FACTS for Oscillation Damping

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FACTS for Oscillation Damping
by N. Mithulananthan, Ph.D.
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EPSM, Energy, Asian Institute of Technology,
December 16, 2004
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Outline

• Introduction
• Oscillation in Power Systems
• Oscillation Control
• FACTS Controllers
• Controller Placement and Input Signals
• Simulation Results
• Conclusions

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Introduction

• Pertinent reasons behind some instability
  problems in power system
• Saddle-node bifurcations
• Limit-induced bifurcations
• Hopf bifurcations
• Saddle-node and certain types of limit-induced
  bifurcations consist of loss of equilibrium
• Hopf bifurcations produce limit cycles that may
  lead to oscillatory instability
• Hopf bifurcations (oscillation problems) more
  likely to occur in heavily loaded systems or
  stressed systems

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Introduction (cont.)

• Hopf bifurcations in power system, could arise
  due one or more of the following reasons
• Voltage controls issues
• Variable net damping
• Frequency dependence of electrical torque
• Some major system collapses related to Hopf
  bifurcation
• Midwestern US disturbance in 1992
• WSCC disturbance in 1996
• With ways of predicting and controlling Hopf
  bifurcations these incidents could have been
  avoided or minimized

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Oscillation in Power Systems

• Power systems are modeled using DAEs
• Bifurcation analysis is based on eigenvalue
  analysis of the linearized system
• DAE systems can be reduced to a set of ODE

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Oscillation in Power Systems (cont.)

• Consider a dynamical power system described by
  DAEs
• As parameters (?,p) vary, the equilibrium points
  change
• The equilibrium points are asymptotically stable
  if all the eigenvalues of the system state matrix
  have negative real parts

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Oscillation in Power Systems (cont.)

• A Hopf bifurcation or oscillation occurs when the
  following conditions are satisfied
• It should be an equilibrium, i.e.
• The Jacobian matrix evaluated at (xo, yo, ?o, po)
  should have a simple pair of purely imaginary
  eigenvalues, i.e. µ?j?
• The rate of change of the real part of the
  critical eigenvalues with respect to a varying
  system parameter should be nonzero, i.e.

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Oscillation in Power Systems (cont.)

• Locus of the critical eigenvalue at a Hopf
  bifurcation
• Hopf bifurcations are typically detected by
  monitoring the eigenvalues of the system state
  matrix

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Oscillation in Power Systems (cont.)

• IEEE 14 bus test system

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Oscillation in Power Systems (cont.)

• IEEE 14-bus test system example
• Loads were modeled using constant PQ, both in
  power flow and stability studies
• All the loads were increased according to

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PV curves at bus 14
Eigenvalues at Hopf Bifurcation base case
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Oscillation in Power Systems (cont.)

• IEEE 14-bus test system example

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Oscillation due to Hopf bifurcation triggered by
line 2-3 outage
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Oscillation Control

• Planning stage
• Introducing different power system controllers
• Take different generation directions
• Operational stage
• Load shedding
• Power system controllers
• Power System Stabilizers (PSS)
• FACTS Controllers
• Static Var Compensators (SVC)
• Static Synchronous Compensators (STATCOM)
• Thyristor Controlled Series Compensators (TCSC)
• Static Series Synchronous Compensators (SSSC)
• Unified Power Flow Controllers (UPFC)

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Oscillation Control (cont.)

• PSS Controller
• An additional control block of a generator
  excitation system
• Use to eliminate power frequency oscillation

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FACTS Controllers

• SVC Controller
• A shunt-connected FACTS controller injects
  capacitive or inductive current so as to maintain
  or control a specific variable
• Use to improve voltage stability, power system
  oscillation damping

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Structure of an SVC controller
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FACTS Controllers (cont.)

• Control block diagram of SVC with additional loop
  for oscillation damping

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FACTS Controllers (cont.)

• STATCOM Controller
• Can be viewed as an ideal synchronous machine
  without inertia
• A STATCOM converts an input dc voltage into
  three-phase output voltage at fundamental
  frequency with rapidly controllable amplitude and
  phase angle

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Structure of STATCOM Controller
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FACTS Controllers (cont.)

• Control block diagram of STATCOM with additional
  loop for oscillation damping (Phase control)

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FACTS Controllers (cont.)

• Damping Enhancement
• If the rotor is accelerating (d?/dt is positive),
  due to kinetic energy built up, the FACTS
  controller is controlled to decrease the
  electrical power output
• If the rotor is decelerating (d?/dt is negative),
  due to loss of kinetic energy, the FACTS
  controller is controlled to increase the
  electrical power output
• Modulation of SVC bus voltage also enhance the
  damping, for which an auxiliary signal is needed

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Placement and Control Input Signal

• The main issues in Hopf bifurcation control using
  various power system controllers are
• Placement
• Best control input signals
• PSS should be placed in the generation side
• Placement Participation factor analysis
• Control input signal Rotor speed deviation
• FACTS controllers can be placed at any locations
  and be designed to use a variety of control
  signal

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Placement and Control Input Signal (cont.)

• Placement depends on the desired objective
• Examples
• Loadabilty improvement Weakest bus in the system
• Transfer capability mid-point of the
  transmission line
• Placement Methodologies
• Eigenvalue based methodology
• Optimization techniques
• Mode controllability index can also be used for
  placing FACTS controllers to damp out the
  oscillatory mode

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Placement and Control Input Signal (cont.)

• Auxiliary signals for power system damping
  enhancement
• Local Signals
• Line current (I)
• Line power flow real/reactive power (P/Q)
• Bus voltage/angle (V/?)
• Remote Signals
• Rotor angle/speed deviation of a remote generator
• Angle/frequency difference between remote
  voltages at the two end of a transmission line
• Mode observability index (OI) can be used to
  select the best control input signal

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Simulation Results

• IEEE 50-machine test system
• This system was developed for stability studies
  in 1991
• An approximate model of an actual power systems
• There are 50 generators, 145 buses and 453
  lines, including 52 fixed tap transformers
• There are 60 loads for a total of 2.83 GW and
  0.80 Gvar
• In the stability analysis
• Six of the generators were modeled in details
  with exciters
• Other generators were represented using their
  swing equations

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Simulation Results (cont.)

• Hopf bifurcation Control in IEEE 50-machine test
  system

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PV curves at bus 92
Oscillation due to Hopf triggered by line 90-92
Outage
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Simulation Results (cont.)

• Hopf bifurcation control using PSS
• Installation location was chosen using
  participation factor analysis
• Generator at bus 93 was found to be the best
  location for a PSS in the base case
• However, this did not resolve the problem due to
  the line outage 90-92

Base Case
Line 90-92 Outage
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Simulation Results (cont.)

• Effect of PSS controller in IEEE 50-machine test
  system

With PSS at bus 93
With PSSs at 93 104
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Oscillation control using PSSs at buses 93 104
Eigenvalues and PV curves with PSSs for line
90-92 outage
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Simulation Results (cont.)

• Hopf Bifurcation control using SVC and STATCOM
  controllers
• Controller was placed using extended eigenvector
  method
• Loadability analysis indicates that the best
  location as bus 107, for improving the distance
  to voltage collapse

Critical Eigenvalue with
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Simulation Results (cont.)

• Hopf Bifurcation control using SVC and STATCOM
  controllers
• Additional input signal was chosen using mode
  observability index

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Simulation Results (cont.)

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Eigenvalues of the system wth SVC (a) with
STATCOM (b) PV curves without any controller,
with SVC and with STATCOM (c)
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Simulation Results (cont.)

• Effect of SVC and STATCOM controllers in IEEE
  50-machine test system

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Oscillation control using SVC at bus 125 with
supplementary control
Oscillation control using STATCOM at bus 125 with
supplementary control
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Simulation Results (cont.)

• Static loading margin with different controllers

Maximum Loading Margin (p.u.)
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Conclusions

• Power system oscillation or Hopf bifurcation can
  be controlled from the generation or transmission
  side
• It is well-known fact that the PSS controllers
  are effective in removing oscillation
• It has been demonstrated that the SVC and STATCOM
  controllers too well suited for these purpose
• Both SVC and STATCOM also improved the distance
  to voltage collapse
• Placement of these FACTS controllers is critical
  and depends on the objective

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Conclusions (cont.)

• General
• Power system performance can be improved using
  various FACTS controllers
• Better utilization of the existing transmission
  facilities and generating reserve can be achieved
  using FACTS controllers
• FACTS controllers are not limited to transmission
  system, can be placed in the distribution system
  as well
• Of course these controllers are very expensive,
  but, there are abundant advantages which would
  make them a viable alternative

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