Flexible DC power transmission system grid following-grid construction hybrid control method

Abstract

The invention discloses a flexible DC power transmission system grid following-grid construction hybrid control method, and the method comprises the following steps: obtaining the voltage and current of a receiving end MMC grid connection point, and carrying out the dq decomposition, thereby obtaining a d-axis component and a q-axis component under a dq rotating coordinate system; acquiring actual values and related reference values of direct-current voltage output by the receiving-end MMC, alternating-current voltage and frequency of a grid-connected point of the receiving-end MMC and average capacitance voltage of a sub-module; sequentially calculating a phase reference value of the receiving end MMC, a d-axis current reference value and a q-axis current reference value of the grid connection point, a d-axis voltage reference value and a q-axis voltage reference value of the differential mode voltage, a direct current reference value and a direct current modulation ratio reference value; and calculating the reference value of the modulation voltage of the receiving end MMC under the abc static coordinate system. The control method has the advantages of adapting to wide-range short-circuit ratio change of the receiving-end power grid, guaranteeing reliable transmission of power under the condition of wind speed change, and still maintaining synchronous operation when the receiving-end alternating-current power grid seriously breaks down.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of large-scale new energy grid-connected flexible DC system control, and in particular to a hybrid grid-following-grid-forming control method for a flexible DC transmission system, a grid-forming control device, a computer device and a storage medium. BACKGROUND

[0002] Flexible DC transmission technology is the preferred solution for large-scale deep-sea wind power grid connection, and its core equipment is the modular multilevel converter (MMC). With the increasing proportion of new energy, large-scale DC is continuously fed in, and the receiving end power grid presents the characteristics of low inertia, weak damping and low short-circuit ratio. The traditional grid-following control is prone to instability and oscillation when connected to a weak power grid, and it is difficult to provide frequency / inertia and voltage support to the receiving end AC power grid. The grid-forming control of flexible DC shows voltage source characteristics, and can actively provide active and reactive power support, which is an effective measure to solve the instability problem of weak power grid. In a point-to-point new energy grid-connected system, the sending end converter station needs to establish a constant AC voltage and frequency on the new energy side, and the receiving end converter station needs to establish a system DC voltage, as shown in Figure 2 .

[0003] The current DC voltage grid-forming control can be divided into DC voltage regulation-based grid-forming control and capacitor energy regulation-based grid-forming control. For the DC voltage regulation-based grid-forming control, its parameter setting is difficult, and power grid frequency fluctuations, wind power changes, etc. are prone to cause DC voltage fluctuations, resulting in DC voltage instability. The capacitor energy regulation-based grid-forming control can effectively avoid the shortcomings of the DC voltage regulation-based grid-forming control, decouple the DC voltage control and capacitor energy, and thus reduce the DC voltage fluctuation. In addition, it can release a certain amount of sub-module capacitor energy to realize frequency support when the receiving end power grid frequency changes. However, its stability under strong power grid is poor, and when the receiving end AC power grid experiences a serious fault, the synchronous controller loses the synchronization ability with the power grid due to insufficient capacitor energy margin, thereby causing instability of the entire new energy grid-connected system. Therefore, it is urgent to propose a capacitor energy regulation-based flexible DC converter grid-forming strategy that can adapt to different power grid strengths and can pass through serious AC faults. SUMMARY

[0004] The main purpose of the present application is to overcome the shortcomings and deficiencies of the prior art, and provide a flexible direct current power transmission system grid-following-grid-forming hybrid control method, device, computer equipment and storage medium. Among them, the sending end MMC adopts island control to establish the voltage and frequency of the new energy side power grid, and the receiving end MMC adopts grid-following-grid-forming hybrid control based on capacitor energy regulation. The flexible direct current system adopting the control method can adapt to different receiving end grid strengths, and the flexible direct current system has certain anti-power disturbance ability and receiving end grid AC fault ride-through capability, and can actively support the receiving end grid.

[0005] In order to achieve the above purpose, the first aspect of the present application discloses a flexible direct current power transmission system grid-following-grid-forming hybrid control method, and a large-scale new energy is connected to an alternating current power grid through a flexible direct current power transmission system. The connected end of a new energy base and a modular multilevel converter (MMC) is a sending end, and the corresponding converter is a sending end MMC. The connected end of an alternating current power grid and a modular multilevel converter is a receiving end. The system topology is as shown in Figure 2 The control block diagram of the receiving end MMC is as shown in Figure 3 The control method comprises the following steps: S1, obtaining the voltage and current of the receiving end MMC grid connection point, the receiving end MMC grid connection point being the access point of the receiving end MMC and the receiving end alternating current power grid, and performing dq decomposition on the voltage and current of the receiving end MMC grid connection point to obtain the d-axis and q-axis components of the voltage and current of the receiving end MMC grid connection point in the dq rotating coordinate system, obtaining the actual values of the direct current voltage output by the receiving end MMC, the alternating current voltage of the receiving end MMC grid connection point, the frequency and the average capacitor voltage of the sub-module, and obtaining the reference values of the direct current voltage of the receiving end MMC, the voltage amplitude of the receiving end MMC grid connection point and the direct current side capacitor energy; S2, calculating the phase reference value of the receiving end MMC ; S3, calculating the d-axis current reference value and the q-axis current reference value of the receiving end MMC grid connection point; S4, calculating the d-axis voltage reference value and the q-axis voltage reference value of the differential mode voltage of the receiving end MMC; S5, calculating the direct current reference value of the receiving end MMC; S6, calculating the direct current modulation ratio reference value m dc of the receiving end MMC; S7, calculating the a-axis voltage reference value , the b-axis voltage reference value and the c-axis voltage reference value ; S8、According to the reference value of the modulation voltage, a corresponding control pulse is generated to realize the control of the receiving end MMC of the flexible DC power transmission system by using the pulse width modulation theory.

[0006] Further, in order to improve the grid connection stability of the receiving end MMC, a phase-locked loop coupling quantity u sq is added to the capacitor energy control loop. a The mixed coefficient k a is used to adjust the proportion of the phase-locked loop and the capacitor energy control loop in the synchronization loop, and the calculation formula of the phase reference value of the receiving end MMC is as follows: (1) Where s is the Laplace operator, is the rated value of the potential angle frequency in the receiving end MMC, k * is the mixed coefficient of the synchronization control loop, W mmc and W mmc are the reference value and the actual value of the capacitor energy of the receiving end MMC, u sq is the q-axis component of the grid connection point voltage of the receiving end MMC, k s is the capacitor energy feedforward coefficient, and k t is the capacitor energy droop coefficient.

[0007] The actual value of the capacitor energy of the receiving end MMC can be calculated by the average value u c_avg of the capacitor voltage of the sub-module of the receiving end MMC.

[0008] Where N is the number of sub-modules of each bridge arm of the MMC, and C sm is the capacitor value of the MMC sub-module.

[0009] The dynamic equation of the capacitor energy of the receiving end MMC can be obtained by rearranging equation (1)

[0010] Where ΔP mmc is the difference between the DC power and the AC power of the receiving end MMC, and U sm is the amplitude of the grid connection point voltage of the receiving end MMC.

[0011] Similar to the rotor motion equation of the synchronous machine, the equivalent inertia J * and the damping D * of the receiving end MMC can be obtained.

[0012] The value range of the mixed coefficient k a is [0, 1], and according to the above formula, the introduction of the mixed coefficient k a can increase the equivalent inertia J* and damping D * , and the larger, the equivalent inertia J * and damping D * . Therefore, in the transient process, the active power support provided by the MMC is larger, and the output phase change is slower, which can help to improve the transient stability of the receiving-end MMC grid connection.

[0013] Further, the d-axis current reference value of the receiving-end MMC grid connection point and the q-axis current reference value The calculation formula is as follows: The d-axis current reference value of the receiving-end MMC grid connection point and the q-axis current reference value The calculation formula is as follows: (2) Where s is the Laplace operator, and are the d-axis and q-axis component reference values of the receiving-end MMC grid connection point current; u d and u q are the d-axis and q-axis voltages of the receiving-end MMC grid connection point; and are the d-axis voltage reference value and q-axis voltage reference value of the receiving-end MMC grid connection point; k pu and k iu are the proportional parameter and integral parameter of the voltage inner loop proportional integral controller.

[0014] Further, the d-axis component reference value of the differential mode voltage of the receiving-end MMC and the q-axis component reference value The calculation formula is as follows: (3) Where s is the Laplace operator, and are the d-axis and q-axis components of the differential mode voltage of the receiving-end MMC; i d and i q are the d-axis and q-axis components of the receiving-end MMC output current; ω is the actual value of the receiving-end AC grid frequency; L is the receiving-end MMC connection reactance value; k pi and k ii are the proportional parameter and integral parameter of the current inner loop proportional integral controller; u d is the d-axis voltage of the receiving-end MMC grid connection point.

[0015] The double inner loop control can realize fast regulation and limiting of the current, and avoid overcurrent of the converter to damage the equipment.

[0016] Further, the calculation formula of the DC component I com_dc of the three-phase common-mode current of the MMC is as follows

[0017] where I dc is the DC current of the receiving end MMC, C dc is the equivalent capacitance of the DC side of the receiving end MMC, and u is the DC voltage of the receiving end MMC. Thus, the DC voltage outer loop proportional integral control can be designed, and the calculation formula of the reference value I of the DC current of the receiving end MMC is as follows: (4) where s is the Laplace operator, I * dc is the reference value of the DC current of the receiving end MMC; and u are the reference value and the actual value of the DC voltage respectively; k pudc and k iudc are the proportional parameter and the integral parameter of the DC voltage outer loop proportional integral controller respectively.

[0018] Further, ignoring the resistance voltage drop of the MMC bridge arm, the common-mode voltage u com_dc of the receiving end MMC and the DC component I com_dc of the common-mode current have the following relationship

[0019] where m dc is the DC modulation ratio of the receiving end MMC, and L arm is the inductance of the MMC bridge arm.

[0020] Thus, the control expression of the DC modulation ratio m dc of the receiving end MMC is as follows: (5) where s is the Laplace operator, I dc is the DC current of the receiving end MMC; k pudc and k iudc are the proportional parameter and the integral parameter of the DC current inner loop proportional integral controller respectively.

[0021] Further, the calculation formula of the a-axis voltage reference value V , the b-axis voltage reference value V and the c-axis voltage reference value V of the modulation voltage of the receiving end MMC in the abc stationary coordinate system are as follows: (6) where m dcIt is the DC modulation ratio of the receiver-side MMC. This is the phase reference value for the receiving-end MMC. Based on the reference value of the receiving-end MMC modulation voltage in the abc stationary three-phase coordinate system, the trigger signals of each switching device in the MMC can be generated, thereby realizing the network control of the receiving-end MMC.

[0022] A second aspect of this invention discloses a hybrid control device for a flexible DC transmission system and its grid-to-grid structure, used to execute the aforementioned hybrid control method for a flexible DC transmission system and its grid-to-grid structure, the hybrid control device comprising: The receiving-end MMC parameter acquisition module is used to acquire the voltage and current of the receiving-end MMC grid connection point, which is the connection point between the receiving-end MMC and the receiving-end AC power grid. The module performs dq decomposition on the voltage and current of the receiving-end MMC grid connection point to obtain the d-axis and q-axis components of the voltage and current of the receiving-end MMC grid connection point in the dq rotating coordinate system. It also acquires the actual values ​​of the DC voltage output by the receiving-end MMC, the AC voltage of the receiving-end MMC grid connection point, the frequency, and the average capacitor voltage of the submodule. Additionally, it acquires the reference values ​​of the DC voltage of the receiving-end MMC, the voltage amplitude of the receiving-end MMC grid connection point, and the DC-side capacitor energy. The first calculation module is used to calculate the phase reference value of the receiving-end MMC. ; The second calculation module is used to calculate the d-axis current reference value at the receiving-end MMC grid connection point. and q-axis current reference value ; The third calculation module is used to calculate the d-axis voltage reference value of the differential-mode voltage of the receiving-end MMC. and q-axis voltage reference value ; The fourth calculation module is used to calculate the DC current reference value of the receiving-end MMC. ; The fifth calculation module is used to calculate the DC modulation ratio reference value m of the receiving-end MMC. dc ; The sixth calculation module is used to calculate the reference value of the modulated voltage of the receiving-end MMC along the a-axis in the abc stationary coordinate system. b-axis voltage reference value and c-axis voltage reference value ; The control pulse generation module is used to generate corresponding control pulses based on the reference value of the modulation voltage and the pulse width modulation theory to realize the control of the receiving end MMC of the flexible DC transmission system.

[0023] The third aspect of the present application discloses a computer device, comprising a processor and a memory for storing a program executable by the processor, and when the processor executes the program stored in the memory, a flexible direct current power transmission system net-following-network-constructing hybrid control method is implemented.

[0024] The fourth aspect of the present application discloses a storage medium, which stores a program, and when the program is executed by a processor, a flexible direct current power transmission system net-following-network-constructing hybrid control method is implemented.

[0025] Compared with the prior art, the present application has the following advantages and beneficial effects: (1) The present application proposes a net-following-network-constructing hybrid control method for a point-to-point new energy through a flexible direct current grid-connected system, and by changing the value of the synchronization ring hybrid coefficient k a , the proportion of the capacitor energy synchronization ring and the phase-locked loop in the synchronization ring can be changed, and then the equivalent inertia and equivalent damping of the receiving end MMC are changed, so that the receiving end MMC can provide a larger inertia during the transient state of the power grid, the output power angle swing speed is slow, and then the transient stability of the flexible direct current power transmission system is improved. According to the net-following-network-constructing hybrid control method of the flexible direct current power transmission system disclosed by the present application, the receiving end MMC shows the characteristics of a direct current voltage source and an alternating current voltage source, and realizes the frequency / voltage support for the receiving end power grid with high new energy penetration rate.

[0026] (2) The present application proposes a net-following-network-constructing hybrid control method suitable for a point-to-point flexible direct current power transmission system for new energy grid connection. Through the innovative control strategy design, the method realizes reliable control of the system under steady-state operation and fault operation conditions. Theoretical analysis and simulation verification show that the flexible direct current transmission system using the control method has the following outstanding advantages: 1) it can adapt to a wide range of short-circuit ratio changes and maintain stable operation of the system; 2) it can effectively cope with wind power fluctuations and ensure smooth power transmission; 3) it can still maintain synchronization operation capability under AC side fault conditions. The research results provide an effective control solution for the safe and stable operation of new energy flexible direct current local systems. BRIEF DESCRIPTION OF DRAWINGS

[0027] In order to more clearly illustrate the technical solutions in the embodiments of the present application, the drawings needed in the embodiment description will be briefly introduced as follows. Obviously, the drawings in the following description are only some embodiments of the present application, and other drawings can be obtained by those skilled in the art without creative labor.

[0028] Figure 1 is a flowchart of the net-following-network-constructing control method of the flexible direct current power transmission system disclosed by the present application; Figure 2is a 1000MW flexible HVDC transmission system topology diagram; Figure 3 is a control block diagram of the receiving end MMC; Figure 4 is a system simulation waveform diagram when the short circuit ratio of the receiving end power grid jumps from 2 to 5 under the control method of the application; Figure 5 is a system simulation waveform diagram when the new energy power fluctuates under the control method of the application; Figure 6 is a system simulation waveform diagram when the active load of the receiving end power grid decreases by 100MW under the control method of the application; Figure 7 is a system simulation waveform diagram before and after the three-phase AC ground fault of the receiving end power grid under the control method of the application; Figure 8 is a system simulation waveform diagram before and after the three-phase AC ground fault of the receiving end power grid under the traditional network construction control based on capacitor energy regulation; Figure 9 is a structural block diagram of the flexible HVDC transmission system network-following-network construction control device disclosed in embodiment 6 of the application; Figure 10 is a structural block diagram of the computer device in embodiment 7 of the application. DETAILED DESCRIPTION

[0029] In order for those skilled in the art to better understand the present application, the technical solutions in the embodiments of the present application will be described clearly and completely below with reference to the drawings in the embodiments of the present application. Obviously, the described embodiments are only part of the embodiments of the present application, not all the embodiments. Based on the embodiments in the present application, all other embodiments obtained by those skilled in the art without creative labor are within the scope of protection of the present application.

[0030] In the present application, the phrase "embodiment" means that the specific features, structures or characteristics described in combination with the embodiment can be included in at least one embodiment of the present application. The appearance of this phrase in various places in the specification does not necessarily mean the same embodiment, nor is it an independent or alternative embodiment to other embodiments. Those skilled in the art explicitly and implicitly understand that the embodiments described in the present application can be combined with other embodiments.

[0031] Embodiment 1 The present embodiment discloses a flexible HVDC transmission system network-following-network construction hybrid control method, which is applied to a system as shown in the accompanying drawings. The hybrid control method comprises the following steps: Figure 2 ​S1, obtain the voltage and current of the receiving end MMC grid-connected point, which is the access point of the receiving end MMC and the receiving end AC power grid, and perform dq decomposition on the voltage and current of the receiving end MMC grid-connected point to obtain the d-axis and q-axis components of the voltage and current of the receiving end MMC grid-connected point in the dq rotating coordinate system, obtain the actual values of the DC voltage output by the receiving end MMC, the AC voltage of the receiving end MMC grid-connected point, the frequency, and the average capacitor voltage of the sub-module, obtain the reference values of the DC voltage of the receiving end MMC, the voltage amplitude of the receiving end MMC grid-connected point, and the DC side capacitor energy; S2, calculate the phase reference value of the receiving end MMC , the calculation formula is as follows: (1) wherein s is a Laplace operator, is the rated value of the potential angle frequency in the receiving end MMC, k a is a mixed coefficient of the synchronous control loop, W * mmc and W mmc are the reference value and the actual value of the capacitor energy of the receiving end MMC, u sq is the q-axis component of the voltage of the receiving end MMC grid-connected point, k s is a capacitor energy feedforward coefficient, k t is a capacitor energy droop coefficient.

[0032] S3, calculate the d-axis current reference value and the q-axis current reference value of the receiving end MMC grid-connected point, the calculation formula is as follows: (2) wherein s is a Laplace operator, and are the d-axis and q-axis component reference values of the current of the receiving end MMC grid-connected point; u d and u q are the d-axis and q-axis voltages of the receiving end MMC grid-connected point; and are the d-axis voltage reference value and the q-axis voltage reference value of the receiving end MMC grid-connected point; k pu and k iu are the proportional parameter and the integral parameter of the voltage inner loop proportional-integral controller.

[0033] S4, calculate the d-axis voltage reference value and the q-axis voltage reference value of the differential mode voltage of the receiving end MMC, the calculation formula is as follows: (3) wherein s is a Laplace operator, and are the d-axis and q-axis components of the received MMC differential-mode voltage, respectively; i d and i q are the d-axis and q-axis components of the received MMC output current, respectively; ω is the actual value of the received AC grid frequency; L is the received MMC connecting reactance value; k pi and k ii are the proportional parameter and integral parameter of the current inner loop proportional-integral controller, respectively; u d is the d-axis voltage of the received MMC grid connection point.

[0034] S5, calculating the DC current reference value of the received MMC , the calculation formula is as follows: (4) wherein s is the Laplace operator, I * dc is the reference value of the received MMC DC current; and are the reference value and actual value of the DC voltage, respectively; k pudc and k iudc are the proportional parameter and integral parameter of the DC voltage outer loop proportional-integral controller, respectively, and s is the Laplace operator.

[0035] S6, calculating the DC modulation ratio m of the received MMC dc , the calculation formula is as follows: (5) wherein s is the Laplace operator, I dc is the received MMC DC current; k pudc and k iudc are the proportional parameter and integral parameter of the DC current inner loop proportional-integral controller, respectively.

[0036] S7, calculating the a-axis voltage reference value , b-axis voltage reference value and c-axis voltage reference value of the modulation voltage of the received MMC in the abc stationary coordinate system, the calculation formula is as follows: (6) wherein m dc is the DC modulation ratio of the received MMC.

[0037] S8, according to the reference value of the modulation voltage, using the pulse width modulation theory, generating the corresponding control pulse to realize the control of the received MMC of the flexible DC power transmission system.

[0038] Embodiment 2 Based on the flexible HVDC system follow-grid-construct-grid hybrid control method disclosed in Embodiment 1, a 1000 MW flexible HVDC transmission system is simulated and verified in this embodiment, and the system topology is shown in Figure 2 The system adopts the control method disclosed in this application, the sending end MMC adopts island control, and the receiving end MMC adopts follow-grid-construct-grid hybrid control based on capacitor energy regulation. The control block diagram of the receiving end MMC is shown in Figure 3 It is assumed that the system has entered a steady state at t = 3 s. At t = 8 s, the short-circuit ratio of the receiving end power grid changes from 2 to 5, and the DC voltage, frequency, output power angle of the receiving end MMC, active power and reactive power of the receiving end MMC are shown in Figure 4 Figure 4 It can be seen that when the receiving end power grid changes from a weak grid to a strong grid, the system can recover to a stable state in a short time. The stable values of the active power output by the MMC, the DC voltage and the frequency of the system after the disturbance are not much different from the original values, while the output power angle of the receiving end MMC jumps from 0.46 to about 0.19, proving that the control method disclosed in this application can adapt to a wide range of short-circuit ratio changes of the receiving end power grid.

[0039] Embodiment 3 Based on the flexible HVDC system follow-grid-construct-grid hybrid control method disclosed in Embodiment 1, a 1000 MW flexible HVDC transmission system is simulated and verified in this embodiment. The system adopts the control method disclosed in this application, the sending end MMC adopts island control, and the receiving end MMC adopts follow-grid-construct-grid hybrid control based on capacitor energy regulation. It is assumed that the system has entered a steady state at t = 3 s. At t = 8 s, the output of the new energy fluctuates, resulting in a decrease of 100 MW of active power transmitted to the receiving end power grid, Figure 5 The waveforms of the DC voltage, frequency, output power angle of the receiving end MMC, and active power of the receiving end power grid under the control disclosed in this application and the construct-grid control based on DC voltage regulation are shown in the simulation graphs, respectively. The solid line refers to the waveform of the control disclosed in this application, and the dashed line refers to the waveform of the construct-grid control based on DC voltage regulation. It can be seen from the simulation graphs that when the wind power fluctuates, the systems adopting the two construct-grid controls can well adapt to the power change and reach a stable operating state in a short time, proving that the two control methods can cope with a certain degree of power fluctuation. However, for the system adopting the construct-grid control based on DC voltage regulation, the frequency is positively correlated with the DC voltage under the action of the DC voltage synchronization ring, so the DC voltage fluctuates greatly after the power disturbance, the DC voltage fluctuation reaches 35 kV, and the DC voltage change rate exceeds 5%, which is easy to cause system stability problems. However, the DC voltage can be decoupled from the capacitor energy by using the control method disclosed in this application, so the DC voltage of the system hardly fluctuates when the system experiences a power disturbance.

[0040] Embodiment 4 ​Based on the hybrid control method of flexible HVDC transmission system disclosed in embodiment 1, a 1000 MW flexible HVDC transmission system is simulated in this embodiment. The system adopts the control method proposed in this paper, the sending end MMC adopts island control, and the receiving end MMC adopts hybrid control based on the regulation of capacitor energy. It is assumed that the system has entered a steady state at t = 3 s. At t = 8 s, a power disturbance occurs, and the load of the receiving end power grid suddenly decreases by 100 MW, Figure 6 The capacitor energy droop coefficient k t = 5 and k t = 25, respectively. The solid line refers to the waveform when k t = 5, and the dashed line refers to the waveform when k t = 25. From the simulation diagram, it can be seen that when the load suddenly decreases, the system can well adapt to the load change and reach a stable operating state in a very short time, proving that the proposed control method can cope with a certain degree of power fluctuation. In addition, when the total output power of the generator in the receiving end power grid suddenly decreases by 100 MW, causing power imbalance in the receiving end power grid, under the synchronous ring effect of the hybrid control based on the regulation of capacitor energy, the MMC quickly releases energy, reduces the power transmitted by the MMC, and provides a certain frequency support for the receiving end power grid. The support effect is related to k t The smaller k t is, the better the frequency support is, which is manifested as a smaller frequency change rate and a smaller frequency increase amplitude. In the frequency-time diagram, the rising speed of the solid line is slower than that of the dashed line, and the highest point is lower.

[0041] Embodiment 5 Based on the hybrid control method of flexible HVDC transmission system proposed in embodiment 1, a simulation model of a 1000 MW flexible HVDC transmission system is constructed in this embodiment. To verify the superiority of the proposed control method, the following two control schemes are set for comparison and analysis: control scheme 1 adopts the control method proposed in this paper, in which the sending end MMC adopts island control mode, and the receiving end MMC adopts hybrid control based on the regulation of capacitor energy; control scheme 2 is used as a control scheme, in which the sending end MMC also adopts island control, but the receiving end MMC adopts traditional grid-forming control based on the regulation of capacitor energy. The system has entered a steady state at t = 2 s, and a three-phase short-circuit ground fault is set in the receiving end power grid at t = 3 s, causing the voltage of the receiving end power grid to drop to 0. The dynamic response characteristics of the system under the two control schemes are shown in Figure 7 and Figure 8 The traditional grid-forming control scheme (scheme 2) shows obvious stability defects under severe faults, as shown in Figure 8As shown, the system appears continuous power angle oscillation, power fluctuation and DC voltage continues to climb phenomenon, eventually leading to system instability, unable to realize fault ride through. The control method (scheme 1) proposed in this paper shows excellent fault ride-through capability. By dynamically adjusting k a The parameter, the system power angle swing amplitude is effectively inhibited. As Figure 7 shown, after the transient process, the system key parameters (including MMC output power angle, output power and DC voltage) can quickly converge to the original stable operating point, and successfully realizes the ride-through of serious AC fault.

[0042] Embodiment 6 As Figure 9 shown, the embodiment provides a grid-following-grid-forming hybrid control device for a flexible DC power transmission system, the hybrid control device comprising: a receiving end MMC parameter acquisition module 901, a first calculation module 902, a second calculation module 903, a third calculation module 904, a fourth calculation module 905, a fifth calculation module 906, a sixth calculation module 907 and a control pulse generation module 908, the specific functions of each module are as follows: The receiving end MMC parameter acquisition module 901 is used for acquiring the voltage and current of the receiving end MMC grid-connected point, which is the access point of the receiving end MMC and the receiving end AC power grid, and performing dq decomposition on the voltage and current of the receiving end MMC grid-connected point to obtain the d-axis and q-axis components of the voltage and current of the receiving end MMC grid-connected point in the dq rotating coordinate system, acquiring the actual values of the DC voltage output by the receiving end MMC, the receiving end MMC grid-connected point AC voltage, the frequency and the sub-module average capacitor voltage, and acquiring the reference values of the DC voltage of the receiving end MMC, the receiving end MMC grid-connected point voltage amplitude and the DC side capacitor energy; The first calculation module 902 is used for calculating the phase reference value of the receiving end MMC ; The second calculation module 903 is used for calculating the d-axis current reference value and the q-axis current reference value of the receiving end MMC grid-connected point; The third calculation module 904 is used for calculating the d-axis voltage reference value and the q-axis voltage reference value of the receiving end MMC differential mode voltage; The fourth calculation module 905 is used for calculating the DC current reference value of the receiving end MMC; The fifth calculation module 906 is used for calculating the DC modulation ratio reference value m dc of the receiving end MMC; The sixth calculation module 907 is used for calculating the a-axis voltage reference value b-axis voltage reference value and c-axis voltage reference value ; The control pulse generation module 908 is used to generate corresponding control pulses based on the reference value of the modulation voltage and the pulse width modulation theory to realize the control of the receiving end MMC of the flexible DC transmission system.

[0043] Example 7 This embodiment provides a computer device, which can be a computer, such as... Figure 10 As shown, the system is connected via a system bus 1001 to a processor 1002, a memory, an input device 1003, a display 1004, and a network interface 1005. The processor provides computing and control capabilities. The memory includes a non-volatile storage medium 1006 and an internal memory 1007. The non-volatile storage medium 1006 stores an operating system, computer programs, and a database. The internal memory 1007 provides an environment for the operation of the operating system and computer programs in the non-volatile storage medium. When the processor 1002 executes the computer programs stored in the memory, it implements the flexible DC transmission system grid-connection hybrid control method proposed in Embodiment 1 above. Large-scale new energy sources are connected to the AC grid through the flexible DC transmission system. The connection end between the new energy base and the modular multilevel converter (MMC) is the sending end. The MMC is referred to as the sending-end MMC. The connection end between the AC grid and the MMC is the receiving end. The control method includes the following steps: S1. Obtain the voltage and current of the receiving-end MMC grid connection point. The receiving-end MMC grid connection point is the connection point between the receiving-end MMC and the receiving-end AC grid. Perform dq decomposition on the voltage and current of the receiving-end MMC grid connection point to obtain the d-axis and q-axis components of the voltage and current of the receiving-end MMC grid connection point in the dq rotating coordinate system. Obtain the actual values ​​of the DC voltage output by the receiving-end MMC, the AC voltage of the receiving-end MMC grid connection point, the frequency, and the average capacitor voltage of the submodule. Obtain the reference values ​​of the DC voltage of the receiving-end MMC, the voltage amplitude of the receiving-end MMC grid connection point, and the DC side capacitor energy. S2. Calculate the phase reference value of the receiving end MMC. ; S3. Calculate the reference value of the d-axis current at the receiving-end MMC grid connection point. and q-axis current reference value ; S4. Calculate the d-axis voltage reference value of the differential mode voltage of the receiving-end MMC. and q-axis voltage reference value ; S5. Calculate the reference value of DC current for the receiving-end MMC. ; S6. Calculate the DC modulation ratio reference value m of the receiving-end MMC. dc ; S7. Calculate the reference value of the modulation voltage of the receiving-end MMC along the a-axis in the abc stationary coordinate system. b-axis voltage reference value and c-axis voltage reference value ; S8. Based on the reference value of the modulation voltage, the corresponding control pulses are generated using pulse width modulation theory to realize the control of the MMC at the receiving end of the flexible DC transmission system.

[0044] Example 8 This embodiment provides a storage medium, which is a computer-readable storage medium, storing a computer program. When the computer program is executed by a processor, it implements a hybrid control method for a flexible DC transmission system connected to the grid and constructed by the grid, as proposed in Embodiment 1 above. Large-scale renewable energy is connected to the AC grid through the flexible DC transmission system. The connection point between the renewable energy base and the modular multilevel converter (MMC) is the sending end. The MMC is referred to as the sending-end MMC. The connection point between the AC grid and the MMC is the receiving end. The receiving-end MMC is referred to as the receiving-end MMC. The control method includes the following steps: S1. Obtain the voltage and current of the receiving-end MMC grid connection point. The receiving-end MMC grid connection point is the connection point between the receiving-end MMC and the receiving-end AC grid. Perform dq decomposition on the voltage and current of the receiving-end MMC grid connection point to obtain the d-axis and q-axis components of the voltage and current of the receiving-end MMC grid connection point in the dq rotating coordinate system. Obtain the actual values ​​of the DC voltage output by the receiving-end MMC, the AC voltage of the receiving-end MMC grid connection point, the frequency, and the average capacitor voltage of the submodule. Obtain the reference values ​​of the DC voltage of the receiving-end MMC, the voltage amplitude of the receiving-end MMC grid connection point, and the DC side capacitor energy. S2. Calculate the phase reference value of the receiving end MMC. ; S3. Calculate the reference value of the d-axis current at the receiving-end MMC grid connection point. and q-axis current reference value ; S4. Calculate the d-axis voltage reference value of the differential mode voltage of the receiving-end MMC. and q-axis voltage reference value ; S5. Calculate the reference value of DC current for the receiving-end MMC. ; S6. Calculate the DC modulation ratio reference value m of the receiving-end MMC. dc ; S7. Calculate the reference value of the modulation voltage of the receiving-end MMC along the a-axis in the abc stationary coordinate system. b-axis voltage reference value and c-axis voltage reference value ; S8, according to the reference value of the modulation voltage, using pulse width modulation theory, generate the corresponding control pulse to realize the control of the receiving end MMC of the flexible DC power transmission system.

[0045] The technical features of the above embodiments can be combined in any manner. In order to make the description concise, not all possible combinations of the technical features in the above embodiments are described, however, as long as the combinations of the technical features do not exist contradictions, they should be considered as the scope of the present disclosure.

[0046] The above embodiments are the preferred embodiments of the present application, but the embodiments of the present application are not limited to the above embodiments, and any changes, modifications, substitutions, combinations, simplifications made without departing from the spirit and principles of the present application are equivalent replacement methods and are included in the protection scope of the present application.

Claims

1. A hybrid control method for a flexible DC transmission system and its grid connection, wherein large-scale renewable energy is connected to the AC grid through the flexible DC transmission system, the connection point between the renewable energy base and the modular multilevel converter (MMC) is the sending end, and the corresponding converter is the sending-end MMC; the connection point between the AC grid and the MMC is the receiving end, and the corresponding converter is the receiving-end MMC, characterized in that... The control method includes the following steps: S1. Obtain the voltage and current of the receiving-end MMC grid connection point. The receiving-end MMC grid connection point is the connection point between the receiving-end MMC and the receiving-end AC grid. Perform dq decomposition on the voltage and current of the receiving-end MMC grid connection point to obtain the d-axis and q-axis components of the voltage and current of the receiving-end MMC grid connection point in the dq rotating coordinate system. Obtain the actual values ​​of the DC voltage output by the receiving-end MMC, the AC voltage of the receiving-end MMC grid connection point, the frequency, and the average capacitor voltage of the submodule. Obtain the reference values ​​of the DC voltage of the receiving-end MMC, the voltage amplitude of the receiving-end MMC grid connection point, and the DC side capacitor energy. S2. Calculate the phase reference value of the receiving end MMC. ; S3. Calculate the reference value of the d-axis current at the receiving-end MMC grid connection point. and q-axis current reference value ; S4. Calculate the d-axis voltage reference value of the differential mode voltage of the receiving-end MMC. and q-axis voltage reference value ; S5. Calculate the reference value of DC current for the receiving-end MMC. ; S6. Calculate the DC modulation ratio reference value m of the receiving-end MMC. dc ; S7. Calculate the reference value of the modulation voltage of the receiving-end MMC along the a-axis in the abc stationary coordinate system. b-axis voltage reference value and c-axis voltage reference value ; S8. Based on the reference value of the modulation voltage, the corresponding control pulses are generated using pulse width modulation theory to realize the control of the MMC at the receiving end of the flexible DC transmission system.

2. The hybrid control method for a flexible DC transmission system based on grid-network integration as described in claim 1, characterized in that, In step S2, the phase reference value of the receiving end MMC The calculation formula is as follows: （1） Where s is the Laplace operator, k is the rated value of the internal potential angular frequency of the receiving-end MMC. a W is the mixing coefficient of the synchronization control loop. * mmc and W mmc These are the reference and actual values ​​of the energy of the receiving-end MMC capacitor, respectively. sq Let k be the q-axis component of the voltage at the receiving end MMC grid connection point. s k is the capacitor energy feedforward coefficient. t This is the capacitor energy droop coefficient.

3. The hybrid control method for a flexible DC transmission system based on grid-network structure according to claim 1, characterized in that, In step S3, the d-axis current reference value of the receiving-end MMC grid connection point and q-axis current reference value The calculation formula is as follows: （2） Where s is the Laplace operator, and These are the reference values ​​for the d-axis and q-axis components of the current at the receiving-end MMC grid connection point, respectively; u d and u q These are the d-axis and q-axis voltages at the receiving-end MMC grid connection point, respectively. and These are the d-axis voltage reference values ​​and q-axis voltage reference values ​​at the receiving-end MMC grid connection point, respectively; k pu and k iu These are the proportional and integral parameters of the voltage inner-loop proportional-integral controller.

4. The hybrid control method for a flexible DC transmission system based on grid-network integration as described in claim 1, characterized in that, In step S4, the reference value u of the d-axis component of the differential mode voltage of the receiving-end MMC is... * diffd and q-axis component reference value u * diffq The calculation formula is as follows: （3） Where s is the Laplace operator, and These are the d-axis and q-axis components of the differential-mode voltage of the receiving-end MMC, respectively; i d and i q These are the d-axis and q-axis components of the MMC output current at the receiving end, respectively; ω is the actual value of the AC grid frequency at the receiving end. L k is the receiving-end MMC connection reactance value. pi and k ii These are the proportional and integral parameters of the inner-loop proportional-integral controller; u d The voltage at the receiving end MMC grid connection point is the d-axis voltage.

5. The hybrid control method for a flexible DC transmission system based on grid-network integration according to claim 1, characterized in that, In step S5, the reference value of the DC current of the receiving end MMC The calculation formula is as follows: （4） Where s is the Laplace operator, This is a reference value for the DC current of the receiving-end MMC; and These are the reference and actual values ​​of the DC voltage, respectively; k pudc and k iudc These are the proportional and integral parameters of the DC voltage outer loop proportional-integral controller.

6. The hybrid control method for a flexible DC transmission system based on grid-network integration as described in claim 1, characterized in that, In step S6, the DC modulation ratio m of the receiving end MMC dc The calculation formula is as follows: （5） Where s is the Laplace operator, I dc k is the DC current of the receiving-end MMC. pudc and k iudc These are the proportional and integral parameters of the DC current inner-loop proportional-integral controller.

7. The hybrid control method for a flexible DC transmission system based on grid-network structure according to claim 1, characterized in that, In step S7, the modulation voltage of the receiving end MMC is the reference value of the a-axis voltage in the abc stationary coordinate system. b-axis voltage reference value and c-axis voltage reference value The calculation formula is as follows: （6） Where, m dc It is the DC modulation ratio of the receiving-end MMC.

8. A flexible DC transmission system and grid-to-grid hybrid control device, used to execute the flexible DC transmission system and grid-to-grid hybrid control method according to any one of claims 1 to 7, characterized in that, The hybrid control device includes: The receiving-end MMC parameter acquisition module is used to acquire the voltage and current of the receiving-end MMC grid connection point, which is the connection point between the receiving-end MMC and the receiving-end AC power grid. The module performs dq decomposition on the voltage and current of the receiving-end MMC grid connection point to obtain the d-axis and q-axis components of the voltage and current of the receiving-end MMC grid connection point in the dq rotating coordinate system. It also acquires the actual values ​​of the DC voltage output by the receiving-end MMC, the AC voltage of the receiving-end MMC grid connection point, the frequency, and the average capacitor voltage of the submodule. Additionally, it acquires the reference values ​​of the DC voltage of the receiving-end MMC, the voltage amplitude of the receiving-end MMC grid connection point, and the DC-side capacitor energy. The first calculation module is used to calculate the phase reference value of the receiving-end MMC. ; The second calculation module is used to calculate the d-axis current reference value at the receiving-end MMC grid connection point. and q-axis current reference value ; The third calculation module is used to calculate the d-axis voltage reference value of the differential-mode voltage of the receiving-end MMC. and q-axis voltage reference value ; The fourth calculation module is used to calculate the DC current reference value of the receiving-end MMC. ; The fifth calculation module is used to calculate the DC modulation ratio reference value m of the receiving-end MMC. dc ; The sixth calculation module is used to calculate the reference value of the modulated voltage of the receiving-end MMC along the a-axis in the abc stationary coordinate system. b-axis voltage reference value and c-axis voltage reference value ; The control pulse generation module is used to generate corresponding control pulses based on the reference value of the modulation voltage and the pulse width modulation theory to realize the control of the receiving end MMC of the flexible DC transmission system.

9. A computer device comprising a processor and a memory for storing a processor-executable program, characterized in that, When the processor executes the program stored in the memory, it implements the flexible DC transmission system and grid-network hybrid control method as described in any one of claims 1 to 7.

10. A storage medium storing a program, characterized in that, When the program is executed by the processor, it implements the flexible DC transmission system and grid-network hybrid control method as described in any one of claims 1 to 7.

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