Frequency parameter setting method of lcc-hvdc in asynchronous interconnected system

Abstract

The application discloses a frequency modulation parameter setting method of LCC-HVDC in an asynchronous interconnected system and relates to the technical field of power grids, and comprises the following steps: based on a common mode frequency model, the influence of droop control and virtual inertia control on the frequency response of a power system is analyzed; a frequency response model of the asynchronous interconnected system is established, and in combination with the analysis result, the direct current instruction is adjusted according to the frequency change to provide power support, and the adjustment of the frequency modulation parameter is completed. The application establishes a frequency response model of the asynchronous interconnected system, can quantitatively describe the frequency dynamics of the sending end and the receiving end system, based on the model, a method for setting the droop control parameter is provided, and the LCC-HVDC system can provide a proper level of power support.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of power grid, more particularly, to a frequency modulation parameter setting method of LCC-HVDC in an asynchronous interconnected system. BACKGROUND

[0002] LCC-HVDC (line commutated converter-high voltage direct current) systems have been widely used around the world, both for large-scale power transmission and as an interconnection device between two asynchronous systems. Traditionally, these HVDC links have mainly focused on steady-state power transmission and have not provided frequency support. However, as the penetration of renewable energy continues to rise and the structure of the power grid gradually shifts from large AC networks to asynchronous internets, there is a significant shortage of frequency regulation resources, increasing the risk of system instability. Given its strong power control capabilities, LCC-HVDC is considered to have significant potential to improve the frequency response performance of power systems.

[0003] Currently, a common method is to integrate a frequency-power (P / f) droop control (DRC) loop in the control loop of the LCC-HVDC system to provide frequency support. Although various frequency control strategies based on DRC have been applied in asynchronous interconnected systems (AIS), these studies do not involve control parameter adjustment for large disturbances, which can lead to insufficient or excessive support during frequency events. In recent years, virtual inertia control (VIC) has also become a key component in providing frequency support. Another more common approach is to combine VIC with DRC to form comprehensive inertia control. However, the introduction of VIC brings new control parameters, increasing the complexity of parameter setting. In addition, unlike physical inertia, VIC cannot provide immediate inertia support to the system. Therefore, its effectiveness in enhancing various frequency characteristics is still unclear.

[0004] In summary, the current strategies for frequency control using LCC-HVDC in asynchronous interconnected systems (AIS) mainly focus on the design of control logic and do not provide a method for adjusting the frequency modulation parameters of the system, which will directly affect the frequency response of the opposite side system. Therefore, how to adjust the frequency modulation parameters of the LCC-HVDC system based on the frequency dynamics of the sending and receiving end systems is a technical problem that needs to be solved by those skilled in the art. SUMMARY

[0005] Therefore, the present application provides a frequency modulation parameter setting method of LCC-HVDC in an asynchronous interconnected system, which solves the problems in the background art.

[0006] To achieve the above purpose, the present application provides the following technical solutions:

[0007] A frequency parameter setting method of LCC-HVDC in an asynchronous interconnected system, comprising the following steps:

[0008] Based on the common-mode frequency model, the influence of droop control and virtual inertia control on the frequency response of the power system is analyzed;

[0009] The frequency response model of the asynchronous interconnected system is established, and the analysis results are combined to adjust the DC current command to provide power support, thereby completing the adjustment of the frequency parameter.

[0010] Optionally, the test system used for analyzing the droop control and the virtual inertia control is modeled in RSCAD and simulated through a real-time digital simulator.

[0011] Optionally, the droop control adjusts the active power according to the frequency deviation, and the virtual inertia control controls the active power through the frequency change rate.

[0012] Optionally, the construction of the common-mode frequency model comprises the following steps:

[0013] The frequency response of different components is:

[0014] ΔP L =-G u (s)Δf(s) (1);

[0015] G u (s)=Js+D+1 / (Ks) (2);

[0016] In the formula: ΔP L represents the active disturbance; G u (s) represents the unified structure of different components, J represents the effective inertia, D represents the effective damping, and K represents the effective regulation coefficient;

[0017] The frequency response of the entire system is obtained by summing the common-mode frequency parameters of each unit:

[0018]

[0019] Wherein:

[0020]

[0021] In the formula: Δf represents the frequency deviation, J cm represents the effective inertia of the entire system, D cm represents the effective damping of the entire system, and K cm represents the effective regulation coefficient of the entire system; subscript i represents the i-th unit in the system, and s represents the Laplace operator; w i represents the weight coefficient obtained by Kron reduction through the network;

[0022] The common mode frequency model incorporating droop control and virtual inertia control is:

[0023]

[0024] Where: K d Indicates the frequency adjustment constant of virtual inertia control, K p Indicates the frequency adjustment constant of droop control.

[0025] Optionally, specific values ​​of J, D, and K are obtained by fitting active power step disturbance test data performed before the unit is put into operation.

[0026] Optionally, when analyzing the impact of droop control and virtual inertia control on the frequency response of the power system, the key indicators considered include: the lowest frequency point Δf nadir , initial frequency change rate RoCoF, quasi-steady-state frequency deviation Δf ss ;

[0027] The expression of the initial frequency change rate RoCoF is:

[0028]

[0029] Therefore, virtual inertia control and droop control do not affect the initial frequency change rate RoCoF, which only depends on the active disturbance ΔP L and effective inertia J;

[0030] When analyzing the quasi-steady-state frequency, the form of the unified structure is modified to:

[0031]

[0032] Quasi-steady-state frequency deviation Δf ss The expression is:

[0033]

[0034] Where: J cm1 Indicates effective inertia, D cm1 represents the effective damping, K cm1 It represents the effective adjustment coefficient, and T0 represents the effective time constant;

[0035] Then, the quasi-steady-state frequency deviation Δf ss Depends on the frequency regulation constant K of the droop control p But the frequency adjustment constant K is not controlled by virtual inertia d the impact of;

[0036] Perform inverse Laplace transform on formula (5) to obtain its time domain expression:

[0037] Δf(t)=-ΔPL e -σt sin(ω d t) (9);

[0038] where σ and ω d represent the undamped natural frequency and the damping coefficient, respectively, and their specific formulas are:

[0039]

[0040] σ = -ζω n (11);

[0041] where ζ and ω n represent the damping ratio and the damped natural frequency, respectively, and their specific formulas are:

[0042]

[0043] By taking the derivative of formula (9) and setting it equal to 0, the time t nadir at which the frequency reaches the minimum point is determined, and the expression is:

[0044]

[0045] Substituting t nadir into formula (9), the analytical expression of the frequency minimum point Δf nadir is obtained:

[0046]

[0047] Then, with the increase of K d and K p , the frequency minimum point Δf nadir increases.

[0048] From the perspective of the frequency minimum point, droop control can effectively replace virtual inertia control.

[0049] Optionally, the rectifier side of the frequency response model of the asynchronous interconnected system adopts constant current control, and the inverter side adopts constant voltage control.

[0050] Optionally, in the frequency response model of the asynchronous interconnected system, the LCC-HVDC system adjusts the DC current command according to the frequency change to provide power support, and the expression is:

[0051] ΔP dc = K I U dcref Δf1 = K dc Δf1 (16);

[0052] where ΔP dc represents the active power change of the LCC-HVDC, KI represents the direct current regulation coefficient, U dcref represents the direct voltage reference value, K dc represents the droop constant of LCC-HVDC; Δf1represents the sending-end system frequency deviation;

[0053] The analytical expression of the frequency response model of the asynchronous interconnected system is:

[0054]

[0055] By choosing the control parameters of K dc , the required frequency dynamics of Δf1and Δf2are obtained;

[0056] Without droop control, K dc is equal to 0, and the frequency dynamics of the inverter side and the rectifier side are decoupled;

[0057]

[0058] In the formula, ω d1 and σ1respectively represent the natural oscillation frequency and the damping coefficient of the sending-end system;

[0059] The specific expressions of the coefficients A-K are as follows:

[0060]

[0061] E = -A 2 -B 2 +C 2 +D 2 (25) ;

[0062] F = 2[(A-C) 2 +(B-D) 2 ][(A-C) 2 +(B+D) 2 ] (26) ;

[0063] G = A(A-C) 2 +A(B 2 +D 2 )-2B 2 C (27) ;

[0064] K = C(A-C) 2 +C(B 2 +D 2 )-2AD 2 (28) ;

[0065] In the formula, A, B and E-K are functions of K dc , and according to the frequency requirements of the sending-end and receiving-end systems, K dcThe extreme values of the formulas (19)-(20) are changed to achieve reasonable frequency support.

[0066] Compared with the prior art, the frequency modulation parameter setting method of LCC-HVDC in an asynchronous interconnected system provided by the present application has the following beneficial effects:

[0067] (1) Based on the common-mode frequency model, it is proved that the virtual inertia control can be replaced by the droop control under the condition that the frequency minimum points are equal, while the initial frequency change rate is not affected and the quasi-steady frequency is improved.

[0068] (2) A frequency response model of the asynchronous interconnected system is established, which can quantitatively describe the frequency dynamics of the sending end and the receiving end system; based on the model, a method for setting the droop control parameters is proposed to ensure that the LCC-HVDC system provides an appropriate level of power support. BRIEF DESCRIPTION OF DRAWINGS

[0069] In order to more clearly illustrate the technical solutions in the embodiments of the present application or the prior art, the drawings needed to be used in the embodiments or the prior art description will be briefly introduced as follows. Obviously, the drawings in the following description are only embodiments of the present application, and other drawings can be obtained by those skilled in the art without creative labor on the basis of the provided drawings.

[0070] Figure 1 The structural diagram of the test system provided by the present application is provided;

[0071] Figure 2 The control block diagram provided by the present application is provided, wherein 2a is the droop control and 2b is the virtual inertia control;

[0072] Figure 3 The power regulation characteristic schematic diagram of the droop control and the virtual inertia control provided by the present application is provided;

[0073] Figure 4 The common-mode frequency model combining the droop control and the virtual inertia control provided by the present application is provided;

[0074] Figure 5 The frequency response model of the asynchronous interconnected system provided by the present application is provided;

[0075] Figure 6 The frequency response of the region one provided by the present application is provided;

[0076] Figure 7 The relationship diagram of the frequency minimum point with K dc provided by the present application is provided;

[0077] Figure 8Simulation results provided by the present application under different power disturbances, wherein 8a is a 100MW load surge of busbar 5, 8b is a 50MW load reduction of busbar 6;

[0078] Figure 9 Simulation results provided by the present application when a 100MW load surge of busbar 6 occurs, wherein 9a is the active output of generators G1-G4, and 9b is the DC power;

[0079] Figure 10 Simulation results provided by the present application under different K dc , wherein 10a is the frequency response of K dc =12.6pu, 10b is the DC power of K dc =12.6pu, 10c is the frequency response of K dc =9.6pu, and 10d is the DC power of K dc =9.6pu. DETAILED DESCRIPTION

[0080] The technical solutions in the embodiments of the present application will be clearly and completely described below with reference to the drawings in the embodiments of the present application. Obviously, the described embodiments are only part of the embodiments of the present application, rather than all the embodiments of the present application. Based on the embodiments in the present application, all other embodiments obtained by those skilled in the art without creative labor fall within the scope of protection of the present application.

[0081] The embodiment of the present application discloses a frequency modulation parameter setting method of LCC-HVDC in an asynchronous interconnected system, comprising the following steps:

[0082] Based on the common-mode frequency model, the influence of droop control and virtual inertia control on the frequency response of the power system is analyzed;

[0083] The frequency response model of the asynchronous interconnected system is established, and the analysis results are combined to adjust the DC current command to provide power support, thereby completing the adjustment of the frequency modulation parameters.

[0084] In the present embodiment, a modified 4G2A system is used as the test system, as shown in Figure 1 The AC tie lines of busbars 7 to 9 are replaced by 400MW active power transmission LCC-HVDC links, G1-G4 are synchronous generators equipped with "Type ESAC4A" excitation systems and "TGOV1" governors, Load1 and Load2 are the loads of busbars 7 and 9 respectively, and C1 and C2 represent reactive power compensation devices. The test system is modeled in RSCAD and simulated through a real-time digital simulator to obtain the simulation results shown in the following parts.

[0085] 1. Analysis of the substitutability of virtual inertia

[0086] In this embodiment, the characteristics of the most common frequency control methods in the power system, namely droop control and virtual inertia control, are analyzed. Then, the effects of virtual inertia control and droop control on the frequency response of the power system are discussed respectively. From the perspective of the lowest frequency point (i.e., the same lowest frequency point), it is proved that virtual inertia control can be effectively replaced by droop control.

[0087] (1) Characteristics of droop control and virtual inertia control

[0088] Reference Figure 2 The control block diagrams of droop control and virtual inertia control shown in a and b are as follows. Δf represents the frequency deviation, K p and K d are the frequency adjustment constants for droop control and virtual inertia control, P DRC and P VIC represent the active power output of droop control and virtual inertia control respectively, and the Laplace operator is represented by s.

[0089] like Figure 2 As shown in the figure, droop control adjusts active power according to frequency deviation, and virtual inertia control controls active power through frequency change rate. Taking frequency drop as an example, Figure 3 The power output characteristics of droop control and virtual inertia control are demonstrated, and the frequency event is divided into the following four stages:

[0090] Phase 1 (0-t1): During this phase, no active power disturbance occurs and the system is in a steady state;

[0091] Phase 2 (t1-t nadir ): At time t1, an active power disturbance occurs, causing the frequency to begin to decrease, and this decrease continues until t nadir At this moment, the frequency reaches its lowest value; at this stage, affected by the frequency drop, both virtual inertia control and droop control begin to release energy to make up for the power shortage. DRC and P VIC The outputs are all less than 0;

[0092] Phase 3 (t nadir -t3):t nadir After that, the frequency starts to improve under the action of the primary frequency modulation; at this stage, Δf is still negative, while df / dt becomes positive. Therefore, P DRC Continue to be negative, indicating that the droop control continues to release power; at the same time, P VIC Turning positive indicates that the virtual inertia control begins to absorb power and prevents frequency recovery.

[0093] (2) Verification of virtual inertia substitutability

[0094] For the frequency response of power systems, three key indicators are usually considered: the frequency nadir Δf nadir , the initial rate of change of frequency RoCoF, and the quasi-steady frequency deviation Δf ss . Next, the effects of droop control and virtual inertia control on the above three parameters are analyzed through the common-mode frequency model.

[0095] The frequency responses of different components (such as synchronous generators, grid-forming inverters, and grid-following inverters) are:

[0096] ΔP L = -G u (s)Δf(s) (1);

[0097] G u (s) = Js + D + 1 / (Ks) (2);

[0098] where ΔP L represents the active power disturbance; G u (s) represents the unified structure of different components, J represents the effective inertia, D represents the effective damping, and K represents the effective regulation coefficient; the specific values of J, D, and K are obtained by fitting the active power step disturbance test data before the unit is put into operation;

[0099] The frequency response of the entire system is obtained by summing the common-mode frequency parameters of each unit:

[0100]

[0101] where:

[0102]

[0103] where Δf represents the frequency deviation, J cm represents the effective inertia of the entire system, D cm represents the effective damping of the entire system, and K cm represents the effective regulation coefficient of the entire system; subscript i represents the i-th unit in the system, and s represents the Laplace operator; w i represents the weight coefficient obtained by network Kron reduction.

[0104] By using the common-mode frequency model, the problem of order explosion caused by heterogeneous units is solved. Next, the effects of droop control and virtual inertia control on the frequency response of power systems will be analyzed through the common-mode frequency model. The common-mode frequency model incorporating droop control and virtual inertia control is shown in Figure 4 , and its analytical expression is:

[0105]

[0106] where: K d represents the frequency regulation constant of the virtual inertia control, K p represents the frequency regulation constant of the droop control.

[0107] A) Initial rate of change of frequency RoCoF

[0108] Before discussing the initial rate of change of frequency RoCoF, it is important to note that the virtual inertia control cannot enhance the instantaneous inertial response as the physical inertia does due to the inherent delay caused by the measurement and control loops in the virtual inertia control, which is typically between 300-400 milliseconds. Therefore, the expression for the initial rate of change of frequency RoCoF is:

[0109]

[0110] It is clear that neither the virtual inertia control nor the droop control affects the initial rate of change of frequency RoCoF, which depends only on the active disturbance ΔP L and the effective inertia J.

[0111] B) Quasi-steady state frequency deviation Δf ss

[0112] When analyzing the quasi-steady state frequency using the common-mode frequency model, the unified structure is modified to:

[0113]

[0114] The expression for the quasi-steady state frequency deviation Δf ss is:

[0115]

[0116] where: J cm1 represents the effective inertia, D cm1 represents the effective damping, K cm1 represents the effective droop coefficient, and T0 represents the effective time constant.

[0117] It can be seen that the quasi-steady state frequency deviation Δf ss depends on the frequency regulation constant K p of the droop control but is not affected by the frequency regulation constant K d of the virtual inertia control; in other words, the virtual inertia control does not improve Δf ss , while the droop control can boost Δf ss as expected.

[0118] C) Frequency nadir Δf nadir

[0119] The frequency nadir Δf nadirIt cannot be directly solved in the frequency domain, so the Laplace inverse transform is performed on formula (5) to obtain its time-domain expression:

[0120] Δf(t) = -ΔP L e -σt sin(ω d t) (9);

[0121] In the formula, σ and ω d represent the undamped oscillation frequency and the damping coefficient respectively, and their specific formulas are:

[0122]

[0123] σ = -ζω n (11);

[0124] In the formula, ζ and ω n represent the damping ratio and the damped oscillation frequency respectively, and their specific expressions are:

[0125]

[0126] By taking the derivative of formula (9) and setting its derivative equal to 0, the time t nadir at which the frequency reaches the minimum point is determined, and the expression is:

[0127]

[0128] Substituting t nadir into formula (9) gives the analytical expression of the frequency minimum point Δf nadir :

[0129]

[0130] It can be seen that as K d and K p increase, the frequency minimum point Δf nadir increases; therefore, both virtual inertia control and droop control help to increase Δf nadir .

[0131] In summary, virtual inertia control has no effect on the initial frequency change rate RoCoF or the quasi-steady frequency deviation Δf ss , it only increases the frequency minimum point Δf nadir , and hinders the frequency recovery by absorbing energy. Therefore, from the perspective of the frequency minimum point, droop control can effectively replace virtual inertia control. Before and after such replacement, Δf nadirThe quasi-steady frequency is enhanced when the initial RoCoF is unchanged. In addition, the virtual inertia control has inherent shortcomings, such as the need for high-precision frequency measurement, inherent time delay, and amplification of measurement errors, which makes it unsuitable for practical applications. Therefore, it is completely feasible to replace the virtual inertia control with droop control.

[0132] 2. Frequency regulation parameter setting method

[0133] In this embodiment, a parameter adjustment method based on droop control is proposed, which not only ensures that the local system is reasonably supported, but also maintains the minimum frequency of the remote system to meet the requirements.

[0134] Taking the supported receiving-end system as an example, Figure 5 The frequency response model of the asynchronous interconnected system based on the common-mode frequency is shown. Subscripts 1 and 2 represent parameters related to regions 1 and 2, respectively, G(s) represents a unified structure as shown in equation (2), K dc represents the droop constant of LCC-HVDC, ΔP dc represents the active power change of LCC-HVDC.

[0135] In this embodiment, the rectifier side uses constant current control, and the inverter side uses constant voltage control. As shown in equation (16), the LCC-HVDC system adjusts the DC current command according to the frequency change to provide power support, and the expression is:

[0136] ΔP dc = K I U dcref Δf1 = K dc Δf1 (16);

[0137] In the equation, K I represents the DC current regulation coefficient, U dcref represents the DC voltage reference value.

[0138] Figure 5 The analytical expression of the frequency response model of the asynchronous interconnected system is shown:

[0139]

[0140] It can be seen that the active power disturbance of the rectifier side affects the frequency of the inverter side through two cascaded transfer functions (TFs). Combined with Figure 5 and equations (17)-(18), the proposed control changes the pole position of TF1 by introducing a droop constant K dc , thereby affecting the dynamics of Δf1. Then, Δf1 affects the frequency of the inverter-side AC network through K dc and the inherent network frequency dynamics TF2. Therefore, by selecting K dcthe control parameters, the required frequency dynamics of Δf1 and Δf2 are obtained; if there is no droop control, K dc is equal to 0, the frequency dynamics of the inverter side and the rectifier side are decoupled;

[0141]

[0142] where ω d1 and σ1 represent the natural oscillation frequency and damping coefficient of the sending end system respectively; the specific formula of these parameters is shown in formula (10)-(14);

[0143] where the specific expressions of the coefficients A-K are as follows:

[0144]

[0145]

[0146] E=-A 2 -B 2 +C 2 +D 2 (25);

[0147] F=2[(A-C) 2 +(B-D) 2 ][(A-C) 2 +(B+D) 2 ] (26);

[0148] G=A(A-C) 2 +A(B 2 +D 2 )-2B 2 C (27);

[0149] K=C(A-C) 2 +C(B 2 +D 2 )-2AD 2 (28);

[0150] It can be seen that A, B and E-K are functions of K dc , and according to the frequency requirements of the sending end and the receiving end system, the extreme values of formula (19)-(20) are changed by adjusting K dc to achieve reasonable frequency support. It should be noted that the extreme values of formula (19)-(20) can be obtained by numerical solution of MATLAB. The above is the entire process of parameter setting of the asynchronous interconnected system through the frequency response model.

[0151] 3. Example analysis

[0152] In this embodiment, the effectiveness of the virtual inertia replaceability analysis and parameter setting method is verified by RTDS simulation results.

[0153] (1) Virtual inertia replaceability analysis

[0154] The reference values of power, frequency and voltage are set as 1800 MW, 60 Hz and 230 kV respectively, K d and K p are both 5pu, and region 1 is the supported side. The inherent delay of virtual inertia control is set as 0.5s. After replacing virtual inertia control with droop control while keeping the frequency nadir unchanged, K p is set as 6.43pu. When a 100MW load sudden reduction occurs at bus 6, the simulation results are shown in Figure 6 , and the specific values of frequency characteristics are shown in Table 1.

[0155] Table 1 Values of frequency characteristics

[0156]

[0157] In Figure 6 , f1 and f 1t are the frequencies of region 1 before and after the equivalent replacement of virtual inertia control. It can be seen that, under the condition that the frequency nadirs are equal, replacing virtual inertia control with droop control maintains the RoCoF of the system while reducing the quasi-steady frequency. In other words, the frequency response performance of the system is improved.

[0158] (2) Effectiveness verification of frequency regulation parameter setting method

[0159] To ensure a reasonable frequency support level, the setting principle designed in this embodiment is to make the frequency nadirs of both ends equal after active power disturbance. Assuming that a 1pu active power disturbance occurs, by solving the extreme values of equations (19) and (20), the relationship between the change of K dc and the frequency nadirs of both ends can be obtained, as shown in Figure 7 .

[0160] As can be seen in Figure 7 , to make the frequency nadirs of both ends equal, K dc should be equal to 9.6pu. It is worth noting that the setting result of K dc is independent of the preset disturbance size, because ΔP L is a common factor in equations (19) and (20) and can be eliminated when setting the frequency nadirs of both ends to be equal. To verify the effectiveness of the parameter setting result, a 100MW load increase and a 50MW load decrease are set at bus 6 respectively, and the simulation results are shown in Figure 8 a and 8b.

[0161] In Figure 8 this case, the minimum frequency of both areas can be ensured to be the same by adjusting the droop coefficient, thus maintaining a reasonable level of frequency support. Taking a 100 MW load increase as an example, the active power output of generators G1-G4 and the DC power are shown in Figure 9 a, 9b. It can be observed that, with the increase of load, the frequency of Area 1 starts to decrease Figure 8 a). To deal with this situation, the DC system reduces its transmission power to prevent the frequency of Area 1 from further decreasing Figure 9 b). Due to the adjustment of DC power, the frequency of Area 2 also decreases, resulting in a decrease in the output power of Area 2 generators Figure 9 a). Based on the above situation, by properly setting the droop coefficient, the unbalanced power of Area 1 is distributed among the generators of Area 1 and Area 2. In other words, the frequency modulation resources are effectively shared between Area 1 and Area 2.

[0162] To further verify the effectiveness of the proposed control strategy, the K dc = 12.6 pu and 9.6 pu cases are set. When a 150 MW active power increase occurs at bus 6, the simulation results are shown in Figure 10 . Figure 10 a, 10b are the simulation results of K dc = 12.6 pu, Figure 10 c, 10d are the simulation results of K dc = 9.6 pu, showing the changes of AC frequency f and HVDC transmission power P dc . From Figure 10 it can be seen that, compared with K dc = 9.6 pu, when K dc = 12.6 pu, the minimum frequency of Area 1 is improved; however, the minimum frequency of Area 2 decreases. This shows that if the parameters are not properly adjusted, there is a risk of over-support. In addition, increasing K dc will result in a larger adjustment of DC active power when responding to the same disturbance, thus increasing the risk of overload of the transmission line.

[0163] The various embodiments in the specification are described in a progressive manner, and each embodiment focuses on the differences from other embodiments. The same or similar parts between the various embodiments can be mutually referred to.

[0164] The foregoing description of the disclosed embodiments enables a person skilled in the art to make or use the application. Modifications of these embodiments will occur to persons of skill in the art, and that the appended claims are intended to cover all such modifications that do not depart from the true spirit and scope of the application. Therefore, the application is not limited to the embodiments shown but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. A method for frequency modulation parameter setting of LCC-HVDC in an asynchronous interconnected system, characterized in that: The following steps are involved: Based on the common mode frequency model, the impact of droop control and virtual inertia control on the frequency response of the power system is analyzed; Establish a frequency response model for the asynchronous interconnected system. Combined with the analysis results, adjust the DC current command according to the frequency change to provide power support and complete the adjustment of the frequency modulation parameters. The construction of the common mode frequency model includes the following steps: The frequency responses of the different components are: （1）； （2）； Where: Indicates active disturbance; Represents a unified structure of different components, J represents the effective inertia, D represents the effective damping, K represents the effective adjustment coefficient; The frequency response of the entire system is obtained by summing the common-mode frequency parameters of each unit: （3）； in: （4）； Where: Indicates the frequency deviation, represents the effective inertia of the entire system, represents the effective damping of the entire system, represents the effective regulation coefficient of the entire system; i Indicates the system i Units, s represents the Laplace operator; represents the weight coefficient obtained by Krona reduction of the network; The common mode frequency model incorporating droop control and virtual inertia control is: （5）； Where: represents the frequency adjustment constant of virtual inertia control, represents the frequency regulation constant of droop control; The analytical expression of the frequency response model of the asynchronous interconnected system is: （17）； （18）； Where: represents the droop constant of LCC-HVDC; Indicates the frequency deviation of the sending end system.

2. The method for frequency modulation parameter setting of LCC-HVDC in an asynchronous interconnected system according to claim 1, characterized in that: The test system used for the analysis of droop control and virtual inertia control was modeled in RSCAD and simulated using a real-time digital simulator.

3. The method for frequency modulation parameter setting of LCC-HVDC in an asynchronous interconnected system according to claim 1, characterized in that: Droop control adjusts the active power according to the frequency deviation, and virtual inertia control controls the active power through the frequency change rate.

4. The method for frequency modulation parameter setting of LCC-HVDC in an asynchronous interconnected system according to claim 1, characterized in that: J 、 D 、 K The specific value of is obtained by fitting the active power step disturbance test data before the unit is put into operation.

5. The method for frequency modulation parameter setting of LCC-HVDC in an asynchronous interconnected system according to claim 1, characterized in that: When analyzing the impact of droop control and virtual inertia control on the frequency response of the power system, the key indicators considered include: the lowest frequency point , initial frequency change rate , quasi-steady-state frequency deviation ; Among them, the initial frequency change rate The expression is: （6）； Then, virtual inertia control and droop control do not affect the initial frequency change rate , depends only on the active disturbance and effective inertia J ; When analyzing the quasi-steady-state frequency, the form of the unified structure is modified to: （7）； Quasi-steady-state frequency deviation The expression is: （8）； Where: represents the effective inertia, represents the effective damping, represents the effective adjustment coefficient, represents the effective time constant; Then, the quasi-steady-state frequency deviation Frequency regulation constant depending on droop control But the frequency adjustment constant is not controlled by virtual inertia the impact of; Perform inverse Laplace transform on formula (5) to obtain its time domain expression: （9）； Where, and They represent the undamped oscillation frequency and attenuation coefficient respectively, and their specific formulas are: （10）； （11）； Where, and They represent the damping ratio and damped oscillation frequency respectively, and their specific expressions are: （12）； （13）； By taking the derivative of formula (9) and setting its derivative equal to 0, we can determine the time when the frequency reaches the lowest point. , the expression is: （14）； Will Substituting into formula (9), we get the lowest frequency point The analytical expression of : （15）； Then, with and The increase in frequency is the lowest point promote; From the perspective of the lowest frequency point, droop control can effectively replace virtual inertia control.

6. The method for frequency modulation parameter setting of LCC-HVDC in an asynchronous interconnected system according to claim 1, characterized in that: The frequency response model of the asynchronous interconnected system adopts constant current control on the rectifier side and constant voltage control on the inverter side.

7. The method for frequency modulation parameter setting of LCC-HVDC in an asynchronous interconnected system according to claim 1, characterized in that: In the frequency response model of the asynchronous interconnected system, the LCC-HVDC system adjusts the DC current command according to the frequency change to provide power support. The expression is: （16）； Where, Indicates the active power change of LCC-HVDC, represents the DC current regulation coefficient, Indicates the DC voltage reference value, represents the droop constant of LCC-HVDC; Indicates the frequency deviation of the sending end system; By selecting The control parameters are obtained based on formulas (17)-(18) and Frequency dynamics; Without droop control, Equal to 0, the frequency dynamics of the inverter side and the rectifier side are decoupled; （19）； （20）； Where: and denote the natural oscillation frequency and attenuation coefficient of the sending-end system respectively; coefficient AK The specific expression is as follows: （21）； （22）； （23）； （24）； （25）； （26）； （27）； （28）； Where A, B and EK are function, according to the frequency requirements of the transmitting and receiving systems, by adjusting To change the extreme values ​​of formulas (19)-(20) to achieve reasonable frequency support.

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