analysis of overexcitation relaying set up in synchronous generators for hydro power plants

7/31/2019 Analysis of overexcitation relaying set up in synchronous generators for hydro power plants

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Analysis of overexcitation relaying set up insynchronous generators for hydro power plants

Erick Fernando Alves, Member, IEEE, and Marco Aur elio de Souza

Abstract This paper seeks to revise the concepts related tothe overexcitation relaying in synchronous generators for hydropower plants and its coordination with the V/Hz limiter of theautomatic voltage regulator (AVR). The methodology for theANSI 24 protection function set up in hydro power plants isrevisited. Finally, problems faced during the commissioning of apower plant in Brazil is discussed.

Index Terms Hydroelectric power generation, overexcitationprotection, protective relaying, AC generator excitation.

I. INTRODUCTION

THE probability of defects occurrence in synchronousgenerators is reduced. Nevertheless, when they happen,

serious damage and long out of service periods are the usualconsequences. Therefore, and by their relevance to the powersystem, generators should be convenient protected, aiming toguarantee the integrity of their various electrical and mechan-ical parts.

Excessive deviations on frequency and voltage cause ther-mal and dielectric stresses that would result in damage withinseconds on power plant equipments. The potential conse-quences of prolonged overexcitation tend to make protectionengineers to adopt conservative adjustments to ANSI 24 func-tion on protection systems.

On the other hand, events in the power system that leads toreactive load rejection, islanding, connection of big consumers,reconnection to the interconnected system, among others re-quire the extrapolation of these limits by a short period. Inthese transient cases, the correct behavior of voltage regulatorsand protection systems of generating units are essential to keepthe power system stable.

In this context, the set up of overexcitation relaying in asynchronous generator and its coordination with the V/Hzlimiter of the AVR is a relevant subject. At the same time,the operational limits of the related equipments should berespected and the contribution to mitigate transients in thepower system maximized. Usually, these are conicting tar-

gets, specially in hydro power plants, where the high inertiaof the unit and the low bandwidth of the speed governor implyadditional limitations.

Section II presents a brief review of the overexcitation issueson generators and transformers and the standards requirementsfor power plant equipments. Section III describes the typicalimplementations of V/Hz limiters on AVRs and section IV,the usual requirements when setting overexcitation protectionfor hydro power plants. Finally, on section V some practicalaspects of the coordination between overexcitation protection

E. F. Alves and M. A. Souza are with Voith Hydro, S ao Paulo, Brazil(e-mail: [email protected], [email protected])

and V/Hz limiter together with problems faced during the com-missioning of two hydro power plants in Brazil are presented.

I I . C ONCEPTS

The overexcitation condition is associated with the basicprinciple of operation from generators and transformers, bothbased on Faradays law of induction. The voltage induced inthe output of generators and transformers is a function of the ux rate of change. Basically, current induces magneticux, and magnetic ux variation induces voltage in the outputterminals. The varying ux is created by either alternating

current owing through the primary winding of a transformeror the direct current in the eld winding of a generator rotor.

E = N ddt

(1)

E = 4 .44 f n B M AX A (2)

Transformers and generators cores are constructed of ironand have the function to couple the magnetic ux of thewindings. They are designed to lead the ux for full loadoperation without saturation and within the heating limits.Heating in magnetic material is caused by hysteresis lossesand Eddy current, and its properties as well as the core area

dene the maximum ux density limits for the equipment.That is, generators and transformers designers have the task of developing equipments to operate in the nominal conditionswithin the employed material limits, considering the applicableeconomic and technical restraints. And with the advance of thedesigning tools, more and more equipments are closer to theirreal rated limit.

During normal operation conditions, all magnetic ux isrestricted to the core, as its permeability is much higherthan of the adjacent structures. When the core saturates, theexcess of ux spills into the surrounding air space and intonon laminated metallic structures around the core. Conversely,these structures are not designed to lead the magnetic ux, and

this condition quickly increases the losses and heating whichmay cause equipment damage.

In generators, induced currents can occur in the end of stator core, leading to induced voltage gradient between thelaminations that can break down the core insulation. If thisoccurs, the stator core will be permanently damaged. Intransformers, the over-ux spills into the insulating spacearound the core, which causes induced currents and heating inleads, structural members and windings. An extensive reviewof this concepts are done in [8] and [7].

Overexcitation limits for generators are not specied bystandards. The ANSI/IEEE standards C50.12 [1] and C37.106


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Fig. 1. Generator limits according ANSI/IEEE C37.106 (apud IEC 600034-3:1996)

[3] determine that generators shall be thermally capable of continuous operation within their capability curves for a vari-ation of +/- 5% in the voltage and +/- 2% in frequency, asshown in Fig. 1.

For transformers, the ANSI/IEEE standards C57.12.00 [2]and C37.106 [3] specify the operational limits in no-loadcondition within 110% of voltage or volts per hertz. At fullload conditions and power factor of 0.8, the transformer shallbe capable of continuous operation with 105% voltage or volts

per hertz at secondary terminal and frequency of at least 95%.

III. E XCITATION S YSTEM V/H Z L IMITER

The V/Hz limiter of an AVR is used to avoid the operation of the electrical equipments in a power plant (generator, step-uptransformer, auxiliary transformers and systems) in an exces-sive ux condition [5] [10] [7]. This is achieved by changingthe maximum allowed generator voltage setpoint accordingto the actual frequency. Usually, at rated frequency the V/Hzlimiter has no inuence. But when frequency decreases, hencethe voltage setpoint. As an example, for a system with ratedfrequency of 60Hz and maximum tolerated V/Hz ratio of 1.1,

a linear type limiter acts as follows: At 60 Hz, the maximum allowed generator voltage is

110%; At 57 Hz, the maximum allowed generator voltage is

104.5%; At 54 Hz, the maximum allowed generator voltage is

99%.Different structures of control are possible for V/Hz lim-

iters. The most usual are presented at Fig. 2, although somevariations exists. The use of proportional or proportional-integral controllers are possible in both structures. Typically, just proportional is used in the structure of Fig. 2a, while

proportional-integral are the usual choice for Fig. 2b. The waythe limiter inuences the main AVR loop also varies from thefollowing types:

Take-over control: a minimum selector is used betweenvoltage setpoint and the limiter output;

Summing point: the limiter output is connected to anadder, subtracting its value from the voltage setpoint;

The choice of the structure and the controller type usuallyis related to the AVR manufacturer standards and the utilityphilosophies. An extensive review of limiters characteristicsis done in [6]. Despite the kind of implementation, themain idea is to reduce the generator output voltage until thevoltage to frequency ratio goes below the threshold, allowingoverexcitation in transients for a specic time and avoidingtripping of the overexcitation protection from the generator,step-up transformer or auxiliary services.

Some excitation systems also present an optional protection:to turn the system off if the frequency drops below a speciedvalue by a certain amount of time and if the generator isnot synchronized. This peculiarity brings in an additional

protection to the unit, as it does not allow the generatorto remain excited ofine with a major failure in the speedgovernor or in the protection system of the power plant.

Even though being widely available in the commercial exci-tation systems, the V/Hz limiter is just a limit function of theAVR setpoint and could not be considered an overexcitationprotection. Furthermore the V/Hz limiter will have no effecton the generator frequency. Consequently, proper protection isdesirable and recommended [10] [11].

Moreover, even considering that most overexcitation eventsoccur ofine, the limiter should be kept in service permanently,so far as instances of overexcitation are possible when thegenerator is synchronized to the power system [7] [9]. Asan example described by Benmouyal [11], in an islandingsituation or during light load with high level of chargingcurrent, the generator could be driven into an under-excitedstate. In this situation, the AVR underexcitation limiter (UEL)will increase the generator output voltage until the generatormoves out of the forbidden under-excited zone. In doing so,the voltage could go to a level high enough that the Volts/Hertzthreshold will be exceeded.

Another example is the situation where an important inter-tie line was switched off and the islanded system has now animbalance between generation and load. As result, frequencyand voltage could drop to the extent that the AVR wouldimmediately boost the generator terminal voltage. However,the frequency deviation would take some minutes to settledown [3], specially in hydro power plants where the highinertia of the unit and the low bandwidth of the speed governorimply additional limitations. In doing so, not just the voltagecould go to a level high enough but also the frequency toa level low enough that the Volts/Hertz threshold will beexceeded.

IV. O VEREXCITATION PROTECTION

First of all, it must be stated that a protection relay is the lastresource level, as it ensures equipment safety even when other


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(a) (b)

Fig. 2. V/Hz limiter structures

elements fail. In the specic case of overexcitation, in the rstlevel there are speed governor and AVR ensuring operationinside the nominal limits for voltage and frequency. In thesecond level there is the AVR V/Hz limiter, which seeks tokeep the electrical equipments operating within its excitationlimits even in abnormal conditions. And in the last level, theV/Hz relaying, ensuring that equipments will be turned off in a

safe condition whether all the other levels fail. Seeing that, astripping philosophy, the eld and the main unit circuit-breakersshould be opened if the unit is synchronized [9] [4].

Secondly, it must be remembered that the application of a protection element is associated to a specic phenomenonand a measured magnitude related to this. By contrast, theux magnitude in a stator or a transformer core is difcult tomeasure and, therefore, overexcitation relaying needs anotheroperation principle. By inspecting the equation 2 is possibleto realize that the magnetic ux is inverse proportional tothe frequency and direct proportional to the voltage. Therebythe amount of ux could be measured indirectly by the ratiobetween voltage and frequency, considering per unit values.

Last but not least, it is important that protection relayoperates for any condition that exceeds equipment limits.On the contrary, this is not possible for overexcitation. Theoverexcitation protection cannot actuate when the equipmentsare operating at the maximum limits. Instead, the setting mustbe below the applicable limit with a security margin to allowrelay and voltage transformer errors [7]. Consequently, to setthe relay near the ideal operation condition is necessary toknow the equipment limits. Usually there are other equipmentsassociated with the generator, as the step up transformer,auxiliary and excitation transformers, so the settings must bebased on the most restrictive equipment. And the right choice

for the rated voltage of these equipments and the relay modelrepresent an important step to avoid operative restriction.Another important issue related to overexcitation is its effect

on the transformer differential relaying [14]: the differentialrelay must be able to identify an overexcitation conditionthrough the harmonic content of the excitation current andto avoid improper tripping.

Finally, distinct protection relays are available in the mar-ket and to know previously their settings characteristics andcurve types helps a lot in choosing the best equipment forapplication. Some relays offer the combination of denite-timeand inverse-time (IEEE curves) characteristic, other relays

Fig. 3. Santa Catarina HPP unit single line diagram

offer user-dened curve (Tailor-made). Table I presents asynthesis of the philosophies from three distinct IEDs. Formore information about them, see [15], [16] and [17].

V. P RACTICAL A SPECTS : TH E SANTA CATARINA HPP

The Santa Catarina Hydro Power Plant 1 (HPP) is equippedwith 2 x 101.3 MVA 13.8kV +/- 5% generators, each oneconnected directly to a step-up transformer of 101.3MVA13.8/138kV +/- 5% in -Y connection and impedance of 0.12pu. The single line diagram of this unit is shown in Fig. 3.

The connection to the Brazilian Interconnected System isdone by four transmission lines around 50 km. Even with arelatively short transmission line, the bus voltage at the HPPsubstation is high, as its is placed in a power exporter region.Synchronism is generally done at a voltage of 1.035 pu. Duringthe design and the commissioning phases, no coordinationbetween AVR and Protection settings where done related tooverexcitation issues.

As result, an improper trip due overexcitation of the Unit1 happened during the assisted operation period of this HPP,causing load rejection at full load. When analyzing the reg-isters available, rstly the commissioning team checked thatan alarm from the protection system was signalized in theSCADA system. However, as no message was generated fromthe AVR and the system was apparently operating in normalconditions, the operator in charge took no action, consideringit could be a malfunction from the recently commissionedprotection system. After 14.5 hours in this situation, the unitwas nally tripped.

1 Fictional name for a power plant in the Santa Catarina State, Brazil. Allthe characteristics and data provided is from real operating equipments.


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Relay Curve characteristic type Frequencyoperationrange

Voltage referencefor V/Hz relaying

Curve example Thermal Constant

A Dual-level dene-time compositewith thermal characteristic (user-dened)

10 - 70 Hz Maximumvoltage of three phase-phasevoltage

Dropout time toemulate the cool-ing time

B Four options: Dual-level dene-time Composite inverse-time

and denite-time Simple inverse-time User-dened inverse-time

15 - 70 Hz Highest value of the three phasevoltage

Emulationheating effectthrough percent-travel operatingcharacteristic

C Two options: Inverse time Customer dened

42 - 75 Hz Phase-phase orpositive sequence(Depends on therelay connection)

Exponential cool-ing process

TABLE ICOMPARISON OF ANSI 24 RELAYING PHILOSOPHIES FROM THREE IED S

Secondly, when comparing the registers from the AVR andthe protection relay, these discrepancies were conrmed: whilein the AVR the threshold for the V/Hz limiter was not reached,in the protection system the pick up for the ANSI 24 protectionwas. The main reason was the difference in the read voltagesfrom AVR and the protection relays, which occurred due:

Measurements took from odd voltage transformers eachone with a different accuracy class;

Different measurements principles. While the AVR usesthe mean value between the phases in the voltage mea-surement circuit, the protection relays uses the max valueof the phases.

Last but not least, AVR and Protection teams formed atask force to avoid this situation to happen again, coordinatingproperly the equipments. In the Santa Catarina HPP the V/Hzlimiter structure is the one shown in Fig. 2a using the take-overcontrol approach with K equals 1.05, the maximum continuousgenerator voltage in pu. The original settings for ANSI 24function could be seen in Table II in the column Beforeand were based on the relay manufacturer recommendationsand on the protection system study. In these, the denite timepickup 24-1 was being used for alarming and the inverse timecurve as well as the denite time pickup 24-2, for tripping.

To avoid a new trip possibility, the temporary remedial

Parameter Before After24-1 V/f Pickup 1.05 1.07

24-1 V/f Time Delay 60.00 s 240 s24-2 V/f Pickup 1.40 1.25

24-2 V/f Time Delay 1.00 s 1.00 s24 V/f = 1.05 Time Delay 20000 s 450 s24 V/f = 1.10 Time Delay 450 s 240 s24 V/f = 1.15 Time Delay 60 s 60 s24 V/f = 1.20 Time Delay 50 s 50 s24 V/f = 1.25 Time Delay 3 s 3 s24 V/f = 1.30 Time Delay 3 s 3 s24 V/f = 1.35 Time Delay 3 s 3 s

TABLE IISETTINGS FROM ANSI 24 FUNCTION IN SANTA CATARINA HPP BASED

ON RELAY A FROM TABLE I

action was to decrease AVR maximum setpoint to 1.04 pu,what brought dispatch restrictions to the unit. In this meantime,for proper coordination, the equipments capability with thesettings of the limiter and the protection function were plottedin a V/Hz x time curve, which is presented in Fig. 4a. The aux-iliary services equipments and the excitation transformer weredesigned to operate continuously with frequency deviationsof +/-5% and voltages of +/-10%, in this way they were notconsidered in the study. Additionally, it was a customer request


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(a) (b)

Fig. 4. Santa Catarina HPP overexcitation coordination curves

that the protection system must generate an alarm when theV/Hz threshold of 1.05 be trespassed.

Finally, new settings for the ANSI 24 relaying were pro-posed, considering the related equipments capability, possi-ble relay settings and the customer requirements. They arepresented in the column After of Table II, where now theinverse time curve is used for alarming and the denite timepickups 24-1 and 24-2 for tripping. The new coordinationcurve could be seen in Fig. 4b. To avoid the problemspresented before and considering the experience from CEMIGdescribed in [13], a minimum distance of 2% was adoptedfrom V/Hz limiter adjustment to ANSI 24 relaying settingsand from this to the most restricting equipment limit.

VI . C ONCLUSIONS

The proper application of protective relaying requiresknowledge of the operating range of each component andan understanding of the interactions of the generating unitand the power system [7]. When a perturbation occurs inthe power system, it is expected that generators help thesystem to return to a stable condition. To this be possible, theymust remain synchronized whenever their operating limits arebeing respected. However, when the integrity of any electricalequipment is violated, it is time for fast and selective actionfrom protection systems. The validity of these underlyingassumptions represents availability and reliability of the power

system.The proper setting of overexcitation protective devices andAVR V/Hz limiter is a relevant subject in this context. Con-sidering this and the concepts emphasized in the previoussections, some conclusion would be summarized:

1) Coordination between AVR V/Hz limiter and the overex-citation protection function is necessary to avoid bothdamage to electrical equipments and improper tripping[7] [10] [3] [11]. In order to do this, equipments capa-bility curves should be available and must be putted ona common base together with the proposed limits of theAVR V/Hz limiter and the relay characteristic curve.

2) The AVR V/Hz limiter settings should allow the genera-tor voltage to reach its rated maximum value. Neverthe-

less, a security margin from the overexcitation protectionlimit should be adopted to avoid improper tripping of theunit. A recommended value is to keep at least 2% lessthan the rst ANSI 24 relay pick up;

3) While setting up an overexcitation protection, the limitof the most restrictive equipment must be adopted. Asecurity factor it is also desired to allow measurementserrors and practical value is to keep at least 2% fromthe most restrictive limit;

4) The exibility in the unit operation and the customer orsystem operator requests would also inuence the adjustments. In this case, the relay choice could determinemore or less options in the setting possibilities.

R EFERENCES

[1] IEEE Std C50.12-2005. IEEE Standard for Salient-Pole 50 Hz and 60 HzSynchronous Generators and Generator/Motors for Hydraulic Turbine Applications rated 5 MVA and Above . New York: IEEE, 2006.

[2] IEEE Std C57.12.00-2000. IEEE Standard General Requirements For Liquid-Immersed Distribution, Power, and Regulating Transformers .New York: IEEE, 2000.

[3] IEEE Std C37.106-2003. IEEE Guide for Abnormal Frequency Protec-tion of Power Generating Plants . New York: IEEE, 2004.

[4] IEEE Std C37.102-2006. IEEE Guide for AC Generator Protection . NewYork: IEEE, 2007.

[5] IEEE Std 421.4-2004. IEEE Guide for the Preparation of ExcitationSystem Specications . New York: IEEE, 2004.

[6] IEEE Task Force on Excitation Limiters. Recommended models for Overexcitation Limiting Devices . IEEE Transactions on Energy Con-version, v. 10, n. 4, p. 706-713, Dec 1995.

[7] REIMERT, D. Protective Relaying for Power Generation Systems . BocaRaton: CRC Press, 2006.

[8] HARLOW, J. H. Electric Power Transformer Engineering . Boca Raton:CRC Press, 2007.

[9] MOZINA, C. (ed.) et al. IEEE Tutorial on the Protection of SynchronousGenerators . Piscataway: IEEE Service Center, 1995. Catalog Number:95 TP 102.

[10] MOZINA, C. (ed.) et al. Coordination of Generator Protection withGenerator Excitation Control and Generator Capability . In: IEEE PowerEngineering Society General Meeting, 2007, Tampa.

[11] BENMOUYAL, G. The Impact of Synchronous Generators ExcitationSupply on Protection and Relays . In: Western Protective Relay Confer-ence, 34, 2007, Spokane.


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[12] SCHARLACH, R. C.; YOUNG, J. Lessons Learned From Generator Event Reports . In: Annual Conference for Protective Relay Engineers,63, 2010, College Station.

[13] ALVES, C. E. et al. Coordenac ao din amica Entre a Protec ao El etrica eo Controle de Geradores Hidr aulicos - Experi encia CEMIG . In: CIGRETechnical Seminar on Protection and Control, 9, 2008, Belo Horizonte.

[14] GUZMAN, A.; DAQING HOU; ZOCHOLL, S.E. Transformer Modeling As Applied to Differential Protection . In: Canadian Conference onElectrical and Computer Engineering, 1996, Calgary.

[15] Siemens. Siprotec Multifunction Machine Protection 7UM62 Manual .

Version 4.6, 2009.[16] SEL Inc. Generator and Intertie Protection Relays SEL-700G Instruction

Manual . Date code 20100521, 2010.[17] ABB. Generator protection IED REG 670 Technical reference manual .

Version 1.1, 2007.

Erick Fernando Alves (S05, M07) was born inS ao Paulo, Brazil in 1981. He received the E.E.bachelor with emphasis in Energy and Automationin 2007 from University of S ao Paulo. He joinedthe Systems Engineering Department of Voith HydroS ao Paulo in 2005 as trainee. Since then, he workedin the control design of hydro power plants, spe-cially with Excitation Systems and Speed Governors.Nowadays he is Lead Engineer of Excitation andProtection Systems at Voith Hydro S ao Paulo.

Marco Aur elio de Souza graduated in E.E. with em-phasis in Power Systems in 2000 and post-graduatedin Protection Systems in 2004 both from FederalUniversity of Itajub a. Since his graduation he hasbeen working in generation and transmission areas,mainly with control and protection design of hydropower plants and high voltage substations. Presentlyhe is Proposal Engineer at Voith Hydro S ao Paulo.

analysis of overexcitation relaying set up in synchronous generators for hydro power plants

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