Mitsubishi Heavy Industries Technical Review Vol. 54 No. 3 (September 2017)
39

Development of Large-capacity Indirect
Hydrogen-cooled Turbine Generator andLatest Technologies Applied to After Sales Service
KAZUHIKO TAKAHASHI*1

MITSURU ONODA*1

KIYOTERU TANAKA*2
SEIJIRO MURAMATSU, P.E.*3

In terms of environmental problems and power generation costs, an increase of the capacityof indirect hydrogen-cooled turbine generators with high efficiency and good maintainability arerequired. Mitsubishi Hitachi Power Systems Co., Ltd. (MHPS) developed High HeatTransmission®, which improves the heat transmission rate of the stator coil insulation byapproximately three times higher compared to conventional one produced with existing technology,
and the feasibility of a 900 MVA class indirect hydrogen-cooled turbine generator, the basic designof which has been completed by actual equipment verification using a 600 MVA class generator,
was confirmed. High Heat Transmission® can be applied not only to new generators, but also toexisting generators, and it can be expected to contribute to an increase of output and the resolutionof problems with generators experiencing overheating of stator coils. We are also promoting theapplication of newly developed technologies to the generator service field, such as inspection andmonitoring technologies of generators.

|1. Introduction
In recent years, the reduction of CO2 emissions and the stable and low-cost supply of electric
power are required to suppress global warming. MHPS is promoting development for an increaseof the capacity of indirect hydrogen-cooled turbine generators with high efficiency and good
maintainability.1 For this development, we implemented higher cooling, lower losses, and lower
vibration design using various design tools such as fluid analysis on the scale of hundreds ofmillions of elements and through vibration analysis with electromagnetic field analysis. We havealso developed High Heat Transmission®, which improved the cooling performance of the statorcoil, and have currently completed factory rotation performance testing using a 600 MVA classgenerator equipped with High Heat Transmission®, where good test results were obtained. In
addition, this paper presents an example case where High Heat Transmission® is applied to an
existing generator. Furthermore, this paper introduces the latest technology lineup in servicing forexisting generators such as insulation diagnosis technology and stator wedge looseness diagnosistechnology.

|2. Technology for increase of capacity of indirect hydrogen-cooled
turbine generator
MHPS selects a cooling system suitable for the capacity band of the generator.
Conventionally, we have used an air cooling system for the small-capacity band (up to 300 MVA),
an indirect hydrogen cooling system for the medium-capacity band (200 to 500 MVA), and a direct
water cooling system for the large-capacity band (500 to 1600 MVA).1 Compared with the direct
water cooling system, the indirect hydrogen cooling system has advantages in terms of operational
performance and maintainability because of the elimination of the need for a stator cooling water
system and its piping. It also has the advantage of better efficiency resulting from lower losses dueto the increased conductor area because hollow conductors for cooling stator coils with water are
not used. MHPS is working on increasing the capacity of indirect hydrogen-cooled turbine
generators through higher cooling, lower losses, and lower vibration design using large scale
analysis and design tools for which the calculation accuracy has been verified1, as well as thedevelopment of High Heat Transmission®, which enhances the cooling performance of stator coils.
*1 Engineering Manager, Power Electric Machine Design Department, Turbomachinery Headquarters, Mitsubishi Hitachi Power Systems, Ltd.
*2 Hitachi Power Systems Service Department, Power Systems Service Headquarters, Mitsubishi Hitachi Power Systems, Ltd.
*3 Engineering Manager, Hitachi Power Systems Service Department, Power Systems Service Headquarters, Mitsubishi Hitachi Power Systems, Ltd.

Mitsubishi Heavy Industries Technical Review Vol. 54 No. 3 (September 2017)40

Figure 1 shows the fluid analysis from the ventilation fan to the periphery of the stator coilend basket using hundreds of millions of elements as an example of large scale analysis. The
analysis results are used for the design study to optimize the structure and to reflect the parametersof the design. Due to the increased generator capacity, the electromagnetic force at the stator coilend increases, so it is necessary to strengthen the support of the stator coil end. For the design ofthe support structure, frequency response analysis with electromagnetic force analysis at the stator
coil end is applied.
Figure 2 shows an example of frequency response analysis with electromagnetic force
analysis. The electromagnetic force analysis is intended for the connection ring side consisting of
the stator coil end and the connection ring, and can simulate various operating conditions of thegenerator because the analysis model includes the stator core and the rotor. Figure 2 shows theanalysis results at the rated load (results of a certain moment) and the electromagnetic forces of thestator coil end and the connection rings are calculated for one cycle and input as a load to thefrequency response analysis. Using these analysis results, the support structure of the stator coil end
is designed so that the vibration value and the stress value of each part are suppressed below thedesign limit.
In the repair or part replacement of generators manufactured by other companies, the design
data of which are difficult to obtain completely, these analysis and design tools estimate detailed
design specifications by comparing the obtained data with the analysis results. In addition, thesetools are used for designing performance improvements based on this estimation.
Actual equipment verification of High Heat Transmission® is described in detail in the nextchapter.

Figure 1 Fluid analysis from ventilation fan toperiphery of stator coil end (flow line distribution)

Figure 2

Frequency response analysis with electromagnetic force analysis at stator coil end

Mitsubishi Heavy Industries Technical Review Vol. 54 No. 3 (September 2017)41|3. Actual equipment verification of High Heat Transmission® on
600 MVA class generator
High Heat Transmission®, which improves the stator coil cooling performance, is a key
technology among the technologies for increasing the capacity of indirect hydrogen-cooled turbine
generators. Here are shown the results of verification of the cooling performance using actual
equipment rotation tests performed with a 600 MVA class turbine generator equipped with MHPS's
proprietary High Heat Transmission®.
The stator coil has a high voltage of several tens of kV and the coil is protected with aninsulation layer. As shown in Figure 3, the heat generated in the strand conductor of the stator coilof the indirect hydrogen-cooled turbine generator is cooled with hydrogen gas through theinsulation layer. The thermal conductivity of the insulation layer is about one-thousandth of that of
the strand conductor, and therefore the improvement of the thermal conductivity of the insulation
layer enhances the cooling performance of the stator coil. High Heat Transmission® improves theheat transmission rate of the insulation layer by about three times higher by adding high thermal
conductive micro filler to the insulation layer. To date, the evaluation of the thermal, electrical, andmechanical basic characteristics and long-term reliability of High Heat Transmission® has been
completed2, and now High Heat Transmission® is in the phase of application to actual equipment.
The heat transmission rate is a parameter indicating the cooling performance obtained by dividing
the thermal conductivity by the insulation thickness of the stator coil. Because the cooling
performance is enhanced not only by the improvement of the thermal conductivity of the insulation,
but also further through the reduction of the insulation thickness due to the improvement of theelectrical insulation characteristics, it is evaluated not by the thermal conductivity, but also by theheat transmission rate which includes the effect of both thermal conductivity and the thickness ofthe insulation layer.

Figure 3 Cross section of stator coilFigure 4 Actual equipment verification test of High Heat Transmission®

To verify the cooling performance when High Heat Transmission® is applied to alarge-capacity indirect hydrogen-cooled generator, an actual equipment verification test was carriedout using a 600 MVA class turbine generator equipped with High Heat Transmission® (Figure 4).
Before the actual equipment verification test, the improvement of the cooling performance of HighHeat Transmission® was predicted by network analysis3 (analysis that models the ventilation andMitsubishi Heavy Industries Technical Review Vol. 54 No. 3 (September 2017)42

the heat transfer path in a generator using a network). The analysis results are indicated by the solidline and the dotted line in Figure 5. With this design, it is expected that the strand temperature riseof the stator coil of High Heat Transmission® can be reduced by about 40% in comparison with thatof conventional insulation. The actual measurement values of the strand temperature rise of thestator coil obtained by the actual equipment verification test are indicated by the circles inFigure 5. The measured values of the temperature rise roughly correspond to the analysis values atboth the inside of the stator slot and the stator coil end, and it was verified that the stator coil ofHigh Heat Transmission® has cooling performance as predicted by the design.

Figure 5 Analysis of stator coil strand temperature and measurement results

From the above, the feasibility of the 900 MVA class indirect hydrogen-cooled turbine
generator1 was confirmed and the improvement of the cooling performance due to High HeatTransmission® was verified.

|4. Application of technologies to service field
MHPS has been actively applying the latest technologies developed for new generators to thegenerator service field, and has developed inspection and monitoring technologies unique to theservice field. Table 1 shows the latest technologies applicable to the generator service field. Thissection presents the items marked with an asterisk in Table 1, including an example in which thedeveloped High Heat Transmission® was applied to an existing generator to reduce the stator coiltemperature.
Table 1 Latest technologies for generator service
No.

Kind of technology

1

Temperature reduction and reliability enhancement ofstator coil2

Quantitative evaluation of remaining insulation life,Optimization of part replacement timing

3

Increase of generator capacity

4

Repair of generators manufactured by other companies

Item*High Heat Transmission®
*Optimization of stator strand arrangement
Inner corona shield (enhancement of insulationcharacteristics)*Diagnosis of stator coil insulation and quantitativeevaluation of remaining insulation life
*Digital Wedge Tapper®
(Diagnosis of stator wedge looseness)
Stator coil end elbow made of stainless steel
Structure for enhancement of generator terminal
cooling
Structure for reduction of stator coil end vibration*Check of interference and fitting of parts using 3Dscans and 3D CAD

4.1 Technologies for temperature reduction and reliability enhancement of stator coil
By reducing the temperature of the stator coil of an existing generator, advantages such as anextension of the insulation life and enhancement of the generator output are expected. Therefore,
even in existing generators, stator coil rewinding has been performed using the stator coil to whichHigh Heat Transmission® and/or the optimization of the strand arrangement are applied, aiming attemperature reduction. For example, we expect an indirect hydrogen-cooled generator of 160 MVA
to reduce the strand temperature of the stator coil by about 40% through the use of High HeatTransmission® and the optimization of strand arrangement. Technologies for optimizing the strand

Mitsubishi Heavy Industries Technical Review Vol. 54 No. 3 (September 2017)43

arrangement include the reduction of the circulating current by improving the strand transposition
angle in the slot from 360° to 540° 4 and the reduction of the eddy current by reducing the strand
thickness.
The effect of High Heat Transmission® is more significant for indirect hydrogen-cooled
turbine generators in comparison with air-cooled turbine generators. This is because the gas
temperature rise of an indirect hydrogen-cooled turbine generator is smaller than that of an
air-cooled turbine generator, and the thermal resistance of the main insulation is dominant over thestator coil temperature rise. For some existing generators, it is estimated that their strand
temperature may exceed the limit temperature according to re-engineering results. In this example
case of the replacement of the stator coil in an existing turbine generator, the strand temperaturerise is estimated to be reduced by 25% due to the application of High Heat Transmission® and thestrand arrangement optimization technology, even though the generator is air-cooled, and thetemperature is expected to be within the limit value. The results of the temperature study using
network analysis are shown in Figure 6 .
With this technology, it becomes possible to improve the performance including thereduction of the temperature, the enhancement of reliability, and the increase of the output simply
by replacing parts such as stator coils without significantly modifying the generator. High HeatTransmission® has been applied not only to various air-cooled and indirect hydrogen-cooled
equipment for thermal power generation, but also to equipment for hydraulic power generation.

Figure 6 Analysis results of stator coil temperature

4.2 Diagnostic technology of stator coil insulation
The insulation layer of the stator coil of the generator is used in an environment where highvoltage, high temperature, and vibrations such as electromagnetic force of the coil are applied. The
insulation layer is one of the main components that determine the reliability of the generator,
because if the insulation layer deteriorates and a ground fault occurs, the generator is tripped. In
addition, the deterioration diagnosis and the remaining life evaluation are important technologies
for the evaluation of the reliability of the generator. The stator wedge is used to firmly fix the stator
coil within the stator slot. Loosening of the stator wedge may cause wear of the insulation layer andresult in insulation breakdown, so the diagnosis of the looseness of the stator wedge in theinsulation layer of the stator coil is important. This section introduces the remaining insulation life
evaluation method and the stator wedge looseness diagnosis technology.
To accurately evaluate the remaining life of the stator coil insulation, the test results ofaccelerated deterioration model bars and actual machine sampling bars were compiled in a databaseand a new quantitative evaluation method of remaining life based on this database was developed.5
As shown in Figure 7 , it was confirmed by a comparison between the values of the remaining
insulation strength estimated using this quantitative evaluation method and the actual values
measured by the test that the accuracy was sufficient to evaluate the remaining insulation life. In
addition, the remaining insulation life of the developed High Heat Transmission® can be evaluatedMitsubishi Heavy Industries Technical Review Vol. 54 No. 3 (September 2017)44

using this method. The remaining insulation life evaluation method has already been applied toactual generators.

Figure 7 Comparison between values estimated by remaining
insulation life evaluation method and measured values

The state of looseness, etc., of the stator wedge is normally evaluated by a sensory test
(hammering sound test) performed by a specialist. Because this is a sensory test, however, a subtle
difference in judgment between specialists may occur, and it was difficult to create a database of
the inspection results without personnel differences. As such, we developed a wedge loosenessdiagnostic device (Digital Wedge Tapper®) that mechanizes the hammering sound test. Figure 8shows the stator wedge looseness diagnosis using Digital Wedge Tapper® and the inspection
results. By performing quantitative evaluation of wedge looseness diagnosis using this device,
variations in judgment between specialists can be eliminated, and the creation of a database has
become easy. Therefore, it is now possible to accurately evaluate the replacement timing of statorwedges.

Figure 8

Stator wedge looseness diagnosis (Digital Wedge Tapper®)

4.3 Technology for repair of generators manufactured by other companies
MHPS is working on the development of technologies for the repair of generators
manufactured by other companies to meet the repair needs of customers, who possess generators
manufactured by other companies that have technical issues. In most cases, design drawings andinformation on such generators that are necessary for repair work cannot be obtained, and fitting toexisting equipment is particularly important. Therefore, we perform 3D measurement using 3Dscans on site, and create a 3D model using 3D CAD in the design process to check for interference.
As target parts, those with complicated shapes such as connection rings and stator coil end
supports, as well as stator coils, can be designed with high accuracy by using three-dimensionalmodels. We have already delivered and installed such parts at sites. Figure 9 shows an example
case of renewing the stator coil of a generator manufactured by other companies. MHPS does not
simply repeat the existing design even for a component such as stator coil. We performMitsubishi Heavy Industries Technical Review Vol. 54 No. 3 (September 2017)45

comprehensive evaluation based on the electrical design, ventilation and temperature design, as
well as the strength and vibration design of the entire generator, and then design, manufacture andsupply parts suitable for the generator.

Figure 9

Replacement of stator coil of generator manufactured by other companies|5. Conclusion
MHPS confirmed the feasibility of 900 MVA class indirect hydrogen-cooled turbine
generators in a temperature verification test using a 600 MVA class generator equipped with HighHeat Transmission®. In addition, we also promote the application of the developed technologies
such as High Heat Transmission® to existing generators, and the improvement of the performance
characteristics such as temperature reduction, the enhancement of reliability and an increase of theoutput are made possible by the replacement of parts including the stator coil, etc. In addition,
MHPS's other latest technologies for the service field have been applied to actual generators andcontribute to the enhancement of reliability.

References
1.
2.

3.
4.
5.

Satake, Y. et al., Development of Large Capacity Turbine Generators for Thermal Power Plants，Mitsubishi Heavy Industries Technical Review Vol. 52 No. 2 (2015)
Onoda, M., et al., Development of High Heat Transmission (HHT) Insulation System for Large Generator
Stator Winding，2016 Annual Conference of Fundamentals and Materials Society IEE Japan，6A-p2-6,(2016)
Hattori, K. et al., State-of-the-art Technology for Large Turbine Generators, CIGRE SC A1 2008 PanelSession (2008)
Takahashi, K. et al., Strand Current Distributions of Turbine Generator Model Coils，IEE Japan, RM-01-154(2001)
Tanaka, K. et al., Prediction of Residual Breakdown Electrical Field Strength of Epoxy-Mica Paper
Insulation Systems for the Stator Winding of Large Generators, IEEE Trans on Dielectrics and ElectricalInsulation, Vol.22 Issue 2 (2015) pp. 1118-1123