Project #148. Generation Interconnection System Impact Study Report

Project #148 Generation Interconnection System Impact Study Report June 05, 2012 Electric Transmission Planning

Table of Contents Table of Contents... 2 Executive Summary... 3 Energy Resource Interconnection Service Results... 3 100 kv Interconnection... 4 Definitions... 6 Energy Resource Interconnection Service... 6 Generator and Interconnection Data... 7 Study Parameters... 8 Senior Queue Generator Assumptions... 8 Steady State Power Flow Analysis... 10 Method... 10 Results... 10 100 kv Interconnection With Project at its full Output.... 10 Mitigation... 11 Transient Stability Analysis... 12 Method... 12 Results... 12 Mitigation... 12 QV Analysis... 13 QV Analysis... 13 Method... 13 Results... 13 Mitigation... 13 Fault Duty Analysis... 14 Method... 14 Results... 14 Mitigation... 14 Conclusions... 15 Mitigation... 15 Next Steps... 15 Attachments... 15 2

Executive Summary NorthWestern Energy ( NWE ) has completed the System Impact Study ( SIS ) Project 148 ( Generation Project ) near Columbus Rapelje, MT. NWE studied the Generation Project as an Energy Resource Interconnection Service ( ERIS ). The SIS is an in-depth analysis that examines the response of the transmission system to a variety of system operating conditions. NWE is responsible for maintaining acceptable system reliability, and must be certain that system reliability is maintained with the addition of the Generation Project. NWE uses tolerance levels outlined by FERC, NERC, and/or WECC. The SIS uses the following types of analyses: Steady-State Power Flow Post Transient Steady-State Power Flow Transient Stability Fault Duty Reactive Margin The results of the SIS demonstrate that with all the senior queue projects and associated mitigation in place, the addition of 80 MW interconnected to NWE 100 kv transmission system near Columbus-Rapelje between Project # 137 Point of Interconnection (POI) and Columbus Auto substation as an Energy Resource will not require additional network upgrades except the interconnection itself. The Generation Project may be required to restrict their output to 52.5 MW under certain operating conditions. For an ERIS application there are two basic questions answered in the study. (1) To identify necessary upgrades to allow full output of the proposed Large Generating Facility and (2) Also identify the maximum allowed output, at the time the study is performed, of the interconnecting Large Generating Facility without requiring additional Network Upgrades. The Customer has waived question 1 of the study for their ERIS application which would identify the necessary network upgrade required to interconnect the Generation Project at its full amount. The Customer has only requested answer to question 2 through this study which would identify the maximum allowed output without requiring additional network upgrades. The findings included in this study do not assure the Interconnection Customer that the planned Generation Project will be allowed to operate at full capacity under all operating conditions. NWE cannot guarantee that future studies (i.e. Operational Studies) will not identify additional problems. Energy Resource Interconnection Service Results ERIS allows the Interconnection Customer to connect the Large Generating Facility to the Transmission System and be eligible to deliver the Large Generating Facility s output using the existing firm or non-firm capacity of the Transmission System on an as available basis. ERIS 3

does not in and of itself convey any right to deliver electricity to any specific customer or Point of Delivery. 100 kv Interconnection A new 100 kv substation between Project # 137 Point of Interconnection (POI) and Columbus Auto substation, depicted in Attachment A - One Line Diagram, was assumed as the POI for Customer's Project. With no additional network upgrades and with the senior queue projects and their mitigation in place, the Generation Project showed thermal issues if the output of the Project is above 52.5 MW. The study did not show any fault duty or transient stability issue with the Generation Project at its full output. Hence no network upgrades necessary to address short circuit issues associated with interconnection facilities is required. With the senior queue projects and their mitigation in place the study showed an N-0 overload on the Columbus Rapelje 161/100 kv auto transformer with the Generation Project at its full output. The study also showed some N-1 and N-2 overloads with the Generation Project at its full output. Due to these system constraints the Customer will have to limit their total output at the POI to 52.5 MW without additional network upgrades under certain operating conditions. NWE cannot guarantee that future studies will not further restrict generation output. The Customer will also be required to curtail their output for certain N-1 outages. The outages and overloads caused by Customer's project are shown in the "Steady State Power Flow Analysis" section. A high-level non-binding cost estimate to interconnect the Generation Project at 80 MW to NWE's 100 kv transmission system without additional network upgrades is summarized in Table I. Without any additional network upgrades the Customer will have to restrict their output to 52.5 MW under certain operating conditions. As the Customer is interconnecting to one of the critical transmission lines a shoe-fly will be necessary in order to interconnect the Customer. The shoefly will make sure that the Generation Project can be interconnected while keeping the critical 100 kv line in service. The cost of implementing a shoe-fly to interconnect the Customer is also included in the interconnection costs. Table I ERIS - 100 kv Interconnection for 80 MW High Level, Non-Binding, Cost Estimate Substation - Interconnection 3,633,000 Transmission 278,785 Relay 660,000 Communications 282,000 Metering 14,000 EMS 10,073 Total $4,877,858 In addition to the cost in Table I, the Interconnection Customer will be responsible for providing the following communications, or if desired, in this case NWE can provide these services for a monthly fee: 1. Telephone circuit to meter 4

2. Data channel to NWE SOCC center 3. Ring down circuit to the generation control center For Customer's information, the mitigation required to allow full output of the Generation Project with senior queue projects and their mitigation in place are listed below (a) Install a new 161/100 kv auto transformer at Columbus Rapelje (approximately $1,600,000). (b) Install 9 MVAR of capacitors at Wilsall to satisfy a minimum reactive margin at the Wilsall substation (approximately $360,000). (c) Install an automatic OMS that will curtail Customer's project for certain N-1 outages (approximately $1,000,000). The costs shown above with the mitigation are very high level. The mitigation shown above is dependent on the senior queue projects, hence a restudy would be required if there are changes to the Generation Interconnection queue. This study assumes that all the mitigation required by the senior queue generation interconnection projects are in place. The study results may change if there are changes to the Generation Project s queue position or the mitigation required by the senior queue projects changes, or if there are any changes to the interconnection specifications provided by the Generation Project to NWE. This study results does not constitute a transmission service request ( TSR ). A TSR study would identify how much transmission capacity is available firm or non-firm on an as available basis. 5

Definitions Energy Resource Interconnection Service Energy Resource Interconnection Service shall mean an interconnection service that allows the interconnection customer to connect its generating facility to the transmission provider's transmission system to be eligible to deliver the facility's electric output using the existing firm or non-firm capacity of the transmission provider's transmission system on an as available basis. Energy Resource Interconnection Service in and of itself does not convey transmission service. Under Energy Resource Interconnection Service ( ERIS ), the Generation Project will be able to inject power from the facility into and deliver power across NWE s transmission system on an as available basis up to the amount of MW identified in the applicable stability and steady state studies to the extent the upgrades initially required to qualify for ERIS have been constructed. No transmission delivery service from the Generation Project is assured, but the Generation Project may obtain point-to-point transmission service, network integration transmission service, or be used for secondary network transmission service, pursuant to NWE s Tariff, up to the maximum output identified in the stability and steady state studies. In those instances, in order for the Generation Project to obtain the right to deliver or inject energy beyond the facility point of interconnection or to improve its ability to do so, transmission delivery service must be obtained pursuant to the provisions of NWE's Tariff. The Generation Project's ability to inject its output beyond the point of interconnection, therefore, will depend on the existing capacity of NWE's transmission system at such time as a Transmission Service Request ( TSR ) is made that would accommodate such delivery. The provision of firm point-to-point transmission service or network integration transmission service may require the construction of additional network upgrades. 6

Generator and Interconnection Data The proposed generator and interconnection data used in the studies was based on the information received from the Interconnection Customer. From the initial application, NWE identified the following project information. Project Name Project 148 Requested MW Output 80 MW at the Point of Interconnection Location On 100 kv line between Project # 137 POI and Columbus Auto Substation. Special Resources/Technology 50 GE 1.6 MW Wind Turbines Proposed Commercial Operation Date December, 2014 Facilities A new 100 kv 3-breaker ring bus substation between Project # 137 POI and Columbus Auto substation. Assumptions o Scheduled Voltage (pu): 1.01 at the Point of Interconnection The generator is assumed to have operational characteristics either through internal or external capabilities to operate throughout a power factor range of.95 leading to.95 lagging at the Point of Interconnection. During the study process, NWE found that if this requirement was not met, system reliability was compromised. 7

Study Parameters In analyzing the Generation Project, NWE utilized PSS/E software to conduct the System Impact Study with the proposed Generation Project. These studies connected the Generation Project to NWE s Transmission System in a computer model to simulate the interaction of the Generation Project with other resources and loads. Two WECC base cases adjusted to include the NWE Transmission System detail representing 2012 light autumn and 2014 heavy summer scenarios were used in this study. Senior Queue Generator Assumptions In addition to existing generators, senior queue resources listed in Table II and the associated mitigation were also considered in this study. Senior queued generation and existing generation dispatch were varied as needed to emulate stress on the system for various scenarios. Table II. Senior Queued Projects Project Number Size (MW) Point of Interconnection 31 396 Wilsall-Shorey Road 230 kv Line 39 22 Billings Steam Plant Switchyard 46 10 Loweth - Two Dot 100 kv line 47 20 69 kv line at Chester 49 23 Rainbow Switchyard 54 100 Wilsall-Shorey Road 230 kv Line 57 85 Bradley Creek Substation 60 20 Bradley Creek - Three Forks S. 100 kv line 61 2 Phillipsburg - Anaconda 25 kv line 63-69 30 (total) 69 kv line near Sumatra (9 requests, 5 MW each) 75 75.6 161 kv line approx 5 mi. N of Bradley Creek sub. 76 75.6 100 kv line approx 5 mi. N of Bradley Creek sub. 81 12 Near 100 kv Rainbow Switchyard 82 Efficiency Improvement Near 100 kv Rainbow Switchyard 88 20 Near Livingston City Substation 89 20 100 kv line between Loweth and Two Dot at Groveland 100 Efficiency Improvement Near 100 kv Rainbow Switchyard 102 18.9 Dutton 69 kv Substation 103 120 Great Falls 230 kv Switchyard 104-106 15 (total) 69 kv line near Sumatra (9 requests, 5 MW each) 114 19.5 Two Dot Substation Distribution 115 460 230 kv line near Judith Gap South 116 0.225 Yellowstone National Park 118 9.2 100 kv line between Harlowton and Broadview 126 25 100 kv line West of Geyser 127 10 69 kv East of Bole Substation 129 0.455 Dry Creek, Montana 133 10 69 kv line East of Bole Substation 134 10 69 kv line East of Bole Substation 8

135 16 100kV line West of Geyser 136 20 Mill Creek Substation 100kV 137 10 Columbus Rapelje Auto Substation 138 1 Mill Creek Substation 230 kv Switchyard 139 10 100kV line between Harlowton Sub and Broadview Switchyard 140 45 6 to 9 miles SSE of Cut Bank, MT 141 9.87 230 kv Interconnection to NWE existing Judith Gap Switchyard 142 9.87 230 kv Interconnection to NWE existing Judith Gap Switchyard 146 20 50 kv line between Big Timber City and Livingston City 9

Steady State Power Flow Analysis The steady-state power flow analysis examines steady state, system normal, operating conditions with no lines out of service (i.e., N-0 Conditions) and with various lines out of service (i.e., N-1 and N-2 conditions). A power flow simulation is completed before and after the addition of the Generation Project to identify any unacceptable thermal overloads and voltage excursions the project may cause. Method NWE simulated an extensive set of N-1 and N-2 outages. Power flow contingencies were simulated for both operating conditions (2012 light autumn and 2014 heavy summer). The local area contingencies were the primary focus, but major transmission line outages around the NWE system were also studied. Results 100 kv Interconnection With Project at its full Output. N-0: The addition of the Generation Project to NWE s Transmission System under N-0 conditions (all lines in service) causes overloads. The overloads seen in the study are shown in table III below. Table III. N-0 Overloads with the Generation Project at 80 MW Thermal Overloads Base case Outage Element Monitored Element 2012 LA NONE Columbus Rapelje 161/100 kv auto transformer % Loading Before Project % Loading After Project 41 117 N-1: The addition of the Generation Project to NWE s Transmission System under N-1 conditions (one line out of service) causes thermal overloads. N-2: The addition of the Generation Project to NWE s Transmission System under credible N-2 conditions (two line out of service) causes thermal overloads. Table IV illustrates the N-1 and N-2 thermal overloads caused by the addition of the Generation Project to NWE's 100 kv transmissions system. 10

Table IV. N-1 and N-2 Thermal Overloads with Generation Project at 80 MW. Thermal Overloads Base case Outage Element Monitored Element % Loading Before Project % Loading After Project at full output 2012LA Chrome 100/50 kv Auto ** ** 2012LA Columbus Rapelje - Project # 137 POI 100 kv line Chrome 100/50 kv Auto ** ** 2012LA Project #137 POI to Project # 148 POI 100 kv line Chrome 100/50 kv Auto ** ** 2012LA Columbus Auto 100/50 kv Auto ** ** 2012LA Columbus Rapelje - Project # 137 POI 100 kv line Columbus Auto 100/50 kv Auto ** ** 2012LA Project #137 POI to Project # 148 POI 100 kv line Columbus Auto 100/50 kv Auto ** ** Chrome 100/50 kv Auto ** ** Columbus Rapelje - Project # 137 POI 100 kv line Chrome 100/50 kv Auto ** ** Project #137 POI to Project # 148 POI 100 kv line Chrome 100/50 kv Auto ** ** Columbus Auto 100/50 kv Auto ** ** Columbus Rapelje - Project # 137 POI 100 kv line Columbus Auto 100/50 kv Auto ** ** Project #137 POI to Project # 148 POI 100 kv line Columbus Auto 100/50 kv Auto ** ** Columbus Auto - Still Water Smelter 100 kv line Columbus Auto 100/50 kv Auto Chrome Junction - Still Water 50 kv line Columbus Auto - Columbus East - Park City 50 kv Line Big Timber 161/50 kv Auto transformer Alkali Creek - Columbus Rapelje 161 kv line Lower Duck Creek - Clyde Park 161 kv line Lower Duck Creek - Big Timber 161 kv line 39% 111% 42% 114% 28% 119% 46% 105% 52% 102% 45% 106% 56% 101% 57% 102% Wilsall - Three Rivers 230 kv line East Gallatin - Bozeman Westside 161 kv line 97% 104% 2012LA Three Rivers - Wilsall 230 kv line + Three Rivers Clyde Park 161 kv line * Monitored Element is a current limiting air-break switch. East Gallatin - Bozeman Westside 161 kv line* 97% 108% ** The overloads on the Monitored Element is a pre-existing issue for which NWE has a planned mitigation. If the Generation Project comes online before the planned mitigation is in place, the Generation Project will have to implement mitigation to alleviate this issue. Mitigation The Generation Project can interconnect 80 MW without additional network upgrades but can only output 52.5 MW under certain operating conditions due to the system constraints shown above. However, the Generation Project along with senior queued projects may be asked to curtail generation in order to relieve thermal overloads for N-1 and N-2 conditions. 11

Transient Stability Analysis When a line fault occurs, the protective relaying must respond by opening circuit breakers to remove the affected transmission line from service. This can result in a system disturbance. The credible worst case fault events must be simulated to determine if the transmission system will recover to acceptable steady state operating conditions. Events that were studied include singlephase and three-phase faults causing either single or multiple line outages or generator failures. The dynamic simulations performed for this project include an assortment of events that are intended to provide a robust test of the impact of the Generation Project. The results from the Transient Stability Analysis are designed to reveal: Whether or not regional electric transmission systems remain stable with each event; Whether or not WECC criteria are met for each outage condition; and Identify where problems are located on the Transmission System. Method NWE simulated an extensive set of 500 kv and non-500 kv faults. The term fault refers to a short-circuit between either a single-phase conductor to ground or all three phases. Due to a senior queued project's Transmission Service Request and the Generation Project's Transmission Service Request, the Colstrip 500 kv upgrade project was included in this analysis. Results All of the events simulated remained stable and met WECC criteria. Mitigation No mitigation is required. 12

QV Analysis The SIS examined the reactive margin at critical buses on NWE s Transmission System. In addition, the QV reactive margin identifies potential voltage collapse issues under maximum operating conditions. This analysis includes the addition of the Generation Project. QV Analysis QV analysis is used to determine the reactive power injection required at predefined bus in order to correct the bus voltage to the required value. The curve is obtained through a series of AC load flow calculations. The voltage at a predefined bus can be calculated for a series of power flows as the reactive load is increased in steps. The point where the power flow becomes nonconvergent is the point where voltage collapse occurs. Method QV curves were obtained for the Wilsall 161 kv bus using the QV analysis tools in PSS/E. These curves were obtained before and after the addition of the Generation Project, and include N-1 contingencies used in the "Steady State Power Flow Analysis." The result is a series of QV curves for each bus, which are used to determine the reactive power margin. The Generation Project is modeled at 52.5 MW with all senior queue projects and their mitigation requirements. Results With the Generation Project modeled at 52.5 MW, the amount of reactive power required to maintain a voltage range of 0.90 p.u to 1.05 p.u. at the Wilsall 161 kv bus after the loss of the Shorey Rd to the Project #54 POI 230 kv line is sufficient. The QV curve for the outage mentioned above with Generation Project @52.5 MW and @ 80 MW is shown in Attachment B. Mitigation No mitigation is required by the Generation Project to interconnect 80 MW. Network mitigation will be required in order for the Generation Project to output the full amount of the request under certain operating conditions. 13

Fault Duty Analysis When a fault occurs on a power line, protective relaying equipment detects the fault current flowing and signals the associated circuit breakers to open. When the circuit breakers open, they must be capable of interrupting the fault current. If the magnitude of the fault current exceeds the interrupt rating of the circuit breakers, the fault may not be cleared, and damage to system equipment and voltage collapse may result. Method To perform a fault duty analysis, busses at or near the POI of the Generation Project are faulted in a PSS/E model to determine the magnitude of fault current anticipated with the Generation Project in service. The results of this analysis determine whether standard circuit breaker fault duty ratings would be exceeded with the addition of the Generation Project. Results The fault current contributed by the Generation Project is negligible and the fault duty at the buses near the POI does not change very much. The breakers in the area are capable of interrupting the fault current. Mitigation No mitigation is required. 14

Conclusions The results of the SIS demonstrate that with all the senior queue projects and associated mitigation in place, the addition of 80 MW interconnected to the 100 kv transmission system between Project # 137 POI and Columbus Auto substation as a Energy Resource Network Resource is feasible without additional network mitigation provided the Generation Project will restrict its output to 52.5 MW under certain operating conditions. The SIS study showed that the Customer can interconnect the full 80 MW but will have to restrict their output to 52.5 MW under certain operating conditions without additional network mitigation other than the interconnection. Mitigation No mitigation is required to interconnect full 80 MW provided the Generation Project will restrict its output to 52.5 MW under certain operating conditions. Network upgrades will be required if the Customer would like to output at 80 MW which is the full amount of the interconnection request. A restudy of this SIS will be required if there are changes to the senior queue generation projects. Next Steps NWE will be scheduling a meeting to discuss the findings of the SIS with the Interconnection Customer. If, after the meeting, the Interconnection Customer wishes to continue with the project, the Generation Project must designate either ERIS or NRIS. A Facility Study specific to the project will then be carried out to determine the final details of the interconnection. This study does not constitute a request for transmission service. The study examined the physics of the electrical system and does not imply that you will receive any transmission required to deliver the generation output to load. You must follow the procedures described in the transmission tariff available on (http://www.oatioasis.com/nwmt/index.html) to request and/or receive transmission service. Attachments Attachment A Project 148 One line Diagram Attachment B QV Curves 15