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

Technical Field

[0001] Embodiments of the present invention relate to a power conversion system and a control method.

Background Art

[0002] Some power conversion systems include, in their main circuits, a transformer and a power conversion device that converts AC power into DC power. The transformer supplies AC power to the power conversion device. The power conversion device includes a rectifier, a capacitive element (such as a smoothing capacitor) for smoothing, and a power converter that uses DC power. By the way, when the main power supply is turned on to the main circuit without pre-charging the capacitive element, a current of a magnitude close to a short-circuit state may flow into the capacitive element. In order to avoid the occurrence of such an event, it has been desired to reduce the inrush current when activating the main circuit of the power conversion system.

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0004] An object of the present invention is to provide a power conversion system and a control method capable of reducing the current flowing when activating the main circuit.

Means for Solving the Problems

[0005] The power conversion system of the embodiment 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 source PS via a circuit breaker. The first power conversion device comprises a rectifier and a capacitive element for smoothing the 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 a 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 the period from when the circuit breaker is off until when it is turned on. The control unit controls the first power conversion device, the second power conversion device, and the circuit breaker. [Brief explanation of the drawing]

[0006] [Figure 1A] A schematic diagram of the power conversion system according to the embodiment. [Figure 1B] A schematic diagram of the control device in the power conversion system of the embodiment. [Figure 2] A flowchart of the initial charging process in the embodiment. [Figure 3] A diagram illustrating the initial charging of an embodiment. [Figure 4] A diagram illustrating the ensuring of phase continuity of the AC voltage in the embodiment. [Figure 5] A diagram illustrating the initial charging of an embodiment. [Modes for carrying out the invention]

[0007] The power conversion system of the embodiment will be described below with reference to the drawings. In the following explanation, components with the same or similar functions will be denoted by the same reference numeral. Furthermore, redundant explanations of these components may be omitted. Note that the term "connected" may be used to simply refer to an electrical connection.

[0008] (First embodiment) The power conversion system will be described with reference to Figures 1A and 1B. Figure 1A is a schematic diagram of the power conversion system 1 of the embodiment. Figure 1B is a schematic diagram of the control device 10 in the power conversion system 1 of the embodiment. The power conversion system 1 receives three-phase AC power from the power supply PS and supplies the converted power to the 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 comprises a primary winding 21, a secondary winding 22, and a tertiary winding 23 that are magnetically coupled to each other. The primary winding 21, the secondary winding 22, and the tertiary winding 23 each comprise windings corresponding to each phase of the three-phase AC. For example, the primary winding 21, the secondary winding 22, and the tertiary winding 23 are described as being symmetrical, but the configuration may be changed without limitation.

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

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

[0013] The second power converter 4 is supplied with control power, and the second power converter 4 uses this control power to generate the third AC power described above. For example, the control power supplied to the second power converter 4 may be generated by transforming the AC voltage of the power source using another transformer 5, which is provided separately from transformer 2. Transformers 2 and 5 have magnetic circuits that are independent of each other. For example, the second power converter 4 includes a rectifier 41, a smoothing capacitor 42, and a power converter 43. The second power converter 4 generates a third AC power by switching the semiconductor switching elements in the power converter 43.

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

[0015] As shown in Figure 1B, the control device 10 comprises a storage unit 11 and a CPU 12. The storage unit 11 includes semiconductor memory 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 converter control unit 125, a second power converter 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 in the DC section of the first power conversion device 3. The information acquisition unit 121 adds the DC voltage in the DC section of the first power conversion device 3 to the voltage detection value in the storage unit 11 as time-series information.

[0018] The AC information acquisition unit 122 acquires information on the AC voltage from the transformer 5 and the voltage sensor 6 and adds it to the voltage detection value 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 so that the phase of the AC voltage of the power supply PS and the phase of the AC voltage of the voltage sensor 6 are aligned based on the information on the AC voltage detection value stored in the storage unit 11.

[0020] The sequence processing unit 124 performs sequence control of various controls required at each stage from the stop state to the operating state and from the operating state to the stop state of the power conversion system 1. For example, during the transition from the stop state to the operating state of the power conversion system 1, it 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 between ON and OFF of its semiconductor switching element.

[0022] The second power converter control unit 126 acquires the magnitude of the output current of the second power converter 4, generates a drive signal for the second power converter 4 based on this, and supplies the drive signal to the second power converter 4 to control the ON and OFF switching of its semiconductor switching element. In addition, during the above control, the second power converter control unit 126 adjusts the phase of the voltage of the third AC power output from the second power converter 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 contacts of the circuit breaker 9 and detects the state of the circuit breaker 9. The circuit breaker control unit 127 also controls the switching between ON and OFF states of the circuit breaker 9.

[0024] The initial charging of the smoothing capacitor 32, which is performed when activating the main circuit of the power conversion system 1 described above, will be explained below with reference to Figures 2 and 3.

[0025] Figure 2 is a flowchart of the initial charging process in the embodiment. Figure 3 is a diagram illustrating the initial charging of an embodiment. Note that the procedure shown in Figures 2 and 3 is just one example and is not limited to it. The timing chart in Figure 3 shows the states of circuit breaker 9, transformer 2, second power converter 4, and first power converter 3, starting from the top.

[0026] (Initial state) First, let's summarize the initial state of power conversion system 1. The power conversion system 1 is in an inactive state before being powered. More specifically, the circuit breaker 9 is open, and the supply of power from the power source to the main circuit, including the transformer 2 and the first power converter 3, is stopped. It is assumed that the smoothing capacitor 32 in the first power converter 3 is in a discharged state.

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

[0028] Furthermore, if any of the following situations occur during the initial charging of the smoothing capacitor 32, the control device 10 will withhold closing the circuit breaker 9 while the following situation is occurring. Alternatively, it may close the circuit breaker 9 after the situation has resolved.

[0029] - When the amplitude of the alternating current flowing through the secondary winding 22 or tertiary winding 23 of transformer 2 is asymmetrical, with positive and negative values. When such asymmetrical conditions are pronounced, it is possible that magnetization bias is occurring in transformer 2 due to a relatively large residual magnetic flux. Switching on the power under these conditions could lead to a magnetization-biased inrush current in transformer 2. Therefore, it is best to wait until the positive and negative currents are balanced before switching on the power.

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

[0031] (Initial charging period) First, let's explain the operation from the state where circuit breaker 9 is off (OFF) until time t2 when circuit breaker 9 is turned on. As described above, circuit breaker 9 is open due to the control of the control device 10, and the supply of power from the power supply PS to the main circuit is stopped.

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

[0033] The control device 10 continues initial charging until time t2 (S12 in Figure 2), and at time t2, it detects the state (S13 in Figure 2). Based on the state detection result, the control device 10 determines whether or not to continue initial charging further (S14 in Figure 2), and continues initial charging further until the condition is met.

[0034] For example, during the initial charging period when the circuit breaker 9 is switched off, the second power converter 4 generates a third AC power under the control of the control device 10 and supplies it to the tertiary winding 23 of the transformer 2. Transformer 2 transforms the third AC power supplied to the tertiary winding 23 into a voltage determined by the turns ratio. This induces voltages in the primary winding 21 and secondary winding 22 of transformer 2. The first power converter 3 uses the voltage induced in the secondary winding 22 to begin the initial charging of capacitor 32. The control device 10 may close the circuit breaker 9 if, after supplying the third AC power to the tertiary winding 23, the identification result of the voltage (amplitude, etc.) applied to any of the windings of transformer 2 satisfies predetermined conditions.

[0035] If the above determination determines that it is unnecessary to continue the initial charging mode, the control device 10 closes the circuit breaker 9 to start supplying power from the power source PS (S15), and ends the initial charging of the second power converter 4 in its initial charging mode (S16). As a result, after closing the circuit breaker 9, the second power converter 4 interrupts the supply of the third AC power from the second power converter 4 to the tertiary winding 23.

[0036] Incidentally, it is known that overcurrent occurs when magnetic saturation occurs in transformer 2. During the initial charging mode described above, the second power converter 4 limits the current value of the third AC power to a level that does not cause magnetic saturation in the core of the transformer 2, under the control of the control device 10. The control device 10 should limit the current flowing through the third AC power to a level that does not cause magnetic saturation in the core of the transformer 2, especially during the initial charging of the capacitor 32. For example, one method for limiting the above-mentioned current value is to apply vector control to the control of the second power converter 4. This allows the second power converter 4 to generate an AC voltage that is adjusted to limit the current value of the third AC power. During this initial charging mode, power is supplied to the first power converter 3 with a limited current value as described above. However, at this stage, the first power converter 3 is not in a state to supply power to its load. Once the initial charging state in the initial charging mode is complete, it is necessary to switch the power supplied to the first power converter 3 to power from the power supply PS.

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

[0038] (Regarding ensuring the continuity of the AC voltage phase when the circuit breaker is closed) Figure 4 is a diagram illustrating the ensuring of phase continuity of the AC voltage in the embodiment. The phase of the AC voltage generated in the primary winding 21 of transformer 2 is matched with the phase of the AC voltage of the power supply PS to ensure the continuity of the AC voltage phase when the circuit breaker is closed. For example, the second power converter 4 can ensure the continuity of the AC voltage phase 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 to the phase of the AC voltage of the power supply PS.

[0039] In addition to the phase difference caused by the second power converter 4 not being synchronized with the power supply PS mentioned above, there may be fixed phase differences between windings depending on the wiring configuration of each winding of the transformer 2. In this case, the control device 10 should include the phase difference due to the wiring configuration of the primary winding 21 and the tertiary winding 23 of the transformer 2 as a condition for phase adjustment, and 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. The second power converter 4 may, for example, supply third AC power with a voltage phase that matches the phase of the AC voltage generated in the primary winding 21 to 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 converter 4 to generate third AC power synchronized with the power supply PS.

[0040] The power conversion system 1 in the above embodiment comprises a transformer 2, a first power conversion device 3, a second power conversion device 4, and a control device 10. The transformer 2 comprises 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 supply PS via a switch. The first power conversion device 3 comprises a rectifier 31 for rectifying and smoothing the AC power received from the secondary winding 22, and a smoothing capacitor 32 (capacitive element). The first power conversion device 3 generates power to be supplied to the load using the smoothed DC power. The second power conversion device 4 generates third AC power by switching semiconductor switching elements. The second power conversion device 4 supplies third AC power to the tertiary winding during the period from when the circuit breaker 9 is off until when the circuit breaker 9 is on. At least during the period from when the circuit breaker 9 is off until when the circuit breaker 9 is on, third AC power is supplied to the tertiary winding 23. The control device 10 (control unit) controls the first power converter 3, the second power converter 4, and the circuit breaker 9. This reduces the current that flows when activating the main circuit.

[0041] By the way, the following are known examples of measures taken during initial charging. The configuration illustrated below is an initial charging method in which the smoothing capacitor is partially charged via a current reducer, etc., and then the current reducer, etc., 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 converter 3. (2) A measure to supply AC power to the AC bus on the power supply side of the first power converter 3 via an AC reactor (ACL) and an electromagnetic contactor (VCS). (3) A measure to supply AC power to the tertiary winding 23 of the transformer 2 via a current-reducing resistor.

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

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

[0045] (Second embodiment) A second embodiment will be described with reference to Figures 1 and 3 mentioned above. Figure 3 is a diagram illustrating the initial charging of an embodiment. The difference between Figure 3 and Figure 2 is that Figure 3 includes an additional section for controlling fan 8 at the bottom. At time t2, the control device 10 completes the initial charging using the initial charging mode of the second power converter 4, closes the circuit breaker 9, and begins 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 converter 4 to the tertiary winding 23 stops.

[0046] At time t4, the control device 10 switches switch 7 to allow power to be supplied from the second power converter 4 to the fan 8. The control device 10 controls the speed of the second power converter 4 using V / f control, etc., to operate the fan 8. This fan 8 is used to cool the transformer 2, the first power converter 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 according to the degree of temperature rise.

[0047] The second power converter 4 may, for example, utilize the fourth AC power to improve the cooling efficiency of at least one of the transformer 2 and the first power converter.

[0048] According to at least one embodiment described above, the power conversion system comprises a transformer, a first power converter, and a second power converter. 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 switch. The first power converter comprises a rectifier and a capacitive element for smoothing the AC power received from the secondary winding. The first power converter generates power to be supplied to the load using the smoothed DC power. The second power converter generates third AC power by switching a semiconductor switching element. The second power converter supplies the third AC power to the tertiary winding at least during the period from when the circuit breaker is off until when it is turned on. The control unit controls the first power converter, the second power converter, and the circuit breaker. This reduces the current that flows when activating the main circuit.

[0049] While several embodiments of the present invention have been described, these embodiments are presented as examples only and are not intended to limit the scope of the invention. These embodiments can be carried out in a variety of other forms, and various omissions, substitutions, and modifications can be made without departing from the spirit of the invention. These embodiments and their variations are included in the scope and spirit of the invention, as well as in the claims and their equivalents. [Explanation of symbols]

[0050] 1…Power conversion system 2… Transformer 3…First power converter 4…Second power converter 8…fan 9... Circuit breaker 10…Control device (control unit) 12…CPU (Control Unit) PS…Power supply

Claims

1. A transformer comprising a primary winding, a secondary winding, and a tertiary winding that are magnetically coupled to each other, wherein the primary winding is connected to a power source via a circuit breaker, A first power converter comprising a rectifier and a capacitive element for smoothing the AC power received from the secondary winding, wherein the first power converter generates power to be supplied to a load using the smoothed DC power, A second power converter that generates a third AC power by switching a semiconductor switching element, the second power converter that supplies the third AC power to the tertiary winding at least during the period from when the circuit breaker is off until when the circuit breaker is turned on, A control unit that controls the first power converter, the second power converter, and the circuit breaker. Equipped with, The control unit, The circuit breaker is switched on after the positive and negative amplitudes of the AC current in the secondary or tertiary winding of the transformer are balanced. Power conversion system.

2. The second power converter limits the current value of the third AC power to a value such that magnetic saturation of the transformer does not occur. The power conversion system according to claim 1.

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

4. The second power converter supplies the third AC power with a voltage phase such that the phase of the AC voltage generated in the primary winding matches the phase of the AC voltage of the power supply, based on the phase difference due to the 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 converter generates the third AC power synchronized with the power supply. The power conversion system according to claim 2.

6. The control unit, After supplying the third AC power to the tertiary winding, the circuit breaker is closed if the voltage across any of the transformer windings is identified as meeting predetermined conditions. The power conversion system according to claim 5.

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

8. The second power converter is A fourth AC power is generated by switching the semiconductor switching element. The fourth AC power is used to improve the cooling efficiency of at least one of the transformer and the first power converter. The power conversion system according to claim 4 or claim 5.

9. A transformer comprising a primary winding, a secondary winding, and a tertiary winding that are magnetically coupled to each other, wherein the primary winding is connected to a power source via a circuit breaker, A first power converter comprising a rectifier and a capacitive element for smoothing the AC power received from the secondary winding, wherein the first power converter generates power to be supplied to a load using the smoothed DC power, A control method for a power conversion system comprising, The second power converter generates a third AC power by switching the semiconductor switching element, and supplies the third AC power to the tertiary winding during the period from when the circuit breaker is off until when it is turned on. Includes, The circuit breaker is switched on after the positive and negative amplitudes of the AC current in the secondary or tertiary winding of the transformer are balanced. Control method.

Images