1. Power System Voltage Control and Stability
Third NSF Workshop on US-Africa Research and Education Collaboration
Abuja, Nigeria, December 13-15, 2004
Power System Voltage Control and Stability
V. Ajjarapu
Iowa State University
Ames, USA

2. Outline Introduction Aim Educators Involved Area of Research
POWER SYSTEM SECURITY AND CONTROL
An Overview
Medium of Interaction
Web based
Travel
Activities Planned

3. Our Department 48 faculty Power: 4 + 2 Over 30 power graduate students
Typical undergraduate power class: 50
Mainly power systems: Security and dynamics , risk analysis, economics and markets
State of the art power electronics lab

4. Aim
Building African collaboration with ISU researchers to build further understanding of different power system case studies.
Developing a roadmap for future research activities in power / energy, information systems technology, environmental and social dynamics, and education pedagogy

5. Educators Involved Prof. George O. Anderson, Univ. of Botswana
Dr. Tanimu Abubakar, Ahmadu Bello University, Zaria, Nigeria
Dr. Venkataramana Ajjarapu (with support from EPNES ISU faculty)
Graduate/Undergraduate Students from these Universities

6. Research : POWER SYSTEM SECURITY AND CONTROL:
Main Area of Expertise : Voltage Stability , Oscillatory Stability, VAR Planning and dispatch , ATC :
Sub Area: Power Electronics and Real Time Control
The group is developing a comprehensive approach for voltage and oscillatory stability analysis and control. This group is also actively involved in the Available Transfer Capability(ATC) evaluation as limited by voltage as well as oscillatory stability. Reactive power requirements and pricing required for deregulated environment. These techniques will be applied to realistic practical power networks.

7. Our Group’s Research Overview
Developing techniques that are very effective for stressed power systems
A single tool can be adapted for cost based security with timing of control information
Available Transfer Capability (ATC) is part of this research
Web Based Security Tools for Distance Education

11. Power Flow, CPF and EQTP
Conventional power flow program calculates steady state solution for a given operating condition.
Continuation power flow (CPF) traces a series of power flow solutions for a given scenario. You can also trace the bottom of the PV curve.
Equilibrium Tracing Program (EQTP) is an extension of CPF. DAE instead of pure power flow equation is used to represent the power system. It has the capability of detecting both voltage and oscillatory instabilities.

12. Security Margin Tracing
Illustration of voltage stability, oscillatory stability and damping ratio boundary tracing
Damping ratio boundary for D= D0
Voltage stability margin boundary
Oscillatory stability margin boundary A1 A2
Predictor
Corrector
Parameter α
Voltage

13. Numerical results for Identification: CPF

14. Example For Voltage Stability Boundary: Reactive Power Variation

15. Optimal Margin Boundary Tracing

16. Basic Formulation Minimize a cost function : f(U)
For a sequence of specified margin levels:
Minimize a cost function : f(U)
Subject to : Equality Conditions : F, G
Inequality constraints : I
Optimality conditions :
Margin boundary conditions : Cobt
Results in a set of nonlinear equations of the form:

17. Formulation (Cont.)
The formulation could automatically generate a whole set of cost based optimal control configurations with each optimal control configuration corresponding to specific margin level that can be realized.

18. Sample Result for OMBT

19. Continuation Based Time Domain Simulation

20. Key Points No divergence due to singularity of
Time step size is adjusted according to the variation of dS/dt
Small time steps for fast dynamics and large time steps for slow dynamics

21. Trajectory Before and After a Line Outage (with load variation)

22. QSS Simulation

23. (A+AT) singular before SNB?
Overall Strategy  t i
Scenario
Credible
Contingency Set
Cj{C1,C2, …,Cm}
Yes
(A+AT) singular before SNB?
j = j +1
Check for (A+AT) singularity No
Trace Eigenvalue for Hopf and Damping Margin
Voltage stability margin
Oscillatory stability margin
Damping ration margin
i = i +1
All Credible
Contingencies
Tested ?
Sufficient
Margin ?
Yes
Preventive
Control
Strategies
Yes No No

24. Research Needs On line monitoring : Fast calculations
Accuracy vs Speed
Timing Information: Fast vs Slow Collapse
Integration of Cost of Voltage and Oscillatory Stability Control in Overall Security Assessment

25. Optimal Strategies for Load Shedding : when, where and how much

26. Capabilities at ISU
A method to quickly identifies voltage and oscillatory instability points.
The change in the instability margin boundaries can be further obtained for any large change in system parameters.
Optimization and Time Domain Simulations are part of Continuation Based Approach
An efficient eigenvalue tracking method is introduced.
Provides damping information
Provides eigenvalue and eigenvector sensitivity information as a by product

27. Capabilities Cont.. Automatic PV Curve Tracing.
Field current and armature current limits are included.
Provides true maximum power point by considering relevant dynamic factors.
There is no need for slack bus
Can solve unsolvable cases created by a line or generator outages

28. Capabilities Cont… Sensitivity Analysis (qualitative)
Screens the contingencies that may lead to voltage collapse
Locates the weak areas , identifies the critical generators
Provides voltage stability margin estimation

29. Capabilities Cont..
Available Transfer Capability as Limited by Voltage and Oscillatory Stability
Simulates simultaneous multi-area transactions
Provides Transaction Sensitivity: Identifies and ranks the transactions that are detrimental to voltage stability
Reactive power pricing /amount /location/ to maintain voltage security

30. Capabilities Cont..
Preventive and Corrective Strategies Against the Voltage Collapse
Strategies for minimum load shedding
Strategies for under-voltage load shedding
Strategies to maintain a certain voltage stability margin

31. Capabilities Cont..
Development of Advanced Algorithms for steady state, dynamic and time domain simulations
A simulation engine that can allow transient and quasi- steady state analysis
to obtain a comprehensive analysis to predict and control instabilities
When , where and how much control

32. Capabilities Cont.. On Line Voltage Stability Monitoring and Control:
Developing methodologies to utilize off line studies for on line implementation

33. Application to Botswana 32 Bus Test System

34. Botswana System (Cont..)
Demand : 243 MW
Supply
Local (coal): 128MW
ESKOM and SAPP : 115MW
Simulation :
Software : Aristo (Advanced Real Time Interactive Simulator for Training and Operation ) by ABB
Dr. Anderson applied this software for Power flow and Fault analysis for 32 bus equivalent system

35. Activities Planned wrt EPNES Project (research)
Task 1: Setting up a test case for Botswana System
Task 2: Study the maximum loadability with respect
various contingencies and scenarios
Task 3: Apply margin sensitivity to find the
locations for shunt capacitors and series
capacitors
Task 4: Develop Automaton for backward algorithm

36. Activities Planned (education)
Botswana group will participate in EE458 course
Develop I hour lecture material for this course

37. EE/Econ 458: Economic systems for planning in electric power
Students completing this course will be able to conduct studies in policy, economics, & engineering associated with transmission expansion; they will enhance their skills using optimization methods for engineering & economic problem-solving.
Evolutionary history of the power industry; present industry structure;
Discrete optimization;
Power system operation;
Electricity markets;
Power system planning and cost-effective solutions;
Cost recovery for transmission investment; basics of public good economics;
Cost-recovery models:
Tariff-based model;
Performance-based model;
Market-driven model;
Tax treatment of transmission investments.
Will offer Fall 2004 on-campus, and via distance-education.

38. Web Based Interaction

39. Characteristics Platform (OS) Independent
Standardized System Input Files
Graphic PV Curves Output
Detailed Simulation Process Output
Margin Sensitivity

40. Simulation Website

41. Simulation Website Cont..
Input Files
IEEE Common Data Format is adopted as the input file for power system topology and parameters
Another standardized control file is used to specify load and generation increase scenario
Sample Test System input files are provided

42. Simulation Website Cont..
Upload Input Files

43. Simulation Website Cont..
Download Sample Systems

44. Multiple Simulation Outputs
Tabular Output

45. Outputs Cont..
Graphic PV Curve

46. Outputs Cont..
Generator Reactive Power Output