PSS2B vs PSS4bTPWRS May2005(IREQ).pdf

IEEE TRANSACTIONS ON POWER SYSTEMS, VOL. 20, NO. 2, MAY 2005 903

IEEE PSS2B Versus PSS4B: The Limits ofPerformance of Modern Power System Stabilizers

I. Kamwa, Fellow, IEEE, R. Grondin, Senior Member, IEEE, and G. Trudel, Member, IEEE

AbstractIEEE Std 421.5 as revised by the IEEE excitationsystem subcommittee will introduce a new type of power systemstabilizer model, the multiband power system stabilizers (PSSs).Although it requires two inputs, like the widely used IEEE PSS2B,an integral of accelerating power PSS introduced at the beginningof the nineties as the first practical implementation of a digital PSS,the underlying principle of the new IEEE PSS4B makes it sharplydifferent. The present paper aims at assessing the two families ofPSSs from the point of view of their relative performance in tack-ling a wide range of system problems, using a single set of so-calledrobust/universal settings. Conclusions are drawn from a largenumber of small- and large-signal analyzes performed on severaltest systems and on an actual Hydro-Qubec system, paying dueaccount to the load models and governor response. Since either ofthe candidate PSSs can easily be tuned to perform acceptably ina standard local and/or inter-area oscillation scenario, emphasiswill be put on comparing them at the inherent limits of the PSSconcept, i.e., considering excessive VAR modulation during largegeneration rejection, fast load pickup on hydro units, and excessivetorsional interactions during faults on large turbine-generators.

Index TermsExcitation system, IEEE PSS2B, IEEE PSS4B,IEEE Std 421.5, inter-area oscillations, power system stability,power system stabilizer (PSS), small-signal stability.

I. INTRODUCTION

FOR MOST utilities around the world, the power systemstabilizer (PSS) is probably the most frequently used de-vice for resolving oscillatory stability problems. Although it wasintroduced and extensively used a long time ago [1][3] anddespite its inherent simplicity, it may still be one of the mostmisunderstood and misused pieces of generator control equip-ment [4]. Following the western U.S. interconnection blackoutsin 1996, it was found that key PSSs were either out of serviceor poorly tuned [5]. Even today, after these problems have beenfixed, large disturbances tend to induce 0.2-Hz low-frequencyoscillations in the grid [6], [31]. In Brazil, the north-south in-terconnection has given rise to a new low-frequency inter-areamode between 0.17 and 0.25 Hz, necessitating a retuning ofPSSs throughout the system [7]. Inter-area oscillations have alsobeen reported on the UCTE/CENTREL interconnection in Eu-rope, at 0.36, 0.26, and even 0.19 Hz [8], [34]. The recent 2003blackout in eastern Canada and the U.S. was equally accompa-nied by severe 0.4-Hz oscillations in several post-contingency

Manuscript received April 29, 2004; revised July 1, 2004. Paper no. TWPRS-002332004.

I. Kamwa and R. Grondin are with Hydro-Qubec, IREQ, Varennes, QC J3X1S1, Canada (e-mail: [email protected]; [email protected]).

G. Trudel is with Hydro-Qubec, Transnergie, Montral, QC H5B1H7,Canada (e-mail: [email protected]).

Digital Object Identifier 10.1109/TPWRS.2005.846197

stages. Thus, it appears that PSS technology still has great daysahead.

Historically, Hydro-Qubec has relied heavily on PSSs toimprove its grid behavior. As of 2000, 80% of the installed38 000-MW generation comprising 26 power plants wasequipped with about 140 PSSs [9]. By retuning and coordi-nating them more effectively, the utility was able to improvethe damping of its dominant 0.6-Hz inter-area mode, gainingabout 400 MW of transfer capability on its main transmissioncorridors. While most of the existing PSSs in the system arepower acceleration analog devices based on a design datingback to 1980, the utility was approached at the beginning of thenineties by manufacturers [10] offering a digital PSS based onthe integral of acceleration power represented as PSS2B PSS inIEEE Std 421.5 [11], [12]. As known today, this modern PSScan easily be tuned as a speed-based PSS [13], while mitigatingtwo major operational problems which had restricted the appli-cation of the old PSS technology utilizing electrical power orterminal frequency [26], namely the excess VAR modulationduring mechanical power reference changes for the first andadverse torsional interactions for the second.

By the year 2000, a novel PSS architecture was proposedin [14] and later included in the revised IEEE Std-421.5 asPSS4B. Using an electrical power transducer for capturinghigh-frequency dynamics, it is able to retain most of thegood properties ascribed to the PSS2B structure in local andtorsional modes. However, by using a separate internal-fre-quency transducer [26] for the inter-area and steady-statedynamics, it provides a distinctive and improved signatureof lower-frequency inter-area modes, with a typical dc phaselead of 90 as compared to 180 for the PSS2B. Additionally,the PSS4B builds on a flexible multiband transfer functionstructure proposed a decade ago [16], to provide many moredegrees of freedom for achieving a robust PSS tuning over awide frequency range, while paying due account to the manyoperational and contradictory trade-offs facing the designer.

The target of this paper is not to devise a novel tuning proce-dure either for the PSS2B or the PSS4B. This worthwhile ob-jective has already been tackled elsewhere [17][21]. Instead,we aim at comparing the two architectures using a comprehen-sive set of test systems and performance metrics, in order forthe reader to easily grasp the fundamental differences betweenthem. A useful side-effect of this task will be the availabilityof reference system data and contingencies, PSS structures andsettings for use as benchmarks by other researchers working toimprove PSS tuning procedures. Therefore, all test systems andMatlab software used in the paper will be freely available bye-mail to anyone who requests them.

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904 IEEE TRANSACTIONS ON POWER SYSTEMS, VOL. 20, NO. 2, MAY 2005

Fig. 1. Standard IEEE stability models of modern PSS. (a) PSS4B (typically R = 1:2). (b) PSS2B (the output washout is optional).

II. CANDIDATE PSSDevising a high-performance PSS, for small signals es-

pecially, is quite easy whatever the specific architecture.Therefore, the real challenge is not to assess whether differentPSSs have good local mode damping but instead, given a similardamping level of local and intra-unit modes, to determine howthey fare on an extended range of inter-area modes and againstlimiting factors common to all PSSs, such as islanding opera-tions, fast mechanical power changes, torsional interaction, etc.Our approach to comparing different PSSs consists thereforein carefully aligning their local-mode damping performance onthe same scale and then assessing their inter-area damping andtheir performance limits in the presence of the usual adversefactors.

A. Stability ModelsThe IEEE models of the two PSS families under study are

shown in Fig. 1. Both have the same external inputs (speedand electrical power). However, while the PSS2B incorporatesa single speed transducer, the PSS4B is equipped with two. Thefirst derives a predominantly low-frequency componentof the rotor speed by low-pass filtering its raw estimate (theinternal frequency [26]), while the second derives a predomi-nantly high-frequency component from Pe. Their frequencyresponses in Fig. 3 show that the PSS2B has its 3-dB low-fre-quency cut-off at 0.025 Hz.

By comparison, the -speed transducer of the PSS4Bhas its 3-dB cut-off frequency at 0.25 Hz. Both PSS2B andPSS4B transducers can be complemented with optional notch

Fig. 2(a). Speed transducers: PSS4B. (b) PSS2B (typicallyM = 5 andN = 1

filters, which are sometimes required on large nuclear unitswith low-frequency torsional modes (under 15 Hz). Fig. 1also highlights some fundamental differences between the twoPSSs with respect to the compensator transfer function. In thePSS2B, the phase lead is provided at all frequencies of interestusing a single cascade of three conventional lead-lag blocks.


KAMWA et al.: IEEE PSS2B VERSUS PSS4B: THE LIMITS OF PERFORMANCE OF MODERN POWER SYSTEM STABILIZERS 905

Fig. 3. Frequency responses of the transducers.

Fig. 4. Conceptual block diagram of the multiband PSS (IEEE PSS4B).

An optional washout block can be added at the output to limitthe PSS reaction to low-frequency phenomena [7], [23].

By contrast, the PSS4B has a separate differential filter toprovide phase lead at low-(0.010.1 Hz), intermediate (0.11Hz) and high-frequency (14 Hz) bands. This is illustrated inFig. 4 where each of the three band filters is shown as a band-pass filter with tunable center frequency and magnitude.

Typically, the phase shift is zero at the center frequency wherethe gain is at maximum. Centering the high-frequency band-pass at 10 Hz will therefore ensure phase lead from 0 to 10Hz. At first glance, the high number of adjustable parametersof the PSS4B may seem to increase the complexity of the PSStuning process, compared to PSS2B, which has only six timeconstants and a gain. However, experience has shown that thehigher number of degrees of freedom allows for a far more flex-ible resolution of the large number of contradictory trade-offsfacing the designer of a PSS to be operated in a large intercon-nected system. In addition, a simplified tuning based on six pa-rameters only, namely the gains and center frequencies of thedifferential filters, is quite satisfactory in most situations [15].

B. SettingsTable I presents the candidate settings for the PSS2B. They

were selected from the most recent literature so as to closelyreflect the best practices in PSS tuning. Setting 2 was proposedby Klein et al. [22], and is included in Kundurs classical book

TABLE IPSS2B

[3] as a typical PSS for a general application. Setting 3 is fromthe Brazilian school [7]. The constants were selected to achievean optimum phase lead between 0.15 and 0.25 Hz, whileminimizing low-frequency interaction through a 3-s washouttime constant. Setting 4 is found on a large hydro-generator inthe Hydro-Qubec system. It is strongly biased toward a gooddamping performance of a dominant 0.6-Hz inter-area mode.

Finally, setting 1 was provided in IEEE Std 421.5 as a typ-ical data set for PSS2B [11], [21]. Since similar data is foundin [2], it faithfully reflects recent North American practices asillustrated in [23]. From a detailed small- and large-signal as-sessment using both test and actual power systems, setting 1turns out to be clearly the best in terms of performance, robust-ness, and generality. It is extremely good in local modes, whileachieving a competitive performance in other oscillatory modesdown to 0.04 Hz. A 10-s washout block can be added to theoutput without significantly altering the overall performance.

Table II proposes four closely related settings for the PSS4B.Setting 1 is the kernel from which the others are derived.Although it achieves a relatively uniform phase advance from0.04 to 7 Hz, its gain is a little high and is studied here just toconfirm that the PSS4B can support a relatively high gain whenneeded. Setting 2 reduces low- and intermediate-frequencygains by 33%, while reducing the high-frequency gain by only20%. This setting is roughly equivalent to PSS2B setting 1, inboth local and torsional modes, and will serve as the basis forcomparing the two architectures. The third setting has roughlythe same gains as setting 2 but with a washout added in thehigh-frequency band for reducing the sensitivity of the PSS4Bto fast mechanical power ramps. The last setting is a simplifiedtuning made of three symmetrical bands specified with only sixparameters and selected to closely mimic setting 2 over a widefrequency range. It is used to demonstrate that the apparentcomplexity of the PSS4B in Fig. 1 can be successfully reducedwithout losing out too much on performance.

The frequency responses of the various PSS4B are plotted inFig. 5, [15], where they are compared with the reference PSS2B.The latter discloses a higher gain and phase beyond 20 Hz. In ad-dition, the various PSSs show quite similar phase shapes in thefrequency range from 0.1 to 5 Hz. However, the PSS4B gainstend to be more aggressive and therefore more effective in the



TABLE IIPSS4B (UNDEFINED CONSTANTS ARE SET AT 0)

Fig. 5. Frequency response benchmarking of four typical PSS settings.

critical inter-area frequencies from 0.1 to 1 Hz. Table III pro-vides numerical values of these frequency responses to ease thecomparisons at typical frequencies of interest. Hence, while allPSS4Bs have only a 81 phase lead at 0.01 Hz, the best PSS2Bprovides a 125 lead. In addition, since the peak gain of all PSSis obtained at 10 Hz, a torsional frequency this low will prob-ably make notch filtering essential. Finally, for similar gain andphase around the local modes (13 Hz), the PSS4B(2) shows asimilar gain as the PSS2B(1) at the lower end of the inter-areamode spectrum (0.10.3 Hz), but 50% more gain at the higherend (0.4 to 0.7 Hz).

To complete this analysis, Fig. 6 illustrates the so-called sen-sitivities of the two-input PSSs. These are transfer functionsfrom a single input to the output, with the second input set atzero. Magnitude response sensitivities are more interesting atasymptotic frequencies, i.e., below 0.1 Hz and above 10 Hz.

TABLE IIISAMPLE FREQUENCY RESPONSES FOR H = 4 s

Fig. 6. Magnitude response sensitivities to-! and -Pe inputs.

Hence, Fig. 6 reveals that above 10 Hz the PSS4B is more sen-sitive to noise in speed transducers while this path is essentiallyinsensitive in a PSS2B, thanks to the ramp tracking filter. Bycontrast, the electrical power path is more sensitive to high-fre-quency noise in PSS2B than in PSS4B because the high-fre-quency band of the latter has a naturally decreasing magnitudeproperty in contrast to the PSS2B multistage lead-lag blockswhose high-frequency asymptotic gain is a high-value constant.For the PSS4B, the low-frequency sensitivity of the electricalpower path is important since it measures the PSS responseto mechanical power steps and ramps. For instance, adding anextra washout in the high-frequency band in setting 3 improvesthe attenuation at 0.01 Hz by a factor of 24 compared to setting1 and a factor of 14 with respect to setting 2.

III. SMALL-SIGNAL ASSESSMENTA. Multiscenario Single-Machine Infinite-Bus Systems

The first target of any PSS installation is the local mode.Therefore, we will start our comparison of the two PSS archi-tectures using a linearized model of the classical single-ma-chine infinite-bus system, in which a constant mechanical powergenerator equipped with a fast static exciter is connected toa large system by two long transmission lines. The selectedbenchmark model is fully documented in Taranto et al. [24]where the numerical values of the state-space representation are



Fig. 7. Assessing various PSSs on a single-machine infinite-bus system [24].

TABLE IVINSTABILITY GAIN (TIMES THE NOMINAL GAIN) AND INSTABILITY

FREQUENCIES OF VARIOUS PSSS

provided for three different configurations: a strong, mediumand weak system characterized respectively by a small, averageand large line reactance. Fig. 7 portrays the damping perfor-mance of PSS2B(1) and PSS4B(2) with respect to the robustPSS originally designed in [24].

For a fair assessment, the PSS of [24] is cascaded withthe same speed transducer found in the -branch of Fig. 2(a)while the PSS2B, as shown in Fig. 2(b), includes a 12.5-mspower transducer time constant. From Fig. 7, it is concludedthat the selected PSS2B and PSS4B fare very well against theoptimized robust PSS designed in [24] using a sophisticatedtechnique. More specifically, the PSS2B is slightly better forthe weak system while the PSS4B is superior for the mediumand strong system. Interestingly, the closed-loop local oscilla-tion frequency is nearly the same for all PSSs. Fig. 7 also shows,on the right, the resulting gain and phase margins.

Overall, the robustness of these controllers is very satisfac-tory, according to conventional criteria (a minimum of 6 dB and30 for gain and phase margins, respectively). Finally, Table IVpresents closed-loop instability gains (in multiples of the nom-inal gain) computed in Matlab using root-locus techniques. Asmentioned by previous authors [1], [21], this gain factor shouldbe greater than 2 in order to provide satisfactory protectionagainst un-modeled high-frequency dynamics. Although thePSS2B shows the largest instability gain factors, both PSS4Bsnevertheless achieve satisfactory margins. Combining Fig. 7and Table I allows us to conclude that, in local mode, a higher

Fig. 8. Benchmark model of a two-area system.

PSS gain is needed to achieve a good performance on weaksystems. However, since higher gains threaten stability marginson strong systems, gain selection reflects a tradeoff betweenperformance and robustness.

B. Multiscenario Four-Machine Two-Area Systems

After the PSSs were shown to be satisfactory in local modesof widely differing shapes, the next step is to confirm their goodperformance on a multimachine system with a strong inter-areamode. For this purpose, we selected a well-known test systemillustrated in Fig. 8, [3], [22].

The advantage of this system is its availability to every one.The data used here comes specifically from [3], [22] with thePSS setting 3 in Table I. Assuming two tie-lines (switches closedin Fig. 8) and two power transfer levels (0 and 400 MW), lin-earized state-space models, including a general-purpose loadrepresentation as in [3], have been obtained for this system usingspecial small-signal software (SDYN) previously reported [25].The accuracy of these small-signal models was confirmed onthe basis of a detailed comparison with the eigenvalue analysisresults provided in ([3], p. 813). Next, a PSS with the same set-tings was installed simultaneously at G1 and G4. No PSS wasinstalled at G2 and G3 in order to better discriminate between avery good and a moderately good PSS setting.

The damping performance of the candidate PSS2B andPSS4B is compared in Fig. 9 against the original PSS specifiedin [3], [22] for this system. The PSS4B(2) is clearly the better,for the inter-area modes as well as for the two local modes.This good performance holds at both 0- and 400-MW powertransfer through the tie-lines. To further discriminate among thevarious PSSs, the original symmetrical system was modifiedto have both a large and a small machine in each area (1200versus 600 MVA). With a PSS still installed on G1 and G4, itturns out that the largest generator of area 1 and the smallestgenerator of area 2 are each equipped with PSSs. This systemre-configuration with the same number of tie-lines and powertransfers significantly changes the local-mode frequencies andshapes, with little impact on the inter-area frequency.

Fig. 10 compares the damping performance of the candidatePSS2B and PSS4B against the original PSS specified in [3],[22]. This time, the PSS4B(2) is the only PSS to provide a satis-factory performance in the inter-area mode (i.e., more than 10%damping in the worst condition). It may be that, since it has 50%more gain at the critical open-loop inter-area frequency (0.6 Hz),it can still achieve a rather good performance using a single large



Fig. 9. Damping performance of the candidate PSS: Symmetrical system(G1 = G2 = G3 = G4 = 900 MVA).

Fig. 10. Damping performance of the candidate PSS: Anti-symmetricalsystem (G1 = G3 = 1200MVA; G2 = G4 = 600MVA).

generator (G1). We also note that in view of the higher frequen-cies of the local modes, the original PSS in [3], [22] fails toachieve the required performance while the PSS2B(1) is essen-tially equivalent to PSS4B(2), as to be expected from their fre-quency responses in Fig. 5.

IV. LARGE-SIGNAL ASSESSMENT

A. Kundur Test SystemIn its basic symmetrical configuration, this system is avail-

able in the Matlab Simpower [30] software as a demo preparedby the present authors for studying the dynamic stability ofa small-size multimachine system. Taking opportunity of thisavailability, we implemented the PSS2B and PSS4B describedin previous sections in G1 and G4 (as in the linearized analysis)in order to assess their behavior following large contingencies

TABLE VLARGE-SIGNAL TESTS ON THE TWO-AREA SYSTEM

(Table V). Two highly stressed scenarios were considered re-spectively with two tie-lines at a 413-MW transfer level and asingle tie-line at a 355-MW transfer level. Turbine-generatorswith default governor settings are assumed everywhere, as wellas constant impedance load models.

Sample results illustrating the PSS performance are shown inFig. 11. While all candidate devices perform similarly well on asingle-tie system, the PSS4B outperforms the PSS2B on a twotie-line system, owing to a larger gain at the corresponding inter-area frequency. To better understand the above results, we havecompleted the small-signal analysis by providing, in Table VIthe modal performance of the candidate PSS on a single-tiesystem, which was missing in Section III-B.

The small-signal model was obtained in this case using asystem identification-based approach [9], [27]. The results showthat in Matlab Simpower the single-tie inter-area frequency is0.44 Hz without a PSS. When PSSs are installed in G1 and G4,PSS4B(2) and PSS2B(1) have a similar performance, althoughthe closed-loop inter-area frequency is quite different. These ob-servations are in line with Fig. 11.

To confirm these Matlab findings, the same-area system wasset in Hydro-Qubec transient stability program (ST600) forwhich governing systems and load models are more realistic.Load models are defined as

for MW and MVAR loads respectively. Consistent with thesummer-load modeling practices in Hydro-Qubec systemstudies, we adopted and , which denotes aconstant-current model for P and constant impedance for .

Under these conditions, a severe fault was applied in themiddle of the tie-lines, followed by the trip of generator G2with no line outage (contingency C3, Table V). The results arepresented in Fig. 12 where the speed deviation reveals that,by trying to over-damp the large under-frequency offset, thePSS2B(1) saturates for several seconds, thereby losing thedynamic control of first the voltage and then the synchronism.By contrast, the PSS4B(2) considerably relaxes the frequencycontrol, which results in a sustained off-nominal frequencyoperation. By doing so, it is able to provide effective damping



Fig. 11. Contingencies C1 and C2: Inter-area angle shift (top) andspeed-deviation of the Kundur test system following a temporary fault atbus G1. (a) One tie-line configuration: Fault at Generator G1, (b) Two tie-linesconfiguration: Fault at Generator G1.

TABLE VIPSS DAMPING PERFORMANCE ON A SINGLE-TIE SYSTEM (C2)

of the inter-area mode with a much faster voltage recovery. In-terestingly, PSS4B(4) turns out to be a sort of tradeoff solutionbetween PSS4B(2) and PSS2B(1). On the other hand, losing60% of load at bus 7 six cycles after the fault is cleared resultsin a significant over-frequency (up to 62.5 Hz), driving thePSS2B(1) into saturation for a while and thus making it unableto retain system stability. As shown in Fig. 12, only PSS4B(1)and PSS4B(2) PSS were successful. Tripping only 50% load(contingency 6) left the system stable for all PSSs shown inFig. 13, although in contrast with PSS4B, the PSS2B(1) wasonly marginally stable in this case.

To better understand how the PSS4B achieves this rather in-tricate behavior, we need to take a closer look at its low-, inter-mediate-, and high-frequency band signals shown in Fig. 13. Fora single contingency without frequency offset like C1 or C2, the

Fig. 12. Major loss of generation and load for contingencies C3 and C5.

Fig. 13. Internal signals of the PSS4B during mild and severe contingencies.(a) PSS signal for contingency C3. (b) Internal IEEE4B(2) signal forcontingency C3.

low-frequency component of the PSS never saturates. The PSSoutput signal can thus be thought of as a linear combination ofits three internal components (VSLL, VSLI, VSLH) as in Fig. 1.However, during islanding and a generator/loss trip as in Fig. 12,the lower/upper limits of the low-frequency band set at % and

% in present case, restrict the PSS from further degrading thevoltage. This reduces the damping of the low-frequency mode



TABLE VIIANALYSIS OF LARGE-SIGNAL TESTS ON THE TWO-AREA SYSTEM

Fig. 14. Major loss of generation: contingency C4.

(compared to the linear case) but, since the other two band sig-nals are not limited, they are still effective and able to achievea substantial damping of inter-area and local modes. However,the single most significant observation is that, when its param-eters are set as in Table I, with only local and inter-area modesin view, the PSS2B is unsatisfactory under this type of contin-gency [23] for which large frequency excursions closely mimican islanding operation [13]. At this point, we should emphasizethat several authors have recently proposed to eliminate this sus-tained saturation following a large frequency offset by cascadingthe PSS with a small washout time-constant when the originalstability signal blocks at its lower limit for a sufficient lengthof time. Although it is true that this nonlinear resetable washoutscheme will mitigate the large voltage dip, it will also detune thePSS for a significant amount of time, during a post-contingencyperiod where it is most needed.

Fig. 12 clearly demonstrates the need for a PSS to damp theinter-area mode. If it is deactivated, either because of hard satu-ration or a programmed washout effect, the result will likely bethe same: no more damping with oscillatory stability at risk. Toillustrate this claim further, we analyzed the sensitivity of thesystem dynamic performance to key factors: PSS output limits,PSS gain and output washout time constant. The results are sum-marized in Table VII. It appears that all the proposed PSS4Bsettings in Table II achieve system stability on contingencies

Fig. 15. Major loss of generation: contingencies C3 and C4 (ST600). (a)Contingency C3: mid tie-line fault + trip of generator G2 and No tie-line trip.(b) Contingency C4: mid tie-line fault + trip of generator G2 + trip of onetie-line.

C3 and C4. By contrast, the PSS2B(1) has to be modified byintroducing a 1-s washout at the output for it to ensure systemstability (Fig. 14). Reducing its gain alone (from 15 to 10) wasdeemed insufficient for this purpose. However, even with a 1-swashout, PSS2B(1) lost stability when the PSS output limit wasreduced to 5%. Under such conditions, PSS4B(4) also loses sta-bility, a 5% dynamic range not being enough for its settings.PSS4B(1) and PSS4B(2) still provide a stable system under con-tingencies C3 and C4.

As a last performance check on the Kundur test system,Fig. 15 illustrates the high robustness of these PSS4B settingswhich, as shown, can ensure stability of two post-contingencynetworks with widely different frequency modes (0.44 Hz forone line trip versus 0.67 Hz for no line trip). In addition, thepower transfer level changes drastically in these contingencies,from MW to MW, without adversely impacting thePSS4B performance.

B. Hydro-Qubec SystemTo put the findings of this study on solid ground, the candidate

PSSs were simulated in the Hydro-Qubec grid [32], [35]. Someimportant characteristics of this system should be stated first, inorder to put the results in appropriate perspective.

Albeit a member of the NPCC, the system is nonethelessisolated from its neighbors, i.e., there is no synchronoustie with the surrounding control areas. It is asynchro-nously linked to its neighbors through five DC links.

The generation is remote from the load with 85% con-centrated at three hydroelectric complexes about 1000 kmfrom the load centers.

The predominantly 735-kV transmission network is veryextensive (more than 11 000 km of 735-kV lines), yet con-centrated in two main corridors: the James Bay network(west) and the North-Coast network (east).

A 450-kV HVDC multiterminal line from James Bay toNew England operated in parallel with the 735-kV acsystem.



Fig. 16. Initial assessment of PSS for a fault at LG2 power plant cleared withno line outage: PSS2B(1) is similar to PSS4B(2) giving Ks1 = 25.

The system under study is representative of the 2003 wintergrid with 853 busses, 1182 lines and about 35 000 MW ofgeneration. Three large stations only were considered for thisPSS study: two hydro-plants, LG2 and Churchill Falls, with5920 and 5500 MVA, respectively, of installed generation,and a 820-MVA nuclear unit close to the major load centers.

Since our intent was not to optimize the PSS response forthese units but rather to assess their generality and robustness,the generic settings in Tables I and II were applied at all plants.However, the PSS2B(1) needed a 50% gain increase to matchthe corresponding PSS4B(2) performance on the Hydro-Qubecsystem whose dominant inter-area mode is located around0.6 Hz. According to Table III, such an increase makes thePSS2B(1) 0.6-Hz gain similar to that of the PSS4B(2). Fig. 16compares these PSSs on a standard contingency consisting ofa solid fault at LG2 power plant cleared with no line outage.Clearly, it appears that for this network the two solutionscan achieve a comparable damping performance on routinecontingencies with only weak frequency excursion. However,since the PSS2B(1) PSS provides about the same dampingof the 0.6-Hz mode with about three to four times more gainthan PSS4B(2) in the 0.010.1 Hz range, the next questionis, its behavior during frequency offset.

Fig. 17 assesses the various PSSs following the trip of LG42500-MW power plant after a solid fault with a two-line outage.Visibly, PSS4B(1) demonstrates a good voltage recovery capa-bility, while at the same time ensuring a robust damping of thedominant inter-area mode. By contrast, PSS2B(1) (with

) over-controlled the frequency, at the expense of voltage sup-port and inter-area mode damping. In Fig. 17, the candidatePSS2B(1) (with ) is also shown modified by a 1.5-swashout. This adjustment led to an overall satisfactory perfor-mance in this case with a similar voltage recovery and dampingof the inter-area mode as the PSS4B(2) (not shown). Taking allthe design factors into consideration, the best PSS2B for thissystem is therefore described by and s.

Fig. 17. LG4 power plant shedding after a standard fault cleared with atwo-line outage (ST600): saturation of the candidate PSS2B.

However, such a compromise should not be left without men-tioning its few pitfalls:

A gain of 25 is somewhat high by the industry standardof to . It should be carefully assessed atcommissioning, since, depending on the manufacturersimplementation, it may lead to a noisy or even unstableclosed-loop.

A 1.5-s washout is too short considering the industry con-sensus of s, with a typical recommended valueof 10 s. Therefore, it can only be applied on systems withno significant inter-area activity below 0.4 Hz (Table III).

We have re-assessed the generality of the final PSS2Bsolution, based on two contingencies involving a moderatefrequency swing, which consists of standard faults at LG2 andChurchill Falls plants respectively, normally cleared with asingle-line outage. According to Fig. 18, all the PSSs includingPSS4B(2) [not shown because it is similar to PSS4B(4)]performed satisfactorily with respect to the inter-area mode.However, the modified PSS2B(1) showed some weaknessesin the low-frequency range (0.040.06 Hz) and a slightlyworse voltage recovery, especially for the Churchill Fallscontingency.

V. FALSIFICATION TESTS

The PSS technology suffers some serious intrinsic pitfallswhich adversely impact its application in two well known situa-tions and which are sometimes invoked as reasons for not usingPSS: fast load pick-up on hydro-generators and torsional inter-actions on thermal units [2], [13], [21], [23]. In the first case,a PSS too sensitive to mechanical power changes (such as anelectrical power-based PSS) will generate undesirable reactivepower variations during normal operator requests for referencepower change, while in the second case, too high PSS gain in tor-sional modes will result in additional mechanical stress on theturbine shaft and, in extreme cases, to an unstable closed-loop



Fig. 18. Final assessment for a fault at LG2 (top) and Churchill Falls (bottom)normally cleared with a single line outage (ST600).

system. Therefore, the assessment of the candidate PSSs per-formed so far cannot be declared completed without giving dueaccount to these two issues.

A. Hydro-Generators Ramping Up/DownTo study this aspect, we considered the typical situation of

a large hydro power plant such as the LG2 plant in the Hydro-Qubec grid, with its sixteen 370-MVA units. For the simulationpart of our stability program ST600, the plant is divided intotwo parts (as in [27]): one generator represented by G449 andthe remaining fifteen generators represented by G49. Then themechanical power reference of the single unit is ramped down to10% from a load-flow value of 90%. With a 1 pu/20 s ramp rate,this action is much faster than what the operator can achieve.

Fig. 19 shows the signals recorded on the single unit whileFig. 20 illustrates the PSS responses on both G449 and G49.On the large machine (G49), all PSS signals stay within 1%variation although the PSS4B(3) is the least sensitive, thanksto the additional washout in its high-frequency band (Table I).By contrast, the large-gain PSS4B(1) is more sensitive, albeitnot as much as the PSS2B(1).

On the small machine, which is also the one affected by thedisturbance (G449), the PSS excursions are larger, whatever thePSS type and setting. However, since the rate of decay of thepeak value is very fast, all the proposed PSSs can be considered

Fig. 19. Ramping down the production of a single unit in a power plant.

Fig. 20. Candidate PSS responses to a typical mechanical power ramp on asingle unit (G449) embedded in a 16-unit power plant (G49).

satisfactory, since they violate the 1% criterion only for a shortperiod of time, during the transition from steady to transientstate and vice-versa.

B. Torsional InteractionsThis type of phenomena can be accurately studied using the

EMTP program [29]. For this purpose, we have considered a820-MVA nuclear unit in the Hydro-Qubec system (Gentilly2). Its full 10-mass representation was set up in EMTP, alongwith the usual 100-bus Hydro-Qubec network model used inoperations planning for electromagnetic transient studies.

Gentilly 2 responses to a three-phase terminal fault are givenin Fig. 21 while the corresponding PSS signals are shown inFig. 22. The latter were obtained by replaying EMTP wave-forms off-line into the PSS transfer functions (in Matlab) inorder to perform meaningful time-domain comparisons of theirresponses with respect to the first two torsional modes whosefrequencies are 10 and 18 Hz. It is seen that PSS2B(1) andPSS4B(2) basically have the same gain at the dominant tor-sional frequency mode (10 Hz), thus confirming the frequencyresponse data in Fig. 2. Furthermore, torsional notch filtersgreatly enhance the PSS signal whatever the PSS type, withoutjeopardizing the phase lead in local mode.



Fig. 21. EMTP-based fault responses of a 820-MVA nuclear unit.

Fig. 22. Off-line replay of the 820-MVA nuclear unit responses to a terminalshort-circuit: with (bottom) and without (top) the two torsional notch filters.

A more stringent test was performed next to assess the feed-back behavior of the PSSs on the so-called IEEE Benchmarknetwork for computer simulation of subsynchronous resonance[28]. For this purpose, the most recent release of the EMTP wasused to implement PSS2B and PSS4B as in Fig. 1, based on itsextensive capabilities for developing user-defined models [29].Results of a terminal fault on the IEEE benchmark network aregiven in Fig. 23.

Although PSS4B(1) provided a very good performancewithout notch filters, we decided for the sake of a fair compar-ison in this paper to attenuate its nominal gain by a factor of

, just to make it have the same local damping as PSS2B(1).Fig. 23 confirms that this was indeed the case, the networkresponse being similar with the two PSSs and much moredamped than in the open-loop network. Without notch filters,the reduced-gain PSS4B signal is less corrupted by torsionaloscillations, the reason being that at a frequency as high as 24Hz (the case here), its frequency response gain is about 50%less than that of PSS2B(1).

To understand why the PSS4B performs so well in this case,its transducers outputs are shown in Fig. 24. While the speed de-viation contains 1% of torsional oscillations, the LF and IF bandspeed transducer contains ten times less. Yet a significantlevel of noise is still visible on the waveforms.

Fig. 23. EMTP results of a terminal short-circuit on the IEEE benchmarknetwork with a 600-MVA, 4-mass turbogenerator: PSS assessment.

Fig. 24. EMTP simulation results of a terminal short-circuit on the IEEEbenchmark network [28]: PSS4B signals.

By contrast, the HF band speed transducer is completelynoise-free. Therefore, the main source of concern for torsionalinteractions in a PSS4B PSS is the speed transducer,which substantially, but not completely, filters the torsionalfrequency. The IF band gain should be limited, unless notchfilters are incorporated in the PSS, which is normally the bestapproach for both PSS4B and PSS2B, when the frequency ofthe dominant torsional mode is below 15 Hz, as on most nuclearand combined-cycle units [23].

VI. CONCLUSIONIn this paper, two modern digital-based PSSs, the IEEE

PSS2B and PSS4B, have been systematically assessed topin-point the main differences in their behavior that can be as-cribed to their intrinsic design characteristics. While the formeris well established in the industry, the latter is a newcomer and



we hope that the results presented here will assist PSS users intheir selection.

In summary, we have shown that the two PSS types canbe tuned to achieve quite a similar performance in local,intra-unit [2] and torsional modes, which is no great surprise,since both use the electrical power signal to capture high-fre-quency dynamics. However, having many more degrees offreedom (Fig. 1) available to modulate its phase lead over awide frequency range allows the PSS4B to better balance itsperformance in inter-area modes from 0.1 to 0.8 Hz.

We also found that while the PSS2B relies on gain reductionor a resetable washout block at the PSS output to improve itsoperation during islanding [13] or large under-frequency devia-tions [21], the PSS4B by contrast combines a washout in its in-termediate-frequency band with adjustable limits in its low-fre-quency band (Table II) to achieve this objective with a better per-formance. Another conclusion is that, in sharp contrast with thePSS4B PSS, the PSS2B PSS equipped with typical settings willlikely saturate and become inefficient when the grid frequencyexceeds 62 Hz, as during the historical August 14, 2003 blackout([33, p. 95, Fig. 6.25]). Finally, the paper provides reference set-tings for the two PSS types, in addition to easily available testsystems and demonstration software which should ease the taskof other researchers seeking benchmark data for assessing theirown schemes.

ACKNOWLEDGMENT

The authors would like to thank Dr. J. Mahseredjian fromIREQ and T. Bakk from the cole Polytechnique of Montralfor their help in the EMTP studies. They are also grateful to ourcolleagues L, Grin-Lajoie and D, Lefebvre for useful discus-sions on the topic of this paper.

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[7] N. Martins, A. A. Barbosa, J. C. R. Ferraz, M. G. dos Santos, A. L.B. Bergamo, C. S. Yung, V. R. Oliveira, and N. J. P. Macedo, Retuningstabilizers for the North-South Brazilian interconnection, in Proc. IEEEPES Summer Meeting, vol. 1, Jul. 1999, pp. 5867.

[8] A. Fischer and I. Erlich, Impact of long-distance power transits on thedynamic security of large interconnected power systems, Proc. 2001IEEE Power Tech. Conf., vol. 2, p. 6, Sep. 2001.

[9] L. Grin-Lajoie, D. Lefebvre, M. Racine, L. Soulires, and I. Kamwa,Hydro-Qubec experience with PSS tuning, panel on system reli-ability as affected by power system stabilizers, in Proc. IEEE PESSummer Meeting, vol. 1, Jul. 1999, pp. 8895.

[10] D. M. L. Crenshaw, C. J. Bridenbauch, A. Murdoch, R. A Lawson, andM. J. DAntonio, Microprocessor-based power system stabilizer anddisturbance recorder, presented at the IEEE Joint Power GenerationConference, Chicago, IL, Mar. 1991.

[11] IEEE Recommended Practice for Excitation System Models for PowerSystem Stability Studies, Aug. 1992. IEEE Standard 421.5, (new draft incirculation since 2000: Approval pending).

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[13] P. Kundur, M. Klein, G. J. Rogers, and M. S. Zywno, Application ofpower system stabilizers for enhancement of overall system stability,IEEE Trans. Power Syst., vol. 4, no. 2, pp. 614626, May 1989.

[14] R. Grondin, I. Kamwa, G. Trudel, J. Taborda, R. Lenstroem, L. Gerin-Lajoie, J. P. Gingras, M. Racine, and H. Baumberger, The multi-bandPSS: A flexible technology designed to meet opening markets, in Proc.CIGR 2000, Paris, France. Paper 39-201.

[15] R. Grondin, I. Kamwa, G. Trudel, L. Grin-Lajoie, and J. Taborda,Modeling and closed-loop validation of a new PSS concept, Themulti-band PSS, presented at the 2003 IEEE/PES General Meeting,Panel Session on New PSS Technologies, Toronto, ON, Canada.

[16] R. Grondin, I. Kamwa, L. Soulires, J. Potvin, and R. Champagne, Anapproach to pss design for transient stability improvement through sup-plementary damping of the common low-frequency, IEEE Trans. PowerSyst,, vol. 7, no. 2, pp. 954963, May 1993.

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[21] A. Murdoch, S. Venkataraman, R. A. Lawson, and W. R. Pearson, Inte-gral of accelerating power type PSS. Parts I and II., IEEE Trans. EnergyConvers., vol. 14, no. 4, pp. 16581672, Dec. 1999.

[22] M. Klein, G. J. Rogers, S. Moorty, and P. Kundur, Analytical investiga-tion of factors influencing power system stabilizer performance, IEEETrans. Energy Convers., vol. 7, no. 3, pp. 382390, Sep. 1992.

[23] A. Murdoch, S. Venkataraman, J. J. Sanchez-Gasca, and R. A. Lawson,Practical application considerations for power system stabilizer (PSS)controls, in Proc. IEEE PES Summer Meeting, vol. 1, Jul. 1999, pp.8387.

[24] G. N. Taranto, J. H. Chow, and H. A. Othman, Robust redesign of powersystem damping controllers, IEEE Trans. Contr. Syst. Technol., vol. 3,no. 3, pp. 290298, Sep. 1995.

[25] J. B Simo and I. Kamwa, Exploratory assessment of the dynamic be-havior of multi-machine system stabilized by a SMES unit, IEEE Trans.Power Syst., vol. 10, no. 3, pp. 15661571, Aug. 1995.

[26] H. Vu and J. C. Agee, Comparison of power system stabilizers fordamping local mode oscillations, IEEE Trans. Energy Convers., vol.8, no. 3, pp. 533540, Sep. 1993.

[27] I. Kamwa and L. Grin-Lajoie, State-space identification-towardMIMO models for modal analysis and optimization of bulk powersystems, IEEE Trans. Power Syst., vol. 15, no. 1, pp. 326335, Feb.2000.

[28] IEEE SSR Working Group,, IEEE second benchmark model for com-puter simulation of subsynchronous resonance, IEEE Trans. PowerSyst., vol. PAS-104, no. 5, pp. 10571066, May 1985.

[29] EMTP-RV Software (2004). [Online]. Available: http://www.emtp.com[30] Matlab SimPower Software (Ver 6.5.1, 2004) [Online]. Available:

www.mathworks.com/products/simpower/[31] The Electric Power Engineering Handbook, L. L. Grigsby, Ed.,

CRC Press/IEEE Press, Boca Raton/Piscataway, FL/MJ, 2001, pp.11-8211-120. Direct analysis of wide area dynamics, J. F. Hauer, W.A. Mittelstadt, M. K. Donnelly, W. H. Litzenberger, R. Adapa.

[32] G. Trudel, S. Bernard, and G. Scott, Hydro-Qubecs defense planagainst extreme contingencies, IEEE Trans. Power Syst., vol. PWRS-8,no. 2, pp. 445451, May 1993.

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[34] H. Breulman, E. Grebe, M. Losing, W. Winter, R. Witzman, P. Dupuis,M. P. Houry, T. Margotin, J. Zerenyi, J. Duzik, J. Machowski, L. Martin,J. M. Rodriguez, and E. Urretavizcaya, Analysis and damping ofinter-area oscillations in the UCTE/CENTREL power system, in Proc.CIGRE 2000, Paris, France. Paper 38-113.

[35] G. Trudel, J.-P. Gingras, and J.-R. Pierre, Designing a reliable powersystem: The Hydro-Qubecs integrated approach, Proc. IEEE, sub-mitted for publication.

Innocent Kamwa (S83M88SM98F05) graduated from Laval Univer-sity, QC, Canada, in 1984, where he also received the Ph.D. degree in electricalengineering in 1988.

Since then, he has been with the Hydro-Qubec Research Institute, Varennes,where he is currently a Principal Researcher with interests in bulk system dy-namic performance. Since 1990, he has held an Associate Professor position inElectrical Engineering at Laval University, where five students have completedtheir Ph.D. under his supervision.

Dr. Kamwa received the 1998 and 2003 IEEE Power Engineering SocietysPrize Paper Awards and is currently serving on the System Dynamic Perfor-mance Committee AdCom. He is a member of CIGR.

Robert Grondin (S77M80SM99) received the B.Sc.A. degree in electricalengineering from Sherbrooke University, Sherbrooke, QC, Canada in 1976 andthe M.Sc. degree from INRS nergie, Varennes, QC, in 1979.

He then joined Hydro-Qubec Research Institute. As a Senior Research En-gineer in the field of power system control and protection, he is leading researchactivities in the field of power system dynamics and defense plans.

Mr. Grondin is a member of the IEEE Power Engineering Society (PES) andof CIGR, and is a registered professional engineer in the province of Qubec,Canada. He received an IEEE PES Paper Award in 2003.

Gilles Trudel received the B.Sc.A. degree in 1978 and the M.Eng. degree in1986 in electrical engineering from cole Polytechnique, Universit de Mon-tral, Montral, QC, Canada.

In 1978, he joined Hydro-Qubec, Montreal, QC, where he was first involvedin the design of control and protection for substations. In 1986, he moved to theSystem Planning Department, where he is now involved in high-voltage networkplanning and the design of special protection systems.

Mr. Trudel is a member of the IEEE Power Engineering Society and is aregistered professional engineer in the province of Qubec.


tocIEEE PSS2B Versus PSS4B: The Limits of Performance of Modern PowI. Kamwa, Fellow, IEEE, R. Grondin, Senior Member, IEEE, and G. I. I NTRODUCTION

Fig.1. Standard IEEE stability models of modern PSS. (a) PSS4B II. C ANDIDATE PSSA. Stability Models

Fig.2(a). Speed transducers: PSS4B. (b) PSS2B (typically ${\rm Fig.3. Frequency responses of the transducers.Fig.4. Conceptual block diagram of the multiband PSS (IEEE PSS4B. Settings

TABLE I PSS2B TABLE II PSS4B (U NDEFINED C ONSTANTS A RE S ET AT 0) Fig.5. Frequency response benchmarking of four typical PSS settTABLE III S AMPLE F REQUENCY R ESPONSES FOR ${\rm H}=4$ s Fig.6. Magnitude response sensitivities to $\Delta$ - $\omega$ III. S MALL -S IGNAL A SSESSMENTA. Multiscenario Single-Machine Infinite-Bus Systems

Fig.7. Assessing various PSSs on a single-machine infinite-bus TABLE IV I NSTABILITY G AIN (T IMES THE N OMINAL G AIN ) AND I NFig.8. Benchmark model of a two-area system.B. Multiscenario Four-Machine Two-Area Systems

Fig.9. Damping performance of the candidate PSS: Symmetrical syFig.10. Damping performance of the candidate PSS: Anti-symmetriIV. L ARGE -S IGNAL A SSESSMENTA. Kundur Test System

TABLE V L ARGE -S IGNAL T ESTS ON THE T WO -A REA S YSTEM Fig.11. Contingencies C1 and C2: Inter-area angle shift (top) aTABLE VI PSS D AMPING P ERFORMANCE ON A S INGLE -T IE S YSTEM (CFig.12. Major loss of generation and load for contingencies C3 Fig.13. Internal signals of the PSS4B during mild and severe coTABLE VII A NALYSIS OF L ARGE -S IGNAL T ESTS ON THE T WO -A REAFig.14. Major loss of generation: contingency C4.Fig.15. Major loss of generation: contingencies C3 and C4 (ST60B. Hydro-Qubec System

Fig.16. Initial assessment of PSS for a fault at LG2 power planFig.17. LG4 power plant shedding after a standard fault clearedV. F ALSIFICATION T ESTS

Fig.18. Final assessment for a fault at LG2 (top) and ChurchillA. Hydro-Generators Ramping Up/Down

Fig.19. Ramping down the production of a single unit in a powerFig.20. Candidate PSS responses to a typical mechanical power rB. Torsional Interactions

Fig.21. EMTP-based fault responses of a 820-MVA nuclear unit.Fig.22. Off-line replay of the 820-MVA nuclear unit responses tFig.23. EMTP results of a terminal short-circuit on the IEEE beFig.24. EMTP simulation results of a terminal short-circuit on VI. C ONCLUSION

IEEE Tutorial Course: Power System Stabilization via Excitation G. Rogers, Power System Oscillations . Boston, MA: Kluwer, 2000.P. Kundur, Power System Stability and Control . New York: McGrawG. R. Brub, L. M. Hajagos, and R. Beaulieu, Practical utility C. W. Taylor, Improving the grid behavior, IEEE Spectrum, vol. C. W. Taylor, C. Erickson, K. E. Martin, R. E. Wilson, and V. VeN. Martins, A. A. Barbosa, J. C. R. Ferraz, M. G. dos Santos, A.A. Fischer and I. Erlich, Impact of long-distance power transitsL. Grin-Lajoie, D. Lefebvre, M. Racine, L. Soulires, and I. KaD. M. L. Crenshaw, C. J. Bridenbauch, A. Murdoch, R. A Lawson, a

IEEE Recommended Practice for Excitation System Models for PowerIEEE Digital Excitation Task Force of the Equipment Working GrouP. Kundur, M. Klein, G. J. Rogers, and M. S. Zywno, Application R. Grondin, I. Kamwa, G. Trudel, J. Taborda, R. Lenstroem, L. GeR. Grondin, I. Kamwa, G. Trudel, L. Grin-Lajoie, and J. TabordaR. Grondin, I. Kamwa, L. Soulires, J. Potvin, and R. Champagne,

Impact of Interactions Among Power Systems Controls, Aug. 1998. K. Bhattacharya, J. Nanda, and M. L. Kotari, Optimization and peT. C. Yang, Applying optimization method to power system stabiliI. Kamwa, L. Trudel, and L. Grin-Lajoie, Robust design and coorA. Murdoch, S. Venkataraman, R. A. Lawson, and W. R. Pearson, InM. Klein, G. J. Rogers, S. Moorty, and P. Kundur, Analytical invA. Murdoch, S. Venkataraman, J. J. Sanchez-Gasca, and R. A. LawsG. N. Taranto, J. H. Chow, and H. A. Othman, Robust redesign of J. B Simo and I. Kamwa, Exploratory assessment of the dynamic beH. Vu and J. C. Agee, Comparison of power system stabilizers forI. Kamwa and L. Grin-Lajoie, State-space identification-toward IEEE SSR Working Group,, IEEE second benchmark model for compute

EMTP-RV Software (2004). [Online] . Available: http://www.emtp.cMatlab SimPower Software (Ver 6.5.1, 2004) [Online] . Available:The Electric Power Engineering Handbook, L. L. Grigsby, Ed., CRCG. Trudel, S. Bernard, and G. Scott, Hydro-Qubec's defense plan

U.S.-Canada Power System Outage Task Force: Final Report on the H. Breulman, E. Grebe, M. Losing, W. Winter, R. Witzman, P. DupuG. Trudel, J.-P. Gingras, and J.-R. Pierre, Designing a reliable

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