David J Hill City University of Hong Kong

1. Bulk Power System Dynamics and Control V IREP2001:OnomichiGLOBAL HYBRID CONTROL OF POWER SYSTEMS David J Hill City University of Hong Kong (on leave from Sydney University) Co-authors: Yi Guo, Mats Larsson, Youyi Wang

2. OUTLINE • Introduction • Global Control Ideas • Global Transient Stability and Voltage Regulation • Emergency Voltage Control • Conclusions

3. GLOBAL CONTROL IDEAS • Introduction • Hybrid Models • Control Elements • Bifurcations and Global Control • Optimal Coordination and Swarming • Issues for Practical Implementation

4. Trends • Environmental limits • Load growth • Deregulation All push the system harder

5. Mathematical Complexity • Stability margins reducing, ie more difficult dynamics (nonlinearity) • Interconnection, ie larger-scale • More uncertainty • System structure changing • No nominal operating point • Less modelling data • Coordinated control with mixed signals, costs and actions (heterogeneity)

6. Specific Features of Complexity • Large-scale network structure • Controls embedded, some with scope for tuning; further design must allow for and enlist • Hierarchical control structure • Control actions largely determined and have diverse timing, cost and priority • Control goals are multi-objective with local and global requirements which vary with operating state • Control interacts with planning

7. Control Challenge In general, we need a high-level version of distributed adaptive control which “swarms” around a complex system attacking problems as they arise, but keeping a meta-view so that other problems are not ignored ie. “reconfigurability” built in

8. dynamic state variables x algebraic state variables ω parameters/controls l = (q, u, u) Hybrid Models

9. Control Elements Those existing controllers and their tunable parameters which are free to adjust for system-wide purposes

10. Bifurcations and Global Control • Power systems have benefited from bifurcation theory • Most nonlinear control methodology does not recognise bifurcations

11. Bifurcation Control • Avoiding the bifurcation • Eliminating the bifurcation • “Delaying” the bifurcation • Stabilisation through bifurcations Can we control across boundaries?

12. What Can Modern Control Do? • Robust control • Adaptive control • Nonlinear control • Fuzzy control • Neural control

13. A Strategy • Bifurcation boundaries define domains of operation where dynamical behaviour is qualitatively different • Design controllers for each region and switch between them

14. Optimal Coordination and Swarming • Nonlinear, multiple controls • Swarming via mi • Optimal coordination via qi

15. Global Control • Global view of nonlinear system • State space segmentation into structurally stable regions • Identification of regional controllers • local models • various control objectives • different regional controller design approaches • Combination and coordination of regional controllers, e.g. scheduling, switching, hierarchical, hybrid control

16. Control Algorithms • Local tunable controllers, eg robust, adaptive etc • Optimal control (hybrid systems) • Staged optimisation • Predictive control • Speed-gradient and passivity • Structure in HJ eqn, etc

17. GLOBAL TRANSIENT STABILITY AND VOLTAGE REGULATION • Introduction • Dynamical Model • Local Controllers • Global Controller Reference: Y Guo, DJHill and Y Wang,Global transient stability and voltage regulation for power systems, IEEE Trans Power Systems, to appear.

18. Introduction • Transient stability and voltage regulation are required at different stages of system operation • Deal with the two problems separately, or employ a switching strategy of two different controllers which causes a discontinuity of system behaviour • Aim to design global control law to co-ordinate the transient stabilizer and voltage regulator, using heterogeneous control strategy • The global control objective is achieved with smooth and robust responses with respect to different transient faults.

19. SMIB Power System Model

20. Local Controllers • Transient controller: • Voltage controller:

21. A Switching Controller (t0 is the fault occuring time, ts is the switching time) Disadvantages: • The switching time is fixed; • Not robust with respect to different faults.

22. Global Controller Design • The fault sequence is NOT known beforehand • The control law in each region is specified to be the usual type developed from model-based (nonlinear) control techniques • The global control law is the above weighted sum of local controllers type, which achieves smooth transitions between the transient period and post-transient period • The controller is globally effective in the presence of different uncertain faults; also the controller is robust with respect to parameter uncertainties

24. Global Controller Design • Operating region membership function:

25. Global Controller Design • Global control law: • Advantages: • Control action is determined by online measurement of power frequency and voltage, which makes it unnecessary to know the fault sequence beforehand • The controller is globally effective in the presence of different uncertain faults • The controller inherits the properties of local controllers, i.e., it is robust with respect to parameter uncertainties

26. Simulations • Temporary fault + permanent fault: Stage 1: The system is in a pre-fault steady state Stage 2: A fault occurs at t=t0 Stage 3: The fault is removed by opening the breakers of the faulted line at t=t1 Stage 4: The transmission lines are restored at t=t3 Stage 5: Another fault occurs at t=t4 Stage 6: The fault is removed by opening the breakers of the faulted line at t=t5 Stage 7: The system is in a post-fault state In the simulations, t0=0.1s,t1=0.25s,t3=1.4s, t4=2.1s, t5=2.25s; l=0.04.

29. EMERGENCY VOLTAGE CONTROL • Introduction • System Modelling • Control Problem Formulation • Tree Search Method • Simulation Results • Other Possibilities Reference: M Larsson, DJHill and G Olsson, Emergency voltage control using searching and predictive control, International J of Electrical Power and Energy Systems, to appear.

30. Coordinated Control Scheme(Popovic, Hill and Wu, presented in Santorini) • Provide voltage regulation • Provide security enhancement • Control actions • reactive power compensation • tap regulation • load control • FACTs • Traditionally, done one by one, trial and error

31. Why coordination • minimum overall effort / cost • maximum control effect • better voltage profile, ie. better quality of supply • Difficulty • Combination of dissimilar controls

32. Optimal scheduling of control actions • Actual control sequence accounts for • combination of dissimilar controls • different response speeds • different dynamic characteristics • priority • Optimal scheduling by • economic cost • availability of controls • When, how to take actions at each step?

33. Problem formulation subject to: (i) controls capability constraints (ii) stability constraints

34. Optimal Scheduling (=0.2)

35. Model Predictive Control approach • Widespread in process control • Multivariable, nonlinear allowed naturally • Constraint handling • Future behaviour predicted for many candidate input sequences • Optimal input sequence selected by (constrained) optimization

37. Optimization by search • All controls are switching actions • Combinatorial optimization problem • Organize control state space in tree structure • Search tree for optimum • Combinatorial explosion • Search heuristics • Similar problem as solved in chess computers!

38. Numerical example

39. Simulation Example (Fig 17)

40. CONCLUSIONS • Complex System Features • Global Control • Possibilities for Power Systems

41. Complex System Features • Control over wide ranges of operating conditions • Nonlinearity, ie control “in the large” • High dimension, ie large-scale • Multiple steady-state solutions • Qualitatively different behavior under different operating conditions • Lack of complete explicitly analytical description • Indices flag proximity to problems, ie bifurcations • ‘Elements’ of control physically based • Accommodate different control objectives • Optimal coordination required

42. Global Control • Global view of nonlinear system • State space segmentation into structurally stable regions • Identification of regional controllers • local models • various control objectives • Optimal combination and coordination of regional controllers, e.g. scheduling, switching, hierarchical, hybrid control • Swarming type adaptive control

43. Possibilities for Power Systems • Power systems are increasing in complexity • Security limits have huge financial implications • Control-based expansion • Modelling, analysis, control might all need to be redone • Develop hybrid, global models and control • Develop swarming type optimal hierarchical control of all available devices • Multi-level swarming, ie devices to system levels, according to where problem is • Adaptively group up the influential and available controls of various types to attack a problem as and when it arises • Project in HK considers power electronic controls.