A transient synchronous stability analysis method under asymmetric fault of a direct current converter

Abstract

The application discloses a kind of transient synchronous stability analysis methods under direct current converter asymmetric fault, it is related to new energy power system stability evaluation technical field, first determine the converter positive sequence voltage amplitude when the output negative sequence power of converter is zero, then determine the asymmetry of the power system where converter is based on the positive sequence voltage amplitude of the grid corresponding to converter and rated grid voltage amplitude, to determine the maximum transmissible positive sequence active power of converter by the asymmetry determined, converter positive sequence voltage amplitude and converter positive sequence active power expression, finally accurately judge whether power system is stable according to the maximum transmissible positive sequence active power of converter and positive sequence reference active power.The scheme excludes the interference of negative sequence component in power system to power system stability detection, improves the accuracy of detection process.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of new energy power system stability evaluation, in particular to a transient synchronous stability analysis method of a direct current converter under asymmetric fault. BACKGROUND

[0002] In recent years, as a reliable and mature topology structure, the modular multilevel converter (MMC) is widely used in the field of high voltage direct current (HVDC) and flexible alternative current transmission systems (FACTS). The network-forming control enables the MMC to generate and maintain the voltage and frequency of the system through the internal control strategy, and is favored due to the advantages of actively participating in the frequency and voltage regulation of the system and responding to the load and fault changes.

[0003] With the vigorous development of offshore wind power, the scenario of sending new energy through MMC is also increasing, so the stability of network-forming MMC under large disturbance has attracted more and more attention. Among them, asymmetric fault is the fault with the highest occurrence probability in the power system, and the analysis of the transient synchronous stability of MMC under asymmetric fault is more complex due to the strong nonlinearity of the system and the coupling between positive and negative sequence components. Therefore, there are few studies on the transient synchronous stability of network-forming MMC under asymmetric fault.

[0004] MMC is commonly used in the receiving end of direct current transmission, as an important hub of power transmission, and analyzing the transient synchronous stability of MMC under asymmetric fault conditions and revealing the transient synchronous instability mechanism are beneficial to the optimization of system control strategy and control parameters, and lay a foundation for enhancing the transient synchronous stability of the system. Therefore, it is necessary to analyze the transient synchronous stability of network-forming MMC under asymmetric fault. SUMMARY

[0005] The purpose of the present application is to provide a transient synchronous stability analysis method of a direct current converter under asymmetric fault, which determines the maximum transmissible positive sequence active power of the converter, the positive sequence reference active power and the positive sequence active power expression of the converter under the condition that the output negative sequence power of the converter is zero, and makes an accurate judgment on the stability of the power system where the converter is located. The present application eliminates the interference of negative sequence components in the power system on the transient synchronous stability analysis of the direct current converter under asymmetric fault, and improves the accuracy of the detection process.

[0006] To solve the above technical problems, the present application provides a transient synchronous stability analysis method of a direct current converter under asymmetric fault, comprising:

[0007] determining a positive sequence voltage amplitude of the converter when the converter outputs zero negative sequence power;

[0008] determining an unbalance degree of a power system in which the converter is located based on a positive sequence voltage amplitude of a power grid corresponding to the converter and a rated power grid voltage amplitude;

[0009] determining a maximum transferable positive sequence active power of the converter by the unbalance degree, the positive sequence voltage amplitude of the converter and an expression of positive sequence active power of the converter;

[0010] judging whether the power system is stable according to the maximum transferable positive sequence active power and a positive sequence reference active power.

[0011] Optionally, the determining of the positive sequence voltage amplitude of the converter when the converter outputs zero negative sequence power comprises:

[0012] performing a preset control on the converter so that the negative sequence power after the preset control is zero;

[0013] determining a positive sequence reactive power of the converter when the negative sequence power is zero, and determining the positive sequence voltage amplitude of the converter when the negative sequence power is zero according to the positive sequence reactive power and an expression of reactive power-voltage control in positive sequence droop control.

[0014] Optionally, the performing of the preset control on the converter so that the negative sequence power after the preset control is zero comprises:

[0015] performing a preset proportional resonance control on the converter based on a positive sequence reference current of the converter, a preset amplitude instruction and a negative sequence reference current so that the negative sequence power after the preset proportional resonance control is zero.

[0016] Optionally, the determining of the unbalance degree of the power system in which the converter is located based on the positive sequence voltage amplitude of the power grid corresponding to the converter and the rated power grid voltage amplitude comprises:

[0017] determining a three-phase voltage amplitude of the converter when the negative sequence power is zero;

[0018] determining the positive sequence voltage amplitude of the power grid by a sum of the three-phase voltage amplitudes;

[0019] taking a ratio of the positive sequence voltage amplitude of the power grid to the rated power grid voltage amplitude as the unbalance degree.

[0020] Optionally, the determining of the maximum transferable positive sequence active power of the converter by the unbalance degree, the positive sequence voltage amplitude of the converter and the expression of positive sequence active power of the converter comprises:

[0021] determining line reactance of the converter;

[0022] determining maximum transmissible positive sequence active power of the converter based on the line reactance, the positive sequence voltage amplitude of the converter, the unbalance degree and the positive sequence active power expression of the converter.

[0023] Optionally, the judging whether the power system is stable according to the maximum transmissible positive sequence active power and the positive sequence reference active power comprises:

[0024] determining the size relationship between the maximum transmissible positive sequence active power and the positive sequence reference active power;

[0025] judging whether the power system is stable based on the size relationship.

[0026] Optionally, the determining the size relationship between the maximum transmissible positive sequence active power and the positive sequence reference active power comprises:

[0027] determining whether the positive sequence reference active power is greater than the maximum transmissible positive sequence active power in a preset time, and taking the determination result as the size relationship.

[0028] Optionally, the judging whether the power system is stable based on the size relationship comprises:

[0029] if the positive sequence reference active power is greater than the maximum transmissible positive sequence active power in the preset time, determining that the power system is unstable;

[0030] if the positive sequence reference active power is not greater than the maximum transmissible positive sequence active power in the preset time, determining that the power system is stable.

[0031] The purpose of the present application is to provide a DC converter asymmetric fault transient synchronous stability analysis method, first determine the converter positive sequence voltage amplitude of the converter when the output negative sequence power is zero, and then determine the unbalance degree of the power system where the converter is based on the corresponding grid positive sequence voltage amplitude and the rated grid voltage amplitude of the converter, so as to determine the maximum transmissible positive sequence active power of the converter through the determined unbalance degree, the positive sequence voltage amplitude of the converter and the positive sequence active power expression of the converter. Finally, according to the maximum transmissible positive sequence active power and the positive sequence reference active power of the converter, the stability of the power system is accurately judged. This scheme excludes the interference of negative sequence components in the power system on the transient synchronous stability analysis of the DC converter under asymmetric fault, and improves the accuracy of the detection process. BRIEF DESCRIPTION OF DRAWINGS

[0032] In order to more clearly illustrate the technical solutions in the embodiments of the present application or the prior art, the accompanying drawings needed to be used in the description of the embodiments or the prior art will be briefly introduced. Obviously, the accompanying drawings in the following description only represent some embodiments of the present application, and all other drawings obtained by those of ordinary skill in the art without creative effort based on the provided drawings also belong to the protection scope of the present application.

[0033] Figure 1 A process flow chart of a transient synchronous stability analysis method of a DC converter under asymmetric fault is provided in the present application.

[0034] Figure 2 A schematic diagram of a MMC weak AC power grid system model is provided in the present application.

[0035] Figure 3 A schematic diagram of a droop control model based on positive and negative sequence separation is provided in the present application.

[0036] Figure 4 A phase diagram under different asymmetric grid voltage drop depths is provided in the present application.

[0037] Figure 5 A positive sequence virtual power angle and positive sequence active power simulation waveform is provided in the present application. DETAILED DESCRIPTION

[0038] The core of the present application is to provide a transient synchronous stability analysis method of a DC converter under asymmetric fault, which determines the maximum transferable positive sequence active power of the converter, the positive sequence reference active power and the positive sequence active power expression of the converter under the condition that the output negative sequence power of the converter is zero, and makes an accurate judgment on the stability of the power system where the converter is located. The present application eliminates the interference of the negative sequence component in the power system on the transient synchronous stability analysis of the DC converter under asymmetric fault, and improves the accuracy of the detection process.

[0039] In order to make the purpose, technical solutions and advantages of the embodiments of the present application more clear, the technical solutions in the embodiments of the present application will be described clearly and completely in combination with the accompanying drawings in the embodiments of the present application. Obviously, the described embodiments are only some of the embodiments of the present application, not all. Based on the embodiments in the present application, all other embodiments obtained by those of ordinary skill in the art without creative effort also belong to the protection scope of the present application.

[0040] To solve the above technical problems, the present application provides a transient synchronous stability analysis method of a DC converter under asymmetric fault, comprising:

[0041] S11: determining the positive sequence voltage amplitude of the converter under the condition that the output negative sequence power of the converter is zero;

[0042] S12: determining the asymmetry of the power system in which the converter is located based on the grid positive sequence voltage amplitude corresponding to the converter and the rated grid voltage amplitude;

[0043] S13: determining the maximum transmissible positive sequence active power of the converter through the asymmetry, the converter positive sequence voltage amplitude and the positive sequence active power expression of the converter;

[0044] S14: judging whether the power system is stable according to the maximum transmissible positive sequence active power and the positive sequence reference active power.

[0045] In the present application, to judge whether the power system in which the converter is located is stable, the positive sequence voltage amplitude of the converter under the condition that the output negative sequence power of the converter is zero needs to be determined first, then the asymmetry of the power system in which the converter is located is determined based on the grid positive sequence voltage amplitude corresponding to the converter and the rated grid voltage amplitude, and then the maximum transmissible positive sequence active power of the converter is determined by substituting the determined asymmetry and the converter positive sequence voltage amplitude into the positive sequence active power expression of the converter, and finally the stability of the power system in which the converter is located is accurately judged according to the maximum transmissible positive sequence active power and the positive sequence reference active power. The present application detects the stability of the power system under the condition that the output negative sequence power of the converter is zero, so the interference of the negative sequence component in the power system on the transient synchronous stability analysis under the asymmetric fault of the DC converter is excluded, and the accuracy of the detection process is improved.

[0046] It should be noted that the purpose of the present application is to solve the problem of MMC transient synchronous instability under asymmetric fault conditions. First, a simplified circuit model of the grid-connected MMC feeding a weak AC system is constructed. Second, an MMC control model is constructed based on positive and negative sequence separation control and droop control. The asymmetry is defined based on the positive sequence voltage and the rated voltage. The positive sequence virtual power angle is defined, and the positive sequence active power expression is obtained using the circuit theorem and combined with the grid-connected MMC control model and the proposed asymmetry to establish the positive sequence virtual power angle nonlinear dynamic mathematical equation of the grid-connected MMC (the positive sequence virtual power angle nonlinear dynamic equation of the converter). Based on the mathematical equation, the mechanism of MMC transient synchronous instability under asymmetric grid voltage sag fault conditions is analyzed, and the transient synchronous stability boundary is proposed. Finally, the influence of asymmetry on the transient synchronous stability boundary is analyzed based on the phase plane method. The present application proposes the transient synchronous stability boundary, which helps to improve the transient synchronous stability of MMC under asymmetric grid voltage sag fault conditions and is beneficial to the safe and stable operation of the system.

[0047] It should also be noted that the stability judgment process of the power system can be realized by the following steps:

[0048] S1, construct a simplified circuit model of grid-connected MMC feeding into a weak AC system, the simplified circuit model of grid-connected MMC feeding into a weak AC system, specifically refers to the MMC connecting to the grid bus through a long line inductance; as shown in the figure, Figure 2 In the weak AC system, the grid-connected MMC is connected to the grid bus through the line inductance Lg, V oabc and I oabc represent the output voltage vector and the output current vector of the MMC, U dc is the DC side voltage value.

[0049] S2, based on the positive and negative sequence separation control and droop control strategy, construct the MMC control model, the positive sequence component in the grid-connected MMC control model adopts the droop control strategy, generates the phase and amplitude instructions of the positive sequence voltage reference value, inputs the positive sequence reference current into the positive sequence voltage loop to obtain the positive sequence reference current, inputs the positive sequence reference current and the negative sequence reference current into the current loop based on PR (Proportional Resonance, proportional resonance) control to obtain the modulation signal, and makes the negative sequence reference current zero to control the MMC output zero negative sequence power, so as to realize the purpose of only existing positive sequence power under asymmetric fault. Because the time scale of the power outer loop of the proportional resonance control is much larger than that of the inner loop, the voltage and current inner loop is regarded as a unified gain with ideal reference tracking in the analysis process. As shown in the figure, Figure 3 The mathematical model of the positive sequence droop control of the MMC based on positive and negative sequence separation is: Wherein, K p + , P + , P ref + are the positive sequence droop coefficient, the positive sequence active power and the positive sequence reference active power of the MMC, ω n + is the positive sequence rated frequency, and ω + is the modulated frequency; K q + , Q + , Q ref + are the positive sequence reactive-voltage droop coefficient, the positive sequence reactive power and the positive sequence reference reactive power of the MMC, V n + is the positive sequence rated voltage, and V + is the final positive sequence voltage.

[0050] S3, define the asymmetry based on the positive sequence voltage and the rated voltage; after the grid three-phase voltage is transformed by the symmetrical component method and the stationary coordinate transformation, the positive sequence voltage can be respectively expressed as:

[0051] Where q=e -jπ / 2is the phase displacement operator, j is a preset parameter, and u αβ is the positive sequence voltage before the static coordinate transformation.

[0052] Further, according to the Clarke transformation, the asymmetric A, B, and C three-phase grid voltage is brought into the expression of the grid positive sequence voltage in the static coordinate system, and simplification can obtain:

[0053] wherein U A , U B , and U C are the amplitudes of the A, B, and C three-phase voltages, ω g is the grid bus frequency; is the A-phase positive sequence voltage, is the B-phase positive sequence voltage, and t is time.

[0054] Further, the asymmetry degree λ is defined as the grid positive sequence voltage amplitude E + compared with the rated grid voltage amplitude, and according to the expression of the grid positive sequence voltage in the static coordinate system and the rated voltage, it can be obtained:

[0055] wherein E and U n are the rated grid voltage amplitudes, and E + is the grid positive sequence voltage amplitude.

[0056] S4, define the positive sequence virtual power angle, use the circuit theorem to obtain the expression of the positive sequence active power, and combine the network type MMC control model and the asymmetry degree to establish the dynamic nonlinear mathematical equation of the positive sequence virtual power angle of the network type MMC; the positive sequence virtual power angle δ + of the MMC is set as the positive sequence phase angle difference between the MMC output voltage and the grid voltage, and the circuit theorem can be used in the simplified circuit model of the network type MMC feeding the weak AC system constructed in S1 to obtain the MMC output positive sequence active power P + :

[0057] wherein V + and X g are the positive sequence output voltage amplitude and line reactance of the MMC, respectively, and δ + is the positive sequence virtual power angle. Further, according to the expression of the output positive sequence active power P + and the asymmetry degree, the maximum transferable positive sequence active power P of the MMC can be obtained as:

[0058] The virtual power angle dynamic nonlinear mathematical equation of the MMC in S4 can be obtained by combining the positive sequence droop control active power ring mathematical model of the MMC, the MMC output positive sequence active power, and the asymmetry degree:

[0059]

[0060] S5, based on the MMC positive sequence virtual power angle dynamic nonlinear mathematical equation established in the S4 step, the mechanism of transient synchronous instability of MMC in asymmetric grid voltage sag fault condition is analyzed, and the transient synchronous stability boundary is proposed; the mechanism of transient synchronous instability of MMC in asymmetric voltage sag fault is that when the grid occurs asymmetric voltage sag fault, the positive and negative sequence power of the network structure MMC based on positive and negative sequence separation is separated and the negative sequence power is controlled to zero, while the maximum positive sequence active power that can be transmitted by the MMC decreases with the asymmetric voltage sag fault, resulting in that the positive sequence reference active power is greater than the maximum positive sequence active power that can be transmitted, i.e. further leading to the maintenance of the final positive sequence virtual power angle will continue to increase to infinity, and the system occurs transient synchronous instability. According to the mechanism of transient synchronous instability, the transient synchronous stability boundary of the droop control MMC based on positive and negative sequence separation when asymmetric grid voltage sag fault occurs can be obtained, i.e.

[0061] S6, the influence of asymmetry on the transient synchronous stability boundary is analyzed based on the phase plane method. As shown in Figure 4 , the analysis of the positive sequence virtual power angle dynamic nonlinear mathematical equation by the phase plane method shows that the more serious the asymmetric voltage sag fault is, the higher the system asymmetry is, the smaller the positive sequence voltage component E + is, the phase diagram is translated upward, and the positive sequence virtual power angle shows an increasing trend; if the degree of asymmetric voltage sag is too deep to make the phase diagram have no equilibrium point, the positive sequence virtual power angle will continue to increase, and the system cannot realize synchronous stability. The deeper the degree of asymmetric grid voltage sag is, the smaller λ is, the closer the MMC fed into the weak AC grid system is to the transient synchronous stability boundary, and the higher the risk of transient synchronous instability of MMC is.

[0062] It should be further pointed out that in order to verify the accuracy of the above steps, Figure 5 the positive sequence virtual power angle waveform and the positive sequence output active power waveform of the network structure MMC are shown in Figure 5 (a)-(b) correspond to the condition of asymmetric grid voltage sag fault, the system satisfies the transient synchronous stability boundary, and the MMC can maintain transient synchronous stability. While Figure 5 (c)-(d) correspond to the condition of deep asymmetric grid voltage sag, the system cannot satisfy the transient synchronous stability boundary, and the MMC occurs transient synchronous instability. The correctness of the theoretical analysis is verified.

[0063] It also needs to be explained that the application proposes an index for measuring the asymmetry degree under the asymmetric fault condition of the MMC connected to the weak AC system, constructs a positive sequence virtual power angle nonlinear dynamic mathematical model of the MMC, and based on the mathematical model, the dynamic characteristics of the positive sequence virtual power angle under the single-phase voltage sag fault of the power grid can be quantitatively analyzed. The application analyzes the mechanism of the transient synchronous instability of the MMC under the asymmetric power grid voltage sag fault, and proposes a transient synchronous stability boundary. Finally, based on the phase plane analysis method, the influence of the asymmetric power grid voltage sag depth on the transient synchronous stability boundary is researched, which is very beneficial to the improvement of the transient synchronous stability of the MMC.

[0064] The embodiment provides a transient synchronous stability analysis method for a DC converter under asymmetric fault, determines the positive sequence voltage amplitude of the converter when the output negative sequence power of the converter is zero, determines the asymmetry degree of the power system in which the converter is located based on the positive sequence voltage amplitude of the corresponding power grid of the converter and the rated power grid voltage amplitude, determines the maximum transmissible positive sequence active power of the converter by the determined asymmetry degree, the positive sequence voltage amplitude of the converter and the positive sequence active power expression of the converter, and finally accurately judges whether the power system is stable according to the maximum transmissible positive sequence active power of the converter and the positive sequence reference active power. The scheme excludes the interference of the negative sequence component in the power system on the transient synchronous stability analysis of the DC converter under asymmetric fault, and improves the accuracy of the detection process.

[0065] On the basis of the above embodiment:

[0066] As an optional embodiment, the positive sequence voltage amplitude of the converter when the output negative sequence power of the converter is zero is determined, and the method comprises the following steps:

[0067] The converter is pre-controlled so that the negative sequence power after the pre-control is zero;

[0068] The positive sequence reactive power of the converter when the negative sequence power is zero is determined, and the positive sequence voltage amplitude of the converter when the negative sequence power is zero is determined according to the positive sequence reactive power and the reactive power-voltage control expression in the positive sequence droop control.

[0069] In the application, in order to determine the positive sequence voltage amplitude of the converter when the output negative sequence power of the converter is zero, the converter is pre-controlled so that the negative sequence power after the pre-control is zero, the positive sequence reactive power of the converter when the negative sequence power is zero is determined, and the positive sequence voltage amplitude of the converter when the negative sequence power is zero is determined according to the determined positive sequence reactive power and the reactive power-voltage control expression in the positive sequence droop control, so that the determination process of the positive sequence voltage amplitude of the converter is carried out when the output negative sequence power of the converter is zero, and the reliability of the scheme is improved.

[0070] As an optional embodiment, the converter is preset controlled to make the preset controlled negative sequence power zero, comprising:

[0071] The converter is preset proportional resonance controlled based on the positive sequence reference current of the converter, the preset amplitude instruction and the negative sequence reference current to make the preset proportional resonance controlled negative sequence power zero.

[0072] In the application, the converter is preset proportional resonance controlled based on the positive sequence reference current of the converter, the preset amplitude instruction and the negative sequence reference current to make the preset proportional resonance controlled negative sequence power zero, through the preset proportional resonance control mode, the interference of the negative sequence component in the power system to the transient synchronous stability analysis under the asymmetric fault of the DC converter is excluded, and the accuracy of the detection process is improved.

[0073] As an optional embodiment, the asymmetry degree of the power system where the converter is located is determined based on the grid positive sequence voltage amplitude corresponding to the converter and the rated grid voltage amplitude, comprising:

[0074] The three-phase voltage amplitude of the converter under the condition that the negative sequence power is zero is determined;

[0075] The grid positive sequence voltage amplitude is determined through the sum of the three-phase voltage amplitudes;

[0076] The ratio of the grid positive sequence voltage amplitude to the rated grid voltage amplitude is taken as the asymmetry degree.

[0077] In the application, the specific process of determining the asymmetry degree of the power system where the converter is located based on the grid positive sequence voltage amplitude corresponding to the converter and the rated grid voltage amplitude is as follows: the three-phase current amplitude of the converter under the condition that the negative sequence power is zero is determined first, then the grid positive sequence voltage amplitude is determined through the sum of the three-phase voltage amplitudes (the sum of the voltage amplitudes of the A, B and C phases), and finally the ratio of the grid positive sequence voltage amplitude to the rated grid voltage amplitude is taken as the asymmetry degree, so that the asymmetry degree of the power system where the converter is located is accurately determined.

[0078] As an optional embodiment, the maximum transmissible positive sequence active power of the converter is determined through the asymmetry degree, the positive sequence voltage amplitude of the converter and the positive sequence active power expression of the converter, comprising:

[0079] The line reactance of the converter is determined;

[0080] The maximum transmissible positive sequence active power of the converter is determined based on the line reactance, the positive sequence voltage amplitude of the converter, the asymmetry degree and the positive sequence active power expression of the converter.

[0081] The specific process for determining the maximum transmissible positive sequence active power of the converter in the application is as follows: first, the line reactance of the converter is determined, and then the determined line reactance, the positive sequence voltage amplitude of the converter and the asymmetry degree are substituted into the positive sequence active power expression of the converter to determine the maximum transmissible positive sequence active power of the converter, thereby improving the accuracy of the determination process of the maximum transmissible positive sequence active power.

[0082] As an optional embodiment, whether the power system is stable is judged according to the maximum transmissible positive sequence active power and the positive sequence reference active power, and the method comprises the following steps of:

[0083] Determining the size relationship between the maximum transmissible positive sequence active power and the positive sequence reference active power.

[0084] Judging whether the power system is stable based on the size relationship.

[0085] In the application, the specific process for judging whether the power system is stable according to the maximum transmissible positive sequence active power and the positive sequence reference active power is as follows: first, the size relationship between the maximum transmissible positive sequence active power and the positive sequence reference active power is determined, and then whether the power system is stable is judged according to the size relationship, thereby ensuring the accuracy of the stability judgment.

[0086] As an optional embodiment, the size relationship between the maximum transmissible positive sequence active power and the positive sequence reference active power is determined, and the method comprises the following steps of:

[0087] Determining whether the positive sequence reference active power is greater than the maximum transmissible positive sequence active power in a preset time, and taking the determination result as the size relationship.

[0088] In the application, the specific process for determining the size relationship between the maximum transmissible positive sequence active power and the positive sequence reference active power is as follows: whether the positive sequence reference active power is greater than the maximum transmissible positive sequence active power in a preset time is determined, and the determined result is taken as the size relationship between the maximum transmissible positive sequence active power and the positive sequence reference active power, thereby accurately obtaining the condition for judging the stability of the power system.

[0089] As an optional embodiment, whether the power system is stable is judged based on the size relationship, and the method comprises the following steps of:

[0090] If the positive sequence reference active power is greater than the maximum transmissible positive sequence active power in a preset time, it is determined that the power system is unstable.

[0091] If the positive sequence reference active power is not greater than the maximum transmissible positive sequence active power in a preset time, it is determined that the power system is stable.

[0092] In the present application, the specific process for judging whether the power system is stable based on the size relationship is as follows: if the positive sequence reference active power is greater than the maximum transferable positive sequence active power in the preset time, it is proved that the power system is unstable; otherwise, if the positive sequence reference active power is not greater than the maximum transferable positive sequence active power in the preset time, it is proved that the power system is stable, thereby improving the reliability of the scheme.

[0093] It should be noted that in this specification, the relational terms such as first and second and the like are used solely to distinguish one entity or action from another, without necessarily requiring or implying that there is any such actual relationship or order between such entities or actions. Moreover, the terms "comprises", "comprising", or any other variations thereof, are intended to cover a non-exclusive inclusion, so that a process, method, article, or apparatus that comprises a list of elements does not include only those elements but can include other elements not expressly listed or inherent to such process, method, article, or apparatus. Without further limitation, an element defined by an "including a" does not exclude the existence of additional identical elements in the process, method, article, or apparatus that includes the element.

[0094] The above description of the embodiments of the present application enables a person skilled in the art to implement or use the present application. Various modifications to the embodiments will be apparent to those skilled in the art, and the general principles defined herein can be implemented in other embodiments without departing from the spirit or scope of the present application. Therefore, the present application will not be limited to the embodiments shown herein, but will conform to the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. A method for transient synchronous stability analysis under DC converter asymmetric fault, characterized in that, The method comprises: determining a positive sequence voltage amplitude of a converter when the converter outputs zero negative sequence power; determining an asymmetry degree of a power system in which the converter is located based on a positive sequence voltage amplitude of a power grid corresponding to the converter and a rated voltage amplitude of the power grid; determining a maximum transmissible positive sequence active power of the converter through the asymmetry degree, the positive sequence voltage amplitude of the converter and an expression of the positive sequence active power of the converter; judging whether the power system is stable according to the maximum transmissible positive sequence active power and a positive sequence reference active power; wherein the determination of the maximum transmissible positive sequence active power of the converter through the asymmetry degree, the positive sequence voltage amplitude of the converter and the expression of the positive sequence active power of the converter comprises: determining a line reactance of the converter; determining the maximum transmissible positive sequence active power of the converter based on the line reactance, the positive sequence voltage amplitude of the converter, the asymmetry degree and the expression of the positive sequence active power of the converter; the expression of the maximum transmissible positive sequence active power is: ； wherein, is the maximum transmissible positive sequence active power, is the asymmetry degree, is the rated grid voltage magnitude, is the positive sequence voltage magnitude, is the line reactance corresponding to the converter.

2. The method of claim 1, wherein the DC converter asymmetric fault transient stability analysis method is characterized by, the determination of the positive sequence voltage amplitude of the converter when the converter outputs zero negative sequence power comprises: performing a preset control on the converter so that the negative sequence power after the preset control is zero; determining a positive sequence reactive power of the converter when the negative sequence power is zero, and determining the positive sequence voltage amplitude of the converter when the negative sequence power is zero according to the positive sequence reactive power and an expression of a reactive power-voltage control in a positive sequence droop control.

3. The method of claim 2, wherein the DC converter asymmetric fault transient stability analysis method is characterized by, the preset control on the converter so that the negative sequence power after the preset control is zero comprises: performing a preset proportional resonance control on the converter based on a positive sequence reference current, a preset amplitude instruction and a negative sequence reference current of the converter so that the negative sequence power after the preset proportional resonance control is zero.

4. The method of claim 1, wherein the DC converter asymmetric fault transient stability analysis method is characterized by, the determination of the asymmetry degree of the power system in which the converter is located based on the positive sequence voltage amplitude of the power grid corresponding to the converter and the rated voltage amplitude of the power grid comprises: determining a three-phase voltage amplitude of the converter when the negative sequence power is zero; determining the positive sequence voltage amplitude of the power grid through a sum of the three-phase voltage amplitudes; taking a ratio of the positive sequence voltage amplitude of the power grid to the rated voltage amplitude of the power grid as the asymmetry degree.

5. The method for transient stability analysis of DC converter asymmetrical fault according to any one of claims 1 to 4, characterized in that, the judgment of whether the power system is stable according to the maximum transmissible positive sequence active power and the positive sequence reference active power comprises: determining a size relationship between the maximum transmissible positive sequence active power and the positive sequence reference active power; judging whether the power system is stable based on the size relationship.

6. The method of transient stability analysis of DC converter asymmetrical fault according to claim 5, characterized in that, the determination of the size relationship between the maximum transmissible positive sequence active power and the positive sequence reference active power comprises: determining whether the positive sequence reference active power is greater than the maximum transmissible positive sequence active power in a preset time, and taking a determination result as the size relationship.

7. The method of transient stability analysis of DC converter asymmetrical fault according to claim 6, characterized in that, the judgment of whether the power system is stable based on the size relationship comprises: if the positive sequence reference active power is greater than the maximum transmissible positive sequence active power in the preset time, it is determined that the power system is unstable. If the positive sequence reference active power is not all greater than the maximum transmissible positive sequence active power within a preset time, it is determined that the power system is stable.

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