The Voltage Ledge - Pterra urban areas Occurred in summer, ... Other aspects of the Voltage Ledge}...
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Ric Austria, Pterra Consulting Ric Austria, Pterra Consulting The Voltage Ledge The Voltage Ledge California ISO California ISO April 27,2007 April 27,2007
Transcript of The Voltage Ledge - Pterra urban areas Occurred in summer, ... Other aspects of the Voltage Ledge}...
Ric Austria, Pterra ConsultingRic Austria, Pterra Consulting
The Voltage LedgeThe Voltage Ledge
California ISOCalifornia ISOApril 27,2007April 27,2007
About Pterra Consulting
A transmission and distribution analysis consulting firm
Based in Albany, NYStarted in 2004
ServicesTechnical analysis, studies and assessmentsExpert witnessStaff supportNERC Compliance
TrainingCourses on power system topics
For more info, please visit us on the web:www.pterra.com
Objectives
Review the nature of Voltage Collapse and some examplesReview a Case Study of the Voltage LedgeDefine the Voltage Ledge and identify relevant phenomena in
Transmission SystemDistribution System
Identify response strategies from a Voltage Ledge
Voltage Collapse
The sudden and precipitous collapse of voltage that occurs typically some minutes after equilibrium has been lost due to a voltage instability.
Fast collapse occurs within the transient periodSlow collapse occurs within the post-transient period or later
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The Voltage Ledge
What is it?An equilibrium state, relatively stable, where voltages are below normal operating levelsAs voltages drop towards collapse, systems may reach this mode, a “ledge”prior to or preventing freefallSystems may operate “on the Ledge” for periods lasting seconds to minutes, and occasionally, hours
Selected System Events
Tokyo, Japan – July 23, 1987Rapid increase in load during the dayLeads to dropping voltage on EHV8000+ MW of load shed
Miami, Florida – Aug 18, 1988Three phase faultSlow voltage recoveryLoss of Load
Southern California – Aug 5, 1997Small plane hits shield wiresVoltage dips to 0.6 at distribution levelHigh air conditioning load
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More System Events
Atlanta, Georgia – July 30, 1999Short circuit in substationSlow voltage recovery 5 generators trip, 1900 MW load shed
New Jersey – July 6, 1999Record heat waveVoltage on EHV goes below 1.0 p.u.Insulation failures in distribution networks
Eastern Seaboard – Aug 13, 2003Line contacts treeCascading outagesWide spread outages
Common Factors to these Events
Voltages stayed at a low level for seconds to minutes prior to collapse or recoveryAffected urban areasOccurred in summer, with high levels of air conditioning loads
A Closer Look at 1 Event
First 2 weeks of July 1999 in the Northeast USAbove average temperaturesRecord system demand
With large air conditioning component
Large power transfers into Eastern Seaboard
July 1999 Event
ObservationsIn New England
Many generating units were out on maintenance or for re-fuelingThis was in preparation for the typical peak load in late July, early August
New York CityHeat related failures in connections, cables and transformers
In Long IslandIncipient voltage collapse in the South Fork areaSome overloaded wires burned downRequested voluntary customer reduction and responded to 5% voltage reduction
July 1999 Event
ObservationsAtlantic Coast Section of PJM
Experienced steep voltage declines in 500 kV system on two occasionsFirst time 500 kV voltage went below 1.0 per unit
In New JerseyTerminator and cable failures in 3 transformers causes shutdown of City Dock substationAt Red Bank substation, 2 transformers fail leading to disconnection of 100,000 customers Over a thousand failed poletop transformers
In the Delmarva PeninsulaCapacity deficiency in local generationSystem operator reports impending voltage collapse
Questions about the July 1999 Event
There was no voltage collapse - why?Transmission voltages were depressed for 2-2½ hours – how?Impacted distribution system more than the transmission system –reasons?Which operator or other actions allowed the system to recover to normal operations?
Transmission System Phenomena
While demand is rising …Increased usage of transmission system
Transmission lines change from supplying VARs to absorbing VARs
Generators delivering VARs approach reactive limits
Transient and steady-state capabilityStatic VAR devices reach limit
Transmission Line Reactive Characteristic
Sample Transmission Usage
Reactive Losses increase
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0450Natural Load
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Generator Reactive Response
Generator reactive capabilitySteady-state – reactive capability decreases as MW output increasesTransient – available for durations of seconds to minutes
Transient Limit
Steady-State Limit
Static VAR Devices
Reactive power from static var devices operating at limit change as the square of voltage
Capacitor banksSVCs
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P-V Curves
P-V curves are a method to identify dropping voltage as a function of load or transfer --- from the transmission POV
July 1999 Event
500 kV voltages
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Voltages were depressed for 2-2.5 hours
July 1999 Event
Hourly Peak Loads
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Normal daily load rise
Voltages recover while load was still rising
Distribution System Phenomena
As voltage drops on the transmission side …
Tap-changing transformers boost voltage on the secondary sideCapacitors switch in to maintain feeder voltagesLoads are initially oblivious …
Distribution Transformers
Are set to maintain a certain voltage on the customer (low voltage) sideTaps adjust automatically to maintain setpoint voltage
Distribution Transformers
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Voltage Drop in The Secondary
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Imbalance in the 3 phases results in larger drop in weaker phase
Distribution Loads
When voltage drops at distribution loads …
Motor loads may stallMotor loads may tripoutOther types of load reduce demandSome loads self-restore
Air Conditioning Load - Characteristics
Low inertia motorsSlows down quickly --- Prone to stalling
Residential air conditioning motors stall if voltage drops below 60% nominal for 5 cycles
Speeds up quickly --- sudden load injections
ProtectionThermal overloadUndervoltage - tripout
Motors
Stalling
Torque at V = 0.7 pu
Torque at V = 1.0 pu
Motors
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Power Factor
High Q demand at start or stall
High starting current
Self-restoring load
Adjust cycle time at low voltageRestores to previous demand even at low voltage conditions
Typical Feeder on the Voltage Ledge
Pole-top Transfomrer
Low Voltage on Circuit
Stalled
Tripped Off
Air Conditioners
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Voltage Ledge
An equilibrium state where:Dynamic effects that would drive down voltage are balanced by effects that recover voltage
Effects that drive down voltage:Loads that have low voltage tolerance that allow them to recover to their normal demand level at lower terminal voltages, such as variable speed motors, or thermostat-controlled loads Motor loads that stall, causing an increase in reactive demand
Voltage Ledge
Effects that recover voltage:The natural response of loads to decrease power demand as terminal voltages decrease The dropout of contactors due to low voltage, most notably in motors, such as air conditioners and pumps
Voltage Ledge
Can be visualized on the P-V curve as a flattening or Ledge near the voltage collapse pointAny effort to increase load, is matched by voltage-dependent effects that reduce load
Ledge is narrower than shown in picture
Voltage Ledge
Other aspects of the Voltage LedgeWhen operators attempt to add reactive power to the region operating on the Voltage Ledge, the VARs may be absorbed without noticeable change in conditionMost of the significant phenomena occurs in the distribution system
The transmission system appears normal, and within voltage criteria!
Consequences
Operating on the Ledge for prolonged periods lead to …
Additional heating on the secondary circuitsInsulation failures on cables and pole-top transformersSmall generators on the feeders may not start up at low voltage… Outages
July 1999 Event
AssessmentCaused by Heat Storm resulting in
Record energy demandLarge cooling component
High imports into the Eastern SeaboardLack of reactive support in the load areasFailures in the distribution system due to high currents (low voltages), high temperatures and humidity
July 1999 Event
AssessmentWhen EHV voltage dropped to 1.0 p.u., operating guidelines did not indicate need for further emergency proceduresVoltage collapse was averted by:
Major contingency NOT occurringOperator response to localize supply –reducing transfers
Responding to the Ledge
On the day of …Reduce power transfersCall on local generation reservesWait until demand reduces due to lower ambient temperatures
A year or so ahead …Provide for local VAR supportAdd static VAR devicesStudy low voltage load shedding
