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

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

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

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

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 = 60-20 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.

Classical SSR: Gas Turbine

Classical SSR: Gas Turbine

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

SSR/SSTI - Definitions

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.

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

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)

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.

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

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