1. Simulation and Analysis Methods for SSR/SSTI/SSCI
PUCT Panel Session
Austin, Nov 19, 2014
Garth Irwin
Electranix Corporation, Winnipeg, Canada

2. Presentation Overview
SSR/SSTI/SSCI - Definitions
SSCI
2009 event of a wind farm near a 345 kV series compensated line
Simulation/Analysis Techniques:
Screening Methods – Impedance Scans
Perturbation Analysis
Advanced Multi-Port Perturbation Analysis
Time Domain Non-Linear Analysis
Mitigation Methods

3. SS Phenomena - Definitions
“SSR”: Sub-Synchronous Resonance
Interaction between the mechanical/torsional masses in a generator (or wind turbine) and the electrical resonance from a series capacitor.
“TA”: Torque Amplification: Increase in peak shaft torques leading to higher fatigue.
“SSTI”: Sub-Synchronous Torsional Interaction
Interactions between the mechanical/torsional masses in a generator (or wind turbine) and a power electronic device (such as an HVDC link, SVC, wind turbine etc…).
“SSCI”: Sub-Synchronous Control Instability
Interactions between a power electronic device (such as an HVDC link, SVC, wind turbine etc…) and a series compensated system.

4. Sub Synchronous Interactions
Device
Series Capacitor
Power Electronics
Gas Turbine or Wind Shaft
---
SSCI
SSR
CI (control interactions can be at any frequency)
SSTI
Gas Turbines
or Wind Shaft

5. SSCI Event in Texas I V

6. Real System SSCI Event Trace
Real System Traces and PSCAD Simulation

7. Wind Projects with Series Compensated Transmission Lines
* Texas 2009 SSCI Event
* North Dakota series capacitor/wind turbines
* Alberta-Montana 230 kV series compensated line
ERCOT CREZ expansion 345 kV series compensated lines
Alberta southern system expansion
Project in Texas with a N-0 radial 345 kV series compensated line
UK large scale transmission expansion
Twin circuit 275 kV series capacitor expansion Australia
...
* Indicates real-system SSCI events.

8. SSCI - Description
Voltages and currents are distorted due to the series capacitor and electrical resonance
Difficult to filter:
Can be close to 60 Hz
Resonant frequency changes
Distorted inputs signals are processed by turbine controls, and ultimately fire IGBTs/power electronics (creating a feedback loop).
Overall controller response can introduce negative damping, resulting in instabilities (growing or sustained oscillations)

9. SSCI - Description
Doubly Fed Induction Generator Wind Turbine

10. SSCI - Description
Cascaded PI Controller – Outer and Inner PI Loops

11. SS – Analysis Methods Overview
Screening Studies
SSR/SSCI: Harmonic Impedance Scans
SSTI: Unit Interaction Factors
Perturbation Analysis
SSR/SSTI: Used to determine generator electrical damping vs freq
SSCI: Used to determine Effective Dynamic Impedance of a power electronic device
Advanced Screening Studied
SSCI: Uses a combination of Harmonic Impedance Scans (linear portion of a system) and Perturbation Analysis (Effective Dynamic Impedance)
Full Time Domain Analysis
SSR/SSTI/SSCI: Uses fully detailed models of all devices

12. SSR/SSCI – 1) Screening Studies
Harmonic Impedance Scans
Determine net system impedance (as seen from behind the generator equivalent impedance) as a function of frequency
Determines approximate frequency of electrical resonance
Impedance “dip” an approximate indicator of the likelihood of SS interactions (large dips indicate “closer to radial” connections – transition from positive to negative reactances)
Limitations:
How should nearby SVC/Statcom/HVDC/non-linear-devices be represented?
What is the equivalent impedance of a wind turbine?
Does SSCI depend on the magnitude of a disturbance/oscillation?
If there are 2 or more wind farms nearby, how does a turbine non-linear controller affect the impedance as seen from the other farm?
Screening Method – used to determine system condition (for later study with more accurate methods).

13. SSR/SSCI – 1) Screening Studies

14. SSR/SSTI 2) Perturbation Analysis
Consider open loop transfer function from generator rotor speed to electrical torque…

15. SSR/SSTI 2) Perturbation Analysis
Enable PSCAD Multi-mass feature on a generator
Force the speed to be 1 pu plus a small oscillation at 5 Hz (this will rock the full system at 5 Hz)
Run until steady state in the time domain
Measure the relative magnitude and angle between the electrical torque and delta W
May require a variable/tuned filter on both Te and W to remove noise and DC, or (better) use FFT methods.
The electrical damping is the real part of dTe/dW
Compute and store the damping factor for this frequency
Increment the frequency to 6 Hz and repeat
Use multiple run features to sweep from 5 Hz to 55 Hz
Plot damping vs frequency

16. SSR/SSTI 2) Perturbation Analysis

17. SSR/SSTI 2) Perturbation Analysis
HVDC links can directly affect torsional damping
Not a problem however, as SSDC stabilizers are easy to design and very effective
Should be studied using detailed models (PLL and main PI controls are critical)
SVCs are usually not an SSR concern, however indirect effects through nearby loads can cause interactions
Interpolation in SVC and HVDC firing controls is essential!
Exciter and governor models are often not validated at torsional frequencies – simplified PSS/E models often are not valid.
Impact with and without mitigation methods can be tested
We always recommend TSRs (torsional stress relays) for all generators near series caps, HVDC links or SVCs.

18. SSR/SSTI 2) Perturbation Analysis

19. SSR/SSTI 2) Perturbation Analysis

20. 3) SSCI Advanced Screening Dynamic Effective Impedance
Perturbation Analysis can be used to determine the Dynamic Effective Impedance of a non-linear device (wind farm):
Perturb voltage with sub-synchronous components
Measure sub-synchronous magnitude and phase of measured terminal current
Impedance (Z) = V/I (performed with complex vectors at each frequency)
Table of Z (R + jX) as a function of frequency
Calculation for wind turbines (Dynamic Effective Impedances) can be added to linear system impedance (including the series capacitor).
Relatively easy for 1 wind farm connected through 1 port to a series compensated system.
Can be applied for a 2 port (2 wind farm) scenario:
Requires 3 system harmonic impedance scans for each system conditions
Solve 3 equations in 3 unknowns to device a 2x2 two port linear network equivalent
Add wind Dynamic Effective Impedances, solve 2x2 network as seen from each device

21. 3) SSCI Advanced Screening Dynamic Effective Impedance

22. Perturbation Analysis Limitations for SSCI
Some devices have a damping characteristic which is “magnitude sensitive”
Non-linearities in the system and device models are not considered

23. SSR/SSTI/SSCI Analysis 4) Complete System Time Domain Analysis
Ultimate Simulation… Model the entire system including multi-mass shaft models, HVDC/SVC/Statcoms, wind farms etc…
Apply a small signal disturbance and measure log-decrement (quantify damping)
Apply faults and observe large signal disturbances (and watch for tripping/ride through)
Time consuming (varying loadflow conditions, contingencies, wind turbine combinations, two segment series capacitors…)
Used in conjunction with Screening Studies (to focus on most-concerning cases)

24. Mitigation Methods SSR SSCI SSTI Description 
Select other transmission or generator options (higher voltage AC lines etc…)
Select Series Compensation Level to Avoid Problems
Use transfer trips to avoid trouble conditions
Design damping controllers (SEDC, SSDC…)
SSCI resistant wind farm controllers

25. Mitigation Methods SSR SSCI SSTI Description 
Add generator step up transformer filters
Add shunt compensation with stabilizers (SVC/Statcom)?
Series Capacitor Bypass Filters
FACTS Devices (TCSC, SSSC, UPFC…)
Combinations of all of the above

26. New Simulation Products
E-TRAN Plus for PSCAD
Parallel Processing of PSCAD Simulations
- break the PSCAD simulation into several cases and run them in parallel talking with each other
E-TRAN Plus for PSS/E
Hybrid Simulation
- PSCAD and Transient Stability simulations are run in parallel talking with each other

27. Thanks!
Garth Irwin Electranix Corporation – Engineering Consultants Winnipeg, Manitoba, Canada