GENERATOR GENERA TOR AND GENERA GENERATOR TOR TRANSFORMERS PROTECTION
General 1.1
Generators Generators are designed designed to run at high load load factor for a large number number of years and permit permit certain certain incidences incidences of abnormal working conditions. The machine and its auxiliaries are supervised by monitoring devices to keep the incidences of abnormal working conditions down to a minimum. Despite the monitoring, electrical and mechanical faults may occur, and the generators must be provided with protective relays, which, in case of a fault, quickly initiate a disconnection of the machine from the system and, if necessary, initiate initiate a complete shutdown of the machine. !ecommendations contained herein for electrical protective system of generator generator and generator transformer transformer are intended to be used for generators of 1" #$% and above. &t describes requirements of various protections, special comments to help in determining applications of these for units of various types and si'es, setting criteria and tripping principles.
1.
The protection requirements of machine used in pumped storage schemes need further attention and the recommendations given below do not cover these details. 1..1 1..1
Generator Generator can can be connected connected to the the primary primary transmiss transmission ion system system in the the followin following g two ways( ways( &. )nit *cheme &n this scheme no switchgear is provided between the generator and generator transformer which are treated as a unit+ a unit transformer is trapped off the interconnection for the supply of power to auxiliary plant. &&. Generator ircuit -reakers *cheme &n this scheme, a generator circuit breaker is provided between the generator and generater transformer.
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&n view of generator unit being a complex system, various electrical electrical ha'ards ha'ards require consideration. These are given below( a. *tat *tator or ins insul ulat atio ion n fail failur ure e b. /vervoltage c. !otor faults d. 0oss 0oss of sync synchr hron onis ism m e. /ver /veru und nder er fre frequ quen ency cy f. /verload g. )nba )nbala lanc nced ed load loadin ing g h. 0oss 0oss of exci excita tati tion on i. !eve !evers rse e pow power er prot protec ecti tion on 2. &nadvertent energisation energisation of generator
1.3
4ollowing are the various various protections protections recommended recommended for for the generator and generator transformer transformer protection. protection.
Type of 4ault G757!%T/! *T%T/! *hort circuits
%symmetry *tator overload
%5*& Device 5o. 89 G 89 GT 1 G :19 G 3= G :1 G
Device 5o.
6rotection 4unctions Generator differential /verall differential #inimum impedance;or alternatively over currentunder voltage< 5egative sequence /verload
7arth fault stator
=3 G1 =3 G 3" G >8 G G9 G
0oss of excitation /ut of step #onitoring -lade fatigue &nter turn fault #ag.circuits @igher voltage %ccidental energisation #onitoring G757!%T/! !/T/! !otor ground G757!%T/! T!%5*4/!#7! *hort circuits
Ground fault /verhang )5&T %)A&0&%!B T!%5*4/!#7! *hort circuit Ground fault
81 G :> G89 GT >> G :> G 9:" G =" G
>:? stator earth fault 1""? stator earth fault 0oss of excitation 6ole slip 0ow forward powerreverse power ;double protection for large generators< #inimum frequency /ver voltage or under current /verfluxing volt@' /vervoltage Dead machine 6T fuse failure
=3 4
!otor earth fault
89 GT :1 GT 89 T :1 5GT 89 5T 89 @$
/verall differential /vercurrent Transformer differential 7arth fault overcurrent !estricted earth fault @$ winding cum overhang differential
89 )%T :1 )%T :1 )%T =3 )%T
Transformer differential /vercurrent !esidual overcurrent !estricted earth fault
The transformers guards like -uchholt' protection, winding temperature high, oil temperature high, etc. are also important and shall be provided as per transformer manufacturerCs recommendations. 1.:
!ecommendations regarding selection of the generator protections for units of different types and si'es are summari'ed in the table given below( 4unctions
differential >:? stator 74 1""? stator 74 &nterturn faults -ackup impedance $oltage controlled / 5egative sequence 4ield 4ailure !everse power 6ole slipping overload overvoltage )nder frequency Dead machine !otar earth fault overfluxing
*team *mall;1 " #$%< B B 5 B 5
Gas #edium;1"E 1"" #$%< B B B5 B B
Turbines 0arge;F1"" #$%< B B B B B
@ydro *mall;1" #$%< B B 5 B 5
Turbines #edium;1"E 1"" #$%< B B B5 B B
0arge;F1"" #$%< B B B B B
B
5
5
B
5
5
B
B
B
B
B
B
B B 5 5 B B 5 B 5
B B 5 5 B B 5 B B
B B B 5 B B B B B
B B 5 B B B 5 B 5
B B 5 B B B 5 B B
B B B B B B B B B
. Detailed requirements .1 Generator stator .1.1 Generator differential protection;89 G< This is a unit type protection, instantaneous in operation, covering the stator winding for phase to phase faults. The generator differential relay is not sensitive to phase earth faults due to high neutral earthing resistance arrangement. %s this protection operates for generator internal faults, opening the generator breaker in classE% eliminates the system inEfeed to the fault;if the unit is synchroni'ed<. 4or all machines of ratings 1" #$% and above, this protection shall be provided. !equirements ( i< ii< iii< iv< v< vi<
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-e triple pole type with individual phase indication @ave operating time less than " milliseconds at times setting -e high or low impedance principle based /perating current : to 1"? of nominal current @igh stability against maximum through fault condition, T saturation, harmonics and D transients -e provided with suitable voltage dependent resistors across the relay to limit the voltage to safe level, in case of high impedance type relay
Generator transformer differential protection ;89 GT< This is a unit type protection with coverage from the generator terminals up to the @$ breaker and the generator transformer @$ terminals. &t will detect phase faults on both sides of the generator transformer and single phase to earth faults of the @$ side only ;the earth fault current on the 0$ side is very small, due to the high neutral earthling resistance<. Generator transformer differential relays have harmonic restraint circuits to prevent incorrect operation when the generator and the unit transformers are energi'ed from the system. 4or all machines of ratings #$% and above, this shall be provided. The protection need not include )nit %uxiliary Transformer ;)%T< in its 'one and )%T should be covered by a separate protection. &n case of breakerEandEhalf switching scheme, the TCs associated with main and tie breakers shall be connected to separate bias winding and these should not be paralleled in order to avoid false operation due to dissimilar T transient response. !equirements ( i< ii< iii<
iv< v< vi<
-e triple pole type with individual phase indication @ave through fault restraint features for external faults with an ad2ustable or multiEbias setting @ave magneti'ing inrush restraint features and also be stable for inrush under normal over fluxing conditions, magneti'ing inrush proof feature shall not be achieved through any intentional time delay e.g. use of timers to block relay operation *hall have unrestrained instantaneous highest overcurrent unit operation which is unaffected by inrush @ave one bias winding per phase and per T input ;@owever )%T T need not have separate input and maybe parallel with the generator T<. @ave an ad2ustable operating current
vii< viii<
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@ave an operating time not greater than " milliseconds at : times setting @ave facility for ratio and phase angle correction eiter through auxiliary transformer or through inbuilt provisions.
-ackup protection for short circuits 4or machines of 1" #$% above this shall be provided. 4or machines of ratings less than 1" #$% voltage restrained over current relay may be used in place of minimum impedance relay.
.1..1 -ackup impedance protection ;1 G< This operates for phase faults in the unit, in the @$ yard or in the ad2acent transmission lines with a suitable delay, for cases when the corresponding main protection fails to operate. 4or faults on the high voltage side of the deltaEstar connected generator transformer, the impedance measured by the relay is influenced by the relay connection, the fault type and generator and system source impedance. 4or this reason a circular characteristic the center of which corresponds to connection position of $T should be used. The impedance relay shall have fuseEfail interlock. !equirements ( i< ii< iii< iv< v<
-e triple pole type -e of single step under impedance type *hall have two ad2ustable definite time delay of ".:E: seconds -e suitable for measuring two or three phase faults within a set distance from the point of installation *hall be able to operate for fault currents down to ". in.
*etting recommendations The impedance relay shall have reach setting to cover the longest @$ outgoing line or 9"? of rated generator load impedance E E
Time relay of step 1 ".: sec Time relay of step shall be set to coordinate with third 'one time of distance relay or back up oc relay of the outgoing line
.1.. /vercurrentundervoltage protection ;:19
-e triple pole type *hould be able to operate when the fault current from the generator terminals becomes low due to excitation system characteristic *hall have undervoltage criteria *hall reset to deEenergi'ed position if underEvoltage criteria disappears
5egative phase sequence protection ;3= G< The negative phase sequence protection safeguards the generator rotor against overheating caused by the induced double frequency ;1"" @'< currents when negative phase sequence currents are present in the stator. The negative phase sequence current can appear due to unEbalanced single phase loads or transmission line unsymmetrical faults. 4or all machines of ratings 1" #$% and above this shall be provided. !equirements(
i< ii< iii< iv< v<
-e triple pole type @ave an alarm unit and a trap unit @ave a continuously ad2ustable negative sequence current setting @ave a definite time setting range of 1E1" seconds associated with an alarm unit @ave alarm unit range covering negative phase sequence current of :E1" ? of generator rated current continuously ad2ustable
vi<
@ave a trip unit with a variable time current characteristic matching with the generator
2
I 2 t
characteristic *etting recommendations The relay should be set to the 56* capacity of the generator. The 56* capability of the machines varies considerably from one machine to another and no required settings are thus suggested. %larm unit shall be set at :"? of continuous withstand capability value of the machine and time delay for alarm can be seconds. .1.:
Generator overload protection ;:1 G<
%s an additional check of the stator winding temperatures, overload relay is used. This may be provided for hydro units where there are high head variations and connected for alarm. &t may also be connected for run back. &n case of thermal set there is no chance of thermal overloading as the machine is provided with number of limiters and therefore is not recommended. !equirements(
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i< -e single pole type ii< -e of definite time overEcurrent type iii< @ave a continuously ad2ustable setting range of :"E""? iv< @ave a dropEoffpickEup ratio greater than >:? v< @ave an ad2ustable time setting range of .: to : seconds Generator stator earth fault protection The high neutral earthling r esistance arrangement limits the generator earth fault current to less than 1" amperes, therefore minimi'ing damage to the core lamination. %lthough a single phase earth fault is not critical, it requires clearance within a short time, due to the following( E E E
&t may develop into a phase to phase fault ;due to presence of ioni'ed air< &f a second earth fault occurs the current is no longer limited by the earthing resistor 4ire may result from the earth arc
Two different types of stator earth fault relays are recommended both installed in the secondary circuit of the generator. .1.=.1 "E>:? stator earth fault protection ;=3 G1< This protection 'one is limited to approximately >:? of the stator winding due to the danger of false tripping. This shall be voltage relay monitoring the voltage developed across the earthing resistor by the neutral return current. This is normally in the back up protection. &t also covers the generator bus, low voltage winding of the unit transformer and the high voltage winding of the )%T. Hhen connected to open delta winding of generation 6T, the protection shall be blocked for 6T fuse failure. 4or faults within 1"? of the generator neutral, the resulting current is not enough to operate the relay.
4or all machines of ratings 1" #$% and above this shall be provided. !equirements( i< ii< iii< iv< v<
*ingle pole type *hall have independently ad2ustable voltage and time setting *uitable to protect >:? of stator winding -e suitable for operating from brokenEdelta voltage transformers or neutral grounding transformer secondary *hall be insensitive to rd harmonic voltage
*etting recommendations 6ickEup voltage of the relay :? of maximum neutral voltage Time delay I ".E".: seconds .1.=. 1""? stator earth fault protection ;=3 G< This protects the whole stator winding and the generator neutral. The relay generally operates on the principle of low frequency signal in2ection into the secondary of the earthing transformer, detecting the corresponding current if an earth fault occurs. The relay is set in terms of insulation resistance. This is normally in the main protection. %lternatively, a protection based on change in magnitudedistribution of rd harmonic voltage caused by an earth fault is used. Hhen provided, this shall have voltage check or current check unit as applicable, to prevent faulty operation of the relay at generator stand still or during the machine running down period. 4or machines above 1"" #$% the protection shall always be provided. 4or machines of smaller si'es the option is left to the utility depending upon the importance of the machine. 4or machines of "" #$% and above low frequency current in2ection based relays are recommended. Hhere >:E1""? relay is separately available, this may be connected for alarm and operator can take the machine out. !equirements( i< ii< iii<
iv< v<
vi<
-e suitable to protect 1""? of stator winding -e insensitive to external faults, transients and inherent harmonic currents. *hall be based on low frequency current in2ection principlealternatively shall operate on the principle of detecting change in the magnitudedistribution of third harmonic voltage caused by an earth fault. @ave continuously ad2ustable time delay range 1E" seconds &f based on in2ection principle *hall continuously monitor ground circuit o *hall continuously monitor in2ection signal and in2ection equipment o *hall be in service at standstill, startup and stop o &f based on rd harmonic voltage principle *hall have under voltage or over current check o
*etting recommendations 1""? stator earth fault relay ;in2ection principle< 6ick up level of the relay :"" ohms
Time delay I seconds ;greater than rd 'one of distance relay< 1""? stator earth fault relay ; rd harmonic principle<
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E *etting ".3: $ E Time delay ." sec E $oltage check unit 8"? of rated voltage E urrent check unit "? of rated current 0oss of excitation protection ;3" G< % complete loss of excitation may occur as a result of unintentional opening of the field breaker, an open circuit or a short circuit of the main field or a fault in %$!. Hhen a generator with sufficient active load loses the field current it goes out of synchronism and starts to run on asynchronously at a speed higher than the system absorbing reactive power for the excitation from the system. )nder these conditions the stator end regions and part od the rotor get over heated. This is recommended for machines of all si'es above 1" #$%. !equirements( i<
ii< iii<
@ave mho characteristic lying in the rd and 3th quadrant of impedance diagram with ad2ustable reach and offset. %lternatively, this protection shall be based on directional current unit with setting range to match generator capability curve. *hall have an under voltage relay andor over current relay as an additional check. *hall have timer with ad2ustable range of 1E1" seconds to distinguish loss of excitation from power swings.
*etting recommendations i<
4or offset mho type relay Diameter of mho circle Ad /ffset of the mho circle from origin Ad o Time delay 1." second o )nder voltage relay 9"? o ii< 4or directional current type relay directional current relay is set to match with the generator capability curve in 3 th quardrand o time delay ".: I 1 second o under voltage 9"? o over current 11" I 11:? o 6ole slipping protection ;>8G< o
.1.8
The loss of field protection shall be supplemented by an additional out of step function which detects all pole slips. 6ole slipping of generators with respect to the system, can be caused by a number of conditions leading to an increase in rotor angular position beyond the generator transient stability limits. *ome of the causes of pole slipping are( a< 0arge network disturbances b< 4aults on electrical network close to the generator c< Heak tie between the network and the generator ;tripping of transmission lines< d< 0oss of generator field ;field winding or excitation supply failure< e< /perating the generator in an excessive under excited mode. This is recommended for machines of 1"" #$% and above. 4or h ydro machines utilities can decide depending on machine parameter.
!equirements i< ii<
*hall be capable of detecting a power swing which can lead to instability in addition to being able to detect an actual pole slip -y varying si'e of the characteristic it shall be possible to ensure that a trip command is given to the circuit breakers in such a way that seperation of poles occurs at a controlled angle at any time.
*etting requirements i<
&f the source of oscillation lies within a generatortransformer unit, the machine has to be isolated from the network after the first slip
4orward reach of relay characteristics shall cover generatorgenerator transformer. Tripping in this 'one shall be in the first pole slip. The reach of this 'one is ".9 AT ii< .1.>
&f the source of oscillation lies outside the unit in the network, the generator should not be switched off or atleast not until several pole slips have occurred. 0ow forward powerreverse power interlock relays ; G9 G< The low forward po wer interlock is recommended for thermal machines and reverse power protection may be used for hydro machines to protect against motoring. Hhen the steam flow through turbine is interrupted by closing the 7*$s or the governor valves, the remaining energy stored in the set is delivered to the system and the machine enters into a motoring condition drawing power from the system to supply its losses while keeping the turbo alternator shaft at synchronous speed. E The low forward power relay detects that the unitis motoring and must therefore be shutdown E Tripping for mechanical faults and abnormal conditions
4or faults in the turbine or boiler, the turbine protection closes the 7*$s. 4or abnormal conditions, the generator protection closes the 7*$s. Hhen the generator develops low forward power, the relay after a short time delay trips the generator breaker. % protection field suppression signal may also be required if the %$! does not have a builtEin facility to reduce the generator field current as the speed decreases, to avoid overfluxing conditions. !equirements i< ii< iii< iv< v< vi<
be single phase power measuring type have a power setting of approximately ".: I 1 ? of rated active power of generator unit have independent time delay relay with setting range of 1 E 1" seconds and " I " seconds respectively on pick up have one more common timer with a pick up setting range : I :" seconds for annunciation that the turbo generator set has started motoring have suitable arrangement for preventing the operation of this protection during startup and synchroni'ing of the unit shall be provided with possibility of angle correction to facilitate measurement of power accurately.
*etting r ecommendations(
.1.1"
E low forward power relay pickup ".": x 6n E timer t1 seconds, t seconds )nder frequency protection The frequency under protection E 6revents the steam turbine and generator from exceeding the permissible operating time at reduced frequencies
E E E
7nsures that the generating unit is separated from the network at a present value of frequency that is less than the final stage of system load shedding. 6revents the %$! from exciting the machine at reduced speeds when some protective relays may not perform at all 6revent over fluxing of the generator. The over fluxing relay is used to protect against small over fluxing for long periods while the over voltage and under frequency relays als protect against large over fluxing for short times
The stator under frequency relay measures the frequency of the stator terminal voltage. Though under frequency tripping is recommended by turbine manufactures, care should be taken by grid operating personnel in ensuring that machines are not run at lower frequencies and instead resort to means like load shedding in the event of overload. !equirements( i< ii< iii<
@ave one alarm stage and two tripping stages *hall have setting of range of 3: @' I :: @' with a least count of ".1 @' for each stage Timer for alarm stage have a range of ".: I : seconds with a least count of ".: second. Timers for each tripping stage shall have a range of 1 to 1" seconds with a least count of ".1 second *hall have under voltage blocking
iv<
*etting r ecommendations( E E E
*tator under frequency relay pickup level 38.: @' Time delay %larm E seconds
4or tripping stages recommendations of turbine manufacturers may be followed. .1.11
&nter turn fault protection ;:> G< &t is generally considered difficult to obtain reliable protection against short circuit of one turn if the stator winding has large number of turns per phase. &nter turn fault protection is recommended only for machines where there is a split winding and all the six terminals are brought out on the neutral side. 4or the generator with split neutral, conventional interEturn fault protective scheme comprises a time delayed low set overEcurrent relay which senses the current flowing in the connection between the neutrals of the stator winding. %lternatively, a split phase differential protection may be used. !equirements( i< ii< iii<
/ver current relay with time delay /ver current relay shall have built in filters to re2ect higher harmonics !ange of over current shall be selected depending on maximum spill current for external fault
*etting recommendations /ver current relay shall be set to maximum unbalanced current in case of external fault. Time delay ". I ".3 seconds .1.1
Generator transformer over fluxing protection ;>> GT< /ver fluxing protection is provided to safeguard the generator, generator transformer and unit transformer against operation at flux densities which may cause accumulative damage to the core.
4rom the fundamental equation $ 3.33 x f x n x ϕ, the level of flux is proportional to the ratio of terminal voltage to frequency ;vf<. This ratio is monitored b y the protective relay. %n over fluxing condition is more likely to occur while the generator is separated from the system and the speed is allowed to drop, but it can also happen with the machine on load if the tap changer of the generator transformer ;@$ side< is on a low tap position and the excitation of the generator is manually increased. &n this case the increased generator terminal voltage may cause over fluxing tripping at nominal frequency. The over fluxing protection operates with a time delay after which the tripping functions are executed. This protection must be provided for generatorEtransformer of si'e 1" #$% and above. !equirements( i< ii< iii< iv< v<
/ver fluxing protection shall be phase to phase connected /perate on the principle of measurement of voltage to frequency ratio @ave inverse time characteristics compatible with generator transformer over fluxing withstand capability for tripping 6rovide an independent alarm with a definite time delay of value of $f between 1""? to 1"? of rated value @ave a high resetting ratio of >8? or better
*etting recommendations( The over fluxing capability of the transformer must be checked and the characteristic matched accordingly for both alarm and trip. .1.1
Generator over voltage protection ;:> G<
