1. INTR INTROD ODUC UCTI TION ON
Generator is an important equipment of the Power Station to provide electric power supply to the industrial and domestic consumers in the country. The growing demand of power supply has also increased the size and capacity of the generators. The maintenance of the uninterrupted power supply to the grid obviously draws one’s attention to the efficient operation and maintenance of the generator.
The efficient operation of generator means the knowledge and fleibility of the various performance parameters of the generator and their limits during its operation under stable operating conditions as well as under fault condition.
This manual covers the various performance parameters and their lim its while the generator is in service! it also covers effects of load variation " the change in generator parameters without eceeding their limits for stable operation.
This manual will prove very useful for the Power Sector Personnel.
1
2.
COMPONENTS OF TURBO-GE -GENERAT RATOR
#.$ G%&%'() The #$* +, turboturbo-gen genera erator tor incorp incorpora orates tes the modern modern featur features es of direct direct cooling with .+. ,ater and /ydrogen gas and fast acting ecitation system.
0urther technical specifications of #$* +, turbo-generator type T/,-#$*-# are as follows1
MAIN PARAMETERS
$. #. 7. 5. 8. ;. 6. :. 9. $*. $$. $#. $7. $7. $5. $5.
+aimum continuous 2, rating +aimum continuous 24( rating 'ated terminal voltage 'ated stator current 'ated power factor 'ated ecitation current at +<' condition Slip ring voltage at +<' condition 'ated speed 'ated frequency %fficiency at +<' condition 'ated /ydrogen gas pressure >gauge? Short circuit ratio &ega &egati tive ve sequ sequen ence ce capa capabi bilit lity y ire irect ctio ion n of rotat rotatio ion n when when viewe viewed d from from slip slip ring ring
#$*3 *** 2, #563 *** 24( $83 68* ( 9*8* ( *.:8 lag #3 ;** ( 7$* 4 73 *** 'P+ 8* /z 9:.5= 7.8 kg@cm# *.59 A#t ≤ 8: (nti clock wise
$8. $;.
end. Phase connection &o. of terminals brought out
ouble Star >BCB? 9 >; >; ne neutral " 7 phase?
OVERALL DIMENTIONS AND WEIGHTS $. #. 7. 5. 8.
Dverall dia of stator )ength of stator )ength of rotor iameter of rotor barrel ,eight of the heaviest
single
2
piece
79;* mm ;8:* mm 966* mm $*68 mm for $68*** kg.
2.
COMPONENTS OF TURBO-GE -GENERAT RATOR
#.$ G%&%'() The #$* +, turboturbo-gen genera erator tor incorp incorpora orates tes the modern modern featur features es of direct direct cooling with .+. ,ater and /ydrogen gas and fast acting ecitation system.
0urther technical specifications of #$* +, turbo-generator type T/,-#$*-# are as follows1
MAIN PARAMETERS
$. #. 7. 5. 8. ;. 6. :. 9. $*. $$. $#. $7. $7. $5. $5.
+aimum continuous 2, rating +aimum continuous 24( rating 'ated terminal voltage 'ated stator current 'ated power factor 'ated ecitation current at +<' condition Slip ring voltage at +<' condition 'ated speed 'ated frequency %fficiency at +<' condition 'ated /ydrogen gas pressure >gauge? Short circuit ratio &ega &egati tive ve sequ sequen ence ce capa capabi bilit lity y ire irect ctio ion n of rotat rotatio ion n when when viewe viewed d from from slip slip ring ring
#$*3 *** 2, #563 *** 24( $83 68* ( 9*8* ( *.:8 lag #3 ;** ( 7$* 4 73 *** 'P+ 8* /z 9:.5= 7.8 kg@cm# *.59 A#t ≤ 8: (nti clock wise
$8. $;.
end. Phase connection &o. of terminals brought out
ouble Star >BCB? 9 >; >; ne neutral " 7 phase?
OVERALL DIMENTIONS AND WEIGHTS $. #. 7. 5. 8.
Dverall dia of stator )ength of stator )ength of rotor iameter of rotor barrel ,eight of the heaviest
single
2
piece
79;* mm ;8:* mm 966* mm $*68 mm for $68*** kg.
;. 6.
tran transp spor orta tati tion on >sta >stato torr with with gas gas cool cooler ers s and and lifting trunnions? ,eight of rotor 5##** kg Total weight of generator #6**** kg
TEMPERA TEMPER ATURE RAT R ATINGS INGS $. #. 7.
5.
mea >measu sure red d by embedded resistance thermometers? +aimum temperature of stator winding >measured by embedded resistance thermometers? +aimum temperature of rotor winding >measured by resistance method?
EF’ $*8° < $*8° <
$$* $$*° <
HYDROGEN IN STATOR CASING $. #. 7. 5. 8. ;. 6.
Perm Permis issi sibl ble e pres pressu sure re norm normal al Pressure variation &omi &omina nall tem tempe pera ratu ture re of cold cold gas gas +inimum percentage purity of /# inside machine Dygen content - maimum uantity of /# ga gas required for initial filing uantity of /# requ require ired d when when gene genera rato torr is running +aimum allowable moisture contents in casing.
7.8 kg@cm# ± *.# kg@cm# 55° < 96= $.#= 7** +7 77; +7 $8 mg@m7 /ydrogen gas.
of
TECHNICAL PARTICULARS $. #. 7. 5. 8. ;. 6. :.
line? &o. of gas coolers uncoupled? 0ly wheel moment of rotor 'atio of S.<. torque to full load torque Fasi Fasic c impu impuls lse e insu insula lati tion on leve levell >wit >with h resp respec ectt to body body?? Fasic impulse insulation level >between turns? calc >calcul ulat ated ed valu value? e? in hot condition. 9. uantity of oil for both shaft seals $*. Dil pressure at inlet of bearings
3
68 +4(' 5 $76*@75** 'P+ #$.$ Ton +# : 593* 593*** ** 4 593*** 4 *.;9 +icro-farad $;* )itre@+in. *.7 to *.8 kg@cm#
$$. 'ated pressure of shaft seal oil $#. eclud >ecluding ing shaft seal? $7. '%(HdI? ii. irect ais transient reactance reactance for positive positive phase phase sequence >Hd’? iii. irect ais synchronous reactance >Hd? iv. &egative phase sequence 'eactance >H #? $5. $5. TA+% A+% Tdo? ii. Time ime cons consta tant nt of peri period od comp compon onen entt of subsubtransient current for 7 ph3 # ph " $ ph short circuits >TdI? iii. Time constant of a periodic component for # ph and 7 ph. $8. $8. S/D' S/D'T T i27I? ii. Sub transient transient current current on # phase phase short circuit circuit >i2#I? iii. iii. SubSub-tra trans nsie ient nt curre current nt on sing single le phas phase e shor shortt circuit >i2$’? iv? Steady state current on 7 phase short circuit >i27?
4
5.$ to 5.8 kg@cm# 7** )itre@+in.
*.#$5 p. u. *.7*8 p. u. #.## p. u. *.#; p. u. 6 Seconds *.$#$ Seconds
*.#9 Seconds
$* p. u. : p. u. $*.8 p. u. $.5 p. u.
5
6
7
8
9
2.2
STATOR
The stator body is totally enclosed gas tight fabricated structure. /# gas coolers are housed longitudinally inside the stator body. Stator core is made up of segmental varnish insulated punchings of <.'.G.D. silicon steel assembled in an inter-leaved manner on core bars. The core consists of several packets separated by steel spacers for radial cooling of core by / # gas and is held in pressed condition by means of heavy non-magnetic steel press rings bolted to the ends of core bars. The core bars are designed to provide elastic suspension of core in stator body to isolate the magnetic vibrations of stator core from foundation of generator.
Stator has a 7 phase double layer short-chorded type windings having two parallel paths. %ach coil side consists of glass insulated solid and hollow conductors with demineralised water passing through hollow conductors. The elementary conductors are roebel transposed in the slot portion of winding to minimize eddy current losses. The over hang portion of the coils is securely lashed with glass chord to bandage rings and special brackets of non-magnetic steel which are pressed @ fied to core press rings.
'ing type distillate headers of copper supported on insulators are provided separately for distillate inlet and outlet in stator on turbine side. The winding ends are solidly soldered into the coil lugs. Andividual bars are provided with water inlet @ outlet connections made of Telfon hoses. The water-cooled terminal bushings are housed inside non-magnetic steel chamber in the lower part of the stator on the slip ring side. The three phase terminals are brought out and si neutral terminals also to facilitate eternal connections.
2.3
ROTOR
The cylinderical type rotor is forged in one piece >shaft and body? from chromium3 nickel3 molybdenum vanadium steel. The rotor >field? winding is made from hard drawn silver bearing copper and is held in position against
10
centrifugal forces by duralumin wedges in the slot portion and by non-magnetic steel retaining rings in the overhang portion.
Gap pick up system is employed for direct hydrogen cooling of rotor windings. There are several groups of ventilation ducts provided on the sides of rotor coils for gas passage. The rotor insulation consist of glass cloth impregnated with epoy resin in the slots pre peg glass cloth for inter turn insulation and block of glass laminate and glass epoy moulded segments for supporting and insulating the end windings. The end windings are held in position against centrifugal forces by retaining rings3 machined from high strength heat-treated non-magnetic alloy steel forgings3 which are shrunk on the rotor body and provided with locking nuts. The centering rings are mounted at the end of the retaining rings support and prevent movement of rotor windings in aial direction due to thermal stresses.
Two propeller type fans are shaft mounted on either end of body of rotor for circulating the gas inside the generator. The field winding is connected to ecitation system through brush gear and slip rings. Two semi-circular hard copper bars insulated from each other and from rotor shaft and placed in the central bore of rotor form current leads between slip rings and field windings.
#.5
H2 GAS COOLERS
0our number gas cooler are mounted longitudinally inside the generator stator body. The gas cooler consists of longitudinally placed cooling tubes made out of admiralty brass with coiled copper wire wound outside for increasing the cooling surface area. The cooling water flows through tube while the hydrogen comes into contact with eternal surface of cooling tubes. 4ent pipes are provided on the slip ring side to remove air from gas coolers while filling them with water.
11
3.
OPERATION OF GENERATOR
3.1 STARTING OF THE GENERATOR (.
Fefore starting3 the following activities are required to be checked and ensured that
i. Anstrument supply to all indicator and recorders is made Eon’ ii.
3.2
RUNNING UP
Dnly after ensuring the completeness and availability of all the above3 the set is required to be started and taken up gradually to full rated speed.
uring the course of running up3 bearing vibrations or rubbing if any inside the generator or abnormal noise need to be carefully investigated. (lso temperatures of bearings3 seal babbit and drain oil are required to be noted down. 12
3.2.1 CHECKS AT RATED SPEED ,hen generator comes to rated speed3 the following items are required to be checked1 i.
Temperature of bearings3 seal babbit and drain oil.
ii.
Performance of brush gear.
iii.
Phase sequence of generator with the help of residual magnetism.
iv.
3.3
Fearings vibrations in all direction.
SYNCHRONIZATION
Fefore synchronization of generator to the desired bus the following activities are required to be carried out1
i?
ii?
iii?
(fter closing of bus side isolator3 and physically verifying the same3 Jnit
iv?
Synchronouscope on@off switch in off and synchronizing check relay KS2%I by pass switch in on KA&
v?
vi?
The voltages in all three phases are required to be checked.
vii?
Af voltage is not coming to the required value3 then giving impulse to field rheostat switch to match the voltage with that of bus.
viii?
13
3.3.2 SYNCHRONIZATION ACTIVITY
i? Switching on synchronous cope is ensured3 if frequency of incoming machine is higher than the system frequency3 synchronous cope will move in clockwise direction and if frequency is lower3 it will move in anticlockwise direction. Speed of rotation depends upon difference in frequencies. The impulse by load change switch to have very slow clockwise rotation is given. ii? ,hen voltage and frequency match3 the synchronous cope moves very slowly in clockwise direction. This position shows that
a. Phase sequence of generated voltage and system voltage is same. b. %ffective values of both the voltages are same. c. 0requency of both voltage is same.
iii. Giving closing impulse to generator circuit breaker at the instance when synchronous cope pointer is in between $$ and $# ED’ clock position and which indicates synchronism by glowing of lamp at generator desk.
iv. $*-#* +, load on machine is taken.
v. Synchronous cope switch to Eoff’ position and return synchronizing switch to Eoff’ position are ensured.
3.4
RAISING LOAD
The following activities need to be carried out1 i? (fter synchronization putting Eanti monitoring’ protection switch to KD&I position " taking load say at #* +,. ii? Dbserving generator voltage and power factor being maintained by (4'3 if selection of ecitation system on auto. Af generator voltage and power factor are not maintained3 then maintaining them by varying field rheostat from generator control desk manually and after maintaining putting ecitation system on Eauto’ mode.
14
iii? Slowly raising the load on generator up to :* +, as per the following guidelines1
)oad a? )oad after synchronizing.
+@c Starting
Soaking time b? )oad
#* +in. #*-:* +,
Soaking time c? )oad
from +@c starting /ot State $8-#* +, L
from
$* +in. 8 #*-:* +, L $8
+,@$* +in.
+,@$* +in
#* +in.
#* +in.
:* +,-$8* +, L 8 :*-$8* +, L 8
Soaking time d? )oad
+,@$* +in.
+,@8 +in
7* +in.
#* +in.
$8* +,-#$* +, L 8 $8* +,-#$* mw L
Total time from * to #$* +,
+,@$* +in.
8 +,@8 +in
79* +in.
##* +in.
>; /rs. 7* +in.?
>7 /rs. 5* +in.?
4oltage and power factor are maintained by (4'.
iv?
v?
(t this load3 generator bearing and seals temperature3 generator winding @ core temperature3 generator T@0 winding temp.3 /# gas temperature3 performance of (4' and cooling water flow to auiliaries are checked. Af generator winding @ core temp. is high3 distillate flow and cooling water flow to gas coolers is adMusted. Af generator T@0 winding temp. is high3 all the cooling fans and pumps position in service condition is ensured.
15
vi?
%citer winding temp. and slip ring sparking are required to be checked. %citer voltage and current within permissible limits are ensured.
vii?
The load as per the guidelines mentioned above is increased and generator winding @ core temp3 Generator T@0 winding temperature and vibrations are checked. Af Generator winding @ core temp. can not be controlled by increasing the cooling water flow to gas coolers3 and distillate to stator water coolers3 the load is to be reduced.
viii? Generator T@0 winding temperature is required to be kept under permissible limits by running cooler fans and pumps.
3.5
ROUTINE OPERATION AND PERIODIC CHECKS
At is important that the generator and its auiliaries be kept under observation during operation. The annunciation system gives warning of abnormal conditions3 but regular observation of different parameters helps the operator to detect any gradual deterioration in the operating conditions and take appropriate corrective action even before any alarm comes. (ll those parameters are required to be noted by operator in the log sheets specially maintained. These are over and above the automatic recording done by instruments. At is recommended that a full inspection of generator should be made after it has been in operation approimately one year after commissioning.
3.5.1 HOURLY CHECKS i?
Temperature /ourly checking of temperature of following are required to be done and any abnormal rise in temperature be reported to the concerned personnel without delay. 'emedial measures need to be taken. a.
Stator winding
b.
Stator core
c.
'otor winding
d.
e.
istillate temperature at inlet and outlet of stator winding 16
f. Fabbit temperature of bearings and seal liners g.
Anlet and outlet temperature of cooling water to gas coolers.
h.
Anlet and outlet temperature of generator bearings and shaft seals oil.
i. Temperature of cooling water at inlet and outlet of stator water coolers and seal oil coolers. M. Generator T@0 winding and oil temperature. k.
!
Jnit auiliary transformer winding temperature.
P"#$$%"# a.
/# gas pressure and purity
b.
ifferential pressure of seal oil and /# gas and pressure of oil after .P.'.
c.
Seal oil and the thrust oil pressure at seals.
d.
istillate pressure at inlet and outlet of stator winding.
e.
f. istillate pressure before and after filter g.
!
)!
Dil pressure at inlet and outlet of seal filter.
F&'($ a.
b.
istillate to stator winding
c.
V*"+,'$
4ibrations at generator front and rear bearings and eciter end bearing are required to be noted in aial3 vertical and transverse direction.
)!
S#/0/ "#$$,), '0 $,&&+,#
Specific resistivity of stator winding cooling distillate is required to be checked.
17
3.5.2 DAILY CHECKS
i?
ii?
aily gas consumption is required to be noted down. (ny abnormal increase in gas consumption indicates leakage for which necessary measures may have to be taken.
iii?
The gas sample to find out whether /# gas is leaking into water circuit is required to be checked. Sample to be taken from gas trap.
iv?
The condition of /# gas driers is eamined and reactivating done if necessary.
v?
Frush gear need to be eamined for1
a.
Jnusual sparking between brushes and slip ring
b.
c.
ust
or
oil
accumulation3
corrective
action
whenever
necessary are taken.
vi?
,ater or oil traces in side stator body is required to be checked and if found drained.
vii?
The conditions of lamps in the signaling system of /# gas cooling seal oil and stator water system is required to be checked.
viii?
Stator water system for presence of any gas is required to be checked.
i?
The leakage rate of /# gas is required to be checked.
?
The /# gas content in oil tank needs to be checked.
i?
The insulation of ecitation system needs to be checked by checking leakage current.
ii?
Test running of the standby (.<. seal oil pump and emergency .<. seal oil pump " checking the interlocks between the two is required to be ensured.
18
3.5.3 WEEKLY CHECKS
i?
The condition of brush gear need to be eamined3 the pressure on brushes is required to be checked.
ii?
Anter locking between working and stand by stator water pump for automatic take over is required to be checked.
iii?
Dperation and calibration of / # gas purity indicator need to be checked.
iv?
ifferential pressure across filters in seal oil and stator water system to ascertain whether they are choked or not need to be checked.
v?
Anter locking between working and stand by /# gas cooler3 booster pump is required to be checked.
vi?
'esistivity of distillate by laboratory testing needs to be checked.
vii?
Purity of /# gas by laboratory testing needs to be checked.
viii? /# gas concentration in bearing chamber is required to be traced. i?
/# gas concentration in bus duct enclosure needs to be checked.
?
The gas sample taken from hydraulic seal for D# gas content is required to be checked.
i?
The bearing vibrations by portable and accurate vibration measuring instruments are required to be checked.
3.5.4 MONTHLY CHECKS
i?
(ll accessible bolts for tightness are required to be checked.
ii?
(ll the protection and signaling circuits need to be checked.
iii?
(ll the alarm contacts are required to be checked.
iv?
The insulation resistance of generator bearing3 shaft seals and connecting oil pipes on slip ring end need to be checked.
v?
The polarity of slip rings needs to be changed once in three months in order to have uniform wear of slip rings.
19
3.5.5 SHUT DOWN CHECKS i?
A.'. of stator winding and rotor winding immediately after shut down is required to be checked.
ii?
iii?
The slip rings and brush gear conditions for any abnormality needs to be checked.
3.5.. PERIODIC CHECKS i?
ii?
iii?
4.
OPERATIONAL LIMITS OF TURBO GENERATOR
4.1
CAPABILITY OF GENERATOR
This generator is capable of delivering #56 +4( continuously at $8.68 24 terminal voltage3 9*8*-(mpere Stator current at 7.8 kg@cm #3 /# gas pressure with cold gas temperature not eceeding 55 ° < and distillate temperature at inlet of stator winding 58° <.
Dutput of generator at various lagging and leading power factor are as per the KGenerator
4.2
VARIATION OF TERMINAL VOLTAGE
Generator can develop rated power at rated power factor when terminal voltage changes within ± 8= of rated value i.e. $5.9: 24 to $;.85 24. The stator current should accordingly be changed within limits of ± 8= of rated value i.e. :;** ampere to 98** ampere.
20
21
uring operation of generator at 9*= of the rated voltage for continuous operation3 stator current should not increase beyond 9*8*.
Terminal 4oltage in $6.7# $6.$6 $6.*$ $;.:8 $;.6 24 Dutput in +4( #$6 Stator current in 6.#5 2(
4.3
$;.85 $8.68 $5.9; $5.$:
##5.6 #7$
#76
#5#
#56
#56
#7#.# ##*
6.8;
:.$5
:.76
:.;
9.*8
9.*8
6.9#
9.*8
FREUENCY VARIATION
Generator can be operated at rated output with a frequency variation of
±8= over the rated value i.e. 56.8 /# to 8#.8 /#. /owever3 the performance of generator with frequency variation is limited by the turbine capability.
4.4
TEMPERATURE OF COOLANTS
Af temperature of cooled /# gas or inlet water to gas coolers increases beyond the rated value3 the unloading of generator has to be carried out as per given curve E+’. The operation of generator with cold gas temperature more than 88° < is not permitted. Dperation of generator with cold gas temperature below #*° < is not recommended.
Similarly if cold distillate temperature at inlet of stator winding increases beyond the rated value3 unloading of generator has to be carried out as per given curve ES’. The operation of generator with cold distillate temperature more than 5:°< is not permitted. Dperation of generator with cold distillate temperature below 78°< is not permitted.
22
23
24
4.5
OVERLOADING Jnder abnormal conditions the generator can be overloaded for short
duration. Permissible values of short time over loads in terms of stator and rotor currents and corresponding duration at rated voltage3 rated power factor and rated parameters of / # gas and distillate are given in table A and AA respectively. T+*&#-I Stator current in 2( Time in minutes
$7.86
$#.;6
$#.##
$$.6;
$$.7$
$*.:;
$*.5$
9.9;
$
#
7
5
8
;
$8
;*
T+*&#-II 'otor current in 2( Time in seconds
4.
8.# #*
7.9 ;*
7.$# #5*
#.68 7;**
OPERATION UNDER UNBALANCED LOAD
The turbo generator is capable of operating continuously on an unbalanced system loading provided that continuous negative phase sequence current during this period shall not eceed 8= of the rated stator current i.e. 58#.8 amperes. At implies that maimum difference between line currents is about $*= of rated value. (t the same time current in maimum loaded phase should not eceed the permissible value for given conditions of operation of turbo generator under balanced loads. Af unbalance eceeds the above permissible limits3 measures should be taken immediately to eliminate or reduce the etent of unbalance within 7 to 8 minutes. An case it is not possible to achieve this3 then machine has to be run down and tripped.
Af negative phase sequence currents reach a value of 6.8= of rated value3 trip-relay will operate and generator will be automatically tripped.
25
4.
ASYNCHRONOUS OPERATION (synchronous operation of the generator on field failure is allowed
depending upon the permissible degree of the voltage dip and acceptability of system from stability point of view. uring the failure of field the field suppression shall be cut off from the circuit and active load of generator shall be decreased to ;*= of the rated value within 7* seconds and to 5*= in the following $.8 minutes. The generator can operate at 5*= rated load asynchronously for a total period of $8 minutes from the instant of ecitation failure. ,ithin this period steps should be taken to establish the reasons of field failure and bring back to normal or set has to be switched over to reserve ecitation if available or shut down.
4.6
MOTORING ACTION +otoring of turbo-generator is permissible within the limitations of
turbine.
5.9
OPERATION AT REDUCED H2 GAS PRESSURE.
side the stator body lower than the rated value of 7.8 kg@cm # is not permitted. /owever3 during emergency the generator can be operated at reduced / # pressure with reduced load continuously and for a short duration as given in table E(G’. Table1 (G /# pressure kg@cm# >g? 7.* #.8 #.* $.8
Dutput of t he g enerator uration of operation +, #**
,ithin this time action should be taken to restore the /# gas pressure to the normal value. Dperation of generator in air medium is not permitted.
5.$*
CAPACITY OF GENERATOR WITH ONE GAS COOLER OUT OF SERVICE.
26
The generator can deliver $68 +4( continuously when one gas cooler is out of service. The operation of generator with more than one cooler out of service is not permitted.
4.11
OPERATION ON LEADING POWER FACTOR Dperation of generator on leading power factor is restrained from the point of
view of stability and establishing aial core flu in core and packets leading to eddy current in end packets and consequent heating. 0igure-# may be referred.
4.12
OPERATION ON LAGGING POWER FACTOR ,hen generator runs at more lagging power factor the total current supplied by
generator increases due to the fact that generator in addition to load current supplied de-magnetizing component of current also.
4.12
ABNORMAL CONDITIONS DURING RUNNING
S. &o. $.
#. 7. 5.
8.
;.
6.
escription of abnormal conditions Generator bearing seal babbit temperature high >t ≥ 68° t ≥ ;*°
generator casing. istillate flow to stator - +achine is required to be loaded to winding low >$:m7 @ hr? $68 +4(
Systems check up and taking measures to increase flow is required. istillate specific resistivity Flowing down distillate and adding fresh low >68 2 Dhm cm? distillate to bring up resistance is required. istillate outlet temperature • istillate flow and load on m@c high >t ≥ :8° t ≥ 68°
27
• •
)oad reduction is required 'eduction in load and tripping
:.
/ot gas temperature high >68°
9.
Stator core high >98°
$*.
'otor winding temperature high >$$*°
$$.
Temperature of cold /# gas low >#*° 55°
$#.
$7. $5.
$8. $;. $6.
temperature
Temperature of cold /# gas too high >t N :8° 7.7 kg@cm#? b. /igh >7.: kg@cm#?
of unit if temperature does not come down3 is required. ,ater flow and temperature at inlet of gas coolers are required to be checked. • ,ater flow to gas cooler and gas temperature is required to be checked. • Jnloading of generator to bring down temperature is required. • 'unning down and tripping of generator is required. •
,ater level in epansion tank low ,ater level in epansion +ake up value adMustment is needed. tank high istillate pressure low • #.5 kg@cm ? and valve position is required. •
28
29
30
31
32
33
34
35
36
37
38
39
40
.
GENERATOR STABILITY 0or better understanding of generator stability3 the following is required
to be understood at first hand. An a simplified form the electrical equivalent circuit of a generator is represented as an ideal voltage source behind the synchronous impedance. Af resistance is neglected3 synchronous impedance reduces to direct ais synchronous reactance. The equivalent circuit is drawn in 0ig. $(.
(resistance neglected? added in series. This equivalent circuit shall be represented as shown in 0ig. $ F.
41
/ere
%
- is the ideal voltage proportional to the field current.
4t
- is the terminal voltage of generator.
4
- is the %/4 bus voltage.
A
- is the load current.
Hd
- Synchronous impedance of generator >resistance neglected?.
Ht
- Synchronous reactance of generator transformer.
The phasor diagram of 0ig. $ F is drawn in 0ig. $ <.
0ig. $ < Power equation can be derived as follows keeping in mind the phasor diagram shown in 0g.$ <. &ow referring ∆ D4% for active power Sin δ ----------- A . Hd O A . Ht
Sin >9* O φ ? ----------------------------%
>$?
+ultiplying both sides of equation &o. >$? by 4A and putting Hd O Ht H
42
Sin δ Sin >9* O φ ? ----------- . 4A 4A . ----------------------------AH % %.4. Sin δ Dr ---------------------- 4A 9* O φ ?
0ig. $ 'eferring to ∆ <( < < Sin φ --------------- ------------ (< AH
43
Dr < AH Sin φ ------------ >$? /ere AH A >Ht O Hd ? &ow referring to ∆ (D D %< #? Fut D< 4 < D C D< Dr < %cos δ - 4 ----------------- >7? 0rom eq. >$? and >7? we get AH Sin φ %5? &ow multiplying eq. >5? by 43 we get 4AH Sin φ %4
4AH Sin φ %4
D'
%.4. 4# 4A Sin φ > ----------- -----------? H H
&ow3 stability is defined as the capability of the generator to remain in synchronism3 following a change in its operating. epending upon the nature of disturbance introducing the instability3 this can be categorized as
a?
Steady state stability
b?
)ong- term oscillation stability >dynamic stability?
c?
Transient stability
+!
S,#+ $,+,# $,+*&,
Stable
Jnstable Qone
Qone Pm PD,%' %)A4%'% *
9*° 'DTD' (&G)%
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,e know that active or real power %. 4 P -------------- . Sin δ H ,hen δ 9*° >%3 4 and H are constant?3 maimum power will be delivered by generator and which will be %. 4. Pm ---------------- . Sin 9*° H %. 4. Pm ---------------H 0or a given machine3 operating at a terminal voltage 43 the synchronous reactance Hd is a constant parameter3 and if the Einternal voltage’ %3 or rotor current3 is kept constant3 power varies as sin δ. (t rated conditions3 δ is about 58-88°. 0rom this position3 a sudden increase in steam throughput3 or >more likely? a sudden demand for more power into the system3 perhaps because of a fault on the lines3 results in an increase in δ and in generated power until a new equilibrium position is reached >0ig. a ?. This is valid if δ is less than 9* ° before the sudden change. Dnce δ is greater than 9*°3 a demand for more power cannot be met by an increase in load angle3 and the generator rotors cannot attain a position of equilibrium >0ig.b?. The rotor then accelerates to Must above-synchronous speed and operates in a non-synchronous mode >Epole slipping’?3 with large power and voltage oscillations which are unacceptable to either the transmission system or the boiler controls. The situation may be retrievable if the voltage regulator can initiate a rapid increase in the field current3 increasing % in the equation3 to prevent instability >0ig. c ?. )oad angles approaching 9* ° are associated with operation at leading power factor3 which is not a normal requirement. /owever3 studies of the transmission system under all credible conditions of loading3 line outages and faults are carried out to ensure that the system will not fall into instability3 and the required values of synchronous reactance and ecitation response are based on these studies3 which
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may recommend different values in different locations. An practice3 because of magnetic saturation3 H is reduced as the load angle moves towards 9*°.
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*!
L'7-,#"8 '$/&&+,' S,+*&, 9D+8/ S,+*&,! The operation of generator beyond 9* ° rotor angle is not stable3 if (4'
is not in action. /owever with fast acting (4's3 it is possible to operate at rotor angles for above 9* ° >often up to $5*°?. This operation is signified by continuous oscillations. /!
T"+$#, S,+*&, ,hen a generating unit is operating on load in the steady state there is
eact balance between the driving torque eerted on the generator shaft by the turbine and the resisting torque arising from the load on the generator. &ow3 the power equation 7%%# P ------------------ Sinδ ,atts QS can be rewritten in the more general form3 7%%# P ------------------ Sin δ ,atts QT ,here QT > QS O Q) ?3 sum of the generator and the load impedances. QT is commonly referred to as the total transfer impedance of the generatorload system. Transient conditions will arise if there is a sudden change in any of the quantities of this general power equation3 i.e. there will be transient disturbance if there is a sudden change in generating driving torque3 ecitation or loading.
)et a steam turbo-generator operating in parallel with other generators be subMected to a sudden load change. The change may be either load reduction caused by increasing Q ) or load increase if Q ) is reduced. Since the steam governor cannot respond instantly to the load change there will eist a power differential between the generator input and output3 and this power differential will either accelerate or decelerate the rotor from its initial steadystate position towards the new power angle required by the changed loading.
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Fy virtue of its inertia3 however3 the rotor will overshoot the stable position and oscillation about this position will occur. So long as the rotor overshoot in the forward swing is not great enough to cause synchronism to be broken3 stability will eventually be regained by the action of the steam turbine governor in adMusting the driving power and by the damping forces acting on the rotor. E:%+& A"#+ S,+*&, C",#"'
,hen the power angle diagram of a generator working on infinite busbars is known3 the conditions under which transient instability would occur can be checked by the method known as the Kequal area stability criterionI.
0or an initial value of QT$ in the power equation3
7%%# P ------------------ Sin δ3 QT the generator would operate at P$ o the power curve ( shown in 0igure >
?. Af there is now a sudden change in the transfer impedance3 say due to
tripping of one of the lines connecting the unit to the mains system3 the generator will begin to operate on a new power curve F whose maimum value is set by the value of Q T#3 the new transfer impedance. There can3 however3 be no sudden change of generator flu or driving power and the voltages %3 %# will remain constant with the initial load angle of δ$ as shown.
irectly the load transfer impedance is changed the generator output drops from P$ on curve ( to P* on curve F. The surplus power3 P $ C P* P$ will be the accelerating power acting on the rotor which will speed up and swing forward from the initial load angle δ$.
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50
)et
θ the angular displacement of the rotor in time t seconds3 a the angular acceleration in electrical degrees per socond #3 f generator frequency3 w angular velocity of rotor in rad @ sec. #π& ----------------------- rad @ sec3 ;* ,R rotor energy in megaMoules + angular momentum of rotor3
# ,R ----------------- . w Therefore ∆P$ angular momentum acceleration3 d#θ + --------dt#
>a?
number of poles & w 7;*° ---------------------------- ----------- elect° @ sec. # ;*
&ow3
7p& elect° @ sec (nd
,R / inertial constant ------------+4(
#/ >+4(? Therefore + ------------ 7p&
>b?
#/>+4(? d#θ ∆P$ + -------------- ----7p& dt#
2
d
Therefore dt
θ
Fut
7p& ∆P$ --------------- elect° @ sec# #/>+4(?
p& $#*f
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2
d
θ
Therefore dt2
7$#*f ∆P$ --------------- elect° @ sec# #/>+4(?
>c?
The acceleration is d #θ @ dt# and if the accelerating power ∆P$ is epressed in per unit values of +4(3 equation >7? becomes3
a
$:* f ∆P$ --------------- elect° @ sec# /
>8;?
(nd θ at#
>8;a?
uring the time t of rotor acceleration the magnetic ais will be displaced by an angle θ to a new load angle δ’ where the generator output and input are again balanced. (t this position of equilibrium the rotor speed will be above synchronism and it will overshoot causing the generator to produce more power than is being supplied by the turbine. Feyond the load and angle δ’$ the etra power is drawn from the rotor itself. Fy its displacement from δ$ to δ’$ the rotor will have stored energy equal to the area ( in 0ig. >
? and in giving up this energy the rotor will slow down until at the
limit of its swing it is again running at synchronous speed. The energy abstracted from the rotor after it has swung through load angle δ’$ must be equal to the energy it has stored in moving from δ$ to δ’$3 i.e. area F area (. An descriptive terms3 area ( may be called accelerating power and area F called braking power. Jnder transient conditions3 if the generator is to remain stable the rotor braking power must be equal to the accelerating power and this is the basis of the equal area stability criterion.
The maimum rotor displacement from which the generator could regain stability would be δ’$ ma $:* - δ’$. This indicates on a curve ( a power level equal to the initial output of the generator and is based on the assumption that during the rotor forward movement there has been no change
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in the input power. An the conditions illustrated by the diagram3 stability would be assured because beyond %3 the limit of the required braking power area3 there is a further area of braking available. This means that the generator could withstand a load change greater than the one shown without the machine becoming unstable. Fut for any change greater than the one shown without the machine becoming unstable. Fut for any increase in the accelerating power ∆P$ with the initial load angle remaining unchanged3 there would be a reduction in δ’$ ma.
At would3 therefore3 be necessary to check
every new condition to ensure that the area of accelerating power can be balanced by an area of braking power. Af δ’$ ma is eceeded the generator output would become less than the input and the power differential would be again positive. The rotor would3 therefore3 be subMected to further acceleration and instability would result.
An an actual system more than one term in the power angle equation will usually be varying and the approach to simple analysis would be to consider one variable at a time3 holding all the other terms constant. An the above3 the transfer impedance Q T was taken as the independent variable.
An this simplified introduction to the comple problem of transient stability some important points may be noted.
a.
The angular acceleration of the rotor is inversely proportional to the inertia constant of the coupled rotating system of the generator and the lower the acceleration during disturbance the more stable will be the generator operation.
b.
The angle of rotor displacement during a disturbance is proportional to the square of the time during which the rotor is subMected to the disturbance or to the effects of it. The shorter the time the less will be the rotor displacement.
c.
The total rotor swing is a direct function of the power differential acting on it.
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d.
An steady state operation the rotor load angle should be at a value which allows ample margin for normal disturbances without the risk of the rotor swinging out of step.
These points show the importance of high inertia constants in generators3 quick acting protective gear3 and the necessity to avoid large step changes in generator power input and loading. Atem >b? shows the main disadvantage of too high transient and subtransient reactances in a generator. These reactances are the measure of the time constants of the machine during transient operation and under fault conditions. The shorter such time constants are the less difficult it is to stabilize the generator during transient load changes and to clear it under fault conditions.
(n etreme case of sudden load change in a generator is a 7-phase short-circuit close to the generator terminals. Jnder such fault conditions the terminal voltage would suffer sever drop and generator output would fall almost to zero. /ence3 the full output from the prime-mover would be available to accelerate the generator rotor. 0or an initially fully loaded generator the rotor acceleration would be high and rotor energy would be stored so rapidly that it would become difficult to maintain stability unless the fault time was made etremely short. An the smaller turbo-generators the worst result following slow fault clearance may be one or two pole-slips in which time steam power to the turbine can be reduced. An the larger machines3 however3 stability would be quickly lost with little chance of regaining synchronism and the main protection of the machine against instability must be the quick action of the circuit-breakers.
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REFERENCES
$. <%GF +(&J() 4D)-A4 #. &/P< +(&J() 7. &PTA +(&J() D& G%&%'(TD' 5. &TP< +anual
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