Phasor Measurement Units and Wide Area Monitoring Systems. From the Sensors to the System

288 Pages·2016·19.64 MB·English

by  Antonello Monti

Phasor Measurement Units and Wide
Area Monitoring Systems
Phasor Measurement Units
and Wide Area Monitoring
Systems
From the Sensors to the System
Edited by
Antonello Monti
Carlo Muscas
Ferdinanda Ponci
AMSTERDAM (cid:129) BOSTON (cid:129) HEIDELBERG (cid:129) LONDON
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Contributors
A.Angioni RWTH Aachen University, Aachen, Germany
P.Castello University ofCagliari, Cagliari, Italy
P.Ferrari University ofBrescia,Brescia,Italy
A.Monti RWTHAachenUniversity, Aachen, Germany
C.Muscas University of Cagliari,Cagliari,Italy
M.Pau RWTHAachenUniversity, Aachen,Germany
P.A. Pegoraro Universityof Cagliari, Cagliari,Italy
L.Peretto University ofBologna, Bologna,Italy
F. Ponci RWTH AachenUniversity, Aachen, Germany
S.Rinaldi University of Brescia, Brescia, Italy
A.Roscoe University ofStrathclyde, Glasgow, United Kingdom
A.Sadu RWTHAachenUniversity, Aachen, Germany
S.Sulis University of Cagliari, Cagliari,Italy
J. Tang ChongqingUniversity, Chongqing, PR China
R. Tinarelli University ofBologna, Bologna,Italy
D.Wilson GE Energy Management, Grid Solutions, Edinburgh,
United Kingdom
ix
Acknowledgment
The editors would like to thank allthe authors that contributed tothis book.
Describing Phasor Measurement Units from the sensor to the system level is
something that we could have not done alone at the right level of quality.
Ithasbeenparticularlyapleasurebecausecolleaguesinthisgrouphavebeen
interacting already for quite some time. This book is the best result of our
annual IEEE Workshop Applied Measurements for Power Systems (AMPS).
At AMPS it has been and remains a satisfaction to discuss the evolution of
the concept of PMUs and their applications. It is so exciting to see this lively
discussion reach maturity in the form of this book. We hope the readers will
perceive this passion.
TheeditorswouldlikealsotothanktheteamatElsevierfortheirsupportinthe
preparationofthemanuscript.Ithasbeenalongjourney.Weappreciatedyour
patiencewhenwewerelateandyourconstantpositiveattitudethathasalways
reinforcedourmotivationtogetitdone.SothanksinparticulartoRaquel,who
drovetheprojectfromthebeginning,toMariana,forhersupportineverystep,
and to Sruthi, for herhelp in thedifficult taskof editing.
xi
CHAPTER 1
Introduction
A. Monti*, C.Muscas†,F. Ponci*
*RWTHAachenUniversity,Aachen,Germany,†UniversityofCagliari,Cagliari,Italy
CONTENTS
1.1 MOTIVATION FOR THE WORK
1.1 Motivation for
the Work..........1
Power systems all around the world are facinga period of significant change.
Differentdriversarepushingforamodernizationoftheinfrastructuretobetter 1.2 What is a
copewiththecurrentoperatingconditions.Oneofthestrongestdriversisthe PMU?...............2
growing role of renewables, or more generally, decentralized energy sources.
1.3 A Short History
ThisisparticularlytrueinEurope,wherethetargetssetbytheEuropeanCom-
ofthe PMU......4
missionarepromotingambitiousplans,inthememberstates,ofrenovationof
thegenerationportfolio.Significantchangesarealsohappeninginotherparts 1.4 Structureof
oftheworld,withwindnowbeingoneofthemostimportantenergygenera- the Book..........5
tion technologies.
References............8
The result of this change in the generation portfolio is a modification in the
requirements inthe monitoring and automation of the powerinfrastructure.
Until now, power grids have operated in a load-driven mode. The basic idea
behindthisisthatloadsarepredictable,eventhoughonlyinastatisticalsense.
Thankstothisstatisticalprediction,generationisscheduled.Giventhatpredic-
tionsareneverperfect,deviationsarecompensatedatrun-time.Suchanapproach
is possible, assuming that the generation is fully controllable and possibly
concentratedinlargepowerplants,sothattheschedulingproblemissolvable.
In the new scenario, we are moving more and more towards a generation-
drivensystem,wheregenerationleadsandtherestofthesystemfollows.Infact,
theoutputpoweroftherenewableenergysourcescannotbeadaptedaseasily
as inthe caseof traditionalpower sources.
This new scenario is creating the need for new solutions and technologies,
among which, for example, storage, electrical, and also thermal technologies
are expectedto play a critical rolein achieving the power balance.
Inanutshell,operatingthepowersystemisgettingmorecomplexandrequires
more sophisticated monitoring for automation technologies. 1
PhasorMeasurementUnitsandWideAreaMonitoringSystems.http://dx.doi.org/10.1016/B978-0-12-804569-5.00001-X
©2016ElsevierInc.Allrightsreserved.
2 CHAPTER 1: Introduction
The first part of the automation that felt this impact was monitoring. All the
operatorscurrentlyrecognizetheneedforamoreaccurateandextensivemon-
itoring of the powergrid.
Transmission systems have been the first to evolve in the direction of more
sophisticated solutions, but distribution grids are expected to receive an even
more clearimpact.
In effect, while transmission grids have been already operating with quite
advancedcontrolroomsforquitesometime,distributiongridsarenowbecom-
ing much more sophisticated than in the past.
Themaingamechangeristhenewroleplayedbydistributiongridsinrelation
togeneration.Inanewscenariocharacterizedbyalargepresenceofdistributed
energysourcesconnectedatlowormediumvoltage,distributiongridsarenow
becomingthe key infrastructure also for the purpose ofgeneration.
This change of role required the definition of the new concept of active
distribution grids.
More advanced monitoring, for both transmission and distribution, basically
means two things:
(cid:129) new algorithms,
(cid:129) new measurement technologies.
Phasormeasurementunits(PMUs)fallinthesecondcategory.PMUsareastep
up in technology when it comes to measurements for power systems. They
introducetwonewfundamentalconceptsthatwerenotpresentatallinpower
systems:
(cid:129) the conceptof synchronized measurementscharacterized by precise
timetags;
(cid:129) theconceptofameasurementthatisbeyondthesimpleideaofrootmean
square(RMS)and brings direct information about the phase.
Thesenewaspectsnotonlyrepresentasignificantenhancementintheconcept
ofmeasurementofanACquantity;theyalsounlockcompletelynewscenarios
and possible applications.
1.2 WHAT IS A PMU?
Whileacompletediscussionofthedefinitionandoftheimplementationofa
PMUispresentedinthefollowingchapters,wewouldlikeheretointroducethe
nonexpert readers tothe basic concepts.
1.2 What is a PMU? 3
Traditionally,devicesutilizedforpowersystemmonitoringhavebeendesigned
toprovidescalarinformation.Whileforpowermeasurementsthisisobvious,
forvoltagesandcurrentsinACsystemsthisusuallymeansreportingtheRMS
value of the quantity.
Themainevolutioninthisfieldhasbeenthetransitionfromanalogandelec-
tromechanicaldevicestodigitalimplementation.Thisdigitalimplementation
allowed the development of more sophisticated measurement options which
are also useful for powerqualitypurposes (eg, total harmonic distortion).
Ontheotherhand,thephaseplaysacriticalroleintheoperationofpowersys-
tems. Phase information has been usually extracted only at the control room
level by means of the state estimation (SE) process. This process uses a large
amount of measurement to extract a coherent picture of the operation of the
grid: this picture is in many cases represented by the voltage profile in terms
ofamplitude and phase.
Such an approach hasthree mainlimitations:
– Theinformationphaseisalwaystheresultofanumericalprocessanditis
never directly measured.
– Theinformationphaseisonlyavailableatthecentrallevelanditcannot
be usedfor localprocessing.
– The refresh rate ofSE is ratherslowand even then, the information is
available only with acoarse detail.
Withanincreasingroleofdynamicsandtheneedtooperategridsclosertotheir
limit,ithasbecomeprogressivelyclearthatitiscriticaltoimprovetheknowl-
edge of the phase quantity as critical assessment of the stability of the whole
system.
Dependingontheapplications,asdescribedinChapters8and9ofthisbook,
this means
– use of the information for more localprocess;
– afaster and more accurate system-level process.
Thisawareness is the root cause ofthe development ofPMUs.
PMUsaremeasurementdevicesabletoextractnotonlytheamplitudebutalso
the phase ofasinusoidal quantity.
Thephaseisestimatedwithreferencetoaglobaltimereference,whichisusu-
allyselectedtobebasedontheGlobalPositioningSystem,whichprovidesan
available and reliable definition of time everywhere.
While thedefinition may sound quite simple, its implementation is not.
4 CHAPTER 1: Introduction
The main hurdle is the formal definition of the quantity we want to measure
and correspondingly of the algorithm needed to extract this quantity from a
sequenceof samples.
In a perfectly sinusoidal system operating in steady state, such a definition is
quite simple and immediate. When we move to a real-life scenario, many
challenges arise:
(cid:129) Whatisthemeaningofaphasorinanormaloperationwhenarealsteady
statecan never be reached?
(cid:129) How dowe extract aphasor quantity froma signal that is typically
changing its frequency over time?
(cid:129) How dowe treat the presence of harmonics thatdetermine anonideal
sinusoidal behavior?
Thesequestionshavebeenandremainattheheartofsignificantresearchefforts
inmanyuniversitiesandresearchcenters.Itshouldbeclarifiedthatovertime,
thisresearchworkhasbroughtanevolutionofthevisionofwhataPMUreally
is. This book aims at explaining the more recent conclusions, but also the
fundamentals.
Allinall,theavailabilityofthisnewtypeofmeasurementhasalsopushedthe
developmentofcompletelynewinfrastructuresuchastheideaofawidearea
monitoringsystem:anetworkofPMUsworkingtogethertoassessthestatusof
anetwork.
Theseconceptshavealreadyfoundsignificantapplicationsintransmissionnet-
works,butareexpectedtofindmoreandmoreapplicationsindistributionnet-
works as well, for the reasons mentioned above.
1.3 A SHORT HISTORY OF THE PMU
TheconceptofaPMUwasintroducedinthe1980s[1].Aresearchgroup,ledby
Prof. Phadke,performed most of the original work at Virginia Tech.
Theneedforsynchronizedsamplingfirstappearedinthedesignofprotection
systems: data samples were used in different substations far apart. This work
resulted in the invention, at Virginia Tech, of the symmetrical component
distance relay.
After this groundwork, the first idea of a PMU was introduced in 1988. The
work had thenits first industrial application at Macrodyne Co.
Afterthispioneeringwork,alotofdemoprojectsweredeveloped,focusingon
theapplicationsofPMUatatransmissionlevel.Correspondingly,thenumber
of manufacturers grew in time up to tens of producers now. Furthermore,
1.4 Structure of the Book 5
modernintelligentelectronicdevicesusedinelectricsubstationsincludenow
PMU functionalities[2].
Inparallel,theIEEEstartedalongandcomplexprocessofstandardization.The
firstversionofaPMUstandardwaspublishedin1995.Theworkwentthrough
further revision, up the current version released in2014.
The IEEE standard leaves PMU manufacturers the choice of design solutions,
giving only specifications under steady state and dynamic test conditions.
It defines the indices, in particular the total vector error, for PMU accuracy
evaluation and comparison. The standard IEEE C37.118.1 introduces two
performanceclasses:aP-class,particularlyintendedforapplicationsrequiring
fastresponses,astheprotectionones,andanM-class,requiringhigheraccuracy
for measurement applications.
Another important standardization milestone is given by the IEEE standard
C37.242, released in 2013, as a guide for PMU calibration, testing, and
installation.
1.4 STRUCTURE OF THE BOOK
Theideabehindthisbookistoprovideacomprehensiveview(fromthesensor
tothesystem)overthecomplextopicofPMUs,Therigorousmethodological
studies and the technological considerations that are necessary to design the
softwareandhardwarepartsoftheinstrument,arestrictlyconnectedtothefea-
turesofthepracticalapplications.Andinturntheapplicationrequirementsare
the basis tospecifythe design itself.
Thereaderisaccompaniedthroughthissubjectfollowingapaththroughseveral
chapters,eachanotheraddressingaspecificsubtopic.Thechaptersaremeantto
bepartofacommonprojectandthustheyarestrictlyrelatedtooneanotherand
cross-referenced; however, for easier access by readers interested only in single
aspects,eachchapterhasalsobeenconceivedtobealmostself-sufficient.
Chapter2statesthegoalofthemeasurementprocess,bydefiningtheconcepts
and the notations of synchronized phasors (synchrophasors), frequency, and
rate of change of frequency (ROCOF), and underlying the differences with
respecttotheclassicalapproachesusedfortheanalysisofvoltageandcurrent
signals in power networks.
ForthereaderswhoareapproachingtheworldofPMUsforthefirsttime,the
readingofthischapterisofmajorimportance,becauseitsetsoutthebasisfor
the following discussion. In particular, the idea of a dynamic synchrophasor,
which is fundamental for PMU operations, is introduced and it is shown
howtheproposeddefinitionsareappropriatetodealwithboththesteady-state