136418087 Calculations of Protective Relay Settings

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ABSTRACTCALCULATIONS OF PROTECTIVE RELAY SETTINGS FOR A UNIT

GENERATOR FOLLOWING CATASTROPHIC FAILUREby

Jaime Anthony YbarraDecember 2011

After a catastrophic failure ofaunit generator system the major com ponents mayneed to be replaced. Many times exact replacement of the failed or damaged componentsmay not be possible . In such a case compone nts with electrical characteristics as close tothe original may be used. Therefore new protective relay settings must be calculated. Inthis thesis, we will examine a type of generator protection re lays, evaluate new settingsand develop a one-line diagram for a 25 MVA generator system. A methodology for thedevelopment of a safe and reliable protections scheme for a unit generator system is alsopresented.


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CALCULATIONS OF PROTECTIVE RELAY SETTINGS FOR A UNITGENERATOR FOLLOWING CATASTROPHIC FAILURE

A THESISPresented to the Department of Electrical Engineering

California State University, Long Beach

In Partial Fulfillmentof the Requirements for the Degree

Master of Science in Electrical Engineering

Committee Members:Hassan Mohamed-Nour, Ph.D (Chair)Mohammad Talebi, Ph.D.Hen-Geul (Henry) Yeh, Ph.D ., P.E.

College D esignee:James Ary, Ph.D.

By Jaime Anthony YbarraB.S., 1999, California State University, Long Beach

December 2011


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UMI Number: 150766

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In the unlikely event that the author did not send a complete manand there are missing pages, these will be noted, Also , if material had to bea note will indicate the deleti

UMI 150766Copyright 2012 by ProQuest L

All rgh ts reserved. This edition of the work is protected aunauthorized c op yn g under Title 17, United States C

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TABLE OF CONTENTSPage

LIST OF TABLES vLIST OF FIGURES viCHAPTER

1. INTRODUCTION 12. GENERATOR COMPON ENTS AN D PROTECTION SCHEME 3

The Transformer 5Short Circuit 6Per Unit Quantities 10One Line Diagram 11Relay and Control Sym bols 14Elementary Diagrams 15

3. UNIT GENERATOR PROTECTION RELAYS 18Volt/Hertz relay (24 ) 18Synchronizing Check Relay (25 ) 20Under Voltage Relay (27 ) 20Directional Reverse Power Relay (32 ) 21Loss of Excitation (Field) Relay (40) 21Negative Sequence or Unbalance Relay (46) 23Stator Temperature Relay (49 ) 24Inadvertent Energization Protection Relay (50) 25Voltage Controlled Over Current Relay (51 V) 25Over Voltage Relay (59) 26Voltage Balance Relay (60) 26Sudden Pressure Relay (63 ) 27Field Ground Relay (64F) 27Oil Level Relay (71) 27Out Of Step Relay (78) 28Frequency Relays (81) 29Lock Out Relay (86) 31

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CHAPTER PageDifferential Relay (87) 31

4. SETTINGS CALCULATIONS AND EXPERIMENTAL RESULTS 33Preliminary Calculations 34Typical Relay Settings Calculations and Verification with Ex perimen t. 3 5

5 CONCLU SIONS 52REFERENCES 54

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LIST OF TABLESPage

Sample Generator Parameters 33Sample Unit Transformer Parameters 34Relay Volt/Hertz Experimental Test Result 37Under Voltage Test Result 38Reverse Power Test Result 39Zone 2 Test Result 41Loss of Excitation Zone1 Reach Test Result 41Current Unbalance Pickups for A, B and C Phases 42Voltage Controlled Over Current Test Results 43Over Voltage Relay Test Result 44Relay Reverse and Forward Reach Z Test Results 46Relay Right Blinder Reach Z Test Result 46Left Blinder Reach Z Test Result 47Equipment Summary Table 48Relay Settings Summ ary Table 49

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LIST OF FIGURESPage

Graphical representation of3phase power generation 4Basic structure of a cylindrical rotor 4Brushless excitation system 5Wye connected windings 53 Phase fault with DC component offset 8Short circuit waveform showing the three transient periods 9Typical electrical symbols 12Waveform output with polarities in phase 13Waveform output with polarities reversed 14Unit connected generator protection with typical relays 16Basic elementary diagram 17Various volts/hertz limit curves 19Generator, transformer and relay plot for volts/hertz relay plot 202 zone protection diagram 22Typical negative sequence relay curve 24Out ofstepprotection zone 29Representation of differential protec tion 31

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FIGURE Page18. Experim ental setup 3619. RMS TIME vs VOL TS of volt/hertz relay (24) operation 3720 . Under voltage (27) relay operation graph 3821. 3 phase vector diagram of reverse power relay (32) 3922. Zone reach impedance and phase angle relationship 4023 . Loss of excitation zone 2 reach test 4024 . Loss of excitation zone1 reach test 4125. Unbalance A, B and C phases 4226 . Voltage control relay (51C) results 4327 . Voltage controlled relay (51C) RMS trip graph 4428. Over voltage relay (59) result plot 4429. Loss of Synchronization protection boundaries 4530. Forward reach results 4531 . Reverse blinder result 4632 . Right blinder result 4633 . Left blinder result 4634 . Sample system one line 50

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CHAPTER 1INTRODUCTION

A generator system is designed to provide electric power to customers reliably.Failure of any electric component such as the generator, unit-transformer or auxiliarytransformers can lead to catastrophic dam age. If any of these com ponen ts are damagedbeyond repair then they must be repaired or replaced. How ever, due to age andcustomized engineered system components exact replacements m ay not be available orthe time for new compo nents to be manufactured m ay not be economically viable. Thegenerator own er or user may have to purchase readably available equipm ent withcapabilities as close as possible to original compo nents. If this is the case new protectivedevice settings mu st be calculated to properly protect the generation . In the event of thereplacement of any of the components the following basic steps are recomm ended:

1. Calculate the new capabilities of the generation system .2. Calculate protective device settings based on new system .3. Deve lop or upda te electric system single line diagra ms (on e-lines) to describe the

basic layout of the electrical system as well as basic information of the majorcomponents.

4. Verify that the relays will operate as program ed or set with sim ulation of faultconditions inherent to that protective device.

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The engineer in charge must produce a system that will provide reliable, economicalpower to the customer as well as maintain a safe system for generator operation andmaintenance personal.

This thesis is focused on recalculation of protective relay settings of a generatorprotection system with replacement comp onents that do not have the same ratings orcapabilities as the original and will require new protective relay settings calculations.Chapter 2 will discuss generating system component and electrical fundamentals as wellas the symbo ls used to describe an electrical system. Chapter 3 will describe protectiverelay types and functions. Chapter 4 covers the calculation of the new protective relaysettings and fault simulation testing of the protective functions with a 3phase powersimulator.

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CHAPTER 2GENERATOR COMPONENTS AND PROTECTION SCHEME

An electric generator is a device, which converts mechanical energy into electricalenergy (see Figure 1). The prime m over provides the rotational mechan ical power into theAC generator. This mechanical power may be derived from fossil fuels, nuclear ormovem ent of water. The mechanical rotational motion is transferred via a shaft to therotating portion of the generator, which is referred to as the rotor. The rotor will containconductors of either copper or aluminum that will have a DC vo ltage applied and p rovidesa current path that will set up a controlled magn etic flux these condu ctors are referred to asthe field windings. The mov ing magn etic flux will induce voltage in the stationary portionof the generator referred to as the stator (see Figure 2) where the am ount of flux beingproduced by the rotor is controlled by a device called the Exciter which controls theamount of current in the in the field windings. The DC current m ay be derived externallyand then transferred to the field windings on the rotor via brushes or the D C m ay begenerated on the rotor itself by the addition a small permanent m agnet A C generator andelectronic circuits that will rectify the A C into DC for use for the field current(see Figure 3).

In a3phase w ye connected generator (see Figure 3) the 3 winding s offset by 120electrical degrees apart and share a com mon point referred to as the neutral.

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ROTATING ELEMENTS

FIGUR E 3. Brushless excitation system [1].

A phaseVolts l ine to

neutral

Volts l ine to l ine

FIGURE 4. Wye connected windings.

The TransformerA transformer allows the conversion of one voltage level to another voltage level.

A higher voltage level allows for lower losses due to lower current levels for a givenamou nt of pow er. Low er voltage levels in turn allow for higher cu rrents to loads for thesame given amount of power. A transformer consists of coils of copper or aluminumwrapped around a common core that readily conducts magn etic lines of force. The

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magn etic lines intersect each other within this core. Math ematically th e relationship isexpressed by the following equation.

N PV S= NPV SWhere N pis the turns of conductor on the primary sideN sis the number of turns of conductor on the secondary sideV sis the voltage of the secondary sideand Vpis the voltage on the primary side.

Like a generator the transformer windings can be configured as a delta wherethere is no intention grounding of the conductors or in wye configuration that isconfigured such that each phase windings end point are connected together at a commonpoint (see Figure 4). This comm on point can be solidly connected to the ground orconnected through imp edance to ground to limit ground fault current. The advantage of adelta connected system is that if there were to be an inadvertent grounding of oneof thephases only a small amount of current will flow and allow the system to stay online untilit can be safely de-e nergized and repaired. How ever, the voltages on the other phaseswill increase thereby stressing the insulation of the cables and equipment. With a wyeconnected system the common point is referred to as the neutral.

Short CircuitA power system is designed to be free of faults as much as possible through

system design, equipm ent selection, installation and ma intenan ce. Ho wev er, even withthese practices faults do occur. Some of these causes can be from insulation failure,moisture or inadvertent contact with conductive material. Regardless of the cause asignificant am ount of current flows to the point of the fault. At the fault location arcing

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and burning will occur as well as mechanical stress to the equipm ent. The system voltagelevels will drop proportionally with the magnitude and distance to the point of the fault.The availa ble short circuit current is the maxim um possib le value of current that canoccur at the location of the fault. The contribution to this ma xim um current comes fromgenerators, synchronou s and induction mo tors. The basic short-circuit equation is shownbelow

j _ rmshe ~~ ^**systemWhere Iscis the short circuit currentVrmsis the rms voltageandZsystem( or X ) is the equivalent system impedance (or reactance).

The system impedance is taken from the point of the fault back to and includingthe source or sources of the fault current for the powe r system . Dur ing a3phase fault thecurrent waveform will be offset by a DC component that shifts the sinusoidal waveform

away from the horizontal axis (see Figure 5). The amount ofDCoffset depends on theX/R ratio which is the imped ance divided by the resistance of the system. A generatorwill have 3 short circuit constants inherent by design (see figure 6) that are used to setvarious protection elements.

These constants are derived by experiment or by analytical m ethods by themanufacture. These constants are defined as follows

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Phase CDC Component

FIGU RE 5. 3 Phase fault with DC comp onent offset [12] .

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A 'd Subtransicnt reactance: Is the reactance ofagenerator at the initiation ofafault and is used in calculations of the initial asym metrical fault cu rrent (see Figure 6).The current continuously decreases lasting approximately 0.05 s after an applied fault [1].

vY d Transient reactance: Is the reactance of a generator betw een the sub transienland synchronous states (see Figure 6.). This reactance is used for the calculation of thefault current during the period between the subtransient and steady state period (seeFigure 6). The current decreases continuously during this period but are assumed to besteady at this value for approximately 0.25s [1].

X Direct axis: The steady-state reactance ofa generator during fault conditionsused to calculate the steady state fault current after the Subtransient and Transientcomponents have decayed away (see Figure 6).

[ Subtransientf\ ^*~ Penod

FIGURE 6. Short circuit waveform showing the three transient perio ds [12].

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Per Unit Q uantitiesA pow er system can be made up of various voltage levels there by making system

calculations difficult. Therefore, to simplify calculations a comm on set of base valuesare selected and the remaining quantities are then scaled to these base valu es. The twocommon base values chosen are voltage and power. The other base values are thencalculated from these two base values by the following equations:

r __ M VABase_3 phase'Base V3 x kV,BaseJLL

7 ^BaseLLBase MVAm v ^Base_3 phaseWhere ZBase is the base impedance in ohms,MVABase_3 phaseis the chosen apparent power baseand kVease_LL is the base line to line voltageFor per phase qu antities are required use line to neutral kV . Once the base values

have been established then the per-unit quantity of a value can be calculated with thefollowing equation:

actual valuePer Unit value = base value

Electrical components in a power system may have different per unit values basedon its own ratings that differ from the chosen base. If this is the case they can be

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converted into the chosen base per unit values. The following eq uation will transform anold per unit impedance value into a new per unit impedance value:

~ . . . . j n -J . _i kVBase aw kV ABasenewPer unit impeda ncene w = Per unit impeda nceoid ( = )L x = KvBasejiew ^^Base_old

Whatever the value for the base voltage and M VA are chosen to be they will bedesignated as1 per unit or1p. u. .

Since it is obvious that electro m echanical and electronic relays cannot d irectlyoperate at high voltages and current magnitudes they must be reduced to a mag nitude that arelay can safely operate. The devices used to reduce the voltage and currents are referredto as potential transformers (PT) and current transformers (CT). This is accomplished bytaking the primary quan tities and scaling it down by a known ratio.

One Line DiagramsA one-line diagram graphically illustrates an electrical power system by

representing a3phase system with single symbol com ponents. It is assumed unless

indicated otherw ise in the drawing that each device w ill have 3 units if they are singlephase devices or1 unit having 3 phase capabilities. For example there will be one CT(current transformer) for each phase for a total of3 ,but a circuit breaker w ill have 3 phasecapabilities per each unit (See Table 1). A three-line diagram assists in the actualconstruction of power equipment. Each compon ent is now displayed as a three phasedevice. This will enable the builders of the system to interconnect the protection and othercomponents.

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3 phase generator

uuuuuun r r r m

Two winding transformer

iT

Mediu m v oltage draw out circuitbreaker

3 phase disconnect switch with fuse

JIQDistribution bus Impedance ground with CT and

resistor for ground fault detection

r ( 0CT and PT with polarity marks in phase CT and PT symbols with reverse polarity Relay symb ol where the # isreplaced with the relay number

A YDelta connection and Wy e connection Open Delta and grounded Wy e connections Control path points to control device 0 0

Circuit breaker close coil Circuit breaker shunt trip co il Circuit breaker charging motor

-Q -Indicating light

1T bNormally open and normally c losed contac t s

FIGURE 7. Typical electrical symbols.

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The symbols for current transformer (CT) and potential transformer (PT) also arereferred to as Voltage Transformers (V T). Both CT and PT have polarities. A polaritymark as indicated by the dots in a one line diagram. Physically on a C T or PT theprimary will be indicated by HI and H2 and XI and X2 for the secondary where HI andXI correspond to the dots in the one-line diagram. The polarity of an instrumenttransformer indicates the phase relationship between the input and the output. On a CTthe current flowing into the polarity mark or HI will result in current flowing out of thesecondary polarity mark or XI with little or no phase shift. Likewise for the PT (seeFigure 7). A 180-degree phase shift will occur if the CT or PT sec on dar y's are connectedor installed in reverse. If this is by design the dots will be reversed in the o ne linediagram (see Figure 8).

If the CT and P T are connected with their polarities reversed they would beindicated w ith the dots in the oppo site side.

15 ,I Pnmay current/voltage Seconday curren t/volta ge

FIGUR E 8. Waveform output with polarities in phase.

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Pnmay current/voltage Set >ndayc j rrer t/voltage

1I

05

I

05

I1

I FIGURE 9. Waveform output with polarities reversed.

Instrument transformers like pow er transformers ma y be connected in multipleways depending on the application. If ungrounded they m ay be connected as a delta or wyeor if grounded they may be connected as an open delta or grounded w ye. No te that opendelta PT only 2 PTs are used with the secondary center phase g rounded.

Relay and Control SymbolsOn a one line diagram a relay will be represented as a circle with the IEEE relay

type number in the center (see Figure 7). The device that is activated wh en a relay operatesthat is, closes its alarm or trip contact a dashed line with arrows is often used to show thedevice that is activated. Placing these sym bols on a one line and interconnecting the singleline elements allows for the representation of any type of electrical system and is thestandard method for the design of electrical systems.

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Elementary DiagramsIn addition to a one line a control logic schematic must also be developed. This

diagram is also referred to as an Elementary Drawing . The Elementary Drawing showsthe actual devices that are being activated. That is, a DC bus will provide the pow er toeither close a circuit breaker or trip it open and power the motor that w ill compress aspring. The contacts from the various relays are also shown on Tab le 1. A circuit breakeropens and closes using stored energy in a compressed (charged) spring. Wh en the circuitbreaker is inserted secondary contacts in the switchgear make contact with pow er andcontrol terminal in the cubicle and a motor in the circuit breaker charges the spring. Whena signal to close is given a close coil (solenoid) is energized and closes the circuit breakerthe same holds for the trip coil. Half the spring energy is used to close the circuit breakerand the other halfisused to trip the circuit breaker. The motor w ill recharge the springsafter the trip operation. These solenoids are referred to as close co ils (CC) or trip coils(TC). The motor is designated by the capital letter M . Indicating lights on theswitchgear panels are used to indicate the status of the circuit breaker. A red light is usedto indicate the circuit breaker is closed and a green light indicates that the c ircuit breaker isopen. Two parallel lines represent contacts. Note that these are contact not capacitors. Anormally open contact or a has empty space between the lines and a normally closedcontact or b contact has a line through it. The state as shown on elementary diagramsis when the circuit breaker is open. W hen the circuit breaker closes the state of thecontact reverses.

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This flux in the unintended areas will create high circulating eddy currents which will inturn generate large am ounts ofheat. This excess heat will degrade lam ination andwinding insulation leading to equipment failure. An exam ple of when over excitationmay occu r is when a gen erator is started and has not come up to full speed and due to avoltage regulator malfun ction or human error the field is applied. Since the voltage is afunction of flux times speed the voltage regulator may attempt to increase the fieldcurrent an attempt to m aintain rated output voltage. Generator voltage regulators mayalso monitor and protect against over excitation and the 24 relay in this case would act asa backup or alarm relay.

To develop 24 relay settings the generator manufacture over excitation limitcurves should be obtained. A sample of limit curves are shown in Figure 11 below

1 5 0 - -

1 3 0- 1 2 0 - o no- -

1 0 0 -

0 0 1

Figure 12. Various volts/hertz limit curves [1].

A transformer is also susceptible to volts/hertz prob lem s. A similar transformercurve can be obtained and the relay set to protect both (see Figure 12).

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MFC GENERATOR MFG 2 GENERATOR MFG 3 GENERATOR

H 1 1 1 1 I j 1 1 1 1 I 1 1 1 1 10 10 10 100 1000 0 01 0 10 10 100 1000 0 01 0 10 10 100 1000TIME (MINUTES) TIME (MINUTES) TIME (MINUTES)

f V l P R O H I B I T E D R E G I O N


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130-

t 120-

t o o .

01

FIGURE 13. Generator, transformer and relay plot for volts/hertz relay plot [1].

Synchronizing Check Relay (25)

A 25 sync check relay is used to check whether or not two separate portions of asystem are of similar phasor quantities such as phase, frequency and mag nitude and arewithin predetermined thresholds. If and when the electrical differences between the twosystems satisfy the threshold conditions an action may be taken. This action in a generationsystem is the closing of a circuit breaker thereby bringing the generator into parallel withthe electrical system. If two electrical systems were brought together and if they aresignificantly different, large currents will flow through th e system s wh ich can exceed thoseexperienced during sudden short circuits. The intense currents and torques produced maycause server damages to the generator stator which may req uire it to be rewoun d.Synchronizing limits that the two systems mu st be with specified limits in order to safelyparallel. Typical limits are circuit breaker closing angle of10,generator side voltagerelative to system 0% to +5% and frequency differences of 0.067 Hz.

Under Voltage Relay (27)The under voltage relay operates when the voltage applied drops below a

predetermined value. Under voltage relays may hav e inverse time characteristics so that20

RELAY CHARACTERISTIC

TIME (MINUTES)


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the system m ay have time to stabilize before any trips or alarms are initiated or they mayhave definite level thresholds. For paralleling a generator and distribution bu s the 27 undervoltage functions is incorporated into the 25 relay.

Directional Reverse power Relay (32)Ifagenerator loses its prime m over it will go into a condition called M otoring

which as the word im plies the generator will now be powered by the external system andthe generator will act as a synchronous motor. This will drive the prime mo ver andpossibly damaging its shaft, couplings, comp ressors.. .etcetera. The manufacture providesthe magnitude of reverse power that the generator system can withstand before damageoccurs. A 32 relay will also contain an adjustable time delay to allow short duration powervariations to stabilize. The manufacture w ill often prov ide the reverse power threshold inprimary watts and the amount of time the generator can mo tor before it is damaged .

Loss of Excitation (Field) Relay (40)Loss of excitation on a synchronous generator will cause the rotor to accelerate and

operate as an induction g enerator. As a result it will draw reactive power from the systeminstead of providing it to the system. Heavy cu rrents will also be induced into the rotorteeth and wedges which w ill cause thermal d amage to the generator if allowed to operate inthis condition. Com mon causes of excitation loss can be operator error, excitation systemfailure, accidental tripping of the field breakers or flashover of the exciter comm utator. Atype of 40 relay is called an offset M HO relay. The following information will be requiredfrom the manufacture to set the protection level: the generator direct ax is reactance Xd,Transient reactance X 'd, line to line voltage, and rated phase current, all in secondaryvalues. The protection characteristics are plot on the R-X plane. W here R is the resistance

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and X is the reactance of the system (see Figure 30). The inner circle referred to as Zone 1will trip the system offline and the outer circle referred to as Zone 2 will alarm beforetripping the system offline.

0 5

-R

. -j

- 2

(v v ^

jDl/ 1

OFFSET

METERO puJ" 2

\

DIAMETER "X,,

R

- 1 - X 1 2

FIGURE 14. 2 zone protection diagram [1].

The following equations are used to calculate the diameters and offsets. Note thatvalues must be secondary ohm s for use in the relay. Time delays are of.1seconds forZone1 and .5 seconds for Zone 2 are suggested. Zone1 (inner circle) is comm only set to1.0 pu of impedance Z.

The offset of the inner and outer circle X is the negative quantity of the half thetransient reactance. The diameter of the outer circle is equal to the direct access transientwith the offset being equal to that of the Zone1 offset. Note that these values are inprimary quantities and mu st be converted into relay base for used w ith the protectionequipment.

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Negative Sequence or Unbalance Relay (46)Negative sequence stator currents, caused by unbalanced faults such as phase-to-

phase, phase-to-ground, double-line-to-ground or load unba lance, induce do uble frequencycurrents into the rotor that may eventually overheat elements not designed to be subjectedto such currents as a result of the11losses. The most serious series unbalance for exampleis an open phase, due to a failed circuit breaker pole. Two lim its are used for 46 relayprotection settings the C ontinues Negative-Sequence current capability limit and the ShortTime Unbalance current lim it. IEEE standards dictate that all generators meet a negativesequence withstand capabilities [5]. For generators continues negative-sequence limitsrange from 5 and 1 0% of rated current. The short time limit is expressed in terms of K,where K= I2t and varies from 5 to 40. The actual values will be provided by the generatormanufacture. These limits will be used to set the relay pickup value s. The relay will havea time delay function to allow downstream circuit breakers to clear the po rtion of thesystem that is causing negative sequence currents. Typical curves (see Figure 15) providethe protection engineer re lay pickup and delay characteristics to coo rdinate trip functions ofprotection dow n the line.

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PER UNIT l2

FIGUR E 15. Typical negative sequence relay curve [1].

Stator Temperature Relay (49)Tem perature Relay supplies a constant current to a remotely located resistive

temperature detector usually installed in the windings, and senses the temperature of thedetector by measuring the voltage across the resistive element. These detectors calledRTD can be made of platinum, copper or nickel. Two pickup settings are comm only

programm ed into a temperature relay, a lower threshold will alarm without shutting downthe system to allow corrective action. The second higher thresho ld w ill trip the system inorder to prevent thermal damage to the generator.

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Inadvertent Energization Protection R elay (50)Inadvertent energization of a generator can result from a circuit breaker flashover or

a breaker that has closed o nto an energized system while the g enerator is at standstill orrotating at slow speeds. The energized generator will act as an induction mo tor and rapidlyaccelerate which can cause extensive dam age if the generator is not de-energizedimm ediately. For inadvertent energization protection3types of relays and a timing deviceare used in tandem. A 27 under-voltage relay, a 81U under-frequency relay and a 50instantaneous over-current relay. When the generator is de-energized the 27 under-voltageand81under frequency relays contacts will be closed thereby enabling the 50instantaneous to operate if current is detected. A timing device is also used w ith thisscheme. It will inhibit the operation of the 50 for a period of time so that it will not operateif there are short term instabilities in voltage or frequency levels and arm the 50 relay w henthe generator is taken out of service.

Voltage Controlled Over Current Relay (51V)Voltage Con trolled Over-Current relay is used to provide protection against a

prolonged fault contribution by the generator. The basic operation of the relay is such thatthe pickup of the over-current un it is not activated until there is a voltage drop due to ashort circuit out in the system. The further the fault is from the g enerator the lower themagnitude of the voltage drop.

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The relay received its input from potential transformers and w orks in conjunctionwith the 60 relay. Ifa60 relay detects a blown fuse it will block th e op eration of the 51Vrelay due to voltage input being lost due to the blown fuse. Typical settings for theovercurrent unit is 50% of the full load current if the activation voltage level is 75% of therated voltage.

Over Voltage Relay (59)The over voltage relay is used to senses above normal voltage m agnitude. Another

important use ofanover voltage relay is for ground fault pro tection in im pedance groundedgenerators. An imp edance wh ich may be a resistor or a step dow n transformer w ith theprimary of the transformer in series with the neutral and the resistor across the secondary sothat a flowing current through the resistor will develop a voltage wh ich will be detected bythe 59 relay.

Voltage Balance Relay (60)A voltage balance relay protects the power system from miss o peration or false

tripping in the event that a fuse blows in the voltage sensing circuit. Tw o sets of PT areused to implement this relay. Under normal conditions all three phase PT outputmagnitudes are equal. If a fuse is blown the relay compares the two inputs and if only oneof the inputs has lost potential then other protective or control functions can be blocked ordisabled. As an example, if one or more PT fuses providing signal to the voltage regulatorfails and the 60 relay detects that the 2nd set of PT remain energized it can disable thevoltage regulator. If the voltage regulator sensed no voltage it may boost the field currentin an attempt to maintain voltage thereby creating an over excitation condition. An alarm isused after the 60 relay has op erated to inform generator operators o f a blown fuse.

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Uttsttfitt

FIGURE 16. Out ofstepprotection zone.

The diameter above does not contain Xsystem since in our sample system since we assumeno system data or stability dataexists. The relay will trip when the relay detects theimpedance between the b linders and inside the circle.

Frequency Relays (81)The two main considerations with the operation of synchronous generators outside

standard frequency ranges are : (1) rapid aging of the mechanical co mp onents during bothunder frequency and over frequency operation and (2) thermal co nsiderations, which willmostly be significant w hen an und er frequency condition exists.Under Frequency Relay (81U)

At lower frequencies than 60 Hz the generator and its prime m over w ill begin toslow down as they attempt to carry the excess load. The reduced ro tation also leads to

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reduced ventilation thereby a reduction in power output. The lower frequency can also leadto over excitation since the flux is inversely proportional to the frequenc y.Over Frequency Relay (810)

Is comm only the result of a sudden reduction in load. During ov er frequencyoperation there is an improvement in ventilation and the flux density need ed for a giventerminal voltage is less and therefore does not produce the same heating as does an underfrequen cy condition. How ever, generator turbines are designed to operate near 60 Hzoutside the designed limit may produce destructive resonance in the rotating mechanicalcomponents.

The relays w ill have thresholds at which they can alarm for a set level and trip if asecond threshold is exceeded. A time delay is also included at to give the system tostabilize before th e generator is separated from the sy stem.

Lock Out Relay (86)A 86 relay is not a protective device in its self but an auxiliary relay when it's

desired that a number of operations be performed simultaneously from the operation of asingle relay. In other words, the 86 lock out relay internally contains multiple contactswhat can be either normally open or normally closed and will change state when aprotective device activates the 86. When a protective relay activates the 86 lock out relaythe changes contact states can be used to trip main circuit breake rs, field circuit breakers,activate alarms system s, etcetera. The protection engineer can specify the function of the86 relay when the protection scheme is developed. The external portion of the 86 relayconsists of a handle which when tripped w ill rotate indicating a trip has occurred.

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30 MVA 13.8kV?

Where XBasejrc is the transformer on the generator M VA and V oltage b ase.Calculate the generator base impedance ZBase_primary_G

7 _14.4fc l /L 2 L B a 5 e _*Base_Primary_G - 2 5 MVAcjase ~ * Z V *

Since relay settings are based on the secondary magnitudes of the CT and PTsconvert the Voltag es, currents and impedances into the relay base

_ 14,4007 _^BaseJLL relay 7 ^ 1 2 0 71 1 4 , 4 0 0 7 _^BaseJLNrelay ~J=X T Tjj 6 9 . 2 8 7_ 1002.4 _ -lBase relay 2 40 ~~

_ 6 9 . 2 8 7 _^Base relay ~~ A *JCA ~ ^

Typical Relay Settings Calculations and Verification with Experiment.The following are the calculations for relays settings. Those that do not require

calculations will reference a figure for the corresponding setting. The ex perim entalverification setup (Figure 17) consists of a waveform and RMS capture device DranetzPower Xploer PX -5, Schweitzer Engineering Laboratories SEL-7 00G protectivegenerator relay and 3-phase power system simulator Megger M PRT R elay Tester. Thesimulator software Megger AVTS 4. 0 will be used to control the 3-phase simulator aswell as vary the phase angles and record the results.

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To convert the primary value into secondary quantities divide the base current value theCT ratio.

U 17.04A =.071ASecondary_reverse_power JA()Since our relay detects the total powe r, mu ltiply the result by 3. The current

required will be .21 A. The currents are rotated 180 degrees (Figu re 20) to simulatecurrent flow into the generator. The results shown on Tab le 5.

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TABLE 6. Zone 2 Test ResultImpedance(Ohms)20 .705

CalcZ(Ohms)20.70Error(%)0.02

MinRange(Ohms)19.66MaxRange(Ohms)21,73

Pass/FailPass

Voltage A j Voltage B J Voltage C J Current A J Current B I I Current C

p _ _ _ j _ ^ _ _ p ^ j _ _ ^ _ | , _ _ p _ _ p_ _ _ _ p _ _ _ ^^^ ^ j p _ _ _ ^ _ _ _

FIGURE 24 . Loss of excitation zone1 reach test.

TABLE 7. Loss of Excitation Zone1Reach Test ResultImpedance(Ohms)18.250

CalcZ(Ohms)18.20Error(%)0.27

MinRange(Ohms)17.29MaxRange(Ohms)

19.11Pass/Fail

PassTestVolts(Volts)69.28

5. Unbalance relay (46): The manufacture will design the generator in accordance

with IEEE standards. For our example the manufacture has provided us with thefollowing information:

Pickup: 7% Time dial (K): = 9 with a linear reset of 4 min utes.The expected results are calculated by taking 7% or the full load amp s which is 4.2A sothat I = ,07 x 4.2A = . 94A but since we are testing 1 phase at a time w hile the other 2phases are held at zero w e must mu ltiply by 3. Therefore the exp ected test current is .294x 3 = .882A. The results are shown graphically in Figure 25, No te that figure 25 has allthree phases superimposed on same graph. The first plot A-phase the current was held

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Time Vs Current

MultiplesofPtctajp

FIGURE 26. Voltage control relay 51C) results.

TABLE 9. Voltage Controlled Over Current Test ResultsMultiplier(X Pickup)

3. 00005.00007. 0000

Applied(Amps)6.0010.0014.00

Time(Seconds)3.75391.35980. 7620

ExpectedJTime(Seconds)3.71971.35730, 7666

Error( )0.910.19-0 .61

MinRange(Seconds)3.551.280 .71

MaxRange(Seconds)3.891.440.82

Pass/FailPassPassPass

Note thatOCunitdoes not occuruntil voltage drops,

FIGURE 27, Voltage controlled relay 51C)RMS trip graph,

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b. Reverse reach current I = 69,28/1. 5 = 46, 19A note that this is exceedstypical 3 phase test equipment. Since we are interested in the current and theimpedance is fixed we can use a lower voltage to calculate a suitable I. Use 20Vand calculate I. 20V/1.5 = 13. 3Ac. Right Blinder the current phases are rotated as to allow the impedancevector approach from right to left as in b above 20 V /1. 2 = 16. 67Ad. Left Blinder the current phases are rotated to allow the impedance toapproach from left to right I is 20 V /1 .2 = 1 6 . 67A

The protection zone are bound by these values (see Figure 29),90

180 11 0Reversereach9.0 7,0LeftBlinder

Forwardreach

s.o f6 i_ J V .. .,_

^ *****

~ J

RightBlinder.0 7 9.0 11 0

Relay will tripinside circle andinside left andright Blinder lines,

270

FIGUR E 29, Loss of Synchronization protection boundaries,

Reverse and forward reach experimental results.VoltageA j

f b lF*Voltage B j[ 20.000V*

Voltage c j

f 240.0 r[ ^ S O O O H T

CurrentA j CurrentB || 13J??A

p E S o W

Current C If nJffK[ m o r

FIGURE 30. Forward reach results.45


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TABLE 15. Relay Settings Summary TableRelay Function25 Over E xcitation27 Under Voltage Trip32 Reverse Power40 Loss of Field (Negative offset MHO)Zone 1DiameterZone 1OffsetZone 2 DiameterZone 2 Offset46 Current Unbalance Relay51V Voltage controlled Over Current Relay59 Over Voltage Trip78 Loss of Synch RelayLeft and right bindersMHO diameterForward reachReverse reach

Pickup105%62.4V. 2 1 A @ 1 2 0 V16.59ft-1.62ft19.07 ft-1.62ft7% , K=92.09A @ 52V76.2V

1.2ft8.03ft6.5 ft1.5 ft

Delay2 seconds50 seconds

4 minute linear resetInverse time curve10 seconds

50 milliseconds

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REFERENCES

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[12] IEEE Pow er & Energy Society.IEEETutorialon the Protection of SynchronousGenerators(Publication 95 TP 102). Piscataway, N J: IEE E, 1995[13] Megg er Inc.Instructional Man ual for MPR T Protective Relay TestSystem,710000.Dallas, TX: Megger Inc., 2010[ 14] Megg er Inc.Instructional M anua l forA VTS4 0Advanced VisualTestSoftware.Dallas, TX: M eggerInc.,2010[15] Schweitzer Engineering Laboratories.Instruction Manua l for SEL-700GGeneratorProtection Relay, 20110324.Pullman, WA: Schweitzer Engineering Laboratories,2011.

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136418087 Calculations of Protective Relay Settings