05546931

1102 IEEE TRANSACTIONS ON ENERGY CONVERSION, VOL. 25, NO. 4, DECEMBER 2010

Coordinated Power Quality Improvementin Multiunit Diesel Power Plants

Carlos A. Platero, Francisco Blazquez, Member, IEEE, Pablo Frıas, and Antonio J. Casado

Abstract—Power quality is a main concern in power systemswith generators driven by internal combustion engines, especiallyby low- and medium-speed diesel engines. The torque in inter-nal combustion engines, which is the superposition of the torquesfrom each cylinder, has periodic oscillations. These torque oscilla-tions, once the generator is connected to the grid, result in powerfluctuations and voltage flicker problems, which clearly reducepower quality. This paper presents a new control strategy to im-prove power quality in multiunit diesel power plants, based on thecompensation of the electrical fluctuations between individual gen-erators. This compensation is performed by properly controllingthe mechanical rotor phase angle between the different generatorsduring the synchronization process. The proposed control strategyis verified using a case study.

Index Terms—Diesel-driven generators, diesel engines, powerquality.

I. INTRODUCTION

D IESEL power plants have traditionally been key sourcesin islanded power systems and also in industrial networks,

as emergency units [1], [2]. Nowadays, diesel power plants arerequired in certain power systems to guarantee the integrationof intermittent power sources, such as wind and solar energy[3]–[5].

In the case of islanded networks, it is necessary to developnew control systems to improve the transient response of a dieselgenerating set during disturbances caused by renewable powerplants [6] and load variations [7].

Nevertheless, the main concern with diesel power plants is thereduced quality of their power output, due to torque oscillations[8], [9]. The torque of an internal combustion engine is the sumof the individual torques from each cylinder. This torque hasperiodic fluctuations of several frequencies, depending on theengine speed, number of cylinders, and firing asymmetries [10].Once the generator is connected to the grid, the engine torque

Manuscript received July 27, 2009; revised April 15, 2010; accepted July 5,2010. Date of publication August 12, 2010; date of current version November19, 2010. Paper no. TEC-00303-2009.

C. A. Platero and F. Blazquez are with the Electrical Engineering Depart-ment, Escuela Tecnica Superior de Ingenieros Industriales, Polytechnic Univer-sity of Madrid, Madrid 28006, Spain (e-mail: [email protected];[email protected]).

P. Frıas is with the Electrical and Power System Department, Universidad Pon-tificia Comillas, Madrid 28015, Spain (e-mail: [email protected]).

A. J. Casado is with the Melilla Diesel Power Plant, Endesa Generacion S. A.,Melilla 52004, Spain (e-mail: [email protected]).

Color versions of one or more of the figures in this paper are available onlineat http://ieeexplore.ieee.org.

Digital Object Identifier 10.1109/TEC.2010.2058116

fluctuations result in output power oscillations, including activeand reactive power, which directly affects voltage and frequency(in the case of islanded networks).

These electrical fluctuations diminish the quality of the powersupplied by diesel power plants. Moreover, when misfiring oc-curs in the internal combustion engine, the mechanical torqueoscillations increase significantly and the electrical power qual-ity is further degraded. In some operating situations, voltageflicker appears because the frequencies of the engines os-cillations are close to those, where the flicker is perceptible[10], [11].

The total electrical fluctuations and the power quality in amultiunit power plant depend on several factors. One of themost important factors is the phase angle between the individualgenerators’ electrical fluctuations.

This paper presents a novel control strategy that reduces theelectrical fluctuations of the multiunit diesel power plants andimproves output power quality. The strategy is based on con-trolling the phase of the mechanical rotors between the differentgenerators during the electrical synchronization process. Theimplementation of the proposed control strategy requires theinstallation of a system to measure and control the mechanicalphase angle between the rotors, since current power plants donot feature such a system [12].

Another key contribution of this control strategy is that powerquality is improved for the power plant as a whole, withoutrequiring the use of techniques to improve the power qualityof each individual unit. Therefore, the proposed control strat-egy is completely compatible with existing quality techniques,which are based on the quality improvement of each individualunit.

The performance of the proposed control strategy depends onthe network to which the generator is connected. A diesel powerplant computer model was developed using the measurementsfrom real-diesel power stations over more than eight years. Thecomputer model was used to verify the applicability of the pro-posed approach. The simulations results show that electricalfluctuations can be easily reduced by a factor of four in mostcases.

This paper is structured as follows. Section II presents a the-oretical approach to diesel power plant oscillations and theirinfluence on the power system quality. Actual measurementsand experiments on diesel power plant operation are presentedin Section III. Section IV describes the basics of the novel con-trol strategy for power quality improvement. Finally, Section Vpresents the results of the simulations to verify the performanceof the approach. These simulations involve a diesel power plantwith two low-speed units.

0885-8969/$26.00 © 2010 IEEE

PLATERO et al.: COORDINATED POWER QUALITY IMPROVEMENT IN MULTIUNIT DIESEL POWER PLANTS 1103

II. INFLUENCE OF DIESEL POWER PLANTS

IN POWER SYSTEM QUALITY

A. Torque Produced by Diesel Engines

As stated earlier, the torque generated by a diesel engine isthe result of the torque of all the cylinders and it has periodic os-cillations at different frequencies. When the engine is balanced,all the cylinders will produce exactly the same torque and theywill be temporally equidistant over the entire power stroke cycleof the engine. In this ideal balanced operation, the torque oscil-lation frequency will be identical to the firing frequency (if Nis the number of cylinders, the oscillation frequency will be Nfor two-stroke engines, and N /2 frequency for four-stroke en-gines) [13]. Real-diesel engines, even new ones, cannot achievea perfectly balanced operation, as the engine cylinders do notproduce exactly the same torque. For instance, when one cylin-der misfires, the torque produced by this cylinder is reduced. Alow-frequency torque harmonic then appears, associated withthe engine power stroke cycle.

In a two-stroke diesel engine, the engine power stroke cyclecorresponds to one shaft revolution, and in the case of a four-stroke diesel engine, the engine power stroke cycle correspondsto two revolutions. Thus, the lowest torque harmonic frequencyis one for two-stroke and half for four-stroke engines.

For the sake of simplicity, this paper focuses on two-strokeengine-driven generators. The results can be still easily appliedto four-stroke engine-driven generators by taking into accountthat the power stroke cycle of the engine corresponds to twoshaft revolutions.

B. Diesel Engine Operations Modes

Modern diesel engines generally allow for operation with acertain firing imbalance. The imbalance is calculated as the dif-ference between the average effective pressure in each cylinderand the average effective pressure in all the cylinders. It is typ-ical practice to run the engine as long as the pressures do notdiffer by more than ±0.50 bar.

In real-engine operations, the most important torque harmon-ics are those associated with the firing asymmetries between thecylinders (order 1 and 2) and those associated with the cylinderfiring frequency (order N ). In measurements of actual dieselpower plant, mainly order 1 torque harmonic was found.

Table I shows a harmonic analysis during normal operation,with the maximum tolerable cylinders pressure imbalance fora 33-MW two-stroke ten-cylinders diesel engine, with a ratedspeed of 102.85 r/min. Two operating scenarios are analyzed:case A represents normal operation with a maximum order 1torque harmonic, and case B represents the normal operationwith a maximum order 2 torque harmonic.

The engine torque can be formulated as the addition of thedifferent harmonics as follows:

Meng (θmec) = Meng0 +N∑

x=1

Mxsin (xθmec + ϕx) (1)

whereMeng engine torque;

TABLE ITORQUE HARMONIC ANALYSIS OF THE 33 MW 10 CYLINDERS 102.85 r/min

TWO-STROKES LOW-SPEED DIESEL ENGINE MAN B&W 10K80 MS

Meng0 engine torque mean value;Mx amplitude of the engine torque harmonic x;N number of engine cylinders;x order of the harmonic;θmec shaft mechanical angle;ϕx phase of the engine torque harmonic x.

C. Conversion of Mechanical Torque Into Electrical Power

The conversion of the different mechanical torque harmonicsinto electrical power depends on their frequencies. In this sec-tion, a simplified description of this phenomenon is presented.

The rotor dynamics formulated as the difference between en-gine torque Meng and generator torque Mgen can be formulatedas a function of the generator load angle δ, the pole pairs p, andthe generator inertia J as following:

Meng − Mgen =J

p

d2δ

dt2. (2)

The generator torque depends on the variation of the loadangle δ and the speed variation of the load angle known as theslip s. In order to analyze the operation of the generator aroundthe mean load angle δ0 , a small-signal analysis is carried out asfollowing:

Mgen = Mgen0 +(

∂M

∂δ

)Δδ +

(∂M

∂s

)Δs. (3)

The generator torque (3) can be simplified as expressed in thefollowing equation:

Mgen = Mgen0 − c · α − kdα

dt(4)

whereMgen generator torque;Mgen0 generator torque mean value;δ generator load angle;δ0 generator load angle mean value;s slip, speed variation of generator load angle;α variation of generator load angle Δδ;c synchronizing torque coefficient;k damping torque coefficient.


Finally, the rotor motion equation (2) can be expressed as afunction of the variation in the generator load angle α, as shownin the following:

Meng − Mgen0 =(

J

p

)d2α

dt2+ k

dα

dt+ c · α. (5)

Using (1), the rotor motion equation can be also formulatedconsidering the engine torque harmonics x

N∑x=1

Mxsin (xθmec + ϕx) =(

J

p

)d2α

dt2+ k

dα

dt+ c · α. (6)

Once the generator is connected to the grid, the speed isclose to the synchronous speed, allowing the engine torque tobe expressed as a time function. Then, for each engine torqueharmonic x, there is a differential angular velocity ν, and thus,

Mxsin (vt + φx) =(

J

p

)d2α

dt2+ k

dα

dt+ c · α. (7)

The vector solution of (7) is the following:

α =Mx

c − (J/p) · ν2 + j · ν · k . (8)

The variation in the load angle α depends on the frequenciesof the harmonics, and has a different phase and amplitude. Thefrequency at which the variation of the generator load angleis only limited by the damping torque coefficient is known asnatural frequency ν0 , which can be formulated as follows:

ν0 =√

p · cJ

. (9)

According to (8) the engine torque harmonic Mx is equal tothe addition of three torques Mb , Md , and Ms (10). The inertiatorque Mb is proportional to the second-time derivative of α.The damping torque Md is proportional to the time derivate ofα, and the synchronizing torque Ms is proportional to α

Mx = −α ·(

J

p

)· ν2 + j · α · ν · k + α · c

= − Mb + Md + Ms. (10)

The sum of the synchronizing and damping torques is theelectromagnetic generator torque ΔMe . Experience shows thatspeed oscillations are negligible compared with torque oscil-lations, if both are formulated in per unit. Moreover, as theengine speed is close to 1 per unit, power oscillations ΔP canbe formulated as follows:

ΔP ∼= Δ Me · Ω ∼= Δ Me. (11)

Consequently, the conversion of the different torque harmon-ics x into electrical power depends on the frequency of the x har-monics. The ratio between the mechanical torque and the activepower is known as the power amplification factor ΔP /ΔM [14]that also depends on the frequency, and can be expressed ac-cording to the following equation:

ΔP

ΔM=

Δ Me

Mx

=c + j · ν · k

c − (J/p) · ν2 + j · ν · k . (12)

Fig. 1. Power amplification factor ΔP /ΔM for different damping torquecoefficients.

Fig. 1 shows the power amplification factor ΔP /ΔM for atypical diesel power plant for different damping torque coeffi-cients k.

The amplification factor ΔP /ΔM is one at zero frequency.This factor increases up to the resonant frequency ν0 , whereit exhibits its maximum value, which is limited only by thedamping torque coefficient. Greater frequencies result in loweramplification factors, which become zero for infinite frequency.

The normal practice is to design the generation unit with anatural frequency ν0 lower than all the excitation frequencies(engine torque harmonics). The harmonic closest to the naturalfrequency ν0 , which is the order 1 engine torque harmonic, isthe most amplified and would have a larger influence on thepower system.

D. Current Technologies and Advances for Improving PowerQuality in Diesel Generators

Current industrial developments on improving the electricalpower quality of internal combustion engine-driven generators,especially in medium- and low-speed diesel power plants, aregeared toward improving of the individual unit power quality.Some of the more common developments are as following.

1) Selection of the inertia: The selection of the inertia is re-lated with the natural frequency of the unit. The excitationfrequencies (diesel engine torque harmonic frequencies)and the natural frequency of the unit should be “far enoughaway” to avoid resonance and undesirable flicker. The rec-ommended margin between the natural frequency and thelowest excitation frequency should be 25% at least, ac-cording to most diesel engine manufacturers. Accordingto [14] and [15], there is a minimum recommended inertiafor operating the generator in satisfactory conditions. Insome cases, additional inertia is added to minimize poweror voltage fluctuations, and not only to avoid resonance.The increasing of the inertia is a useful method for mini-mizing all types of fluctuations.

2) Damping: For combustion engine generators, the damp-ing is suitable for transient, as well as for steady-stateoperation, since the generator load angle is continuouslyoscillating around the average load angle. The drawback


Fig. 2. Active power, voltage, and frequency recorded on a low-speed two-strokes 45.9 MVA/102.8 r/min diesel engine-driven generator.

to the damping is the energy consumption through copperlosses in the damping windings [16].

3) Power system stabilizer: The power system stabilizer(PSS) is used to enhance the damping of power systemoscillations though the generator excitation current con-trol [17]. In diesel power plants, the PSS normally is set todampen the active power fluctuations produced by the en-gine oscillatory torque. The main disadvantage of the PSSis that the damping properties depend on various param-eters, such as power system impedance and the operatingpoint of the synchronous machine [18].

III. ANALYSIS OF DIESEL POWER PLANTS POWER QUALITY

Measurements in actual diesel power plants served to verifythe theoretical approach used to analyze the power quality prob-lems. The first quality problem is the presence of low-frequencyharmonics, mainly of order 1, related to the diesel engine powerstroke cycle frequency, in the active power and, in general, in allthe electrical parameters of the unit: voltage, current, reactivepower, etc. The second quality problem is the high correlationbetween the amplitude of the electrical oscillations and the pres-sure imbalance between the cylinders of the diesel engines.

A. Steady-State Operation

The existence of low-frequency electrical fluctuations isclearly evident in all the diesel-driven generators, even in com-pletely brand new units, as shown in Fig. 2. Data were recordedduring the commissioning of the unit, when active power fluc-tuations were around 9% of rated power with a period cor-responding to the engine power cycle (order 1). In addition,voltage and frequency fluctuations were also observed. Otherhigher frequency harmonics are insignificant in the electricalmeasurements, as expected.

It is remarkable that the data for the generator in Fig. 2 is fora new unit. This fact is a confirmation that the most significantharmonics are of the lowest excitation frequency, order 1, as itwas described in the previous section.

Fig. 3. Active power and current rms record of a medium-speed four-stroke11.6 MVA/600 r/min diesel engine-driven generator under normal operation.

Fig. 4. Active power and rms current recorded on a medium-speed four-stroke11.6 MVA/600 r/min diesel engine-driven generator under forced imbalanceoperation.

B. Artificial Imbalance Operation

For analysis purposes, an artificial imbalance in the fuel inputof the cylinders was established in different diesel power plants.The imbalance was created by pushing or pulling the actuatorof one fuel pump in order to increase or decrease the quantity ofheavy fuel injected into one cylinder. The results of these exper-iments were that the electrical fluctuations corresponding to thepower stroke frequency, harmonic 1, increased considerably.

For example, one test is presented in Figs. 3 and 4, whichshow the measurements for normal operation and forced im-balance operation, respectively, of a 11600 kVA/9280 kW, 14cylinders, 600 r/min four-stroke diesel engine-driven generator.This generator is placed in an autonomous power system with atotal installed generation capacity of 88 MW.

The figures reveal a 53% increase in the active power oscil-lations, from 374 to 575 kW, just by pushing 5% of the totallength of the actuator of one fuel pump more than the rest, inorder to inject more fuel into this cylinder. It is remarkable howa 5% modification in the fuel injection in one of the engine’s 14cylinders can increase power oscillations by 53%.


Another important issue to consider is that the amplitude ofthe electrical oscillations is closely related with the amplitudeof the torque harmonic as expected.

IV. GENERATORS ROTORS SYNCHRONIZATION:PRINCIPLE OF OPERATION

The proposed control strategy improves the power quality ofthe whole power plant, and is based on the compensation of theelectrical fluctuations of the different generators, through thecontrol of the mechanical rotors phase angle during the elec-trical synchronization to the grid. The mechanical phase anglebetween two generators rotors is “fixed” after the synchroniza-tion to the grid, and it is a random, noncontrolled magnitude,because only electrical parameters are taken into considerationwhen synchronizing generators to the power system. This rotorphase angle is going to be included in the electrical synchro-nization process, and the optimal mechanical rotor phase will beselected so as to minimize the electrical oscillations, thus result-ing in the best power quality and, as a consequence, improvingthe power quality of the entire power plant.

A. Different Possibilities of Synchronization

When two generators with the same number of pole pairsp are coupled, their armature voltages are in phase, as are thenorth and south poles of their rotors. Then, there are p differ-ent possibilities for connecting these two generators to the grid,depending on the relative pole or rotor positions. In the case offour-stroke engine-driven generators, there are 2p synchroniza-tion possibilities, because the engine power stroke correspondsto two shaft revolutions.

B. Rotor-Phase Synchronization

The power quality of a multiunit diesel power plant dependson several factors, one of them being the phase angle betweenthe electrical fluctuations of the individual generators, whichis equivalent to the order 1 torque harmonic phase angle. Thepower quality, therefore, depends on the mechanical phase an-gle between the rotors of the different power plant generators.Conventional electrical synchronization systems only take intoconsideration electrical parameters, namely the generator ter-minal voltage and bus voltage. Closing the generator breakersafely requires complying with electrical synchronization con-ditions. In addition to electrical synchronization conditions, thisnew control strategy selects the best from among all of thep synchronization possibilities, taking into account the powerquality resulting from the synchronization of the lowest fre-quency torque harmonic, so as to compensate for the electricaloscillations between the generators.

Thus, the rotors of the different units should be synchronizedin a fixed relative position, depending on the phase of the order1 torque harmonics. Consequently, it is necessary to know themechanical phase angle between the rotors. This is achieved byinstalling a sensor in the shaft of each generator. The simplestmethod involves sensor that sends a signal when the shaft isin a specific position. This could be an inductive proximity

Fig. 5. Example of active power and shaft position reference records in thediesel power plant computer model.

sensor similar to those used for the engine speed measurement.The time between the signals from two generator sensors isproportional to the mechanical phase between the two rotors,because the units are operating at rated speed. These signals,the shafts position references, will be recorded along with theengine power, as shown in Fig. 5.

This makes it possible to obtain the relative position of theactive power fluctuations, and consequently, the relative posi-tion of the order 1 engine torque harmonics, which is the newparameter to be synchronized.

Other more complex systems for measuring the relative po-sition of the rotors could be used, but the considered system,is enough for obtaining the necessary information to do theoptimal synchronization of the generators.

If the proposed method is used, the power oscillations couldbe completely cancelled only when the order 1 torque harmonicsamplitude are the same. However, in real operation, it is ratherunusual that the harmonics amplitude are identical, because itdepends on the cylinder pressure imbalance or the operationpoints of the generators. Nevertheless, even if the amplitudeswere different, the best power quality from the power plantwould be obtained with the proposed method.

C. Optimal Synchronization-Phase Estimates

The objective is to synchronize the order 1 torque harmonicsin antiphase, meaning the optimal phase angle is 180◦. Selectingthe optimal displacement between the shafts reference positionsrequired taking individual previous records of active power andshaft position for the different generators, similar to this shownin Fig. 6, with the information from the individual records, it ispossible to calculate the optimal time between the shaft positionreferences for obtaining 180◦.

The optimal time between the shaft position references TX ,can be calculated as follows:

TX =T

2+ TG1 − TG2 (13)


Fig. 6. Optimal synchronization time calculation for a two-unit power plant.

whereTG 1 and TG 2 time between the shaft reference position sig-

nal and the maximum active power, for units1 and 2, respectively [seconds];

T revolution period time [seconds];TX optimal synchronization time [seconds].As there are only p different possibilities of synchronization,

the time between the shaft position references cannot be exactlythe optimal synchronization time TX , but it will be selected theclosest to the optimal.

D. Rotor-Phase Synchronizer Block Diagram

The rotor-phase synchronizer was used to record the activepower and the shaft position reference signals of the differentunits during a previous rated load operation tests, as shown inFig. 5. Using the information from these prior tests, the syn-chronizer calculates the optimal time between the shaft positionreferences TX for the next synchronization of any of the units.

Fig. 7 shows the block diagram of the rotor-phase synchro-nizer based on the measurement of active power. The conven-tional generator synchronizer is in the upper part of the diagram,and the additional rotor-phase synchronization is at the bottom.

In Fig. 7, actual mechanical angle between generators is cal-culated as the difference of the time t between the referencegenerator signals. Measured time t is compared with the optimalsynchronization time TX , which has been previously calculated.The difference Δt is the input of a proportional-integral (PI) reg-ulator. This regulator modifies the speed set point n, in order toselect between the different p possibilities of synchronization.

Fig. 7. Rotor-phase synchronizer block diagram.

The optimal synchronization conditions are when Δt is closeto zero. Then, the conventional generator synchronizer sendsthe closing command to the generator breaker.

The synchronization procedure for a generic n-machinespower plant is similar to the previous two-machine case. Then – 1 machines, which are already online can be considered as asingle-equivalent generator. The active power of the equivalentgenerator can be calculated as the sum of the active power ofthe individual n – 1 machines, or measured at the power plantgrid connection point. On the other hand, the reference signalfor the shaft position of the equivalent generator could be theshaft position of any of the n – 1 machines already online.

Therefore, it is possible to measure and record the activepower and shaft position of the equivalent generator, as it hasbeen presented in Fig. 5 for an individual generator. The analy-sis of the power output of the equivalent generator identifiesthe power oscillations of the whole power plant. Then, thesynchronization of the new generator (machine n) will try tocompensate the total power oscillations. The calculation of theoptimal synchronization phase has been previously presented inSection IV-C.

V. CASE STUDY

A two-unit low-speed diesel power plant model was devel-oped using Simpower (MATLAB and Simpower are trademarksof Mathworks).

The engines considered are the 33-MW low-speed MANB&W 10K80 MS diesel engine with a torque distribution shownin Table I. The technical data of the engines and generators areprovided in the Appendix.

The computer model was validated and tested to check forproper operations with different engine torque harmonics, reso-nant frequency, power amplification factor, etc. For the valida-tion of power plant model, measurements taken from real-powerplants were used. In Fig. 8, a simulation of the diesel power plantmodel connected to a noninfinite bus (1000 MVA) is shown.


Fig. 8. Active power and voltage of the diesel engine-driven generator of thediesel power plant model.

A. Cases Considered

The phase angles between the order 1 torque harmonics con-sidered in the simulation range from 0◦ to 180◦, being the worstand the best cases, respectively. The remaining higher frequencytorque harmonics are considered as well, but always in phase,because if the compensation of harmonic 1 is achieved, the worstcase is that the phase of the remaining harmonics will be zero.

In actual operations, diesel engines do not have exactly iden-tical cylinder pressure imbalances. Therefore, different ampli-tudes of order 1 engine torque harmonics were considered forengine 1 and for engine 2. The engine torque ratios consideredin the simulations were 100%, 75%, 50%, 25%, and 0%.

The case study considered different possible scenarios for adiesel power plant.

1) Diesel power plant connected to an infinite bus.2) Diesel power plant connected to noninfinite bus (short-

circuit power from 1000 to 10 000 MVA).3) Diesel power plant operating with another generator (gen-

erator power from 8 to 800 MVA).4) Islanded operation of the diesel power plant.All these scenarios were combined with different engine order

1 torque harmonic ratios and with different rotor mechanicalphase angles, resulting in hundreds of simulation runs.

B. Simulations Results

The results of the simulations show that the best compensa-tion, and consequently the best power quality, is attained whenthe phase displacement of the lowest frequency torque harmon-ics is 180◦ antiphase mode.

Another expected conclusion is that the power quality is betterif the order 1 torque harmonics have similar amplitudes. Incase of different order 1 torque harmonics amplitudes, the bestpower quality from the power plant is still achieved in antiphaseoperation mode.

Fig. 9. Power plant and individual unit active power at 30◦ rotor phase anglein infinitive bus operation.

Fig. 10. Power plant and individual unit active power at 160◦ rotor phaseangle in infinitive bus operation.

Fig. 11. Power plant active power at 10◦ rotor phase angle in infinitive busoperation harmonics analysis.

For example, Figs. 9 and 10 clearly show the power plant ac-tive power fluctuation reductions when synchronizing at a 160◦

rotor phase angle, as compared to a 10◦ angle in an infinite busoperation with large units power fluctuations. We can observemore than five times reduction in the active power fluctuation.

The harmonics analysis of the results for the aforementionedcases are enclosed in the Figs. 11 and 12. It can be observedthe compensation of the order 1 harmonic, while the higherorder harmonics remain constant, according to the simulationhypotheses.

Fig. 13 includes the results of the diesel power plant con-nected to a noninfinite bus (1500 MVA), with three rotor phasedifferences: 0◦, 90◦, and 160◦. A reduction greater than fourtimes in voltage fluctuations is observed.


Fig. 12. Power plant active power at 160◦ rotor phase angle in infinitive busoperation harmonics analysis.

Fig. 13. Generation bus voltage of diesel power plant model at 0◦, 90◦, and160◦ rotor phase angles in noninfinite bus operation (1500 MVA).

A brief summary of all the simulations is included in Table II.This table presents the amplitudes of the electrical oscillations(peak-to-peak), for the two most representative operation situa-tions: infinite bus and islanded operation.

The analysis of these simulations shows the improvementsachieved in the proposed approach, synchronization with at 180◦

compared with 0◦ for different scenarios:1) voltage fluctuations reduced by four times;2) frequency fluctuations reduced by 3.8 times;3) power fluctuations reduced by 12 times.Other simulations were conducted for new power plants

considering reduced inertia in the generators, while keepingthe electrical fluctuations constant. In some cases, the inertiacould be easily reduced by 15%, for obtaining similar electricalfluctuations.

TABLE IISIMULATION RESULTS

Fig. 14. Simulated power plant active power fluctuations amplitude in infinitebus operation.

For all the scenarios and magnitudes analyzed, the simulationresults are similar to those shown in Fig. 14, which presentspower plant power fluctuations in the case of an infinite grid. Asstated earlier, the best compensation is obtained if the engineorder 1 torque harmonics phase is 180◦ out of phase and theratio is 1, meaning they have similar amplitudes.

VI. CONCLUSION

A new control strategy for synchronizing internal engine-driven generators to the power system and which takes intoconsideration the mechanical generator rotor phase angle hasbeen developed.


This synchronization control strategy allows increasing thepower quality of multiunit diesel power plants, reducing all elec-trical fluctuations by compensating for the individual generatorelectrical fluctuations.

Moreover, this control strategy is compatible with all ex-isting techniques, as it is based on the control of the genera-tors’ mechanical rotor phases, which actually is a noncontrolledparameter.

The benefits of the rotor synchronization were evaluated un-der different operating conditions using a computer model ofa two-unit low-speed diesel power plant. The results served tovalidate the proposed control strategy, and revealed improvedpower quality under all possible operating conditions.

APPENDIX

See Tables III and IV as following.

TABLE IIITECHNICAL DATA OF MAN B&W 10K80 MS TWO-STROKES LOW-SPEED

DIESEL ENGINES

TABLE IVTECHNICAL DATA DIESEL ENGINE-DRIVEN GENERATORS

ACKNOWLEDGMENT

The authors would like to thank for the contributions ofMr. F. Vacas of ENDESA Power Generation, Spain, Mr Coverof Grand Bahamas Power Ltd. and Mr. Amaya of Alstom PowerS. A.

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Carlos A. Platero was born in Madrid, Spain, in 1972. He received the Diplomaand Ph.D. degrees in electrical engineering from the Polytechnic University ofMadrid, Madrid, Spain, in 1996 and 2007, respectively.

He joined Electrical Engineering Power Plant Division, ABB GeneracionS. A., in 1996, where he was involved in design and commissioning of hydroand diesel power plants for Power Plants Engineering Department, EndesaGeneracion S. A. Since 2002, he has been teaching in the Electrical EngineeringDepartment, Polytechnic University of Madrid, where he is engaged in EnergyResearch Group.

Francisco Blazquez (M’07) was born in Toledo, Spain, on April 9, 1972. Hereceived the Diploma degree in industrial engineering and the Ph.D. degree inelectrical engineering from Universidad Politecnica de Madrid, Madrid, Spain,in 1997 and 2004, respectively.

Since 1999, he has been a Professor in Electrical Engineering Department,Universidad Politecnica de Madrid. His current research interests include elec-trical machine design and wind power generation.


Pablo Frıas received the M.S. and Ph.D. degrees in electrical engineering fromthe Pontificia Comillas University ICAI, Madrid, Spain, in 2001 and 2008,respectively.

He is currently a Researcher at the Institute for Applied Research, Universi-dad Pontificia Comillas, where he also teaches in the Power System Department,Engineering School (ICAI). He has participated in many international researchprojects and in several consultancy projects with electricity utilities in Spainand South America. His research interests include electric machines, ancillaryservices in power systems, and distributed generation.

Antonio J. Casado was born in Melilla, Spain, in 1974. He received the Dipl.Eng. degree in electronic engineering from the Pontificia Comillas UniversityICAI, Madrid, Spain, in 2002. He is currently working toward the Ph.D. degreein electrical machines from Polytechnic University of Madrid, Madrid.

He is also a Chief of electrical, instrumentation, and control systems main-tenance at Melilla Diesel Power Plant, Endesa Generacion S. A., Melilla, SpainHis research interests include electrical machines, diesel generation, and powersystems maintenance.

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