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Overview

Wind Farm Challenges In Weak Transmission Systems An Introduction to Weak System Interconnections Andrew L. Isaacs Electranix Corporation WINDPOWER Conference, Chicago IL May 8, 2013. Overview. What is a weak system? Useful metric to identify weak systems: SCR What could possibly go wrong?

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Overview

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  1. Wind Farm Challenges In Weak Transmission SystemsAn Introduction to Weak System InterconnectionsAndrew L. IsaacsElectranix CorporationWINDPOWER Conference, Chicago ILMay 8, 2013

  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

  3. What is a strong system?

  4. What is a weak system?

  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

  6. Is the network relativelyweak or strong? • The size of the wind farm relative to the strength of the system is a useful metric… • We calculate a relative metric called “Short Circuit Ratio”, or SCR. This is not a perfect metric… only a guide. • Relatively Weak = Low SCR • Relatively Strong = High SCR

  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

  8. Why is low SCR a problem? For example: for a typical Type 3 turbine manufacturer…

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

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

  11. Control tuning, or increased SCR? 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

  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… $$)

  14. What if you have a real problem? 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!)

  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/Z60)/(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.

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