Willow Glen Short Circuit Study

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Importance of Reactive Power Management, Voltage
Stability and FACTS Applications in todays
Operating Environment Sharma Kolluri Manager of
Transmission Planning Entergy Services
Inc Engineering Seminar Organized by IEEE
Mississippi Section Jackson State
University August 20, 2010

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Outline

• Introduction
• VAR Basics
• Voltage Stability
• FACTS
• Applications at Entergy
• Summary
• .

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Voltage Profile during Aug 14th Blackout

• Voltages decay to almost 60 of normal voltage.
  This is probably the point that load started
  dropping off.
• However, the recovery is too slow and generators
  are not able to maintain frequency during this
  condition.
• Many generators trip, load shedding goes into
  effect, and then things just shut down due to a
  lack of generation.

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A Near Fast Voltage Collapse in Phoenix in 1995
North American Electric Reliability Council,
System Disturbances, Review of Selected 1995
Electric System Disturbances in North America,
March 1996.
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Recommendation23

• Strengthen Reactive Power and Control Practices
  in all NERC Regions
• Reactive power problem was a significant factor
  in the August 14 outage, and they were also
  important elements in the several of the earlier
  outages
• -Quote form the outage report

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Reactive Power
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Laws of Reactive Physics

• System load is comprised of resistive current
  (such as lights, space heaters) and reactive
  current (induction motor reactance, etc.).
• Total current IT has two components.
• IR resistive current
• IQ reactive current
• IT is the vector sum of IR IQ
• IT IR jIQ

IT
IQ
IR
North American Electric Reliability Corporation
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Laws of Reactive Physics

• Complex Power called Volt Amperes (VA) is
  comprised of resistive current IR and reactive
  current IQ times the voltage.
• VA VIT V (IR jIQ) P jQ
• Power Factor (PF) Cosine of angle between P
  and VA
• P VA times PF
• System Losses
• Ploss IT2 R (Watts)
• Qloss IT2 X (VARs)

VA
Q
P
North American Electric Reliability Corporation
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Reactive Physics VAR loss

• Every component with reactance, X VAR loss IT2
  X
• Z is comprised of resistance R and reactance X
• On 138kV lines, X 2 to 5 times larger than R.
• One 230kV lines, X 5 to 10 times larger than R.
• On 500kV lines, X 25 times larger than R.
• R decreases when conductor diameter increases. X
  increases as the required geometry of phase to
  phase spacing increases.
• VAR loss
• Increases in proportion to the square of the
  total current.
• Is approximately 2 to 25 times larger than Watt
  loss.

North American Electric Reliability Corporation
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Reactive Power for Voltage Support
Reactive Loads
VARs flow from High voltage to Low voltage
import ofVARs indicate reactivepower deficit
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Reactive Power Management/Compensation

• What is Reactive Power Compensation?
• Effectively balancing of capacitive and inductive
  components of a power system to provide
  sufficient voltage support.
• Static and dynamic reactive power
• Essential for reliable operation of power system
• prevention of voltage collapse/blackout
• Benefits of Reactive Power Compensation
• Improves efficiency of power delivery/reduction
  of losses.
• Improves utilization of transmission
  assets/transmission capacity.
• Reduces congestion and increases power transfer
  capability.
• Enhances grid reliability/security.

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Transmission Line Real and Reactive Power Losses
vs. Line Loading

• Source B. Kirby and E. Hirst 1997,
  Ancillary-Service Details Voltage Control,
• ORNL/CON-453, Oak Ridge National Laboratory, Oak
  Ridge, Tenn., December 1997.

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Static and Dynamic VAR Support

• Static Reactive Power Devices
• Cannot quickly change the reactive power level as
  long as the voltage level remains constant.
• Reactive power production level drops when the
  voltage level drops.
• Examples include capacitors and inductors.
• Dynamic Reactive Power Devices
• Can quickly change the MVAR level independent of
  the voltage level.
• Reactive power production level increases when
  the voltage level drops.
• Examples include static VAR compensators (SVC),
  synchronous condensers, and generators.

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Voltage Stability
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Common Definitions

• Voltage stability - ability of a power system to
  maintain steady voltages at all the buses in the
  system after disturbance.
• Voltage collapse - A condition of a blackout or
  abnormally low voltages in significant part of
  the power system.
• Short term voltage stability - involves the
  dynamics of fast acting load components such as
  induction motors, electronically controlled
  loads, and HVDC converters.
• Long term voltage stability - involves slower
  acting equipments such as tap-changing
  transformer, thermostatically controlled loads,
  and generator limiters.

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What is Voltage Instability/Collapse?

• A power system undergoes voltage collapse if
  post-disturbance voltages are below acceptable
  limits
• voltage collapse may be due to voltage or angular
  instability
• Main factor causing voltage instability is the
  inability of the power systems to maintain a
  proper balance of reactive power and voltage
  control

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Voltage Instability/Collapse

• The driving force for voltage instability is
  usually the load
• The possible outcome of voltage instability
• loss of loads
• loss of integrity of the power system
• Voltage stability timeframe
• transient voltage instability 0 to 10 secs
• long-term voltage stability 1 10 mins

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Voltage stability causes and analysis

• Causes of voltage instability
• Increase in loading
• Generators, synchronous condensers, or SVCs
  reaching reactive power limits
• Tap-changing transformer action
• Load recovery dynamics
• Tripping of heavily loaded lines, generators
• Methods of voltage stability analysis
• Static analysis methods
• Algebraic equations, bulk system studies, power
  flow or continuation power flow methods
• Dynamic analysis methods
• Differential as well as algebraic equations,
  dynamic modeling of power system components
  required

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Generator Capability Curve
Over-excitation Limit
Lagging (Over-excited)
0.8 pf line
Stator Winding Heating Limit
- Per unit MVAR (Q)
Normal Excitation (Q 0, pF 1)
MW
Turbine Limit
Leading (Under-excited)
Under-excitation Limit
Stability Limit
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P-V Curve
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Q-V Curve
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Key Concerns
Voltage (pu)
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Possible Solutions for Voltage Instability

• Install/Operate Shunt Capacitor Banks
• Add dynamic Shunt Compensation in the form of
  SVC/STATCOM to mitigate transient voltage dips
• Add Series Compensation on transmission lines in
  the problem area
• Implement UVLS Scheme
• Construct transmission facilities

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Voltage Collapse
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Fault Induced Delayed Voltage Recovery (FIDVR)

• FIDVR Definition
• Load Models

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Fault Induced Delayed Voltage Recovery (FIDVR)

• What is it?
• After a fault has cleared, the voltage stays at
  low levels (below 80) for several seconds
• Results in dropping load / generation or fast
  voltage collapse
• 4 key factors drive FIDVR
• Fault Duration
• Fault Location
• High load level with high Induction motor
  load penetration
• Unfavorable Generation Pattern

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Load characteristics

• The accuracy of analytical results depends on
  modeling of power system components, devices, and
  controls.
• Power system components - Generators, excitation
  systems, over/under excitation limiters, static
  VAr systems, mechanically switched capacitors,
  under load tap changing transformers, and loads
  among others.
• Loads are most difficult to model.
• Complex in behavior varying with time and
  location
• Consist of a large number of continuous and
  discrete controls and protection systems
• Dynamics of loads, especially, induction motors
  at low voltage levels should be properly modeled.

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Induction motor characteristics

• Impact of fault on transmission grid
• Depressed voltages at distribution feeders and
  motor terminals
• Reduction of electrical torque by the square of
  the voltage resulting in slow down of motors
• The slow down depends on the mechanical torque
  characteristics and motor inertias
• With fault clearing

Fig. 1 Induction motor characteristics

• Partial voltage recovery
• Slowed motors draw high reactive currents,
  depressing voltage magnitudes
• Motor will reaccelerate to normal speed if,
  electrical torquegtmechanical torque
• else, the motors will rundown, stall, and trip
• The problem is severe in the summer time with
  large proportion of air conditioner
• motors

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Air conditioner motor characteristics

• Characteristics
• Main portion (80-87) consumed by compressor
  motor
• Electromagnetic contactor drop out between
  (43-56) of the nominal voltage and reclose above
  drop out voltage
• Stalling at (50-73) of the nominal voltage
• Thermal overload protection act if motors stall
  for 5-20 seconds
• The operation time of thermal over load (TOL)
  protection relay is inversely proportional to the
  applied voltage at the terminal
• Air conditioner should be modeled to analyze the
  short term voltage stability problem
• Quite important for utilities in the Western
  interconnection

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Load modeling

• Old models Loads are represented as lumped load
  at distribution feeder
• Does not consider the electrical distance between
  the transmission bus and the end load components
• The diversity in composition and dynamic behavior
  of various electrical loads is not modeled
• Modeling
• WECC interim model
• 20 of the load as generic induction motor load
• 80 constant current P and constant impedance Q

Fig. 2 Traditional load model
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Composite load modeling

• Representation of distribution equivalent
• Feeder reactance
• Substation transformer reactance
• Parameters of various load components
• Discharge lighting
• Electronic Loads
• Constant Impedance loads
• Motor loads
• Distribution Capacitor

Fig. 3 Composite load model structure
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FACTS
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What is FACTS?

• Alternating Current Transmission Systems
  Incorporating Power Electronic Based and Other
  Static Controllers to Enhance Controllability and
  Increase Power Transfer Capability.
• power semi-conductor based inverters
• information and control technologies

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Major FACTS Controllers

• Static VAR Compensator (SVC)
• Static Reactive Compensator (STATCOM)
• Static Series Synchr. Compensator (SSSC)
• Unified Power Flow Controller (UPFC)
• Back-To-Back DC Link (BTB)

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FACTS Applications
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Static VAr compensator (SVC)

• Variable reactive power source
• Can generate as well as absorb reactive power
• Maximum and minimum limits on reactive power
  output depends on limiting values of capacitive
  and inductive susceptances.
• Droop characteristic

Fig. 4 Schematic diagram of an SVC
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Static compensator (STATCOM)

• Voltage source converter device
• Alternating voltage source behind a coupling
  reactance
• Can be operated at its full output current even
  at very low voltages
• Depending upon manufacturer's design, STATCOMs
  may have increased transient rating both in
  inductive as well as capacitive mode of operation

Fig. 5 Schematic diagram of STATCOM
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Technology Applications at Entergy
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Technology Applications at Entergy to Address
Reactive Power Issues

• Large Shunt Capacitor Banks
• UVLS
• Series Compensation
• SVC
• Coordinated Capacitor Bank Control
• DVAR
• AVR

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Determining Reactive Power Requirements in the
Southern Part of the Entergy System for Improving
Voltage Security A Case Study
Sharma Kolluri Sujit Mandal Entergy Services
Inc New Orleans, LA Panel on Optimal Allocation
of Static and Dynamic VARS for Secure Voltage
Control 2006 Power Systems Conference and
Exposition Atlanta, Georgia October 31, 2006
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Areas of Voltage Stability Concern
North Arkansas
Mississippi
West of the Atchafalaya Basin (WOTAB)
Southeast Louisiana
Western Region
Amite South/DSG
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Study Objective

• Identify Voltage Stability Problems in the DSG
  area
• Determine the proper mix of reactive power
  support to address voltage stability problem
• Determine size and location of static and dynamic
  devices.

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Downstream of Gypsy Area - Critical Facilities
Little Gypsy-South Norco 230kV line
Waterford-Ninemile 230kV line
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DSG Issues

• Area load growth
• 1.6 projected for 2003 - 2013
• Weather normalized to 100º F
• Projected peak load 3800 MW
• Area power factor - Low
• 94 at peak load
• Worst double contingency
• Loss of the Waterford to Ninemile 230 kV
  transmission line and one of the 230 kV
  generating units at Ninemile or Michoud

Michoud
Ninemile
New Orleans area voltage profile on June 2,
2003 (with 2 major generators offline)

• Area Problems
• Thermal overloads of underlying 115 kV and 230 kV
  transmission system
• Depressed voltages throughout New Orleans metro
  area potentially leading to voltage collapse and
  load shedding

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Various Steps Used for Determining Reactive Power
Requirements

• Step 1 Problem identification
• Step 2 Determining total reactive power
  requirements
• Step 3 Sizing and locating dynamic devices
• Step 4 Sizing and locating static shunt
  devices
• Step 5 Verification of reactive power
  requirements

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Tools Techniques Used

• Various tools and techniques used for analysis
  purposes
• PV analysis using PowerWorld
• Transient stability using PSS/E Dynamics
• Mid-term stability using PSS/E Dynamics
• PSS/E Optimal Power Flow
• Detailed Models used
• Motor models and appropriate ZIP model for
  dynamic analysis
• Tap-changing distribution transformers,
  overexcitation limiters, self-restoring loads
  modeled in mid-term stability study

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Criteria/Requirements
Voltage (pu)
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Steady State AnalysisResults
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PV CurveNinemile Unit 4 out-of-serviceTrip
Ninemile Unit 5 and Waterford Ninemile 230 kV
line
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Dynamic Analysis
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Stability Simulation Ninemile Unit 4
out-of-serviceTrip Ninemile Unit 5 and Waterford
Ninemile 230 kV line
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Process for Determining Reactive Power
Requirements

• Approx 700 MVAr of reactive power shortage
  identified in the DSG
• How much static and how much dynamic?
• Criteria for determining static and dynamic
  requirements
• Voltage at critical buses should recover to 1 pu
  in several seconds
• Voltage at critical buses should recover to 0.9
  pu within 1.5 - 2 seconds
• Voltage should not dip below 0.7 pu for more than
  20 cycles
• Generator reactive power output should be below
  Qmax
• Factors considered in sizing static/dynamic
  devices
• Short circuit levels, size location of the
  stations, number and existing size of cap banks,
  back-to-back switching, etc

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SVC Size and Location

• Sites considered
• Ninemile 230 kV
• Gretna 115 kV
• Paterson 115 kV
• Size
• 300 MVAR
• 500 MVAR

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Steps to locate Static Shunt Devices

• Static shunt requirements 400 MVAR
  approximately
• Options available to locate the static shunt
  devices on the transmission or distribution
  systems
• OPF Program used to come up with size and
  location of shunt devices

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OPF Application

• PSS/E OPF Program used
• Objective Function Minimize adjustable shunts
• OPF simulated for critical contingencies

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List of Shunt Capacitor Banks Banks Recommended
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Simulation Results with the Capacitors and SVC
Ninemile Unit 4 out-of-serviceTrip Ninemile
Unit 5 and Waterford Ninemile 230 kV line
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SVC Performance Ninemile Unit 4
out-of-serviceTrip Ninemile Unit 5 and Waterford
Ninemile 230 kV line
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Summary

• Process for determining static and dynamic
  reactive power requirements discussed
• OPF program utilized for sizing/locating static
  shunt capacitor banks
• Results verified using mid-term stability
  simulations
• Study recommendation 400 MVAR of static shunt
  devices and 300 MVAR of dynamic shunt compensation

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Ninemile SVC Configuration
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External Device ControlSingle line diagram of
SVC and MSC
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SVC Ninemile
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SVC Ninemile
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Porter 0/300Mvar SVC
SVC Topology 2 x 75MVAr TSC 1 x 150MVAr TSC
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Porter Static Var Compensator (SVC)
Maintains system voltage by continuously varying
VAR output to meet system demands Controls
capacitor banks on the transmission system to
match reactive output to the load requirements.
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Porter SVC
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Series Capacitor Dayton Bulk 230kV Station
The Capacitor offsets reactance in the line,
making it appear to the system to be half of its
actual length. Power flows are redirected over
this larger line, unloading parallel lines and
increasing transfer capability.
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DSMES Unit
Stores Energy in a superconducting coil
Automatically releases energy to the system when
needed to ride through voltage dips caused by
faults. This unit improves power quality and
reduces customer loss of production.
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Industry Issues

• Coordination of reactive power between regions
• No clearly defined requirements for reactive
  power reserves
• Proper tools for optimizing reactive power
  requirements
• Incentive to reduce losses

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Summary

• The increasing need to operate the transmission
  system at its maximum safe transfer limit has
  become a primary concern at most utilities
• Reactive power supply or VAR management is an
  important ingredient in maintaining healthy power
  system voltages and facilitating power transfers
• Inadequate reactive power supply was a major
  factor in most of the recent blackouts

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Questions?
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Under Voltage Load Shed Logic - Western Region

• TD Planning
• April 2010

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Western Region Overview
230 kV Tie Lines
Generation
Load Center
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Load Projection

• 2010 peak 1770 MW
• 2012 peak 1852 MW

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Sample PV Curve ResultLewis Creek Unit 1
China-Porter 230kV Out - 2010
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2010 Summer PV Curve Analysis
Approved Construction Plan Projects
included Relocate Caney Creek 138kV
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Dynamic Analysis Results
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Results 2010 case without load shed
Case 3 Voltages (pu) Goslin 0.810 Conroe
0.855 Cleveland 0.909 Jacinto 0.924 Dayton
0.944 Huntsville 0.944
Case 4 Voltages (pu) Goslin 0.757 Conroe
0.800 Dayton 0.913 Huntsville 0.928
Cleveland 0.928 Rivtrin 0.941
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2010 Summer Conditions - Dynamics Analysis

• Lewis Creek Unit 1 outaged in the base case
• 50 induction motor load is modeled
• Result Shed Load Block 1 (183 MW)

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• Observations for 2010 Summer Peak Conditions
• Existing load shed logic in Western Region OK for
  2010 Summer conditions
• Voltage at some critical buses drop below 0.7 pu
  for more than 20 cycles Potential of motor load
  tripping
• Conclusions for 2010 Summer
• Reducing load shed blocks to 180 70 MW in
  Western Region has no negative impact

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Results 2010 case with load shed (Block 1)
Case 3 Voltages (pu) Goslin 0.872 Conroe
0.902 Cleveland 0.934 Jacinto 0.948 Dayton
0.966 Huntsville 0.968
Case 4 Voltages (pu) Goslin 0.827 Conroe
0.855 Dayton 0.939 Cleveland 0.951
Huntsville 0.954 Jacinto 0.964
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Conclusions and Recommendations

• Retain the exiting UVLS logic
• Change the load blocks
• Block one 180 MW
• Block two 70 MW (existing size 111 MW)

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Proposed Load Shed Logic
Voltage _at_ 4/8 buses lt0.90 pu
Armed all time
Drop load
OEL at Lewis Creek units
One or more Lewis Creek units in-service?
Voltage _at_ 4/8 buses lt 0.92 pu
Time Delay 3 seconds
Load Blocks Block 1 175 MW Alden 50 MW Metro
35 MW Oakridge30 MW Goslin 60 MW Block 2 75
MW In the vicinity of Block 1
Monitored Buses Metro 138kV Goslin 138kV Alden
138kV Oakridge 138kV Huntsville 138kV Rivtrin 138
kV Poco 138 kV Conroe 138 kV
Reset the Process for next LVSH block
Load Blocks Block 1 175 MW Block 2 75 MW
The above conditions need to be met for 3 scans
to trigger load shedding