Stability detection method and device for DRU-MMC hybrid DC power transmission system, and medium

Abstract

The invention discloses a stability detection method and device for a DRU-MMC hybrid direct current power transmission system and a medium, and the method comprises the steps: carrying out the calculation based on the disturbance operation data of the DRU-MMC hybrid direct current power transmission system after disturbance according to a DRU-MMC small signal model, and obtaining a stability characteristic value capable of reflecting the state and oscillation frequency of the system; wherein the DRU-MMC small signal model is established according to equations such as a DRU converter station linearization state-space equation and a direct current submarine cable linearization state-space equation. According to the stability detection method and device for the DRU-MMC hybrid direct-current power transmission system and the medium provided by the invention, the disturbance data of the DRU-MMC hybrid direct-current power transmission system is calculated by using the preset DRU-MMC small-signal model, the stability characteristic value can be directly and effectively obtained, and the stability of the DRU-MMC hybrid direct-current power transmission system is improved. The related linearization processing of the DRU-MMC small signal model enables the original possible complex nonlinear relationship to be simplified near a specific working point, and can solve the problem that the stability of the DRU-MMC hybrid DC power transmission system is difficult to effectively detect.

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 prospects for development, and 70% of the world's potential offshore wind resources are located in deep waters with a depth of more than 60 meters. With the innovation of wind power technology, offshore wind power projects are gradually developing towards deep seas with richer wind energy resources; however, compared with offshore 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-connected methods of offshore wind power. The existing offshore wind power is mainly connected to the grid using a flexible DC system, in which the modular multilevel converter (MMC) 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, and has low construction costs and huge economic advantages, and has received widespread attention in recent years; therefore, the characteristics of the two can be combined to construct an offshore wind power uncontrolled rectifier DC transmission system including offshore grid-type wind farms-offshore DRU converter stations-DC submarine cables-onshore MMC converter stations-onshore AC systems, so as to achieve 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 internal control strategies and startup methods of wind farms, and there is a lack of in-depth research on the small-signal stability of the system. At the same time, in terms of the stability of DC transmission systems, existing technologies mainly focus on the small-signal model research of double-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, so it is 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 DC transmission system after the disturbance;

[0006] 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 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 operation 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 calculate based on these data, so that the small signal model can capture the slight changes in the dynamic operation of the system, and then accurately evaluate the stability and oscillation frequency of the system, so as to obtain the 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 is helpful to evaluate the stability and recovery ability of the system. The linearized state-space equations of the MMC converter station describe in detail the evolution of the internal state of the MMC converter station over time. Through the analysis of the 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 of the MMC converter station control system can accurately describe the dynamic behavior of the MMC converter station control system, including the evolution of the system state, the input-output relationship, and the effect of the control strategy, which provides a reliable theoretical basis for the analysis, design, and optimization of the system.

[0008] Compared with the prior art, 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 possibly 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 the operating data of the DRU-MMC hybrid DC transmission system;

[0011] Based on the initial state space equation, the linearized state space equations of the AC voltage and current phase angle 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 of the DRU-MMC hybrid direct current transmission system into a global unified coordinate system to obtain a unified coordinate equation system;

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

[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 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 to achieve the description of the system variables in a unified framework, which 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.

[0016] As a preferred solution, based on the initial state space equation, the linearized state space equations of 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, which are 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] According to the operation data, a relationship between current and impedance base values ​​on both sides of the AC power transmission system and the DC power transmission system is established to obtain a first relationship;

[0019] Substitute the power formula and the first relationship into the initial state space equation for linear expansion calculation to obtain the linearized state space equation related to the AC voltage and current phase angle under a unified scale.

[0020] This preferred solution can ensure the consistency of power calculation between the AC power transmission system and the DC power transmission system by setting the power base value uniformly, making the power comparison between different systems more accurate and intuitive. By establishing the relationship between the current and impedance base values ​​on both sides, the electrical connection between the AC power transmission system and the DC power transmission system can be expressed by a unified mathematical expression, which helps to simplify the complexity of the state space equation.

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

[0022] According to the operation data of the DRU-MMC hybrid DC power transmission system, a voltage and current relationship between the AC and DC sides of the DRU is established under ideal conditions;

[0023] In consideration of the equivalent inductance, the voltage-current relationship is subjected to voltage drop adjustment to obtain the initial state space equation regarding 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 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.

[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 a 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 operation data of the DRU-MMC hybrid DC power transmission system, and establishing a delay equation group of a first-order inertia link according to the operation data;

[0028] 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.

[0029] This preferred solution controls the d-axis through DC voltage, which can effectively adjust the DC voltage of the MMC converter station and keep it within the set range, which helps to maintain the stability of the DC transmission system; and 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 to improve the flexibility and response speed of the system. In actual control systems, there is often a certain delay due to factors such as signal transmission and equipment response; by establishing a delay equation group of the first-order inertia link and linearizing the main controller model based on these equation groups, these delay factors can be effectively considered, making the linearized model closer to the dynamic characteristics of the actual system.

[0030] As a preferred solution, a main controller model is established according to the operation data of the DRU-MMC hybrid DC power transmission system, specifically:

[0031] According to the outer loop control intermediate quantity 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 outer loop control expression is established;

[0032] According to 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, an inner loop control expression is established;

[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 power 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 can more accurately describe the delay phenomenon existing in the actual system by introducing the voltage measurement delay time constant, the current measurement delay time constant and the converter delay time constant, and establishing the first-order inertia link based on these delays. This makes the model closer to the dynamic characteristics of the actual system and improves the detection accuracy of the model.

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

[0043] Based on the detection 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 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 the operating data of the DRU-MMC hybrid DC transmission system;

[0046] Based on the initial state space equation, the linearized state space equations of the AC voltage and current phase angle 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 of the DRU-MMC hybrid direct current transmission system into a global unified coordinate system to obtain a unified coordinate equation system;

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

[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, the linearized state space equations of 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, which are 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] According to the operation data, a relationship between current and impedance base values ​​on both sides of the AC power transmission system and the DC power transmission system is established to obtain a first relationship;

[0053] Substitute the power formula and the first relationship into the initial state space equation for linear expansion calculation to obtain the linearized state space equation related to the AC voltage and current phase angle under a unified scale.

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

[0055] According to the operation data of the DRU-MMC hybrid DC power transmission system, a voltage and current relationship between the AC and DC sides of the DRU is established under ideal conditions;

[0056] In consideration of the equivalent inductance, the voltage-current relationship is subjected to voltage drop adjustment to obtain the initial state space equation regarding 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 a 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 operation data of the DRU-MMC hybrid DC power transmission system, and establishing a delay equation group of a first-order inertia link according to the operation data;

[0060] 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.

[0061] As a preferred solution, a main controller model is established according to the operation data of the DRU-MMC hybrid DC power transmission system, specifically:

[0062] According to the outer loop control intermediate quantity 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 outer loop control expression is established;

[0063] According to 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, an inner loop control expression is established;

[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, on which a computer program is stored. The computer program is called and executed by a computer to implement the stability detection method of a DRU-MMC hybrid DC transmission system as described above. BRIEF DESCRIPTION OF THE DRAWINGS

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

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

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

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

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

[0076] Figure 6 is a second change diagram of the output after the DC voltage reference value is changed provided in an embodiment of the present application;

[0077] Figure 7 is a third variation diagram of the output amount after the DC voltage reference value is changed provided in an embodiment of the present application;

[0078] Figure 8 This is a first change diagram of the output after the AC voltage amplitude at the DRU side is changed, provided in an embodiment of the present application;

[0079] Fig. 9 This is a second change diagram of the output after the AC voltage amplitude at the DRU side is changed, provided in an embodiment of the present application;

[0080] Fig.10 This is a third change diagram of the output amount after changing the AC voltage amplitude at the DRU side provided in an embodiment of the present application;

[0081] Fig.11 This is a first change diagram of the output after the AC voltage amplitude on the MMC side is changed, provided in an embodiment of the present application;

[0082] Fig.12 This is a second change diagram of the output after the AC voltage amplitude on the MMC side is changed, provided in an embodiment of the present application;

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

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

[0085] Fig.15 It is a structural schematic 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 the present application to clearly and completely describe the technical solutions in the embodiments of the present application. Obviously, the described embodiments are only part of the embodiments of the present application, not all of the embodiments. Based on the embodiments in the present application, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of this application.

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

[0088] A stability detection method for 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] Embodiment 1:

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

[0091] S1. Obtain the 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 after the disturbance in real time.

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

[0095] The structure mainly considers four parts: DRU converter station, DC system, MMC converter station and MMC converter station control system; Figure 2 As shown, the hybrid DC system uses the voltage across PCC1 and PCC2 as input and the current across PCC2 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, and the group of equations is obtained by linearizing the operation 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 coordinate transformation expressions, S2.10 is the process of solving the DRU-MMC small signal model by combining coordinate transformation expressions, linearized state space equations of DRU converter stations, linearized state space equations of DC submarine cables, linearized state space equations of MMC converter stations, and linearized state space equations of MMC converter station control systems, S2.11 is the process of calculating the stability eigenvalue according to the DRU-MMC small signal model, S2.12 is the process of verifying the step response of the DRU-MMC small signal model, and S2.13 is the process of applying eigenvalue analysis to the DRU-MMC small signal model, specifically:

[0098] S2.1. Based on the operation 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 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 voltage and current on the AC side, L r is the transformer inductance, U dc1 and U dc2 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 embodiment, please refer to Figure 3 , Figure 3 is a detailed internal structure diagram of a DRU provided in an embodiment of the present application, showing the detailed internal structure of a DRU converter station; Figure 3 As shown in the figure, in order to suppress the harmonics on the DC side, the DRU converter uses 12-pulse uncontrolled rectification as the basic commutation unit, which is composed of 12 diodes. The DRU converter uses diodes as commutation units and does not contain a control system. That is, it uses the unidirectional conductivity of the diode 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 voltage-current relationship under ideal conditions 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.

[0110] S2.2. The relationship between the electrical quantities on the DRU side can be known from the voltage-current relationship, the actual DC voltage relationship of the DRU 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. Therefore, based on the 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 to obtain the first relationship;

[0113] Substitute the K value, power formula and the first relation into the voltage-current relation, the actual DC voltage relation of the DRU 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 are the DRU AC side voltage reference value and DC side voltage reference value, Iacbr and I dcb 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, K is the ratio of the DRU DC side to AC side voltage reference values, 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] In this embodiment S2.2, by uniformly setting the power base value, the consistency of power calculation between the AC power transmission system and the DC power transmission system can be ensured, making the power comparison between different systems more accurate and intuitive. By establishing the relationship between the current and impedance base values ​​on both sides, the electrical connection between the AC power transmission system and the DC power transmission system can be expressed by a unified mathematical expression, which helps to simplify the complexity of the state space equation.

[0128] S2.3, transforming the input and output of the DRU-MMC hybrid DC transmission 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 DC filter inductor linearized state space equation;

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

[0131] Among them, the unified coordinate equation system is:

[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 inductance, I dc1 is the DC side current of DRU; 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 by amplitude and phase, in order to facilitate the subsequent establishment of the state space model of the whole system with other modules such as wind farms, it is necessary to convert the input variable ΔU r , And output ΔI r , Transform to the global unified coordinate system ΔU sxy1 With 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 directly correspond to each other, 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 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 to achieve the description of the system variables in a unified framework, which 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 includes DC resistance, DC inductance and DC capacitance. Since a long-distance offshore DC transmission cable is used, the capacitance effect cannot be ignored, so the initial equation group of the DC submarine cable is established;

[0140] It can be seen from the initial equation group 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 equation group;

[0141] Among them, the initial equation system is:

[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. Since the internal characteristics of MMC are relatively complex, the MMC characteristic equation can be simplified by using a simplified end model, that is, ignoring the dynamic behaviors such as the voltage fluctuation of the submodule capacitor and the internal circulation, so as to make the establishment of the state space model easier. Among them, the DC side electrical quantity uses the DC side base value for per-unit conversion, and the AC side electrical quantity uses the AC side base value for per-unit conversion, thereby establishing the first set of equations for the MMC converter station;

[0147] The first set of equations is linearized to obtain a simplified linearized state space equation 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] Where 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 main controller of the DRU-MMC hybrid DC transmission system, the MMC main controller adopts the dq decoupling control strategy. In order to achieve the 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;

[0154] Since the main controller includes two outer loop control modules of the d and q axes and two inner loop control modules, that is, it includes four PI links, the outer loop control expression is established by measuring the attenuation value of the voltage at the end of the DC submarine cable according to the outer loop control intermediate quantity of the d and q axes;

[0155] According to the inner loop control intermediate quantity of the d-axis and the 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 an outer loop control expression, an inner loop control expression and a 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 at 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 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 value of the d-axis outer ring control and the intermediate value of the q-axis outer ring control, and z 12 and z 13 They are the intermediate control amount of the inner loop of the d-axis and the intermediate control amount of the inner loop 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 reference values ​​of the dq axis current on the AC side, U cidref and U ciqref 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 embodiment, please refer to Figure 4 , Figure 4 : is a structural diagram of the MMC control system provided in the embodiment of the present application, which shows the structure of the MMC control system; the MMC control system includes two parts: a main controller and a submodule controller, wherein the main controller outputs the converter AC voltage reference value to the submodule controller, and the submodule controller realizes the stable operation of the MMC by controlling the switching of the submodules. The specific control block diagram is shown in Figure 4 As shown, in the small signal model of the control system, the role of the main controller is mainly considered.

[0166] In this embodiment S2.6, the d-axis is controlled by the DC voltage, which can effectively adjust the DC voltage of the MMC converter station and keep it within the set range, which helps to maintain the stability of the DC transmission system; and by controlling the q-axis with a fixed AC voltage or a fixed reactive power, the AC side voltage and reactive power can be adjusted according to system requirements, thereby improving the flexibility and response speed of the system;

[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 and ensure 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, 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, a first delay equation is established;

[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 are the d-axis voltage and q-axis voltage of the AC grid connection point, I idm and I iqm are the d-axis current and q-axis current on the AC side, U cid and U ciq They 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 described more accurately;

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

[0180] S2.8. Based on the delay equations of the first-order inertia link, the main controller model is linearized. 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; wherein, the linearized state space equations of the MMC converter station control system include an outer loop control equation group, an inner loop control equation group, and a phase-locked loop module equation group;

[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, because 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, it involves the input variables and output variables in the DRU-MMC hybrid DC transmission system. In order to establish the state space equation of the whole 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. By combining 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, an initial small signal model containing 20th-order state variables is established; wherein, the state variable ΔX of the small signal model of the DRU-MMC hybrid DC transmission system 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, namely the DRU-MMC hybrid DC transmission system small signal model, is obtained;

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

[0194]

[0195] Where 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 DC transmission system respectively; A, B1, B2, C1, C2, D11, D12, D21 and D22 all represent the coefficient matrices before variables.

[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 unified 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 the DRU-MMC small signal model is established, the input variables and step values ​​can be set in MATLAB to simulate small disturbance signals, and then compared 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 the present 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 the DC voltage reference value:

[0203] To apply this embodiment, please refer to Figure 5 , Figure 6 and Figure 7 , Figure 5 , Figure 6 and Figure 7 They are respectively a first change diagram, a second change diagram and a third change diagram of the output amount after the DC voltage reference value is changed provided in the embodiment of the present application, which represent the change of the output amount after the DC voltage reference value is changed;

[0204] At t = 0.5s, the DC reference voltage U dcref From 500kV to 495kV, add a step value of -0.01 per unit value, and set the same step value 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 , Fig. 9 and Fig.10 , Figure 8 , Fig. 9 and Fig.10 They are respectively a first change diagram, a second change diagram and a third change diagram of the output amount after the AC voltage amplitude on the DRU side is changed provided in the embodiment of the present application, which represent the change of the output amount 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 as the 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 , Fig. 9 and Fig.10 As shown:

[0209] Depend on Figure 8 , Fig. 9 and Fig.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 Fig.11 and Fig.12 , Fig.11 and Fig.12 They are respectively output quantity change diagrams after changing the AC voltage amplitude on the MMC side provided in the embodiments of the present application, indicating the change of the output quantity 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: Fig.11 and Fig.12 As shown:

[0212] Depend on Fig.11and Fig.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, each electrical quantity has a good response match under the step response, that is, the system can correctly respond to the dynamic changes of the system operation after a small disturbance, which verifies 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 whether there are characteristic roots crossing the real axis based on the calculation results to determine whether its operating state is stable, and read its oscillation frequency based on its imaginary part.

[0215] To apply this embodiment, please refer to Fig.13 , Fig.13 is an influence diagram of the short circuit ratio on the system stability provided by the embodiment of the present application, which shows the influence of the short circuit ratio on the system stability;

[0216] A two-terminal power supply system is established in the small signal model, and the short circuit ratio (SCR) is set to 8. When the strength of the AC power grid, i.e. the short circuit ratio (SCR), decreases, there are eigenvalue modes 13 and 14 moving in the positive direction. When the short circuit ratio is set to 8, the real parts of all characteristic roots of the system 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: Fig.13 shown.

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

[0218] In PSCAD / EMTDC, a time domain simulation is performed. At 7s, the short-circuit ratio SCR of the power supply at both ends drops from 8 to 3 and 2 respectively, and the system oscillates. The results are as follows: Figure 8 When SCR = 3, the oscillation frequency is about 31.35Hz, and the corresponding eigenvalue changes from -21.56±j40.72×2π to 7.08±j31.95×2π; when SCR = 2, the oscillation frequency is about 26.82Hz, 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 operation status by calculating the system characteristic values, and can be used to judge the system stability; at the same time, due to the input voltage and output current interface it provides, it can be further connected to wind farms and other systems to perform characteristic value analysis of the entire system.

[0220] Overall, this embodiment 1 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 calculate based on these data, so that the small signal model can capture the slight changes in the dynamic operation of the system, and then accurately evaluate the stability and oscillation frequency of the system, so as to obtain the 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 is helpful to evaluate the stability and recovery ability of the system. The linearized state-space equations of the MMC converter station describe in detail the evolution of the internal state of the MMC converter station over time. Through the analysis of the 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 of the MMC converter station control system can accurately describe the dynamic behavior of the MMC converter station control system, including the evolution of the system state, the input-output relationship, and the effect of the control strategy, which provides a reliable theoretical basis for the analysis, design, and optimization of the system;

[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 to a global unified coordinate system through coordinate transformation, which is convenient for connecting with systems such as wind farms, and is beneficial for subsequent small signal stability analysis of complex systems.

[0223] Embodiment 2:

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

[0225] Wherein, 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, 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, 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 after the disturbance in real time.

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

[0230] The structure mainly considers four parts: DRU converter station, DC system, MMC converter station and MMC converter station control system; Figure 2 As shown, the hybrid DC system uses the voltage across PCC1 and PCC2 as input and the current across PCC2 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 space unit, a transformation unit, a fourth unit, a feature value 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, and the variable The conversion unit is the process of establishing the coordinate transformation expression. The fourth unit is the process of solving the DRU-MMC small signal model by combining 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. The eigenvalue unit is the process of calculating the stability eigenvalue according to 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, which is specifically:

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

[0234] The initial unit is also used to adjust the voltage drop of the voltage-current relationship according to the commutation voltage drop formula of the phase-controlled rectifier circuit under the condition that the equivalent inductance of the converter transformer exists, so as 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 according to the actual DC voltage relationship of the DRU;

[0236] The voltage-current relationship is:

[0237]

[0238] The actual DC voltage relationship of 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 voltage and current on the AC side, L r is the transformer inductance, U dc1 and U dc2 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 embodiment, please refer to Figure 3 , Figure 3 is a detailed internal structure diagram of a DRU provided in an embodiment of the present application, showing the detailed internal structure of a DRU converter station; Figure 3 As shown in the figure, in order to suppress the harmonics on the DC side, the DRU converter uses 12-pulse uncontrolled rectification as the basic commutation unit, which is composed of 12 diodes. The DRU converter uses diodes as commutation units and does not contain a control system. That is, it uses the unidirectional conductivity of the diode 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 system variables. However, in the actual system, 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 know the relationship between the electrical quantities on the DRU side from the voltage-current relationship, the actual DC voltage relationship of the DRU, 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 respectively. Therefore, based on the 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 power transmission system and the DC power transmission system based on the power formula and the K value, and obtain a first relationship;

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

[0249] The linearization unit is also used to perform linear expansion processing on 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 according to 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 are the DRU AC side voltage reference value and DC side voltage reference value, I acbr and I dcb 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, K is the ratio of the DRU DC side to AC side voltage reference values, 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] The linearization unit of this embodiment can ensure the consistency of power calculation between the AC power transmission system and the DC power transmission system by uniformly setting the power base value, making the power comparison between different systems more accurate and intuitive. By establishing the relationship between the current and impedance base values ​​on both sides, the electrical connection between the AC power transmission system and the DC power transmission system can be expressed by a unified mathematical expression, which helps to simplify the complexity of the state space equation.

[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 according to the linearized state space equation, the unified coordinate equation group and the linearized state space equation of the DC filter inductor;

[0266] Among them, the unified coordinate equation system is:

[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 inductance, I dc1 is the DC side current of DRU; 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 whole 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 With 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 directly correspond to each other, except for the DC side current I 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 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 to achieve the description of the system variables in a unified framework, which 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.

[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 the long-distance offshore DC transmission cable is used, the capacitance effect cannot be ignored, so the initial equation group of the DC submarine cable is established;

[0275] The second unit is also used to know that the variables are linearly related from 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 equation system is:

[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 equation using a simplified end model due to the complex internal characteristics of the MMC, that is, ignoring the dynamic behaviors such as the voltage fluctuation of the submodule capacitor and the internal circulation, so as to make the establishment of the state space model easier. Among them, the DC side electrical quantity uses the DC side base value for per-unit conversion, and the AC side electrical quantity uses the AC side base value for per-unit conversion, thereby establishing 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] Where 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. In order 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 a constant reactive power control, and generate a converter AC voltage reference value through the dq decoupling link;

[0289] The model unit is also used for establishing an outer loop control expression by measuring the attenuation value of the voltage at the end of the DC submarine cable according to the outer loop control intermediate quantity of the d-axis and q-axis, because the main controller includes two outer loop control modules of the d and q axes and two inner loop control modules, that is, includes four PI links;

[0290] The model unit is also used to establish an inner loop control expression according to 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 according to the AC grid connection point voltage;

[0292] The model unit is also 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 at 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 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 value of the d-axis outer ring control and the intermediate value of the q-axis outer ring control, and z 12 and z 13 They are the intermediate control amount of the inner loop of the d-axis and the intermediate control amount of the inner loop 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 reference values ​​of the dq axis current on the AC side, U cidref and U ciqref 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 embodiment, please refer to Figure 4 , Figure 4 : is a structural diagram of the MMC control system provided in the embodiment of the present application, which shows the structure of the MMC control system; the MMC control system includes two parts: a main controller and a submodule controller, wherein the main controller outputs the converter AC voltage reference value to the submodule controller, and the submodule controller realizes the stable operation of the MMC by controlling the switching of the submodules. The specific control block diagram is shown in Figure 4 As shown, in the small signal model of the control system, the role of the main controller is mainly considered.

[0302] The model unit of this embodiment controls the d-axis through the DC voltage, which can effectively adjust the DC voltage of the MMC converter station and keep it within the set range, which helps to maintain the stability of the DC transmission system; and by controlling the q-axis with a fixed AC voltage or a fixed reactive power, the AC side voltage and reactive power can be adjusted according to system requirements, thereby improving the flexibility and response speed of the system;

[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 and ensure the synchronous operation of the MMC converter station and the AC grid.

[0304] A delay unit, for establishing 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, for establishing 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, used for linearizing the first delay equation and the second delay equation together 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 are the d-axis voltage and q-axis voltage of the AC grid connection point, I idm and I iqm are the d-axis current and q-axis current on the AC side, U cid and U ciq They 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 are often certain delays due to factors such as signal transmission and device response. By establishing a delay equation group for the first-order inertia link and linearizing the main controller model based on these equation groups, these delay factors can be effectively considered, making the linearized model closer to the dynamic characteristics of the actual system.

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

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

[0325] The coordinate transformation expression is:

[0326]

[0327] It should be noted that since the MMC control system is connected to the external system, it involves the input variables and output variables in the DRU-MMC hybrid DC transmission system. In order to establish the state space equation of the whole 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 20-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 small signal model of the DRU-MMC hybrid DC transmission system 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 DC transmission 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] Where 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 DC transmission system respectively; A, B1, B2, C1, C2, D11, D12, D21 and D22 all represent the coefficient matrices before variables.

[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, adding 0 to the subscript of the parameter unified variable represents the steady-state point operating value; and adding △ before the parameter represents the change or increment of the parameter.

[0335] The verification unit is used to set the input variables and step values ​​in MATLAB after the DRU-MMC small signal model is established, that is, to simulate the small disturbance signal, and compare it 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 for applying 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 the present application, which indicates 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 the DC voltage reference value:

[0341] To apply this embodiment, please refer to Figure 5 , Figure 6 and Figure 7 , Figure 5 , Figure 6 and Figure 7 They are respectively a first change diagram, a second change diagram and a third change diagram of the output amount after the DC voltage reference value is changed provided in the embodiment of the present application, which represent the change of the output amount after the DC voltage reference value is changed;

[0342] At t = 0.5s, the DC reference voltage U dcref From 500kV to 495kV, add a step value of -0.01 per unit value, and set the same step value 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 , Fig. 9 and Fig.10 , Figure 8 , Fig. 9 and Fig.10 They are respectively a first change diagram, a second change diagram and a third change diagram of the output amount after the AC voltage amplitude on the DRU side is changed provided in the embodiment of the present application, which represent the change of the output amount 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 as the 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 , Fig. 9 and Fig.10 As shown:

[0347] Depend on Figure 8 , Fig. 9 and Fig.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 Fig.11 and Fig.12 , Fig.11 and Fig.12 They are respectively a first change diagram and a second change diagram of the output amount after the AC voltage amplitude on the MMC side is changed provided in the embodiment of the present application, which represent the change of the output amount 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: Fig.11 and Fig.12 As shown:

[0350] Depend on Fig.11 and Fig.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, each electrical quantity has a good response match under the step response, that is, the system can correctly respond to the dynamic changes of the system operation after a small disturbance, which verifies 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 Fig.13 , Fig.13 is an influence diagram of the short circuit ratio on the system stability provided by the embodiment of the present application, which shows 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, and the short circuit ratio (SCR) is set to 8. When the strength of the AC power grid, that is, the short circuit ratio (SCR), decreases, there are eigenvalue modes 13 and 14 moving in the positive direction. When the short circuit ratio is set to 8, the real parts of all characteristic roots of the system 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: Fig.13 shown.

[0355] The analysis unit is also used to apply this embodiment, see Fig.14 , Fig.14 is a diagram of an oscillation waveform of a system after a short-circuit ratio is changed provided in an embodiment of the present application, showing an oscillation waveform of the system after a 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 drops from 8 to 3 and 2 respectively, and the system oscillates. The results are as follows Figure 8 When SCR = 3, the oscillation frequency is about 31.35Hz, and the corresponding eigenvalue changes from -21.56±j40.72×2π to 7.08±j31.95×2π; when SCR = 2, the oscillation frequency is about 26.82Hz, 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 operation status by calculating the system characteristic values, and can be used to judge the system stability; at the same time, due to the input voltage and output current interface it provides, it can be further connected to wind farms and other systems to perform characteristic value 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 calculate based on these data, so that the small signal model can capture the slight changes in the dynamic operation of the system, and then accurately evaluate the stability and oscillation frequency of the system, so as to obtain the 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 is helpful to evaluate the stability and recovery ability of the system. The linearized state-space equations of the MMC converter station describe in detail the evolution of the internal state of the MMC converter station over time. Through the analysis of the 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 of the MMC converter station control system can accurately describe the dynamic behavior of the MMC converter station control system, including the evolution of the system state, the input-output relationship, and the effect of the control strategy, which provides a reliable theoretical basis for the analysis, design, and optimization of the system;

[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 to a global unified coordinate system through coordinate transformation, which is convenient for connecting with systems such as wind farms, and is beneficial for subsequent small signal stability analysis of complex systems.

[0361] Embodiment three:

[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 running, the device where the computer-readable storage medium is located is controlled to execute the stability detection method of a DRU-MMC hybrid DC power transmission system;

[0363] Wherein, the stability detection method of a DRU-MMC hybrid DC transmission system, if implemented in the form of a software functional unit and used as an independent product, can be stored in a computer-readable storage medium. Based on such an 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, and the computer program can implement the steps of the above-mentioned various method embodiments when executed by the processor. 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, etc. The computer-readable medium may include: any entity or device capable of carrying 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), electric carrier signal, telecommunication signal and software distribution medium, etc.

[0364] The above are preferred embodiments 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 power transmission system, characterized in that: include: Obtain the disturbance operation data of the DRU-MMC hybrid DC transmission system after the disturbance; 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 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 operation equations of the DRU-MMC hybrid DC transmission system.

2. The stability detection method of a DRU-MMC hybrid DC power transmission system according to claim 1, characterized in that: The linearized state space equation of the DRU converter station is specifically: Establishing initial state space equations related to AC voltage and current phase angles based on the operating data of the DRU-MMC hybrid DC transmission system; Based on the initial state space equation, the linearized state space equations of the AC voltage and current phase angle 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; Converting the input and output of the DRU-MMC hybrid direct current transmission system into a global unified coordinate system to obtain a unified coordinate equation system; The inductor current formula of the DRU-MMC hybrid DC power transmission system is linearized to obtain a DC filter inductor linearized state space equation; 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.

3. The stability detection method of a DRU-MMC hybrid DC power transmission system according to claim 2, characterized in that: Based on the initial state space equation, the linearized state space equations of the AC voltage and current phase angle 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, which are 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; According to the operation data, a relationship between current and impedance base values ​​on both sides of the AC power transmission system and the DC power transmission system is established to obtain a first relationship; Substitute the power formula and the first relationship into the initial state space equation for linear expansion calculation to obtain the linearized state space equation related to the AC voltage and current phase angle under a unified scale.

4. The stability detection method of a DRU-MMC hybrid DC power transmission system according to claim 2, characterized in that: According to the operation data of the DRU-MMC hybrid DC transmission system, the initial state space equations of the AC voltage and current phase angle are established, which are as follows: According to the operation data of the DRU-MMC hybrid DC power transmission system, a voltage and current relationship between the AC and DC sides of the DRU is established under ideal conditions; In consideration of the equivalent inductance, the voltage-current relationship is subjected to voltage drop adjustment to obtain the initial state space equation regarding the AC voltage and current phase angle.

5. The stability detection method of a DRU-MMC hybrid DC power transmission system according to claim 1, characterized in that: 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 power 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; Establishing a main controller model according to the operation data of the DRU-MMC hybrid DC power transmission system, and establishing a delay equation group of a first-order inertia link according to the operation 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.

6. A stability detection method for a DRU-MMC hybrid DC power transmission system as claimed in claim 5, characterized in that: A main controller model is established according to the operation data of the DRU-MMC hybrid DC transmission system, specifically: According to the outer loop control intermediate quantity 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 outer loop control expression is established; According to 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, an inner loop control expression is established; 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.

7. The stability detection method of a DRU-MMC hybrid DC power transmission system according to claim 5, characterized in that: The delay equations of the first-order inertia link are established according to 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.

8. A stability detection device for a DRU-MMC hybrid DC power transmission system, characterized in that: Including data module and detection module; Wherein, the data module is used to obtain the disturbance operation data of the DRU-MMC hybrid DC transmission system after the disturbance; Based on the detection 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 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.

9. A stability detection device for a DRU-MMC hybrid DC power transmission system according to claim 8, characterized in that: The linearized state space equation of the DRU converter station is specifically: Establishing initial state space equations related to AC voltage and current phase angles based on the operating data of the DRU-MMC hybrid DC transmission system; Based on the initial state space equation, the linearized state space equations of the AC voltage and current phase angle 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; Converting the input and output of the DRU-MMC hybrid direct current transmission system into a global unified coordinate system to obtain a unified coordinate equation system; The inductor current formula of the DRU-MMC hybrid DC power transmission system is linearized to obtain a DC filter inductor linearized state space equation; The linearized state space equation of the DRU converter station is established according to the linearized state space equation, the unified coordinate equation group and the DC filter inductor linearized state space equation.

10. The stability detection device of a DRU-MMC hybrid DC power transmission system according to claim 9, characterized in that: Based on the initial state space equation, the linearized state space equations of 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; According to the operation data, a relationship between current and impedance base values ​​on both sides of the AC power transmission system and the DC power transmission system is established to obtain a first relationship; Substitute the power formula and the first relationship into the initial state space equation for linear expansion calculation to obtain the linearized state space equation related to the AC voltage and current phase angle under a unified scale.

11. The stability detection device of a DRU-MMC hybrid DC power transmission system according to claim 9, characterized in that: According to the operation data of the DRU-MMC hybrid DC transmission system, the initial state space equations of the AC voltage and current phase angle are established, which are as follows: According to the operation data of the DRU-MMC hybrid DC power transmission system, a voltage and current relationship between the AC and DC sides of the DRU is established under ideal conditions; In consideration of the equivalent inductance, the voltage-current relationship is subjected to voltage drop adjustment to obtain the initial state space equation regarding the AC voltage and current phase angle.

12. The stability detection device of a DRU-MMC hybrid DC power transmission system according to claim 8, characterized in that: 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 power 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; Establishing a main controller model according to the operation data of the DRU-MMC hybrid DC power transmission system, and establishing a delay equation group of a first-order inertia link according to the operation 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.

13. A stability detection device for a DRU-MMC hybrid DC power transmission system according to claim 12, characterized in that: A main controller model is established according to the operation data of the DRU-MMC hybrid DC transmission system, specifically: According to the outer loop control intermediate quantity 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 outer loop control expression is established; According to 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, an inner loop control expression is established; 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.

14. The stability detection device of a DRU-MMC hybrid DC power transmission system according to claim 12, characterized in that: The delay equations of the first-order inertia link are established according to 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.

15. A storage medium, characterized in that: The storage medium stores a computer program, which is called and executed by a computer to implement a stability detection method for a DRU-MMC hybrid direct current transmission system as described in any one of claims 1 to 7.

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