ipts-2013-05-ruester-enu.pdf

8/13/2019 IPTS-2013-05-Ruester-ENU.pdf

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OMICRON2013International Protection Testing Symposium

Early Detection of Power System Oscillations for Improved StabilityAssessment

Dr. Christian Rster / Gerd Kaufmann, A. Eberle, Germany

Abstract

Todays biggest challenges to the stability of theelectricity grid come from the proliferation ofdecentralized generation and from economicpressure to maximize the utilization of lines.

To maintain stable operation, improved techniquesfor the early detection of critical system events areneeded. Because of this, new measurementtechnologies such as power quality analyzers aswell as wide area measurement systems for theassessment of dynamic power systems stability

are progressively introduced.This paper discusses the application of a noveltechnique for high resolution oscillation monitoringand comments how results can be used for thestability assessment of power systems. We giveexamples of detailed analyses of critical systemoscillation events that can occur in grids at times ofhigh dynamic stress.

Furthermore we comment on the benefits ofsecondary injection testing for the developmentand application of our measurement device.Typical system disturbances can have very long

durations and can be non-periodic as well as non-deterministic. Because of this, standard test toolsare not entirely suitable all the time. We show howthese limitations can be overcome by employing atest tool that makes use of the highly flexible CMEngine programming interface.

Low frequency oscillations andstability assessment

Wide area synchronous power systems offer greatbenefits for todays deregulated energy markets.

Flexible energy transfer across geographical andpolitical borders as well as lower costs through thepooling of generation capacities are some of thekey advantages of an interconnected grid. On theother hand the resulting massive exchange ofelectrical power over large distances represents achallenge for the stability of the grid, especially onthe transmission level.

In many interconnected grids relatively few andweak interconnecting lines have to carry a largeshare of the total power flow. As there is anobvious pressure to maximize the utilization ofthese lines, they are repeatedly operated close to

their stability limit. In this limit, the presence of lowfrequency oscillations on weak interconnectionscan rapidly develop into a fundamental bottleneckfor power transfer. Interarea oscillations (0,1-1Hz)

are present in virtually all large power systems onthe transmission level. They have often beenobserved to increase in amplitude during times ofhigh stress in the grid. A very recent and dramaticexample for this can be found in an official reportabout the wide area blackouts that happened on30th and 31st of July 2012 in the Northern Indiaelectricity grid [1]. Summarizing events during oneof the overall largest blackout incidents ever, thereport clearly states the presence of unusual andpoorly damped low frequency oscillations before,during and after the blackout. Such active or

reactive power swings are known to causeunwanted protection relay action and trippingswhich in turn can aggravate stability problems in analready stressed system, eventually leading toblackouts.

In order to relieve stress in the most critical areasof their grid, many national operators increasinglyrely on the introduction of active network elementssuch as FACTS devices or HVDC connections.While these are proven technologies that operatevery efficiently, this development inherentlyincreases the number and size of capacitiveelements embedded inside an otherwise

predominantly inductive electrical grid. Thisintroduction of capacitors into a grid also bearssome drawbacks. The most dangerous problemcan be the simultaneous introduction ofundesirable low frequency oscillations(~1040Hz) that may even lead to the destructionof turbine-generator shafts in bad cases. This well-known phenomenon has been given the namesubsynchronous resonance (SSR) [2-4].

A relatively new phenomenon which howeverbecomes more and more important is theoccurrence of SSR-like oscillations in wind farms.This arises from the need for increasedtransmission capacity to transport a bulk amount ofwind power over long distances. In this case,series compensation is an established means ofenhancing the power transfer capability of existingtransmission lines and is being increasinglyconsidered for integrating large wind power plants.A very similar phenomenon can happen when aturbine-generator (TG) unit is connected to a highvoltage direct current (HVDC) system which alsohas capacitive components. In [5], this effect isdescribed as (SSTI).

Though complex, the above basic mechanisms are

fairly well understood. In a perfectly stable grid,without any dynamic influences, all of these lowfrequency oscillations are fairly easily controllable.


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monitoring in power systems. Furthermore, thetypical transient nature of these disturbances isdemonstrated which has consequences for thedesign of an efficient equipment test suite.

Critical oscillation event caused by

sudden loss of generation capacityDuring this incident, multiple generators from acoal fired power station tripped simultaneously.Sudden losses in generation capacity constituteconsiderable stress for any kind of grid and areprone to excite oscillatory disturbances. Thissudden loss of generation capacity specificallyaffected two long and correspondingly weaktransmission corridors connected to the region inwhich the loss happened. One corridorsresponsibility is to supply a load center within thesame country, the other one is an interconnection

to neighboring nations which also considerablydepend on power imports from the region.

In this case, a comprehensive automatic underfrequency load shedding (UFLS) scheme was inplace. Since the generation loss lead to a drop infrequency exceeding 0,5 Hz, the UFLS relays wereactivated. Within the time frame of minutes, anequivalent amount of load was shed and theelectrical balance was restored.

In terms of oscillation monitoring, the brief period ofheavily disturbed operation is crisply resolvedwithin the GDASys damping monitor recordsshown in Figure 3 (blue data).

Fig. 3 Time series of amplitudes measured during amajor oscillation event caused by generationloss. Comparison of Wavelet based dampingmonitor and conventional sliding FFT results isgiven.

This oscillation event manifested itself with verylarge oscillation amplitudes exceeding 2% (0,02p.u) at a nearby measurement location. It can beseen that as far as oscillatory disturbances areconcerned, the event was over in less than oneminute, with amplitudes clearly exceeding anestablished alarm level before returning to ambientconditions.A criticality analysis based on oscillationamplitudes, their durations and damping reveals

that the large oscillations in the beginning of theevent are negatively damped and exceed aduration criterion of 10s. The largest oscillationamplitudes are found for a mode at 0,6 Hz. Thelongest sustained oscillations however exist for the0,35 Hz and 0,7 Hz interarea modes, with

oscillation durations exceeding 34 seconds in bothcases. The frequencies of the interarea modes arewell known in this grid as they are related to thetwo transmission corridors mentioned above. Fromthis it can be concluded that these transmissioncorridors were most strongly affected by the event.

Despite the event lasting only for a very short time,the device was able to resolve the multiple modesindividually, some of which even happenedsimultaneously. This superior response time is dueto fundamental advantages of the employedwavelet analysis algorithms of the device anddistinguishes it from typically used Fourier

techniques that have difficulties in thesubsynchronous range. The high time resolution ofthe Wavelet analysis is revealed by thecomparison with a FFT employing a sliding 100stime window (red data in Figure 8). The blueWavelet data rises immediately at what is knownfrom post mortem analysis to be the onset ofoscillations. In comparison, the response of the red100s FFT is naturally delayed and artificiallybroadened. In fact, when FFT amplitudes exceedthe alarm level, the actual event is already over.

Subsynchronous resonancemonitoring for generator protection

As pointed out above, classical SSR oscillationscan for example occur due to an interaction of arotating machine with the electrical grid, in which aseries capacitor is installed to compensate theinductance of a long transmission line. Theprinciple is shown in Figure 4.

A dangerous situation can occur if the mechanicalshaft oscillations of the turbine-generator set lieclose to the electrical resonance frequency definedby the capacitor and its grid environment. A typicalrange for the mechanical oscillations is 15-35 Hz. Ifunder transient conditions the electrical oscillationis excited, e.g. dynamically during a fault, there canbe an electromechanical resonance resulting inserious generator damage.

In the practical case analyzed with GDASys, somemechanical oscillations had already beenmeasured by shaft sensors on a very substantialgenerator. For safety reasons, this generator wasdecommissioned at first. Since a series capacitorwas known to be active close by, the likelihood ofSSR as the origin of shaft oscillations wasappreciated as high. However, without a definite

measurement of the corresponding electricaloscillations, other causes could not be ruled out.Subsequently, a measurement campaign based onthe GDASys damping monitor was carried out.


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Over the course of a few months, no substantialoscillations in the expected frequency range weredetected.

Fig. 4 Illustration of the technical background for theoccurrence of SSR. Illustrations taken from [10]

Strategically, this is an important result in that itsuggests that no fundamental SSR problems seemto be causing the mechanical generatoroscillations. Also based on such this result, other

root causes (e.g. simply a mechanical problem ofthe generator itself) seem more likely and it is clearthat their analysis should be pursued with higherpriority.

Device testing with OMICRONCMC 256 and CM Engine interface

The case studies described above show that therange of disturbances that can be analyzed withGDASys is very large and comprises diverseeffects. Most conventional testing tools are not

suitable because they either only offer quasi-staticsignals or transient signals that are too limited withrespect to their maximum duration. Also the natureof low frequency oscillations is such thatdisturbances are modulated onto the 50 Hz carriersignal. This is different from the more well-knownadditive oscillations that describe harmonics in thepower quality testing field. To address theselimitations, a customized testing interface wasdeveloped for the GDASys network analyzers. It isdescribed in detail in [11]. The most importantaspects and some test results are pointed outbelow.

Test signal generator details

The test interface relies on the real time playbackfeature that is available in the OMICRON CMC x56devices. The user interface was developed usingVB.NET and is shown in Figure 5. All parameterswhich are relevant for test signal generation can becomfortably adjusted in the panel on the left. Thesignal output is visualized in the graph section ofthe program.

Fig. 5 User interface of test signal generator

Currently, five different program sectionscontaining qualitatively different test signals areavailable to the user:

1. dU/dt for the simulation of voltage collapseprocesses for example preceeding ablackout

2. df/dt for the simulation of slow frequencydrift processes even over the course ofmultiple hours

3. Fingerprint analysis for the simulation ofspecific sets low frequency oscillations indifferent frequency bands that occur at thesame time, e.g. 8Hz and its "harmonics"

4. Lyapunov for the simulation of heavilyexpanding, broad band oscillatoryprocesses

5. Damped oscillation events for thesimulation of well-defined single lowfrequency oscillations with specific

frequency, amplitude, damping andduration

6. CSV for the playback of custom transientrecords

Signals generated in program sections 1-5 all relyon specific analytic functions that are coded intothe application. The corresponding formulasconsist of terms for a user definable referencesignal ("50 Hz") and (for oscillatory signals) time-dependent modulation and amplification factors.With respect to the modulation, both frequencymodulation as well as voltage (amplitude)

modulation is selectable.In addition it is possible to define a prefix timeduring which the undisturbed, clean 50Hz signal isoutput. This is necessary to accommodate for


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setting times of the device under test, for examplewindow lengths used in Fourier analysis.

The resulting analytic signals are output by theCMC 256 hardware and are limited by themaximum available sample rate of 10kHz. Due tothe Nyquist-Shannon sampling theorem the

devices under test should thus not exceed theupper frequency limit of 5 kHz.

In case actual disturbance records of transientsignal are available, these can be played backusing program section "CSV". The CSVfunctionality copes well even with very long high-resolution records with files sizes in the hundredsof Megabytes.

Usage of real time playback and CMEngine interface

The interface "CM Engine" is a program library thatgives low level access to the CMC x56 test sets.Using standard programming languages such asC/C++, C', Visual Basic and others, the developercan create custom test programs in a quick andstraightforward way. The interface allows tomanipulate the test sets outputs (voltages,currents, relays) and also to read out its binary andanalogue inputs. In this way, freely programmablewaveforms can be output by the test set.

However, only limited pattern memory is availablefor the creation of these waveforms. In the case ofCMC 256, 393216 samples are available. In

general the maximum time duration of transientsignals is limited by the total number of samples,the sample rate and the number of channels. Forexample, when using 6 channels at a 10 kHzsample rate, the maximum duration of the transientsignal is 6553ms.

In order to overcome this limitation, the CM Engineinterface allows to use the pattern memory as aring buffer. Samples that have been already beenoutput can be overwritten with new samples "onthe fly", i.e. while a live output routine is currentlyrunning. When the "end" of the sample memory isreached, new data can already be accessed at the

"start" of the ring buffer without ever stopping thereal time output. This functionality was realizedusing the "real-time playback" function and itallows for much longer signal durations.

Long waveforms are especially useful for reducingotherwise inevitable quantization artifacts andincreases the realism of the output. One exampleis the dV/dt section of the test signal generator(voltage drift). Here, the instantaneous voltageamplitude is linearly adjusted after only 10-4 s.Compared to the standard voltage ramp function oftest universe 2.3 (10-1 s or longer), this creates amuch smoother waveform.

Example Test: Oscillations in SSRfrequency range

One of the key features of GDASys is the largefrequency bandwidth that is available for oscillationmonitoring. This allows the user to investigate

several dynamic phenomena in the grid thatotherwise would not be observable using standardtechnologies such as synchrophasors. Oneexample is SSR, with characteristic disturbancefrequencies lying in the range of 15-35 Hz. SinceSSR oscillations are simply modulations of thestandard 50Hz (or 60 Hz) carrier, program section5 ("damped oscillation events") can be used to testthe device sensitivity in this band.

Fig. 6 Time domain visualization of test signal "dampedoscillation" in test signal generator application.

Figure 6 shows the time domain visualization of adamped voltage oscillation as displayed in thepreview graph area of the test signal generatorapplication. It follows the simple analytical

description of an amplitude modulated sinusoid

with

The symbols have the following meaning:

AAmplitude of the damping event (p.u.) damping factor (p.u.) f frequency of the damped oscillation fn(hypothetical) natural frequency of the

corresponding undamped oscillation

Please note that the signal plotted is only thedisturbance itself, i.e. it is demodulated from thecarrier. The adjustable parameters used in thisexample are as follows:

Total duration Td=200s: here, 100s areused for a negatively damped oscillation,100s for a positively damped but otherwiseidentical oscillation

Amplitude A=0,02 p.u.: Due to a chosenbase signal amplitude of 100 V, this means

the voltage varies between the peak values102 V and 98 V

Natural frequency fn=2,25 Hz Damping factor d=0,01 p.u.

tfeAtA tfn 2cos)( 2

21

ffn


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This corresponds to an actual dampedfrequency of f=2,249887 Hz.

The measurement test was conducted using thetest tool, an OMICRON CMC 256 and a DA-Box2000 as the measurement instrument. Severaldamped oscillations were created with the test

signal generator: Oscillations ranging from 7Hz to 35Hz

(natural frequency) Negative and positive damping at each

frequency of +/- 0,01 p.u. Varying disturbance amplitudes from

0,02.pu. down to 0,0001 p.u.

Some measurement results are shown in Figure 7.In the upper panel, the detected oscillationfrequencies are plotted in the analysis softwareversus test time. Each dot corresponds to adetected damped oscillation of equal frequency

and damping. It can be seen that both amplitudeas well as frequency modulated oscillations up to35 Hz were measured.

Fig. 7 Measurement results of damped oscillation testin the SSR frequency range (1535Hz)

The lower panel shows the corresponding detecteddamping values versus test time. It can be clearlyseen that in accordance with the test signalschematic (see Figure 6), at each a negatively andpositively damped oscillations of +/- 0,01 p.u. weremeasured. A variation of the oscillation amplitudeshowed that oscillation amplitudes as low as0,0001 p.u. could still be resolved.

This means that characteristic SSR disturbancefrequencies can be picked up even already atspurious amplitude levels. The device can serve asan early detector for these oscillations, long beforedangerous amplitudes are reached.

Summary

In summary we have argued that low frequencyoscillations are present in any power system.Almost by definition the occurrence of lowfrequency oscillations indicate "disturbances", in

the sense of a transient deviation from the stablesteady state operating point of the grid and itsnetwork elements. In the vast majority of situations

this is no problem at all - as long as oscillations arewell damped and amplitudes remain low. However,some effects can affect system stability (e.g. inter-area oscillations) or damage equipment (SSR)when oscillation amplitudes reach dangerouslevels. This can happen particularly quickly and

dramatically during times of high stress and duringfaults in the grid, as the measurement example"dangerous oscillations after loss of generation"has shown. It is generally advisable to detect lowfrequency oscillations as sensitively and thus asearly as possible in order to be able to preventserious damage.

In the second part of the paper we have arguedthat realistic disturbance patterns are not readilyavailable using standard test signal generatorsbecause they have some unique functionalproperties. We have shown how the flexible CMEngine interface can be used to realize a highly

customizable test suite that addresses the specifictest needs of our oscillation monitoring device.

Literature

[1] "Report on the grid disturbance on 30th July2012 and grid disturbance on 31st July 2012"available on www.cercind.gov.in

[2] J. W. Ballance and S. Goldberg,"Subsynchronous Resonance in SeriesCompensated Transmission Lines", IEEETrans. on PAS, pp 1649-1658, Sep./Oct.

1973.

[3] D. N. Walker, C. E. Bowler, R. L. Jackson,and D. A. Hodges,"Results ofSubsynchronous Resonance Test atMohave", IEEE Trans. on PAS, pp 1878-1889, Sep./Oct. 1975.

[4] R. G. Farmer, E. Katz, and A. L.Schwalb,"Navajo Project on SubsynchronousResonance Analysis and Solutions", IEEETrans. on PAS, pp 1226-1232, July/Aug.,1977.

[5] Yin Chin Choo et al., "SubsynchronousTorsional Behaviour of a Hydraulic Turbine-Generator Unit Connected to a HVDC", 2008Australasian Universities Power EngineeringConference (AUPEC'08)

[6] www.a-eberle.de and www.cprd.info

[7] W. Haussel, M. Hofbeck, T.Sybel, M. Fette,"Collapse Protection System - Basics andApplications", Proceedings of the SouthAfrican Power System Protection Conference2004

[8] M. Hofbeck, L. Mayer, T.Sybel, M. Fette, B.Werther, I. Winzenick, "Collapse PredictionRelay CPR-D - Theory and Applications -Implementation into SCADA-Systems to


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Support Security Stability Assessment",Proceedings of the South African PowerSystem Protection Conference 2008

[9] M. Hofbeck, T.Sybel, M.Fette, I. Winzenick,"Measurements of the Dynamical Status ofHigh-Voltage-Networks with CPR-D",

Proceedings of the South African PowerSystem Protection Conference 2008

[10] Source: presentation Dr. M.S.R. Murtyhttp://ee.sharif.edu/~egysysanalysis1/ssr.pdf

[11] Oliver Skrbinjek (Steweag-Steg GmbH),"berprfung von sekundrtechnischenEinrichtungen mittels langer transienterSignalausgabe"

About the Authors

Dr. Christian Rster receivedhis Ph.D. in Physics in 2005from the University ofWuerzburg in Germany. Hisprofessional career includesmore than ten years ofexperience as an industrialresearcher in various high techindustries. The technicalbackground he gained during

this time includes the development of electricalmeasurement systems, sensors and

communication networks. At A. Eberle GmbH heworks as product manager for wide areameasurement systems and disturbance recorders.

Gerd Kaufmann received hisMasters degree (Dipl. Ing.) inpower engineering from theTechnical University ofIlmenau in Germany. Histhesis work on efficient high-power inverter designs wasdone at ABB Switzerland. In2006 he joined A. Eberle

GmbH where he currentlyworks as the product manager for earth faultprotection devices. Over the years he has gainedconsiderable experience in the commissioning ofthe A.Eberle product range world-wide.

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