Power assist device

Abstract

The present invention supplies a necessary and sufficient amount of power at an appropriate timing when supplying power from a capacitor, a battery, or the like at the time of starting of a load.　The present invention is a power assist device that is connected to an output system of an AC generator for driving an engine and supplies assist power to a load, wherein the power assist device comprises a capacitor, a power conversion unit for converting DC power discharged by the capacitor into AC power and generating assist power, and a control unit for controlling the assist power generated by the power conversion unit, and the control unit detects the output frequency of the output system, synchronizes the assist power with the output frequency of the AC generator, detects output variation in the output system due to the load power, and controls the assist power in accordance with the output variation.

Description

Power Assist Device

[0001] The present invention relates to a power assist device.

[0002] Generally, when electrically connecting a load to an engine-driven generator, the generator is selected with a sufficient generating capacity to take into account the large amount of power required to start the load (so-called inrush power). For example, when using a submersible pump, if the electric capacity of the electric motor (load) connected to the generator is 15 kW, it is recommended to select a generator with a generating capacity of 60 kVA, which is about three times that amount; selecting a generator with a generating capacity of 25 kVA will result in the generator being unable to start.

[0003] However, even if the load requires a large amount of power at startup, the electrical load decreases after startup, so if an engine-driven generator with a large generating capacity is selected, it will continue to operate at a low load of about 30% of the load at startup.As a result, if it is operated for a long period of time, it is difficult for the engine's combustion temperature to rise, and there is a concern that unburned fuel and carbon will accumulate in the muffler, leading to a decrease in output or malfunction.

[0004] Furthermore, when multiple types of loads are connected to a single generator, if a generator is selected taking into consideration the maximum load for each type of load, it is possible that the maximum load of one model will be significantly higher than that of the other models. In such a case, the rated capacity of the selected generator will be over-specified overall, resulting in a large generator body and weight, making it an undesirable choice in terms of cost and portability.

[0005] To address this situation, it has been proposed to utilize a capacitor or an engine starting battery mounted on the generator, and use the electricity stored in these for the inrush power required to start the load, thereby keeping the rated capacity of the generator low (see Patent Document 1 below).

[0006] Japanese Patent Application Laid-Open No. 2017-011917

[0007] According to the above-mentioned conventional technology, the capacitor or battery mounted on the generator must have sufficient stored power capacity to supply the power (inrush power) required when starting the load. For this reason, the AC current output from the generator is converted to DC current and first used to fully charge the capacitor or battery, and then the DC current discharged from the capacitor or battery is converted to AC current of a desired frequency and supplied to the load.

[0008] According to such conventional technology, it is necessary to install a large-capacity capacitor or battery in the generator to accommodate the inrush power when starting the load, but the capacitor or battery becomes large and heavy, making it difficult to ensure space saving and portability of the generator. In addition, because the generator requires an inverter and a large-capacity capacitor, the generator itself becomes expensive and cannot be applied to existing generators that do not have an inverter, etc.

[0009] Furthermore, because the capacitor and battery mounted on the generator are charged by the engine running, the load cannot be started even if the engine is running until the capacitor and battery mounted on the generator are sufficiently charged. This creates the problem that the load cannot be started immediately after the engine starts running.

[0010] The present invention addresses these problems by reducing the rated capacity of an engine-driven generator to ensure compactness and portability when starting a load, shortening the time from starting the engine to starting the load, and reducing the size and weight of the capacitor or battery by supplying sufficient power at the appropriate timing when supplying power from the capacitor or battery when starting the load.

[0011] In order to solve the above problems, the present invention has the following configuration: A power assist device that is connected to an output system of an engine-driven AC generator and supplies assist power to a load, the power assist device comprising: a storage battery, a power conversion unit that converts DC power discharged from the storage battery into AC power to generate assist power, and a control unit that controls the assist power generated by the power conversion unit, wherein the control unit detects the output frequency of the output system and synchronizes the assist power with the output frequency of the AC generator, and detects output fluctuations of the output system associated with load power and controls the assist power in accordance with the output fluctuations.

[0012] With a power assist device having these characteristics, when starting a load with an engine-driven generator, the rated capacity of the generator can be reduced, ensuring compactness and portability. It also makes it possible to shorten the time from starting the engine to starting the load. Furthermore, when supplying power from a capacitor or battery to start the load, the necessary and sufficient power can be supplied at the appropriate timing, thereby making the capacitor or battery smaller and lighter.

[0013] A block diagram showing the configuration of a power assist device according to an embodiment of the present invention. A control flow diagram for assist control performed by the power assist device. A control flow diagram for follow-up control of assist current. A control flow diagram for charge / discharge control performed by the power assist device. A graph showing waveforms of the assist power of the assist device, the output power of the generator, and the load power of the electric motor that drives the submersible pump near the start of operation. A graph showing waveforms of the assist power of the assist device, the output power of the generator, and the load power of the electric motor that drives the submersible pump near the start of pump operation.

[0014] Hereinafter, embodiments of the present invention will be described with reference to the drawings. Hereinafter, with regard to the same description in different drawings, duplicated description in each drawing will be omitted as appropriate.

[0015] As shown in Fig. 1, a power assist device 1 according to an embodiment of the present invention is a device that is connected to an output system 100A of an engine-driven generator 100 and supplies assist power to a load 200. In Fig. 1, thick lines indicate power lines, and thin dashed lines indicate signal lines.

[0016] In the illustrated example, a generator-side connection terminal 1A and a load-side connection terminal 1B are provided, with a power line between them connecting to an output system 100A. The generator 100 is connected to the generator-side connection terminal 1A, and the load 200 is connected to the load-side connection terminal 1B, so that the power assist device 1 is externally connected between the generator 100 and the load 200. However, the power assist device 1 is not limited to this, and it can also be configured as an integral part of the generator 100 or as a built-in device inside the generator 100.

[0017] The power assist device 1 basically comprises a power storage device 2, a power conversion unit 3, and a control unit 4. The power storage device 2 is a source of assist power that the power assist device 1 supplies to the load 200, and is composed of a capacitor or a battery (secondary battery) that can charge and discharge DC power. As an example, a lithium ion capacitor or a lithium ion battery can be used, but an appropriate power storage device can be used depending on the power capacity required for the assist power.

[0018] The power conversion unit 3 converts DC power discharged by the battery 2 into AC power to generate assist power. Conversely, it converts AC power output by the generator 100 into DC power to output DC power for charging the battery 2. The power conversion unit 3 can be configured with a bidirectional DC / DC converter 30, a bidirectional three-phase inverter 31, a magnet contactor 32, and the like.

[0019] The bidirectional DC / DC converter 30 has a function of boosting the output of the battery 2 or lowering the output of the bidirectional three-phase inverter 31. The bidirectional three-phase inverter 31 converts the boosted DC output of the bidirectional DC / DC converter 30 into three-phase AC and outputs it, or converts the three-phase AC output from the generator 100 into DC and outputs it to the bidirectional DC / DC converter 30. When the magnet contactor 32 is in the OFF state, the output system 100A is disconnected from the power assist device 1, and when the magnet contactor 32 is in the ON state, the output system 100A and the power assist device 1 are connected to each other.

[0020] A charge / discharge switch 5 is provided between the battery 2 and the power conversion unit 3. As an example, an IGBT for charge / discharge control can be used as the charge / discharge switch 5. By controlling the switching of the charge / discharge switch 5 to the discharge side, a power line from the battery 2 to the power conversion unit 3 is energized, and by controlling the switching of the charge / discharge switch 5 to the storage side, a power line from the power conversion unit 3 to the battery 2 is energized. In addition, by turning off the charge / discharge switch 5, charging and discharging of the battery 2 is stopped.

[0021] The control unit 4 mainly controls the assist power generated by the power conversion unit 3, and also controls the charging and discharging of the battery 2. The control unit 4 includes a power storage management board 40 that manages abnormalities in the battery 2, a DC / DC control board 41 that controls the bidirectional DC / DC converter 30, and a DC / AC control board 42 that controls the bidirectional three-phase inverter 31.

[0022] The power storage management board 40 outputs an abnormality signal of the battery 2 to the DC / DC control board 41. In addition, the power storage management board 40 itself may be provided with a calculation element and a temperature sensor, and signals of the charge state of the battery 2, the cell temperature, and other operating states may be transmitted to the DC / DC control board 41.

[0023] The DC / DC control board 41 controls the switching of the charge / discharge switch 5 and controls the step-up / step-down of the bidirectional DC / DC converter 30 based on signals output from the power storage management board 40 and signals from the DC / AC control board 42. The DC / DC control board 41 also functions as the main control board of the power assist device 1, and connects the DC / DC control board 41 to other control boards, or to the operation unit 6 or display unit 7 via communication lines, thereby managing the control between the communication lines.

[0024] The output current of the output system 100A measured by an ammeter 10 installed on the power line between the generator-side connection terminal 1A and the load-side connection terminal 1B, and the output voltage of the output system 100A measured by a voltmeter installed at the generator-side connection terminal 1A are input to the DC / AC control board 42. The DC / AC control board 42 controls the ON / OFF of the magnet contactor 32, and controls the bidirectional three-phase inverter 31 based on the input output current and output voltage of the output system 100A, thereby controlling the assist current supplied to the load 200.

[0025] In the power assist device 1, a circuit breaker 11 is provided on the power line between the generator side connection terminal 1A and the load side connection terminal 1B. The assist power output from the power conversion unit 3 is connected to the output system 100A via the circuit breaker 11, and the output of the generator 100 is transmitted to the power conversion unit 3 via the circuit breaker 11. The circuit breaker 11 sets upper limits for the assist current connected to the output system 100A and the current transmitted from the generator 100 to the power assist device 1.

[0026] Hereinafter, the control performed by the control unit 4 regarding the supply of assist power (assist control) and the control regarding the charging and discharging of the battery 3 (charge and discharge control) will be specifically described with reference to the control flows shown in FIGS.

[0027] [Assist Control] The main flow of assist control is shown in Figure 2. Assist control is basically performed separately for each phase (RST phase) of the three-phase AC. When control is initiated by connecting the generator 100 to the generator-side connection terminal 1A and the load 200 to the load-side connection terminal 1B, Determination 1 (Step S01) is performed with the magnetic contactor 32 turned off. In Determination 1, the stability of the output voltage is determined based on the output voltage of the output system 100A input to the control unit 4. If the output voltage value and the output voltage frequency are normal, the output voltage is determined to be stable (Step S01: YES). If either the output voltage value or the output voltage frequency is abnormal, the output voltage is determined to be unstable (Step S01: NO). If Determination 1 determines that the output voltage is unstable, the output voltage is continuously measured and Determination 1 is repeated until the output voltage is determined to be stable.

[0028] If it is determined that the output voltage is stable (step S01: YES), synchronization processing (step S02) is performed with the magnet contactor 32 turned OFF. In synchronization processing, the bidirectional three-phase inverter 31 is controlled to synchronize with the frequency of the output voltage (output frequency) in the output system 100A, and a current synchronized with the frequency of the output voltage (output frequency) is output from the bidirectional three-phase inverter 31. After synchronization processing, the magnet contactor 32 is turned ON to start interconnection between the output system 100A and the power assist device 1 (step S03). This state is called the grid-connected mode.

[0029] In the grid-connected mode, the output fluctuation of the output system 100A due to the load power (all or any of the output current, output voltage, and output frequency) is monitored, and the output fluctuation of the output system 100A is determined in decision 2 (step S04). If there is an output fluctuation in decision 2 (step S04: YES), assist starts (step S05). This state is called the assist mode. If there is no output fluctuation in decision 2 (step S04: NO), monitoring of the output fluctuation of the output system 100A continues, and the determination of whether there is an output fluctuation is repeated.

[0030] The determination of the presence or absence of output fluctuation in decision 2 is made based on the output voltage and output current or output frequency of the output system 100A. For example, if it is determined that the output voltage is less than a set value (value A) and the output current exceeds a set value (value B), it is determined that there is an output fluctuation (decision 2: YES) and the system transitions to assist mode. Also, if the output voltage is equal to or less than the set value (value A) or the output current is equal to or less than the set value (value B), it is determined that there is no output fluctuation (decision 2: NO) and the system continues in grid-connected mode.

[0031] As a specific example of judgment 2 for determining the assist mode, when the voltage effective value of one of the RST phases becomes equal to or less than a predetermined value from the zero cross point of each of the three phases (RST phase) in the output voltage and output current of the output system 100A, and the output current of the generator 100 flows at a certain level or more, the assist mode is entered.

[0032] As described above, when the output fluctuation of the output system 100A due to the load power is detected and the assist mode is entered (assist start: step S05), the assist power is controlled in accordance with this output fluctuation. Specifically, a target current value of the assist current is set for each cycle of the output voltage (step S06), and the assist current is controlled to follow the target current value (step S07).

[0033] The target current value is set (step S06) using the voltage change of the immediately preceding output voltage. For example, the target voltage value for one cycle is calculated from the output voltage one or more cycles ago, and the target current value is instantaneously calculated from the waveform that matches the voltage cycle. As an example, the target current value AI is calculated using the following formula (1). The load virtual impedance (LI) here is set and stored in advance if the load is fixed, or the normal waveform of the output system 100A with the load connected is recorded and calculated from the recorded data. AI = (NV - LV) / LI (1) AI: target current value NV: effective voltage value in normal state LV: effective voltage value one cycle ago under load LI: virtual impedance of the load

[0034] The assist current tracking control (step S07) basically involves setting a target current value AI for each voltage cycle of the output voltage so as to track output fluctuations in the output system 100A, and PWM-controlling the assist current. As shown in FIG. 3 , the assist current tracking control (step S07) involves determining whether the calculated target current value AI of the assist current is smaller than a maximum specified value IR (Decision 4: step S11). If the target current value AI is smaller than the maximum specified value IR (step S11: YES), the target value of the assist current is set to the target current value AI (step S12). If the target current value AI is equal to or greater than the maximum specified value IR (step S11: NO), the assist current target value is set to the maximum specified value IR (step S14). Then, by PWM-controlling with the set target value, the assist current is controlled to a constant value for one cycle (step S13), and the assist current is controlled to track voltage fluctuations in each cycle.

[0035] After the assist current tracking control (step S07) has been performed for a predetermined time, the supply of assist power that is no longer needed due to smooth start of the load is stopped. In decision 3 (step S08), a decision is made as to whether to stop the assist. The decision to stop the assist is made based on whether the output fluctuations have been smoothed as a result of the control. For example, if the output voltage is greater than a set value (C value), the output current is less than a set value (D value), and the output frequency is close to the normal frequency, it is determined that the output fluctuations have been smoothed (step S08: YES), and the assist is stopped (step S09). In other cases (step S08: NO), the assist current is controlled again (steps S06 and onward).

[0036] After the assist is stopped (step S09), if there is an end instruction from the operation unit 6 or the like (step S10: YES), the control is ended, and if there is no end instruction (step S10: NO), the control transitions to the grid-connected mode (step S04 and following) after the grid-connection starts.

[0037] [Charge / Discharge Control] The control flow for charge / discharge control of the battery 2 is shown in Figure 4. The control unit 4 determines the remaining capacity of the battery 2 based on the charge / discharge amount, and compares the remaining capacity of the battery 2 with the dischargeable capacity in decision 10 of step S20. The dischargeable capacity here is preset depending on the storage capacity of the battery 2, etc. If the remaining capacity of the battery 2 exceeds the dischargeable capacity (step S20: YES), decision 11 of step S21 determines whether the battery 2 is currently in the discharge mode. If the battery 2 is not currently in the discharge mode (step S21: NO), the control unit 4 switches the battery 2 to the discharge mode (step S22) and controls the charge / discharge switch 5 to switch to the discharge side to start discharging (step S23). If the battery 2 is currently in the discharge mode (step S21: YES) or if discharging has started in step S23, constant voltage control is performed at the voltage value boosted by the bidirectional DC / DC converter 30 (step S24).

[0038] The discharge mode is a state in which it is possible to transition to the aforementioned assist mode, and when an output fluctuation of the output system 100A is detected in the discharge mode of the storage battery 2 (when the assist control decision 2 is YES), the mode transitions to the assist mode.

[0039] On the other hand, if it is determined in step S20 that the remaining capacity of the capacitor 2 is equal to or less than the set dischargeable capacity (step S20: NO), the charge / discharge switch 5 is turned off to stop discharging (step S30). Then, it is determined in step S31 that there is an output fluctuation in the output system 100A.

[0040] A specific example of decision 12 is to determine whether the output current of the generator 100 is below a set value (E value), and if the output current of the generator 100 is below the set value (E value) (YES in step S31), it is determined that there is no output fluctuation, and then in decision 13 in step S32, it is determined whether the current mode is the charging mode, and if the current mode is not the charging mode (NO in step S32), it is set to the charging mode (step S33), and charging is started by switching the charge / discharge switch 5 to the charging side (step S34). Then, if the current mode is the charging mode (YES in step S32) or if charging has started in step S34, constant output control is performed at the voltage value stepped down by the bidirectional DC / DC converter 30 (step S35).

[0041] When the constant output control is performed in step S35, the output power of the generator 100 is controlled to a set value by the control unit 4 and supplied to the battery 2 via the charge / discharge switch 5. Thereafter, abnormalities in the battery 2 are monitored by a signal from the battery storage management board 40, and a determination is made in step S36 as to whether the bidirectional DC / DC converter is fully charged (step S36: YES). If the battery is fully charged, the charge / discharge switch 5 is shut off to stop charging (step S37), and if the battery is not fully charged (step S36: NO), the processing from step S31 onwards is repeated.

[0042] Furthermore, if it is determined in judgment 12 of step S31 that the output current of the generator 100 is equal to or greater than the set value (value E) (step S31: NO), that is, if it is determined that there is output fluctuation, in this case, charging is stopped (step S38), the mode is switched to discharge mode (step S39), and the mode transitions to assist mode (step S40).

[0043] After transitioning to assist mode (step S40), if there is an end instruction from the operation unit 6 or the like (step S25: YES), the discharge mode is maintained and processing is terminated, and if there is no end instruction (step S25: NO), the discharge mode is maintained and processing from step S20 (decision 10) onwards is repeated.

[0044] [Example of Use] A description will be given of an example of use of the power assist device 1. First, when assist power is not being supplied, a commercial AC power source is connected to the generator-side connection terminal 1A of the power assist device 1, the control unit 4 is set to the charge mode, the charge / discharge switch 5 is switched to the charge side, and the AC output of the commercial AC power source is converted to a DC output to fully charge the capacitor 2.

[0045] Then, an engine-driven generator 100 is connected to the generator-side connection terminal 1A of the power assist device 1, and the submersible pump motor is connected to the load-side connection terminal 1A. Here, the rated output of the motor driving the submersible pump is set to 15 kW, and the power generation capacity of the generator 100 is set to 25 kVA. In this case, if an attempt is made to drive the submersible pump motor directly with the generator 100 without using the power assist device 1, the effective power of the generator 100 will not increase due to inrush power, and the submersible pump will not be able to start.

[0046] In contrast, when the power assist device 1 is connected to the output system 100A of the generator 100, the control unit 4 detects a drop in output voltage and an increase in output current (output fluctuation) due to inrush power and starts assisting, supplying an assist current to the submersible pump motor (load) that is synchronized with the output voltage of the generator 100 and corresponds to the output fluctuation of the output system 100A. As a result, the supply of assist current suppresses the drop in output voltage after the occurrence of inrush power, and the submersible pump motor operates normally at rated output. Then, several seconds after startup, the effect of the inrush power on the output voltage gradually disappears, so the supply of assist current gradually decreases, and eventually the control unit 4 stops assisting. As a result, the output voltage gradually recovers.

[0047] 5 and 6 are graphs showing monitored waveforms (voltage waveform and current waveform) of the assist power of the power assist device 1, the output power of the generator 100, and the load power of the electric motor that drives the submersible pump. Fig. 5 shows the waveforms immediately after the load power supply to the electric motor of the submersible pump is turned on while starting the power assist device 1 and the generator 100, and Fig. 6 shows the waveforms several seconds after that. Note that one phase (e.g., the R phase) of the three-phase AC (RST phase) is monitored here.

[0048] After the load power supply is turned on, the power assist device 1 starts assisting one cycle after the output voltage and outputs an assist current. The target current value for the assist current at this time is calculated using the above formula (1) from the measured value of the output voltage one cycle before, and the assist current is controlled to follow fluctuations in the output voltage using PWM control based on this target current value. The output voltage immediately after the load power supply is turned on temporarily drops due to the influence of the load power, but the output voltage recovers as an assist current corresponding to the voltage change is supplied to the load 200.

[0049] As shown in the figure, the assist power in the power assist device 1 is output in synchronization with the output voltage of the generator 100, and the phases of the current (assist current) and voltage of the assist power are approximately the same. As a result, the power assist device 1 outputs assist power with a power factor of approximately 1, and can efficiently supply the assist current to the load 200 with little power loss.

[0050] The load current gradually decreases and stabilizes as the assist current is steadily supplied. As the load current stabilizes, the output voltage of the generator 100 gradually increases, and the submersible pump starts when the electric motor is operating steadily using power from the generator 100. Before the submersible pump starts, the assist current gradually decreases, and after the pump starts, the assist by the power assist device 1 stops.

[0051] As explained above, by using the power assist device 1, when operating a load with the engine-driven generator 100, it is possible to select an engine-driven generator 100 having a power generation capacity that matches the output capacity of the load. This makes it possible to reduce the size and space of the generator 100 used, and therefore reduces the CO ２ This allows for reductions in emissions and fuel consumption. Furthermore, during normal operation after the load is started, the generator 100 can be operated near the rated output range, which prevents unburned fuel and soot from accumulating in the muffler and prevents a decrease in the generator 100's output.

[0052] Furthermore, compared to conventional techniques that require an inverter and a storage battery inside the generator to handle inrush power, the generator 100 itself can be an existing one, and by charging the storage battery 2 of the power assist device 1 in advance using an AC power source or the like, an assist current can be supplied to the load immediately after the load is started by the engine-driven generator 100. This shortens the time from engine start to load start. Furthermore, because the assist current supplied from the power assist device 1 to the load can supply the necessary and sufficient power to the load at the appropriate timing, the capacity of the storage battery 2 can be minimized, allowing for a reduction in size and weight of the device, including the storage battery 2.

[0053] 1: Power assist device 1A: Generator side connection terminal 1B: Load side connection terminal 2: Storage battery 3: Power conversion unit 4: Control unit 5: Charge / discharge switch 6: Operation unit 7: Display unit 10: Ammeter 11: Circuit breaker 30: Bidirectional DC / DC converter 31: Bidirectional three-phase inverter 32: Magnet contactor 40: Storage management board 41: DC / DC control board 42: DC / AC control board 100: Generator 100A: Output system 200: Load

Claims

1. A power assist device that is connected to an output system of an engine-driven AC generator and supplies assist power to a load, comprising: a storage battery; a power conversion unit that converts DC power discharged from the storage battery into AC power to generate assist power; and a control unit that controls the assist power generated by the power conversion unit, wherein the control unit detects the output frequency of the output system and synchronizes the assist power with the output frequency of the AC generator, and detects output fluctuations of the output system associated with load power and controls the assist power in accordance with the output fluctuations.

2. The power assist device according to claim 1, wherein the control unit controls the power conversion unit so that the power factor of the assist power becomes approximately 1.

3. The power assist device according to claim 1, wherein the control unit detects output fluctuations in the output system, and sets a target current value for each voltage cycle based on the immediately preceding voltage change in the output system to follow and control the assist current.

4. The power assist device according to claim 3, wherein the target current value is set in advance or calculated from a virtual impedance of the load determined from a normal waveform of the output system.

5. The power assist device according to claim 1, wherein a charge / discharge switch is provided between the capacitor and the power conversion unit, and the control unit controls the charge / discharge switch to perform a charge mode in which the capacitor is charged with power from the AC generator when the assist power is not being supplied.

6. The power assist device according to claim 5, wherein when the output fluctuation is detected in the charge mode of the capacitor, the control unit switches the charge / discharge switch to the discharge mode and controls the assist power.

7. The power assist device according to claim 1, wherein the power conversion unit comprises a bidirectional DC / DC converter and a bidirectional three-phase inverter.

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