Analysis and Mitigation of Subsynchronous Resonance Involving
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Analysis and Mitigation of Subsynchronous Resonance Involving Type-III Wind Turbines Ignacio Vieto and Jian Sun Rensselaer Polytechnic Institute 1 Outline • • • • • • Wind Power Conversion Subsynchronous Resonance Impedance Models Impedance Based Analysis of SSR SSR Damping Summary CFES Annual Conference 02-26-2015 2 Wind Power Generation Type-I Wind Turbine Fixed Speed SCIG PMSG Type-II Wind Turbine WRIG Type-IV Wind Turbine Type-III Wind Turbine WRIG CFES Annual Conference 02-26-2015 3 Type-III System, Detail CFES Annual Conference 02-26-2015 4 ERCOT 2009 SSR 50% Compensation Lines Highlighted Lon Hill 1. Ajo-Nelson Tripped 2. Ajo Wind Farm is Radially Connected to Rio Hondo 3. Voltages Increase Up to 1.95 pu Nelson Line fault ia ib Ajo ic Edinburg va vb vc Rio Hondo Massimo Bongiorno, ‘The impact of Wind Farms on Subsynchronous Resonance in Power Systems’, Gothia Power AB CFES Annual Conference 02-26-2015 5 DFIG Sub-Synchronous Resonance RSC + Machine • • • Most significant turbine component for SSR. RSC is also the best prospect of using turbines own controls for reducing SSR. Converter interacts with the power grid though induction machine. π π πΏππ πΏππ Power Grid π π πΏπ GSC • πππ CFES Annual Conference 02-26-2015 • High impedance compared to RSC Not significant for SSR studies or mitigation strategies 6 DFIG Currents during SSR Stator Currents 0.88 pu/div 0 ia ib ib ic ic 60 Hz 8 Hz 20 Ii/I1 (%) Ii/I1 (%) ia 10 8 6 4 2 Rotor Currents 40 60 Frequency (Hz) 80 100 10 8 6 4 2 0 12 Hz 20 64 Hz 40 60 80 100 Frequency (Hz) CFES Annual Conference 02-26-2015 7 Turbine Models Below fundamental, Positive Sequence πππ πππ ππ 2 π ′π π πΏπ + πΏ′π + π π + + π»ππ ππ1 − π + π πΎππ π π π π π π π π ππ1 π = ππ π πΌ 1 − 1π πΊππΏ ππ1 − π πππ πππ π»ππ ππ1 − π + π πΎππ 1π + 1 ππ π1π 2ππ π Above fundamental, Positive Sequence ππ2 π = π π π π ′ π π (πΏπ + πΏ′π ) + π π + + ππ ππ π ππ (π ) ππ (π ) ππ ∗ π1π 1− πΊ π − ππ1 2ππ (π ) ππΏ ππ ππ 2 π»ππ π − ππ1 − ππΎππ πππ πππ π» π − ππ1 − ππΎπ πΌ1π π1π ∗ +1 Negative Sequence πππ πππ ππ 2 π ′ π π (πΏπ + πΏ′π ) + π π + + π»ππ π + π π1 + π πΎππ ππ ππ (π ) ππ π ππ π = ∗ π1π ππ πΌ 1− πΊππΏ π + π π1 πππ πππ π»ππ π + π π1 + π πΎππ 1π + 1 ππ π1π 2ππ π CFES Annual Conference 02-26-2015 8 System Parameters Parameter Value Parameter Value ππ 690 V πππ 1500 V f 60 Hz π π 0.929 mβ¦ πΌπ 3740 π π 5.8 mβ¦ Turns Ratio 0.34:1 πΏππ 54.7 µH Pole Pairs 3 πΏππ 462 µH Mac. Speed 2160 RPM πΏπ 1.6 mH πΎππ 0.0518 πΎπ −8.2 ∗ 10−3 πΎππ 3.8462 πππ 2.5 ∗ 10−4 πΎππ 0.7886 πΎππ 2495 RSC BW 100 Hz RSC PM 60° PLL BW 50 Hz PLL PM 45° πΏππππ 0.5 pu Compensation 50% CFES Annual Conference 02-26-2015 9 DFIG Model for SSR Magnitude (db) Phase (deg) πππ πππ ππ 2 π ′π π πΏπ + πΏ′π + π π + + π»ππ ππ1 − π + π πΎππ π π π π π π π π ππ1 π = ππ π πΌ 1 − 1π πΊππΏ ππ1 − π πππ πππ π»ππ ππ1 − π + π πΎππ 1π + 1 ππ π1π 2ππ π ~0° Frequency Frequency Red: Positive Sequence Turbine Purple: Grid Impedance Black: Negative Sequence Turbine Dots: Measured Line: Model CFES Annual Conference 02-26-2015 10 Impedance Ratio Nyquist Plot ππ π ππ1 π Stator Currents ~0° Phase Margin CFES Annual Conference 02-26-2015 11 SSR Range Phase (deg) Magnitude (db) DFIG Model Analysis Frequency SSR Range Frequency Red: Positive Sequence Turbine Dark Red: Positive Sequence Machine Passives • In the SSR range machine inductance dominates. • SSR falls into converter control bandwidth: • Turbine impedance can be modified through RSC controls. CFES Annual Conference 02-26-2015 12 SSR Damping Methods • Static Var Compensator at wind farm terminals – (H. Xie and M. Oliveira, 2014) • STATCOM damping at wind farm terminals – (A. Moharana, R. Varma and R. Seethapathy, 2014) • Damping through power electronics controls – GSC damping using series capacitor voltage (H. Mohammadpour and E. Santi, 2015) – RSC damping using dq-axis rotor voltages (A. Leon and J. Solsona, 2015) CFES Annual Conference 02-26-2015 13 Wind Farm SVC SSR Damping SSR damping Power Grid SVC Damping OFF Damping ON • Controls susceptance based on local variables, e.g. line power • Susceptance response is non linear, and depends on operating conditions • Needs the installation TSC • With enough information, its effective over a wide power and wind speed spectrum. Hailian Xie, “Mitigation of SSR in Presence of Wind Power and Series compensation by SVC”, 2014 POWERCON, Chengdu CFES Annual Conference 02-26-2015 14 STATCOM SSR Damping Wind Farm STATCOM Control Power Grid STATCOM • • • • STATCOM is tuned to stabilize electrical modes of the complete system. Requires detailed knowledge of the wind farm parameters and operating conditions. Fine-tuning of parameters is done by hand using EMT simulations. STATCOM gives a lot of flexibility to adjust system eigenvalues and stabilize it. A. Moharana, “SSR Alleviation by STATCOM in Induction-Generator-Based Wind Farm Connected to Series Compensated Lines”, 2014 Trans. Sust. Energ. CFES Annual Conference 02-26-2015 15 Damping Though Turbine Controls Local variables • • • Obtain chosen control signals. No damping Damping approach determines which measurements to use and where to inject signal. Can be applied to RSC or GSC. Does not need external equipment. Damping Signal can be inserted at any corresponding part of the controller. Capacitor voltage compensation Injected at GSC voltage regulator H. Mohammadpour, “SSR Damping Controller Desing and Optimal Placement in RSC and GSC of Series Compensated DFIG Based Wind Farm”, 2015 Trans. Sust. Energ. CFES Annual Conference 02-26-2015 16 Virtual Impedance Damping • Approach: – Use rotor dq-axis currents (already measured) – Obtain voltage drop for currents through an equivalent impedance. πππ = πΌππ ⋅ ππ π – Inject voltage terms into controller output. • Modify turbine impedance to add phase margin • Does not need eigenvalue analysis of turbine system • Impedance of systems with complex/unknown controls can be scanned instead of modelled • Virtual impedance can be filtered and narrowed to not affect fundamental or higher frequencies CFES Annual Conference 02-26-2015 17 Virtual Rotor Impedance Damping π π ππ πΏππ πΏππ πΌππ − abc πΌππ − dq πππΏ − ππ abc dq πππΏ − ππ Σ − ππ π = πΎππ + πΎππ /π Σ π1 πΎππ + πΎππ /π Σ Magnitude (dB) πππ Power Grid π π πππ πππ πΌππ πΌππ Σ − Σ Σ − πΎππ − ππ π πΎππ πΌππ πΌππ ππ π π π π π 1+ 1+ π1 π2 π π1 π2 πΌππ πΌππ CFES Annual Conference 02-26-2015 frequency 18 Damped Impedance Magnitude (db) Phase (deg) Turbine impedance with a negative virtual resistance as damping 20° Frequency Frequency Red: Positive Sequence Turbine Purple: Grid Impedance Black: Negative Sequence Turbine Dots: Measured Line: Model CFES Annual Conference 02-26-2015 19 Virtual Stator Impedance πΏππ πΌππ − abc πΌππ − dq πππΏ − ππ abc dq πππΏ − ππ Σ − πΏππ Σ πΎππ + πΎππ /π ππ ππ π = ππ π π1 πΎππ + πΎππ /π Σ πππ πππ πΌππ πΌππ Σ − Σ Σ − πΎππ − ππ π πΎππ πΌππ πΌππ ππ π π π π π 1+ 1+ π1 π2 Magnitude (dB) π π πππ Power Grid π π πΌππ πΌππ CFES Annual Conference 02-26-2015 frequency 20 Undamped Resonance, Zoomed 10 Hz Resonance Current (A) Total Output Currents (2kA/div) Strong grid Fault Compensated weak grid Time (s) CFES Annual Conference 02-26-2015 21 Undamped Resonance Current (A) Total Output Currents (2kA/div) Fault Time (s) CFES Annual Conference 02-26-2015 22 Damped Resonance Current (A) Total Output Currents (2kA/div) Fault Time (s) CFES Annual Conference 02-26-2015 23 SSR, Subcynchronous Operation Magnitude (db) Phase (deg) The SSR phenomenon is present also when the generator operates below synchronous speed. Frequency Frequency Red: Positive Sequence Turbine Black: Negative Sequence Turbine Purple: Grid Impedance Line: Model CFES Annual Conference 02-26-2015 24 Damped Simulation, SuperS Oper. 18 Hz Resonance Current (A) Total Output Currents (2kA/div) SSR Control OFF SSR Control ON Time (s) CFES Annual Conference 02-26-2015 25 SSR Control d-axis Output Voltage (V) SSR Control output when connected to an ideal grid (no resonance) Steady-state output: 211V SSR Control Output (0.1/div) Time (s) CFES Annual Conference 02-26-2015 26 Damping Stator Impedance Magnitude (db) Phase (deg) Shaping stator impedance is more drastic, but to implement it, controls needs to consider machine airgap Frequency Frequency Red: Virtual Rotor Resistance Black: Virtual Stator Resistance Purple: Grid Impedance CFES Annual Conference 02-26-2015 27 Summary • SSR in type-III wind turbines was explained and analyzed using turbine impedance. • Turbine impedance was modified through controllers to damp the resonance. • Future Work: – More robust control actions that can emulate impedances on rotor or stator side – Use damping as part of stability studies in heavily compensated weak grids. CFES Annual Conference 02-26-2015 28 End of Presentation This work is supported in part by the National Science Foundation under Award #ECCS1002265 CFES Annual Conference 02-26-2015 29
