of synchronism for this extreme event. In order to evaluate these results as a potential triggering event for cascading, a follow-up analysis was performed along with tripping all Rawhide generators which pulled out of synchronism. The follow-up analysis demonstrated a stable system with satisfactory damping characteristics and no cascading. (The critical clearing time for Rawhide generation to remain stable for this extreme event is 20 cycles for the 2020 Heavy Summer case.) Simulating the same disturbance but with a 30-cycle SLG fault, which changes this event to a Category C8, results in a stable system with satisfactory damping characteristics. a) All 230kV transmission lines exiting the Rawhide plant have redundant pilot relaying schemes and therefore will reduce the likelihood of this extreme event from occurring. 2. Category D4 a 3 fault on the Rawhide 230 kv bus with a stuck breaker (BKR 2122) and a Breaker Failure (BF) delayed clearing time of 18 cycles. BKR 2122 BF relaying disconnects all Rawhide peaking Units A, B, C, D, and F from the 230 kv bus. Rawhide Unit 1 pulled out of synchronism for this extreme event. In order to evaluate these results as a potential triggering event for cascading, a follow-up analysis was performed along with tripping Rawhide Unit 1 which pulled out of synchronism. The follow-up analysis demonstrated a stable system with satisfactory damping characteristics and no cascading. (The critical clearing time for Rawhide Unit 1 to remain stable for this extreme event is 15 cycles for the 2020 Heavy Summer case.) Simulating the same disturbance but with an 18-cycle SLG fault, 6 Delayed Clearing is defined in the NERC Glossary of Terms as Fault clearing consistent with correct operation of a breaker failure protection system and its associated breakers, or of a backup protection system with an intentional time delay. Page 13 of 881

which changes this event to a Category C9, results in a stable system with satisfactory damping characteristics. a) Possible action to reduce the likelihood of the event Evaluate reducing the clearing times for both BF and Circuit Switcher Failure relaying at Rawhide. Prior Outage Initial Conditions The purpose of the Prior Outage Initial Conditions (POIC) analysis is to evaluate the strength of the transmission system to withstand forced outage contingencies during the prior outage of a facility susceptible to a long-term repair period that might span across a summer peak in the planning horizon. Powerflow studies were performed and the results documented herein for the 2020 Heavy Summer scenario to assess the impacts of a damaged underground transmission cable, a damaged autotransformer, or a damaged generator for which longer repair periods could occur in the PRPA transmission system. Four 230 kv underground transmission cable circuits, one 345/230 kv transformer, nine 230/115 kv transformers, and Rawhide Unit 1 were studied for prior outage scenarios. The same Category A, B, C and D contingencies simulated for the near-term and longer-term planning horizon assessments were also simulated for this POIC assessment. There were no N- 1-0 Category A problems. The system performance for N-1-1 Category B contingencies had a small number of criteria violations which could be mitigated by monitoring transformer alarms or by shedding some load if necessary. The worst N-1-2 or More Category C problem was the prior outage of a Fordham-Fort St.Vrain 230 kv Line underground cable section followed by the forced double-circuit tower outage of the Longs Peak-County Line 115 kv Line and the Longs Peak-Meadow-Del Camino 3-Terminal 115 kv Line which reduced the Longmont/Del Camino/Brighton area voltages to 0.50 per unit supported only by the Beaver Creek-Erie 230 kv Line and the Estes-Longmont Northwest 115kV Line. PRPA Sub-Area Reactive Power Assessment The purpose of this reactive power assessment is to verify that PRPA will continue to satisfy the requirements of its Reactive Power Supply Guidelines in the planning horizon. The PRPA Sub- Area is a boundary metering system within the PSCo Balancing Authority Area necessary for the operations and measurement of load and losses on a real-time basis in the PRPA system. A PRPA Sub-Area reactive power assessment was performed and the results documented herein for the 2020 Heavy Summer scenario. The results demonstrate PRPA has the capability to meet the peak Sub-Area reactive power demand in the ten-year planning horizon with a margin for dynamic reserve using its own reactive power supply facilities installed inside the Sub-Area metered boundaries. The greatest reactive power demand occurs in the summer season. During off-peak times the Sub-Area may be importing vars from elsewhere but internal reactive power facilities are available if necessary to adjust the interchange within safe voltage operating limits. The following new PRPA reactive power supply facilities are included in the Ten-Year Transmission Plan: 70 Mvar from two 35-Mvar 115 kv shunt capacitors at the Horseshoe 230/115 kv Substation in 2012 90 Mvar of charging from underground cable on three 230 kv line projects in Longmont, Loveland, and Fort Collins in 2011 Page 14 of 881

In the 2020 Heavy Summer scenario the PRPA Sub-Area is exporting 87 Mvar with all shunt capacitors in-service and Rawhide generating a net plant 84 Mvar. See Exhibit 8 for the PRPA Sub-Area reactive power assessment results. Short-Circuit Analysis The purpose of a short-circuit analysis in the planning horizon is to determine whether planned facilities will cause the short-circuit rating of existing BES equipment to be exceeded. Shortcircuit studies were performed by PRPA System Engineering for the 2018 Heavy Summer system topology and the results documented in a PRPA 2018 System Forecast Fault Current Study memo dated December 4, 2009. All fault currents are within BES equipment ratings. The planned system topology in the 2018 Heavy Summer scenario studied in 2009 is similar to the 2020 Heavy Summer planned system topology in 2010 for the Foothills Area. Other Exhibits See Exhibit 9 for all steady-state thermal and voltage Matrix study results for each planning horizon assessment and for the POIC assessment. For all transient stability results 7 reference Exhibit 10 for a tabular summary and Exhibit 11 for plots of generator angles and rotor speeds, bus voltages and bus frequencies for all simulated disturbances. VII. Conclusions The PRPA Ten-Year Transmission Plan (2011-2020) ensures a transmission system designed for the reliable delivery of electricity to its municipal owners in Estes Park, Fort Collins, Longmont, and Loveland, Colorado and to other PRPA transmission customers. The PRPA transmission system is planned such that it will meet the NERC and WECC performance requirements and can be operated to supply projected customer demands and firm transfers at all demand levels over the range of forecasted system demands under the conditions defined for Categories A, B, C, and D of the NERC Standards TPL-001 through -004. The PRPA and Foothills transmission systems are steady-state thermal or steady-state voltage limited systems (not stability limited) in the operating horizon and throughout the ten-year planning horizon. 7 Transient Stability results are provided in both tables and graphical plots. Stability results for machines which are disconnected from the transmission system may appear similar to those of a machine that pulls out of synchronism and should be disregarded. Page 15 of 881

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