Check out this presentation from GE Energy Consulting's Bruce English on strategies for protecting turbines from the challenge of SSR resulting from the growing amount of renewable energy being introduced to electric power systems around the world.

1. Reactive Power Solutions
Subsynchronous Oscillations (SSO):
Risk Analysis, Protection, and Mitigation Techniques
GE
Digital Energy

2. GE Guest Speaker
Bruce English
GE Energy Consulting, Technical Director
• Over 15 years of T&D experience, FACTS equipment design, ratings, and system
studies
• Served as vice-chair IEEE Series Capacitor Working Group,
• Leading technical expert for recent PUCT/ERCOT panel session on the topic of
subsynchronous interactions between series capacitor banks and power generation.
• Holds 4 patents for electric power systems equipment inventions.

3. GE Energy Consulting Involvement in SSR
Failed Rotor at Mohave
Tuoketuo Power Plant,
China
Mohave: 1st SSR event involving series caps
• Unit was radial on-line with 7 of 8 SC modules
• 30 Hz oscillation grow over many seconds until shaft failure
occurred : Dec ’70 & Oct ’71
• GE EC team determined root cause & solution
Over the past 40 years GE EC has:
• Developed analysis tools, protection and mitigation
concepts
• Performed SSR analysis on over well over 100 turbine-
generators
• Designed protection and mitigation systems for a full range
of SSR issues
• Analysis and solutions provided for GE and non-GE units
world-wide

4. Discussion Topics
• Introduction to torsional stability, subsynchronous
resonance (SSR)
• Introduction to electrical and controls stability,
subsynchronous controls interaction (SSCI)
• SSR and SSCI protection and mitigation
techniques

5. Torsional Modes - 2 Mass Example
Zero mode - T-G oscillates
as a single mass (e.g. synchronous
speed, power swings)
Torsional Mode (mode 1) - two
masses oscillate against each other,
causes cyclical twisting on shaft
Frequency and mode shape of torsional modes is
dependent on mass of each section (M1, M2) and stiffness
of shaft (K12)
M1 M2
Dashed Line Indicates Mode shape
K12

6. Torsional Modes - 3 Mass Example
M1 M2K12 M3K23
Zero Mode
Mode 1
Mode 2
Dashed Line Indicates Mode shape
Number of torsional Modes = # masses -1

7. Torsional Vibrations in Turbine-Generator Rotors
Exc
LP-B
LP-A
HP
Gen
Mode 1
18.5 Hz
Mode 2
22.0 Hz
Mode 3
33.1 Hz
Mode 4
43.8 Hz
• Frequencies and mode shapes
determined by shaft geometry
• Very low inherent damping
• Electrical system influences damping via
electrical torque in generator
Mode shapes show relative movement of each turbine section for that torsional mode

8. Phenomena That Create High Torsional Stress
• Series Capacitors (SSR)
• Nearby Faults
• High-Speed Reclosing
• HVDC Interaction
• Steel Mills, Arc Furnaces, Cycloconverter Drives
• Synchronizing Out-of-Phase
• Off Nominal Frequency Operation

9. Mechanics of Interaction With Series Capacitors
Δw Δv
Te
Vg
wG
AirGap
Stator
Winding
Ig
Generator
Rotor
AC Line
Series
Capacitor
ΔTE Δi
Feedback Loop:
Excites
Electrical
Resonance
EE
E
E
jSD
v
iT
iT
jBG
v
i
Z
v
i
v














w
w DE = damping torque
(in-phase with w)
SE = synchronizing torque
(90º out of phase with w)

10. Electrical System Damping
T-G connected to an uncompensated
transmission system
Electrical system provides positive
damping at all subsynchronous
frequencies
Total damping of each mode is the sum
of the electrical and T-G mechanical
torsional
Uncompensated Line
0
Positive Damping
for All Torsional Modes
100 20 30 40 50 60 Hz
Torsional Frequency
TorsionalDamping
Positive Damped
Oscillations
0 5 10 15
Time (sec.)
TorsionalStress
0

11. Impact of Series Capacitors
Uncompensated Line
0
Positive Damping
for All Torsional Modes
100 20 30 40 50 60 Hz
Torsional Frequency
TorsionalDamping
Positive Damped
Oscillations
0 5 10 15
Time (sec.)
TorsionalStress
0
Series-Compensated Line
Large Negative
Damping Due to
L-C Resonance
0
100 20 30 40 50 60 Hz
Torsional Frequency
TorsionalDamping
0 5 10 15
Time (sec.)
TorsionalStress
0
Unstable Growing
Torsional Oscillation

12. Damping Torque Plot
Electrical System Damping Torque
Mode 2
Mechanical
damping
Mode 1
Mechanical
damping Mode 3
Mechanical
damping
Mode 4
Mechanical
damping
Mode 3 Unstable at Low Load
Electrical System Damping Torque
Mode 2
Mechanical
damping
Mode 1
Mechanical
damping
No load
100% load
Mode 3
Mechanical
damping
Mode 4
Mechanical
damping
Mode 3 Unstable at Low Load
Resonance caused by series capacitors

13. Transient Torque Amplification

14. SSR Transient Torque Amplification
Stored energy (0.5CV2) discharges
through all available paths.
Discharge through positive sequence
path of generator becomes a transient
torque on generator mass through air
gap.
Single-phase faults generally not an
issue for transient torque.
More-radial (fewer parallel discharge
paths) means more stored energy
dissipated through transient torque
discharge path.
X”dR(w)
Series
Capacitor RSYS1 XSYS1
RSYS2
XSYS2
RSYS3 XSYS3
Line CB
Isolate Fault
Fault
Transient Torque
Discharge Path
Parallel Discharge
Path

15. Transient Torque Example
3-phase fault and line trip on high voltage side of generator
HP-LPA shaft torque without (red) and with (blue) series capacitors

16. Fatigue Curve for T-G Shaft Section
• # of cycles of torque to failure for a shaft
section (cyclical torque)
• Shaft material, outer and inner
diameters, machining
• Failure: onset of surface cracking
• Fatigue accumulates over the life of the
shaft
• Torques below the endurance limit do
not result in accumulated fatigue
• At very high shaft torques other
phenomena can fail shaft
• Couplings can slip
• Shaft can become “plastic” and kink
Shaft Endurance Limit

17. Introduction to Electrical and Controls
Stability
Subsynchronous Controls Interaction (SSCI)

18. (Over) Simplified Electrical Stability
 te
L
R
LC
t
w

w 
sin
2
11 

Electrically-Stable, R>0 Electrically-Unstable,
R<0
SSCI stability risk: growing voltage and current oscillations,
electrical damage due to TOC and TOV limitations of all
equipment in resonant path.

19. Induction Generator Effect
Source of negative resistance: s < 0
• Mechanical overspeed
• Subsynchronous currents through stator
windings

20. Dual-Fed Asynchronous Generators (Type-3
WTG)
Use of VSC to introduce positive rotor resistance:
 
2
R
BUDGET
R
EXC
EFFT
i
VA
i
V
RR 

21. SSR and SSCI Protection and Mitigation

22. The Difference Between “Protection” and “Mitigation”
Protection
• Applied to all units with even remote
risk of SSR
• Detects actual stress on the shaft
itself, regardless of the cause
• Trips unit or series cap to prevent
damage to the turbine-generator
Reduces Risk by avoiding Known
Causes of high torsional stress
Protects machine from All Causes
of high torsional stress
Mitigation
• If SSR exposure is high enough, mitigation
may be required
• At Series Capacitors:
• Operation, % Xc, Bank Design
• At Plant:
• Damping controller, Blocking filters
Analysis shows SSR risk/exposure: System and generation conditions
where unit is exposed to unstable SSR or high transient torque

23. Examples of Protection and Mitigation
SSR Risk
Very low (e.g. n-4) Very High (e.g. n-0)Moderate (e.g. n-2)
Protection - TSR
SC Bank Bypassing (SPS or RAS)
Supplemental Damping (SEDC)
Modify Compensation Level
Modify Bank Design
(TCSC, Damping Filter)
Blocking Filter
@ Generator

24. SSR Protection
With low SSR Risk:
– SSR mitigation not required
– TSR substantially eliminates risk due to SSR in rare,
extreme or unanticipated conditions
With higher SSR Risk:
– SSR mitigation may be required
– TSR is backup for failure of SSR mitigation equipment
– TSR protects for unanticipated conditions
Torsional Stress Relay (TSR)

25. Mitigation - Operating Procedures
Topology/Power-Based Switching Schemes
– Example: Bypass series capacitors when critical units are at low load
levels (low torsional damping)
– Only effective for SSR stability issues at low unit loading
– May not provide required system performance (e.g. system stability)
Used effectively at the Jim Bridger, Coronado, AEP West/Lower Rio Grand
ValleySystem and Mohave plants in the US along with torsional protection and
mitigation

26. Protection and Mitigation Example
• Unit radial on series compensated line
• Failure of SPS to bypass series capacitors
• TSR issues a trip based on torsional
instability
• Low fatigue damage to T-G shaft
Recorded Turbine-Generator Torsional Instability

27. Mitigation During System Planning Stage
Reduced Compensation Levels
• Can eliminate or
minimize problem
• May allow use of only
torsional protection
• My not be feasible due
to system performance
requirements (e.g.
system stability)
• Multiple switching steps,
power-flow based
automatic selective
bypassing schemes
Note: Sometimes increasing
compensation levels helps, depending on
specific torsional mode frequencies

28. Mitigation at the Generator - SEDC
• Increase torsional damping by
modulation field voltage
• Acts through excitation system
• Will not mitigate extreme SSR
conditions or transient torque
alone
• Can be very effective with other
mitigation (e.g. SEDC +
Blocking Filters at Navajo,
SEDC + Operating procedures
at Bridger )
Without SEDC
With SEDC
Mitigation - Supplementary Excitation Damping Control (SEDC)

29. SSR Mitigation at the Generator - Blocking
Filters
• Located at generating station, in series with generator transformer
• Tuned to block current at complement of torsional frequencies
• One filter stage per torsional mode protected
• Can mitigate both SSR stability and transient torque amplification problems
• Mitigation for a wide range of generation and transmission conditions
Passive SSR Blocking Filters at Generator

30. SSR Blocking Filter - For 4 Modes (Navajo)
Filter blocks current
at frequencies
corresponding to
shaft torsional
oscillations

31. Effect of SSR Filter on Transient Torque
(a) Base Case without SSR Filter (b) Case with SSR Filter

32. Navajo Generating Station

33. Blocking Filters for all Three Units

34. Mitigation at Series Capacitor - Passive
XC1 XC2
Circulating
CurrentPassive
Bypass
Filter
Mitigation
RBPF
XLBPF
XCBPF
• Detunes resonance at subsynchronous
frequencies
• Can eliminate both SSR stability and
transient torque amplification problems -
Most effective for higher frequency torsional
modes
• Severe SSR interactions require larger filters
• Adds positive/stabilizing series-resistance at
lower frequencies to mitigate SSCI (electrical
stability) issues
Passive Damping Filter for SSR Mitigation

35. Mitigation at Series Capacitor - Active
• Bypassed
XTCSC = Small Inductive
• Inserted, No Thyristor Current
XTCSC = 1.0 pu
(only capacitor)
• Inserted, with Vernier Control
XTCSC > 1.0 pu
(circulating some thyristor current)

36. SSR Performance of TCSC
Closing the TCSC Bypass Breaker;
No Series Compensation
Inserting Five Modules with
Thyristor Gating Blocked
Inserting Five Modules with
Normal Vernier Operation
Boardman Slatt Buckley
Time in Seconds
Test 13
Speed
Deviation
in %
0.44
0.3
0.2
0.1
0
-0.1
-0.2
-0.3
-0.44
0 5 10 15 20 25 30
 = .36
Speed
Deviation
in %
0.44
0.3
0.2
0.1
0
-0.1
-0.2
-0.3
-0.44
0 5 10 15 20 25 30
Boardman Slatt Buckley5 Modules
Time in Seconds
Test 18
Speed
Deviation
in %
0.44
0.3
0.2
0.1
0
-0.1
-0.2
-0.3
-0.44
0 5 10 15 20 25 30
 = 0
Boardman Slatt Buckley5 Modules
Time in Seconds
Test 27
Speed
Deviation
in %
0.44
0.3
0.2
0.1
0
-0.1
-0.2
-0.3
-0.44
0 5 10 15 20 25 30
 = .35

37. Mitigation at the Series Capacitor Bank
Passive damping filters can be significantly less-expensive as-compared to TCSC.
Regular fixed banks with RAS control less-expensive than that. Keep your options
open!
“Per-Unit”
Cost
TCSC: 2.5 to
4.0
PDF: 1.3 to 2.2
FSC: 0.8 to 1.2
Relative Cost-Comparison

38. Mitigation at the Series Capacitor Bank –
Transient Torque Comparison
Steady-state linearization of nonlinear actively-controlled thyristor valve operation is an
oversimplification that ignores important transients.
Use of damping torque plots to compare TCSC to PD filters can be misleading.
Steady-State Transient Torque
Steady-State Versus Transients

39. Best Mitigation Scheme
Selection of the best mitigation scheme for given transmission system depends
on many factors, including:
• Value of power transfer
• Cost of SSR mitigation strategies
• Operational constraints imposed by SSR mitigation
• Cost of alternatives to series compensation (e.g., additional transmission line)

40. You May Have Missed...
Available Now on Digital Energy Website
www.gedigitalenergy.com
Whitepaper:
The Next Fifty Years of Series Capacitors – an the Last
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Webinars:
• Dynamic Stability with Synchronous Condenser
• Planning Strategies for Effective Series Compensation
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41. Questions?
Bruce English
GE Energy Consulting
Technical Director
Opportunity to ask Bruce questions

42. THANK YOU!