20180208PAC to Staff Attachment Bridger D.2 SSR.pdf

Gateway West D.2
Project SSR/SSCI Study
Final Report
Preparedfor:PacifiCorpand Power Engineers
Prepared by:Robert D'Aquila
General Electric International,Inc.
Energy Consulting Group
October 24,2017
Gateway West D.2 Project SSR/SSCI Study
Forward
This project evaluates subsynchronous resonance (SSR)and subsynchronous control interaction (SSCI)
issues associated with the Gateway West D.2 project.This report was prepared by General Electric
International,Inc.(GEII),acting through its Energy Consulting group,based in Schenectady,New York.
Questions and any correspondence concerning this document should be referred to:
Robert D'Aquila
1 River Road
Building 53,Room 302H
Schenectady,New York 12345
Phone:[phone]
E-mail:[e-mail]
Copyright 2017 General Electric Company,U.S.A
Gateway West D.2 Project SSR/SSCI Study
Legal Notices
This report was prepared by General Electric International,Inc.as an account of work sponsored by Power
Engineers.General Electric International,Inc.,nor any person acting on their behalf:
1)Makes any warranty or representation,expressed or implied,with respect to the use of any
information contained in this report,or that the use of any information,apparatus,method,or
process disclosed in the report may not infringe privately owned rights.
2)Assumes any liabilities with respect to the use of or for damage resulting from the use of any
information,apparatus,method,or process disclosed in this report.
GE Energy Consulting i
Gateway West D.2 Project SSR/SSC1 Study
CONTENTS
1 EXECUT I VESUMM A RY ................................................................................1-1
2 PROJECTOVERV I EW ...............................................................................2 -1
2.1 Transmission System and Series Capacitor Operation........................................................................2-2
3 BRI DGERSS R..................................................................................................................3 -1
3.1 Br i dgerTors i on a I Modes....................................................................................................................3 -1
3.2 Tra nsm i ss i onCond it i ons ..............................................................................................................................3 -1
3.3 Benchma rk i ng A g a i nstPerv i ousBr i d ge rSSR A n a lys i s .............................................................................3 -3
3.4 SSRSt a b i Ii tyw i thG atew a yD .2 .....................................................................................................................3 -5
3.5 Benchmark System Limiting Conditions and SSR Margin......................................................................3-10
3.6 Possible Modifications to RCSC Switching Scheme.................................................................................3-13
4 SSCI SCREEN I NG .................................................................................................................4-1
4.1 SSCI forG a tew a yD .2..................................................................................................4-1
4.2 SSCI with Full Gateway Project...................................................................................4-5
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,,Gateway West D.2 Project EXECUTlVESUMMARYSSR/SSci Study
1 EXECUTIVE SUMMARY
Pacificorp is planning the developmentof Gateway West Segment D.2 by 2020.This project
includes a new 500 kV line from Aeolus to Anticline and a new 345 kV line between Bridger
and Anticline.The project is intended to deliver wind generation in Eastern Wyomingtowards
the west.Power will flow west from Aeolus to Bridger,and across the Bridger 345 kV lines.
The three 345 kV transmission lines from the Jim Bridger Plant west to Populus and to
Threemile Knoll are series compensated.The series capacitors are needed to maintain
stabilityof the Jim Bridger units,particularlywhen the plant is operating at high power output.
However,the series capacitors cause subsynchronousresonance (SSR)issues for the Bridger
units.
The SSR risk is managed using protection and mitigation equipment at the plant as well as an
automatic series capacitor switching scheme.The switching scheme inserts the series
capacitors based on flow on individual 345 kV lines,outputof individual Bridger units and total
plant output.This scheme is successful in large part because the flow on the Bridger 345 kV
lines is very closely tied to plant output.
AII other things being equal,a large steam turbine generator is least susceptible to SSR when
operating at high load.This is due to the high mechanical torsional damping caused by the
steam flow.When the Bridger units are operating at high load (i.e.Iowest SSR e×posure),the
flow on the 345 kV lines to the west is high and the series capacitors are needed for system
stability.The series capacitors can be inserted to maintain system stability without significant
SSR risk.
As unit load decreases,mechanical torsional damping reducers and the units become more
susceptible to SSR.With the low line flow,the series capacitors are not needed to maintain
stability.The series capacitors are bypassed to reduce SSR risk without a significant impact
on system stability.
Althoughthe Gateway D.2 project does not include series compensation,it will affect the SSR
condition at Bridger,both directly and indirectly.The new transmission line will change the
impedance seen by the Bridge generation.This will shift the e×isting resonances.Additionally,
the flow on the Bridger 345 kV line will not be tied directlyto Bridger plant output.The lines
could have high flow (and require series capacitors for stability)even when the Bridger units
are at low load and most susceptible to SSR.
The primary objective of this study is to assess the SSR stability analysis of the Bridger units
with Gateway D.2,review of the current series capacitor bypass scheme and determine if the
scheme can or should be changed.
Results show that Gateway D.2 improves the SSR condition for the Bridger units.With the
improved SSR,the following changes to the Remotely Controlled Series capacitor (RCSC)logic
may allow for more fle×ible operation of the series capacitors without significantlyincreasing
risk to the Bridger units.
The current Locally Controlled Series capacitor (LCSC)logic should remain unchanged.
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,,Gateway West D.2 Project EXECUTlVESUMMARYSSR/SSci Study
·Retain the existing 135-185MW Iimit on individual unit generation.
With respect to the total plant MW Iimit,there are two options to consider:
a.Higher SSR Risk Option:Remove the existing total plant MW Iimit.This will
increase SSR risk a little above the current risk.With the higher SSR risk,the
TSR line/au×iliarytrip function should be enabled.This function is used to trip
the source of SSR (RCSC in this case)when torsional oscillations are detected.
This reduces the chances of a generator trip.
b.Lower SSR Risk Option:Lower the e×isting 1325-1475 MW plant limit to about
650-800 MW.This option will require at least two units to be operating before
the RCSC is inserted.Enabling the au× trip function would not be critical,but
would reduce risk of generator trips.
Add minimum line flow logic to the RCSC banks (if not already part of the RCSC logic).
This will only allow the RCSC banks to be switched in when flow is above a minimum
level (possibly the same MW Ievel and same contacts as LCSC MW flow contacts).This
logic will help to bypass the RCSC upon loss of the Bridger-Anticline line with low
Bridger generation.
There is existing logic that will bypass the RCSC if the SEDC on any on-line unit is out of service.
This logic was disabled for Unit 2 after the blocking filter was installed.With the above
changes,this logic should be enabled for all units.
The Gateway D.2 project will also introduce the risk of subsynchronouscontrol interaction
(SSCl).SSCI is a condition where an electrical system resonance caused by series capacitors
can cause wind turbine controlsto be unstable.Wind plants connected directlyto Aeolus 230
kV will have some SSCI risk with the D.2 project,but the risk is low.However,as the Gateway
project e×pands to include additional 500 kV lines and series capacitors,the SSCI risk is high.
Wind plant owners and developers of future plants in the area should be made aware of the
future SSCI risk in order to select appropriate wind turbinesand controls.
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,,Gateway West D.2 Project Project Overview'SSR/SSCI study
2 PROJECT OVERVIEW
Pacificorp is planning the developmentof Gateway West Segment D.2 by 2020.This
project includes a new 500 kV line from Aeolus to Anticline and a new 345 kV line
between Bridger and Anticline.The first phase of this project does not include series
compensation.
The project is intended to deliver wind generation in Eastern Wyoming towardsthe west.
Power will flow west from Aeolus to Bridger,and across the Bridger 345 kV lines.
The three 345 kV transmission lines from the Jim Bridger Plant west to Populus and to
Threemile Knoll are series compensated.The series capacitors are needed to maintain
stability of the Jim Bridger units,particularly when the plant is operating at high power
output.However,the series capacitors cause subsynchronousresonance (SSR)issues
for the Bridger units.
The SSR risk is managed using protectionand mitigation equipmentat the plant as well
as an automatic series capacitor switching scheme.The switching scheme inserts the
series capacitors based on flow on individual 345 kV lines,outputof individual Bridger
units and total plant output.This scheme is successful in large part because the flow on
the Bridger 345 kV lines is very closely tied to plant output.
AII other things being equal,a large steam turbine generator is least susceptible to SSR
when operating at high load.This is due to the high mechanical torsional damping
caused by the steam flow.When the Bridger units are operating at high load (i.e.Iowest
SSR e×posure),the flow on the 345 kV lines to the west is high and the series capacitors
are needed for system stability.The series capacitors are inserted to maintain system
stability without significantSSR risk.
As unit load decreases,mechanical torsional damping reducers and the units become
more susceptible to SSR.With the low line flow,the series capacitors are not needed to
maintain stability.The series capacitors are bypassed to reduce SSR risk without a
significantimpact on system stability.
Althoughthe Gateway D.2 project does not include series compensation,it will affect the
SSR condition at Bridger,both directly and indirectly.The new transmission line will
change the impedance seen by the Bridge generation.This will shift the e×isting
resonances.Additionally,the flow on the Bridger 345 kV line will not be tied directly to
Bridger plant output.The lines could have high flow (and require series capacitors for
stability)even when the Bridger units are at low load and most susceptible to SSR.
The primary objective of this study is to assess the SSR stability analysis of the Bridger
units with Gateway D.2,review of the current series capacitor bypass scheme and
determine if the scheme can or should be changed.
The Gateway D.2 project may also introduce the risk of subsynchronous control
interaction (SSCl).SSCI is a condition where an electrical system resonance caused by
series capacitors can cause wind turbine controls to be unstable.This study includes a
screening for possible SSCI for wind plants near the Aeolus with the new project.SSCI
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,,Gateway West D.2 Project Project Overview'SSR/SSci Study
screening is presented for the D.2 system as well as for the currently-planned full
Gateway system.
2.1 Transmission System and Series capacitor Operation
Figure 2-1 is a simplified one-line diagram of the current transmission system west of
Bridger (without D.2).This diagram shows the three Bridger 345 kV line with their series
capacitors.
The series capacitors have been sized and segmented based on SSR constraints of the
Bridger units.The segments are inserted and bypassed based on transmission line flow
and Bridger unit operation.The locally controlled series capacitors (LCSC)are inserted
first,based on line flow (measured at the series capacitor,hence "locally controlled").
The remotelycontrolled series capacitors (RCSC)are switched based individual unit and
total Bridger plant generation.They will be inserted after the LCSC are inserted.
The following is a description of the series capacitor switching logic,per the PacifiCorp
document "East &West of Populus Series Compensation Rev 5.doc×".
East of Populus Series Compensation:-
The series compensationin the Populusto Bridger #1 &#2 and Three Mile Knoll to Bridger 345 kV
lines (#4,#5 &#6)are each split into two segments.The 'L'segment in series capacitors 4,5,6 (see
Fig.2)is switched or bypassed by local control and segment 'R'in series capacitors 4,5,6 (See Fig.
2)is switched by remote control signal based on Bridger generation levels.For Populus to Bridger
#1 &#2 345 kV lines,respective locally controlled series capacitor is bypassed if the line flow is
less than or equal to 260 MW and is inserted if the line flow is more than or equal to 350 MW.For
3 Mile Knoll to Bridger 345 kV line,the respective locally controlled series compensation is
bypassed if the line flow is 266 MW or less and is inserted if the line flow is 350 MW or more.
The remotely controlled series compensation segment is switched based upon the Jim Bridger's net
generation.It is bypassed if the total net Jim Bridger generation is 1325 MW or less and inserted if
the 1475MW or more.If any of the Bridger unit is loaded less than 135 MW,the remote series
capacitors are also bypassed.If only three Jim Bridger units are online and total net generation is
1475 MW or more,the remote series capacitor is inserted.If 4th UniÍ iS brought online,the remote
series capacitors are bypassed until the 4th Unit iS loadedup to 185 MW.
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,,Gateway West D.2 Project Project Overview'SSR/SSCI Study
POPULUS PROJECT SERIES CAPACITOR
CONFIGURATION
Goshen
3 MileKnoll
Kinport 28.5 Ohms 28.5 Ohms
15.9 Ohms L R
3Borah Bridger
5.Ohms POpUIUS
13.96
O1h
s Value)/
#2
20 s O
#1
L R Remotely Controlled
#2
20.59 25.74
#1 Ohms Ohms
LEGEND:-
WEST OF ExistingPOPULUS
New
Normal/30Minute Emergency Rating
15.9 Ohms Borah -Populus 345kV Line
1 13.96 Ohms (New #1 Series Compensation @ BorahValue)
Ben Lomond 2 15.9 Ohms Borah -Populus 345kV Line
#2 Series Compensation @ Borah
3 15.9 Ohms Kinport -Populus 345kV Line
Series Compensation @ Kinport
4 20.59 Ohms Populus -Bridger 345kV Line #2
25.74 Ohms Series Compensation @ Populus
Terminal
5 20.59 Ohms Populus -Bridger 345kV Line
25.74 Ohms #1 Series Compensation @ Populus
6 28.5 Ohms 3 Mile Knoll -Bridger 345kV Line
28.5 Ohms Series Compensation @ 3 Mile Knoll
Figure 2-1 System and Series capacitors West of Bridger (without Gateway D.2)
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,,Gateway West D.2 Project Bridger SSR'SSR/SSci Study
3 BRlDGER SSR
3.1 Bridger Torsional Modes
Measured torsionalfrequencies for the Bridger are listed in Table 3-1.Units 3 and 4 have
full GE turbines (HP/IP,LPA,LPB and generator rotor).The original GE HP/IP turbine on
Unit 1 has been replaced with a Hitachi HP/IP turbine.This changed the torsional
frequencies slightlyfrom Units 3 &4.
In 2013,the original GE HP/IP,LPA and LPB turbines on Unit 2 were replaced with Hitachi
turbines.This caused a larger shift in torsional frequencies.Without mitigation,Unit 2
with the new Hitachi turbineswould have SSR issues.An SSR blocking filter was installed
on the unit at the time of turbine change.
Table 3-1 Jim Bridger Units Torsional Modes
Mode Unit 1(H-HP/IP,Unit 2 (H)*Unit 3 (GE)Unit 4 (GE)
GE LPA &LPB)
1 18.87 18.09 19.11 19.04
2 32.28 29.91 32.51 32.49
3 43.10 36.06 43.28 43.28
*Unit 2 has Hitachi HP/IP,LPA and LPB turbines and an SSR blocking filter
Given these torsional frequencies,Units 1,3 &4 assumed identical from an SSR
standpoint.The following generation conditions were studied:
SSR analysis of Units 1,3 &4:
With one,two and three units on line
With and without Unit 2 on line
SSR analysis of Unit 2:
With none,one,two and three other Bridger units on line
In total,10 different generation/SSR conditions were evaluated.
3.2 Transmission Conditions
SSR analysis was performed for wide range of system conditions,as listed in Table 3-2.
Each of these conditions were evaluatedwith the RCSC inserted and bypassed and with
the D.2 project in and out of service.They were studied for the ten different generation
conditions listed above.
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,,Gateway West D.2 Project Bridger SSR'SSR/SSClstudy
Table 3-2 Systerr Conditions studied for Bridger SSR
Borah StKinportline/caps
Pop 1,Pop 2,3MK Line/cap (Borah 1,Borah 2,Kinport)Lines South (Ben Lmn
status(1=LCSC+RCSC,2=(1=Scinserted,2=SC :L&2,Terminal,1=line
Case LCSC Only,O =line tripped)bypassed,O =line tripped)in,O =line tripped)
CO1 L111 X111 S111
CO2 L111 X211 S111
CO3 L111 X221 S111
CO4 L111 X222 S111
CO5 L111 X011 S111
CO6 L111 X021 S111
CO7 L111 X111 SO11
CO8 L111 X111 SO10
C11 L110 X111 S111
C12 L110 X211 S111
C13 L110 X221 S111
C14 L110 X222 S111
C15 L110 X011 S111
C16 L110 X021 S111
C17 L110 X111 SO11
C18 L110 X111 SO10
C21 L112 X111 S111
C22 L112 X211 S111
C23 L112 X221 S111
C24 L112 X222 S111
C25 L112 X011 S111
C26 L112 X021 S111
C27 L112 X111 SO11
C28 L112 X111 SO10
C31 LO11 X111 S111
C32 LO11 X211 S111
C33 LO11 X221 S111
C34 LO11 X222 S111
C35 LO11 X011 S111
C36 LO11 X021 S111
C37 LO11 X111 SO11
C38 LO11 X111 SO10
C41 L211 X111 S111
C42 L211 X211 S111
C43 L211 X221 S111
C44 L211 X222 S111
C45 L211 X011 S111
C46 L211 X021 S111
C47 L211 X111 SO11
C48 L211 X111 SO10
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,,Gateway West D.2 Project Bridger SSR'SSR/SSCI Study
3.3 BenchmarkingAgainst Pervious Bridger SSR Analysis
The last Bridger SSR analysis performed by GE was in 2012,associates with the Populus
project and the Unit 2 turbine replacement/SSR blocking filter project.Figure 3-1shows
results from the 2012 analysis.This plot shows Bridger damping torque with Units 1,3
and 4 on line and the RCSC inserted.
Figure 3-2 shows SSR damping for the same system/Bridgerconditions using the new
model supplied by PacifiCorp for this project.However,the line from Bridger to Anticline
is open,effectivelyremoving the D.2 project.Results are very close to the 2012 analysis.
This new model with the Bridger-Anticline line open can serve as a benchmarkto gauge
the impact of the D.2 project.
Note that there is a very slight upward shift in the damping torque dips for the current
system model.The dips are closer to the 2nd ÍOFSÍOOO TOde and reduce the SSR stability
margin by a very slight amount.This is likely due to a small change in series capacitor
ohms on the banks west of Bridger.
These plots show the Bridger torsional modes as I bars.The top of the I bar represent
no-load mechanical damping and the bottom represents full load damping.The
mechanical damping is based on measurements without SEDC (supplemental e×citation
damping controller).The Bridger SEDC increase mechanical damping by roughly a
factor of two for Units 1,3 &4.The increase in damping for Unit 2 with the Hitachi
turbinesand SSR damping filter even greater (over 3×).The increased damping from the
SEDC is considered margin and is not included in SSR stability studies.
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,,Gateway West D.2 Project Bridger SSRO'SSR/SSCI study
GE Units -3 Units on line,RCSC Inserted Benchmark w/o Filter
2
1
O
-4
-5
-6
10 15 20 25 30 35 40 45 50
Rotor Frequency (Hz)
Figure 3-1 Results from 2012 SSR Study
Current System w/o D2,RCSC In service
1
-4
-5
10 15 20 25 30 35 40 45 50
Rotor Frequency (Hz)
Figure 3-2 Results with Current Model,Bridger-Anticline Line Out of Service
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,,Gateway West D.2 Project Bridger SSR'SSR/SSCI Study
3.4 SSR Stabilitywith GatewayD.2
This section shows the impact of the D.2 project on Bridger SSR stability using SSR
damping torque plots.The plots are grouped as follows:
Figure 3-3 Units 1,3&4 with RCSC inserted
Figure 3-4 Unit 2 with RCSC inserted
Figure 3-5 Units 1,3&4 with RCSC bypassed
Figure 3-6 Unit 2 with RCSC bypassed
The figures show SSR for the benchmarksystem in the top plot and for the D.2 system
in the bottom plot.The I bars for each torsional mode span from no-load to full-load
mechanical damping.An additional point is shown on the I bar that represents an
estimate of mechanical damping at 25%Ioading,roughly the cut off point for
insertion/bypassof the RCSC.
Figure 3-3 and Figure 3-4 show a lot of unstable SSR cases with the RCSC inserted,both
for the benchmarksystem and the D.2 system.AII the unstable conditions are mitigated
by RCSC switching scheme.This is evident in the plots with RCSC bypassed.
The Gateway D.2 project improves Bridger SSR for all study conditions.There are still
unstable SSR conditions with the RCSC inserted butthe electrical damping is higher than
for the benchmark system.Even with the D.2 project,RCSC switching logic will be
needed.However,the D.2 project may provide an opportunityto relax some of the RCSC
constraints.
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,,Gateway West D.2 Project Bridger SSR'SSR/SSCI study
Current System w/o D2,RCSC In service
-4
-5
10 15 20 25 30 35 40 45 50
Rotor Frequency (Hz)
D2 System,RCSC in service
1
0
4
5
10 15 20 25 30 35 40 45 50
Rotor Frequency (Hz)
Figure 3-3 Units 1,3 &4 SSR,RCSC Inserted,Benchmark (top)and D.2 (bottom)systems
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,,Gateway West D.2 Project Bridger SSR'SSR/SSCI study
Unit 2,w/o D2,RCSC Inserted
1
E4
5
6
7
10 15 20 25 30 35 40 45 50
Frequency (Hz)
Unit 2,D2 in service,RSCS in service
E4
5
6
10 15 20 25 30 35 40 45 50
Frequency (Hz)
Figure 3-4 Unit 2 SSR,RCSC Inserted,Benchmark (top)and D.2 (bottom)Systems
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Current System w/o D2,RCSC o/s
1
0
4
10 15 20 25 30 35 40 45 50
Rotor Frequency (Hz)
D2 System with RCSC o/s
03
4
5
10 15 20 25 30 35 40 45 50
Frequency (Hz)
Figure 3-5 Units 1,3 &4 SSR,RCSC Bypassed,Benchmark (top)and D.2 (bottom)systems
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,,Gateway West D.2 Project Bridger SSR'SSR/SSCI study
Unit 2,w/o D2,RCSC Bypassed
1
'
2
E4
5
6
7
10 15 20 25 30 35 40 45 50
Rotor Frequency (Hz)
Unit 2,D2 In,RCSC Bypassed
2
1
E4
5
6
7
8
10 15 20 25 30 35 40 45 50
Rotor frequency (Hz)
Figure 3-6 Unit 2 SSR,RCSC Bypassed,Benchmark (top)and D.2 (bottom)Systems
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,,Gateway West D.2 Project Bridger SSR'SSR/SSCI Study
3.5 Benchmark System LimitingConditions and SSR Margin
For the benchmarksystem with the existing RCSC scheme,the banks would only be in
service with three or four Bridger units on line.The minimum load per individual unit
(above 185 MW to insert,below 135 MW to bypass)and total plant load limit (above 1475
MW to insert,below 1325 MW to bypass)ensure all conditions are stable.Limiting
conditions are shown in the figures below.The conditions shown are
Three units on line,2 at full load (~525MW),one at ~275 MW,1325 MW total
generation,RCSC inserted:Damping torque plots with three units on line and the RCSC
inserted are shown in Figure 3-7.The top plot shows SSR for the units with GE turbines
(1,3 &4)and the bottom plot shows SSR for Units 2 with the SSR blocking filter.Damping
for the full set of contingencies is shown.
The red bo×es on the mode 2 I bar appro×imate the mechanical damping at 275 MW.
AII cases are SSR stable.There are some system conditions that show very little margin,
but they are stable.These plots do not account for the added margin of the SEDC and
the shared damping between units (the unit at 275 MW will appear to have higher
damping,due to the higher damping of the units operating at full load),so the actual
margin is higher.
Four units on line,3 at 400 MW,one at 150 MW:Figure 3-8 shows damping torque
plots for all conditions with four Bridger units on line.The red boxes appro×imate the
mechanical damping 150 MW.AII cases are SSR stable with margin.
These two e×amples serve as a reference to the current SSR stability margin.
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,,Gateway West D.2 Project Bridger SSR'SSR/SSCI study
Current System w/o D2,RCSC In service,3 units total
1
4
5
10 15 20 25 30 35 40 45 50
Rotor Frequency (Hz)
Unit 2,w/o D2,RCSC Inserted -Unit 2 with 2 other units on line (3 units total)
CLE4
6
7
10 15 20 25 30 35 40 45 50
Frequency (Hz)
Figure 3-7 Benchmark System (w/o D.2)with Three Bridger Units On line.SSR for Untis 1,
3&4 (top)and Unit 2 (bottom)
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Current System w/o D2,RCSC In service,4 units total
1
E
4
10 15 20 25 30 35 40 45 50
Rotor Frequency (Hz)
Unit 2,w/o D2,RCSC Inserted -Unit 2 with 3 other units on line (4 units total)
1
0
E4
O
5
6
7
10 15 20 25 30 35 40 45 50
Frequency (Hz)
Figure 3-8 Benchmark System (w/o D.2)with Four Bridger Units On line.SSR for Untis 1,
3&4 (top)and Unit 2 (bottom)
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3.6 Possible Modifications to RCSC SwitchingScheme
Given that the Gateway D.2 project will improve the Bridger SSR condition,there may
opportunityto rela×some of the RCSC constraints.The plots below show operation with
1,2,3 &4 units on line,all with RCSC inserted.The appro×imate mechanical damping at
150 MW is indicated by a black dash on the |Bar.The minimum load level for mode 2
SSR stability is indicate with a red bo×.Each set of plots is described below.
Figure 3-9 shows SSR damping with one unit on line (Ul,3 or 4 on top plot,U2 on bottom
plot).Ul,3 &4 would be stable at the minimum RCSC load leve.U2 would only be stable
at loading above about 300 to 400 MW.The limiting conditions for U2 are loss of one of
the Bridger-west 345 kV lines.Under normal transmission conditions U2 would be stable
at the minimum RCSC load level.
Figure 3-10 shows SSR damping with two units on line (Ul,3 &4 on top plot,U2 on
bottom plot).The units would be SSR stable at roughly 200 MW generation,just above
the current RCSC level.
Figure 3-11 shows SSR damping with three units on line (Ul,3 &4 on top plot,U2 on
bottom plot).Figure 3-12 shows SSR damping with four units on line (Ul,3 &4 on top
plot,U2 on bottom plot).With three or four units on line,the units would be SSR stable
with all operating at or above the current RCSC level.
The limiting conditions for SSR tend to be loss of one of the Bridger-west 345 kV lines or
bypassing of the west of Populus series capacitors banks.Figure 3-13 shows SSR
damping for the normal system condition (n-0)for all combinationsof units on line (1,2,
3 &4 units on line).Under normal transmission conditions,the units would be SSR stable
when operating above the RCSC limit.
With the improved SSR condition,the following changes to the RCSC logic may allow for
more flexible operation of the series capacitors without significantly increasing risk to
the Bridger units.Note that the current LCSC logic is assumed to remain unchanged.
Retain the existing 135-185MW Iimit on individual unit generation.
With respect to the total plant MW Iimit,there are two options to consider:
a.Higher SSR Risk Option:Remove the e×isting total plant MW Iimit.This
will increase SSR risk a little above the current risk.With the higher SSR
risk,the TSR line/auxiliarytrip function should be enabled.This function
is used to trip the source of SSR (RCSC in this case)when torsional
oscillations are detected.This reduces the chances of a generatortrip.
b.Lower SSR Risk Option:Lower the e×isting 1325-1475 MW plant limit to
about 650-800 MW.This option will require at least two units to be
operating before the RCSC is inserted.Enabling the au× trip function
would not be critical,but would reduce risk of generatortrips.
Add minimum line flow logic to the RCSC banks (if not already part of the RCSC
logic).This will only allow the RCSC banks to be switched in when flow is above
a minimum level (possibly the same MW Ievel and same contacts as LCSC MW
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Gateway West D.2 Project Bridger SSRSSR/SSCI Study
flow contacts).This logic will help to bypass the RCSC upon loss of the Bridger-
Anticline line with low Bridger generation.
There is e×isting logic that will bypass the RCSC if the SEDC on any on-line unit is out
of service.This logic was disabled for Unit 2 after the blocking filter was installed.
With the above changes,this logic should be enabled for all units.
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,,Gateway West D.2 Project Bridger SSR'SSR/SSCI Study
D2 System,RCSC in service,1 unit on line (U1,3 or 4)
1
0
1
4
10 15 20 25 30 35 40 45 50
Rotor Frequency (Hz)
Unit 2,D2 in service,RSCS in service with no other units on line
E4
10 15 20 25 30 35 40 45 50
Frequency (Hz)
Figure 3-9 D.2 System with One Bridger Unit On line.SSR for Untis 1,3&4 (top)and
Unit 2 (bottom)
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,,Gateway West D.2 Project Bridger SSRO'SSR/SSCI study
D2 System,RCSC in service,2 units on line
1
4
5
10 15 20 25 30 35 40 45 50
Rotor Frequency (Hz)
Unit 2,D2 in service,RSCS in service with 10ther unit on line
1
E4
6
7
10 15 20 25 30 35 40 45 50
Frequency (Hz)
Figure 3-10 D.2 System with Two Bridger Unit On line.SSR for Untis 1,3&4 (top)and
Unit 2 (bottom)
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,,Gateway West D.2 Project Bridger SSR'SSR/SSCI study
D2 System,RCSC in service,3 units on line
01
03
4
5
10 15 20 25 30 35 40 45 50
Rotor Frequency (Hz)
Unit 2,D2 in service,RSCS in service with 2 other units on line
1
E4
5
6
7
10 15 20 25 30 35 40 45 50
Frequency (Hz)
Figure 3-11 D.2 System with Three Bridger Unit On line.SSR for Untis 1,3&4 (top)and
Unit 2 (bottom)
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,,Gateway West D.2 Project Bridger SSRO'SSR/SSCI Study
D2 System,RCSC in service,4 units on line
4
5
10 15 20 25 30 35 40 45 50
Rotor Frequency (Hz)
Unit 2,D2 in service,RSCS in service with 3 other units on line
1
7
10 15 20 25 30 35 40 45 50
Frequency (Hz)
Figure 3-12 D.2 System with Four Bridger Unit On line.SSR for Untis 1,3&4 (top)and
Unit 2 (bottom)
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,,Gateway West D.2 Project Bridger SSR'SSR/SSCI StudyO
D2 System,RCSC in service,All Generation Conditions,Base System
4
5
10 15 20 25 30 35 40 45 50
Rotor Frequency (Hz)
Unit 2,D2 in service,RSCS in service,All Generating conditions,Base System
1
0
E4
6
7
10 15 20 25 30 35 40 45 50
Frequency (Hz)
Figure 3-13 D.2 System with 1,2,3&4 Bridger Units On line,Base System (n-0)SSR for
Untis 1,3&4 (top)and Unit 2 (bottom)
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,,Gateway West D.2 Project SSCl Screening'SSR/SSci study
4 SSCI SCREENING
SubsynchronousControl Interaction (SSci)is a condition where an electrical system
resonance caused by series capacitors can cause wind turbine controlsto be unstable.
SSCI screening is performed using impedance scan techniques.This typically involves
calculatingthe impedance looking into the grid from the he wind plant interconnection
transformer (POl bus).The impedance of the wind plant (including POl transformer,
collector system and effective impedance of the wind turbinesand controls)is added in
series to the system impedance.
The impedance is calculated over a range of frequency,typically from 5 to 55 Hz.The
scans are performed for a range of system conditions,including different synchronous
generation dispatches and transmission outages.The impedance scans are evaluated
for unstable or lightlydamped electrical system resonances which flag possible unstable
SSCI conditions.These conditions would be reviewed with the wind turbine vendor and
possibly require electro-magnetictransient (e.g.PSCAD,ATP or EMTP)analysis.
A brief SSCI screening analysis has been performed for the D.2 system and for the full
Gateway system.The analysis only considers the system impedance,not the wind plant
impedance.Results are presented in the following sections
4.1 SSCI for GatewayD.2
Since the Gateway West line will not be series compensated in the D.2 phase,any
electrical system resonances and possible SSCI conditions would be caused by the series
capacitors on the 345 kV lines west of Bridger.Scans were first run looking into the
Aeolus 230 kV bus with all 230 kV lines disconnected.This puts the Aeolus 230 kV bus
radial to Bridger and gives a screen of electrical resonances that wind plants could be
e×posed to.The wind plants would see the impedance from the collector system to
Aeolus 230 kV and the effective impedance of the wind turbine and it controlsadded to
the Aeolus impedance.
The analysis was performed with none,one,two,three and four units on line.Since
Bridger 2 has an SSR blocking filter,analysis with one unit was performed with U1 only
and with U2 only.Figure 4-1shows a plot of the resistance and reactance looking into
Aeolus 230 kV for each generation scenario.
The resonance caused by the series capacitors is seen as reversals in reactance and
sharp spikes in resistance.Concerning conditions for SSCI are when the reactance dips
from positive to negative and back to positive.The frequencywhere reactance crosses
from negative to positive is the resonant frequency.
The resistance at that point gives an indication of the stability of the interaction.A low
or negative resistance raises a flag for possible instabilities.While the system resistance
is positive for all test conditions,it is possible that the wind plant can add negative
effective resistance,due to the wind turbine and controls.Therefore,this analysis
considers any condition with a zero-crossing reactance a possible SSCI risk.
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,,Gateway West D.2 Project SSCl Screening'SSR/SSCI Study
In this case,there is one study condition with a possible SSCI risk.When all Bridger units
are off line there is a resonance at 25 Hz (orange reactance trace crosses from negative
to positive).
The supplied data set shows a wind plant at Q0706_POI 230 kv,connected radially to
the Aeolus 230 kv).This plant would have the lease numberout outages to make it radial
to Aeolus 230 kV vs.Other wind plants in Eastern Wyoming.Impedance scans were
performed looking into the POl bus for the base (n-0)condition and three outages:
Aeolus-Freezout line open,Aeolus-Shirly lines open and both set of lines out making the
plant radial to Aeolus.Analysis was performed with no Bridger units on line.Plots of
resistance and reactance are shown in Figure 4-2.
The radial condition (green reactance trace)has an impedance similar to that looking
into Aeolus 230 kv,but with the added impedance of the line from Q0706 to Aeolus.This
shifts the resonance to 24 Hz and raises the reactance to the point where is just slightly
positive over the entire frequencyrange.While technicallynot an SSCI risk,SSCI should
still be considered for the Q0706 plant.
Other plants further from Aeolus would likely show even lower SSCI risk,due to the higher
impedance and more intermediate lines between those plants and Aeolus.
This analysis shows that with the Gateway D.2 project,wind plants connecting near
Aeolus have low (but not zero)SSCI risk.The wind plants would have to be radial to
Aeolus and all Bridger generation would have to be off line to be at risk.New and existing
plant owners should be made aware of this risk.
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Gateway West D.2 Project .SSCl ScreeningSSR/SSCI study
0.06
0.05 -No BridgerUnitsR
-1Bridger(1,3or4)R
-2 Bridger Units R
0.04
-3 Bridger Units R
-4 Bridger Units R
-1BridgerUnit(U2)R
c 0.03
0.02
0.01
0
5 10 15 20 25 30 35 40 45 50 55
Electricial Frequency (hz)
0.08
0.07 -NoBridgerUnitsX
1Bridger(1,30r4)X
0.06
-2 Bridger Units X
-3 Bridger Units X
-4 Bridger Units X
0.05 -1 Bridser unit (u2)x
0.04
0.03
0.02
0.01
0
5 10 15 20 25 30 35 40 45 50 55
-0.01 Electricial Frequency (hz)
Figure 4-1 R (top)and X (bottom)Looking into Aolus 230 kV Bus with 0,1,2,3 &4 Bridger
Units on Line (230 kV lines from Aolus disconnected)
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Gateway West D.2 Project .SSCl ScreeningSSR/SSCI study
0.06
Base(n-0)R
0.05
-Radial to Bridger R
-Trip Freezout Lines R
-Trip Shirly Lines R
0.04
c 0.03
0.02
0.01
0
5 10 15 20 25 30 35 40 45 50 55
Electricial Frequency (hz)
0.08
Base (n 0)X0.07 Radial to Bridger X
-Trip Freezout Lines X
0.06 -TripShirlyLinesX
0.02
0.01 ,
O
5 10 15 20 25 30 35 40 45 50 55
Electricial Frequency (hz)
Figure 4-2 R (top)and X (bottom)Looking into Q0706 POI 230 kV Bus with no Bridger
Units on Line,Base System and Three Outages
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,,Gateway West D.2 Project SSCl Screening'SSR/SSCI Study
4.2 SSCI with Full GatewayProject
SSCI was performed on a system modeling the final Gateway project as it is currently
planned.The intent here is to identify possible SSCI conditions in future.This will allow
wind plant developers to be aware of possible SSTI and select appropriate technology
to avoid issues in the future.
The full Gateway system model provided includes Gateway West and South 500 kV lines
with series compensation.The Aeolus 230 kV station has a wind plant (Q707)connected
through 230/34.5 kV POl transformer.The Q0706 wind plant is connected to Aeolus
through a 230 kV line (same as in the D.2 model).
Since there is a plant connected directlyto the Aeolus 230 kV bus,all impedance scans
are performed looking into that bus.Scans were performed for the following outage
conditions:
1.Base System (n-0)
2.Outage of two Aeolus-Sherly 230 kV lines
3.Outage of Aeolus-Freezout 230 kV line
4.Outage of Aeolus-Windstar230 kV line
5.Outage of Aeolus-Freezout and Aeolus-Windstar230 kV lines
6.AII 230 kV line out,Aeolus 230 radial to 500 kV Gateway lines
Analysis was performed with all Bridger units off line and all Bridger units on line.The
wind plants and SVC connected to Aeolus were disconnected.Figure 4-3 shows a plot
of the reactance looking into Aeolus 230 kV for the conditions listed above (the radial
condition #6 is not included in these plots).The upper set of plots has all Bridger units
off line and the lower plot has all four Bridger units on line.There are several resonance
points for the base (n-0)system.These become deeper with more lines out of service.
The resonances are in the 30hz to 37Hz range.
Figure 4-4 shows the reactance for radial condition (#6)with no Bridger generation on
line.There are several sharp resonance conditions that are SSCI concerns.
This analysis confirmsthat wind plants near Aeolus will have a significantSSCI risk when
the full Gateway 500 kV project is built.New and existing wind plant developers should
be made aware of the SSCI risk.
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Gateway West D.2 Project .SSCl ScreeningSSR/SSCI study
0.04
-BaseX
0.03 ShidyLinesOutX
Freezout&WindstarX
-FreezoutX
0.02 --WindstarX
0.01
0
5 10 15 20 25 3 35 40 45 50 55
-0.01
-0.02
-0.03 Electricial Frequency (hz)
0.04
-BaseX
0.03 ShirlyLinesOutX
Freezout&WindstarX
-FreezoutX
0.02 -windstarx
0.01
0
5 10 15 20 25 35 40 45 50 55
-0.01
-0.02
-0.03 Electricial Frequency (hz)
Figure 4-3 Aolus 230 kV Reactance for Conditions 1through 5,No Bridger Units On Line
(top)and Four Bridger Units On Line (bottom)
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,,Gateway West D.2 Project SSCl Screening'SSR/SSCI Study
0.2
0.15 Radial X
0.1
0.05
co
5 10152025303540455055
-0.05
-0.1
-0.15
-0.2 Electricial Frequency (hz)
Figure 4-4 Aolus 230 kV Reactance forRadial Condition (#6),No Bridger Units On Line
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