A method, system, device and medium for controlling a steady-state operating range of an MMC

Abstract

This invention provides a method, system, device, and medium for controlling the steady-state operating range of a modular multilevel converter (MMC). The method includes: using the maximum arm capacitor voltage within the acquired steady-state operating range of the MMC as a target capacitor peak voltage; setting the target capacitor peak voltage as a reference value; and controlling the steady-state operating range of the MMC based on preset operating constraints. This invention controls the expansion of the steady-state operating range of the MMC by employing a constant capacitor voltage peak method and preset operating constraints. This facilitates the accurate calculation and control of the steady-state operating range of the modular multilevel converter's high-voltage direct current transmission system, while maintaining the number of sub-modules and the cost of the MMC within the system.

Description

Technical Field

[0001] This invention belongs to the field of flexible DC transmission technology, specifically relating to a method, system, equipment, and medium for controlling the steady-state operating range of MMC. Background Technology

[0002] Currently, modular multilevel converters (MMCs) are widely studied and considered the optimal choice for HVDC transmission systems. In MMC-based HVDC systems, the boundaries of the steady-state operating range of the MMC are crucial for power system stability. However, research on MMC-based HVDC systems largely focuses on control methods, modulation strategies, loss assessment, and analysis methods. The steady-state operating range problem of MMC-based HVDC systems, which is critical for power system stability, remains largely unresolved. Existing literature does not provide comprehensive constraints on the steady-state operating range of MMC-based HVDC systems. The main constraints proposed in existing literature are: 1) the instantaneous reference voltage of each arm should fall within the range determined by the sum of the voltages of all submodule capacitors; 2) the maximum instantaneous value of the arm current should not exceed a certain value; 3) the maximum instantaneous value of the submodule capacitor voltage should not exceed a certain value. Any increase in these restrictions typically means an increase in the cost of the MMC-based HVDC system. Furthermore, other modular multilevel converter parameters such as capacitor voltage ripple are not taken into account. In addition, existing methods for extending the steady-state operating range of modular multilevel converters mainly focus on increasing the number of submodules, second harmonic current injection, and zero-sequence voltage injection. However, increasing the number of submodules will increase costs, and zero-sequence voltage injection will increase system power loss, thus limiting the application scope of the above methods. Summary of the Invention

[0003] To overcome the shortcomings of the prior art, this invention proposes a method for controlling the steady-state operating range of MMC, comprising:

[0004] The maximum bridge arm capacitor voltage within the obtained steady-state operating range of the MMC is taken as the target capacitor peak voltage.

[0005] The peak voltage of the target capacitor is set as a reference value, and the steady-state operating range of the MMC is controlled based on preset operating constraints.

[0006] Preferably, setting the target capacitor peak voltage as a reference value and controlling the steady-state operating range of the MMC based on preset operating constraints includes:

[0007] Based on the reference value and the initial bridge arm capacitor voltage at each operating point in the steady-state operating range of the MMC, the correction value of the DC component corresponding to the initial bridge arm capacitor voltage at each operating point is obtained.

[0008] Based on the correction value of the DC component corresponding to the initial bridge arm capacitor voltage at each operating point, calculate the DC component of the bridge arm capacitor voltage corresponding to the reference value.

[0009] The steady-state operating range of the MMC is controlled based on the DC component of the bridge arm capacitor voltage corresponding to the reference value and the preset operating constraints.

[0010] Preferably, the formula for calculating the correction value of the DC component corresponding to the initial bridge arm capacitor voltage at each operating point is as follows:

[0011] Δu cap0 (i d i q )=U cap0 -max[u cap (i d i q )]

[0012] Where, Δu cap0 (i d i q ) represents the correction value for the DC component of the initial bridge arm capacitor voltage at each operating point in the MMC; U cap0 Indicates the peak voltage of the target capacitor; u cap (i d i q ) represents the DC component of the initial bridge arm capacitor voltage at each operating point within the steady-state operating range of the MMC; i d Indicates the d-axis component of the MMC AC voltage; i q This represents the q-axis component of the MMC AC voltage.

[0013] Preferably, the calculation formula for the DC component of the bridge arm capacitor voltage corresponding to the reference value is as follows:

[0014] U cap (i d i q )=u cap0 +Δu cap0 (i d i q )

[0015] Among them, U cap (i d i q ) represents the DC component of the bridge arm capacitor voltage corresponding to the reference value; u cap0 This represents the initial bridge arm capacitor voltage at each operating point within the steady-state operating range of the MMC.

[0016] Preferably, the operating constraints include: the effective value of the AC current is less than or equal to the rated value of the AC current and the effective value of the bridge arm current is less than or equal to the rated value of the bridge arm current, the peak value of the modulation wave is equal to a preset modulation wave threshold, the minimum value of the bridge arm output voltage is less than or equal to a preset voltage threshold, and the capacitor voltage ripple is less than or equal to a preset ripple threshold.

[0017] Preferably, the expression corresponding to the effective value of the alternating current being less than or equal to the rated value of the alternating current is as follows:

[0018]

[0019] Among them, i d Indicates the d-axis component of the MMC AC voltage; i q I1 represents the q-axis component of the MMC AC voltage; I1 represents the rated value of the AC current.

[0020] Preferably, the expression corresponding to the effective value of the bridge arm current being less than or equal to the rated value of the bridge arm current is as follows:

[0021]

[0022] Among them, i dc Indicates the DC current of the MMC; i a I represents the phase a AC current; I2 represents the rated value of the bridge arm current.

[0023] Preferably, the expression corresponding to the peak value of the modulation wave being equal to the preset modulation wave threshold is as follows:

[0024]

[0025] Where, m pa This represents the modulation wave of the upper bridge arm of phase a in the MMC; i d Indicates the d-axis component of the MMC AC voltage; i q I represents the q-axis component of the MMC AC voltage; d Indicates the rated active current; I q A represents the rated reactive current; A represents the preset modulation threshold.

[0026] Preferably, the expression corresponding to the minimum output voltage of the bridge arm being less than or equal to a preset voltage threshold is as follows:

[0027] min(u pa )≤C

[0028] Among them, u pa C represents the output voltage of the bridge arm; C represents the preset voltage threshold.

[0029] Preferably, the expression corresponding to the capacitor voltage ripple being less than or equal to a preset ripple threshold is as follows:

[0030]

[0031] Among them, u cap Indicates the voltage across the bridge arm capacitor; u cap0 B represents the initial bridge arm capacitor voltage at each operating point within the steady-state operating range of the MMC; B represents the preset ripple threshold.

[0032] Preferably, the bridge arm capacitor voltage u cap The corresponding calculation formula is as follows:

[0033]

[0034] Where N represents the number of upper bridge arm submodules of the MMC; i m Indicates the peak value of the AC phase current; C sm ω represents the capacitance value of the MMC submodule; k1 represents the angular velocity of phase a; k2 represents the first coefficient; k3 represents the third coefficient.

[0035] Based on the same inventive concept, the present invention also provides a control system for the steady-state operating range of MMC, comprising:

[0036] Peak voltage setting module: used to take the maximum bridge arm capacitor voltage in the acquired MMC steady-state operating range as the target capacitor peak voltage;

[0037] Steady-state range control module: used to set the peak voltage of the target capacitor as a reference value and control the steady-state operating range of the MMC based on preset operating constraints.

[0038] Preferably, the steady-state range control module is specifically used for:

[0039] Based on the reference value and the initial bridge arm capacitor voltage at each operating point in the steady-state operating range of the MMC, the correction value of the DC component corresponding to the initial bridge arm capacitor voltage at each operating point is obtained.

[0040] Based on the correction value of the DC component corresponding to the initial bridge arm capacitor voltage at each operating point, calculate the DC component of the bridge arm capacitor voltage corresponding to the reference value.

[0041] The steady-state operating range of the MMC is controlled based on the DC component of the bridge arm capacitor voltage corresponding to the reference value and the preset operating constraints.

[0042] Preferably, the calculation formula for the correction value of the DC component corresponding to the initial bridge arm capacitor voltage at each operating point in the steady-state range control module is as follows:

[0043] Δu cap0 (i d i q )=U cap0 -max[u cap (i d i q )]

[0044] Where, Δu cap0 (i d i q ) represents the correction value for the DC component of the initial bridge arm capacitor voltage at each operating point in the MMC; U cap0 Indicates the peak voltage of the target capacitor; u cap (i d i q ) represents the DC component of the initial bridge arm capacitor voltage at each operating point within the steady-state operating range of the MMC; i d Indicates the d-axis component of the MMC AC voltage; i q This represents the q-axis component of the MMC AC voltage.

[0045] Preferably, the calculation formula for the DC component of the bridge arm capacitor voltage corresponding to the reference value in the steady-state range control module is as follows:

[0046] U cap (i d i q )=u cap0 +Δu cap0 (i d i q )

[0047] Among them, U cap (i d i q ) represents the DC component of the bridge arm capacitor voltage corresponding to the reference value; u cap0 This represents the initial bridge arm capacitor voltage at each operating point within the steady-state operating range of the MMC.

[0048] Preferably, the operating constraints in the steady-state range control module include: the effective value of the AC current is less than or equal to the rated value of the AC current and the effective value of the bridge arm current is less than or equal to the rated value of the bridge arm current, the peak value of the modulation wave is equal to the preset modulation wave threshold, the minimum value of the bridge arm output voltage is less than or equal to the preset voltage threshold, and the capacitor voltage ripple is less than or equal to the preset ripple threshold.

[0049] Preferably, the expression corresponding to the effective value of the AC current being less than or equal to the rated value of the AC current in the steady-state range control module is as follows:

[0050]

[0051] Among them, i dIndicates the d-axis component of the MMC AC voltage; i q I1 represents the q-axis component of the MMC AC voltage; I1 represents the rated value of the AC current.

[0052] Preferably, the expression for the effective value of the bridge arm current being less than or equal to the rated value of the bridge arm current in the steady-state range control module is as follows:

[0053]

[0054] Among them, i dc Indicates the DC current of the MMC; i a I represents the phase a AC current; I2 represents the rated value of the bridge arm current.

[0055] Preferably, the expression corresponding to the modulation wave peak value being equal to the preset modulation wave threshold value in the steady-state range control module is as follows:

[0056]

[0057] Where, m pa This represents the modulation wave of the upper bridge arm of phase a in the MMC; i d Indicates the d-axis component of the MMC AC voltage; i q I represents the q-axis component of the MMC AC voltage; d Indicates the rated active current; I q A represents the rated reactive current; A represents the preset modulation threshold.

[0058] Preferably, the expression corresponding to the minimum output voltage of the bridge arm in the steady-state range control module being less than or equal to a preset voltage threshold is as follows:

[0059] min(u pa )≤C

[0060] Among them, u pa C represents the output voltage of the bridge arm; C represents the preset voltage threshold.

[0061] Preferably, the expression corresponding to the capacitor voltage ripple being less than or equal to a preset ripple threshold in the steady-state range control module is as follows:

[0062]

[0063] Among them, u cap Indicates the voltage across the bridge arm capacitor; u cap0 B represents the initial bridge arm capacitor voltage at each operating point within the steady-state operating range of the MMC; B represents the preset ripple threshold.

[0064] Preferably, the bridge arm capacitor voltage u in the steady-state range control module cap The corresponding calculation formula is as follows:

[0065]

[0066] Where N represents the number of upper bridge arm submodules of the MMC; i m Indicates the peak value of the AC phase current; C sm ω represents the capacitance value of the MMC submodule; k1 represents the angular velocity of phase a; k2 represents the first coefficient; k3 represents the third coefficient.

[0067] Based on the same inventive concept, the present invention also provides a computer device, comprising: one or more processors;

[0068] Memory, used to store one or more programs;

[0069] When the one or more programs are executed by the one or more processors, a method for controlling the steady-state operating range of MMC as described above is implemented.

[0070] Based on the same inventive concept, the present invention also provides a computer-readable storage medium having a computer program stored thereon, wherein when the computer program is executed, it implements the control method for the steady-state operating range of MMC as described above.

[0071] Compared with the closest existing technology, the present invention has the following beneficial effects:

[0072] This invention provides a method, system, device, and medium for controlling the steady-state operating range of a modular multilevel converter (MMC). The method includes: using the maximum arm capacitor voltage within the acquired steady-state operating range of the MMC as a target capacitor peak voltage; setting the target capacitor peak voltage as a reference value; and controlling the steady-state operating range of the MMC based on preset constraints. This invention employs constant capacitor voltage peak control and controls the steady-state operating range of the MMC by applying preset operating constraints to the operating point of constant capacitor voltage peak control. This expands the steady-state operating range, facilitating precise calculation and control of the steady-state operating range of the modular multilevel converter's high-voltage direct current transmission system, while maintaining the number of sub-modules and the cost of the MMC within the system. Attached Figure Description

[0073] Figure 1 A flowchart illustrating a method for controlling the steady-state operating range of an MMC provided by the present invention;

[0074] Figure 2 This is a main circuit diagram of a modular multilevel converter provided in an embodiment of the present invention;

[0075] Figure 3 This is a schematic diagram of the bridge arm capacitor voltage and bridge arm output voltage of the MMC in an embodiment of the present invention;

[0076] Figure 4 This is a schematic diagram of the modulation wave of MMC in an embodiment of the present invention;

[0077] Figure 5 This is a schematic diagram of the steady-state operating range of MMC when the constant capacitor voltage peak control method is not used in the embodiments of the present invention;

[0078] Figure 6 This is a schematic diagram of the bridge arm capacitor voltage and bridge arm output voltage of the MMC operating at point A5 in an embodiment of the present invention;

[0079] Figure 7 This is a schematic diagram of the modulation wave of MMC operating at point A5 in an embodiment of the present invention;

[0080] Figure 8 This is a schematic diagram of the bridge arm capacitor voltage and bridge arm output voltage of the MMC operating at point A9 in an embodiment of the present invention;

[0081] Figure 9 This is a schematic diagram of the modulation wave of MMC operating at point A9 in an embodiment of the present invention;

[0082] Figure 10 This is a schematic diagram of the bridge arm capacitor voltage at point A5 in an embodiment of the present invention;

[0083] Figure 11 This is a schematic diagram of the bridge arm capacitor voltage at point A2 in an embodiment of the present invention;

[0084] Figure 12 This is a schematic diagram of the steady-state operating range of MMC when the constant capacitor voltage peak control method is used in an embodiment of the present invention;

[0085] Figure 13 This is a schematic diagram of the bridge arm output voltage and bridge arm capacitor voltage at point B5 when the constant capacitor voltage peak control method is used in an embodiment of the present invention.

[0086] Figure 14 This is a schematic diagram of the modulation wave at point B5 when the constant capacitance voltage peak control method is used in an embodiment of the present invention;

[0087] Figure 15 This is a schematic diagram of the bridge arm current at point B5 when the constant capacitor voltage peak control method is used in an embodiment of the present invention;

[0088] Figure 16 This is a schematic diagram of the AC current at point B5 when the constant capacitor voltage peak control method is used in an embodiment of the present invention;

[0089] Figure 17 This is a schematic diagram of the bridge arm output voltage and bridge arm capacitor voltage at point B6 when the constant capacitor voltage peak control method is used in an embodiment of the present invention.

[0090] Figure 18 This is a schematic diagram of the modulation wave at point B6 when the constant capacitor voltage peak control method is used in an embodiment of the present invention;

[0091] Figure 19 This is a schematic diagram of the bridge arm current at point B6 when the constant capacitor voltage peak control method is used in an embodiment of the present invention;

[0092] Figure 20 This is a schematic diagram of the AC current at point B6 when the constant capacitor voltage peak control method is used in an embodiment of the present invention;

[0093] Figure 21 In an embodiment of the present invention, when a constant capacitor voltage peak control method is used, from B... 12 A schematic diagram of the bridge arm capacitor voltage to point B8;

[0094] Figure 22 In an embodiment of the present invention, when a constant capacitor voltage peak control method is used, from B... 12 Schematic diagram of bridge arm output voltage to point B8;

[0095] Figure 23 In an embodiment of the present invention, when a constant capacitor voltage peak control method is used, from B... 12 Schematic diagram of the modulation wave to point B8;

[0096] Figure 24 In an embodiment of the present invention, when a constant capacitor voltage peak control method is used, from B... 12 Schematic diagram of bridge arm current to point B8;

[0097] Figure 25 In an embodiment of the present invention, when a constant capacitor voltage peak control method is used, from B... 12 A schematic diagram of the alternating current to point B8;

[0098] Figure 26 This is a schematic diagram of the control system structure for the steady-state operating range of MMC provided by the present invention. Detailed Implementation

[0099] The specific embodiments of the present invention will be further described in detail below with reference to the accompanying drawings.

[0100] Example 1:

[0101] This invention provides a method for controlling the steady-state operating range of an MMC, the flowchart of which is shown below. Figure 1 As shown, it includes:

[0102] Step 1: Take the maximum bridge arm capacitor voltage in the obtained steady-state operating range of MMC as the target capacitor peak voltage;

[0103] Step 2: Set the peak voltage of the target capacitor as a reference value, and control the steady-state operating range of the MMC based on preset operating constraints.

[0104] Specifically, step 2 includes:

[0105] Based on the reference value and the initial bridge arm capacitor voltage at each operating point in the steady-state operating range of the MMC, the correction value of the DC component corresponding to the initial bridge arm capacitor voltage at each operating point is obtained.

[0106] Based on the correction value of the DC component corresponding to the initial bridge arm capacitor voltage at each operating point, calculate the DC component of the bridge arm capacitor voltage corresponding to the reference value.

[0107] Based on the DC component of the bridge arm capacitor voltage corresponding to the reference value and the preset operating constraints, the steady-state operating range of the MMC is controlled.

[0108] The formulas for calculating the correction values ​​of the DC components of the initial bridge arm capacitor voltages at each operating point are as follows:

[0109] Δu cap0 (i d i q )=U cap0 -max[u cap (i d i q )]

[0110] Where, Δu cap0 (i d i q ) represents the correction value for the DC component of the initial bridge arm capacitor voltage at each operating point in the MMC; U cap0 Indicates the peak voltage of the target capacitor; u cap (i d i q ) represents the DC component of the initial bridge arm capacitor voltage at each operating point within the steady-state operating range of the MMC; i d Indicates the d-axis component of the MMC AC voltage; i q This represents the q-axis component of the MMC AC voltage.

[0111] The formula for calculating the DC component of the bridge arm capacitor voltage corresponding to the reference value is as follows:

[0112] U cap (i d i q )=u cap0 +Δu cap0 (i d i q )

[0113] Among them, U cap (i d i q ) represents the DC component of the bridge arm capacitor voltage corresponding to the reference value; u cap0 This represents the initial bridge arm capacitor voltage at each operating point within the steady-state operating range of the MMC.

[0114] The operating constraints include: the effective value of the AC current is less than or equal to the rated value of the AC current and the effective value of the bridge arm current is less than or equal to the rated value of the bridge arm current, the peak value of the modulation wave is equal to the preset modulation wave threshold, the minimum value of the bridge arm output voltage is less than or equal to the preset voltage threshold, and the capacitor voltage ripple is less than or equal to the preset ripple threshold.

[0115] The expression corresponding to the effective value of the alternating current being less than or equal to the rated value of the alternating current is as follows:

[0116]

[0117] Among them, i d Indicates the d-axis component of the MMC AC voltage; i q I1 represents the q-axis component of the MMC AC voltage; I1 represents the rated value of the AC current.

[0118] The expression corresponding to the bridge arm current effective value being less than or equal to the bridge arm current rated value is as follows:

[0119]

[0120] Among them, i dc Indicates the DC-side current of the MMC; i a Ia represents the phase a AC current; I2 represents the rated value of the bridge arm current.

[0121] Active power P on DC and AC sides da and P ac The calculation formula is as follows:

[0122] P ac =u d i d +u q i q =u2i d =P dc =U dc i dc

[0123] Among them, u d The d-axis represents the AC voltage of the MMC; u q The q-axis represents the AC voltage of the MMC; u2 represents the secondary winding voltage of the transformer; i d Indicates the d-axis component of the MMC AC voltage; i qU represents the q-axis component of the MMC AC voltage; dc The DC voltage of the MMC; i dc This refers to the DC current of the MMC;

[0124] Based on the effective value of the bridge arm current, the expression for the bridge arm current is derived as follows:

[0125]

[0126] Among them, i arm This indicates the effective value of the bridge arm current;

[0127] Considering the upper bridge arm of phase a, its output voltage is calculated as follows:

[0128] u pa =0.5U dc -u a

[0129] Among them, u pa Indicates the output voltage; U dc Indicates the DC voltage of the MMC; u a This represents the AC voltage of phase a;

[0130] Phase a AC voltage u a The expression is as follows:

[0131]

[0132] Among them, U m and Let A and B represent the amplitude and phase of phase a, respectively. In steady state, the relationship between voltage and current in the MMC can be expressed as:

[0133]

[0134] Among them, u d u q These represent the d-axis and q-axis components of the MMC AC voltage, respectively, and u2 represents the secondary winding voltage of the transformer; i d i q These are the d-axis and q-axis components of the MMC AC current, respectively; X L This indicates the reactance value of the series reactor;

[0135] The reactance value X of the series reactor L The corresponding calculation formula is as follows:

[0136]

[0137] Among them, X T L represents the transformer leakage inductance; ω represents the angular velocity of phase a; L arm Indicates the bridge arm inductance;

[0138] The amplitude and phase of the AC phase voltage of the MMC are calculated as follows:

[0139]

[0140] Among them, U m and These represent the amplitude and phase of phase a, respectively; u d u q These represent the d-axis and q-axis components of the MMC AC voltage, respectively; the expression corresponding to the modulation wave peak value being equal to the preset modulation wave threshold is as follows:

[0141]

[0142] Where, m pa This represents the modulation wave of the upper bridge arm of phase a in the MMC; i d Indicates the d-axis component of the MMC AC voltage; i q I represents the q-axis component of the MMC AC voltage; d Indicates the rated active current; I q A represents the rated reactive current; A represents the preset modulation threshold; preferably, A = 0.95.

[0143] The expression corresponding to the minimum output voltage of the bridge arm being less than or equal to a preset voltage threshold is as follows:

[0144] min(u pa )≤C

[0145] Among them, u pa This represents the bridge arm output voltage; C represents the preset voltage threshold, preferably C = 0.05U. dc ;

[0146] The minimum output voltage of the bridge arm is min(u pa The corresponding calculation formula is as follows:

[0147]

[0148] Among them, min(U arm ) represents the minimum voltage output capability of the bridge arm; Δu pa Indicates the minimum output voltage of the bridge arm (min(u)) pa ()) and minimum voltage output capability of bridge arm min(U) arm The margin between ); U dc u1 represents the DC voltage of the MMC; u2 represents the secondary winding voltage of the transformer; i d i q These are the d-axis and q-axis components of the MMC AC current, respectively; X L This indicates the reactance value of the series reactor;

[0149] The expression corresponding to the capacitor voltage ripple being less than or equal to a preset ripple threshold is as follows:

[0150]

[0151] Among them, u cap Indicates the voltage across the bridge arm capacitor; u cap0 B represents the initial bridge arm capacitor voltage at each operating point within the steady-state operating range of the MMC; B represents the preset ripple threshold, preferably B = 10%.

[0152] The bridge arm capacitor voltage u cap The corresponding calculation formula is as follows:

[0153]

[0154]

[0155] Where N represents the number of upper bridge arm submodules of the MMC; i m Indicates the peak value of the AC phase current; C sm Indicates the capacitance value of the MMC submodule; ω represents the angular velocity of phase a; k1 represents the first coefficient; k2 represents the second coefficient; k3 represents the third coefficient; U dc ω represents the DC voltage of the MMC; ω represents the angular velocity of phase a. It is the phase of the alternating current; U m The amplitude of phase a; The phase of phase a;

[0156] Based on the above formula, the expression for the modulation wave of the upper bridge arm of phase a can be derived as follows:

[0157]

[0158] Where, m pa This represents the modulation wave of the upper bridge arm of phase a in the MMC; U dc u1 represents the DC voltage of the MMC; u2 represents the secondary winding voltage of the transformer; i d Indicates the d-axis component of the MMC AC voltage; i q Represents the q-axis component of the MMC AC voltage; X L The reactance of the series reactor is represented by ω; the angular velocity of phase a is represented by u. cap0 Indicates the initial bridge arm capacitor voltage at each operating point within the steady-state operating range of the MMC; N represents the number of bridge arm submodules on the MMC; i m Indicates the peak value of the AC phase current; C sm This represents the capacitance value of the MMC submodule; k1 represents the first coefficient; k2 represents the second coefficient; k3 represents the third coefficient;

[0159] The specific control process of the MMC steady-state operating range control method provided in this invention is illustrated by a specific embodiment: the MMC main circuit diagram for HVDC is as follows. Figure 2 As shown; the MMC contains six bridge arms, each consisting of N sub-modules and a series reactor, and each phase unit consists of an upper bridge arm and a lower bridge arm. Wherein, U s X represents the grid voltage. T L represents the transformer leakage inductance. arm This indicates the bridge arm inductance. pa and u na These represent the output voltages of the upper and lower arms of phase a, respectively. Additionally, i ap and i an These refer to the currents in the upper and lower arms of phase a, respectively. U dc i is the DC voltage of the MMC. a i b and i c i is the alternating current of the MMC. dc This represents the DC current of the MMC. The primary and secondary winding voltages of the transformer are u1 and u2, respectively; a schematic diagram of the MMC bridge arm capacitor voltage and bridge arm output voltage is shown below. Figure 3 As shown, the corresponding modulation wave schematic diagram is as follows: Figure 4 As shown;

[0160] The effective value of alternating current i ac It can be obtained from the following formula:

[0161]

[0162] Where S refers to the apparent power of MMC, i d and i q The d-axis and q-axis values ​​represent the alternating current.

[0163] The effective value of the bridge arm current can be obtained by the following formula:

[0164]

[0165] Among them, i dc This refers to the current on the DC side of the MMC; i arm This indicates the effective value of the bridge arm current;

[0166] Active power P on DC and AC sides dc and P ac The calculation is as follows:

[0167] P ac =u d i d +u q i q =u2i d =Pdc =U dc i dc

[0168] Among them, u d The d-axis represents the AC voltage of the MMC; u q The q-axis represents the AC voltage of the MMC; u2 represents the secondary winding voltage of the transformer; i d Indicates the d-axis component of the MMC AC voltage; i q U represents the q-axis component of the MMC AC voltage; dc The DC voltage of the MMC; i dc This refers to the DC current of the MMC;

[0169] Furthermore, when the MMC operates in steady state, it is assumed that the phase of the system-side voltage U1 is 0. Based on the above equation, the expression for the bridge arm current can be derived as follows:

[0170]

[0171] Where u2 represents the secondary winding voltage of the transformer; U dc The DC voltage of the MMC; i d Indicates the d-axis component of the MMC AC voltage; i q Represents the q-axis component of the MMC AC voltage;

[0172] Considering the upper bridge arm of phase a, its output voltage is calculated as follows:

[0173] u pa =0.5U dc -u a

[0174] Among them, U dc This refers to the DC voltage of the MMC; u a The phase a AC voltage can be expressed as:

[0175]

[0176] Among them, U m and Let ω be the amplitude and phase of phase a, respectively, and ω be the angular velocity of phase a.

[0177] In steady state, the relationship between voltage and current in an MMC can be expressed as:

[0178]

[0179] Among them, u d u q These represent the d-axis and q-axis components of the MMC AC voltage, respectively, and u2 is the secondary winding voltage of the transformer; i d i qThese are the d-axis and q-axis components of the MMC AC current, respectively.

[0180] X L The reactance value of a series reactor is expressed by the following formula:

[0181]

[0182] Among them, X T L represents the transformer leakage inductance; ω represents the angular velocity of phase a; L arm Indicates the bridge arm inductance;

[0183] The amplitude and phase of the AC phase voltage of the MMC are calculated as follows:

[0184]

[0185] Among them, U m and These represent the amplitude and phase of phase a, respectively; u d u q These represent the d-axis and q-axis components of the MMC AC voltage, respectively.

[0186] Based on the above formula, the minimum output voltage of the bridge arm can be derived.

[0187]

[0188] Among them, min(U arm ) represents the minimum voltage output capability of the bridge arm; Δu pa Indicates the minimum output voltage of the bridge arm (min(u)) pa ()) and minimum voltage output capability of bridge arm min(U) arm The margin between ); U dc u1 represents the DC voltage of the MMC; u2 represents the secondary winding voltage of the transformer; i d i q These are the d-axis and q-axis components of the MMC AC current, respectively; X L This indicates the reactance value of the series reactor;

[0189] In this embodiment, the bridge arm submodule uses a half-bridge submodule (HBSM), min(U arm ) should be set to 0; Δu pa Set to 0.05U dc Based on this, the voltage of the transformer's secondary winding can be designed to be 200kV.

[0190] When the MMC is running stably, the bridge arm capacitor voltage u cap It should be higher than the output voltage u pa To avoid overmodulation;

[0191]

[0192] Where, m pa It is the modulation wave of the upper bridge arm of phase a; u cap Indicates the voltage across the bridge arm capacitor; u pa This indicates the output voltage of the bridge arm.

[0193] To study the modulated wave, the analytical solution for the capacitor voltage must first be derived; the instantaneous currents of the upper and lower bridge arms of phase a can be calculated as follows:

[0194]

[0195] Among them, i dc i represents the DC current of the MMC. a Indicates the alternating current of phase a, i a It can be represented as:

[0196]

[0197] Among them, i m and These are the amplitude and phase of the alternating current, respectively, and can be expressed as:

[0198]

[0199] Based on the bridge arm output voltage u pa and bridge arm current i pa i d Indicates the d-axis component of the MMC AC voltage; i q Represents the q-axis component of the MMC AC voltage;

[0200] The bridge arm capacitor voltage can be derived as follows:

[0201]

[0202] Among them, u cap U is the voltage across the bridge arm capacitor. cap0 To eliminate the DC component of the bridge arm capacitor voltage when CCPV is not used, the CCPV control method mentioned in this embodiment is the constant capacitor voltage peak control method proposed in this invention, where N is the number of bridge arm submodules on the MMC, and i m Where ω is the peak value of the AC phase current, C is the angular velocity of the AC current, and ω is the peak value of the AC phase current. sm These are the capacitance values ​​for the submodule. The expressions for k1, k2, and k3 are as follows:

[0203]

[0204] The expression for the modulation wave of the upper bridge arm of phase a can be derived as follows:

[0205]

[0206] Where u2 represents the secondary winding voltage of the transformer; i d i q These are the d-axis and q-axis components of the MMC AC current, respectively; X L Indicates the reactance value of the series reactor, and N represents the number of upper bridge arm submodules of the MMC; i m Indicates the peak value of the AC phase current; C sm Indicates the capacitance value of the MMC submodule; ω represents the angular velocity of phase a; k1 represents the first coefficient; k2 represents the second coefficient; k3 represents the third coefficient; U dc This is the DC voltage of the MMC; It is the phase of the alternating current; U m The amplitude of phase a; The phase of phase a;

[0207] Under rated conditions, the maximum value of the modulated wave is designed to be 0.95;

[0208]

[0209] Among them, I d and I q These represent the rated active and reactive currents, respectively. d i q These are the d-axis and q-axis components of the MMC AC current, respectively.

[0210] When the MMC is operating stably, the capacitor voltage ripple should be limited to within 10%. The capacitor voltage ripple can be represented as follows:

[0211]

[0212] Among them, u cap U represents the voltage across the bridge arm capacitor. cap0 This represents the DC component of the bridge arm capacitor voltage when CCPV is not used.

[0213] When the MMC operates under rated conditions, the capacitor voltage ripple will reach its maximum value. Therefore, when the MMC operates under rated conditions (I... d I q When operating under these conditions, the capacitor voltage ripple is designed to be 10%. Based on the above formula, the DC component of the submodule capacitor voltage and the capacitance value can be designed to be 384.5kV and 15.6mF, respectively; one bridge arm has 183 submodules, as shown in Table 1.

[0214] In summary, the steady-state operating range of MMC is limited by five conditions: 1) maximum modulation wave; 2) capacitor voltage ripple; 3) minimum bridge arm output voltage; 4) AC current rating; 5) bridge arm current rating.

[0215] The boundaries and constraints of the steady-state operating range are shown in Table 2. The current direction in the main circuit diagram indicates that positive reactive power represents inductive reactive power, while negative reactive power represents capacitive reactive power. The steady-state operating range without CCPV control shows that inductive reactive power is constrained by the maximum modulation wave (A3-A5-A6, max(m)=0.95), and capacitive reactive power is constrained by the minimum bridge arm output voltage (A8-A9-A1, min(uap=0.05U). dc The boundaries of A2-A3 and A6-A7 are constrained by the capacitor ripple voltage, while the boundaries of A1-A2 and A7-A8 are constrained by the bridge arm current.

[0216] Table 1. Boundaries of Steady-State Operating Range

[0217]

[0218] When the MMC operates in Synchronous Static Var Compensator (STATCOM) mode, the active power is 0. In this case, the modulation wave of the upper arm of phase a can be rewritten as:

[0219]

[0220] In the formula:

[0221]

[0222] A schematic diagram of the steady-state operating range of MMC without using the constant capacitor voltage peak control method is shown below. Figure 5 As shown, when the MMC operates at point A5, the peak modulation waveform reaches its maximum value of 0.95. According to the above calculation formula, the reactive power can be calculated as 0.1961 pu. Therefore, the coordinates of A5 are (0, 0.1961). Furthermore, the figure shows the operating point A9, where the trough of the output voltage of the upper arm of phase a reaches its minimum value of 0.05U. dc Similarly, the reactive current can be calculated to be -0.3289, therefore the coordinates of A9 are (0, -0.3298), and the calculated peak value of the modulation wave is 0.9387. Furthermore, according to the above formula, the amplitudes of the AC phase voltages at A5 and A9 can be calculated as U... m5 =0.9412 and U m9 =1.0989, where u d =1. The AC voltage of the MMC at A9 is greater than the AC voltage at A5; however, the peak value of the modulation wave at A5 is greater than that at A9. This is because at point A5, the minimum capacitor voltage coincides with the peak value of the bridge arm output voltage, resulting in a higher peak value of the modulation wave. The corresponding bridge arm capacitor voltage and bridge arm output voltage diagram is shown below. Figure 6 As shown, the corresponding modulation wave schematic diagram is as follows: Figure 7As shown in the diagram. At point A9, the peak value of the capacitor voltage coincides with the peak value of the bridge arm output voltage; therefore, the peak value of the modulation wave is lower than that at A5. The corresponding schematic diagram of the bridge arm capacitor voltage and bridge arm output voltage is shown in the diagram. Figure 8 As shown, the corresponding modulation wave schematic diagram is as follows: Figure 9 As shown;

[0223] When extending the steady-state operating range of the MMC, it can be shifted upwards by increasing the number of submodules, and the steady-state operating range of the MMC can be expanded accordingly. However, this method increases the number of submodules, thereby increasing the cost of the submodules. To extend the steady-state operating range, this paper proposes CCPV control. When the MMC operates at rated inductive reactive power and zero active power (A5), the peak voltage of the bridge arm capacitor is 392kV. Figure 10 As shown. When the MMC operates at rated inductive reactive power and rated active power (A2), the peak voltage of the bridge arm capacitor is 424kV, as... Figure 11 As shown. Since the peak voltage of the bridge arm capacitor at point A2 reaches 424kV, operation at 424kV is safe, and the peak voltage of the bridge arm capacitor at other operating points can also reach 424kV. Therefore, the curve with a modulation wave peak value of 0.95 can be shifted upward, and the steady-state operating range of the MMC can be expanded accordingly.

[0224] The proposed CCPV control method can adjust the peak capacitor voltage to a constant value. Using CCPV control, the DC component of the capacitor voltage under various operating conditions can be obtained as follows:

[0225] U cap (i d i q )=u cap0 +Δu cap0 (i d i q )

[0226] When the DC component in the capacitor voltage is adjusted to U cap (i d i q When ), the peak voltage of the bridge arm capacitor can be controlled to U. cap0 U cap (i d i q This refers to the DC component of the capacitor voltage under various operating conditions when using CCPV control. In the constant capacitor voltage peak control method, the reference value of the capacitor voltage can be set to U. cap (i d i q ), in its expression, u cap0 Δu represents the DC component of the bridge arm capacitor voltage when CCPV is not used. cap0 (i d iq The value representing the correction for the DC component of the capacitor voltage can be calculated as follows:

[0227] Δu cap0 (i d i q )=U cap0 -max[u cap (i d i q )]

[0228] Among them, U cap0 This indicates the peak voltage of the bridge arm capacitor when using a CCPV capacitor; u cap (i d i q This refers to the capacitor voltage under various operating conditions when the CCPV is not used, which can be calculated using the above formula. cap (i d i q The value can be calculated offline based on the parameters of MMC and the above formula. Based on this, the reference value for capacitor voltage control can be obtained through table lookup and interpolation.

[0229] The effect of using constant capacitor voltage peak (CCPV) control is to extend the steady-state operating range of the MMC while keeping the number of submodules and the cost of the MMC constant. This is illustrated in the following diagram.

[0230] Figure 12 The steady-state operating range (S2) is given by using CCPV control. Table 2 shows the boundaries of the steady-state operating range. Compared with not using CCPV control, the steady-state operating range using the CCPV control method is expanded; at the operating point B6 (0, 0.7843), the reactive power of the MMC reaches 0.7843 pu, the ripple component reaches its 10% limit, and the modulation peak value is below 0.95.

[0231] Table 2 Steady-state operating range boundaries when using CCPV control

[0232]

[0233] In operating region S3, the capacitor voltage ripple exceeded 10%; however, the peak capacitor voltage remained U. cap0 The value remains unchanged. Therefore, it is safe for the MMC to operate within the S3 range. When the MMC operates at point B5, the inductive reactive power reaches 0.8765 pu, while the capacitor voltage ripple reaches 11.46%, and the peak value of the bridge arm capacitor voltage remains at U. cap0 constant.

[0234] A simulation model was built in the simulation software PSCAD based on the parameters in Table 3 to verify the steady-state operating range of the MMC and the control strategy of extending the steady-state operating range through CCPV. In the MMC simulation model, the rated DC voltage and rated DC current are 400kV and 3.125kA, respectively; the rated active and reactive power are 1250MW and 250Mvar, respectively. Based on the above parameters, the transformer secondary winding voltage can be designed to be 200kV (effective line voltage); the active current and reactive current can be calculated to be 5.10kA and 1.02kA, respectively; the AC current and arm current are 3.67kA and 2.11kA, respectively. Each arm has 183 sub-modules, each with a capacitor voltage of 2.1kV and a capacitance of 15.6mF. The arm reactor is 46.2mH. For MMC control, Nearest Level Approximation Modulation (NLM) is used in the arm to balance the capacitor voltage in the arm and distribute the IGBT trigger pulses.

[0235] Table 3 MMC Parameters

[0236]

[0237] Simulation results of MMC controlled by CCPV running in B5 are as follows: Figures 13-16 As shown; the simulation results when running in B6 are as follows Figures 17-20 As shown. The capacitor voltage ripple (Δu) can be obtained from the simulation results. cap ), bridge arm current (i am ), alternating current (i ac ), peak modulation wave (max(m)) and peak bridge arm capacitor voltage (max(u)) cap As shown in Table 4, at point B6, the capacitor voltage ripple reaches the 10% limit, while the simulation result is 10.2%, close to the design value. At point B5, the design value for the modulation peak value is 0.95, while the simulation result is 0.9510, close to its limit value. When using CCPV control, the peak voltage of the bridge arm capacitor (max(u)) is... cap The design value is 424kV, while the peak voltage of the bridge arm capacitors at operating points B5 and B6 (max(u)) is... cap The simulation results are 424.6 and 424.7 kV, respectively, which are close to the design values.

[0238] Table 4. Parameter Comparison of Design and Simulation Results Using CCPV Control

[0239]

[0240] *, Design value; #, Simulation value

[0241] When using CCPV control, the active power increases (from B). 12The simulation results up to B8 are as follows: Figures 21-25 As shown. When the active power changes, the peak value of the bridge arm capacitor voltage (max(u)) cap At 424kV, it remains almost unchanged, such as Figure 21 As shown, the output voltage diagram of the bridge arm is as follows. Figure 22 As shown; the peak modulation value is not greater than 0.95, within the allowable range, such as... Figure 23 As shown. Bridge arm current (i arm ) and alternating current (i ac It increases with the increase of active power, such as Figures 24-25 As shown. Therefore, the MMC operating range when using the CCPV control method has been verified to be accurate.

[0242] Therefore, the steady-state operating range of MMC can be extended through CCPV control, while keeping the number of submodules and the cost of MMC constant.

[0243] The CCPV control method proposed in this invention allows the peak capacitor voltage to be adjusted through CCPV control, keeping it constant under various operating conditions. As illustrated in the embodiments, CCPV control can be used to extend the steady-state operating range of the MMC, enabling the steady-state operating range of the modular multilevel converter (MMC) high-voltage DC transmission system to be accurately calculated and controlled, while keeping the number of sub-modules and the cost of the MMC unchanged.

[0244] Example 2:

[0245] Based on the same inventive concept, this invention also provides a control system for the steady-state operating range of MMC, the structural composition of which is shown in the schematic diagram below. Figure 26 As shown, it includes:

[0246] Peak voltage setting module: used to take the maximum bridge arm capacitor voltage in the acquired MMC steady-state operating range as the target capacitor peak voltage;

[0247] Steady-state range control module: used to set the peak voltage of the target capacitor as a reference value and control the steady-state operating range of the MMC based on preset operating constraints.

[0248] The steady-state range control module is specifically used for:

[0249] Based on the reference value and the initial bridge arm capacitor voltage at each operating point in the steady-state operating range of the MMC, the correction value of the DC component corresponding to the initial bridge arm capacitor voltage at each operating point is obtained.

[0250] Based on the correction value of the DC component corresponding to the initial bridge arm capacitor voltage at each operating point, calculate the DC component of the bridge arm capacitor voltage corresponding to the reference value.

[0251] The steady-state operating range of the MMC is controlled based on the DC component of the bridge arm capacitor voltage corresponding to the reference value and the preset operating constraints.

[0252] The calculation formulas for the correction values ​​of the DC components of the initial bridge arm capacitor voltage at each operating point in the steady-state range control module are as follows:

[0253] Δu cap0 (i d i q )=U cap0 -max[u cap (i d i q )]

[0254] Where, Δu cap0 (i d i q ) represents the correction value for the DC component of the initial bridge arm capacitor voltage at each operating point in the MMC; U cap0 Indicates the peak voltage of the target capacitor; u cap (i d i q ) represents the DC component of the initial bridge arm capacitor voltage at each operating point within the steady-state operating range of the MMC; i d Indicates the d-axis component of the MMC AC voltage; i q This represents the q-axis component of the MMC AC voltage.

[0255] The calculation formula for the DC component of the bridge arm capacitor voltage corresponding to the reference value in the steady-state range control module is as follows:

[0256] U cap (i d i q )=u cap0 +Δu cap0 (i d i q )

[0257] Among them, U cap (i d i q ) represents the DC component of the bridge arm capacitor voltage corresponding to the reference value; u cap0 This represents the initial bridge arm capacitor voltage at each operating point within the steady-state operating range of the MMC.

[0258] The operating constraints in the steady-state range control module include: the effective value of the AC current is less than or equal to the rated value of the AC current and the effective value of the bridge arm current is less than or equal to the rated value of the bridge arm current, the peak value of the modulation wave is equal to the preset modulation wave threshold, the minimum value of the bridge arm output voltage is less than or equal to the preset voltage threshold, and the capacitor voltage ripple is less than or equal to the preset ripple threshold.

[0259] The expression for the effective value of the AC current being less than or equal to the rated value of the AC current in the steady-state range control module is as follows:

[0260]

[0261] Among them, i d Indicates the d-axis component of the MMC AC voltage; i q I1 represents the q-axis component of the MMC AC voltage; I1 represents the rated value of the AC current.

[0262] The expression for the bridge arm current RMS value being less than or equal to the bridge arm current rated value in the steady-state range control module is as follows:

[0263]

[0264] Among them, i dc Indicates the DC current of the MMC; i a I represents the phase a AC current; I2 represents the rated value of the bridge arm current.

[0265] The expression corresponding to the modulation wave peak value being equal to the preset modulation wave threshold in the steady-state range control module is as follows:

[0266]

[0267] Where, m pa This represents the modulation wave of the upper bridge arm of phase a in the MMC; i d Indicates the d-axis component of the MMC AC voltage; i q I represents the q-axis component of the MMC AC voltage; d Indicates the rated active current; I q A represents the rated reactive current; A represents the preset modulation threshold.

[0268] The expression corresponding to the minimum output voltage of the bridge arm being less than or equal to a preset voltage threshold in the steady-state range control module is as follows:

[0269]

[0270] Among them, u pa C represents the output voltage of the bridge arm; C represents the preset voltage threshold.

[0271] The expression corresponding to the capacitor voltage ripple being less than or equal to the preset ripple threshold in the steady-state range control module is as follows:

[0272]

[0273] Among them, u cap Indicates the voltage across the bridge arm capacitor; u cap0B represents the initial bridge arm capacitor voltage at each operating point within the steady-state operating range of the MMC; B represents the preset ripple threshold.

[0274] The bridge arm capacitor voltage u in the steady-state range control module cap The corresponding calculation formula is as follows:

[0275]

[0276] Where N represents the number of upper bridge arm submodules of the MMC; i m Indicates the peak value of the AC phase current; C sm ω represents the capacitance value of the MMC submodule; k1 represents the angular velocity of phase a; k2 represents the first coefficient; k3 represents the third coefficient.

[0277] Example 3:

[0278] Based on the same inventive concept, this invention also provides a computer device, which includes a processor and a memory. The memory stores a computer program, which includes program instructions. The processor executes the program instructions stored in the computer storage medium. The processor may be a Central Processing Unit (CPU), or other general-purpose processors, digital signal processors (DSPs), application-specific integrated circuits (ASICs), field-programmable gate arrays (FPGAs), or other programmable logic devices, discrete gate or transistor logic devices, discrete hardware components, etc. It is the computing and control core of the terminal, suitable for implementing one or more instructions, specifically suitable for loading and executing one or more instructions in the computer storage medium to implement corresponding method flows or corresponding functions, thereby realizing the steps of the control method for the steady-state operating range of MMC in the above embodiments.

[0279] Example 4:

[0280] Based on the same inventive concept, this invention also provides a storage medium, specifically a computer-readable storage medium (Memory), which is a memory device in a computer device used to store programs and data. It is understood that the computer-readable storage medium here can include both the built-in storage medium in the computer device and extended storage media supported by the computer device. The computer-readable storage medium provides storage space that stores the terminal's operating system. Furthermore, this storage space also stores one or more instructions suitable for loading and execution by a processor. These instructions can be one or more computer programs (including program code). It should be noted that the computer-readable storage medium here can be high-speed RAM or non-volatile memory, such as at least one disk storage device. The processor can load and execute one or more instructions stored in the computer-readable storage medium to implement the steps of the MMC steady-state operating range control method in the above embodiments.

[0281] Those skilled in the art will understand that embodiments of the present invention can be provided as methods, systems, or computer program products. Therefore, the present invention can take the form of a completely hardware embodiment, a completely software embodiment, or an embodiment combining software and hardware aspects. Furthermore, the present invention can take the form of a computer program product embodied on one or more computer-usable storage media (including, but not limited to, disk storage, CD-ROM, optical storage, etc.) containing computer-usable program code.

[0282] This invention is described with reference to flowchart illustrations and / or block diagrams of methods, apparatus (systems), and computer program products according to embodiments of the invention. It will be understood that each block of the flowchart illustrations and / or block diagrams, and combinations of blocks in the flowchart illustrations and / or block diagrams, can be implemented by computer program instructions. These computer program instructions can be provided to a processor of a general-purpose computer, special-purpose computer, embedded processor, or other programmable data processing apparatus to produce a machine, such that the instructions, which execute via the processor of the computer or other programmable data processing apparatus, generate instructions for implementing the flowchart illustrations and / or block diagrams. Figure 1 One or more processes and / or boxes Figure 1 A device that provides the functions specified in one or more boxes.

[0283] These computer program instructions may also be stored in a computer-readable storage medium that can direct a computer or other programmable data processing device to function in a particular manner, such that the instructions stored in the computer-readable storage medium produce an article of manufacture including instruction means, which are implemented in a process Figure 1 One or more processes and / or boxes Figure 1 The function specified in one or more boxes.

[0284] These computer program instructions may also be loaded onto a computer or other programmable data processing equipment to cause a series of operational steps to be performed on the computer or other programmable equipment to produce a computer-implemented process, thereby providing instructions that execute on the computer or other programmable equipment for implementing the process. Figure 1 One or more processes and / or boxes Figure 1 The steps of the function specified in one or more boxes.

[0285] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and not to limit its scope of protection. Although the present invention has been described in detail with reference to the above embodiments, those skilled in the art should understand that after reading the present invention, they can still make various changes, modifications or equivalent substitutions to the specific implementation methods of the application, but these changes, modifications or equivalent substitutions are all within the scope of protection of the claims pending approval.

Claims

1. A method of controlling the steady-state operating range of an MMC, characterized by, include: The maximum bridge arm capacitor voltage within the obtained steady-state operating range of the MMC is taken as the target capacitor peak voltage. The peak voltage of the target capacitor is set as a reference value, and the steady-state operating range of the MMC is controlled based on preset operating constraints. The operating constraints include: the effective value of the AC current is less than or equal to the rated value of the AC current and the effective value of the bridge arm current is less than or equal to the rated value of the bridge arm current, the peak value of the modulation wave is equal to the preset modulation wave threshold, the minimum value of the bridge arm output voltage is less than or equal to the preset voltage threshold, and the capacitor voltage ripple is less than or equal to the preset ripple threshold.

2. The method of claim 1, wherein, The step of setting the peak voltage of the target capacitor as a reference value and controlling the steady-state operating range of the MMC based on preset operating constraints includes: Based on the reference value and the initial bridge arm capacitor voltage at each operating point in the steady-state operating range of the MMC, the correction value of the DC component corresponding to the initial bridge arm capacitor voltage at each operating point is obtained. Based on the correction value of the DC component corresponding to the initial bridge arm capacitor voltage at each operating point, calculate the DC component of the bridge arm capacitor voltage corresponding to the reference value. Based on the DC component of the bridge arm capacitor voltage corresponding to the reference value and the preset operating constraints, the steady-state operating range of the MMC is controlled. The formulas for calculating the correction values ​​of the DC components of the initial bridge arm capacitor voltages at each operating point are as follows: ； wherein, represents the correction value of the DC component of the initial bridge arm capacitor voltage of each operating point in the MMC; represents the target capacitor peak voltage; represents the DC component of the initial bridge arm capacitor voltage of each operating point in the steady-state operating range of the MMC; represents the MMC AC voltage axis component; represents the MMC AC voltage axis component; The formula for calculating the DC component of the bridge arm capacitor voltage corresponding to the reference value is as follows: ； in, This represents the DC component of the bridge arm capacitor voltage corresponding to the reference value. This represents the initial bridge arm capacitor voltage at each operating point within the steady-state operating range of the MMC.

3. The method as described in claim 1, characterized in that, The expression corresponding to the effective value of the alternating current being less than or equal to the rated value of the alternating current is as follows: ； in, Indicates MMC AC voltage Axial components; Indicates MMC AC voltage Axial components; This indicates the rated value of the alternating current.

4. The method as described in claim 1, characterized in that, The expression corresponding to the bridge arm current effective value being less than or equal to the bridge arm current rated value is as follows: ； in, This indicates the DC current of the MMC; express Phase alternating current; This indicates the rated current of the bridge arm.

5. The method as described in claim 1, characterized in that, The expression corresponding to the modulation wave peak value being equal to the preset modulation wave threshold is as follows: ； in, In MMC The modulated wave of the upper bridge arm; Indicates MMC AC voltage Axial components; Indicates MMC AC voltage Axial components; Indicates the rated active current; Indicates the rated reactive current; This indicates the preset modulation threshold.

6. The method as described in claim 1, characterized in that, The expression corresponding to the minimum output voltage of the bridge arm being less than or equal to a preset voltage threshold is as follows: ； in, Indicates the output voltage of the bridge arm; This indicates the preset voltage threshold.

7. The method as described in claim 1, characterized in that, The expression corresponding to the capacitor voltage ripple being less than or equal to a preset ripple threshold is as follows: ； in, Indicates the voltage across the bridge arm capacitor; This represents the initial bridge arm capacitor voltage at each operating point within the steady-state operating range of the MMC; This indicates the preset ripple threshold.

8. The method as described in claim 7, characterized in that, The bridge arm capacitor voltage The corresponding calculation formula is as follows: ； in, Indicates the number of bridge arm submodules on the MMC; Indicates the peak value of the AC phase current; Indicates the capacitance value of the MMC submodule; express angular velocity of the phase; Indicates the first coefficient; Indicates the second coefficient; This represents the third coefficient.

9. A control system for the steady-state operating range of MMC, characterized in that, include: Peak voltage setting module: used to take the maximum bridge arm capacitor voltage in the acquired MMC steady-state operating range as the target capacitor peak voltage; Steady-state range control module: used to set the peak voltage of the target capacitor as a reference value and control the steady-state operating range of the MMC based on preset operating constraints; The operating constraints include: the effective value of the AC current is less than or equal to the rated value of the AC current and the effective value of the bridge arm current is less than or equal to the rated value of the bridge arm current, the peak value of the modulation wave is equal to the preset modulation wave threshold, the minimum value of the bridge arm output voltage is less than or equal to the preset voltage threshold, and the capacitor voltage ripple is less than or equal to the preset ripple threshold.

10. The system as described in claim 9, characterized in that, The steady-state range control module is specifically used for: Based on the reference value and the initial bridge arm capacitor voltage at each operating point in the steady-state operating range of the MMC, the correction value of the DC component corresponding to the initial bridge arm capacitor voltage at each operating point is obtained. Based on the correction value of the DC component corresponding to the initial bridge arm capacitor voltage at each operating point, calculate the DC component of the bridge arm capacitor voltage corresponding to the reference value. Based on the DC component of the bridge arm capacitor voltage corresponding to the reference value and the preset operating constraints, the steady-state operating range of the MMC is controlled. The calculation formulas for the correction values ​​of the DC component of the initial bridge arm capacitor voltage at each operating point in the steady-state range control module are as follows: ； in, This represents the correction value for the DC component of the initial bridge arm capacitor voltage at each operating point in the MMC. Indicates the peak voltage of the target capacitor; This represents the DC component of the initial bridge arm capacitor voltage at each operating point within the steady-state operating range of the MMC. Indicates MMC AC voltage Axial components; Indicates MMC AC voltage Axial components; The calculation formula for the DC component of the bridge arm capacitor voltage corresponding to the reference value in the steady-state range control module is as follows: ； in, This represents the DC component of the bridge arm capacitor voltage corresponding to the reference value. This represents the initial bridge arm capacitor voltage at each operating point within the steady-state operating range of the MMC.

11. A computer device, characterized in that, include: One or more processors; Memory, used to store one or more programs; When the one or more programs are executed by the one or more processors, a method for controlling the steady-state operating range of MMC as described in any one of claims 1 to 8 is implemented.

12. A computer-readable storage medium, characterized in that, It contains a computer program, which, when executed, implements a method for controlling the steady-state operating range of an MMC as described in any one of claims 1 to 8.

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