1514 IEEE TRANSACTIONS ON POWER DELIVERY, VOL. 30, NO. 3, JUNE 2015

Synchrophasor Measurements Under theIEEE Standard C37.118.1-2011 With

Amendment C37.118.1aWorking Group H11 on the Synchrophasor Standard, C37.118.1 of the Relay Communications

Subcommittee of the IEEE Power System Relaying Committee

AbstractSynchrophasor Standards have evolved since therst one was introduced in 1995, IEEE 1344-1995. IEEE StandardC37.118-2005 introduced the evaluation of measurement accuracyunder steady state conditions and interference rejection. It alsodened a messaging system for communication of phasor data.In 2009, the IEEE started a joint project with IEC to harmonizereal time communications dened in C37.118 with the IEC 61850communication standard. This led to the need to split the C37.118into two standards, one for communication and one for mea-surements. This paper presents an overview of IEEE StandardC37.118.1-2011, which includes the measurement part of the pre-vious standard. This standard strengthens the 2005 steady-staterequirements and extends them to include measurement perfor-mance under dynamic system conditions. Frequency and rate ofchange of frequency (ROCOF) measurement was dened andrequirements for these measurements were also added. Amend-ment IEEE Std. C37.118.1a-2014 modied some performancerequirements. These changes are also included in this paper.Index TermsFrequency, frequency error (FE), phasor, phasor

measurement unit (PMU), rate of change of frequency (ROCOF),ROCOF error (RFE), synchronized phasor, synchrophasor, totalvector error (TVE).

I. INTRODUCTION

T HE synchrophasor standard IEEE C37.118-2005 [1]was separated into two parts. The rst part, IEEEStd.118.1-2011 Standard for Synchrophasor Measurements forPower Systems, deals with the measurement of synchrophasorsand related performance requirements [2]. The second part,IEEE Standard C37.118.2-2011 Standard for SynchrophasorData Transfer for Power Systems, addresses the real-timetransfer of synchrophasor data over communication systems[3].The standard was separated into two parts because:

Manuscript received June 12, 2014; revised September 29, 2014 andNovember 27, 2014; accepted January 05, 2015. Date of publication March02, 2015; date of current version May 20, 2015. This work was prepared byworking group H-11 of the Relay Communications Subcommittee of the IEEEPower System Relaying Committee of the Power Engineering Society. Paperno. TPWRD-00690-2014.K. E. Martin, Chair, is with the Phasor Measurement Systems, KenM Con-

sulting, Portland, OR 97213 USA and also with the Electric Power Group,Pasadena, CA 91101 USA.Color versions of one or more of the gures in this paper are available online

at http://ieeexplore.ieee.org.Digital Object Identier 10.1109/TPWRD.2015.2403591

1) the technology has matured to where individual topic areasneed more specialized treatment

2) separation facilitates adoption and harmonization of thiswork with International Electrotechnical Commission(IEC) standards.

The IEC structure for standards separates topic areas into dif-ferent standards. IEEE tends to group by application, often in-cluding several topics related to an application area in one stan-dard. Both IEEE's and IEC's methods have their advantages buta structure compatible with both organizations simplies adop-tion by both groups. The two subject areas of measurement andcommunications are addressed by different technical commit-tees within IEC.The IEEE and the IEC initiated a joint project in 2009 to har-

monize the IEEE Standard C37.118 data transfer with the IEC61850 communications standard. That work was completed in2012 with TR 61850-90-5, which describes how to communi-cate synchrophasor data using 61850 methods while fulllingall of the functions that were developed under IEEE StandardC37.118. Concurrently, IEEE and IEC are working on a joint de-velopment process for an IEC standard on phasor measurementunit (PMU)measurements. This joint standard will be publishedas IEC 60255-118-1.Subsequent to publication, detailed evaluation of the

signal-processing model performance, presented in Annex C, aswell as testing of PMUs, revealed deciencies in meeting someperformance requirements as specied. The problem areaswere related to error limits and response times of frequencyand ROCOF measurements. The standard was amended byIEEE Standard C37.118.1a-2014 to ensure that the performancerequirements can be met [4]. The amendment relaxes therelevant parameters, claries a few requirements, and correctstypographical errors. The requirements described in this paperinclude these modications.This paper does not follow the same order as the standard.

It focuses on specic changes and provides background dis-cussion regarding these changes. Section II covers reportingrequirements. Section III discusses performance classes. Sec-tions IV and V cover steady-state and dynamic performance.Frequency measurement considerations are discussed in Sec-tion VI. Section VII reviews latency requirements. Section VIIIis an overview of the standard's annexes. Section IX lists con-siderations for the next standard and Section X concludes thispaper.

0885-8977 2015 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission.See http://www.ieee.org/publications_standards/publications/rights/index.html for more information.

MARTIN: SYNCHROPHASOR MEASUREMENTS UNDER THE IEEE STANDARD C37.118.1-2011 1515

TABLE IREQUIRED PMU REPORTING RATES (FPS)

II. REPORTING REQUIREMENTSFor consistency, PMU measurements are to be transmitted

in time-tagged samples (frames) at a xed rate, called the re-porting rate in frames per second (fps). (The term framesper second is used in this context to differentiate from samplesof the waveform values where the term samples per secondis used.) Since phase angle is measured relative to a time ref-erence, angles from different measurements can only be com-pared if they are made at the same time. Fixed reporting timesand rates allows ready angle comparisons. Reports can be sentwith various specied rates, evenly spaced in time as detailed inClause 5.4. One of the reports must be coincident with the UTChour (xx:00:00). Note that the data are inevitably transmittedslightly after the time stamp on the data. The revised standardhas extended the reporting rates that PMUmanufacturers are re-quired to support. Table I lists the required rates.Higher or lower reporting rates than those presented in Table I

are permitted. Higher rates may be useful for control systems orsystem dynamic analysis. Lower rates may be useful for super-visory control and data acquistion (SCADA) systems reporting,such as power ow and state estimation. A user may requestcompliance with the appropriate sections of the standard at theseadditional rates, but this is not required for compliance with thestandard.Synchrophasors with reporting rates lower than 10 frames/s

are not subject to the dynamic requirements of the standard.Anti-alias ltering at very low reporting rates results in longreporting delays. Low reporting rate measurements are intendedas a snapshot with which the system as a whole can be assessed.Unltered lower sample rates could be obtained by selectingevery th sample from a higher rate stream.

III. DISCUSSION OF P & M CLASSSynchronized phasor measurements were rst implemented

using a one-cycle discrete Fourier transform (DFT) where thewaveform was sampled at the rate of 720 samples/s and syn-chrophasors were calculated at each sample [5]. Reporting mea-surements at this rate exceeded most communication capability,so the rst standard detailed reporting rates at even fractions ofthe power system frequency. This has been continued in the cur-rent standards (Table I).Reporting rate reduction using every th sample of a

higher measurement rate (e.g., taking every 48th sample of a720-frames/s rate to achieve 15 frames/s) is easy to implementbut may cause aliasing of signals present in the original data.Additional ltering can provide an antialias function, but in-troduces delay in reporting. From these initial observations, itwas clear that one approach was not sufcient for all uses ofsynchrophasors.

The concept of different classes of performance was intro-duced with IEEE Standard C37.118-2005. Classes were differ-entiated as performance levels 0 and 1. Compared to the re-quirements for level 1, level 0 had relaxed requirements forharmonics and out-of-band signal rejection, and narrowed fre-quency and magnitude ranges for measurement performance.Level 1 was intended for use by controls and applications thatare sensitive to harmonics and to small signals that could bealiased. Level 0 was for applications insensitive to small sig-nals but needed minimum latency.Since these requirements were described as levels of perfor-

mance and level 1 had tighter requirements than level 0, manyusers interpreted the higher the level, the better the performanceof their applications. Some users were specifying level 1 whenlevel 0 was better suited to their requirements.In the 2011 standard, the label levels was changed to

classes. Rather than using a number (that seemed to implyquality) for the class, a letter was chosen to indicate the typeof targeted application. P was chosen for the class of mea-surement that would be made with minimal delay but lessimmunity to out-of-band interference. P class measurementis intended to support applications (e.g., high-speed controls)that generally require minimal delay in responding to dynamicchanges. M was chosen for the class of measurements thatrequires out-of-band ltering to avoid signal aliasing wherelonger latency is acceptable. M class measurement is intendedto support applications (e.g., certain control functions) that aresensitive to signal aliasing but can tolerate longer delays.It should be noted that the letters chosen for classes are only

an indicator of the differences in performance requirements.They might be used as a general guideline in choosing a per-formance class, but certainly should not be used without fullconsideration of the specic application requirements. A de-manding application may have performance requirements be-yond what is specied for either class. The system designershould always choose the class with the specic system and ap-plication in mind.

IV. STEADY-STATE REQUIREMENTSThe 2005 standard dened performance requirements under

steady-state signal conditions. It introduced the total vector error(TVE) method of evaluating the measured synchrophasor value.Total vector error (TVE) combines the evaluation of the angleand magnitude errors of the synchrophasor estimate as a singleerror value. This method of evaluation was retained to main-tain compatibility with the previous standard and its simplicityin specifying error limits. Separate phase angle and magnitudelimits could be added to future revisions.Phase angle is determined by the reference time in con-

junction with the estimation algorithm. Its accuracy dependson time synchronization, analog input components, digitalconversion, and signal processing. To avoid problems withspecifying internal operations or requirements that may notbe testable (for example, some PMUs use an internal globalpositioning system (GPS) receiver and do not provide access tothe timing signal), the Working Group H11 (WGH11) decidedto only specify the result. This leaves it to the manufacturer tooptimize their design and allocation of the error budget to meet

1516 IEEE TRANSACTIONS ON POWER DELIVERY, VOL. 30, NO. 3, JUNE 2015

requirements. It should be noted that timing signal accuracy caneasily be measured to 1 s which is about 0.02 of the phasorangle that is much less than PMU accuracy requirements. Thus,a time source for PMUs that require an external clock caneasily be checked to an accuracy that assures insignicant errorcontribution.Signal frequency requirements ensure the measurements will

be accurate over a range of input frequency. The 2005 stan-dard required 1% TVE over a range of frequency variation of0.5 Hz for level 0 and 5 Hz for level 1, independent of re-

porting rate . The 0.5 Hz variation was deemed too narrowfor a large number of applications, so the new range for P classis now 2 Hz, still independent of . It was also found thatthe 5-Hz range created difcult ltering requirements and, infact, exceeds the Nyquist criterion for 10 frames/s reporting.The range of operation depends on lter design requirementswhich, in turn, depends on the reporting rate. Keeping the oper-ating range wide requires variation based on the reporting rate.The ranges of frequency performance for the M class are 2 Hzfor 10 frames/s, 5 for 25 frames/s, and5 Hz for 25 frames/s.An additional requirement in the new standard is that compli-

ance with this frequency variation test must be achieved at 0 Cand 50 C in addition to nominal room temperature C).The signal magnitude requirements retain the same 2005

limit of 1% TVE for phasors, but separates voltage and currentrequirements.. Based on a nominal voltage or current valuespecied by the manufacturer, the voltage magnitude test rangeis 80% to 120% for P class, and 10% to 120% for M class.The current magnitude range is 10% to 200% for both classes.These ranges are expected to cover voltages and currents undernormal to high-stressed operating conditions, which is the rangeover which synchrophasors are intended to be used. Relayshandle fault level currents for fault identication and location.(Note that the long PMU measurement windows also reducefault calculation effectiveness.) Further PMU developmentmay extend these ranges in the future, but will require betterunderstanding about estimation under extreme unbalance andnonlinear conditions.The phase angle range requirements have not changed, but

the test conditions have been modied to simplify testing. Thismodied test uses a slowly varying angle that is produced by anoffnominal frequency of less than 0.25 Hz, eliminating testingwith separate step changes as required before.The harmonic distortion requirements were the least-changed

requirement. The requirements for P class PMUs are the sameas for level 0: 1% phasor TVE with 1% harmonics from the 2ndto the 50th. The requirements for M class are the same as forlevel 1: 1% phasor TVE with 10% harmonics from the 2nd tothe 50th. For conformance testing, all 49 harmonics will be ap-plied individually with the amplitude of each at the speciedlevel. The WGH11 discussed requiring this test to be done withmultiple harmonics at one time, to more accurately simulatehow harmonics appear in the power systems and to possibly re-duce the testing time. Issues of signal crest factor and rise timesand their impact on measurement were raised. Different powersystem operating scenarios produce different combinations ofharmonics. The question of whether the test limits should apply

to the total power of the harmonics or the individual amplitudeswas raised. Without general agreement on these questions, theWGH11 decided the best approach is still to perform the testwith one harmonic applied at a time. The 2014 amendment sus-pended the ROCOF error requirement for harmonic testing ofMclass PMUs, after testing of actual PMUs indicated that this re-quirement might be a driving factor in PMU design which couldimpair the phasor measurement capability. P class PMUs stillhave a ROCOF error limit for harmonic tests, though relaxed to0.4 Hz/s.In some applications, a PMU must be able to lter inter-

fering signals that could be aliased into the measurement. Anout-of-band interference test is dened to test this capability.For M class tests, the interfering signal magnitude is 10% of thefundamental, the same as the level 1 requirement for the 2005standard, but the allowable TVE error limit was increased from1% to 1.3%. The test was eliminated for P class. The 2005 and2011 standards both dene interfering signal frequencies to beany frequency such that

where is the nominal system frequency and is the Nyquistfrequency (i.e., one half) of the reporting rate .The 2011 standard further claries that these interfering

signal tests must include frequencies down to 10 Hz and up totwice the nominal frequency . Experience has shown thatthe critical frequencies are those close to the lter limits, whichare at the Nyquist frequency distance from the nominal systemfrequency . Frequencies 10 Hz and have not beenand are not expected to be a problem, so further testing shouldnot be needed. The standard requires that when measuring theinterference effects on a positive sequence phasor from thePMU, the interfering signal harmonics must be positive se-quence. The need to make the interharmonics positive sequencearises from the fact that a PMU calculates symmetrical compo-nents from the phase components. Any nonpositive-sequencecomponent of the interharmonics would be highly attenuatedin the computation of the positive-sequence phasor.A signicant change in verifying ltering compliance is that

power system signal frequency must be varied over a range of10% of the Nyquist frequency of the reporting rate. For ex-

ample, for a reporting rate of 25 fps, the test must be performedwith power system frequency at various points from 47.5 to 52.5Hz rather than just at the nominal 50 Hz. (The standard doesnot specify which or how many points must be tested.) This re-quirement was added to ensure the ltering is sharp enough tohandle conditions where the power system is off nominal fre-quency (which it usually is). While the better approach may berequiring a single frequency range for all reporting rates, thelink to reporting rates assures it will not add difculty for lterdesign.The biggest change to the steady state requirements is the

addition of specications for frequency and ROCOF. Fre-quency Error (FE) and ROCOF error (RFE) are evaluated asthe absolute difference between the measured value and thevalue specied for the given test. The requirements are keyedto the phasor requirements, so the conformance tests only need

MARTIN: SYNCHROPHASOR MEASUREMENTS UNDER THE IEEE STANDARD C37.118.1-2011 1517

to be run once. The frequency and ROCOF measurements aremade only during the signal frequency, harmonic rejection,and OOB interference rejection tests. The 2014 amendmentalso suspended the ROCOF error limit for the out-of-bandinterfering signals test.

V. DYNAMIC REQUIREMENTS AND THEIR TESTSThe original standard, IEEE Std. 1344-1995, specied

sampling and timing requirements and assumed that all imple-mentations would use the same estimation algorithm [6]. Thiswould ensure comparable measurements from different PMUs.The succeeding standard, IEEE Std. C37.118-2005, did not as-sume use of the same estimation methods, so it established theTVE metric for evaluating phasor measurement performance.However, it addressed performance only under steady stateconditions.A principal use for synchrophasor measurements is observing

power system dynamics. Consequently, requirements for PMUperformance under dynamic conditions is now part of the stan-dard, and verifying conformance is important.Dynamic PMU testing had been established using modulated

signals and step tests [7][11]. WGH11 determined that confor-mance tests should also reect the sort of phenomena that mightbe observed on the real power system. From that, they narrowedthe test types to modulation, ramp, and step. These tests reason-ably characterize PMU performance.WGH11 decided also to limit the dynamics over which a

PMU will be tested. With limited dynamics, the signal willchange nearly linearly across a short measurement window.If the change across the window is nearly linear and the es-timation algorithm is symmetrical, the phasor estimate willclosely represent the actual phasor value at the center of thewindow. Thus, the most common estimators using a time tagat the center of the sampling window should meet the dynamicrequirements in the new standard.Consider that the phasor measurement process produces an

estimated phasor value for the basic input signal, which nomi-nally is sinusoidal at 50 or 60 Hz. However, in reality this basicsignal can be above or below nominal system frequency due tothe generation-load balance as well as deviation due to any dy-namic effects such as oscillation and switching. The reportedphasor value reects an estimate of the amplitude, phase angle,frequency and ROCOF of the sinusoid at the instant recorded inthe time tag. The time series of samples represents the dynami-cally changing waveform subject to signal frequency limitationsbased on the Nyquist sampling limit of the phasor reporting rate.Tests that might conrm that measurements from different

PMUs are comparable include linearity, bandwidth, frequencytracking, response time, settling time, overshoot, noise, and dis-tortion as follows. Linearity is tested with steady-state tests. The measurement bandwidth test compares the output with

the input over a range of modulation frequency. For thistest, the input-power signal is modulated in phase or ampli-tude. This is the only test that measures the PMU's abilityto track a changing ROCOF.

The ramp of the system frequency test monitors all mea-surements during a ramp in the fundamental frequency to

Fig. 1. Step change in amplitude illustrating the points of measurementassessment.

ensure they can follow such changes as can occur in apower system. This test is useful in exposing a reportingtime error in the frequency estimate.

Response time and overshoot use a step change in inputamplitude or phase to see how quickly the PMU responds,and how far it goes beyond the nal value (overshoot) asshown in Fig. 1.

Since synchrophasor estimates are the result of lteringfunctions, settling time is included with the response timemeasurement, as the overall response is an importantparameter.

Noise and distortion are included in the overall assessmentby using the worst case TVE for each test as the test result.

The rst dynamic test is a modulation test. Some powersystem oscillations appear as a modulation of the AC powersignal, so this test simulates actual operational phenomena.Simulations showed that a typical power swing would createsimultaneous amplitude and phase modulation. Therefore,IEEE Std. C37.118.1-2011 used this simultaneous modulationfor the test. However, due to interactions between amplitudeand phase modulation in linear PMU algorithms, this proveddifcult to properly evaluate. IEEE Standard C37.118.1a-2014amended the procedure into separate amplitude and phasemodulation tests.The primary purpose of these tests is to determine if the mea-

surement bandwidth complies with the requirements. The band-width of an instrument is commonly stated as the point wherethe response is measured as down 3 dB (about 30%) from a ref-erence value. The test modulation is 10% of the signal, so a 30%reduction in the measured value would produce a 3% TVE error,which is the requirement in this test.For this test, the AC signal amplitude or phase is modulated

with a cosine signal, and the measurement is compared with theinput signal characteristics. Amplitude modulation is illustrated

1518 IEEE TRANSACTIONS ON POWER DELIVERY, VOL. 30, NO. 3, JUNE 2015

Fig. 2. The 60 Hz waveform with 10% amplitude modulation.

by Fig. 2. The modulation is applied at a series of frequenciesstarting at 0.1 Hz and increasing to amaximum that is the greaterof Fs/5 or 5 Hz for M class, or Fs/10 or 2 Hz for P class, whereFs is the phasor reporting rate. The modulation frequency forthe test is limited to a fraction of the sample rate because l-tering and algorithm frequency response will of necessity rolloff the response considerably below the Nyquist rate of Fs/2.The response is evaluated with the TVE, FE, and RFE criterionusing the highest error value over the greater of 2 full cycles ofmodulation or 5 seconds. This assures that the test will captureanomalies and noise in the measurement. This test is repeatedat enoughmodulation frequency points to determine an accurateresponse curve to be sure it remains below the limit.The second dynamic test is a ramp of system frequency. This

test determines how closely the measurements track the inputduring a constant rate of change of frequency. The frequencyis ramped from below nominal to above nominal over rangesspecied according to class and reporting rate.In the power system, a frequency ramp will occur after a

sudden change in generation or load, such as loss of a large loador system islanding. For the test, the ramp rate is 1.0 Hz/s,which was chosen as a worst case that might occur in a largepower system. WGH11 recognized that a small power systemwould have a faster ramp rate, but there was little recorded in-formation to draw on which would indicate a rate that would beuniversally applicable. Since most phasor system implementa-tion was being done in large power systems and there was goodagreement on 1 Hz/s, this value was used. As synchrophasoruse expands, higher rates can be specied to meet applicationrequirements in future revisions of the standard.TVE, FE and RFE are used to measure error, and the highest

values are used for determining compliance. Modulation testsalso show the ability to track changes, so this test may seemsomewhat redundant. However, the ramp of system frequencytests covers a wider range of frequencies than do the modulationtests. Some methods used for compensating phasor algorithmsmay be susceptible to errors during frequency changes and theramp of system frequency test is included to be sure these willnot cause problems.

The third dynamic test is a step in amplitude or phase. In apower system a step is generated when a switch operates or afault occurs. PMUs are not expected to produce accurate mea-surements during rapid changes like steps (because the estima-tion time windows are too long to reproduce short transients), sothe output is evaluated for response time, delay time, and over-shoot/undershoot. These tests are focused on system changes,not faults and so they are limited to 10% amplitude or 10 phase.A true step change for this test is impossible to create. Most

equipment used to produce these test signals synthesize themfrom a digital representation. The minimum step change is onesample interval. Further, since the output is basically a 50/60 Hzsine wave, lters that smooth the digital steps also slow the risetime. So a step may end up with a rise time considerably lowerthan a true step. For example, a simulator with a 10 kHz outputrate has 100 output intervals and with ltering the outputstep may approach 500 rise time. While test signal rise timecould have an effect on measurement, dening it opens up awhole new area whichWGH11 decided should be left to confor-mance certication, such as the IEEE Conformity AssessmentProgram (ICAP). This allows adapting equipment and methodsas they are available as well as tending to the many small detailsrequired in a comprehensive test program that may not be in-cluded in the standard. It simplies test adaptation as equipmentevolves. It also assures uniformity of process and application ofthe requirements. So only the test signals and performance re-quirements are specied in this standard.The response time (Fig. 1) is evaluated as the length of time

from when the measurement estimate leaves the specied TVE,FE or RFE limit at the prestep level until it enters and stayswithin the specied TVE, FE or RFE limit at the new value.This gives a good measure of how quickly the measurementwill re-settle to the new value after a sudden change. Phasorresponse times must be within for P class and 7/Fs for Mclass, shown in Table 9 of the amendment ( and Fs dened asbefore).Delay time evaluates how closely the time of the measure-

ment (measurement time stamp) corresponds to the actual stepchange. The requirement of 1/4 of the reporting time interval en-sures that themeasured step appears very close to the actual timeof occurrence. The overshoot/undershoot requirement assuresreasonableness of resultsexcessive overshoot in a PMU canmask, or appear as, meaningful responses in the power systemitself.Determining these parameters by simply evaluating the re-

sponse curve drawn by linearly interpolating the reported phasorvalues can be difcult. The response time is typically compa-rable to the reporting interval, and details of the PMU responsecan be missed.To obtain a more complete picture of the step change re-

sponse, the standard describes a technique for taking repeatedsteps and interleaving the results. This technique is based on thefact that the phasor estimates are made at exactly the same timeseach second. By shifting the stepped input by fractions of the re-porting interval, the output reports will occur at different pointson the measurement response. The output reports can then be in-terleaved according to the input shifts to form a more completeresponse curve. The standard suggests using a shift of 1/10 of

MARTIN: SYNCHROPHASOR MEASUREMENTS UNDER THE IEEE STANDARD C37.118.1-2011 1519

the reporting interval, because in testing, this shift adequatelyrevealed device performance. The process is described in moredetail in [11] and [12].Frequency and ROCOF are also tested with the same tests

and evaluated using the FE and RFE criterion. Only the re-sponse time is evaluated, but using longer limits to allow forthe noise amplication caused by differentiation. These limitsare (f) and (ROCOF) for P class. The M class limitis the greater of 14/Fs and for both f & ROCOF, as shownin Table 10 of the amendment. Response time was set to theserather long limits to ensure that current measurement techniquescould comply. F & ROCOF are new requirements and the es-timation techniques are probably a generation behind those ofphasors. Phasors, f, and ROCOF are reported in a single time-tagged measurement set which creates difculty adding moreltering, as further described in the next section. WGH11 ap-proach was to have relaxed limits for these new requirements toensure availability of compliant equipment and to move to moreexacting limits when the technology will support them.

VI. FREQUENCY MEASUREMENT

A. Frequency Measurement Discussion

Heuristic development of the concept of power system fre-quency is founded on the notion of rotating machines deter-mining a single value of frequency for the interconnected net-work. As power engineers, we are accustomed to thinking of thefrequency of the power system as a useful guide to whether thegeneration and load are properly matched as well as the overallsystem stability. Even small power system disturbances producedeviations from the target system frequency, so the machinesmust be made to track the system frequency, which they dovia a portfolio of automatic control mechanisms (inertial, auto-matic generation control, power system stabilizers, etc.)The introduction of PMUs 25 years ago revealed that the in-

stantaneous frequency was often slightly different at each mea-surement point. Fig. 3 shows the frequency measured at fourpoints in a real power system disturbance. It shows the dynamicnature of the power system and presents an excellent representa-tion of how generators aggregate in their response to an event.On average, the frequencies must agree (or generator pole slip-ping will occur).The original PMUs measured frequency by differencing

phase angles determined by the phasor values. This can pro-duce smooth frequency estimates by averaging over a longinterval or a more responsive but noisier estimate using shorterinterval. The longer average produces an estimate closer toa rotor angle and the latter a dynamic measurement such asillustrated in Fig. 3 [13]. WGH11 decided that simulation ofrotor frequency would be difcult to achieve on a uniform basisand that observing the dynamic power system behavior wasvaluable. Moreover the frequency measurement is much moreuseful if time aligned with the phasor values. Consequently thedenition of frequency as a derivative was adopted as describedin the following section. Testing requires the measurement istime aligned to the timetag. Note that while the standard offersa method for deriving frequency and ROCOF based on phase

Fig. 3. Instantaneous frequency measured by four identical PMUs at differentlocations in the power system.

angle differencing, it leaves the actual implementation up tothe PMU vendor.Note also that a fast measurement like this may be sub-

sequently ltered, for instance, to estimate rotor angle; but ina slow measurement where this ltering is already in place,the lost information can never be regained. Both frequency andROCOF data may be smoothed by ltering or post-processing.Any further processing and longer time windows in the PMUitself may increase delay in obtaining the measurement, inher-ently reducing its usefulness.B. Frequency Defined in the StandardPMUs measure phase angle relative to a cosine wave at nom-

inal frequency synchronized to UTC time, which is generallyderived from the GPS network. Most PMU implementations de-rive the frequency from the PMU estimates of the phase angleat an internal rate higher than the reporting rate. Frequency es-timation could be done differently: for example, using algo-rithms based on zero crossings of waveforms [14]. IEEE Stan-dard C37.118.1-2011 standard denes frequency as the deriva-tive of the phase angle and ROCOF as the derivative of fre-quency. As stated in IEEE Standard C37 118.1 (Clause 5.2),Given a sinusoidal signal

Frequency is dened

(1)

And ROCOF is dened

(2)

These derivative calculations can be done using phasorvalues computed at the point-on-wave rate data to improveperformance. This reduces the delay time between the phasorvalues and frequency and ROCOF as well as reducing mea-surement noise.Estimation of derivatives by differencing small sample steps

has been shown to create high noise due to quantized steps.However in this case, the data used to compute the steps is

1520 IEEE TRANSACTIONS ON POWER DELIVERY, VOL. 30, NO. 3, JUNE 2015

highly correlated, so the close spacing of the measurement sam-ples actually reduces the noise. Suppose the waveform sam-pling is every s and the three phase-angle estimates at times, , and are used to estimate frequency and

ROCOF. Due to the window of calculation they are based on al-most the same data. For instance, in the 50/s estimator describedin the reference model, each PMU estimate is based on 143samples. Assume that the three phase-angle estimates are deter-mined using three sets of 143 samples, offset by 1 sample(i.e., one sample interval, which is 1/800 s here). This re-quires a total of 145 samples. 141 of the samples will be usedby all three of the estimates, with slightly different weighting;and two samples at the ends of this group will be used by onlyone or two of the estimates, with very small weighting. Noisepresent in any of the samples as well as the desired signal willpropagate to the phase-angle estimates to almost the same de-gree for all three estimates. The smaller , the closer to iden-tical the three estimates become, and the more the signal (in-cluding noise embodied in it) will cancel in the frequency andROCOF calculations. In the limiting case, if is set to zero,all three estimates will be identical (even with truncation in themath, which otherwise does not cancel).The sample data in this analysis includes errors in the signal,

analog front end, timing, and analog-to-digital sampling. Theseare correlated in the phasor estimation and minimized. Howevererrors caused by truncation or round off in the phasor estima-tion algorithm itself (dependent on the CPU/DSP word lengthand other implementation considerations) do not cancel sincethey are created independently for each estimate after the wave-form samples are taken and are not correlated. Also noise that isinband or leaks through the phasor lters, such as those intro-duced by the OOB test, can cause frequency and ROCOF errors.C. Assessment of Frequency PerformanceThe tests described in the standard use input signals are low-

noise sinusoids with limited, well-dened changes in magnitudeand phase. The standard notes that the real power system sig-nals consist of noisy, rapidly changing values. Artifacts are in-troduced by nonlinear loads, line switching, reactor switching,and others. This situation is further exacerbated by the prolif-eration of nonsynchronous power sources, such as wind, solar,and HVdc.As rst- and second-order derivatives of phase angle,

frequency and ROCOF may be adversely affected by thesereal-world conditions. Though they are theoretically invaluablequantities for power system protection and control, the standardnotes that they should be used with caution.PMU frequency and ROCOF estimates are not intended to re-

place IEEE Standard C37.2 device 81 (under or over frequency)or 81R (rate-of-change of frequency) elements as found in con-ventional protection and control schemes for power networks.In order to comply with the requirement of having a singletime-tag for a set of measurements from a PMU (a PMU re-porting message), their performance and accuracy are dictatedby the phasor ltering system, the primary purpose of which hasbeen to achieve adequate synchrophasor measurements. For usein any application, the PMU frequency and ROCOF measure-ments performance and accuracy should be checked against

the required specications. Further work and development areneeded to rene the PMU's frequency and ROCOF evaluationsand to better dene the range of their potential applications.

VII. LATENCY REQUIREMENTSA. Latency Overview

Latency is important because in order to act upon informa-tion, the actor (whether human or machine) must know how oldthe input information is that it is acting upon, and how long itwill take to see the response to that action. Since a feedbacksystem takes information from the output and applies it to theinput, the age of the information that is fed back must be knownin order to control the system dynamic response. If latencies inthe feedback information become longer than the system designboundaries, system damping and stability can be affected.Synchrophasor reports carry with them the time of measure-

ment of the information they contain. If the receiver of syn-chrophasor information has an accurate source of time, it candetermine the latency of the information by comparing the syn-chrophasor time stamp to the actual time. In this sense, syn-chrophasor information is particularly well suited for use in dis-crete feedback systems.Limits to latency allow feedback system designers to rely on

the information they are acting upon to be no older than the la-tency limit. Accurate knowledge of latency may in some casesallow predictive estimation [15], which would project measure-ments to the moment when a control decision is being made.B. Latency Limits in the Standard

The synchrophasor standard denes two limits that relate tolatency and requires two tests to determine if the PMU is per-forming within those limits.The rst of the two limits appears in Table 9 of clause 5.5.8

Dynamic compliance-performance under step changes in phaseand magnitude. The limit is Delay Time and is required tobe

where is the reporting rateThis limit is required for both classes of PMU for all reporting

rates. It is the time difference between the actual time of a step inphase ormagnitude of the input signal and the time that the PMUoutput reaches 50% of the step size. This requirement limits themaximum inaccuracy of the time stamp relative to a change inthe input being reported in the synchrophasor output.The second limit on latency appeared in Table 11 of Clause

5.5.9, PMU reporting latency compliance. The limit is Max-imum PMU Reporting Latency (in seconds) and is limited tono more than 2/(Reporting Rate) for P class and 7/(ReportingRate) for M Class. PMU reporting latency is the intervalfrom the time reported by the time stamp to the time that thesynchrophasor information becomes available at the output ofthe PMU. This requirement assures the data transmission fromthe PMU has bounded delay, as is required for real-time appli-cations. IEEE Std. C37.118.1-2011 described Measurementreporting latency in ambiguous terms. Amendment IEEE Std.C37.118.1a-2014 replaced Measurement reporting latency

MARTIN: SYNCHROPHASOR MEASUREMENTS UNDER THE IEEE STANDARD C37.118.1-2011 1521

with PMU reporting latency and cleared up the ambiguitiesin the description.C. Latency: in ConclusionA feedback system designer will be interested in limits to

latency of the entire control system being designed.With respectto the latencies within the PMU, the designer can be ensuredthat any PMU compliant with this standard will not exceed theconstraints of the aforementioned two limits.

VIII. ANNEXESSix informative Annexes are provided. The three most signif-

icant ones are discussed in this section.Annex C of IEEE Standard C37.118.1-2011 provides the ref-

erence models for the M- and P-class PMUs. The purpose of areference model is to be sure that all performance requirementscan be met. The reference models are provided as a disclosureof what was used to conrm the requirements, not as a recom-mended implementation. In fact, these models were kept simplewith the idea that better implementations could be made thatwould more easily meet the requirements.During development, severalWGH11members implemented

the reference models in software and tested them to be sureit would meet every requirement. Since these implementationswere in software only, an allowance of at least 50% was madeto be sure real PMUs could meet all requirements. Results werecompared to be sure the implementations agreed and all limitsweremet. Amendment IEEE Standard C37.118.1a-2014 revisedtheM-class lter coefcients for better immunity to out-of-bandinterference at the expense of longer latency. Also, the equationsfor frequency and ROCOF estimation were revised to eliminatea time offset in these estimates, thereby better aligning themwith the synchrophasor estimates.Annex D discusses time stamping and time quality. Of par-

ticular note, it includes an extension of IRIG-B that providesyear, leap seconds, and local time offsets (including daylightsaving time). The extension also includes time-quality codesthat allow the PMU to assess the time code accuracy relativeto a UTC-traceable source. This extension is an update of thewidely used prole introduced in the IEEE 1344 standard, andshould be used instead of the previous versions. The extension isnot included in the IRIG standards specication nor other IEEEdocuments.Annex F is an adaptation of the work of our colleagues

in China, in particular, the WAMS & Time SynchronizationWorking Group of SAC 82, Beijing, China [16]. The annexshows how the internal angle of the generator may be calculatedfrom the terminal conditions. It shows how the angular offset ofa shaft encoder can be calibrated via the no-load PMU results.

IX. FUTURE WORKIEEE Power Systems Relaying Committee WGH11, which

authored the IEEE Synchrophasor standards, has joined forceswith IEC technical committee 95 to revise the synchrophasormeasurement standard and publish it as joint standard IEC/IEEE60255-118-1. The agenda of the joint body includes:1) reduction of the required number of reporting rates;2) considering adding and higher reporting rates;

3) further clarication of latency requirements;4) resolve several ambiguities in clause 5.5:

a) relative phase of harmonics;b) step-response time interpolation;c) step overshoot reference level;d) phase-angle test is redundant with the frequency

range test;e) increments in the out-of-band interfering signals test;f) maximum PMU reporting latency might not be found

with only 1000 reports.5) temperature tests are the only requirement that make the

standard a device standard rather than a function stan-dard; consider the reference to other environmental teststandards and suggest how to test the PMU function underthose standards;

6) consider testing a PMU function with test signals in digitalform (as from a merging unit);

7) consider either new, higher accuracy classes, or setting per-formance levels within the M and P class;

8) reconsider the denition of frequency.The joint working group estimates this effort will take until

2017 to complete.

X. CONCLUSIONSThis paper has outlined the development of synchrophasor

standards spanning almost two decades. Standards have pro-gressed from basic denitions to include requirements for fre-quency and rate-of-change of frequency measurements. Presentparallel efforts between the IEEE and IEC have been facili-tated and harmonized by the separation of the measurementperformance requirements into C37.118.1-2011 described here,and IEEE Standard. C37.118.2-2011, which details the messageformat.The content of this standard has been reviewed including

and denitions, reporting requirements, introduction ofperformance classes (M and P), and performance requirementsunder steady-state and dynamic conditions. The most impor-tant annexes were reviewed, particularly the reference model inAnnex C. Some of the factors considered in drafting this stan-dard and its amendments have been discussed. Implications andlimitations have been presented.Potential problems in measurement have been mentioned,

particularly in actual power systems with attendant noise andsystem artifacts that are not present in the standardized test sig-nals. Future work will focus on rening the performance re-quirements as outlined in Section IX, with particular attentionto measurements in more realistic environments.

ACKNOWLEDGMENTWGH11 members are K. E. Martin

(Chair), A. R. Goldstein , M. G. Adamiak,G. Antonova, M. Begovic, G. Benmouyal, G. Brunello ,B. Dickerson , Y. Hu , M. Jalali, H. Kirkham ,M. Kezunovic, A. Kulshrestha, R. Midence, M. Patel,J. Murphy , K. Narendra, D. Ouellette , G. Stenbakken ,V. Skendzic , E. Udren, Z. and Zhang

Principle contributors to this paper

1522 IEEE TRANSACTIONS ON POWER DELIVERY, VOL. 30, NO. 3, JUNE 2015

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