Modular multilevel converter dual-port network configuration control method

Abstract

The application discloses a kind of modular multilevel converter double-port network configuration control method belonging to the technical field of converter control.It includes the following steps: obtaining MMC AC output voltage additional amount Δu ac,d / q , and then calculating the reference value u ac,ref,d / q of MMC AC side output voltage;Obtaining MMC DC output voltage additional amount Δu dc , and then calculating the reference value u dc,ref of MMC DC side output voltage;u ac,ref,d / q And u dc,ref Respectively sent to AC side and DC side voltage and current double closed loop control link, respectively obtain MMC modulation signal differential mode amount u diff,d / q And common mode amount u sum,j At the output end of two links;According to u diff,d / q And u sum,j , using bridge arm output voltage reference value calculation method to obtain the expected value of MMC bridge arm capacitor module output voltage, and then generate the trigger pulse signal of all switching devices in MMC through modulation part.The application is simple to operate, low in computing resource requirement, high in control algorithm implementability and strong in engineering applicability.

Description

Technical Field

[0001] This invention relates to the field of converter control technology, and in particular to a two-port network control method for a modular multilevel converter. Background Technology

[0002] With the continuous growth of my country's new energy installed capacity, the demand for flexible DC transmission technology in large-capacity power transmission is constantly increasing. Among them, modular multilevel converters (MMCs) have become the mainstream choice due to their flexible structure and superior performance. In flexible DC transmission systems, the AC and DC systems typically present a segmented structure, meaning that multiple AC subgrids are interconnected through DC links. Therefore, MMC converter control methods are needed to ensure the safe and stable operation of the system.

[0003] Existing control methods for MMCs can be broadly categorized into active-reactive power (PQ) control, DC voltage-reactive power (Udc-Q) control, and voltage-frequency (Vf) control. However, regardless of the control method employed, the AC and DC sides are incompatible in terms of grid connection capabilities. Specifically, when PQ control is used on the AC side, the MMC DC voltage needs to be provided by an external DC network. When Udc-Q control is used, the AC side of the converter essentially operates in grid-connected mode, requiring a stable common coupling point voltage from the AC network. When Vf control is used, the converter can provide AC side voltage support, but the DC side lacks grid connection capability.

[0004] Current single-port grid-connected converter control methods impose prerequisites on the operational characteristics of the deployed power grid, namely, the non-grid-connected port of the converter must be connected to an equivalent voltage source with voltage support capability. In fact, this requires coordinated control modes among single-port grid-connected converters. For example, in a flexible DC transmission system, if the sending-end AC side operates under Vf control, the receiving-end converter must be responsible for stabilizing the DC bus voltage, making it exhibit voltage source characteristics for the sending-end system. When the network operational characteristics need to change, such as from a voltage source to a current source, the MMC converter must undergo changes in its operating control mode and parameters through upper-level dispatching to ensure system stability. This results in inflexible converter control and poor adaptability to AC / DC grid operation. Currently, very little research has been conducted on MMC dual-port grid-connected operation control methods, but these lack the ability to align the dq rotating coordinate system, making it impossible to achieve decoupled control of active and reactive power without a phase-locked loop; they also lack the ability to control the energy of the MMC bridge arm capacitors, failing to ensure stable system operation under three different operating conditions. Therefore, a modular multilevel converter dual-port network control method is needed to solve the above problems. Summary of the Invention

[0005] The purpose of this invention is to propose a two-port network control method for a modular multilevel converter, comprising the following steps:

[0006] Using the energy control method and reactive power control method of the AC side of the MMC bridge arm capacitor, the additional AC output voltage Δu of the MMC is obtained. ac,d / q Then, the reference value u of the MMC AC side output voltage is calculated. ac,ref,d / q ;

[0007] The DC output voltage increment Δu of the MMC is obtained using the DC-side energy control method of the MMC bridge arm capacitor. dc Then, the reference value u of the DC-side output voltage of the MMC can be calculated. dc,ref ;

[0008] will u ac,ref,d / q and u dc,ref The voltage and current signals are respectively sent to the AC and DC side dual closed-loop control loops, and the differential modulus u of the MMC modulation signal is obtained at the output of the two loops. diff,d / q and common modulus u sum,j ;

[0009] According to u diff,d / q and u sum,j The expected output voltage of the MMC bridge arm capacitor module is obtained by using the bridge arm output voltage reference value calculation method, and then the trigger pulse signal of all switching devices in the MMC is generated by the modulation section.

[0010] Furthermore, the MMC AC side output voltage reference value u ac,ref,d / q The calculation formula is:

[0011] u ac,ref,d / q =u ac,base,d / q +Δu ac,d / q

[0012] Among them, u ac,base,d / q It is the MMC AC voltage reference value output by the AC side synchronous control loop.

[0013] Furthermore, the MMC DC-side output voltage reference value u dc,ref The calculation formula is:

[0014] u dc,ref =u dc,base +Δu dc

[0015] Among them, u dc,base It is the MMC DC voltage reference value output by the DC-side synchronous control loop.

[0016] Furthermore, the energy control method for the AC side of the MMC bridge arm capacitor is expressed as follows:

[0017] Δu ac,d =G ac,w (s)(W m -Wref )

[0018]

[0019] Where, Δu ac,d It is the additional AC output voltage of the d-axis MMC, G ac,w (s) represents the PI controller transfer function in the AC-side energy control method, W m This is the measured value of the bridge arm capacitor energy, W. ref This is the reference value for the energy of the bridge arm capacitor, u p,j and u n,j These are the total voltage values ​​of the upper and lower bridge arm capacitors of phase j, respectively; C eq It is the total equivalent capacitance of N modules on the bridge arm.

[0020] Furthermore, when the bridge arm capacitor energy measurement value W m Reference value W of bridge arm capacitor energy ref When errors occur, the MMC converter will automatically adjust the active power output on the AC side to stabilize the energy of the bridge arm capacitor.

[0021] Furthermore, the calculation method for the bridge arm output voltage reference value is expressed as follows:

[0022]

[0023] Among them, u diff,j Indicate u diff,d / q The differential modulus of the MMC modulated signal in the three-phase coordinate system obtained after the inverse Park transform, u j,p and u j,n These represent the reference values ​​for the output voltage of the upper and lower bridge arms in phase j, respectively.

[0024] The beneficial effects of this invention are as follows:

[0025] 1. The MMC dual-terminal grid control method proposed in this invention can simultaneously achieve voltage support capabilities on both the AC and DC sides, avoiding the need for external power supply for the voltage of the opposite port in traditional single-port control, and also avoiding the problem of pre-allocation of converter operating modes. Even if either sub-network loses external power support, the proposed MMC control can ensure stable system operation, significantly improving the flexibility of AC / DC sub-network interconnection; at the same time, it can also achieve decoupling control of active and reactive power without phase-locked loop.

[0026] 2. The MMC dual-terminal network control method proposed in this invention has the ability to independently control the energy of the MMC capacitor, which means that the voltage amplitude of the bridge arm capacitor module can be directly controlled, significantly improving the safety of MMC operation and avoiding exceeding the device withstand voltage limit. At the same time, the expanded control freedom also significantly improves the control flexibility of MMC.

[0027] 3. The MMC dual-end network control method proposed in this invention relies entirely on the electrical measurements required by traditional single-end control, without the need for any additional measuring elements and devices, resulting in a high cost advantage.

[0028] 4. The MMC dual-end network control method proposed in this invention is simple and efficient. Compared with the traditional single-end control, it only adds a few PI controllers. It is simple to operate, has low requirements for computing resources, and the control algorithm has high feasibility and strong engineering applicability. Attached Figure Description

[0029] Figure 1 This is a flowchart of the modular multilevel converter dual-port network control method of the present invention;

[0030] Figure 2 This is a topology diagram of an MMC converter;

[0031] Figure 3 This is a block diagram of the dual-terminal network control for an MMC converter.

[0032] Figure 4 A schematic diagram of the phase alignment method for the dq rotating coordinate system;

[0033] Figure 5 The control block diagram for the MMC AC side energy control method and reactive power control method;

[0034] Figure 6 This is a schematic diagram of a voltage vector;

[0035] Figure 7 This is a control block diagram of the MMC DC-side energy control method;

[0036] Figure 8 Three operating conditions for MMC dual-terminal grid converters;

[0037] Figure 9 The waveform of the DC side switching from being connected to a current source to being connected to a voltage source after 5 seconds, with the AC side connected to a voltage source;

[0038] Figure 10 The waveform shows the AC side switching from a current source to a voltage source after 5 seconds, with the DC side connected to a voltage source. Detailed Implementation

[0039] This invention proposes a two-port network control method for a modular multilevel converter. The invention will be further described below with reference to the accompanying drawings and specific embodiments.

[0040] Figure 1 This is a flowchart of the modular multilevel converter two-port network control method of the present invention; as follows: Figure 2 As shown, the topology of the MMC converter is given. Figure 3A block diagram of the two-terminal network control of the MMC converter is given. Where, u g,d / q and i g,d / q It is the voltage measurement value u at the MMC common coupling point (PCC). g,abc and current measurement value i g,abc The voltage and current components along the dq axis in the dq rotating coordinate system; θ is the phase quantity during the transformation between the abc coordinate system and the dq rotating coordinate system; u dc ω0 is the measured DC port voltage of the converter; ω0 is the reference angular frequency output by the AC side synchronization control loop; i sum,j This refers to the DC current of the MMC bridge arm, calculated as follows:

[0041]

[0042] In the formula, and These are the current measurements of the upper and lower arms of one phase of the MMC bridge.

[0043] Traditional AC-side grid control methods include droop control and virtual synchronization control. These AC-side synchronization control loops output the converter PCC point voltage reference value u. ac,base The component u on the dq axis ac,base,d and u ac,base,q ,like Figure 4 As shown, where V is u ac,base Three-phase composite vector. In traditional VF control, the AC side voltage and current dual closed-loop control loop tracks u. ac,base,d and u ac,base,q Components, thereby achieving the MMC output voltage u g,d / q Control. However, due to the absence of a phase-locked loop, V cannot be guaranteed to coincide with the d-axis, i.e., u. g,q ≠0. In the dq rotating coordinate system phase alignment method, the reference rotation phase θ is generated through ω0. base Based on this, an additional phase Δθ can be output through a PI controller, which can make θ = θ base The new rotating coordinate system d'q' corresponding to +Δθ satisfies u g,q =0. Therefore, according to the formula for calculating output power in a rotating coordinate system:

[0044]

[0045] It can be seen that at this time, the AC side current control loop can independently control the active and reactive power output of the MMC by tracking the d-axis and q-axis current reference values.

[0046] The control method of the present invention is specifically implemented as follows:

[0047] Step 1: Using the MMC bridge arm capacitor AC side energy control method and reactive power control method, obtain the MMC AC output voltage increment Δu. ac,d / q Then, the reference value u of the MMC AC side output voltage is calculated. ac,ref,d / q .

[0048] Figure 5 This is a control block diagram for the MMC AC side energy control method and reactive power control method. Wherein, W m and W ref These are the measured and reference values ​​of the total energy of the MMC bridge arm capacitors; Q m and Q ref These are the measured and reference values ​​of reactive power on the AC side of the MMC; PI represents the proportional-integral controller.

[0049] Based on this, the AC-side energy control method can be expressed as:

[0050] Δu ac,d =G ac,w (s)(W m -W ref )

[0051] Among them, G ac,w (s) represents the PI controller transfer function in the AC-side energy control method. W m The calculation method is as follows:

[0052]

[0053] Among them, u p,j and u n,j These are the total voltage values ​​of the upper and lower bridge arm capacitors of one phase, respectively; C eq It is the total equivalent capacitance of N modules on the bridge arm.

[0054] MMC AC side output voltage reference value u ac,ref,d / q The calculation formula is:

[0055] u ac,ref,d / q =u ac,base,d / q +Δu ac,d / q

[0056] Among them, u ac,base,d / q It is the MMC AC voltage reference value output by the AC side synchronous control loop.

[0057] This method will generate an additional voltage component Δu along the d-axis. ac,d To correct the reference value u of the dual closed-loop control loop for AC side voltage and current input. ac,ref,d ,like Figure 6 As shown (Note: when the coordinate systems dq in the figure are phase aligned, u...) g,q =0), when the measured value of bridge arm capacitor energy Wm Compared with reference value W ref When errors occur, the MMC converter will automatically adjust the active power output on the AC side to stabilize the energy of the bridge arm capacitor.

[0058] Step 2: Obtain the additional DC output voltage Δu of the MMC using the DC-side energy control method of the MMC bridge arm capacitor. dc Then, the reference value u of the DC-side output voltage of the MMC can be calculated. dc,ref .

[0059] Figure 7 This is a control block diagram for the MMC DC-side energy control method. The DC-side energy control principle is similar to that of the AC-side energy control, based on the measured energy value W of the bridge arm capacitor. m Compared with reference value W ref Inter-error generates additional voltage Δu dc This, combined with the DC-side synchronous control loop, outputs a DC voltage reference value u. dc,base The DC-side output voltage reference value u is calculated. dc,ref =u dc,base +Δu dc .

[0060] Step 3: Put u ac,ref,d / q and u dc,ref The voltage and current signals are respectively sent to the AC and DC side dual closed-loop control loops, and the differential modulus u of the MMC modulation signal is obtained at the output of the two loops. diff,d / q and common modulus u sum,j ,like Figure 3 As shown.

[0061] In the AC side voltage and current dual closed-loop control loop, the differential modulus u of the output MMC modulation signal is decoupled. diff,d / q .

[0062] In the DC-side voltage and current dual closed-loop control loop, firstly based on the reference value u dc,ref With the measured value u dc The error between them is processed by a PI circuit to generate a DC current increment Δi. dc Then divide by 3 to get the current increment of each bridge arm, and then compare it with the bridge arm current i. sum,j Generate bridge arm current reference values, and finally generate the common mode u of the three bridge arms of the MMC based on the current control loop. sum,j .

[0063] Step 4: Based on u diff,d / q and u sum,jThe desired output voltage of the MMC bridge arm capacitor module is obtained using a bridge arm output voltage reference value calculation method. This is then used to generate trigger pulse signals for all switching devices in the MMC via a modulation section. The bridge arm output voltage reference value calculation method can be expressed as follows:

[0064]

[0065] Where u diff,j Indicate u diff,d / q The differential modulus of the MMC modulated signal in the three-phase coordinate system obtained after Park inverse transform. j,p and u j,n These represent the reference values ​​for the output voltage of the upper and lower bridge arms in a single phase, respectively.

[0066] As described above, both the AC and DC sides have synchronous control mechanisms that generate AC and DC voltage reference values, ensuring that the MMC AC and DC ports simultaneously possess voltage construction capabilities. Furthermore, the AC and DC energy control sections add additional adjustments to the reference values ​​and dynamically adjust the active power output of the MMC AC and DC ports to stabilize the capacitor module energy at the set energy reference value, ensuring stable MMC operation. Clearly, the proposed dual-terminal grid control method enables the MMC converter to operate stably in three scenarios without changing any control parameters or operating modes: one side connected to a voltage source, one side connected to a current source, and both sides simultaneously connected to voltage sources. Figure 8 As shown.

[0067] like Figure 9 , Figure 10 As shown, the simulation verification results of the MMC two-terminal network control method are presented. Figure 9 When the AC side of the MMC is connected to a voltage source, the DC side switches from being connected to a current source to being connected to a voltage source after 5 seconds. Figure 10 The DC side of the MMC is connected to a voltage source, and the AC side switches from being connected to a current source to being connected to a voltage source after 5 seconds. The control topology and parameter settings of the MMC converter remain completely consistent in both experiments, with only the external power supply being adjusted. Simulation results show that the control method proposed in this invention... Figure 8 The converter can operate stably under all three operating conditions shown. The MMC maintains stable AC and DC side voltages, and the energy of the MMC capacitor module remains stable at 1 p.u., with AC and DC power changing synchronously. Clearly, the two experiments demonstrate that the method proposed in this invention can achieve a single control method that enables the MMC to operate in both AC and DC grid configurations simultaneously, verifying the correctness and effectiveness of the theoretical analysis.

[0068] The MMC dual-terminal network control method proposed in this invention has the ability to independently control the energy of MMC capacitors, that is, to directly control the voltage amplitude of the bridge arm capacitor module, which significantly improves the safety of MMC operation and avoids exceeding the device withstand voltage limit. At the same time, the expanded control freedom also significantly improves the control flexibility of MMC.

Claims

1. A two-port network control method for a modular multilevel converter, characterized in that, Includes the following steps: Using the energy control method and reactive power control method of the AC side of the MMC bridge arm capacitor, the additional AC output voltage Δu of the MMC is obtained. ac,d / q Then, the reference value u of the MMC AC side output voltage is calculated. ac,ref,d / q ; The DC output voltage increment Δu of the MMC is obtained using the DC-side energy control method of the MMC bridge arm capacitor. dc Then, the reference value u of the DC-side output voltage of the MMC can be calculated. dc,ref ; will u ac,ref,d / q and u dc,ref The voltage and current signals are respectively sent to the AC and DC side dual closed-loop control loops, and the differential modulus u of the MMC modulation signal is obtained at the output of the two loops. diff,d / q and common modulus u sum,j ; According to u diff,d / q and u sum,j The expected output voltage of the MMC bridge arm capacitor module is obtained by using the bridge arm output voltage reference value calculation method, and then the trigger pulse signal of all switching devices in the MMC is generated by the modulation section.

2. The modular multilevel converter dual-port network control method according to claim 1, characterized in that, MMC AC side output voltage reference value u ac,ref,d / q The calculation formula is: u ac,ref,d / q =u ac,base,d / q +Δu ac,d / q Among them, u ac,base,d / q It is the MMC AC voltage reference value output by the AC side synchronous control loop.

3. The modular multilevel converter dual-port network control method according to claim 1, characterized in that, MMC DC-side output voltage reference value u dc,ref The calculation formula is: u dc,ref =u dc,base +Δu dc Among them, u dc,base It is the MMC DC voltage reference value output by the DC-side synchronous control loop.

4. The modular multilevel converter dual-port network control method according to claim 1, characterized in that, The energy control method for the AC side of the MMC bridge arm capacitor is expressed as follows: Δu ac,d =G ac,w (s)(W m -IN ref ) Where, Δu ac,d It is the additional AC output voltage of the d-axis MMC, G ac,w (s) represents the PI controller transfer function in the AC-side energy control method, W m This is the measured value of the bridge arm capacitor energy, W. ref This is the reference value for the energy of the bridge arm capacitor, u p,j and u n,j These are the total voltage values ​​of the upper and lower bridge arm capacitors of phase j, respectively; C eq It is the total equivalent capacitance of N modules on the bridge arm.

5. The modular multilevel converter dual-port network control method according to claim 4, characterized in that, When the bridge arm capacitor energy measurement value W m Reference value W of bridge arm capacitor energy ref When errors occur, the MMC converter will automatically adjust the active power output on the AC side to stabilize the energy of the bridge arm capacitor.

6. The modular multilevel converter dual-port network control method according to claim 1, characterized in that, The method for calculating the bridge arm output voltage reference value is expressed as follows: Among them, u diff,j Indicate u diff,d / q The differential modulus of the MMC modulated signal in the three-phase coordinate system obtained after the inverse Park transform, u j,p and u j,n These represent the reference values ​​for the output voltage of the upper and lower bridge arms in phase j, respectively.

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