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 (vajjarap@iastate.edu) Iowa State University Ames, USA

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

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

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

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

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.

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

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.

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

Numerical results for Identification: CPF

Example For Voltage Stability Boundary: Reactive Power Variation

Optimal Margin Boundary Tracing

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:

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.

Sample Result for OMBT

Continuation Based Time Domain Simulation

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

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

QSS Simulation

(A+AT) singular before SNB? Overall Strategy  t 0 1 2 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

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

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

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

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

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

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

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

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

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

Application to Botswana 32 Bus Test System

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

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

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

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.

Web Based Interaction

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

Simulation Website http://design-2.ece.iastate.edu/newsim/index.php

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

Simulation Website Cont.. Upload Input Files

Simulation Website Cont.. Download Sample Systems

Multiple Simulation Outputs Tabular Output

Outputs Cont.. Graphic PV Curve

Outputs Cont.. Generator Reactive Power Output