Sub-Synchronous Interactions with Wind Turbines Sub-Synchronous Control Interaction StudiesRelated 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: 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
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.
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 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