Power conversion device and control method thereof

Abstract

To provide a power conversion device with which it is possible to suppress the voltage changes of a DC capacitor in each phase, under a condition that the voltage of a power grid to which a delta connection part is connected is unbalanced.SOLUTION: Provided is a power conversion device comprising a delta connection unit in which one or a plurality of series connected bridge cells are delta connected, and a control device that controls a current flowing in the delta connection unit by the plurality of bridge cells. The bridge cells include a plurality of semiconductor switch groups, each including a plurality of series connected semiconductor switches, and a capacitor that is connected in parallel to the plurality of semiconductor switch groups. The control device chooses to control a phase-to-phase balance of the voltage of the capacitor by a circulating current flowing in the delta connection unit, or by an inverse phase current flowing in each phase of the delta connection unit, on the basis of a condition under which the voltage of the power grid to which the delta connection unit is connected is unbalanced.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present disclosure relates to a power conversion device and a control method thereof. [Background technology]

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

[0003] The bridge cell has a plurality of semiconductor switch groups each including a plurality of semiconductor switches connected in series, and a DC capacitor connected in parallel to the plurality of semiconductor switch groups. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Patent No. 5537235 Summary of the Invention [Problem to be solved by the invention]

[0005] In an MMC, for example, if an unbalanced fault occurs in the power system, non-zero active power will flow into each phase, which can cause fluctuations in the voltage of the DC capacitor.As a countermeasure, there is a control method that flows a circulating current in the delta connection to maintain the orthogonal relationship between the AC voltage and AC current of each phase, thereby maintaining the phase balance of the DC capacitor voltage.

[0006] However, under conditions where the voltage of the power system to which the delta connection is connected is unbalanced, it may not be possible to maintain the phase balance of the DC capacitor voltage even if the phase balance is controlled by the circulating current.

[0007] The present disclosure provides a power conversion device and a control method thereof that can suppress voltage fluctuations in DC capacitors of each phase under conditions where the voltage of a power system to which a delta connection part is connected becomes unbalanced. [Means for solving the problem]

[0008] In one aspect of the present disclosure, a delta connection section in which one or a plurality of series-connected bridge cells are delta-connected; a control device that controls the current flowing in the delta connection by the plurality of bridge cells, The bridge 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 switches between controlling the inter-phase balance of the capacitor voltage by the circulating current flowing in the delta connection section and controlling it by the negative-phase current flowing in each phase of the delta connection section, based on the condition under which the voltage of the power system to which the delta connection section is connected becomes unbalanced.

[0009] In another aspect of the present disclosure, A control method for a power conversion device including a delta connection part in which one or a plurality of bridge cells connected in series are delta-connected, the control method controlling a current flowing through the delta connection part by the plurality of bridge cells, The bridge 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, A control method for a power conversion device is provided, which switches between controlling the interphase balance of the capacitor voltage by the circulating current flowing in the delta connection or by the negative-phase current flowing in each phase of the delta connection, based on the condition under which the voltage of the power system to which the delta connection is connected becomes unbalanced. [Effects of the Invention]

[0010] According to the present disclosure, it is possible to provide a power conversion device and a control method thereof that can suppress voltage fluctuations in DC capacitors of each phase under conditions where the voltage of a power system to which a delta connection part is connected becomes unbalanced. [Brief explanation of the drawings]

[0011] [Figure 1] 1 is a diagram illustrating a configuration example of a power conversion device according to an embodiment; [Figure 2] FIG. 1 is a control block diagram showing a method for controlling the interphase balance of the voltage of a DC capacitor using a circulating current. [Figure 3] FIG. 10 is a control block diagram showing a control method for switching between controlling the interphase balance of the voltage of a DC capacitor by a circulating current and controlling it by a negative-phase current. DETAILED DESCRIPTION OF THE INVENTION

[0012] Hereinafter, embodiments of the present disclosure will be described with reference to the drawings.

[0013] FIG. 1 is a diagram showing an example of the configuration of a power conversion device according to an embodiment, illustrating an example of the circuit configuration of a delta-connected MMC. The MMC is applicable to, for example, a static synchronous compensator (STATCOM) and a high-voltage direct current (HVDC) transmission system. The power conversion device 100 shown in FIG. 1 includes a delta connection unit 10 and a control device 102.

[0014] The delta connection section 10 is a circuit in which one or more bridge cells connected in series are delta-connected. In the example shown in FIG. 1, three bridge cells 11-k, 11v-k, and 11w-k (where k = 1 to 3) are provided on each phase of the delta connection section 10 and are connected in series via a pair of AC output terminals a and b. k represents the number of bridge cells connected in series in each phase of the delta connection section 10 and is an integer equal to or greater than 1. In other words, the number of bridge cells in each phase of the delta connection section 10 is not limited to multiple and may be one. In FIG. 1, the reactor component on each phase of the delta connection section 10 is represented by L.

[0015] The multiple bridge cells 11 (11-k, 11v-k, and 11w-k) 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 bridge cells 11 has its first AC output terminal a connected to the second AC output terminal b of one of the bridge cells adjacent to it, and its second AC output terminal b connected to the first AC output terminal a of the other bridge cell adjacent to it.

[0016] The plurality of bridge cells 11-k, 11v-k, and 11w-k are each delta-connected via a reactor component L and are connected to the power grid 101. The connection to the power grid 101 may be via a transformer (not shown). A circulating current flows within the delta connection. The control device 102 can adjust the negative-phase reactive current by controlling the circulating current flowing through the delta connection unit 10 by switching the plurality of bridge cells 11.

[0017] Each of the bridge cells 11 includes a power conversion circuit having a plurality of semiconductor switches and a drive circuit unit that operates the power conversion circuit. The bridge cells 11 have the same configuration. The semiconductor switch is, for example, a switching element including a transistor and a diode connected in antiparallel 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 bridge cell 11 has the function of converting DC power in the capacitor C into AC power and outputting it to a pair of AC output terminals a, b, and the function of converting AC power input from the pair of AC output terminals a, b into DC power and supplying it to the capacitor C.

[0019] Each bridge cell 11 includes a pair of AC output terminals a and b, a capacitor C, a power conversion circuit 12, and a drive circuit unit (not shown) (for example, a GDU (Gate Drive Unit) and a power supply circuit).

[0020] The capacitor C is a capacitance element connected to the pair of AC output terminals a and b via the power conversion circuit 12.

[0021] The power conversion circuit 12 is an inverter circuit connected between the capacitor C and a pair of AC output terminals a and b, and converts power bidirectionally between DC and AC. The power conversion circuit 12 is connected in parallel to the capacitor C. FIG. 1 illustrates a full-bridge circuit having a plurality of semiconductor switches 13.

[0022] The power conversion circuit 12 has a plurality of semiconductor switch groups 14, each including a plurality of semiconductor switches 13 connected in series. The plurality of semiconductor switch groups 14 are connected in parallel to a capacitor C. The power conversion circuit 12 shown in FIG. 1 has a full-bridge configuration in which a first semiconductor switch group including a plurality of semiconductor switches 13 connected in series and a second semiconductor switch group including a plurality of semiconductor switches 13 connected in series are connected in parallel. A first AC output terminal a is connected to a connection point between the semiconductor switch 13 of the first upper arm and the semiconductor switch 13 of the first lower arm. A second AC output terminal b is connected to a connection point between the semiconductor switch 13 of the second upper arm and the semiconductor switch 13 of the second lower arm.

[0023] The plurality of semiconductor switches 13 illustrated in FIG. 1 are IGBTs with diodes connected in antiparallel, but may also be switching elements having a switching function, such as MOSFETs or thyristors.

[0024] At least one of the semiconductor switch and the anti-parallel diode is preferably an element including a wide bandgap semiconductor such as SiC (silicon carbide), GaN (gallium nitride), Ga2O3 (gallium oxide), or diamond. By using a wide bandgap semiconductor as a switching element, the effect of reducing loss in the switching element is enhanced. The semiconductor switch may be an element including a semiconductor such as Si (silicon). Similarly, by using an element including a wide bandgap semiconductor as 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 bridge cell 11 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 12, and more specifically, a gate drive circuit that drives the gates of the plurality of semiconductor switches 13 that are configured in the power conversion circuit 12. The GDU drives the plurality of semiconductor switches 13 that are configured in the power conversion circuit 12 based on power supplied from the capacitor C via a power supply circuit.

[0027] The GDU turns on or off a corresponding one of the semiconductor switches 13 by applying a voltage between the gate and emitter of the corresponding semiconductor switch in accordance with a control signal from the control device 102. This operation generates a square-wave voltage between a pair of AC output terminals a and b of the bridge cell 11.

[0028] The control device 102 is a controller that generates control signals (e.g., PWM signals (pulse width modulated signals)) that turn on or off the multiple semiconductor switches 13 in accordance with a carrier period Tc (the reciprocal of the carrier frequency) common to the multiple bridge cells 11. The control device 102 has a memory and a processor (e.g., a CPU (Central Processing Unit)). Each function of the control device 102 is realized by the processor operating in accordance with a program stored in the memory. The functions of the control device 102 may be realized by an FPGA (Field Programmable Gate Array) or an ASIC (Application Specific Integrated Circuit).

[0029] The power conversion device 100 can output a multilevel voltage waveform with reduced harmonics and a voltage equal to or higher than the withstand voltage of the semiconductor switch 13 by causing the control device 102 to output voltage waveforms with mutually different phases from each of the multiple bridge cells 11. Therefore, the power conversion device 100 can be applied to, for example, a reactive power compensator or a DC power transmission system directly connected to an extra-high voltage grid.

[0030] Furthermore, in an MMC, if an unbalanced fault occurs in the power system 101, for example, a steadily non-zero active power may flow into each phase, causing fluctuations in the voltage of the capacitor C. To address this, the control device 102 of the power conversion device 100 controls the circulating current (also called the zero-phase current) flowing through the delta connection 10, thereby maintaining the orthogonal relationship between the AC voltage and AC current of each phase and maintaining the phase balance of the voltage of the capacitor C.

[0031] Here, in FIG. 1, the line voltage v of the power system 101 S (v Suv ,v Svw ,v Swu ) is the current i flowing from the MMC to the power grid 101. u ,i v ,i w ) The current (inverter current) flowing through each phase of the delta connection 10 is (i uv ,i vw ,i wu ) The voltage of the capacitor C in each bridge cell 11 is (v Cku ,v Ckv ,v Ckw ) In this example, k is 1, 2, and 3.

[0032] The current (i u ,i v ,i w ) and the current (i uv ,i vw ,i wu ) and the relationship shown in Equations 1 to 3 holds.

[0033]

number

[0034]

number

[0035]

number

[0036]

number

[0037] FIG. 2 is a control block diagram showing a method for controlling the interphase balance of the voltage of a DC capacitor by using a circulating current (zero-phase current).

[0038] The positive-phase dq-axis component of the voltage (system voltage) of the power system 101 to which the delta connection 10 is connected is defined as v + d ,v + q , the negative-phase dq-axis components of the system voltage are v - d ,v - q , the positive-phase dq-axis components of the inverter current are i + d ,i + q , and the imaginary unit is j. In this case, the command value i of the circulating current (zero-phase current) to be passed through the delta connection 10 in order to suppress the DC voltage fluctuation of the MMC (voltage fluctuation of the capacitor C) is * 0 is

[0039]

number

[0040] In the case of Equation 5, when the positive-sequence voltage and the negative-sequence voltage of the power grid 101 are equal, the circulating current (zero-sequence current) to be passed through the delta connection 10 becomes infinite, and the phase balance of the voltage of the capacitor C cannot be controlled by the zero-sequence current. This is because, when the positive-sequence voltage and the negative-sequence voltage of the power grid 101 are equal, the denominator of the right-hand side of Equation 5 becomes zero. As a result, the voltage of the capacitor C of each phase may fluctuate significantly. If an overvoltage occurs due to the voltage fluctuation of the capacitor C, the protection circuit will stop the operation of the power conversion device 100.

[0041] As a countermeasure, when the positive-sequence voltage and the negative-sequence voltage of the power system 101 are equal, the control device 102 switches the control of the phase balance of the voltage of the capacitor C from control by the circulating current (zero-sequence current) to control by the negative-sequence current.

[0042] The positive-phase dq-axis component of the voltage (system voltage) of the power system 101 to which the delta connection 10 is connected is defined as v + d ,v + q , the negative-phase dq-axis components of the system voltage are v - d ,v - q , the positive-phase dq-axis components of the inverter current are i + d ,i + q , and the imaginary unit is j. In this case, the command value (i -* d ,i -* q )teeth,

[0043]

number

[0044]

number

[0045] Fig. 3 is a control block diagram showing a control method for switching between controlling the inter-phase balance of the voltage of a DC capacitor by a circulating current and controlling it by a negative-sequence current. The control block shown in Fig. 3 is realized by a control device 102. The control block shown in Fig. 3 includes a positive-sequence / negative-sequence calculation unit 31, a zero-sequence current command calculation unit 32, a negative-sequence current command calculation unit 33, a control switching unit 34, a three-phase current command generation unit 35, and a current control unit 36.

[0046] The positive and negative phase calculation unit 31 calculates the system voltage (v Suv ,v Svw ,v Swu The positive-phase / negative-phase calculation unit 31 performs three-phase / two-phase conversion of the system voltage (v Suv ,v Svw ,v Swu ) is converted into a positive-phase dq-axis voltage (v + d ,v + q Similarly, the positive and negative phase calculation unit 31 generates the system voltage (v Suv ,v Svw ,v Swu ) is converted into a 3-phase / 2-phase voltage by the dq-axis voltage converter block, and then the moving average is calculated by the moving average block. - d ,v - q )

[0047] The zero-phase current command calculation unit 32 calculates the positive-phase dq-axis voltage (v + d ,v + q ), negative-phase dq-axis voltage (v - d ,v -q ) and positive-phase dq-axis current command (i +* d ,i +* q ) and calculate the command value i of the circulating current (zero-phase current) based on Equation 5. * Calculates 0.

[0048] Positive phase dq axis current command (i +* d ,i +* q ) represents a command value of the positive-phase dq-axis component of the current (inverter current) flowing through each phase of the delta connection 10, and is a predetermined value.

[0049] The negative-phase current command calculation unit 33 calculates the positive-phase dq-axis voltage (v + d ,v + q ), negative-phase dq-axis voltage (v - d ,v - q ) and positive-phase dq-axis current command (i +* d ,i +* q ) and calculate the command value (i -* d ,i -* q ) is calculated.

[0050] The control switching unit 34 controls the positive-phase dq-axis voltage (v + d ,v + q ) and negative-phase dq-axis voltage (v - d ,v - q ), the coefficient K is set to 1 or 0. For example, when the positive-phase component and the negative-phase component of the voltage (system voltage) of the power system 101 are different [(v + d v + d +v + q v + q )≠(v -d v - d +v - q v - q )], the control switching unit 34 sets the coefficient K to 0. In this case, the zero-phase current command i * 0 is the value calculated by Equation 5, and the negative-phase dq-axis current command (i -* d ,i -* q ) is 0. On the other hand, when the positive and negative components of the system voltage are equal, [(v + d v + d +v + q v + q )=(v - d v - d +v - q v - q )], the control switching unit 34 sets the coefficient K to 1. In this case, the zero-phase current command i * 0 becomes 0, and the negative-phase dq-axis current command (i -* d ,i -* q ) is the value calculated using Equation 6 and Equation 7.

[0051] The three-phase current command generator 35 generates a positive-phase dq-axis current command (i +* d ,i +* q ) and negative-phase dq-axis current command (i -* d ,i -* q ) or based on the positive-phase dq-axis current command (i +* d ,i +* q ) and zero-phase current command i * Based on the three-phase current command (i * uv ,i* vw ,i * wu ) is generated. * uv ,i * vw ,i * wu ) represents the command value (target value) of the current (inverter current) flowing through each phase of the delta connection 10.

[0052] When the positive-sequence voltage and the negative-sequence voltage of the power grid 101 are different, the negative-sequence dq-axis current command (i -* d ,i -* q ) is 0. Therefore, the three-phase current command generating unit 35 performs inter-phase balancing of the voltage of the capacitor C by controlling the zero-phase current. The three-phase current command generating unit 35 generates the positive-phase dq-axis current command (i +* d ,i +* q ) is subjected to an inverse phase dq transformation, and the result of the inverse phase dq transformation is converted into a two-phase / three-phase * uv ,i * vw ,i * wu The three-phase current command generator 35 generates the three-phase current command (i * uv ,i * vw ,i * wu ) to the zero-phase current command i * The value with 0 superimposed is used as the new three-phase current command (i * uv ,i * vw ,i * wu )

[0053] On the other hand, when the positive-sequence voltage and the negative-sequence voltage of the power system 101 are equal, the zero-phase current command i *0 becomes 0. Therefore, the three-phase current command generation unit 35 performs inter-phase balancing of the voltage of the capacitor C by controlling the negative-phase current. The three-phase current command generation unit 35 generates the negative-phase dq-axis current command (i -* d ,i -* q The three-phase current command generator 35 converts the negative-phase dq-axis current command (i -* d ,i -* q ) is converted into a positive-phase dq-axis current command (i +* d ,i +* q The three-phase current command generator 35 superimposes the negative-phase dq-axis current command (i -* d ,i -* q ) is converted into a positive-phase dq-axis current command (i +* d ,i +* q ) is superimposed on the three-phase current command (i * uv ,i * vw ,i * wu )

[0054] In this way, the control device 102 switches between controlling the interphase balance of the voltage of the capacitor C by the circulating current flowing through the delta connection 10 or by the negative-phase current flowing through each phase of the delta connection 10, based on the conditions under which the voltage of the power system 101 becomes unbalanced.

[0055] The current control unit 36 ​​controls the three-phase current (i uv ,i vw ,i wu ) is generated by the three-phase current command generating unit 35. * uv ,i * vw ,i * wu) so that the voltage command value (v * uv ,v * vw ,v * wu )

[0056] The control device 102 generates the voltage command value (v * uv ,v * vw ,v * wu ) and controls the switching of the semiconductor switch 13 in the bridge cell 11 of each phase.

[0057] As described above, in this embodiment, when the condition that the positive-sequence voltage and the negative-sequence voltage of the power grid 101 are equal is not satisfied, the control device 102 controls the inter-phase balance of the voltage of the capacitor C using a command value of the circulating current (zero-phase current). On the other hand, when the condition that the positive-sequence voltage and the negative-sequence voltage of the power grid 101 are equal is satisfied, the control device 102 controls the inter-phase balance of the voltage of the capacitor C using a command value of the negative-sequence current. As a result, even under a system imbalance condition in which the positive-sequence voltage and the negative-sequence voltage of the power grid 101 are equal, the voltage fluctuation of the capacitor C of each phase of the power conversion device 100 can be suppressed. As a result, the power conversion device 100 can be prevented from stopping due to an overvoltage caused by the voltage fluctuation of the capacitor C, and the operation of the power conversion device 100 can be continued. Furthermore, the power conversion device 100 can be made smaller and less expensive without increasing the capacitance of the capacitor C.

[0058] Although the embodiments have been described above, the technology of the present disclosure is not limited to the above-described embodiments, and various modifications and improvements are possible, such as combinations with or substitutions for part or all of other embodiments. [Explanation of symbols]

[0059] 10 Delta connection 11,11-k,11v-k,11w-k bridge cell 12 Power Conversion Circuit 13 Solid State Switch 14 Semiconductor switches 100 Power conversion device 101 Power system 102 Control device a,b AC output terminal C capacitor L reactor component

Claims

1. a delta connection section in which one or a plurality of series-connected bridge cells are delta-connected; a control device that controls the current flowing in the delta connection by the plurality of bridge cells, The bridge 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 switches between controlling the inter-phase balance of the capacitor voltages by the circulating current flowing in the delta connection or by the negative-phase current flowing in each phase of the delta connection, based on a condition under which the voltage of the power grid to which the delta connection is connected becomes unbalanced.

2. 2. The power conversion device according to claim 1, wherein the control device controls the inter-phase balance by the circulating current when the condition is not met, and controls the inter-phase balance by the negative-phase current when the condition is met.

3. the control device calculates a command value for the circulating current; The power conversion device according to claim 2 , wherein the command value of the circulating current becomes infinite when the condition is met.

4. The positive-phase dq-axis components of the voltage of the power system are expressed as v ＋ ｄ , v ＋ ｑ , the negative-phase dq-axis components of the voltage of the power system are v － ｄ , v － ｑ , the positive dq axis components of the current flowing through each phase of the delta connection are i ＋ ｄ , i ＋ ｑ , where j is the imaginary unit, The circulating current command value i ＊ ０ teeth, [Equation 5] The power converter of claim 3 , wherein:

5. the control device calculates a command value for the negative-phase current; The command value of the negative-phase current (i －＊ ｄ , i －＊ ｑ )teeth, [Equation 6] [Equation 7] The power converter of claim 4, wherein:

6. The power conversion device according to claim 1 , wherein the condition is that a positive-phase voltage and a negative-phase voltage of the power grid are equal to each other.

7. A control method for a power conversion device including a delta connection part in which one or a plurality of bridge cells connected in series are delta-connected, the control method controlling a current flowing through the delta connection part by the plurality of bridge cells, The bridge 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, A control method for a power conversion device, which switches between controlling the inter-phase balance of the capacitor voltage by a circulating current flowing in the delta connection or by a negative-phase current flowing in each phase of the delta connection based on a condition under which the voltage of the power grid to which the delta connection is connected becomes unbalanced.

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