reverse power protection of alternator.pdf

Reverse Power Protection of An Alternator

INTRODUCTION Today’s complex power scenario demands uninterrupted supply for consumer. Therefore generator is the ultimate ultimate source of today’s today’s life and its function is of heart of power syste system. m. Hence Hence it needs the maximum maximum protec protectio tion n from the occurring faults. The generator used is a synchronous machine which can either run as generator generator or motor depending upon the form of input energy energy.. These generator generator are synchronized with the bus-bar that is the grid. Thes Thesee are are number number of faul faults ts which which may may cause cause heavy heavy damage damage to generator and simultaneously to the economical situation. The faults are described in the table. Some Abnormal condition and protection system.

Sr.

Abnormal

No. Condition . Thermal ov over

Effects

!ver

heating

loading continuous windi nding

Protection

of

and and

insulati ation resi esistor tor

overloading failure failure. ".

of

therm hermom omet eter er

embedded in in stator stator slots.

of coolant. #xtern ernal faults fe fed $nbalanced $nbalanced loading loading stresses stresses %egative phase se&uence by generator

on

winding

#xcessive

and heating

prolonged short circuit. '.

stator Thermocouples

shaft. protection

for

for machine protection

large overload

for

small

generator. Stato ator fa fault ults pha phasse windin winding g burnt burnt out( out( weldin welding g )iased differential to phase( phase to of core lamination

protection sensitive earth

earth inter turn

fault protection( inter turn

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*.

+otor #arth fault

Single

fault

does

fault protection. not +otor earth

fault

harmful( second fault causes protection unbalanced unbalanced magnetic magnetic forces forces caus causin ing g dama damage ge to shaf shaft( t( ,.

oss

of

bearing etc. field /enerator runs as induction oss

trip trippi ping ng of fiel field d gener nerator circuit breaer

driv drivin ing g

field eld excit excitat atio ion n

of

field

or

failure production curre current nt

from bus-bar speed increases 0.

slightly. of #ffect depend upon type of +everse power protection

1otoring

generator that is it prime motor and the power by turns 2.

as

such draw drawn n

motor. !ver voltage

from rom

the

during motoring. 3nsulation failure

directional

power

bus bus-bar -bar relays direct the reversal of power. ightening connected

arrestor ne a r

connected near

4.

!ver fluxing

Heating of core

generator terminal. !ver fluxing protection by

6.

$nder fre&uency

volts5

hertz relay. failure of blades of Steam 7re&uency relays. turbines.

7or this range of phase angle power factor is negative that is 8osφ 8osφ is negative. 9:;3cosφ 9:;3cosφ.

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#ventually power becomes negative which is termed as reverse power. CONDITION FOR REVERSE POWER

7ollowing are the possible circumstances in which reverse power condition may arise. i< =hen number of units are operating in parallel and failure of one unit may result in feeding power to the suffered unit from healthy units. ii< =hen one of the unit from all connected parallel units is overloaded and its conse&uence would be reduced speed.

>ue to reduction of speed( fre&uency

decreases and ultimately emf generated goes on decreasing and finally a critical stage is reached where is machine ?alternator< is driven as motor. iii< =hen load is suddenly thrown off( terminal voltage of bus-bar increases instantly. 3f this condition is not detected properly again alternator will run as synchronous motor.

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WHAT IS REVERSE POWER ? =hen the input to the turbine is stopped the generator continuous to rotate as synchronous motor taing power from the bus-bar. >uring the motoring action of the generator the power flows from the bus-bars to the machine and this condition is called as REVERSE POWER.

GRAPICA! REPRESENTATION i" Normal Condition #$ >uring normal condition angle between load current and

terminal voltage lies between @@ to 6@@. 7or a typical inductive load the voltage current waveform is shown below.

;

3

@ φ  6@°

φ

7or this range of phase angle power factor ?cosφ< is positive and therefore ?9:;3 cosφ< power is of forward in nature. ii" Re%erse &o'er condition #$ >uring abnormal condition the angle of

commencement of two &uantities exceeds 6@@. The waveform for this condition is shown below.

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6@@  φ  "2@@

φ iii" O&eratin( times and %ario)s c*aracters.

As the fault level increases rapidly fault clearing becomes a must. The static relays do not have moving parts in their measuring circuits( hence relay timer of low values can be achieved ?  cycle(  B cycle < such low relay times are impossible with conventional electro mechanical relay system which compromise measuring relay plus several auxiliary relays. ;ariety of characteristics can be obtained with static relays( therefore selectivity( stability and ade&uateness can be achieved.

1easurement of several

&uantities such as negative phase se&uence component( fre&uency( harmonics( impedance etc. can be performed by static measuring unit.

i%" Re&resentin( times and o%ers*oots

)y using special circuits( the resting times and overshoot time can be reduced( thereby the selectivity can be improved.

%" Remo%e +ac,)& and -onitorin(

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Static relays restricted by power line carrier can be used for remote bac up and networ monitoring. 3n centrally monitored systems( the bacup protection is monitored by the digital computer. The switching is carried out in such se&uence that the stability is improved.

%i" Static Relas can /t*in,0

8omplex protection scheme employ logic circuits ?ogic means the process of reasoning( induction or deduction<.

Suppose several conditions are

imposed on a protective system such that for certain conditions( the relay should operate and for some other conditions( the relay should remain stable in such cases( logic gates can be operated.

EFFECTS OF REVERSE POWER

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>uring reverse power condition alternator runs as synchronous motor and the turbine acts as a load. 1otoring protection is mainly for the benefit of the prime mover and load coming on the generator bus-bar while motoring reerse power protection measures the power flow from bus-bars to the generator running as a motor. %ormally the power taen in most cases is low of the order of "C to @C of noted power( power factor and current depends on excitation level. A single element directional power rely( sensing the direction of power flow in any one direction( phase is sufficient. The setting depend on the type of prime mover. et up see effects of motoring on different types of turbine. 1" STEA- T2R+INE

)ac pressure steam turbines sets should be protected by sensitive reverse power protection. The blades of steam turbine gets overheated &uicly as the steam gets trapped( if rotated in opposite direction due to windage. 3n steam turbine the steam acts lie coolant of the turbine blades and maintain them at constant pressure and temperature. 3f the steam flows stops the blades get overheated due to windage. 3n condensing type steam turbine the heating of blades is slower hence reverse power protection may not be necessary. 7or large turbo generator with bac pressure type( non condensing steam turbines( sensitive reverse power protection with sensitivity of the order @.,C rated power is preferred. The relay should have directional stability for the entire relay operating zone.

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3" RECIPROCATING P2-P #

1otoring is harmful to the engine( hence the reverse power protection is sensitive and engine must be disconnected from generator shaft during motoring.

4" 5DRA2!IC T2R+INE #

The water is generally fitted with mechanical devices which detects the low water flow because such a flow causes cavitation. However reverse power protection may be provided to operate for motoring power less than 'C of rated power.

6" GAS T2R+INE #

The gas turbine driven generator should not be permitted to operate as a motor because the gas turbine offers a load of @ to ,@C of full load during motoring. The 7actor to be considered are as follows. < 8apability of prime mover to run as load. "< oad current drawn while motoring. The reverse power protection is generally set for @C of rated power in reverse condition. 1otoring of synchronous machine does not change the direction of rotation of rotor.

STATIC RELAYS

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#lectromagnetic relays have almost considered place to static relays because of the latter’s number of advantages. Static relays ?Solid State +elay< is an electrical relay in which the response is developed by electronic5magnetic5optical or other components without mechanical motion of component.

1" ADVANTAGE OF STATIC RE!A5S

The static relays compared to the corresponding electromagnetic relay have many advantages.

i" !o' Po'er Cons)m&tion

Static +elays provide less burden on current transformers and potential transformers as compared to electromagnetic relays. 3n other words the power consumption is much lower than conventional one reduced consumption has the following merits. 8urrent transformers and potential transformers of less D;A rating. Accuracy of current transformer and potential transformer is increased. Air gapped current transformer can be used. 9roblem arising out of current transformer saturation are avoided overall reduction in cost of current transformer and potential transformer. ii" No -o%in( Contact

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Solid state devices do not have moving contacts. Therefore there are no problems of contact bounce( arcing( contact erosion etc. in the static relay circuit.

iii" O&eratin( times and %ario)s c*aracters

As the fault level increases rapidly fault clearing becomes a must. The static relays do not have moving parts in their measuring circuits( hence relay timer of low values can be achieved ?  cycle(  B cycle < such lo relay times are impossible with conventional electro mechanical relay system which compromise measuring relay plus several auxiliary relays. ;ariety of characteristics can be obtained with static relays( therefore selectivity( stability and ade&uateness can be achieved.

1easurement of several

&uantities such as negative phase se&uence component( fre&uency( harmonics( impedance etc. can be performed by static measuring unit.

i%" Re&resentin( times and o%ers*oots

)y using special circuits( the resting times and overshoot time can be reduced( thereby the selectivity can be improved.

%" Remote +ac,)& and -onitorin(

Static relays restricted by power line carrier can be used for remote bac up and networ monitoring.

Govt. Poly. Washim.

@


3n centrally monitored systems( the bacup protection is monitored by the digital computer. The switching is carried out in such se&uence that the stability is improved.

%i" Static Relas can /t*in,0

8omplex protection scheme employ logic circuits ?ogic means the process of reasoning( induction or deduction<.

Suppose several conditions are

imposed on a protective system such that for certain conditions( the relay should operate and for some other conditions( the relay should remain stable in such cases( logic gates can be operated.

%ii" Effects of Vibration and s*oc,s

1ost of the component in static relays( including the auxiliary relay in the output stages are relatively indifferent to vibrations and shocs. The ris of unwanted tripping is( therefore less with static relays as compared to the electromechanical relays. This aspects maes the static relays. $ne&ually suitable for earth &uaes prone areas( ships( vehicles( locomotive.

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!A-INATION OF STATIC RE!A5 i" A)7iliar Volta(e Re8)irement

This disadvantage is not of any importance now as auxiliary voltage can be obtained from station battery supply and conveniently changed to suit local re&uirement.

ii" Volta(e Transient

The static relays are sensitive to voltage spies or voltage transients. Such transients are caused by operation of breaer and isolator in the primary circuits of current transformer and potential transformer( severe over voltage are also caused by breaing of control circuit relay contact etc.

Such voltage spies of small

duration can damage the semiconductor components and can also cause nulloperation of relays. Special measures are taen in static relays to overcome this difficulty. These include( use of filter circuit in relays( screening the cable connected to relays.

iii" Tem&erat)re De&endence of Static Relas

The characteristics of semiconductors are influenced by ambient temperature for example the amplification factor of a transistor( the forward voltage drop of diode etc. change with temperature variation.

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Accurate measurements of relays should not be affected by temperature variation. +elay should be accurate over wide range of temperature. This difficulty is overcome by the following measures. 3ndividual component in circuit are used in such a way that change in characteristics of components does not effect of the characteristics of complete relay. Temperature compensation is provided by means of thermistor circuit( digital measuring techni&ues etc. Thus modern static relays are designed to suit wide limits of temperature.

i%" Price

The price of static relays is higher than the e&uivalent electro mechanical types. The prices of semi-conductor components have been reduced during past years and a further reduction is expected.

%" 3n electro mechanical relays the pic up of relay are rest of relays does not effect

the relay characteristics since the operation is based on the comparison between operating tor&ues.

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DESIGN AND APPLICATION OF COMPONENTS DESIGN OF TRANSFOR-ER

Therefore three transformer in the circuit( two of them have three winding on each core having primary consisting of two winding current coil and pressure coil and that of secondary consisting of only one winding. Another remaining one transformer is for dual supply which is a simple potential transformer.

RATTING OF TRANSFOR-ER S&ecial transformer Potential transformer

Primar

9.c."'@ v.c.c.,A "'@ v

Secondar

4v A 0-@-0 v "@@ mA

$nder the design aspects of the first two transformer ?Special transformer< As it is not possible theoretically to design such a transformer by simple design. =e have to mae it possible by trail and error method. Step wise it is such that first( we have to design simple potential transformer of "'@ v54 v and lamp. Then turns per volt are calculated. Secondly

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design a current transformer for , A5 A. Turns per volt are calculated. Then taing their average( the specification are calculated. The design of small transformer that is ?0-@-0
DESIGN DATA A" Data &resentation of s&ecial transformer

9ressure coil

- "'@ v

8urrent coil turns

- 4*

8urrent coil

-,A

Secondary winding turns

- 4*

Secondary coil

-4v A

9ressure coil conductor

E ', swg

Turns per volt ?T#<

- @.'*

8urrent coil conductor

E "" swg

8ross section area of core

- ." s&. inch amination

-, %ms( %/!

9ressure coil turns

- "*@@

)obin

- , %( %ylon "nd

Stac

- .", s&. inch

Identification of PC, CC, SW

9ressure coil

-

)lue colour sleeve

8urrent coil

-

/reen colour sleeve

Secondary winding

-

+ed colour sleeve

+" Data s&ecification of small transformer rated

"'@ v50-@-0 v and "@@ mA 9rimary winding

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-

"'@ v

,


Secondary winding

-

0-@-0 v

Turns per volt

-

".24

9rimary turns

-

,",@

Secondary turns

-

'*4-@-'*4

9rimary winding conductor

-

** swg

Secondary winding conductor-

** swg

aminations

-

" %ms

Stac

-

@.2, s&. inch

Identification of Primary Winding, Secondary Winding, Tap

9rimary winding

-

)lac

Secondary winding

-

+ed

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5 ! P P 2 S R E W O P ! A 2 D

DESIGN AND OPERATION OF D2A! POWER S2PP!5 Govt. Poly. Washim.

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Data S&ecification

Step down transformer

-

"'@ v 5'" v ?0-@-0 v<

+ectifier )ridge

-

/ 3( =o* /( @,"4

8apacitors

-

@@@ µf( ,@ v

1onolithic 7ixed ;oltage +egulator < 38 24" "< 38 26"

F2NCTION OF CO-PONENT Transformer 9

7or stepping down the voltage( we have decided to design center tapped 0-@-0 volts secondary because we re&uire ± " v

+rid(e Rectifier 9

3t is used to convert input A8 into proportionate >8 ? not pure but unidirectional one<. 3ts output voltage is '" v >8

Ca&acitors 9

These are used filtering pulsated output voltage of bridge rectifier and for voltage division. Therefore voltage across each capacitor : 0 v

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-onolit*ic Volta(e Re()lators 9

As our circuit needs F " v and E " v for operation of op-Amp( we used. 38 24" for F " v supply 38 26" for E " v supply 7or giving constant F " volt 1erits of 38 ;oltage +egulators i) Facilities for building positive and negative regulates voltage ii) Battle ripple rejection The transformer step down the incoming "'@ v Ac to the re&uired '" v. This step down Ac voltage will be rectified in bridge rectifier. The capacitor will remove ripples from rectified output. This >8 is now an input to the combination of three terminal regulators. 38 24" will give regulated F " v. And 38 26" will give regulated E " v with respect to common point that is ground point.

The ground point is

achieved by tie the common points of 24" and 26" to the center tap of transformer winding that is secondary winding.

OTER CO-PONENT AND TEIR F2NCTIONS 1" Ca&acitor Connected Across Rela Coil

Rating – 470 µ f, 25 v

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3t is connected across relay coil which filters the >8 voltage and completely removes chattering of relay contacts and suppress bac emf induced in relay coil.

3" Diode connected bet'een IC :61;s &in < and transistor base

Rating I 4007 3t is used to bac A8 and to pass only >8 voltage to transistors base.

4" Transistor

Rating !" #00 $%&%) 3t is high gain current amplifier transistor. 3ts base has been given the output of 38 2* through '.' D Ω resistance. 3t drives relay actuation provided to comparator has given voltage signal.

6" 8apacitor connected across output terminal of rectifier bridges  and " are for

filtering purpose.

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R O T A R A P O C E G A T ! O V

VOLTAGE COMPARATOR ELEMENT

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IN STATIC RELAY Volta(e Com&arator Element in Static Rela 9

The strategic element used in out proGect is a voltage comparator which gives actuating signal to relay under abnormal condition. The function of comparator that is voltage comparator is to compare the voltage at one of its input against the voltage at the other. Also to produce either a high or low output voltage( depending on which input has higher level. The comparator circuit for voltage comparator using on op-amp is as shown in fig. )y using such type of comparator we can get three types of output. ;o : ;! for ;p  ;n ;o : ;!H for ;p  ;n ;o : @

for

;p  ;n

=here ;! and ;!H denote low and high output levels. Thus if we introduce the differential input voltage ;d : ;p E ;n the above e&uation can be rewrite as ;o : ;! for ;d  @ ;o : ;!H for ;d  @ The input output characteristic for comparator is as shown in following graph.

;! ;!H ;d Govt. Poly. Washim.

""


;!

Thus ;p( ;n and ;d can assume all possible values while ;o is restricted only two valve ;! and ;!H 3n comparator application ;d is usually in the range of m; to volts. The voltage comparator may be inverting or non inverting type depending upon the output from comparator that is if output voltage is high it is said to be non inverting comparator and if it is low the comparator is said to be inverting comparator.

RELAY AND CONTACTOR RE!A5 AND CONTACTOR

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=e have used a simple attracted armature type relay for tripping circuit under abnormal condition. +elay is a device by means of which an electric circuit is controlled by change in other circuit. The +elay has an energizing coil when the fault occurs input to relay is sufficient( the coil energized and it pulls off the plate which opens the contacts and hence the contactors are opened and protective machine is disconnected from the supply.

Attracted armature relay responds to both ac as well as dc actuating

&uantities. These are essentially very fast relays. +elay used in circuit has two set of contacts( one is %ormally close ?%8< whereas other is normally open ?%!< +ating ? " ;( "@@ Ω < =e have provided an indication facility for tripping under abnormal condition. A small buzzer will not give sound when abnormal condition is reached.

CONTACTOR

Although in our simulated model we haven’t used contactor it comes into picture for breaing the circuit when abnormal condition is reached. A contactor is a mechanical switching device having one position of rest ?corresponding to the position of main contacts either open or closed< operated otherwise than by hand( capable of maing( carrying

and breaing overload

conditions #lectromagnetic contactors are popular among all types of conductor.

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=e have used contactor with * %o. contacts and  %8 contact. 8onductor has main contact as well as auxiliary contact the main contacts are used for switching on and off the power circuit whereas auxiliary contacts are used in the control circuit. The contactor coil is to be energized from a source of supply. The difference in electromagnetic contactor and a relay is that a contactor has three main contacts and a few auxiliary contacts whereas relay contact has only auxiliary contacts.

WORKING

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WOR=ING OPERATION #

The main criterion principle of our mechanism is to mae the synchronous generator to dead state. =hen its prime mover faults that is when starts running as the synchronous motor drawing power from bus-bar. %ow under these conditions we now that synchronous generator and the bus-bar are synchronized in all aspects. All the parameter of the synchronous generator as well as the bus-bar are in phase. 3 the normal state the magnetic flux created by pressure coil and current coil of transformer one are subtractive and that of transformer two are additive so at the secondary side output voltage of transformer one will be less than transformer two’s secondary. The output of both transformer is rectified by separate bridges and their positive volt terminals are connected to comparator’s ?!p-amp< pin no." and ' that is >8 output of transformer one is connected to pin no.' and that of transformer two is connected to pin no." and as we have seen in normal condition ?that is alternator is supplying power to bus-bar< output of transformer two is higher than transformer one which is connected to inverting terminal of 38 2*. So at the pin no 0 of 38 2*( output voltage will be negative and as we are using transistor 8 @@ ?%-9-%< with output of 38 2* connected to its base. 3t will not conduct and coil will not getting energized( so its %c contacts will remain in that state only. Suppose due to any reason Alternators terminal voltage is less than that of bus-bar voltage( it will act as a load on bus-bar and on the verge of motoring at that instant due to reversal of power( that is currents direction is negative with

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respect to voltage. 1agnetic flux created by transformer one’s pressure and current coil will be additive in nature and that of transformer two will be differentiative. 3n this case positive terminal of rectifier bridge ?8onnected to secondary of transformer one< will be at higher potential with respect to second bridge. So potential at pin no three of 38 2* is high and that of pin no." is low( so output of !p-amp 38 2* of pin no.0 will be positive and transistor 8 @@ %-9-% will conduct because %-9-% transistors base will get Fve potential and relay coil will get energized and will open %c contacts of relay. These are coupled to contacts of conductor. So contactor will brea the circuit of alternators. Thus it gets disconnected from bus-bar. 3n our simulated model instead of using alternator we have utilized basic property of single phase Ac supply. )y reversing phase and neutrals of current coil with respect to pressure coil. =e obtained both normal as well as abnormal condition and further operation taes place as stated above that is during abnormal condition relay open its close contacts and buzzer will not gives sound. >uring normal condition relay closes its contact and buzzer will gives sound.

CONCLUSION Govt. Poly. Washim.

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Actually when the power is reversed( there is no harm to the generator as it is synchronized machine( so when it receives the power from the grid( it starts rotating as motor. The main and serve damage is to the turbine that is the prime mover and generator bus-bar as load coming while motoring. After connecting this protection circuit to generator unit we can avoid waste of the in repair or replacement in turbine or prime mover and also economical loss. 3t also avoids cascade tripping of generating units and ultimately maintaining the system stability to higher extent.

REFERENCES < Switch /ear and 9rotection

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-

S. S. +ao

-

T. S. 1. +ao

"< 9ower System 9rotection

'< #lectrical 1achine >esign -

A. D. Sawhney

-

D. +. )otar

*< 3ntegrated 8iecuit

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"6

reverse power protection of alternator.pdf

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