JP7803203B2 - Power conversion device and control method thereof - Google Patents

Description

This disclosure relates to a power conversion device and a control method thereof.

The modular multilevel converter (MMC) is a next-generation transformerless power converter suitable for large-capacity, high-voltage applications. MMCs can be applied, for example, to static synchronous compensation (STATCOM) systems and high-voltage direct current (HVDC) systems. Delta-connected MMCs, in particular, have attracted attention because they can pass a circulating current through the delta connection and output a negative-phase reactive current. 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).

A bridge cell has multiple semiconductor switch groups, each containing multiple semiconductor switches connected in series, and a DC capacitor connected in parallel to the multiple semiconductor switch groups.

Patent No. 5537235

In an MMC, for example, if an unbalanced fault occurs in the power system, non-zero active power will steadily flow into each phase, which can cause fluctuations in the DC capacitor voltage. One countermeasure is to pass a circulating current through the delta connection to maintain the orthogonal relationship between the AC voltage and AC current of each phase, thereby maintaining the phase-to-phase balance of the DC capacitor voltage.

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 using circulating current.

This disclosure provides a power conversion device and a control method for the same that can suppress voltage fluctuations in the DC capacitors of each phase when the voltage of the power system to which the delta connection is connected becomes unbalanced.

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.

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.

This disclosure provides a power conversion device and a control method thereof that can suppress voltage fluctuations in the DC capacitors of each phase when the voltage of the power system to which the delta connection is connected becomes unbalanced.

1 is a diagram illustrating a configuration example of a power conversion device according to an embodiment; 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. 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.

Embodiments of the present disclosure will be described below with reference to the drawings.

Figure 1 shows an example of the configuration of a power conversion device in one embodiment, illustrating an example of the circuit configuration of a delta-connected MMC. The MMC can be applied to, for example, a static var compensator (STATCOM) or a high-voltage direct current (HVDC) transmission system. The power conversion device 100 shown in Figure 1 includes a delta-connected section 10 and a control device 102.

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 Figure 1, each phase of the delta connection section 10 has three bridge cells 11-k, 11v-k, and 11w-k (where k = 1 to 3) 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 greater than or equal to 1. In other words, the number of bridge cells in each phase of the delta connection section 10 is not limited to multiple and can be as few as one. Note that in Figure 1, the reactor component on each phase of the delta connection section 10 is represented by L.

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.

Multiple bridge cells 11-k, 11v-k, and 11w-k are each delta-connected via reactor component L and connected to the power grid 101. Connection to the power grid 101 may also 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 section 10 by switching the multiple bridge cells 11.

Each of the multiple bridge cells 11 has a power conversion circuit with multiple semiconductor switches and a drive circuit unit that operates the power conversion circuit. The multiple bridge cells 11 have the same configuration. The semiconductor switch is a switching element that has, for example, a transistor and a diode connected in anti-parallel to the transistor. Specific examples of transistors include an IGBT (Insulated Gate Bipolar Transistor) and a MOSFET (Metal Oxide Semiconductor Field Effect Transistor).

Each bridge cell 11 has the function of converting DC power in capacitor C into AC power and outputting it to a pair of AC output terminals a and b, and the function of converting AC power input from the pair of AC output terminals a and b into DC power and supplying it to capacitor C.

Each bridge cell 11 has a pair of AC output terminals a, b, a capacitor C, a power conversion circuit 12, and a drive circuit unit (not shown) (e.g., a GDU (Gate Drive Unit) and a power supply circuit, etc.).

Capacitor C is a capacitive element connected to a pair of AC output terminals a and b via power conversion circuit 12.

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. Figure 1 shows an example of a full-bridge circuit having multiple semiconductor switches 13.

The power conversion circuit 12 has multiple semiconductor switch groups 14, each including multiple semiconductor switches 13 connected in series. The multiple 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 multiple semiconductor switches 13 connected in series and a second semiconductor switch group including multiple semiconductor switches 13 connected in series are connected in parallel. A first AC output terminal a is connected to the 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 the connection point between the semiconductor switch 13 of the second upper arm and the semiconductor switch 13 of the second lower arm.

The multiple semiconductor switches 13 shown in Figure 1 are IGBTs with diodes connected in antiparallel, but they may also be switching elements with switching functions such as MOSFETs or thyristors.

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 applying a wide bandgap semiconductor to the 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 applying an element including a wide bandgap semiconductor to the diode, the effect of reducing loss in the diode is enhanced. The diode may be an element including a semiconductor such as Si (silicon).

Each bridge cell 11 is equipped with a drive circuit unit (not shown), such as a GDU and a power supply circuit.

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 multiple semiconductor switches 13 that are configured in the power conversion circuit 12. The GDU drives the multiple semiconductor switches 13 that are configured in the power conversion circuit 12 based on the power supplied from the capacitor C via the power supply circuit.

The GDU applies a voltage between the gate and emitter of a corresponding one of the multiple semiconductor switches 13 in accordance with a control signal from the control device 102, thereby turning the corresponding semiconductor switch on or off. This operation generates a square-wave voltage between a pair of AC output terminals a and b of the bridge cell 11.

The control device 102 is a controller that generates control signals (e.g., PWM signals (pulse-width modulated signals)) that turn on or off multiple semiconductor switches 13 in accordance with a carrier period Tc (the inverse of the carrier frequency) common to 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 also be realized by an FPGA (Field Programmable Gate Array) or an ASIC (Application Specific Integrated Circuit).

The power conversion device 100 can output a multi-level voltage waveform with reduced harmonics and a voltage equal to or greater than the withstand voltage of the semiconductor switch 13 by having the control device 102 cause each of the multiple bridge cells 11 to output voltage waveforms with different phases. Therefore, the power conversion device 100 can be applied to, for example, reactive power compensation devices and DC transmission systems that are directly connected to extra-high voltage grids.

Furthermore, in an MMC, if an unbalanced fault occurs in the power system 101, for example, non-zero active power may steadily flow into each phase, causing fluctuations in the voltage of capacitor C. As a countermeasure, the control device 102 of the power conversion device 100 controls the circulating current (also called 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-to-phase balance of the voltage of capacitor C.

1 , the line voltage _vS of the power system 101 is ( _vSuv , _vSvw , _vSwu ). The current i flowing from the MMC to the power system 101 is ( _iu , _iv , _iw ). The current (inverter current) flowing through each phase of the delta connection 10 is ( _iuv , _ivw , _iwu ). The voltage of the capacitor C in each bridge cell 11 is ( _vCku , _vCkv , _vCkw ). In this example, k is 1, 2, and 3.

The relationship shown in equations 1 to 3 holds between the current (i _u , i _v , i _w ) flowing from the power conversion device 100 to the power system 101 and the current (i _uv , i _vw , i _wu ) flowing into each phase of the delta connection 10.

At this time, the circulating current _i0 flowing through the delta connection 10 is expressed as in Equation 4.

Figure 2 is a control block diagram showing a method for controlling the interphase balance of DC capacitor voltages using circulating currents (zero-phase currents).

Let v ⁺ _d , v ⁺ _q be the positive-phase dq-axis components of the voltage (system voltage) of the power system 101 to which the delta connection 10 is connected, v ^- _d , v ^- _q be the negative-phase dq-axis components of the system voltage, i ⁺ _d , i ⁺ _q be the positive-phase dq-axis components of the inverter current, and j be the imaginary unit. In this case, the command value i ^* ₀ of the circulating current (zero-phase current) to be passed through the delta connection 10 in order to suppress DC voltage fluctuations of the MMC (voltage fluctuations of the capacitor C) is given by

It is expressed by:

In the case of Equation 5, when the positive-sequence voltage and negative-sequence voltage of the power system 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 capacitor C cannot be controlled by the zero-sequence current. This is because when the positive-sequence voltage and negative-sequence voltage of the power system 101 are equal, the denominator of the right-hand side of Equation 5 becomes zero. As a result, the voltage of capacitor C of each phase may fluctuate significantly. If an overvoltage occurs due to voltage fluctuations in capacitor C, the protection circuit will stop operation of the power conversion device 100.

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

Let v ⁺ _d , v ⁺ _q be the positive-phase dq-axis components of the voltage (system voltage) of the power system 101 to which the delta connection unit 10 is connected, v ^- _d , v ^- _q be the negative-phase dq-axis components of the system voltage, i ⁺ _d , i ⁺ _q be the positive-phase dq-axis components of the inverter current, and j be the imaginary unit. In this case, the command values (i ^{- *} _d , i ^{- *} _q ) of the negative-phase dq-axis components of the inverter current to suppress DC voltage fluctuations of the MMC (voltage fluctuations of capacitor C) are given by

The command values (i ^−* _d , i ^−* _q ) of the negative-phase dq-axis components of the inverter current represent the command values of the dq-axis components of the negative-phase current flowing through each phase of the delta connection 10 .

Figure 3 is a control block diagram showing a control method for switching between controlling the interphase balance of DC capacitor voltages using circulating currents and using negative-phase currents. The control block shown in Figure 3 is implemented by a control device 102. The control block shown in Figure 3 includes a positive-phase/negative-phase calculation unit 31, a zero-phase current command calculation unit 32, a negative-phase current command calculation unit 33, a control switching unit 34, a three-phase current command generation unit 35, and a current control unit 36.

The positive-phase/negative-phase calculation unit 31 performs three-phase/two-phase transformation of the system voltages ( _vSuv , _vSvw , _vSwu ) using a three-phase/two-phase transformation block. The positive-phase/negative-phase calculation unit 31 performs positive-phase dq transformation on the result of the three-phase/two-phase transformation of the system voltages ( _vSuv , _vSvw , _vSwu ) using a positive-phase dq transformation block and performs moving averaging using a moving average block to generate positive-phase dq-axis voltages (v ⁺ _d , v ⁺ _q ). Similarly, the positive-phase/negative-phase calculation unit 31 performs negative-phase dq transformation on the result of the three-phase/two-phase transformation of the system voltages ( _vSuv , _vSvw , _vSwu ) using a negative-phase dq transformation block and performs moving averaging using a moving average block to generate negative-phase dq-axis voltages (v ^- _d , v ^- _q ).

The zero-phase current command calculation unit 32 calculates the command value i ^* 0 of the circulating current (zero-phase current) based on equation 5 using the positive-phase dq-axis voltages (v ⁺ _d , v ⁺ _q ), the negative-phase dq-axis voltages (v ^- _d , v ^- _q ) and the positive-phase dq-axis current commands (i + ^* _d , i ^{+ *} _{q )} .

The positive-phase dq-axis current commands (i ^{+ *} _d , i ^{+ *} _q ) represent command values of the positive-phase dq-axis components of the current (inverter current) flowing through each phase of the delta connection 10, and are predetermined values.

The negative-phase current command calculation unit 33 uses the positive-phase dq-axis voltages (v ⁺ _d , v ⁺ _q ), the negative-phase dq-axis voltages (v ^- _d , v ^- _q ) and the positive-phase dq-axis current commands (i ^{+ *} _d , i ^{+ *} _q ) to calculate the command values (i ^{- *} _d , i ^{- *} _q ) of the negative-phase dq-axis components of the inverter current based on equations 6 and 7.

The control switch 34 sets the coefficient K to 1 or 0 based on the positive-phase dq-axis voltage (v ⁺ _d , v ⁺ _q ) and the negative-phase dq-axis voltage (v ^- _d , v ^- _q ). For example, if the positive-phase component and the negative-phase component of the voltage (system voltage) of the power grid 101 are different [(v ⁺ _{d v} ⁺ d + ^{v +} _q v ⁺ _q ) ≠ (v ^- _d ^{v -} _{d +} v - q v ^- _q )], the control switch 34 sets the coefficient K to 0. In this case, the zero-phase current command i ^* ₀ output from the control switch 34 is the value calculated by Equation 5, and the negative-phase dq-axis current command (i ^{- *} _d , i ^{- *} _q ) output from the control switch 34 is 0. On the other hand, when the positive-sequence component and the negative-sequence component of the system voltage are equal [( _v ⁺ _dv ⁺ _d + v ⁺ qv ⁺ _q ) = (v ^- _dv ^- _d + _v ^- qv ^- _q )], the control switch unit 34 sets the coefficient K to 1. In this case, the zero-phase sequence current command i ^* ₀ output from the control switch unit 34 becomes 0, and the negative-sequence dq-axis current commands (i- ^* _d , i- ^* _q ) output from the control switch unit 34 become values calculated by Equations 6 and 7.

The three-phase current command generating unit 35 generates three-phase current commands (i ^* _{uv, i*vw} , i ^* _wu ) based on the positive-phase dq-axis current commands (i ^+* _d , i ^+* _q ) and the negative-phase dq-axis current commands (i- ^* _d , i- ^* _q ), or based on the positive-phase dq-axis current commands (i ^+* _{d ,} i ^{+ *} _q ) and the zero-phase current command i ^* ₀ . The three-phase current commands (i ^* _uv , i ^* _vw , i ^* _wu ) represent command values (target values) of the currents (inverter currents) flowing through the respective phases of the delta connection unit 10.

When the positive-phase voltage and negative-phase voltage of the power system 101 are different, the negative-phase dq-axis current commands (i- ^* _d , i- ^* _q ) input to the three-phase current command generation unit 35 are 0. Therefore, the three-phase current command generation unit 35 balances the phases of the voltages of the capacitor C by controlling the zero-phase current. The three-phase current command generation unit 35 performs a negative-phase dq transformation on the positive-phase dq-axis current commands (i ^+* _d , i ^+* _q ) and then performs a two-phase/three-phase transformation on the result of the negative-phase dq transformation to generate three-phase current commands (i ^* _uv , i ^* _vw , i ^* _wu ). The three-phase current command generating unit 35 outputs a value obtained by superimposing the zero-phase current command i ^* ₀ on the three-phase current command (i ^* _uv , i ^* _vw , i ^* _wu ) as a new three-phase current command (i ^* _uv , i ^* _vw , i ^* _wu ).

On the other hand, when the positive-sequence voltage and the negative-sequence voltage of the power system 101 are equal, the zero-sequence current command i ^* ₀ input to the three-phase current command generation unit 35 is 0. Therefore, the three-phase current command generation unit 35 balances the phases of the voltage of the capacitor C by controlling the negative-sequence current. The three-phase current command generation unit 35 performs positive-sequence dq transformation on the negative-sequence dq-axis current commands (i- ^* _d , i- ^* _q ). The three-phase current command generation unit 35 superimposes the positive-sequence dq-axis current commands (i ^+* _d , i ^+* _q ) on the commands obtained by performing positive-sequence dq transformation on the negative-sequence dq-axis current commands (i- ^* _d , i- ^* _q ). The three-phase current command generating unit 35 performs a positive-phase dq conversion on the command obtained by performing a positive-phase dq conversion on the negative-phase dq-axis current command (i- ^* _d , i- ^* _q ), superimposes the positive-phase dq-axis current command (i ^+* _d , i ^+* _q ), and then performs a negative-phase dq conversion on the result of the negative-phase dq conversion to generate a three-phase current command (i ^* _uv , i ^* _vw , i ^* _wu ).

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

The current control unit 36 generates voltage command values (v* _uv , v _*vw , v* _wu ) for the bridge cells 11 of each phase so that the detected or estimated values of the three-phase currents (iuv, _ivw , _iwu ) follow the three-phase current commands (i ^* _uv , i ^* _vw , i ^* _{wu )} generated by the three ^{- phase} current command generating unit ³⁵ .

The control device 102 controls the switching of the semiconductor switches 13 in the bridge cells 11 of each phase in accordance with the generated voltage command values (v ^* _uv , v ^* _vw , v ^* _wu ).

In this manner, in this embodiment, when the condition that the positive-sequence voltage and the negative-sequence voltage of the power system 101 are equal is not met, the control device 102 controls the inter-phase balance of the voltage of capacitor C using a command value for 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 system 101 are equal is met, the control device 102 controls the inter-phase balance of the voltage of capacitor C using a command value for the negative-sequence current. As a result, even under system imbalance conditions where the positive-sequence voltage and the negative-sequence voltage of the power system 101 are equal, voltage fluctuations of 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 shutting down due to overvoltage caused by voltage fluctuations of capacitor C, and 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 capacitor C.

Although the embodiments have been described above, the technology of the present disclosure is not limited to the above embodiments. Various modifications and improvements are possible, such as combining or substituting part or all of the other embodiments.

10 Delta connection section 11, 11-k, 11v-k, 11w-k Bridge cell 12 Power conversion circuit 13 Semiconductor switch 14 Group of semiconductor switches 100 Power conversion device 101 Power system 102 Control device a, b AC output terminal C Capacitor L Reactor component

Claims (7)

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. The power conversion device of claim 1, wherein the control device controls the inter-phase balance using the circulating current when the condition is not met, and controls the inter-phase balance using the negative-phase current when the condition is met. 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. When the positive-phase dq-axis components of the voltage of the power system are v ⁺ _d and v ⁺ _q , the negative-phase dq-axis components of the voltage of the power system are v ^- _d and v ^- _q , the positive-phase dq-axis components of the current flowing through each phase of the delta connection are i ⁺ _d and i ⁺ _q , and the imaginary unit is j,
The command value i ^* ₀ of the circulating current is
The power converter of claim 3 , wherein: the control device calculates a command value for the negative-phase current;
The negative-phase current command value (i ^−* _d , i ^−* _q ) is expressed as follows:
The power converter of claim 4, wherein: A power conversion device according to any one of claims 1 to 5, wherein the condition is that the positive-phase voltage and negative-phase voltage of the power grid are equal. 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.