Mitsubishi Heavy Industries Technical Review Vol. 60 No. 1 (March 2023)
1

Next-Generation Gas Turbine Controlfor High Ramp Rate and Stable Combustion
MAKOTO KISHI*1

RYUJI TAKENAKA*2

RYOICHI HAGA*2

KAZUSHIGE TAKAKI*3

SHUHEI TODA*4

KOSHIRO FUKUMOTO*4

In order to deal with power output fluctuations caused by the increase in renewable energy,
gas turbines for power generation are required to handle high ramp rates more than ever before.
To enable high ramp rates, it is essential to achieve stable combustion under transient conditions
and to improve controllability of the power output and turbine inlet temperature. In addition,
keeping the turbine inlet temperature high even during transient conditions enables higher
efficiency and lower CO2 emissions. To realize these goals, Mitsubishi Heavy Industries, Ltd.
(MHI) has developed a next-generation gas turbine control that combines a turbine inlet
temperature estimation method based on a physical model and model predictive control thatperforms anticipatory control. This report outlines this technology and describes its verification
status at our power plant validation facility.

|1. IntroductionBeginning with the adoption of the Paris Agreement at the 21st Session of the Conference ofthe Parties to the United Nations Framework Convention on Climate Change (COP21) in 2015, themovement toward carbon neutrality has accelerated worldwide. The United States, the EU, andJapan have declared their goal of achieving carbon neutrality by 2050. In response to this trend, theshare of natural energy, including renewable energy, in the total electricity generated in Japanexceeded 20% in 2021, which was a significant increase from 12% in 2014. The increased use ofrenewable energy contributes to CO2 reduction. However, the amount of electricity generated using
renewable energy depends on environmental conditions from time to time and is subject to large
fluctuations, which is a disadvantage. In order to absorb these fluctuations in power generation,
thermal power plants are required to operate flexibly such as high ramp rates, and the gas turbine
combined cycle (GTCC), which emits the least amount of CO2 among thermal power plants, isrequired to realize higher ramp rate operation than ever before.
On the other hand, stable operation of a gas turbine, one of the main components of a GTCC,
requires precise control. If the fuel and air input balance is not appropriate, there is a risk of causing
serious equipment damage due to combustion instability or temperature limit exceedance. Whenthe ramp rate is increased, it becomes difficult to achieve stable operation with proper control offuel and air. Therefore, we have developed a next-generation gas turbine control based on model
predictive control.
The following chapter 2 provides an overview of the next-generation gas turbine control,
chapter 3 describes the simulation and verification status at the power plant validation facility, andchapter 4 provides a summary.

*1Manager, CIS Department, Digital Innovation Headquarters, Mitsubishi Heavy Industries, Ltd.

*2Engineering Manager, Takasago Plant Engineering Department, GTCC Business Division, Energy Transition & Power*3CIS Department, Digital Innovation Headquarters, Mitsubishi Heavy Industries, Ltd.

*4Gas Turbine Engineering Department, GTCC Business Division, Energy Transition & Power Headquarters, Energy Systems,Headquarters, Energy Systems, Mitsubishi Heavy Industries, Ltd.

Mitsubishi Heavy Industries, Ltd.

Mitsubishi Heavy Industries Technical Review Vol. 60 No. 1 (March 2023)
2|2. Next-generation gas turbine control
Figure 1 shows the configuration of our large stationary gas turbine. The gas turbine consistsof the main components of a compressor, combustor, and turbine. Compressed air is combusted togenerate over 1,600°C and over 2 MPaG, high-temperature and high-pressure combustion gas,
which is then converted into power by the turbine.
The existing gas turbine control method and the next-generation gas turbine control method
currently under development are overviewed below.

Figure 1

2.1(1)

Configuration of stationary gas turbine

Overview and issues of existing gas turbine control method
Existing gas turbine control method
During normal operation, gas turbines for power generation are operated at a constant
speed to keep the power output following the demand. The power output is increased byincreasing the fuel flow rate with the fuel flow control valve and the intake air flow rate with
the inlet guide vane (IGV) at the compressor inlet, thereby increasing the working combustion
gas flow rate and the combustion temperature of the gas turbine.
Figure 2 shows a conceptual diagram of the existing gas turbine control. The fuel flowrate is controlled by feedback control so that the power output follows the target value, and thefuel flow rate command is determined. The air flow rate is controlled by determining theopening of the IGV according to the preset table setting with respect to the power output.
The fuel distribution control of the combustor nozzle distributes the total fuel flow
determined by the upper control to the multiple fuel systems. Fuel commands to each system
are calculated by multiplying the total fuel flow rate command by the distribution ratio setting,which is calculated according to the table setting based on the turbine inlet temperature.
However, since the turbine inlet temperature is very high and cannot be measured, a simple
estimate calculated from the performance map using key state quantities such as power output
and IGV opening is used.

Figure 2

Conceptual diagram of existing gas turbine control

Mitsubishi Heavy Industries Technical Review Vol. 60 No. 1 (March 2023)
3(2)Issues with existing gas turbine control
Issues with the existing gas turbine controls are listed below.
(i) Simultaneous attainment of stable combustion control and high ramp rate
The combustor of a gas turbine can achieve stable combustion when the fueldistribution is set appropriately according to the operating conditions. If the fuel distribution
is not appropriate, there is a risk of combustion instability. As shown in (1), the fueldistribution is calculated using simple estimates based on the key state quantities andperformance map. However, in the case of high-ramp rate operation, the accuracy of turbine
inlet temperature estimation deteriorates, resulting in deviations from the appropriate fueldistribution according to the turbine inlet temperature.
When the load on the gas turbine changes, the injection of fuel and air is determined so
as to achieve the preset operating line (power output and turbine inlet temperature settings),
but as the ramp rate increases, the actual operating line deviates from the target line,
increasing the risk that stable operation cannot be achieved (Figure 3).

Figure 3 Deviation from target turbine inlet temperature
operating line(ii) Improvement of power output following performance near the rated output
When the power output rises near the rated load, the power output following
performance deteriorates because the power output rise rate is suppressed so that the turbine
inlet temperature does not exceed the upper limit from the standpoint of equipment
protection (Figure 4).
(iii) Realization of high-efficiency operation under partial load conditions and reduction of CO2emissions
One method for achieving high-efficiency operation and reducing CO2 emissions is tokeep the turbine inlet temperature high even under partial load conditions. With theconventional control method, it is necessary to conduct partial load operation with a margin
with respect to the upper temperature limit due to concerns about excessive turbine inlet
temperature rise (Figure 5).

Mitsubishi Heavy Industries Technical Review Vol. 60 No. 1 (March 2023)
4Figure 4 Deterioration of power output following performance near the rated
power output

Figure 5 High efficiency operating line of current operating line ofturbine inlet temperature operation

2.2

Next-generation gas turbine control

In order to solve the issues described in section 2.1, we are developing a next-generation gas
turbine control to achieve high following performance and stable operation even at high ramp rates.
Figure 6 shows a conceptual diagram of the next-generation gas turbine control. While theconventional control (described in section 2.1) does not control the turbine inlet temperature
directly because it was difficult to evaluate accurately in real time, the feedback control issimultaneously used to improve the control accuracy of the turbine inlet temperature, in addition tothe power output, for the next-generation gas turbine control. The technological features of thismethod are (1) accurate turbine inlet temperature calculation using turbine inlet temperature
estimation based on a physical model, and (2) improved following performance using model
predictive control, which predicts future behavior. The details are explained below.

Mitsubishi Heavy Industries Technical Review Vol. 60 No. 1 (March 2023)
5Figure 6(1)Turbine inlet temperature estimation based on physical model
We developed a method to accurately calculate the turbine inlet temperature, which is an
important state quantity as an indicator of combustion control. The unique feature of thismethod is that it combines two physical models to achieve both static accuracy and transient
behavior accuracy. Figure 7 shows an overview of this method.
The first physical model calculates an accurate turbine inlet temperature by calculating
the heat balance using sensor values for the entire gas turbine, such as exhaust gas temperatureand gas turbine power output. However, this physical model has the disadvantage that its
accuracy can only be guaranteed in a stabilized state due to the large delay in sensormeasurement. The second physical model calculates the turbine inlet temperature without delay
by calculating the heat balance around the combustor. However, this physical model has errors
in the model equation for fuel flow rate calculation, so that the accuracy of the absolute value isinsufficient. By correcting the error in the calculated fuel flow rate in the second physicalmodel with the true value of the fuel flow rate calculated from the stable state data in the firstphysical model, it becomes possible to determine the turbine inlet temperature with highaccuracy in both stable and transient operations.
By using the turbine inlet temperature estimates calculated with this method, which arehighly accurate and free from transient deviations, as input values for fuel distribution control
and model predictive control as described in (2) below, stable combustion and high-ramp rate
operation can be achieved.

Figure 7(2)Conceptual diagram of next-generation gas turbine control

Turbine inlet temperature estimation based on physical model

Anticipatory control of gas turbine power output and turbine inlet temperature based onmodel predictive control

Mitsubishi Heavy Industries Technical Review Vol. 60 No. 1 (March 2023)
6

Model predictive control is a method of calculating control inputs by repeating
optimization calculations at each step, taking into account constraint conditions while
predicting future behavior based on a "prediction model" that represents the behavior of thetarget. Using an analogy of driving a car, it is easy to understand the process by imagining aseries of actions of predicting future behavior based on the shape of the curve in front of you
and the driving status of the car in front of you, tracing the center of the lane, and operating theaccelerator pedal and steering wheel early to avoid colliding with the guardrails or cars in front(Figure 8).

Figure 8 Analogy between driving a car and model predictive
control

Details of applying model predictive control to a gas turbine are described below. A
target trajectory is set for the gas turbine power output and turbine inlet temperature up to a few
seconds into the future. The future behavior of the gas turbine is predicted using a "prediction
model" that represents the behavior of the gas turbine to follow this target trajectory, and thefuel flow rate command and IGV opening command are calculated at each step by optimizing
an evaluation function that considers the trade-off between the target trajectory following
performance and the stability (Figure 9). This method enables the gas turbine power output andturbine inlet temperature to follow the target values with high accuracy.

Figure 9

Model predictive control for gas turbine

Mitsubishi Heavy Industries Technical Review Vol. 60 No. 1 (March 2023)
7|3. Verification using simulation and verification at power plantvalidation facility
To verify the operation of the next-generation gas turbine control system developed inchapter 2, we conducted simulation verification using dynamic response analysis and verificationof the actual system at our power plant validation facility in Takasago Machinery Works.

3.1

Verification using simulation

We have dynamic analysis technology to calculate the behavior of plant pressure,temperature, flow rate, etc., in a time series. Using a general-purpose analysis tool(MATLAB/Simulink), we constructed dynamic analysis models of the gas turbine, conventional
gas turbine control, and next-generation gas turbine control, and simulated various operations,
including load changes, to verify the operation and control performance (Figure 10).
In addition to the above desktop analysis, HILS (Hardware in the Loop Simulation)verification was conducted as the next-step verification. This is a real-time simulation using acontroller for the actual equipment, and it was verified that the control algorithm could beimplemented on the actual controller as specified and that the intended operation could be realized.

Figure 10 Simulation analysis (accuracy verification of turbine inlet temperatureestimation)

3.2

Verification using actual equipment

Following the simulation using verification, we conducted an actual equipment
demonstration test of the next-generation gas turbine control using our power plant validation
facility in Takasago Machinery Works.
As a result, we verified that the deviation of the turbine inlet temperature estimation from thetarget value was significantly reduced in load change operation at a higher ramp rate (20%/min)compared to conventional operation (5-10%/min) and the upper temperature limit was quickly
reached without transiently exceeding it when reaching the rated temperature, and that the gas
turbine power output quickly increased to the limit output constrained by the upper turbine inlet
temperature limit and the desired operation (Figure 11).

Mitsubishi Heavy Industries Technical Review Vol. 60 No. 1 (March 2023)
8Figure 11

Test result at power plant validation facility|4. Conclusion
This report introduced the next-generation gas turbine control, our approach for high-ramp
rate operation of gas turbines for power generation, for which demand is expected to increase in thefuture.
The next-generation gas turbine control system combines a turbine inlet temperature
estimation method based on a physical model and model predictive control that anticipates thefuture to achieve both stable combustion control and high ramp rates. We have verified this method
using our power plant verification facility in Takasago Machinery Works and reported the results inthis report.
Moving forward, we will increase the functionality of this control by expanding theoperational conditions under which it can be used, and will conduct long-term verification throughelectric supply operation using the power plant verification facility to promote efforts toward its
commercialization.

References(1)(2)

Jan M. Maciejowski et al., Predictive Control with Constraints
Kazuki Morimoto et al., Validation Results of 1650°C Class JAC Gas Turbine at T-point 2 Demonstration
Plant, Mitsubishi Heavy Industries Technical Review, Vol.58 No.1 (2021)