――――――――――――――――――――――――――* 2-4, Suehiro-Cho, Tsurumi-Ku, Yokohama 230-0045

IMPACTS ON TURBINE GENERATOR DESIGN BY THE APPLICATION OF INCREASED THERMAL CONDUCTING STATOR INSULATION

M. TARI, K. YOSHIDA, S. SEKITO*, H. HATANO 　　　　　　　　　　　　　　　　 R. BRÜTSCH, A. LUTZ Toshiba Corporation 　　　　　　　　　　　　　　　　　　　　Von Roll Isola

Japan 　　　　　　　　　　　　　　　　　　　　 Switzerland

SUMMARYThe increase of power density (capacity /weight ratio)of indirect-cooled generators can be achieved byimproving the thermal conductivity of stator coilinsulation. Of several concepts, this is the most reliablefrom the viewpoint of a long lifetime. This newlydeveloped HTC (High Thermal Conducting) insulationsystem has achieved double the thermal conductivity byreplacing a part of the epoxy resin with a high thermalconducting filler, while keeping almost the samebreakdown and voltage endurance characteristicscompared to conventional insulation systems.By adopting this HTC insulation system, the maximumunit capacity of air/hydrogen indirect-cooled generatorscan be increased up to 400 MVA and 600 MVArespectively. Lower electrical losses in coils anddecreased windage loss due to a reduced cooling gascan contribute to high generator efficiency, because ofthe lower coil working temperature. An industrial use350 MVA class generator with HTC insulation showsremarkable reduction of coil temperature in shop tests.At rewinding of operating generators, this increasesgenerator output by 10 - 15 % without any change ofcoil design, except for the insulation system.

KEYWORDSTurbine - Generator - Stator - Winding – Insulation -Cooling - Thermal Conductivity - Control

1. INTRODUCTIONRecently, there have been rapidly increasing commercialapplications of electric power generating systemsincorporating gas turbines. Gas turbine drivengenerators require a simple structure, improvedoperationability, high reliability, low life cycle cost, andshort production period. Turbine generators withindirectly air/hydrogen-cooled stator coils have

experienced successful long-term operations, and theiroperationability and reliability have been certificated.And also, these advantageous features respond torequirements. In particular, the unit capacityenlargement of turbine generators with long and largeindirect-cooled stator coils is becoming a critical issue.This has been brought about by the recent demand forincreasing unit capacity of gas turbines that characterizefrequent start/stop operations and fast loading to fullpower. To achieve unit capacity enlargement whilemaintaining high reliability, the world’s first highthermal conducting (hereinafter called HTC)　statorinsulation system has been developed and it has beensuccessfully applied to industrial and large capacityturbine generators [1].This paper introduces the impacts on generator designby the application of increased thermal conductingstator insulation and its advantages.

2. BASIC CONCEPTS FOR UNIT CAPACITYENLARGEMENT

The unit capacity enlargement of indirect-cooledgenerators is limited mainly by the current capacity ofstator coils. While coil insulation is required to provideappropriate dielectric characteristics that are maintainedthroughout the life of generators, conventionalinsulations act as a thermal barrier. Therefore, thesekinds of insulation are a critical factor for generatorswith indirect-cooled stator design where the heatgenerated in the conductors is transmitted through theinsulation walls.In response to two consistent features, several designconcepts are proposed, as shown in Fig.1. These arereduced insulation wall, global vacuum pressureimpregnation (GVPI), HTC insulation and high thermalclass insulation. Coil temperature decrease along withincreased electrical stress as a result of reducedinsulation thickness is shown in Fig.2.

Less insulation thickness has a minor effect on loweringthe coil temperature, having a rather small impact ongenerator capacity change while increasing electricalstress. For example, even if the electrical stress isincreased 1.2 times of standard design, the coiltemperature reduction will remain only 5K while themachine life is shorten. On the contrary, the doubledthermal conductivity of stator insulation could ensurethe remarkable decrease of coil temperature more than10K.Meanwhile, GVPI system is a reasonable concept fromthe viewpoint of thermal transmitting in slot, becausethe initial coil/slot gaps are eliminated. This system iswell suited to small and medium capacity generatorswith rather short coils. However, for large generatorswith long and large coils, the initial coil/slot contactcondition seems rather difficult to maintain over anumber of years due to thermal expansions andshrinkages of the coils, resulting from frequent start/stopand adjustable load operations [2].For high thermal class generators, long stator coilsoperated at elevated temperature will have the harmfulinfluence on coil end support structure and slot packingsystem due to increased thermal expansion [3].Furthermore, insulation deterioration will be acceleratedby high thermo-mechanical stresses resulting inelectrical and mechanical wearing at rapid and transienttemperature changes during adjustable load and frequentstart/stop operations on gas turbine driven generators.

Apart from these design concepts, HTC insulationdesigned at F-class/B-rise is adopted with top prioritygiven to long-term reliability. The increased thermalconductivity of the main insulation effectivelysuppresses the coil temperature rise without any designchange including wall thickness. As shown in Fig.3,HTC insulation will reduce the temperature differencebetween the internal and the external surfaces of theinsulation layer. When the measured temperature is keptat the same level as the conventional design generators,the stator current could be larger and the generatorcapacity could be increased without changing frame size.On the other hand, if the stator current is kept as sameas the conventional generators and quantity of coolinggas is reduced in proportion to stator coil losses, theconductor temperature could be effectively downed asshown in Fig.4, resulting in higher generator efficiency.This design concept will provide remarkable designflexibility along with greatly improved design freedomrange.

3. CONCEPTUAL DESIGN OF HTC INSULATIONSYSTEM

Indirect-cooled coils generally operated at highertemperatures than direct-cooled ones must have higherreliability under thermal cycling.

4. High working temperature Accelerated thermo-mechanical deterioration

Decreased coil temp.3. Increased thermal conductivity

2. Global VPI insulation Changing coil slot contact

1. Reduced wall thickness Shorter machine life

Fig.1 Design concepts for upgrading/uprating

Fig.2 Coil temperature decrease with doubled thermalconductivity and increased electrical stress

Electrical stress (p.u.)

40

50

60

70

Coi

l tem

pera

ture

rise

(K)

1.0 1.40.8 1.2

Standard designConventionalHTC

80

Fig.3 Impacts of HTC insulation on generator

Tem

pera

ture

ETD

HTC insulation

Conventional

HTC

Therefore, development of HTC system is finallydecided to take into consideration of the maximumutilization of the same material composition and VPIprocess for half-turn coils as the existing F/B insulationsystem, whose reliability has been proven by manyapplications to indirect-cooled generators and long-termoperational experiences. This means that impregnatingresin, manufacturing process and coil assemblingprocedure would be kept as the same, giving substantialadvantages from the viewpoints of productivity andeconomy.The target of the newly developed HTC insulationsystem is to double the thermal conductivity of theinsulation, while preserving the well-provencharacteristics inherent in conventional ones (Fig.5).

Among the major compositions of conventionalinsulation, the thermal conductivity of resin is by far thelowest when compared with mica paper and glass cloth.Therefore, epoxy resin was to be partly replaced by ahigh thermal conducting filler. This improvementinvolving the modification of mica tape only is the mostadvantageous for the common use of existingmanufacturing facilities for the conventional insulationsystem. However, there was a requirement to make surethat modification of mica tape necessitated by adoptionof the filler would not influence any physical propertiesother than thermal conductivity.

However, for development, in order to prevent loss offiller during taping and to maintain impregnabilityduring impregnating, type of filler, filler grain size, fillerquantity and its arrangement have been optimizedthrough comprehensive experimental studies.

4. DEVELOPMENT AND EVALUATION OF HTCINSULATION SYSTEM

4.1 Evaluation tests on HTC mica tapeIn the course of mica tape development, the fillerquantity and the manufacturing process were optimizedto avoid any change of other characteristics. Conformitywith the existing coil manufacturing process in respectto tapingability, impregnability and compatibility withthe impregnating resin were confirmed.The thermal conductivity of the insulation is influencedwith taping tension, pressing process after impregnationand final physical dimensions in addition to mica tapequality itself [4]. Measurements of thermal conductivityon specimens taken from industrial use coils showedaround twice the conventional value and meet with thebasic concept (Fig.6), and the conductivity distributionalong the longitudinal direction also showed stablevalues.

To test the ductility of insulation with filler added, thesurface strain at bending rupture of the insulation layerswas measured. The results showed no change fromconventional insulation system and enough ductilityagainst excessive electromagnetic forces at the instanceof line faults (Fig.7).

Fig.5 Basic concept of HTC insulation system

3

10Voltage endurance life (p.u.)

0.01 0.1 10

1

2

Ther

mal

con

duct

ivity

(p.u

.)

TargetNot acceptable

Acceptable

Conventional

Fig.6 Thermal conductivity of HTC insulation coils

0

1

2

3

0 5 10 15 20 25 30 35 40Coil number

Ther

mal

con

duct

ivity

(p.u

.)

Fig.4 Simulated coil temperature distribution

0

10

20

30

40

50

60

Position in slot

Tem

pera

ture

Ris

e (K

) ConventionalHTC

ConductorCore Core

Ins. Ins.+Spacer

Furthermore, the mechanical strength of HTC insulationwas equivalent to that of the conventional insulation atworking temperature.

4.2 Evaluation tests on HTC insulation coilsHTC insulation coils were assembled into 600 MVAclass model slots, and heated by AC current to conductthermal cycling at 40-155 °C for evaluating long-termreliability as shown in Fig.8. The tanδ change of HTCinsulation coils after 1,000 thermal cycles was smallcomparing with initial figures, and the resistance againstrepeated thermal cycling was evaluated successfully asshown in Fig.9.

Furthermore, in order to evaluate dielectriccharacteristics after adding filler, voltage endurancetests were conducted on industrial use coils includinglarge size coils exposed to the thermal cycling discussedabove. The test results showed the same or bettercharacteristics compared with conventional insulationover a wide range of electrical stress, and met with thebasic concept (Fig.10).

4.3 Evaluation tests on HTC insulation generatorTemperature measurements were conducted on a 350MVA class hydrogen-cooled turbine generator withHTC insulation coils at shop tests. In comparison with aconventional insulation generator of the same frame size,HTC insulation generator showed a remarkablereduction in coil temperature.Not only was the coil surface temperature lower, butalso the conductor temperature measured by a specialmethod was much lower in comparison with theconventional generator. Furthermore, the temperaturedifference between the internal and the external surfacesof HTC insulation was found to be smaller as shown inFig.11.

In addition, sudden three phase short-circuit tests wereconducted to evaluate the resistance against excessive

Fig.8 Thermal cycling test of HTC insulation coils

Fig.9 Tanδ change due to thermal cycling

0

0.5

1.0

1.5

2.0

0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6Applied Voltage (p.u.)

Tan δ

(%)

InitialAfter 1,000 cycles

Fig.11 Temperature tests at rated stator currentthree phase short circuit

-4000 -2000 0 2000 4000 6000 8000 10000 12000 14000Duration (s)

0

20

40

60

80

100

Tem

pera

ture

rise

(K)

0 81624324048566472

Arm

atur

e cu

rren

t (kA

)Conductor temp. conv. ETD temp. conv. Gas temp. conv.Conductor temp. HTC ETD temp. HTC Gas temp. HTC

Armature Current

Fig.7 Surface strain at bending rupture test

0

0.5

1.0

1.5

0 40 80 120 160

Stra

in c

apab

ility

at f

ract

ure

(%)

Temperature (°C)

HTCConventional

Fig.10 Voltage endurance test

105100 101 102 103 1040.3

0.5

1.0

1.5

2.0

Time (h)

Elec

trica

l Stre

ss (p

.u.)

HTCConventional

1,500Hz

After 1,000cycles

electromagnetic forces. Detailed inspections on the endwinding insulation surface after the tests showed noabnormality.

5. ADVANTAGES OF HTC INSULATIONGENERATORS

The newly developed HTC insulation can improvethermal conductivity of insulation while maintainingelectrical stresses and coil temperatures at the samelevel as conventional insulation generators. Byappropriate combination of HTC insulation technologywith an improved generator ventilation system andoptimized frame structure, the maximum unit capacityof indirect-cooled generators is remarkably increased asshown in Fig.12.

The power density and efficiency of the generatorscould be attained at a level comparable with those ofwater-cooled generators. The increased power densityof HTC insulation generators is shown in Fig.13, and isfound to be equivalent to that of water-cooledgenerators.

HTC insulation generators with indirect-cooled coils donot need to provide cooling hydrogen gas or waterpassages in the coils, thus contributing to high reliability

because of the simplified coil structure. Moreover, nonecessity of auxiliary equipment such as a stator water-cooling unit will be profitable from the viewpoint ofeconomy.

6. DESIGN IMPACTS WITH HTC INSULATIONHTC insulation is found to reduce the temperaturedifference between the internal and the external surfacesof the insulation wall without any change of insulationthickness and coil structure. Therefore, stator currentcan be increased, resulting in uprating of the generator.For the same generator capacity, reduction in coil lossesand windage loss accompanied by reduced quantity ofcooling gas will improve generator efficiency.Furthermore, during operations at peak load specified tothe output characteristics of gas turbines, coil conductortemperatures are higher than at base load operations.With some applicable standards, the peak output of agenerator is restricted with the estimated maximumpermissible temperature of the conductors. Because thedifference between the maximum conductor temperatureand ETD coil temperature in HTC insulation generatoris decreased, its peak capacity can be increased morethan that of conventional insulation generators.In the case of increasing unit capacity with the sameframe size, HTC insulation generators offer highreliability because they can be designed for the sameelectrical stress and the same temperature asconventional generators.The fact that mechanical deterioration caused by rapidlyincreasing requirements for frequent start/stop andvarious load operations for load adjustment is nowgenerally recognized as the critical insulationdeterioration factor. This learnt to be resulted ininsulation deterioration accompanied by high coiltemperature, high electrical stress and rigid coilassembling in slots. It is also clear that the basic conceptof HTC insulation generators ensures long-termreliability under these operations.The various advantages achieved by HTC insulationtechnology are not limited to newly designed generators,but also apply to operating generators at rewinding ofstator coils as listed below:

• For the same generator capacity in kW- Larger apparent capacity (larger reactive power)- Improved generator efficiency

• For increasing generator capacity- Increased generator capacity with minimum design

change (10-15%)- Simplified cooling system (from direct-cooling to

indirect-cooling)- Reduced electricity power supplied to auxiliary

equipment by eliminating of coil coolant and itscooling unit

Furthermore, site rewinding by using half-turn coilswith VPI process allows shortened rewinding period forwhole winding, applicable partial rewinding, easyremoval of existing coils and no transportation of aheavy stator to the factory.

0

0.5

1.0

1.5

2.0

2.5

0 100 200 300 400 500 600 700 800 900Capacity P (MVA)

Out

put c

oeffi

cien

t C (p

.u.)

HTC insulation

Stator water-cooled

Indirectlyhydrogen-cooled

P=C・D2･L･n(n: rounds per minute)

Fig.13 Generator output coefficient

Statorwater-cooled

Hydrogen-cooled

Air-cooled

0 200 400 600 800 1000Unit capacity (MVA)

Olddesign

Fig.12 Enlarged capacity range for indirect-cooled generators

Olddesign

Newdesign

Newdesign

Newdesign

7. CONCLUSIONThe newly developed HTC insulation is compatiblewith well-established coil manufacturing processes, andalso applicable to proven design at the same electricalstress and coil temperatures as much experiencedconventional insulation generators. Furthermore, theapplications of such advanced HTC insulationremarkably increase the maximum unit capacity ofindirect-cooled turbine generators, providing high totalcost merits from the viewpoints of design,manufacturing and efficiency.Stator rewinding of operating generators enablesincreased generator capacity and improved efficiencywithout any change of coil design, except for theinsulation system.The newly developed HTC insulation system canminimize design limitations as discussed above, andprovide distinguished flexibility in the design stage.Various requirements from customers could be surelyresponded because of the improved aspects of thistechnology. This advanced technology would guaranteemuch profit to customers.

8. REFFERENCES[1] M. Tari, K. Yoshida, S. Sekito, R. Brütsch, J. Allison,A. Lutz, “HTC Insulation Technology Drives RapidProgress of Indirect-Cooled Turbo Generator UnitCapacity”, IEEE PES Summer Meeting, Vancouver, July2001.[2] G. Griffith, S. Tucker, J. Milsom and G. Stone,“Problems with Modern Air-Cooled Generator StatorWinding Insulation”, IEEE Electrical Magazine, Vol.16,No.6, pp6-10, 2000.[3] R. E. Joho, “Study of Stressing Turbo generatorsbeyond their Established Thermal Limits”, CIGRE SC11Meeting Preliminary Report, Stockholm, June 2001.[4] M. L. Miller, F. T. Emery, “Thermal Conductivity ofHigh Voltage Stator Coil Groundwall Insulation”, EICConference, Chicago, October 1997.