Vienna September 15th, Niall McMahon, Trinity College Dublin. Reducing Mechanical Complexity with AI-inspired Control

Transcription

1 Reducing Mechanical Complexity with AI-inspired Control Niall McMahon, Trinity College Dublin Vienna September 15th, 2016

2 Control is (Very) Important In the 1980s many wind turbines were stall-regulated fixed-pitch machines. The rotors were usually equipped with integral overspeed protection such as ailerons, tip brakes, flip tips and other similar devices. Nowadays, most machines have variable pitch systems (active or passive), in addition to mechanical braking systems.

3 Control is Expensive Control systems add complexity and cost to wind turbine systems.

4 Reducing Complexity Reduces Cost Fewer complicated components. Fewer mechanisms and potential failure points. Relative ease of manufacture and assembly. Reduced maintenance requirements.

5 How Can We Reduce Mechanical Complexity? Eliminating mechanisms, i.e. Pitch systems. Fixed pitch systems are often coupled to induction generators, e.g. Gaia-Wind 133. Mechanical brake systems. These are usually left in place, with some exceptions, e.g. first generation Ampair Possible replacements for these systems include blade-mounted control and electrodynamic braking. i.e. short-circuiting the generator.

6 IEC r3 Guidelines Section 8: Protection and Shutdown System The SWT shall be designed in order to keep all parameters within their design limits under all design load cases. This shall be achieved through an active and/or passive protection system included in the design. There shall be means to prevent the rotational speed design limit η max from being exceeded.

7 IEC r3 Guidelines The protection system shall be designed to be fail-safe. It shall be able to protect the SWT from any single failure or fault in a power source or in any non-safe-life component within the control and protection system... A failure of the control, power, or protection system shall not allow the turbine to exceed the η max rotational speed or go into an unsafe state of operation.

8 Electrodynamic Braking Has Been Attempted Several manufacturers have attempted to build electrodynamic braking-based systems that comply with the standards, e.g. Ampair A6000 (first generation) Southwest Skystream Evance R9000 With mixed results...

9 There Have Been Failures Taken from the Dorset Echo (c) The car in the photograph was damaged before the machine fell over.

10 Electrodynamic Braking Failures Electrodynamic braking is hard to do right: 1. Wind turbine rotor torques can quickly exceed generator short-circuit torques, even at wind speeds close to rated; 2. The energy dumped into the generator during braking is significant and can cause rapid heating to high temperatures.

11 Torque [Nm] Peak short-circuit torque (300 STK 2M) Simple short-circuit torque curve (500 STK 2M) Rotor torque: 2 m/s 4 m/s 6 m/s 8 m/s 10 m/s 12 m/s 14 m/s 16 m/s 18 m/s 20 m/s 22 m/s Rate of Rotation [RPM]

12 Peak Short-Circuit Torque (300 STK 2M) Simple SC Torque [Nm] Increasing short-circuit resistance --> Rate of Rotation [RPM]

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15 Typical Failure 1. Wind speed increases. 2. Rotor torque exceeds generator capacity. 3. Accelerates to maximum λ (8+). Overspeed. 4. Something breaks: 4.1 Transient high energy discharge into generator. 4.2 Temperature increases rapidly. 4.3 Magnets come off. 4.4 Tower fails at base. 4.5 Other bad things...

16 Goal Maintain rate of rotation below critical value while maintaining generator health.

17 System Specification 4 m rotor diameter. No gearbox. Synchronous permanent magnet machine. 4 kw nominal output with 5 kw dump load. Alxion 300 STK 2M type generator architecture.

18 Braking Strategy Adjust speed with: 1. Dump load. 2. Short-circuit load, tuned by varying the short-circuit resistance. 3. Emergency stop system (pin etc....) Simplified model. Monitor: 1. Ω - rotor speed. 2. U - wind speed. 3. T - generator internal temperature (T 2 = E 2mc + T 1).

19 Torques (Nm) Rotor Torques 2 m/s 4 m/s 6 m/s 8 m/s 10 m/s 12 m/s 14 m/s 16 m/s 18 m/s 20 m/s 22 m/s 24 m/s Rotational Speed (RPM)

20 Braking Constraints Do not release short-circuit if danger of overspeed. Release when safe. Stop if opportunity. Maintain energy transfer (power) to generator windings/magnets within safe limits. Do not brake while peak braking torque exceeded by rotor. Account for system response time.

21 Basic Control 1. Maintain operation close to nominal with load/dump load until U = 12 m/s. 2. If RPM exceeds maximum, halt system using electrodynamic brake. Wait until cool and re-start. 3. If U > 12 m/s, bring system to a halt. Wait until cool and re-start.

22 Intelligence - Step 1: Hard Coded Rules 1. As U increases, calculate rolling averages, Ū. Estimate error bars about averages at different scales, +/ ɛ. 2. As Ū + ɛ exceeds 12 m/s, stop machine. 3. If RPM exceeds upper critical value, stop machine.

23 Intelligence - Step 1: Hard Coded Rules 4. If U or RPM exceed max. generator short-circuit capability: If it is likely that the wind will drop again very soon after, If structural limits are note exceeded at that wind speed/rpm, Then allow to free-wheel and wait until the wind drops. (Uncontrolled-but-safe overspeed.) Then take the rotor speed all the way down to zero.

24 Intelligence - Step 2: Machine Learning 1. Train artificial neural network to recognise patterns in wind at that site and on generator capability. 2. System anticipates braking opportunities. 3. System anticipates uncontrolled-but-safe overspeed opportunities.

25 Challenges These are many and include: 1. May not be possible to do with real systems. 2. Real generator short-circuits have transient effects, simplified away here. 3. Effective cooling may pose a problem for real generators. 4. Cost benefit must be at least 10% over regular systems to compensate for lost winds at the high end.

26 Hardware and Software Python running on a Raspberry Pi 3.

27 References See for summary article.

28 Acknowledgements With thanks to: Sustainable Energy Authority of Ireland. David Sharman and Peter Burton, formerly of Ampair. Henry Rice and others at TCD. And, in particular, Kurt Leonhartsberger, FH Technikum Wien and Kleinwindkraft 2016 and sponsors.

29 Thank You Questions?