Flexible direct current system comprehensive coordination control method giving consideration to frequency active support and direct current overvoltage suppression

Abstract

The invention relates to the technical field of new energy control, and discloses a flexible DC system comprehensive coordination control method giving consideration to frequency active support and DC overvoltage suppression, and the method comprises the steps: obtaining a sending end frequency deviation delta fS, a receiving end system frequency deviation delta fR and a DC voltage deviation delta Udc of a flexible DC interconnection system; when the absolute value of the sending end frequency deviation is greater than the frequency dead zone value, an MMC adaptive control strategy for supporting the sending end system frequency stability is executed; when the absolute value of the frequency deviation of the receiving end system is greater than 0, inertia support control is carried out; and after inertia support control, if the absolute value of the direct-current voltage deviation is greater than the direct-current voltage dead zone value UdcH, an MMC adaptive control strategy giving consideration to receiving end system frequency support and direct-current overvoltage suppression functions is executed. According to the invention, the active frequency support and fault ride-through capability of the flexible DC power transmission system can be improved.

Description

Technical Field

[0001] The present application relates to the field of new energy control technology, and in particular to a comprehensive coordinated control method for a flexible DC system that takes into account both active frequency support and DC overvoltage suppression. Background Art

[0002] In the context of large-scale development and utilization of new energy, the use of flexible direct current transmission technology to realize the centralized transmission of new energy and asynchronous interconnection between the transmission and receiving power grids is a future development trend. As the power electronics characteristics of flexible direct current interconnection systems become increasingly prominent, they show characteristics such as low inertia, weak damping, and reduced power regulation capabilities. Different disturbance conditions can easily cause power imbalance problems on the AC and DC sides of flexible direct current systems, posing severe challenges to the safety and stability of AC frequency and DC voltage. Therefore, it is necessary to explore new control strategies for flexible direct current systems, improve their power balancing capabilities, and actively support system frequency stability while ensuring DC voltage safety.

[0003] In terms of frequency support control, the control methods of MMC-HVDC are mainly divided into grid-following control and grid-building control. Grid-following control achieves synchronization with the system through a phase-locked loop, and often uses additional control to achieve frequency support for the system. For example, based on the communication-free control strategy of DC voltage / frequency droop, the rotor kinetic energy of the wind farm is used to achieve inertia support. There is a technology that adjusts the DC voltage command value of the receiving end converter station through frequency changes and uses DC capacitors to achieve inertia support. There is a technology that achieves frequency modulation by adjusting the DC voltage, but only uses a simple coupling relationship. There is a technology that proposes an improved virtual synchronous generator control strategy for the flexible direct current system to dynamically adjust the droop coefficient. There is a technology that proposes a coordinated control strategy for wind power, energy storage and flexible direct current system to jointly support the system inertia, but there is an economic problem of configuring energy storage. There is a technology that introduces mutually coupled inertia and damping dynamic characteristics between interconnected power grids for frequency support, but this solution can only provide frequency support for a single-side power grid. The grid-building control adopts a power synchronization strategy similar to that of the synchronous machine, and synchronization can be achieved without a phase-locked loop. A technology has proposed a four-dimensional MMC control strategy for wind power and AC weak grid that can be grid-connected at both ends, further improving the MMC's support capabilities for AC systems. A technology has designed a virtual synchronous control strategy with DC voltage control, which allows the tracking speed and virtual inertia to be set separately, thereby improving the frequency support capability.

[0004] In terms of suppressing DC overvoltage caused by AC faults at the receiving end, existing research mainly focuses on two aspects: configuring energy-consuming devices and rapidly reducing the sending-end output power. The former is the most direct and effective solution to the DC overvoltage problem. However, its power control accuracy is closely related to the number of its groups, and the project cost and land occupation problems are prominent. For example, some technologies have respectively proposed a scheme of installing dissipative resistors on the DC line and AC bus and their switching strategies to suppress the DC overvoltage caused by faults. Some other technologies have proposed a new DC energy-consuming topology structure and control method for the VSC-HVDC system of offshore wind power, effectively solving the DC overvoltage problem. The latter is to reduce the sending-end output power based on the fast communication method, frequency increase method or voltage reduction method to maintain the power balance of the system. Some technologies have proposed a fault ride-through coordinated control strategy for the external transmission of wind power through MMC-HVDC based on fast communication, effectively solving the DC overvoltage problem. Some technologies reduce the sending-end output power according to the frequency change to avoid the rise of DC voltage. Some technologies reduce the sending-end output power according to the local DC voltage change to effectively suppress the DC overvoltage. Some technologies have proposed a DC fault ride-through coordinated control strategy for the grid connection of wind power through overhead line MMC-HVDC, effectively suppressing the DC overvoltage by using power transfer and wind farm load shedding.

[0005] To sum up, the existing improvements of the flexible DC system control strategy are mostly oriented to single regulation requirements, and there are few reports on the comprehensive control strategy that takes into account frequency support and DC voltage safety. In addition, the safety constraint of the DC voltage in the existing control methods directly restricts the utilization of the capacitor energy margin of the sub-module, and the inertia support capacity of the MMC-HVDC system has not been fully developed. At the same time, the traditional droop control will lead to a strong coupling relationship in the flexible DC interconnected grid, expanding the scope of disturbance influence, and it is difficult to fully exert the ability of the sending and receiving end systems to locally absorb unbalanced power, resulting in problems such as unreasonable utilization of frequency regulation resources. Summary of the Invention

[0006] The purpose of this application is to provide a comprehensive coordinated control method for a flexible DC system that takes into account active frequency support and DC overvoltage suppression, so as to improve the active frequency support and fault ride-through capabilities of the flexible DC transmission system.

[0007] In order to achieve the above purpose, the following technical solutions are adopted:

[0008] This application provides a comprehensive coordinated control method for a flexible DC system that takes into account active frequency support and DC overvoltage suppression. The method includes:

[0009] Obtain the sending-end frequency deviation Δf of the flexible DC interconnected system S , the receiving-end system frequency deviation Δf R and the DC voltage deviation ΔU dc ;

[0010] When the absolute value of the sending - end frequency deviation is greater than the frequency dead - zone value, implement the MMC adaptive control strategy to support the frequency stability of the sending - end system. While supporting the frequency stability of the sending - end, reduce the impact of disturbances on the receiving - end power grid as much as possible;

[0011] When the absolute value of the receiving - end system frequency deviation is greater than 0, perform inertia - support control;

[0012] After the inertia - support control, if the absolute value of the DC voltage deviation is greater than the DC voltage dead - zone value U dcH , then implement the MMC adaptive control strategy that takes into account both the frequency support of the receiving - end system and the suppression of DC over - voltage. While ensuring the safety of the DC voltage, improve the frequency stability of the receiving - end.

[0013] Furthermore, the MMC adaptive control strategy for supporting the frequency stability of the sending - end system is expressed as:

[0014]

[0015] In the formula: f H is the frequency dead - zone value; Δf S is the sending - end system frequency deviation; x i , x i,max are the VSC - HVDC system state variables and their limit values respectively; K p and K i are the proportional and integral link coefficients respectively; ΔP i , ΔP max are the additional power regulation amounts and their upper limit values after the sending - end frequency exceeds f H respectively; P ref0 , P ref are the active - power reference values before and after the start of the frequency - support control strategy respectively.

[0016] Furthermore, the frequency dead - zone value is determined as follows:

[0017] Establish the frequency - deviation expression corresponding to the maximum power regulation margin of the synchronous machine's primary frequency regulation, which is expressed as:

[0018] Δf s，max =ΔP s，max ·σ

[0019] In the formula: ΔP s,max is the maximum power regulation margin of the primary frequency regulation; σ is the droop coefficient of the primary frequency regulation; Δf s,max is the sending - end frequency deviation corresponding to the maximum power regulation margin of the primary frequency regulation;

[0020] Let the frequency dead - zone value f H =Δf s,max .

[0021] Furthermore, the inertia support control includes a receiving-end converter station control sub-strategy and a sending-end converter station control sub-strategy;

[0022] The receiving-end converter station control sub-strategy includes:

[0023] According to the receiving system frequency f R The total number of sub-modules N put into use in the receiving end converter station is adjusted adaptively based on the change information of R , by introducing the frequency change rate, changing N at the beginning of the disturbance R , in order to improve the system's ability to suppress frequency mutations, and introduce low-pass and high-pass filters in the differential and proportional links respectively, so that N R After the frequency stabilizes, the number of submodules put into operation will automatically return to the normal operation of the receiving converter station;

[0024] The control sub-strategy of the sending-end converter station includes:

[0025] According to the DC voltage and the receiving system frequency f R Coupling relationship, the DC voltage deviation ΔU dc Introduced into the modulation link of the sending-end converter station, adaptively adjust the total number of sub-modules N put into use in the sending-end converter station S , so that it and the receiving converter station can jointly support the system inertia, and resume normal operation after the system frequency stabilizes.

[0026] Furthermore, according to the receiving system frequency f R The total number of sub-modules N put into use in the receiving end converter station is adjusted adaptively based on the change information of R The control process is expressed as:

[0027]

[0028] Where: N R0 k is the number of submodules put into operation when the receiving converter station is operating normally; pD and k dD are the coefficients of the proportional and differential links under the frequency reduction condition.

[0029] Furthermore, low-pass and high-pass filters are introduced in the differential and proportional links respectively, so that N R The control process of the number of submodules put into operation when the frequency is automatically restored to the normal operation of the receiving converter station after the frequency is stabilized is expressed as:

[0030]

[0031]

[0032] Where: N' max The maximum number of MMC submodules input; (df / dt) maxis the maximum value constraint of the rate of change of frequency; Δf max is the maximum deviation of the grid frequency.

[0033] Furthermore, the coupling relationship between the DC voltage and the receiving-end system frequency f R is expressed as:

[0034] U dcref = U dc0 + k c Δf R

[0035] In the formula: U dcref is the reference value of the DC voltage set by the receiving-end converter station; k c is the coupling coefficient between U dc and f R ; Δf R is the frequency deviation of the receiving-end system; U dc0 is the initial value of the DC voltage;

[0036] The value of k c is determined through the following formula:

[0037]

[0038] In the formula: ΔU dcmax , Δf max are respectively the maximum deviations of the DC voltage and the grid frequency.

[0039] Furthermore, the control process of adaptively adjusting the total number N S of sub-modules put into operation in the sending-end converter station is expressed as:

[0040]

[0041] In the formula: U dcH is the dead zone value of the DC voltage; N S0 is the number of sub-modules put into operation when the sending-end converter station operates normally; k pDS is the proportional coefficient under the condition of frequency drop; N' max is the maximum value of the number of MMC sub-modules put into operation.

[0042] Furthermore, the control process of the MMC adaptive control strategy that takes into account the frequency support of the receiving-end system and the suppression of DC overvoltage is expressed as:

[0043]

[0044] In the formula: U dcH is the dead zone value of the DC voltage; x i , x i,max are respectively the state quantity of the HVDC system and its limit value; K p and Ki are proportional and integral link coefficients respectively; ΔP iu , ΔP umax are the power additional regulation amount and its upper limit after the DC voltage exceeds the frequency modulation dead zone; P ref0 , P ref They are the active power reference values ​​before and after the frequency support control strategy is started.

[0045] Furthermore, the DC voltage dead zone value U is determined by the following formula: dcH :

[0046] U dcH =k c ΔP s，max ·σ

[0047] Where: k c For U dc With f R The coupling coefficient of ΔP s,max is the maximum power regulation margin of primary frequency modulation; σ is the regulation coefficient of primary frequency modulation.

[0048] The beneficial effects of this application are:

[0049] 1) Aiming at the frequency support demand of the sending-end system and according to the regulation margin of the sending-end synchronous machine, the frequency regulation control strategy and dead zone value of the sending-end MMC are designed so that it can adaptively switch the damping regulation and primary frequency regulation functions according to the degree of power disturbance, while supporting the frequency stability of the sending end and minimizing the impact of the disturbance on the receiving-end power grid.

[0050] 2) In view of the inertia support demand of the receiving system, the coupling relationship between the receiving grid frequency and the DC voltage and the number of sub-modules put into operation in the converter station is established. At the same time, considering the modulation ratio margin, a strategy to improve the inertia support capacity of the flexible DC system is proposed, and key control parameters are designed to maximize the utilization of the energy margin of the flexible DC system.

[0051] 3) In view of the frequency support and fault ride-through requirements of the receiving-end system, according to the regulation margin of the receiving-end synchronous machine, the sending-end MMC coordinated control strategy and its dead zone value that take into account both frequency support and DC overvoltage suppression functions are designed, so that it can adaptively switch inertia support, damping regulation, primary frequency modulation and DC overvoltage suppression functions according to the degree of power disturbance, realize the reasonable call of the sending-end system regulation capability, and improve the receiving-end frequency stability while ensuring the DC voltage safety. BRIEF DESCRIPTION OF THE DRAWINGS

[0052] Figure 1Structural diagram of a flexible DC interconnected system provided by an embodiment of the present application; wherein, A, sending-end MMC control strategy for supporting the frequency stability of the sending-end system; B, inertia support control; C, sending-end MMC control strategy for taking into account the frequency support of the receiving-end system and DC overvoltage suppression;

[0053] Figure 2 Flowchart of a comprehensive coordinated control method for a flexible DC system that takes into account active frequency support and DC overvoltage suppression provided by an embodiment of the present application;

[0054] Figure 3 Schematic diagram of reducing the reference voltages of the front and rear bridge arms of the DC component provided by an embodiment of the present application;

[0055] Figure 4 Simulation results of the sending-end grid frequency under the condition that the sending-end grid cuts off 100 MW of load provided by an embodiment of the present application;

[0056] Figure 5 Simulation results of the output power of the sending-end synchronous generator under the condition that the sending-end grid cuts off 100 MW of load provided by an embodiment of the present application;

[0057] Figure 6 Simulation results of the active power regulation amount of the sending-end MMC under the condition that the sending-end grid cuts off 100 MW of load provided by an embodiment of the present application;

[0058] Figure 7 Simulation results of the power fed into the sending-end MMC under the condition that the sending-end grid cuts off 100 MW of load provided by an embodiment of the present application;

[0059] Figure 8 Simulation results of the sending-end grid frequency under the condition that the sending-end grid cuts off 170 MW of load provided by an embodiment of the present application;

[0060] Figure 9 Simulation results of the output power of the sending-end synchronous generator under the condition that the sending-end grid cuts off 170 MW of load provided by an embodiment of the present application;

[0061] Figure 10 Simulation results of the active power regulation amount of the sending-end MMC under the condition that the sending-end grid cuts off 170 MW of load provided by an embodiment of the present application;

[0062] Figure 11 Simulation results of the power fed into the sending-end MMC under the condition that the sending-end grid cuts off 170 MW of load provided by an embodiment of the present application;

[0063] Figure 12 Simulation results of the receiving-end grid frequency under the condition that the receiving-end grid cuts off 50 MW of load provided by an embodiment of the present application;

[0064] Figure 13The simulation results of the DC voltage deviation under the condition that the receiving - end power grid cuts off 50MW of load provided by the embodiments of this application;

[0065] Figure 14 The simulation results of the number of sub - modules put into operation of the sending - end converter station under the condition that the receiving - end power grid cuts off 50MW of load provided by the embodiments of this application;

[0066] Figure 15 The simulation results of the number of sub - modules put into operation of the receiving - end converter station under the condition that the receiving - end power grid cuts off 50MW of load provided by the embodiments of this application;

[0067] Figure 16 The simulation results of the average voltage of the sub - module capacitors under the condition that the receiving - end power grid cuts off 50MW of load provided by the embodiments of this application;

[0068] Figure 17 The simulation results of the equivalent capacitance under the condition that the receiving - end power grid cuts off 50MW of load provided by the embodiments of this application;

[0069] Figure 18 The simulation results of the output power of the receiving - end synchronous generator under the condition that the receiving - end power grid cuts off 50MW of load provided by the embodiments of this application;

[0070] Figure 19 The simulation results of the active power regulation amount of the sending - end MMC under the condition that the receiving - end power grid cuts off 50MW of load provided by the embodiments of this application;

[0071] Figure 20 The simulation results of the power fed into the sending - end MMC under the condition that the receiving - end power grid cuts off 50MW of load provided by the embodiments of this application;

[0072] Figure 21 The simulation results of the receiving - end power grid frequency under the condition that the receiving - end power grid increases the load by 55MW provided by the embodiments of this application;

[0073] Figure 22 The simulation results of the DC voltage deviation under the condition that the receiving - end power grid increases the load by 55MW provided by the embodiments of this application;

[0074] Figure 23 The simulation results of the number of sub - modules put into operation of the sending - end converter station under the condition that the receiving - end power grid increases the load by 55MW provided by the embodiments of this application;

[0075] Figure 24 The simulation results of the number of sub - modules put into operation of the receiving - end converter station under the condition that the receiving - end power grid increases the load by 55MW provided by the embodiments of this application;

[0076] Figure 25 The simulation results of the average voltage of the sub - module capacitors under the condition that the receiving - end power grid increases the load by 55MW provided by the embodiments of this application;

[0077] Figure 26 The simulation result of the AC outlet voltage under the condition that the load of the receiving-end power grid in the embodiment of this application increases by 55MW;

[0078] Figure 27 The simulation result of the frequency of the receiving-end power grid under the condition that the receiving-end power grid in the embodiment of this application cuts off a load of 100MW;

[0079] Figure 28 The simulation result of the DC voltage deviation under the condition that the receiving-end power grid in the embodiment of this application cuts off a load of 100MW;

[0080] Figure 29 The simulation result of the output power of the receiving-end synchronous generator under the condition that the receiving-end power grid in the embodiment of this application cuts off a load of 100MW;

[0081] Figure 30 The simulation result of the active power regulation amount of the sending-end MMC under the condition that the receiving-end power grid in the embodiment of this application cuts off a load of 100MW;

[0082] Figure 31 The simulation result of the power fed into the sending-end MMC under the condition that the receiving-end power grid in the embodiment of this application cuts off a load of 100MW;

[0083] Figure 32 The simulation result of the frequency of the receiving-end power grid under the condition that the receiving-end power grid in the embodiment of this application cuts off a load of 180MW;

[0084] Figure 33 The simulation result of the DC voltage deviation under the condition that the receiving-end power grid in the embodiment of this application cuts off a load of 180MW;

[0085] Figure 34 The simulation result of the output power of the receiving-end synchronous generator under the condition that the receiving-end power grid in the embodiment of this application cuts off a load of 180MW;

[0086] Figure 35 The simulation result of the active power regulation amount of the sending-end MMC under the condition that the receiving-end power grid in the embodiment of this application cuts off a load of 180MW;

[0087] Figure 36 The simulation result of the power fed into the sending-end converter station under the condition that the receiving-end power grid in the embodiment of this application cuts off a load of 180MW;

[0088] Figure 37 The simulation result of the DC voltage under the condition of an AC fault occurring at the receiving end in the embodiment of this application;

[0089] Figure 38The simulation results of the power fed into the sending - end MMC under the condition of the receiving - end AC fault in the embodiments of this application;

[0090] Figure 39 The simulation results of the power output by the receiving - end MMC under the condition of the receiving - end AC fault in the embodiments of this application;

[0091] Figure 40 The simulation results of the number of sub - modules put into operation in the sending - end converter station under the condition of the receiving - end AC fault in the embodiments of this application;

[0092] Figure 41 The simulation results of the number of sub - modules put into operation in the receiving - end converter station under the condition of the receiving - end AC fault in the embodiments of this application;

[0093] Figure 42 The simulation results of the equivalent capacitance of the sending - end converter station under the condition of the receiving - end AC fault in the embodiments of this application;

[0094] Figure 43 The simulation results of the equivalent capacitance of the receiving - end converter station under the condition of the receiving - end AC fault in the embodiments of this application. Detailed implementation manners

[0095] The following uses specific specific examples to illustrate the implementation manners of this application. Those skilled in the art can easily understand other advantages and effects of this application from the content disclosed in this specification. This application can also be implemented or applied through other different specific implementation manners. Various details in this specification can also be modified or changed based on different viewpoints and applications without departing from the spirit of this application. It should be noted that, without conflict, the following embodiments and the features in the embodiments can be combined with each other.

[0096] The following combines the drawings and embodiments to further describe the specific implementation manners of this application in detail.

[0097] To improve the frequency active support and fault - ride - through capabilities of the flexible DC transmission system, based on the analysis of the frequency support requirements on the AC side and the DC voltage support requirements, the embodiments of this application provide a comprehensive coordinated control method for a flexible DC system that takes into account both frequency active support and DC over - voltage suppression, as Figure 1As shown in the figure, it is a structural diagram of a flexible DC interconnected system provided by an embodiment of the present application. This method can operate based on this flexible DC interconnected system, realizing wide-area coordinated complementarity of multiple types of regulation resources while ensuring the safe operation of the flexible DC system. Generally speaking, aiming at the frequency support requirements of the sending-end system, an adaptive control strategy for the frequency support of the sending-end MMC considering the frequency modulation dead zone is proposed, and the frequency modulation dead zone value is designed according to the maximum power regulation margin of the sending-end frequency modulation resources, which can reduce the disturbance influence range while supporting the stability of the sending-end frequency. Aiming at the frequency support and fault ride-through requirements of the receiving-end system, the coupling relationship between the frequency of the receiving-end system, the DC voltage, and the number of sub-modules put into operation at the converter station is established. Without affecting the AC outlet characteristics of the MMC, considering the modulation ratio margin, an active inertia support control method for the flexible DC system based on the improvement of the control dimension is proposed. On this basis, an adaptive control strategy for the sending-end MMC that takes into account both the frequency support of the receiving-end system and the suppression of DC overvoltage is designed, and the DC voltage dead zone value is designed in combination with the maximum power regulation margin of the receiving-end frequency modulation resources, enabling it to adaptively switch functions such as inertia support, damping regulation, primary frequency modulation, and DC overvoltage suppression according to the disturbance conditions, effectively improving the frequency support and fault ride-through capabilities of the flexible DC system. Finally, a simulation model of the flexible DC interconnected power grid is constructed based on the real-time digital simulation platform to verify the effectiveness of the proposed control strategy.

[0098] As Figure 2 shown, it is a flowchart of a comprehensive coordinated control method for a flexible DC system that takes into account both active frequency support and DC overvoltage suppression provided by an embodiment of the present application. This method can be implemented by the following steps S10 to S40.

[0099] S10. Obtain the sending-end frequency deviation Δf of the flexible DC interconnected system S , the receiving-end system frequency deviation Δf R and the DC voltage deviation ΔU dc .

[0100] In this embodiment, the method of obtaining the above parameters in step S10 can be to collect data through corresponding sensors and then perform simple calculations based on general calculation formulas in the art. Therefore, the method of obtaining the sending-end frequency deviation Δf S , the receiving-end system frequency deviation Δf R and the DC voltage deviation ΔU dc will not be introduced in detail here.

[0101] S20. When the absolute value of the sending-end frequency deviation is greater than the frequency dead zone value, execute the MMC adaptive control strategy to support the stability of the sending-end system frequency, and reduce the influence of the disturbance on the receiving-end power grid as much as possible while supporting the stability of the sending-end frequency.

[0102] The MMC adaptive control strategy for supporting the frequency stability of the sending-end system is one of the important control strategies proposed in this application. The design principle and implementation process of the MMC adaptive control strategy for supporting the frequency stability of the sending-end system will be introduced in detail below.

[0103] Please combine Figure 1 , and the flexible DC system structure is as Figure 1 shown. The sending-end converter station adopts constant active power and reactive power control, and the receiving-end converter station adopts constant DC voltage and reactive power control. In the flexible DC system, AC unbalanced power will cause frequency safety and stability problems, and DC unbalanced power will cause DC voltage safety problems. The support requirements for the frequencies and DC voltages at the sending and receiving ends of the flexible DC system will be analyzed in detail below.

[0104] Analysis of frequency support requirements: Since the analysis of frequency support requirements at the sending and receiving ends is similar, only the receiving end will be analyzed in this article. According to the maximum power regulation margin and unbalanced power of the receiving-end synchronous generator, its frequency support is divided into the following two scenarios.

[0105] Scenario 1: If after the power disturbance, the unbalanced power of the receiving-end system satisfies the constraint of Equation (1), that is, the maximum power regulation margin of the receiving-end synchronous generator can completely absorb the unbalanced power. At this time, the frequency regulation task of the receiving-end system can be solely borne by the synchronous generator, and the flexible DC system only participates in the frequency dynamic regulation. However, due to the relatively slow primary frequency regulation response speed of the synchronous generator, in the initial stage of the disturbance, it is necessary to further explore the inertia support potential of the MMC-HVDC system and actively participate in suppressing the system frequency change rate to provide response time for the primary frequency regulation of the synchronous generator. In this scenario, the unbalanced power at steady-state frequency is completely borne by the synchronous generator, and the flexible DC system plays the role of inertia support and damping control during this process.

[0106] ΔP unb ≤ΔP max (1)

[0107] In the formula: ΔP max is the maximum power regulation margin of the receiving-end synchronous generator, and ΔP unb is the unbalanced power of the receiving-end system.

[0108] Scenario 2: If after the power disturbance, the unbalanced power of the receiving-end system does not satisfy the constraint of Equation (1), that is, the maximum power regulation margin of the synchronous generator cannot completely regulate the unbalanced power. At this time, the sending-end system needs to adjust its fed-in power to jointly maintain the frequency stability of the receiving-end system. In this scenario, the unbalanced power at steady-state frequency is borne by both the synchronous generator and the frequency regulation resources of the sending-end system, and the flexible DC system plays the role of inertia support and primary frequency regulation during this process.

[0109] Analysis of DC voltage support requirements:

[0110] Regarding the power surplus problem caused by the receiving - end fault, according to the law of conservation of energy, it can be obtained that the voltage U of the VSC - HVDC system dc and the surplus power ΔP(t) satisfy the relational expression (2) at any time t. Thus, it is obtained that there is a proportional relationship between the equivalent capacitance of each MMC and the surplus power it transfers. According to Equation (3), the surplus power ΔP i (t) that needs to be transferred within the i - th MMC can be known, and thus the distribution characteristics of the surplus power can be known.

[0111]

[0112] In the formula: M represents the number of MMCs in the VSC - HVDC system, C eq,i represents the equivalent capacitance of the i - th MMC, and t 0 is the receiving - end fault time.

[0113] For a single converter station, the relationship between the equivalent capacitance voltage on the DC side and its input and output powers can be expressed as:

[0114]

[0115] In the formula: P in,i represents the input power on the AC side, and P out,i represents the output power on the DC side.

[0116] It can be seen that when there is a surplus power ΔP(t) in the VSC - HVDC system, the rising rate and amplitude of U dc are mainly determined by the surplus power, the size of the equivalent capacitance, and the duration of the surplus power. Therefore, three schemes, namely reducing the value of the surplus power, increasing the equivalent capacitance, and shortening the duration of the surplus power, can be used to suppress the DC over - voltage.

[0117] Regarding the frequency support requirement of the sending - end system, an adaptive control strategy for the sending - end MMC to support the frequency stability of the sending - end system is proposed. As Figure 1 shown, the principle expression of its control strategy is as shown in Equation (5):

[0118]

[0119] In the formula: f H is the frequency dead - zone value; Δf S is the frequency deviation of the sending - end system; x i and x i,max are the state variables of the VSC - HVDC system and their limit values respectively; K p and K i are the proportional and integral link coefficients respectively; ΔP i and ΔP max are the additional power adjustment amount and its upper limit value after the sending - end frequency exceeds f H respectively; P ref0 and Pref They are the active power reference values before and after the start of the frequency support control strategy, respectively.

[0120] It can be seen that f H directly determines the degree to which the sending-end MMC participates in the frequency regulation of the sending-end system and is the key to realizing the coordinated cooperation of multiple types of frequency regulation resources in the flexible DC interconnected power grid. Therefore, combined with the power regulation margin of the sending-end synchronous machine, a design method for f H is proposed to reduce the impact of sending-end disturbances on the receiving-end system while giving full play to the regulation ability of the synchronous machine.

[0121] The frequency deviation expression corresponding to the maximum power regulation margin of the synchronous machine's primary frequency regulation is shown in Equation (6):

[0122] Δf s，max = ΔP s，max ·σ (6)

[0123] In the formula: ΔP s,max is the maximum power regulation margin of the primary frequency regulation; σ is the droop coefficient of the primary frequency regulation; Δf s,max is the sending-end frequency deviation corresponding to the maximum power regulation margin of the primary frequency regulation, and f H = Δf s,max .

[0124] It can be seen that through the reasonable design of f H , a reasonable distribution of the frequency regulation power between the synchronous generator set and the flexible DC system can be achieved, that is, after the synchronous machine reaches the upper limit of its frequency regulation ability, the flexible DC system is used to absorb the unbalanced power of the sending-end system, reducing the influence range of power disturbances while maintaining the safe and stable operation of the system.

[0125] S30. When the absolute value of the frequency deviation of the receiving-end system is greater than 0, inertia support control is carried out.

[0126] Inertia support control is also one of the important control strategies proposed in this application. The design principle and implementation process of inertia support control will be introduced in detail below.

[0127] First, the inertia support ability is analyzed. The electric potential energy stored in the sub-module capacitors in the MMC-HVDC system can be used as the energy source for inertia support. In this paper, the energy ΔE that the sub-module capacitors can support c is shown in Equation (7):

[0128]

[0129] In the formula: U SM and U SM0 are the sub-module capacitor voltage and its steady-state value, respectively; C SMis a single sub-module capacitor; N is the total number of sub-module capacitors in each arm of the converter station; E is the energy storage of the sub-module capacitor; E 0 is the energy storage of the sub-module capacitor when the sub-module capacitor voltage reaches the steady-state value.

[0130] For the frequency rising condition:

[0131] U SM The maximum value that can be tolerated in a short time can reach 1.5 times its rated value. Combining the system parameters in Equation (7) and Table 1, it can be calculated that when using the traditional control strategy, since the maximum value that the sub-module capacitor voltage can reach is 1.05U SM0 , the maximum energy that the sub-module capacitor can absorb is 10.12 MJ; when the maximum value of the sub-module capacitor voltage is 1.5U SM0 , the maximum energy that the sub-module capacitor can absorb is 123.36 MJ, which is about 11 times higher than that of the traditional inertia control strategy.

[0132] For the frequency falling condition:

[0133] Under normal circumstances, the system modulation ratio m is less than 1. As Figure 3 shown, there is a certain adjustment margin for the DC component in the arm reference voltage, so U dc can be further reduced, broadening the working range of the capacitor voltage and enhancing the inertia support ability of the MMC-HVDC system, while not affecting the AC outlet characteristics of the MMC. Combining the system parameters in Equation (7) and Table 1, it can be calculated that when using the traditional control strategy, since the minimum value that the sub-module capacitor voltage can reach is 0.95U SM0 , the maximum energy that the sub-module capacitor can release is 9.62 MJ; when considering the modulation ratio margin (m = 0.82), and at the same time, the MMC has about 8% redundant sub-modules in actual engineering, which can be put into operation while reducing U dc . Therefore, from the equation U dc = NU SM , it can be known that the minimum value that the sub-module capacitor voltage can reach is 0.76U SM0 , and the maximum energy that the sub-module capacitor can release is 41.68 MJ, which is about 4 times higher than that of the traditional inertia control strategy.

[0134] Based on the above analysis, it can be seen that the traditional inertia support control strategy fails to make full use of the energy margin of the sub-module capacitor. Therefore, this application designs a multi-dimensional control strategy (inertia support control) to enhance the inertia support ability of the flexible DC system, as Figure 1 shown.

[0135] In order to fully develop the active inertia support ability of the MMC-HVDC system and utilize the DC voltage to transmit disturbance information, by adopting the f / U as Figure 1 shown dcDroop control, establish U dc The coupling relationship with the receiving-end system frequency f R provides a prerequisite for the sending-end converter station to achieve active inertia support, and at the same time provides a way to make full use of the modulation ratio margin during frequency drops.

[0136] U dcref = U dc0 + k c Δf R (8)

[0137] In the formula: U dcref is the DC voltage reference value of the receiving-end converter station; k c is the coupling coefficient between U dc and f R ; Δf R is the frequency deviation of the receiving-end system.

[0138] It can be seen that if k c takes too large a value and causes U dc to reach its limit value, it will cause f R to not be fully feedback to the sending-end converter station. Therefore, considering the system safe operation constraints, the value of k c is designed as:

[0139]

[0140] In the formula: ΔU dcmax , Δf max are the maximum deviations of the DC voltage and the grid frequency respectively.

[0141] Furthermore, the equivalent inertia time constant of the receiving-end converter station can be obtained as:

[0142]

[0143] It can be seen that H MMC is directly proportional to C eq and ΔU dcmax . From the formula C eq = 12C SM / N, it can be seen that the magnitude of C eq can be changed by changing N through the nearest level approximation modulation. ΔU dcmax can utilize the modulation ratio margin to further reduce the lower limit value of the DC voltage.

[0144] According to the change information of f R , adaptively adjust the total number of sub-modules N R put into the receiving-end converter station. As shown in formula (11), by introducing the frequency change rate, N R, to improve the system's ability to suppress frequency mutations. In addition, low-pass and high-pass filters are introduced into the differential and proportional links respectively, which can make N R automatically return to N after the frequency stabilizes 0 . In this embodiment, the control parameter design for the frequency drop condition is taken as an example. Considering the maximum value N' max of the number of MMC sub-modules put into operation is 1.08 p.u., so the maximum value constraint (df / dt) max and Δf max can be used to design k dD and k pD respectively, as shown in equations (12) and (13), and with the limiter link, it can ensure that U dc and U SM do not exceed the limit (under the condition of m = 0.82, the lower limit value of U dc is 0.82U dc0 , the lower limit value of U SM is 0.76U SM0 ), as Figure 1 shown, while maintaining the safe operation of the system, its inertia support ability is improved.

[0145]

[0146]

[0147]

[0148] In the formula: N R0 is the number of sub-modules put into operation at the receiving-end converter station during normal operation; k pD and k dD are the coefficients of the proportional and differential links respectively under the frequency drop condition.

[0149] For the sending-end converter station, according to the coupling relationship between U dc and f R , ΔU dc is introduced into the modulation link of the sending-end converter station to adaptively adjust the total number N S of sub-modules put into operation in the sending-end converter station, so that it jointly supports the system inertia with the receiving-end converter station, and returns to normal operation after the system frequency stabilizes, as shown in equation (14):

[0150]

[0151] In the formula: N S0 is the number of sub-modules put into operation at the sending-end converter station during normal operation; k pDS is the proportional coefficient under the frequency drop condition.

[0152] S40. After the inertia support control, if the absolute value of the DC voltage deviation is greater than the DC voltage dead zone value U dcH , the MMC adaptive control strategy that takes into account both the frequency support of the receiving-end system and the suppression of DC overvoltage is executed to improve the frequency stability of the receiving end while ensuring the safety of the DC voltage.

[0153] The MMC adaptive control strategy that takes into account both the frequency support of the receiving-end system and the suppression of DC overvoltage is also one of the important control strategies proposed in this application. The design principle and implementation process of the MMC adaptive control strategy that takes into account both the frequency support of the receiving-end system and the suppression of DC overvoltage will be introduced in detail below.

[0154] Since the process of the sub-module capacitor providing inertia support is short and unsustainable, in order to maintain the receiving-end grid frequency and DC voltage within a safe range, it is necessary to reasonably utilize the regulation ability of the sending-end system to ensure the safe and stable operation of the system. Therefore, considering the coupling relationship between U dc and f, a coordinated control strategy for the sending-end MMC that takes into account both the frequency support of the receiving-end system and the suppression of DC overvoltage is proposed, as shown in Figure 1 the dotted box. The principle expression of its control strategy is shown in Equation (15):

[0155]

[0156] In the formula: U dcH is the DC voltage dead zone value.

[0157] In terms of the frequency support of the receiving end, U dcH directly determines the degree to which the sending-end MMC participates in the frequency regulation of the receiving-end system and is the key to realizing the coordinated cooperation of various types of regulation resources in the flexible DC interconnected power grid. Therefore, in combination with the power regulation margin of the receiving-end synchronous machine, a design method for U dcH is proposed to meet the frequency support requirements mentioned above and reduce the disturbance influence range while ensuring frequency stability.

[0158] According to the coupling relationship between ΔU dc and Δf R in Equation (8), substituting Equation (6) into Equation (8) can obtain U dcH , as shown in Equation (16):

[0159] U dcH = k c ·ΔP s，max ·σ (16)

[0160] In terms of the suppression of DC overvoltage, when an AC fault occurs at the receiving end of the flexible DC power grid, U dc rises rapidly. Under the active inertia support control, the number of sub-modules put into operation is greatly reduced, the equivalent capacitance is increased, and the DC overvoltage is suppressed. When ΔUdc >U dcH Finally, the unbalanced power after the fault occurs is absorbed by the sending-end MMC, which further reduces the DC voltage during the period when the unbalanced power exists and effectively improves the fault ride-through capability of the receiving end.

[0161] Based on the above analysis, combined Figure 2 The specific implementation process of the comprehensive coordinated control strategy of the flexible DC system is shown in the figure. Considering that the sending-end MMC has both additional frequency and DC voltage control modes, in order to avoid mutual interference between the two, a Δf based S and ΔU dc The control mode selection strategy that reaches the corresponding dead zone value priority is as follows: Figure 1 As shown in the dashed box, when one mode is activated, the other mode is in an inactive state, so that it can meet the frequency support and fault ride-through requirements under different disturbance conditions of the system, and achieve optimal utilization of the interconnected power grid control resources while maintaining system safety and stability.

[0162] The following example will conduct simulation verification on the method proposed in this application to prove its feasibility and progress.

[0163] This embodiment is built in Matlab / simulink as follows Figure 1 The electromagnetic transient simulation model of the flexible DC system is shown in FIG. 1 , and the real-time digital simulator RTLAB is used for simulation. The specific simulation parameters are shown in Table 1.

[0164] Table 1 Flexible DC system parameters

[0165]

[0166]

[0167] The sending and receiving end grids of the flexible DC system include two synchronous units with a rated capacity of 900MW, and their primary frequency regulation coefficient σ is 0.05, the maximum power regulation margin is 144MW, the load size is 1030MW, and f H 0.004pu, U dcH It is 0.02pu, and the system modulation ratio m is 0.82.

[0168] The following will focus on the two main working conditions of the sending-end system frequency support and the receiving-end system frequency support and fault ride-through, and compare and analyze the response effects of different control strategies through simulation to verify the effectiveness and performance of the proposed comprehensive coordinated control strategy.

[0169] Sending end system frequency support simulation verification:

[0170] The specific control mode is set as follows: Mode 1 The receiving end MMC adopts the traditional f / Udc Droop control, no additional control is adopted for the sending - end MMC; in Mode 2, the receiving - end MMC adopts the traditional f / U dc Droop control, the sending - end MMC adopts the traditional P / f droop control; in Mode 3, the method proposed in this paper is adopted.

[0171] Simulation analysis of Scenario 1:

[0172] To verify the effectiveness of the proposed coordinated control strategy after power perturbation in Scenario 1, at t = 50 s, a load of 100 MW is cut off from the sending - end power grid for simulation verification. The simulation results are as Figures 4 to 7 shown.

[0173] It can be seen from the simulation results that after the load is cut off, in Mode 3, the frequency rises, resulting in the sending - end frequency deviation exceeding f H , under the action of the frequency support control of the sending - end MMC, its active power regulation amount gradually increases. The sending - end system increases its feeding power and jointly bears the unbalanced power with the sending - end synchronous machine to suppress the system frequency rise, as Figure 5 、 Figure 6 and Figure 7 shown. Until the frequency drops and the sending - end system frequency deviation is lower than f H after that, the power regulation amount of the sending - end system gradually decreases to 0, and only the sending - end synchronous machine regulates the unbalanced power. At steady state, the sending - end frequency deviation is lower than 0.004 p.u (i.e., f H ), as Figures 4 to 7 shown. The flexible DC system shows the role of damping regulation. Compared with Mode 2, during the frequency regulation process, the maximum rising amplitude of the feeding power of the sending - end system in Mode 3 increases from 10 MW to 21 MW, and the maximum frequency deviation decreases by 0.0004 p.u (i.e., 0.02 Hz). At steady state, the sending - end system does not participate in frequency regulation, as Figure 4 、 Figure 6 、 Figure 7 shown. While supporting the sending - end frequency stability, it reduces the influence range of the perturbation, verifying the effectiveness of the proposed control strategy in Scenario 1 and its damping regulation performance.

[0174] To verify the effectiveness of the proposed coordinated control strategy after power perturbation in Scenario 2, at t = 50 s, a load of 170 MW is cut off from the sending - end power grid for simulation verification. The simulation results are as Figures 8 to 11 shown.

[0175] It can be seen from the simulation results that after the load is cut off from the sending - end power grid, the power perturbation exceeds the maximum power regulation margin of the sending - end synchronous machine. In Mode 1, the frequency - modulation resources are limited, and the sending - end system does not participate in frequency regulation, resulting in system frequency instability, as Figure 8 shown. In Mode 3, the frequency rises, resulting in the sending - end frequency deviation exceeding f HThe dynamic response characteristics when reaching its maximum deviation are the same as those in Scenario 1, except that the regulation margin of the sending-end synchronous machine is limited, and f H is designed according to its power regulation margin. Therefore, after the synchronous machine reaches its maximum power regulation margin of 144 MW, the sending-end converter station participates in regulating the remaining unbalanced power (assuming 0.0076 p.u. at steady state and the active power regulation amount not being fully released). As shown in Figure 9 , Figure 10 , Figure 11 , the sending-end frequency deviation will be maintained at 0.004 p.u. (i.e., f H ), as shown in Figure 8 , and the flexible DC system exhibits the function of primary frequency regulation. Compared with Mode 2, in Mode 3, the active power regulation amount of the sending-end MMC is larger during power disturbances, effectively suppressing the frequency rise trend and alleviating the frequency regulation burden of the synchronous machine. At steady state, the system frequency drops from 1.0076 p.u. to 1.004 p.u., as shown in Figure 8 , verifying the effectiveness of the proposed control strategy and its primary frequency regulation performance under Scenario 2.

[0176] Verification of the receiving-end system frequency support and fault ride-through simulation.

[0177] Verification of the receiving-end system frequency support simulation:

[0178] The specific control modes are set as follows: In Mode 1, the receiving-end MMC adopts the traditional f / U dc droop control, and the sending-end MMC does not adopt additional control; in Mode 2, the receiving-end MMC adopts the traditional f / U dc droop control, and the sending-end MMC adopts the traditional P / f droop control; in Mode 3, the method proposed in this paper is adopted.

[0179] Simulation analysis of Scenario 1:

[0180] To verify the effectiveness of the proposed coordinated control strategy under Scenario 1 after power disturbances, a simulation verification is carried out by disconnecting 50 MW of load from the receiving-end power grid at t = 50 s, and the simulation results are shown in Figures 12 to 20 .

[0181] According to the simulation results, it can be seen that after the load is disconnected, the rise in the receiving-end power grid frequency will cause the DC voltage deviation to rise in real time in a linked manner, as shown in Figure 12 and Figure 13 . Since the DC voltage does not exceed U dcH , the flexible DC system only provides inertia support under the action of the inertia active support control strategy based on the improvement of the control dimension. Compared with Mode 1 and Mode 2, the number of sub-modules invested in the sending-end and receiving-end converter stations decreases by 12.3% and 12.7% respectively, as shown in Figure 14 and Figure 15 , and the maximum value of the sub-module capacitor voltage increases by 16.25%, asFigure 16 and Figure 17 As shown, more energy margin of the capacitor is utilized, and the maximum frequency change rate drops from 0.0016 p.u / s to 0.0008 p.u / s, effectively suppressing the frequency change rate at the initial stage of power disturbance. As Figure 12 shown, the effectiveness of the inertia active support control strategy based on the improvement of control dimension is verified.

[0182] In Mode 2, U is not set dcH , and the active power regulation amount of the sending-end MMC will gradually increase after power disturbance, reducing the feeding power of the sending-end converter station. As Figure 19 、 Figure 20 shown. In Mode 3, the DC voltage deviation does not exceed U dcH , as Figure 13 shown. The sending-end MMC does not participate in frequency regulation, and only the receiving-end synchronous machine bears the unbalanced power. As Figure 18 、 Figure 19 、 Figure 20 shown, the flexible DC system only presents the role of inertia support. Compared with Mode 2, Mode 3 slows down the frequency rising trend at the initial stage of power disturbance, follows the principle of "the disturbance of the receiving-end system should not affect the sending-end system as much as possible", and reduces the disturbance influence range while maintaining the system frequency stability.

[0183] To further verify the performance of the proposed inertia active support control strategy under the condition of frequency drop, a load of 55 MW is added to the receiving-end power grid at t = 50 s, and the proposed inertia active support control strategy in this paper is compared with several common inertia active support control strategies. Control 1 in the figure is the traditional f / U dc droop control, Control 2 is the inertia active support control strategy considering the U dc safety constraint, and Control 3 is the inertia active support control strategy considering the U dc reducible margin proposed in this paper. The simulation results are as Figures 21 to 26 shown.

[0184] It can be seen from the simulation results that after adding the load, the frequency drop of the receiving-end power grid will cause the DC voltage deviation to drop in real-time linkage. Compared with Control 1 and Control 2, the DC voltage deviation of Control 3 is larger, and while reducing U dc , the AC outlet voltage remains unchanged. As Figure 21 、 Figure 22 、 Figure 26 shown. Compared with Control 1, the number of sub-modules put into operation at the sending and receiving-end converter stations of Control 2 and Control 3 increases by 8%, as Figure 23 、 Figure 24 shown, and the minimum values of the sub-module capacitor voltages drop by 8.9% and 13.28% respectively, as Figure 25As shown, Control 3 makes more use of the energy margin of the capacitor, and the maximum frequency change rate drops from 0.0013 p.u / s under Control 2 and 0.0024 p.u / s under Control 1 to 0.0008 p.u / s, effectively suppressing the frequency change rate at the initial stage of power disturbance. As Figure 21 shown, the effectiveness of the inertia active support control strategy proposed in this paper is verified.

[0185] In addition, to further verify the damping regulation performance of the proposed coordinated control strategy, a simulation verification is carried out by removing 100 MW of load from the receiving-end power grid at t = 50 s. The simulation results are as Figures 27 to 31 shown.

[0186] It can be seen from the simulation results that after the frequency in Mode 3 rises and causes the DC voltage deviation to exceed U dcH , under the action of the frequency support control of the sending-end MMC, its active power regulation amount gradually increases, the sending-end system reduces its feeding power, and jointly bears the unbalanced power with the receiving-end synchronous machine to suppress the system frequency rise. As Figure 28 , 29 , 30, 31 shown. Until the frequency drops and causes the DC voltage deviation to be lower than U dcH , the power regulation amount of the sending-end system gradually decreases to 0, and only the receiving-end synchronous machine regulates the unbalanced power. At steady state, the DC voltage deviation is lower than 0.02 p.u (i.e., U dcH ), as Figure 27 , 28 , 29, 30 shown. The VSC-HVDC system presents the functions of inertia support and damping regulation. Compared with Mode 2, the maximum decrease amplitude of the feeding power of the sending-end system during the frequency regulation process in Mode 3 increases from 18 MW to 25 MW, the maximum frequency deviation decreases by 0.0002 p.u (i.e., 0.01 Hz), and the sending-end system does not participate in frequency regulation at steady state. As Figure 27 , 30 , 31 shown, while maintaining the frequency safety and stability of the receiving-end system, the influence range of the disturbance is reduced, verifying the effectiveness of the proposed control strategy in Scenario 1 and its damping regulation performance.

[0187] Scenario 2 simulation analysis:

[0188] To verify the effectiveness of the proposed coordinated control strategy in Scenario 2 after power disturbance, a simulation verification is carried out by removing 180 MW of load from the receiving-end power grid at t = 50 s. The simulation results are as Figures 32 to 36 shown.

[0189] It can be seen from the simulation results that after the load is removed from the receiving-end power grid, the power disturbance exceeds the maximum power regulation margin of the receiving-end synchronous machine. In Mode 1, the frequency modulation resources are limited, and the sending-end system does not participate in frequency regulation, resulting in system frequency instability. As Figure 31As shown. The increase in the frequency of Mode 3 causes the DC voltage deviation to exceed U dcH When it reaches its maximum deviation, the dynamic response characteristic is the same as that in Scenario 1. The difference is that the regulation margin of the receiving-end synchronous machine is limited, and U dcH is designed according to its power regulation margin. Therefore, after the synchronous machine reaches its maximum power regulation margin of 144 MW, the sending-end system participates in regulating the remaining unbalanced power (assuming 0.0089 p.u. at steady state and the active regulation amount not being fully released). As shown in Figure 34 、 35 、36, the DC voltage deviation will be maintained at 0.02 p.u. (i.e., U dcH ), as shown in Figure 33 . The flexible DC system exhibits the functions of inertia support and primary frequency regulation. Compared with Mode 2, in Mode 3, the active power regulation amount of the sending-end MMC is larger during power disturbances, effectively suppressing the frequency increase trend and alleviating the frequency regulation burden of the synchronous machine. At steady state, the system frequency drops from 1.0044 p.u. to 1.004 p.u., as shown in Figure 32 , verifying the effectiveness of the proposed coordinated control strategy and its primary frequency regulation performance under Scenario 2.

[0190] Verification of the fault ride-through simulation of the receiving-end system:

[0191] At t = 50 s, a fault occurs on the receiving-end AC side with a duration of 100 ms. The specific control method settings are as follows: No additional control is adopted in Mode 1; the overvoltage suppression strategy proposed in this paper is adopted in Mode 2. The simulation results are as shown in Figures 37 to 43 .

[0192] According to the simulation results, it can be seen that after the fault occurs, U dc rises rapidly. Compared with Mode 1, the number of sub-modules put into operation at the sending-end and receiving-end converter stations in Mode 2 decreases by 6% and 30% respectively, increasing the equivalent capacitance of the converter stations. After the fault occurs, the inertia of the sending-end and receiving-end converter stations is increased, and the maximum DC voltage rise rate drops from 3.38 p.u. / s to 1.92 p.u. / s, effectively suppressing the DC voltage rise rate at the initial stage of the fault, as shown in Figure 37 、 40 、41、42、43. When the DC voltage deviation exceeds U dcH , the sending-end MMC absorbs the unbalanced power existing after the fault occurs. Compared with Mode 1, the maximum decrease amplitude of the power fed into the sending-end system in Mode 2 during the power regulation process increases from 89 MW to 529 MW, and the DC voltage amplitude drops by 0.28 p.u. (i.e., 140 kV), effectively suppressing the DC voltage amplitude, as shown in Figure 37 、 38 , verifying the effectiveness of the proposed DC overvoltage suppression function.

[0193] In summary, in view of the AC frequency and DC voltage safety and stability problems faced by the flexible DC interconnected system, this application proposes a comprehensive coordinated control strategy and control parameter design method for a flexible DC system that takes into account both frequency active support and DC voltage safety, achieving coordinated and optimized utilization of the capacitor energy margin of sub-modules and various types of regulation resources while ensuring the safe and stable operation of the system. Simulation analysis proves that this application can achieve the following beneficial effects:

[0194] 1) In terms of inertia support, through the coupling relationship between DC voltage, the number of sub-modules put into operation, and the frequency of the receiving-end power grid, without changing the MMC AC outlet characteristics, considering the modulation ratio margin, the working range of the capacitor voltage is further broadened, realizing the full utilization of the capacitor energy margin of the flexible DC system without communication, and significantly improving the inertia support ability of the MMC-HVDC system.

[0195] 2) In terms of frequency deviation regulation, according to Δf, ΔU dc dynamically adjust the power fed into the sending-end MMC, and combined with the reasonable design of the dead zone value, enable the flexible DC system to adaptively switch the inertia support, damping regulation, and primary frequency modulation functions according to the degree of power disturbance, realizing the full utilization of the local resource regulation ability, and reducing the disturbance influence range while ensuring the system frequency stability.

[0196] 3) In terms of DC overvoltage suppression, by greatly reducing the number of sub-modules put into operation, increasing the equivalent capacitance, and reducing the power fed into the sending-end MMC, jointly suppress the rising rate and amplitude of the DC voltage, effectively improving the receiving-end AC fault ride-through ability of the flexible DC system.

[0197] The above embodiments are only used to illustrate this application and are not intended to limit this application. Those of ordinary skill in the relevant technical fields can also make various changes and modifications without departing from the spirit and scope of this application. Therefore, all equivalent technical solutions also belong to the scope of this application, and the patent protection scope of this application shall be defined by the claims.

Claims

1. A comprehensive coordinated control method for a flexible DC system taking into account both active frequency support and DC overvoltage suppression, characterized in that: The method comprises: Obtain the frequency deviation Δf at the sending end of the flexible DC interconnection system S , receiving system frequency deviation Δf R and DC voltage deviation ΔU dc ; When the absolute value of the frequency deviation at the sending end is greater than the frequency dead zone value, the MMC adaptive control strategy that supports the frequency stability of the sending end system is executed, while supporting the frequency stability of the sending end and minimizing the impact of the disturbance on the receiving end power grid; When the absolute value of the frequency deviation of the receiving system is greater than 0, inertia support control is performed; After inertia support control, if the absolute value of the DC voltage deviation is greater than the DC voltage dead zone value U dcH , the MMC adaptive control strategy that takes into account both the frequency support of the receiving system and the DC overvoltage suppression function is implemented, which improves the frequency stability of the receiving end while ensuring the safety of the DC voltage.

2. The method for comprehensive coordinated control of a flexible DC system taking into account both active frequency support and DC overvoltage suppression as claimed in claim 1, characterized in that: The MMC adaptive control strategy for supporting the frequency stability of the sending-end system is expressed as: Where: f H is the frequency dead zone value; Δf S is the frequency deviation of the sending end system; x i 、x i,max are the state variables and their limiting values ​​of the flexible DC system respectively; K p and K i are proportional and integral link coefficients respectively; ΔP i , ΔP max The sending frequency exceeds f H The power additional adjustment amount and its upper limit after P ref0 , P ref They are the active power reference values ​​before and after the frequency support control strategy is started.

3. The method for comprehensive coordinated control of a flexible DC system taking into account both active frequency support and DC overvoltage suppression as claimed in claim 1, characterized in that: The frequency deadband value is determined as follows: The frequency deviation expression corresponding to the maximum power regulation margin of the synchronous machine primary frequency regulation is established as follows: Δf s，max =ΔP s，max ·s Where: ΔP s,max is the maximum power adjustment margin of primary frequency modulation; σ is the adjustment coefficient of primary frequency modulation; Δf s,max The frequency deviation at the sending end corresponding to the maximum power regulation margin of primary frequency modulation; Let the frequency dead zone value f H =Δf s,max .

4. The method for comprehensive coordinated control of a flexible DC system taking into account both active frequency support and DC overvoltage suppression as claimed in claim 1, characterized in that: The inertia support control includes a receiving-end converter station control sub-strategy and a sending-end converter station control sub-strategy; The receiving-end converter station control sub-strategy includes: According to the receiving system frequency f R The total number of sub-modules N put into use in the receiving end converter station is adjusted adaptively based on the change information of R , by introducing the frequency change rate, changing N at the beginning of the disturbance R , in order to improve the system's ability to suppress frequency mutations, and introduce low-pass and high-pass filters in the differential and proportional links respectively, so that N R After the frequency stabilizes, the number of submodules put into operation will automatically return to the normal operation of the receiving converter station; The control sub-strategy of the sending-end converter station includes: According to the DC voltage and the receiving system frequency f R Coupling relationship, the DC voltage deviation ΔU dc Introduced into the modulation link of the sending-end converter station, adaptively adjust the total number of sub-modules N put into use in the sending-end converter station S , so that it and the receiving converter station can jointly support the system inertia, and resume normal operation after the system frequency stabilizes.

5. The method for comprehensive coordinated control of a flexible DC system taking into account both active frequency support and DC overvoltage suppression as claimed in claim 4, characterized in that: According to the receiving system frequency f R The total number of sub-modules N put into use in the receiving end converter station is adjusted adaptively based on the change information of R The control process is expressed as: Where: N R0 k is the number of submodules put into operation when the receiving converter station is operating normally; pD and k dD are the coefficients of the proportional and differential links under the frequency reduction condition.

6. The method for comprehensive coordinated control of a flexible DC system taking into account both active frequency support and DC overvoltage suppression as claimed in claim 5, characterized in that: Introduce low-pass and high-pass filters in the differential and proportional links respectively, so that N R The control process of the number of submodules put into operation when the frequency is automatically restored to the normal operation of the receiving converter station after the frequency is stabilized is expressed as: Where: N' max The maximum number of MMC submodules input; (df / dt) max is the maximum value constraint of the frequency change rate; Δf max is the maximum deviation of the grid frequency.

7. The method for comprehensive coordinated control of a flexible DC system taking into account both active frequency support and DC overvoltage suppression as claimed in claim 4, characterized in that: DC voltage and receiving system frequency f R The coupling relationship is expressed as: U dcref =U dc0 +k c Δf R Where: U dcref Set the DC voltage reference value for the receiving converter station; k c For U dc With f R The coupling coefficient of R is the frequency deviation of the receiving system; U dc0 is the initial value of DC voltage; Determine k by the following formula c The value of: Where: ΔU dcmax , Δf max are the maximum deviations of DC voltage and grid frequency respectively.

8. The method for comprehensive coordinated control of a flexible DC system taking into account both active frequency support and DC overvoltage suppression as claimed in claim 4, characterized in that: Adaptively adjust the total number of submodules N put into use in the sending-end converter station S The control process is expressed as: Where: U dcH is the DC voltage dead zone value; N S0 k is the number of submodules put into operation when the sending-end converter station is operating normally; pDS is the proportional coefficient under the frequency reduction condition; N' max The maximum number of MMC submodules that can be used.

9. The method for comprehensive coordinated control of a flexible DC system taking into account both active frequency support and DC overvoltage suppression as claimed in claim 1, characterized in that: The control process of the MMC adaptive control strategy taking into account both the receiving-end system frequency support and the DC overvoltage suppression function is expressed as follows: Where: U dcH is the DC voltage dead zone value; x i 、x i,max are the state variables and their limiting values ​​of the flexible DC system respectively; K p and K i are proportional and integral link coefficients respectively; ΔP iu , ΔP umax are the power additional regulation amount and its upper limit after the DC voltage exceeds the frequency modulation dead zone; P ref0 , P ref They are the active power reference values ​​before and after the frequency support control strategy is started.

10. The method for comprehensive coordinated control of a flexible DC system taking into account both active frequency support and DC overvoltage suppression as claimed in claim 9, characterized in that: Determine the DC voltage dead zone value U by the following formula dcH : U dcH =k c ·ΔP s，max ·s Where: k c For U dc With f R The coupling coefficient of ΔP s,max is the maximum power regulation margin of primary frequency modulation; σ is the regulation coefficient of primary frequency modulation.

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