ieee standard for synchrophasors for power systeme

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IEEE Transactions on Power Delivery, Vol. 13, No. 1, January 1998 73IEEE STANDARD FOR SYNCHROPHASORS FOR POWER SYSTEME

Prepared by W orking Group H-8of the Relay Communications Subcommitteeof the IEEE Power System Relaying Comm ittee

Working group members are:K. E.Martin, Chairman,G. Benmouyal, Vice Chairman,M. G.Adamiak, M. Begovic, R.. 0.Bum ett, Jr.,K. R. Carr, A. Cobb, J. A. Kusters, S. H. Horowitz, G. R. Jensen, G. L. Michel, R. .I. Murphy,A. G. Phadke, M. S. Sachdev, J. S. Thorp.

Abstract: IEEE Standard 1344, Synchrophasors for PowerSystems, was completed in 1995. It sets parameters required toensure that phasor measurementwill be made and communicated ina consistent manner. It specifies requirements for the timing signalused for phasor synchronization and the time code needed for inputto a measurement unit. GPS is the recommended time source andIRIG-B is the basic format used for time communication. Thestandard requires correlating phasors computed fromunsynchronized and synchronized sampling to a common basis.Timetagging accurately and consistently is essential for wide areacomparison of phase. The standard specifies information exchangeand control message formats. These include data output,configuration, and command messages. It includes 7 annexes thatdiscuss the concepts covered in the body of the standard.

Keywords: Synchrophasor, Synchronized phasor, Phasor

source. The standard describes the measurements in relationto a Phasor Measurem ent Unit (PMU ). It addressessynchronization of data sampling, data-to-phasorconversions, and formats for timing inpu t and phasor datatime, accuracy, hardware, software, or a phasor computationprocess.

YIG P S R y e i v e r IT i m e & S y n c

P haso r Measu remen t

D at a O u t p u tOVERVIEW

Figure 1. PM U functional block diagram.IEEE 1344-1995 is a standard for synchronized phasormeasurements in substations. A phasor is a vectorrepresentation of the magnitude and phase angle of an ACwaveform. Phase angle between sites can be determinedwhen the measurements are synchronized to a com mon time

PE-068-PWRD-1-03-1997 A paper recommended and approved bythe IEEE Power System Relaying Committee of the IEEE PowerEngineering Society for publication in the IEEE Transactions on PowerDelivery. Manuscript submitted February 7, 1997; made available forprinting March 26, 1997.

The purpose for the standard is to define synchronizedphasor measurements in substations so that measurementequipment can be readily interfaced with other systems.Power system measurements are steadily migrating fromanalog to digital systems. Low cost, high-powermicroprocessors allow construction of measurement unitsthat can d igitize AC power waveforms and compute phasorsin real time. Wide area phase comparisons can be made fromthis phasor data through the use of high precision timedissemination that is now readily available. As a result,phasor measurements are being explored using a variety ofhardware and software approaches. This standard specifiesdata formats and synchronization requirements to allowcorrela ting phasors from v arious sources imd com paring themwith similar data from different measurement systems. It hassections that address synchronizing signals, time inputformat, synchronization of the phasor measurement process,and data output formats. Details are discussed in the sevenannexes.

0885-8977/98/$10.00 0 1997 IEEE


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14

SYNCHRONIZATION AND T IME

A phasor represents both the m agnitude and phase angle ofthe AC power signal. Phase angle is measured in reference tothe time of measurement. Comparison of phasors measuredthroughout an interconnected grid requires a comm on timingreference provided by a synchronizing source. Thesynchronizing source may be local or global as long as itsupplies all sites included in the region of comparison.

The Global Position System (GPS) provides timesynchronization that satisfies these requirements. GPS is aUS Department of Defense radio-navigation systemconsisting of 24 satellites arrayed to provide at least 4satellite visibility at all times, Each satellite transmits anavigation signal from which a receiver can decode timesynchronized to within 0.2 ps of Coordinated Universal Time(UTC), the world standard. The inherent availability,redundancy, reliability, and accuracy make it a system wellsuited for synchronized phasor measurem ent systems.

GP S is currently the only regional synchronizing signalsource that is accurate enough for phasor measurement. Thestandard time broadcast systems do not have sufficientaccuracy. Radio broadcasts, two way radio transfer, fiberoptic, UHF, or microwave systems could be used forlocalized systems but require development of specializedinterfaces to deal with signal delays. These other alternativesare costly and are not currently competitive with GPSreceivers.IEEE 1344 requires the synchronizing source to bereferenced to within 1 ps of UTC. This will ensure thegreatest compatibility among PMUs. The standard alsorequires the reliability to exceed 99.87% (one hour of outageper month). 1 ps accuracy is achievable from GPS and meetsthe requ irements of all foreseeable data needs. It corresponds

to an angular accuracy of 0.022 degree for a 60 Hz systemand 0.018 degree for a 50 Hz system which allows an errorbudget for most application s which require a ngle accuracy of0.1 degree or less.

Time inputThe standard specifies that time should be providedreferenced to UTC and as an IRIG-B code (Inter-RangeInstrumentation Group time code) or using a new highprecision format. If time is provided as modulated IRIG-Ban additional 1 pulse-per-second (PPS) is required for 1 pssynchronization. The control bits are used to enhancestandard IRIG-B with year, time zone, and other special

flags. Control bit designations and the high precision formatare described in detail in Annex F of the standard.After an extensive investigation of other alternatives, IRIG-B was chosen as the basic format. It is the most commonform of time synchronization in sub-station equipment, andno other standardized codes provide all that it lacks.Standard IRIG codes have n o provision for leap seconds, leap

years, local time offsets, daylight savings time, year num ber,or time quality factors. They do have a nu mber of spare bitsthat can be used to present the missing information. Giventhe wide use of IFUG-B and its flexibility of implementationthrough the av ailability of spare bit positions, the only goodchoice was to use this as the base of transmitting an accuratetime reference. IRIG-B is normally transmitted either as a10 0 Hz level shift, or used to modulate a 1 kHz AM carrier.Neither of these are suitable for timing needs that approach 1microsecond. As a result, a modified Manchester codingscheme was chosen. The coding scheme is easilytransmitted by fiber-optic, has defined coding edges to reachsub microsecond timing precision, and can be easilyimplemented with existing electronic devices.

PHASOR MEASUREMENT CONVENTIONSData sampling

The sample taken on the 1 second mark is sample numberzero (0). Using the conven tion that the signalcorresponds to the phasor V eh, he phasor computed from awindow beginning at a positive voltage maximum is realwhile one com puted from a window beginning at a positivegoing voltage zero is negative imaginary as shown in figure2.

v(t) = JZ v cos (mot + c p )

ISample NO. o ISample NO.o


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75The choice of sampling frequency, the algorithms used forphasor calculation, and the choice of the time-tag all affectthe apparent system responses represented by phasors.Different choices of these parameters can yield somewhatdifferent phasor response characteristics during transientcond itions. Phasors produced by different systems can beaccurately compared only for steady state calculations.Comparison for transient conditions requires a detailed

knowledge o f these characteristics and their consequences.

Figure 2 - Convention for phasor representationIf the 1 PPS signal occurs at time to, the m easured phasorcorresponding o a signal v(t) = JZ V cos (coot + 9)with afrequency coo is V rdOoto+ 9). For steady state signals atoff-nominal frequency o , the measured phasor with time-

tag corresponding to the 1 PPS instant to is V d o l t o + q).The behavior of the measured phasor between the 1 PPSpoints and the response to non-steady-state (transient)conditions will vary with the algorithms used, and isdiscussed in a nnex C o f the standard.Time-tags

Phasor timetags consist of a four (4) byte second-of-century (SOC) word in Network Time Protocol (NTP) formatand a two (2) byte sample count (SMPCNT) integer. Thissecond-of-century number is the UTC time in secondsfigured from midnight of January 1, 1900. NTP can bedetermined by m ultiplying the n umber of seconds per day bythe days since 1/1/1900. Leap seconds do not alter the scale;a leap second insertion results in two consecutive secondshaving the same SOC number. The sample number is thetwo byte integer between 0 and N-1 where N is the samplerate. In systems where phaso rs are derived fromsynchronized samples, this is the sample number of thesample in the data window used in the phasor calculation.This is independent of the length of the data window used inthe phasor calculation.Phasor Calculation

If synchronized phasors are calculated from synch ronizeddata samples, the data sampling is required to besynchronized to within 1 ps of UTC time. Data sampling isrequired to be phase locke d to GPS with an integer number ofsamples each second that are evenly spaced throughout theone second interval. The samples are required to benumbered from 0 to N-1 where N is the sample rate. Sample0 is the first sample in the new second coincides with the onesecond roll-over. It is recommended that the sam ple rate bechosen from the list of Least Common Multiple samplefrequencies as defined in IEEE Standard C37.111-1991(COMTRADE).

The standard also allows for synchronized phasorscalculated from non-synchronized data samples with theprovision that the ca lculating process is required to determin ean equivalent UTC time and sam ple number as described forsynchronized sampling techniques. The phasor thusdetermined is required to have the sam e magnitude and phaseangle as a phasor derived from synchronized samples overthe same data window with the same time stamp and samplenumber.

For exam ple, figure 3 illustrates how the choice of timetagaffects the apparent phasor response. The illustration is abalanced 3 phase system w hich undergo'es a -0.1 rad phaseangle step at P0.05 sec. In this example, a DFT transformwith a 12 sample data window and a 720 sample/secondsample rate was used. The three plots ishow the differenceinduced by taking the phasor timetag from the data sample atthe beginning (front), the middle, or the end (back) of thedata window.

I I II I I I I I.t++++x~r"mI

1.46 I I I0.01 0.02 0.03 0.04 0.05 0.06 0.07Time [sec]

Figure 3. Response of the positive sequence phasor to a step anglechange of -0.1 radSeveral observations may be m ade:0 Using the timetag at the front of the window m akes thephasor appear to begin responding to the transient before itoccurs. It is, how ever, close to the true response when theevent is over.0 The timetag at the middle of the data window yields a

response that is closest on an average to the true e vent, butstill starts to react before the event.The timetag at the end of the windlow giv es a responsethat is most like other transducers in that it begins to respondas soon as the event occu rs and slews tlo the new value. Ittakes the longest to reach the final value. This is the choicespecified in the standard.There is an error due to length of time the phasorresponds to the input. This is similar to the frequency

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76response in an analog system, but is of a fixed length which * stored in the configu ration file. The data frame containsis a function of th ed ai a window. The length o f the datawindow can be altered to suit a particular application. Ashorter window yields a faster response in transientconditions but higher noise and greater data handlingcapability due to a higher data rate.There is a transient error whose properties aredependent upon the filtering algorithm used. This effect canbe minimized fo r a certain transient (or a class of transients),but only at the exp ense of additional computational burdenand a po orer result for another class of transients.

The effects of harmonics, transducer non-linearities, andother factors which m ay degrade the overall response are notincluded in this example. Real-time delays relating to datacomputation and transmissions are also not shown.Generally, the variety of approaches to phasor calculationwill yield similar but differen t results during transient systembehavior. Any approach will have its strong and weakpoints. The system designer will have to choose techniquesthat best suit the range of applications of primary interest.Data analysis will have to account for differences whencomparing phasors derived using differen t techniqu es.DATA FORMATSND MESSAGES

The PMU described in the standard transmits three types ofinformation--data, configu ration, and header. Data includesstatus, phasor, and other computed information derived frommeasurements. Configuration information is a machinereadable, binary file stored in the PMU and describes set-upand operational parameters. Header information is an A SCIIfile that provides user information that may includeconfiguration information. These three information types aretransmitted in frames that are described in detail in thestandard.The general syntax of all message frames is as follows:The f i s t 4 bytes of each frame are the SOC time mark. Inaddition to prov iding the basic one second resolution timetag,the 4 yte SOC can be used for frame synchronization. It isfollowed by the two byte SMPCN T word which identifies theframe type and gives the sam ple number (for data frames) orthe frame count. The sample number gives sub-secondtiming information for data frames. The three mostsignificant bits of the SMPCNT word determine the frame

type. Only three of he eightpossible frame types are definedleaving room for future dermition. Every frame is terminatedby a tw o byte CRC word which allows positive errordetection.The Data Frame is show n in figure 4. Each frame is a datascan corresponding to the time-tag and sample number. Thedata is in binary form at with no delimiters. Inform ation forparsing and converting the information to engineering units is

measured data and is identified by having the three mostsignificant bits of the SM PCNT (third word, 5th byte) equ alto zero. Following SMPCNT the status word (STAT)includes flags for time, data, and trigger status in the high 5bits and gives the byte cou nt for the fram e in the low 11 bits.

[socl /-xzEq[sTAT]irsttransmitted MSB 4 LS B 2 2WI last transmitted2 < byteslword

Figure 4 - Data frame for phasor dataFollowing STAT, phasors are sent in two word (4 byte)format. The words are real and imaginary values respectively

in 16bit integer format (twos complement) if the data is inrectangular coordinates. If polar coordinates are used, thephaso rs are magnitude in unsigned 16bit integer and an gle inradians x in the range - 7 ~o + n, respectively. After thelast phasor, frequency deviation (FREQ) from nominal andrate-of-change of frequency (DFREQ) are in 16 bit integerformat. Dev iation is scaled in milliHertz (H z x lom3)whichallows a range of -32 to +32 Hz from nom inal, adequate forall but the most severe disturban ce. DFR EQ is in Hz persecond x Digital channel data (DIG) is contained in 2byte segments, one for each 16 bit chann el. The cyclicredundancy check word (CRC) is the last two bytes. TheCRC uses the CCITT specified error checking polynomial,16+,12+,5+1.

In order to parse the data frame, a central system mustacquire the configuration file. There are two types ofconfigu ration files: a system configuration file CFG-1, and adata configuration file CFG-2. The system configu ration fileindicates all inputs available on the PMU. The dataconfiguration file indicates the information actually beingsent, since only a subset of that av ailable may be transmittedby the PMU in the data frame. Configuration fileinformation kcludes:

(1) Station name and identification(2) Number of phasors(3) Num ber of digital channels(4) Channel and phasor nam es, units, andconversion factors( 5 ) Nominal line frequency( 6 ) Transmission period (data ;ate)


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7700402-8, U.S. Government Printing Ojffice, Washington,D.C. 20402.[3] IRIG Standard 200-89, Secretariat, RangeCommanders Council, White Sands Missile Range, NewMexico 88002,1989.

[4] GPS Interface Control Document, IRN-200B-PR-001, 1July 1992.[ 5 ] IEEE C37.111-1991, IEEE Standard Common

Format for Transient Data Exchange (COMTRADE) forPower Systems, June 199 1.Network T ime Protocol (Version 3) Specification,Implementation, and Analysis, D. L. Mills, DARPANetwork Working Group, RFC- 13051, University ofDelaware, March, 1992.Power System Relaying Committee Working GroupH7, Syn chronized Sampling and Phasor M easurements forRelaying and Control, IEEE Trans. on ID, Vol. 9, No . 1,Jan. 1994.

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[7]

The header file is an ASCII file containing PMU and inputparameters such as station, line, scaling, and filteringinformation. It is meant to be human readable and does nothave a fixed form at. The file is sent as reques ted in a framedformat described in the standard .

The header and configuration files may be transmittedduring real time data transmissions (if sufficient bandwidthexists), so the information will be interleaved with dataframes. Since these files may be too large to send in oneblock between data frames, they can are broken into multipleframes for transmission.

The PMU may be setup and controlled from a host.Commands are passed in received message frames. Thestandard specifies a minimum command set, leaving most ofthe message set for future expansion. Like the other frames,this frame starts with the same synch ronizing SOC timetag,and ends with a CRC termination.ANNEXES

This Standard contains seven Annexes which provideadditional information to the concepts and practicesdeveloped in the main body of the text. The first annex is abibliography. Three annexes discuss details of samplingaccuracies, the effects of various time tagging conventions,and the merits of using non-synchr onized sampling. Twoannexes discuss various systems for time s ynchro nization andprovides details on the choice of time codes. Ano ther annexprovides detailed examples of phasor data form ats.CONCLUSIONS

Phasor measurements add a new dimension to powersystem monitoring and controls. They provide directmeasurement of the fundamental units of voltage and currentincluding both magnitude and phase angle. This com pactmeasurement set is easily co mmunicated in real time whichenhances its utility for real time system s. It provides all thefundamental power system measurements as well as system-wide phase angles. State estimation, protective relaying, andstability controls can be greatly improved with adaptivetechniques using phasor measurements. This standardprovides a framework for defining phasor measurements andintegrating them w ith other applications.

BIBLIOGRAPHY[11 The Fast Fourier Transform, E.O. Brigham,Prentice Hall, Inc., New Y ork, 1974, p105.[Z ] 1992 Federal Radionavigation Plan, DoT-VNTSC-RSPA-92-2/DoD-4650.5, tack No. 008-047-

ieee standard for synchrophasors for power systeme

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