A frequency control method for a VSC-MTDC interconnection system based on a virtual synchronous machine

Abstract

The application provides a VSC-MTDC interconnection system frequency control method based on a virtual synchronous machine, and steps are as follows: according to the power balance relationship between a converter and a synchronous generator, a virtual synchronous machine control strategy considering a virtual speed regulator is designed; an adaptive droop control strategy considering a power margin is designed; firstly, initial droop coefficients are determined according to the principle that the droop coefficients are inversely proportional to the rated capacity of the converter station, and then the droop coefficients are adaptively corrected according to the available power margin of each converter station; the AC side frequency and the DC side voltage are coupled, the relationship between the AC frequency and the DC side active power is established, and the power input and output of the voltage source converter station is adjusted to reduce the frequency disturbance of the AC system. The application can not only effectively suppress the frequency fluctuation, improve the inertia level and primary frequency modulation capacity of the AC side, but also realize power support between each AC power grid, and improve the power distribution capacity of the DC system.

Description

Technical Field

[0001] This invention relates to the technical field of multi-terminal flexible DC transmission, and in particular to a frequency control method for a VSC-MTDC interconnection system based on a virtual synchronous machine. Background Technology

[0002] With the continuous adjustment of the energy structure, the penetration rate of high-capacity power electronic devices in the power grid is gradually increasing, showing a trend of power electronization in the four fields of power supply, grid, load, and energy storage. In terms of power transmission systems, the multi-terminal high-voltage direct current (VSC-MTDC) transmission system based on voltage source converters is one of the important development directions of long-distance power transmission technology in the future, and is gradually becoming the best choice for wind farm grid connection.

[0003] With the rapid increase in the proportion of wind power in the power generation structure, it has not only impacted traditional power systems but also attracted significant attention in the field of power electronics. On the one hand, wind farms based on power electronics technology possess strong frequency regulation capabilities and can participate in system frequency regulation together with other power sources. On the other hand, the system's rotational inertia is decreasing, leading to significant system frequency fluctuations caused by faults. VSC-MTDC systems, however, possess rapid decoupling capabilities between active and reactive power, eliminating issues such as reactive power compensation and commutation failure. Therefore, appropriate control strategies can be designed to improve the stability of AC / DC systems. Existing control strategies for VSC-MTDC systems mainly include three types: master-slave control, margin control, and DC voltage droop control. Among these, DC voltage droop control can coordinate the distribution of unbalanced power across multiple converter stations, thereby improving DC voltage levels, and is currently the most widely used inter-station coordinated control strategy. However, DC systems based on these control methods still exhibit weak damping and low inertia, making it difficult to provide inertial support to the AC side. When the system is subjected to load disturbances, the AC side frequency often fluctuates significantly. References [W.Wang, Y.Li, Y.Cao, U.] [and C. Rehtanz, "Adaptive Droop Control of VSC-MTDC System for Frequency Support and Power Sharing," in IEEE Transactions on Power Systems, vol. 33, no. 2, pp. 1264-1274, March 2018, doi: 10.1109 / TPWRS.2017.2719002.] By adding AC frequency control to the upper-level control of the converter station, the DC system can respond quickly to changes in AC side frequency and reduce frequency deviation; however, this may cause large fluctuations in DC voltage. The literature [FDBianchi and JLDomnguez-Garca, "Coordinated Frequency Control Using MT-HVDC Grids With Wind Power Plants," in IEEE Transactions on Sustainable Energy, vol.7, no.1, pp.213-220, Jan.2016, doi:10.1109 / TSTE.2015.2488098.] coordinates the active power output of each converter station based on the frequency difference and frequency change rate of multiple AC grids, thereby improving the dynamic response of the AC side frequency. The literature [Miao Z, Fan L, Osborn D, et al. Wind Farms With HVdc Delivery in Inertial Response and Primary Frequency Control[J]. IEEE Transactions on Energy Conversion, 25.] participates in the primary frequency regulation of the system by adjusting the power delivered from wind farms to the DC side, thereby adjusting the blade angle of wind turbines to increase or decrease the power captured from the wind. However, there is a risk of wind turbine stall and subsequent difficulty in restoring rotor speed.

[0004] In recent years, virtual synchronous machine (VSG) technology has received widespread attention from scholars both domestically and internationally. Power electronic converters can utilize this technology to acquire the inertia and damping characteristics of synchronous generators, simulating the active power frequency regulation and reactive power voltage regulation processes of synchronous machines in terms of external characteristics, thereby improving the stability of AC systems. This is of great reference value for VSC-MTDC systems to effectively avoid the shortcomings of existing control strategies in AC frequency fluctuations and steady-state response. The literature [WYZhang, K. Rouzbehi, A. Luna, GBGharehpetian, and P. Rodriguez, “Multi-terminal HVDC grids with inertia mimicry capability,” IET Renewable Power Generation, vol.10, no.6, pp.752–760, Jul.2016.] proposes a VSG control strategy for VSC-MTDC systems connected to weak power grids. This control strategy embeds the VSG core algorithm in the upper-level control of the converter, actively supporting the system frequency. Literature [YJCao, WYWang, Y.Li, Y.Tan, C.Chen, L.He, U. [and C. Rehtanz, “A virtual synchronous generator control strategy for VSC-MTDC systems,” IEEE Transactions on Energy Conversion, vol. 33, no. 2, pp. 750–761, Jun. 2018.] Based on the use of VSG control in VSC-MTDC systems, this paper also studies the method of AC frequency suppression and the allocation method of frequency regulation reserve. Reference [Liu Zhongyuan, Wang Weiqing, Wang Haiyun, Yuan Chengyu, Wang Liang, Li Yongqin, Ding Wenbin. Control strategy for receiving-end converter of VSC-HVDC transmission system based on VSG technology [J]. Electric Power Construction, 2019, 40(02): 100-108.] proposes a VSC control strategy based on VSG technology, using fuzzy PI droop control to balance the power distribution between AC and DC systems, improving system inertia and mitigating frequency and voltage fluctuations under disturbances. In summary, most current stability studies on VSC-MTDC systems with VSGs only consider the operating status of the AC side and the power characteristics of the receiving-end converter, while ignoring the dynamic processes of the DC system itself and the coordinated distribution of unbalanced power between stations. Summary of the Invention

[0005] To address the technical problems of DC side's inability to respond to AC side frequency changes and unreasonable power distribution between stations under conventional droop control, this invention proposes a frequency control method for VSC-MTDC interconnected systems based on a virtual synchronous machine. This method can not only effectively suppress frequency fluctuations and improve AC side inertia level and primary frequency regulation capability, but also enable power support between different power grids, thereby improving the DC system's power distribution capability.

[0006] To achieve the above objectives, the technical solution of the present invention is implemented as follows: a frequency control method for a VSC-MTDC interconnection system based on a virtual synchronous machine, comprising the following steps:

[0007] Step 1: Based on the power balance relationship between the converter and the synchronous generator, the DC grid is regarded as the prime mover and the converter as the synchronous generator. A virtual synchronous machine control strategy considering the virtual speed governor is designed.

[0008] Step 2: Design an adaptive droop control strategy that takes power margin into account: First, determine the initial droop coefficient based on the principle that the droop coefficient is inversely proportional to the rated capacity of the converter station, and then correct the droop coefficient based on the available power margin of each converter station.

[0009] Step 3: Couple the AC side frequency with the DC side voltage to establish the relationship between the AC frequency and the DC side active power. Reduce the frequency disturbance of the AC system by quickly adjusting the input and output power through the converter station.

[0010] Preferably, the power balance relationship between the converter and the synchronous generator is as follows:

[0011] In an AC system, the power balance of a synchronous generator is represented by the rotor motion equation:

[0012]

[0013] In the formula, H is the inertia coefficient of the synchronous generator; ω is the rotor angular frequency; and P... m P e These are the mechanical power and electromagnetic power output by the prime mover, respectively.

[0014] When the mechanical power P output by the synchronous generator m and electromagnetic power P e When there is an imbalance, the rotor speed will change, and the rotor's kinetic energy will compensate for part of the power loss. At the same time, the speed governor will adjust the power input of the prime mover according to the rotor speed.

[0015] In a DC system, the capacitor balance equation on the DC side of the converter station is:

[0016]

[0017] In the formula, Nm S represents the number of DC capacitors in the converter station; C represents the DC-side capacitor of the converter station; S represents the number of DC capacitors in the converter station. vsc U is the rated capacity of the converter station. dc This is the measured value of the DC side voltage of the converter station; P in P represents the input power of the converter station. out This refers to the output power of the converter station.

[0018] When there is unbalanced power in the DC system, the DC-side capacitor C of the converter will charge and discharge, and the DC voltage U will change. dc It will change accordingly until the system reaches a new equilibrium state.

[0019] Preferably, the additional power command generated by the virtual synchronous machine control strategy considering the virtual speed governor consists of two parts:

[0020] ΔP vsc =ΔP vsg +ΔP vg ;

[0021] Where: ΔP vsc Total additional power; ΔP vsg Additional power generated for the virtual synchronizer VSG; ΔP vg Additional power generated by the virtual speed controller.

[0022] Additional power ΔP vsg The rotor equation obtained by simulating a synchronous generator is as follows:

[0023]

[0024] In the formula: J is the moment of inertia; D is the damping coefficient; ω ref The rated angular frequency;

[0025] The additional power generated by the virtual speed controller is: ΔP vg =k m (ω-ω ref );

[0026] Where, k m This is the active power-frequency droop factor.

[0027] Preferably, the active-frequency droop coefficient k m The calculation method is as follows:

[0028] By comparing the rotor motion equation with the capacitor balance equation, the DC voltage U on the DC side is obtained. dc Coupling relationship with AC side frequency:

[0029]

[0030] Integrating both sides of the above equation indefinitely, we get:

[0031]

[0032] In the formula, H vsc Let m represent the virtual inertial constant, where m is the integration constant for the indefinite integral;

[0033] The rotor angular frequency ω = ω ref U dc =U dcref Substitute the values ​​and find the integral constant:

[0034]

[0035] According to the integral constant, we can obtain:

[0036]

[0037] According to the calculation method of the inertial time constant of a synchronous generator, the virtual inertial constant is: In the formula: U dcn This refers to the rated voltage of the DC capacitor in the converter station.

[0038] DC voltage U in converter station droop control dc The relationship with active power is as follows:

[0039] U dc =U dcref +k(P s -P sref );

[0040] In the formula: U dc U is a DC voltage. dcref is the reference value for DC voltage; k is the voltage droop coefficient; P s P represents the active power of the converter station. sref This is a reference value for active power;

[0041] The coupling relationship between the active power of the voltage source converter station and the AC side frequency can then be obtained:

[0042]

[0043] Then the active-frequency droop coefficient k m The expression is:

[0044]

[0045] The total additional power is:

[0046]

[0047] For a given VSC-MTDC system, the additional power ΔP generated by the virtual speed governor vg With the droop coefficient k m Inversely proportional to the rated capacity S of the converter station vsc Proportional; that is, the virtual speed controller can automatically adjust the output power of the converter station according to the AC side frequency and suppress AC side frequency deviation;

[0048] After a load disturbance occurs in the AC system, the converter station can simulate the power characteristics of a synchronous generator and automatically adjust the active power command value P of the converter station. sref To compensate for the frequency difference in the AC system and suppress low-frequency oscillations; the damping coefficient D and the active-frequency droop coefficient k m Both are the ratio of power deviation to frequency deviation.

[0049] Preferably, the droop coefficient of each converter station is calculated as follows:

[0050]

[0051] In the formula: k0 is the initial droop coefficient, which is determined based on the principle that the droop coefficient is inversely proportional to the rated capacity of the converter station; P max The rated capacity of the converter station is α; the power influence factor is P. s This represents the active power of the converter station.

[0052] Preferably, the initial droop coefficient k0 is determined based on the principle that the droop coefficient is inversely proportional to the rated capacity of the converter station; the power influence factor α ranges from [0.5 to 1.5].

[0053] Preferably, the implementation method of step three is as follows: based on the coupling relationship between the active power of the voltage source converter station and the AC side frequency: When the AC side frequency fluctuates, the active power P of the converter station... s This will change accordingly, thereby reducing frequency disturbances on the AC side by adjusting the input and output power of the converter station.

[0054] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0055] By utilizing the DC equations of the VSC-MTDC system, a virtual synchronous machine control module considering a virtual speed governor is designed by coupling the AC frequency and DC voltage of the power grid. This module is then introduced into the Pf droop control, proposing a VSG control strategy with inertial response and primary frequency regulation capability. This allows the VSC-MTDC to simulate the inertia, damping, and primary frequency regulation function of a synchronous generator, improving the frequency response and stability of low-inertia systems. Secondly, considering that the application of the VSG control strategy can disrupt the power balance on the DC side during frequency regulation, thereby altering the output power of the converter station, a multi-station coordinated control strategy combining VSG with adaptive droop control considering power margin is proposed. Tuning methods for key parameters are also provided. Finally, a three-terminal VSC-MTDC system is constructed based on PSCAD / EMTDC and simulated. The simulation results verify the effectiveness of the proposed control strategy.

[0056] This invention applies VSG technology to converters, endowing them with inertia and droop characteristics to simulate the active power frequency regulation process of synchronous machines from an external characteristic perspective. Based on this, an adaptive droop control (ADC) strategy with coordinated inter-station power distribution is proposed to achieve multi-site control. The ADC strategy not only adaptively adjusts the droop coefficient according to the real-time power margin of each converter station, but also inherits the advantage of conventional droop control that requires no communication, thus rationally allocating unbalanced power in the DC system. Simultaneously, it enhances the power mutual support capability of the wind power grid-connected system and improves its operational stability. Finally, simulation verification of the strategy shows that this control strategy can effectively suppress frequency fluctuations, improve the AC side inertia level and primary frequency regulation capability, and enable power support between different grids, thereby improving the DC system's power distribution capability. Attached Figure Description

[0057] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0058] Figure 1 This is an analogy diagram of the converter and synchronous generator models of the present invention.

[0059] Figure 2 This diagram compares the working principles of a virtual speed controller and a traditional speed controller.

[0060] Figure 3 This is a block diagram of the VSC-MTDC control based on VSG control according to the present invention.

[0061] Figure 4 This is the control principle diagram of a DC voltage droop controller.

[0062] Figure 5 This is the overall control block diagram of the VSC-MTDC system of the present invention.

[0063] Figure 6 This is a model diagram of a three-terminal VSC-MTDC system.

[0064] Figure 7 The following is a simulation comparison diagram of the present invention and the commonly used method under the increase of active load in AC3, where (a) is the frequency of AC1, (b) is the frequency of AC2, (c) is the frequency of AC3, (d) is the active power of VSC1, (e) is the active power of VSC2, (f) is the active power of VSC3, and (g) is the DC voltage of the system.

[0065] Figure 8 The following is a simulation comparison of the reduction of active load on AC2 using the present invention and the commonly used method. (a) is the frequency of AC1, (b) is the frequency of AC2, (c) is the frequency of AC3, (d) is the active power of VSC1, (e) is the active power of VSC2, (f) is the active power of VSC3, and (g) is the DC voltage of the system.

[0066] Figure 9 The following is a simulation comparison diagram of the present invention and the commonly used method when a single-phase short circuit occurs in AC1. (a) is the frequency of AC1, (b) is the frequency of AC2, (c) is the frequency of AC3, and (d) is the DC voltage of the system. Detailed Implementation

[0067] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0068] A frequency control method for a VSC-MTDC interconnect system based on a virtual synchronous machine, comprising the following steps:

[0069] Step 1: Based on the power balance relationship between the converter and the synchronous generator, the DC grid is regarded as the prime mover and the converter as the synchronous generator. A virtual synchronous machine control strategy considering the virtual speed governor is designed.

[0070] Converters and synchronous generators have similar power balance relationships, such as Figure 1As shown. The inverter potential e is equivalent to the internal potential of the synchronous generator, the reactance X is equivalent to the armature reactance of the synchronous generator, R is equivalent to the armature resistance, and the high-pass filter at the fundamental frequency is equivalent to a pure capacitor C. The mechanical power of the VSC is provided by the DC side. Taking PCC as the reference point, if the voltage at the PCC bus is U∠δ°, then the AC bus voltage of the voltage source converter VSC is E∠δ°. The output mechanical power of the prime mover is P. m Its output electromagnetic power is P e .

[0071] In an AC system, the power balance of a synchronous generator can be expressed by the classical rotor motion equations:

[0072]

[0073] In the formula, H is the inertia coefficient of the synchronous generator; ω is the rotor angular frequency.

[0074] In a DC system, the capacitor balance equation on the DC side of the converter station can be expressed as:

[0075]

[0076] In the formula, N m S represents the number of DC capacitors in the converter station; C represents the DC-side capacitor of the converter station; S represents the number of DC capacitors in the converter station. vsc U is the rated capacity of the converter station. dc This is the measured value of the DC side voltage of the converter station; P in P is the input power of the converter station. out This refers to the output power of the converter station.

[0077] From equation (1), it can be seen that when the mechanical power P output by the synchronous generator... m and electromagnetic power P e When there is an imbalance, the rotor speed will change, and the rotor's kinetic energy will compensate for part of the power deficit. At the same time, the speed governor will adjust the power input of the prime mover according to the rotor speed. As can be seen from equation (2), when there is an imbalance in the DC system, the DC side capacitor C of the converter will charge and discharge, and the DC voltage U dc It will also change accordingly until the system reaches a new equilibrium state.

[0078] Therefore, based on the above similarity, the DC grid is considered as the prime mover and the converter as the synchronous generator, and a VSG control strategy considering a virtual speed governor is designed. The additional power command generated by the VSG control strategy consists of two parts.

[0079] ΔP vsc =ΔP vsg +ΔP vg (3)

[0080] Where: ΔPvsc Total additional power; ΔP vsg Additional power generated for the virtual synchronizer VSG; ΔP vg Additional power generated by the virtual speed controller.

[0081] Additional power ΔP vsg It can be obtained by simulating the rotor equation of a synchronous generator, and its expression is:

[0082]

[0083] In the formula: J is the moment of inertia; D is the damping coefficient; ω ref This is the rated angular frequency.

[0084] In order to obtain the additional power command ΔP generated by the virtual speed controller vg By comparing equation (1) and equation (2), the DC voltage U on the DC side is obtained. dc Relationship with AC side frequency:

[0085]

[0086] Where: H vsc This represents the virtual inertia constant.

[0087] Integrating both sides of equation (5) indefinitely, we get

[0088]

[0089] In the formula: m is the integration constant of the indefinite integral.

[0090] The rotor angular frequency ω = ω ref U dc =U dcref Substituting into equation (6), the integral constant can be obtained:

[0091]

[0092] Where, ω ref This is the rated value of the AC power grid angular frequency. U dcref The reference value for the DC side voltage is 200kV, which is used in this invention.

[0093] Substituting equation (7) into equation (6), we get

[0094]

[0095] Based on the calculation method for the inertial time constant of a synchronous generator, the expression for the virtual inertial constant is:

[0096]

[0097] In the formula: Udcn This is the rated voltage of the DC capacitor in the converter station.

[0098] The DC voltage U in the converter station droop control dc The relationship with active power is

[0099] U dc =U dcref +k(P s -P sref (10)

[0100] In the formula: U dc and U dcref These are the measured and reference values ​​of the DC voltage, respectively; k is the voltage droop coefficient; P s and P sref These are the measured and reference values ​​of the active power of the voltage source converter station, respectively.

[0101] Combining equations (8) and (10), we can obtain the coupling relationship between the active power of the voltage source converter station and the AC side frequency.

[0102]

[0103] Then the active-frequency droop coefficient k m The expression is

[0104]

[0105] The additional power command value generated by the virtual speed controller can be obtained further:

[0106] ΔP vg =k m (ω-ω ref (13)

[0107] From equations (12) and (13), it can be seen that for a given VSC-MTDC system, the additional power ΔP generated by the virtual speed controller is... vg With the droop coefficient k m Inversely proportional to the rated capacity S of the converter station vsc It is directly proportional. That is, the virtual speed governor can automatically adjust the output power of the converter station according to the AC side frequency to suppress AC side frequency deviation. Figure 2 This section compares the control principles of traditional speed governors and virtual speed governors. Traditional speed governors work by changing the system frequency, which in turn alters the rotor speed and generator output, thus suppressing system frequency changes. Virtual speed governors, on the other hand, change the DC voltage of the system based on rotor speed changes, which in turn alters the converter station's output power, thereby suppressing system frequency changes. Figure 2 It can be seen that the virtual speed controller can automatically adjust the active power output of the converter station according to the AC side frequency change, thereby reducing the AC side frequency deviation.

[0108] Combining equations (4) and (13), we can obtain

[0109]

[0110] As shown in equation (14), after a load disturbance occurs in the AC system, the converter station can simulate the power characteristics of a synchronous generator, automatically adjust the active power command value, compensate for the frequency difference in the AC system, and suppress low-frequency oscillations in the system. From another perspective, the damping coefficient D and the active-frequency droop coefficient k... m Although they have different physical meanings, they are both ratios of power deviation to frequency deviation and occupy the same position in control. However, from an overall perspective, the damping coefficient D effectively increases the original power-frequency droop coefficient k. m This also causes the power distribution of the converter to no longer be proportional to the droop factor, reducing the accuracy of power distribution. Figure 3 A control block diagram of the VSC-MTDC system based on VSG is presented. The module containing the virtual synchronizer control strategy is the virtual synchronizer control module.

[0111] In a VSC-MTDC parallel system, each converter station automatically distributes unbalanced power based on its droop coefficient when power fluctuates. However, conventional droop control strategies suffer from problems such as non-independent power distribution and poor DC voltage waveforms. Therefore, a suitable power distribution strategy needs to be adopted in the upper-level control of the converter station to solve these problems.

[0112] Step 2: Design an adaptive droop control strategy that takes power margin into account: First, determine the initial droop coefficient based on the principle that the droop coefficient is inversely proportional to the rated capacity of the converter station, and then adjust the droop coefficient according to the available power margin of each converter station.

[0113] In the VSC-MTDC system studied, the virtual synchronous machine control module enables the converter station to have primary frequency regulation capability and can provide the system with a certain amount of inertia and damping to deal with the transient and dynamic stability problems of the power grid. However, according to equation (14), the introduction of VSG only taps into the advantages of the converter station participating in AC side frequency regulation, but does not enable it to play a role in reasonably allocating unbalanced power.

[0114] Conventional DC voltage droop control uses multiple converter stations with power distribution capabilities to adjust the system DC voltage value according to the droop coefficient. This achieves rapid distribution of unbalanced power without switching control modes. Its controller structure is as follows: Figure 4 As shown.

[0115] It is assumed that N converter stations in the DC system adopt the conventional voltage droop control strategy. When a power disturbance ΔP occurs in the DC network, each droop-controlled converter station automatically finds a new equilibrium point through the droop curve. Let the variation of DC voltage be ΔU when the system reaches a new steady state dc , then the unbalanced power ΔP borne by converter station i (0<i<N) i and the DC voltage variation ΔU dc has the following relationship

[0116]

[0117] where: k i is the droop coefficient of the i-th converter station. For the entire DC network, the sum of the unbalanced power borne by N droop-controlled converter stations (converter stations adopting droop control) shall be equal to the total system power disturbance ΔP, that is

[0118]

[0119] By combining equations (15) and (16) to substitute the DC voltage variation ΔU d c , we can obtain

[0120]

[0121] It can be known from equation (17) that when the droop coefficient k i is fixed, the unbalanced power borne by any converter station is inversely proportional to its droop coefficient. That is, the larger the droop coefficient, the smaller the unbalanced power borne by the converter station. In general, the droop coefficient of each converter station is inversely proportional to the capacity of the converter station, that is, a converter station with larger capacity bears more unbalanced power.

[0122] To solve the problem of unreasonable power distribution among converter stations, this paper designs an adaptive droop control strategy considering power margin. First, the initial droop coefficient k is determined according to the principle that the droop coefficient is inversely proportional to the rated capacity of the converter station. Then the droop coefficient is automatically corrected according to the available power margin of each converter station. According to the above idea, the droop coefficient of converter station i can be set as:

[0123]

[0124] where: P max is the rated capacity of the converter station; α is the power influence factor. Considering that the initial active power command values of converter stations are different, an excessively large α easily leads to excessive correction of the droop coefficient and weakens the ability of the converter station to regulate unbalanced power; an excessively small α easily causes the problem of full-load regulation of the converter station. Based on the above comprehensive analysis, the value range of the power influence factor α is [0.5-1.5].

[0125] From equation (18), it can be seen that when the active power P s When the value is greater than 0, as the output power increases, the available power margin of the converter station gradually decreases, and its droop coefficient gradually increases, effectively reducing the unbalanced power borne by the converter station during dynamic adjustment. Similarly, when the active power P... s When the droop coefficient is less than 0, as the output power increases, the available power margin of the converter station gradually increases, while the corresponding droop coefficient gradually decreases, effectively increasing the unbalanced power borne by the converter station during dynamic adjustment. The overall control strategy block diagram of the VSC-MTDC system is shown below. Figure 5 As shown.

[0126] The virtual synchronous machine control strategy allows the converter station to simulate the operating characteristics of a synchronous generator, improving the inertia level on the AC side. By adaptively adjusting the droop coefficient, the unbalanced power on the DC side is rationally allocated, and power support can be provided between different AC grids, further reducing frequency variations.

[0127] Step 3: Couple the AC side frequency with the DC side voltage to establish the relationship between the AC frequency and the DC side active power. Reduce the frequency disturbance of the AC system by quickly adjusting the input and output power through the converter station.

[0128] According to equation (5), the coupling relationship between AC side frequency and DC side voltage can be obtained. According to equation (11), the relationship between AC frequency and DC side active power can be obtained. According to equation (11), when AC side frequency fluctuates, the active power of the converter station will also change accordingly. Therefore, by adjusting the input and output power of the converter station, the frequency disturbance on the AC side can be reduced.

[0129] To verify the effectiveness of the control strategy proposed in this invention, a simulation platform based on PSCAD / EMTDC was established as follows: Figure 6 The three-terminal VSC-MTDC system model is shown. Figure 7 AC1 to AC3 form an active AC system. Converters VSC1 and VSC2 are rectifier stations, and VSC3 is an inverter station. The simulation parameters of the entire system are shown in Table 1. PU in the figure refers to the conventional droop control of PU.

[0130] Table 1 System Simulation Parameters

[0131]

[0132] The most common fault in power systems is load variation. At t=5s, the load on AC3 of the AC system suddenly increases by 10%. The simulation results of the control strategy proposed in this invention and the conventional droop control strategy are as follows: Figure 7 As shown.

[0133] Under a conventional droop control strategy, since the AC side frequency and DC voltage are not coupled, the VSC-MTDC system shows no response when the load suddenly increases, and the DC voltage remains constant. Figure 7 (g) The output power of each converter station remains unchanged, see Figure 7 (d), (e), (f). The increase in AC load can only be regulated by the generator set's own frequency regulation performance and the load's own power frequency characteristics, resulting in large frequency fluctuations. Figure 7 (c) It can be seen that the lowest point of frequency f3 is 49.18Hz, and the quasi-steady-state amplitude is 49.61Hz.

[0134] Under the control strategy of this invention, when the AC3 frequency f3 begins to decrease, the output power of the converter station VSC3 changes accordingly. This is because, under the action of the VSG control strategy, VSC3 responds to the AC system frequency change by actively increasing its output power to reduce the amplitude of the frequency change. Figure 7 (c) It can be seen that under the action of the VSG control strategy, the frequency deviation is significantly reduced compared with the conventional droop control, the lowest frequency point is reduced by 0.38Hz, and the quasi-steady-state amplitude is reduced by 0.13Hz.

[0135] Furthermore, the droop coefficient of each converter station is automatically corrected according to equation (18), maximizing the utilization of its own capacity to provide power support to the fault-end system. Therefore, the frequency change Δf1 of AC1 changes by 0.053Hz, and the frequency change Δf2 of AC2 changes by 0.058Hz; the active power change ΔP1 of converter station 1 changes by 5.36MW, and the active power change ΔP2 of converter station 2 changes by 4.45MW. Although the changes in the output power of converter stations VSC1 and VSC2 cause a decrease in the frequency of AC1 and AC2 systems, both operate stably within a reasonable range, while the system DC voltage remains stable at around 198.8kV, decreasing by only 0.6%. It is evident that the control strategy presented in this paper significantly improves the frequency response of the AC system during sudden increases in the load on the sending-end grid.

[0136] At t=5s, the load on AC2 of the AC system suddenly decreases by 10%. The simulation results of the control strategy proposed in this invention and the conventional droop control strategy are as follows: Figure 8 As shown, when the active power load of AC2 in the AC system suddenly decreases, the system frequency rises. Under the conventional droop control strategy, the DC side cannot provide frequency support for AC2, resulting in a large fluctuation in the AC2 frequency f2, with a peak frequency of 50.60Hz and a quasi-steady-state amplitude of 50.29Hz.

[0137] Under the control strategy of this invention, when the AC system load decreases and the frequency increases, converter station VSC2 responds to the AC system frequency deviation and actively adjusts its output power to support the AC side frequency. Simultaneously, converter station VSC1 and VSC3 adaptively adjust their power allocation ratio based on their own power margins, rather than a constant 1:2, to provide power support to VSC2 and alleviate frequency fluctuations in the active AC system AC2. Figure 8 As shown, the peak frequency f2 of AC2 is 50.32Hz and the quasi-steady-state amplitude is 50.19Hz, and its frequency response is better than that of the former.

[0138] At t=4s, a single-phase short-circuit fault occurs in AC1, lasting 0.05s. Simulation results for the proposed control strategy and the conventional droop control strategy are as follows: Figure 9 As shown, under the conventional droop control strategy, when a short-circuit fault occurs in AC1, its frequency fluctuation amplitude is large, with the peak frequency f1 of AC1 reaching 50.62Hz, which seriously affects the normal operation of the system.

[0139] Under the control strategy of this invention, when a single-phase short-circuit fault occurs in AC1, the fault is caused by... Figure 9 As can be seen from (a) and (d), the peak frequency f1 of AC1 is 50.5Hz. Compared to the conventional droop control strategy, the peak frequency is reduced by 0.12Hz. Figure 9 It can be seen that after adopting the control strategy proposed in this invention, the frequency response and voltage response can quickly enter a stable state after the fault is cleared.

[0140] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A frequency control method for a VSC-MTDC interconnection system based on a virtual synchronous machine, characterized in that, The steps are as follows: Step 1: Based on the power balance relationship between the converter and the synchronous generator, the DC grid is regarded as the prime mover and the converter as the synchronous generator. A virtual synchronous machine control strategy considering the virtual speed governor is designed. Step 2: Design an adaptive droop control strategy that takes power margin into account: First, determine the initial droop coefficient based on the principle that the droop coefficient is inversely proportional to the rated capacity of the converter station, and then correct the droop coefficient based on the available power margin of each converter station. Step 3: Couple the AC side frequency with the DC side voltage to establish the relationship between the AC frequency and the DC side active power. Reduce the frequency disturbance of the AC system by quickly adjusting the input and output power through the converter station. The additional power command generated by the virtual synchronous machine control strategy considering the virtual speed governor consists of two parts. composition: ； Where: ΔP vsc Total additional power; ΔP vsg Additional power generated for the virtual synchronizer VSG; ΔP vg Additional power generated by the virtual speed controller; Additional power ΔP vsg The rotor equation obtained by simulating a synchronous generator is as follows: ； In the formula: J is the moment of inertia; D is the damping coefficient; The rated angular frequency; The additional power generated by the virtual speed governor is: ; Where, k m The active power-frequency droop factor; Active-frequency droop coefficient k m The expression is: ； Among them, H vsc S represents the virtual inertial constant. vsc N is the rated capacity of the converter station. m U is the number of DC capacitors in the converter station; C is the DC-side capacitance of the converter station; k is the voltage droop coefficient; U dc This is the measured value of the DC side voltage of the converter station; U dcref This is a reference value for DC voltage.

2. The frequency control method for a VSC-MTDC interconnection system based on a virtual synchronous machine according to claim 1, characterized in that, The power balance relationship between the converter and the synchronous generator is as follows: In an AC system, the power balance of a synchronous generator is represented by the rotor motion equation: ； In the formula, H is the inertia coefficient of the synchronous generator; P is the rotor angular frequency. m P e These are the mechanical power and electromagnetic power output by the prime mover, respectively. When the mechanical power P output by the synchronous generator m and electromagnetic power P e When there is an imbalance, the rotor speed will change, and the rotor's kinetic energy will compensate for part of the power loss. At the same time, the speed governor will adjust the power input of the prime mover according to the rotor speed. In a DC system, the capacitor balance equation on the DC side of the converter station is: ； In the formula, N m S represents the number of DC capacitors in the converter station; C represents the DC-side capacitor of the converter station; S represents the number of DC capacitors in the converter station. vsc U is the rated capacity of the converter station. dc This is the measured value of the DC side voltage of the converter station; P in P represents the input power of the converter station. out This refers to the output power of the converter station; When there is unbalanced power in the DC system, the DC-side capacitor C of the converter will charge and discharge, and the DC voltage U will change. dc It will change accordingly until the system reaches a new equilibrium state.

3. The frequency control method for a VSC-MTDC interconnection system based on a virtual synchronizer according to claim 2, characterized in that, The active-frequency droop coefficient k m The calculation method is as follows: By comparing the rotor motion equation with the capacitor balance equation, the DC voltage U on the DC side is obtained. dc Coupling relationship with AC side frequency: ； Integrating both sides of the above equation indefinitely, we get: ； In the formula, H vsc Let m represent the virtual inertial constant, where m is the integration constant for the indefinite integral; Rotor angular frequency = U dc =U dcref Substitute the values ​​and find the integral constant: ； According to the integral constant, we can obtain: ； According to the calculation method of the inertial time constant of a synchronous generator, the virtual inertial constant is: In the formula: U dcn This refers to the rated voltage of the DC capacitor in the converter station. DC voltage U in converter station droop control dc The relationship with active power is as follows: ； In the formula: U dcref is the reference value for DC voltage; k is the voltage droop coefficient; P s P represents the active power of the converter station. sref This is a reference value for active power; The coupling relationship between the active power of the voltage source converter station and the AC side frequency can then be obtained: ； Then the active-frequency droop coefficient k m The expression is: ； The total additional power is: ； For a given VSC-MTDC system, the additional power ΔP generated by the virtual speed governor vg With the droop coefficient k m Inversely proportional to the rated capacity S of the converter station vsc Proportional; that is, the virtual speed controller can automatically adjust the output power of the converter station according to the AC side frequency and suppress AC side frequency deviation; After a load disturbance occurs in the AC system, the converter station can simulate the power characteristics of a synchronous generator and automatically adjust the active power command value P of the converter station. sref It compensates for frequency differences in the AC system and suppresses low-frequency oscillations in the system; Damping coefficient D and active-frequency droop coefficient k m Both are the ratio of power deviation to frequency deviation.

4. The frequency control method for a VSC-MTDC interconnection system based on a virtual synchronous machine according to any one of claims 1-3, characterized in that, The calculation method for the droop coefficient of each converter station is as follows: ； In the formula: k0 is the initial droop coefficient; P max α is the maximum capacity of the converter station; α is the power influence factor; P s This represents the active power of the converter station.

5. The frequency control method for a VSC-MTDC interconnection system based on a virtual synchronous machine according to claim 4, characterized in that, The initial droop coefficient k0 is determined based on the principle that the droop coefficient is inversely proportional to the rated capacity of the converter station; the power influence factor α ranges from 0.5 to 1.

5.

6. The frequency control method for a VSC-MTDC interconnection system based on a virtual synchronous machine according to claim 3, characterized in that, The implementation method of step three is as follows: based on the coupling relationship between the active power of the voltage source converter station and the AC side frequency: When the AC side frequency fluctuates, the active power P of the converter station... s This will change accordingly, thereby reducing frequency disturbances on the AC side by adjusting the input and output power of the converter station.

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