Receiving end station control method, system and equipment for offshore wind power flexible direct delivery

Abstract

The invention provides a receiving end station control method, system and device for flexible direct transmission of offshore wind power, and the method comprises the steps: obtaining the parameters of an MMC in a receiving end station when the offshore wind power is transmitted through flexible direct transmission, and carrying out the AC side control of the receiving end station based on the parameters, and obtaining the upper and lower bridge arm output reference current of the MMC; performing direct current side control on the receiving end station based on the parameters to maintain the stability of the positive and negative electrode outlet voltage of the MMC; based on the parameters and the MMC upper and lower bridge arm output reference current, performing AC side control on the receiving end station to obtain MMC upper and lower bridge arm output reference voltage; modulating the upper bridge arm and the lower bridge arm of the MMC according to the output reference voltage of the upper bridge arm and the lower bridge arm of the MMC, and outputting the actual voltage of the upper bridge arm and the lower bridge arm of the MMC; the MMC is equivalently formed by splicing two completely identical H-MMC bodies. The receiving end station operation principle based on the MMC structure is reconstructed, the limitation of traditional MMC coupling control is broken through, and branch bridge arm independent control over MMC upper and lower bridge arm current and output reference voltage is achieved.

Description

Technical Field

[0001] This invention belongs to the fields of electrical engineering and flexible DC transmission technology, specifically relating to a receiving-end station control method, system and equipment for offshore wind power flexible DC transmission. Background Technology

[0002] In recent years, the installed capacity of offshore wind power has grown rapidly, while the development space for near-shore wind power is limited. Offshore wind power, with its richer wind energy resources, will inevitably become the future trend of wind power development. In the development of wind power, flexible DC transmission technology has solved the problem of limited transmission distance of AC cables, and can isolate the mutual influence between the onshore power grid and offshore wind farms, making it an effective way to reliably connect offshore wind power to the grid.

[0003] However, in the Modular Multilevel Converter (MMC) used in flexible DC transmission, the capacitors are distributed on the DC side of the sub-modules, and the three-phase arms are connected in parallel to a common DC bus. It is difficult to achieve complete voltage uniformity across the arms, which easily leads to circulating current problems. Typically, the circulating current in the MMC arms mainly consists of a second-harmonic negative-sequence AC component and a balanced three-phase DC component. When the grid voltage is unbalanced, the voltage fluctuations in the MMC arms become complex, and the power distribution between phases is difficult to balance. The resulting positive, negative, and zero-sequence circulating current components and asymmetrical DC circulating currents inevitably exacerbate the differences in current and thermal stress between the MMC phases. This can increase system losses and shorten device lifespan, and in severe cases, trigger lockout protection or even directly cause overcurrent damage to switching devices. The reason for this is that existing control methods control the differential-mode output voltage of the upper and lower arms of the three phase units of the MMC. The upper and lower arms of each phase unit are in a coupled control state, making independent control of each arm impossible and unable to cope with rapidly rising fault currents in the arms. To address this issue, a novel receiving-end station control method is urgently needed to achieve precise MMC regulation and ensure the safe and stable operation of offshore wind power flexible DC transmission systems. Summary of the Invention

[0004] To address the challenge of independently controlling the upper and lower arms of the MMC (Multi-Channel Wind Controller) in existing technologies, this invention proposes a receiving-end station control method for offshore wind power flexible DC transmission, comprising: The parameters of the modular multilevel converter (MMC) in the receiving-end converter station when offshore wind power is transmitted via flexible DC transmission are obtained, and the AC side control of the receiving-end converter station is performed based on the parameters to obtain the output reference current of the upper and lower arms of the MMC. Based on the parameters, the stability of the MMC positive and negative electrode outlet voltages is maintained by DC-side control of the receiving-end converter station; After the positive and negative terminal output voltages of the MMC stabilize, based on the parameters and the output reference currents of the upper and lower bridge arms of the MMC, the output reference voltages of the upper and lower bridge arms of the MMC are obtained by controlling the AC side of the receiving-end converter station. The upper and lower arms of the MMC are modulated according to the reference voltages output by the upper and lower arms of the MMC, and the actual voltages of the upper and lower arms of the MMC are output. The MMC is equivalent to two identical half-modular multilevel converters (H-MMCs) spliced ​​together.

[0005] Preferably, the parameters include: the overall average energy of the half-modular multilevel converter of the upper arm of the MMC, the overall average energy of the half-modular multilevel converter of the lower arm of the MMC, the overall energy of a single arm, the energy setting value of a single arm, the reactive power reference value of the MMC, the valve-side d-axis voltage of the transformer connected to the MMC, the valve-side q-axis voltage of the transformer connected to the MMC, the reference values ​​of the 0-axis current of the upper and lower arms of the MMC, the actual output voltage of the positive and negative terminals of the DC side of the MMC, the reference voltage of the positive and negative terminals of the DC side of the MMC, the line voltage of the transformer valve side, the average value of the three-phase voltage of the transformer valve side, the actual output current of the upper and lower arms of the MMC, and the reactance of the arm reactor.

[0006] Preferably, the step of obtaining the MMC upper and lower arm output reference currents by controlling the AC side of the receiving-end converter station based on the parameters includes: The overall average energy of the half modular multilevel converter that serves as the upper arm of the MMC and the overall average energy of the half modular multilevel converter that serves as the lower arm of the MMC are respectively subtracted from the single arm energy setting value to obtain the first and second deviation values. By inputting the first and second deviation values ​​into the PI controller to control the AC side of the receiving-end converter station, the reference values ​​of the active current output of the upper and lower arms of the MMC are obtained. Based on the MMC reactive power reference value, the valve-side d-axis voltage and q-axis voltage of the transformer, the reactive current reference values ​​of the upper and lower bridge arms of the MMC are calculated. Based on the reference values ​​of the active current output of the upper and lower bridge arms of the MMC, the reference values ​​of the reactive current output of the upper and lower bridge arms of the MMC, the reference value of the 0-axis current, and the set phase angle of the phase-locked loop output, the reference current output of the upper and lower bridge arms of the MMC is determined by coordinate transformation.

[0007] Preferably, maintaining the stability of the MMC positive and negative electrode outlet voltages by controlling the DC side of the receiving-end converter station based on the parameters includes: The actual output voltages of the positive and negative terminals of the MMC DC side and the reference output voltages of the positive and negative terminals of the MMC DC side are input to the PI controller to calculate the deviation between the output and the reference voltage. Based on the deviation between the output and the reference voltage, the PI controller adjusts the reference voltage of the positive and negative terminals of the MMC DC side to ensure that the actual output voltage of the positive and negative terminals of the MMC DC side is consistent with the corresponding reference voltage of the positive and negative terminals of the MMC DC side, thereby maintaining the stability of the output voltage of the positive and negative terminals of the MMC.

[0008] Preferably, the step of obtaining the MMC upper and lower arm output reference voltages by controlling the AC side of the receiving-end converter station based on the parameters and the MMC upper and lower arm output reference currents includes: Based on the transformer valve side line voltage and the average value of the three-phase voltage on the transformer valve side, calculate the voltage at the reactor connection point in the upper and lower arms of the MMC; By introducing the overall energy of the single bridge arm into the AC side control of the receiving-end converter station, the output deviation current of the upper and lower bridge arms of the MMC is calculated by the PI controller. Based on the reference output current of the upper and lower bridge arms of the MMC, the actual output current of the upper and lower bridge arms of the MMC, the output deviation current of the upper and lower bridge arms of the MMC, and the reactance of the bridge arm reactor, calculate the voltage difference across the reactor in the upper and lower bridge arms of the MMC. Based on the actual output voltage of the positive and negative terminals of the MMC DC side, the voltage difference across the reactors in the upper and lower arms of the MMC, and the voltage at the connection point of the reactors in the upper and lower arms of the MMC, the output reference voltage of the upper and lower arms of the MMC is determined by limiting.

[0009] Preferably, the step of introducing the overall energy of the single bridge arm into the AC side control of the receiving-end converter station, and calculating the output deviation current of the upper and lower bridge arms of the MMC by the PI controller, includes: The difference between the overall energy of the single bridge arm and the set energy value of the single bridge arm is used to obtain the third deviation value; The third deviation value is input to the PI controller for AC side control of the receiving-end converter station, and the output deviation current of the upper and lower bridge arms of the MMC is obtained.

[0010] Preferably, the step of determining the output reference voltage of the upper and lower bridge arms of the MMC based on the actual output voltage of the positive and negative terminals of the MMC DC side, the voltage difference across the reactors in the upper and lower bridge arms of the MMC, and the voltage at the connection point of the reactors in the upper and lower bridge arms of the MMC by limiting includes: The actual output voltage of the DC side positive terminal of the MMC is successively compared with the voltage difference across the reactor in the upper arm of the MMC and the voltage at the connection point of the reactor in the upper and lower arms of the MMC, and the output reference voltage of the upper arm of the MMC is obtained by limiting the amplitude. The voltage difference at the connection point of the reactor in the upper and lower arms of the MMC is successively subtracted from the voltage difference across the reactor in the lower arm of the MMC and the actual output voltage of the negative terminal of the DC side of the MMC, and then the output reference voltage of the lower arm of the MMC is obtained by limiting the amplitude.

[0011] Based on the same inventive concept, the present invention also provides a receiving-end station control system for offshore wind power flexible DC transmission, including: an AC side control module, a DC side control module, a reference voltage calculation module and a bridge arm modulation module; The AC-side control module is used to acquire the parameters of the modular multilevel converter (MMC) in the receiving-end converter station when offshore wind power is transmitted via flexible DC transmission, and to obtain the upper and lower bridge arm output reference current of the MMC by performing AC-side control on the receiving-end converter station based on the parameters. The DC-side control module is used to maintain the stability of the MMC positive and negative terminal outlet voltages by performing DC-side control on the receiving-end converter station based on the parameters. The reference voltage calculation module is used to obtain the reference voltage of the upper and lower bridge arms of the MMC based on the parameters and the reference current output of the upper and lower bridge arms of the MMC after the positive and negative terminal outlet voltages of the MMC have stabilized, by controlling the AC side of the receiving-end converter station. The bridge arm modulation module is used to modulate the upper and lower bridge arms of the MMC according to the reference voltage output of the upper and lower bridge arms of the MMC, and output the actual voltage of the upper and lower bridge arms of the MMC. The MMC is equivalent to two identical half-modular multilevel converters spliced ​​together.

[0012] Based on the same inventive concept, the present invention also provides an electronic device, comprising: at least one processor and a memory; wherein the memory and the processor are connected via a bus; The memory is used to store one or more programs; When the one or more programs are executed by the at least one processor, a receiving-end station control method for offshore wind power flexible DC transmission as described above is implemented.

[0013] Based on the same inventive concept, the present invention also provides a readable storage medium having an executable program stored thereon, which, when executed, implements a receiving-end station control method for offshore wind power flexible DC transmission as described above.

[0014] Compared with the prior art, the beneficial effects of the present invention are as follows: This invention provides a receiving-end station control method, system, and device for offshore wind power transmission via flexible DC transmission. The method includes: acquiring parameters of a modular multilevel converter (MMC) in the receiving-end converter station during offshore wind power transmission via flexible DC transmission; obtaining reference currents for the upper and lower arms of the MMC by performing AC-side control on the receiving-end converter station based on the parameters; maintaining the stability of the positive and negative outlet voltages of the MMC by performing DC-side control on the receiving-end converter station based on the parameters; after the positive and negative outlet voltages of the MMC stabilize, obtaining reference voltages for the upper and lower arms of the MMC by performing AC-side control on the receiving-end converter station based on the parameters and the reference currents for the upper and lower arms of the MMC; modulating the upper and lower arms of the MMC according to the reference voltages for the upper and lower arms of the MMC, and outputting the actual voltages of the upper and lower arms of the MMC; the MMC is equivalent to two identical half-modular multilevel converters spliced ​​together. This invention, based on the operating principle of a semi-modular multilevel converter (MMC), reconstructs the operating principle of a receiving-end converter station based on an MMC structure. It overcomes the limitations of traditional MMC coupled control, enabling independent control of the current in the upper and lower arms of the MMC, and consequently, independent control of the output reference voltage of the upper and lower arms. This suppresses circulating current components, reduces differences in interphase current and thermal stress, lowers system losses, and extends device lifespan. It is adaptable to offshore wind power flexible DC transmission scenarios, ensuring large-scale, long-distance stable grid connection of wind power and providing technical support for the construction of new power systems. Attached Figure Description

[0015] Figure 1 A typical topology diagram of a new energy transmission system via flexible direct transmission. Figure 2 This is a schematic diagram of the H-MMC structure; Figure 3 A simplified equivalent structure diagram of H-MMC in the form of a controlled source; Figure 4 This is a flowchart of a receiving-end station control method for offshore wind power flexible DC transmission according to the present invention; Figure 5 Topology diagram of the receiving-end converter station in the form of a controlled source; Figure 6 This is a flowchart of step 1 of embodiment 1 of the present invention; Figure 7 This is the control logic diagram for the reference value of the active current output of the upper arm of the MMC in this invention. Figure 8 This is the control logic diagram for the reference value of the active current output of the lower arm of the MMC in this invention. Figure 9 This is the logic diagram for controlling the positive and negative DC voltages at the MMC outlet of the present invention. Figure 10 This is a flowchart of step 3 in Embodiment 1 of the present invention; Figure 11 This is the adjustment logic diagram for the average value of the actual three-phase output voltage of the receiving-end converter station according to the present invention; Figure 12 This is a logic diagram of the output deviation current of each phase of the H-MMC at the receiving end of the present invention. Figure 13 This is the logic diagram for calculating the voltage difference across the MMC bridge arm reactor in the receiving-end converter station of the present invention; Figure 14 This is a flowchart illustrating the generation process of the output reference voltage for each phase of the MMC upper arm in the receiving-end converter station of the present invention. Figure 15 This is a flowchart illustrating the generation process of the output reference voltage of each phase of the MMC lower arm in the receiving-end converter station of the present invention. Figure 16 This is a schematic diagram of the basic structure of a receiving-end station control system for offshore wind power flexible DC transmission according to the present invention. Figure 17 This is a schematic diagram of an electronic device structure provided by the present invention. Detailed Implementation

[0016] like Figure 1 The diagram shows a typical topology of a new energy power plant's direct transmission system. PCC is the common connection point for the AC system of the new energy power plant. L eq and R eq These are the equivalent inductance and equivalent resistance of an AC line, respectively. S 1 and S 2 represents the sending-end converter station and the receiving-end converter station, respectively, and AC represents the receiving-end AC system. Sending-end converter station S 1. A grid-type control method with constant AC voltage amplitude and frequency is adopted for the receiving-end converter station. S 2. A grid-following control strategy is adopted, with constant DC-side voltage and AC-side reactive power. Large-scale new energy sources exhibit significant characteristics such as "randomness, volatility, and intermittency," and their transmission through the sending-end converter station... S 1. After collection and AC-DC conversion, power is transmitted via flexible DC transmission; the DC transmission line transmits energy through the receiving-end converter station. S 2. After DC-AC conversion, power is absorbed in AC form.

[0017] A complete MMC structure can be considered as being composed of two identical half-modular multilevel converters (H-MMCs) joined together. For example... Figure 2 The diagram shown is a schematic representation of the H-MMC structure. Among them, u sabc For AC power, T is the transformer.L 0 represents the bridge arm inductance, SM i (i=1,2,…,N) are sub-modules based on a half-bridge structure. v j ( j =a, b, c (the same below) are the internal potentials of the three-phase bridge arm of H-MMC, respectively, e j These are the output voltages of the three-phase bridge arms of the H-MMC. i j These are the three-phase bridge arm currents of the H-MMC. L It is a DC reactor. U dc The voltage is the DC side voltage. The active and reactive power input from the AC system to the H-MMC can satisfy formula (1).

[0018] Formula (1) In the formula, P represents active power and Q represents reactive power. v abc The internal potential of the A / B / C phase bridge arm, U sabc For exchange source u sabc The effective value of the voltage, δ 1. δ 2 are the AC source u sabc and internal potential v abc phase angle, V abc Internal potential v abc The effective value of the voltage, X for u sabc and v abc The equivalent impedance between them.

[0019] like Figure 3 The diagram shown is a simplified equivalent structure of an H-MMC in a controlled-source configuration. In the diagram, V This is the effective value of the internal potential. i dc The output current is DC. When the system is running stably, it can be determined based on the internal potential of the three-phase bridge arms. v j and DC output voltage V dc Calculate the bridge arm output reference voltage e jref Bridge arm output reference voltage e jref The following formula (2) can be satisfied.

[0020] Formula (2) Based on the H-MMC three-phase bridge arm output reference voltage e jref By using MMC modulation and submodule capacitor voltage balance control, the voltage of the H-MMC three-phase bridge arm can be balanced. e j The output.

[0021] This invention, based on the H-MMC operating principle, reconstructs the operating principle of the receiving-end converter station based on the MMC structure. It breaks through existing control methods for flexible DC transmission systems and innovatively proposes a receiving-end station control method for offshore wind power flexible DC transmission. This method enables independent control of the upper and lower arm currents of the MMC, thereby achieving independent control of the output reference voltages of the upper and lower arms of the MMC, significantly improving the safe and stable operation of the offshore wind power flexible DC transmission system. Furthermore, the method provided by this invention is not limited to offshore wind power; it is also applicable to other renewable energy access scenarios.

[0022] To better understand the present invention, the following description, in conjunction with the accompanying drawings and embodiments, will further illustrate the content of the present invention.

[0023] Example 1: A control method for receiving-end stations used in flexible DC transmission of offshore wind power is illustrated in the flowchart below. Figure 4 As shown, it includes: Step 1: Obtain the parameters of the modular multilevel converter (MMC) in the receiving-end converter station when offshore wind power is transmitted via flexible DC transmission, and obtain the reference output current of the upper and lower arms of the MMC by controlling the AC side of the receiving-end converter station based on the parameters. Step 2: Based on the parameters, maintain the stability of the MMC positive and negative electrode outlet voltages by performing DC-side control on the receiving-end converter station; Step 3: After the positive and negative terminal output voltages of the MMC stabilize, based on the parameters and the reference currents output by the upper and lower arms of the MMC, the reference voltages output by the upper and lower arms of the MMC are obtained by controlling the AC side of the receiving-end converter station. Step 4: Modulate the upper and lower arms of the MMC according to the reference voltage output of the upper and lower arms of the MMC, and output the actual voltage of the upper and lower arms of the MMC. The MMC is equivalent to two identical half-modular multilevel converters spliced ​​together.

[0024] Controlled source type receiving-end converter station topology such as Figure 5 As shown in the figure, u abc This refers to the three-phase voltage on the transformer valve side. u jThese are the voltages at the connection points of the upper and lower bridge arm reactors of the MMC, respectively. i jp For the upper arm current of the MMC, i jn For the lower arm current of the MMC, e jp This is the sum of the output voltages of the capacitors in the MMC upper bridge arm submodules. e jn This is the sum of the output voltages of the capacitors in the MMC lower bridge arm submodules. v jp To output internal potential for the upper arm of the MMC, v jn For the output internal potential of the lower bridge arm of the MMC, L p and L n These are the positive DC line reactance and negative DC line reactance of the MMC, respectively. i dcp and i dcn DC current at the positive and negative terminals of MMC respectively. u p and u n These represent the DC reactor line-to-ground voltages of the upper and lower bridge arms, respectively. Δ and Y of the transformer represent the delta and star connections, respectively, with O being the neutral point. Based on known AC voltage sources... u sabc and DC line positive and negative current i dcp , i dcn Designing the control method for the AC / DC side and bridge arm output reference voltage of the MMC is the key to ensuring the stable operation of the receiving-end converter station.

[0025] In this invention, the MMC of the receiving-end converter station can be considered as two identical H-MMCs spliced ​​together. In the controlled-source type receiving-end converter station, the active power from the sending-end converter station is absorbed by the upper and lower H-MMCs of the receiving-end converter station and then fed into the receiving-end AC system. This allows for AC-side control of the receiving-end converter station. Based on the active current reference value and the reactive current reference value, the upper and lower bridge arm output reference currents of the receiving-end converter station are calculated.

[0026] Step 1 detailed process, such as Figure 6 As shown, it includes: Step 1.1 Calculate the overall average energy of the half-modular multilevel converter, which serves as the upper arm of the MMC. E avp And the overall energy average of the half-modular multilevel converter as the lower arm of the MMC. E avn, respectively with the single bridge arm energy setting value E set By taking the difference, we obtain the first deviation value Δ. E p Second deviation value Δ E n .

[0027] Step 1.2 involves inputting the first and second deviation values ​​into a PI controller to control the AC side of the receiving-end converter station, thereby obtaining reference values ​​for the active current output of the upper and lower arms of the MMC; the control logic for this step is as follows: Figure 7 and Figure 8 As shown. It should be understood that the PI controller is existing technology, which uses a linear combination of the proportional and integral values ​​of the first and second deviations to form the control quantity, thereby obtaining the reference values ​​of the d-axis current output of the upper and lower H-MMC bridge arms. i dpref and i dnref The reference values ​​of the d-axis current output of the upper and lower H-MMC bridge arms are the reference values ​​of the active current output of the upper and lower MMC bridge arms.

[0028] Step 1.3 Based on the MMC reactive power reference value, the valve-side d-axis voltage and q-axis voltage of the transformer, calculate the MMC upper and lower bridge arm output reactive current reference values; The instantaneous reactive power in the two-phase rotating dq coordinate system can satisfy the following formula (3).

[0029] Formula (3) In the formula, u d and u q These are the d-axis voltage and the q-axis voltage, respectively. i d and i q These are the d-axis current and q-axis current, respectively. When the flexible DC transmission system is in steady-state operation and the phase-locked loop is synchronously phase-locked, u q The value is 0. At this time, the reference values ​​of the reactive current output of the upper and lower arms of the MMC at the receiving end converter station can be calculated according to formula (3). The reference value of the reactive current output of the lower arm of the MMC at the receiving end converter station can be directly proportional to the reactive power reference value and inversely proportional to the d-axis voltage on the transformer valve side. The reference value of the reactive current output of the upper arm of the MMC at the receiving end converter station can be opposite to the reference value of the reactive current output of the lower arm of the MMC at the receiving end converter station. The reference values ​​of the reactive current output of the upper and lower arms of the MMC at the receiving end converter station can satisfy the following formula (4).

[0030] Formula (4) In the formula, i qpref This is the reference value for the reactive current output of the upper arm of the MMC at the receiving end converter station. i qnref This is the reference value for the reactive current output of the lower arm of the MMC at the receiving-end converter station. Q ref This is the MMC reactive power reference value.

[0031] Step 1.4 Based on the reference values ​​of the active current output of the upper and lower bridge arms of the MMC, the reference values ​​of the reactive current output of the upper and lower bridge arms of the MMC, the reference value of the 0-axis current, and the set phase angle of the phase-locked loop output, the reference current output of the upper and lower bridge arms of the MMC is determined by coordinate transformation.

[0032] The reference value for the zero-axis current of the MMC upper arm can be determined based on the DC current of the MMC positive terminal. i dcp It has been determined that the reference value for the zero-axis current of the lower arm of the MMC can be determined based on the DC current of the negative terminal of the MMC. i dcn It has been confirmed. In such cases... Figure 5 In the controlled-source type receiving-end converter station shown, based on the circuit topology and arm current composition, the reference value of the 0-axis current of the upper H-MMC arm is... i 0pref for i dcp / 3, Reference value for zero-axis current of lower H-MMC bridge arm i 0nref for- i dcp / 3. Based on the active current reference value, reactive current reference value and 0-axis current reference value, the output reference current of the upper and lower arms of the MMC of the receiving-end converter station can satisfy the following formula (5).

[0033] Formula (5) In the formula, i jkref ( k =n, p, the same below) are the bridge arm output reference currents of the upper and lower H-MMC respectively; θ This is the phase angle output of the phase-locked loop.

[0034] Step 2 specifically includes: Specifically, in such Figure 5 In the controlled-source type of the receiving-end converter station shown, the main function of DC-side control is to maintain the stability of the positive and negative output voltages of the MMC at the receiving-end converter station. Therefore, based on the basic principles of automatic control, a control system can be designed as follows: Figure 9 The diagram shows the DC voltage control logic for the positive and negative terminals of the MMC output. The actual output voltages of the MMC positive and negative terminals are then... V dckWith the corresponding DC reference voltage V dckref After comparison, the deviation value Δ can be used. V dck It adjusts itself to ensure that the actual output voltage is consistent with the corresponding reference voltage.

[0035] Step 3, specific process, such as Figure 10 As shown, it includes: Step 3.1 Based on the transformer valve side line voltage and the average value of the three-phase voltage on the transformer valve side, calculate the voltage at the reactor connection point in the upper and lower arms of the MMC; In such Figure 5 In the controlled-source type of the receiving-end converter station shown, the transformer valve side is connected in a delta configuration, meaning the transformer valve side voltage is in a "floating ground" state. This causes the three-phase-to-ground voltage on the transformer valve side to change due to system operating conditions, thus affecting the MMC arm output reference voltage. To ensure the accuracy of the MMC arm output reference voltage, it is necessary to calculate the voltage at the connection points of the reactors in the upper and lower MMC arms in real time. u a , u b and u c .

[0036] Specifically, any two of the three sets of line voltages on the transformer valve side can be measured. The actual value of any two of the three sets of line voltages on the transformer valve side and the average value of the three-phase voltages on the transformer valve side can then be used. u av The voltage at the connection point of the upper and lower arm reactors of the MMC at the receiving-end converter station was determined. u a , u b and u c Voltage at the connection point of the upper and lower arm reactors of the MMC at the receiving-end converter station. u a , u b and u c The following formula (6) can be satisfied.

[0037] Formula (6) In the formula, u ab , u bc and u av These represent the line voltages between phases A and B, the line voltages between phases B and C, and the average values ​​of the three phases A, B, and C on the transformer valve side, respectively.

[0038] In actual system operation, in order to maintain the average value of the three-phase voltage on the transformer valve side u av Under ideal operating conditions, the average value of the actual three-phase output voltage can be compared with the reference value. u avref After comparison, the system uses the deviation value to adjust itself, ultimately achieving... u av and u avref Maintain consistency.

[0039] Specifically, such as Figure 11 As shown, the reference value of the average three-phase voltage on the transformer valve side can be used. u avref Actual value of the three-phase voltage average on the transformer valve side u av The difference Δ u av Perform PI control to make Δ u av Equal to or approaching zero, making u av and u avref Maintain consistency.

[0040] Step 3.2 By introducing the overall energy of the single bridge arm into the AC side control of the receiving-end converter station, the output deviation current of the upper and lower bridge arms of the MMC is calculated by the PI controller; To achieve overall energy balance in the MMC, the surplus or deficit of energy in a single MMC arm can be incorporated into the control of that arm's output current. For example... Figure 12 As shown, the overall energy of each single bridge arm of the upper and lower H-MMC phases of the receiving-end converter station can be monitored in real time. E jk With single bridge arm energy setting value E set The difference Δ E jk The third deviation value Δ E jk After being controlled by the PI controller, the output deviation current Δ of each phase of the upper and lower H-MMCs at the receiving end converter station can be obtained. i jk It should be understood that the PI controller is existing technology, meaning that the deviation value Δ... E jk The proportional and integral terms are linearly combined to form the control quantity, namely the bridge arm output deviation current Δ. i jk .

[0041] Step 3.3 Based on the reference output current of the upper and lower bridge arms of the MMC, the actual output current of the upper and lower bridge arms of the MMC, the output deviation current of the upper and lower bridge arms of the MMC, and the reactance of the bridge arm reactor, calculate the voltage difference across the reactor in the upper and lower bridge arms of the MMC. During operation, a deviation exists between the reference current and the actual output current of the MMC arm in the receiving-end converter station, resulting in a voltage difference across the arm reactor. The calculation method for the voltage difference across the MMC arm reactor in the receiving-end converter station is as follows: Figure 13 As shown, the real-time bridge arm output reference current of the receiving-end converter station can be obtained from this. i jkref Compared with the actual output value i jk and bridge arm output deviation current Δ i jk The difference Δ i Then, based on the relationship between voltage and current, the bridge arm reactor is calculated. L Voltage difference Δ across 0 u L0jk .

[0042] Specifically, the reference current can be output from the upper arm of the MMC at the receiving-end converter station in real time. i jpref Compared with the actual output value i jp and the output deviation current Δ of the upper bridge arm of the MMC i jp The difference Δ i Then, based on the relationship between voltage and current, the upper arm reactor is calculated. L 0 Voltage difference across terminals u L0jp The reference current can be output from the lower arm of the MMC at the receiving-end converter station in real time. i jnref Compared with the actual output value i jn and the MMC lower bridge arm output deviation current Δ i jn The difference Δ i Then, based on the relationship between voltage and current, the lower arm reactor is calculated. L 0 Voltage difference across terminals u L0jn .

[0043] The voltage across the bridge arm reactor can satisfy the following formula (7).

[0044] u L = LΔi / ΔT Formula (7) in, uL The voltage across the bridge arm reactor is Δ. T The sampling period.

[0045] Step 3.4 Based on the actual output voltage of the positive and negative terminals of the MMC DC side, the voltage difference across the reactors in the upper and lower arms of the MMC, and the voltage at the connection point of the reactors in the upper and lower arms of the MMC, the output reference voltage of the upper and lower arms of the MMC is determined by limiting.

[0046] Based on the basic operating principles of MMC, in such Figure 5 In the controlled-source type receiver-end converter station topology shown, the actual output voltage of the bridge arm is... e jk This should be the sum of the capacitor voltages of all sub-modules in the bridge arm, selectively and orderly activated based on the bridge arm output reference voltage. For example... Figure 14 The diagram shows the generation process of the output reference voltage for each phase of the upper arm of the MMC in the receiving-end converter station. The output reference voltage for each phase of the upper arm of the MMC in the receiving-end converter station can be obtained from the actual output voltage at the positive terminal outlet of the MMC. V dcp The voltage difference across the corresponding upper bridge arm reactor u L0jp and voltage at the connection point of the upper and lower bridge arm reactors u j To calculate.

[0047] Specifically, the actual output voltage of the MMC positive terminal outlet of the receiving-end converter station can be... V dcp The voltage difference across the corresponding upper bridge arm reactor u L0jp Divide the difference, and then subtract the voltage at the connection point of the upper and lower bridge arm reactors. u j The resulting difference, after being limited, is used as the output reference voltage for the upper arm of the receiving-end MMC. e jpref The limiting operation is the same as that of the sending-end converter station. Based on the output voltage characteristics of the half-bridge submodule capacitors, the upper and lower limits of the bridge arm output reference voltage should be set to... NV c and 0, where V c This is the power supply voltage.

[0048] like Figure 15 The diagram shows the generation process of the output reference voltage for each phase of the lower arm of the MMC in the receiving-end converter station. The output reference voltage for each phase of the lower arm of the MMC in the receiving-end converter station can be obtained from the voltage at the connection point of the reactors of the upper and lower arms of the MMC. u j Voltage difference across the corresponding bridge arm reactor u L0jnand the actual output voltage of the MMC negative terminal outlet V dcn To achieve this.

[0049] Specifically, the voltage at the connection point of the upper and lower bridge arm reactors of the MMC can be... u j The voltage difference across the corresponding lower bridge arm reactor u L0jn Divide the difference, then subtract the actual output voltage of the MMC negative terminal. V dcn The resulting difference, after being subjected to a limiting operation, is used as the output reference voltage of the lower arm of the receiving-end MMC. e jnref The limiting operation is the same as that of the sending-end converter station. Based on the output voltage characteristics of the half-bridge submodule capacitors, the upper and lower limits of the bridge arm output reference voltage should be set to... NV c And 0.

[0050] The beneficial effects of this invention are as follows: 1. Breaking through the limitations of traditional MMC coupling control, it enables independent control of the current of the upper and lower bridge arms of the MMC, thereby enabling independent control of the output reference voltage of the upper and lower bridge arms of the MMC, suppressing circulating current components, reducing the difference between phase current and thermal stress, reducing system losses, and extending the service life of devices.

[0051] 2. An energy deviation feedback mechanism for the bridge arm is introduced. Through PI adaptive adjustment, it can quickly respond to grid imbalance conditions and fault currents, thereby improving the transient stability of the flexible DC transmission system.

[0052] 3. Adaptable to offshore wind power flexible DC transmission scenarios, ensuring large-scale, long-distance stable grid connection of wind power, and providing technical support for the construction of new power systems.

[0053] Example 2: Based on the same inventive concept, this invention also provides a receiving-end station control system for offshore wind power flexible DC transmission, which is applied to a configuration of a new energy power plant, a sending-end converter station, a receiving-end converter station, and a receiving-end AC system connected in sequence. The sending-end converter station adopts… V / f The power output is controlled by a "current + voltage" control method at the receiving end converter station for power absorption. New energy from the renewable energy plant is collected at the sending-end converter station and converted from AC to DC, then transmitted as DC power to the receiving-end converter station. Energy transmitted via DC lines is converted from DC to AC at the receiving-end converter station and then transmitted as AC power to the receiving-end AC system. Taking wind farms as an example, the renewable energy plant corresponds to a wind farm station, thus transmitting power as DC. A basic structural diagram of a receiving-end station control system for flexible DC transmission of offshore wind power is shown below. Figure 16As shown, it includes: an AC side control module, a DC side control module, a reference voltage calculation module, and a bridge arm modulation module; The AC-side control module is used to acquire the parameters of the modular multilevel converter (MMC) in the receiving-end converter station when offshore wind power is transmitted via flexible DC transmission, and to obtain the upper and lower bridge arm output reference current of the MMC by performing AC-side control on the receiving-end converter station based on the parameters. The DC-side control module is used to maintain the stability of the MMC positive and negative terminal outlet voltages by performing DC-side control on the receiving-end converter station based on the parameters. The reference voltage calculation module is used to obtain the reference voltage of the upper and lower bridge arms of the MMC based on the parameters and the reference current output of the upper and lower bridge arms of the MMC after the positive and negative terminal outlet voltages of the MMC have stabilized, by controlling the AC side of the receiving-end converter station. The bridge arm modulation module is used to modulate the upper and lower bridge arms of the MMC according to the reference voltage output of the upper and lower bridge arms of the MMC, and output the actual voltage of the upper and lower bridge arms of the MMC. The MMC is equivalent to two identical half-modular multilevel converters spliced ​​together.

[0054] This invention, based on the operating principle of a semi-modular multilevel converter (MMC), reconstructs the operating principle of a receiving-end converter station based on an MMC structure. It overcomes the limitations of traditional MMC coupled control, enabling independent control of the current in the upper and lower arms of the MMC, and consequently, independent control of the output reference voltage of the upper and lower arms. This suppresses circulating current components, reduces differences in interphase current and thermal stress, lowers system losses, and extends device lifespan. It is adaptable to offshore wind power flexible DC transmission scenarios, ensuring large-scale, long-distance stable grid connection of wind power and providing technical support for the construction of new power systems.

[0055] Example 3: like Figure 17 As shown, the present invention also provides an electronic device, which may be a computer device, a microcontroller device, a smart mobile device, etc. The electronic device in this embodiment may include a processor, a memory, a transceiver component, etc. The memory, processor, and transceiver component are connected via a bus; the memory can be used to store executable programs, and an exemplary executable program may include instructions; the processor is used to execute the instructions stored in the memory. The memory can also be used to store data, which can be accessed and / or modified when instructions are executed.

[0056] The processor may be a central processing unit (CPU), or it may be 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 core and control core of the terminal, and it is suitable for implementing one or more instructions. Specifically, it is suitable for loading and executing one or more instructions in the storage medium to realize the corresponding method flow or corresponding function, so as to realize the steps of the receiving station control method for offshore wind power flexible DC transmission in the above embodiments.

[0057] This invention, based on the operating principle of a semi-modular multilevel converter (MMC), reconstructs the operating principle of a receiving-end converter station based on an MMC structure. It overcomes the limitations of traditional MMC coupled control, enabling independent control of the current in the upper and lower arms of the MMC, and consequently, independent control of the output reference voltage of the upper and lower arms. This suppresses circulating current components, reduces differences in interphase current and thermal stress, lowers system losses, and extends device lifespan. It is adaptable to offshore wind power flexible DC transmission scenarios, ensuring large-scale, long-distance stable grid connection of wind power and providing technical support for the construction of new power systems.

[0058] Example 4: Based on the same inventive concept, this invention also provides a readable storage medium, specifically an electronic device readable storage medium (Memory). This readable storage medium is a memory device within an electronic device used to store programs and data. It is understood that the storage medium here can include both built-in storage media within the electronic device and extended storage media supported by the electronic device. The storage medium provides storage space, which 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 executable programs (including program code). It should be noted that the storage medium here can be high-speed RAM or non-volatile memory, such as at least one disk storage device. Loading and executing one or more instructions stored in the storage medium by the processor can implement the steps of a receiving-end station control method for offshore wind power flexible DC transmission in the above embodiments.

[0059] This invention, based on the operating principle of a semi-modular multilevel converter (MMC), reconstructs the operating principle of a receiving-end converter station based on an MMC structure. It overcomes the limitations of traditional MMC coupled control, enabling independent control of the current in the upper and lower arms of the MMC, and consequently, independent control of the output reference voltage of the upper and lower arms. This suppresses circulating current components, reduces differences in interphase current and thermal stress, lowers system losses, and extends device lifespan. It is adaptable to offshore wind power flexible DC transmission scenarios, ensuring large-scale, long-distance stable grid connection of wind power and providing technical support for the construction of new power systems.

[0060] 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.

[0061] 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.

[0062] 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.

[0063] 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 1The steps of the function specified in one or more boxes.

[0064] The above are merely embodiments of the present invention and are not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention are included within the scope of the claims of the present invention pending approval.

Claims

1. A receiving-end station control method for offshore wind power flexible DC transmission, characterized in that, include: The parameters of the modular multilevel converter (MMC) in the receiving-end converter station when offshore wind power is transmitted via flexible DC transmission are obtained, and the AC side control of the receiving-end converter station is performed based on the parameters to obtain the output reference current of the upper and lower arms of the MMC. Based on the parameters, the stability of the MMC positive and negative electrode outlet voltages is maintained by DC-side control of the receiving-end converter station; After the positive and negative terminal output voltages of the MMC stabilize, based on the parameters and the output reference currents of the upper and lower bridge arms of the MMC, the output reference voltages of the upper and lower bridge arms of the MMC are obtained by controlling the AC side of the receiving-end converter station. The upper and lower arms of the MMC are modulated according to the reference voltages output by the upper and lower arms of the MMC, and the actual voltages of the upper and lower arms of the MMC are output. The MMC is equivalent to two identical half-modular multilevel converters spliced ​​together.

2. The method as described in claim 1, characterized in that, The parameters include: the overall average energy of the half-modular multilevel converter of the upper arm of the MMC, the overall average energy of the half-modular multilevel converter of the lower arm of the MMC, the overall energy of a single arm, the energy setting value of a single arm, the reactive power reference value of the MMC, the valve-side d-axis voltage of the transformer connected to the MMC, the valve-side q-axis voltage of the transformer connected to the MMC, the reference values ​​of the 0-axis current of the upper and lower arms of the MMC, the actual output voltage of the positive and negative terminals of the DC side of the MMC, the reference voltage of the positive and negative terminals of the DC side of the MMC, the line voltage of the transformer valve side, the average value of the three-phase voltage of the transformer valve side, the actual output current of the upper and lower arms of the MMC, and the reactance of the arm reactor.

3. The method as described in claim 2, characterized in that, The process of obtaining the MMC upper and lower arm output reference currents by controlling the AC side of the receiving-end converter station based on the parameters includes: The overall average energy of the half modular multilevel converter that serves as the upper arm of the MMC and the overall average energy of the half modular multilevel converter that serves as the lower arm of the MMC are respectively subtracted from the single arm energy setting value to obtain the first and second deviation values. By inputting the first and second deviation values ​​into the PI controller to control the AC side of the receiving-end converter station, the reference values ​​of the active current output of the upper and lower arms of the MMC are obtained. Based on the MMC reactive power reference value, the valve-side d-axis voltage and q-axis voltage of the transformer, the reactive current reference values ​​of the upper and lower bridge arms of the MMC are calculated. Based on the reference values ​​of the active current output of the upper and lower bridge arms of the MMC, the reference values ​​of the reactive current output of the upper and lower bridge arms of the MMC, the reference value of the 0-axis current, and the set phase angle of the phase-locked loop output, the reference current output of the upper and lower bridge arms of the MMC is determined by coordinate transformation.

4. The method as described in claim 2, characterized in that, The step of maintaining stable MMC positive and negative terminal outlet voltages by controlling the DC side of the receiving-end converter station based on the parameters includes: The actual output voltages of the positive and negative terminals of the MMC DC side and the reference output voltages of the positive and negative terminals of the MMC DC side are input to the PI controller to calculate the deviation between the output and the reference voltage. Based on the deviation between the output and the reference voltage, the PI controller adjusts the reference voltage of the positive and negative terminals of the MMC DC side to ensure that the actual output voltage of the positive and negative terminals of the MMC DC side is consistent with the corresponding reference voltage of the positive and negative terminals of the MMC DC side, thereby maintaining the stability of the output voltage of the positive and negative terminals of the MMC.

5. The method as described in claim 2, characterized in that, The method of obtaining the MMC upper and lower bridge arm output reference voltages based on the parameters and the MMC upper and lower bridge arm output reference currents by controlling the AC side of the receiving-end converter station includes: Based on the transformer valve side line voltage and the average value of the three-phase voltage on the transformer valve side, calculate the voltage at the reactor connection point in the upper and lower arms of the MMC; By introducing the overall energy of the single bridge arm into the AC side control of the receiving-end converter station, the output deviation current of the upper and lower bridge arms of the MMC is calculated by the PI controller. Based on the reference output current of the upper and lower bridge arms of the MMC, the actual output current of the upper and lower bridge arms of the MMC, the output deviation current of the upper and lower bridge arms of the MMC, and the reactance of the bridge arm reactor, calculate the voltage difference across the reactor in the upper and lower bridge arms of the MMC. Based on the actual output voltage of the positive and negative terminals of the MMC DC side, the voltage difference across the reactors in the upper and lower arms of the MMC, and the voltage at the connection point of the reactors in the upper and lower arms of the MMC, the output reference voltage of the upper and lower arms of the MMC is determined by limiting.

6. The method as described in claim 5, characterized in that, The process involves introducing the overall energy of the single bridge arm into the AC side control of the receiving-end converter station, and calculating the output deviation current of the upper and lower bridge arms of the MMC via a PI controller, including: The difference between the overall energy of the single bridge arm and the set energy value of the single bridge arm is used to obtain the third deviation value; The third deviation value is input to the PI controller for AC side control of the receiving-end converter station, and the output deviation current of the upper and lower bridge arms of the MMC is obtained.

7. The method as described in claim 5, characterized in that, The method of determining the output reference voltage of the upper and lower bridge arms of the MMC based on the actual output voltage of the positive and negative terminals of the MMC DC side, the voltage difference across the reactors in the upper and lower bridge arms of the MMC, and the voltage at the connection point of the reactors in the upper and lower bridge arms of the MMC through limiting includes: The actual output voltage of the DC side positive terminal of the MMC is successively compared with the voltage difference across the reactor in the upper arm of the MMC and the voltage at the connection point of the reactor in the upper and lower arms of the MMC, and the output reference voltage of the upper arm of the MMC is obtained by limiting the amplitude. The voltage difference at the connection point of the reactor in the upper and lower arms of the MMC is successively subtracted from the voltage difference across the reactor in the lower arm of the MMC and the actual output voltage of the negative terminal of the DC side of the MMC, and then the output reference voltage of the lower arm of the MMC is obtained by limiting the amplitude.

8. A receiving-end station control system for offshore wind power flexible DC transmission, characterized in that, include: AC side control module, DC side control module, reference voltage calculation module and bridge arm modulation module; The AC-side control module is used to acquire the parameters of the modular multilevel converter (MMC) in the receiving-end converter station when offshore wind power is transmitted via flexible DC transmission, and to obtain the upper and lower bridge arm output reference current of the MMC by performing AC-side control on the receiving-end converter station based on the parameters. The DC-side control module is used to maintain the stability of the MMC positive and negative terminal outlet voltages by performing DC-side control on the receiving-end converter station based on the parameters. The reference voltage calculation module is used to obtain the reference voltage of the upper and lower bridge arms of the MMC based on the parameters and the reference current output of the upper and lower bridge arms of the MMC after the positive and negative terminal outlet voltages of the MMC have stabilized, by controlling the AC side of the receiving-end converter station. The bridge arm modulation module is used to modulate the upper and lower bridge arms of the MMC according to the reference voltage output of the upper and lower bridge arms of the MMC, and output the actual voltage of the upper and lower bridge arms of the MMC. The MMC is equivalent to two identical half-modular multilevel converters spliced ​​together.

9. An electronic device, characterized in that, include: At least one processor and memory; The memory and processor are connected via a bus; The memory is used to store one or more programs; When the one or more programs are executed by the at least one processor, a receiving-end station control method for offshore wind power flexible DC transmission as described in any one of claims 1-7 is implemented.

10. A readable storage medium, characterized in that, It contains an execution program, which, when executed, implements a receiving-end station control method for offshore wind power flexible DC transmission as described in any one of claims 1-7.

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