Power conversion system and power conversion control method

Abstract

The power conversion system of this embodiment includes multiple converters and a control unit. The multiple converters are arranged in parallel phase-to-phase configuration for each phase of a multiphase AC power system. The control unit uses multiple carrier signals with a predetermined phase difference to perform PWM control on the multiple converters. At predetermined time intervals, a predetermined number of even-numbered timings are determined within one cycle of a specific carrier signal. At each of these even-numbered timings, the control unit detects the primary-side current of the multiple converters. The control unit uses the detected primary-side current value and a primary-side current reference value to generate a voltage reference for the PWM control, and updates the voltage reference at each of the four or more even-numbered timings. The control unit uses the specific carrier signal and the updated voltage reference to control a specific converter among the multiple converters.

Description

Technical Field

[0001] The embodiments of the present invention relate to a power conversion system and a power conversion control method. Background Technology

[0002] A power conversion system comprises a power converter with switching elements and is interconnected with a multiphase AC power system. Switching ripples are generated on the power system side due to the operation of the power converter, and it is desirable to further reduce these switching ripples. Therefore, the power conversion system is required to further improve the control responsiveness of current control on the power system side.

[0003] Existing technical documents

[0004] Patent documents

[0005] Patent Document 1: Japanese Patent Application Publication No. 2015-43660 Summary of the Invention

[0006] The problem that the invention aims to solve

[0007] The problem to be solved by the present invention is to provide a power conversion system and power conversion control method that can further improve the control responsiveness of current control on the power system side.

[0008] Methods for solving problems

[0009] The power conversion system of this embodiment is interconnected with a multiphase AC power system. The power conversion system includes multiple converters and a control unit. The multiple converters are arranged in parallel phase-to-phase configuration for each phase of the power system. The control unit controls the multiple converters using multiple carrier signals having a predetermined phase difference with each other via carrier comparison-type PWM control. Within one cycle of a specific carrier signal among the multiple carrier signals, the control unit determines at least four even-numbered timing intervals at predetermined time intervals, and detects the primary-side current of each of the multiple converters during these even-numbered timing intervals. The control unit uses the detected primary-side current value and a primary-side current reference value to generate a voltage reference for the PWM control, and updates the voltage reference during these even-numbered timing intervals. The control unit uses the specific carrier signal and the updated voltage reference to control a specific converter among the multiple converters. Attached Figure Description

[0010] Figure 1 This is a diagram illustrating an example of a power conversion system implemented in this way.

[0011] Figure 2A This is a circuit diagram showing the winding structure of a transformer in an embodiment.

[0012] Figure 2B This is a circuit diagram showing the winding structure of a transformer in an embodiment.

[0013] Figure 3A This is a circuit diagram showing the internal structure of the unit converter in the implementation method.

[0014] Figure 3B This is a circuit diagram showing the internal structure of the unit converter in the implementation method.

[0015] Figure 3C This is a circuit diagram showing the internal structure of the unit converter in the implementation method.

[0016] Figure 4 This is a simplified structural diagram of the power conversion system according to the first embodiment.

[0017] Figure 5 This is a diagram illustrating the timing of current sampling and voltage reference updates in the first embodiment.

[0018] Figure 6 It is used for Figure 4 The diagram illustrates the simulation results of the circuit structure.

[0019] Figure 7 This is a diagram illustrating the timing of current sampling and voltage reference updates in a modified example of the first embodiment.

[0020] Figure 8 This is a structural diagram of a power conversion system model in the second embodiment, which connects five converters in parallel.

[0021] Figure 9 This is a structural diagram of the converter control unit according to the second embodiment.

[0022] Figure 10 This diagram illustrates the timing of current sampling and voltage reference updates in the third embodiment. Detailed Implementation

[0023] Hereinafter, a power conversion system and power conversion control method according to embodiments will be described with reference to the accompanying drawings. The power conversion system described below supplies desired alternating current (AC) power to an electric motor (motor), which is an example of a load. The power conversion system of the embodiments includes a power converter formed in a manner interconnected with an AC power system. The connection described in the embodiments includes electrical connection.

[0024] First, the overall electrical structure of the power conversion system 1 will be described. Figure 1 This is a diagram illustrating an example of a power conversion system 1 according to an embodiment. The power conversion system 1 includes, for example, a transformer 201, a power converter 30, and a control unit 601.

[0025] The power conversion system 1 has AC input terminals R, S, and T, and is connected to the AC power supply 101 (power system) via these terminals. The power conversion system 1 has AC output terminals U, V, and W, and is connected to the motor 401 via these terminals.

[0026] The primary winding of transformer 201 is connected to the AC input terminals R, S, T, and the AC input side of power converter 30 is connected to the secondary winding of transformer 201. The AC output side of power converter 30 is connected to the AC output terminals U, V, W.

[0027] The control unit 601 controls the switching elements built into the unit converters 30U, 30V, and 30W. The unit converters 30U, 30V, and 30W are collectively referred to as multiple unit converters 30X.

[0028] Next, the various parts of the power conversion system 1 will be described.

[0029] Transformer 201 has a primary winding and three sets of secondary windings. The primary winding is configured as a three-phase star connection. The secondary winding is configured as a three-phase open delta connection with mutual insulation. Three-phase AC power is supplied to transformer 201 from AC power source 101. Transformer 201 transforms the voltage of the AC power supplied from AC power source 101 to a desired voltage based on the turns ratio, and supplies the transformed AC power to multiple unit converters 30X.

[0030] The power converter 30 includes multiple unit converters 30X. In this embodiment, the multiple unit converters 30X include three first-phase unit converters 30U (30U1, 30U2, 30U3), three second-phase unit converters 30V (30V1, 30V2, 30V3), and three third-phase unit converters 30W (30W1, 30W2, 30W3). Each unit converter 30X is a single-phase converter that converts the single-phase AC power supplied from the secondary winding of the transformer 201 into DC power, and then converts the converted DC power into single-phase AC power of the desired frequency and voltage for output. Each unit converter 30X has the same circuit structure. Details will be described later.

[0031] The first phase of the first group of the secondary side of transformer 201 is connected to the input of unit converter 30U1. The second phase of the first group of the secondary side of transformer 201 is connected to the input of unit converter 30V1. The third phase of the first group of the secondary side of transformer 201 is connected to the input of unit converter 30W1.

[0032] The first phase of the second group on the secondary side of transformer 201 is connected to the input of unit converter 30U2. The second phase of the second group on the secondary side of transformer 201 is connected to the input of unit converter 30V2. The third phase of the second group on the secondary side of transformer 201 is connected to the input of unit converter W2.

[0033] The first phase of the third group on the secondary side of transformer 201 is connected to the input of unit converter 30U3. The second phase of the third group on the secondary side of transformer 201 is connected to the input of unit converter 30V3. The third phase of the third group on the secondary side of transformer 201 is connected to the input of unit converter W3.

[0034] The outputs of the three first-phase unit converters 30U1, 30U2, and 30U3 are connected in series in this order. The three second-phase unit converters 30V1, 30V2, and 30V3 and the three third-phase unit converters 30W1, 30W2, and 30W3 are also connected in series in the same phase order.

[0035] The outputs of unit converter 30U1 on the side not connected to unit converter 30U2, the outputs of unit converter 30V1 on the side not connected to unit converter 30V2, and the outputs of unit converter 30W1 on the side not connected to unit converter 30W2 are interconnected to form the neutral point of the three-phase AC load circuit.

[0036] The output of unit converter 30U3 on the side not connected to unit converter 30U2 is connected to AC output terminal U, and is connected to the U-phase winding of motor 401 via AC output terminal U. The output of unit converter 30V3 on the side not connected to unit converter 30V2 is connected to AC output terminal V, and is connected to the V-phase winding of motor 401 via AC output terminal V. The output of unit converter 30W3 on the side not connected to unit converter 30W2 is connected to AC output terminal W, and is connected to the W-phase winding of motor 401 via AC output terminal W.

[0037] Control unit 601 controls multiple unit converters 30X. For example, control unit 601 includes a first control unit 610, a second control unit 620, and a third control unit 630. First control unit 610 controls the multiple unit converters 30X used as converters (described later). First control unit 610 includes, for example, a converter control unit 611. Second control unit 620 controls the multiple unit converters 30X used as inverters (described later). Second control unit 620 includes, for example, an inverter control unit 621. Third control unit 630 monitors the control state of power conversion system 1 and makes adjustments for stable control. For example, third control unit 630 includes a timer that generates an interrupt signal with a predetermined period.

[0038] For example, the converter control unit 611 is based on the current sensor 301 described later ( Figure 3A The detected value indicates that the secondary winding of transformer 201 is flowing through it. Figure 2A , Figure 2B The current information of the unit converter 30X is sent to each unit converter 30X to control the switching elements contained in each unit converter 30X.

[0039] With this structure, the power conversion system 1 can convert the AC power supplied from the AC power source 101 into three-phase AC power of the desired frequency and voltage and supply it to the motor 401. Details of the converter control unit 611 will be described later.

[0040] Figure 2A and Figure 2B This is a circuit diagram showing the winding structure of the transformer 201 in the embodiment. The primary winding (input side) is star (Y) connected, but this is only one example; a delta (Δ) connection can also be used. The secondary winding (output side) is an open winding that electrically separates the three-phase winding into a single phase.

[0041] exist Figure 2B In the example, if the neutral point of the star connection is set to N, the voltages applied between RN, SN, and TN are doubled according to the turns ratio, appearing between Rs1-Na1, Ss1-Nb1, and Ts1-Nc1 of the first group on the secondary side. The Rs1-Na1, Ss1-Nb1, and Ts1-Nc1 of the first group on the secondary side correspond to the first, second, and third phases of the first group on the secondary side, respectively.

[0042] The same applies to the second and third groups on the secondary side. For the Rs2-Na2, Ss-Nb2, and Ts-Nc2 intervals of the second group on the secondary side, and the Rs3-Na3, Ss3-Nb3, and Ts3-Nc3 intervals of the third group on the secondary side, the voltages also exhibit a doubling of the turns ratio, similar to the first group. The Rs2-Na2, Ss2-Nb2, and Ts2-Nc2 intervals of the second group on the secondary side correspond to the first, second, and third phases of the second group on the secondary side, respectively. Similarly, the Rs3-Na3, Ss3-Nb3, and Ts3-Nc3 intervals of the third group on the secondary side correspond to the first, second, and third phases of the third group on the secondary side, respectively.

[0043] Figures 3A-3C This is a circuit diagram showing the internal structure of the unit converter 30X (X: U, V, W) according to the implementation method. Figure 3AAs shown, the unit converter 30X has its input terminals set to IN1 and IN2, and its output terminals set to OUT1 and OUT2. For example, the unit converter 30X includes a current sensor (current transformer) 301 and a unit converter body 302. The current sensor 301 detects the current flowing between the input terminals IN1 and IN2. The magnitude of this current is represented by ix. x is an identifier for the level, and can be a natural number, for example.

[0044] exist Figure 3B and Figure 3C The diagram shows structural examples of the unit converter 30X as a so-called two-level converter and a so-called three-level converter, and either one can be used.

[0045] Figure 3B The unit converter body 302 shown includes a two-level converter with alternating AC side potentials in two stages. The unit converter 30X has single-phase converters on both the input and output sides, with their DC sections connected back-to-back. An energy storage element such as capacitor C1 is connected to the DC section. The voltage Vdcx is the voltage applied to the terminals of capacitor C1 (called the capacitor voltage).

[0046] Figure 3C The unit converter body 303 shown includes a diode-clamped three-level converter configured with three levels of AC side potential variation. Its DC section has capacitors CP and CN. In the three-level converter, the voltages of the two capacitors CP and CN can be controlled to be balanced. When simply referred to as capacitor voltage, it refers to the total voltage (Vdcx) of capacitors CP and CN.

[0047] Figure 3B and Figure 3C The switching element used is an IGBT (Insulated-Gate Bipolar Transistor), but other switching elements can also be used. Examples of other switching elements include MOSFET (Metal-Oxide-Semiconductor Field-Effect Transistor), GTO (Gate Turn-Off) thyristor, and GCT (Gate Commutated Turn-Off) thyristor. Additionally, if the switching element does not have a built-in body diode, a FWD (Free-Wheeling Diode) can be used in anti-parallel configuration.

[0048] If the flow of electricity (power flow) during operation is considered to be from the input side (R, S, T side) to the output side (U, V, W side), then the single-phase converter on the input side performs forward conversion (conversion from AC to DC), and the single-phase converter on the output side performs reverse conversion (conversion from DC to AC). Therefore, the single-phase converter on the input side is called a converter, and the single-phase converter on the output side is called an inverter.

[0049] For example, Figure 3B The unit converter body 302 shown can also be configured as a circuit unit that separates the converter 3021 and the inverter 3022. In this case, the capacitor C1 can be separate from the converter 3021 and the inverter 3022, or it can be housed in either circuit unit. Alternatively, the unit converter body 302 can also be configured to house the converter 3021, the inverter 3022, and the capacitor C1 in one circuit unit.

[0050] Figure 3C The unit converter body 303 shown can also be configured as a circuit unit that separates the converter 3031 and the inverter 3032. In this case, the capacitors CP and CN can be separate from the converter 3031 and the inverter 3032, or they can be housed in either circuit unit. Alternatively, the unit converter body 303 can also be configured to house the converter 3031, the inverter 3032, and the capacitors CP and CN in one circuit unit.

[0051] In the following explanation, for the sake of simplicity, the combination of the capacitors (C1, CP, CN) and inverters (3032, 3032) described above is regarded as a DC power supply, which makes the unit converter 30X equivalent to the combination of converters (3021, 3031) and DC power supply.

[0052] In this embodiment, the converter control unit 611 generates the gate pulses of the converters (3021, 3031) of the unit converter 30X by the following method.

[0053] • In the PWM control of the converters (3021, 3031), a triangular wave carrier signal (referred to as a triangular wave PWM carrier signal) is used. For example, the triangular wave PWM carrier is defined to include a first period in one cycle where the amplitude increases at a specified rate of change and a second period where the amplitude decreases at a specified rate of change, the lengths of the first period and the second period being equal to each other.

[0054] • Set a phase difference to make the phases of the triangular wave PWM carriers at each stage inconsistent.

[0055] Specifically, the basic triangular wave PWM carrier is shifted so that the phase of each stage of the triangular wave PWM carrier has a predetermined phase difference. In the case of an N-stage structure, the phase difference is set to 180 / N (deg / stage), and the phase of each stage of the triangular wave PWM carrier is determined sequentially. For example, the phase of the first-stage triangular wave PWM carrier is set to 0, coinciding with the phase of the basic triangular wave PWM carrier. The phase of the second-stage triangular wave PWM carrier is set to the phase of the basic triangular wave PWM carrier plus 180 / N (deg / stage). The phase of the third-stage triangular wave PWM carrier is set to the phase of the basic triangular wave PWM carrier plus 180×2 / N (deg / stage). The phase of each stage is determined similarly thereafter.

[0056] • To give Figure 3A The voltage references of input IN1 and input IN2 of the unit converter 30X shown are reversed (referred to as single-phase single-pole modulation).

[0057] Furthermore, the inverter control unit 621 can also implement control with the same phase difference set in the PWM control of the inverters (3022, 3032) using the same triangular wave PWM carrier as described above. Alternatively, the inverter control unit 621 can also apply other control methods.

[0058] To simplify the explanation, the following example illustrates a two-level unit converter with a two-stage structure. Figure 4 This is a simplified structural diagram of the power conversion system 1 according to the implementation method. For example, Figure 4 The DC side of the equivalent circuit of the power conversion system 1 shown is set as an ideal power source, and the AC side is set as a single-phase circuit. Unit converters 30S1 and 30S2 are equivalent to unit converter 30X.

[0059] Transformer 201 is exemplified as an open star-wound transformer, but it is replaced with an equivalent circuit structure for explanation. The voltage and current equations in the above circuit structure are shown in equations (1) to (3).

[0060] [Formula 1]

[0061]

[0062] [Equation 2]

[0063]

[0064] [Formula 3]

[0065]

[0066] In equations (1) to (3) above, voltage v1 and voltage v2 represent the input voltages of unit converters 30S1 and 30S2, and voltage vs represents the system voltage of the AC power supply. Current i1 and current i2 represent the input currents of unit converters 30S1 and 30S2, and current is represents the system current of the AC power supply. L is the equivalent reactance corresponding to transformer 201, etc.

[0067] Add both sides of the above equations (2) and (3) respectively, apply equation (1) to rearrange them, and convert the result into an integral system to obtain equation (4).

[0068] [Formula 4]

[0069]

[0070] Equation (4) above indicates that the combined voltage (v1+v2) based on the input voltages of unit converters 30S1 and 30S2 can control the system current is.

[0071] Reference Figure 5 Explain the timing of current sampling and voltage reference updates.

[0072] Figure 5 This is a diagram illustrating the timing of current sampling and voltage reference updates in the implementation method.

[0073] exist Figure 5 The upper layer of the timing diagram shows the changes in two triangular wave PWM carriers (e.g., referred to as the first unit carrier Car1 (first carrier signal) and the second unit carrier Car2 (second carrier signal)), the voltage reference vA* of branch A, and the voltage reference vB* of branch B. For example, the difference between voltage reference vA* and voltage reference vB* corresponds to the input voltage v1. The lower layer of the timing diagram shows the change in the synthesized voltage (v1 + v2). Time intervals t1 to t17 are defined as specified periods (Tsmp), representing the timing for current sampling and voltage reference updates, respectively.

[0074] As described above, the phase shift of the carrier (called the unit carrier) of each stage of the unit converter 30X is approximately one-quarter of a cycle. Figure 5 In the example shown, the phase of the second unit carrier Car2 is delayed by about a quarter of the phase of the first unit carrier Car1, which is one cycle of the triangular wave PWM carrier.

[0075] By applying voltage references with opposite polarities to branch A and branch B as described above, the polarity of the voltage references applied to the inputs IN1 and IN2 of the converter is reversed. Here, for the sake of simplicity, the voltage references of unit converters 30S1 and 30S2 are an example of voltage references obtained by reversing the polarity with the same amplitude.

[0076] For example, the timing of current sampling and voltage reference updating of the first-stage unit converter 30S1 can be determined based on the timing of each vertex of the first unit carrier Car1 and the second unit carrier Car2, as well as the intermediate points (hereinafter referred to as intermediate points) between two adjacent vertices in the time axis direction. The timing of current sampling and voltage reference updating of the second-stage unit converter 30S2 can also be implemented using the above-described timing. Furthermore, at the vertices of each unit carrier, there are points that are positive peaks (extreme values) and points that are negative peaks (extreme values), either one or both. For example, the timing can be determined by associating adjacent positive peaks (extreme values) with each other, or adjacent negative peaks (extreme values) with each other.

[0077] For example, the timing of the vertex of the first unit carrier Car1 is Figure 5 The timing of t1, t5, t9, t13, and t17 in the second unit carrier Car2 is as follows. Figure 5 The timing of each intermediate point is t3, t7, t11, and t15. Figure 5 The timestamps t2, t4, t6, t8, t10, t12, t14, and t16 are used. In this embodiment, the timings t1 to t17 are the timings for current sampling and voltage reference updates for each unit converter 30X.

[0078] As described above, since the phase difference between the unit carriers of each stage is one-quarter of a cycle of the triangular wave PWM carrier, the timing of current sampling and voltage reference updating of the first-stage unit converter 30S1 is consistent with the timing of current sampling and voltage reference updating of the second-stage unit converter 30S2. Accordingly, a structure is formed in which current sampling and voltage reference updating are performed at each vertex and each midpoint of each unit carrier of unit converters 30S1 and 30S2.

[0079] For example, it is possible to generate a synthesized voltage that presents a multi-valued voltage based on the magnitudes of two triangular wave PWM carriers (first unit carrier Car1 and second unit carrier Car2), the voltage reference vA* of branch A, and the voltage reference vB* of branch B.

[0080] When the voltage reference vA* is less than both the first unit carrier Car1 and the second unit carrier Car2, and the voltage reference vB* is greater than both the first unit carrier Car1 and the second unit carrier Car2, the converter control unit 611 controls each switching element of each converter to make the combined voltage (v1 + v2) become (-2E).

[0081] When the voltage reference vA* is less than the first unit carrier Car1 and greater than the second unit carrier Car2, and the voltage reference vB* is greater than both the first unit carrier Car1 and the second unit carrier Car2, the converter control unit 611 controls each switching element of each converter to make the combined voltage (v1 + v2) become (-E).

[0082] When both voltage reference vA* and voltage reference vB* are greater than the second unit carrier Car and less than the first unit carrier Car1, or when both voltage reference vA* and voltage reference vB* are less than both the first unit carrier Car1 and the second unit carrier Car1, the converter control unit 611 controls each switching element of each converter to make the combined voltage (v1 + v2) become (-0).

[0083] When the voltage reference vA* is greater than both the first unit carrier Car1 and the second unit carrier Car2, and the voltage reference vB* is less than the first unit carrier Car1 and greater than the second unit carrier Car2, or when the voltage reference vB* is less than both the first unit carrier Car1 and the second unit carrier Car2, and the voltage reference vA* is less than the first unit carrier Car1 and greater than the second unit carrier Car2, the converter control unit 611 controls each switching element of each converter to make the combined voltage (v1 + v2) become (+E).

[0084] When the voltage reference vA* is greater than both the first unit carrier Car1 and the second unit carrier Car2, and the voltage reference vB* is less than both the first unit carrier Car1 and the second unit carrier Car2, the converter control unit 611 controls each switching element of each converter to make the combined voltage (v1 + v2) become (+2E).

[0085] Here, we focus on the relationship between the voltage reference vA* and the composite voltage (v1 + v2) for each interval (called the Tsmp interval) divided according to the period Tsmp. The magnitude of the composite voltage (v1 + v2) in each Tsmp interval varies discretely with the passage of time, taking multiple values.

[0086] The converter control unit 611 controls each converter in PWM mode by averaging the instantaneous values ​​of the synthesized voltage (v1+v2) in each Tsmp interval with the voltage reference vA* in each Tsmp interval. That is, by making equation (4) true in each Tsmp interval as described above, equation (5) below also becomes true.

[0087] [Formula 5]

[0088]

[0089] Because equation (5) holds true in each Tsmp interval, therefore Figure 4 The two parallel-connected converters 3021 achieve current controllability comparable to a three-phase two-level inverter by repeatedly sampling the current and updating the voltage reference with a period of Tsmp.

[0090] According to the control method described above, the converter control unit 611 is as follows: Figure 5 As shown, the voltage reference is updated multiple times within half a cycle of the triangular wave PWM carrier. Therefore, the voltage reference vA* and the triangular wave PWM carrier (first unit carrier Car1) cross multiple times within half a cycle of the triangular wave PWM carrier. In other words, within one cycle of the triangular wave PWM carrier, the timing involving current sampling and voltage reference updates occurs at least four times in an even number of times.

[0091] In addition, Figure 5 In the example shown, since the comparison result between the voltage reference and the triangular wave PWM carrier is illustrated as is, a narrow pulse width is included. In practical applications, for narrow pulse widths (e.g., narrow pulses that do not meet the specified width), the generation of narrow pulse widths can be limited by methods such as not switching the switching elements of the converter 3021.

[0092] Reference Figure 6 illustrate Figure 4 The results of the simulation of the circuit structure.

[0093] Figure 6 It is used for Figure 4 The diagram illustrates the simulation results of the circuit structure. Figure 6 Image (a) shows the sampled values ​​(I_U1_S: solid line) of the detected current (CNV current) of the first-stage converter (I_U1: dashed line). Figure 6 (b) shows the sampled values ​​(I_U2_S: solid line) of the detected current (CNV current) of the second-stage converter (I_U2: dashed line). Figure 6 The variation of the system current is shown in (c). The amplitude of the system current is as described above. Figure 6 The sampling results (I_U1_S) of the current of the first-stage converter shown in (a) and Figure 6 The sum of the current sampling results (I_U2_S) of the second-stage converter shown in (b) is given. Figure 6 The switching ripple of the system current is shown in (c) is smaller than the sampling results of the current of each converter.

[0094] When current sampling and voltage reference updates are performed at each apex and midpoint of each unit carrier, the above equation (5) holds true in an N-stage circuit structure. In this case, the carrier frequency fcar and period Tsmp of each stage are related by the following equation (6). As shown in equation (6), as the stage number N increases, the period Tsmp becomes shorter, and correspondingly, an improvement in control characteristics is expected.

[0095] [Formula 6]

[0096]

[0097] Furthermore, the circuit structure at level N is equivalent to the circuit structure with N converters connected in parallel on the secondary side of transformer 201.

[0098] According to the embodiment, the multiple unit converters 30X of the power conversion system 1 are arranged in parallel phase to phase of the AC power supply 101 (power system). The converter control unit 611 (control unit) controls the multiple unit converters 30X using multiple carrier signals having a predetermined phase difference with each other through carrier comparison type PWM control. The converter control unit 611 determines a timing interval of more than four even times with a period Tsmp (a predetermined time interval) within one cycle of a specific carrier signal among the multiple carrier signals, and detects the current on the primary side of the multiple unit converters 30X at each of the four or more even-numbered timing intervals. The converter control unit 611 uses the detected value ix of the primary side current and the value of the primary side current reference to generate a voltage reference for PWM control, and updates the voltage reference at each of the four or more even-numbered timing intervals. The converter control unit 611 uses the specific carrier signal and the updated voltage reference to control a specific unit converter among the multiple unit converters 30X. As a result, the power conversion system 1 can further improve the control responsiveness of current control on the AC power supply 101 side.

[0099] <Modifications of the First Embodiment>

[0100] A variation of the first embodiment will be described.

[0101] In the first embodiment, an example was described in which current sampling and voltage reference updates were repeatedly performed at each vertex and each intermediate point of each unit carrier. Instead, in this modified example, compared with the example of the first embodiment, the intermediate points of each unit carrier were removed from the timing related to control, and an example was described in which current sampling and voltage reference updates were repeatedly performed at each vertex of each unit carrier.

[0102] Figure 7 This is a diagram illustrating the timing of current sampling and voltage reference updates in a modified example of the first embodiment.

[0103] Furthermore, it can be seen that, in the aforementioned... Figure 4 In the circuit, there exists a path (called the circulating current path) that allows the current to circulate within the power converter 30 without affecting the system current is. If expressed as an arithmetic expression, taking the difference between equations (2) and (3) and converting it to an integral system yields the following equation (7).

[0104] [Formula 7]

[0105]

[0106] Equation (7) above indicates that the circulating current (i1-i2) can be controlled by the differential voltage (v1-v2) of each converter in the power converter 30.

[0107] Based on the above, even if current sampling and voltage reference updates are not performed at both the apex and midpoint of each unit carrier, control can still be performed in the same manner as in the first embodiment by performing current sampling and voltage reference updates at the apex of each unit carrier. Furthermore, in this modified example, the period Tsmp can be made longer compared to the example of the first embodiment. While extending the period Tsmp may sometimes reduce the responsiveness of current control, it can reduce the concentration (density) of computational processing within each period Tsmp.

[0108] Furthermore, the timing of each vertex of each unit carrier can be transformed into the timing of each intermediate point of each unit carrier. Thus, according to this variation, by performing current sampling at the timing of each vertex or intermediate point of each unit carrier, the switching ripple of the system current is can be made smaller than the sampling results of the current of each stage of the converter.

[0109] Comparing the first embodiment with its variations, the responsiveness can be improved for the system current is by applying the method of the first embodiment compared to the method of the variations of the first embodiment. Even by adjusting the length of the period Tsmp to change the timing density of the current sampling, the converter-based system current is can be stably controlled.

[0110] In contrast, the same relationship does not hold for the control of the circulating current as for the system current is. Therefore, it is necessary to reduce the control gain related to the control of the circulating current. As shown in equations (5) and (7) above, the control gain can be determined independently for both the system current is and the circulating current. For example, it is possible to adjust the control gain for the control of the circulating current to be reduced while maintaining the control gain for the control of the system current is constant.

[0111] <Second Implementation Method>

[0112] The second embodiment will be described.

[0113] In the second embodiment, an example of extending the method shown in the first embodiment to a converter with N parallel connections will be described. The system current is becomes an addition of N currents corresponding to the number of parallel connections of each converter. Since each converter is controlled individually, the values ​​of the currents of each converter are inconsistent. In order to mitigate the influence of the deviation of the currents of each converter, the average of the N currents can be calculated in a way that makes the weights of the current values ​​of each converter consistent, and the system current is is controlled based on this. In addition, the remaining (N-1) components correspond to the components of the circulating current. Various methods can be considered for the distribution of the components of the circulating current, and one example is shown in equations (8) to (13) below.

[0114] In equations (8) to (13) shown below, for example, using a n This represents the current component corresponding to the AC side of each converter. For example, n is the identification number of each converter. The system current is, because it is electrically equal to the zero phase, is represented by the first letter z. The circulating current is represented by the first letter c1…c of circulate. n－1 This indicates that the component a of the current corresponding to each converter will be represented. n Converted to system current z and circulating current c1… cn－1 This process is called the zc transformation. The inverse zc transformation is called the inverse zc transformation. For example, from a n The components of each current are the currents before the zc conversion. From z and c1…c n－1 The components of each current are the currents after zc conversion.

[0115] For example, equations (8) and (9) are examples of conversion formulas when the number of parallel converters is set to two (N=2). Equation (8) is the formula for zc conversion, and equation (9) is the formula for inverse zc conversion.

[0116] [Formula 8]

[0117]

[0118] [Formula 9]

[0119]

[0120] Equations (10) and (11) are examples of conversion formulas when the number of parallel converters is set to three (N=3). Equation (10) is the formula for zc conversion, and Equation (11) is the formula for inverse zc conversion.

[0121] [Formula 10]

[0122]

[0123] [Equation 11]

[0124]

[0125] Equations (12) and (13) are examples of conversion formulas when the number of parallel converters is set to five (N=5). Equation (12) is the formula for zc conversion, and Equation (13) is the formula for inverse zc conversion.

[0126] [Equation 12]

[0127]

[0128] [Equation 13]

[0129]

[0130] By using the zc conversion described above, the components of the current after zc conversion can be divided into system current z and circulating current c1…c. n－1 The components are determined by adjusting the system current z and the circulating current c1…c after zc conversion. n－1 The components of the current control are adjusted, and the current of N parallel converters is controlled based on the adjustment results.

[0131] Reference Figure 8 and Figure 9 More specifically, an example of a current control system with five converters connected in parallel (N=5) will be described. Figure 8 This is a structural diagram of a power conversion system model in the second embodiment, which connects five converters in parallel.

[0132] The power converter 30 includes unit converters 30U1 to 30U5, unit converters 30V1 to 30V5, and unit converters 30W1 to 30W5.

[0133] Figure 9 This is a structural diagram of the converter control unit 611 according to the second embodiment. Figure 9 The diagram shows the converter control unit 611 and the associated power converter 30.

[0134] The power converter 30 outputs the current values ​​detected by each stage converter. i_1^uvw represents the current value of the first-stage converter. The current value of the first-stage converter represents the magnitude of the current detected by the current sensor 301 within the first-stage converter. The same applies below. Furthermore, the string following "_" (underscore) indicates a subscript character, and the string following "^" (hat) indicates a superscript character. i_2^uvw represents the current value of the second-stage converter. Similarly, i_5^uvw represents the current value of the fifth-stage converter. For example, the current values ​​(i1u, i1v, i1w) detected by the aforementioned unit converters 30U1, 30V1, and 30W1 respectively... Figure 8 The sum is denoted as i_1^uvw. The same applies to i_2^uvw through i_5^uvw. Furthermore, Figure 8 In this context, v1u~v5u represents the input voltage of the unit converters 30U1~30U5 in phase U.

[0135] The converter control unit 611 includes, for example, a dqO conversion unit 6111, a ZC conversion unit 6112, a current control unit 6113, an inverse ZC conversion unit 6114, an inverse dqO conversion unit 6115, a PWM control unit 6116, a gate pulse generation unit 6117, and a current reference generation unit 6118.

[0136] The dqO conversion unit 6111 performs general dqO conversion based on the current values ​​of each stage of the converter using the phase reference θO. dqO conversion is a coordinate transformation that converts components of a stationary coordinate system into components of a rotating coordinate system, i.e., the dqO coordinate system, which is rotated according to the phase reference θO. For example, i_1^dqz represents the current vector after dqO conversion of the first-stage converter. i_2^dqz represents the current vector after dqO conversion of the second-stage converter. Similarly, i_5^dqz represents the current vector after dqO conversion of the fifth-stage converter. i_1^dqz to i_5^dqz represent the vectors of the current components in the dqO coordinate system.

[0137] The ZC conversion unit 6112 performs ZC conversion on the current components in the dqO coordinate system after the dqO conversion. For example, i_z^dqz is the current vector of the system current z after the ZC conversion. i_c1^dqz to i_c4^dqz are the current vectors of the circulating current after the ZC conversion.

[0138] The current reference generation unit 6118 generates a current reference based on the magnitude of the DC voltage at each stage. This current reference can also be generated using general methods.

[0139] The current control unit 6113 (referred to as ACR in the figure) performs current control based on the current reference generated by the current reference generation unit 6118 and determined in accordance with the control cycle, and generates a voltage reference. The current vector after zc conversion includes the aforementioned i_z^dqz and i_c1^dqz to i_c4^dqz in its elements. For example, v_z^dqz* represents the voltage reference corresponding to the system current z after zc conversion. v_c1^dq*z to v_c4^dqz* represent the voltage reference corresponding to the circulating current after inverse zc conversion.

[0140] As described above, the current control unit 6113 can implement the following current control: the current control includes a calculation using a current vector and an Nth power matrix, wherein the current vector contains the magnitude of a system current is and the magnitudes of (N-1) circulating currents in the element. The current control unit 6113 can also implement the following current control: the current control includes a calculation using a current vector and an Nth power matrix, wherein the current vector contains a system current is and (N-1) circulating currents in the element. Through this calculation, the current control unit 6113 can generate a voltage vector containing the composite voltage (v_z^dqz*) on the AC power supply 101 side and (N-1) voltage references corresponding to the (N-1) circulating currents, and implement current control including this calculation.

[0141] The inverse ZC conversion unit 6114 performs inverse ZC conversion on the voltage reference generated by the current control unit 6113, and generates voltage references in the dqO coordinate system corresponding to each stage. v_1^dqz*~v_5^dqz* represent the voltage references of each stage after the inverse ZC conversion.

[0142] The inverse dqO conversion unit 6115 performs inverse dqO conversion on the voltage references of each stage of the dqO coordinate system generated by the inverse ZC conversion unit 6114 to generate the voltage references of each stage of the uvw coordinate system. v_1^uvw*~v_5^uvw* represent the voltage references of each stage of each phase after the inverse dqO conversion.

[0143] The PWM control unit 6116 includes a carrier generation unit 6116c. Based on the triangular carrier generated by the carrier generation unit 6116c and the voltage references of each stage generated by the inverse dqO conversion unit 6115, gate signals for controlling each switch of the power converter 30 are generated, and gate pulses based on the gate signals are supplied to each switch of the power converter 30 via the gate pulse generation unit 6117. This controls the on / off state of each switch of the power converter 30.

[0144] The above series of transformations are explained with reference to equations (14) to (17) below. The expansion of equations (14) to (17) corresponds to the voltage reference in the form of dqO obtained from the current of each parallel converter after the dqO transformation. Since there is no mathematical difference between the components of the d-axis, q-axis, and O-axis in the dqO axis coordinate system, the d-axis component is exemplified and used as a representative for explanation. In addition, for the sake of simplicity, the case where the number of parallel connections N is set to 2 will be explained.

[0145] Equation (14) is used to define the current reference vector I^d*, current FBK vector I^d, and voltage reference vector V^d* of each parallel converter related to the d-axis component.

[0146] [Formula 14]

[0147]

[0148] Let the aforementioned zc transformation matrix be M_zc, and the inverse zc transformation matrix be M_zc^(-1). In addition, if we assume that the current control is only proportional control, then the current control gain matrix M_ACR is defined by the following equation (15).

[0149] [Formula 15]

[0150]

[0151] In equation (15) above, K_z is the current control gain for the system current is, and K_c1 is the current control gain for the circulating current.

[0152] The above definition is used in equation (16) below. Figure 9 Calculation of the voltage reference vector V^d*.

[0153] [Formula 16]

[0154]

[0155] If we replace the above equation (16) with the following equation (17), it can be seen that it is equivalent to calculating the voltage reference vector V^d* by controlling the current differential (I^(d*) - I^d) in the form of a parallel converter.

[0156] [Equation 17]

[0157]

[0158] According to this embodiment, by utilizing the zc transformation, the system current z and the circulating current c1…c are adjusted using the matrix M_ACR after the zc transformation. n－1 The components of the system current z and the circulating current c1…c n－1Adjusting the components becomes easier.

[0159] By first calculating the transformation matrix M' using equation (17), the transformation matrix M' can be used for calculations, which can reduce the computational load of each control cycle.

[0160] <Third Implementation Method>

[0161] The third embodiment will be described.

[0162] In the third embodiment, an example of applying the above method to a converter with three converters connected in parallel will be described. The structure of this case is equivalent to... Figure 1 The structure.

[0163] Reference Figure 10 The timing of current sampling and voltage reference updates in the third embodiment will be explained. Figure 10 This is a diagram illustrating the timing of current sampling and voltage reference updates in the third embodiment.

[0164] With the aforementioned Figure 5 The difference lies in the number of carriers: by changing the number of parallel carriers from two to three, the number of carriers is also changed from two to three. This adds a third carrier, Car3, which is delayed by a specified phase from the second carrier, Car2. The phase differences between the carriers are equal, as described above. In this case, the timing of current sampling and voltage reference updates becomes eight times per carrier cycle. Since there are three parallel carriers, the combined voltage (v1+v2+v3) becomes seven levels: -3E, -2E, -E, 0, +E, +2E, and +3E.

[0165] According to this embodiment, even when the number of converters connected in parallel is three, the desired synthesized voltage described above can still be generated. By increasing the number of converters connected in parallel in this way, the ratio of the step size of the change in the quantized synthesized voltage value to the full amplitude becomes smaller, thus helping to reduce ripple noise.

[0166] <Fourth Implementation Method>

[0167] The fourth embodiment will be described.

[0168] In the fourth embodiment, an example of applying the above method to a system other than the drive system of the drive motor 401 will be described.

[0169] First application example:

[0170] The aforementioned Figure 4The illustrated structure can be applied to a static synchronous compensator (STATCOM). It is presented as a schematic structure of the main circuit of the power conversion system 1A, not an equivalent circuit. In the case of a STATCOM, multiple unit converters 30X function as self-excited inverters. In other words, it can also be configured as an inverter circuit without the unit converters 30X.

[0171] The structure is defined as described above. Furthermore, as a STATCOM, the switching of various operating modes, such as capacitor operation, no-load operation, and reactor operation, can be achieved through the compensation of invalid power by the power conversion system 1A. For example, the control unit of the inverter (equivalent to the aforementioned converter control unit 611) can compensate for the invalid power of the AC power supply (power system) 101 by adjusting the circulating current flowing in the multiple unit converters 30X to switch the above operating modes.

[0172] Second application example:

[0173] Furthermore, the battery can be connected to its DC side, configuring the power conversion system 1A as a battery power storage device. In this case, the battery can be charged using DC power converted from AC power, or the DC power stored in the battery can be discharged from the battery as converted AC power.

[0174] According to the above implementation method, the same control method can be applied to systems other than the drive system.

[0175] According to at least one embodiment described above, a power conversion system is interconnected with a multiphase AC power system. The power conversion system includes multiple converter bodies and a control unit. The multiple converter bodies are arranged in parallel phase-to-phase configuration for each phase of the power system. The control unit controls the multiple converter bodies using multiple carrier signals having a predetermined phase difference with each other via carrier comparison-type PWM control. The control unit determines at least four even-numbered timing intervals within a cycle of a specific carrier signal among the multiple carrier signals, and detects the current on the primary side of each of the multiple converter bodies at these even-numbered timing intervals. The control unit generates a voltage reference for the PWM control using the detected value of the primary side current and the value of the primary side current reference, and updates the voltage reference at these even-numbered timing intervals. The control unit controls a specific converter body among the multiple converter bodies using the specific carrier signal and the updated voltage reference. Thus, the power conversion system can further improve the control responsiveness of current control on the power system side.

[0176] Furthermore, the aforementioned control unit 601 (computer) includes, for example, a storage unit, a CPU (central processing unit), a drive unit, and an acquisition unit. The storage unit, CPU, drive unit, and acquisition unit are connected within the control unit, for example, via a BUS. The storage unit contains semiconductor memory. The CPU contains a processor that executes desired processing according to a software program. The drive unit generates control signals for each part of the power conversion system 1 according to the control of the CPU. The acquisition unit acquires the detection results of each current sensor and voltage sensor. For example, the CPU of the control unit 601 controls the main circuit of each phase via the drive unit based on the detection results of the current and voltage sensors acquired by the acquisition unit. The control unit 601 can implement part or all of its processing through software program processing as described above, or it can implement part or all of its processing through hardware. Additionally, the control unit 601 can be appropriately partitioned to ensure circuit insulation.

[0177] Several embodiments of the present invention have been described, but these embodiments are provided by way of example and are not intended to limit the scope of the invention. These embodiments can be implemented in various other ways, and various omissions, substitutions, and modifications can be made without departing from the spirit of the invention. These embodiments and their variations are included within the scope and spirit of the invention, and are also included within the scope of the invention as described in the claims and its equivalents.

[0178] Explanation of reference numerals in the attached figures

[0179] 1 Power conversion system; 201 Transformer; 30 Power converter; 30S, 30U, 30V, 30W, 30X unit converters; 302, 3021, 3031 converters; 303, 3022, 3032 Inverters; C1, CP, CN capacitors; 401 Motor; 101 Power system; 601 Control unit; 611 Converter control unit; 621 Inverter control unit; 201 Transformer; 6111 dqO conversion unit; 6112 ZC conversion unit; 6113 Current control unit; 6114 Inverse ZC conversion unit; 6115 Inverse dqO conversion unit; 6116 PWM control unit; 6117 Gate pulse generation unit; 6118 Current reference generation unit

Claims

1. A power conversion system interconnected with a multiphase AC power system, characterized in that, have: A plurality of converters, each comprising a plurality of switches, wherein the plurality of converters are arranged in parallel phase-to-phase configuration for each phase of the power system; and The control unit uses multiple carrier signals with a predetermined phase difference to control the multiple converters via carrier comparison-type PWM control; The control unit determines an even number of timing intervals at more than four times within a cycle of a specific carrier signal among the plurality of carrier signals, and detects the current on the primary side of the plurality of converters during each of the four or more even-numbered timing intervals. The control unit uses the detected value of the primary side current and the value of the primary side current reference to generate a voltage reference for the PWM control, and updates the voltage reference at even-numbered timing intervals at least four times. The control unit uses the specific carrier signal and the updated voltage reference to control a specific converter among the plurality of converters.

2. The power conversion system according to claim 1, characterized in that, Among the four or more even-numbered timings, there is a first timing related to the timing when the amplitude of any one of the plurality of carrier signals reaches its peak value.

3. The power conversion system according to claim 1, characterized in that, The even-numbered timings of the four or more times include a second timing related to the timing between the first timings in which the amplitudes of the plurality of carrier signals respectively reach their peak values.

4. The power conversion system according to claim 3, characterized in that, The second timing is determined at the midpoint between two timings in which the amplitudes of the plurality of carrier signals that are adjacent to each other along the time axis reach their peak values.

5. The power conversion system according to claim 3, characterized in that, The second timing is determined between a first peak timing when the first carrier signal preceding the first carrier signal becomes the peak value and a second peak timing when the second carrier signal following the first carrier signal becomes the peak value.

6. The power conversion system according to claim 1, characterized in that, The number of even-numbered timings (four or more) is determined based on the number of the plurality of converters configured in parallel.

7. The power conversion system according to claim 1, characterized in that, The number of the multiple converters arranged in parallel is N, where N is an integer greater than or equal to 2. The control unit determines this through current control, which includes calculations using a current vector and an Nth power matrix. The current vector contains elements of the magnitude of the system current flowing through the power system and the magnitude of the (N-1) circulating currents flowing through the N converters.

8. The power conversion system according to claim 7, characterized in that, The control unit implements current control, which includes the operation of using a current vector that includes a system current and (N-1) circulating currents in its elements.

9. The power conversion system according to claim 7, characterized in that, The control unit implements current control, which includes the operation of generating a voltage vector that includes, in its elements, a composite voltage on the power system side and a voltage reference corresponding to the (N-1) circulating currents.

10. The power conversion system according to claim 1, characterized in that, The control unit includes: The dqO conversion unit performs dqO conversion of information into current components in a rotating coordinate system based on the detected current values ​​of the primary side of each stage of the converters arranged in parallel phase for each phase of the power system, using a phase reference θO. The ZC conversion unit performs ZC conversion based on the current components after the dqO conversion, dividing them into components of the system current flowing through the power system and components of the circulating current. The current control unit generates a voltage reference by current control based on a determined current reference, the component of the system current after the ZC conversion, and the component of the circulating current. The inverse ZC conversion unit performs an inverse ZC conversion on the voltage reference related to the inverse ZC conversion, and generates the voltage reference of the rotating coordinate system corresponding to each stage; The inverse dqO conversion unit uses the phase reference θO to perform an inverse dqO conversion related to the inverse conversion of the dqO conversion on the voltage references of the rotating coordinate system at each stage, generating voltage references for each stage in the stationary coordinate system; and The PWM control unit generates gate signals for controlling the switching elements of the specific converter based on the triangular wave carrier signal and the voltage references of each stage.

11. The power conversion system according to claim 1, characterized in that, The power conversion system includes a transformer with a three-phase star connection for the primary winding and a three-phase open delta connection for the secondary winding, which are mutually insulated. The plurality of converters are connected to the secondary winding. Three-phase AC power is supplied from the power system to the primary winding.

12. A power conversion system interconnected with a multiphase AC power system, characterized in that, have: A first converter and a second converter are configured in parallel with each phase of the power system; and The control unit uses multiple carrier signals, including a first carrier signal and a second carrier signal having a predetermined phase difference between them, to control the first converter and the second converter through carrier comparison type PWM control; The control unit determines an even number of timing intervals at more than four times within one cycle of the first carrier signal, and detects the current on the primary side of the first converter and the second converter during these four or more even-numbered timing intervals. The control unit uses the detected value of the primary side current and the value of the primary side current reference to generate a voltage reference for the PWM control, and updates the voltage reference at even-numbered timing intervals at least four times. The control unit uses the first carrier signal and the updated voltage reference to control the first converter, and uses the second carrier signal and the updated voltage reference to control the second converter.

13. A power conversion control method, which is a power conversion control method for a power conversion system interconnected with a multiphase AC power system, characterized in that, In a power system where multiple converters are connected in parallel phase to phase for each phase, a carrier comparison type PWM control method is used to control multiple switches contained in the multiple converters using multiple carrier signals with a specified phase difference. This method includes the following process: The computer determines at least four even-numbered timing intervals within a cycle of a specific carrier signal from the plurality of carrier signals, and detects the primary-side current of the plurality of converters during each of these even-numbered timing intervals. A voltage reference for PWM control is generated by a computer using the detected value of the primary-side current and the value of the primary-side current reference, and the voltage reference is updated at four or more even-numbered timing intervals. A specific converter among the plurality of converters is controlled by a computer using the specific carrier signal and the updated voltage reference.

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