Power conversion device and its control method

Abstract

The present invention provides a power conversion device that can be connected to a power system in a Y connection mode and can compensate for reverse power. The power conversion device converts direct current into three-phase alternating current, or converts three-phase alternating current into direct current, and includes: a cluster, which is a cluster composed of at least one converter unit for each phase in each phase, the terminals on the DC side of the converter units are connected in parallel, and the terminals on the AC side of the converter units are connected to the power system in a star connection mode; and an inter-phase power control unit, which makes the direct current of the clusters of each phase unequal according to the reverse power on the power system side.

Description

Technical Field

[0001] The present invention relates to a power conversion device and a control method thereof, and more particularly, to a power conversion device that converts input direct current into three-phase commercial alternating current and outputs it, and a control method thereof. Background Art

[0002] In equipment connected to a power system with high voltage and extra-high voltage, a high voltage of the power system (for example, 3.3 kV or 6.6 kV) is applied to semiconductor switches. Since semiconductor switches with high withstand voltage are costly and have large losses, sometimes a multi-cell power conversion device using low withstand voltage semiconductor switches is used.

[0003] The multi-cell power conversion device includes a plurality of converter units, each of which is composed of a conversion circuit (for example, a single-phase two-level full-bridge inverter, etc.) using low withstand voltage semiconductor elements. By connecting the output terminals of the respective converter units in series, components such as low withstand voltage semiconductor switches can be used. Therefore, compared with a power conversion device composed of a single converter unit and a high withstand voltage semiconductor switch, the manufacturing cost can be reduced (for example, refer to Patent Document 1).

[0004] As an application example of the multi-cell power conversion device, a PCS (Power Conditioning System) of a solar power generation device is known. It uses an isolated DC / DC converter and a single-phase inverter to form a conversion unit, and can significantly miniaturize and lighten the transformer. The PCS can not only convert the direct current output from the solar power generation panel into commercial alternating current, but also be connected to a power system (high voltage or extra-high voltage). In the case of grid connection, it is obliged to continue operating even if an instantaneous voltage drop occurs. However, in the case where the voltage of the power system instantaneously drops to an unbalanced state due to a single-phase short circuit, a two-phase short circuit, etc., during the instantaneous voltage drop, reverse power sometimes flows according to the output current of the power conversion device. For example, when the power system becomes unbalanced, if the output current to the system remains three-phase balanced, the power of each phase becomes unbalanced, so reverse power is required.

[0005] Prior Art Documents

[0006] Patent Documents

[0007] Patent Document 1: Japanese Patent No. 6496608 Gazette

[0008] Patent Document 2: Japanese Patent No. 5537235 Gazette Summary of the Invention

[0009] Technical Problem to be Solved by the Invention

[0010] Figure 1 shows the output power when a three-phase balanced current flows through a power conversion device in a state where the voltage of the power system is three-phase unbalanced. As an example, the case of outputting a three-phase balanced alternating current Figure 1 (a) Figure 1 is described.

[0011] The voltage component with the phase rotation of the power system voltage in the positive direction Figure 1 (b) is set as:

[0012] [Equation 1]

[0013]

[0014] The voltage component with the phase rotation in the reverse direction Figure 1 (c) is set as:

[0015] [Equation 2]

[0016]

[0017] The former is the positive-phase voltage on the object coordinate, and the latter is the reverse voltage. Here, it is assumed that there is no zero-phase voltage on the object coordinate. As the three-phase balanced alternating current flowing through the power system Figure 1 (d), only the positive-phase component),

[0018] [Equation 3]

[0019]

[0020] is as shown in Equation 3 above.

[0021] The balanced part of the power output to the power system, i.e., the positive-phase power Figure 1 (e), is the product of the positive-phase voltage and the forward current. In contrast, the unbalanced part, i.e., the reverse power Figure 1 (f), is the product of the reverse-phase voltage and the positive-phase current (and the product of the positive-phase voltage and the reverse-phase current). In this case, the power flowing into the power system becomes a waveform that includes reverse power in addition to the forward power Figure 1 (g).

[0022] In order not to generate this reverse power, if the three-phase current is made unbalanced so that the output power of each phase becomes balanced, it becomes an overcurrent due to voltage drop and voltage unbalance rate, which may affect the equipment on the power system side. Therefore, considering the problem of overcurrent, it is preferable to make a three-phase balanced current flow.

[0023] Figure 2Fig. 0 shows a schematic structure of a conventional multi-unit power conversion device. In the multi-unit power conversion device 10, for each of the U, V, and W phases, clusters 11U, 11V, and 11W composed of a plurality of converter units are provided. The input terminals of each cluster are connected in parallel and connected to a DC power source such as a solar power generation device or a storage battery. The output terminals of each cluster are connected to the power system 14 in a Y connection (star connection) manner. In each converter unit, an isolated DC / DC converter, i.e., a DC-DC converter 12, and a single-phase inverter, i.e., a DC-AC converter 13, are cascaded. The input terminals of each converter unit are connected in parallel, and the output terminals are connected in series and connected to one phase of the power system 14.

[0024] When the multi-unit power conversion device 10 outputs reverse power, the input power to each cluster flows in equally (e.g., 200 kW). In contrast, the output power from each cluster becomes unbalanced. Therefore, there is a discrepancy in the average value of (the sum of the positive-phase power and the reverse-phase power), the input-output power of each cluster. The capacitors of each converter unit are charged and discharged by the difference in the input-output power. If the average of the difference in the input-output power is 0, the capacitor voltage becomes a constant value including the ripple caused by power pulsation. However, when the average of the difference in the input-output power is not 0, that is, when the average value of the input power and the average value of the output power are inconsistent, the capacitor voltages of the converter units included in each cluster continue to rise or fall.

[0025] Therefore, it is known that the output terminals of each cluster are connected to the power system in a Δ connection (delta connection) manner, and circulating current is used to compensate for the reverse power (for example, refer to Patent Document 2). However, in the case of the Δ connection method, the line voltage of the power system 14 (e.g., between the U and V phases) is applied to the converter units connected in series. In contrast, in the Y connection method, the phase voltage of the power system 14 (e.g., the U phase) is applied to the converter units connected in series. Therefore, if the number of stages of the converter units is the same, compared with the Y connection method, the applied voltage in the Δ connection method becomes 1 / 2 √3 times higher, and the withstand voltage required for the applicable components becomes higher, resulting in a problem of increased device cost.

[0026] Technical solutions adopted to solve the technical problems

[0027] An object of the present invention is to provide a power conversion device and its control method that can be connected to a power system in a Y connection manner and can compensate for reverse power.

[0028] For the above object, an embodiment of the present invention is a power conversion device that converts direct current into three-phase alternating current or converts three-phase alternating current into direct current, characterized by including: a cluster, which is a cluster composed of at least one converter unit for each phase of each phase, the terminals on the DC side of the converter units are connected in parallel, and the terminals on the AC side of the converter units are connected to the power system in a star connection; and an inter-phase power control unit, which makes the direct current of the clusters of each phase unequal according to the reverse-phase power on the power system side.

[0029] Advantages of the Invention

[0030] According to the present invention, by connecting to the power system in a Y connection and adding an unbalanced part to the input power on the DC side of each cluster, the reverse-phase power can be compensated without causing continuous increase or decrease in the capacitor voltage of each converter unit. Therefore, the reliability of the power conversion device can be maintained and the cost can be reduced. Description of the Drawings

[0031] Figure 1 It is a diagram showing the output power when a three-phase balanced current flows from the power conversion device in a state where the voltage of the power system is three-phase unbalanced.

[0032] Figure 2 It is a diagram showing a brief structure of an existing multi-unit power conversion device.

[0033] Figure 3 It is a diagram showing a multi-unit power conversion device according to an embodiment of the present invention.

[0034] Figure 4 It is a diagram showing the control system of the multi-unit power conversion device of this embodiment.

[0035] Figure 5 It is a diagram showing the structure of the positive and reverse phase separation unit of this embodiment.

[0036] Figure 6 It is a diagram showing the structure of the inter-phase power control unit according to Embodiment 1.

[0037] Figure 7 It is a diagram showing the structure of the inter-phase power control unit according to Embodiment 2.

[0038] Figure 8 It is a diagram showing the structure of the inter-phase power control unit according to Embodiment 3.

[0039] Figure 9 It is a diagram showing the structure of the inter-phase power control unit according to Embodiment 4. Detailed Embodiments

[0040] Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings.

[0041] (Multi-unit power conversion device)

[0042] Figure 3 In the multi-unit power conversion device, a multi-unit power conversion device according to an embodiment of the present invention is shown. In the multi-unit power conversion device 20, for each of the U, V, and W phases, clusters 21U, 21V, and 21W each composed of a plurality of converter units are provided. The input terminals on the DC side of each cluster are connected in parallel and connected to a DC power source such as a solar power generation device or a storage battery. The output terminals on the AC side of each cluster are connected to the power system 24 in a Y connection. In each converter unit, an isolated DC / DC converter, i.e., a DC-DC converter 22, and a single-phase inverter, i.e., a DC-AC converter 23, are cascaded. The input terminals on the DC side of each converter unit are connected in parallel, and the output terminals on the AC side are connected in series and connected to one phase of the power system 24. In the present embodiment, the DC side is used as the input and the AC side is used as the output for description, but a storage battery can also be connected to the DC side, and power can also be transmitted from the power system on the AC side to the DC side.

[0043] The DC-DC converter 22 is configured to include: an inverter circuit IN1 that converts direct current into high-frequency alternating current; a high-frequency transformer T that converts the output of the alternating current into a specified alternating voltage; and a converter circuit CN1 composed of transistors that convert the converted alternating current into direct current of a specified voltage. A DC-AC converter 23 is connected via a capacitor C1 for filtering the DC output. The DC-AC converter 23 is composed of an inverter circuit IN2, and the inverter circuit IN2 is composed of a transistor bridge that converts direct current into alternating current.

[0044] In the present embodiment, when the output power from each cluster becomes unbalanced (the sum of the positive-phase power and the negative-phase power), the distribution of the input power on the DC side of each cluster is adjusted according to the negative-phase power on the AC side. The distribution of the input power is adjusted only between clusters, and the input power of the converter units included in the same cluster is equally distributed.

[0045] Here, the positive-phase voltage, negative-phase voltage, positive-phase current, and negative-phase current of the power system are shown below.

[0046] [Mathematical formula 4]

[0047]

[0048]

[0049]

[0050]

[0051] At this time, the positive-phase power of each phase becomes:

[0052] [Mathematical formula 5]

[0053]

[0054]

[0055]

[0056] The negative-phase power of each phase becomes as follows.

[0057] [Mathematical formula 6]

[0058]

[0059]

[0060] For example, if it is set as:

[0061] [Mathematical formula 7]

[0062] Positive-phase voltage amplitude (Line-to-line voltage conversion )

[0063] Negative-phase voltage amplitude

[0064] Positive-phase current amplitude

[0065] Negative-phase current amplitude I N0 = 0 [A]

[0066] Then the positive-phase power and negative-phase power of each phase become as follows.

[0067] [Mathematical formula 8]

[0068]

[0069]

[0070]

[0071]

[0072]

[0073]

[0074] As Figure 3As shown, an unbalanced portion is added to the input power on the DC side of each cluster. Thus, the average value of the input power of each cluster is made to coincide with the average value of the output power, and the reverse power can be compensated. Further, since the average value of the input power of each cluster coincides with the average value of the output power, the capacitor voltage of the converter unit included in each cluster is maintained constant.

[0075] Figure 4 FIG. shows a control system of the multi-unit power conversion device according to the present embodiment. In order to adjust the distribution of the input power of each cluster, the multi-unit power conversion device 20 includes an inter-phase power control unit 31 and a positive / negative phase separation unit 32. The positive / negative phase separation unit 32 separates the positive-phase voltage, negative-phase voltage, positive-phase current, and negative-phase current shown in Equation (1) based on the measurement results of the system voltage and system current. The inter-phase power control unit 31 calculates the negative-phase power (Equation (3)) of each cluster based on the output of the positive / negative phase separation unit 32, and calculates the current correction value for each cluster based on the negative-phase power and the DC bus voltage. The current command from the control unit 33 is equally sent to each cluster, and the current correction value from the inter-phase power control unit 31 is added thereto to adjust the input power for each cluster.

[0076] Figure 5 FIG. shows the structure of the positive / negative phase separation unit according to the present embodiment. The measured system voltage and system current are respectively converted from the abc phase to the dq axis, the positive-phase part and the negative-phase part are separated, and the positive-phase voltage, negative-phase voltage, positive-phase current, and negative-phase current are calculated.

[0077] (Embodiment 1)

[0078] Figure 6 FIG. shows the structure of the inter-phase power control unit according to Embodiment 1. The inter-phase power control unit 31 multiplies the positive-phase current, which is the output of the positive / negative phase separation unit 32, by the negative-phase voltage, multiplies the negative-phase current by the positive-phase voltage, and adds the two multiplication results to calculate the negative-phase power P of Equation (3) for each of the U, V, and W phases. UN 、P VN 、P WN . The negative-phase power is divided by the measurement result of the voltage between the terminals on the DC side, i.e., the DC bus voltage, to calculate the current correction value of the current command supplied to each of the clusters 21U, 21V, and 21W. The current correction value corresponds to the unbalanced portion of the input power applied to Figure 3 shown.

[0079] The current command from the control unit 33 of the multi-unit power conversion device is equally distributed to each cluster. The current correction value is added to the current command supplied to each cluster, thereby adding an unbalanced portion corresponding to the negative-phase power to the input power of each cluster.

[0080] (Embodiment 2)

[0081] Figure 7 In it, the structure of the inter-phase power control unit related to Embodiment 2 is shown. Since the reverse power in Equation (1) all includes a vibration component (30 kW × cos2ωt), in the vibration component removal unit 34, the vibration component obtained by the following equation

[0082] [Mathematical formula 9]

[0083]

[0084] can also be subtracted from the current correction value added to each cluster.

[0085] (Embodiment 3)

[0086] Figure 8 In it, the structure of the inter-phase power control unit related to Embodiment 3 is shown. In Embodiment 1 and Embodiment 2, a correction value is added to the current command for the DC-DC converter 22, and an unbalanced part is added to the input power, thereby compensating for the reverse power. In Embodiment 3, in the feedback control for the DC-AC converter 23, a correction value is added to the voltage command provided to each cluster 21U, 21V, 21W.

[0087] The control unit 41 measures the voltage between the terminals of the filter capacitor C1 of each converter unit of the DC-AC converter 23, and calculates the voltage command based on the average value of the capacitor voltages of all units. The voltage command is equally distributed to each cluster, and a current correction value is added to this voltage command respectively to adjust the input power of each cluster. In addition, the phase voltage command can also be calculated based on the average value of the capacitor voltages of each cluster (each phase), that is, the phase average value, and inter-stage balance control is performed to calculate the unit voltage command for each cluster. At this time, the above current correction value is added to each of the unit voltage commands for each cluster.

[0088] In addition, in Embodiment 3, in the inter-phase balance control unit 42, inter-phase balance control for making the phase average value close to the average value of all units is performed. The inter-phase balance control is a control for correcting the deviation of the capacitance, detection error, and the change of the capacitor voltage accompanying the switching in each cluster and each converter.

[0089] On the other hand, the control performed by the inter-phase power control unit 31 is feed-forward control, and power equivalent to the reverse power is provided from the DC-DC converter 22 according to the reverse power to be output by the DC-AC converter 23. Looking at each cluster, the average power provided by the DC-DC converter 22 is consistent with the average power provided by the DC-AC converter 23, and the capacitor voltage becomes constant. Therefore, the inter-phase power control and the inter-phase balance control can be controlled separately.

[0090] (Embodiment 4)

[0091] Figure 9 In this, the structure of the inter-phase power control unit according to Embodiment 4 is shown. In Embodiment 3, the control unit 41 measures the voltage between the terminals of the filter capacitor C1 of each converter unit of the DC-AC converter 23, performs feedback control, sends out the unit voltage command for each cluster, and in the inter-phase balance control unit 42, performs inter-phase balance control to make the phase average value approach the average value of all units.

[0092] In Embodiment 4, using this inter-phase balance control, an unbalanced portion corresponding to the reverse power is added to the input power of each cluster. That is, a current correction value is respectively added to the current commands supplied to each cluster to suppress the variation of the filter capacitor C1 accompanied by the reverse power. In the inter-phase balance control unit 51, the current correction value is calculated based on the phase average value and the average value of all units of the voltage between the terminals of the filter capacitor C1, thereby making the average power supplied by the DC-DC converter 22 consistent with the average power supplied by the DC-AC converter 23. Thus, the reverse power can be compensated without causing a continuous increase or decrease in the capacitor voltage of each converter unit.

[0093] The inter-phase balance control unit 51 indirectly obtains the power allocated to each phase based on the capacitor voltage variation accompanied by the reverse power, so it can take into account the control performed by the inter-phase power control unit 31. However, since it is based on PI control, it may not be able to track the instantaneous change of external disturbances such as the instantaneous voltage drop of the system voltage, but it can cope with stable variations.

[0094] According to this embodiment, for each of the U, V, and W phases, there is a cluster composed of at least one converter unit. In a power conversion device in which the terminals on the DC side of the converter units are connected in parallel, the AC side terminals of the converter units can be connected to the power system in a Y connection mode, and the voltage between the terminals of the filter capacitor can be maintained and the reverse power can be output. Therefore, the reliability of the power conversion device can be maintained and the cost can be reduced.

[0095] (Other Embodiments)

[0096] In Embodiments 1 to 3, as the DC-DC converter 22, a DAB (Dual Active Bridge) converter was taken as an example for explanation, but an LLC converter can also be applied.

[0097] In this embodiment, a power conversion device applicable to a PCS, which converts the input direct current into alternating current and outputs it, has been described. Obviously, bidirectional power conversion can be performed. Therefore, for example, it can also be applied to a power conversion device that provides direct current (-48V) from a high-voltage system AC power supply (6600V) to communication equipment in a communication equipment room.

[0098] Description of Reference Numerals

[0099] 10, 20 Multi - unit Power Conversion Device

[0100] 11U, 11V, 11W, 21U, 21V, 21W Clusters

[0101] 12, 22 DC - DC Converter

[0102] 13, 23 DC - AC Converter

[0103] 14, 24 Power System

[0104] 31 Inter - phase Power Control Unit

[0105] 32 Positive and Negative Phase Separation Unit

[0106] 33, 41 Control Unit

[0107] 34 Vibration Component Removal Unit

[0108] 42, 51 Inter - phase Balance Control Unit

[0109] CN1, CN2 Converter Circuit

[0110] IN1, IN2 Inverter Circuit

[0111] C1, C2, C3 Capacitor Ca

[0112] T High - frequency Transformer

Claims

1. A power conversion device, The power conversion device converts direct current into three-phase alternating current or converts three-phase alternating current into direct current. The power conversion device is characterized by comprising: Clusters, each of which is composed of at least one converter unit for each phase. The terminals on the DC side of the converter units are connected in parallel, and the terminals on the AC side of the converter units are connected to the power system in a star connection; An inter-phase power control unit that makes the DC power on the DC side of the clusters of each phase unequal according to the reverse power on the power system side; And A positive and reverse phase separation unit that measures the system voltage and system current of the power system and calculates the positive phase voltage, reverse phase voltage, positive phase current, and reverse phase current. The inter-phase power control unit calculates the reverse power according to the positive phase voltage, the reverse phase voltage, the positive phase current, and the reverse phase current, divides the reverse power by the voltage between the terminals on the DC side to calculate a current correction value, and adds the current correction value to the current command for the clusters of each phase.

2. The power conversion device according to claim 1, characterized in that It further includes a vibration component removal unit that subtracts the vibration component of the reverse power from the current correction value.

3. The power conversion device according to claim 1, wherein It further includes: A unit for measuring the DC voltage of the converter unit; And An inter-phase balance control unit that performs control to make the phase average value of the DC voltage of the converter units in the cluster approach the average value of all the converter units of all the DC voltages.

4. A control method for a power conversion device, The power conversion device converts direct current into three-phase alternating current or converts three-phase alternating current into direct current and includes clusters, each of which for each phase includes at least one converter unit. The terminals on the DC side of the converter units are connected in parallel, and the terminals on the AC side of the converter units are connected to the power system in a star connection. The control method for the power conversion device is characterized by including: A step of measuring the system voltage and system current of the power system and calculating the reverse power; A step of calculating a current correction value corresponding to the unequal part of the DC power on the DC side of the clusters of each phase according to the reverse power; And A step of adding the current correction value to the current command for the clusters of each phase.

5. The control method for a power conversion device according to claim 4, characterized in that It further includes a vibration component removal step of subtracting the vibration component of the reverse power from the current correction value.

Images