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“Importance of Reactive Power Management, Voltage Stability and FACTS
Applications in today’s Operating Environment”
Sharma KolluriManager of Transmission Planning
Entergy Services Inc
Engineering SeminarOrganized 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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• 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.
Voltage Profile during Aug 14th Blackout
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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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Recommendation#23
• 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 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
IR
IQ
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
P
Q
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 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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MW
Stator Winding Heating Limit
Turbine Limit
- P
er u
nit
MV
AR
(Q
) +
0.8 pf line
Under-excitation Limit
Lag
gin
g
(Ove
r-ex
cite
d)
Lea
din
g
(Un
der
-exc
ited
)
Normal Excitation (Q = 0, pF = 1)
Over-excitation Limit
Stability Limit
Generator Capability Curve
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P-V Curve
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Q-V Curve
200
Q-V Curve with Detailed Load Model
Peak Load with Fixed Taps
-80
-60
-40
-20
0
20
40
60
80
100
120
0.5 0.6 0.7 0.8 0.9 1 1.1 1.2 1.3 1.4 1.5
Voltage (p.u.)
Mv
ars
Base Case
Contingency
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Key Concerns
Minimize motor tripping
Limit UVLS activation
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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Villa Rica 500 Pos Seq Volts at Bowen
0.0 4.0 8.0 12.0 16.0 20.0 24.0 28.0 32.0 215000.0 220000.0 225000.0 230000.0 235000.0 240000.0 245000.0 250000.0 255000.0 260000.0 265000.0 270000.0 275000.0 280000.0 285000.0 290000.0 295000.0 300000.0 305000.0 310000.0 315000.0 320000.0 325000.0
7/ 30/ 99 @ 19:25:40.458 hrs CDT
Flt #1
Flt #2
Flt #3
Seconds
Phase- Gnd Voltage Voltage profile at
Plant Bowen 500kV
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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
– Partial voltage recovery– Slowed motors draw high reactive currents, depressing voltage magnitudes– Motor will reaccelerate to normal speed if, electrical torque>mechanical torque
else, the motors will rundown, stall, and trip– The problem is severe in the summer time with large proportion of air conditioner
motors
Fig. 1 Induction motor characteristics
Speed – per unit
Electric torque
1.0
Constant load torque
Square-law load torque
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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
Distribution Capacitor
Distribution Bus
OLTC
Transmission Bus
Lumped Load (ZIP load)
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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
Bus 3
Distribution Capacitor
Transmission Bus
Bus 1
Bus 2
Distribution Bus
Substation Capacitor
OLTC
Distribution Feeder
Feeder Equivalent
Static Loads(Constant impedance,
constant current, constant impedance loads)
Dynamic Loads(Small motor, Large
motor, trip motor loads)
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FACTS 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
Voltage ControlPower System Stability
SSSC
S/S
UPFCPower Generation
Load
IncreasedTransmission Capacity
Inter-area ControlInter-tie Reliability
Power Flow ControlSystem Reliability
ImprovedPower Quality
EnhancedImport Capability
Inter-connectedRTO System
Inter-connectedPower System
BTB
BTB
STATCOM
S/S
S/S
STATCOMLoad
Load
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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
XC
TCR
XL
I
V
Firing angle control
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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
Transformer
DC-AC switching converter
IX
System bus
Cs
Vdc
V
E
Fig. 5 Schematic diagram of STATCOM
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Technology Applications at Technology Applications at
EntergyEntergy
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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
West of the Atchafalaya Basin(WOTAB)
North Arkansas
Southeast Louisiana
Western Region
Amite South/DSG
Mississippi
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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
Waterford-Ninemile 230kV line
Ninemile Units1 - 50 MW2 - 60 MW3 - 128 MW4 - 740 MW5 - 750 MW
115 kV
230 kV
Michoud Units1 - 65 MW2 - 240 MW3 - 515 MW
115 kV
- 230 kV
Little Gypsy-South Norco 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
• 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
New Orleans area voltage profile on June 2, 2003
(with 2 major generators offline)
Michoud
Ninemile
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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
Minimize motor tripping
Improve post-fault voltage
Voltage (pu)
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Steady State AnalysisResults
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PV CurveNinemile Unit 4 out-of-service
Trip Ninemile Unit 5 and Waterford – Ninemile 230 kV line
Without Waterford - 9Mile 230kV line 3of5 & Michoud unit 3 PV curves
0.8
0.85
0.9
0.95
1
1.05
3300 3350 3400 3450 3500 3550 3600 3650 3700 3750 3800
Load in DSG (MW)
Vo
ltag
e (p
.u.)
Berman 230kV
Market 230kV
Tricou 230kV
Almonaster 230kV
PARIS 230kV
Gretna 115kV
Delta 115kV
9mile 230kV
cc
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Dynamic Analysis
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Stability Simulation Ninemile Unit 4 out-of-service
Trip 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
Optimal size and
location
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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-service
Trip Ninemile Unit 5 and Waterford – Ninemile 230 kV line
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SVC Performance Ninemile Unit 4 out-of-service
Trip 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
SN = 300 MVA, uk = 9.5 %
3AC 60Hz 230kV
3AC 60Hz 15.5kV
CTSC2
LTSC2
TSC 2
V2
VR2
TSC 1
CTSC3
LTSC3
TSC 3
V3 VR3
CTSC1
LTSC1
V1
VR1
= 75 MVAr = 75 MVAr = 150 MVAr
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External Device ControlSingle line diagram of SVC and MSC
TSC 1-3
MSC 1
Ninemile 230kV
MSC 10
.................
Different voltage levels
(115 kV)
SVC system
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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
T&D 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
0.8
0.85
0.9
0.95
1
1.05
1.1
1350 1450 1550 1650 1750 1850 1950 2050 2150
-100
-50
0
50
100
150
200
Dayton
Rivtrin
Conroe
Poco
Jacinto
Cleveland
Huntsville
Goslin
Lewis Creek
Pelican Road
Cypress
Frontier
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2010 Summer PV Curve Analysis
Approved Construction Plan Projects included:*Relocate Caney Creek 138kV
ScenariosP Limit (MW)
With 3% Margin (MW)
Voltage (4/8 Buses) (pu)
Lewis Creek U1 out 2385 2313 0.84 – 0.89
Lewis Creek U1 + China-Jacinto out 2260 2192 0.83 – 0.89
Lewis Creek U1 + Grimes-Crockett out 2230 2163 0.86 – 0.91
Lewis Creek U1 + China-Porter out 2065 2003 0.85 – 0.93
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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 @ 4/8 buses <0.90 pu
Voltage @ 4/8 buses < 0.92 pu
Armed all time
Drop load
Time Delay 3 seconds
Reset the Process for next
LVSH block
OEL at Lewis Creek
units
Load Blocks:Block 1: 175 MWBlock 2: 75 MW
One or more Lewis Creek
units in-service?
The above conditions need to be met for 3 scans to trigger load shedding
Monitored Buses:Metro 138kVGoslin 138kVAlden 138kVOakridge 138kVHuntsville 138kVRivtrin 138 kVPoco 138 kVConroe 138 kV
Load Blocks:Block 1: 175 MWAlden: 50 MWMetro: 35 MWOakridge:30 MWGoslin: 60 MW
Block 2: 75 MWIn the vicinity of Block 1