Frequency modulation parameter setting method for LCC-HVDC in asynchronous interconnection system

Abstract

The invention discloses a frequency modulation parameter setting method for LCC-HVDC in an asynchronous interconnection system, and relates to the technical field of power grids, and the method comprises the following steps: analyzing the influence of droop control and virtual inertia control on the frequency response of a power system based on a common-mode frequency model; and establishing a frequency response model of the asynchronous interconnection system, combining an analysis result, adjusting a direct current instruction according to frequency change to provide power support, and completing adjustment of frequency modulation parameters. According to the method, a frequency response model of an asynchronous interconnection system is established, the frequency dynamic states of a transmitting end system and a receiving end system can be quantitatively described, and a method for setting droop control parameters is provided based on the model to ensure that an LCC-HVDC system provides a proper level of power support.

Description

Technical Field

[0001] The present invention relates to the technical field of power grids, and more particularly to a method for setting frequency modulation parameters of LCC-HVDC in an asynchronous interconnected system. Background Art

[0002] LCC-HVDC (Line Converter-High Voltage Direct Current) systems are 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 focused on steady-state power transmission and have not provided frequency support. However, as the penetration of renewable energy continues to rise and the grid structure gradually shifts from large AC networks to asynchronous interconnections, there has been a significant shortage of frequency regulation resources, increasing the risk of system instability. Given its powerful power control capabilities, LCC-HVDC is considered to have significant potential to improve the frequency response performance of power systems.

[0003] Currently, a common approach is to integrate a frequency-power (P / f) droop control (DRC) loop into 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 the adjustment of control parameters for large disturbances, which may lead to under-support or over-support during frequency events. In recent years, virtual inertial control (VIC) has also become a key component for providing frequency support. Another more common approach is to combine VIC with DRC to form a comprehensive inertial control. However, the introduction of VIC brings new control parameters and increases the complexity of parameter setting. In addition, unlike physical inertia, VIC cannot provide instant inertial support to the system. Therefore, its effectiveness in enhancing various frequency characteristics remains unclear.

[0004] In summary, the current strategy of using LCC-HVDC for frequency control in asynchronous interconnected systems (AIS) mainly focuses on the design of control logic, and does not provide a method for adjusting the frequency modulation parameters of the system, which will directly affect the frequency response of the opposite system. Therefore, how to adjust the frequency modulation parameters of the LCC-HVDC system based on the frequency dynamics of the transmitting and receiving systems is a technical problem that technicians in this field need to solve urgently. Summary of the invention

[0005] In view of this, the present invention provides a method for setting frequency modulation parameters of LCC-HVDC in an asynchronous interconnected system, which solves the problems existing in the background technology.

[0006] In order to achieve the above object, the present invention provides the following technical solutions:

[0007] A method for setting frequency modulation parameters of LCC-HVDC in an asynchronous interconnected system comprises 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] A frequency response model of the asynchronous interconnected system is established. Combined with the analysis results, the DC current command is adjusted according to the frequency change to provide power support and complete the adjustment of the frequency modulation parameters.

[0010] Optionally, the test system used for the analysis of droop control and virtual inertial control is modeled in RSCAD and simulated by a real-time digital simulator.

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

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

[0013] The frequency responses of the different components are:

[0014] ΔP L =-G u (s)Δf(s) (1);

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

[0016] Where: ΔP L Represents 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 adjustment coefficient;

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

[0018]

[0019] in:

[0020]

[0021] 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, K cm represents the effective regulation coefficient of the entire system; the subscript i represents the i-th unit in the system, s represents the Laplace operator; w i represents the weight coefficient obtained by Cron reduction of the network;

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

[0023]

[0024] Where: K d represents 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] Among them, the expression of the initial frequency change rate RoCoF is:

[0028]

[0029] Then, neither virtual inertia control nor droop control affects 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 Denotes effective inertia, D cm1 represents the effective damping, K cm1 represents the effective adjustment coefficient, 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] In the formula, σ and ω d They represent the undamped oscillation frequency and attenuation coefficient respectively, and their specific formulas are:

[0039]

[0040] σ=-ζω n (11);

[0041] In the formula, ζ and ω n They represent the damping ratio and damped oscillation frequency respectively, and their specific expressions are:

[0042]

[0043] By taking the derivative of formula (9) and setting its derivative equal to 0, we can determine the time t when the frequency reaches the lowest point. nadir , the expression is:

[0044]

[0045] t nadir Substituting into formula (9), we get the lowest frequency point Δf nadir The analytical expression of is:

[0046]

[0047] Then, as K d and K p The increase in frequency, the lowest point Δf nadir promote;

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

[0049] Optionally, 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.

[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] In the formula, ΔP dc Indicates the active power change of LCC-HVDC, KI Indicates the DC current regulation coefficient, U dcref Indicates the DC voltage reference value, K dc represents the droop constant of LCC-HVDC; Δf1 represents the frequency deviation of the sending end system;

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

[0054]

[0055] By selecting K dc The control parameters are used to obtain the required frequency dynamics of Δf1 and Δf2;

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

[0057]

[0058] Where: d1 and σ1 represent the natural oscillation frequency and attenuation coefficient of the sending-end system, respectively;

[0059] The specific expression of coefficient AK is as follows:

[0060]

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

[0062] F=2[(AC) 2 +(BD) 2 ][(AC) 2 +(B+D) 2 ] (26);

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

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

[0065] In the formula, A, B and EK are K dc The function of K is adjusted according to the frequency requirements of the transmitting and receiving systems. dcTo change the extreme values ​​of formulas (19)-(20) to achieve reasonable frequency support.

[0066] It can be seen from the above technical solutions that, compared with the prior art, the present invention discloses a method for setting frequency modulation parameters of LCC-HVDC in an asynchronous interconnected system, which has the following beneficial effects:

[0067] (1) Based on the common mode frequency model, it is proved that when the lowest frequency point is equal, virtual inertia control can be replaced by droop control without affecting the initial frequency change rate and improving the quasi-steady-state frequency;

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

[0069] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are only embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on the provided drawings without paying creative work.

[0070] Figure 1 A structural diagram of the test system provided by the present invention;

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

[0072] Figure 3 A schematic diagram of power regulation characteristics of droop control and virtual inertia control provided by the present invention;

[0073] Figure 4 A common mode frequency model combining droop control and virtual inertia control provided by the present invention;

[0074] Figure 5 A frequency response model of the asynchronous interconnection system provided by the present invention;

[0075] Figure 6 The frequency response of region 1 provided by the present invention;

[0076] Figure 7 The frequency minimum point provided by the present invention varies with K dc Schematic diagram of the changing relationship;

[0077] Figure 8The simulation results under different power disturbances provided by the present invention are shown in Figure 8a, where bus 5 experiences a 100MW load surge, and bus 6 experiences a 50MW load reduction.

[0078] Fig. 9 The simulation result when a 100MW load sudden increase occurs on the bus 6 provided by the present invention, wherein 9a is the active output of the generators G1-G4, and 9b is the DC power;

[0079] Fig.10 Different K provided by the present invention dc The simulation results are as follows, where 10a is K dc =12.6pu frequency response, 10b is K dc = 12.6 pu of DC power, 10c is K dc =9.6pu frequency response, 10d is K dc =9.6pu DC power. DETAILED DESCRIPTION

[0080] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0081] The embodiment of the present invention discloses a method for setting frequency modulation parameters of LCC-HVDC in an asynchronous interconnection 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] A frequency response model of the asynchronous interconnected system is established. Combined with the analysis results, the DC current command is adjusted according to the frequency change to provide power support and complete the adjustment of the frequency modulation parameters.

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

[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, and 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 diagram of droop control and virtual inertia control shown in a and b, Δf represents the frequency deviation, K p and K d are the frequency regulation constants of droop control and virtual inertia control, P DRC and P VIC They 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 drop, and this drop continues until t nadir At this moment, the frequency reaches the minimum value; at this stage, affected by the frequency drop, both the virtual inertia control and the 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 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 starts to absorb power and prevents frequency recovery.

[0093] (2) Verification of virtual inertia substitutability

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

[0095] The frequency response of different components (such as synchronous generator, grid-connected inverter and grid-following inverter) is:

[0096] ΔP L =-G u (s)Δf(s) (1);

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

[0098] Where: ΔP L Represents active disturbance; G u (s) represents the unified structure of different components, J represents effective inertia, D represents effective damping, and K represents 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] in:

[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, K cm represents the effective regulation coefficient of the entire system; the subscript i represents the i-th unit in the system, s represents the Laplace operator; w i represents the weight coefficient obtained by Cron reduction of the network.

[0104] By using the common mode frequency model, the order explosion problem caused by the heterogeneity of the units is solved. Next, the impact of droop control and virtual inertia control on the frequency response of the power system 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 2. Figure 4 As shown, its analytical expression is:

[0105]

[0106] Where: K d represents the frequency adjustment constant of virtual inertia control, K p Indicates the frequency adjustment constant of droop control.

[0107] A). Initial frequency change rate RoCoF

[0108] Before discussing the initial rate of change of frequency RoCoF, it should be noted that virtual inertial control cannot enhance the transient inertial response like physical inertia due to the inherent delay caused by the measurement and control links in virtual inertial control. This delay is generally between 300-400 milliseconds. Therefore, the expression of the initial rate of change of frequency RoCoF is:

[0109]

[0110] Obviously, neither virtual inertia control nor droop control affects the initial frequency change rate RoCoF, which only depends on the active disturbance ΔP L And 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 form of the unified structure is modified to:

[0113]

[0114] Quasi-steady-state frequency deviation Δf ss The expression is:

[0115]

[0116] Where: J cm1 Denotes effective inertia, D cm1 represents the effective damping, K cm1 represents the effective adjustment coefficient, 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 of the droop control p But the frequency adjustment constant K is not controlled by virtual inertia d In other words, virtual inertial control does not improve Δf ss , while droop control can improve Δf as expected ss .

[0118] C). Frequency lowest point Δf nadir

[0119] Frequency minimum point Δf nadirIt is not possible to directly solve it analytically in the frequency domain. Therefore, 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 They represent the undamped oscillation frequency and attenuation coefficient respectively, and their specific formulas are:

[0122]

[0123] σ=-ζω n (11);

[0124] In the formula, ζ and ω n They represent the damping ratio and 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, we can determine the time t when the frequency reaches the lowest point. nadir , the expression is:

[0127]

[0128] t nadir Substituting into formula (9), we get the lowest frequency point Δf nadir The analytical expression of is:

[0129]

[0130] It can be seen that with the increase of K d and K p The increase in frequency, the lowest point Δf nadir Therefore, both virtual inertia control and droop control contribute to improving Δf nadir .

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

[0132] 2. Frequency adjustment 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 ensures that the lowest frequency point of the remote system meets the requirements.

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

[0135] In this embodiment, the rectifier side adopts constant current control, and the inverter side adopts constant voltage control. As shown in formula (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 formula, K I Indicates the DC current regulation coefficient, U dcref Indicates the DC voltage reference value;

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

[0139]

[0140] It can be seen that the active power disturbance on the rectifier side affects the frequency on the inverter side through two cascaded transfer functions (TF). Figure 5 And equations (17)-(18), the proposed control introduces the droop constant K dc to change the pole position of TF1, thus affecting the dynamics of Δf1. Then, Δf1 is dc The inherent network frequency dynamics TF2 affects the frequency of the inverter side AC network. Therefore, by selecting K dcThe control parameters are used to obtain the required frequency dynamics of Δf1 and Δf2; if there is no droop control, K dc Equal to 0, the frequency dynamics of the inverter side and the rectifier side are decoupled;

[0141]

[0142] Where: d1 and σ1 represent the natural oscillation frequency and attenuation coefficient of the sending end system respectively; the specific formulas of these parameters are shown in equations (10)-(14);

[0143] Among them, the specific expression of coefficient AK is as follows:

[0144]

[0145]

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

[0147] F=2[(AC) 2 +(BD) 2 ][(AC) 2 +(B+D) 2 ] (26);

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

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

[0150] It can be seen that A, B and EK are K dc The function of K is adjusted according to the frequency requirements of the transmitting and receiving systems. dc To change the extreme values ​​of formulas (19)-(20) to achieve reasonable frequency support. It should be noted that the extreme values ​​of formulas (19)-(20) can be obtained by numerical solution in MATLAB. The above is the whole process of parameter tuning of asynchronous interconnection system through frequency response model.

[0151] 3. Example Analysis

[0152] In this embodiment, the analysis of the substitutability of virtual inertial control and the effectiveness of the parameter setting method are verified through RTDS simulation results.

[0153] (1) Analysis of the substitutability of virtual inertia

[0154] The base values ​​of power, frequency and voltage are set to 1800MW, 60Hz and 230kV respectively. d and K p The values ​​are all 5pu, area 1 is the supported side, and the inherent delay of virtual inertial control is set to 0.5 seconds. When the lowest frequency point remains unchanged, the virtual inertial control is replaced by droop control. p When a 100MW load reduction occurs at bus 6, the simulation results are as follows: Figure 6 The specific values ​​of the frequency characteristic quantities are shown in Table 1.

[0155] Table 1 Frequency characteristic value

[0156]

[0157] exist Figure 6 In the 1t They are the frequencies of region 1 before and after the equivalent replacement of virtual inertial control. It can be seen that under the condition that the lowest frequency point is equal, replacing virtual inertial control with droop control maintains the RoCoF of the system and reduces the quasi-steady-state frequency. In other words, the frequency response performance of the system is improved.

[0158] (2) Verification of the effectiveness of the frequency regulation parameter setting method

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

[0160] exist Figure 7 It can be seen that in order to make the lowest frequency points at both ends the same, K dc It should be equal to 9.6pu. It is worth noting that K dc The tuning result is independent of the preset disturbance size, because ΔP L As a common factor in equations (19) and (20), it can be eliminated when the lowest frequencies at both ends are set equal. In order to verify the effectiveness of the parameter setting results, a load increase of 100MW and a load reduction of 50MW are set on bus 6 respectively. The simulation results are shown in Figure 2. Figure 8 a, as shown in 8b.

[0161] exist Figure 8 In the process, no matter whether the load increases or decreases suddenly, by properly adjusting the droop coefficient, the lowest frequency points at both ends can always be ensured to be the same, thereby maintaining a reasonable frequency support level. Taking a 100MW load increase as an example, the active power output and DC power of generators G1-G4 are as follows: Fig. 9 a, 9b. It can be observed that as the load increases, the frequency of region 1 begins to decrease ( Figure 8 a). In response to this situation, the DC system reduces its transmission power to prevent the frequency in region 1 from dropping further ( Fig. 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 the generator in area 2 ( Fig. 9 a) Based on the above situation, by appropriately setting the droop coefficient, the unbalanced power in area 1 is distributed between the generators in area 1 and area 2. In other words, the frequency regulation resources are effectively shared between area 1 and area 2.

[0162] In order to further verify the effectiveness of the proposed control strategy, the K dc The simulation results are as follows: Fig.10 shown. Fig.10 a、10b is K dc =12.6pu simulation results, Fig.10 c、10d is K dc =9.6pu simulation results, showing the AC frequency f and HVDC transmission power P dc From Fig.10 It can be seen that with K dc =9.6pu, when K dc When K is 12.6 pu, the frequency minimum point in region 1 is improved; however, the frequency minimum point in region 2 decreases. This indicates that there is a risk of over-support if the parameters are not adjusted appropriately. In addition, increasing K dc This will result in a larger adjustment of DC active power when responding to the same disturbance, thereby increasing the risk of overload of the transmission line.

[0163] The various embodiments in this 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 referenced to each other.

[0164] The above description of the disclosed embodiments enables one skilled in the art to implement or use the present invention. Various modifications to these embodiments will be apparent to one skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of the present invention. Therefore, the present invention will not be limited to the embodiments shown herein, but rather to the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. A method for setting frequency modulation parameters of LCC-HVDC in an asynchronous interconnected system, characterized in that: The following steps are involved: 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; A frequency response model of the asynchronous interconnected system is established. Combined with the analysis results, the DC current command is adjusted according to the frequency change to provide power support and complete the adjustment of the frequency modulation parameters.

2. The method for setting frequency modulation parameters of LCC-HVDC in an asynchronous interconnection 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 setting frequency modulation parameters of LCC-HVDC in an asynchronous interconnection 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 setting frequency modulation parameters of LCC-HVDC in an asynchronous interconnection system according to claim 1, characterized in that: The construction of the common mode frequency model includes the following steps: The frequency responses of the different components are: ΔP L =-G u (s)Δf(s)(1); G u (s)=Js+D+1 / (Ks)(2); Where: ΔP L Represents 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 adjustment coefficient; The frequency response of the entire system is obtained by summing the common-mode frequency parameters of each unit: in: 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, K cm represents the effective regulation coefficient of the entire system; the subscript i represents the i-th unit in the system, s represents the Laplace operator; w i represents the weight coefficient obtained by Cron reduction of the network; The common mode frequency model incorporating droop control and virtual inertia control is: Where: K d represents the frequency adjustment constant of virtual inertia control, K p Indicates the frequency adjustment constant of droop control.

5. The method for setting frequency modulation parameters of LCC-HVDC in an asynchronous interconnection system according to claim 4, characterized in that: The specific values ​​of J, D, and K are obtained by fitting the active power step disturbance test data conducted before the unit is put into operation.

6. The method for setting frequency modulation parameters of LCC-HVDC in an asynchronous interconnection system according to claim 4, 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 Δf nadir , initial frequency change rate RoCoF, quasi-steady-state frequency deviation Δf ss ; Among them, the expression of the initial frequency change rate RoCoF is: Then, neither virtual inertia control nor droop control affects the initial frequency change rate RoCoF, which only depends on the active disturbance ΔP L and effective inertia J; When analyzing the quasi-steady-state frequency, the form of the unified structure is modified to: Quasi-steady-state frequency deviation Δf ss The expression is: Where: J cm1 Denotes effective inertia, D cm1 represents the effective damping, K cm1 represents the effective adjustment coefficient, T0 represents the effective time constant; 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 Perform inverse Laplace transform on formula (5) to obtain its time domain expression: Δf(t)=-ΔP L e -σt sin(ω d t)(9); In the formula, σ and ω d They represent the undamped oscillation frequency and attenuation coefficient respectively, and their specific formulas are: σ=-live n (11); In the formula, ζ and ω n They represent the damping ratio and damped oscillation frequency respectively, and their specific expressions are: By taking the derivative of formula (9) and setting its derivative equal to 0, we can determine the time t when the frequency reaches the lowest point. nadir , the expression is: t nadir Substituting into formula (9), we get the lowest frequency point Δf nadir The analytical expression of is: Then, as K d and K p The increase in frequency, the lowest point Δf nadir promote; From the perspective of the lowest frequency point, droop control can effectively replace virtual inertia control.

7. The method for setting frequency modulation parameters of LCC-HVDC in an asynchronous interconnection 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.

8. The method for setting frequency modulation parameters of LCC-HVDC in an asynchronous interconnection 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: ΔP dc =K I U dcref Δf1=K dc Δf1(16); In the formula, ΔP dc Indicates the active power change of LCC-HVDC, K I Indicates the DC current regulation coefficient, U dcref Indicates the DC voltage reference value, K dc represents the droop constant of LCC-HVDC; Δf1 represents the frequency deviation of the sending end system; The analytical expression of the frequency response model of the asynchronous interconnected system is: By selecting K dc The control parameters are used to obtain the required frequency dynamics of Δf1 and Δf2; Without droop control, K dc Equal to 0, the frequency dynamics of the inverter side and the rectifier side are decoupled; Where: d1 and σ1 represent the natural oscillation frequency and attenuation coefficient of the sending-end system, respectively; The specific expression of coefficient AK is as follows: E=-A 2 -B 2 +C 2 +D 2 (25); F=2[(A-C) 2 +(B-D) 2 ][(A-C) 2 +(B+D) 2 ] (26); G=A(A-C) 2 +A(B 2 +D 2 )-2B 2 C (27); K=C(A-C) 2 +C(B 2 +D 2 )-2AD 2 (28); In the formula, A, B and EK are K dc function, by adjusting K according to the frequency requirements of the transmitting and receiving systems dc To change the extreme values ​​of formulas (19)-(20) to achieve reasonable frequency support.

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