PROJECT #164
GENERATION INTERCONNECTION
REVISED SYSTEM IMPACT STUDY REPORT
01/12/2016
REGIONAL ELECTRIC TRANSMISSION PLANNING
1
Table of Contents
Executive Summary ................................................................................................................................. 3
Interconnection and Project Information ................................................................................................. 4
Study Parameters .................................................................................................................................... 5
Steady-State Power Flow Analysis ........................................................................................................... 6
Transient Stability Analysis ...................................................................................................................... 8
Voltage Stability .................................................................................................................................... 10
Short Circuit .......................................................................................................................................... 12
Cost Estimates ....................................................................................................................................... 13
Conclusion............................................................................................................................................. 14
2
Executive Summary
NorthWestern Energy (NWE) has completed the revised System Impact Study (SIS) work for Project #164
(Customer) for an interconnection of a 300.9 MW wind farm (Generation Project) to be connected to
the existing Colstrip 230 kV substation. The proposed Commercial Operation Date (COD) for the project
has been changed from 12/1/2016 to 09/01/2017.
The project was studied as a large generation Energy Resource Interconnection Service (ERIS). This SIS is
an in-depth analysis of the interconnection facilities necessary to connect the Generation Project and
examines the impact of the interconnection to the local transmission system under 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 new Generation Projects. NWE uses
standards and guidelines outlined by FERC, NERC, and/or WECC.
The SIS examined the following:
•
•
•
•
•
Steady-State Power Flow
Post-Transient Steady-State Power Flow
Transient Stability
Voltage Stability
Short Circuit
The results of the revised SIS confirm that, with all senior queue projects and associated mitigation in
place, the addition of the Generation Project can connect as an energy resource to NWE’s 230 kV
transmission system at Colstrip without additional network system improvements beyond the Point of
Interconnection (POI). The study showed that if Colstrip Units 1 & 2 were online at full output, the
additional output from the Generation Project could cause N-1 overloads on the nearby system. This
study does not provide for or guarantee that firm or non-firm transmission service will be available.
Operating procedures will need to be developed to limit the combined generation output at the 230 kV
Colstrip substation, which may require the project to be held at low levels or completely offline.
As an ERIS, the interconnection allows the Customer to connect its Generation Project to NWE’s system
to be eligible to deliver the Generation Project’s electric output using existing firm or non-firm capacity
on NWE’s system on an as available basis. ERIS in and of itself does not convey transmission service.
NWE cannot guarantee the Customer that the Generation Project will be allowed to operate at full or
reduced output capacity under any and/or all operating conditions, on a firm or non-firm basis. If
transmission service is desired, a Transmission Service Request (TSR) is required. NWE cannot
guarantee that future analysis for firm or non-firm transmission service (i.e. TSR or operational studies)
will not identify addition problems or system constraints that require mitigation or reduced operation.
The high-level, non-binding good faith estimate of the upgrades or construction necessary to
interconnect the Generation Project is roughly $1.36 million.
3
Interconnection and Project Information
The proposed interconnection and generator data used in the study was based on information received
from the Customer. From the initial application and subsequent dialog with the Customer, NWE
identified the following project information.
Project Name:
Project Number:
Location:
Total Size:
Proposed COD:
Scheduled Voltage (p.u.):
Individual Generator Data
Type:
Max Output:
MVA Rating:
Quantity:
Power Factor:
Step Up Transformer 1
Capacity:
Voltage Ratio:
Impedance:
X/R:
Taps:
Quantity:
Step Up Transformer 2
Capacity:
Voltage Ratio:
Impedance:
X/R:
Taps:
Quantity:
1
Project #164
164
230 kV Colstrip substation - Rosebud and Custer County, Montana
300.9 MW
09/01/2017
1.00 at the POI1
General Electric 1.70 MW wind turbine
1.73 MW
1.92 MVA
177
0.95 leading to 0.95 lagging1
1.89 MVA
0.69/34.5 kV1
5.75%
7.5
2 @ +/- 2.5% per tap
177
340 MVA1
34.5/230 kV1
7.00%1
1001
4 @ +/- 2.5% per tap1
1
Assumed
Should any equipment or assumed value change prior to operation, NWE must be notified and new
study work may need to be completed before operation of the Generation Project commences.
A conceptual one-line diagram of the Point of Interconnect can be seen in Error! Reference source not
found..
4
Study Parameters
In analyzing the Generation Project, NWE utilized both PowerWorld and PSS®E software to conduct the
SIS. These studies simulated the interaction of the Generation Project with other resources and loads
on the transmission system. The two cases used for the analysis were the Western Electricity
Coordinating Council (WECC) 2015 Light Summer (15ls1ap) and 2020 Heavy Summer (20hs2ap) base
cases. Each case was modified to include NWE’s detailed system as well as projected 2017 system
loading. Projects that are currently in construction and that will be finished by the Generation Project’s
COD were also included. Current network resources as well as any senior queue NRIS or ERIS with a firm
TSR were dispatched at their designated amounts. All senior queue resources were considered in this
study. A list of the senior queue projects can be seen in Error! Reference source not found..
A total of 16 cases were created and analyzed, each with varying levels of generation dispatch and path
flows in the surrounding area. A summary of the cases is shown detailed in Table 1. A more detailed
description of each case can be viewed in Error! Reference source not found..
Table 1: Case Descriptions
Case
2017ls_1_a
2017ls_1_b
2017ls_2_a
2017ls_2_b
2017ls_3_a
2017ls_3_b
2017ls_4_a
2017ls_4_b
2017hs_1_a
2017hs_1_b
2017hs_2_a
2017hs_2_b
2017hs_3_a
2017hs_3_b
Description
2017 Light Summer, normal dispatch, pre-project, Path 8 at 2200 MW E-W
2017 Light Summer, normal dispatch, post-project, Path 8 at 2500 MW E-W
2017 Light Summer, Colstrip Unit 1 offline, pre-project, Path 8 at 2000 MW E-W
2017 Light Summer, Colstrip Unit 1 offline, post-project, Path 8 at 2200 MW E-W
2017 Light Summer, system wind resources down, pre-project, Path 8 at 2000 MW E-W
2017 Light Summer, system wind resources down, post-project, Path 8 at 2200 MW E-W
2017 Light Summer, Colstrip Units 1 & 2 offline, pre-project
2017 Light Summer, Colstrip Units 1 & 2 offline, post-project
2017 Heavy Summer, normal dispatch, pre-project
2017 Heavy Summer, normal dispatch, post-project
2017 Heavy Summer, Colstrip Unit 1 offline, pre-project
2017 Heavy Summer, Colstrip Unit 1 offline, post-project
2017 Heavy Summer, system wind resources down, pre-project
2017 Heavy Summer, system wind resources down, post-project
5
Steady-State Power Flow Analysis
The steady-state power flow analysis examined the steady-state, system normal operating conditions
with no lines out of service (i.e. N-0 conditions) as well as various credible outages (i.e. N-1 and N-2
conditions) around the immediate area of the Generation Project.
Power flow simulations compared the system before and after the addition of the Generation Project to
identify any unacceptable thermal overloads and voltage issues that the project may cause.
Assumptions
The following assumptions were made for the post-contingency steady-state analysis:
•
•
•
•
•
•
•
•
All manually operated voltage control devices were fixed to pre-contingency status.
Phase shifting transformers were locked to pre-contingency tap position.
Automatically controlled Load Tap Changers and switched shunts were allowed to move
Generators which manually control a high side remote bus were set to regulate the voltage at
the terminal bus at pre-contingency voltage set point. Generators with automatically controlled
remote regulation (i.e. LDC) were allowed to control high side bus.
Automatically controlled remedial action schemes (RAS), relay devices, and/or load shedding
schemes were allowed to operate.
No lines were allowed to be loaded over 100% of their emergency rating. If there was no
emergency rating given to the line, lines were not permitted to exceed 100% of their normal
rating.
Voltage ratings listed on the PEAK RC website were monitored and were to be maintained postcontingency.
Project is assumed capable of controlling the POI to +/- 0.95 power factor.
Contingencies
The following is a list of the contingencies used in the power flow analysis:
•
•
•
•
•
•
•
•
•
•
•
Colstrip – Broadview 500 kV A line
Colstrip – Broadview 500 kV A line + Colstrip Unit 4 Trip
Broadview – Garrison 500 kV #1 Line
Broadview – Garrison 500 kV #1 Line + Colstrip Unit 4 Trip
Broadview – Garrison 500 kV #1 and #2 Lines + Colstrip Units 1, 3, and 4 Trip
Garrison – Taft 500 kV #1 Line
Garrison – Taft 500 kV #1 Line + Colstrip Unit 4 Trip
Garrison – Taft 500 kV #1 and 2 Lines + Colstrip Units 1, 3, and 4 Trip
Colstrip 500/230 kV #4 Transformer
Colstrip 230/115 kV #2 Transformer
Colstrip – Hardin 230 kV Line
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•
•
•
•
•
South Huntley – Billings Steam Plant 230 kV Line
South Huntley – Crossover 230 kV Line
Crossover 230 kV Phase Shifting Transformer
Loss of Colstrip Unit 4
Loss of Generation Project
Results
The complete set of power flow results can be viewed in Error! Reference source not found.. The
following is a summary of the results.
N-0 – System Normal
The study indicated that with all facilities in service, there were no N-0 overloads or voltage violations
associated with the project.
N-1 – Single Element Contingency
There were two N-1, single element contingencies where the Generation Project caused thermal
overloads under certain system dispatch.
•
•
With the addition of the Generation Project, the loss of a single Colstrip – Broadview 500 kV line
can cause overloads on the parallel Colstrip – Broadview 500 kV line as well as the South
Huntley – Crossover 230 kV and South Huntley – Billings Steam 230 kV lines. These overloads
are highly dependent on the total generation output from the Colstrip substation.
With the addition of the Generation Project, the loss of a single Colstrip 500/230 kV transformer
can cause overloads on the parallel Colstrip 500/230 kV transformer as well as the South
Huntley – Crossover 230 kV and South Huntley – Billings Steam 230 kV lines. These overloads
are highly dependent on the total generation output from the Colstrip substation as well as Path
80 imports.
N-2 – Multiple Element Contingency
Analysis of the credible N-2, multiple element contingencies showed no overloads or voltage violations
caused by the Generation Project.
Mitigation
No mitigation is needed at this time. However, there may be times when the Generation Project would
not be capable of producing full output. Those scenarios would include, but would not be limited to; full
Colstrip unit 1 and 2 output, moderate to heavy northbound Path 80 flows, outage of one of the Colstrip
to Broadview 500 kV lines, and an outage of one of the two Colstrip 500/230 kV transformers.
Operating procedures will need to be developed to limit the combined generation output at the 230 kV
Colstrip substation such that these overloads will not occur.
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Customer is also responsible for providing a power factor at the Point of Interconnect of 0.95 lagging to
0.95 leading. If Generation Project is unable to do so, the Customer is responsible for installing reactive
compensation devices in order to be able to provide the reactive support necessary.
In order for the Customer to move their power on a firm or non-firm basis to the system, the Customer
will need to secure firm transmission rights through the Transmission Service Request process and
mitigation may be necessary at that time.
Transient Stability Analysis
The transient stability analysis examined the dynamic response of various credible outages (i.e. N-1 and
N-2 conditions) around the immediate area of the Generation Project. The analysis examined both
single-phase and three-phase faults with normal clearing time.
Transient stability simulations compared the system before and after the addition of the Generation
Project to identify any dynamic stability issues that the project may cause.
Assumptions
The following assumptions were made for transient stability analysis:
•
•
•
•
•
•
•
•
Automatically controlled remedial action schemes (RAS), relay devices, and/or load shedding
schemes were allowed to operate and modeled appropriately.
A detailed Acceleration Trend Relay (ATR) model was used.
A no-event 1 second simulation was ran prior to applying the event.
Series capacitors nearby to fault were bypassed during the fault.
A double Palo Verde outage was simulated to ensure case was adequate for use in analysis.
System must show positive system damping within 20 second simulation time.
All machines in the WECC interconnection must remain in synchronism and relative rotor angles
must not diverge.
Generation Project must not cause excess ATR tripping to the Colstrip Generation Facility.
Contingencies
The following is a list of the contingencies used in the transient stability analysis:
•
•
•
•
•
•
•
Colstrip – Broadview 500 kV A line
Broadview – Garrison 500 kV #1 Line with Unsuccessful Reclose
Broadview – Garrison 500 kV #1 Line with Successful Reclose
Broadview – Garrison 500 kV #1 and 2 Lines with Unsuccessful Reclose
Broadview – Garrison 500 kV #1 and 2 Lines with Successful Reclose
Garrison – Taft 500 kV #1 Line
Garrison – Taft 500 kV #1 and 2 Lines
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•
•
•
•
•
•
Taft – Bell 500 kV Line
Taft – Dworshak 500 kV Line
Taft – Hot Springs 500 kV Line
Colstrip 500/230 kV #4 Transformer
Garrison 500/230 kV Transformer
Loss of Generation Project
Results
The complete set of power flow results can be viewed in Error! Reference source not found.. The
following is a summary of the results.
N-0 – System Normal
The study indicated that with all facilities in service, there were no N-0 dynamic stability issues caused
by the Generation Project.
N-1 – Single Element Contingency
The study indicated that with all facilities in service, there were no N-1 dynamic stability issues caused
by the Generation Project. There was no appreciable difference in ATR tripping when comparing cases
with similar path flows.
N-2 – Multiple Element Contingency
Analysis of the credible N-2, multiple element contingencies showed no overloads or voltage violations
caused by the Generation Project. There was no appreciable difference in ATR tripping when comparing
cases with similar path flows.
Mitigation
No mitigation is necessary at this time. However, the Customer will be required to participate in a
generation tripping scheme for major 500 kV contingencies if it is found that the Generation Project
causes transient stability issues or increased ATR tripping in the future.
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Voltage Stability
The voltage stability analysis examined the reactive margin at critical buses around the area of the
Generation Project. For this study, a V-Q analysis was performed which finds the minimum amount of
reactive power resource required to maintain voltage at the critical buses.
Assumptions
The following assumptions were made for transient stability analysis:
•
•
•
•
•
•
•
•
•
Generation Project was increased to 102.5% nameplate rating for N-1 contingencies and 105%
nameplate for N-2 contingencies.
All manually operated voltage control devices were fixed to pre-contingency status.
Phase shifting transformers were locked to pre-contingency tap position.
Automatically controlled switched shunts were allowed to move
Automatically controlled Load Tap Changers were locked into pre-contingency position
Generators which manually control a high side remote bus were set to regulate the voltage at
the terminal bus at pre-contingency voltage set point. Generators with automatically controlled
remote regulation (i.e. LDC) were allowed to control high side bus.
Automatically controlled remedial action schemes (RAS), relay devices, and/or load shedding
schemes were allowed to operate.
Positive VAR margin must be maintained.
A transient, sub-transient, and synchronous reactance value of 0.28 p.u was assumed for the
Generation Project.
Contingencies
The following is a list of the contingencies used in the transient stability analysis:
•
•
•
•
•
•
•
•
•
•
•
•
•
Colstrip – Broadview 500 kV A line
Colstrip – Broadview 500 kV A line + Colstrip Unit 4 Trip
Broadview – Garrison 500 kV #1 Line
Broadview – Garrison 500 kV #1 Line + Colstrip Unit 4 Trip
Broadview – Garrison 500 kV #1 and #2 Lines + Colstrip Units 1, 3, and 4 Trip
Garrison – Taft 500 kV #1 Line
Garrison – Taft 500 kV #1 Line + Colstrip Unit 4 Trip
Garrison – Taft 500 kV #1 and 2 Lines + Colstrip Units 1, 3, and 4 Trip
Colstrip 500/230 kV #4 Transformer
Colstrip 230/115 kV #2 Transformer
Colstrip – Hardin 230 kV Line
South Huntley – Billings Steam Plant 230 kV Line
South Huntley – Crossover 230 kV Line
10
•
•
•
Crossover 230 kV Phase Shifting Transformer
Loss of Colstrip Unit 4
Loss of Generation Project
Results
The complete set of power flow results can be viewed in Error! Reference source not found.. The
following is a summary of the results.
N-0 – System Normal
The study indicated that with all facilities in service, there were no N-0 voltage stability issues caused by
the Generation Project.
N-1 – Single Element Contingency
The study indicated that with all facilities in service, there were no N-1 voltage stability issues caused by
the Generation Project.
N-2 – Multiple Element Contingency
The study indicated that with all facilities in service, there were no N-2 voltage stability issues caused by
the Generation Project.
Mitigation
No mitigation required for voltage stability issues.
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Short Circuit
The short circuit analysis examined the increase of fault current contribution into the system from the
Generation Project. The fault currents were then analyzed to determine whether or not the interrupt
ratings of the existing circuit breakers on NWE’s system were exceeded.
Results
Table 2 below shows the results of the study on buses near the Generation Project.
Table 2: Short Circuit Fault Currents
Bus
Colstrip 500 kV
Colstrip 230 kV
Colstrip 115 kV
Colstrip 69 kV
Minimum
Interrupt
Rating (Amps)
40,000
40,000
40,000
40,000
Pre Project
3-ph (Amps)
Post Project
3-ph (Amps)
Pre Project
L-G (Amps)
Post Project
L-G (Amps)
11,660
18,216
9,490
3,726
12,180
20,390
9,671
3,731
14,770
23,899
10,020
3,772
15,191
26,504
10,106
3,767
The analysis shows that that the fault currents are significantly less than the minimum interrupt ratings
at the nearby substations and that the Customer may interconnect their project without any need for
any circuit breaker replacements.
Mitigation
There is no mitigation required for short circuit issues. If there are any changes to the Generation
Project that would significantly alter the fault duty contribution, a re-study will be needed and
mitigation may be required at that time.
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Cost Estimates
Table 3 below summarizes the high-level, non-binding good faith estimates required to interconnect the
Generation Project.
Table 3: High-Level, Non-Binding Good Faith Estimate of Costs to Interconnect
Department
Description
No new land is needed, project will connect with existing 230 kV
Substation. New transmission or distribution build outs are needed.
The Customer will construct the line to our site. New permits are
Lands
needed based on the use of existing facilities and a disturbance of dirt
that is less than one acre in size. No zoning or building permits are
required for changes within our existing substation fence.
Customer is providing OPGW on the transmission line to the collector
substation and NWE is responsible only for termination inside the
NWE substation.
Communications Customer is providing all telecommunications equipment at the
collector substation.
Customer is providing the required geographically diverse
communications path.
Transmission
EMS
Relay
Substation
Metering
Estimate
$500
$45,600
Customer is building the 230 kV line from the collector bus to the
substation.
$0
The Generation Project will need to be added to the EMS SCADA
system, databases, and displays. Additional one-time licensing fee
from the EMS software vendor is also required.
$16,900
Standard line relaying will be installed to clear faults on the line
heading to the Generation Project. Breaker failure protection will also
be installed.
$114,200
An upgrade to the existing 230 kV Substation will be needed to
connect the Generation Project. Upgrade may include Power Circuit
Breakers, dead-end towers, etc.
$1,173,450
A standard billing revenue class watt-hour meter will be required.
$14,400
Total $1,365,050
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Conclusion
The results of this study indicate that the Customer can interconnect the Generation Project to the
system without any system upgrades outside of the POI. This allows interconnection only, and does not
provide for Firm or Non-Firm transmission service. There were new overloads observed that were
caused by the addition of the Generation Project’s additional output to the system under certain system
dispatch scenarios, but the Customer is not responsible for mitigating network transmission system
problems. The high-level, non-binding good faith estimate of the upgrades or construction necessary to
interconnect the Generation Project is roughly $1.36 million.
This study is for an Energy Resource product and does not constitute a request for Transmission Service
or imply transmission rights. In order for the Customer to move power on a firm, or non-firm basis
through the system, the Customer will need to secure those rights through the Transmission Service
Request process. The Customer must follow the procedures described in the NWE Open Access
Transmission Tariff (OATT) to request and/or receive Transmission Service, available on the following
website: http://www.oatioasis.com/NWMT/index.html
Next Steps
NWE will schedule a meeting to discuss the findings of the SIS with the Customer. Should the Customer
wish to move on the GIA process, a Facility Study specific to the Project will then be carried out to
determine the final details and requirements of the interconnection.
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