2. Sub-Synchronous Interactions with Wind Turbines
Sub-Synchronous Control Interaction Studies Related to the CREZ Project
Austin, January 26, 2010
Garth Irwin

3. Overview Kennedy Wind Farm and Series Compensation
System Description
Event from November, 2009
Electranix Study Scope
Introduction to SSR, SSTI…
SS Control Instabilities with Wind Turbines
Turbine Manufacturers and Experience with Series Compensation
Possible Discussion Points

4. Kennedy Wind Farm Studies
Recent Event:
trip of a 345 kV line left the Kennedy wind farm radial into a series compensated line
Study Scope:
Develop a detailed PSCAD model of the system
Use detailed models from the turbine manufacturer (Mitsubishi)
Validate the model, reproduce DFR traces
Investigate solutions
Extend study conclusions to CREZ

5. SSR - Definitions SSR: Sub-Synchronous Resonance
An electrical resonance in the 5-55 Hz range, often created by series capacitors.
Classical problem – series capacitors near gas turbines or thermal generators

6. SSR – Series Capacitors
Electrical Resonance
A resonance between the series capacitor and the system effective impedance (inductance L). Fe: ( 1 / 2∏ *√LC )
SSR is a direct concern if the frequency of the electrical resonance (Fe) correlates with the complement of a mechanical mode of oscillation
ie Fe = 20 Hz and Fm1=40 Hz (40 Hz = for a 60 Hz system).
System Conditions and Contingencies
The system equivalent inductance changes:
Load and generators come on/off daily/seasonally, transmission line or transformer outages and future system changes
Radial configurations (where the generator is radial with the series capacitor) have less damping and higher interactions.

7. Classical SSR: Gas Turbine

8. Classical SSR: Gas Turbine

9. Classical SSR: Torsional Modes
Generator Torsional Modes:
Oscillations of one generator mass against 1 or more other masses.
If a generator has 5 masses, there will be 5 modes of oscillation:
0 mode: typically 1-2 Hz, all masses move together, familiar with lumped inertia models in PSS/E etc… using a single H inertia.
The other 4 modes involve oscillations between the masses (ie they oscillate against each other) – the more masses that participate, the lower the frequency.
Eigenvalue analysis of the shaft system will determine the torsional modes
Wind Farms also have torsional modes, but usually at lower frequencies
Manufacturers are quite secretive about shaft, gearbox and blade dynamics

10. SSR/SSTI - Definitions

11. SSTI/”SSCI” - Definitions
SSTI: Sub-Synchronous Torsional Interactions
Interactions between a power electronic device (such as an HVDC link, SVC, wind turbine etc…) and the mechanical mass system of a generator.
“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.

12. Sub Synchronous Interactions
Device
Series Cap
Power Electronics
Gas Turbine
---
SSCI
SSR
SSTI
Gas Turbines

13. SSTI/SSCI
Voltages and Currents are distorted due to the electrical resonance
May be difficult to filter as they are close to 60 Hz
Distorted inputs signals are processed by controls
Ultimately firing thyristors or IGBTs
Overall controller response can introduce negative damping, resulting in instabilities (growing or sustained oscillations)

14. Wind Turbine Experience with Series Compensated Systems
Experience with Radial Series Comp. Systems
Turbine Type
Notes
Vendor#1 ?
DFIG
Vendor#2
Yes
Ongoing research
Vendor#3 No
Full Converter
Seems immune
Vendor#4
Vendor#5
Vendor#6
Not aware of any problems
Vendor#7
Vendor#8
Vendor#9
Notes: Vendors have updated turbine designs and may offer full converter topologies...
This information is Confidential and should not be published.

15. Wind Projects with Series Compensated Transmission Lines
PacifiCorp Gateway System Expansion
500 kV Lines, series capacitors, SVCs
Alberta Southern System Expansion
230 kV Lines, series capacitors, SVCs
Wind Project in Texas
Radial 345 kV line
Proposed 500 kV Project in PacifiCorp
UK Large Scale Transmission Expansion

16. Possible Discussion Points???
Changes to Wind Interconnection Standard
“Series capacitor safe” turbines
Alternatives to Series Capacitors
Higher voltage lines
TCSC (thyristor controlled series capacitors)
Operating restrictions, selective bypassing
Wind turbine modeling
Detailed models (using real controller code)
System models and EMT model development methods
Study Requirements