What do you think about this system response?

1. What do you think about this system response? Rotor Angle Time

2. How about this response? Rotor Angle Time

3. Compare these two responses Rotor Angle Time

4. What about these responses? Rotor Angle Time

5. Compare these instabilities Rotor Angle Time

6. Steady-state = stable equilibrium • things are not changing • concerned with whether the system variables are within the correct limits

7. Transient Stability • "Transient" means changing • The state of the system is changing • We are concerned with the transition from one equilibrium to another • The change is a result of a "large" disturbance

8. Primary Questions • 1. Does the system reach a new steady state that is acceptable? • 2. Do the variables of the system remain within safe limits as the system moves from one state to the next?

9. Main Concern: synchronism of system synchronous machines • Instability => at least one rotor angle becomes unbounded with respect to the rest of the system • Also referred to as "going out of step" or "slipping a pole"

10. Additional Concerns: limits on other system variables • Transient Voltage Dips • Short-term current & power limits

11. Time Frame • Typical time frame of concern • 1 - 30 seconds • Model system components that are "active" in this time scale • Faster changes -> assume instantaneous • Slower changes -> assume constants

12. Primary components to be modeled • Synchronous generators

13. Traditional control options • Generation based control • exciters, speed governors, voltage regulators, power system stabilizers

14. Traditional Transmission Control Devices • Slow changes • modeled as a constant value

15. FACTS Devices • May respond in the 1-30 second time frame • modeled as active devices

16. May be used to help control transient stability problems

17. Kundur's classification of methods for improving T.S. • Minimization of disturbance severity and duration • Increase in forces restoring synchronism • Reduction of accelerating torque by reducing input mechanical power • Reduction of accelerating torque by applying artificial load

18. Commonly used methods of improving transient stability • High-speed fault clearing, reduction of transmission system impedance, shunt compensation, dynamic braking, reactor switching, independent and single-pole switching, fast-valving of steam systems, generator tripping, controlled separation, high-speed excitation systems, discontinuous excitation control, and control of HVDC links

19. FACTS devices = Exciting control opportunities! • Deregulation & separation of transmission & generation functions of a utility • FACTS devices can help to control transient problems from the transmission system

20. 3 Minute In-Class Activity • 1. Pick a partner • 2. Person wearing the most blue = scribe Other person = speaker • 3. Write a one-sentence definition of "TRANSIENT STABILITY” • 4. Share with the class

21. Mass-Spring Analogy • Mass-Spring System

22. Equations of motion • Newton => F = Ma = Mx’’ • Steady-state = Stable equilibrium = Pre-fault • SF = -K x - D x’ + w = Mball x’’ = 0 • Can solve for x

23. Fault-on system • New equation of motion • SF = -K x - D x’ + (Mball + Mbird)g = (Mball + Mbird) x’’ • Initial Conditions? • x = xss x’ = 0

24. How do we determine x(t)? • Solve directly • Numerical methods • (Euler, Runge-Kutta, etc.) • Energy methods

25. Simulation of the Pre-fault & Fault-on system responses

26. Post-fault system • "New" equation of motion • SF = -K x - D x’ + w = Mball x’’ • Initial Conditions? • x = xc x’ = xc’

27. Simulation of the Pre-fault, Fault-on, and Post-fault system responses

28. Transient Stability? • Does x tend to become unbounded? • Do any of the system variables violate limits in the transition?

29. Power System Equations Start with Newton again .... T = I a We want to describe the motion of the rotating masses of the generators in the system

30. The swing equation • 2H d2d = Paccwo dt2 • P = T w • a = d2d/dt2, acceleration is the second derivative of angular displacement w.r.t. time • w = dd/dt, speed is the first derivative

31. Accelerating Power, Pacc • Pacc = Pmech - Pelec • Steady State => No acceleration • Pacc = 0 => Pmech = Pelec

32. Classical Generator Model • Generator connected to Infinite bus through 2 lossless transmission lines • E’ and xd’ are constants • d is governed by the swing equation

33. Simplifying the system . . . • Combine xd’ & XL1 & XL2 • jXT = jxd’ + jXL1 || jXL2 • The simplified system . . .

34. Recall the power-angle curve • Pelec = E’ |VR| sin( d ) XT

35. Use power-angle curve • Determine steady state (SEP)

36. Fault study • Pre-fault => system as given • Fault => Short circuit at infinite bus • Pelec = [E’(0)/ jXT]sin(d) = 0 • Post-Fault => Open one transmission line • XT2 = xd’ + XL2 > XT

37. Power angle curves

38. Graphical illustration of the fault study

39. Equal Area Criterion • 2Hd2d = Paccwo dt2 • rearrange & multiply both sides by 2dd/dt • 2 ddd2d = wo Paccdd dt dt2 H dt => d {dd}2 = wo Pacc dd dt {dt } H dt

40. Integrating, • {dd}2 = wo Pacc dd{dt} H dt • For the system to be stable, d must go through a maximum => dd/dt must go through zero. Thus . . .dm • wo Pacc dd = 0 = { dd }2 • H { dt } • do

41. The equal area criterion . . . • For the total area to be zero, the positive part must equal the negative part. (A1 = A2) • Pacc dd = A1 <= “Positive” Area • Pacc dd = A2 <= “Negative” Area dcl do dm dcl

42. For the system to be stable for a given clearing angle d, there must be sufficient area under the curve for A2 to “cover” A1.

43. In-class Exercise . . . • Draw a P-d curve • For a clearing angle of 80 degrees • is the system stable? • what is the maximum angle? • For a clearing angle of 120 degrees • is the system stable? • what is the maximum angle?

44. Clearing at 80 degrees

45. Clearing at 120 degrees

46. What would plots of d vs. t look like for these 2 cases?