210 Turbo Generator

1

1. INTRODUCTION

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 ones attention to the efficient operation and maintenance of the generator.

The efficient operation of generator means the knowledge and flexibility 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 limits while the generator is in service; it also covers effects of load variation & the change in generator parameters without exceeding their limits for stable operation.

This manual will prove very useful for the Power Sector Personnel.

2. COMPONENTS OF TURBO-GENERATOR

2.1 GENERAL

The 210 MW turbo-generator incorporates the modern features of direct cooling with D.M. Water and Hydrogen gas and fast acting excitation system.

Further technical specifications of 210 MW turbo-generator type THW-210-2 are as follows:

MAIN PARAMETERS

1.Maximum continuous KW rating210, 000 KW

2.Maximum continuous KVA rating247, 000 KVA

3.Rated terminal voltage15, 750 A

4.Rated stator current9050 A

5.Rated power factor0.85 lag

6.Rated excitation current at MCR condition2, 600 A

7.Slip ring voltage at MCR condition310 V

8.Rated speed3, 000 RPM

9.Rated frequency50 Hz

10.Efficiency at MCR condition98.4%

11.Rated Hydrogen gas pressure (gauge)3.5 kg/cm2

12.Short circuit ratio0.49

13.Negative sequence capabilityI2t ( 58

14.Direction of rotation when viewed from slip ring end.Anti clock wise

15.Phase connectionDouble Star (YY)

16.No. of terminals brought out9 (6 neutral & 3 phase)

OVERALL DIMENTIONS AND WEIGHTS

1.Overall dia of stator3960 mm

2.Length of stator6580 mm

3.Length of rotor 9770 mm

4.Diameter of rotor barrel1075 mm

5.Weight of the heaviest single piece for transportation (stator with gas coolers and lifting trunnions)175000 kg.

6.Weight of rotor42200 kg

7.Total weight of generator270000 kg

TEMPERATURE RATINGS

1.Class of insulation of generator windingB

2.Maximum temperature of stator core (measured by embedded resistance thermometers)105( C

3.Maximum temperature of stator winding (measured by embedded resistance thermometers)105( C

4.Maximum temperature of rotor winding (measured by resistance method)110( C

HYDROGEN IN STATOR CASING

1.Permissible pressure normal

Pressure variation3.5 kg/cm2( 0.2 kg/cm2

2.Nominal temperature of cold gas44( C

3.Minimum percentage purity of H2 inside machine97%

4.Oxygen content - maximum1.2%

5.Quantity of H2 gas required for initial filing300 M3

6.Quantity of H2 required when generator is running336 M3

7.Maximum allowable moisture contents in casing.15 mg/m3 of Hydrogen gas.

TECHNICAL PARTICULARS

1.Charging capacity (line)75 MVAR

2.No. of gas coolers4

3.Critical speed of rotor (uncoupled)1370/3400 RPM

4.Fly wheel moment of rotor21.1 Ton M2

5.Ratio of S.C. torque to full load torque8

6.Basic impulse insulation level (with respect to body)49,000 V

7.Basic impulse insulation level (between turns)49,000 V

8.Capacitance of stator winding (calculated value) in hot condition.0.69 Micro-farad

9.Quantity of oil for both shaft seals160 Litre/Min.

10.Oil pressure at inlet of bearings0.3 to 0.5 kg/cm2

11.Rated pressure of shaft seal oil4.1 to 4.5 kg/cm2

12.Consumption of oil per generator bearing (excluding shaft seal)300 Litre/Min.

13.REACTANCES

i. Director axis sub transient reactance for positive phase sequence (Xd)0.214 p. u.

ii. Direct axis transient reactance for positive phase sequence (Xd) 0.305 p. u.

iii. Direct axis synchronous reactance (Xd)2.22 p. u.

iv. Negative phase sequence Reactance (X2)0.26 p. u.

14.TIME CONSTANTS

i. Field time constant with open circuited stator winding (Tdo)7 Seconds

ii. Time constant of period component of sub-transient current for 3 ph, 2 ph & 1 ph short circuits (Td)0.121 Seconds

iii. Time constant of a periodic component for 2 ph and 3 ph.0.29 Seconds

15.SHORT CIRCUIT CURRENTS

i. Sub-transient current on 3 phase short circuit (iK3)10 p. u.

ii. Sub transient current on 2 phase short circuit (iK2) 8 p. u.

iii. Sub-transient current on single phase short circuit (iK1)10.5 p. u.

iv) Steady state current on 3 phase short circuit (iK3)1.4 p. u.

2.2 STATOR

The stator body is totally enclosed gas tight fabricated structure. H2 gas coolers are housed longitudinally inside the stator body. Stator core is made up of segmental varnish insulated punchings of C.R.G.O. 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 H2 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 3 phase double layer short-chorded type windings having two parallel paths. Each 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 / fixed to core press rings.

Ring 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. Individual 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 six neutral terminals also to facilitate external connections.

2.3 ROTOR

The cylinderical type rotor is forged in one piece (shaft and body) from chromium, nickel, molybdenum vanadium steel. The rotor (field) winding is made from hard drawn silver bearing copper and is held in position against 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 epoxy resin in the slots pre peg glass cloth for inter turn insulation and block of glass laminate and glass epoxy moulded segments for supporting and insulating the end windings. The end windings are held in position against centrifugal forces by retaining rings, machined from high strength heat-treated non-magnetic alloy steel forgings, 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 axial 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 excitation 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.

2.4 H2 GAS COOLERS

Four 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 external surface of cooling tubes. Vent pipes are provided on the slip ring side to remove air from gas coolers while filling them with water.

3. OPERATION OF GENERATOR

3.1 STARTING OF THE GENERATORA. Before starting, the following activities are required to be checked and ensured that

i. Instrument supply to all indicator and recorders is made on

ii. Clearance from C&I and electrical division that all instruments and equipments are available.

iii. All the flags on relays are reset and annunciation circuit is healthy.

iv. Supply of oil to generator bearings and shaft seals available and make sure that quality, temperature and flow are normal.

v. H2 is filled with desired purity and rated pressure of 3.5 kg/cm2 in side the generator.

vi. H2 gas coolers are charged with desired quality of water and temperature and flow of this water is normal.

vii. Differential pressure and thrust oil regulators are set maintaining the differential pressure 0.5 kg/cm2 & pressure 2 kg/cm2 respectively.

viii. Stator water-cooling system is charged with distillate of desired quality & flow & type of this D.M. water is normal.

ix. Protection and metering circuit is healthy.

x. The generator circuit breaker is open.

xi. The field breaker is open.

xii. Mode of excitation on manual position or Auto position.

3.2 RUNNING UP

Only after ensuring the completeness and availability of all the above, the set is required to be started and taken up gradually to full rated speed.

During the course of running up, bearing vibrations or rubbing if any inside the generator or abnormal noise need to be carefully investigated. Also temperatures of bearings, seal babbit and drain oil are required to be noted down.

3.2.1 CHECKS AT RATED SPEED

When generator comes to rated speed, the following items are required to be checked:

i. Temperature of bearings, seal babbit and drain oil.

ii. Performance of brush gear.

iii. Phase sequence of generator with the help of residual

magnetism.

iv. Bearings vibrations in all direction.

3.3 SYNCHRONIZATION

Before synchronization of generator to the desired bus the following activities are required to be carried out:

i) Closing of bus side isolator by switchyard control room

ii) Close transformer side isolator from switchyard control room.

iii) After closing of bus side isolator, and physically verifying the same, Unit Control Board to be informed.

iv) Synchronouscope on/off switch in off and synchronizing check relay SKE by pass switch in on IN CIRCUIT position to be ensured.

v) Closing field breaker and giving closing impulse for 5 seconds and releasing the switch when indicating lamp showing the closing of field breaker glows.

vi) The voltages in all three phases are required to be checked.

vii) If voltage is not coming to the required value, then giving impulse to field rheostat switch to match the voltage with that of bus.

viii) Checking the frequency of generator, if the frequency of generator is not equal to that of bus, giving impulse to speed changer to bring frequency approx. equal.

3.3.2 SYNCHRONIZATION ACTIVITY

i) Switching on synchronous cope is ensured, if frequency of incoming machine is higher than the system frequency, synchronous cope will move in clockwise direction and if frequency is lower, 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) When voltage and frequency match, 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. Effective values of both the voltages are same.

c. Frequency of both voltage is same.

iii. Giving closing impulse to generator circuit breaker at the instance when synchronous cope pointer is in between 11 and 12 O clock position and which indicates synchronism by glowing of lamp at generator desk.

iv. 10-20 MW load on machine is taken.

v. Synchronous cope switch to off position and return synchronizing switch to off position are ensured.

3.4 RAISING LOADThe following activities need to be carried out:

i) After synchronization putting anti monitoring protection switch to ON position & taking load say at 20 MW.

ii) Observing generator voltage and power factor being maintained by AVR, if selection of excitation system on auto. If generator voltage and power factor are not maintained, then maintaining them by varying field rheostat from generator control desk manually and after maintaining putting excitation system on auto mode.

iii) Slowly raising the load on generator up to 80 MW as per the following guidelines:

LoadM/c Starting from Cold StateM/c starting from Hot State

a) Load after synchronizing.

Soaking time10 15 MW

20 Min.15-20 MW

10 Min.

b) Load

Soaking time20-80 MW @ 5 MW/10 Min.

20 Min.20-80 MW @ 15 MW/10 Min

20 Min.

c) Load

Soaking time80 MW-150 MW @ 5 MW/10 Min.

30 Min.80-150 MW @ 5 MW/5 Min

20 Min.

d) Load150 MW-210 MW @ 5 MW/10 Min.

150 MW-210 mw @ 5 MW/5 Min

Total time from 0 to 210 MW390 Min.

(6 Hrs. 30 Min.)220 Min.

(3 Hrs. 40 Min.)

Voltage and power factor are maintained by AVR.

iv) Changing over 6.6 KV auxiliary bus from station supply to unit auxiliary supply taking both the UATs into circuit.

v) At this load, generator bearing and seals temperature, generator winding / core temperature, generator T/F winding temp., H2 gas temperature, performance of AVR and cooling water flow to auxiliaries are checked. If generator winding / core temp. is high, distillate flow and cooling water flow to gas coolers is adjusted. If generator T/F winding temp. is high, all the cooling fans and pumps position in service condition is ensured.

vi) Exciter winding temp. and slip ring sparking are required to be checked. Exciter voltage and current within permissible limits are ensured.

vii) The load as per the guidelines mentioned above is increased and generator winding / core temp, Generator T/F winding temperature and vibrations are checked. If Generator winding / core temp. can not be controlled by increasing the cooling water flow to gas coolers, and distillate to stator water coolers, the load is to be reduced.

viii) Generator T/F winding temperature is required to be kept under permissible limits by running cooler fans and pumps.

3.5 ROUTINE OPERATION AND PERIODIC CHECKS

It is important that the generator and its auxiliaries be kept under observation during operation. The annunciation system gives warning of abnormal conditions, 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. All 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. It is recommended that a full inspection of generator should be made after it has been in operation approximately one year after commissioning.

3.5.1 HOURLY CHECKS

i) Temperature

Hourly checking of temperature of following are required to be done and any abnormal rise in temperature be reported to the concerned personnel without delay. Remedial measures need to be taken.

a. Stator winding

b. Stator core

c. Rotor winding

d. Cold and Hot H2 gas

e. Distillate temperature at inlet and outlet of stator winding

f. Babbit temperature of bearings and seal liners

g. Inlet and outlet temperature of cooling water to gas coolers.

h. Inlet 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.

j. Generator T/F winding and oil temperature.

k. Unit auxiliary transformer winding temperature.

ii) Pressure

a. H2 gas pressure and purity

b. Differential pressure of seal oil and H2 gas and pressure of oil after

D.P.R.

c. Seal oil and the thrust oil pressure at seals.

d. Distillate pressure at inlet and outlet of stator winding.

e. Cooling water pressure at inlet and outlet of H2 gas coolers, stator

water cooler and seal oil cooler.

f. Distillate pressure before and after filter

g. Oil pressure at inlet and outlet of seal filter.

iii) Flows

a. Cooling water to H2 gas coolers.

b. Distillate to stator winding

c. Cooling water to stator water coolers and seal oil coolers.

iv) Vibrations

Vibrations at generator front and rear bearings and exciter end bearing are required to be noted in axial, vertical and transverse direction.

v) Specific resistivity of distillate

Specific resistivity of stator winding cooling distillate is required to be checked.

3.5.2 DAILY CHECKS

i) Cleanliness of surroundings of various equipments needs to be ensured.

ii) Daily gas consumption is required to be noted down. Any abnormal increase in gas consumption indicates leakage for which necessary measures may have to be taken.

iii) The gas sample to find out whether H2 gas is leaking into water circuit is required to be checked. Sample to be taken from gas trap.

iv) The condition of H2 gas driers is examined and reactivating done if necessary.

v) Brush gear need to be examined for:

a. Unusual sparking between brushes and slip ring

b. Chattering of the brushes

c. Dust or oil accumulation, corrective action whenever necessary are taken.

vi) Water 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 H2 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.

ix) The leakage rate of H2 gas is required to be checked.

x) The H2 gas content in oil tank needs to be checked.

xi) The insulation of excitation system needs to be checked by checking leakage current.

xii) Test running of the standby A.C. seal oil pump and emergency D.C. seal oil pump & checking the interlocks between the two is required to be ensured.

3.5.3 WEEKLY CHECKS

i) The condition of brush gear need to be examined, the pressure on brushes is required to be checked.

ii) Inter locking between working and stand by stator water pump for automatic take over is required to be checked.

iii) Operation and calibration of H2 gas purity indicator need to be checked.

iv) Differential pressure across filters in seal oil and stator water system to ascertain whether they are choked or not need to be checked.

v) Inter locking between working and stand by H2 gas cooler, booster pump is required to be checked.

vi) Resistivity of distillate by laboratory testing needs to be checked.

vii) Purity of H2 gas by laboratory testing needs to be checked.

viii) H2 gas concentration in bearing chamber is required to be traced.

ix) H2 gas concentration in bus duct enclosure needs to be checked.

x) The gas sample taken from hydraulic seal for O2 gas content is required to be checked.

xi) The bearing vibrations by portable and accurate vibration measuring instruments are required to be checked.

3.5.4 MONTHLY CHECKS

i) All accessible bolts for tightness are required to be checked.

ii) All the protection and signaling circuits need to be checked.

iii) All the alarm contacts are required to be checked.

iv) The insulation resistance of generator bearing, 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.

3.5.5 SHUT DOWN CHECKS

i) I.R. of stator winding and rotor winding immediately after shut down is required to be checked.

ii) Calibration of H2 purity recorder is required to be done.

iii) The slip rings and brush gear conditions for any abnormality needs to be checked.

3.5.6. PERIODIC CHECKS

i) Cleaning of the tubes of gas coolers, if needed.

ii) Cleaning of the tubes of seal oil and stator water coolers, if required.

iii) Calibration of H2 gas purity indicators with purge gas or with gas of known purity is required to be checked.

4. OPERATIONAL LIMITS OF TURBO GENERATOR

4.1 CAPABILITY OF GENERATORThis generator is capable of delivering 247 MVA continuously at 15.75 KV terminal voltage, 9050-Ampere Stator current at 3.5 kg/cm2, H2 gas pressure with cold gas temperature not exceeding 44( C and distillate temperature at inlet of stator winding 45( C.

Output of generator at various lagging and leading power factor are as per the Generator Capability Curve cited in Figure A.

4.2 VARIATION OF TERMINAL VOLTAGE

Generator can develop rated power at rated power factor when terminal voltage changes within ( 5% of rated value i.e. 14.98 KV to 16.54 KV. The stator current should accordingly be changed within limits of ( 5% of rated value i.e. 8600 ampere to 9500 ampere.

During operation of generator at 90% of the rated voltage for continuous operation, stator current should not increase beyond 9050.

Terminal Voltage in KV

17.3217.1717.0116.8516.716.5415.7514.9614.18

Output in MVA

217224.7231237242247247232.2220

Stator current in KA

7.247.567.928.148.378.69.059.059.05

4.3 FREQUENCY VARIATION

Generator can be operated at rated output with a frequency variation of (5% over the rated value i.e. 47.5 H2 to 52.5 H2. However, the performance of generator with frequency variation is limited by the turbine capability.

4.4TEMPERATURE OF COOLANTS

If temperature of cooled H2 gas or inlet water to gas coolers increases beyond the rated value, the unloading of generator has to be carried out as per given curve M. The operation of generator with cold gas temperature more than 55( C is not permitted. Operation of generator with cold gas temperature below 20( C is not recommended.

Similarly if cold distillate temperature at inlet of stator winding increases beyond the rated value, unloading of generator has to be carried out as per given curve S. The operation of generator with cold distillate temperature more than 48(C is not permitted. Operation of generator with cold distillate temperature below 35(C is not permitted.

4.5OVERLOADING

Under 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 voltage, rated power factor and rated parameters of H2 gas and distillate are given in table I and II respectively.

Table-I

Stator current in KA

13.5712.6712.2211.7611.3110.8610.419.96

Time in minutes1234561560

Table-II

Rotor current in KA5.23.93.122.75

Time in seconds20602403600

4.6 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 exceed 5% of the rated stator current i.e. 452.5 amperes. It implies that maximum difference between line currents is about 10% of rated value. At the same time current in maximum loaded phase should not exceed the permissible value for given conditions of operation of turbo generator under balanced loads. If unbalance exceeds the above permissible limits, measures should be taken immediately to eliminate or reduce the extent of unbalance within 3 to 5 minutes. In case it is not possible to achieve this, then machine has to be run down and tripped.

If negative phase sequence currents reach a value of 7.5% of rated value, trip-relay will operate and generator will be automatically tripped.

4.7 ASYNCHRONOUS OPERATION

Asynchronous 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. During the failure of field the field suppression shall be cut off from the circuit and active load of generator shall be decreased to 60% of the rated value within 30 seconds and to 40% in the following 1.5 minutes. The generator can operate at 40% rated load asynchronously for a total period of 15 minutes from the instant of excitation failure. Within 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 excitation if available or shut down.

4.8 MOTORING ACTION

Motoring of turbo-generator is permissible within the limitations of turbine.

4.9 OPERATION AT REDUCED H2 GAS PRESSURE.

Continuous operation of the turbo-generator with H2 gas pressure in side the stator body lower than the rated value of 3.5 kg/cm2 is not permitted. However, during emergency the generator can be operated at reduced H2 pressure with reduced load continuously and for a short duration as given in table AG.

Table: AG

H2 pressure kg/cm2 (g)Output of the generator MW Duration of operation

3.0200Continuously

2.5170Not exceeding 5 hours



Within this time action should be taken to restore the H2 gas pressure to the normal value.

Operation of generator in air medium is not permitted.

4.10 CAPACITY OF GENERATOR WITH ONE GAS COOLER OUT OF SERVICE.

The generator can deliver 175 MVA 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

Operation of generator on leading power factor is restrained from the point of view of stability and establishing axial core flux in core and packets leading to eddy current in end packets and consequent heating. Figure-2 may be referred.

4.12OPERATION ON LAGGING POWER FACTOR

When 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. Consequently stator winding temperature and rotor winding temperature of generator increases. Figure-1 may be referred.

4.12 ABNORMAL CONDITIONS DURING RUNNING

S. No.Description of abnormal conditionsCorrective Action to be taken.

1.Generator bearing seal babbit temperature high (t ( 75( C)Inlet oil temperature and oil flow rate need to be checked.

2.Generator bearing outlet temperature high (t ( 60( C)Oil inlet temperature and oil flow rate need to be checked.

3.Pressure of liquid in generator casing. Whether seal oil or gas cooler or stator water leaking is required to be checked.

4.Distillate flow to stator winding low (18m3 / hr)- Machine is required to be loaded to 175 MVA

Systems check up and taking measures to increase flow is required.

5.Distillate specific resistivity low (75 K Ohm cm)

Blowing down distillate and adding fresh distillate to bring up resistance is required.

6.Distillate outlet temperature high (t ( 85( C)

Distillate flow and load on m/c need to be checked.

Load on m/c is required to be

checked.

7.Stator winding temperature high (t ( 75( C) Load reduction is required

Reduction in load and tripping of unit if temperature does not come down, is required.

8.Hot gas temperature high (75( C)Water flow and temperature at inlet of gas coolers are required to be checked.

9.Stator core temperature high (95( C) Water flow to gas cooler and gas temperature is required to be checked.

Unloading of generator to bring down temperature is required.

Running down and tripping of generator is required.

10.Rotor winding temperature high (110( C) Cold gas temperature and H2 pressure is required to be checked.

Running down and tripping if temperature does not come down, is required.

11.Temperature of cold H2 gas low (20( C)Reducing flow through coolers is required.

12.Temperature of cold H2 gas high (44( C) Unloading of generator is required.

Inlet water temperature and flow to gas coolers need to be checked.

13.Temperature of cold H2 gas too high (t > 85( C)Running down and tripping of generator is required

14.H2 pressure in generator casing

a. Low (3.3 kg/cm2)

b. High (3.8 kg/cm2) Making up of H2 is required and tripping of generator if fall continues, is required.

Venting of H2 is required.

15.Water level in expansion tank lowChecking and making up is needed.

16.Water level in expansion tank highMake up value adjustment is needed.

17.Distillate pressure low (2.4 kg/cm2 ) Checking of pump operation and valve position is required.

Checking of any blockage in system is required.

( Pacross filters is required to be checked.

7. GENERATOR STABILITY

For better understanding of generator stability, the following is required to be understood at first hand. In a simplified form the electrical equivalent circuit of a generator is represented as an ideal voltage source behind the synchronous impedance. If resistance is neglected, synchronous impedance reduces to direct axis synchronous reactance. The equivalent circuit is drawn in Fig. 1A.

ACTIVE POWER

With present day system of unit connected generator and generator transformer, the equivalent circuit will have generator transformer reactance Xt (resistance neglected) added in series. This equivalent circuit shall be represented as shown in Fig. 1 B.

Here

E- is the ideal voltage proportional to the field current.

Vt- is the terminal voltage of generator.

V- is the EHV bus voltage.

I- is the load current.

Xd- Synchronous impedance of generator (resistance neglected).

Xt- Synchronous reactance of generator transformer.

The phasor diagram of Fig. 1 B is drawn in Fig. 1 C.

Fig. 1 C

Power equation can be derived as follows keeping in mind the phasor diagram shown in Fg.1 C.

Now referring ( OVE for active power

Sin (

Sin (90 + ( )

----------- = ----------------------------- (1)

I . Xd + I . Xt

E

Multiplying both sides of equation No. (1) by VI and putting Xd + Xt = X

Sin (

Sin (90 + ( )

----------- . VI = VI . -----------------------------

IX

E

E.V. Sin (Or ---------------------- = VI Cos ( Since Sin (90 + ( ) = Cos (

X

E.V. Sin (Or ---------------------- = VI Cos ( = P

X

( is also known as power angle, rotor angle, and load angle etc. where P is active Power generated by generator.

REACTIVE POWER

For calculation of reactive power figure 1 C can be modified as shown in Fig. 1 D.

Fig. 1 D

Referring to ( CAD

CD

CD

Sin ( = --------------- = -------------

AC

IX

Or CD = IX Sin ( ------------ (1) Here IX = I (Xt + Xd )

Now referring to ( AOD

OD = EC Cos ( ------------------ (2)

But OC = V

CD = OD OC

Or CD = Ecos ( - V ----------------- (3)

From eq. (1) and (3) we get

IX = Sin ( = ECos ( - V ------------- (4)

Now multiplying eq. (4) by V, we get

VIX Sin ( = EV Cos ( - V.V

ORVIX Sin ( = EV Cos ( - V2

E.V.

V2

OR VI Sin ( =( ----------- Cos () - (-----------) = Q

X

X

Now, stability is defined as the capability of the generator to remain in synchronism, following a change in its operating. Depending upon the nature of disturbance introducing the instability, this can be categorized as

a) Steady state stability

b) Long- term oscillation stability (dynamic stability)

c) Transient stability

a) Steady state stability

Stable

Unstable Zone

Zone

Pm

POWER DELIVERED

0

90(

ROTOR ANGLE

We know that active or real power

E. V

P = -------------- . Sin (

X

When ( = 90( (E, V and X are constant), maximum power will be delivered by generator and which will be

E. V.

Pm = ---------------- . Sin 90(

X

E. V.

Pm = ----------------

X

For a given machine, operating at a terminal voltage V, the synchronous reactance Xd is a constant parameter, and if the internal voltage E, or rotor current, is kept constant, power varies as sin (. At rated conditions, ( is about 45-55(.

From this position, a sudden increase in steam throughput, or (more likely) a sudden demand for more power into the system, perhaps because of a fault on the lines, results in an increase in ( and in generated power until a new equilibrium position is reached (Fig. a ).

This is valid if ( is less than 90( before the sudden change. Once ( is greater than 90(, a demand for more power cannot be met by an increase in load angle, and the generator rotors cannot attain a position of equilibrium (Fig.b). The rotor then accelerates to just above-synchronous speed and operates in a non-synchronous mode (pole slipping), 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 current, increasing E in the equation, to prevent instability (Fig. c ).

Load angles approaching 90( are associated with operation at leading power factor, which is not a normal requirement. However, studies of the transmission system under all credible conditions of loading, line outages and faults are carried out to ensure that the system will not fall into instability, and the required values of synchronous reactance and excitation response are based on these studies, which may recommend different values in different locations. In practice, because of magnetic saturation, X is reduced as the load angle moves towards 90(.

b) Long-term oscillation Stability (Dynamic Stability)

The operation of generator beyond 90( rotor angle is not stable, if AVR is not in action. However with fast acting AVRs, it is possible to operate at rotor angles for above 90( (often up to 140(). This operation is signified by continuous oscillations.

c) Transient Stability

When a generating unit is operating on load in the steady state there is exact balance between the driving torque exerted on the generator shaft by the turbine and the resisting torque arising from the load on the generator. Now, the power equation

3EE2P = ------------------ Sin( Watts

ZS

can be rewritten in the more general form,

3EE2P = ------------------ Sin ( Watts

ZTWhere ZT = ( ZS + ZL ), sum of the generator and the load impedances. ZT is commonly referred to as the total transfer impedance of the generator-load system. Transient conditions will arise if there is a sudden change in any of the quantities of this general power equation, i.e. there will be transient disturbance if there is a sudden change in generating driving torque, excitation or loading.

Let a steam turbo-generator operating in parallel with other generators be subjected to a sudden load change. The change may be either load reduction caused by increasing ZL or load increase if ZL is reduced. Since the steam governor cannot respond instantly to the load change there will exist a power differential between the generator input and output, and this power differential will either accelerate or decelerate the rotor from its initial steady-state position towards the new power angle required by the changed loading. By virtue of its inertia, however, 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 broken, stability will eventually be regained by the action of the steam turbine governor in adjusting the driving power and by the damping forces acting on the rotor.

Equal Area Stability Criterion

When the power angle diagram of a generator working on infinite bus-bars is known, the conditions under which transient instability would occur can be checked by the method known as the equal area stability criterion.

For an initial value of ZT1 in the power equation,

3EE2P = ------------------ Sin (,

ZTthe generator would operate at P1 o the power curve A shown in Figure ( ). If there is now a sudden change in the transfer impedance, say due to tripping of one of the lines connecting the unit to the mains system, the generator will begin to operate on a new power curve B whose maximum value is set by the value of ZT2, the new transfer impedance. There can, however, be no sudden change of generator flux or driving power and the voltages E, E2 will remain constant with the initial load angle of (1 as shown.

Directly the load transfer impedance is changed the generator output drops from P1 on curve A to P0 on curve B. The surplus power, P1 P0 = P1 will be the accelerating power acting on the rotor which will speed up and swing forward from the initial load angle (1.

Let ( = the angular displacement of the rotor in time t seconds,

a = the angular acceleration in electrical degrees per socond2,

f = generator frequency,

w = angular velocity of rotor in rad / sec.

2(N

= ----------------------- rad / sec,

60

WJ = rotor energy in megajoules

M = angular momentum of rotor,

2 WJ

= ----------------- .

w

Therefore (P1 = angular momentum x acceleration,

d2(

= M ---------

( a )

dt2

number of poles N

Now,

w = 360( x ---------------------------- x ----------- elect( / sec.

2

60

= 3pN elect( / sec

WJAndH = inertial constant = -------------

MVA

2H (MVA)

Therefore M = -------------

(b)

3pN

2H(MVA) d2(

(P1 = M -------------- x -----

3pN dt2

3pN x (P1

Therefore = --------------- elect( / sec2

2H(MVA)

ButpN = 120f

3x120fx (P1

Therefore = --------------- elect( / sec2

(c)

2H(MVA)

The acceleration is d2( / dt2 and if the accelerating power (P1 is expressed in per unit values of MVA, equation (3) becomes,

180 x f x (P1

a = --------------- elect( / sec2

(56)

H

And ( = at2

(56a)

During the time t of rotor acceleration the magnetic axis will be displaced by an angle ( to a new load angle ( where the generator output and input are again balanced. At 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. Beyond the load and angle (1 the extra power is drawn from the rotor itself. By its displacement from (1 to (1 the rotor will have stored energy equal to the area A in Fig. ( ) 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 (1 must be equal to the energy it has stored in moving from (1 to (1, i.e. area B = area A. In descriptive terms, area A may be called accelerating power and area B called braking power. Under transient conditions, 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 maximum rotor displacement from which the generator could regain stability would be (1 max = 180 - (1. This indicates on a curve A 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 in the input power. In the conditions illustrated by the diagram, stability would be assured because beyond E, the limit of the required braking power area, 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. But for any change greater than the one shown without the machine becoming unstable. But for any increase in the accelerating power (P1 with the initial load angle remaining unchanged, there would be a reduction in (1 max. It would, therefore, be necessary to check every new condition to ensure that the area of accelerating power can be balanced by an area of braking power. If (1 max is exceeded the generator output would become less than the input and the power differential would be again positive. The rotor would, therefore, be subjected to further acceleration and instability would result.

In 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 time, holding all the other terms constant. In the above, the transfer impedance ZT was taken as the independent variable.

In this simplified introduction to the complex 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 subjected 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.

d. In 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 generators, quick acting protective gear, and the necessity to avoid large step changes in generator power input and loading. Item (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.

An extreme case of sudden load change in a generator is a 3-phase short-circuit close to the generator terminals. Under such fault conditions the terminal voltage would suffer sever drop and generator output would fall almost to zero. Hence, the full output from the prime-mover would be available to accelerate the generator rotor. For 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 extremely short. In 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. In the larger machines, however, 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.

REFERENCES

1. CEGB MANUAL VOL-IV

2. NHPC MANUAL

3. NPTI MANUAL ON GENERATOR

4. NTPC Manual

Unit P. F.

Lead P.F.

Lag P. F.

0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 0.85 0.8 0.7 0.6 0.4 0.3 0.2 0.1

FIELD CURRENT

FIG.3 SHOWING THE EFFECT OF FIELD CURRENT ON STATOR

CURRENT WITH TG RUNS AT DIFFERENT P. Fs.d2(

dt

STATOR CURRENT

d2(

dt2

PAGE 53

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210 Turbo Generator