Masahiko Tsukakoshi joined the DriveSystems Department of Toshiba in 1995. Hetransferred to the same section of ToshibaMitsubishi-Electric Industrial SystemsCorporation (TMEIC), in Tokyo, Japan, in2005. His current research project concernsindustrial applications, for example, hot-stripmills and compressors of oil and gas plants.His research interests are the effectiveuse of large inverter systems and control

algorithms of drive systems. Mr. Tsukakoshi received B.S. and M.S. degrees (Electrical

Engineering, 1993, 1995) from Meiji University, Kawasaki. He is amember of the Institute of Electrical Engineering of Japan (IEEJ).

Mostafa Al Mamun joined the DriveSystems Department of Toshiba Mitsubishi-Electric Industrial Systems Corporation, inTokyo, Japan, in 2008. His current researchprojects concern power electronics, especiallythe development of drive systems forgeneral industry and power system analysis.Dr. Al Mamun’s research interests areenvironmental energy engineering, windpower, load forecasting, distributed power

generations, and application of artificial neural networks topower systems. He was also involved in the promotion ofrenewable energy, a COE research project of the Ministry ofEducation, Japan.Dr. Al Mamun obtained his Diploma degree (Electrical

Engineering, 2001) from Ibaraki National College of Technology,and B.S. and M.S. degrees (Electrical and Electronics Engineering,2003, 2005) from Tokyo University of Agriculture and Technology.He received his Ph.D. degree (Electronics and InformationEngineering, 2008) from the same university. He is a member ofJSER of Japan and IEEE of the US.

Kazunori Hashimura is a ProjectEngineering and Management Specialistat Toshiba Mitsubishi-Electric IndustrialSystems Corporation, in Tokyo, Japan. Heis currently managing motor and driveprojects in various industries and applicationsin the US, Europe, and Australia. The projectsinclude those in oil and gas industries,such as the motors and drives on offshoreplatforms. He is also involved in the

development and testing programs for the new large motor-drivesystem for the company.Mr. Hashimura received his B.S. and M.S. degrees (Mechanical

Engineering, 1995, 1997) from Georgia Institute of Technology.

193

NOVEL TORQUE RIPPLE MINIMIZATION CONTROL FOR 25MW VARIABLESPEED DRIVE SYSTEM FED BY MULTILEVEL VOLTAGE SOURCE INVERTER

byMasahiko TsukakoshiDrive Systems Department

Mostafa Al Mamun Drive Systems Department

Kazunori Hashimura Motor and Drive Engineering Department

Hiromi Hosoda Drive Systems Department

Toshiba Mitsubishi-Electric Industrial Systems Corporation

Tokyo, Japan

Junichi Sakaguchi General Manager

Research Institute of Technology Innovation & Strategy

Chiyoda Corporation

Yokohama, Japan

andLazhar Ben-Brahim

Professor, Electrical Engineering Department

Qatar University

Doha, Qatar

Hiromi Hosoda is a Chief Engineer ofToshiba Mitsubishi-Electric Industrial SystemsCorporation Drive System department, inTokyo, Japan. He joined the Department ofPower Electronics of Toshiba Corporation,Japan, where he has been engaged in thedevelopment of motor drives. Now, he isworking as a project leader of the largeVSI drives development.Mr. Hosoda received a B.E. degree

(Electrical Engineering, 1974) from Shizuoka University.

Junichi Sakaguchi joined ChiyodaCorporation, Japan, in 1974 and becamea General Manager in the MechanicalEngineering Department in 1996. Hewas promoted to Senior General Managerof the Detail Engineering Division in1999. Mr. Sakaguchi continued as anexecutive officer for the departments ofCorporate Planning (2001-2003) andTechnology and Engineering (2003-2005).

At present he is a Fellow and General Manager in theResearch Institute of Technology Innovation & Strategy, inYokohama, Japan.Mr. Sakaguchi received B.S. and M.S. degrees (Mechanical

Engineering, 1972, 1974) from the University of Tokyo.

Lazhar Ben-Brahim is currently aProfessor at the Electrical EngineeringDepartment of Qatar University, in Doha,Qatar, and Industrial Electronics Chairfor RasGas Company (LNG plant). From1991 to 1997, he was with ToshibaCorporation, where he was engaged inresearch and development of powerelectronics and electric drive systems. Hewas Head of the Department of Industrial

Technology from 1998 to 2005. His research interests includecontrol applications, motor drives, instrumentation, sensors, andpower electronics.Dr. Lazhar Ben-Brahim received his B.S. and M.S. degrees

(Electrical Engineering, 1985, 1986) from the University of Tunis,Tunisia. He received his Ph.D. degree (Electrical and ComputerEngineering, 1991) from Yokohama National University, inYokohama, Japan. He is a senior member of IEEE.

ABSTRACT

Continuous improvements in the power rating and switchingcharacteristics of power semiconductor devices have enabled theuse of power electronics converters in high power variable speeddrives (VSDs). These multimegawatt drives are needed for drivinglarge capacity compressors in liquefied natural gas (LNG) plants.However, the generated harmonics and their associated torqueripples may result in serious drawbacks in the application ofVSDs in the oil and gas industry. The torque ripples may leadto torsional vibrations that may in turn cause damage to theload-motor coupling. To overcome these drawbacks, a new speedcontrol technique, which is based on a synchronized pulse widthmodulation (PWM) control method, is proposed. A 25 MW fivelevel VSD system was developed to verify the new approach usingtwo experimental tests, namely, back-to-back and full load tests.The tests validated the feasibility of the proposed method inreducing the torsional vibration.

INTRODUCTION

LNG is in great demand globally because it is a clean fuel that isfriendly to the environment. To obtain LNG, the natural gas is chilledto !162�C to produce a clear liquid that occupies up to 600 times lessspace than the corresponding gas. To achieve the necessary cryogenictemperatures, refrigerating turbocompressors are traditionally drivenby industrial heavy-duty gas turbines (GTs). Besides their lowefficiency, GT need regular maintenance. Furthermore, the necessaryshutdown periods and the unscheduled outages interrupt the LNGproduction and reduce LNG plant productivity. As electrical drivessuch as VSDs are maintenance free and more efficient than GT,efforts are being made by major LNG plant operators, contractorsand manufacturers to develop VSDs suitable for LNG compressors.On the other hand, VSDs have been used in various industries

such as steel and paper mills. In the megawatt capacity ranges,these industries prefer multilevel voltage source inverter (VSI) overthe load commutated inverter (LCI) as a power converter for VSDapplications. VSIs are preferred due to their lower harmonics,better power factor, and smaller torque ripples at the motor side.These same features make the VSI fed VSD systems the mostattractive solution for driving LNG plant compressors. A newcontrol method is proposed to reduce the harmonics and the torqueripples of a VSI-based VSD drive system.From previous experience, the installation of a VSI-based drive

system for a large capacity compressor of an LNG plant led toseveral technical issues related to ripples and torsional vibrations ina centrifugal LNG compressor train with a gearbox. Kita, et al.(2007), reported that the torsional vibration was transferred to thelateral vibration at the gear mesh. Based on the knowledge ofprevious coupling failure of a compressor driven by a VSD fed bya three-level inverter, the following two methods were implementedto solve the problem:

• Synchronized pulse width modulation control for the outputfrequency and

• V/F constant control (Shimakawa and Kojo, 2007).

The compressor train has been operating properly after theimplementation of the above-mentioned two methods. The objective of this study was to improve the new techniques

and to apply them to a five-level large capacity inverter instead ofa three-level inverter, which resulted in even higher performances.An improved control method, based on a fixed pulse pattern, wasalso applied to further improve the waveform of the five-levelinverter. The effectiveness of the improvements was validated bytorque ripple measurement during the motor combined experimentaltest. The authors built a 25 MW motor drive system and evaluatedthe system in a back-to-back test using a 7.2 kV 30 MVA VSI bankalong with a 25 MW synchronous motor (SM). A power recoverysystem with a synchronous generator (SG) and a regenerative PWMinverter were used to load the high power SM. The relationshipbetween the VSI output voltage pulse pattern and the torque ripplewas examined. The effectiveness of the implemented controlmethod was also experimentally verified.

DRIVE SYSTEM APPLICATION BACKGROUND

Stable speed and torque control is essential for a largeadjustable-speed motor-driven compressor train. In order toachieve the stable control, however, highly advanced controltechniques are required. In this section, will be introduced theinfluence of and issues involved with motor speed control of anadjustable speed drive that were experienced.

Fuel Gas Compressor Systemand Analysis of a Coupling Failure

A schematic diagram of the studied fuel gas compressor systemis shown in Figure 1. A 13.65 MW induction motor was driven by

PROCEEDINGS OF THE THIRTY-NINTH TURBOMACHINERY SYMPOSIUM • 2010194

a PWM converter and a PWM inverter. A gear was used to increasethe speed. Two compressors (low pressure compressor and highpressure compressor) were driven at the increased speed.

Figure 1. Fuel Gas Compressor System.

It was found that the speed feedback showed continuousvibration with a measured vibration frequency of 14 Hz, which isthe resonant frequency of the compressor system. The resonancewas caused by torsional and torque ripple onsite. As a result, the LPcompressor coupling was broken after 2000 hours of operation.Figure 2 shows an actual photograph of the coupling failurebetween a gear and an LP compressor (Kocur and Corcoran, 2008).

Figure 2. Photographs of Coupling Failure of an LP Compressor.(Kocur and Corcoran, 2008)

The PWM control of the inverter was not synchronized to theoutput frequency, resulting in a side band wave, which was thecause of the torque ripple. Torsional vibration of the compressorshaft caused the speed signal ripple. The vector control was basedon the speed signal and it amplified the speed vibration. Themechanical system of the compressor was relatively weakcompared with that of a rolling mill system and the coupling wasbroken with a small torque ripple in several months of operation.The authors overcame this problem by analyzing the failure and

applying two methods, that is:

• A synchronized carrier with the output frequency and• V/F constant control.

The distortion in the motor control with the feedback functionaffected the stable operation of the compressor train. The unstablecontrol caused by the improved motor control indicates accuratevector control and converter phase control in the compressor trainwith the small damping involve some risk. On the other hand, thesynchronized control in the PWM inverter is an effective methodfor the large adjustable speed motor in the compressor train.According to the analysis, it was verified that the V/F control forthe synchronized PWM inverter efficiently controlled the motorused in the compressor train in which the load fluctuated gradually.The motor driven compressor train could be successfully stabilizedby this motor control method (Shimakawa and Kojo, 2007).

VSI Versus Conventional LCI Drive

The basic configuration of an LCI is shown in Figure 3. The LCI isa conventional drive system using thyristors and has a low switchingfrequency device. It is controlled by aid of the grid voltage or motorvoltage. LCI has higher grid harmonics and larger motor torque ripple.

Figure 3. Single Line Diagram of a Load Commutated Inverter.

Compared with an LCI, a VSI has the newest high frequencyswitching device and can be controlled independently of the powergrid voltage or motor voltage. The advantages of a VSI over LCIare smooth startup and small motor torque ripple. Figure 4 showsa single line diagram of a VSI.

Figure 4. Single Line Diagram of a Voltage Source Inverter.

VSIs offer considerable advantages over the widely used LCIdrives considering the following cases:

• Because a thyristor has to be turned off by the aid of the powergrid or motor voltage, the grid and motor characteristics requirespecial design to realize proper LCI operation. Therefore, a speciallydesigned motor and additional power compensating equipment arenecessary. Also, a VSI can be designed independently of the powergrid and motor conditions due to the high controllability of selfturned-off devices. A VSI drive can be applied to a standard motorwithout any modified grid.

• An LCI has a low power factor with lower order harmoniccurrent. Therefore, power factor correction equipment andharmonic filter capacitors are necessary to maintain the powersystem quality. The power factor correction equipment andharmonic filters have to be carefully designed to avoid parallel andseries power resonance. However, a highly reliable power systemcan be obtained from a VSI even in the case of a weak powersupply system, because no capacitor is required.

PRINCIPLE OF THREE-LEVELINVERTER AND FIVE-LEVEL INVERTER

In a two-level inverter, which is commonly used for low voltage(LV) alternating current (AC) drives, the output voltage waveformis produced by using PWM with two voltage levels. In a three-levelinverter, which is commonly used for higher power VSI, the outputvoltage and the current waveforms are improved due to the greaternumber of voltage levels. The total harmonic distortion (THD) isalso reduced compared with two-level inverters. The efficiency ofthree-level inverters at full load is also higher than that of two-levelinverters, which means better energy handling of the system. Abetter efficiency at rated power also means a smaller heat sink andbetter reliability. The efficiency of three-level inverters at smallpower is also improved (Ikonen, et al., 2005). Figure 5(a) and (b)show the main circuit configurations of two-level and three-levelinverters, respectively.

Figure 5. Development of Main Circuit Technology.

Figure 5(c) shows a VSI with an even higher number of voltagelevel and a five-level inverter with gate commutated turn-offthyristor (GCT). The diode converter portion has three layers of 12

195NOVEL TORQUE RIPPLE MINIMIZATION CONTROL FOR 25MW VARIABLESPEED DRIVE SYSTEM FED BY MULTILEVEL VOLTAGE SOURCE INVERTER

phases diode rectifier, which is built without any fuses for acompact structure. The input circuit breaker protects the diodesfrom short circuit by using a current sensor device at the input ofthe rectifier.The five-level inverter output voltage and current waveforms are

more sinusoidal than the waveforms of the three-level inverter andthe obtained THD is lower. An LCI has commonly been used forlarge capacity compressor applications in the oil and gas industry.However, five-level technology can increase the voltage level andyield large capacity VSI drives, which are more suitable for drivingthe compressors of LNG plants (Tsukakoshi, et al., 2009).

ADVANTAGES OF FIXED PULSE PATTERN

Improved control by employing a fixed pulse pattern is introducedin this section. Unlike the sinusoidal PWM and space vectorPWM techniques, where the pulses are controlled by the currentcontroller and changed in every cycle, in the fixed pulse patterntechnique, the same pulse pattern is output in every cycle. Thispulse pattern is described below.With the conventional PWM control, the controller changes the

pulse width at every instant to regulate the VSI current output andkeep it sinusoidal. Then the pulse pattern changes cycle by cycleand asynchronous to the output frequency. In the VSI, the inverteroutputs a voltage according to the pulse pattern, which means thatthe output voltage also changes its shape cycle by cycle. Thefrequency spectrum of such an asynchronous voltage shapegenerally contains harmonic components in a wide frequency band,including low frequency components that may cause problems withmechanical oscillation.In order to avoid the above risk, the authors developed the

controller to apply the fixed pulse pattern. The principle of thepattern generation is well-known and has been described in theliterature when VSIs were first developed. However, the algorithmrequired very high control performance and resources that couldnot be realized by the commercial products available at that time.These days, owing to the development of CPUs and memories,fixed pulse pattern control can be implemented in practicalcontrollers and can be used online in real time.According to a pulse control theory, one can make a pulse

pattern that eliminates low order harmonics. By repeating such apulse pattern every cycle, the frequency spectrum of the outputvoltage should not contain low frequency components. The pulsepatterns are calculated offline by computer taking the outputvoltage value as the parameter. Then, the calculation results areinstalled in the memory of the VSI controller. The controllerchooses appropriate pulse pattern data according to the voltagerequired to drive the motor and outputs the pattern synchronizedwith the frequency.Figure 6 shows an example of a fixed pulse pattern. Figures 6

(a) and (b) show the pulse patterns for legs A and B of theU-phase of the five-level VSI in Figure 5. The patterns wereprepared to eliminate low frequency components, as describedabove. The same pulse pattern is applied to both legs but thetiming is shifted. Because the output voltage shape is the sameas the pulse pattern, each leg outputs three voltage levels. Figure6 (c) shows the U-phase output voltage with five output voltagelevels, which can be obtained by substituting the output voltageof leg B with that of leg A. Since the substitution is a linearoperation, the voltage shape does not contain low frequencycomponents since the original voltage shape of each leg doesnot contain them. Figure 6 (d) shows the V-phase outputvoltage. The V-phase output voltage shape is the same as theU-phase voltage with a 120 degree phase shift to produce thesymmetrical three-phase output voltage from the VSI. Figure 6(e) indicates the line-to-line output voltage between the U-phaseand the V-phase, showing nine voltage levels (Tsukakoshi, et al.,2009). Again, the synthesized line-to-line voltage does notcontain low frequency components.

Figure 6. Output Voltages of Five-Level Switching Circuit.

Figures 7 and 8 show a comparison of the harmonic frequencycomponents in the VSI output power with the asynchronous pulsepattern and the fixed pulse pattern, respectively, with respect to themotor frequency.

Figure 7. Characteristics of the Harmonic Frequency Componentsin the VSI Output Power with the Asynchronous Pulse Pattern withRespect to the Motor Frequency.

Figure 8. Characteristics of the Harmonic Frequency Componentsin the VSI Output Power with the Fixed Pulse Pattern with Respectto the Motor Frequency.

Though the asynchronous pulse pattern operation contains a goodnumber of harmonic components, this problem can be overcome byusing the fixed pulse pattern obtained in this study. In these figures,the red line indicates that the motor frequency behavior correspondsto the harmonic frequency contained in the VSI output power.Generally, long mechanical structures like the compressor trains

have the lowest resonance frequency of around 20 Hz or less. Thus,the harmonics of this frequency range should be focused, as indicatedby the rectangular region in Figure 7 and Figure 8. For example, inFigure 7, the motor frequency around 42.5 Hz indicates many loworder harmonics at frequencies from 0 Hz to 20 Hz in the inverteroutput power. However, the low order harmonics at these frequenciesare almost completely eliminated in Figure 8 by employing theproposed fixed pulse pattern. For other motor frequency, Figure 7indicates many low frequency harmonics. On the other hand, Figure

PROCEEDINGS OF THE THIRTY-NINTH TURBOMACHINERY SYMPOSIUM • 2010196

8 indicates very few low frequency harmonics. The higher orderharmonics do not negatively influence the motor and drive system.Experimental results obtained using a fixed pulse pattern will beshown later in the “RESULTS AND EVALUATION” section.

THE TEST SYSTEM OF ACTUALDIMENSIONS RATED AT 30 MVA

Test Setup

Figure 9 shows a photograph of the test setup rated at 30 MVA,used for system performance evaluation. Tests conducted underactual operating conditions are essential for evaluating theperformance of the five-level inverter drive system controlled bythe proposed fixed pulse pattern. Figure 9 shows the photograph ofthe test setup rated at 30 MVA for this purpose. A five-level GCTVSI rated at 30 MVA and a two-pole motor rated at 25 MW weredesigned and manufactured for the evaluation. The details of theVSI and the motor will be explained in the following sections. TheVSI output voltage harmonics, torque ripple and so on measured inthe test setup are described in the following sections. Figure 9 alsoshows the load, constructed of a generator and inverters.

Figure 9. Test Setup for System Performance.

Figure 10 shows a schematic diagram of the test setup. Thefive-level VSI powers the motor that is connected to the generatorthrough a gearbox. The five-level GCT VSI controls the motor speed.The torque of the load, namely, the generator is controlled by theIEGT VSIs (Ichikawa, et al., 2004) to simulate the actual fieldoperation up to full load condition. The feature of the test setup is thepower regeneration. The electrical power taken from the generatorpasses through the IEGT VSIs and returns to the power system. Thistechnique reduces the power consumption even for the full 30 MVAtest, supplying only the make up power from the electrical losses,enabling a test under actual operation conditions (Tsukakoshi, et al.,2005). The generator for the load is rated at 25 MW and is of thesynchronous type with four poles. The four parallel IEGT VSIshandle 32 MVA and 25 MW. A speed gear with a ratio 2:1 isconnected between the motor and the generator to match the speeds.

Figure 10. Schematic Diagram of 30 MVA VSI Test System.

Configuration of the Drive System(Five-Level GCT VSI)

The five-level GCT VSI system is configured with three pairsof single-phase three-level GCT inverters that operate as athree-phase five-level inverter. The main circuit unit and maincircuit board can be interconnected with other drive systems.The system has been developed into a 30 MVA inverter that

produces a 7.2 kV output voltage. Because the capacity of asingle-bank five-level GCT inverter is 30 MVA, it is possible toobtain 120 MVA by connecting four banks together. When appliedto a compressor, it is not necessary for the variable frequency drive(VFD) to have a power regeneration ability. Therefore, a diodeconverter circuit is introduced to achieve a smaller footprint andlower cost.Figure 11(a) shows the structure of a prototype five-level GCT

inverter. Figure 11(b) shows the configuration of the main powerblock in a five-level GCT inverter. It was developed with mainswitching devices, such as GCTs, freewheel diodes connected inparallel and coupling diodes. The specifications of the proposedinverter are given in Table 1.

Figure 11. (a) Outline of Five-Level GCT Inverter (Prototype) and(b) GCT Power Unit.

Table 1. Specification of Five-Level GCT Inverter.

197NOVEL TORQUE RIPPLE MINIMIZATION CONTROL FOR 25MW VARIABLESPEED DRIVE SYSTEM FED BY MULTILEVEL VOLTAGE SOURCE INVERTER

Configuration of the Electric Motor

Figure 12 shows a cross-sectional diagram of the motor.Though the basic configuration is the same as a two-pole turbinegenerator, it is necessary to consider the following matterscomparing to a generator:

• Speed fluctuation• Torque ripple• Harmonic losses• Surge voltage

Figure 12. Cross-sectional Diagram of the Motor.

Figure 13 shows a diagram of an elastically-supported stator. Itis designed in such a way that the 2n-element (electromagneticoscillation at twice the power supply frequency) of the stator is nottransmitted through the frame.

Figure 13. Diagram of Stator.

Figure 14 shows a diagram of a rotor. It is configured with areverse phase excitation system so that it is possible to startup fromzero speed. The soundness of each part in terms of endurancestrength was confirmed by repeatedly applying centrifugal forceand fretting the mating part by torque ripples.

Figure 14. Diagram of Rotor.

Shaft Torque Measurement

Figure 15 shows the shaft torque measurement point in the testsystem. Torsional distortion of a high speed flexible joint wasmeasured by using a strain gauge and telemeter, and the shafttorque was calculated.

Figure 15. Shaft Torque Measurement Point.

Torsional Vibration Analysis of the Motor

Method of Analysis

Torsional vibration analysis was performed to examine the torqueripple due to the shaft, especially to analyze the strength of theelastic joint. Figure 16 shows an analytical model for the test systemwith a four-pole generator acting as a load connected through thegear. As the motor rotor is long in the axial direction, it is dividedinto 10 of inertia by considering the torsional modes of the rotor.The analytical model is divided into 24 mass points between themotor and gear and between the gear and generator. Vibrationanalysis was performed using a commercially available technicalcomputing software with this model. The lowest torsional vibrationin which the motor and generator are set in a reverse phase vibrationmode was 17.1 Hz. In addition, the torsional natural frequency was67.4 Hz and 200 Hz at an operating frequency of 200 Hz.

Figure 16. Torsional Vibration Analysis Model.

Figure 17 shows the results of the torsional inherence mode for17.1 Hz, 67.4 Hz and 200 Hz. The results show the distribution ofthe amplitude of vibration. For low frequencies, the normalizedmodal vector increases gradually with increasing number of inertia.

Figure 17. Torsional Inherence Mode.

PROCEEDINGS OF THE THIRTY-NINTH TURBOMACHINERY SYMPOSIUM • 2010198

Test Condition

The measurements were conducted assuming a rated outputfrequency of 60 Hz by varying the load from zero to the maximum(100 percent). With the same load conditions, the authors alsoconducted measurements at 70 percent of the rated outputfrequency (42 Hz). Detailed results are shown in the next section.

RESULTS AND EVALUATION

Inverter Voltage Waveform and FFT Analysis

Figure 18 shows the test results of U-V line-to-line no-loadvoltage waveform for 60 Hz five-pulse operation. The fast Fouriertransform (FFT) analysis of these test results is shown in Figure 19.

Figure 18. Five-Pulse 60 Hz Line-to-Line Voltage (U-V).

Figure 19. Comparison of Theoretical and Experimental Values forFive-Pulse No-Load Voltage by FFT Analysis.

In Figure 19, the theoretical values and the test results of zeroizedlow order harmonics by FFT analysis are compared showing thatboth values are approximately the same. The cancellation of the loworder harmonics using only the inverter is one of the superior VSIcharacteristics realized by this advanced control method.

Results of Torsional Vibration Analysis of the Motor

Figure 20 shows a three-dimensional illustration of the shafttorque at 5 percent load condition. From the figure, it is clear thattorque ripple on the shaft is caused at the specific frequencies of17.1 Hz and 67.4 Hz regardless of the rotational speed. When the2n-element corresponds to those specific frequencies, the shafttorque ripple becomes maximum.

Figure 20. Three-Dimensional Illustration of the Shaft Torque Ripple.

The 2n-element of the output frequency of the drive equipmentis the main cause of vibration, and no mechanical excitation is foundfor other low order elements. However, a very small excitation occursbecause of the influence of 2n-element, which can be neglected.Figures 21 and 22 show the results of shaft torque ripple and

frequency analysis at the rated speed (3600 rotations per min) with100 percent load. From Figure 21, the maximum shaft torque rippleis 7.87 kN-m (about 5 percent) by assuming zero as the middle point.The influence of coupling and the axial strength is small. Figure 22shows that the maximum shaft torque is 2.5 percent at 17 Hz.

Figure 21. Waveform of Shaft Torque Ripple (3600 RPM, 100Percent Load).

Figure 22. Frequency Analysis (3600 RPM, 100 Percent Load).

Figure 23 shows the relation between shaft torque ripple andload. Here, the element of the shaft torque ripple is considered tobe the first-order specific frequency element. From the result, theshaft torque ripple increases in proportion to the load. However, thetorque ripple is smaller because the fixed pulse pattern is employed.

Figure 23. Relation Between Shaft Torque Ripple and Load.

Torque ripple from the load as a generator and three-levelinverters are included in the test results. From these results, it ispossible to calculate the torque ripple for a five-level inverter andthe motor; it was calculated to be less than 1.5 percent. Figure 24shows the test results for shaft torque ripple and frequency response.

Figure 24. Test Results for Shaft Torque Ripple and Frequency Response.

199NOVEL TORQUE RIPPLE MINIMIZATION CONTROL FOR 25MW VARIABLESPEED DRIVE SYSTEM FED BY MULTILEVEL VOLTAGE SOURCE INVERTER

Here, the generated torque ripple was 1.5 percent, the modaldamping ratio was 1 percent, and the number of torque generatingpoints was 6 to 16 for motor excitation and 24 for generator excitation.The measured shaft torque ripple was lower than the frequency

response curve for generator excitation shown by the black pointsin Figure 24. Considering the motor side inverter as a five-levelinverter, the torque ripple can be calculated from the above values.The result gives a low-frequency torque ripple of less than 1.5 percent.In Figure 24, the red line indicates the estimated generator

excitation line. When the mechanical excitation occurs by thegenerator, the frequency versus torque response characteristic canbe estimated according to the model of Figure 16. On the otherhand, the response by the motor when mechanical excitation isperformed is shown in the blue line. The torque ripple in theexcitation source was calculated to be 1.5 percent and the modaldamping constant was calculated to be 1 percent. In Figure 16, atotal of 24 mass points was used for the generator and excitation ofthese mass points by the generator was treated as the excitationsource in the calculation. Moreover, the number of mass points forthe motor was from 6 to 16, and excitation of these mass points bythe motor was treated as the excitation source in the calculation.The torque ripple measurements conducted under various test

conditions are plotted in Figure 24. From the results, most of thepoints are located under the blue line. Some of these are presumedto be a ripple generated from the generator, which imitates the loadthough it is above the blue line, and there are measurements nearthe red line. Some are above the blue line, which is close to the redline. The generated torque ripple is caused by the generatormodeled as the load. Therefore, from these test results, thegenerated torque ripples in the motor and the five-level inverter arethought to be only the elements below the blue line. If there is noexcitation from the load side, it should be possible to obtain asmaller torque ripple of less than 1.5 percent.

CONCLUSION

A new method of torque minimization in VSI systems that issuitable for large compressor applications is proposed. Besides itstechnical advantages, the proposed system, when used as the primemover for a large compressor train, will result in significantfinancial benefits for the end user. In this study, a strategy for howto avoid mechanical resonance by the synchronous control methodis described. The effectiveness of the proposed method was madepossible by the use of a fixed pulse pattern that was synchronizedwith the operating frequency of the drive system. The authorssucceeded in experimentally evaluating the system prior to its usein the field by conducting a challenging task, which involved anexperiment on a large system. Reduced torque ripple was

confirmed through these experiments. In this study, the authorsachieved satisfactory results from the experimental tests, and theyexpect that their findings will be implemented in practical systemsin the near future.

REFERENCES

Kocur, J. A., Jr., Corcoran, J. P., 2008, “VFD Induced CouplingFailure,” Thirty-Seventh Turbomachinery Symposium, CaseStudy #09, Turbomachinery Laboratory, Texas A&M University,College Station, Texas.

Ichikawa, K., Tsukakoshi, M., and Nakajima, R., 2004, “HigherEfficiency Three-Level Inverter Employing IEGTs,” AppliedPower Electronics Conference and Exposition, Anaheim,California, 3, pp. 1663-1668.

Ikonen, M., Laakkonen, O., and Kettunen, M., 2005, “Two-Leveland Three-Level Converter Comparison in Wind PowerApplication,” Department of Electrical Engineering, LappeenrantaUniversity of Technology, FI-53851, Lappeenranta, Finland, p. 3.

Kita, M., Hataya, T., and Tokimasa, Y., 2007, “Study of a RotorDynamic Analysis Method That Considers Torsional andLateral Coupled Vibrations in Compressor Trains with aGearbox,” Proceedings of the Thirty-Sixth TurbomachinerySymposium, Turbomachinery Laboratory, Texas A&MUniversity, College Station, Texas, pp. 31-38.

Shimakawa, T. and Kojo, T., 2007, “The Torsional TorqueFluctuations of a Compressor Train with a Vector ControlPWM Inverter,” Thirty-Sixth Turbomachinery Symposium,Case Study #01, Turbomachinery Laboratory, Texas A&MUniversity, College Station, Texas.

Tsukakoshi, M., Al Mamun, M., Hashimura, K., and Hosoda, H.,2009, “Performance Evaluation of a Large Capacity VSDSystem for Oil and Gas Industry,” 2009 IEEE EnergyConversion Congress and Exposition, San Jose, California,pp. 3485-3492.

Tsukakoshi, M., Mukunoki, M., and Nakamura, R., 2005, “HighPerformance IEGT Inverter for Main Drives in the SteelIndustry,” IPEC2005, Nigata, Japan, S15-2.

ACKNOWLEDGEMENTS

The authors thank the Motor Department of ToshibaMitsubishi-Electric Industrial Systems Corporation, for providingmotor test results, including permission to reproduce them here.The contributions to this project from many persons, all of whomcannot be listed here, are highly appreciated.

PROCEEDINGS OF THE THIRTY-NINTH TURBOMACHINERY SYMPOSIUM • 2010200