Three-phase series voltage-sharing control method for a novel ac / dc converter

Abstract

The application discloses a three-phase series voltage balance control method of a novel AC / DC converter, which is applied to a modular multilevel converter, the converter comprising three single-phase power units in series connection, a transformer and a DC filter, each single-phase power unit being composed of a cascade module chain and a commutation bridge, the cascade module chain comprising a plurality of power sub-modules in series connection, and each sub-module containing a sub-module capacitor; the method comprises the following steps: detecting the average voltage of three-phase sub-modules in real time, triggering the voltage balance control when detecting the imbalance, controlling the output voltage of the cascade module chain to contain harmonic voltage, the frequency of the harmonic voltage being equal to the series resonance frequency of the DC valve side filter inductance and the DC filter capacitor, adjusting the phase of three-phase harmonic voltage, so that the harmonic voltage of the phase with lower voltage and the harmonic current act on the sub-modules to charge the sub-modules, and the harmonic voltage of the phase with higher voltage acts on the sub-modules to discharge the sub-modules, thereby realizing voltage balance, and the control is exited when the three-phase voltage is balanced and the three-phase harmonic voltage is all zero; and the application can realize the consistency of the average voltage of three-phase sub-module capacitors.

Description

Technical Field

[0001] This invention belongs to the field of power electronics and flexible DC transmission technology, and particularly relates to a novel three-phase series voltage equalization control method for AC / DC converters. Background Technology

[0002] Modular multilevel converters (MMCs) are currently the mainstream topology for flexible DC transmission, but they suffer from the problem of large capacitor usage, resulting in large converter size, weight, and high cost. Modular commutator converters (MCCs) are a new type of AC / DC converter with significant advantages, including a substantial reduction in submodule capacitor usage, resulting in lightweight and low-cost characteristics. However, the unique structure of MCCs presents new challenges: under three-phase unbalanced grid conditions, the active power interacting with the AC grid in the three-phase series units of an MCC is inconsistent, leading to voltage imbalance in the submodule capacitors of the three-phase series power units. Compared to MCCs, MMCs, with their three-phase converter units connected in parallel on the DC side, naturally achieve self-balancing of the three-phase submodule voltages. MCCs, with their three-phase series connection, require active control to achieve three-phase balance. Existing research lacks studies on three-phase balancing strategies for MCCs. Therefore, this patent proposes a three-phase series balancing control suitable for MCCs to address the aforementioned problems. Summary of the Invention

[0003] To address the problems mentioned in the background section, this invention discloses a novel three-phase series voltage equalization control method for AC / DC converters, which can achieve consistency in the average voltage of the three-phase sub-module capacitors. To achieve the above objective, the technical solution adopted by this invention is as follows: A novel three-phase series voltage equalization control method for an AC / DC converter is characterized by its application to a modular multilevel converter. The modular multilevel converter comprises three series-connected single-phase power units, a transformer, and a DC filter. Each single-phase power unit consists of a cascaded module chain and a commutation bridge connected in series on the DC side. The cascaded module chain includes multiple series-connected power sub-modules, each containing a sub-module capacitor. The method includes the following steps: S1, real-time detection of the average voltage of the three-phase sub-module; when an imbalance in the voltage of the three-phase sub-module is detected, three-phase voltage equalization control is triggered. S2, control the output voltage of the cascaded module chain so that its output voltage includes harmonic voltage, and the frequency of the harmonic voltage is equal to the series resonant frequency of the DC valve-side filter inductor and the DC filter capacitor. S3 adjusts the phase of the three-phase harmonic voltage so that the phase with the lower average voltage of the submodule generates power from the interaction of its output harmonic voltage and harmonic current to charge the submodule, and the phase with the higher average voltage of the submodule generates power from the interaction of its output harmonic voltage and harmonic current to discharge the submodule, thereby achieving a balance of the three-phase submodule voltage. S4 detects the average voltage of the three-phase sub-modules in real time. When the voltage of the three-phase sub-modules is balanced and the three-phase harmonic voltages output by the cascaded module chain are all zero, the three-phase voltage equalization control is exited.

[0004] Furthermore, the DC filter includes a DC valve-side filter inductor, a DC grid-side filter inductor, and a DC filter capacitor.

[0005] Furthermore, the DC grid-side filter inductor is used to block harmonics, and the DC valve-side filter inductor and DC filter capacitor resonate to form a low-impedance path.

[0006] Furthermore, the requirement for determining the voltage imbalance of the three-phase sub-modules in step S1 is: to detect the average voltage of the three-phase sub-modules, and to determine the voltage imbalance of the three-phase sub-modules when the average voltages of the three-phase sub-modules are inconsistent.

[0007] Furthermore, in step S3, the three-phase harmonic voltages are connected in series, and their vector sum is the composite harmonic voltage. The composite harmonic voltage acts on the stray resistance of the DC-side harmonic equivalent circuit to generate harmonic current.

[0008] Furthermore, the stray resistance includes copper busbar resistance, power semiconductor device resistance, and filter equivalent resistance.

[0009] Furthermore, in step S3, the phase of the harmonic voltage is adjusted to change the angle between the output harmonic voltage and the harmonic current of each phase, thereby controlling the charging and discharging power of each phase submodule.

[0010] This invention also provides a novel three-phase series voltage equalization control device for an AC / DC converter, characterized in that it employs the aforementioned novel three-phase series voltage equalization control method for an AC / DC converter, and the device comprises: The voltage detection unit is used to detect the average voltage of the three-phase submodule in real time, and trigger the three-phase voltage equalization control when an imbalance in the voltage of the three-phase submodule is detected. The harmonic injection unit is used to control the output voltage of the cascaded module chain so that the output voltage includes harmonic voltage, and the frequency of the harmonic voltage is equal to the series resonant frequency of the DC valve-side filter inductor and the DC filter capacitor. The phase adjustment unit is used to adjust the phase of the three-phase harmonic voltage so that the phase with the lower average voltage of the submodule uses the power generated by the interaction of the output harmonic voltage and harmonic current to charge the submodule, and the phase with the higher average voltage of the submodule uses the power generated by the interaction of the output harmonic voltage and harmonic current to discharge the submodule, thereby achieving the balance of the three-phase submodule voltage. The control unit exits and is used to monitor the average voltage of the three-phase sub-modules in real time. When the voltage of the three-phase sub-modules is detected to be balanced and the three-phase harmonic voltages output by the cascaded control module chain are all zero, the three-phase voltage equalization control exits.

[0011] The present invention has the following beneficial effects: This patent enables the MCC to balance the voltage of the three-phase submodule capacitors when the three-phase grid is unbalanced, without affecting the output voltage of the DC port and AC port. Attached Figure Description

[0012] Figure 1 This is a flowchart of the three-phase series voltage equalization control method for the novel AC / DC converter described in a specific embodiment.

[0013] Figure 2 This is a topology diagram of the three-phase series connection of the novel AC / DC converter described in a specific embodiment.

[0014] Figure 3 The equivalent topology diagram of the three-phase series connection of the novel AC / DC converter described in a specific embodiment is shown.

[0015] Figure 4 This is a schematic diagram of the DC-side harmonic equivalent circuit of the novel AC / DC converter described in a specific embodiment.

[0016] Figure 5 This is a schematic diagram of the harmonic voltage phase relationship in a specific embodiment. Detailed Implementation

[0017] To facilitate understanding by those skilled in the art, the present invention will be further described below in conjunction with embodiments and accompanying drawings.

[0018] This embodiment provides a novel three-phase series voltage equalization control method for an AC / DC converter. The controlled object of this invention is as follows: Figure 2As shown, the MCC converter, or Modular Multilevel Converter, is a new type of AC / DC converter. The MCC converter includes three series-connected single-phase power units, a transformer, and a DC filter. The DC filter includes a DC valve-side filter inductor, a DC grid-side filter inductor, and a DC filter capacitor. Each single-phase power unit consists of a cascaded module chain and a commutation bridge. The cascaded module chain includes multiple series-connected power sub-modules, each containing a sub-module capacitor. The goal of the control strategy in this embodiment is to achieve consistency in the average voltage of the three-phase sub-module capacitors. The cascaded module chain can output arbitrary waveform voltages through the switching of half-bridge sub-modules; therefore, the cascaded module chain can be equivalent to a controlled voltage source. See details... Figure 2 For further information, please refer to [link / reference]. Figure 3 Furthermore, considering the controlled voltage source and DC filter circuit separately, we can obtain the following: Figure 3 The DC-side harmonic equivalent circuit is shown. Figure 3 In this configuration, the DC valve-side filter inductor and DC filter capacitor resonate, providing a low-impedance path near the resonant frequency. Therefore, the voltage output of the controlled voltage source near the resonant frequency can be controlled, generating harmonic currents in the controlled voltage source-DC valve-side filter inductor-DC filter capacitor path, thereby achieving power exchange in the three-phase cascaded module chain. Simultaneously, the DC grid-side filter inductor can block harmonics. (See also...) Figure 1 The method in this embodiment includes the following steps: S1, Three-phase submodule voltage imbalance detected, specifically including:

[0019] The control system monitors the average voltage of the three-phase sub-modules in real time. When it detects an imbalance in the sub-module voltages, it triggers three-phase voltage equalization control.

[0020] S2 controls the generation of harmonic voltages in the cascaded module chain, specifically including: The output voltages Usa, Usb, and Usc of the cascaded control module chain are controlled to include harmonic voltages Usha, Ushb, and Ushc, and the frequency of the harmonic voltages is equal to the series resonant frequency of the DC valve-side filter inductor and the DC filter capacitor.

[0021] S3, adjusts the phase of harmonic voltage, specifically including: The three-phase harmonic voltages are connected in series, and their vector sum is equal to Ush = Usha + Ushb + Ushc. The synthesized harmonic voltage Ush acts on the stray resistance R of the harmonic equivalent circuit on the DC side of the MCC, generating a harmonic current Ish = Ush / R. The stray resistance R includes the resistance of the copper busbar, the resistance of the power semiconductor device, and the equivalent resistance of the filter. Without changing Ush, adjust Usha, Ushb, and Ushc respectively, so that for a certain phase with a lower average voltage of the sub-module, the output harmonic voltage Usha or Ushb or Ushc and the power Usha×Ish or Ushb×Ish or Ushc×Ish generated by the action of the harmonic current Ish charge the sub-module. Conversely, for a certain phase with a lower average voltage of the sub-module, the power generated by the action of its output harmonic voltage and the harmonic current discharges the sub-module. Thus, the balance of the three-phase voltages of the sub-module is achieved.

[0022] S4. It is detected that the three-phase sub-module voltages are balanced and the harmonic voltage is zero, specifically including: The control system continuously detects the average voltages of the three-phase sub-modules. When it is found that the sub-module voltages are already balanced, and the three-phase harmonic voltages Usha = 0, Ushb = 0, and Ushc = 0 output by the control cascade module chain of the control system, the three-phase voltage equalization control is exited.

[0023] Refer to Figure 5 , the DC filter capacitor is 40 μF, the valve-side DC filter inductor is 10 mH, and the grid-side DC filter inductor is 2 mH. Then the resonant frequency can be calculated to be approximately 177.9 Hz. Control the frequencies of Usha, Ushb, and Ushc to be all 177.9 Hz. For example: when the magnitude relationship among the three-phase average sub-module voltages Usma, Usmb, Usmc, and the average value Usm of the three-phase sub-module voltages is Usma < Usm < Usmc < Usmb, control the phase relationship of the harmonic voltages to satisfy Figure 5 . Figure 5 In, the included angle between the synthesized harmonic voltage vector Ush and Ushb and Ushc is an acute angle, and the included angle between Ush and Ushb is less than the included angle between Ush and Ushc. Therefore, under the action of the loop resistance, the power generated by the action of the harmonic current Ish and Ushb is larger and discharges the capacitor of the B-phase sub-module, and the power generated by the action of the harmonic current Ish and Ushc is smaller and discharges the capacitor of the C-phase sub-module; the included angle between the synthesized harmonic voltage vector Ush and Usha is an obtuse angle. Therefore, under the action of the loop resistance, the power generated by the action of the harmonic current Ish and Usha charges the capacitor of the A-phase sub-module. To sum up, the voltage of the A-phase sub-module gradually increases, the voltages of the B- and C-phase sub-modules gradually decrease, and the voltage reduction speed of the B-phase sub-module is faster than that of the C-phase, and thus the balance of the three-phase sub-module voltages is finally achieved.

[0024] The above embodiments are merely illustrative of the technical concept of the present invention and should not be construed as limiting the scope of protection of the present invention. Any modifications made to the technical solution based on the technical concept proposed in this invention shall fall within the scope of protection of this invention.

Claims

1. A novel three-phase series voltage equalization control method for an AC / DC converter, characterized in that, This method is applied to a modular multilevel converter, which includes three single-phase power units connected in series, a transformer, and a DC filter. Each single-phase power unit consists of a cascaded module chain and a commutation bridge connected in series on the DC side. The cascaded module chain includes multiple power sub-modules connected in series, and each power sub-module contains a sub-module capacitor. The method includes the following steps: S1, real-time detection of the average voltage of the three-phase sub-module; when an imbalance in the voltage of the three-phase sub-module is detected, three-phase voltage equalization control is triggered. S2, control the output voltage of the cascaded module chain so that its output voltage includes harmonic voltage, and the frequency of the harmonic voltage is equal to the series resonant frequency of the DC valve-side filter inductor and the DC filter capacitor. S3 adjusts the phase of the three-phase harmonic voltage so that the phase with the lower average voltage of the submodule generates power from the interaction of its output harmonic voltage and harmonic current to charge the submodule, and the phase with the higher average voltage of the submodule generates power from the interaction of its output harmonic voltage and harmonic current to discharge the submodule, thereby achieving a balance of the three-phase submodule voltage. S4 detects the average voltage of the three-phase sub-modules in real time. When the voltage of the three-phase sub-modules is balanced and the three-phase harmonic voltages output by the cascaded module chain are all zero, the three-phase voltage equalization control is exited.

2. The three-phase series voltage equalization control method for the novel AC / DC converter according to claim 1, characterized in that, The DC filter includes a DC valve-side filter inductor, a DC grid-side filter inductor, and a DC filter capacitor.

3. The three-phase series voltage equalization control method for the novel AC / DC converter according to claim 2, characterized in that, The DC grid-side filter inductor is used to block harmonics, and the DC valve-side filter inductor and DC filter capacitor resonate to form a low-impedance path.

4. The three-phase series voltage equalization control method for the novel AC / DC converter according to claim 1, characterized in that, The requirement for determining the voltage imbalance of the three-phase submodules in step S1 is: detect the average voltage of the three-phase submodules, and when the average voltages of the three-phase submodules are inconsistent, it is determined that the voltage of the three-phase submodules is unbalanced.

5. The three-phase series voltage equalization control method for the novel AC / DC converter according to claim 1, characterized in that, In step S3, the three-phase harmonic voltages are connected in series, and their vector sum is the composite harmonic voltage. The composite harmonic voltage acts on the stray resistance of the DC side harmonic equivalent circuit, generating harmonic current.

6. The three-phase series voltage equalization control method for the novel AC / DC converter according to claim 5, characterized in that, The stray resistance includes copper busbar resistance, power semiconductor device resistance, and filter equivalent resistance.

7. The three-phase series voltage equalization control method for the novel AC / DC converter according to claim 1, characterized in that, In step S3, the phase of the harmonic voltage is adjusted to change the angle between the output harmonic voltage and the harmonic current of each phase, thereby controlling the charging and discharging power of each phase submodule.

8. A novel three-phase series voltage equalization control device for an AC / DC converter, characterized in that, The device employs a three-phase series voltage equalization control method for a novel AC / DC converter as described in any one of claims 1-7, the device comprising: The voltage detection unit is used to detect the average voltage of the three-phase submodule in real time, and trigger the three-phase voltage equalization control when an imbalance in the voltage of the three-phase submodule is detected. The harmonic injection unit is used to control the output voltage of the cascaded module chain so that the output voltage includes harmonic voltage, and the frequency of the harmonic voltage is equal to the series resonant frequency of the DC valve-side filter inductor and the DC filter capacitor. The phase adjustment unit is used to adjust the phase of the three-phase harmonic voltage so that the phase with the lower average voltage of the submodule uses the power generated by the interaction of the output harmonic voltage and harmonic current to charge the submodule, and the phase with the higher average voltage of the submodule uses the power generated by the interaction of the output harmonic voltage and harmonic current to discharge the submodule, thereby achieving the balance of the three-phase submodule voltage. The control unit exits and is used to monitor the average voltage of the three-phase sub-modules in real time. When the voltage of the three-phase sub-modules is detected to be balanced and the three-phase harmonic voltages output by the cascaded control module chain are all zero, the three-phase voltage equalization control exits.

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