A stability detection method, device and medium for a DRU-MMC hybrid DC transmission system

Abstract

The present invention discloses a stability detection method, device, and medium for a DRU-MMC hybrid DC transmission system. The method comprises: based on the disturbance operation data of the DRU-MMC hybrid DC transmission system after a disturbance, calculating according to a DRU-MMC small-signal model a stability characteristic value that can reflect the system state and oscillation frequency; wherein the DRU-MMC small-signal model is established based on equations such as the linearized state-space equation of the DRU converter station and the linearized state-space equation of the DC submarine cable. The present invention proposes a stability detection method, device, and medium for a DRU-MMC hybrid DC transmission system. The method uses a preset DRU-MMC small-signal model to calculate the disturbance data of the DRU-MMC hybrid DC transmission system, and can directly and effectively obtain the stability characteristic value. In addition, the related linearization processing of the DRU-MMC small-signal model simplifies the originally complex nonlinear relationship near a specific operating point, which can solve the problem of difficulty in effectively detecting the stability of the DRU-MMC hybrid DC transmission system.

Description

Technical Field

[0001] The present invention relates to the technical field of power system stability, and in particular to a stability detection method, device and medium for a DRU-MMC hybrid direct current transmission system. Background Art

[0002] Offshore wind power has broad development prospects, and 70% of the world's potential offshore wind resources are located in deepwater areas with a depth of more than 60 meters. With the innovation of wind power technology, offshore wind power projects are gradually developing in the deep sea with richer wind energy resources. However, compared with near-shore wind power that can be directly transmitted at industrial frequency, offshore wind power has problems such as long distances resulting in higher AC cable costs, capacitance effects, and questionable reliability. More consideration needs to be given to the control methods and grid connection methods of offshore wind power. Existing offshore wind power mainly adopts flexible direct current (HVDC) system for grid connection. The modular multilevel converter (MMC) in it can improve the power density and efficiency in power conversion, thereby ensuring the efficient transmission of offshore wind power. The diode uncontrolled rectifier unit (DRU) is suitable for building offshore platforms. It has low cost and huge economic advantages and has attracted widespread attention in recent years. Therefore, the characteristics of both can be combined to construct an offshore wind power uncontrolled rectifier HVDC transmission system including offshore grid-type wind farm-offshore DRU converter station-HVDC submarine cable-onshore MMC converter station-onshore AC system, realizing the low-cost transmission of offshore wind power.

[0003] Current research on DRU-based offshore wind power DC grid-connected systems mainly focuses on the study of wind farm internal control strategies and startup methods, and lacks in-depth research on the small-signal stability of the system. At the same time, in terms of DC transmission system stability, existing technologies mainly focus on the small-signal model research of two-terminal MMC flexible DC transmission systems and conduct detailed analysis of the internal dynamic characteristics of MMC. However, since the DRU side does not contain a control system, its operating characteristics are also quite different from those of MMC, making it difficult to effectively detect the stability of the DRU-MMC hybrid DC transmission system. Summary of the Invention

[0004] The present invention provides a stability detection method, device and medium for a DRU-MMC hybrid DC power transmission system, so as to solve the problem that it is difficult to effectively detect the stability of the DRU-MMC hybrid DC power transmission system.

[0005] Obtain the disturbance operation data of the DRU-MMC hybrid HVDC system after the disturbance;

[0006] Based on the disturbance operation data, a stability characteristic value that can reflect the system state and oscillation frequency is calculated according to the DRU-MMC small signal model; wherein, the DRU-MMC small signal model is established based on a group of equations including the linearized state space equations of the DRU converter station, the linearized state space equations of the DC submarine cable, the linearized state space equations of the MMC converter station, and the linearized state space equations of the MMC converter station control system, and the group of equations is obtained by linearizing the operating equations of the DRU-MMC hybrid DC transmission system.

[0007] The present invention obtains the disturbance operation data of the system after the disturbance, and uses the DRU-MMC small signal model to perform calculations based on these data, so that the small signal model can capture the slight changes in the dynamic operation process of the system, and then accurately evaluate the stability and oscillation frequency of the system, thereby obtaining characteristics that can accurately reflect the stability of the system. Among them, the DRU-MMC hybrid DC transmission system mainly needs to consider the four parts of the DRU converter station, the DC system, the MMC converter station, and the MMC converter station control system; because the linearized state space equation of the DRU converter station itself includes the change law of its internal state variables and the interaction with the external system, it can accurately describe the dynamic behavior of the DRU converter station under different working conditions. The linearized state space equation of the DC submarine cable can accurately reflect the impedance characteristics of the line such as resistance and inductance. When the system is disturbed, the linearized equation of the submarine cable line can describe its contribution to the dynamic response of the system, which helps to evaluate the stability and recovery capability of the system. The linearized state-space equations for the MMC converter station describe in detail the evolution of the station's internal state over time. By analyzing these equations, the stability of the MMC converter station can be assessed, including its ability to maintain stable operation under various operating conditions. The linearized state-space equations for the MMC converter station control system accurately describe the dynamic behavior of the MMC converter station control system, including the evolution of system states, input-output relationships, and the effectiveness of control strategies. This provides a reliable theoretical foundation for system analysis, design, and optimization.

[0008] Compared with the existing technology, the present invention uses a preset DRU-MMC small signal model to calculate the disturbance data of the DRU-MMC hybrid DC transmission system, which can directly and effectively obtain the stability characteristic value. In addition, the relevant linearization processing of the DRU-MMC small signal model simplifies the originally complex nonlinear relationship near a specific operating point, thereby improving the accuracy of the analysis. Therefore, it can solve the problem of difficulty in effectively detecting the stability of the DRU-MMC hybrid DC transmission system.

[0009] As a preferred solution, the linearized state space equation of the DRU converter station is specifically:

[0010] Establishing initial state-space equations related to AC voltage and current phase angles based on operating data of the DRU-MMC hybrid DC transmission system;

[0011] Based on the initial state-space equation, linearized state-space equations related to the AC voltage and current phase angles are obtained by normalizing the variables on both sides of the AC transmission system and the DC transmission system in the DRU-MMC hybrid DC transmission system;

[0012] Converting the input and output quantities of the DRU-MMC hybrid direct current transmission system into a global unified coordinate system to obtain a unified coordinate equation system;

[0013] Linearizing the inductor current formula of the DRU-MMC hybrid DC transmission system to obtain a linearized state space equation of the DC filter inductor;

[0014] The DRU converter station linearized state space equation is established according to the linearized state space equation, the unified coordinate equation group and the DC filter inductor linearized state space equation.

[0015] This preferred solution can accurately describe the dynamic relationship between the various system variables by establishing the initial state space equations related to the AC voltage and current phase angle. The variables on both sides of the AC transmission system and the DC transmission system are normalized and the state space equations are linearized, which greatly simplifies the complexity and analysis difficulty of the system. The input and output quantities are converted to a global unified coordinate system, which realizes the description of the various system variables in a unified framework, helping to reduce the errors and complexity caused by coordinate conversion. The DC filter inductance formula is linearized to obtain the DC filter inductance linearized state space equation, which helps to more accurately describe the dynamic characteristics of the DC side filter inductance.

[0016] As a preferred solution, based on the initial state-space equation, by normalizing the variables on both sides of the AC transmission system and the DC transmission system in the DRU-MMC hybrid DC transmission system, a linearized state-space equation for the AC voltage and current phase angle is obtained, specifically:

[0017] The power base values ​​of the AC transmission system and the DC transmission system in the DRU-MMC hybrid DC transmission system are uniformly set to obtain a power formula;

[0018] Establishing a relationship between current and impedance base values ​​on both sides of the AC power transmission system and the DC power transmission system based on the operating data to obtain a first relationship;

[0019] Substituting the power formula and the first relationship into the initial state space equation for linear expansion calculation, the linearized state space equation of the AC voltage and current phase angle under a unified scale is obtained.

[0020] This preferred solution ensures consistency in power calculations between AC and DC transmission systems by establishing a unified power base value, making power comparisons between the different systems more accurate and intuitive. By establishing a relationship between the current and impedance base values ​​on both sides, the electrical connection between the AC and DC transmission systems can be expressed using a unified mathematical expression, helping to simplify the complexity of the state-space equations.

[0021] As a preferred solution, based on the operating data of the DRU-MMC hybrid DC transmission system, the initial state space equations for the AC voltage and current phase angle are established, specifically:

[0022] Based on the operating data of the DRU-MMC hybrid DC transmission system, establish the voltage and current relationship between the AC and DC sides of the DRU under ideal conditions;

[0023] Taking into account the equivalent inductance, the voltage-current relationship is adjusted for voltage drop to obtain the initial state space equation related to the AC voltage and current phase angle.

[0024] In this preferred solution, the ideal voltage-current relationship is the basis for system modeling, which reflects the basic relationship between the various variables in the system. However, in actual systems, due to the influence of various factors, these relationships will be offset to a certain extent. Therefore, by considering the equivalent inductance and adjusting the voltage drop, the characteristics of the actual system can be more accurately reflected, thereby improving the accuracy of the model.

[0025] As a preferred solution, the linearized state space equation of the MMC converter station control system is specifically:

[0026] In the MMC main controller of the DRU-MMC hybrid DC power transmission system, the d-axis is controlled by the DC voltage, and the q-axis is controlled by a constant AC voltage or a constant reactive power;

[0027] Establishing a main controller model according to the operating data of the DRU-MMC hybrid direct current transmission system, and establishing a delay equation group of a first-order inertia link according to the operating data;

[0028] Based on the delay equations of the first-order inertia link, the main controller model is linearized to obtain the linearized state space equations of the MMC converter station control system.

[0029] This preferred solution controls the d-axis via DC voltage, effectively regulating the DC voltage of the MMC converter station and keeping it within a set range, thus helping to maintain the stability of the DC transmission system. Furthermore, by controlling the q-axis with a constant AC voltage or reactive power, the AC voltage and reactive power can be adjusted according to system requirements, improving the system's flexibility and response speed. In actual control systems, delays often occur due to factors such as signal transmission and device response. By establishing a set of delay equations for the first-order inertia link and linearizing the main controller model based on these equations, these delay factors can be effectively accounted for, making the linearized model more closely resemble the dynamic characteristics of the actual system.

[0030] As a preferred solution, a main controller model is established based on the operating data of the DRU-MMC hybrid DC transmission system, specifically:

[0031] An outer loop control expression is established based on the outer loop control intermediate quantities of the d-axis and the q-axis by measuring the attenuation value of the voltage at the end of the DC submarine cable;

[0032] An inner loop control expression is established by measuring the attenuation value of the AC side current reference value according to the inner loop control intermediate quantity of the d-axis and the q-axis;

[0033] According to the AC grid connection point voltage, establish the phase-locked loop control expression;

[0034] The main controller model is composed of the outer loop control expression, the inner loop control expression and the phase-locked loop control expression.

[0035] In this preferred solution, the establishment of the phase-locked loop control expression enables the system to accurately track the voltage phase of the AC grid connection point, ensuring the synchronous operation of the MMC converter station and the AC grid.

[0036] As a preferred solution, a delay equation group of the first-order inertia link is established according to the operating data, specifically:

[0037] Based on the voltage measurement delay time constant and the current measurement delay time constant, a first delay equation is established by adding a first-order inertia link between the measured value and the actual value of the AC voltage and current at the MMC grid connection point;

[0038] Based on the converter delay time constant, a second delay equation is established by adding a first-order inertia link between the reference value and the actual value of the MMC converter voltage.

[0039] The first delay equation and the second delay equation are linearized together to obtain the delay equation group.

[0040] This preferred solution introduces voltage measurement delay time constants, current measurement delay time constants, and converter delay time constants, and establishes a first-order inertia link based on these delays. This allows for a more accurate description of the delays present in real systems. This makes the model more closely aligned with the dynamic characteristics of the actual system, improving the model's detection accuracy.

[0041] The present invention also provides a stability detection device for a DRU-MMC hybrid DC power transmission system, comprising a data module and a detection module;

[0042] The data module is used to obtain the disturbance operation data of the DRU-MMC hybrid direct current transmission system after the disturbance;

[0043] Based on the detection data, a stability characteristic value that can reflect the system state and oscillation frequency is calculated according to the DRU-MMC small signal model; wherein, the DRU-MMC small signal model is established according to a group of equations including the linearized state space equations of the DRU converter station, the linearized state space equations of the DC submarine cable, the linearized state space equations of the MMC converter station, and the linearized state space equations of the MMC converter station control system, and the group of equations is obtained by linearizing the operating equations of the DRU-MMC hybrid DC transmission system.

[0044] As a preferred solution, the linearized state space equation of the DRU converter station is specifically:

[0045] Establishing initial state-space equations related to AC voltage and current phase angles based on operating data of the DRU-MMC hybrid DC transmission system;

[0046] Based on the initial state-space equation, linearized state-space equations related to the AC voltage and current phase angles are obtained by normalizing the variables on both sides of the AC transmission system and the DC transmission system in the DRU-MMC hybrid DC transmission system;

[0047] Converting the input and output quantities of the DRU-MMC hybrid direct current transmission system into a global unified coordinate system to obtain a unified coordinate equation system;

[0048] Linearizing the inductor current formula of the DRU-MMC hybrid DC transmission system to obtain a linearized state space equation of the DC filter inductor;

[0049] The DRU converter station linearized state space equation is established according to the linearized state space equation, the unified coordinate equation group and the DC filter inductor linearized state space equation.

[0050] As a preferred solution, based on the initial state-space equation, by normalizing the variables on both sides of the AC transmission system and the DC transmission system in the DRU-MMC hybrid DC transmission system, a linearized state-space equation for the AC voltage and current phase angle is obtained, specifically:

[0051] The power base values ​​of the AC transmission system and the DC transmission system in the DRU-MMC hybrid DC transmission system are uniformly set to obtain a power formula;

[0052] Establishing a relationship between current and impedance base values ​​on both sides of the AC power transmission system and the DC power transmission system based on the operating data to obtain a first relationship;

[0053] Substituting the power formula and the first relationship into the initial state space equation for linear expansion calculation, the linearized state space equation of the AC voltage and current phase angle under a unified scale is obtained.

[0054] As a preferred solution, based on the operating data of the DRU-MMC hybrid DC transmission system, the initial state space equations for the AC voltage and current phase angle are established, specifically:

[0055] Based on the operating data of the DRU-MMC hybrid DC transmission system, establish the voltage and current relationship between the AC and DC sides of the DRU under ideal conditions;

[0056] Taking into account the equivalent inductance, the voltage-current relationship is adjusted for voltage drop to obtain the initial state space equation related to the AC voltage and current phase angle.

[0057] As a preferred solution, the linearized state space equation of the MMC converter station control system is specifically:

[0058] In the MMC main controller of the DRU-MMC hybrid DC power transmission system, the d-axis is controlled by the DC voltage, and the q-axis is controlled by a constant AC voltage or a constant reactive power;

[0059] Establishing a main controller model according to the operating data of the DRU-MMC hybrid direct current transmission system, and establishing a delay equation group of a first-order inertia link according to the operating data;

[0060] Based on the delay equations of the first-order inertia link, the main controller model is linearized to obtain the linearized state space equations of the MMC converter station control system.

[0061] As a preferred solution, a main controller model is established based on the operating data of the DRU-MMC hybrid DC transmission system, specifically:

[0062] An outer loop control expression is established based on the outer loop control intermediate quantities of the d-axis and the q-axis by measuring the attenuation value of the voltage at the end of the DC submarine cable;

[0063] An inner loop control expression is established by measuring the attenuation value of the AC side current reference value according to the inner loop control intermediate quantity of the d-axis and the q-axis;

[0064] According to the AC grid connection point voltage, establish the phase-locked loop control expression;

[0065] The main controller model is composed of the outer loop control expression, the inner loop control expression and the phase-locked loop control expression.

[0066] As a preferred solution, a delay equation group of the first-order inertia link is established according to the operating data, specifically:

[0067] Based on the voltage measurement delay time constant and the current measurement delay time constant, a first delay equation is established by adding a first-order inertia link between the measured value and the actual value of the AC voltage and current at the MMC grid connection point;

[0068] Based on the converter delay time constant, a second delay equation is established by adding a first-order inertia link between the reference value and the actual value of the MMC converter voltage.

[0069] The first delay equation and the second delay equation are linearized together to obtain the delay equation group.

[0070] The present application also provides a storage medium having a computer program stored thereon. The computer program is called and executed by a computer to implement the stability detection method of the DRU-MMC hybrid direct current transmission system as described above. BRIEF DESCRIPTION OF THE DRAWINGS

[0071] Figure 1 1 is a flow chart of a stability detection method for a DRU-MMC hybrid DC transmission system provided in an embodiment of the present application;

[0072] Figure 2 This is a structural diagram of the DRU-MMC hybrid DC transmission system provided in an embodiment of the present application;

[0073] Figure 3 This is a detailed internal structure diagram of the DRU provided in an embodiment of the present application;

[0074] Figure 4 This is a structural diagram of the MMC control system provided in an embodiment of the present application;

[0075] Figure 5 This is a first change diagram of the output quantity after changing the DC voltage reference value provided by an embodiment of the present application;

[0076] Figure 6 This is a second graph showing changes in output after a DC voltage reference value is changed, as provided in an embodiment of the present application;

[0077] Figure 7 This is a third graph showing changes in output after changing the DC voltage reference value provided by an embodiment of the present application;

[0078] Figure 8 This is a first change diagram of the output after changing the AC voltage amplitude on the DRU side provided by an embodiment of the present application;

[0079] Figure 9 This is a second change diagram of the output after changing the AC voltage amplitude on the DRU side provided in an embodiment of the present application;

[0080] Figure 10 This is a third change diagram of the output after changing the AC voltage amplitude on the DRU side provided by an embodiment of the present application;

[0081] Figure 11 This is a first change diagram of the output after changing the AC voltage amplitude on the MMC side provided by an embodiment of the present application;

[0082] Figure 12 This is a second graph showing changes in output after the AC voltage amplitude on the MMC side is changed, as provided in an embodiment of the present application;

[0083] Figure 13 This is a diagram showing the effect of the short-circuit ratio on system stability provided by an embodiment of the present application;

[0084] Figure 14 This is a diagram of the system oscillation waveform after the short-circuit ratio is changed according to an embodiment of the present application;

[0085] Figure 15 This is a structural diagram of a stability detection device for a DRU-MMC hybrid DC transmission system provided in an embodiment of the present application. DETAILED DESCRIPTION

[0086] The following will be combined with the drawings in the embodiments of this application to clearly and completely describe the technical solutions in the embodiments of this application. Obviously, the embodiments described are only part of the embodiments of this application, not all of the embodiments. Based on the embodiments in this application, all other embodiments obtained by ordinary technicians in this field without making creative efforts are within the scope of protection of this application.

[0087] In the description of this application, it should be understood that the terms "first," "second," "third," and "fourth" are used for descriptive purposes only and should not be understood to indicate or imply relative importance or implicitly specify the number of the technical features indicated. Thus, a feature defined as "first," "second," "third," and "fourth" may explicitly or implicitly include one or more of such features. In the description of this application, unless otherwise specified, "several" means two or more.

[0088] The stability detection method of a DRU-MMC hybrid DC transmission system provided in an embodiment of the present application is mainly used in situations where a small signal model of the DRU-MMC hybrid DC transmission system needs to be established based on DRU and MMC mathematical equations, so as to accurately reflect the operating characteristics of the DRU-MMC hybrid DC transmission system after a small disturbance.

[0089] Example 1:

[0090] See also Figure 1 The embodiment of the present application provides a stability detection method for a DRU-MMC hybrid DC transmission system, including S1 to S2, and the specific implementation steps are as follows:

[0091] S1. Obtain disturbance operation data of the DRU-MMC hybrid DC transmission system after the disturbance.

[0092] Step S1 of the embodiment of the present application is specifically as follows:

[0093] The data acquisition system is used to monitor and record the disturbance operation data of the DRU-MMC hybrid DC transmission system in real time after the disturbance.

[0094] To apply this example, please refer to Figure 2 , Figure 2 2 is a structural diagram of a DRU-MMC hybrid DC transmission system provided in an embodiment of the present application, illustrating the structure of the DRU-MMC hybrid DC transmission system in the first embodiment of the present application;

[0095] The structure mainly considers the four parts of DRU converter station, DC system, MMC converter station and MMC converter station control system; Figure 2 As shown in Figure 1, the hybrid DC system uses the voltage across PCC1 and PCC2 as input and the current across them as output.

[0096] S2. Based on the disturbance operation data, the stability characteristic value that can reflect the system state and oscillation frequency is calculated according to the DRU-MMC small signal model; wherein, the DRU-MMC small signal model is established based on a group of equations including the linearized state space equations of the DRU converter station, the linearized state space equations of the DC submarine cable, the linearized state space equations of the MMC converter station, and the linearized state space equations of the MMC converter station control system. The group of equations is obtained by linearizing the operating equations of the DRU-MMC hybrid DC transmission system.

[0097] Step S2 of the embodiment of the present application includes S2.1 to S2.13; wherein, S2.1 is the process of establishing the initial state space equation, S2.2 is the process of establishing the linearized state space equation based on the initial state space equation, S2.3 is the process of establishing the linearized state space equation of the DRU converter station based on the linearized state space equation, S2.4 is the process of establishing the linearized state space equation of the DC submarine cable, S2.5 is the process of establishing the linearized state space equation of the MMC converter station, S2.6 is the process of establishing the main controller model, S2.7 is the process of establishing the delay equation group, and S2.8 is the process of establishing the linearized state space equation of the MMC converter station control system based on the main controller model and the delay equation group. S2.9 is the process of establishing the coordinate transformation expression. S2.10 is the process of solving the DRU-MMC small signal model by simultaneously solving the coordinate transformation expression, the linearized state space equation of the DRU converter station, the linearized state space equation of the DC submarine cable, the linearized state space equation of the MMC converter station, and the linearized state space equation of the MMC converter station control system. S2.11 is the process of calculating the stability eigenvalue based on the DRU-MMC small signal model. S2.12 is the process of verifying the step response of the DRU-MMC small signal model. S2.13 is the process of applying the eigenvalue analysis to the DRU-MMC small signal model. Specifically:

[0098] S2.1. Based on the operating data of the DRU-MMC hybrid DC transmission system, establish the voltage and current relationship between the AC and DC sides of the DRU under ideal conditions;

[0099] Taking into account the equivalent inductance of the converter transformer, the voltage-current relationship is adjusted for voltage drop according to the commutation voltage drop formula of the phase-controlled rectifier circuit to obtain the actual DC voltage relationship of the DRU.

[0100] The initial state space equations for the AC voltage and current phase angles are calculated based on the actual DC voltage relationship of the DRU;

[0101] The voltage-current relationship is:

[0102]

[0103] The actual DC voltage relationship of the DRU is:

[0104]

[0105] The initial state space equation is:

[0106]

[0107] Among them, U r and I r are the voltage and current amplitudes on the AC side, and are the voltage phase and current phase on the AC side, is the phase angle difference between the AC side voltage and current, L r is the transformer inductance, U dc1 and U dc2 They are the voltage at the beginning and end of the DC submarine cable, I dc is the DC cable current, R dc is the equivalent resistance of the DC submarine cable, P r and Q r They are the transmitted active power and the transmitted reactive power, U0 is the DC side voltage of DRU without considering the commutation voltage drop, X r is the transformer equivalent inductance, U dc is the DC voltage at the DRU side, ΔU dc is the DRU switching voltage drop.

[0108] To apply this example, please refer to Figure 3 , Figure 3 : is a detailed internal structure diagram of the DRU provided in the embodiment of the present application, showing the detailed internal structure of the DRU converter station; Figure 3 As shown in the figure, to suppress DC-side harmonics, the DRU converter uses a 12-pulse uncontrolled rectifier as its basic commutation unit, consisting of 12 diodes. The DRU converter uses diodes as commutation units and does not include a control system. This means that the unidirectional conductivity of the diodes is used to achieve AC-DC power conversion, and its commutation is completely dependent on the AC-side voltage waveform.

[0109] In this embodiment S2.1, the ideal voltage-current relationship is the basis for system modeling, which reflects the basic relationship between the various system variables. However, in actual systems, due to the influence of various factors, these relationships will be offset to a certain extent. Therefore, by considering the equivalent inductance and adjusting the voltage drop, the characteristics of the actual system can be more accurately reflected, thereby improving the accuracy of the model.

[0110] S2.2. The relationship between the various electrical quantities on the DRU side can be understood from the voltage-current relationship, the DRU actual DC voltage relationship, and the initial state-space equation. Since the DRU connects the AC transmission system and the DC transmission system, the variables on both sides need to be normalized separately. Therefore, based on operating data, the power base values ​​of the AC transmission system and the DC transmission system in the DRU-MMC hybrid DC transmission system are uniformly set to obtain the power formula;

[0111] According to the system AC voltage and DC voltage reference values, set the voltage base values ​​on both sides and make

[0112] Based on the power formula and K value, the relationship between the current and impedance base values ​​on both sides of the AC transmission system and the DC transmission system is established, and the first relationship is obtained;

[0113] Substitute the K value, power formula, and the first relationship into the voltage-current relationship, the DRU actual DC voltage relationship, and the initial state space equation to obtain the normalized DRU characteristic equation.

[0114] The DRU characteristic equation is linearized and expanded to obtain the transition equation;

[0115] According to the DRU characteristic equation and transition equation, the linearized state space equations of the AC voltage and current phase angles under unified scale are calculated;

[0116] The power formula is:

[0117]

[0118] The first relation is:

[0119]

[0120] The DRU characteristic equation is:

[0121]

[0122] The transition equation is:

[0123]

[0124] The linearized state space equation is:

[0125]

[0126] Among them, S acbr and S dcb They are the DRU AC side power reference value and DC side power reference value, U acbr and U dcb They are the DRU AC side voltage reference value and DC side voltage reference value, Iacbr and I dcb They are the DRU AC side current reference value and DC side current reference value, Z acbr and Z dcb are the DRU AC side impedance reference value and DC side impedance reference value respectively, K is the ratio of the DRU DC side to AC side voltage reference value, U dc0 is the DC voltage before the filter inductor, I dc1 and I dc2 They are the DC current at the beginning and end of the cable respectively.

[0127] By uniformly setting the power base value, Example S2.2 ensures consistency in power calculations between AC and DC transmission systems, making power comparisons between the different systems more accurate and intuitive. By establishing a relationship between the current and impedance base values ​​on both sides, the electrical connection between the AC and DC transmission systems can be expressed using a unified mathematical expression, helping to simplify the complexity of the state-space equations.

[0128] S2.3, transforming the input and output quantities of the DRU-MMC hybrid HVDC system into a global unified coordinate system to obtain a unified coordinate equation system;

[0129] The inductor current formula of the DRU-MMC hybrid DC transmission system is linearized to obtain the linearized state space equation of the DC filter inductor.

[0130] The linearized state space equation of the DRU converter station is established based on the linearized state space equation, the unified coordinate equation group and the linearized state space equation of the DC filter inductor;

[0131] Among them, the unified coordinate equations are:

[0132]

[0133] The linearized state space equation of the DC filter inductor is:

[0134]

[0135] Among them, L dcr is the DRU DC side filter inductor, I dc1 is the DRU DC side current; U dc0 is the voltage before the filter, i.e. the DC voltage at the DRU side; U dc1 is the voltage after the filter, that is, the voltage at the beginning of the DC cable.

[0136] It should be noted that for S2.3, since the DRU system is connected to the external system, it involves the input variables and output variables in the DRU-MMC hybrid DC transmission system. Since the voltage and current of the original characteristic equation of the DRU are described in terms of amplitude and phase, in order to facilitate the subsequent establishment of the state space model of the entire system with other modules such as wind farms, it is necessary to convert the input ΔU r 、 and output ΔI r 、 Transform to the global unified coordinate system ΔU sxy1 and ΔI sxy1 .

[0137] Moreover, for S2.2~S2.3, since the DRU converter station does not contain a control module and does not have an inertia link, that is, when a small disturbance occurs in the system, the electrical quantities correspond directly, except for the DC side current I passing through the filter inductor. dc1 In addition, it does not contribute to the system state space variables, so its characteristic equation is mainly used to derive the relationship between the wind farm system and the input and output variables.

[0138] Overall, the present embodiments S2.1 to S2.3 can accurately describe the dynamic relationship between the system variables by establishing the initial state space equations for the AC voltage and current phase angles. The variables on both sides of the AC transmission system and the DC transmission system are normalized and the state space equations are linearized, which greatly simplifies the complexity and analysis difficulty of the system. The input and output quantities are converted to a global unified coordinate system, which realizes the description of the system variables in a unified framework, and helps to reduce the errors and complexity caused by coordinate conversion. The DC filter inductance formula is linearized to obtain the DC filter inductance linearized state space equation, which helps to more accurately describe the dynamic characteristics of the DC side filter inductance.

[0139] S2.4. To facilitate the writing of state-space equations, the DC submarine cable line is replaced by a π-type equivalent circuit. The DC line consists of DC resistance, DC inductance, and DC capacitance. Since long-distance DC transmission cables are used, the effect of capacitance cannot be ignored. Therefore, the initial set of equations for the DC submarine cable is established.

[0140] It can be seen from the initial equations that all variables are linearly related, so the linearized state space equation of the DC submarine cable can be directly transformed according to the initial equations.

[0141] Among them, the initial equations are:

[0142]

[0143] The linearized state space equation of DC submarine cable is:

[0144]

[0145] Among them, L dc 、R dc and C dc They are the inductance, resistance and capacitance of the DC submarine cable respectively.

[0146] S2.5. Due to the complex internal characteristics of the MMC, the MMC characteristic equations can be simplified using a simplified terminal model. This ignores dynamic behaviors such as submodule capacitor voltage fluctuations and internal circulating currents, making the establishment of the state-space model easier. The DC-side electrical quantities are converted to per-unit values ​​using the DC-side base values, and the AC-side electrical quantities are converted to per-unit values ​​using the AC-side base values. This allows the establishment of the first set of equations for the MMC converter station.

[0147] The first set of equations is linearized to obtain the simplified linearized state space equations of the MMC converter station;

[0148] Among them, the first set of equations is:

[0149]

[0150] The linearized state space equation of the MMC converter station is:

[0151]

[0152] Among them, N is the number of MMC submodules, U c is the MMC submodule capacitor voltage, ω c is the AC frequency on the MMC side; C sm is the submodule capacitance, L arm is the inductance of each bridge arm.

[0153] S2.6. In the MMC master controller of the DRU-MMC hybrid DC transmission system, the MMC master controller adopts a dq decoupling control strategy. To achieve connection with the DRU converter station, the d-axis must control the DC voltage to maintain a stable value to ensure the transmission of active power in the DC link. The q-axis can adopt a constant AC voltage or constant reactive power control, and the converter AC voltage reference value is generated through the dq decoupling link.

[0154] Since the main controller includes two outer-loop control modules for the d and q axes and two inner-loop control modules, i.e., four PI links, the outer-loop control expression is established based on the outer-loop control intermediate quantities of the d and q axes and by measuring the attenuation value of the DC submarine cable terminal voltage.

[0155] According to the inner loop control intermediate quantity of the d-axis and q-axis, the inner loop control expression is established by measuring the attenuation value of the AC side current reference value;

[0156] According to the AC grid connection point voltage, establish the phase-locked loop control expression;

[0157] The main controller model is composed of the outer loop control expression, the inner loop control expression and the phase-locked loop control expression;

[0158] Among them, the outer loop control expression is:

[0159]

[0160] The inner loop control expression is:

[0161]

[0162] The phase-locked loop control expression is:

[0163]

[0164] Among them, U dc2ref is the voltage reference value at the end of the DC submarine cable, Q i is the reactive power of the MMC grid connection point, Q iref is the reactive power reference value of the MMC grid connection point, U i is the voltage amplitude of the MMC grid connection point, U iref is the reference value of the voltage amplitude at the MMC grid connection point, U id and U iq They are the d-axis voltage and q-axis voltage of the AC grid connection point, I id and I iq are the d-axis current and q-axis current on the AC side, L i is the equivalent inductance of the MMC AC side, z 10 and z 11 They are the intermediate amount of the outer ring control of the d-axis and the intermediate amount of the outer ring control of the q-axis, 12 and z 13 They are the intermediate amount of the inner loop control of the d-axis and the intermediate amount of the inner loop control of the q-axis, K p10 , K i10 , K p11 and K i11 is the outer loop control PI parameter of the d-axis and q-axis, K p12 , K i12 , K p13 and K i13 is the inner loop control PI parameter of the d-axis and q-axis, I idref and I iqref They are the AC side dq axis current reference values, U cidref and U ciqref They are the d-axis voltage reference value and q-axis voltage reference value of the converter bridge arm, x1 is the intermediate quantity of the phase-locked loop control, ω s is the phase-locked angular velocity, θ s is the phase-locked loop phase angle.

[0165] To apply this example, please refer to Figure 4 , Figure 4 The MMC control system structure diagram provided by the embodiment of the present application shows the structure of the MMC control system; the MMC control system includes two parts: the main controller and the sub-module controller, wherein the main controller outputs the converter AC voltage reference value to the sub-module controller, and the sub-module controller realizes the stable operation of the MMC by controlling the switching of the sub-modules. The specific control block diagram is shown in FIG. Figure 4 As shown in Figure 2, in the small signal model of the control system, the role of the main controller is mainly considered.

[0166] In this embodiment, step S2.6 controls the d-axis via DC voltage, effectively regulating the DC voltage of the MMC converter station and keeping it within a set range, thus helping to maintain the stability of the DC transmission system. Furthermore, by controlling the q-axis with a constant AC voltage or a constant reactive power, the AC voltage and reactive power can be adjusted according to system requirements, improving the system's flexibility and response speed.

[0167] In addition, the establishment of the phase-locked loop control expression enables the system to accurately track the voltage phase of the AC grid connection point, ensuring the synchronous operation of the MMC converter station and the AC grid.

[0168] S2.7. Based on the voltage measurement delay time constant and the current measurement delay time constant, a first delay equation is established by adding a first-order inertia link between the measured value and the actual value of the AC voltage and current at the MMC grid connection point;

[0169] Based on the converter delay time constant, a first delay equation is established by adding a first-order inertia link between the reference value and the actual value of the MMC converter voltage;

[0170] Linearizing the first delay equation and the second delay equation together to obtain a delay equation group;

[0171] Among them, the first delay equation is:

[0172]

[0173] The first delay equation is:

[0174]

[0175] The delay equations are:

[0176]

[0177] Among them, T mu is the voltage measurement delay time constant, T mi is the current measurement delay time constant, T dis the converter delay time constant, U idm and U iqm They are the d-axis voltage and q-axis voltage of the AC grid connection point, I idm and I iqm They are the d-axis current and q-axis current on the AC side, U cid and U ciq are the d-axis voltage and q-axis voltage of the converter bridge arm respectively.

[0178] In this embodiment S2.7, by introducing a voltage measurement delay time constant, a current measurement delay time constant, and a converter delay time constant, and establishing a first-order inertia link based on these delays, the delay phenomenon existing in the actual system can be more accurately described.

[0179] Moreover, in actual control systems, there is often a certain delay due to factors such as signal transmission and device response. By establishing a set of delay equations for the first-order inertia link and linearizing the main controller model based on these equations, these delay factors can be effectively taken into account, making the linearized model closer to the dynamic characteristics of the actual system.

[0180] S2.8. Linearize the main controller model based on the delay equations of the first-order inertia link. Since the DC voltage, AC voltage, and reactive power reference values ​​are all constants, they can be omitted during the linearization process. This results in the linearized state-space equations of the MMC converter station control system. The linearized state-space equations of the MMC converter station control system include the outer-loop control equations, the inner-loop control equations, and the phase-locked loop module equations.

[0181] Among them, the outer loop control equations are:

[0182]

[0183] The inner loop control equations are:

[0184]

[0185] The phase-locked loop module equations are:

[0186]

[0187] S2.9. Since the MMC side relies on the phase-locked loop to track the system phase angle, it is necessary to consider the offset between the phase-locked loop angle and the system phase angle, so the coordinate transformation expression is established accordingly.

[0188] The coordinate transformation expression is:

[0189]

[0190] It should be noted that since the MMC control system is connected to the external system and involves the input and output variables in the DRU-MMC hybrid DC transmission system, in order to establish the state space equation of the entire system, it is necessary to transform the voltage and current on the MMC AC side from the dq coordinate system to the globally unified xy coordinate system.

[0191] S2.10. Establish an initial small-signal model containing 20th-order state variables by using the simultaneous coordinate transformation expression, the linearized state-space equation of the DRU converter station, the linearized state-space equation of the DC submarine cable, the linearized state-space equation of the MMC converter station, and the linearized state-space equation of the MMC converter station control system; wherein, the state variable ΔX of the DRU-MMC hybrid DC transmission system small-signal model is [ΔI dc ,ΔI dc1 ,ΔI dc2 ,ΔU dc1 ,ΔU dc2 ,ΔU c ,ΔU cid ,ΔU ciq ,ΔI id ,ΔI iq ,ΔU gdm ,ΔU gqm ,ΔI idm ,ΔI iqm ,Δz1,Δz2,Δz3,Δz4,Δx1,Δθ]; intermediate variable ΔW=[ΔI dc1 ,ΔQ2,Δω c ,ΔU cidref ,ΔU ciqref ,ΔI idref ,ΔI iqref ]; the input variable is the bus voltage ΔU between PCC1 and PCC2 in the global unified coordinate system sxy1 , ΔU sxy2 The output variable is the bus current ΔI between PCC1 and PCC2 in the global unified coordinate system. sxy1 , ΔI sxy2 ;

[0192] By eliminating the intermediate variables of the initial small signal model, the DRU-MMC small signal model is obtained, namely the DRU-MMC hybrid HVDC system small signal model;

[0193] Among them, the expression of the DRU-MMC small signal model is:

[0194]

[0195] Among them, pX represents the first-order derivative of the state variable X, I sxy1 with I sxy2 are the output currents at both ends of the DRU-MMC hybrid DC transmission system, Uxsy1 with U xsy2 are the input voltages at both ends of the DRU-MMC hybrid HVDC system respectively; A, B1, B2, C1, C2, D11, D12, D21 and D22 represent the pre-variable coefficient matrices.

[0196] S2.11. Based on the disturbance operation data, the stability characteristic value that can reflect the system state and oscillation frequency is calculated according to the DRU-MMC small signal model.

[0197] It should be noted that in Example 1 of the present application, for the convenience of expression, adding 0 to the subscript of the parameter uniform variable represents the steady-state point operating value; and adding △ before the parameter represents the change or increment of the parameter.

[0198] S2.12. After establishing the DRU-MMC small-signal model, you can simulate small disturbance signals by setting input variables and step values ​​in MATLAB, and compare them with the electromagnetic transient simulation results in PSCAD / EMTDC to analyze the correctness of the small-signal model.

[0199] To apply this embodiment, please refer to Table 1, which is a table of main parameters of the DRU-MMC simulation model provided in the embodiment of this application, indicating the parameter values ​​when the DRU-MMC small signal model is simulated.

[0200] Table 1 Main parameters of DRU-MMC simulation model

[0201]

[0202] 1. Comparison of step response when changing DC voltage reference value:

[0203] To apply this example, please refer to Figure 5 、 Figure 6 and Figure 7 , Figure 5 、 Figure 6 and Figure 7 The first, second, and third graphs respectively represent the output quantity changes after the DC voltage reference value is changed, according to an embodiment of the present application, and represent the changes in the output quantity after the DC voltage reference value is changed.

[0204] At t = 0.5s, the DC reference voltage U dcref The voltage is reduced from 500kV to 495kV, that is, a step value of -0.01 is added to the per unit value. At the same time, the same step value is set in the PSCAD / EMTDC electromagnetic transient model. The corresponding results are as follows: Figure 5 、 Figure 6 and Figure 7 shown.

[0205] Depend on Figure 5、 Figure 6 and Figure 7 It can be seen that when changing the DC voltage reference value U dcref Afterwards, the electrical quantities in the PSCAD / EMTDC simulation results are almost consistent with the MATLAB calculation results. Due to the use of new input variables, the step response mainly verifies the correctness of the A matrix in the linearized state space equation.

[0206] 2. Comparison of step response when changing input voltage:

[0207] (1) To apply this embodiment, please refer to Figure 8 、 Figure 9 and Figure 10 , Figure 8 、 Figure 9 and Figure 10 The first, second, and third graphs are respectively diagrams of the output after the AC voltage amplitude on the DRU side is changed, as provided in the embodiment of the present application, showing the change in the output after the AC voltage amplitude on the DRU side is changed;

[0208] At t = 0.5s, the AC voltage amplitude on the DRU side is reduced from 220kV to 217.8kV, that is, a step value of -0.01 is added to the per-unit value. At the same time, the same step is set in the PSCAD / EMTDC electromagnetic transient model. The corresponding results are as follows Figure 8 、 Figure 9 and Figure 10 As shown:

[0209] Depend on Figure 8 、 Figure 9 and Figure 10 It can be seen that after changing the voltage amplitude on the DRU side, the electrical quantities in the PSCAD / EMTDC simulation results are almost consistent with the MATLAB calculation results. Since the DRU side voltage is used as the input variable, the step response mainly verifies the correctness of the B1 matrix in the linearized state-space equation.

[0210] (2) To apply this embodiment, please refer to Figure 11 and Figure 12 , Figure 11 and Figure 12 They are respectively diagrams of output change after changing the AC voltage amplitude on the MMC side provided by the embodiments of the present application, showing the change of output after changing the AC voltage amplitude on the MMC side;

[0211] At t = 0.5s, the AC voltage amplitude on the MMC side is reduced from 220kV to 209kV. At the same time, the same step is set in the PSCAD / EMTDC electromagnetic transient model. The corresponding results are as follows: Figure 11 and Figure 12 As shown:

[0212] Depend on Figure 11and Figure 12 It can be seen that after changing the voltage amplitude on the MMC side, the electrical quantities in the PSCAD / EMTDC simulation results are almost consistent with the MATLAB calculation results. Since the MMC side voltage is used as the input variable, the step response mainly verifies the correctness of the B2 matrix in the linearized state-space equation.

[0213] From the above results, it can be seen that the PSCAD simulation curve in the figure after the step response is almost consistent with the MATLAB calculation curve, that is, the electrical quantities have a good response match under the step response, that is, the system can correctly respond to the dynamic changes of the system operation after small disturbances, verifying the correctness of the DRU-MMC small signal model.

[0214] S2.13. Calculate the eigenvalues ​​of the small signal model A array established in step S2.12. Analyze the calculation results to see if there are any characteristic roots crossing the real axis to determine whether its operating state is stable. At the same time, read its oscillation frequency based on its imaginary part.

[0215] To apply this example, please refer to Figure 13 , Figure 13 1 is an influence diagram of the short circuit ratio on the system stability provided by an embodiment of the present application, showing the influence of the short circuit ratio on the system stability;

[0216] In the small signal model, a two-terminal power supply system is established, and the short circuit ratio (SCR) is set to 8. When the AC power grid strength, i.e., the short circuit ratio (SCR), decreases, the eigenvalue modes 13 and 14 move in the positive direction. When the short circuit ratio is set to 8, the real parts of all the system's characteristic roots are negative; when the short circuit ratio is set to 3, the real parts of modes 13 and 14 become positive, and the system oscillates; when the short circuit ratio is set to 2, the real parts of modes 13 and 14 increase further, and the oscillation frequency decreases slightly. The specific results are as follows: Figure 13 shown.

[0217] To apply this example, please refer to Figure 14 , Figure 14 is a diagram of the system oscillation waveform after the short-circuit ratio is changed according to an embodiment of the present application, showing the oscillation waveform of the system after the short-circuit ratio is changed;

[0218] In the time domain simulation in PSCAD / EMTDC, the short circuit ratio SCR of the power supply at both ends dropped from 8 to 3 and 2 respectively at 7s, and the system oscillated. The results are as follows Figure 8 As shown in the figure, when SCR = 3, the oscillation frequency is around 31.35 Hz, and the corresponding eigenvalue changes from -21.56 ± j40.72 × 2π to 7.08 ± j31.95 × 2π. When SCR = 2, the oscillation frequency is around 26.82 Hz, and the corresponding eigenvalue changes to 20.11 ± j27.53 × 2π. It can be seen that the simulation results are almost consistent with the calculation results.

[0219] It can be seen that the DRU-MMC small signal model established in the present invention can reflect the system operating status by calculating the system eigenvalues, and can be used to judge the system stability; at the same time, due to the input voltage and output current interfaces it provides, it can be further connected to systems such as wind farms to perform eigenvalue analysis of the entire system.

[0220] Overall, this embodiment has the following beneficial effects:

[0221] The present invention obtains the disturbance operation data of the system after the disturbance, and uses the DRU-MMC small signal model to perform calculations based on these data, so that the small signal model can capture the slight changes in the dynamic operation process of the system, and then accurately evaluate the stability and oscillation frequency of the system, thereby obtaining characteristics that can accurately reflect the stability of the system. Among them, the DRU-MMC hybrid DC transmission system mainly needs to consider the four parts of the DRU converter station, the DC system, the MMC converter station, and the MMC converter station control system; because the linearized state space equation of the DRU converter station itself includes the change law of its internal state variables and the interaction with the external system, it can accurately describe the dynamic behavior of the DRU converter station under different working conditions. The linearization equation of the DC submarine cable line can accurately reflect the impedance characteristics of the line such as resistance and inductance. When the system is disturbed, the linearization equation of the submarine cable line can describe its contribution to the dynamic response of the system, which helps to evaluate the stability and recovery capability of the system. The linearized state-space equations for the MMC converter station describe in detail the evolution of the station's internal state over time. By analyzing these state-space equations, the stability of the MMC converter station can be evaluated, including whether it can maintain a stable operating state under various operating conditions. The linearized state-space equations for the MMC converter station control system can accurately describe the dynamic behavior of the MMC converter station control system, including the evolution of system states, input-output relationships, and the effectiveness of control strategies. This provides a reliable theoretical basis for system analysis, design, and optimization.

[0222] Moreover, in the present invention, the input quantities for establishing the linearized equation on the DRU side are the voltage amplitude and phase angle on the AC side, and there is no need to perform dq transformation on its characteristic equation, thereby simplifying the model establishment process; in addition, when establishing the simultaneous equations, the input voltage and output current at both ends of the DRU-MMC hybrid DC transmission system are unified into a global unified coordinate system through coordinate transformation, which is convenient for connecting with systems such as wind farms and is conducive to subsequent small-signal stability analysis of complex systems.

[0223] Example 2:

[0224] See also Figure 15 , an embodiment of the present application provides a stability detection device for a DRU-MMC hybrid DC transmission system, comprising a data module 10 and a detection module 20;

[0225] The data module 10 is used to obtain the disturbance operation data of the DRU-MMC hybrid DC transmission system after the disturbance;

[0226] Based on the detection module 20, a stability characteristic value that can reflect the system state and oscillation frequency is calculated according to the DRU-MMC small signal model; wherein, the DRU-MMC small signal model is established based on a group of equations including the linearized state space equations of the DRU converter station, the linearized state space equations of the DC submarine cable, the linearized state space equations of the MMC converter station, and the linearized state space equations of the MMC converter station control system, and the group of equations is obtained by linearizing the operating equations of the DRU-MMC hybrid DC transmission system.

[0227] In one embodiment, the data module 10 is specifically:

[0228] The data acquisition system is used to monitor and record the disturbance operation data of the DRU-MMC hybrid DC transmission system in real time after the disturbance.

[0229] To apply this example, please refer to Figure 2 , Figure 2 2 is a structural diagram of a DRU-MMC hybrid DC transmission system provided in an embodiment of the present application, showing the structure of the DRU-MMC hybrid DC transmission system in the second embodiment;

[0230] The structure mainly considers the four parts of DRU converter station, DC system, MMC converter station and MMC converter station control system; Figure 2 As shown in Figure 1, the hybrid DC system uses the voltage across PCC1 and PCC2 as input and the current across them as output.

[0231] In one embodiment, the detection module 20 includes an initialization unit, a linearization unit, a first unit, a second unit, a third unit, a model unit, a delay unit, a spatial unit, a transformation unit, a fourth unit, a eigenvalue unit, a verification unit, and an analysis unit;

[0232] Among them, the initial unit is the process of establishing the initial state space equation, the linearization unit is the process of establishing the linear state space equation based on the initial state space equation, the first unit is the process of establishing the linear state space equation of the DRU converter station based on the linear state space equation, the second unit is the process of establishing the linear state space equation of the DC submarine cable, the third unit is the process of establishing the linear state space equation of the MMC converter station, the model unit is the process of establishing the main controller model, the delay unit is the process of establishing the delay equation group, the space unit is the process of establishing the linear state space equation of the MMC converter station control system based on the main controller model and the delay equation group, The conversion unit is the process of establishing a coordinate transformation expression. The fourth unit is the process of solving the DRU-MMC small signal model by simultaneously solving the coordinate transformation expression, the DRU converter station linearized state space equation, the DC submarine cable linearized state space equation, the MMC converter station linearized state space equation, and the MMC converter station control system linearized state space equation. The eigenvalue unit is the process of calculating the stability eigenvalue based on the DRU-MMC small signal model. The verification unit is the process of verifying the step response of the DRU-MMC small signal model. The analysis unit is the process of applying the eigenvalue analysis to the DRU-MMC small signal model, specifically:

[0233] The initialization unit is used to establish the voltage and current relationship between the AC and DC sides of the DRU under ideal conditions based on the operating data of the DRU-MMC hybrid DC transmission system;

[0234] The initial unit is also used to adjust the voltage-current relationship according to the commutation voltage drop formula of the phase-controlled rectifier circuit, taking into account the equivalent inductance of the converter transformer, to obtain the actual DC voltage relationship of the DRU;

[0235] The initial unit is also used to calculate the initial state space equations related to the AC voltage and current phase angle based on the actual DC voltage relationship of the DRU;

[0236] The voltage-current relationship is:

[0237]

[0238] The actual DC voltage relationship of the DRU is:

[0239]

[0240] The initial state space equation is:

[0241]

[0242] Among them, U r and I r are the voltage and current amplitudes on the AC side, and are the voltage phase and current phase on the AC side, is the phase angle difference between the AC side voltage and current, L r is the transformer inductance, U dc1 and U dc2 They are the voltage at the beginning and end of the DC submarine cable, I dc is the DC cable current, R dc is the equivalent resistance of the DC submarine cable, P r and Q r They are the transmitted active power and the transmitted reactive power, U0 is the DC side voltage of DRU without considering the commutation voltage drop, X r is the transformer equivalent inductance, U dc is the DC voltage at the DRU side, ΔU dc is the DRU switching voltage drop.

[0243] To apply this example, please refer to Figure 3 , Figure 3 : is a detailed internal structure diagram of the DRU provided in the embodiment of the present application, showing the detailed internal structure of the DRU converter station; Figure 3 As shown in the figure, to suppress DC-side harmonics, the DRU converter uses a 12-pulse uncontrolled rectifier as its basic commutation unit, consisting of 12 diodes. The DRU converter uses diodes as commutation units and does not include a control system. This means that the unidirectional conductivity of the diodes is used to achieve AC-DC power conversion, and its commutation is completely dependent on the AC-side voltage waveform.

[0244] In the initial unit of this embodiment, the ideal voltage-current relationship is the basis for system modeling, which reflects the basic relationship between the various variables of the system. However, in actual systems, due to the influence of various factors, these relationships will be offset to a certain extent. Therefore, by considering the equivalent inductance and adjusting the voltage drop, the characteristics of the actual system can be more accurately reflected, thereby improving the accuracy of the model.

[0245] The linearization unit is used to determine the relationship between various electrical quantities on the DRU side from the voltage-current relationship, the DRU actual DC voltage relationship, and the initial state-space equation. Since the DRU connects the AC transmission system and the DC transmission system, the variables on both sides need to be normalized separately. Therefore, based on operating data, the power base values ​​of the AC transmission system and the DC transmission system in the DRU-MMC hybrid DC transmission system are uniformly set to obtain the power formula;

[0246] The linearization unit is also used to set the voltage base values ​​on both sides according to the system AC voltage and DC voltage reference values, and to

[0247] The linearization unit is further used to establish a relationship between the current and impedance base values ​​on both sides of the AC transmission system and the DC transmission system based on the power formula and the K value, thereby obtaining a first relationship.

[0248] The linearization unit is also used to substitute the K value, power formula and the first relationship into the voltage-current relationship, the DRU actual DC voltage relationship and the initial state space equation to obtain the normalized DRU characteristic equation.

[0249] The linearization unit is also used to linearize and expand the DRU characteristic equation to obtain the transition equation;

[0250] The linearization unit is also used to calculate the linearized state space equations of the AC voltage and current phase angles under a unified scale based on the DRU characteristic equation and the transition equation;

[0251] The power formula is:

[0252]

[0253] The first relation is:

[0254]

[0255] The DRU characteristic equation is:

[0256]

[0257] The transition equation is:

[0258]

[0259] The linearized state space equation is:

[0260]

[0261] Among them, S acbr and S dcb They are the DRU AC side power reference value and DC side power reference value, U acbr and U dcb They are the DRU AC side voltage reference value and DC side voltage reference value, I acbr and I dcb They are the DRU AC side current reference value and DC side current reference value, Z acbr and Z dcb are the DRU AC side impedance reference value and DC side impedance reference value respectively, K is the ratio of the DRU DC side to AC side voltage reference value, U dc0 is the DC voltage before the filter inductor, I dc1 and I dc2 They are the DC current at the beginning and end of the cable respectively.

[0262] By uniformly setting the power base value, the linearization unit in this embodiment ensures consistency in power calculations between AC and DC transmission systems, making power comparisons between the different systems more accurate and intuitive. By establishing a relationship between the current and impedance base values ​​on both sides, the electrical connection between the AC and DC transmission systems can be expressed using a unified mathematical expression, helping to simplify the complexity of the state-space equations.

[0263] The first unit is used to transform the input and output quantities of the DRU-MMC hybrid DC transmission system into a global unified coordinate system to obtain a unified coordinate equation system;

[0264] The first unit is also used to linearize the inductor current formula of the DRU-MMC hybrid DC transmission system to obtain the linearized state space equation of the DC filter inductor;

[0265] The first unit is also used to establish the linearized state space equation of the DRU converter station based on the linearized state space equation, the unified coordinate equation group and the DC filter inductor linearized state space equation;

[0266] Among them, the unified coordinate equations are:

[0267]

[0268] The linearized state space equation of the DC filter inductor is:

[0269]

[0270] Among them, L dcr is the DRU DC side filter inductor, I dc1 is the DRU DC side current; U dc0 is the voltage before the filter, i.e. the DC voltage at the DRU side; U dc1 is the voltage after the filter, that is, the voltage at the beginning of the DC cable.

[0271] It should be noted that for the first unit, since the DRU system is connected to the external system, it involves the input variables and output variables in the DRU-MMC hybrid DC transmission system. Since the voltage and current of the original characteristic equation of the DRU are described in terms of amplitude and phase, in order to facilitate the subsequent establishment of the state space model of the entire system with other modules such as wind farms, it is necessary to convert the input ΔU r 、 and output ΔI r 、 Transform to the global unified coordinate system ΔU sxy1 and ΔI sxy1 .

[0272] Moreover, for the linearization unit and the first unit, since the DRU converter station does not contain a control module and does not have an inertia link, that is, when a small disturbance occurs in the system, the electrical quantities correspond directly, except for the DC side current I passing through the filter inductor. dc1 In addition, it does not contribute to the system state space variables, so its characteristic equation is mainly used to derive the relationship between the wind farm system and the input and output variables.

[0273] Overall, the initial unit, linearization unit, and first unit of this embodiment can accurately describe the dynamic relationship between the various system variables by establishing the initial state-space equations related to the AC voltage and current phase angle. The variables on both sides of the AC transmission system and the DC transmission system are normalized and the state-space equations are linearized, which greatly simplifies the complexity and analysis difficulty of the system. The input and output quantities are converted to a global unified coordinate system, which realizes the description of the various system variables in a unified framework, helping to reduce the errors and complexity caused by coordinate conversion. The DC filter inductance formula is linearized to obtain the DC filter inductance linearized state-space equation, which helps to more accurately describe the dynamic characteristics of the DC side filter inductance.

[0274] The second unit is used to replace the DC submarine cable line with a π-type equivalent circuit to facilitate the writing of state-space equations. The DC line includes DC resistance, DC inductance, and DC capacitance. Since long-distance DC transmission cables are used, the effect of capacitance cannot be ignored, so the initial set of equations for the DC submarine cable is established.

[0275] The second unit is also used to know that the variables are all linearly related according to the initial equations, so the linearized state space equation of the DC submarine cable can be directly transformed according to the initial equations;

[0276] Among them, the initial equations are:

[0277]

[0278] The linearized state space equation of DC submarine cable is:

[0279]

[0280] Among them, L dc 、R dc and C dc They are the inductance, resistance and capacitance of the DC submarine cable respectively.

[0281] The third unit is used to simplify the description of the MMC characteristic equations using a simplified terminal model due to the complex internal characteristics of the MMC. This model ignores dynamic behaviors such as submodule capacitor voltage fluctuations and internal circulating currents, making the establishment of the state-space model easier. DC-side electrical quantities are converted to per-unit values ​​using DC-side base values, and AC-side electrical quantities are converted to per-unit values ​​using AC-side base values. This allows the establishment of the first set of equations for the MMC converter station.

[0282] The third unit is also used to linearize the first set of equations to obtain a simplified linearized state space equation of the MMC converter station;

[0283] Among them, the first set of equations is:

[0284]

[0285] The linearized state space equation of the MMC converter station is:

[0286]

[0287] Among them, N is the number of MMC submodules, U c is the MMC submodule capacitor voltage, ω c is the AC frequency on the MMC side; C sm is the submodule capacitance, L arm is the inductance of each bridge arm.

[0288] The model unit is used in the MMC main controller of the DRU-MMC hybrid DC transmission system. The MMC main controller adopts a dq decoupling control strategy. To achieve connection with the DRU converter station, the d-axis must control the DC voltage to maintain a stable value to ensure the transmission of active power in the DC link. The q-axis can adopt a constant AC voltage or constant reactive power control and generate the converter AC voltage reference value through the dq decoupling link.

[0289] The model unit is also used to establish an outer loop control expression based on the outer loop control intermediate quantities of the d-axis and q-axis by measuring the attenuation value of the DC submarine cable terminal voltage. Since the main controller includes two outer loop control modules for the d and q axes and two inner loop control modules, that is, it includes four PI links;

[0290] The model unit is also used to establish an inner loop control expression based on the inner loop control intermediate quantity of the d-axis and the q-axis by measuring the attenuation value of the AC side current reference value;

[0291] The model unit is also used to establish a phase-locked loop control expression based on the AC grid connection point voltage;

[0292] The model unit is further used to form a main controller model by an outer loop control expression, an inner loop control expression and a phase-locked loop control expression;

[0293] Among them, the outer loop control expression is:

[0294]

[0295]

[0296] The inner loop control expression is:

[0297]

[0298] The phase-locked loop control expression is:

[0299]

[0300] Among them, U dc2ref is the voltage reference value at the end of the DC submarine cable, Q i is the reactive power of the MMC grid connection point, Q iref is the reactive power reference value of the MMC grid connection point, U i is the voltage amplitude of the MMC grid connection point, U iref is the reference value of the voltage amplitude at the MMC grid connection point, U id and U iq They are the d-axis voltage and q-axis voltage of the AC grid connection point, I id and I iq are the d-axis current and q-axis current on the AC side, L i is the equivalent inductance of the MMC AC side, z 10 and z 11 They are the intermediate amount of the outer ring control of the d-axis and the intermediate amount of the outer ring control of the q-axis, 12 and z 13 They are the intermediate amount of the inner loop control of the d-axis and the intermediate amount of the inner loop control of the q-axis, K p10 , K i10 , K p11 and K i11 is the outer loop control PI parameter of the d-axis and q-axis, K p12 , K i12 , K p13 and K i13 is the inner loop control PI parameter of the d-axis and q-axis, I idref and I iqref They are the AC side dq axis current reference values, U cidref and U ciqref They are the d-axis voltage reference value and q-axis voltage reference value of the converter bridge arm, x1 is the intermediate quantity of the phase-locked loop control, ω s is the phase-locked angular velocity, θ s is the phase-locked loop phase angle.

[0301] To apply this example, please refer to Figure 4 , Figure 4 The MMC control system structure diagram provided by the embodiment of the present application shows the structure of the MMC control system; the MMC control system includes two parts: the main controller and the sub-module controller, wherein the main controller outputs the converter AC voltage reference value to the sub-module controller, and the sub-module controller realizes the stable operation of the MMC by controlling the switching of the sub-modules. The specific control block diagram is shown in FIG. Figure 4 As shown in Figure 2, in the small signal model of the control system, the role of the main controller is mainly considered.

[0302] The model unit in this embodiment controls the d-axis through DC voltage, effectively adjusting the DC voltage of the MMC converter station and keeping it within a set range, helping to maintain the stability of the DC transmission system. Furthermore, by controlling the q-axis with a constant AC voltage or a constant reactive power, the AC side voltage and reactive power can be adjusted according to system requirements, improving the system's flexibility and response speed.

[0303] In addition, the establishment of the phase-locked loop control expression enables the system to accurately track the voltage phase of the AC grid connection point, ensuring the synchronous operation of the MMC converter station and the AC grid.

[0304] A delay unit is used to establish a first delay equation by adding a first-order inertia link between the measured value and the actual value of the AC voltage and current at the MMC grid connection point based on the voltage measurement delay time constant and the current measurement delay time constant;

[0305] A delay unit is configured to establish a first delay equation by adding a first-order inertia link between a reference value and an actual value of the MMC converter voltage based on a converter delay time constant;

[0306] A delay unit is used to linearize the first delay equation and the second delay equation to obtain a delay equation group;

[0307] Among them, the first delay equation is:

[0308]

[0309] The first delay equation is:

[0310]

[0311] The delay equations are:

[0312]

[0313] Among them, T mu is the voltage measurement delay time constant, T mi is the current measurement delay time constant, T d is the converter delay time constant, U idm and Uiqm They are the d-axis voltage and q-axis voltage of the AC grid connection point, I idm and I iqm They are the d-axis current and q-axis current on the AC side, U cid and U ciq are the d-axis voltage and q-axis voltage of the converter bridge arm respectively.

[0314] The delay unit of this embodiment introduces a voltage measurement delay time constant, a current measurement delay time constant, and a converter delay time constant, and establishes a first-order inertia link based on these delays, which can more accurately describe the delay phenomenon existing in the actual system.

[0315] Moreover, in actual control systems, there is often a certain delay due to factors such as signal transmission and device response. By establishing a set of delay equations for the first-order inertia link and linearizing the main controller model based on these equations, these delay factors can be effectively taken into account, making the linearized model closer to the dynamic characteristics of the actual system.

[0316] The spatial unit is used to linearize the main controller model based on the delay equations of the first-order inertia link. Since the DC voltage, AC voltage, and reactive power reference values ​​are all fixed values, they can be directly omitted during the linearization process, thereby obtaining the linearized state space equations of the MMC converter station control system. The linearized state space equations of the MMC converter station control system include the outer loop control equations, the inner loop control equations, and the phase-locked loop module equations.

[0317] Among them, the outer loop control equations are:

[0318]

[0319]

[0320] The inner loop control equations are:

[0321]

[0322] The phase-locked loop module equations are:

[0323]

[0324] The transformation unit is used to establish a coordinate transformation expression because the MMC side relies on the phase-locked loop to track the system phase angle, and the offset between the phase-locked loop angle and the system phase angle needs to be considered;

[0325] The coordinate transformation expression is:

[0326]

[0327] It should be noted that since the MMC control system is connected to the external system and involves the input and output variables in the DRU-MMC hybrid DC transmission system, in order to establish the state space equation of the entire system, it is necessary to transform the voltage and current on the MMC AC side from the dq coordinate system to the globally unified xy coordinate system.

[0328] The fourth unit is used to establish an initial small signal model containing 20th-order state variables by using the simultaneous coordinate transformation expression, the linearized state space equation of the DRU converter station, the linearized state space equation of the DC submarine cable, the linearized state space equation of the MMC converter station, and the linearized state space equation of the MMC converter station control system; wherein the state variable ΔX of the DRU-MMC hybrid DC transmission system small signal model is [ΔI dc ,ΔI dc1 ,ΔI dc2 ,ΔU dc1 ,ΔU dc2 ,ΔU c ,ΔU cid ,ΔU ciq ,ΔI id ,ΔI iq ,ΔU gdm ,ΔU gqm ,ΔI idm ,ΔI iqm ,Δz1,Δz2,Δz3,Δz4,Δx1,Δθ]; intermediate variable ΔW=[ΔI dc1 ,ΔQ2,Δω c ,ΔU cidref ,ΔU ciqref ,ΔI idref ,ΔI iqref ]; the input variable is the bus voltage ΔU between PCC1 and PCC2 in the global unified coordinate system sxy1 , ΔU sxy2 The output variable is the bus current ΔI between PCC1 and PCC2 in the global unified coordinate system. sxy1 , ΔI sxy2 ;

[0329] The fourth unit is also used to obtain a DRU-MMC small signal model, i.e., a DRU-MMC hybrid HVDC system small signal model, by eliminating intermediate variables of the initial small signal model;

[0330] Among them, the expression of the DRU-MMC small signal model is:

[0331]

[0332] Among them, pX represents the first-order derivative of the state variable X, I sxy1 with I sxy2are the output currents at both ends of the DRU-MMC hybrid DC transmission system, U xsy1 with U xsy2 are the input voltages at both ends of the DRU-MMC hybrid HVDC system respectively; A, B1, B2, C1, C2, D11, D12, D21 and D22 represent the pre-variable coefficient matrices.

[0333] The eigenvalue unit is used to calculate the stability eigenvalue that can reflect the system state and oscillation frequency based on the disturbance operation data and the DRU-MMC small signal model.

[0334] It should be noted that in Example 2 of the present application, for the convenience of expression, 0 is added to the subscript of the parameter uniform variable to indicate the steady-state point operating value; and △ is added before the parameter to indicate the change or increment of the parameter.

[0335] The verification unit is used to simulate small disturbance signals by setting input variables and step values ​​in MATLAB after the DRU-MMC small signal model is established, and compare the results with the electromagnetic transient simulation results in PSCAD / EMTDC to analyze the correctness of the small signal model.

[0336] The verification unit is also used to apply this embodiment. Please refer to Table 1. Table 1 is a main parameter table of the DRU-MMC simulation model provided in the embodiment of this application, which represents the parameter values ​​when the DRU-MMC small signal model is simulated.

[0337] Table 1 Main parameters of DRU-MMC simulation model

[0338]

[0339]

[0340] 1. Comparison of step response when changing DC voltage reference value:

[0341] To apply this example, please refer to Figure 5 、 Figure 6 and Figure 7 , Figure 5 、 Figure 6 and Figure 7 The first, second, and third graphs respectively represent the output quantity changes after the DC voltage reference value is changed, according to an embodiment of the present application, and represent the changes in the output quantity after the DC voltage reference value is changed.

[0342] At t = 0.5s, the DC reference voltage U dcref The voltage is reduced from 500kV to 495kV, that is, a step value of -0.01 is added to the per unit value. At the same time, the same step value is set in the PSCAD / EMTDC electromagnetic transient model. The corresponding results are as follows: Figure 5 、 Figure 6 and Figure 7 shown.

[0343] Depend on Figure 5 、 Figure 6 and Figure 7 It can be seen that when changing the DC voltage reference value U dcref Afterwards, the electrical quantities in the PSCAD / EMTDC simulation results are almost consistent with the MATLAB calculation results. Due to the use of new input variables, the step response mainly verifies the correctness of the A matrix in the linearized state space equation.

[0344] 2. Comparison of step response when changing input voltage:

[0345] (1) To apply this embodiment, please refer to Figure 8 、 Figure 9 and Figure 10 , Figure 8 、 Figure 9 and Figure 10 The first, second, and third graphs are respectively diagrams of the output after the AC voltage amplitude on the DRU side is changed, as provided in the embodiment of the present application, showing the change in the output after the AC voltage amplitude on the DRU side is changed;

[0346] At t = 0.5s, the AC voltage amplitude on the DRU side is reduced from 220kV to 217.8kV, that is, a step value of -0.01 is added to the per-unit value. At the same time, the same step is set in the PSCAD / EMTDC electromagnetic transient model. The corresponding results are as follows Figure 8 、 Figure 9 and Figure 10 As shown:

[0347] Depend on Figure 8 、 Figure 9 and Figure 10 It can be seen that after changing the voltage amplitude on the DRU side, the electrical quantities in the PSCAD / EMTDC simulation results are almost consistent with the MATLAB calculation results. Since the DRU side voltage is used as the input variable, the step response mainly verifies the correctness of the B1 matrix in the linearized state-space equation.

[0348] (2) To apply this embodiment, please refer to Figure 11 and Figure 12 , Figure 11 and Figure 12 The first and second graphs are respectively a change graph of the output after the AC voltage amplitude on the MMC side is changed according to an embodiment of the present application, and represent the change of the output after the AC voltage amplitude on the MMC side is changed;

[0349] At t = 0.5s, the AC voltage amplitude on the MMC side is reduced from 220kV to 209kV. At the same time, the same step is set in the PSCAD / EMTDC electromagnetic transient model. The corresponding results are as follows: Figure 11 and Figure 12 As shown:

[0350] Depend on Figure 11 and Figure 12 It can be seen that after changing the voltage amplitude on the MMC side, the electrical quantities in the PSCAD / EMTDC simulation results are almost consistent with the MATLAB calculation results. Since the MMC side voltage is used as the input variable, the step response mainly verifies the correctness of the B2 matrix in the linearized state-space equation.

[0351] From the above results, it can be seen that the PSCAD simulation curve in the figure after the step response is almost consistent with the MATLAB calculation curve, that is, the electrical quantities have a good response match under the step response, that is, the system can correctly respond to the dynamic changes of the system operation after small disturbances, verifying the correctness of the DRU-MMC small signal model.

[0352] The analysis unit is used to calculate the eigenvalues ​​of the small signal model A array established by the verification unit. It can be used to analyze whether there are characteristic roots crossing the real axis based on the calculation results to determine whether its operating state is stable, and at the same time read its oscillation frequency based on its imaginary part.

[0353] The analysis unit is also used to apply this embodiment, see Figure 13 , Figure 13 1 is an influence diagram of the short circuit ratio on the system stability provided by an embodiment of the present application, showing the influence of the short circuit ratio on the system stability;

[0354] The analysis unit is also used to establish a two-terminal power supply system in the small signal model, setting the short circuit ratio (SCR) to 8. When the AC power grid strength, i.e., the short circuit ratio (SCR), decreases, the eigenvalue modes 13 and 14 move in the positive direction. When the short circuit ratio is set to 8, the real parts of all the system's characteristic roots are negative; when the short circuit ratio is set to 3, the real parts of modes 13 and 14 become positive, and the system oscillates; when the short circuit ratio is set to 2, the real parts of modes 13 and 14 increase further, and the oscillation frequency decreases slightly. The specific results are as follows: Figure 13 shown.

[0355] The analysis unit is also used to apply this embodiment, see Figure 14 , Figure 14 is a diagram of the system oscillation waveform after the short-circuit ratio is changed according to an embodiment of the present application, showing the oscillation waveform of the system after the short-circuit ratio is changed;

[0356] The analysis unit is also used for time domain simulation in PSCAD / EMTDC. At 7s, the short-circuit ratio SCR of the power supply at both ends dropped from 8 to 3 and 2 respectively, and the system oscillated. The results are shown in the figure. Figure 8 As shown in the figure, when SCR = 3, the oscillation frequency is around 31.35 Hz, and the corresponding eigenvalue changes from -21.56 ± j40.72 × 2π to 7.08 ± j31.95 × 2π. When SCR = 2, the oscillation frequency is around 26.82 Hz, and the corresponding eigenvalue changes to 20.11 ± j27.53 × 2π. It can be seen that the simulation results are almost consistent with the calculation results.

[0357] It can be seen that the DRU-MMC small signal model established in the present invention can reflect the system operating status by calculating the system eigenvalues, and can be used to judge the system stability; at the same time, due to the input voltage and output current interfaces it provides, it can be further connected to systems such as wind farms to perform eigenvalue analysis of the entire system.

[0358] Overall, the second embodiment has the following beneficial effects:

[0359] The present invention obtains the disturbance operation data of the system after the disturbance, and uses the DRU-MMC small signal model to perform calculations based on these data, so that the small signal model can capture the slight changes in the dynamic operation process of the system, and then accurately evaluate the stability and oscillation frequency of the system, thereby obtaining characteristics that can accurately reflect the stability of the system. Among them, the DRU-MMC hybrid DC transmission system mainly needs to consider the four parts of the DRU converter station, the DC system, the MMC converter station, and the MMC converter station control system; because the linearized state space equation of the DRU converter station itself includes the change law of its internal state variables and the interaction with the external system, it can accurately describe the dynamic behavior of the DRU converter station under different working conditions. The linearization equation of the DC submarine cable line can accurately reflect the impedance characteristics of the line such as resistance and inductance. When the system is disturbed, the linearization equation of the submarine cable line can describe its contribution to the dynamic response of the system, which helps to evaluate the stability and recovery capability of the system. The linearized state-space equations for the MMC converter station describe in detail the evolution of the station's internal state over time. By analyzing these state-space equations, the stability of the MMC converter station can be evaluated, including whether it can maintain a stable operating state under various operating conditions. The linearized state-space equations for the MMC converter station control system can accurately describe the dynamic behavior of the MMC converter station control system, including the evolution of system states, input-output relationships, and the effectiveness of control strategies. This provides a reliable theoretical basis for system analysis, design, and optimization.

[0360] Moreover, in the present invention, the input quantities for establishing the linearized equation on the DRU side are the voltage amplitude and phase angle on the AC side, and there is no need to perform dq transformation on its characteristic equation, thereby simplifying the model establishment process; in addition, when establishing the simultaneous equations, the input voltage and output current at both ends of the DRU-MMC hybrid DC transmission system are unified into a global unified coordinate system through coordinate transformation, which is convenient for connecting with systems such as wind farms and is conducive to subsequent small-signal stability analysis of complex systems.

[0361] Example 3:

[0362] An embodiment of the present application provides a computer-readable storage medium, wherein the computer-readable storage medium includes a stored computer program, wherein when the computer program is executed, the device containing the computer-readable storage medium is controlled to execute the stability detection method of a DRU-MMC hybrid direct current transmission system;

[0363] Wherein, if the stability detection method of the DRU-MMC hybrid DC transmission system is implemented in the form of a software functional unit and used as an independent product, it can be stored in a computer-readable storage medium. Based on this understanding, the present invention implements all or part of the processes in the above-mentioned embodiment method, and can also be completed by instructing the relevant hardware through a computer program. The computer program can be stored in a computer-readable storage medium. When the computer program is executed by the processor, it can implement the steps of the above-mentioned various method embodiments. Wherein, the computer program includes computer program code, and the computer program code can be in source code form, object code form, executable file or some intermediate form. The computer-readable medium may include: any entity or device that can carry the computer program code, recording medium, U disk, mobile hard disk, magnetic disk, optical disk, computer memory, read-only memory (ROM, Read-Only Memory), random access memory (RAM, Random Access Memory), electrical carrier signal, telecommunication signal and software distribution medium, etc.

[0364] The above is a preferred embodiment of the present invention. It should be pointed out that for ordinary technicians in this technical field, several improvements and modifications can be made without departing from the principles of the present invention. These improvements and modifications are also considered to be within the scope of protection of the present invention.

Claims

1. A stability detection method for a DRU-MMC hybrid DC transmission system, characterized in that: include: Obtain the disturbance operation data of the DRU-MMC hybrid HVDC system after the disturbance; Based on the disturbance operation data, a stability characteristic value that can reflect the system state and oscillation frequency is calculated according to the DRU-MMC small signal model; wherein the DRU-MMC small signal model is established according to a set of equations including a linearized state space equation of a DRU converter station, a linearized state space equation of a DC submarine cable, a linearized state space equation of an MMC converter station, and a linearized state space equation of an MMC converter station control system, and the set of equations is obtained by linearizing the operating equations of the DRU-MMC hybrid DC transmission system; The linearized state space equation of the DRU converter station is specifically: Based on the operating data of the DRU-MMC hybrid DC transmission system, initial state-space equations for the AC voltage and current phase angle are established; based on the initial state-space equations, the variables on both sides of the AC transmission system and the DC transmission system in the DRU-MMC hybrid DC transmission system are normalized to obtain linearized state-space equations for the AC voltage and current phase angle; the input and output quantities of the DRU-MMC hybrid DC transmission system are converted to a global unified coordinate system to obtain a unified coordinate equation system; the inductor current formula of the DRU-MMC hybrid DC transmission system is linearized to obtain a linearized state-space equation for the DC filter inductor; and the linearized state-space equation for the DRU converter station is established based on the linearized state-space equation, the unified coordinate equation system, and the linearized state-space equation for the DC filter inductor. The linearized state space equation of the MMC converter station control system is specifically: In the MMC main controller of the DRU-MMC hybrid DC transmission system, the d-axis is controlled by a DC voltage, and the q-axis is controlled by a constant AC voltage or a constant reactive power; a main controller model is established according to operating data of the DRU-MMC hybrid DC transmission system, and a delay equation group of a first-order inertia link is established according to the operating data; based on the delay equation group of the first-order inertia link, the main controller model is linearized to obtain the linearized state space equation of the MMC converter station control system.

2. The stability detection method of a DRU-MMC hybrid DC power transmission system according to claim 1, characterized in that: Based on the initial state-space equation, the linearized state-space equations for the AC voltage and current phase angles are obtained by normalizing the variables on both sides of the AC transmission system and the DC transmission system in the DRU-MMC hybrid DC transmission system. Specifically, The power base values ​​of the AC transmission system and the DC transmission system in the DRU-MMC hybrid DC transmission system are uniformly set to obtain a power formula; Establishing a relationship between current and impedance base values ​​on both sides of the AC power transmission system and the DC power transmission system based on the operating data to obtain a first relationship; Substituting the power formula and the first relationship into the initial state space equation for linear expansion calculation, the linearized state space equation of the AC voltage and current phase angle under a unified scale is obtained.

3. The stability detection method of a DRU-MMC hybrid DC power transmission system according to claim 1, characterized in that: Based on the operating data of the DRU-MMC hybrid DC transmission system, the initial state space equations for the AC voltage and current phase angle are established, specifically: Based on the operating data of the DRU-MMC hybrid DC transmission system, establish the voltage and current relationship between the AC and DC sides of the DRU under ideal conditions; Taking into account the equivalent inductance, the voltage-current relationship is adjusted for voltage drop to obtain the initial state space equation related to the AC voltage and current phase angle.

4. The stability detection method of a DRU-MMC hybrid DC power transmission system according to claim 1, characterized in that: A main controller model is established based on the operating data of the DRU-MMC hybrid DC transmission system, specifically: An outer loop control expression is established based on the outer loop control intermediate quantities of the d-axis and the q-axis by measuring the attenuation value of the voltage at the end of the DC submarine cable; An inner loop control expression is established by measuring the attenuation value of the AC side current reference value according to the inner loop control intermediate quantity of the d-axis and the q-axis; According to the AC grid connection point voltage, establish the phase-locked loop control expression; The main controller model is composed of the outer loop control expression, the inner loop control expression and the phase-locked loop control expression.

5. The stability detection method of a DRU-MMC hybrid DC power transmission system according to claim 1, characterized in that: The delay equations of the first-order inertia link are established based on the operating data, specifically: Based on the voltage measurement delay time constant and the current measurement delay time constant, a first delay equation is established by adding a first-order inertia link between the measured value and the actual value of the AC voltage and current at the MMC grid connection point; Based on the converter delay time constant, a second delay equation is established by adding a first-order inertia link between the reference value and the actual value of the MMC converter voltage. The first delay equation and the second delay equation are linearized together to obtain the delay equation group.

6. A stability detection device for a DRU-MMC hybrid DC transmission system, characterized in that: Including data module and detection module; The data module is used to obtain the disturbance operation data of the DRU-MMC hybrid direct current transmission system after the disturbance; Based on the detection data, a stability characteristic value that can reflect the system state and oscillation frequency is calculated according to the DRU-MMC small signal model; wherein the DRU-MMC small signal model is established according to a system of equations including a linearized state-space equation of a DRU converter station, a linearized state-space equation of a DC submarine cable, a linearized state-space equation of an MMC converter station, and a linearized state-space equation of an MMC converter station control system, and the system of equations is obtained by linearizing the operating equations of the DRU-MMC hybrid DC transmission system; The linearized state space equation of the DRU converter station is specifically: Based on the operating data of the DRU-MMC hybrid DC transmission system, initial state-space equations for the AC voltage and current phase angle are established; based on the initial state-space equations, the variables on both sides of the AC transmission system and the DC transmission system in the DRU-MMC hybrid DC transmission system are normalized to obtain linearized state-space equations for the AC voltage and current phase angle; the input and output quantities of the DRU-MMC hybrid DC transmission system are converted to a global unified coordinate system to obtain a unified coordinate equation system; the inductor current formula of the DRU-MMC hybrid DC transmission system is linearized to obtain a linearized state-space equation for the DC filter inductor; and the linearized state-space equation for the DRU converter station is established based on the linearized state-space equation, the unified coordinate equation system, and the linearized state-space equation for the DC filter inductor. The linearized state space equation of the MMC converter station control system is specifically: In the MMC main controller of the DRU-MMC hybrid DC transmission system, the d-axis is controlled by a DC voltage, and the q-axis is controlled by a constant AC voltage or a constant reactive power; a main controller model is established according to operating data of the DRU-MMC hybrid DC transmission system, and a delay equation group of a first-order inertia link is established according to the operating data; based on the delay equation group of the first-order inertia link, the main controller model is linearized to obtain the linearized state space equation of the MMC converter station control system.

7. The stability detection device of a DRU-MMC hybrid DC power transmission system according to claim 6, characterized in that: Based on the initial state-space equation, the linearized state-space equations for the AC voltage and current phase angles are obtained by normalizing the variables on both sides of the AC transmission system and the DC transmission system in the DRU-MMC hybrid DC transmission system. Specifically, The power base values ​​of the AC transmission system and the DC transmission system in the DRU-MMC hybrid DC transmission system are uniformly set to obtain a power formula; Establishing a relationship between current and impedance base values ​​on both sides of the AC power transmission system and the DC power transmission system based on the operating data to obtain a first relationship; Substituting the power formula and the first relationship into the initial state space equation for linear expansion calculation, the linearized state space equation of the AC voltage and current phase angle under a unified scale is obtained.

8. The stability detection device of a DRU-MMC hybrid DC power transmission system according to claim 6, characterized in that: Based on the operating data of the DRU-MMC hybrid DC transmission system, the initial state space equations for the AC voltage and current phase angle are established, specifically: Based on the operating data of the DRU-MMC hybrid DC transmission system, establish the voltage and current relationship between the AC and DC sides of the DRU under ideal conditions; Taking into account the equivalent inductance, the voltage-current relationship is adjusted for voltage drop to obtain the initial state space equation related to the AC voltage and current phase angle.

9. The stability detection device of a DRU-MMC hybrid DC power transmission system according to claim 6, characterized in that: A main controller model is established based on the operating data of the DRU-MMC hybrid DC transmission system, specifically: An outer loop control expression is established based on the outer loop control intermediate quantities of the d-axis and the q-axis by measuring the attenuation value of the voltage at the end of the DC submarine cable; An inner loop control expression is established by measuring the attenuation value of the AC side current reference value according to the inner loop control intermediate quantity of the d-axis and the q-axis; According to the AC grid connection point voltage, establish the phase-locked loop control expression; The main controller model is composed of the outer loop control expression, the inner loop control expression and the phase-locked loop control expression.

10. The stability detection device of a DRU-MMC hybrid DC power transmission system according to claim 6, characterized in that: The delay equations of the first-order inertia link are established based on the operating data, specifically: Based on the voltage measurement delay time constant and the current measurement delay time constant, a first delay equation is established by adding a first-order inertia link between the measured value and the actual value of the AC voltage and current at the MMC grid connection point; Based on the converter delay time constant, a second delay equation is established by adding a first-order inertia link between the reference value and the actual value of the MMC converter voltage. The first delay equation and the second delay equation are linearized together to obtain the delay equation group.

11. A storage medium, characterized in that: The storage medium stores a computer program, which is called and executed by a computer to implement the stability detection method of a DRU-MMC hybrid direct current transmission system as described in any one of claims 1 to 5.

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