Mitsubishi Heavy Industries Technical Review Vol. 55 No. 2 (June 2018)
1Engine Generator Torque Estimation Technique Using
Mechanical System-Electrical Power System
Coupled Analysis

MASATO MITSUHASHI*1 YOSHIKATSU IKAWA*2
RYO SASE*2

OSAMU NAKAKITA*3

Shaft systems of engine generators have been designed conventionally by analyzing thegenerated torque of the engine body and the load torque of the generator individually, mainly forthe steady state. However to deal with the increase in regeneration energy and the instability of theelectrical power system in recent years, transient analysis and evaluation of the power system aretaking on increasing importance. This paper reports on technology to estimate torque undertransient conditions of the power system and upon the occurrence of an accident using coupledanalysis of the mechanical system and the electrical power system of an engine generator.

|1. Introduction
In the past, the shaft system of an engine generator was designed by analyzing the generated
torque of the engine body and the load torque of the generator individually, and designed mainly
for the steady state. In the design process of a shaft system, the torsional vibration from the engine
body to the generator is calculated to confirm that there is no resonance point in the operation
range, and static strength and lifespan evaluation are performed based on the steady load torque andthe maximum load torque.
However, in recent years, there are an increasing number of small-scale systems that
introduce large amounts of regeneration energy. In addition, load fluctuations and behaviors that
affect electrical power systems as seen in developing countries have been increasing. The numberof system accidents occurring in developing countries is much larger than that in Japan. Therefore,evaluation under transient conditions and upon the occurrence of accidents is also required.
In this report, in order to estimate the torque generated on the engine shaft under transientconditions and upon the occurrence of accidents, which is the basis data of the shaft systemevaluation, engine shaft torque estimation technology using mechanical system-electrical powersystem coupled analysis through simulation is presented.

|2. Overview of mechanical system-electrical power system coupledsimulation
2.1 Target product
The target products are engine generators manufactured by Mitsubishi Heavy Industries
Engine & Turbocharger, Ltd., which has a lineup ranging from 180 kW to 15.4 MW. The fuel isdiesel oil or gas. The power supply voltage ranges from a low voltage of 200 VAC to a highvoltage of 11 kVAC, and the power supply frequency is 50 or 60 Hz. In Japan and developed
countries, the need for gas engine generators is high, but in developing countries, there are many
areas where the gas supply infrastructure is not well established and the demand for diesel engine
generators remains strong. In developing countries, the introduction of regeneration energy has also
been increasing, and such a largely fluctuating power source is connected to a weak system in somecases, making systematic disturbances likely to occur.
*1
*2
*3

Chief Staff Manager, Electricity & Applied physics Research Department, Research & Innovation Center
CIS Department, ICT Solution Headquarters
Chief Staff Manager, Engine & Energy Division, Engine Engineering Department, Mitsubishi Heavy Industries Engine & Turbocharger, Ltd.

Mitsubishi Heavy Industries Technical Review Vol. 55 No. 2 (June 2018)
2This paper reports on the examination results for a 7.5 to 10 MW class, 18-cylinder diesel
engine generator aimed for use overseas.

2.2 Overview of simulation analysis model
In conventional engine generator simulation, as shown in Figure 1, the inertia of the engine
and the inertia of the generator are integrated into the generator model, and the fluctuation of thevoltage and frequency is obtained using the engine output as the input, or conversely, using thetorque generated by the generator as the input, to simulate the dynamic characteristics of theengine.

Figure 1Overview of general simulation model

In the simulation presented here, the torque generated in the electric system is applied as theload torque of the engine at any given time to obtain the dynamic characteristics of the engine.
Figure 2 presents the overview of the mechanical system-electrical power system coupledsimulation model.

Figure 2

Overview of mechanical system-electrical power system coupled simulation model

The mechanical system models of the engine body and the generator consist of a spring-masssystem and consider the torque generated by each cylinder due to explosion, the absolute dampingof the engine (proportional to the angular velocity of the engine), and the relative damping(proportional to the difference between the angular velocities at the front and back of the target
crank). The governor that adjusts the rotation speed and the output, as well as the actuator that
supplies the fuel, are modeled, and not only the influence of the response of the control system, butalso the engine shaft torque of the mechanical system can be calculated. The input from theelectrical power system to the mechanical system is the input of the electric torque obtained by thegenerator model.
The electrical power system model of the generator adopts a synchronous generator model,

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using general generator parameters such as the transient reactance, transient time constant, etc. In
addition, an AVR (automatic voltage regulator) for voltage control is also incorporated in themodel. In order to keep the engine shaft rotation speed and the generator shaft rotation speed thesame, this model uses the engine rotation speed as an input to the generator model.
The generator model and the electrical power system model are connected via a circuit
breaker and a transformer, and the electrical power system model consists of a constant voltage
source, a transmission line and a power load as an infinite bus. In general, design condition datasuch as back impedance, etc., can be obtained, but in many cases, it is difficult to obtain detailedtransmission line parameters. However, since the transmission line distance could be obtained forthis case, the transmission line parameters are determined by using the published resistance andreactance value per distance of a similar transmission line.
In the simulation study, by intentionally changing the parameters of the constant voltage
source model of the bus, generating bus frequency fluctuation and causing an accident such as ashort circuit (three-phase short circuit or line-to-line short circuit), etc., in the middle of thetransmission line, a change is imposed to the bus to obtain the torque applied to the shaft of theengine generator.
MATLAB/Simulink SimPowerSystems1 is used as the analysis tool.

|3. Results of simulation
3.1 Model validation
As a verification of the model, the results of a line-to-line short-circuit (two-phase shortcircuit) generated at the generator output terminal are given in Figure 3. The instantaneous current
change was about 10 times the rated current. This is roughly in agreement with the typical literature
value2 of a line short circuit and indicates that the model was reasonable. In addition, it was foundthat the instantaneous engine shaft torque generated at this time was about 7.5 times the rated
torque. Based on this model and on-site record data, the event that occurred at the site and theengine shaft torque were estimated. The results are provided in the next section.

Figure 3 Verification of simulation model (line-to-line short circuit)

3.2

Case study

In the actual plant, the electrical power system is not redundant, and there are many system
accidents, resulting in frequent power outages. An electrical power system accident also has amajor influence on the engine generator supplying the electrical power, which is one cause ofengine trouble. In this simulation analysis, a form of an accident occurring in the electrical powersystem of the site was estimated and reproduced in the simulation on the basis of the record data(three-phase voltage), and the electrical torque and the mechanical torque in that state wereMitsubishi Heavy Industries Technical Review Vol. 55 No. 2 (June 2018)
4obtained.
As an example of the simulation, first, Figure 4 illustrates the three-phase voltage of thegenerator output terminal at the time when a line short circuit occurs in the bus system of the site.
When confirming the change in the voltage, it is considered that a high-resistance three-phase shortcircuit accident has occurred because the three-phase voltage is in the same phase and at the samepotential and the voltage is not 0 V.
Figure 5 makes a comparison between the on-site record data and the results of reproducing
the data in the simulation. In the simulation, the parameters of the accident occurrence model are
adjusted so that the voltage fluctuation in the simulation coincides with the actual voltage
fluctuation based on the measured accident data of the line short circuit, and the calculation is
performed while switching the parameters for each accident continuation time. For this reason,
there is a slight difference in the accident parameter switching section. However, as a whole, thesimulation reproduces the line short-circuit phenomenon of the actual equipment.

Figure 4

On-site record data of accident (three-phase voltage)

Figure 5Comparison between on-site record data and simulation results (three-phase voltage)

Figure 6 and Figure 7 present the changes in the electric torque, generator shaft torque andengine shaft torque in this state. With respect to the generator shaft and engine shaft torque, theinstantaneous change, the influence of the response delay of the mechanical system and the controlsystem, as well as the amplification situation of the torque caused by the resonance point, areindicated. Instantaneously, a torque about 4 times the steady torque is generated.

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Figure 6

Torque estimated using simulation

Figure 7

Torque estimated using simulation (magnified view)|4. Conclusion
By using mechanical system-electrical power system coupled simulation, the estimation ofengine generator shaft torque in a transient state was made possible. The torque presented as an
example here is the torsion torque and the generator shaft torque at the flywheel connecting theengine and the generator, but by modeling the engine generator in further detail and by coupledanalysis of the power system, it is also possible to evaluate the torque of more detailed parts inside
the engine. Moreover, since this is simulation of the transient state, it can be applied to the tuningof the control system and the transient areas can also be quantitatively evaluated from both themechanical system and control system. Based on the simulation model, we will estimate the causesof fluctuation events in an electrical power system and quantitatively evaluate the influence on theengine generator, and strive to further improve product reliability.

References
1. Mathematical computing software by MathWorks
2. Shoyama, E., Furuyama, M., Operating Characteristics of Large Turbine Generators, Vol. 54 No. 4 (1972)
3. Kozo Aratame, Basic technical computing of electric power system, Denkishoseki, 1996, Vol.1
4. Kozo Aratame, Applied technical computing of electric power system, Denkishoseki, 1997, Vol.1