Frequency adaptive cooperative support control method and system of flexible DC power transmission system

Abstract

The invention belongs to the field of active frequency support of grid-connected electronic power equipment, and particularly discloses a frequency adaptive cooperative support control method and system of a flexible direct current power transmission system. The method comprises the following steps: collecting the frequency change rate of a receiving-end alternating-current power grid, obtaining the maximum frequency change rate of the receiving-end alternating-current power grid when disturbance occurs, and calculating the maximum frequency difference caused by the disturbance according to an expansion frequency response model; based on the maximum frequency difference caused by disturbance, adaptively determining a direct-current voltage-frequency droop coefficient, and obtaining a direct-current voltage instruction signal; a direct-current voltage deviation signal is detected at a sending-end converter station side, and an energy instruction signal of the sending-end converter station is obtained based on energy-direct-current voltage droop control, so that the sending-end converter station carries out cooperative support on the frequency of a receiving-end alternating-current power grid. According to the adaptive direct current voltage-frequency droop coefficient provided by the invention, the supporting capability can be fully utilized in each disturbance, and the supporting effect is improved.

Description

Technical Field

[0001] The present application belongs to the field of active frequency support for grid-connected electronic power equipment, and more specifically, to a frequency adaptive collaborative support control method and system for a flexible direct current transmission system. Background Art

[0002] Modular Multilevel Converter-based High Voltage Direct Current Transmission (MMC-HVDC) has the characteristics of good power quality and high power supply reliability. It has become the main technical means for the current Shagohuang new energy base and offshore wind power centralized transmission and asynchronous interconnection of power grids. Under the traditional vector control method, MMC-HVDC dynamically decouples the active power-frequency of the AC system at both ends and cannot actively participate in the inertia support and frequency adjustment of the system. With the rapid development of new energy, the power system faces severe challenges of reduced inertia and weakened frequency stability. In order to cope with the above problems, it is necessary to explore new frequency support methods for MMC so that it can participate in maintaining the safety and stability of the system frequency.

[0003] For grid-following MMCs, existing frequency support methods mainly consider introducing measured frequency information for additional control, so that the DC voltage of the MMC is associated with the system frequency or the frequency change rate, and the DC voltage command is adjusted when the frequency fluctuates, so that the MMC releases or absorbs more energy for support during the disturbance.

[0004] Furthermore, in order to maximize the frequency support of the receiving-end AC system, the inverter station and the rectifier station of the MMC-HVDC system need to cooperate, and the cooperation is divided into communication mode and non-communication mode. The communication mode has the defects of high probability of failure of long communication lines and difficult maintenance; the non-communication mode often realizes information exchange between the converter stations at both ends through the DC voltage-frequency droop relationship. However, the existing non-communication coordination method is to adjust a DC voltage-frequency droop coefficient through preliminary calculation. This coefficient is usually adjusted according to the most severe disturbance situation and is not suitable for all actual disturbances. Often when facing smaller disturbances, the energy of the MMC-HVDC system cannot be fully utilized for the frequency support of the AC power grid. Summary of the invention

[0005] In view of the defects of the prior art, the purpose of the present application is to provide a frequency adaptive collaborative support control method and system for a flexible direct current transmission system, aiming to solve the problem that when the inverter station and the rectifier station of the existing MMC-HVDC system are coordinated without communication, a set DC voltage-frequency droop coefficient is used, resulting in the energy of the MMC-HVDC system cannot be fully utilized for the frequency support of the AC power grid.

[0006] To achieve the above objectives, in a first aspect, the present application provides a frequency adaptive collaborative support control method for a flexible direct current transmission system, comprising the following steps: Step S1: collecting the frequency change rate of the receiving-end AC power grid, obtaining the maximum frequency change rate of the receiving-end AC power grid when the disturbance occurs, and calculating the maximum frequency difference caused by the disturbance according to the extended frequency response model; Step S2: Based on the maximum frequency difference caused by the disturbance, adaptively determine the DC voltage-frequency droop coefficient to obtain a DC voltage command signal; Step S3: Based on the DC voltage command signal, a DC voltage deviation signal is detected at the sending-end converter station side, and based on the energy-DC voltage droop control, an energy command signal of the sending-end converter station is obtained, and the energy command signal is applied to the energy control loop of the sending-end converter station, so that the sending-end converter station can coordinately support the frequency of the receiving-end AC power grid; Among them, a DC line is set between the sending-end converter station and the receiving-end converter station; the other side of the sending-end converter station is connected to the sending-end power grid, and the other end of the receiving-end converter station is connected to the receiving-end power grid.

[0007] Further preferably, the method for obtaining the frequency change rate of the receiving-end AC power grid in step S1 is: The port voltage signal of the receiving-end power grid in the three-phase stationary coordinate system is transformed into the port voltage of the receiving-end power grid in the two-phase rotating coordinate system through Park coordinates; The q-axis voltage signal in the receiving-end power grid port voltage is input into the PI controller to obtain the real-time angular frequency of the receiving-end power grid; Differentiate the real-time angular frequency of the receiving grid and divide by 2 Then the frequency change rate of the receiving AC power grid is obtained.

[0008] Further preferably, the extended frequency response model is:

[0009]

[0010]

[0011]

[0012] in, is the maximum frequency change rate, is the maximum frequency difference caused by the disturbance; is the load disturbance power; is the total inertia of the receiving system; is the damping coefficient of the synchronous generator; is the droop factor of the synchronous generator speed governor; is the reheater time constant; is the work ratio coefficient of the high pressure cylinder of the reheat turbine; is the inertia constant of the synchronous generator; is the capacity of the MMC-HVDC system; is the capacity of the synchronous generator; Virtual inertia coefficient set for MMC virtual inertia control; is the unit adjustment factor of the synchronous generator; is the frequency deviation; It is the maximum frequency change rate of the receiving AC power grid when the disturbance occurs.

[0013] Further preferably, in step S2, the DC voltage-frequency droop coefficient is:

[0014] in, is the maximum DC voltage variation allowed by MMC-HVDC; is the DC voltage-frequency droop coefficient; is the maximum frequency difference caused by the disturbance.

[0015] Further preferably, in step S3, the energy control method in the energy-DC voltage droop control specifically comprises the following steps: The command value and the measured value of the energy of the sending-end converter station are subtracted, and then the difference signal is subjected to PI control operation to obtain a DC current command signal; The DC current command signal is subtracted from the actual DC current signal, and then the difference signal is subjected to PI control to output a DC component control signal of the bridge arm voltage; Based on the DC component control signal of the bridge arm voltage, the command signals of the upper bridge arm and lower bridge arm voltages are obtained, and after modulation and voltage balancing algorithm, they are converted into switching signals to control the energy output of the sending-end converter station.

[0016] Further preferably, the relationship between the energy and the DC voltage of the sending-end converter station in step S3 is:

[0017] in, is the energy-DC voltage droop coefficient of the sending-end converter station; is the deviation of DC voltage.

[0018] In a second aspect, the present application provides a frequency adaptive collaborative support control system for a flexible direct current transmission system, comprising: The maximum frequency difference acquisition module is used to collect the frequency change rate of the receiving-end AC power grid, obtain the maximum frequency change rate of the receiving-end AC power grid when the disturbance occurs, and calculate the maximum frequency difference caused by the disturbance according to the extended frequency response model; A voltage command acquisition module, used to adaptively determine a DC voltage-frequency droop coefficient based on a maximum frequency difference caused by a disturbance, and acquire a DC voltage command signal; The sending-end collaborative support module is used to detect the DC voltage deviation signal at the sending-end converter station side based on the DC voltage command signal, obtain the energy command signal of the sending-end converter station based on the energy-DC voltage droop control, and apply it to the energy control loop of the sending-end converter station, so that the sending-end converter station can collaboratively support the frequency of the receiving-end AC power grid; Among them, a DC line is set between the sending-end converter station and the receiving-end converter station; the other side of the sending-end converter station is connected to the sending-end power grid, and the other end of the receiving-end converter station is connected to the receiving-end power grid.

[0019] Further preferably, the maximum frequency difference acquisition module includes: A coordinate converter, used for converting the port voltage signal of the receiving-end power grid in a three-phase stationary coordinate system into the port voltage of the receiving-end power grid in a two-phase rotating coordinate system through Park coordinates; The receiving-end phase-locked PI controller is used to perform PI operation on the q-axis voltage signal in the receiving-end power grid port voltage to obtain the real-time angular frequency of the receiving-end power grid; Differential operator, used for real-time angular frequency differential operation of the receiving-end power grid; Divider, used to divide the real-time angular frequency of the receiving power grid by 2 after differential operation Then the frequency change rate of the receiving AC power grid is obtained.

[0020] Further preferably, the extended frequency response model of the maximum frequency difference acquisition module is:

[0021]

[0022]

[0023] in, RoCoF max is the maximum frequency change rate, is the maximum frequency difference caused by the disturbance; is the load disturbance power; is the total inertia of the receiving system; is the damping coefficient of the synchronous generator; is the droop factor of the synchronous generator speed governor; is the reheater time constant; is the work ratio coefficient of the high pressure cylinder of the reheat turbine; is the inertia constant of the synchronous generator; is the capacity of the MMC-HVDC system; is the capacity of the synchronous generator; Virtual inertia coefficient set for MMC virtual inertia control; is the unit adjustment factor of the synchronous generator; is the frequency deviation; It is the maximum frequency change rate of the receiving AC power grid when the disturbance occurs.

[0024] Further preferably, the DC voltage-frequency droop coefficient in the voltage instruction acquisition module is:

[0025] in, is the maximum DC voltage variation allowed by MMC-HVDC; is the DC voltage-frequency droop coefficient; is the maximum frequency difference caused by the disturbance.

[0026] Further preferably, the delivery-end collaborative support module includes: A first adder, used for subtracting the command value and the measured value of the energy of the sending-end converter station; A first PI controller, used for performing a PI control operation on the difference signal obtained by the first adder to obtain a DC current command signal; A second adder, used for making a difference between the DC current command signal and the actual DC current signal; A second PI controller, used for performing PI control on the difference signal obtained by the second adder, and outputting a bridge arm voltage DC component control signal; The signal converter is used to obtain the command signals of the upper bridge arm and lower bridge arm voltages based on the DC component control signal of the bridge arm voltage. After modulation and voltage balancing algorithm, the command signals are converted into switch signals to control the energy output of the sending-end converter station.

[0027] Further preferably, the relationship between the energy and the DC voltage of the sending-end converter station in the sending-end collaborative support module is:

[0028] in, is the energy-DC voltage droop coefficient of the sending-end converter station; is the deviation of DC voltage.

[0029] In general, the above technical solutions conceived by this application have the following beneficial effects compared with the prior art: The present application provides a frequency adaptive collaborative support control method for a flexible direct current transmission system, and establishes an extended system frequency response model (Extended System Frequency Response Model, ESFR) including MMC-HVDC. Compared with the traditional frequency response model that only considers the synchronous generator and its speed regulation system, the proposed model takes into account the inertia control effect of the MMC, and the frequency response prediction is more accurate; that is, through the extended frequency response model, the maximum frequency difference of the entire disturbance process can be quickly predicted based only on the maximum frequency change rate that appears in the initial stage of the disturbance, and the disturbance information is grasped a priori, which provides a basis for completing adaptive collaborative support.

[0030] The present application provides a frequency adaptive collaborative support control method for a flexible direct current transmission system, and proposes a method for adaptively adjusting a direct current voltage-frequency droop coefficient according to actual disturbance conditions. The previous method for setting the droop coefficient cannot fully utilize its own energy for support when the frequency disturbance is small. In contrast, the adaptive direct current voltage-frequency droop coefficient provided by the present application can fully utilize the support capacity in each disturbance, thereby improving the support effect.

[0031] The present application provides a frequency adaptive collaborative support control method for a flexible direct current transmission system, which does not require communication lines, transmits frequency information through direct current voltage, and provides the inertia support capability of the MMC-HVDC system in a communication collaborative manner, thereby overcoming the problems of high probability of failure and difficult maintenance of long communication lines. BRIEF DESCRIPTION OF THE DRAWINGS

[0032] Figure 1 This is one of the flow charts of the frequency adaptive collaborative support control method of the flexible direct current transmission system provided in the embodiment of the present application; Figure 2 It is a topological diagram of two AC power systems interconnected through a flexible DC transmission system provided in an embodiment of the present application; Figure 3 It is a structural diagram of a phase-locked loop and frequency change rate measurement applied in the frequency adaptive collaborative support control method of a flexible direct current transmission system provided in an embodiment of the present application; FIG4 (a) is a control structure diagram of a receiving-end converter station provided in an embodiment of the present application; FIG4( b ) is a control structure diagram of a sending-end converter station provided in an embodiment of the present application; FIG5 (a) is a frequency response original model of an extended system including MMC-HVDC established in an embodiment of the present application; FIG5( b ) is a simplified frequency response model of an extended system including MMC-HVDC established in an embodiment of the present application; FIG6 (a) is a block diagram of an adaptive DC voltage-frequency droop control structure provided in an embodiment of the present application; FIG6 (b) is a block diagram of the sending-end MMC energy-DC voltage droop structure provided in an embodiment of the present application; FIG. 7 ( a ) is a comparison and verification diagram of the system frequency response of the ESFR model provided in an embodiment of the present application and the electromagnetic transient simulation model; FIG. 7( b ) is a comparison and verification diagram of the synchronous machine power response of the ESFR model and the electromagnetic transient simulation model provided in the embodiment of the present application; FIG7( c ) is a comparison and verification diagram of the MMC power response of the ESFR model and the electromagnetic transient simulation model provided in the embodiment of the present application; Figure 8 The ESFR model established in the embodiment of the present application is based on RoCoF 0 predictions Δf max Comparison and verification diagram of the results with the electromagnetic transient simulation model; FIG9 (a) is a diagram of system frequency simulation effects provided by an embodiment of the present application when the method of the present application is not used and when the method of the present application is used after a sudden increase in the load on the receiving-end power grid; FIG9( b ) is a diagram of the DC voltage simulation effect when the method of the present application is not used and when the method of the present application is used after a sudden increase in the load on the receiving-end power grid, provided by an embodiment of the present application; FIG9( c ) is a diagram showing the simulation effect of the active power output of the MMC-HVDC without and with the method of the present application after a sudden increase in the load on the receiving-end power grid, provided by an embodiment of the present application. DETAILED DESCRIPTION

[0033] In order to make the purpose, technical solution and advantages of the present application more clearly understood, the present application is further described in detail below in conjunction with the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain the present application and are not used to limit the present application.

[0034] The term "and / or" in this article is a description of the association relationship of associated objects, indicating that there can be three relationships. For example, A and / or B can represent: A exists alone, A and B exist at the same time, and B exists alone. The symbol " / " in this article indicates that the associated objects are in an or relationship, for example, A / B means A or B.

[0035] The terms “first”, “second” and the like in the specification and claims herein are used to distinguish different objects rather than to describe a specific order of the objects.

[0036] In the embodiments of the present application, words such as "exemplary" or "for example" are used to indicate examples, illustrations or descriptions. Any embodiment or design described as "exemplary" or "for example" in the embodiments of the present application should not be interpreted as being more preferred or more advantageous than other embodiments or designs. Specifically, the use of words such as "exemplary" or "for example" is intended to present related concepts in a specific way.

[0037] The embodiments of the present application are described below in conjunction with the drawings in the embodiments of the present application.

[0038] like Figure 1 As shown, an embodiment of the present application provides a frequency adaptive collaborative support control method for a flexible direct current transmission system, specifically: Step S1: the receiving-end converter station detects and obtains the frequency change rate of the receiving-end AC power grid, records the maximum frequency change rate when the disturbance occurs, and calculates the maximum frequency difference caused by the disturbance according to the extended frequency response model; Step S2: According to the maximum frequency difference result calculated by the model, the DC voltage-frequency droop coefficient is adaptively determined, and the DC voltage command signal is determined accordingly, and sent to the DC voltage controller; Step S3: The sending-end converter station detects and obtains the DC voltage deviation signal, and sends it to the energy-DC voltage droop controller to obtain the energy command signal of the sending-end converter station, which acts on the energy control loop of the sending-end converter station, so that the sending-end converter station performs collaborative support.

[0039] Before specifically introducing the frequency adaptive collaborative support control method for the flexible DC transmission system provided by the present application, the following introduces two aspects of improvements involved in the present application: The first improvement: using an improved phase-locked loop structure to obtain frequency changes when disturbances occur: like Figure 2 The figure shows a topological structure diagram of a flexible DC transmission system connecting two AC power grid systems. This application is implemented by designing a control method for converter stations at both ends; Figure 3 The present invention is a structural diagram of the phase-locked loop provided in the receiving-end converter station and the sending-end converter station of the frequency adaptive collaborative support control method of the flexible DC transmission system proposed in this application, and the corresponding operation of the frequency change rate measurement is derived in the phase-locked loop; the phase-locked loop is used to achieve the phase synchronization between the output voltage of the converter station and the grid connection point voltage, and to obtain the frequency and phase angle signals of the AC power grid, on this basis, the frequency change rate signal can be further calculated; specifically, the receiving-end converter station and the sending-end converter station are MMC converter stations; the port voltage signal of the MMC converter station (modular multilevel converter) connected to the power grid is measured ; The port voltage signal in the three-phase stationary coordinate system is transformed by Park coordinate transformation Transformed into the port voltage in the two-phase rotating coordinate system ; Among them, the coordinate transformation angle signal Provided by the phase-locked loop output phase signal; the q-axis voltage signal Input PI controller to get the real-time angular frequency of the system , real-time angular frequency After integration, the phase of the grid-connected point voltage is obtained , real-time angular frequency Differentiate and divide by 2 Then we get the frequency change rate of the system df / dt (ie Rate of Change of Frequency, RoCoF).

[0040] In the second aspect, the station-level control involved in the frequency adaptive collaborative support control method of the flexible DC transmission system provided by the present application is introduced: Fig. 4 (a) and Fig. 4 (b) are station-level control structure diagrams of the proposed frequency adaptive collaborative support control method for the flexible DC transmission system; wherein, the station-level control of the receiving-end converter station consists of three aspects: DC voltage control, AC voltage control and circulating current suppression control; at the sending-end converter station, the station-level control consists of four aspects: active power control, reactive power control, circulating current suppression control and energy control; wherein, active / DC voltage control and reactive / AC voltage control belong to the AC side control link of MMC, and circulating current suppression control and energy control belong to the DC side control link of MMC, wherein, active / DC voltage control, reactive / AC voltage control and circulating current suppression control are no different from those of the traditional vector controlled MMC converter station; The active power control consists of an active power outer loop and a d-axis current inner loop; and P They are the command value and the measured value of active power respectively. The difference signal obtained by subtracting the two is sent to the active power loop PI controller to generate the d-axis current command signal , and the actual current signal of the d-axis After the difference is made, the difference signal is sent to the d-axis current loop PI controller to generate the differential mode d-axis voltage control signal ; Reactive power control consists of a reactive power outer loop and a q-axis current inner loop: and Q They are the command value and the measured value of reactive power respectively. The difference signal obtained by subtracting the two is sent to the reactive power loop PI controller to generate the q-axis current command signal , and the actual current signal of the q axis After the difference is made, the difference signal is sent to the q-axis current loop PI controller to generate the differential mode q-axis voltage control signal ; The control signal generated above and After the inverse Park coordinate transformation, the bridge arm voltage differential mode voltage control signal of the abc three-phase is obtained. ( j =a, b, c); the circulating current of MMC is the negative sequence double frequency component, and the three-phase circulating current ( j =a, b, c) After coordinate transformation, the dq components in the dq-2 coordinate system are obtained , , in order to suppress it to 0, the command value , All are set to 0, and after subtracting the measured value, they are sent to the circulating current suppression PI controller to output the common-mode d and q axis voltage control signals. , After the inverse Park coordinate transformation, the bridge arm voltage common mode voltage control signal of the abc three-phase is obtained. ( j =a,b,c); Energy control in the sending-end converter station is a new link added in this application to achieve system coordination. Specifically, and The command value and measured value of the energy of the receiving converter station are sent to the sending converter station energy ring PI controller to output the DC current command signal. , and measure the actual DC current signal After the difference is made, the difference signal is integrated into the DC current loop PI controller to output the DC component control signal of the bridge arm voltage. As shown in Figure 4 (a), according to the control signal obtained in the above process, the command signal of the upper and lower bridge arm voltages can be obtained. , , follow these rules:

[0041] Generated , After further modulation and voltage-balancing algorithms, it can be turned into a switch signal to control the operation of the MMC; Based on the station-level control of the flexible DC transmission system proposed above, the frequency adaptive collaborative support control method is described in detail below; the receiving-end converter station is directly connected to the receiving-end AC power grid, and can directly detect the receiving-end power grid frequency and frequency change rate; there is a DC line between the sending-end converter station and the receiving-end converter station, and the sending-end converter station cannot directly sense the receiving-end power grid frequency. The non-communication collaborative frequency support control between them is realized by two droop controls, with DC voltage as the channel for information transmission; Based on the above two points, the frequency adaptive collaborative support control method provided by the present application is described in detail below, specifically including the following steps: Step 1: Establish a frequency response model of the extended system including MMC-HVDC to predict the frequency response process; As shown in Figure 5 (a), the established extended frequency response model containing MMC is shown in Figure 5 (a). is the active power command value of the synchronous generator; For the increased mechanical power; is the load disturbance power; The power added to MMC; is the inertia constant of the synchronous generator; D is the damping coefficient of the synchronous generator; Virtual inertia coefficient set for MMC virtual inertia control; R is the droop factor of the synchronous generator speed governor; is the work ratio coefficient of the high pressure cylinder of the reheat turbine; is the reheater time constant; is the mechanical power gain coefficient; By simplifying the extended system frequency response model in Figure 5 (a), we can obtain the simplified model shown in Figure 5 (b); is the capacity of the synchronous generator; is the capacity of the MMC-HVDC system, where the inertia constant of the MMC-HVDC is converted according to the capacity; is the unit adjustment factor of the synchronous generator, equal to and R The ratio of According to the simplified model shown in Figure 5(b), it can be deduced that:

[0042] in, is the frequency deviation; Maximum rate of change of system frequency Appears at the moment after the load disturbance occurs; Based on the above, it can be further deduced that:

[0043] in,

[0044] Similarly, the expression of the maximum frequency difference can be deduced as:

[0045] The calculation formulas for each parameter are as follows:

[0046] In summary, it can be concluded that there is a linear relationship between the maximum frequency difference and the maximum frequency change rate of the system. As long as the information of the synchronous generator and the MMC in the power grid is known, when the maximum frequency change rate is measured, the maximum frequency difference can be calculated according to the extended frequency response model, that is:

[0047] in, is the phase angle of the system; is the natural frequency of the system; Step 2: Implementing adaptive DC voltage-frequency droop control at the DC voltage control front end of the receiving-end converter station; After measuring the frequency change of the AC power grid at the receiving converter station and recording the maximum frequency change rate, the corresponding maximum frequency difference is calculated according to the extended frequency response model. Δf max Based on this, the DC voltage-frequency droop coefficient can be adaptively determined. D Uf :

[0048] in, The maximum DC voltage variation allowed by MMC-HVDC is generally set to a certain value, such as 10%; The DC voltage is controlled by the receiving converter station so that it forms the following droop relationship with the frequency of the receiving AC system:

[0049] It is particularly important to note that in the traditional method of setting the droop coefficient, the droop coefficient is always kept at a certain value, and is set according to the most severe situation and the maximum disturbance to release all energy, so the support capacity cannot be fully released in the case of smaller disturbances. Compared with the traditional method of setting the droop coefficient, the adaptive droop control method proposed in this application can adjust the droop coefficient according to the actual situation of each disturbance, so it can adapt to all disturbance sizes and fully release the MMC support potential; Step 3: Implement energy-DC voltage droop control at the energy control front end of the sending-end converter station; The energy of the MMC at the sending end and the DC voltage form the following droop relationship:

[0050] From the above, the relationship between the energy of the MMC at the sending end and the frequency of the AC system at the receiving end will be:

[0051] It can be seen that the energy of the sending-end MMC is also related to the system frequency; In summary, the block diagram of the inter-station droop cooperative control of the frequency adaptive cooperative support control method of the flexible DC transmission system is shown in Figure 6 (a) and Figure 6 (b). By adding the above two droop controls, the cooperative control between the sending-end converter station and the receiving-end AC system without the need for a communication line is realized, so that the sending-end converter station can also participate in the inertia support of the system in real time.

[0052] Example 2 To further illustrate the control effect of a new energy power generation inertia phase-locked control method with fast frequency response proposed in this application, a specific implementation example is described below: This implementation method takes a 1100MW flexible DC transmission system connecting two AC power grid systems as an example to conduct simulation research. Since the focus is on the supporting capacity of the flexible DC transmission system for the receiving system, the sending system is simplified to an infinite system. The parameters of the flexible DC transmission system and the receiving system are shown in Table 1. The schematic diagram of the system is shown in Figure 2 As shown; Table 1

[0053] Verification Aspect 1: Expanding the Accuracy of the ESFR Model Scenario I: Comparison of the time domain response of the ESFR model with MMC-HVDC proposed in this application and the electromagnetic transient simulation; Scenario II: Comparison of the prediction of the maximum frequency difference by the ESFR model containing MMC-HVDC proposed in this application with the results of electromagnetic transient simulation.

[0054] Figures 7(a) to 7(c) are time domain response comparison and verification diagrams of the established ESFR model and the electromagnetic transient simulation model. A 55MW (5%) load disturbance was put into operation at 30s. Figure 7(a) shows the frequency response of the system. Figure 7(b) shows the power response of the synchronous machine. Figure 7(c) shows the power response of the MMC. It can be seen from Figures 7(a) to 7(c) that the time domain response waveform obtained by using the proposed ESFR model is basically consistent with the time domain response waveform obtained by using the electromagnetic transient simulation, and the correspondence is good, indicating that the established ESFR model is accurate. Table 2 is a comparison table of the prediction of the maximum frequency difference by the established ESFR model and the results of electromagnetic transient simulation under different load disturbance levels; Figure 8 is a comparison chart of the maximum frequency difference predicted by the established ESFR model and the electromagnetic transient simulation results; from Table 2 and Figure 8 It can be seen that the maximum frequency difference predicted by the proposed ESFR model is very close to the result obtained by electromagnetic transient simulation, and the correspondence is good, which shows that the established ESFR model is accurate and the result of predicting the maximum frequency difference by the ESFR model is accurate.

[0055] Table 2

[0056] Verification Aspect 2: Support Effect of the Proposed Technical Solution In the simulation, there was originally 1000MW of active load in the receiving grid, and at 20s, the active load suddenly increased by 50MW (5%) to verify the frequency support capability of the flexible DC transmission system using the proposed method. Specifically, the scenarios in the embodiment are set as follows: Scenario I: No frequency support strategy is used; Scenario II: Using the traditional non-decoupled cooperative control strategy; Scenario III: Using the proposed frequency adaptive collaborative support control method for a flexible DC transmission system.

[0057] Figures 9 (a) to 9 (c) are relevant simulation effect diagrams of not using the method of the present application and using the method of the present application after a sudden increase in the load of the receiving-end power grid; Figure 9 (a) shows the change of the system frequency; Figure 9 (b) shows the change of the DC voltage; Figure 9 (c) shows the change of the output active power of the MMC-HVDC; As can be seen from Figure 9 (b) and Figure 9 (c), the flexible DC transmission system without frequency support method has almost no active response; Figure 9 (a) shows the change of system frequency; Figure 9 (b) shows the change of DC voltage; Figure 9 (c) shows the change of MMC-HVDC output active power; As can be seen from Figure 9 (b) and Figure 9 (c), the flexible DC transmission system without the frequency support method has almost no active response; the traditional collaborative control method has the ability to use the MMC energy on both sides to generate more active power for support during the frequency disturbance, but because the load disturbance is not the most serious, the DC voltage has not dropped by 10%, so there is still energy in the MMC-HVDC that has not been released for support; using the frequency adaptive collaborative support control method for a flexible DC transmission system proposed in this application, under the same disturbance, the flexible DC transmission system has an adaptive droop coefficient, and the DC voltage drops by about 10%, which is close to the limit. The surplus energy of the MMC-HVDC can be fully released, so that the active power peak provided by the MMC-HVDC is higher, the support time is longer, and the frequency support capability is improved.

[0058] FIG9 (a) shows the frequency simulation effect diagram of the receiving-end power grid without using the method of the present application and with using the method of the present application after a sudden increase in the load on the receiving-end power grid; it can be seen that under the same load disturbance, the frequency change rate is the largest and the lowest frequency minimum point is the lowest when the frequency support method is not used; the traditional non-decoupling method has a certain improvement in the frequency response; and using the frequency adaptive collaborative support control method for a flexible direct current transmission system proposed in the present application, since the support capacity of the MMC-HVDC system is fully utilized, the system frequency change rate is further reduced and the lowest frequency point is further improved.

[0059] Example 3 The present application provides a frequency adaptive collaborative support control system for a flexible direct current transmission system, comprising: The maximum frequency difference acquisition module is used to collect the frequency change rate of the receiving-end AC power grid, obtain the maximum frequency change rate of the receiving-end AC power grid when the disturbance occurs, and calculate the maximum frequency difference caused by the disturbance according to the extended frequency response model; A voltage command acquisition module, used to adaptively determine a DC voltage-frequency droop coefficient based on a maximum frequency difference caused by a disturbance, and acquire a DC voltage command signal; The sending-end collaborative support module is used to detect the DC voltage deviation signal at the sending-end converter station side based on the DC voltage command signal, obtain the energy command signal of the sending-end converter station based on the energy-DC voltage droop control, and apply it to the energy control loop of the sending-end converter station, so that the sending-end converter station can collaboratively support the frequency of the receiving-end AC power grid; Among them, a DC line is set between the sending-end converter station and the receiving-end converter station; the other side of the sending-end converter station is connected to the sending-end power grid, and the other end of the receiving-end converter station is connected to the receiving-end power grid.

[0060] Further preferably, the maximum frequency difference acquisition module includes: A coordinate converter, used for converting the port voltage signal of the receiving-end power grid in a three-phase stationary coordinate system into the port voltage of the receiving-end power grid in a two-phase rotating coordinate system through Park coordinates; The receiving-end PI controller is used to perform PI operation on the q-axis voltage signal in the receiving-end power grid port voltage to obtain the real-time angular frequency of the receiving-end power grid; Differential operator, used for real-time angular frequency differential operation of the receiving-end power grid; Divider, used to divide the real-time angular frequency of the receiving power grid by 2 after differential operation Then the frequency change rate of the receiving AC power grid is obtained.

[0061] Further preferably, the extended frequency response model of the maximum frequency difference acquisition module is:

[0062]

[0063] in, RoCoF max is the maximum frequency change rate, is the maximum frequency difference caused by the disturbance; is the load disturbance power; is the total inertia of the receiving end AC system; is the damping coefficient of the synchronous generator; is the droop factor of the synchronous generator speed governor; is the reheater time constant; is the work ratio coefficient of the high-pressure cylinder of the reheat turbine.

[0064] Further preferably, the DC voltage-frequency droop coefficient in the voltage instruction acquisition module is:

[0065] in, is the maximum DC voltage variation allowed by MMC-HVDC; is the DC voltage-frequency droop coefficient; is the maximum frequency difference caused by the disturbance.

[0066] Further preferably, the delivery-end collaborative support module includes: A first adder, used for subtracting the command value and the measured value of the energy of the sending-end converter station; A first PI controller, used for performing a PI control operation on the difference signal obtained by the first adder to obtain a DC current command signal; A second adder, used for making a difference between the DC current command signal and the actual DC current signal; A second PI controller, used for performing PI control on the difference signal obtained by the second adder, and outputting a bridge arm voltage DC component control signal; The signal converter is used to obtain the command signals of the upper bridge arm and lower bridge arm voltages based on the DC component control signal of the bridge arm voltage. After modulation and voltage balancing algorithm, the command signals are converted into switch signals to control the energy output of the sending-end converter station.

[0067] Further preferably, the relationship between the energy and the DC voltage of the sending-end converter station in the sending-end collaborative support module is:

[0068] in, is the energy-DC voltage droop coefficient of the sending-end converter station; is the deviation of DC voltage.

[0069] In summary, compared with the prior art, the present application has the following advantages: An extended frequency response model of the system including MMC-HVDC was established, and the inertia effect control of MMC was taken into account, so that the frequency response prediction was more accurate; the maximum frequency difference of the entire disturbance process can be quickly predicted through the model, providing a basis for completing adaptive collaborative support; a method for adaptively adjusting the DC voltage-frequency droop coefficient according to the actual disturbance situation was proposed. The previous method of setting the droop coefficient could not fully use its own energy for support when the frequency disturbance was small; in contrast, the adaptive method proposed in this application can fully utilize the support capacity at each disturbance, thereby improving the support effect. No communication line is required, and frequency information is transmitted through DC voltage. The inertia support capacity of the MMC-HVDC system is improved in a non-communication collaborative manner, overcoming the problem of high probability of failure and difficult maintenance of long communication lines.

[0070] The above is only a specific implementation of the present application, but the protection scope of the present application is not limited thereto. Any person skilled in the art who is familiar with the present technical field can easily think of changes or substitutions within the technical scope disclosed in the present application, which should be included in the protection scope of the present application. Therefore, the protection scope of the present application should be based on the protection scope of the claims.

Claims

1. A frequency adaptive collaborative support control method for a flexible direct current transmission system, characterized in that: The following steps are involved: Step S1: collecting the frequency change rate of the receiving-end AC power grid, obtaining the maximum frequency change rate of the receiving-end AC power grid when the disturbance occurs, and calculating the maximum frequency difference caused by the disturbance according to the extended frequency response model; Step S2: Based on the maximum frequency difference caused by the disturbance, adaptively determine the DC voltage-frequency droop coefficient to obtain a DC voltage command signal; Step S3: Based on the DC voltage command signal, a DC voltage deviation signal is detected at the sending-end circulating station side, and based on the energy-DC voltage droop control, an energy command signal of the sending-end converter station is obtained, and the energy command signal is applied to the energy control loop of the sending-end converter station, so that the sending-end converter station can coordinately support the frequency of the receiving-end AC power grid; Among them, a DC line is set between the sending-end converter station and the receiving-end converter station; the other side of the sending-end converter station is connected to the sending-end power grid, and the other end of the receiving-end converter station is connected to the receiving-end power grid.

2. The frequency adaptive collaborative support control method according to claim 1, characterized in that: The method for obtaining the frequency change rate of the receiving-end AC power grid in step S1 is: The port voltage signal of the receiving-end power grid in the three-phase stationary coordinate system is transformed into the port voltage of the receiving-end power grid in the two-phase rotating coordinate system through Park coordinates; The q-axis voltage signal in the receiving-end power grid port voltage is input into the PI controller to obtain the real-time angular frequency of the receiving-end power grid; Differentiate the real-time angular frequency of the receiving grid and divide by 2 Then the frequency change rate of the receiving AC power grid is obtained.

3. The frequency adaptive collaborative support control method according to claim 1, characterized in that: The expanded frequency response model in step S1 is: in, is the maximum frequency change rate, is the maximum frequency difference caused by the disturbance; is the load disturbance power; is the total inertia of the receiving system; is the damping coefficient of the synchronous generator; is the droop factor of the synchronous generator speed governor; is the reheater time constant; is the work ratio coefficient of the high pressure cylinder of the reheat turbine; is the inertia constant of the synchronous generator; is the capacity of the MMC-HVDC system; is the capacity of the synchronous generator; Virtual inertia coefficient set for MMC virtual inertia control; is the unit adjustment factor of the synchronous generator; is the frequency deviation; It is the maximum frequency change rate of the receiving AC power grid when the disturbance occurs.

4. The frequency adaptive collaborative support control method according to claim 3, characterized in that: The DC voltage-frequency droop coefficient in step S2 is: in, is the maximum DC voltage variation allowed by MMC-HVDC; is the DC voltage-frequency droop coefficient; is the maximum frequency difference caused by the disturbance.

5. The frequency adaptive collaborative support control method according to claim 3 or 4, characterized in that: The energy control method in the energy-DC voltage droop control in step S3 specifically includes the following steps: The command value and the measured value of the energy of the sending-end converter station are subtracted, and then the difference signal is subjected to PI control operation to obtain a DC current command signal; The DC current command signal is subtracted from the actual DC current signal, and then the difference signal is subjected to PI control to output a DC component control signal of the bridge arm voltage; Based on the DC component control signal of the bridge arm voltage, the command signals of the upper bridge arm and lower bridge arm voltages are obtained, and after modulation and voltage balancing algorithm, they are converted into switching signals to control the energy output of the sending-end converter station.

6. The frequency adaptive collaborative support control method according to claim 5, characterized in that: The relationship between the energy and DC voltage of the sending-end converter station in step S3 is: in, is the energy-DC voltage droop coefficient of the sending-end converter station; is the deviation of DC voltage.

7. A frequency adaptive collaborative support control system for a flexible direct current transmission system, characterized in that: include: The maximum frequency difference acquisition module is used to collect the frequency change rate of the receiving-end AC power grid, obtain the maximum frequency change rate of the receiving-end AC power grid when the disturbance occurs, and calculate the maximum frequency difference caused by the disturbance according to the extended frequency response model; A voltage command acquisition module, used to adaptively determine a DC voltage-frequency droop coefficient based on a maximum frequency difference caused by a disturbance, and acquire a DC voltage command signal; The sending-end collaborative support module is used to detect the DC voltage deviation signal at the sending-end converter station side based on the DC voltage command signal, obtain the energy command signal of the sending-end converter station based on the energy-DC voltage droop control, and apply it to the energy control loop of the sending-end converter station, so that the sending-end converter station can collaboratively support the frequency of the receiving-end AC power grid; Among them, a DC line is set between the sending-end converter station and the receiving-end converter station; the other side of the sending-end converter station is connected to the sending-end power grid, and the other end of the receiving-end converter station is connected to the receiving-end power grid.

8. The frequency adaptive collaborative support control system according to claim 7, characterized in that: The maximum frequency difference acquisition module includes: A coordinate converter, used for converting the port voltage signal of the receiving-end power grid in a three-phase stationary coordinate system into the port voltage of the receiving-end power grid in a two-phase rotating coordinate system through Park coordinates; The receiving-end phase-locked PI controller is used to perform PI operation on the q-axis voltage signal in the receiving-end power grid port voltage to obtain the real-time angular frequency of the receiving-end power grid; Differential operator, used for real-time angular frequency differential operation of the receiving-end power grid; Divider, used to divide the real-time angular frequency of the receiving power grid by 2 after differential operation Then the frequency change rate of the receiving AC power grid is obtained.

9. The frequency adaptive collaborative support control system according to claim 8, characterized in that: The extended frequency response model of the maximum frequency difference acquisition module is: in, RoCoF max is the maximum frequency change rate, is the maximum frequency difference caused by the disturbance; is the load disturbance power; is the total inertia of the receiving system; is the damping coefficient of the synchronous generator; is the droop factor of the synchronous generator speed governor; is the reheater time constant; is the work ratio coefficient of the high pressure cylinder of the reheat turbine; is the inertia constant of the synchronous generator; is the capacity of the MMC-HVDC system; is the capacity of the synchronous generator; Virtual inertia coefficient set for MMC virtual inertia control; is the unit adjustment factor of the synchronous generator; is the frequency deviation; It is the maximum frequency change rate of the receiving AC power grid when the disturbance occurs.

10. The frequency adaptive collaborative support control system according to claim 9, characterized in that: The DC voltage-frequency droop coefficient in the voltage command acquisition module is: in, is the maximum DC voltage variation allowed by MMC-HVDC; is the DC voltage-frequency droop coefficient; is the maximum frequency difference caused by the disturbance; The relationship between the energy and DC voltage of the sending-end converter station in the sending-end collaborative support module is: in, is the energy-DC voltage droop coefficient of the sending-end converter station; is the deviation of DC voltage.

Images