2. Overview What is a weak system? Useful metric to identify weak systems: SCR What could possibly go wrong? Is a weak system so bad? What can you do? Risk management Mitigation alternatives Discussion and Questions Slide 2

3. What is a strong system? Slide 3

4. What is a weak system? Slide 4

5. Is the network weak or strong? Imagine putting a fault on the system, and measuring the current in the fault… strong systems will have lots of current! We calculate an MVA number based on this current, called “Short Circuit MVA”, or SCMVA STRONG SYSTEM = Big SCMVA Weak System = Little SCMVA Slide 5

6. Is the network relatively weak or strong? Slide 6

7. Why is low SCR a problem? If the wind plant is relatively large with respect to the system strength (low SCR): Wind plant has a large capacity to affect, or move the system Fast power electronics require a steady voltage and frequency to operate in a stable way Conventional power systems study tools may not be sufficiently detailed to represent controls in weak systems Slide 7

8. Why is low SCR a problem? For example: for a typical Type 3 turbine manufacturer… Slide 8 SCR ValueConcerns? Less than 1.5-Power electronics can’t maintain control… wind plant will not run at full power. -Conventional study tools (Transient stability) may not run. Less than 2.5-Potential for control problems... Wind plant may trip inappropriately, or interact badly with the external network -Conventional study tools may not be accurate Higher than 2.5-Wind plant will likely perform predictably

9. Problems… SCR < 1.5 Slide 9 Real Power Reactive Power Simulation starts at red line…

10. Problems… 1.5 < SCR < 2.5 Slide 10 Wind plant starts, but trips after fault Wind plant starts, but interacts with system after fault

11. Control tuning, or increased SCR? Slide 11 Wind plant recovers from fault, but hold your nose! Wind plant recovers cleanly after fault

12. What to do? Manage Risk! Find out SCR early in project planning. It’s an easy calculation! Guide interconnection study process to ensure case lists represent potential problems If necessary, do detailed studies. Electromagnetic transient (EMT) tools can provide great visibility into precise wind farm behavior. Manufacturers generally have EMT capability, but may need some advance notice. Avoid study delays! Get manufacturer involved at an early stage to optimize controls for low SCR Slide 12

13. What if you have a real problem? If SCR is too low, and studies are showing concerns, you can increase the SCMVA to increase the SCR. Synchronous condensers increase SCMVA and system inertia. (BE CAREFUL… you can make new problems!) Additional network infrastructure (More lines increase SCMVA… $$) Slide 13

14. What if you have a real problem? Slide 14 Sync Condensers may introduce angular instability…

15. What if you have a real problem? If new lines or sync condensers are out of the question: Lots of study and heroic control tuning measures (You will make your consultants and manufacturers nervous) Special protection and remedial action (Reliability standards may prevent this) Smaller wind farm! (Directly increase SCR… it will work!) Slide 15

16. Thank You!

17. Cigre B4-62 Addition: Challenges as we see them! General trend toward weak systems, reduced inertia, complex electronics Lack of Industry Awareness Planning departments and wind developers sometimes completely unaware Utilities who are aware still have trouble imposing effective standards for planning studies (lack of guidelines and standards) Problems with SCR metric Doesn’t properly account for nearby wind and power electronics (eg. STATCOMS) Doesn’t measure detrimental effect of shunt capacitors. Could we use 60 Hz impedance- based metric rather? Eg… (V*V/Z 60 )/(Plant MW) Effective Strength Ratio (ESR)? Relative Strength Ratio (RSR)? Model Quality and Availability Transient stability algorithms/models may be inadequate to predict problems (Simplified controls, protections, neglecting PLL, etc) EMT type models are still poor quality industry wide, although getting better Independent consultants are required (to meet NDA req’ts for EMT models)… unacceptable for building EMT into regular planning practice. Slide 17