Minimum inertia demand evaluation method for direct current delivery system considering multi-frequency modulation resource support characteristic difference

Abstract

The invention belongs to the technical field of inertia evaluation, and discloses a direct current delivery system minimum inertia demand evaluation method considering multi-frequency modulation resource support characteristic difference, and the method comprises the steps: firstly analyzing the frequency response characteristics of each frequency modulation resource in a direct current delivery system in a typical control mode; a system frequency response model considering wind power, photovoltaic and extra-high voltage direct current frequency response characteristic differences is established, and an influence mechanism of different frequency modulation resources on system frequency characteristics is disclosed. Secondly, constructing a system equivalent inertia calculation model considering a wind power and photovoltaic inertia support effect, and providing a minimum inertia demand evaluation method based on an initial frequency change rate as a constraint; and then, considering the influence of a direct current frequency limit controller on the maximum frequency deviation, designing a direct current delivery system minimum inertia evaluation scheme considering the frequency dynamic safety constraint so as to support efficient, stable and continuous delivery of new energy power generation.

Description

Technical Field

[0001] The present invention relates to the technical field of inertia assessment, and more specifically, to a method for assessing minimum inertia requirements of a DC transmission system taking into account differences in supporting characteristics of multiple frequency modulation resources. Background Art

[0002] In view of the scarcity of frequency regulation resources in the "double high" system under active disturbance, the inertia support and frequency regulation of the system involving new energy and high voltage direct current has received extensive attention, and relevant literature has conducted detailed discussions and research on this. Existing technologies have studied the role of high-proportion new energy in frequency regulation, and proposed a control strategy for multiple frequency regulation resources to jointly participate in virtual inertia support and primary frequency regulation.

[0003] The document "MIN Yong, CHEN Lei, LIU Ruikuo, et al. Analysis on characteristics of inertia and inertial response in power system frequency dynamics[J]. Proceedings of the CSEE, 2023, 43(3): 855-868." deeply analyzes the coordination problem between synchronous inertia, virtual inertia of asynchronous motor and inertia of doubly fed wind generator (DFIG).

[0004] Reference "Yan Gangui, Zhang Lijue, Zhao Weizhe, et al. Strategic Studies on Frequency-modulated Controlling of Photovoltaic-Energy Storage System Based on Virtual Synchronous Generator[J]. Journal of Northeast Electric Power University, 2017, 37(3): 1-6. Yan Gangui, Zhang Lijue, Zhao Weizhe, et al. Strategic Studies on Frequency-modulated Controlling of Photovoltaic-Energy Storage System Based on Virtual Synchronous Generator[J]. Journal of Northeast Electric Power University, 2017, 37(3): 1-6." By adopting a virtual synchronization control strategy for the energy storage inverter, the photovoltaic and energy storage systems can work together, and the overall performance is similar to that of a synchronous generator to achieve stable external output.

[0005] The document "Zhang Haizheng, Zhang Xing, Li Ming, et al. A control strategy for active standby photovoltaic virtual synchronization [J]. Power System Technology, 2019, 43(2): 514-520." proposed a control strategy for photovoltaic virtual synchronization, aiming to enable traditional photovoltaic inverters to have the functions of frequency and voltage regulation, and further improve the stability and reliability of photovoltaic power generation systems.

[0006] The document "Chen Yiping, Cheng Zhe, Zhang Kun, et al. Frequency Regulation Strategy for Island Operation of HVDC Transmission System [J]. Proceedings of the CSEE, 2013, 33(4): 96-102." shows that the periodic oscillation phenomenon of the system in terms of frequency can be effectively alleviated by properly adjusting the range of primary frequency regulation and the dead zone value (frequency limit controller, FLC) of UHVDC transmission.

[0007] The literature "Shen Zhipeng, Zhu Jiebei, Li Bin, et al. Bilateral inertia and damping simulation control scheme based on flexible DC transmission system [J]. High Voltage Technology, 2021, 47(10): 3505-3518.", "Wu Xian, Li Zifeng, Li Weidong, et al. Active frequency response control strategy of back-to-back DC interconnection system [J]. High Voltage Technology, 2022, 48(1): 232-244." proposes a bilateral virtual inertia control scheme based on ultra-high voltage DC transmission system, which can provide the required inertia support for the AC system.

[0008] Due to the diversified development of the system inertia support source, the system's demand for minimum inertia has also changed. At present, relevant literature has studied the minimum inertia evaluation of the system.

[0009] The paper "Wu, L., Infield, et al. Towards an Assessment of Power System Frequency Support From Wind Plant-Modeling Aggregate Inertial Response [J]. IEEE Transactions on Power Systems: A Publication of the Power Engineering Society, 2013, 28 (3)" estimates the equivalent virtual inertia of wind turbines in system frequency regulation based on an improved frequency response model (system frequency response, SFR). However, it is limited to the wind farm level and fails to fully reflect the changes in the equivalent inertia of the power system under non-synchronous power supply frequency regulation.

[0010] The document "Zhou Tao, Chen Zhong, Wang Yi, et al. Evaluation of the minimum inertia of the power system considering the frequency response of asynchronous motors [J]. Power System Protection and Control, 2022, 50(20): 22-31." established a frequency response model considering the influence of asynchronous motors. By integrating frequency characteristic constraints and applying genetic algorithms, the evaluation of the minimum inertia requirement of the power grid containing asynchronous motors was realized.

[0011] The document "Wang Baocai, Sun Huadong, Li Wenfeng, et al. Evaluation of minimum inertia of power system considering dynamic frequency constraints [J]. Proceedings of the CSEE, 2022, 42(1): 114-127." analyzes the system with and without speed regulation under frequency deviation constraints, and establishes a system frequency response model based on the characteristics of each power supply to evaluate the minimum inertia of the system.

[0012] The literature "Li Dongdong, Sun Yaru, Xu Bo, et al. Minimum inertia and primary frequency regulation capacity evaluation method for new energy high penetration power system considering frequency stability [J]. Power System Protection and Control, 2021, 49(23): 54-61." takes the minimum frequency limit as the dynamic frequency stability constraint condition, establishes a system minimum inertia and primary frequency regulation capacity evaluation model under large disturbances, and estimates the system minimum inertia and primary frequency regulation capacity respectively.

[0013] The document "GU Huajie, YAN Ruifeng, SAHAT K. Minimum synchronous inertiality requirement of renewable power systems [J]. IEEE Transactions on Power Systems, 2018, 33(2): 1533-1543." proposes an economic dispatch algorithm with synchronous inertia constraints to ensure that the minimum synchronous inertia required for frequency control is met.

[0014] The above existing literature has not comprehensively considered the impact of differences in the supporting characteristics of multiple frequency regulation resources on the dynamic safety of system frequency, and is difficult to apply to weakly supported DC transmission systems containing multiple types of frequency regulation resources. Therefore, conducting an assessment of its minimum inertia demand is of great significance for the safe, efficient and continuous transmission of new energy bases. Summary of the invention

[0015] In order to solve the above problems existing in the prior art, the present invention provides a method for evaluating the minimum inertia requirement of a DC transmission system taking into account the differences in supporting characteristics of multiple frequency regulation resources, so as to support efficient, stable and continuous transmission of new energy power generation.

[0016] The present invention provides a method for evaluating the minimum inertia requirement of a DC transmission system taking into account the differences in supporting characteristics of multiple frequency modulation resources, the method comprising:

[0017] Construct a system frequency response model that takes into account the differences in frequency response characteristics of wind power, photovoltaic power, and UHV DC;

[0018] Constructing a system equivalent inertia calculation model that takes into account the inertia support of wind power and photovoltaic power generation, and calculating the equivalent inertia time constant based on the system equivalent inertia calculation model;

[0019] Calculating the minimum inertia of the system frequency response model under the frequency change rate constraint; wherein the frequency change rate is determined by the equivalent inertia time constant and the active disturbance;

[0020] In the case of considering DC FLC control, by adjusting the size of the equivalent inertia time constant, the maximum frequency deviation of the system frequency response model meets the maximum frequency deviation constraint, and the corresponding equivalent inertia time constant is used as the minimum inertia under the maximum frequency deviation constraint;

[0021] The minimum inertia under the frequency change rate constraint and the minimum inertia under the frequency maximum deviation constraint are compared, and the larger value is used as the minimum inertia of the system frequency response model.

[0022] Furthermore, a system frequency response model is constructed that takes into account the differences in frequency response characteristics of wind power, photovoltaic power, and UHV DC, including:

[0023] Establish frequency response models for multiple types of FM resources;

[0024] Based on the frequency response models of the multi-type frequency modulation resources, a frequency response model of the new energy base through the direct current transmission system is established;

[0025] The frequency response model of the DC transmission system of the new energy base is subjected to aggregate equivalent processing to obtain a system frequency response model.

[0026] Furthermore, the frequency response models of the multi-type frequency modulation resources include a synchronous generator frequency response model, a wind power generation frequency response model, a photovoltaic power generation frequency response model and an ultra-high voltage direct current frequency response model:

[0027] The frequency response transfer function of the generator frequency response model is as follows:

[0028]

[0029] In the formula, B P represents the primary frequency modulation droop coefficient; Δf represents the system frequency change; k b Represents the mechanical power gain coefficient; F HP Indicates the work coefficient of the prime mover's high-pressure cylinder; T RH Represents the reheat time constant; ΔP Grepresents the power change of synchronous generator set; s represents the Laplace operator;

[0030] The wind power generation frequency response model adopts the frequency differential link to realize the inertia response of the wind turbine set, and its frequency transfer function is shown in formula (2):

[0031]

[0032] In the formula, H ω represents the rotor inertia time constant; k df Indicates the inertia coefficient; ΔP Wa Indicates the power change of inertia control;

[0033] The wind power generation frequency response model adopts a frequency ratio link to achieve a primary frequency modulation response, and its frequency transfer function is shown in formula (3):

[0034] ΔP Wb =k pf Δf(3)

[0035] In the formula, k pf Indicates the primary frequency modulation coefficient; ΔP Wb Indicates the power change of droop control;

[0036] The DC capacitor voltage equation of the photovoltaic power generation frequency response model is:

[0037]

[0038] In the formula, C s Indicates DC capacitance; U dc , U dc0 Respectively represent DC voltage and voltage rating; P dc Indicates DC output power; P g represents the photovoltaic power output power, and t represents the time;

[0039] Linearizing equation (5) yields:

[0040]

[0041] Where ΔP pv , ΔU dc They represent the change in photovoltaic power generation and DC voltage respectively;

[0042] According to formula (5) and formula (6), the photovoltaic power generation frequency response expression of the photovoltaic power generation frequency response model is:

[0043]

[0044] Among them, H pvrepresents the virtual inertia time constant; k pv Indicates photovoltaic control parameters;

[0045] The control equation of the UHV DC frequency response model is:

[0046]

[0047] In the formula, K Dp Represents proportional gain; K Di Indicates the integral gain; m i Indicates the state quantity of the DC system; ΔP DC Indicates the change of UHV DC power; f DH Represents the dead zone value in the high voltage DC system control strategy; m min Indicates the minimum value of the DC system state quantity; ΔP DC Indicates the minimum value of UHV DC power.

[0048] Furthermore, the frequency response model of the DC transmission system of the new energy base is subjected to aggregation equivalent processing to obtain a system frequency response model, including:

[0049] By using the aggregation equivalence principle, the frequency modulation resource parameters in the frequency response model of the DC transmission system of the new energy base are made to meet the following requirements:

[0050]

[0051] In the formula, χ represents the equivalent parameter factor after system aggregation; Indicates the types of frequency-regulating units, including synchronous generators, wind power generation, photovoltaic power generation, and high-voltage direct current; Indicates the frequency regulation unit category is The number of units corresponding to the time; The category of the i-th frequency regulation unit before aggregation is Unit parameters; The category of the i-th frequency regulation unit before aggregation is The unit capacity.

[0052] Furthermore, when Δf does not exceed f DH When UHV DC does not participate in the frequency regulation of the system, the frequency response transfer function G of the system frequency response model is sys (s) is:

[0053]

[0054] In the formula, G k When k in (s) is G, W, pv, or D, it represents the frequency response transfer function of synchronous generator, wind power generation, photovoltaic power generation, and UHV DC. syseq represents the equivalent inertia time constant, λk The proportion of each frequency regulation resource’s active power to the total frequency regulation active power, D sys represents the equivalent damping of the system.

[0055] When Δf exceeds f DH When UHV DC participates in the frequency support of the system, the frequency change analytical expression of the system frequency response model is:

[0056]

[0057] in,

[0058] ΔP Z (s) = ΔP L (s)-f DH G D (s)(12)

[0059] In the formula, G D (s) represents the frequency response transfer function of UHV DC, Δf(s) represents the frequency change, ΔP Z (s) represents the introduced intermediate variable;

[0060] Based on the initial value theorem, when the system is disturbed by ΔP L When the frequency is adjusted by the new energy, the initial frequency change rate R0 is calculated as shown in formula (13):

[0061]

[0062] Where Δf(t) represents the frequency change;

[0063] When the system frequency reaches the maximum deviation, the system frequency change rate will be zero, that is:

[0064]

[0065] In the formula, t max Indicates the time when the frequency reaches the lowest point;

[0066] Determine the maximum frequency deviation Δf of the system max The expression is shown in formula (15):

[0067] Δf max ={Δf(t)t=t max ,Δf(t)=L -1 (G sys (s)ΔP Z}(15)

[0068] Where, L -1 represents the inverse Laplace transform;

[0069] According to equations (13) and (15), the relationship between the system's initial frequency change rate and the maximum frequency deviation and the system's equivalent inertia is determined.

[0070] Furthermore, the system generalized kinetic energy expression based on the system equivalent inertia calculation model is shown in formula (16):

[0071]

[0072] In the formula, E G , Z Gi are the kinetic energy of the synchronous generator and the start and stop status of the i-th synchronous unit respectively; E W , H Wq , S Wq , Z Wq are the kinetic energy provided by wind power generation, the virtual inertia time constant, capacity and start / stop status of the qth wind power generation; E PV , H PVj , S PVj , Z PVj are the energy form of photovoltaic power generation, the virtual inertia time constant, capacity and start / stop status of the jth photovoltaic unit; N G 、N W 、N pv are the number of synchronous units, wind power generation, and photovoltaic power generation units respectively;

[0073] The equivalent inertia time constant of the new energy base through the DC transmission system is obtained from formula (16):

[0074]

[0075] In the formula, H syseq represents the equivalent inertia time constant.

[0076] Further, calculating the minimum inertia of the system frequency response model under the frequency change rate constraint includes:

[0077] The rotor motion equation of the system frequency response model is determined as:

[0078]

[0079] According to formula (18), the maximum frequency change rate RoCoF of the power system after the disturbance is taken as the constraint condition, and the expression of the minimum inertia of the system based on the RoCoF constraint is obtained as follows:

[0080]

[0081] In the formula, H min_R represents the minimum inertia under the frequency change rate constraint, ΔP maxrepresents the maximum disturbance power, R _max Indicates the maximum rate of frequency change.

[0082] Furthermore, in the case of considering DC FLC control, by adjusting the size of the equivalent inertia time constant, the maximum frequency deviation of the system frequency response model satisfies the maximum frequency deviation constraint, and the corresponding equivalent inertia time constant is used as the minimum inertia under the maximum frequency deviation constraint, including:

[0083] Set the range of DC FLC dead zone value, based on the system frequency response model, to obtain the maximum frequency deviation of the system;

[0084] When the maximum frequency deviation of the system satisfies the maximum frequency deviation constraint, the current equivalent inertia time constant is used as the minimum inertia under the maximum frequency deviation constraint;

[0085] When the maximum frequency deviation of the system does not meet the maximum frequency deviation constraint, adjust the size of the equivalent inertia time constant so that the maximum frequency deviation meets the maximum frequency deviation constraint. At this time, the equivalent inertia time constant is the minimum inertia H under the maximum frequency deviation constraint. min_Δfmax .

[0086] Furthermore, the minimum inertia of the system frequency response model is calculated by the following formula:

[0087] H min =max{H min_R ,H min_Δfmax} (twenty one)

[0088] In the formula, H min_R represents the minimum inertia under the frequency change rate constraint, H min_Δfmax It represents the minimum inertia under the maximum frequency deviation constraint, H min Represents the minimum inertia of the system frequency response model.

[0089] The present invention has at least the following beneficial effects:

[0090] The present invention proposes a quantitative evaluation method for the minimum inertia requirement for a large-scale new energy base via a DC transmission system, taking into account the dynamic safety constraints of the system frequency and the differences in the response characteristics of various frequency modulation resources. The effect is reflected in:

[0091] 1) In terms of frequency response modeling, frequency response models of multiple types of frequency modulation resources under typical control methods are established, which can accurately characterize their frequency response characteristics; the frequency response aggregation model of the DC transmission system established based on the aggregation principle fully considers the differences in the supporting characteristics of multiple frequency modulation resources and clarifies the dominant equipment in different frequency response stages.

[0092] 2) In terms of minimum inertia requirement assessment, a minimum inertia requirement assessment method for the DC transmission system is designed by considering the safety constraints of the maximum frequency change rate and the maximum frequency deviation. This method effectively takes into account the influence of new energy and DC frequency control links and parameters, and can comprehensively quantify the minimum inertia requirement while ensuring the dynamic safety of the system frequency. BRIEF DESCRIPTION OF THE DRAWINGS

[0093] Figure 1 This is a structural diagram of a large-scale new energy base DC transmission system in the prior art;

[0094] Figure 2 is the power system frequency response curve;

[0095] Figure 3 is a schematic diagram of a frequency response model of a synchronous generator in an embodiment of the present invention;

[0096] Figure 4 is a schematic diagram of a wind power generation frequency response model in an embodiment of the present invention;

[0097] Figure 5 is a schematic diagram of a photovoltaic power generation frequency response model in an embodiment of the present invention;

[0098] Figure 6 is a block diagram of a DC FLC control in an embodiment of the present invention;

[0099] Figure 7 It is a schematic diagram of a frequency response model of a new energy base through a DC transmission system in an embodiment of the present invention;

[0100] Figure 8 is a schematic diagram of an equivalent single-machine system frequency response model in an embodiment of the present invention;

[0101] Fig. 9 are different H in the embodiments of the present invention. syseq The frequency deviation diagram of the following system;

[0102] Fig.10 Schematic diagram of frequency curves of the system in different frequency modulation modes in an embodiment of the present invention;

[0103] Fig.11 is a flowchart of minimum inertia evaluation of a power system in an embodiment of the present invention;

[0104] Fig.12 Schematic diagram of frequency curves of different simulation models in an embodiment of the present invention;

[0105] Fig.13 Schematic diagram of system frequency curves under different frequency modulation modes in an embodiment of the present invention;

[0106] Fig.14For different f in the embodiments of the present invention DH Schematic diagram of the impact on the power system;

[0107] Fig.15 Schematic diagram of minimum inertia requirement under RoCoF constraint in an embodiment of the present invention;

[0108] Fig.16 For different f in the embodiments of the present invention DH H under the maximum frequency deviation constraint min_Δfmax Schematic diagram;

[0109] Fig.17 For different f in the embodiments of the present invention DH Corresponding to H syseq Schematic diagram of the frequency deviation curve. DETAILED DESCRIPTION

[0110] In order to enable those skilled in the art to better understand the technical solution of the present invention, the present invention is described in detail below in conjunction with the accompanying drawings and specific embodiments. The embodiments of the present invention are further described in detail below in conjunction with the accompanying drawings and specific embodiments, but are not intended to limit the present invention. For the various steps described herein, if there is no necessity for a causal relationship between each other, the order in which they are described as examples herein should not be regarded as a limitation, and those skilled in the art should know that they can be adjusted in order, as long as the logic between them is not destroyed, resulting in the inability to implement the entire process.

[0111] The transmission of large-scale new energy bases via direct current is the main mode of large-scale development of new energy in deserts, Gobi and deserts. The minimum inertia demand assessment is the basis for realizing the low inertia risk perception, control and resource allocation of direct current transmission systems. Based on this, this embodiment takes the direct current transmission system of large-scale new energy bases as the background to carry out the study on the minimum inertia demand assessment of the transmission system taking into account the differences in frequency support characteristics of multiple frequency modulation resources. In terms of frequency response modeling, the frequency response characteristics of various frequency modulation resources in the direct current transmission system under typical control methods are analyzed, and a system frequency response model considering the differences in frequency support characteristics of wind power, photovoltaics and ultra-high voltage direct current is established. In terms of minimum inertia demand assessment, a system equivalent inertia model considering the inertia support of wind power and photovoltaics is constructed. With the frequency change rate and the maximum frequency deviation as constraint indicators, a minimum inertia demand assessment method for direct current transmission systems considering the differences in support characteristics of multiple types of frequency modulation resources is proposed, and its comprehensive assessment process is designed. Finally, the effectiveness and accuracy of the proposed evaluation method are verified through the MATLAB / Simulink simulation platform.

[0112] This embodiment proposes an application scenario, which is to implement minimum inertia demand assessment in a large-scale new energy base via a DC transmission system structure. Figure 1 As shown in the figure, the rectifier station at the sending end is connected to the synchronous generator, wind power generation, and photovoltaic power generation; the inverter station at the receiving end is connected to the AC grid; Rd represents the resistance of the DC line; XL represents the line reactance. Due to the lack of support from large-scale hydropower and thermal power sources, synchronous generators, wind power generation, photovoltaic power generation, and UHV DC systems are required to jointly support the system frequency in order to improve the frequency safety and stability of the weakly supported DC transmission system.

[0113] When a large-scale new energy base experiences active power disturbance through the DC transmission system, the active power balance of the system is broken, and the dynamic process of the system's frequency response can be divided into three stages, such as Figure 2 As shown in the figure, the maximum frequency change rate and the maximum frequency deviation are important indicators for evaluating the frequency safety and stability of the system, and they appear in the inertia response stage and the primary frequency modulation stage respectively, which are the frequency dynamic response processes that need to be analyzed in this paper.

[0114] In the inertia response stage, in addition to the inertia response of synchronous generators, wind power generation and photovoltaic power generation can provide inertia response similar to that of synchronous generators through control improvement to suppress the frequency change rate. When the system frequency exceeds the primary frequency regulation dead zone, it will enter the primary frequency regulation stage. Wind power generation, photovoltaic power generation and UHV DC will adjust their active power according to the frequency change, and work with synchronous generators to maintain the safety and stability of the system frequency to avoid large fluctuations.

[0115] Based on the above application scenario, an embodiment of the present invention proposes a method for evaluating the minimum inertia requirement of a DC transmission system taking into account the differences in supporting characteristics of multiple frequency modulation resources. The method includes the following steps S1 to S5, which are described in detail as follows.

[0116] S1. Construct a system frequency response model that takes into account the differences in frequency response characteristics of wind power, photovoltaic power, and ultra-high voltage direct current.

[0117] In some embodiments, step S1 specifically includes steps S11 to S13, which are described in detail as follows.

[0118] S11. Establish frequency response models for multiple types of frequency modulation resources.

[0119] In this embodiment, in order to accurately characterize the frequency response characteristics of the DC transmission system of a large-scale new energy base, frequency response models of synchronous generators, wind power generation, photovoltaic power generation, and ultra-high voltage DC transmission systems are constructed respectively, so as to form a frequency response model of multiple types of frequency modulation resources. It should be noted that only the influence of the active power-frequency control link of multiple types of frequency modulation resources is considered here, and the influence of new energy and ultra-high voltage DC switch characteristics is ignored.

[0120] Regarding the synchronous generator frequency response model, the model retains the connection between the prime mover and the speed regulator, while ignoring the influence of other non-active-frequency coupling links. In view of the fact that the introduction of the frequency additional controller may cause frequency fluctuations, the dead zone value of its primary frequency regulation is set to ±0.033Hz, and the upper and lower limits of the adjustment power are ±0.1pu. The synchronous generator frequency response model is as follows Figure 3 As shown, its frequency response transfer function is as follows:

[0121]

[0122] In the formula, B P represents the primary frequency modulation droop coefficient; Δf represents the system frequency change; k b Represents the mechanical power gain coefficient; F HP Indicates the work coefficient of the prime mover's high-pressure cylinder; T RH Represents the reheat time constant; ΔP G represents the power change of the synchronous generator set; s represents the Laplace operator.

[0123] For the wind power frequency response model, since the grid-following converter uses a phase-locked loop (PLL) to keep synchronization with the system, its support for the grid frequency is mainly achieved through additional frequency control, which enables it to have virtual inertia support and primary frequency regulation functions. In view of the fast control characteristics of the rotor side response of the wind turbine, the frequency differential link is used to realize the inertia response of the wind turbine, and its frequency transfer function is shown in formula (2):

[0124]

[0125] In the formula, H ω represents the rotor inertia time constant; k df Indicates the inertia coefficient; ΔP Wa Indicates the power change of inertia control.

[0126] Since the droop control is affected by mechanical characteristics, resulting in a long frequency response time, a frequency proportional link is used to achieve a primary frequency modulation response, and its frequency transfer function is shown in formula (3):

[0127] ΔP Wb =k pf Δf(3)

[0128] In the formula, k pf Indicates the primary frequency modulation coefficient; ΔP Wb Indicates the power change amount of droop control.

[0129] Combining inertial control and droop control, the wind power generation frequency response model can be obtained as follows: Figure 4 shown.

[0130] Among them, ΔP W Indicates the change in wind power generation. Figure 4 The wind power generation frequency response transfer function can be obtained as follows:

[0131]

[0132] For the photovoltaic power generation frequency response model, the DC capacitor voltage equation of bipolar power generation is derived based on the virtual inertia control strategy of photovoltaic power generation:

[0133]

[0134] In the formula, C s Indicates DC capacitance; U dc , U dc0 Respectively represent DC voltage and voltage rating; P dc Indicates DC output power; P g Represents the photovoltaic power output power.

[0135] Linearizing the above formula (5) yields:

[0136]

[0137] Where ΔP pv , ΔU dc They represent the change in photovoltaic power generation and DC voltage respectively.

[0138] According to equations (5) and (6), the frequency response expression of photovoltaic power generation can be obtained as follows:

[0139]

[0140] Among them, H pv represents the virtual inertia time constant; k pv Represents the photovoltaic control parameters. The photovoltaic power generation frequency response model is as follows Figure 5 shown.

[0141] For the UHV DC frequency response model, UHV DC transmission mainly uses FLC control to achieve its frequency support function. Its control structure is as follows: Figure 6 When the system experiences a power disturbance and the frequency deviation exceeds the dead zone value set by the control strategy, the control link begins to intervene, using the frequency deviation signal as input, and further affects the UHV DC transmission power through the proportional-integral control link, thereby reducing the unbalanced power of the sending-end system after the disturbance. When the frequency drops to the dead zone again, the UHV DC regulation amount will gradually decrease until it is completed and exits operation.

[0142] In order to ensure safe and stable operation of the system and prevent the disturbance at the sending end from affecting the receiving end power grid, the setting of the DC FLC dead zone value should take into account the full utilization of other frequency regulation resources in the primary frequency regulation reserve capacity. The control principle expression is shown in formula (8):

[0143]

[0144] In the formula, K Dp Represents proportional gain; K Di Indicates the integral gain; m i Indicates the state quantity of the DC system; ΔP DC Indicates the change of UHV DC power; f DH Represents the dead zone value in the HVDC system control strategy.

[0145] S12. Based on the frequency response model of multiple types of frequency modulation resources, a frequency response model of the new energy base through the DC transmission system is established.

[0146] Since various frequency modulation resources have different degrees of frequency support in the dynamic process of system frequency response, the study comprehensively considers the differences in the support characteristics of various frequency modulation resources and comprehensively explores the frequency response characteristics, control parameters, dead zone value range and the influence of the limiting link of different equipment. Based on this, a frequency response model of the DC transmission system of the new energy base is constructed, such as Figure 7 As shown in the figure, λ G , W , pv , D They are the coefficients of synchronous generator frequency regulation unit, wind power generation frequency regulation unit, photovoltaic power generation frequency regulation unit and UHV DC frequency regulation unit, that is, the proportion of the active power of each frequency regulation resource to the total frequency regulation active power.

[0147] S13, performing aggregation equivalent processing on the frequency response model of the DC transmission system of the new energy base to obtain a system frequency response model.

[0148] Using the aggregation equivalence principle, the frequency modulation resource parameters satisfy:

[0149]

[0150] In the formula, χ represents the equivalent parameter factor after system aggregation; Indicates the types of frequency-regulating units, including synchronous generators, wind power generation, photovoltaic power generation, and high-voltage direct current; Indicates the frequency regulation unit category is The number of units corresponding to the time; is the i-th station before aggregation Frequency regulation unit parameters; is the i-th station before aggregation Frequency regulation category unit capacity.

[0151] To simplify the analysis, Figure 7 The frequency response model of the multi-machine system shown in the figure is subjected to aggregate equivalent processing, and the following is obtained: Figure 8 The system frequency response model under the frequency modulation of multiple resources is shown.

[0152] When Δf does not exceed f DH When , UHV DC does not participate in the frequency regulation of the system. At this time, the system frequency response transfer function is:

[0153]

[0154] In the formula, G k When k in (s) is G, W, pv, or D, it represents the frequency response transfer function of synchronous generator, wind power generation, photovoltaic power generation, and UHV DC. The UHV DC link is not considered here. syseq represents the equivalent inertia time constant, λ k The proportion of each frequency regulation resource’s active power to the total frequency regulation active power, D sys represents the equivalent damping of the system.

[0155] When Δf exceeds f DH When UHV DC participates in the frequency support of the system, the frequency change of the system is:

[0156]

[0157] in,

[0158] ΔP Z (s) = ΔP L (s)-f DH G D (s)(12)

[0159] In the formula, G k When k is D in (s), it represents the frequency response transfer function of UHV DC. ΔP Z (s) represents the introduced intermediate variable;

[0160] From the above section 1.2.4, we can know that f DH It does not affect the initial frequency change rate. Therefore, when analyzing the initial frequency change rate of the system, the dead zone of the DC FLC is ignored. Based on the initial value theorem, when the system is disturbed by ΔP L When , the initial frequency change rate R0 under the participation of new energy in frequency modulation can be calculated as shown in formula (13):

[0161]

[0162] Where Δf(t) represents the frequency change;

[0163] When the equivalent inertia of the power system decreases, the system's ability to regulate frequency changes decreases, resulting in an increase in the absolute value of the maximum frequency deviation. When the system frequency reaches the maximum deviation, the system's frequency change rate will be zero, that is:

[0164]

[0165] Where, t max Indicates the time when the frequency reaches the lowest point;

[0166] Then the maximum frequency deviation of the system is Δf max The expression is shown in formula (15):

[0167] Δf max ={Δf(t)t=t max ,Δf(t)=L -1 (G sys (s)ΔP Z}(15)

[0168] Where, L -1 represents the inverse Laplace transform;

[0169] According to equations (13) and (15), it can be concluded that there is a close relationship between the system's initial frequency change rate and maximum frequency deviation and the system equivalent inertia. Therefore, when evaluating the minimum inertia requirement of the DC transmission system, it is necessary to comprehensively consider the initial RoCoF and the maximum frequency deviation constraints to ensure the safe and stable operation of the system.

[0170] S2. Construct a system equivalent inertia calculation model that takes into account the inertia support of wind power and photovoltaic power generation, and calculate the equivalent inertia time constant based on the system equivalent inertia calculation model.

[0171] When multiple resources participate in frequency regulation, the equivalent inertia of the system is composed of the inertia of the synchronous generator and the virtual inertia provided by the new energy. The generalized kinetic energy of the system can represent the collection of different energy forms that affect inertia in the power system. In this paper, the expression of the generalized kinetic energy of the system is shown in formula (16):

[0172]

[0173] In the formula, E G , Z Gi are the kinetic energy of the synchronous generator and the start and stop status of the i-th synchronous unit respectively; E W , H Wq , S Wq , Z Wqare the kinetic energy provided by wind power generation, the virtual inertia time constant, capacity and start / stop status of the qth wind power generation; E PV , H PVj , S PVj , Z PVj are the energy form of photovoltaic power generation, the virtual inertia time constant, capacity and start / stop status of the jth photovoltaic unit; N G 、N W 、N pv are the number of synchronous units, wind power generation, and photovoltaic power generation units respectively;

[0174] From formula (16), the equivalent inertia time constant of the new energy base through the DC transmission system can be obtained as:

[0175]

[0176] In the formula, H syseq represents the equivalent inertia time constant.

[0177] S3. Calculate the minimum inertia of the system frequency response model under the frequency change rate constraint; wherein the frequency change rate is determined by the equivalent inertia time constant and the active disturbance.

[0178] In this embodiment, the RoCoF corresponding to the moment when the system encounters active disturbance is usually the largest, and the size of RoCoF is determined by the system equivalent inertia time constant and the active disturbance. The rotor motion equation of the system can be expressed as:

[0179]

[0180] According to formula (18), if the maximum RoCoF of the power system after the disturbance is taken as the constraint condition, the minimum inertia H of the system under the RoCoF constraint is min_R The expression is as follows:

[0181]

[0182] Where ΔP max represents the maximum disturbance power, R _max Indicates the maximum rate of frequency change.

[0183] Then the minimum inertia evaluation nominal value expression of the system is

[0184]

[0185] According to formula (19), the system inertia requirement is proportional to the disturbance power. For a power grid with stricter RoCoF restrictions, the system inertia requirement under disturbance will be greater.

[0186] S4. Considering the DC FLC control, by adjusting the size of the equivalent inertia time constant, the maximum frequency deviation of the system frequency response model satisfies the maximum frequency deviation constraint, and the corresponding equivalent inertia time constant is used as the minimum inertia under the maximum frequency deviation constraint.

[0187] In this embodiment, according to the grid guidelines, the absolute value of the maximum deviation of the system frequency should not exceed 1 Hz. Therefore, when evaluating the minimum inertia requirement of the system, this should be used as a constraint condition. By adjusting the equivalent inertia time constant in the system frequency response model, the maximum frequency deviation is matched with ±1 Hz. At this time, the equivalent inertia of the system is the minimum inertia H under the maximum frequency deviation constraint condition. min_Δfmax .

[0188] based on Figure 8 The system frequency response model shown in Figure 1 analyzes the frequency response characteristics of different inertia conditions under power shortage disturbance, such as Fig. 9 As shown in Figure 2. When the sending end system has an active power shortage, the system frequency will drop. As the equivalent inertia time constant H syseq As the frequency increases, the lowest point of the frequency gradually rises, and the frequency stability of the system also improves accordingly.

[0189] Considering the differences in frequency modulation characteristics of each resource, the frequency response characteristics under different frequency modulation methods are analyzed, such as Fig.10 As shown. When only renewable energy participates in frequency regulation, the degree of system frequency drop is more significant. For large-scale renewable energy bases through the DC transmission system, the participation of DC significantly improves the frequency characteristics of the power system of renewable energy units during the frequency regulation process, and significantly improves the lowest frequency point. Frequency regulation 1 and frequency regulation 2 represent two cases with different DC FLC dead zone values. The results show that when the DC FLC dead zone value is increased, the lowest frequency point will decrease, and the system's demand for minimum inertia will also increase accordingly. Therefore, when evaluating the minimum inertia demand under the constraint of maximum frequency deviation, it is necessary to comprehensively consider the differences in the supporting characteristics of multiple frequency regulation resources to improve the accuracy and practicality of its evaluation.

[0190] S5. Compare the minimum inertia under the frequency change rate constraint and the minimum inertia under the frequency maximum deviation constraint, and take the larger value as the minimum inertia of the system frequency response model.

[0191] When the inertia level of the system is relatively low, it is easy to cause RoCoF to exceed the limit, and RoCoF becomes the main constraint indicator. However, when the inertia level and primary frequency regulation capability are both low, the maximum frequency deviation is most likely to exceed the limit, and the maximum frequency deviation becomes the dominant constraint indicator. Therefore, for the DC transmission system of a large-scale new energy base, it is necessary to consider both RoCoF and the maximum frequency deviation constraints to determine its minimum inertia requirement, that is, the minimum inertia requirement of the system should take the larger value under these two constraints, and the expression is as follows:

[0192] H min =max{H min_R ,H min_Δfmax} (twenty one)

[0193] In the formula, H min_R represents the minimum inertia under the frequency change rate constraint, H min_Δfmax It represents the minimum inertia under the maximum frequency deviation constraint, H min Represents the minimum inertia of the system frequency response model.

[0194] In an exemplary embodiment, the system minimum inertia evaluation process considering the frequency change rate and the maximum frequency deviation constraints is as follows: Fig.11 The main steps of the system minimum inertia requirement evaluation are as follows:

[0195] Step 1: For RoCoF constraints: Based on the improved SFR model and the preset RoCoF constraints, calculate the minimum inertia H of the power system min_RoCoF .

[0196] It should be noted that the “improved SFR model” mentioned here is the system frequency response model constructed in this embodiment.

[0197] Step 2, regarding the maximum frequency deviation constraint: first set the range of the DC FLC dead zone value, and based on the improved SFR model, obtain the maximum frequency deviation of the system. If the maximum frequency deviation constraint is not met, go to step 3; if it is met, jump to step 4.

[0198] Step 3: By adjusting the equivalent inertia time constant H syseq The size of the maximum frequency deviation satisfies the maximum frequency deviation constraint. The equivalent inertia time constant at this time is the minimum inertia of the system under the maximum frequency deviation constraint H min_Δfmax .

[0199] Step 4: H min_RoCoF With H min_Δfmax Compare the two and take the larger value as the minimum inertia of the system.

[0200] In order to verify the effectiveness of the minimum inertia requirement assessment method proposed in the present invention, an improved SFR simulation model and a model of a new energy DC transmission system were built on the MATLAB / Simulink platform.

[0201] In this system, the load rated power is 450MW, and the resources involved in system frequency regulation include: synchronous generators with a total rated capacity of 900MW, 200 double-fed wind turbines with a rated capacity of 1.5MW, photovoltaics with a total capacity of 150MW, and converter stations with a total capacity of 1500MVA. In addition, the rated frequency of the system is set to 50Hz, and the action setting value of low-frequency load reduction f UFLS is 49Hz, so the maximum frequency deviation allowed by the system can be calculated as Δf max =-1Hz. The equivalent inertia time constants of synchronous generator, wind power generation and photovoltaic power generation are 5s, 1s and 1.5s respectively. Other specific parameters are shown in Table 1.

[0202] Table 1 Simulation parameters

[0203]

[0204] In order to verify the feasibility of the improved SFR model proposed in this paper, a power shortage of 300MW occurs in the power system at t=40s. The comparison curve of system frequency is as follows: Fig.12 The calculation results obtained by different simulation models are shown in Table 2.

[0205] Table 2 Data comparison of different simulation models

[0206]

[0207]

[0208] According to Table 2, in the post-steady-state stage of the frequency response, the steady-state frequency deviations obtained by the Simulink model and the SFR model are almost the same due to the regulation of the DC FLC. Although there are some deviations, the frequency dynamic characteristics of the two models are almost the same in the initial stage of the disturbance. Therefore, the evaluation of the minimum inertia requirement of the system will not be affected.

[0209] By comparison Fig.12 From the results in Table 2, it can be found that there are errors in the simulation results of the SFR model and the Simulink detailed model in terms of the initial frequency change rate, the lowest frequency point, and the steady-state frequency, which are 6.6%, 0.3%, and 0.3% respectively.

[0210] In summary, through the simulation comparison of the two models, it is calculated that the error is small and within an acceptable range. Therefore, the frequency response model of the aggregation system constructed in this paper is feasible.

[0211] Quantitative analysis and verification of system frequency characteristics with the participation of each frequency modulation unit: This embodiment makes a quantitative comparison of the frequency characteristics of each frequency modulation unit, focuses on the frequency deviation and power output, and clearly emphasizes the advantages of common frequency modulation.

[0212] The simulation analysis was conducted on three operation modes: wind power frequency regulation alone, wind-solar combined frequency regulation, and wind-solar-DC combined frequency regulation. Fig.13 and Table 3 show the details.

[0213] according to Fig.13 According to the analysis results in Table 3, new energy units and high-voltage direct current have played a positive role in frequency regulation, significantly improving the system's inertia response capability and frequency regulation capability. Specifically, the minimum frequency value increased by 0.0845Hz, the steady-state frequency deviation increased by 0.3183Hz, and the absolute value of the frequency change rate was almost the same. Through quantitative data analysis, it is shown that it is feasible for direct current to assist new energy in frequency regulation, which significantly improves the frequency characteristics of the power system of new energy units in the frequency regulation process and effectively reduces the absolute value of the steady-state frequency deviation.

[0214] Table 3 Comparison of frequency characteristics under different operating modes

[0215]

[0216] Quantitative analysis and verification of system frequency characteristics under different dead zone values ​​of DC FLC:

[0217] By setting the proportional control coefficient in the additional control unit of the UHV DC frequency to a certain value, f DH In order to study the variation of system frequency with f DH The dead zone values ​​are set to 0.16, 0.20, and 0.24 respectively. Fig.14 The simulation results of frequency deviation under different dead zone values ​​are shown, and the simulation results are statistically analyzed. The results are shown in Table 4.

[0218] Table 4 Frequency conditions under different fDH

[0219]

[0220] According to the results in Table 4, the same system is subjected to the same disturbance. DH As the frequency decreases, the absolute value of the maximum frequency deviation will also decrease accordingly. The steady-state frequency deviation value is related to f DH At the same time, it can be seen that f DHThe change of will hardly delay the time when the frequency reaches the lowest point, but it changes the maximum frequency deviation, that is, it affects the evaluation of the minimum inertia of the system under the constraint of the maximum frequency deviation.

[0221] This embodiment also performs minimum inertia requirement evaluation verification, including minimum inertia verification based on RoCoF constraints and minimum inertia verification based on maximum frequency deviation constraints.

[0222] Minimum inertia verification based on RoCoF constraints: The relationship between the minimum inertia requirement of the power system and the active disturbance size and the initial frequency change rate is as follows: Fig.15 As shown in the figure, it can be seen that when |R_ max When the constraint of | is reduced, the disturbance power increases, and the system's demand for inertia also increases accordingly. Under the premise of keeping the active disturbance unchanged, as the RoCoF constraint is reduced, the minimum inertia H of the power system min_R This shows that there is an inverse proportional relationship between the two.

[0223] Considering the inertia evaluation under the RoCoF constraint, it is necessary to meet the requirement that the absolute value of the initial frequency change rate is not greater than the preset critical value indicator. Based on the reference "YANRF, MASOODNA, KUMARSAHAT, et al. The anatomy of the 2016 South Australia blackout: a catastrophic air vent in a high renewable network [J]. IEEE Transactions on Power Systems, 2018, 33(5): 5374-5388.", it is recommended to set the RoCoF constraint of the power system to 2Hz / s. For the convenience of comparative analysis, the power system unit penetration ratio is set unchanged. When the load level ΔPL is 300MW, it can be calculated that Hmin_R is 2.5s.

[0224] Minimum inertia verification based on frequency maximum deviation constraint: at different f DH Under the corresponding maximum frequency deviation constraint, the simulation results of the required minimum inertia are as follows: Fig.16 As shown in the figure, it can be observed that when the active disturbance size and f are fixed DH Under the condition of min_Δfmax It has the same growth relationship with the maximum frequency deviation constraint. In addition, when the maximum frequency deviation constraint remains unchanged, increasing the active disturbance size will also increase the system's H min_Δfmax In different f DH Under the same active disturbance and fixed maximum frequency deviation constraint, the system's H min_Δfmax With the f DHincreased with the increase of .

[0225] Depend on Fig.16 It can be seen that when ΔP L When the maximum frequency deviation is -1Hz, f DH The corresponding minimum inertia time constants of the system are 2.98s, 3.08s, and 3.31s, which are 0.16, 0.20, and 0.24, respectively, and meet the maximum frequency deviation constraint of the system. At the same time, the initial RoCoF of the system is -1.26, -1.22, and -1.13, respectively, which meets the RoCoF constraint.

[0226] In order to verify the reliability of the improved SFR model in this scenario, a comparative simulation was carried out with the Simulink detailed model. Fig.17 Shows that when different f DH The corresponding H syseq The system frequency response characteristic curves under each simulation model are 2.98s, 3.08s, and 3.31s respectively. As can be seen from the figure, the frequency response characteristic curves under the two models are almost the same, which shows that the simulation results of the constructed models are effective.

[0227] In summary, considering that the steady-state frequency deviation usually does not exceed 0.2Hz, it is most ideal to set the minimum inertia time constant below 3.08s. Fig.17 In the above figure, the maximum frequency deviation and initial frequency change rate of the system meet the requirements of safe operation of the system frequency. Therefore, the feasibility of the minimum inertia demand assessment method for the DC transmission system of the new energy base proposed in this paper can be verified.

[0228] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modification, equivalent substitution or improvement made within the spirit and principle of the present invention should be included in the protection scope of the present invention.

Claims

1. A method for evaluating the minimum inertia requirement of a DC transmission system taking into account the differences in supporting characteristics of multiple frequency modulation resources, characterized in that: The method comprises: Construct a system frequency response model that takes into account the differences in frequency response characteristics of wind power, photovoltaic power, and UHV DC; Constructing a system equivalent inertia calculation model that takes into account the inertia support of wind power and photovoltaic power generation, and calculating the equivalent inertia time constant based on the system equivalent inertia calculation model; Calculating the minimum inertia of the system frequency response model under the frequency change rate constraint; wherein the frequency change rate is determined by the equivalent inertia time constant and the active disturbance; In the case of considering DC FLC control, by adjusting the size of the equivalent inertia time constant, the maximum frequency deviation of the system frequency response model meets the maximum frequency deviation constraint, and the corresponding equivalent inertia time constant is used as the minimum inertia under the maximum frequency deviation constraint; The minimum inertia under the frequency change rate constraint and the minimum inertia under the frequency maximum deviation constraint are compared, and the larger value is used as the minimum inertia of the system frequency response model.

2. The method according to claim 1, characterized in that: Construct a system frequency response model that takes into account the differences in frequency response characteristics of wind power, photovoltaic power, and UHV DC, including: Establish frequency response models for multiple types of FM resources; Based on the frequency response models of the multi-type frequency modulation resources, a frequency response model of the new energy base through the direct current transmission system is established; The frequency response model of the DC transmission system of the new energy base is subjected to aggregate equivalent processing to obtain a system frequency response model.

3. The method according to claim 2, characterized in that The frequency response models of the multi-type frequency modulation resources include a synchronous generator frequency response model, a wind power generation frequency response model, a photovoltaic power generation frequency response model and an ultra-high voltage direct current frequency response model: The frequency response transfer function of the generator frequency response model is as follows: In the formula, B P represents the primary frequency modulation droop coefficient; Δf represents the system frequency change; k b Represents the mechanical power gain coefficient; F HP Indicates the work coefficient of the prime mover's high-pressure cylinder; T RH Represents the reheat time constant; ΔP G represents the power change of synchronous generator set; s represents the Laplace operator; The wind power generation frequency response model adopts the frequency differential link to realize the inertia response of the wind turbine set, and its frequency transfer function is shown in formula (2): In the formula, H ω represents the rotor inertia time constant; k df Indicates the inertia coefficient; ΔP Wa Indicates the power change of inertia control; The wind power generation frequency response model adopts a frequency ratio link to achieve a primary frequency modulation response, and its frequency transfer function is shown in formula (3): ΔP Wb =k pf Δf (3) In the formula, k pf Indicates the primary frequency modulation coefficient; ΔP Wb Indicates the power change of droop control; The DC capacitor voltage equation of the photovoltaic power generation frequency response model is: In the formula, C s Indicates DC capacitance; U dc , U dc0 Respectively represent DC voltage and voltage rating; P dc Indicates DC output power; P g represents the photovoltaic power output power, and t represents the time; Linearizing equation (5) yields: Where ΔP pv , ΔU dc They represent the change in photovoltaic power generation and DC voltage respectively; According to formula (5) and formula (6), the photovoltaic power generation frequency response expression of the photovoltaic power generation frequency response model is: Among them, H pv represents the virtual inertia time constant; k pv Indicates photovoltaic control parameters; The control equation of the UHV DC frequency response model is: In the formula, K Dp Represents proportional gain; K Di Indicates the integral gain; m i Indicates the state quantity of the DC system; ΔP DC Indicates the change of UHV DC power; f DH Represents the dead zone value in the high voltage DC system control strategy; m min Indicates the minimum value of the DC system state quantity; ΔP DC Indicates the minimum value of UHV DC power.

4. The method according to claim 3, characterized in that The frequency response model of the DC transmission system of the new energy base is subjected to aggregate equivalent processing to obtain a system frequency response model, including: By using the aggregation equivalence principle, the frequency modulation resource parameters in the frequency response model of the DC transmission system of the new energy base are made to meet the following requirements: In the formula, χ represents the equivalent parameter factor after system aggregation; Indicates the types of frequency-regulating units, including synchronous generators, wind power generation, photovoltaic power generation, and high-voltage direct current; Indicates the frequency regulation unit category is The number of units corresponding to the time; The category of the i-th frequency regulation unit before aggregation is Unit parameters; The category of the i-th frequency regulation unit before aggregation is The unit capacity.

5. The method according to claim 4, characterized in that When Δf does not exceed f DH When UHV DC does not participate in the frequency regulation of the system, the frequency response transfer function G of the system frequency response model is sys (s) is: In the formula, G k When k in (s) is G, W, pv, or D, it represents the frequency response transfer function of synchronous generator, wind power generation, photovoltaic power generation, and UHV DC. syseq represents the equivalent inertia time constant, λ k It indicates the proportion of each frequency regulation resource’s active power to the total frequency regulation active power, D sys represents the equivalent damping of the system; When Δf exceeds f DH When UHV DC participates in the frequency support of the system, the frequency change analytical expression of the system frequency response model is: in, ΔP Z (s)=ΔP L (s)-f DH G D (s) (12) In the formula, G D (s) represents the frequency response transfer function of UHV DC, Δf(s) represents the frequency change, ΔP Z (s) represents the introduced intermediate variable; Based on the initial value theorem, when the system is disturbed by ΔP L When the frequency is adjusted by the new energy, the initial frequency change rate R0 is calculated as shown in formula (13): Where Δf(t) represents the frequency change; When the system frequency reaches the maximum deviation, the system frequency change rate will be zero, that is: Where, t max Indicates the time to reach the maximum frequency point; Determine the maximum frequency deviation Δf of the system max The expression is shown in formula (15): Δf max ={Δf(t)|t=t max ,Δf(t)=L -1 (G sys (s)ΔP Z } (15) Where, L -1 represents the inverse Laplace transform; According to equations (13) and (15), the relationship between the system's initial frequency change rate and the maximum frequency deviation and the system's equivalent inertia is determined.

6. The method according to claim 1, characterized in that The generalized kinetic energy expression of the system based on the system equivalent inertia calculation model is shown in formula (16): In the formula, E G , Z Gi are the kinetic energy of the synchronous generator and the start and stop status of the i-th synchronous unit respectively; E W , H Wq , S Wq , Z Wq are the kinetic energy provided by wind power generation, the virtual inertia time constant, capacity and start / stop status of the qth wind power generation; E PV , H PVj , S PVj , Z PVj are the energy form of photovoltaic power generation, the virtual inertia time constant, capacity and start / stop status of the jth photovoltaic unit; N G 、N W 、N pv are the number of synchronous units, wind power generation, and photovoltaic power generation units respectively; The equivalent inertia time constant of the new energy base through the DC transmission system is obtained from formula (16): In the formula, H syseq represents the equivalent inertia time constant.

7. The method according to claim 1, characterized in that Calculating the minimum inertia of the system frequency response model under the frequency change rate constraint, including: The rotor motion equation of the system frequency response model is determined as: According to formula (18), the maximum frequency change rate RoCoF of the power system after the disturbance is taken as the constraint condition, and the expression of the minimum inertia of the system based on the RoCoF constraint is obtained as follows: In the formula, H min_R represents the minimum inertia under the frequency change rate constraint, ΔP max represents the maximum disturbance power, R _max Indicates the maximum rate of frequency change.

8. The method according to claim 1, characterized in that In the case of considering DC FLC control, by adjusting the size of the equivalent inertia time constant, the maximum frequency deviation of the system frequency response model satisfies the maximum frequency deviation constraint, and the corresponding equivalent inertia time constant is used as the minimum inertia under the maximum frequency deviation constraint, including: Set the range of DC FLC dead zone value, based on the system frequency response model, to obtain the maximum frequency deviation of the system; When the maximum frequency deviation of the system satisfies the maximum frequency deviation constraint, the current equivalent inertia time constant is used as the minimum inertia H under the maximum frequency deviation constraint. min_Δfmax ; When the maximum frequency deviation of the system does not meet the maximum frequency deviation constraint, adjust the size of the equivalent inertia time constant so that the maximum frequency deviation meets the maximum frequency deviation constraint. At this time, the equivalent inertia time constant is the minimum inertia H under the maximum frequency deviation constraint. min_Δfmax .

9. The method according to claim 1, characterized in that: The minimum inertia of the system frequency response model is calculated by the following formula: H min =max{H min_R ,H min_Δfmax } (21) In the formula, H min_R represents the minimum inertia under the frequency change rate constraint, H min_Δfmax It represents the minimum inertia under the maximum frequency deviation constraint, H min Represents the minimum inertia of the system frequency response model.

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