Modern Power Grid for Regulatory Assistance Project

1. A SURVEY OF ADVANCED TECHNOLOGIES ON THE MODERN GRID
Kabeed Mansur and Kevin Porter, Exeter Associates
Draft: NOT FOR CITATION OR ATTRIBUTION
Introduction
A fullymodernizedpowergridisessential forproviding electricservice thatisreliable,economical,and
secure.We define the moderngridforthe purposesof thispaperasa twodimensionalsystem. These
twodimensionsare:
 The Physical Dimension –thisreferstothe physical infrastructure (physical grid) of the modern
grid,whichiscomprisedof three sectors,namely:Generation,Transmission,andDistribution.
 The Operational Dimension –thisreferstosystemoperations(operational grid) whichisthe
domainof the systemoperator.The primaryfunctionof the systemoperatoristo keepthe
systeminbalance,thatis,ensuringthatsupply(i.e.,generation)anddemand(i.e.,load) are in
equilibriumatall times.
The goal of thispaper isto analyze the setof advancedtechnologiescurrentlydeployedonthe modern
gridand understandhowtheyrelate tothe physical gridaswell ashow theysupportthe operational
grid.
Our analysisisorganizedintothree sections:
 SectionI – Overviewof the Energy Infrastructure: Thissectionexaminesatthe physical
infrastructure of the moderngrid,andfocuson the functional andphysical characteristics(i.e.,
hardware,equipment,devices,etc.) of eachsector.
 SectionII – Overviewof SystemOperations: Inthissectionwe will definesystemoperations,
and,the functional role of the systemoperator.
 SectionIII – Overviewof Advanced Grid Technologies:Thissectionwill be the crux of our
analysis.The core technologiesdeployedonthe moderngrid,withrespecttosensingand
measurementtechnologies,advancedcontrol methods,andcommunicationtechnologies.

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I. Overview of the EnergyInfrastructure
The electric grid is comprised of three sectors: generation, transmission, and distribution.
Generation produces electricity, transmission carries this electricity via high voltage power lines, and
distributiondeliversthe electricity toenduse customers. Figure 1.1 shows how these three sectors are
connected on the grid: Figure 1.1
Source: Universityof Idaho, Principles of Sustainability (Ch.6)
Generation:
The generation sector is responsible for the bulk production of electric power. Generators produce
electricity byconvertingprimaryenergysourceslike fossil fuels, nuclear, hydro, wind, and solar power
into electric energy.
Generatorsare connectedtothe transmissiongridviaa‘stepup’transformer. Step up transformers are
responsible forincreasingthe voltage of the electricity exiting the generation station in order to match
the voltage level of the transmission line.
With respect to system operations, generating units are classified into three categories: Baseload,
Intermediate,andPeakingunits.Baseloadunits are used to meet the constant (i.e. base) power needs
of the system and to this end they are inflexible (that is, their volumetric electric output cannot be
changed) and run continuously (24 hours a day). Intermediate units are typically operated for an
extendedperiodof time tocovermorning(mid-morningtoevening).These unitsare usedbecause their
operational flexibilityallowsthemtobe rampedupand down in response to load fluctuations. Peaking
unitsare typicallyoperated/broughton-line whenthe systemdemandisnear itspeak.Peakingunits are
similartointermediate units,intermsof operational flexibility; however, due to higher variable costs,
these unitsare onlyused during peak demand hours (early afternoon to early evening). Peaking units
run for a limited numberof hoursperyearand may be restricted in the number of operating hours due
1
Figure 1.1 - Energy Infrastructure of the Modern Grid

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to environmental restrictions. Figure 1.2 shows which generation resources are used most often, with
respect to each of the categories outlined above:
Figure 1.2 Baseload, Intermediate & Peaking Plants
_______________
Source: ilsr.org/political-and-technical-advantages-distrubted-generation/
Transmission
The transmissionsystemisresponsible for carryingbulkelectricpoweroverlongdistances.Inthis
processelectricityismovedfromacentral generatingunittoaninterconnectionwithanelectrical
distributionsystem.Tothisend,we canthinkof the transmissionsystemactingasthe electrical highway
connectingsupply(generation) todemand(load).
The transmissionsystem(seeFigure 1.3) iscomposedof highvoltage powerlines(>60kV),stations,and
substations.Stationsandsubstationstypicallyhousethe followingequipment/devices:
 Transformers are usedtochange the voltage level upordown. Generationside transformers
increase the voltage level of the electrical output, whiledemandsidetransformersdecreasethe
voltage;
 Switchgears includescircuitbreakersandothertypesof switchesthatcanbe usedto turn on or
off parts of the transmissionnetworkinordertoprotectthe systemandmaintainreliability;
 MeasurementInstrumentation aresensorsthatare usedto collectvoltage,current,andpower
data for monitoringandcontrol purposes;and
 CommunicationsEquipment are usedtotransmitthe data collectedfromsensorsand
measurementinstrumentstothe systemoperator,and,thisequipmentcanalsobe usedto
remotelycontrol switchgears.

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Figure 1.3 Transmission Process Flow Diagram
_______________
Source: ilsr.org/political-and-technical-advantages-distrubted-generation/
Withrespectto topology,the transmissionsystemhasa meshnetworktype configuration –thismeans
that there existsmultiple pathwaysbetweenanytwopointsonthe transmissionnetwork(see Figure
1.4).
Figure 1.4 Topological Illustration of Transmission Network Mesh
_______________
Source: http://webpage.pace.edu/ms16182p/networking/mesh.png

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The meshtopologyof the transmissionsystemallowsforgreaterredundancy,thisredundancyinturn
allowsthe powersystemtobe able tosupply powertoloadsinthe eventthata transmissionlineor
generatingunitgoesoffline.
The amount of powera transmissionlinecarriesisnotunlimited. There existsthree primaryconstraints
withrespecttothe capacityof a transmissionline:
• Thermal Constraints1
refertothe max temperature atransmissionline canhandle.Tothisend,
the temperature inatransmissionline isafunctionof thatlines’lossrate.Lossesincrease the
temperature of atransmissionline causingthe line tostretchandsag, at some maximum
temperature,the sagissufficientenoughtoreduce the lines’capacityfactor;
• Voltage StabilityConstraint2
voltage stabilityisdefinedasthe abilityof apowersystemto
maintainsteadyvoltagesatall buses afteradisturbance event. The voltage stabilityconstraint
arisesdue to reactance of a transmissionline,thisinturncausesthe voltage atthe far endof
the line todrop belowsome allowablelevel;and
• Transient Constraint3
referstothe tolerance thresholdof a transmissionline withrespectto
changesinthe powerflowingthrough,if atransmissionlineexceedsitstransienttolerance it
will cause generators tofall outof synchwitheach other.
Figure 1.5 illustrates graphicallythe limitingfactorsmostcommonlyassociatedwiththese three
constraints,withrespecttoshort,medium, andlongrange transmissionlines.
1
Source: MIT, Future of the Electrical Grid, Ch. 2
2 Ibid
3 Ibid

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Figure 1.5 Common Limits to Transmission Carrying Capacity
_______________
Source: MIT, Future of the Electrical Grid, Ch. 2
Distribution (workin progress)
The distributionsystemservesactsasthe interconnectorbetweenthe transmissionsystemandend-use
consumption.Distributiontypicallyreferstoelectricsystemswithvoltageslowerthan60kV.A
distributionsystemiscomposedof the followingelements:
 DistributionLines
 Transformers
 Voltage Regulators
 Witches
 CircuitBreakers
 AutomaticsReclosers
 PowerCapacitors
 MonitoringSystems
 Service Drops

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II. Overview of System Operations
The objective of system operations is to ensure that the electrical system is operating reliably; to that
end,systemoperatorsmustkeepsupply(generation) anddemand(load)in balance at all times, as well
as maintain and control system voltages and frequencies. In order to carry out these functions system
operators must:
 Forecast demand in the day-ahead;
 Schedule generation (or its applicable demand response) to match forecasted demand;
 Schedule reserves and other ancillary services;
 Schedule use of the transmission system;
 Communicate schedulestoneighboring operators so that power flows across interconnections
can be anticipated;
 Manage and control the electrical system by correcting supply and demand imbalances in real
time; and
 Correct any system disturbances, and restore power in the event an outage occurs.
Load Forecasting and Scheduling
System operators are responsible for scheduling generation in order to meet the expected system
demand.Todo this,the schedulingprocessstartsbyfirstforecastingsystemdemand, otherwise known
as load forecasting.Generally,systemoperatorsprepare loadforecastsusingstatistical modelsbasedon
historical demand and current weather forecasts. System operators then use the output from these
models in order to develop hourly, day-ahead demand forecasts. The models are rerun during the
operating day such that forecasts can be continually adjusted based on changes in weather or other
exogenous variables that affect demand.
Once the system operator has prepared the load forecast, they then take an inventory of all available
generationresources.Basedonthis inventory, system operators schedules available generation on an
hour-by-hour basis in order to match the expected system load as well as meet system reserve4
requirements.
The system operator schedules the available generation based on a principle known as least-cost
dispatch5
. Figure 2.1 presents a simple example to help illustrate/model this process.
4 Reserves – Generation capacity thatis availableto the system operator if needed, but that is not currently
generating electricity.
5 Least-Cost Dispatch - The operation of generation facilities to produceenergy atthe lowest costto reliably serve
consumers,recognizingany operational limits of generation and transmission facilities.

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Figure 2.1 - SIMPLE EXAMPLE OF SCHEDULING
Unit 1
400 MW @
$45/MWh
Unit 2
100 MW @
$35/MWh
Unit 3
100 MW @
$40/MWh Unit 4
200 MW @
$25/MWh
Unit 5
200 MW @
$55/MWh
400 MW
Load Forecast
In this simple model, system operations has forecast 400 MW of load for the hour. In attempting to optimize scheduling, the system operator
w ould prefer to schedule:
Unit 4 $25 200 MW
Unit 2 $35 100 MW
Unit 3 $40 100 MW
Unfortunately, this is clearly not feasible given the limited capacity of transmission line B (only 200 MW). So the optimized dispatch, subject to
constraints w ill be:
Generation Schedule
Unit 4 $25 200 MW
Unit 2 $35 100 MW
Unit 3 $40 100 MW
Transmission Schedule
Line A 200 MW
Line B 200 MW
In addition to the units scheduled for energy, the operator w ill also need reserves. Since the greatest single loss contingency is 200 MW on
Line B, the operator w illneed to schedule 200 MW of reserves. The safest place to obtain the reserves is from Unit 5, since the loss of Line A
w ould create a situation w here reservesfrom Unit 1 w ould not be available to the system. If Unit 5 is scheduled for reserves, this w ould also
necessitate scheduling Line C for 200 MW to ensure the transmission capacity is available if supply from Unit 5 is needed.

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Reserves
Reserves may be divided into spinning (synchronized to the grid and able to operate within a short
periodof time,suchas 10 minutes) andnon-spinning(not synchronized to the grid but able to respond
and reachfull output within a defined period of time, such as 30 minutes). The amount of receivers is
usuallyset to meet a reliability standard, such as minimizing unserved energy to one day in ten years.
Reserve levels vary, but a general range is from 10-15 percent of peak demand.

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III. Sensing and MeasurementTechnologies
In order for system operators to carry out their operational functions they require continuous
information regardingthe state of the electricsystem.Tothisend,the efficientoperationof the modern
electric grid is made possible through a series of sensors and measurement devices which collect
informationfromdifferentpartsof the physical grid and relay this information to the system operator.
In Table 3.1 we identifythe critical informationcollectedbysensorsandmeasurementdevices installed
on the physical grid.
Table 3.1 Grid Information Collected by Service and Measurement Devices
Generation Transmission Distribution
Generation equipment information
regardingavailability (e.g., online,not
online)
Information from sensors
monitoringthe state of high-voltage
power lines
Power usage information collected
from customer meters
Information from sensors monitoring
the interconnections with the
transmission grid
Information from sensors
monitoringthe state of devices in
the transmission substations
Information from sensors monitoring
the state of devices in the
distribution substations
Availablecapacity for individual
generators particularly important with
respect to variablegeneration (i.e.,
wind and solar).
Information from phasor
measurement units (PMUs)
monitoringpower flow on the
transmission grid
Information from sensors monitoring
the state of distribution feeders (a
transmission line carrying electricity
to a distribution point)
_______________
Source: CISCO, Smart Grid – Leveraging Intelligent Communication to Transform the Power Infrastructure
Thissectionfocusesontwo specific sensing and measurement technologies which are widely used on
the modern electric grid, namely:
1. Supervisory Control and Data Acquisition Systems (SCADA)
2. Phasor Measurement Units (PMUs)
SCADA Systems
SCADA systems are used by system operators to collect real time data in order to monitor and
control generation, transmission and distribution equipment. This section will consider only the data
acquisition dimension of SCADA systems – the supervisory control dimension will be covered in the
section corresponding to advanced control methods and technologies.
The data acquisition portion of SCADA allows system operators to remotely monitor electrical
quantities such as voltage and current in real time. SCADA systems accomplish this function by using
sensordevicesinstalledongeneratorsandatdistributionsubstationstomeasure state variables such as

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voltage,current,andpowerlevels.Thisdataisthencollectedbydevicesknownas remoteterminal units
(RTUs).
SCADA systems also contain a backend software application known as a state estimator. The
state estimator receives the measurement data from the SCADA RTUs. The state estimator then
processesthe usingadvancedalgorithmsand“calculatesall the load flows and critical voltage points in
the system, and calibrates them to real time values….[the] state estimator uses all available
measurements,knownfacts,andotherrelevant information to calculate the best possible estimate of
the true status(“state”) of the powersystem.Forexample, the state estimator is used to calculate new
power flow conditions, such as voltages and currents, to help system operators predict “what if”
scenarios”.6
Figure 3.2 presents a conceptual model of the SCADA data acquisition process.
Figure 3.2 Data Needed for SCADA Systems
_______________
Source: CISCO, Smart Grid – Leveraging Intelligent Communication to Transform the Power Infrastructure
PMUs
PMUs are usedto “estimate voltages and currents at substations, generators, and load center…system
frequency and other quantities are also measured.” Combined with known line characteristics, PMU
measurements can determine instantaneous power flows throughout the system…PMUs [can also
provide muchfaster] datathan SCADA systems,whichresultsinhigher-resolutioninformationabout the
statusof the grid. Therefore, measurementsfromall PMUscan be synchronizedusingGPStime signals,
[thus] allowing for a more accurate assessment of the status of the grid.”7
PMUs alsomeasure the electrical wavesonhighvoltage AC transmission lines. To understand how this
metrichelpsthe systemoperatorcarryout itsfunctionwithrespecttoreliable operation,itis important
to understand what a phasor is. To this end, a phasor is a complex number which represents both the
magnitude andphase angle of the sine wavesgeneratedbythe flow of electronsthrough a high voltage
AC transmissionline.Thus,if alarge number of PMUs can be installed on the transmission grid, system
operators can compare - in real time - the shapes of these electrical waves at various points on the
6 Source: Blume, Electric Power System Basics, Ch. 9
7 Source: MIT, Future of Electric Grid, Ch. 2

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transmission grid. System operators can then use this data to help measure the state of the power
system as well as respond to system conditions in a rapid and dynamic way. 8
Advanced Control Methodsand Technologies
Advancedcontrol methods and technologies refer to devices and algorithms that use backend
analytics to help diagnose, evaluate and predict different conditions on the electric grid. Advanced
control methods and technologies also have the ability to autonomously take corrective actions to
mitigate, eliminate and prevent outages or other grid reliability issues.9
In this section two prominent control methods and technologies currently in use are reviewed:
1. SCADA systems
2. Governor Control (GC)
SCADA Systems
In the prior section we looked at the data acquisition dimension of SCADA systems, in this
sectionwe considerthe systemssupervisorycontrol dimension.The basicfunctionof the SCADA system,
withrespecttosupervisorycontrol,isto operate all critical equipment in each substation from a single
control center.
8
Source: Fang, Smart Grid – The New and Improved Power Grid: A Survey.
9 Source: Enose, Advanced Technologies Implementation Framework for a Smart Grid

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Figure 3.3 – Inputs and Outputs to a SCADA System
_______________
Source: Modern SCADA Philosophy in Power System Operation].
SCADA systemsinoperational control centersallow forthe centralizedmonitoringandcontrol of various
devicesandequipmentonthe physical grid,specifically equipment housed at distribution substations.
The control function of the SCADA system relies on the data acquisition process. In general, the
supervisorycontrol processbeginswiththe systemoperatorreceiving actionable information from the
stateestimatorand usingthisinformation tosend either an automated or operator-driven supervisory
commandto an RTU, the RTU thenpassesthiscommandontothe specified field devices. Field devices,
alsocalledremote station control devices, have the ability to control local operations such as opening
and closing valves or circuit breakers based on a set of received SCADA control commands.10
Governor Control (AGC)
The electrical grid can become unstable if the balance between supply and demand, i.e., the
voltage andfrequency mayexceedallowableboundswhichcaninturn result in damaged equipment as
well asservice interruptions.Tothis end, the balance between supply and demand, in the short run, is
maintained by generators equipped with governor control.
The governor is a device that controls the mechanical power driving a generator via the valve
limiting the amount of steam, water, or gas flowing to the turbine. The governor acts in response to
locally measured changes in the generator’s output frequency relative to some established system
standard; the standard in the U.S. is 60 Hz. If the electrical load on the generator is greater than the
mechanical power driving it, the generator maintains power balance by converting some of its kinetic
energyintoextraoutput power—butslowsdowninthe process.Onthe otherhand,if the electrical load
10
Source: Modern SCADA Philosophy in Power System Operation

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is less than the mechanical power driving the generator, the generator absorbs the extra energy as
kinetic energy and speeds up. This behavior is known as “inertial response.”
The frequencyof the ACvoltage producedby a generatorisproportional toitsrotational speed.
Therefore,changesingeneratorrotational speed canbe trackedby the generator’soutputfrequency. A
decreasing frequency indicates that real power consumption is greater than generation, while an
increasing frequency indicates that generation is exceeding power consumption. Any changes in
frequency are sensed within a fraction of a second, and the governor responds within seconds by
altering the position of the valve— increasing or reducing the flow to the turbine. If the frequency is
decreasing,the valve will be openedfurthertoincrease the flow andprovide more mechanical powerto
the turbine,hence increasingthe generator’soutputpower,bringingdemand andsupplyinbalance and
stabilizingthe speedof the generatoratthisreducedlevel.The speedof the generatorwillstayconstant
at this level as long as the mechanical power driving it balances its electrical load. 11
the power system.
Communication Technology (workin progress)
Communication technologies refer to the specific communication networks used to connect various
parts, systems, and devices on the modern. Communication networks allow for real time exchange of
system data between the physical grid and the operational grid. Therefore, without proper
communication networks, system operators would not be able to effectively monitor and control the
power system.
We will organize ouranalysishereintothree subsections:
 CommunicationSystemElements
 CommunicationSystemSchemes
CommunicationSystemElements
Communicationsystemsforthe modernpowergridare designedtocarryout a single core function,
namely,transferdataandinformationbetweenvariouslayersof the powersystem.Tothisend,a
typical communicationsystemcontainsthree elements:
 Transmitter- a set of equipmentusedtogenerate andtransmitelectromagneticwavescarrying
messagesorsignals
 Channel- is a particulartype of mediathroughwhicha message issent andreceived
 Receiver – a device thatreceivesandextractsthe informationcontainedinthe electromagnetic
signalssentbya transmitterinformation
Figure 3.4 showsthe informationflowthroughatypical communicationsystem.
11 Source: MIT, Future of the Electric Grid, Appendix B.2

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Figure 3.4
Surveyof CommunicationSystemsonthe ModernGrid
In thissectionwe lookatthe communicationsystemthatare currentlyusedonthe modernpowergrid,
to thisend,we will consider,inturn,the followingsystems:
 PowerLine Carrier
 PacketSwitching
Power Line Carrier (PLC)
PLC isa communicationsystemthatusespowerlines(transmissionanddistribution) asthe primary
communicationchannel tosendandreceive data.A PLCnetworkstructure isdividedintwomainparts.
1. PLC networkparallel tothe mediumvoltage grid
2. PLC networkparallel tothe lowvoltage grid.
The borderand endcomponentsof the networkare showninfigure 3.5.
MediumVoltage
PLC Network
Low Voltage
PLC Network

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 MediumVoltageHead End: It enablesthe communicationbetweenthe Backbone orthe main
communicationsnetworkandthe PLCnetwork.
 MediumVoltageModem: it isthe interface betweenaMediumVoltage PLCNetworkandaLow
Voltage PLCnetworkonthe MediumVoltage side.
 Low VoltageHead End: It representsthe endof the Low Voltage PLCnetworkandisa gateway
to the MediumVoltage networkwhichcanbe PLCor otherwise.The low voltage headendis
normallyplacedonthe distributiontransformerwhichactsas a natural low pass filterforthe
highfrequencysignal injectedinthe network.
 Low VoltageRepeater: In case of linesof significantdistancesbetweenthe HeadEndandthe
NetworkTerminationUnititwill be necessarytoplace RepeaterUnitsalongthe line inordernot
to lose the highfrequencysignal.
 Network TerminationUnit(NTU):It is the interface betweenthe clientequipmentandthe low
voltage PLCnetwork.The NTU isnormallyplacedatthe clientpremises.
Packet SwitchingNetwork
A packetswitchingnetwork(PSN) isacommunicationsystemthatworksbygroupingandsendingdata
froma transmissionnode toareceivingnode inthe formof small packets,where eachpacketcontains
specificdetailslikeasource IP address,destinationIPaddress.Moreover,PSN usesthe broadband
Ethernetas itscommunicationchannel.Itisthe mostcommonlyused communicationsystemonthe
modernpowergridbecause of itsabilitytoprovide real time supportforSCADA systems.

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