WO2010002051A1

WO2010002051A1 - Engine-generator provided with super capacitor

Info

Publication number: WO2010002051A1
Application number: PCT/KR2008/003964
Authority: WO
Inventors: Seng Gi Seol
Original assignee: Seoul National University Industry Foundation
Priority date: 2008-07-04
Filing date: 2008-07-04
Publication date: 2010-01-07

Abstract

The present invention relates to an engine-generator system which drives an engine so as to generator power. The engine-generator includes an engine, a generator, a rectifier, an inverter, and a super capacitor. The super capacitor is connected between positive and negative terminals of an output terminal of the rectifier, and is electrically connected in parallel to the inverter. When the magnitude of a load which receives power from the engine-generator increases instan¬ taneously, electric energy stored in the super capacitor is discharged so as to be supplied to the load. Therefore, it is possible to enhance the response speed of the engine-generator system in response to the variation in load magnitude. Further, it is possible to solve the problem of the conventional engine-generator system where a mechanical response (that is, control of engine speed) in response to the variation in load magnitude is slow.

Description

ENGINE-GENERATOR PROVIDED WITH SUPER CAPACITOR

Technical Field

[1] The present invention relates to an engine-generator system, and more particularly, to an engine-generator system which drives an engine by using a super capacitor, thereby generating power.

[2]

Background Art

[3] Currently, engine-generators are being widely used in a variety of industrial sites.

The engine-generators are widely used for supplying a power with constant voltage/ constant frequency at a construction site, a remote place, a delivery equipment (for example, large- sized truck or harbor container crane) or a ship, where a utility power source is unavailable.

[4] FIG. 1 is a diagram showing a conventional engine-generator system. The engine- generator system includes an engine 110 and an engine-generator 100 comprising a wire- wound synchronous generator 120 which is driven by an output of the engine so as to generate power. The power output from the generator 120 is supplied to an electrical load 200.

[5] The electrical load 200 of the engine-generator system is an electrical system which has a very fast response characteristic and a large variation in load magnitude. On the other hand, since the engine-generator 100 using the engine 110 is a mechanically driven system, the response speed of the engine-generator 100 is relatively low. Therefore, a variation in output voltage and frequency of the engine-generator 100 occurs inevitably.

[6] As shown in FIG. 1, the conventional engine-generator includes a speed governor

112 which adjusts the amount of fuel supplied to the engine by controlling a throttle valve 114 in response to the rotation speed w_φm of the engine, thereby adjusting the speed (output frequency) of the engine to be constant. The engine-generator further includes an automatic voltage regulator (AVR) 122 which controls the exciting current of the generator by using a field winding 124 in response to the output voltage of the generator, thereby adjusting an output voltage to be constant.

[7] However, in the above-described control devices, variation in output voltage and frequency occurs inevitably because of a slow response characteristic of the mechanical system, when the electrical load changes abruptly.

[8] To reduce such a variation, an engine-generator having a larger capacity than the maximum rating of an electrical load is installed so as to make the response to the abruptly changing electrical load less sensitive. However, in case of typical or light load, such a configuration lowers operation efficiency, thereby increasing the operation cost of the generation system. Further, since an engine-generator having a larger capacity than a load should be installed, the initial installation cost increases.

[9] Most generators are operated for a much longer time under a light-load or no-load condition than around the rating load. Therefore, the enhancement of generation efficiency under a light-load condition is a very important factor with respect to the enhancement of the overall operation efficiency of the engine-generator. Nowadays, as the fuel cost increases continuously, the enhancement of generation efficiency under a light-load condition becomes more important.

[10] The conventional engine-generator should continuously rotate at a constant speed where a rated output can be obtained, in order to maintain the output frequency of the generator to be constant even when no load or a light load is applied. In this case, however, fuel consumption with respect to output significantly increases because of an engine characteristic.

[11] In a vehicle which produces a mechanical output through an engine, a transmission is controlled in response to the mechanical output of the engine, in order to adjust the operation speed of the engine where fuel efficiency is optimized, thereby improving the fuel efficiency. However, in an engine-generator which should always rotate at a constant speed, such an operation is impossible. As a result, the efficiency of the engine-generator under a no-load or light-load condition is several times less than that of the engine-generator under a rated- load condition.

[12] The engine-generator needs to adjust field currents so as to prevent a variation in terminal voltage caused by a drop of internal impedance which occurs when an output varies under a constant speed. To this end, the wire- wound synchronous generator 122 provided with the field winding 124 should be used. The structure of the engine- generator is complex, because of the filed winding and the mechanical system which supplies electricity to the field winding. Further, the efficiency of the generator is lower than that of a synchronous generator using a permanent magnet.

[13] To solve such a problem, a variable- speed engine-generator has been developed. As shown in FIG. 2, a typical variable-speed engine-generator obtains DC power through a diode rectifier 300 connected to an output terminal of a permanent-magnet AC generator 120, and converts the DC power into AC power with a constant voltage/ constant frequency through an inverter 400, thereby supplying the AC power to an electric load 200. Further, a capacitor 310 which is provided between the output terminals of the rectifier can control the speed of the engine by controlling a throttle valve 144 in response to the voltage value between the output terminals. Therefore, the engine speed can be adjusted in response to an electric load, which makes it possible to enhance fuel efficiency when no load or a light load is applied.

[14] FIG. 3 is a diagram showing an example of a three-phase constant voltage/constant frequency (CVCF) inverter 400 which is used in an engine-generator system.

[15] Because the output voltage of the variable- speed engine-generator system is proportional to the rotation speed, the DC voltage which is an output of the diode rectifier 300 decreases when the engine speed is decreased to optimize the fuel efficiency of the engine 110 under a light-load condition. Therefore, the engine-generator should be designed in such a manner that the inverter 400 outputs a rated voltage even at the minimum operation speed of the engine 110. In this configuration, when the engine is operated at a rated speed for the rated output, the voltage rating of the inverter 400 and the diode rectifier 300 increases excessively in comparison with the AC output voltage. As a result, the capacity of the inverter 400 and the diode rectifier 300 increases, and the price thereof also increases.

[16] In this case, the variation in output voltage of the diode rectifier 300 in response to the variation in engine rotation speed can be reduced to some degree by using the wire- wound AC generator 122 (refer to FIG. 1) having a field winding. However, the wire- wound AC generator 122 has a complex structure and low efficiency, and the price of the generator is high.

[17] The engine-generator shown in FIG. 1 and the variable-speed engine-generator shown in FIG. 2 should have a separate starting motor (not shown) for driving the engine 110. Therefore, the price increases, and the structure becomes complicated because of the starting motor.

[18] Recently, with the development of the power electronics technologies and the reduction in price of power semiconductors, an engine-generator system has emerged, in which the diode rectifier 300 (refer to FIG. 2) of the variable- speed engine-generator system is substituted with a pulse width modulation (PWM) boost rectifier 302, as shown in FIG. 4. FIG. 5 is a diagram showing the configuration of a three-phase PWM boost rectifier which is used in the variable- speed engine-generator system of FIG. 4.

[19] In the above-described engine-generator system, when a voltage is provided in a DC terminal 310 at the initial startup, the PWM boost converter 302 is used as an inverter without a separate starting motor, and the generator 120 is driven as an electric motor so as to drive the engine 110. Even when the speed of the engine changes from 0 to the idle speed, the voltage of the DC terminal 310 can be maintained constant. Therefore, it is possible to optimize the voltage rating of the CVCF inverter which outputs an AC voltage.

[20] As described, all the conventional engine-generator systems obtain an electrical output from the mechanical engine 110. Therefore, when the electric load 200 changes abruptly, an AC output voltage is varied due to a response (i.e., the output of the engine-generator) delay of the mechanically driven system.

[21] In particular, the variable- speed engine-generator is operated at a considerably lower speed than the engine operation speed of a rated output, in order to improve the fuel efficiency of the engine under a light load. In this case, even when the electric load increases rapidly, the speed of the engine does not increase rapidly. Therefore, an AC output voltage of the inverter decreases rapidly, and the load may be cut off from power or may not operate properly. Therefore, if the variation in electric load is small or predictable, the variable-speed engine-generator system can remarkably improve the fuel efficiency of the engine. But, if the load is unpredictable or the load variation cannot be controlled, e.g., in an emergency generator or a generator used in construction equipments, the use of the variable-speed engine-generator system is limited inevitably because of the disadvantage that the variation in output voltage is large, even though it has an excellent merit of improving fuel-efficiency.

[22]

Disclosure of Invention Technical Problem

[23] Therefore, the present invention has been made in view of the above problems, and it is an object of the present invention to provide an engine generator which enhances the response speed in response to the variation in load magnitude.

[24]

Technical Solution

[25] In accordance with an aspect of the present invention, the above and other objects can be accomplished by the provision of an engine-generator system comprising an engine; a generator that is driven by an output of the engine so as to generate power; a rectifier that converts the power output from the generator into DC power; an inverter that is electrically connected to an output terminal of the rectifier and converts the DC power output from the rectifier into AC power to supply to a load; and a super capacitor that is charged and discharged with electric energy.

[26] The super capacitor is connected between positive (+) and negative (-) terminals of the output terminal of the rectifier and is electrically connected in parallel to the inverter. When the magnitude of a load which receives power from the engine- generator increases momentarily, the electric energy stored in the super capacitor is discharged so as to be supplied to the load.

[27] Alternatively, the super capacitor may be connected between the neutral point of the generator and the positive (+) or negative (-) terminal of the output terminal of the rectifier.

Advantageous Effects [28] According to the present invention, when the magnitude of a load momentarily increases, the electric energy stored in the super capacitor is discharged so as to be supplied to the load. This makes it possible to enhance the response speed of the engine-generator system in response to the variation in load magnitude. Further, it is possible to solve the problem of the conventional engine-generator system where a mechanical response (i.e., control of engine speed) in response to a variation in load magnitude is slow.

[29]

Brief Description of Drawings

[30] The above and other objects, features and other advantages of the present invention will be more clearly understood from the following detailed description taken in conjunction with the accompanying drawings, in which:

[31] FIG. 1 is a diagram showing the configuration of a conventional engine-generator system;

[32] FIG. 2 is a diagram showing the configuration of a conventional variable- speed engine-generator system;

[33] FIG. 3 is a diagram showing an example of an inverter which is used in an engine- generator system;

[34] FIG. 4 is a diagram showing the configuration of a variable-speed engine-generator system in which a separate starting motor does not need to be provided;

[35] FIG. 5 is a diagram showing the configuration of a three-phase PWM boost rectifier which is used in the variable- speed engine-generator system of FIG. 4;

[36] FIG. 6 is a diagram showing the configuration of an engine-generator system according to a first embodiment of the invention;

[37] FIG. 7 is a diagram showing the configuration of an engine-generator system according to a second embodiment of the invention;

[38] FIG. 8 is a diagram showing the configuration of a DC/DC converter used in the engine-generator system according to the second embodiment of the invention;

[39] FIG. 9 is a diagram showing the configuration of an engine-generator system according to a third embodiment of the invention;

[40] FIG. 10 is a diagram showing the configuration of an engine-generator system according to a fourth embodiment of the invention; and

[41] FIG. 11 is a diagram showing an example where the engine-generator system according to the invention is used as an uninterruptible power supply (UPS). Best Mode for Carrying out the Invention

[42] Hereinafter, an engine-generator system provided with a super capacitor according to the present invention will be described with reference to the drawings. However, the present invention is not limited to the following embodiments. For convenience of description, like reference numerals will be attached to the same or similar components in the respective embodiments, and the duplicated descriptions of the same or similar components will be omitted.

[43] FIG. 6 is a diagram showing the configuration of an engine-generator system according to a first embodiment of the invention.

[44] The engine-generator system according to the first embodiment of the invention includes an engine 110, a generator 120 which is driven by an output of the engine 110 so as to generate power, a PMW boost rectifier 302 which converts the power output from the generator 120 into DC power, an inverter 400 which is electrically connected to an output terminal of the rectifier 302 and converts the DC power output from the rectifier 302 into AC power to supply to a load 200, and a super capacitor 500 which can be charged and discharged with electric energy. A construction including the engine 110 and the generator 120 is called the engine-generator 100.

[45] The super capacitor 500 is connected between positive (+) and negative (-) terminals

1 and 2 of the output terminal of the rectifier, and is electrically connected in parallel to the inverter 400 (FIG. 3 shows an example of the inverter 400). The super capacitor 500 can be charged with voltages of the DC terminals 1 and 2 of the boost rectifier 302, when the engine-generator 100 has surplus power.

[46] When the magnitude of the electric load 200 increases rapidly and a mechanical response of the engine-generator 100 is delayed, so that a power shortage occurs instantaneously, the energy stored in the super capacitor 500 is supplied to the load 200, thereby solving the power shortage problem.

[47] Further, even when an electric load is momentarily above the rated output of the generator, the power for the load may be additionally supplied to the load, utilizing the energy of the super capacitor. Therefore, the capacity of the engine-generator can be set so as to correspond to the maximum load in a steady state, not the instantaneous peak load. Therefore, the capacity of the engine-generator can be significantly reduced.

[48] In the engine-generator shown in FIG. 6, the energy of the super capacitor 500 is proportional to the square of a DC-terminal voltage (E_cap = l/2xCV²). Therefore, in order that the energy of the super capacitor 500 is consumed to 3/4 with respect to the overall energy thereof in a state where the super capacitor 500 is fully charged (that is, 3/4 of the maximum capacity of the super capacitor 500 is discharged), the DC-terminal voltage should be varied to 1/2 of the voltage when the super capacitor 500 is fully charged. Even in this case, when an AC power with a constant voltage is output, the rated voltage of the inverter 400 during a rated- speed operation for rated output excessively increases in comparison with an AC output voltage, as in the variable speed engine-generator system using the diode rectifier 300 (refer to FIG. 2). Therefore, the capacity and price of the inverter 400 increases inevitably.

[49] The engine-generator system further includes an engine-speed governor 112. The engine-speed governor 112 controls a throttle valve 114 in response to an engine rotation speed w_φm, thereby adjusting an amount of fuel supplied to the engine. Then, the speed (output frequency) of the engine is adjusted constant, thereby reducing a variation in output voltage and frequency of the engine-generator 100.

[50] FIG. 7 is a diagram showing the configuration of an engine-generator system according to a second embodiment of the invention.

[51] In this embodiment, a DC/DC converter 510 is inserted between DC terminals 1 and

2 of the super capacitor 500 such that an inverter 400 with a small capacity can be used. The DC/DC converter 510 can change the magnitude of a DC voltage so as to deliver power in both directions.

[52] In this configuration, the DC/DC converter 510 can properly maintain the voltage difference between input and output. Therefore, voltages of the DC terminals 1 and 2 can be optimally maintained regardless of the charging or discharging of the super capacitor 500. Further, the charging and discharging of the super capacitor 500 can be controlled independently by the DC/DC converter 510 regardless of the voltages of the DC terminals 1 and 2.

[53] FIG. 8 is a diagram showing a both-direction DC/DC converter 510 used in the engine-generator system according to the second embodiment of the invention. As shown in FIG. 8, one terminal of the DC/DC converter 510 is connected to the DC terminals 1 and 2, and the other terminal thereof is connected to the super capacitor 500 through a smoothing inductor 512. The smoothing inductor 512 removes pulsating components of zero-phase-sequence currents of the super capacitor 500 and the generator 120. Therefore, although the voltage of the super capacitor 500 varies depending on the energy charging or discharging, a desired amount of energy can be supplied to the DC terminals with a constant voltage, or can be received.

[54] By using the super capacitor 500, the engine-generator system according to this embodiment can overcome various disadvantages of the variable speed engine-generator system and improve fuel efficiency of the engine-generator when no load or a light load is applied. However, since the DC/DC converter 510 is provided additionally, the price of the engine-generator system increases.

[55] FIG. 9 is a diagram showing the configuration of an engine-generator system according to a third embodiment of the invention.

[56] In this embodiment, a super capacitor 500 is connected between the neutral point 3 of a stator winding of a generator 120 and a positive (+) or negative (-) terminal 1 or 2 of a DC terminal, in order to improve fuel efficiency of the engine-generator when no load or a light load is applied, without using the DC/DC converter 510 (refer to FIG. 7). In FIG. 9, the super capacitor 500 is connected between the neutral point 3 and the negative terminal 2, for the convenience of description.

[57] In this configuration, although a separate DC/DC converter is not provided, the charging and discharging of energy stored in the super capacitor 500 can be controlled effectively.

[58] The super capacitor 500 can be charged when the engine-generator 100 has surplus power because the magnitude of a load 200 is smaller than the output of the engine- generator 100. The super capacitor 500 can control the charging and discharging of the super capacitor 500 by controlling a zero phase sequence voltage of each phase of a boost rectifier 302.

[59] As voltages of the respective phases of the boost rectifier 302 are controlled by a controller (not shown) for controlling an operation of the boost rectifier 302, a zero- phase- sequence current (i.e., 1/3 of the sum of currents flowing in the respective phases) can be controlled, and a current which is three times of the zero- phase- sequence current flows into the super capacitor 500. Therefore, as the current flowing in the super capacitor is controlled by controlling the zero phase sequence voltage of each phase, the charging and discharging of the super capacitor 50 can be controlled.

[60] When the magnitude of the load 200 increases or decreases, the voltage of each phase of the boost converter 302 can be controlled by adjusting the PWM pattern of the boost converter 302 through a controller (not shown) of the boost converter 302. As a result, the charging and discharging of the super capacitor 500 can be controlled.

[61] When the magnitude of the electric load 200 increases rapidly and a mechanical response of the engine-generator 100 is delayed so that a power shortage occurs instantaneously, the energy stored in the super capacitor 500 is supplied to the load 200, thereby solving the power shortage problem.

[62] In the engine-generator system according to this embodiment, engine speed is controlled by controlling a throttle valve 114, so that the engine-generator 100 is operated at the condition where the fuel efficiency is optimized depending on the electrical output.

[63] When an electric load increases rapidly in a state where the engine speed is lowered under a no-load or light- load condition, the zero-phase-sequence current (that is, a current flowing in the super capacitor) of the generator is controlled by controlling the zero phase sequence voltage of the boost converter 302, so that electric energy is supplied to the load 200 through the discharging of the super capacitor 500. Thereafter, when the throttle valve 114 is adjusted in such a manner that the rotation speed of the engine approaches a speed where an engine-generator output corresponding to the increasing load is generated at the optimal fuel efficiency, the zero phase sequence voltage is adjusted again so as to stop the discharging of the super capacitor 500. If necessary, the output of the engine-generator 100 may be set to be greater than the load so as to generate surplus power, and the super capacitor 500 may be charged with the surplus power.

[64] If the magnitude of the load 200 decreases rapidly while the engine-generator 100 supplies a rated output to the load 200, the surplus power between the output of the engine-generator 100 and the load 200 is absorbed by the super capacitor 500. Further, the speed of the engine 110 is decreased to a speed which provides optimal fuel efficiency with respect to the decreasing load. Then, even when the magnitude of the load 200 is varied rapidly, the DC-terminal voltage is not varied with respect to an excess output of the engine-generator 100.

[65] When a permanent-magnet generator 120 is connected to a typical boost rectifier

302, the neutral point of the generator is not connected to anywhere. Therefore, when the respective phases of the generator are balanced (that is, when the impedances of the phases are equal to each other), the zero phase sequence voltage and current of each phase of the generator is non-existent, even when the boost rectifier 302 generates any voltage. That is, the sum of currents flowing in the respective phases is instantaneously 0 at all times, in accordance with the Kirchhoffs current law.

[66] In the above-described embodiment, however, when the neutral point 3 of the generator is connected to the DC terminal (the negative (-) terminal 2 in the above embodiment) through the super capacitor 500, the boost rectifier 302 generates a zero phase sequence voltage, and a zero-phase-sequence current passing through the super capacitor 500 is generated by the zero phase sequence voltage. Therefore, by properly controlling the zero phase sequence voltage of the boost rectifier 302, the magnitude and direction of currents flowing in the super capacitor 500 can be controlled.

[67] Further, a zero phase sequence inductance (leakage inductance) of a generator 120 winding is used as an inductance between the super capacitor 500 and the boost rectifier 302. Therefore, it is possible to reduce pulsating components of currents of the super capacitor 500, thereby reducing a loss of the super capacitor 500.

[68] FIG. 10 is a diagram showing the configuration of an engine-generator system according to a fourth embodiment of the invention.

[69] In this embodiment, a smoothing inductor 530 is connected in series to the super capacitor 500, in order to reduce an operation state loss caused by pulsating components of zero-phase-sequence currents flowing in windings of the super capacitor 500 and the generator 120. Instead of the smoothing inductor 503, a smoothing filter composed of an inductor and a capacitor may be used.

[70] Further, in order to reduce losses of the generator 120 and the super capacitor 500 caused by a zero-phase- sequence current in a state where the super capacitor 500 is not charged or discharged but is in a standby mode, an electromagnetic contactor 520 or a solid state relay (not shown) formed of power semiconductor may be connected in series to the super capacitor 500.

[71] FIG. 11 is a diagram showing an example where the engine-generator according to the invention is used as an uninterruptible power supply (UPS).

[72] In this embodiment, the engine-generator according to the fourth embodiment of the invention is configured so as to be used as a UPS.

[73] That is, the electric load 200 is connected to a commercial power source 330 through the inverter 400 and the diode rectifier 320 so as to be driven. The engine-generator according to the invention is connected to the electric load 200 in parallel to the commercial power source 330 such that power can be supplied to the electric load 200 even when the commercial power source 330 is interrupted.

[74] In this configuration, the super capacitor 500 is charged with surplus power of the commercial power source 330 through the PWM boost rectifier 302 so as to prepare for a case where the commercial power source is interrupted. When the commercial power source 330 is interrupted (most instantaneous power interruptions last for 1-2 seconds), the energy stored in the super capacitor 500 is discharged through the boost rectifier 302 so as to be supplied to the DC terminal. Therefore, power can be supplied to the load 200 for a short time during which power is not supplied from the commercial power source 330.

[75] When the power interruption is extended, the energy stored in the super capacitor

500 is discharged through the PWM boost rectifier 302 such that the generator 120 can be operated in an electric-motor mode so as to drive the engine 110. Then, a mechanical output of the engine 110 is converted through the generator 120, the boost rectifier 302, and the inverter 400 for generating a constant voltage/constant frequency such that power can be supplied to the load 200. Simultaneously, until the engine- generator 100 is driven so as to generate an output required by the load 200, the energy stored in the super capacitor 500 is supplied to the DC terminal 310 through the PWM boost rectifier 302 such that power required by the load can be supplied without interruption.

[76] After the commercial power source 330 is restored to the normal condition, the super capacitor 500 may be charged through the boost rectifier 310. When the electric load is less than the output of the engine-generator 100, the super capacitor 500 may be charged with surplus power of the generator even during the power interruption.

[77] Even when the engine-generator 100 supplies power to the electric load 200, the super capacitor 500 can be charged with some of the momentary maximum power of the load 200, and the electric energy stored in the super capacitor is supplied to the electric load 200 during the power interruption. Therefore, it is possible to reduce the capacity of the engine-generator 100 which operates during the power interruption.

[78] In the above-described embodiments, the three-phase three winding engine-generator

100 is used. If necessary, however, a variety of engine-generators such as two-phase, four-phase, five-phase, and six-phase engine-generators may be used. Further, if necessary, a multi winding engine-generator with six or more windings may be used. Accordingly, it can be easily appreciated by those skilled in the art that a rectifier, an inverter, and so on with a proper form can be used.

[79] Although the preferred embodiments of the present invention have been disclosed for illustrative purposes, those skilled in the art will appreciate that various modifications, additions and substitutions are possible, without departing from the scope and spirit of the invention as disclosed in the accompanying claims.

[80]

Industrial Applicability

[81] The engine-generator according to the present invention may be widely used for supplying a constant voltage/constant frequency of power at a construction site, a remote place, or a delivery equipment (for example, large-sized truck or harbor container crane) or ship, where a commercial power source may be unavailable.

Claims

[1] An engine-generator system comprising: an engine; a generator that is driven by an output of the engine so as to generate power; a rectifier that converts the power output from the generator into DC power; an inverter that is electrically connected to an output terminal of the rectifier and converts the DC power output from the rectifier into AC power to supply to a load; and a super capacitor that is charged and discharged with electric energy, wherein the super capacitor is connected between positive (+) and negative (-) terminals of the output terminal of the rectifier and is electrically connected in parallel to the inverter.

[2] The engine-generator system as set forth in claim 1, wherein a DC/DC converter, which changes the magnitude of a DC voltage, is interposed between the output terminal of the rectifier and the super capacitor.

[3] The engine-generator system as set forth in claim 1 or 2, wherein, when the magnitude of the load increases, the electric energy stored in the super capacitor is discharged from the super capacitor so as to be supplied to the load.

[4] The engine-generator system as set forth in claim 3, wherein the charging and discharging of the super capacitor is controlled by zero-phase-sequence current control of the generator.

[5] The engine-generator system as set forth in claim 1 or 2, wherein a smoothing inductor or smoothing filter is connected in series to the super capacitor, in order to remove pulsating components of zero-phase-sequence currents of the super capacitor and the generator.

[6] The engine-generator system as set forth in claim 1 or 2, wherein an electromagnetic contactor or a solid state relay is connected in series to the super capacitor so as to remove a zero-phase- sequence current of the generator while the super capacitor is not charged/discharged but is in a standby mode.

[7] The engine-generator system as set forth in claim 1 or 2, wherein the rectifier is a pulse- width modulation (PWM) rectifier.

[8] The engine-generator system as set forth in claim 7, wherein, in order to drive the engine when the engine is stopped, the PWM rectifier discharges the energy stored in the super capacitor such that the generator can operate as an electric motor.

[9] The engine-generator system as set forth in claim 1 or 2, further comprising: an engine- speed governor that controls the rotation speed of the engine by con- trolling a throttle valve such that fuel efficiency is optimized, the throttle valve receiving the voltage magnitude of the output terminal of the rectifier and the rotation speed of the engine and then adjusting the amount of fuel supplied to the engine.

[10] An engine-generator system comprising: an engine; a generator that is driven by an output of the engine so as to generate power; a rectifier that converts the power output from the generator into DC power; an inverter that is electrically connected to an output terminal of the rectifier and converts the DC power output from the rectifier into AC power to supply to a load; and a super capacitor that is charged and discharged with electric energy, wherein the super capacitor is connected to a neutral point between the generator and a positive (+) or negative (-) terminal of the output terminal of the rectifier.

[11] The engine-generator system as set forth in claim 10, wherein, when the magnitude of the load increases, the electric energy stored in the super capacitor is discharged from the super capacitor so as to be supplied to the load.

[12] The engine-generator system as set forth in claim 11, wherein the charging and discharging of the super capacitor is controlled by zero-phase-sequence current control of the generator.

[13] The engine-generator system as set forth in claim 10, wherein a smoothing inductor or smoothing filter is connected in series to the super capacitor, in order to remove pulsating components of zero-phase-sequence currents of the super capacitor and the generator.

[14] The engine-generator system as set forth in claim 10, wherein an electromagnetic contactor or a solid state relay is connected in series to the super capacitor so as to remove a zero-phase-sequence current of the generator while the super capacitor is not charged/discharged but is in a standby mode.

[15] The engine-generator system as set forth in claim 10, wherein the rectifier is a pulse- width modulation (PWM) rectifier.

[16] The engine-generator system as set forth in claim 15, wherein, in order to drive the engine when the engine is stopped, the energy stored in the super capacitor is discharged by the PWM rectifier such that the generator can operate as an electric motor.

[17] The engine-generator system as set forth in claim 10, further comprising: an engine- speed governor that controls the rotation speed of the engine by controlling a throttle valve such that fuel efficiency is optimized, the throttle valve receiving the voltage magnitude of the output terminal of the rectifier and the rotation speed of the engine and then adjusting the amount of fuel supplied to the engine.