Controller of power generating system and method for controlling power generating system

Abstract

To provide a controller of a power generating system and a control method that can reduce losses while causing a single motor generator to function as both a starter and an electric generator.SOLUTION: The controller of the power generating system includes a processing circuit. The processing circuit is capable of executing a first control mode and a second control mode of controlling a first power converter such that DC power of a DC wire is converted into AC power and the AC power is supplied to a synchronous motor generator for starting an engine from a stop state. The second control mode includes at least one of frequency reduction processing of, during a period in which the engine has a rotational speed less than a reference rotational speed, setting a carrier frequency in the first power converter at a second carrier frequency lower than the first carrier frequency in the first control mode, and voltage reduction processing of setting a DC voltage target value of the DC wire at a second DC voltage target value lower than the first DC voltage target value in the first control mode.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present disclosure relates to a controller for a power generation system and a method for controlling a power generation system. [Background technology]

[0002] There is known a power generation system that includes an engine as a drive source for vehicles, ships, aircraft, etc., and in which a motor generator that generates electricity using the driving force of the engine is mechanically connected to the engine. Furthermore, Patent Document 1 below proposes using the motor generator not only for generating electricity, but also as a starter for starting the engine. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] JP 2001-334998 A Summary of the Invention [Problem to be solved by the invention]

[0004] However, having one motor-generator function as both a generator and a starter poses the following problems. When the motor-generator functions as a starter, a large torque is required to rotate the engine. For this reason, the motor-generator is required to have a low frequency but a large current. On the other hand, when the motor-generator functions as a generator, it needs to be able to handle high-speed engine rotation. When the engine rotates at high speed, the induced voltage becomes high. For this reason, the motor-generator is required to have a high frequency and a large voltage.

[0005] Patent Document 1 does not particularly disclose how to configure a power converter that supplies power to a motor generator and receives power generated by the motor generator in order to simultaneously achieve these two functions. If the power converter is optimized for low voltage, low frequency, and large current, it cannot handle high voltage and high frequency. If the power converter is optimized for high voltage and high frequency, switching loss increases at low voltage, low frequency, and large current. If one control mode is used to handle the large current and large voltage required for the motor generator, the power converter will become large.

[0006] The present disclosure is intended to solve the above-mentioned problems, and aims to provide a power generation system controller and a power generation system control method that can reduce losses while allowing a single motor generator to function as both a starter and a generator. [Means for solving the problem]

[0007] A controller of a power generation system according to one embodiment of the present disclosure is a controller of a power generation system including an engine, a synchronous motor-generator mechanically connected to the engine and capable of generating electricity by utilizing the driving force of the engine, and a first power converter electrically connected to the synchronous motor-generator, the controller including a processing circuit, the processing circuit being capable of executing a first control mode in which the first power converter is controlled to convert AC power in an AC wiring between the synchronous motor-generator and the first power converter to DC power and output the DC power to the DC wiring, and a second control mode in which the first power converter is controlled to convert the DC power in the DC wiring to AC power and supply the AC power to the synchronous motor-generator in order to start the engine from a stopped state, the second control mode including at least one of a frequency reduction process in which, at least when the engine is in a stopped state, a carrier frequency in the first power converter is set to a second carrier frequency lower than the first carrier frequency in the first control mode, and a voltage reduction process in which a DC voltage target value in the DC wiring is set to a second DC voltage target value lower than the first DC voltage target value in the first control mode.

[0008] A control method for a power generation system according to another aspect of the present disclosure is a control method for a power generation system including an engine, a synchronous motor / generator mechanically connected to the engine and capable of generating power by utilizing the driving force of the engine, and a first power converter electrically connected to the synchronous motor / generator, the control method switching between a first control mode in which the first power converter is controlled to convert AC power in an AC wiring between the synchronous motor / generator and the synchronous motor / generator into DC power and output the DC power to the DC wiring, and a second control mode in which the first power converter is controlled to convert the DC power in the DC wiring into AC power and supply the AC power to the synchronous motor / generator in order to start the engine from a stopped state, and in the second control mode, at least when the engine is in a stopped state, a carrier frequency in the first power converter is set to a second carrier frequency lower than the first carrier frequency in the first control mode, or a DC voltage target value in the DC wiring is set to a second DC voltage target value lower than the first DC voltage target value in the first control mode. Effect of the Invention

[0009] According to the present disclosure, it is possible to reduce losses while allowing one motor generator to function as both a starter and a generator. [Brief description of the drawings]

[0010] [Figure 1] FIG. 1 is a block diagram showing a schematic configuration of a power generation system according to an embodiment of the present disclosure. [Diagram 2] FIG. 2 is a diagram illustrating an example of a circuit configuration of the first power converter illustrated in FIG. [Diagram 3] FIG. 3 is a diagram showing a schematic configuration of a processing circuit in the power generation system shown in FIG. [Figure 4] FIG. 4 is a block diagram illustrating an example of a voltage target value generating circuit according to the present embodiment. [Diagram 5] FIG. 5 is a time chart of various parameters related to the power generation system in the second control mode of the present embodiment. [Figure 6]FIG. 6 is a time chart of various parameters related to the power generation system in the second control mode of one modified example. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0011] Hereinafter, embodiments of the present disclosure will be described with reference to the drawings. In the following, elements having the same or similar functions are denoted by the same reference numerals throughout the drawings, and redundant description thereof will be omitted.

[0012] Fig. 1 is a block diagram showing a schematic configuration of a power generation system according to an embodiment of the present disclosure. As shown in Fig. 1, the power generation system 1 according to the present embodiment includes an engine 2 and a synchronous motor generator 3. The engine 2 is configured as a driving source for a moving body such as a vehicle, a ship, or an aircraft. The engine 2 includes, for example, a gas turbine engine. Note that the engine 2 may be a reciprocating engine.

[0013] The synchronous motor-generator 3 is mechanically connected to the engine 2. As a result, the synchronous motor-generator 3 can rotate in synchronization with the rotation of the output shaft 4 of the engine 2. The synchronous motor-generator 3 may be directly connected to the output shaft 4 of the engine 2, or may be connected to the output shaft 4 of the engine 2 via a power transmission mechanism such as a gear. The synchronous motor-generator 3 is configured to be able to generate power by using the driving force of the engine 2. In this embodiment, the synchronous motor-generator 3 is controlled by sensorless control in which the rotation angle is not detected by a rotation angle sensor. The synchronous motor-generator 3 has a cylindrical rotor to accommodate the high-speed rotation of the engine 2. However, the synchronous generator 3 is not limited to this, and may have, for example, a salient pole rotor.

[0014] Furthermore, the power generation system 1 includes a first power converter 5 electrically connected to the synchronous motor generator 3. The first power converter 5 and the synchronous motor generator 3 are connected by an AC wiring 6. The first power converter 5 is connected to a predetermined power source 8 via a second power converter 7. The first power converter 5 and the second power converter 7 are connected by a DC wiring 9. A capacitor 10 that defines a DC voltage in the DC wiring 9 is electrically connected to the DC wiring 9. Note that, for the sake of explanation, FIG. 1 shows an example in which the capacitor 10 is arranged independently of the first power converter 5 and the second power converter 7. However, the capacitor 10 may be included in either one or both of the first power converter 5 and the second power converter 7. The first power converter 5 converts the AC power generated by the synchronous motor generator 3 into DC power and supplies it to the DC wiring 9, and converts the DC power in the DC wiring 9 into AC power and supplies it to the synchronous motor generator 3.

[0015] The power source 8 may be, for example, an AC power source system such as an AC system inside a vehicle, a commercial AC system, or a microgrid, or may be, for example, a DC power source system such as a battery disposed inside or outside the vehicle. That is, the power source wiring 11 between the second power converter 7 and the power source 8 is AC wiring when the power source 8 is an AC power source system, and is DC wiring when the power source 8 is a DC power source system. The second power converter 7 converts the DC power of the DC wiring 9 into power to be supplied to the power source 8, and converts the power source power from the power source 8 into DC power to be supplied to the DC wiring 9. The second power converter 7 is an AC-DC converter when the power source 8 is an AC power source system, and is a DC-DC converter when the power source 8 is a DC power source system.

[0016] Furthermore, the power generation system 1 includes a controller 12. The controller 12 controls the first power converter 5 and the second power converter 7. The controller 12 includes a processing circuit 13 that performs various signal processing. The processing circuit 13 includes a computer such as a microcontroller, a personal computer, or a programmable logic controller (PLC). For example, the processing circuit 13 includes a processor, a memory, and peripheral circuits. Moreover, the processing circuit 13 of the controller 12 may be configured by a plurality of divided processing circuits.

[0017] The processor includes, for example, a CPU or an MPU. The memory includes volatile memory such as a ROM, a RAM, and a register, and non-volatile memory such as a flash memory. The memory stores a control program for the power converters 5, 7 in advance. The memory includes peripheral circuits, an input / output interface, and the like. The processor, memory, and peripheral circuits transmit data to each other via a bus. The processor executes the control program to perform arithmetic processing (described later) based on various information stored in the memory.

[0018] It should be noted that the functions of the elements disclosed herein can be performed using circuits or processing circuits including general purpose processors, special purpose processors, integrated circuits, ASICs (Application Specific Integrated Circuits), conventional circuits, or combinations thereof configured or programmed to perform the disclosed functions. A processor is considered a processing circuit or circuit because it includes transistors and other circuits. In this specification, a circuit, unit, or means is hardware that performs the recited functions or hardware that is programmed to perform the recited functions. The hardware may be hardware disclosed in this specification or other known hardware that is programmed or configured to perform the recited functions. In the case where the hardware is a processor that is considered a type of circuit, the circuit, unit, means, or part is a combination of hardware and software, and the software is used to configure the hardware or processor.

[0019] FIG. 2 is a diagram showing an example of a circuit configuration of the first power converter shown in FIG. 1. In the example of FIG. 2, the first power converter 5 is configured as a PWM (Pulse Width Modulation) inverter circuit. In this case, the first power converter 5 is realized by, for example, a three-phase bridge circuit. The first power converter 5 converts DC power provided by a DC wiring 9 into three-phase AC power and supplies it to the synchronous motor generator 3. That is, the AC wiring 6 includes three connection wirings 6a, 6b, and 6c. The connection wirings 6a, 6b, and 6c are connected to the three terminals 3a, 3b, and 3c of the synchronous motor generator 3, respectively. The windings of the stator connected to the three terminals 3a, 3b, and 3c, respectively, correspond to the a-phase winding, the b-phase winding, and the c-phase winding.

[0020] The first power converter 5 includes a plurality of switching elements T for passing AC current through each of the connection wirings 6a and 6c. The switching elements T are switched between on and off states based on a drive signal Sd1 from the controller 12. In the switching elements T, the on state means a conductive state between the main terminals of the switching elements T, and the off state means a cut-off state between the main terminals of the switching elements T. The switching elements T are realized by, for example, transistors. For example, in the example of FIG. 2, the switching elements T are IGBTs or bipolar transistors. In this case, a diode D is connected in inverse parallel to the switching elements T. The switching elements T may be field-effect transistors such as MOSFETs. In this case, the diode D does not need to be connected to the switching elements T.

[0021] In this embodiment, the switching element T includes six switching elements T1U, T1D, T2U, T2D, T3U, and T3D. The switching elements T1U and T1D are connected in series to form a switching element pair, and the switching element pair is connected in parallel to the capacitor 10. Similarly, the switching elements T2U and T2D are connected in series to form a switching element pair, and the switching element pair is connected in parallel to the capacitor 10. Similarly, the switching elements T3U and T3D are connected in series to form a switching element pair, and the switching element pair is connected in parallel to the capacitor 10.

[0022] A connection wire 6a is connected to an intermediate wire 51 between the switching elements T1U and T1D. A connection wire 6b is connected to an intermediate wire 52 between the switching elements T2U and T2D. A connection wire 6c is connected to an intermediate wire 53 between the switching elements T3U and T3D.

[0023] The processing circuit 13 of the controller 12 generates a gate signal corresponding to each switching element T. The gate signal is a PWM modulated, i.e., pulse width modulated, signal. A combination of the gate signals for the multiple switching elements T becomes the drive signal Sd1.

[0024] The processing circuit 13 is capable of executing a first control mode in which the synchronous motor-generator 3 functions as a generator, and a second control mode in which the synchronous motor-generator 3 functions as a starter for the engine 2.

[0025] In the first control mode, the processing circuit 13 controls the first power converter 5 to convert the AC power in the AC wiring 6 between the synchronous motor-generator 3 and the first power converter 5 into DC power and output it to the DC wiring 9. In the first control mode, the processing circuit 13 estimates the angular speed and phase of the magnetic poles of the rotor of the synchronous motor-generator 3 from the actual current flowing through the synchronous motor-generator 3 and a target voltage value in the AC wiring 6. The processing circuit 13 generates a drive signal Sd1 for controlling the first power converter 5 functioning as a rectifier so as to follow the estimated angular speed and phase.

[0026] In the second control mode, the processing circuit 13 controls the first power converter 5 to convert the DC power of the DC wiring 9 into AC power and supply the AC power to the synchronous motor-generator 3 in order to start the engine 2 from a stopped state. In the second control mode, the processing circuit 13 generates a drive signal Sd1 for controlling the first power converter 5 so that AC currents having a sinusoidal waveform with a phase difference of 120 degrees flow through the connection wirings 6a, 6b, and 6c. The rotor of the synchronous motor-generator 3 is driven to rotate by such AC currents flowing through the connection wirings 6a, 6b, and 6c. The processing circuit 13 can change the speed of the rotor of the synchronous motor-generator 3 by adjusting the drive signal Sd1.

[0027] In this embodiment, the processing circuit 13 executes a first start control, which is a sensorless vector control, when the angular velocity of the synchronous motor 3 is equal to or greater than a predetermined reference angular velocity, and executes a second start control different from the first start control while the angular velocity of the synchronous motor 3 is less than a first angular velocity reference value ωa from the start of startup of the synchronous motor 3. For example, the first angular velocity reference value ωa can be set to an angular velocity corresponding to a rotation speed at which the engine 2 can operate autonomously.

[0028] For example, the second startup control may be V / f control using a current target value. In this case, the processing circuit 13 generates the current target value using a preset value or an arithmetic expression. The processing circuit 13 generates a voltage target value using, for example, a PID calculation so that the actual current matches the current target value. The processing circuit 13 generates a voltage target value such that the ratio of the output voltage to the frequency of the first power converter 5 is constant.

[0029] In order to control the first power converter 5 by the processing circuit 13 of the controller 12, the power generation system 1 includes a voltage detector 14 that detects a DC voltage in the DC wiring 9 and a current detector 15 that detects an AC current in the AC wiring 6. The current detector 15 includes a first detector 15a that detects a current Ia in the connection wiring 6a and a second detector 15c that detects a current Ic in the connection wiring 6c. The current Ib in the connection wiring 6b can be calculated from the vector sum of the current Ia in the connection wiring 6a and the current Ic in the connection wiring 6c.

[0030] Fig. 3 is a diagram showing a schematic configuration of a processing circuit in the power generation system shown in Fig. 1. The processing circuit 13 includes an estimation unit 21 and a signal generation unit 22 as control blocks. The estimation unit 21 estimates the angular velocity and phase of the magnetic poles in the rotor of the synchronous motor-generator 3. To this end, the estimation unit 21 has a dq conversion circuit 23, a model application circuit 24, and an estimation circuit 25.

[0031] The dq conversion circuit 23 outputs a d-axis actual current Id and a q-axis actual current Iq from the instantaneous value of the current flowing through the AC wiring 6 detected by the current detectors 15a and 15c, using an estimated magnetic pole phase θm, which will be described later. The model application circuit 24 estimates the current flowing through the synchronous motor-generator 3, using a model calculation formula that models the synchronous motor-generator 3. The model calculation formula is stored in advance in the memory of the processing circuit 13.

[0032] The d-axis actual current Id and the q-axis actual current Iq are input to the model application circuit 24. Furthermore, the d-axis voltage target value Vod and the q-axis voltage target value Voq generated by a voltage target value generation circuit 26 described later are input to the model application circuit 24. Furthermore, the estimated induced voltage Em estimated by an estimation circuit 25 is input to the model application circuit 24. The model application circuit 24 outputs the d-axis estimated current Imd and the q-axis estimated current Imq by substituting these values ​​Id, Iq, Vod, Voq, and Em into the model computational equation and solving the model computational equation.

[0033] The estimation circuit 25 estimates the estimated induced voltage Em, the estimated angular velocity ωm, and the estimated phase θm from the d-axis estimated deviation ΔId between the d-axis actual current Id and the d-axis estimated current Imd and the q-axis estimated deviation ΔIq between the q-axis actual current Iq and the q-axis estimated current Imq using a predetermined estimation algorithm. The d-axis estimated deviation ΔId is obtained by subtracting the d-axis estimated current Imd from the d-axis actual current Id. The q-axis estimated deviation ΔIq is obtained by subtracting the q-axis estimated current Imq from the q-axis actual current Iq.

[0034] As described above, the estimated induced voltage Em estimated by the estimation circuit 25 is substituted into the model calculation formula in the model application circuit 24. The estimated phase θm is used in the dq conversion circuit 23 and a drive signal generation circuit 27, which will be described later.

[0035] The signal generating unit 22 generates a drive signal Sd1 for the first power converter 5. To this end, the signal generating unit 22 has a voltage target value generating circuit 26 and a drive signal generating circuit 27. The voltage target value generating circuit 26 generates voltage target values ​​Vod, Voq using the d-axis actual current Id, the q-axis actual current Iq, and the estimated angular velocity ωm estimated by the estimation circuit 25.

[0036] 4 is a block diagram showing a schematic example of a voltage target value generation circuit in this embodiment. The voltage target value generation circuit 26 includes PID calculation circuits 28, 29, and 31 and a selection circuit 30. The voltage target value generation circuit 26 generates an angular velocity deviation Δω, which is a deviation between an estimated angular velocity ωm and an angular velocity target value ωo. The angular velocity deviation Δω is obtained by subtracting the estimated angular velocity ωm from the angular velocity target value ωo. The PID calculation circuit 28 performs PID calculation on the angular velocity deviation Δω to calculate a first q-axis current target value Ioq_1.

[0037] Furthermore, the voltage target value generating circuit 26 generates a power deviation ΔP, which is the deviation between the power generation P generated by the synchronous motor-generator 3 and the power generation target value Po. Here, the power generation P is a negative value when the synchronous motor-generator 3 generates power. The power generation P is calculated, for example, from the currents Ia, Ib, and Ic of each phase and the voltages Voa, Vob, and Voc of each phase. That is, the power generation P is expressed as P=Voa×Ia+Vob×Ib+Voc×Ic. The voltages Voa, Vob, and Voc of each phase are derived from the DC voltage V detected by the voltage detector 14, the estimated phase θm, and the drive signal Sd1. The power generation P may be calculated from the d-axis actual current Id, the q-axis actual current Iq, and the voltage target values ​​Vod and Voq. In this case, the power generation P is expressed as P=K(Vod×Id+Voq×Iq) using a coefficient K based on the dq conversion. The power deviation ΔP is obtained by subtracting the generated power P from the generated power target value Po. The PID calculation circuit 29 performs PID calculation on the power deviation ΔP to calculate the second q-axis current target value Ioq_2.

[0038] The selection circuit 30 compares the first q-axis current target value Ioq_1 with the second q-axis current target value Ioq_2, and outputs the larger one as the q-axis current target value Ioq. The d-axis current target value Iod is set according to the torque target value, the rotation speed target value, or the like. The voltage target value generation circuit 26 generates a d-axis target deviation ΔIod between the d-axis current target value Iod and the d-axis actual current Id, and a q-axis target deviation ΔIoq between the q-axis current target value Ioq and the q-axis actual current Iq. The d-axis target deviation ΔIod is obtained by subtracting the d-axis actual current Id from the d-axis current target value Iod. The q-axis target deviation ΔIoq is obtained by subtracting the q-axis actual current Iq from the q-axis current target value Ioq.

[0039] The PID calculation circuit 31 performs PID calculation on the d-axis target deviation ΔIod to calculate the d-axis voltage target value Vod, and performs PID calculation on the q-axis target deviation ΔIoq to calculate the q-axis voltage target value Voq.

[0040] The drive signal generation circuit 27 generates a drive signal Sd1 from the voltage target values ​​Vod, Voq and the estimated phase θm. The drive signal generation circuit 27 performs an inverse dq transformation on the voltage target values ​​Vod, Voq using the estimated magnetic pole phase θm to calculate three-phase voltage target values ​​voa, vob, voc. Furthermore, the drive signal generation circuit 27 generates a drive signal Sd1 by pulse-width modulating the three-phase voltage target values ​​voa, vob, voc based on a predetermined carrier frequency fc. In this embodiment, the carrier frequency fc when the drive signal generation circuit 27 executes the pulse-width modulation is configured to be changeable.

[0041] Here, the induced voltage of the synchronous motor-generator 3 is designed based on the time of power generation. As a result, the torque constant is low and the current at the start of the synchronous motor-generator 3 is large. For this reason, in the second control mode in which the synchronous motor-generator 3 functions as a starter for the engine 2, it becomes necessary to improve the drive efficiency and suppress the rise in the internal temperature of the synchronous motor-generator 3.

[0042] In response to this, the processing circuit 13 executes a frequency reduction process and a voltage reduction process in the second control mode. As the frequency reduction process, the processing circuit 13 sets the carrier frequency fc to a second carrier frequency fc2 lower than the first carrier frequency fc1 set in the first control mode during a period in which the rotation speed R of the engine 2 is lower than the reference rotation speed Rj. Also, as the voltage reduction process, the processing circuit 13 sets the DC voltage target value Vo in the DC wiring 9 to a second DC voltage target value Vo2 lower than the first DC voltage target value Vo1 in the first control mode during a period in which the rotation speed R of the engine 2 is lower than the reference rotation speed Rj.

[0043] Fig. 5 is a time chart of various parameters related to the power generation system in the second control mode of the present embodiment. Fig. 5 shows a time chart of the rotation speed R of the engine 2, the DC voltage V of the DC line 9, the carrier frequency fc of the first power converter 5, and the power generation P of the synchronous motor generator 3.

[0044] When starting the second control mode, the processing circuit 13 controls the second power converter 7 to step down the power supply voltage in the power supply 8 and charge the capacitor 10 so that the DC voltage V of the DC line 9 becomes the second DC voltage target value V2. For this reason, the processing circuit 13 generates a drive signal Sd2 such that the DC voltage target value Vo becomes the second DC voltage target value Vo2 at time t1, and transmits the drive signal Sd2 to the second power converter 7.

[0045] At time t2, the DC voltage V of the DC wiring 9 becomes the second DC voltage target value Vo2. The DC voltage V is maintained at the second DC voltage target value Vo2 from time t2 until time t7 after time t6 when the engine 2 starts the independent operation described later. The processing circuit 13 outputs a drive signal Sd1 for converting the DC power of the DC wiring 9 into AC power by the first power converter 5 and supplying it to the synchronous motor generator 3 in order to rotate and drive the synchronous motor generator 3 from time t3 after time t2. That is, in the example of FIG. 5, the processing circuit 13 executes the voltage reduction process from time t3 when the engine 2 is stopped to time t7 determined based on time t6 when the engine 2 reaches the rotation speed Ra when the engine 2 starts the independent operation. In this example, the reference rotation speed Rj in the voltage reduction process is determined based on the rotation speed Ra when the engine 2 starts the independent operation.

[0046] At time t3, the processing circuit 13 adopts a second carrier frequency fc2 lower than the first carrier frequency fc1 during power generation as the carrier frequency fc when generating the drive signal Sd1 from the voltage target values ​​Vod and Voq. When power is supplied from the first power converter 5 to the synchronous motor-generator 3 via the AC wiring 6 by the drive signal Sd1, the synchronous motor-generator 3 starts to rotate, and accordingly the engine 2 also starts to rotate.

[0047] When the rotation speed R of the engine 2 reaches a predetermined ignition rotation speed Ri, the engine 2 is ignited, and a driving force is generated from the engine 2. In the example of FIG. 5, the first power converter 5 is controlled so that the rotation speed R of the engine 2, i.e., the angular speed of the synchronous motor generator 3, is constant as a period required for the engine 2 to ignite during a predetermined period from time t4 when the engine 2 reaches the ignition rotation speed Ri. In this embodiment, the rotation speed R of the engine 2 is calculated from an estimated angular speed ωm of the magnetic poles of the synchronous motor generator 3. Alternatively, the processing circuit 13 may obtain the rotation speed R of the engine 2 from an engine speed detector that detects the rotation speed R of the engine 2.

[0048] After time t5, which is a predetermined period after time t4, the driving force generated by the engine 2 gradually increases, and accordingly, the driving force transmitted by the synchronous motor-generator 3 to the output shaft 4 gradually decreases. In the graph of the generated power P in FIG. 5, negative values ​​indicate power consumption, i.e., power for the torque generated by the synchronous motor-generator 3. For this reason, FIG. 5 shows that the torque generated by the synchronous motor-generator 3 decreases from time t5, and becomes zero at time t6. At the time t6, the engine 2 enters an independent operation. The rotation speed of the engine 2 at which the independent operation starts is indicated as the independent rotation speed Ra in FIG. 5. After time t6, the synchronous motor-generator 3 enters a power generating state by the driving force of the engine 2.

[0049] In the example of FIG. 5, the carrier frequency fc is maintained at the second carrier frequency fc2 from time t3 until time t4 when the engine 2 is ignited, which will be described later. That is, the processing circuit 13 executes the frequency reduction process from time t3 when the engine 2 is stopped to time t4 when the engine 2 reaches the rotation speed Ri when igniting. In this example, the reference rotation speed Rj in the frequency reduction process is the ignition rotation speed Ri. In this example, when the rotation speed R of the engine 2 becomes equal to or higher than the ignition rotation speed Ri, the processing circuit 13 increases the carrier frequency fc from the second carrier frequency fc2 to the first carrier frequency fc1. In this embodiment, the processing circuit 13 increases the carrier frequency fc from the second carrier frequency fc2 to the first carrier frequency fc1 in a ramp shape from time t5. In the example of FIG. 5, the timing when the carrier frequency fc reaches the first carrier frequency fc1 is before the engine 2 becomes independent, that is, before time t6 when the synchronous motor generator 3 switches from the power supply state to the power generation state. However, the carrier frequency fc may reach the first carrier frequency fc1 after time t6.

[0050] The reference rotation speed Rj that determines the timing to increase the carrier frequency fc is not limited to the above. For example, the reference rotation speed Rj [rpm] that is the reference for increasing the carrier frequency fc in the frequency reduction process can be set within the range of the following formula. fc2 / 60PN <Rj<fc2 / 60PM Here, P indicates the number of pole pairs of the synchronous motor-generator, and N and M indicate the period of the carrier frequency included in one rotation of the synchronous motor-generator. For example, N is selected from 9 to 20, and M is selected from 2 to 5.

[0051] The processing circuit 13 may determine the timing for increasing the carrier frequency fc, i.e., the reference rotation speed Rj, based on the current flowing through the synchronous motor / generator. That is, in the second control mode, the processing circuit 13 may increase the carrier frequency fc from the second carrier frequency fc2 to the first carrier frequency fc1 when the current flowing through the synchronous motor / generator 3 falls within a predetermined range. The predetermined range serving as a reference for increasing the carrier frequency fc may be determined based on a current value corresponding to the ignition rotation speed Ri, or may be determined based on a current value at time t6 when the engine 2 switches to independent operation and the synchronous motor / generator 3 switches from a power supply state to a power generation state, i.e., 0. The predetermined range may be determined by setting the reference current value as a lower limit, intermediate value, or upper limit, or a value offset by a predetermined amount from the reference current value may be set as the lower limit, intermediate value, or upper limit. This allows the time when the engine 2 is ignited or the time when the independent operation starts to be detected from the current flowing through the synchronous motor / generator 3.

[0052] Furthermore, when the current flowing from the first power converter 5 to the synchronous motor-generator 3 via the AC wiring 6 becomes equal to or less than a predetermined value, the processing circuit 13 stops controlling the first power converter 5 for a predetermined period. As described above, the torque generated by the synchronous motor-generator 3 becomes zero at time t6. At this time, no current flows through the synchronous motor-generator 3. For this reason, it becomes difficult for the processing circuit 13 to estimate the angular velocity and the like using the model application circuit 24 and the estimation circuit 25 around time t6.

[0053] Therefore, when the current flowing through the synchronous motor-generator 3 becomes equal to or less than a predetermined value, the processing circuit 13 stops the control of the first power converter 5. As a result, all switching elements T in the first power converter 5 are turned off. Even if the switching elements T are turned off, the rotation speed of the synchronous motor-generator 3 increases with an increase in the rotation speed R of the engine 2. In this way, by stopping the control of the first power converter 5 when the current flowing through the synchronous motor-generator 3 becomes equal to or less than a predetermined value, it is possible to prevent loss of synchronism due to estimation errors in angular velocity, etc. During the control stop, electricity is generated by the rotation of the synchronous motor-generator 3, and the electricity is supplied to the DC wiring 9 via the diode D.

[0054] Furthermore, the processing circuit 13 controls the second power converter 7 so as to maintain the DC voltage V of the DC line 9 at the second DC voltage target value Vo2 even during the control suspension. After the control is suspended as described above, the processing circuit 13 controls the first power converter 5 in the first control mode.

[0055] At this time, the estimated induced voltage Em input to the model application circuit 24 is a predetermined fixed value or a value immediately before the control is stopped. Therefore, the d-axis estimated current Imd and the q-axis estimated current Imq output by the model application circuit 24 are not highly accurate. Therefore, the estimated phase θm estimated by the estimation circuit 25 may not match the actual phase of the magnetic pole of the synchronous motor-generator 3. As a result, the on / off timing of the switching element T that operates based on the drive signal Sd1 may not follow the phase of the synchronous motor-generator 3, and appropriate rectification processing may not be performed.

[0056] In response to this, by maintaining the DC voltage V of the DC wiring 9 at the second DC voltage target value Vo2, which is lower than the first DC voltage target value Vo1, at the start of execution of the first control mode, the current flowing through the synchronous motor-generator 3 can be kept low at the start of execution of the first control mode, and it is possible to prevent an excessive current from flowing through the DC wiring 9 due to a disturbance in the rectification process after control of the first power converter 5 is resumed.

[0057] The processing circuit 13 increases the DC voltage V of the DC wiring 9 from the second DC voltage target value Vo2 to the first DC voltage target value Vo1, starting from time t7 after the start of the first control mode. For example, time t7 is the time when the current flowing from the synchronous motor generator 3 to the first power converter 5 is equal to or greater than a reference value greater than 0. That is, in the second control mode, when the current flowing through the synchronous motor generator 3 falls within a predetermined range including the reference value, the processing circuit 13 increases the DC voltage target value from the second DC voltage target value Vo2 to the first DC voltage target value Vo1. This makes it possible to easily detect the switch from the second control mode to the first control mode, and to maintain the DC voltage of the DC wiring 9 at the second DC voltage target value Vo2 at the start of execution of the first control mode, and then to increase it to the first DC voltage target value Vo1 at an appropriate timing thereafter.

[0058] In this embodiment, the processing circuit 13 increases the DC voltage target value from the second DC voltage target value Vo2 to the first DC voltage target value Vo1 in a ramp manner starting from time t7. In the example of Fig. 5, the timing at which the DC voltage V reaches the first DC voltage target value Vo1 is before time t8 at which the engine 2 operates at a steady rotation speed.

[0059] In the first control mode after time t8, when the synchronous motor-generator 3 is dominant over the engine 2, there is usually no large fluctuation in the rotation speed R of the engine 2, and therefore the angular speed deviation Δω input to the PID calculation circuit 28 of the voltage target value generation circuit 26 is also close to 0. Therefore, the second q-axis current target value Ioq_2 based on the power deviation ΔP is dominant as the q-axis current target value Ioq selected by the selection circuit 30. On the other hand, when the engine 2 is dominant over the synchronous motor-generator 3, the angular speed deviation Δω becomes large, and the first current target value Ioq_1 based on the angular speed deviation Δω is dominant as the q-axis current target value Ioq selected by the selection circuit 30.

[0060] In this way, in the second control mode, the processing circuit 13 performs control according to the rotation speed of the synchronous motor generator 3, and in the first control mode, performs control according to the power output from the first power converter 5 to the DC wiring 9.

[0061] According to this embodiment, in the second control mode in which the synchronous motor-generator 3 functions as a starter for the engine 2, the drive signal Sd1 is generated using the second carrier frequency fc2, which is a lower carrier frequency fc compared to the first control mode in which the synchronous motor-generator 3 functions as a generator using the driving force of the engine 2. The carrier frequency fc in the first power converter 5 and the loss in the synchronous motor-generator 3 are in a proportional relationship. Therefore, when the angular velocity, i.e., the frequency, of the synchronous motor-generator 3 is in a low state upon starting the engine 2, the carrier frequency fc is also lowered, thereby making it possible to reduce the loss while performing appropriate control.

[0062] Furthermore, according to this embodiment, in the second control mode, the DC voltage V in the DC wiring 9 is a lower voltage than in the first control mode. When the change rate of the DC voltage V per unit time is increased at a constant rate, the loss in the synchronous motor-generator 3 is proportional to the square of the voltage. Therefore, when the induced voltage of the synchronous motor-generator 3 is in a low state upon starting the engine 2, the DC voltage is also lowered, thereby making it possible to reduce the loss while performing appropriate control.

[0063] In this way, during a predetermined period including the stopped state of the engine 2 when the second control mode is executed, a frequency reduction process is executed to set the carrier frequency fc in the first power converter 5 to a second carrier frequency fc2 lower than the first carrier frequency fc1 in the first control mode, and a voltage reduction process is executed to set the DC voltage target value in the DC wiring 9 to a second DC voltage target value Vo2 lower than the first DC voltage target value Vo1 in the first control mode, thereby making it possible to reduce losses in the synchronous motor-generator 3 while allowing one synchronous motor-generator 3 to function as both a starter and a generator.

[0064] Furthermore, according to this embodiment, the first power converter 5 can be made smaller. In addition, in the case of sensorless control as in this embodiment, the synchronous motor-generator 3 can also be made smaller. The size, i.e., the dimensions and weight, of the first power converter 5 depend on the capacity. In the first power converter 5, the larger the capacity, the larger the dimensions and weight. The capacity of the first power converter 5 is generally proportional to the value of (voltage) x (current).

[0065] In the case where the synchronous motor-generator 3 has both the functions of a generator and a starter as in this embodiment, a large current is required to generate a large torque in the second control mode in which the synchronous motor-generator 3 functions as a starter, while a high voltage is required to accommodate the induced voltage generated in the first control mode in which the synchronous motor-generator 3 functions as a generator. That is, in the first control mode, the synchronous motor-generator 3 also rotates at a high speed in accordance with the high speed rotation of the engine 2, and the induced voltage becomes high. Therefore, the capacity required for the first power converter 5 is (large current corresponding to low speed rotation and large torque) x (high voltage corresponding to high speed rotation), and a large capacity is required.

[0066] In contrast, according to the present embodiment, by lowering the DC voltage V in the second control mode requiring low speed and high torque, the capacity required in the first power converter 5 can be set to the larger of (large current corresponding to low speed rotation and high torque)×(low voltage corresponding to low speed rotation) and (low current during high speed power generation)×(high voltage corresponding to high speed rotation). In other words, this difference makes it possible to reduce the size of the first power converter 5.

[0067] In addition, the carrier frequency fc also greatly affects the dimensions and weight of the first power converter 5. The carrier frequency fc depends on the frequency output by the first power converter 5, and in order to increase the frequency output by the first power converter 5, the carrier frequency fc must also be increased. In addition, when a large current flows at a high carrier frequency, loss increases. As a result, the amount of heat generated in the first power converter 5 increases, and the cooler for cooling the first power converter 5 also becomes larger. In contrast, in this embodiment, in the second control mode in which a large current flows, the carrier frequency fc is lowered, thereby making it possible to reduce loss and, as a result, to reduce the size of the first power converter 5.

[0068] Furthermore, according to this embodiment, when the second control mode is started, the second power converter 7 is controlled to step down the power supply voltage in the power supply 8 and charge the capacitor 10 connected to the DC wiring 9 so that the DC voltage V of the DC wiring 9 becomes the second DC voltage target value Vo2. That is, the second power converter 7 is configured to operate as a step-down converter. For example, the second power converter 7 is a step-up / step-down DC-DC converter. This makes it possible to easily switch the DC voltage V of the DC wiring 9 between the first DC voltage target value Vo1 and the second DC voltage target value Vo2.

[0069] Furthermore, according to this embodiment, when the rotation speed R of the engine 2 becomes equal to or higher than the ignition rotation speed Ri, the carrier frequency fc is controlled to increase from the second carrier frequency fc2 to the first carrier frequency fc1. This makes it possible to increase the carrier frequency fc2 to the first carrier frequency fc1 in accordance with an increase in torque after ignition of the engine 2. Also, by increasing the carrier frequency fc in a ramp manner, it is possible to suppress the occurrence of losses in the synchronous motor-generator 3.

[0070] Although the embodiments of the present disclosure have been described above, the present disclosure is not limited to the above-described embodiments, and various improvements, changes, and modifications are possible without departing from the spirit and scope of the present disclosure.

[0071] [Other embodiments] For example, in the above embodiment, an example has been given of the mode in which both the frequency reduction process and the voltage reduction process are executed in the second control mode, but only one of the frequency reduction process and the voltage reduction process may be executed.

[0072] In the above embodiment, the synchronous motor-generator 3 is controlled by sensorless control in which the rotation angle is not detected, but the above disclosed control is also applicable to a case in which the rotation angle of the synchronous motor-generator 3 is detected by a rotation angle sensor and the rotation speed is controlled based on the rotation angle. For example, in the second control mode, the processing circuit 13 may increase the carrier frequency fc from the second carrier frequency fc2 to the first carrier frequency fc1 when the detected rotation angle of the synchronous motor-generator 3 becomes equal to or greater than a value corresponding to the ignition rotation speed Ri of the engine 2.

[0073] In the above embodiment, the first q-axis current target value Ioq_1 is generated from the angular velocity deviation Δω, which is the deviation between the estimated angular velocity ωm and the angular velocity target value ωo. That is, in the above embodiment, the control mode in which the rotation speed control and the generated power control are combined is exemplified. Alternatively, the first q-axis current target value Ioq_1 may be generated by multiplying the torque target value To by a predetermined torque constant. That is, in the configuration of the present disclosure, it is possible to adopt a control mode in which the torque control and the generated power control are combined.

[0074] In the above embodiment, the first power converter 5 is controlled so that there is a period in which the rotation speed of the engine 2 is constant at the ignition rotation speed Ri, but the present invention is not limited to this. FIG. 6 is a time chart of various parameters related to the power generation system in the second control mode of one modified example. FIG. 6 also shows a time chart of the parameters R, V, fc, and P similar to FIG. 5. Depending on the characteristics of the engine 2, a constant rotation speed may not be required at the time of ignition. In such a case, as shown in the time chart of FIG. 6, the rotation speed of the engine 2 may be controlled to increase in a ramp shape from the start of rotation of the engine 2 until the engine 2 reaches a steady operation. In this case, the torque control described above can be easily applied.

[0075] In the above embodiment, in the second control mode, when the current flowing through the synchronous motor-generator 3 becomes equal to or less than a predetermined value, the control of the first power converter 5 is stopped. However, the present invention is not limited to this. If the rotation angle of the synchronous motor-generator 3 is detected and the rotation speed control is performed based on the rotation angle, such control is not necessary. Also, in the sensorless control, instead of the above embodiment, reactive power may be supplied to the synchronous motor-generator 3. That is, in the second control mode, when the current flowing from the first power converter 5 to the synchronous motor-generator 3 through the AC wiring 6 becomes equal to or less than a predetermined value, the processing circuit 13 may control the first power converter 5 to supply reactive power from the first power converter 5 to the synchronous motor-generator 3 for a predetermined period, and execute the first control mode after the predetermined period has elapsed.

[0076] By supplying reactive power, it is possible to flow active power, i.e., a current that does not contribute to the generated power, to the synchronous motor-generator 3. Therefore, the angular speed of the synchronous motor-generator 3 can be estimated using this current.

[0077] In the above embodiment, the carrier frequency fc is increased from the second carrier frequency fc2 to the first carrier frequency fc1 in a ramp-like manner, but the present invention is not limited to this. For example, the carrier frequency fc may be increased in a step-like manner.

[0078] In addition, in the above embodiment, an example has been given in which one controller 12 controls both the first power converter 5 and the second power converter 7, but instead, the controller that controls the first power converter 5 and the controller that controls the second power converter 7 may be different controllers. In addition, the controller 12 that controls the first power converter 5 or the second power converter 7 may be configured as the same controller as the controller that controls the engine 2, or may be configured as a different controller.

[0079] [Summary of this disclosure] [Item 1] A controller of a power generation system according to one embodiment of the present disclosure is a controller of a power generation system including an engine, a synchronous motor-generator mechanically connected to the engine and capable of generating electricity by utilizing the driving force of the engine, and a first power converter electrically connected to the synchronous motor-generator, the controller including a processing circuit, the processing circuit being capable of executing a first control mode in which the first power converter is controlled to convert AC power in an AC wiring between the synchronous motor-generator and the synchronous motor-generator into DC power and output the DC power to the DC wiring, and a second control mode in which the first power converter is controlled to convert the DC power in the DC wiring into AC power and supply the AC power to the synchronous motor-generator in order to start the engine from a stopped state, the second control mode including at least one of a frequency reduction process in which a carrier frequency in the first power converter is set to a second carrier frequency lower than the first carrier frequency in the first control mode during a period in which the engine speed is lower than a reference speed, and a voltage reduction process in which a DC voltage target value in the DC wiring is set to a second DC voltage target value lower than the first DC voltage target value in the first control mode.

[0080] According to the above configuration, in the second control mode in which the synchronous motor-generator functions as a starter for the engine, a drive signal is generated using a lower carrier frequency, or the DC voltage V in the DC wiring 9 is a lower voltage, compared to the first control mode in which the synchronous motor-generator functions as a generator using the driving force of the engine. When the angular velocity, i.e., frequency, of the synchronous motor-generator is low upon starting the engine, the carrier frequency or the DC voltage is also lowered, thereby making it possible to reduce losses while performing appropriate control. Therefore, it is possible to reduce losses in the synchronous motor-generator while making one motor-generator function as both a starter and a generator.

[0081] [Item 2] In the controller for the power generation system of item 1, the power generation system may include a capacitor electrically connected to the DC distribution and defining a DC voltage in the DC distribution, and a second power converter electrically connected between the DC distribution and a predetermined power source, the second control mode may include the voltage reduction process, and the processing circuit may control the second power converter to step down a power source voltage in the power source and charge the capacitor when starting the second control mode so that the DC voltage of the DC distribution becomes the second DC voltage target value. This makes it possible to easily switch the DC voltage of the DC distribution between a first DC voltage target value and a second DC voltage target value.

[0082] [Item 3] In the power generation system controller of item 1 or 2, the second control mode may include the frequency reduction process, and the processing circuit may acquire a rotation speed of the engine or the synchronous motor / generator in the second control mode, and when the rotation speed becomes equal to or higher than an ignition rotation speed set as a rotation speed at which the engine ignites, increase the carrier frequency from the second carrier frequency to the first carrier frequency. This makes it possible to increase the carrier frequency from the second carrier frequency to the first carrier frequency as the torque increases after the engine ignition.

[0083] [Item 4] In the power generation system controller of item 1 or 2, the second control mode may include the frequency reduction process, and the processing circuit may increase the carrier frequency from the second carrier frequency to the first carrier frequency when the current flowing through the synchronous motor-generator falls within a predetermined range in the second control mode. This makes it possible to detect the time of engine ignition or the start of independent operation from the current flowing through the synchronous motor-generator, and to increase the carrier frequency from the second carrier frequency to the first carrier frequency as the torque increases after the engine ignition.

[0084] [Item 5] In the controller of the power generation system according to item 3 or 4, the processing circuit may increase the carrier frequency from the second carrier frequency to the first carrier frequency in a ramp manner. This makes it possible to suppress the occurrence of losses in the synchronous motor generator.

[0085] [Item 6] In the controller of the power generation system of any one of items 1 to 5, the second control mode may include the voltage reduction process, and the processing circuit may increase the DC voltage target value from the second DC voltage target value to the first DC voltage target value when the current flowing through the synchronous motor-generator falls within a predetermined range in the second control mode. This makes it possible to increase the DC voltage target value in the DC wiring from the second DC voltage target value to the first DC voltage target value at an appropriate timing.

[0086] [Item 7] In the controller of the power generation system of item 6, the processing circuit may increase the DC voltage target value from the second DC voltage target value to the first DC voltage target value in a ramp manner.

[0087] [Item 8] In the controller of the power generation system according to any one of items 1 to 7, the synchronous motor-generator may be controlled by sensorless control in which detection of a rotation angle is not performed.

[0088] [Item 9] In the power generation system controller of item 8, the processing circuit may stop control of the first power converter for a predetermined period when the current flowing from the first power converter to the synchronous motor-generator via the AC wiring becomes equal to or less than a predetermined value in the second control mode, and execute the first control mode after the predetermined period has elapsed. This makes it possible to prevent loss of synchronism due to estimation errors of angular velocity, etc.

[0089] [Item 10] In the controller of the power generation system of item 8, the second control mode may include the voltage reduction process, and the processing circuit may control the second power converter to maintain the DC voltage of the DC wiring at the second DC voltage target value until a predetermined period of time has elapsed since the current flowing through the synchronous motor-generator becomes equal to or less than the predetermined value. In this way, even if a disturbance occurs in the rectification process after the control of the first power converter is resumed and an excessive current flows in the DC wiring, the current can be absorbed by flowing in a capacitor connected to the DC wiring.

[0090] [Item 11] In the controller of the power generation system of item 8, in the second control mode, when the current flowing from the first power converter to the synchronous motor-generator via the AC wiring becomes equal to or lower than a predetermined value, the processing circuit may control the first power converter to supply reactive power from the first power converter to the synchronous motor-generator for a predetermined period of time, and execute the first control mode after the predetermined period has elapsed.

[0091] [Item 12] In the controller of any one of the power generation systems of items 1 to 11, the processing circuit may perform control according to the rotation speed of the synchronous motor-generator in the second control mode, and may perform control according to the power output from the first power converter to the DC wiring in the first control mode.

[0092] [Item 13] A control method for a power generation system according to another aspect of the present disclosure is a control method for a power generation system including an engine, a synchronous motor / generator mechanically connected to the engine and capable of generating electricity by utilizing the driving force of the engine, and a first power converter electrically connected to the synchronous motor / generator, the control method switching between a first control mode in which the first power converter is controlled to convert AC power in an AC wiring between the synchronous motor / generator and the synchronous motor / generator into DC power and output the DC power to the DC wiring, and a second control mode in which the first power converter is controlled to convert the DC power in the DC wiring into AC power and supply the AC power to the synchronous motor / generator in order to start the engine from a stopped state, and in the second control mode, during a period in which the engine speed is less than a reference speed, a carrier frequency in the first power converter is set to a second carrier frequency lower than the first carrier frequency in the first control mode, or a DC voltage target value in the DC wiring is set to a second DC voltage target value lower than the first DC voltage target value in the first control mode. [Explanation of symbols]

[0093] 1. Power generation system 2 Engine 3. Synchronous motor generator 5. First power converter 6 AC wiring 7 Second power converter 8 Power 9 DC wiring 10 Capacitor 12 Controller 13 Processing circuit

Claims

1. A controller for a power generation system including an engine, a synchronous motor-generator mechanically connected to the engine and capable of generating power by utilizing a driving force of the engine, and a first power converter electrically connected to the synchronous motor-generator, A processing circuit is provided, the processing circuit is capable of executing a first control mode in which the first power converter is controlled to convert AC power in an AC wiring between the synchronous motor-generator and the synchronous motor-generator into DC power and output the DC power to the DC wiring, and a second control mode in which the first power converter is controlled to convert DC power in the DC wiring into AC power and supply the AC power to the synchronous motor-generator in order to start the engine from a stopped state, the second control mode includes at least one of a frequency reduction process of setting a carrier frequency in the first power converter to a second carrier frequency lower than the first carrier frequency in the first control mode during a period when the engine speed is lower than a reference speed, and a voltage reduction process of setting a DC voltage target value in the DC wiring to a second DC voltage target value lower than the first DC voltage target value in the first control mode.

2. The power generation system includes: a capacitor electrically connected to the DC line, the capacitor defining a DC voltage on the DC line; a second power converter electrically connected between the DC wiring and a predetermined power source, the second control mode includes the voltage reduction process, 2. The controller for a power generation system according to claim 1, wherein, when starting the second control mode, the processing circuit controls the second power converter to step down a power supply voltage in the power supply and charge the capacitor so that the DC voltage of the DC wiring becomes the second DC voltage target value.

3. the second control mode includes the frequency reduction process, 2. The controller of a power generation system according to claim 1, wherein in the second control mode, the processing circuit acquires a rotation speed of the engine or the synchronous motor-generator, and when the rotation speed becomes equal to or higher than an ignition rotation speed set as a rotation speed at which the engine ignites, increases the carrier frequency from the second carrier frequency to the first carrier frequency.

4. the second control mode includes the frequency reduction process, 2. The controller of the power generation system according to claim 1, wherein the processing circuit increases the carrier frequency from the second carrier frequency to the first carrier frequency when a current flowing through the synchronous motor generator falls within a predetermined range in the second control mode.

5. The power generation system controller of claim 3 or 4, wherein the processing circuit ramps up the carrier frequency from the second carrier frequency to the first carrier frequency.

6. the second control mode includes the voltage reduction process, 2. The controller for a power generation system according to claim 1, wherein, in the second control mode, when a current flowing through the synchronous motor-generator falls within a predetermined range, the processing circuit increases the DC voltage target value from the second DC voltage target value to the first DC voltage target value.

7. The controller of claim 6 , wherein the processing circuit ramps up the DC voltage target value from the second DC voltage target value to the first DC voltage target value.

8. 3. The controller for a power generation system according to claim 1, wherein the synchronous motor-generator is controlled by a sensorless control in which a rotation angle is not detected.

9. 9. The controller for a power generation system according to claim 8, wherein, in the second control mode, when a current flowing from the first power converter to the synchronous motor-generator via the AC wiring becomes equal to or lower than a predetermined value, the processing circuit stops control of the first power converter for a predetermined period, and executes the first control mode after the predetermined period has elapsed.

10. the second control mode includes the voltage reduction process, 9. The controller of the power generation system according to claim 8, wherein the processing circuit controls the second power converter so as to maintain the DC voltage of the DC wiring at the second DC voltage target value until a predetermined period of time has elapsed since the current flowing through the synchronous motor-generator becomes equal to or less than the predetermined value.

11. 9. The controller for a power generation system according to claim 8, wherein, in the second control mode, when a current flowing from the first power converter to the synchronous motor-generator via the AC wiring becomes equal to or lower than a predetermined value, the processing circuit controls the first power converter to supply reactive power from the first power converter to the synchronous motor-generator for a predetermined period, and executes the first control mode after the predetermined period has elapsed.

12. 3. The controller for a power generation system according to claim 1 or 2, wherein the processing circuit executes control according to a rotation speed of the synchronous motor-generator in the second control mode, and executes control according to power output from the first power converter to the DC wiring in the first control mode.

13. A control method for a power generation system including an engine, a synchronous motor-generator mechanically connected to the engine and capable of generating power by using a driving force of the engine, and a first power converter electrically connected to the synchronous motor-generator, a first control mode in which the first power converter is controlled to convert AC power in an AC wiring between the synchronous motor / generator and the synchronous motor / generator into DC power and output the DC power to the DC wiring, and a second control mode in which the first power converter is controlled to convert DC power in the DC wiring to AC power and supply the AC power to the synchronous motor / generator in order to start the engine from a stopped state, a second carrier frequency in the first control mode that is lower than a first carrier frequency in the first control mode, or a DC voltage target value in the DC wiring that is lower than a first DC voltage target value in the first control mode, during a period in which the engine speed is lower than a reference speed in the second control mode.

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