Squirrel cage induction generator based wind farm connected with a single power converter to a HVDC grid. Lluís Trilla PhD student

Transcription

1 Squirrel cage induction generator based wind farm connected with a single power converter to a HVDC grid Lluís Trilla PhD student

2 Current topology of wind farm Turbines are controlled individually Wind farm is connected directly to the grid 2/28

3 HVDC link Turbines are controlled individually Wind farm is connected to a power converter Transmission to the main grid is done in HVDC Useful in remote wind farms (usually offshore) 3/28

4 Proposed topology Turbines are controlled together poorer performance No individual converters less power loss, better reliability Transmission in HVDC offshore converter becomes necessary 4/28

5 Wind power generator Induction Generator Permanent Magnet Synchronous Generator Doubly Fed Induction Generator 5/28

6 Control scheme 6/28

7 Scalar control Same voltage and frequency for each generator Control of an aggregated model of the wind farm Aggregation of generator speeds 7/28

8 Aggregated model Different types of aggregation have been considered : Mean Quadratic mean Maximum 8/28

9 Results on power generation (overall power) Wind speed base is 12 m/s, each wind turbine has a incoming wind of a 100%, 90% and 80% respectively Power measured at the generator, not considering power loss Greater power extraction using conventional topology The difference in power extracted (3%) is similar to the power loss at the converter Slight difference in power generation between aggregations 9/28

10 Operating points (mean speed aggregation) Reference torque is the same for the whole set of machines Their operating points are moved away from the optimum The wind farm is controlled as one single generator 10/28

11 Results on power generation (individual power) Power generated by each machine using different aggregations WT1 WT2 WT3 11/28

12 Cp analysis Cp is not the maximum for all the turbines The more variation in incoming winds between turbines the further from optimum is the performance 12/28

13 Same wind speed Wind speed is the same for the whole set All the generators have the same output power using individual or common converter 13/28

14 Same wind speed Cp coefficient becomes optimum for all the turbines The whole set is working at the maximum power extraction operating point Same power generated as using individual converters but less power loss in the system 14/28

15 Onshore converter Onshore converter keeps the DC link voltage constant It can provide reactive power depending on the grid code requirements 15/28

16 Effects of wind speed variations (slope) Incoming wind speeds are 100%, 90% and 80% for each wind turbine respectively The controller modifies the electrical frequency when there is a speed variation 16/28

17 Effects of wind speed variations (slope) The overall power generated by the system increments The voltage of the DC link is kept constant by the onshore converter 17/28

18 Effects of wind speed variations (gust) Incoming wind speeds are 100%, 90% and 80% of 12 m/s for each wind mill respectively Pitch control reacts when the generator speed exceeds the nominal value 18/28

19 Effects of wind speed variations (gust) Power generated is limited by the pitch controller Onshore converter delivers the power when a DC link voltage variation is detected 19/28

20 Control scheme for LFRT 20/28

21 Line fault ride through The grid voltage drops to a 10% of its nominal value for 500 ms Alpha signal is applied when the DC link voltage reachs a predetermined limit 21/28

22 Line fault ride through DC link voltage oscilation during the voltage sag, it stabilizes rapidly Voltage variation in detail, over 1.05 p.u. alpha signal is activated 22/28

23 Line fault ride through Reference torque is limited during the voltage sag The same torque is referenced to all the generators, in some cases exceeding the mechanical torque 23/28

24 Line fault ride through The power generated is limited during the voltage sag Detail of the current injected to the grid during the voltage sag 24/28

25 Line fault ride through As the torque is set to zero during the voltage sag the angular speed of the generator increases rapidly Pitch controller will keep the speed in the safe area 25/28

26 Line fault ride through During the fault the torque is set to zero, then the slip is zero as well Frequency becomes equal to the average angular speed in order to get a zero slip operating point. 26/28

27 Conclusions Wind farm control can be performed appropriately by a single converter. Adaptation to wind variations and line fault ride through performance have been tested via simulation. Power converter losses are avoided using this topology but it results in a poorer performance in wind power extraction. DC link voltage is kept in the safe area. Better reliability, the accessibility is low in remote areas (offshore) and there is less probability of failure if there are less elements Future work: Analysis of wind farm voltage stability 27/28

28 Thanks for your attention Questions?