Power System Voltage Control and Stability

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Power System Voltage Control and Stability
Third NSF Workshop on US-Africa Research and
Education Collaboration Abuja, Nigeria, December
13-15, 2004

• V. Ajjarapu (vajjarap_at_iastate.edu)
• Iowa State University
• Ames, USA

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Outline

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

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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

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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

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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

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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.
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Our Groups 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

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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.

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Security Margin Tracing
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• Numerical results for Identification CPF

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Example For Voltage Stability Boundary Reactive
Power Variation
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Optimal Margin Boundary Tracing
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Basic Formulation
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  

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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.
• 
  

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Sample Result for OMBT
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Continuation Based Time Domain Simulation
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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

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Trajectory Before and After a Line Outage (with
load variation)
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QSS Simulation
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Overall Strategy
Credible Contingency Set CjC1,C2, ,Cm
j j 1
Yes
Check for (AAT) singularity
No
Trace Eigenvalue for Hopf and Damping Margin
Yes
Yes
No
No
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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

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Optimal Strategies for Load Shedding when,
where and how much
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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

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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

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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

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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

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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

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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

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Capabilities Cont..

• On Line Voltage Stability Monitoring and Control
  
• Developing methodologies to utilize off line
  studies for on line implementation

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Application to Botswana 32 Bus Test System
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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

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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
  

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Activities Planned (education)

• Botswana group will participate in EE458 course
• Develop I hour lecture material for this course

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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.
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Web Based Interaction

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Characteristics

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

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Simulation Website

• http//design-2.ece.iastate.edu/newsim/index.php

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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

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Simulation Website Cont..

• Upload Input Files

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Simulation Website Cont..

• Download Sample Systems

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Multiple Simulation Outputs

• Tabular Output

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Outputs Cont..

• Graphic PV Curve

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Outputs Cont..

• Generator Reactive Power Output