Full Converter Wind Turbine Operating under Weak Grid Conditions Daniel Mueller, Walter Kuehn Frankfurt University of Applied Sciences Germany 1

Grid Code Requirements 2 Wind turbines must be able to provide 41% of their rated real power as reactive power Installation of FACTS devices Using oversized converter → here Grid voltage support through reactive current injection Proportional reactive current infeed depending on AC voltage at the PCC

Proportional Voltage Control 3 Reactive current support pattern can vary within boundaries Minimum gradient: k = 2 Maximum gradient: k = 10 Real power has to be curtailed to provide reactive power WT controls have to deal with load rejection Matlab

Limiting Turbine Power Real power curtailment leads to unbalance between rectifier and inverter power Generator power has to be reduced to prevent DC overvoltage 1.Mechanical torque reduction via pitch angle → slow 2.Rotor flux oriented controls allow direct manipulation of the electric torque producing current component → fast 4 Matlab

Wind Turbine Controls 5 TurbineFull ConverterGrid Reduce Generator Power when Real Power is Curtailed Proportional Reactive Current Infeed Visio

Test Setup 6 SCIGRECINV 5 mH 7 / 33 kV 100 mH 10 MVA Synchronous Machine providing AFLC 30 MVA Synchronous Machine 5 MW Wind Turbine with Full Converter Variable Line Reactance

Test Setup The grid reactance is ramped up from 0.45 p.u. to 0.9 p.u. → Corresponds to SCR of 2.2 and 1.11 for rated voltages and turbine power  Case A: k = 2  Case B: k = 10 7

Case A – Grid Quantities 8 Transmission Angle Grid Short Circuit Ratio (measured) Reactive Power at the PCC AC Voltage at the PCC PSCAD & Matlab

Case A – Grid Quantities Low gain of the AC voltage control leads to fast declining voltage at increasing grid reactance Depressed voltage reduces the maximum stable angle to ≈ 70° 9 Transmission Angle AC Voltage at the PCC PSCAD & Matlab angle margin 30 deg. 7 deg. SCR = 1.5 SCR = 1.1

Case A – Turbine Quantities 10 Inverter Real Power (MW base)WT DC Voltage Generator Electrical Torque (MVA base)Generator Real Power (MW base) PSCAD & Matlab

Case A – Turbine Quantities At depressed voltage inverter power is determined by the VSC current limit 11 Inverter Real Power Generator Electrical TorqueGenerator Real Power PSCAD & Matlab Real Power SCR = 1.5

Case A – Turbine Quantities Real power limitation at the inverter leads to increasing DC voltage Generator torque is immediately reduced when DC voltage exceeds a threshold of 1 p.u. kV 12 WT DC Voltage Generator Electrical Torque PSCAD & Matlab Torque Reduction Threshold

Case A – Turbine Quantities Generator power follows the inverter power 13 Inverter Real Power Generator Real Power WT DC Voltage PSCAD & Matlab

Case B – Grid Quantities 14 Reactive Power at the PCC Transmission Angle AC Voltage at the PCC PSCAD & Matlab Grid Short Circuit Ratio (measured)

Case B – Grid Quantities 15 AC Voltage at the PCC Higher AC voltage than in case A Maximum stable angle increases to almost 90° Power curtailment starts at about 65 deg. Transmission Angle PSCAD & Matlab SCR = 1.1 angle margin 25 deg.

Real Power Curtailment at Decreasing SCR Conclusion: Case B real power is by 15 % higher than Case A real power at SCR = 1.1=> higher utilization of transmission capacity Stability margin: Case B = 25 deg. / Case A = 7 deg. => Can transient stability be ensured? 16 Inverter Real Power - Case BInverter Real Power – Case A PSCAD & Matlab Real power curtailed at SCR < 1.5Real power curtailed at SCR < 1.1 SCR = 1.5 SCR = 1.1

Transient Stability 17 Probably yes! How? Through a basket of measures comprising DC power order reduction DC power order limitation DC power modulation Transient battery storage PMU implementation and some other means For this see you at the Wind & Solar Integration Workshop in Berlin in Nov. 2014