Power Conversion Equipment

Abstract

To provide a power conversion device capable of reducing harmonic current flowing through a converter cell.SOLUTION: A power conversion device comprises a delta connection unit in which one or a plurality of converter cells connected in series is delta-connected, and a control device controlling cyclic current flowing through the delta connection unit by the plurality of converter cells. The converter cells have a plurality of semiconductor switch groups including a plurality of semiconductor switches connected in series, and a capacitor connected to the plurality of semiconductor switch groups in parallel. The control device limits a current command of the plurality of converter cells by using orthogonal two shaft current commands of a positive phase and a reverse phase, and a cyclic current command which is a command of the cyclic current.SELECTED DRAWING: Figure 3

Description

[Technical field]

[0001] The present disclosure relates to a power conversion device. [Background technology]

[0002] Modular multilevel converters (MMCs) are next-generation transformerless power converters suitable for large-capacity, high-voltage applications. MMCs can be applied to, for example, static static compensation (STATCOMs) and high-voltage direct current (HVDC) systems. In particular, delta-connected MMCs have attracted attention because they can output a negative-phase reactive current by passing a circulating current through the delta connection. Delta-connected MMCs have a delta connection inside, and one or multiple converter cells connected in series are provided on each phase of the delta connection (see, for example, Patent Document 1).

[0003] Meanwhile, in a reactive power compensator that suppresses voltage fluctuations in a power system, a current limiting circuit is known that limits the instantaneous value of the output current to a predetermined value (see, for example, Patent Document 2). When a reactive power compensator outputs a negative-phase-sequence current and performs three-phase unbalanced operation, the peak values ​​of the currents of the three phases become uneven. Furthermore, when a three-phase balanced operation is performed, the offset third harmonic is output, and the peak value of the actual current value does not match the peak value of the current command value. Therefore, from the viewpoint of device protection, it is necessary to monitor the actual output current value of the reactive power compensator, and quickly narrow down the current command value when any one of the phase currents exceeds the limit set value.

[0004] The current limiting circuit disclosed in Patent Document 2 is a circuit that generates a limiting signal to be multiplied with the current command value of the reactive power compensator from a three-phase full-wave rectified signal of the output current of the reactive power compensator to perform this current limiting. When the three-phase full-wave rectified signal of the output current exceeds the current limit setting value, the current limiting circuit reduces the limiting signal to be multiplied with the current command value of the reactive power compensator, thereby limiting the current command value. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] International Publication No. 2012 / 099176 [Patent Document 2] Patent No. 3334005 Summary of the Invention [Problem to be solved by the invention]

[0006] However, in an MMC, since a circulating current flows through the delta connection, even if the AC output current to the power grid is limited as in Patent Document 2, harmonics may occur in the converter cell current (current flowing through the converter cell), which includes the circulating current.

[0007] The present disclosure provides a power conversion device capable of reducing harmonic currents flowing through a converter cell. [Means for solving the problem]

[0008] In one aspect of the present disclosure, a delta connection portion in which one or a plurality of converter cells connected in series are delta-connected; The circulating current flowing in the delta connection is Included in delta connection A control device that controls the multiple converter cells, The converter cell includes a plurality of semiconductor switch groups each including a plurality of semiconductor switches connected in series, and a capacitor connected in parallel to the plurality of semiconductor switch groups, The control device uses each of the orthogonal two-axis current commands of the positive phase and the negative phase and a circulating current command which is a command for the circulating current, Included in delta connection A power converter is provided that limits current commands for a plurality of converter cells. Effect of the Invention

[0009] According to one aspect of the present disclosure, it is possible to reduce harmonic currents flowing through a converter cell. [Brief description of the drawings]

[0010] [Figure 1] 1 is a diagram illustrating a configuration example of a power conversion device according to an embodiment; [Diagram 2] FIG. 2 is a diagram illustrating an example of the configuration of a control device. [Diagram 3] FIG. 4 is a diagram illustrating an example of the configuration of a current command unit. [Figure 4] 5A and 5B are diagrams illustrating examples of operational waveforms of a current command unit. [Diagram 5] FIG. 1 illustrates a conventional current limiting circuit. [Figure 6] FIG. 1 is a diagram showing an example of operational waveforms of a conventional current limiting circuit applied to a delta-connected MMC. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0011] Hereinafter, embodiments of the present disclosure will be described with reference to the drawings.

[0012] Fig. 1 is a diagram showing an example of the configuration of a power conversion device in one embodiment, and shows an example of the circuit configuration of a delta-connected MMC. The MMC is applicable to, for example, a static static compensation (STATCOM) system and a high voltage direct current (HVDC) system. The power conversion device 400 shown in Fig. 1 includes a delta connection unit 402 and a control device 401.

[0013] The delta connection unit 402 is a circuit in which one or a plurality of converter cells connected in series are delta-connected. The delta connection unit 402 includes a plurality of clusters 50 (50UV, 50VW, 50WU) and a plurality of reactors 51 (51UV, 51VW, 51WU).

[0014] The UV-phase cluster 50UV includes a plurality of converter cells 52UV connected in series via a pair of AC output terminals a and b. 1 ,52UV 2 ,···52UV x FIG. 1 shows three converter cells 52UV 1 ,52UV 2 ,52UV 3The VW-phase cluster 50VW includes a plurality of converter cells 52VW connected in series via a pair of AC output terminals a and b. 1 ,52VW 2 ,···52VW x FIG. 1 shows three converter cells 52VW 1 ,52VW 2 ,52VW 3 The WU-phase cluster 50WU includes a plurality of converter cells 52WU connected in series via a pair of AC output terminals a and b. 1 ,52WU 2 ,···52WU x FIG. 1 shows three converter cells 52WU 1 ,52WU 2 ,52WU 3 is illustrated. x represents the number of converter cells connected in series in each cluster, and is an integer equal to or greater than 1. In other words, the number of converter cells in one cluster may be as small as one.

[0015] Multiple Converter Cells 52 (52UV 1 ~52UV x ,52VW 1 ~52VW x ,52WU 1 ~52WU x ) each have a pair of AC output terminals a, b and are connected in series via the pair of AC output terminals a, b. Each of the multiple converter cells 52 has its first AC output terminal a connected to the second AC output terminal b of one of the converter cells adjacent to it, and its second AC output terminal b connected to the first AC output terminal a of the other converter cell adjacent to it.

[0016] The clusters 50UV, 50VW, and 50WU are delta-connected via reactors 51UV, 51VW, and 51WU, and are connected to the grid 300. The connection to the grid 300 may be via a transformer (not shown). A circulating current flows in the delta connection. The control device 401 can adjust the negative-phase reactive current by controlling the circulating current flowing in the delta connection portion 402 by switching the multiple converter cells 52.

[0017] Each of the converter cells 52 has a power conversion circuit having a plurality of switching elements and a drive circuit unit that operates the power conversion circuit. The converter cells 52 have the same configuration. The switching element is, for example, a semiconductor switch having a transistor and a diode connected in anti-parallel to the transistor. Specific examples of the transistor include an IGBT (Insulated Gate Bipolar Transistor) and a MOSFET (Metal Oxide Semiconductor Field Effect Transistor).

[0018] Each converter cell 52 has a function of converting the DC power in the capacitor 54 into AC power and outputting it to a pair of AC output terminals a, b, and a function of converting the AC power input from the pair of AC output terminals a, b into DC power and supplying it to the capacitor 54.

[0019] Each converter cell 52 includes a pair of AC output terminals a, b, a capacitor 54, a power conversion circuit 53, and a drive circuit section (not shown) (for example, a GDU (Gate Drive Unit) and a power supply circuit).

[0020] The capacitor 54 is a capacitive element connected to the pair of AC output terminals a, b via the power conversion circuit 53.

[0021] The power conversion circuit 53 is an inverter circuit that is connected between the capacitor 54 and a pair of AC output terminals a, b and converts power bidirectionally between DC and AC. The power conversion circuit 53 is connected in parallel to the capacitor 54. A full bridge circuit having a plurality of switching elements 56 is illustrated in FIG.

[0022] The power conversion circuit 53 has a plurality of semiconductor switch groups 55 each including a plurality of semiconductor switches connected in series. The plurality of semiconductor switch groups 55 are connected in parallel to a capacitor 54. The power conversion circuit 53 shown in FIG. 1 has a full-bridge configuration in which a first semiconductor switch group including a plurality of switching elements 56 connected in series and a second semiconductor switch group including a plurality of switching elements 56 connected in series are connected in parallel. A first AC output terminal a is connected to a connection point between the switching element 56 of the first upper arm and the switching element 56 of the first lower arm. A second AC output terminal b is connected to a connection point between the switching element 56 of the second upper arm and the switching element 56 of the second lower arm.

[0023] The multiple switching elements 56 illustrated in FIG. 1 are IGBTs with diodes connected in anti-parallel, but may be switching elements having a switching function such as MOSFETs or thyristors.

[0024] At least one of the switching element and the anti-parallel diode is made of SiC (silicon carbide), GaN (gallium nitride), or Ga 2 O 3 It is preferable that the switching element includes a wide band gap semiconductor such as gallium oxide (gallium oxide) or diamond. By applying a wide band gap semiconductor to a switching element, the effect of reducing loss in the switching element is enhanced. The switching element may be an element including a semiconductor such as Si (silicon). Similarly, by applying an element including a wide band gap semiconductor to a diode, the effect of reducing loss in the diode is enhanced. The diode may be an element including a semiconductor such as Si (silicon).

[0025] Each converter cell 52 includes a drive circuit unit (not shown) such as a GDU and a power supply circuit.

[0026] The GDU is a drive circuit that drives the power conversion circuit 53, and more specifically, is a gate drive circuit that drives the gates of multiple switching elements 56 configured in the power conversion circuit 53. The GDU drives the multiple switching elements 56 configured in the power conversion circuit 53 based on power supplied from the capacitor 54 via a power supply circuit.

[0027] The GDU turns on or off a corresponding switching element among the multiple switching elements 56 by applying a voltage between the gate and emitter of the corresponding switching element in accordance with a control signal from the control device 401. By such an operation, a square-wave voltage is generated between a pair of AC output terminals a, b of the converter cell 52.

[0028] The control device 401 is a controller that generates control signals (e.g., PWM signals (pulse width modulated signals)) that turn on or off a plurality of switching elements 56 in accordance with a carrier period Tc (the inverse of a carrier frequency) common to the plurality of converter cells 52. The control device 401 has a memory and a processor (e.g., a CPU (Central Processing Unit)). Each function of the control device 401 is realized by the processor operating according to a program stored in the memory. The functions of the control device 401 may be realized by a FPGA (Field Programmable Gate Array) or an ASIC (Application Specific Integrated Circuit).

[0029] The power conversion device 400 can output a multilevel voltage waveform having a voltage equal to or higher than the withstand voltage of the switching elements and with reduced harmonics by causing the control device 401 to output voltage waveforms with mutually different phases from each of the multiple converter cells 52. Therefore, the power conversion device 400 is applicable to, for example, a reactive power compensation device or a DC transmission system directly connected to an extra-high voltage system.

[0030] Furthermore, in an MMC, when compensating for a negative-sequence current, a steadily non-zero active power flows into each phase, which may cause fluctuations in the voltage of capacitor 54. As a countermeasure, control device 401 of power conversion device 400 controls the circulating current (zero-sequence current) flowing through delta connection 402 to maintain the orthogonal relationship between the AC voltage and AC current of each phase, thereby maintaining the voltage balance of capacitor 54.

[0031] 2 is a diagram showing an example of the configuration of a control device 401. The control device 401 shown in FIG.

[0032] The current command unit 420 outputs two-axis current commands (I d ,I q ,I nd ,I nq ) to the converter cell current command (iuv * ,ivw * ,iwu * The converter cell current command is a command value for a current (converter cell current) flowing through one or more converter cells 52 of each phase.

[0033] The converter control unit 410 calculates the deviation between the converter cell current command for each phase and the detected value of the converter cell current for each phase, and generates a voltage command value for each phase such that each deviation becomes zero. The converter control unit 410 supplies each converter cell 52 with a control signal (e.g., a PWM signal) that turns on or off a plurality of switching elements 56 in each converter cell 52 according to the voltage command value for each phase.

[0034] Fig. 3 is a diagram showing an example of the configuration of the current command unit 420. The current command unit 420 shown in Fig. 3 has dq inverse transform units 37 and 38, a circulating current command calculation unit 39, a two-axis instantaneous value calculation unit 40, adders 47 and 48, a multiplier 41, a maximum value detection unit 45, an asymmetric filter 46, a divider 43, and an upper and lower limiter 42.

[0035] The current command unit 420 receives two orthogonal axes current commands (I d ,I q ,I nd ,I nq The current command unit 420 obtains a positive-sequence current command (I d ,I q ) is inversely converted by the dq inverse converter 37 to obtain a positive-phase AC current command and a negative-phase current command (I nd ,I nq ) is converted by the dq inverse conversion unit 38 into a negative-phase AC current command, and the circulating current command (i z ) are added by adders 47 and 48. The current command unit 420 adds these together to derive the converter cell current commands before limiting (iuv, ivw, iwu). The current command unit 420 multiplies the converter cell current commands before limiting (iuv, ivw, iwu) by the limiting signal K by the multiplier 41 to derive the converter cell current commands after limiting (iuv * ,ivw * ,iwu * Next, the method of deriving each signal will be described.

[0036] The line voltage v of the delta connection 402 uv ,v vw ,v wu of,

[0037]

number

[0038] The current i flowing into each phase of the delta connection 402 uv ,ivw ,i wu of,

[0039]

number

[0040] A circulating current command i, which is a command for the circulating current to be passed through the delta connection 402 z of,

[0041]

number

[0042] At this time, by passing a circulating current through the delta connection 402, the effective power of each phase becomes zero. z0 and φ z0 teeth,

[0043]

number

[0044] Therefore, the circulating current command i z teeth,

[0045]

number

[0046] Next, the current i uv ,i vw ,i wu When the dq inverse transform (including 3-phase / 2-phase transform) is performed,

[0047]

number

[0048] and,

[0049]

number

[0050]

number

[0051] Since equation (10) is equal to equation (6),

[0052]

number

[0053] Therefore, the circulating current command i z teeth,

[0054]

number

[0055] Therefore, the converter cell current command (iuv, ivw, iwu) before limiting is

[0056]

number

[0057] The two-axis instantaneous value calculation unit 40 calculates, from the result of the above equation (13), according to the following equation (14):

[0058]

number

[0059] The maximum value detection unit 45 detects the maximum value Ia of the AC current command peak values ​​Iuv, Ivw, Iwu, and the detected maximum value Ia is input to the asymmetric filter 46 and input to the divider 43 through the asymmetric filter 46.

[0060] The asymmetric filter 46 increases the output signal Iaf with a small time constant when the maximum value Ia is increasing, and decreases the output signal Iaf with a large time constant when the maximum value Ia is decreasing. The output signal Iaf of the asymmetric filter 46 is compared with a predetermined current command limit value 44 (e.g., 100%).

[0061] The divider 43 outputs a value (divided value s) obtained by dividing a predetermined current command limit value 44 by the output signal Iaf. The divided value s is limited to 100[%] to 0[%] by the upper and lower limiter 42, and the limit signal K that is the output thereof is input to the multiplier 41. The upper and lower limiter 42 limits the input signal (in this case, the divided value s) to a range between an upper limit of 100[%] and a lower limit of 0[%].

[0062] The converter cell current commands (iuv, ivw, iwu) before the limit are input to the multiplier 41. The multiplier 41 multiplies the converter cell current commands (iuv, ivw, iwu) by the limit signal K (by multiplying by K) to obtain the converter cell current commands (iuv * ,ivw * ,iwu * ) to output.

[0063] When the output signal Iaf is smaller than the current command limit value 44, the output of the divider 43 is 1 or more, but is limited to 100% by the upper and lower limiter 42. Therefore, the limit signal K to the multiplier 41 becomes 100% and the output of the multiplier 41 becomes equal to the converter cell current commands (iuv, ivw, iwu) before the limit. In other words, the current command is not limited.

[0064] On the other hand, when the output signal Iaf becomes larger than the current command limit value 44, the output of the divider 43 becomes a value smaller than 1 according to the magnitude of the output signal Iaf, and is not limited by the upper and lower limiter 42, but is input as is to the multiplier 41 as a limit signal K smaller than 1. The multiplier 41 multiplies the converter cell current commands (iuv, ivw, iwu) before limiting by the limit signal K and outputs the result. Therefore, the converter cell current commands (iuv * ,ivw * ,iwu * ) are limited to values ​​smaller than the converter cell current commands (iuv, ivw, iwu) before limiting.

[0065] Converter cell current command after limiting (iuv * ,ivw * ,iwu * ) the converter cell current is controlled, thereby reducing the harmonic current flowing through the converter cell.

[0066] Fig. 4 is a diagram showing an example of operational waveforms of the current command unit 420. In contrast to this, Fig. 6 is a diagram showing an example of operational waveforms of the conventional current limiting circuit 10 shown in Fig. 5, and shows the current command limiting result when the current limiting circuit 10 disclosed in Patent Document 2 is applied to a delta-connected MMC (Iq changes from 100% to 150%).

[0067] As shown in Fig. 6, when the current command changes suddenly and exceeds 100% of the rated current, distortion (harmonics) occurs in the current command after limiting. When the rated current is exceeded, it is necessary to limit the current command instantly to protect the device. For this reason, in Fig. 6, the time constant of the asymmetric filter 106 when the rectified signal Xn increases is set to 0 ms (no filter), and the time constant of the asymmetric filter 106 when it decreases is set to 10 ms.

[0068] On the other hand, Fig. 4 shows the current command limited waveforms due to the positive and negative phase orthogonal two-axis current commands and the circulating current command (Iq changes from 100% to 150%). The time constant of the asymmetric filter 46 is the same as that in Fig. 6. Fig. 4 shows that when the converter cell current command changes abruptly and exceeds the rated current of 100%, the distortion (harmonics) generated in the converter cell current command after limiting is reduced.

[0069] In addition, since the method of this embodiment limits the current command from the instantaneous value of the peak value, it is possible to simplify the control by eliminating the asymmetric filter 46.

[0070] Although the embodiments have been described above, the technology of the present disclosure is not limited to the above-described embodiments. Various modifications and improvements, such as combinations with or substitutions for part or all of other embodiments, are possible. [Explanation of symbols]

[0071] 50, 50UV, 50VW, 50WU cluster 51, 51UV, 51VW, 51WU Reactor 52UV 1 ,52UV 2 ,52UV x Converter Cell 52VW1 ,52VW 2 ,52VW x Converter Cell 52WU 1 ,52WU 2 ,52WU x Converter Cell 53 Power Conversion Circuit 54 Capacitor 55 Semiconductor Switches 56 Switching element 300 lines 400 Power Converter 401 Control device 402 Delta connection 410 Converter control section 420 Current command section a,b AC output terminal

Claims

1. a delta connection portion in which one or a plurality of converter cells connected in series are delta-connected; a control device that controls a circulating current flowing through the delta connection by a plurality of converter cells included in the delta connection, The converter cell includes a plurality of semiconductor switch groups each including a plurality of semiconductor switches connected in series, and a capacitor connected in parallel to the plurality of semiconductor switch groups, The control device limits current commands of a plurality of converter cells included in the delta connection portion by using each of orthogonal two-axis current commands of positive phase and negative phase and a circulating current command that is a command for the circulating current.

2. 2. The power conversion device according to claim 1, wherein the control device derives peak values ​​of the orthogonal two-axis current commands based on the orthogonal two-axis current commands and the circulating current command, and limits current commands of a plurality of converter cells included in the delta connection portion by using the peak values.

3. 3. The power conversion device according to claim 1, wherein the control device generates a limiting signal for limiting current commands of a plurality of converter cells included in the delta connection portion, using the orthogonal two-axis current commands and the circulating current command.

4. The power conversion device according to claim 1 , wherein the control device derives the circulating current command based on a negative-phase current command among the orthogonal two-axis current commands.

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