Power conversion system and control method

Abstract

This power conversion system comprises a transformer, a first power conversion device, a second power conversion device, and a control unit. The transformer comprises a primary winding, a secondary winding, and a tertiary winding that are magnetically coupled to each other. The primary winding of the transformer is connected to a power supply PS via a circuit breaker. The first power conversion device includes a rectifier for rectifying and smoothing AC power received from the secondary winding, and a smoothing capacitive element. The first power conversion device uses the smoothed DC power to generate power to be supplied to a load. The second power conversion device generates third AC power by switching a semiconductor switching element. The second power conversion device supplies the third AC power to the tertiary winding at least during a period from when the circuit breaker is opened to when the circuit breaker is closed. The control unit controls the first power conversion device, the second power conversion device, and the circuit breaker.

Description

Power conversion system and control method

[0001] FIELD OF THE INVENTION An embodiment of the present invention relates to a power conversion system and a control method.

[0002] Some power conversion systems include a transformer and a power converter that converts AC power into DC power in their main circuit. The transformer supplies AC power to the power converter. The power converter includes a rectifier, a smoothing capacitive element (e.g., a smoothing capacitor), and a power converter that uses DC power. However, if a main power source is applied to the main circuit without precharging the capacitive element, a current nearly equal to that of a short circuit may flow into the capacitive element. To avoid such an event, it is desirable to reduce the inrush current when activating the main circuit of the power conversion system.

[0003] Japanese Patent Application Publication No. 2002-345258

[0004] An object of the present invention is to provide a power conversion system and a control method that can reduce the current that flows when activating a main circuit.

[0005] According to an embodiment, the power conversion system includes a transformer, a first power conversion device, a second power conversion device, and a control unit. The transformer includes a primary winding, a secondary winding, and a tertiary winding that are magnetically coupled to each other. The primary winding of the transformer is connected to a power source PS via a circuit breaker. The first power conversion device includes a rectifier and a smoothing capacitive element for rectifying and smoothing AC power received from the secondary winding. The first power conversion device generates power to be supplied to a load using the smoothed DC power. The second power conversion device generates third AC power by switching a semiconductor switching element. The second power conversion device supplies the third AC power to the tertiary winding at least during a period from when the circuit breaker is turned off to when the circuit breaker is turned on. The control unit controls the first power conversion device, the second power conversion device, and the circuit breaker.

[0006] 1 is a schematic configuration diagram of a power conversion system according to an embodiment; 2 is a schematic configuration diagram of a control device in the power conversion system according to an embodiment; 3 is a flowchart of an initial charging process according to an embodiment; 4 is a diagram for explaining initial charging according to an embodiment; 5 is a diagram for explaining ensuring phase continuity of AC voltage according to an embodiment; and 6 is a diagram for explaining initial charging according to an embodiment.

[0007] Hereinafter, a power conversion system according to an embodiment will be described with reference to the drawings. In the following description, components having the same or similar functions will be assigned the same reference numerals. Duplicate descriptions of those components may be omitted. Note that being electrically connected may simply be referred to as being "connected."

[0008] (First embodiment) A power conversion system will be described with reference to Figures 1A and 1B. Figure 1A is a schematic configuration diagram of a power conversion system 1 of the embodiment. Figure 1B is a schematic configuration diagram of a control device 10 in the power conversion system 1 of the embodiment. The power conversion system 1 receives three-phase AC power from a power source PS and supplies the converted power to a load.

[0009] The power conversion system 1 includes, for example, a transformer 2, a first power conversion device 3, a second power conversion device 4, and a control device 10. The power conversion system 1 may further include a transformer 5, a voltage sensor 6, a switch 7, a fan 8, and a circuit breaker 9.

[0010] The transformer 2 includes a primary winding 21, a secondary winding 22, and a tertiary winding 23 that are magnetically coupled to one another. The primary winding 21, the secondary winding 22, and the tertiary winding 23 each include a winding corresponding to each phase of a three-phase AC. For example, the primary winding 21, the secondary winding 22, and the tertiary winding 23 will be described as being symmetrical to one another, but the present invention is not limited to this and the configuration may be changed.

[0011] The transformer 2 is, for example, a three-phase AC transformer. Its primary winding 21 is connected to a power supply PS via a circuit breaker 9. Its secondary winding 22 is connected to the first power conversion device 3. Its tertiary winding 23 is connected to the second power conversion device 4.

[0012] The first power conversion device 3 rectifies and smoothes the AC power received from the secondary winding 22 of the transformer 2. For example, the first power conversion device 3 includes a rectifier 31 for the above purpose, a smoothing capacitor 32 (capacitive element), and a power conversion device 33. The AC power received from the secondary winding 22 is rectified by the rectifier 31 of the first power conversion device 3 and smoothed by the smoothing capacitor 32 to generate DC power. The power conversion device 33 uses the smoothed DC power to generate power to be supplied to a load under the control of the control device 10. Note that the rectifier 31 may be configured as a converter capable of generating a reverse power flow.

[0013] The second power conversion device 4 is supplied with control power, and uses the control power to generate the third AC power. For example, the control power supplied to the second power conversion device 4 may be generated by transforming the AC voltage of a power supply using a transformer 5 provided separately from the transformer 2. The magnetic paths of the transformer 2 and the transformer 5 are configured to be independent of each other. For example, the second power conversion device 4 includes a rectifier 41, a smoothing capacitor 42, and a power conversion device 43. The second power conversion device 4 generates the third AC power by switching a semiconductor switching element included in the power conversion device 43.

[0014] The output of the second power conversion device 4 is connected to a switch 7. The switch 7 connects the output of the second power conversion device 4 to the tertiary winding of the transformer 2 or connects the output of the second power conversion device 4 to the fan 8. This switching is controlled by a control device 10. The control of the fan 8 will be described in detail later. In response to this, the transformer 2 generates a desired AC voltage in the primary winding 21 and the secondary winding 22 based on the third AC power supplied to the tertiary winding 23.

[0015] 1B, the control device 10 includes a storage unit 11 and a CPU 12. The storage unit 11 includes a semiconductor memory and the like, and stores voltage detection value data, threshold data, programs, and the like.

[0016] The CPU 12 includes an information acquisition unit 121, an AC information acquisition unit 122, a voltage phase adjustment unit 123, a sequence processing unit 124, a first power conversion device control unit 125, a second power conversion device control unit 126, and a circuit breaker control unit 127.

[0017] The information acquisition unit 121 acquires information indicating the states of the first power conversion device 3 and the second power conversion device 4 from the first power conversion device 3 and the second power conversion device 4. The information indicating the states of the first power conversion device 3 and the second power conversion device 4 may include the DC voltage of the DC section of the first power conversion device 3. The information acquisition unit 121 adds the DC voltage of the DC section of the first power conversion device 3 to the voltage detection values ​​in the storage unit 11 as time-series information.

[0018] The AC information acquisition unit 122 acquires AC voltage information from the transformer 5 and the voltage sensor 6 and adds it to the voltage detection values ​​in the storage unit 11 as time-series information.

[0019] The voltage phase adjustment unit 123 adjusts the phase of the voltage of the third AC power generated by the second power conversion device 4 based on the information of the AC voltage detection value stored in the memory unit 11 so that the phase of the AC voltage of the power source PS and the phase of the AC voltage of the voltage sensor 6 are aligned.

[0020] The sequence processing unit 124 controls the sequence of various controls required at each stage from a stopped state to an operating state and from an operating state to a stopped state of the power conversion system 1. For example, the process of changing the power conversion system 1 from a stopped state to an operating state includes initial charging of the first power conversion device 3, control of the circuit breaker 9, and the like.

[0021] The first power conversion device control unit 125 acquires the magnitude of the output current of the first power conversion device 3, generates a drive signal for the first power conversion device 3 based on this, supplies the drive signal to the first power conversion device 3, and controls the switching of its semiconductor switching element between ON and OFF.

[0022] The second power conversion device control unit 126 acquires the magnitude of the output current of the second power conversion device 4, generates a drive signal for the second power conversion device 4 based on the magnitude of the output current, and supplies the drive signal to the second power conversion device 4 to control the switching on and off of its semiconductor switching elements. During the above control, the second power conversion device control unit 126 adjusts the phase of the voltage of the third AC power output from the second power conversion device 4 based on the phase information adjusted by the voltage phase adjustment unit 123.

[0023] The circuit breaker control unit 127 acquires the state of the auxiliary contact of the circuit breaker 9 to detect the state of the circuit breaker 9. The circuit breaker control unit 127 also controls the switching of the circuit breaker 9 between ON and OFF.

[0024] Hereinafter, with reference to FIGS. 2 and 3, the initial charging of the smoothing capacitor 32 that is carried out when the main circuit of the power conversion system 1 is activated will be described.

[0025] Fig. 2 is a flowchart of the initial charging process according to the embodiment. Fig. 3 is a diagram for explaining the initial charging according to the embodiment. Note that the procedures shown in Figs. 2 and 3 are merely examples, and the present invention is not limited to these. From the top of Fig. 3, the states of the circuit breaker 9, the transformer 2, the second power conversion device 4, and the first power conversion device 3 are shown in timing charts.

[0026] (Initial State) First, the initial state of the power conversion system 1 will be summarized. The power conversion system 1 is in an inactive state before power is supplied. More specifically, the circuit breaker 9 is open, and power supply from the power source to the main circuit including the transformer 2 and the first power conversion device 3 is stopped. It is assumed that the smoothing capacitor 32 provided in the first power conversion device 3 is in a discharged state.

[0027] It is assumed that the power supply PS that supplies power to the power conversion system 1 is capable of supplying AC power of a predetermined rated voltage and has sufficient AC power supply capacity. For the sake of simplicity, if the impedance of the power supply and wiring is sufficiently low and the smoothing capacitor 32 in a discharged state has not been initially charged, an inrush current may occur.

[0028] If the following situation occurs during the initial charging of the smoothing capacitor 32, the control device 10 will suspend closing the circuit breaker 9 while the situation is occurring, or may close the circuit breaker 9 after the situation is resolved.

[0029] - When the amplitude of the AC current flowing through the secondary winding 22 or tertiary winding 23 of the transformer 2 is asymmetrical in positive and negative. If such an asymmetrical situation is evident, there is a possibility that the residual magnetic flux of the transformer 2 is relatively large and that a magnetization bias has occurred. If the transformer 2 is turned on under such a situation, there is a possibility that a magnetization bias inrush current will occur in the transformer 2. Therefore, it is advisable to wait until the positive and negative currents are balanced before turning on the transformer.

[0030] When the current flowing through the secondary winding 22 or the tertiary winding 23 of the transformer 2 is relatively large. In such a situation, the initial charging of the capacitor 32 may be incomplete. Therefore, it is advisable to wait until the amplitude of the current (such as the maximum value of the instantaneous value or the moving average value of the absolute value) becomes smaller than a predetermined value before applying the current.

[0031] (Initial charging period) First, the operation from the state in which the circuit breaker 9 is turned off (OFF) to time t2 at which the circuit breaker 9 is turned on will be described. As described above, the circuit breaker 9 is opened by the control of the control device 10, and the supply of power from the power source PS to the main circuit is stopped.

[0032] At time t1, the control device 10 controls the second power conversion device 4 to supply the third AC power and start initial charging of the capacitor 32 (S11 in FIG. 2). This control mode controlled by the control device 10 is called an initial charging mode. The second power conversion device 4 starts supplying power (third AC power) for initial charging of the capacitor 32 under the control of the control device 10.

[0033] The control device 10 continues the initial charging until time t2 (S12 in FIG. 2), and detects the state at time t2 (S13 in FIG. 2). Based on the state detection result, the control device 10 determines whether to further continue the initial charging (S14 in FIG. 2), and further continues the initial charging until the condition is satisfied.

[0034] For example, during an initial charging period with the circuit breaker 9 turned off, the second power conversion device 4 generates third AC power under the control of the control device 10 and supplies it to the tertiary winding 23 of the transformer 2. The transformer 2 transforms the third AC power supplied to the tertiary winding 23 to a voltage determined by the turns ratio. This induces a voltage in the primary winding 21 and the secondary winding 22 of the transformer 2. The first power conversion device 3 starts initial charging of the capacitor 32 using the voltage induced in the secondary winding 22. The control device 10 may close the circuit breaker 9 when the identification result of the voltage (e.g., amplitude) applied to any winding of the transformer 2 after the third AC power is supplied to the tertiary winding 23 satisfies a predetermined condition.

[0035] When it is determined that the initial charging mode does not need to be continued, the control device 10 closes the circuit breaker 9 to start power supply from the power source PS (S15) and ends the initial charging in the initial charging mode of the second power conversion device 4 (S16). As a result, after closing the circuit breaker 9, the second power conversion device 4 interrupts the supply of the third AC power from the second power conversion device 4 to the tertiary winding 23.

[0036] It is known that magnetic saturation in the transformer 2 can cause an overcurrent. During the initial charging mode, the second power conversion device 4 is controlled by the control device 10 to limit the current value of the third AC power to a value that prevents magnetic saturation in the core of the transformer 2. The control device 10 may limit the current flowing by the third AC power to a value that prevents magnetic saturation in the core of the transformer 2, particularly during the initial charging of the capacitor 32. For example, one method for limiting the current value may be to apply vector control to the second power conversion device 4. This allows the second power conversion device 4 to generate an AC voltage adjusted to limit the current value of the third AC power. During the initial charging mode, power is supplied to the first power conversion device 3 with the current value limited as described above. However, at this stage, the first power conversion device 3 is not in a state where it can supply power to its load. Once the initial charging state in the initial charging mode is completed, it is necessary to switch the power supplied to the first power conversion device 3 to power from the power source PS.

[0037] Furthermore, if the continuity of AC power supplied to the first power conversion device 3 cannot be ensured when switching from the initial charging state in the initial charging mode to power from the power source PS (main power), the magnitude of the current flowing through the transformer 2 will fluctuate. In other words, a situation in which the continuity of AC power is ensured means a situation in which at least the phase of the AC voltage is continuous and the amplitude of the AC voltage is equal.

[0038] (Regarding Ensuring Phase Continuity of AC Voltage When Circuit Breaker is Closed) Fig. 4 is a diagram for explaining ensuring phase continuity of AC voltage in an embodiment. The phase of the AC voltage generated in the primary winding 21 of the transformer 2 is matched with the phase of the AC voltage of the power supply PS to ensure phase continuity of AC voltage when the circuit breaker is closed. For example, the second power conversion device 4 may ensure phase continuity of AC voltage by supplying third AC power with a voltage phase that matches the phase of the AC voltage generated in the primary winding 21 of the transformer 2 with the phase of the AC voltage of the power supply PS.

[0039] In addition to the phase difference due to the second power conversion device 4 not being synchronized with the power supply PS, a fixed phase difference between the windings of the transformer 2 may occur depending on the connection configuration of the windings. In this case, the control device 10 may include the phase difference due to the connection configuration of the primary winding 21 and the tertiary winding 23 of the transformer 2 in the phase adjustment conditions, and may match the phase of the AC voltage generated in the primary winding 21 of the transformer 2 with the phase of the AC voltage of the power supply PS. For example, the second power conversion device 4 may supply the third AC power with a voltage phase that matches the phase of the AC voltage generated in the primary winding 21 with the phase of the AC voltage of the power supply PS, based on the phase difference due to the connection configuration of the primary winding 21 and the tertiary winding 23 of the transformer 2. This allows the second power conversion device 4 to generate the third AC power synchronized with the power supply PS.

[0040] The power conversion system 1 in the above embodiment includes a transformer 2, a first power conversion device 3, a second power conversion device 4, and a control device 10. The transformer 2 includes a primary winding 21, a secondary winding 22, and a tertiary winding 23 that are magnetically coupled to each other. The primary winding 21 of the transformer 2 is connected to a power source PS via a switch. The first power conversion device 3 includes a rectifier 31 and a smoothing capacitor 32 (capacitive element) for rectifying and smoothing the AC power received from the secondary winding 22. The first power conversion device 3 generates power to be supplied to a load using the smoothed DC power. The second power conversion device 4 generates third AC power by switching a semiconductor switching element. The second power conversion device 4 supplies the third AC power to the tertiary winding 23 during a period from when the circuit breaker 9 is turned off to when the circuit breaker 9 is turned on. The third AC power is supplied to the tertiary winding 23 at least during a period from when the circuit breaker 9 is turned off to when the circuit breaker 9 is turned on. The control device 10 (controller) controls the first power converter 3, the second power converter 4, and the circuit breaker 9. This makes it possible to reduce the current that flows when activating the main circuit.

[0041] The following is a known example of a comparative measure for initial charging. The configuration shown below is an initial charging method in which a smoothing capacitor is charged to a certain extent via a current reducer or the like, and then the current reducer or the like is short-circuited.

[0042] (1) A measure using an initial charging circuit (e.g., a soft start circuit) that supplies DC power to the DC bus in the first power conversion device 3. (2) A measure that supplies AC power to the AC bus on the power supply side of the first power conversion device 3 via an AC reactor (ACL) and a electromagnetic contactor (VCS). (3) A measure that supplies AC power to the tertiary winding 23 of the transformer 2 via a current-reducing resistor.

[0043] Although there are configurations for initially charging the capacitor 32 as described above, in any of these methods, it is not easy to ensure continuity of the AC voltage waveform output from the secondary winding 22 of the transformer 2 when starting to supply main power to the main circuit. Therefore, an inrush current occurs in the capacitor 32 when starting to supply main power to the main circuit.

[0044] In contrast to this, in the present embodiment, the second power conversion device 4 generates the third AC power by switching on and off the semiconductor switching elements. Therefore, the second power conversion device 4 supplies the third AC power to the tertiary winding 23 at least during the period from when the circuit breaker 9 is turned off until when the circuit breaker 9 is turned on. This makes it possible to adjust the amplitude of the AC voltage waveform and provide protection by limiting the current.

[0045] Second Embodiment A second embodiment will be described with reference to the above-mentioned Figs. 1 and 3. Fig. 3 is a diagram for explaining the initial charging according to the embodiment. Fig. 3 differs from Fig. 2 in that a column for controlling the fan 8 is added to the bottom of Fig. 3. At time t2, the control device 10 completes the initial charging in the initial charging mode of the second power conversion device 4, and then closes the circuit breaker 9 to start supplying power from the power source PS. At time t3 after the circuit breaker 9 is closed, the supply of the third AC power from the second power conversion device 4 to the tertiary winding 23 stops.

[0046] At time t4, the control device 10 switches the switch 7 so that power can be supplied from the second power conversion device 4 to the fan 8. The control device 10 controls the speed of the second power conversion device 4 by V / f control or the like to operate the fan 8. This fan 8 is used to cool the transformer 2, the first power conversion device 3, etc. By controlling the speed of the fan 8 based on the detected ambient temperature, it becomes possible to adjust the speed of the fan 8 depending on the degree of temperature rise.

[0047] The second power conversion device 4 may use the fourth AC power to increase the cooling efficiency of at least one of the transformer 2 and the first power conversion device, for example.

[0048] According to at least one embodiment described above, the power conversion system includes a transformer, a first power conversion device, and a second power conversion device. The transformer includes a primary winding, a secondary winding, and a tertiary winding that are magnetically coupled to each other. The primary winding of the transformer is connected to a power source PS via a switch. The first power conversion device includes a rectifier and a smoothing capacitive element for rectifying and smoothing AC power received from the secondary winding. The first power conversion device generates power to be supplied to a load using the smoothed DC power. The second power conversion device generates third AC power by switching a semiconductor switching element. The second power conversion device supplies the third AC power to the tertiary winding at least during a period from when the circuit breaker is turned off to when the circuit breaker is turned on. The control unit controls the first power conversion device, the second power conversion device, and the circuit breaker. This reduces the current flowing when activating the main circuit.

[0049] Although several embodiments of the present invention have been described, these embodiments are presented as examples and are not intended to limit the scope of the invention. These embodiments can be implemented in various other forms, and various omissions, substitutions, and modifications can be made without departing from the spirit of the invention. These embodiments and their modifications are included within the scope and spirit of the invention, as well as within the scope of the invention described in the claims and their equivalents.

[0050] REFERENCE SIGNS LIST 1... Power conversion system 2... Transformer 3... First power conversion device 4... Second power conversion device 8... Fan 9... Circuit breaker 10... Control device (control unit) 12... CPU (control unit) PS... Power supply

Claims

1. a transformer including a primary winding, a secondary winding, and a tertiary winding that are magnetically coupled to one another, the primary winding being connected to a power source via a circuit breaker; a first power conversion device including a rectifier and a smoothing capacitive element for rectifying and smoothing the AC power received from the secondary winding, the first power conversion device generating power to be supplied to a load using the smoothed DC power; a second power conversion device that generates third AC power by switching a semiconductor switching element, and that supplies the third AC power to the tertiary winding at least during a period from when the circuit breaker is turned off to when the circuit breaker is turned on; a control unit that controls the first power conversion device, the second power conversion device, and the circuit breaker; Equipped with The control unit The circuit breaker is turned on after the positive and negative amplitudes of the AC current in the secondary winding or the tertiary winding of the transformer are balanced. Power conversion systems.

2. the second power conversion device limits the current value of the third AC power to a magnitude that does not cause magnetic saturation of the transformer. The power conversion system of claim 1 .

3. the second power conversion device generates an AC voltage adjusted to limit a current value of the third AC power. The power conversion system of claim 2 .

4. the second power conversion device supplies the third AC power with a voltage phase that matches the phase of the AC voltage generated in the primary winding with the phase of the AC voltage of the power source, based on a phase difference due to a connection configuration of the primary winding and the tertiary winding of the transformer. The power conversion system according to claim 3 .

5. The second power conversion device generates the third AC power synchronized with the power supply. The power conversion system of claim 2 .

6. The control unit After the third AC power is supplied to the tertiary winding, if an identification result of a voltage applied to any winding of the transformer satisfies a predetermined condition, the circuit breaker is closed. The power conversion system according to claim 5 .

7. The second power conversion device is After the circuit breaker is closed, the supply of the third AC power from the second power conversion device to the tertiary winding is interrupted. The power conversion system according to claim 4 or 5.

8. The second power conversion device is generating a fourth AC power by switching the semiconductor switching element; The fourth AC power is used to increase the cooling efficiency of at least one of the transformer and the first power conversion device. The power conversion system according to claim 4 or 5.

9. a transformer including a primary winding, a secondary winding, and a tertiary winding that are magnetically coupled to one another, the primary winding being connected to a power source via a circuit breaker; a first power conversion device including a rectifier and a smoothing capacitive element for rectifying and smoothing the AC power received from the secondary winding, the first power conversion device generating power to be supplied to a load using the smoothed DC power; A control method for a power conversion system comprising: generating third AC power by switching a semiconductor switching element of a second power conversion device, and supplying the third AC power to the tertiary winding during a period from when the circuit breaker is turned off to when the circuit breaker is turned on; Including, The circuit breaker is turned on after the positive and negative amplitudes of the AC current in the secondary winding or the tertiary winding of the transformer are balanced. Control method.