Mitsubishi Heavy Industries Technical Review Vol. 58 No. 3 (September 2021)
1Development and Verification of 350 MVA Class
Large-Capacity Air-Cooled Generators

TAKAHIRO MIZUNO*1TADAAKI KAKIMOTO*2

HIROHIDE MURAYAMA*2 SEIJIRO MURAMATSU*3
KAZUHIKO TAKAHASHI*3 YUKIHIRO YAMAMOTO*3

With the rise of renewable energy, there has been increasing demand for gas turbines thatcan start rapidly and follow the fluctuating load as peak power sources. As a solution to meet thedemand, Mitsubishi Power, Ltd. (Mitsubishi Power) offers sets of 300 MW class gas turbine powergeneration equipment. In some cases, the generator used for the equipment may be desired to bean air-cooled generator that is easy to operate and maintain and we have been developing a 350
MVA class air-cooled generator. This time, we have confirmed that all the performances of thedeveloped generator satisfy the standards by conducting actual equipment verification. Thisgenerator has achieved a capacity increase of about 1.5 times that of Mitsubishi Power’s
conventional air-cooled generators.

|1. Introduction
In the global trend toward decarbonization, the movement to utilize renewable energy isaccelerating. On the other hand, solar power generation and wind power generation have problems
with output fluctuations, thus there has been increasing demand for gas turbines that can start
rapidly and follow the fluctuating load as a peak power source that adjusts the balance between
supply and demand.
We offer sets of 300 MW class gas turbine power generation equipment. For this capacity
class, hydrogen indirect cooling generators are generally used. In some cases, however, air-cooled
generators that are easy to operate and maintain may be desired in some cases.
We proceeded with the development of a 350 MVA class air-cooled generator and haverecently completed actual equipment verification. This report describes the capacity increasing
technology applied to the generator and its verification, as well as the shop test results.

|2. Cooling method for turbine generators
Since the amount of heat generated inside a turbine generator increases as the capacity of thegenerator increases, methods for cooling the stator coil, including air cooling, hydrogen indirect
cooling and water direct cooling, have been developed depending on to the capacity in order to deal
with larger capacities.
Figure 1 shows the relationship between the stator coil cooling method of turbine generatorsthat our manufactured and the generator capacity. Generally, the small capacity class (up to 230MVA) is dealt with by the air cooling method, the medium capacity class (200 MVA to 750 MVA)
by the hydrogen indirect cooling method and the large capacity class (500 MVA to 1400 MVA) by
the water direct cooling method.
We developed a 350 MVA class air-cooled generator (hereinafter referred to as thedeveloped generator) and have recently completed actual equipment verification and confirmed that
all the performances of the developed generator satisfy the standards. The developed generator has
achieved a capacity increase of about 1.5 times that of our conventional air-cooled generators*1

Power Electric Machine Design Department, Mitsubishi Power, Ltd.

*2

Manager, Power Electric Machine Design Department, Mitsubishi Power, Ltd.

*3

Chief Staff Manager, Power Electric Machine Design Department, Mitsubishi Power, Ltd.

Mitsubishi Heavy Industries Technical Review Vol. 58 No. 3 (June 2021)
2(hereinafter referred to as the preceding generator).
Compared to other cooling methods, the air cooling method features the advantages of asimpler system configuration, better maintainability and higher safety due to non-use of hydrogen(Figure 2). This time, we succeeded in increasing the capacity of the air-cooled generator, which
resulted in more options for our customers.

Figure 1 Relationship between stator coil cooling method and generator
capacity

Figure 2 Comparison of system configurations between
differences in generator cooling methods|3. Completion of 350 MVA class large-capacity air-cooled generator
3.1 Design policy
The capacity P of a generator is generally expressed by the following formula.
P ∝ K × D2 × L × N(P: Generator capacity, K: Output coefficient (electrical loading, magnetic loading, etc.), D: Stator
core inner diameter, L: Stator core length and N: Rotation speed)
When the rotation speed N is the same, the capacity of the generator can be increased byincreasing the output coefficient K and increasing the size of the generator (D2 x L).
Table 1 compares the specifications between the preceding generator and the developedgenerator. In order to achieve a 50% increase in the capacity compared to the preceding generator,
the design policy of increasing the output coefficient by 20% and the size by 27% was set.
In order to achieve this, it was important to improve the cooling performance and reduce theloss that occurred in various areas of the generator, so we applied a wide range ofcapacity-enhancing technologies. In addition to these, evaluation of design details such as strengthand vibration for the structure of various areas were carried out and the feasibility of the design wasconfirmed.

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Table 1: Comparison of main specifications between 350 MVA class air-cooledgenerator and preceding generator
Preceding generator

Developed generator

Highest-capacity air-cooled generator in the past

350 MVA class air-cooled generator

Cooling medium

Air

Air

Capacity (MVA)

230

349

Voltage (kV)20

21

Current (A)

6 640

9 595

Power factor

0.9 (lagging)

0.85(lagging)

Frequency (Hz)60

60

Rotation speed (min-1)

3 600

3 600

Capacity (p.u)

1.00

1.52

Output coefficient K (p.u)

1.00

1.20

Size D2 x L (p.u)

1.00

1.27

3.2 Applied technologies and verification
In order to increase the capacity and efficiency of the air-cooled generator, various design
technologies were applied to the developed generator (Figure 3). As examples of these, specificitems are described below.

Figure 3

350 MVA class air-cooled generator(1) Basic design set by multi-objective optimization
For the developed generator, we applied efficiency enhancing technologies used in thedevelopment of hydrogen indirect cooling generators in order to reduce internal loss. One suchtechnology is the basic design of the generator set by multi-objective optimization.
Generator loss is roughly divided into mechanical loss, iron loss, direct load loss, fieldcoil resistance loss, stray load loss, etc. Each loss is closely related to the electromagnetic
properties of the generator and to important design factors such as cooling, mechanical strengthand electrical insulation and cannot be reduced independently in many cases.
The technology for selecting the optimum design solution in this complicated trade-off
relationship is the conventionally developed multi-objective optimization calculation system(1).
This system proposes a rational and objective optimal design plan by conducting, incombination with applying a genetic algorithm, a large-scale parameter survey including
simultaneous calculations for cost-evaluating the material cost, manufacturing man-hours, etc.,
in addition to technical evaluation of the generator characteristics, efficiency, cooling,mechanical strength, etc. Figure 4 gives an evaluation of the relationship between efficiency
and cost as an example of multi-objective optimization calculation.
We performed optimization calculations for the developed generator using this system toselect a basic design plan superior in terms of cost, while also achieving the required efficiency.

Mitsubishi Heavy Industries Technical Review Vol. 58 No. 3 (June 2021)
4Figure 4

Example of multi-objective optimization calculation(2) Improvement of cooling performance
If the temperature inside the generator increases and the insulation of the coil is exposed
to high temperatures for a long period of time, the insulation performance degrades due tothermal deterioration, which may ultimately lead to dielectric breakdown. Therefore, generators
are designed so that the temperatures of its various areas remain within the limit value.
Air has a lower specific heat and thermal conductivity than these of hydrogen, so its
function as a refrigerant is inferior. In addition, the high density of air leads to the increase of
mechanical loss, which leads to a significant cooling air temperature rise at the internal fan andthen causes the problem where the temperature of the refrigerant increases. In consideration ofthese issues, it is necessary to improve the cooling performance in order to realize alarger-capacity air-cooled generator.
Therefore, we adopted for the developed generator the inner cooler cooling method,which has been applied to actual equipment since 1998 and has accumulated many
achievements. Figure 5 is the airflow circuit of the inner cooler cooling method. The cooling
air is boosted by the fan, cools the structure such as the shaft end coil with part of it and thenheads for the inner cooler. In the case of an air-cooled generator, the air temperature has risen at
this point and it is re-cooled by the inner cooler. After that, the air that cooled the stator joins
together with the cooling air discharged from the rotor and finally returns to the main cooler. As
described above, the inner cooler method characteristically has two or more coolers in series ina single airflow loop and has a feature that the cooling air can be introduced to the desiredplace. We also adopted a duct structure that does not easily cause drift and a design that allows
the cooler area to be effectively used was made.

Figure 5

Inner cooler airflow circuit

Mitsubishi Heavy Industries Technical Review Vol. 58 No. 3 (June 2021)
5(3) Reduction of stator core end temperature
As shown in Table 1, the electric current through the developed generator is greatercompared to that through the preceding generator. Since the loss caused by end leakage
magnetic flux increases due to the greater current, it is necessary to evaluate overheating of thestator core end. The stator core end of the large-capacity generator has a structure in which acore stepping and a copper shield plate are installed and their shapes are optimized to reduce
the end leakage magnetic flux. In adopting this structure for the developed generator, magnetic
field analysis and temperature analysis of the stator end using the three-dimensional finite
element method were carried out and it was confirmed that the temperature of the stator coreend was within the limit value. Figure 6 depicts the magnetic field analysis model of the stator
core end and the result.

Figure 6

Magnetic field analysis of stator core end(4) Reduction of vibration at stator coil end
Since the electromagnetic force at the stator coil end increases as the current increases, itis necessary to have a resonance preventing structure with which the natural frequency of thestator coil end does not coincide with the electromagnetic vibration frequency during rated
operation (twice the rated frequency). The support structure was designed using
electromagnetic force-vibration response analysis (Figure 7) so that the stress value of various
areas was less than the permissible value and using natural frequency analysis so that thenatural frequency was sufficiently far from the electromagnetic vibration frequency.

Figure 7 Electromagnetic force-vibration response
analysis of stator coil end(5) Evaluation of shrink-fitting stress of retaining ring at rotor coil end
The rotor body has slots and the rotor coil winding are inserted therein. In order toprevent the winding from coming off due to centrifugal force, a rotor wedge is inserted into theMitsubishi Heavy Industries Technical Review Vol. 58 No. 3 (June 2021)
6slot. For the rotor coil end where the winding are not held in the slots, a retaining ring that isstrong enough to withstand the centrifugal force is shrink-fitted. It is necessary to determine ashrink-fit margin sufficient enough to prevent the ring from coming off the rotor during therated rotation in which centrifugal force occurs.
The stress distribution differs depending on the presence or absence of centrifugal force
between a stationary state and during rated rotation. As a result, in the case of peak power
supply causing a large number of starts and stops, repeated stress occurs and sufficient strength
against fatigue fracture is required.
The developed generator was designed to satisfy the fatigue strength by evaluating thestress amplitude of various areas (Figure 8) using the three-dimensional finite element method
to review the structure of the shrink-fitted area as a stress concentration relaxation measure.

Figure 8

Stress analysis of rotor end(6) Temperature evaluation under various starting conditions
For starting a gas turbine, thyristor starting, which drives a turbine generator as anelectric motor, is used. When a gas turbine is used as the peak power source, special operation
modes such as rapid start-up at a speed that is about 5 times higher than usual and continuous
turning are required. For the developed generator, the temperature of various areas inside thegenerator was evaluated under these starting conditions to confirm the integrity at thyristor
starting.
As an example, Figure 9 gives the analysis result of the rotor body temperature in thethyristor starting mode using the finite element method. The current waveform flowing throughthe stator coil at thyristor starting contains multiple harmonic components, which form anasynchronous magnetic field with respect to the rotor. Since the rotor of a turbine generator isconductive, an eddy current is induced on the rotor surface when an asynchronous magnetic
field is generated, causing a loss. The developed generator uses a structure that considers
thyristor starting, including the insertion of a damper winding in order to avoid the occurrenceof local overheating due to eddy currents.

Figure 9Analysis of rotor body temperature in thyristor startingMitsubishi Heavy Industries Technical Review Vol. 58 No. 3 (June 2021)
7(7) Actual equipment verification by shop rotation test
After manufacturing and assembling at the shop, a rotation test was conducted to verify
the performance. We measured generator parameters such as reactances and time constants inaddition to basic characteristics such as no-load saturation characteristics and three-phaseshort-circuit characteristics and confirmed that the specifications and the requirements of thestandards were satisfied.
We also installed temperature sensors and vibration sensors in various areas of thegenerator to measure and evaluate the integrity. Figure 10 presents the evaluation of the stator
coil temperature as an example. As shown in this figure, the measured maximum temperatureand the calculated value of that area were in good agreement and the high accuracy of
calculation and prediction was confirmed, as was the integrity.
In order to verify the efficiency enhancing technologies, we performed various loss
measurements according to the measurement method specified in the standards and calculated
the conventional efficiency. Table 2 compares the efficiency of the developed generator withthat of the preceding generator. The developed generator achieved a high efficiency of 98.82%,
which is equal to or higher than that of the preceding generator, while also achieving a large
capacity, thereby confirming the effectiveness of the applied technologies. In addition, since thedeveloped generator uses a non-oriented electromagnetic steel sheet as the core material, it isexpected that further loss reduction and efficiency enhancement can be realized by using ahigh-grade electromagnetic steel sheet.

Figure 10 Comparison of measured maximum temperatureand calculation result of stator coil
Table 2Comparison of conventional efficiency between 350 MVA class air-cooled
generator and preceding generator
Preceding generator

Highest-capacity air-cooled generator in thepast
Conventional efficiency(％)
98.81
Core material

Oriented electromagnetic steel sheet

Developed generator
350 MVA class air-cooled generator
98.82
Non-oriented electromagnetic steel sheet|4. Conclusion
We completed the development and actual equipment verification of a 350 MVA classair-cooled generator and confirmed that all the performances satisfied the standards. For thedevelopment, we adopted a method to improve the cooling performance and applied efficiency
enhancing technologies that are also applied to hydrogen indirect cooling generators. In addition,
for the detailed design evaluation of various areas, we evaluated the design feasibility using
analysis techniques such as the three-dimensional finite element method.
In order to verify the applied technologies, we performed actual equipment verification
through shop rotation tests. As a result of the verification, in addition to satisfying the requirements
such as the specifications and standards, the developed generator achieved a high efficiency of98.82%, which is equal to or higher than that of the preceding generator.

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Going forward, we will further expand the lineup of air-cooled generators and expand theirscope of application.

References(1)

Hirohide Murayama et. al, Efficiency Enhancement and Actual Machine Verification of Indirect
Hydrogen-cooled Turbine Generators, Mitsubishi Heavy Industries Technical Review Vol.56 No.3 (2019)