MMC-based AC-DC Multi-port Flexible Interconnection Device and Control Method

Abstract

The present invention relates to the field of power conversion technology. The present invention discloses an MMC AC-DC multi-port flexible interconnection device, a control method, and a distribution network system; provides a medium-voltage DC port and multiple AC ports; consists of three parts, including a four-arm modular multilevel converter (MMC), multiple cascaded H-bridges (CHBs) connected between different AC ports, and multiple groups of resonant modules. The midpoint lead of the fourth arm of the MMC is connected to the midpoint of multiple groups of three-phase resonant LC. The power flow control of different AC feeders is achieved through the CHBs connected between different AC ports. The converter of the present invention can be used for the flexible interconnection of multiple AC and DC feeder systems in a distribution network, and can significantly reduce the cost and volume of the converter.

Description

Technical Field

[0001] The present invention relates to the field of power transformation technology, and specifically to an AC-DC multi-port flexible interconnection device, a control method, and a distribution network system based on MMC. Background Art

[0002] With the rapid development of distributed renewable energy and the proposal of concepts such as smart grids, the AC-DC hybrid power grid has become one of the important development trends of future power grid forms. As a key device connecting the AC power grid and the DC power grid, the AC-DC multi-port flexible interconnection device is gradually becoming a new research hotspot in the field of hybrid distribution networks.

[0003] Currently, the research on flexible interconnection devices mainly focuses on aspects such as optimal configuration and fault handling, while the research on topological structures is less. Engineering applications mainly use two topological structures: the back-to-back cascaded H-bridge converter type (BTB-CHB type) and the back-to-back modular multilevel converter type (BTB-MMC type). These two topological structures are currently relatively mature in technology, but both use a large number of switching devices and passive components, resulting in high prices and large floor areas; when power is transmitted between different port feeders, the current passes through a large number of switching devices, thereby reducing the operating efficiency of the equipment; the BTB-CHB type converter can only provide two AC ports, and cannot further expand the number of ports, nor can it provide DC ports. Summary of the Invention

[0004] The purpose of the present invention is to provide an AC-DC multi-port flexible interconnection device, a control method, and a distribution network system based on MMC; the multi-port interconnection device can not only provide DC ports and multiple AC ports at the same time, but also can conveniently expand the number of AC ports by increasing the number of three-phase CHBs, thereby reducing the volume and cost of the converter.

[0005] The purpose of the present invention can be achieved by the following technical solutions:

[0006] In a first aspect, an AC-DC multi-port flexible interconnection device based on MMC includes:

[0007] One DC port, n AC ports, n≥2; a four-leg MMC, the AC end of the four-leg MMC is connected to any AC port, and the DC end of the four-leg modular multilevel converter is connected to the DC port; a CHB, bridging any two AC ports; a three-phase resonant module is connected in series on the AC port, and the three-phase resonant module is used for medium-frequency resonance.

[0008] Further, the three-phase resonant module is connected in a star shape, and the midpoint lead of the fourth leg of the four-leg MMC is connected to the midpoints of multiple groups of three-phase resonant modules.

[0009] Further, the sub - modules of the four - leg MMC include half - bridge type SMs and / or full - bridge type SMs.

[0010] Further, the three - phase resonant module is a capacitor, an inductor, or / and a resistor element with medium - frequency resonance function, or an active module with corresponding functions.

[0011] Further, a CHB is connected in series between one end of the four - leg MMC and the AC port.

[0012] Further, multiple cross - connected CHBs are connected end to end, and multiple groups of three - phase resonant modules can be combined into a single group of three - phase resonant modules.

[0013] In a second aspect, a control method for a multi - port flexible interconnection device includes the multi - port flexible interconnection device of the first aspect, and further includes:

[0014] The control strategy of the four - leg MMC, which includes: AC port power flow control, SM capacitor voltage balancing control, and mixed - frequency modulation strategy;

[0015] The basic control strategy of the four - leg MMC is mainly responsible for ensuring that the average value of the DC capacitor voltage of the SM is stable at the rated value, and at the same time controlling the power factor of the corresponding AC port. The SM capacitor voltage balancing control strategy ensures that the voltage of each SM capacitor is stable at the rated value. The modulation strategy uses mixed - frequency modulation, that is, the modulation signal contains both power - frequency modulation components and medium - frequency modulation components. The medium - frequency modulation signal of the four - leg MMC is given by open - loop.

[0016] The control strategy of the CHB is divided into two parts: power - frequency control and medium - frequency control. The power - frequency control adopts a constant - power control strategy to control the active power and reactive power of the power grid at the corresponding AC port; the medium - frequency control adopts a constant DC voltage control strategy to ensure the constancy of the CHB DC capacitor voltage; a mixed - frequency modulation strategy is adopted, and the fundamental - wave modulation signal of the mixed - frequency modulation strategy is generated by power - frequency control, and the medium - frequency signal is generated by medium - frequency control.

[0017] Further, the resonant frequency of the resonant module is set to the medium - frequency signal frequency of the medium - frequency control.

[0018] Further, the medium - frequency signal of the medium - frequency control is a three - phase zero - sequence modulation signal, and its signal form can be a sine signal or a square - wave signal, etc.

[0019] In a third aspect, a distribution network system includes a converter, and the converter includes the multi - port flexible interconnection device of the first aspect.

[0020] Advantages of the present invention:

[0021] 1. The AC - DC multi - port flexible interconnection device based on MMC proposed by the present invention can provide a DC port and multiple AC ports simultaneously, and it is convenient to expand the number of AC ports.

[0022] 2. The AC-DC multi-port flexible interconnection device based on MMC proposed by the present invention requires fewer switching devices and passive components compared with the existing solutions. Therefore, it has the advantages of lower cost, smaller volume, higher power density, etc. Description of the Drawings

[0023] Figure 1 It is the topology structure of the three-phase flexible multi-port converter of the present invention;

[0024] Figure 2 It is the derivative topology structure of the three-phase flexible multi-port converter of the present invention;

[0025] Figure 3 It is the improved topology for reducing the harmonic of the zero-sequence voltage in the grid side of the present invention;

[0026] Figure 4 It is the various optional structures of the three-phase resonant module of the present invention;

[0027] Figure 5 It is the steady-state working vector diagram of the single-phase flexible multi-port converter of the present invention;

[0028] Figure 6 It is the control strategy of the four-leg MMC;

[0029] Figure 7 It is the control strategy of the cross-connected CHB;

[0030] Figure 8 It is the waveform diagram of the grid voltage and current of the standardized AC port 1;

[0031] Figure 9 It is the waveform diagram of the grid voltage and current of the standardized AC port 2;

[0032] Figure 10 It is the waveform diagram of the grid voltage and current of the standardized AC port 3;

[0033] Figure 11 It is the waveform diagram of the standardized DC bus voltage;

[0034] Figure 12 It is the waveform diagram of the DC capacitor voltage of the upper and lower bridge arms of phase A of the MMC;

[0035] Figure 13 It is the waveform diagram of the DC capacitor voltage of the upper and lower bridge arms of phase D of the MMC;

[0036] Figure 14 It is the waveform diagram of the DC capacitor voltage of the CHB on the side of the standardized AC port 1;

[0037] Figure 15It is the waveform diagram of the DC capacitor voltage of the CHB connected across AC port 1 and AC port 2 after per-unit conversion;

[0038] Figure 16 It is the waveform diagram of the DC capacitor voltage of the CHB connected across AC port 1 and AC port 3 after per-unit conversion;

[0039] Figure 17 It is the waveform diagram of the high-frequency current on the primary side of the high-frequency transformer;

[0040] Figure 18 It is the waveform diagram of the medium-frequency zero-sequence circulating current of the resonant LC on the AC port 2 side;

[0041] Figure 19 It is the waveform diagram of the medium-frequency zero-sequence circulating current of the resonant LC on the AC port 3 side. Specific embodiments

[0042] Next, the technical solutions in the embodiments of the present invention will be clearly and completely described in conjunction with the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without making creative efforts belong to the scope of protection of the present invention.

[0043] In some embodiments, the present application provides a flexible AC-DC multi-port interconnection device based on an MMC. The three-phase topological structures are respectively as Figure 1 shown. The topology includes a four-leg MMC (four-leg modular multilevel converter), multiple cascaded H-bridges (CHBs) connected across different ports, and multiple groups of three-phase resonant modules (such as resonant LC).

[0044] The four-leg MMC is connected to AC port 1 and a DC bus is led out on the DC side. The topology can provide multiple AC ports. A CHB is connected across AC port 1 and AC port 2 to adjust the power flow of AC port 2. A CHB is connected across AC port 1 and AC port 3 to adjust the power flow of AC port 3. Similarly, a CHB is connected across AC port 1 and AC port N to adjust the power flow of AC port N. By analogy with this connection method, multiple AC ports can be expanded according to requirements.

[0045] In addition to the A, B, and C phase legs of the traditional MMC, the four-leg MMC topology also has an additional fourth leg, and the structure of this fourth leg is exactly the same as that of the other three legs. The fourth leg can be used to lead out the neutral line.

[0046] The basic control strategy of the four - leg MMC is mainly responsible for ensuring that the average value of the DC capacitor voltage of the SM is stabilized at the rated value, while controlling the power factor of the corresponding AC port. The SM capacitor voltage balancing control strategy ensures that the voltage of each SM capacitor is stabilized at the rated value. The modulation strategy adopts mixed - frequency modulation, that is, the modulation signal contains both power - frequency modulation components and intermediate - frequency modulation components. The intermediate - frequency modulation signal of the four - leg MMC is given by open - loop.

[0047] Both the four - leg MMC of the topology and the bridged CHB adopt mixed - frequency modulation, and intermediate - frequency three - phase zero - sequence modulation signals are superimposed on the power - frequency modulation signals. A corresponding set of resonant LC is set at each of the AC ports from port 2 to port N, and its resonant frequency is the same as the frequency of the intermediate - frequency modulation signal in the mixed - frequency modulation. Each set of three - phase resonant LC is connected in star, and a lead is drawn from the mid - point to the mid - point of the fourth leg of the MMC to provide a path for the intermediate - frequency zero - sequence circulating current. Since there is no zero - sequence current in a three - phase three - wire system, the mixed - frequency modulation will not affect the feeder currents of each AC port, and the intermediate - frequency zero - sequence circulating current only flows inside the device through the resonant LC.

[0048] Figure 2 It is a topological structure derived from the above - mentioned topology. Multiple bridged CHBs are connected end to end, and multiple sets of three - phase resonant LC are combined into a single set of three - phase resonant LC, which can further reduce the volume and weight of the equipment.

[0049] Figure 3 It is an improved topology for reducing the intermediate - frequency zero - sequence voltage harmonics on the grid side. A set of three - phase CHB is connected in series at AC port 1, and the open - loop control of the AC port of this CHB outputs an intermediate - frequency zero - sequence voltage with the same amplitude and opposite phase as the intermediate - frequency zero - sequence voltage generated by the device, which can cancel the zero - sequence voltage harmonics introduced by the original device at the AC port.

[0050] Figure 4 (a)-(g) are various optional structures of the three - phase resonant module. In addition to the several structures listed, any series - parallel combination of capacitive, inductive, and resistive elements with intermediate - frequency resonance functions or active modules can be used as the optional structure of the three - phase resonant module of the topology.

[0051] Figure 5 It is the steady - state operating voltage - current vector diagram of the single - phase AC - DC multi - port flexible interconnection device. Taking the application scenario including two AC ports, AC port 1 and AC port 2, as an example, the left figure is the vector diagram of the power flow from the AC grid into AC port 2, and the right figure is the vector diagram of the device feeding back power to the grid through AC port 2. Taking the vector diagram of the power flow from the AC grid into AC port 2 in the left figure as an example, where θ is the voltage phase difference between AC port 2 and 1, U A and U U are the grid voltages of AC port 1 and 2 respectively, and U CHBis the voltage of the CHB across the AC ports 1 and 2, i CHB is the power frequency component of the current flowing through the CHB across the AC ports 1 and 2, i A and i U are the grid currents of the AC ports 1 and 2 respectively, i LC is the power frequency component of the current flowing through the corresponding resonant LC of the AC port 2.

[0052] Figure 6 is the control strategy of the four - leg MMC. The control strategies of the A, B, and C phase legs of the MMC are the same as those of the conventional MMC. It includes the AC port power flow control strategy, the SM capacitor voltage control strategy, and the mixed - frequency modulation strategy.

[0053] In the case of this application, the four - leg MMC uses a PI controller in the dq coordinate system and adopts a double - closed - loop control strategy. The outer loop is the SM DC capacitor voltage control loop. By comparing the average value of the SM capacitor voltage with the reference value, a current reference signal is generated through the controller. The inner loop is the current loop. The controller tracks the control output of the voltage outer loop and controls the power factor of the corresponding AC port. Finally, the basic power frequency sine modulation signals of the upper and lower bridge arms of the three - phase are obtained.

[0054] The SM capacitor voltage balancing algorithm is responsible for ensuring the balance of all SM capacitor voltages. By sorting all SM voltages and determining according to the current direction at this time whether each SM needs to be inserted or removed. When the current flows into the SM, the SM with a lower voltage is inserted and the SM with a higher voltage is removed; when the current flows out of the SM, the SM with a higher voltage is inserted and the SM with a lower voltage is removed.

[0055] The AC power frequency modulation signal at the mid - point of the fourth leg of the MMC is 0, that is, the voltage at the mid - point of the fourth leg always remains at 0V. Only the SM capacitor voltage balance control function is required to ensure the balance of all SM capacitor voltages of the fourth leg.

[0056] The modulation strategy adopts mixed - frequency modulation, that is, the modulation signal contains both power frequency modulation components and intermediate frequency modulation components. The intermediate - frequency zero - sequence modulation signal is open - loop superimposed on the power frequency sine modulation signal output by the current loop of the MMC controller.

[0057] Figure 7The control strategy for the CHB across different ports is divided into two parts: power frequency control and intermediate frequency control. The power frequency control adopts constant power control, that is, a single current loop is used to control the corresponding grid-side current. The intermediate frequency control adopts fixed DC voltage control. The outer loop is a DC voltage loop. By comparing the average value of the DC voltage with the set value, the amplitude command value of the intermediate frequency circulating current is generated through the controller. The amplitude command value is multiplied by the unit intermediate frequency AC signal to obtain the command value of the intermediate frequency circulating current. The current loop is the intermediate frequency circulating current loop, which is responsible for controlling the phase difference between the intermediate frequency circulating current and the intermediate frequency zero-sequence voltage of the four-arm MMC to be in the same or opposite direction, so as to realize the maximum power to charge or discharge the DC capacitor to ensure the stability of the DC capacitor voltage.

[0058] To verify the effectiveness of this topology and its control method, the topology and control method of this multi-port converter are simulated and verified. The simulation parameters are shown in Table 1.

[0059] Table 1 Simulation parameters of the compact AC-DC multi-port flexible interconnection device

[0060]

[0061]

[0062] According to the parameters in Table 1, the multi-port converter based on MMC is simulated and verified. The simulation results are as Figures 8 to 19 shown.

[0063] Figure 8 and 9 10 are the waveform diagrams of the grid voltage and current of the AC ports after per-unit conversion. Among them, for AC ports 1 and 3, the current flows into the grid, so the voltage and current are in the same phase. While for AC port 2, the current flows into the device, so the voltage and current are in the opposite direction.

[0064] Figure 11 is the DC bus voltage, and the DC voltage is stable at the rated value of 20 kV.

[0065] Figure 12 is the waveform diagram of the DC capacitor voltages of the upper and lower bridge arms of phase A of the MMC. The capacitor voltages are stable and fluctuate near the rated value, and the capacitor voltages are balanced.

[0066] Figure 13 is the waveform diagram of the DC capacitor voltages of the upper and lower bridge arms of phase D of the MMC. The capacitor voltages are balanced with each other and have small fluctuations.

[0067] Figure 14 is the waveform diagram of the currents of the upper and lower bridge arms of phase A of the MMC. In addition to the fundamental current component and the second-harmonic circulating current component, the main components of this current also include the intermediate frequency zero-sequence current component.

[0068] Figure 15This is the D-phase upper and lower bridge arm SM current waveform of the MMC. This current component has no fundamental current component and only has an intermediate frequency zero-sequence current component.

[0069] Figure 16 , 17 They are the DC capacitor voltages of CHB connected across ports 1 and 2 and CHB connected across ports 1 and 3. The voltages are both stable at the rated values, and the voltage balancing effect is good.

[0070] Figure 18 and 19 is the current of the resonant LC corresponding to AC port 2 and AC port 3. Since the resonant frequency is 500 Hz, its power frequency current component is very small, and the main current component is the 500 Hz medium frequency zero sequence current component.

[0071] According to the above simulation waveforms, the power of each AC and DC port of the device is controlled to the rated value, and the voltage of each SM DC capacitor is stabilized at the rated value, achieving the control purpose and expected effect, verifying the effectiveness and feasibility of the MMC-based AC / DC multi-port flexible interconnection device and control strategy.

[0072] The technical means disclosed in the scheme of the present invention are not limited to the technical means disclosed in the above-mentioned implementation mode, but also include technical schemes composed of any combination of the above-mentioned technical features. It should be pointed out that for ordinary technicians in this technical field, several improvements and modifications can be made without departing from the principle of the present invention, and these improvements and modifications are also regarded as the protection scope of the present invention.

[0073] In the description of this specification, the description with reference to the terms "one embodiment", "example", "specific example", etc. means that the specific features, structures, materials or characteristics described in conjunction with the embodiment or example are included in at least one embodiment or example of the present invention. In this specification, the schematic representation of the above terms does not necessarily refer to the same embodiment or example. Moreover, the specific features, structures, materials or characteristics described can be combined in any one or more embodiments or examples in a suitable manner.

[0074] The above shows and describes the basic principles, main features and advantages of the present invention. Those skilled in the art should understand that the present invention is not limited to the above embodiments, and the above embodiments and descriptions are only for explaining the principles of the present invention. Without departing from the spirit and scope of the present invention, the present invention may have various changes and improvements, and these changes and improvements all fall within the scope of the present invention to be protected.

Claims

1. Based on the MMC AC-DC multi-port flexible interconnection device, including: One DC port, n AC ports, n≥2; characterized in that A four-arm MMC, the AC side of the four-arm MMC is connected to any AC port, and the DC side of the four-arm modular multilevel converter is connected to the DC port; CHB, bridging any two AC ports; A three-phase resonant module is connected in series on the AC port, and the three-phase resonant module is used for medium-frequency resonance; The three-phase resonant module is connected in a star shape, and the midpoint lead of the fourth arm of the four-arm MMC is connected to the midpoints of multiple groups of three-phase resonant modules; The sub-modules of the four-arm MMC include half-bridge type SM and / or full-bridge type SM; The three-phase resonant module is a capacitor, inductor or / and resistor element with medium-frequency resonance function, or an active module with medium-frequency resonance function; A CHB is connected in series between one end of the four-arm MMC and the AC port; Multiple bridged CHBs are connected end to end, and multiple groups of three-phase resonant modules can be combined into a single group of three-phase resonant modules.

2. A control method for a multi-port flexible interconnection device, characterized in that, Applied to the multi-port flexible interconnection device of claim 1, further including: The control strategy of the four-arm MMC, which includes: the basic control strategy of the four-arm MMC, the SM capacitor voltage balance control strategy and the mixed-frequency modulation strategy; The basic control strategy of the four-arm MMC is responsible for ensuring that the average value of the DC capacitor voltage of the SM is stable at the rated value, and at the same time controlling the power factor of the corresponding AC port; the SM capacitor voltage balance control strategy ensures that the capacitor voltage of each SM is stable at the rated value; the modulation strategy adopts mixed-frequency modulation, that is, the modulation signal contains both power-frequency modulation components and medium-frequency modulation components; the medium-frequency modulation signal of the four-arm MMC is given by open loop; The control strategy of the CHB, its control strategy is divided into two parts: power-frequency control and medium-frequency control. The power-frequency control adopts a constant power control strategy to control the active power and reactive power of the corresponding AC port grid; the medium-frequency control adopts a constant DC voltage control strategy to ensure that the DC capacitor voltage of the CHB is constant; the mixed-frequency modulation strategy is adopted, and the fundamental wave modulation signal of the mixed-frequency modulation strategy is generated by power-frequency control, and the medium-frequency signal is generated by medium-frequency control.

3. The control method according to claim 2, wherein The resonant frequency of the resonant module is set to the medium-frequency signal frequency of the medium-frequency control.

4. The control method according to claim 3, characterized in that The medium-frequency signal of the medium-frequency control is a three-phase zero-sequence modulation signal, and its modulation signal waveform is a sine signal or a square wave signal.

5. A distribution network system, characterized in that, Including a converter, the converter includes the multi-port flexible interconnection device of claim 1.

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