A power interaction analysis method for AC / DC systems considering hybrid DC infeed

Abstract

The present invention discloses a method for analyzing power interaction of an AC / DC system considering hybrid DC feed-in, characterized by comprising the following steps: S1, calculating a power interaction factor of the hybrid DC feed-in system; S2, judging the power bifurcation characteristics of the hybrid DC feed-in system according to the difference between the short-circuit ratio and the critical short-circuit ratio; S3, analyzing the influence of the active and reactive power injected into the AC side of the flexible DC in the hybrid feed-in system on the critical short-circuit ratio index and power stability. The method is suitable for evaluating the power interaction intensity of a system with hybrid DC feed-in, and the proposed short-circuit ratio index has a clear physical meaning and a clear boundary value, can accurately judge the power stability of a hybrid DC multi-feed system, and has guiding significance for the planning and safe and stable operation of DC projects.

Description

Technical Field

[0001] The present invention relates to the field of methods for evaluating power bifurcation characteristics of an AC / DC hybrid system in a static state, and in particular to a method for analyzing power interaction of an AC / DC system considering hybrid DC feeding. Background Art

[0002] In the existing technology, for the receiving-end power grid considering a single conventional high-voltage direct current landing point, the short-circuit ratio SCR can be used as an indicator to measure the strength of the receiving-end power grid. By calculating the difference between the short-circuit ratio SCR and the critical short-circuit ratio CSCR, the power bifurcation characteristics of the system under rated operation can be measured. The multi-infeed short-circuit ratio MISCR indicator proposed by the CIGRE DC Working Group has been widely recognized at home and abroad. This indicator takes into account the interaction between multiple feeds and is an extension of the original SCR concept. The latest version of the power system guidelines points out that the DC transmission power needs to match the grid strength, but the current evaluation indicators for the strength of DC multi-infeed systems are often based on empirical short-circuit ratios and lack clear mechanisms and boundary values. In a hybrid multi-infeed receiving system combining flexible HVDC and conventional HVDC transmission, there is a certain coupling relationship between DC converter stations and between DC converter stations and AC grids, which makes it difficult to quantify power interactions. For example, a Chinese patent document entitled "A Method and System for Determining Voltage Stability of a Multi-infeed AC / DC System," with publication number CN109167400A, discloses the weighting of multi-DC loop infeed interaction factors to determine stability. Summary of the Invention

[0003] To this end, the present invention provides a power interaction analysis method for an AC / DC system considering hybrid DC feed-in, which overcomes the shortcomings of the existing technology. It can make the physical meaning of the short-circuit ratio of the receiving end of the AC / DC system considering hybrid DC feed-in clear, and has a clear intensity distinction boundary. It can accurately determine the power interaction intensity of the receiving end power grid considering hybrid DC feed-in, based on which the power bifurcation characteristics of the DC system can be effectively analyzed.

[0004] In order to achieve the above object, the present invention provides the following technical solutions:

[0005] A method for analyzing power interaction of an AC / DC system considering hybrid DC infeed includes the following steps:

[0006] S1. Calculate the power interaction factor of the hybrid DC feed-in system;

[0007] S2. judging the power bifurcation characteristics of the hybrid DC feed-in system according to the difference between the short-circuit ratio and the critical short-circuit ratio;

[0008] S3. Analyze the impact of active and reactive power injected into the AC side of a hybrid infeed system from flexible DC on the critical short-circuit ratio and power stability. Accurately determine the power interaction intensity of the receiving grid considering hybrid DC infeed.

[0009] Preferably, S1 includes calculating a power bifurcation Jacobian matrix equation suitable for power stability analysis of a hybrid DC feed-in system based on a quasi-steady-state model and power flow calculation equation of the AC / DC hybrid system.

[0010] The power bifurcation Jacobian matrix equation is specifically expressed as follows:

[0011] ;

[0012] in:

[0013] ;

[0014] ;

[0015] ;

[0016] ;

[0017] In the above expression, the active power of the DC side of conventional DC LCC-HVDC is expressed as Indicates voltage Indicates current Indicates the shut-off angle γ Indicates and commutation reactance Χ express, φ Indicates the power factor of conventional DC. Conventional DC AC voltage is expressed as U 1 ∠δ 1 express, k is the ratio of the voltage reference value of the DC side to the AC side of the conventional DC; the active power of the DC side of the flexible DC VSC-HVDC is Indicates reactive power Indicates conventional DC / AC voltage U 2 ∠δ 2 express, Represents the admittance between nodes m and n, J PdId The subscripts are all partial derivatives between the DC side and AC measured variables. The closest power equation can be obtained.

[0018] Preferably, the power interaction factor is calculated by the power bifurcation Jacobian matrix equation The power interaction factor is the Jacobian matrix in the power bifurcation equation J power bifurcation Based on this, it is reduced to the order using the Schur complement formula, which can be expressed as follows:

[0019] .

[0020] Preferably, S2 calculates the determinant of the power interaction factor and simplifies it to obtain an equation characterizing the power interaction of the hybrid DC feed-in system, which is specifically in the following form:

[0021] ;

[0022] ;

[0023] ;

[0024] ω represents the angular frequency of the system, Bc represents the admittance of the filter capacitor, and the active power of the DC side of the conventional DC LCC-HVDC is expressed as The apparent power injected into the AC side by the flexible direct current VSC-HVDC is represented by S2, and the active power is represented by Indicates reactive power Denotes, det(B) represents the determinant of the system admittance matrix, It represents the admittance between nodes m and n. Its physical meaning is clear.

[0025] Preferably, S2 further includes defining the short-circuit ratio of the hybrid DC feed system, converting the power interaction characteristic equation into an explicit function of the short-circuit ratio, and solving it to obtain the critical short-circuit ratio of the hybrid DC feed system. Specifically, SCR and CSCR are expressed as follows:

[0026] ;

[0027] ;

[0028] .

[0029] Facilitates calculation of system stability.

[0030] Preferably, S3 includes performing qualitative analysis using sensitivity analysis. The specific expression of the sensitivity analysis method is as follows:

[0031] ;

[0032] ;

[0033] According to the above analysis, we can get that The greater the active power injected into the AC side by the flexible DC, the greater the coefficient a affecting the critical short-circuit ratio CSCR, the greater the critical short-circuit ratio, the smaller the difference between the short-circuit ratio and the critical short-circuit ratio, the closer the system is to the power bifurcation point, and the smaller the stability margin; The greater the reactive power injected by the flexible DC into the AC side, the smaller the coefficient a affecting the critical short-circuit ratio CSCR, the smaller the critical short-circuit ratio, the greater the difference between the short-circuit ratio and the critical short-circuit ratio, the farther the system is from the power bifurcation point, and the greater the power stability margin.

[0034] Preferably, the determination method in S2 includes determining the system stability margin, where the system stability margin is the difference between the short circuit ratio and the critical short circuit ratio. When the short circuit ratio (SCR) of the hybrid DC feed-in system is greater than the critical short circuit ratio (CSCR), the system does not lose power controllability; when the short circuit ratio (SCR) is less than the critical short circuit ratio (CSCR), the system loses power controllability. The difference between the SCR and the CSCR reflects the system stability margin, and the greater the difference between the SCR and the CSCR, the farther the system operating point is from the power bifurcation point, and the better the power stability.

[0035] The embodiments of the present invention have the following advantages:

[0036] (1) A method for evaluating the power interaction intensity of hybrid DC feed-in systems is proposed. The proposed short-circuit ratio indicator has a clear physical meaning and a distinct boundary value. It can accurately determine the power stability of hybrid DC multi-feed systems and has guiding significance for the planning and safe and stable operation of DC projects. BRIEF DESCRIPTION OF THE DRAWINGS

[0037] To more clearly illustrate the technical solutions of the present invention or the prior art, the following briefly introduces the drawings required for the embodiments or the prior art descriptions. Obviously, the drawings described below are merely illustrative, and those skilled in the art can derive other implementation drawings based on the provided drawings without inventive effort.

[0038] The structures, proportions, sizes, etc. illustrated in this specification are intended only to complement the contents disclosed herein and to facilitate understanding and reading by persons familiar with the art. They are not intended to limit the conditions under which the present invention may be implemented and therefore have no substantive technical significance. Any structural modifications, changes in proportions, or adjustments in sizes shall remain within the scope of the technical contents disclosed herein without affecting the efficacy and objectives of the present invention.

[0039] Figure 1 It is a schematic diagram of the method steps of the present invention.

[0040] Figure 2 This is a schematic diagram of an AC / DC hybrid system model considering hybrid DC feed in the present invention.

[0041] Figure 3 This is the DC standard model structure of CIGRE adopted in the present invention.

[0042] Figure 4 Schematic diagram showing the error comparison between PSCAD / EMTDC simulation and the calculation method of the present invention under different short-circuit ratios in the simulation verification of an embodiment of the present invention.

[0043] In the picture:

[0044] 1-Power bifurcation point obtained by simulation; 2-Short circuit ratio = critical short circuit ratio; 3-Short circuit ratio = 2.2; 4-Short circuit ratio = 2.3. DETAILED DESCRIPTION

[0045] The following describes the implementation of the present invention using specific embodiments. Those skilled in the art will readily understand the other advantages and benefits of the present invention from the disclosure herein. Obviously, the embodiments described are only a portion of the present invention, not all of it. All other embodiments derived by persons of ordinary skill in the art based on the embodiments of the present invention without inventive effort are intended to fall within the scope of protection of the present invention.

[0046] like Figure 1-4 As shown, in a preferred embodiment, the present invention discloses a method for analyzing power interaction of an AC / DC system considering hybrid DC feed, comprising the following steps:

[0047] S1. Calculate the power interaction factor of the hybrid DC feed-in system;

[0048] S2. judging the power bifurcation characteristics of the hybrid DC feed-in system according to the difference between the short-circuit ratio and the critical short-circuit ratio;

[0049] S3. Analyze the impact of active and reactive power injected into the AC side of a hybrid infeed system from flexible DC on the critical short-circuit ratio and power stability. Accurately determine the power interaction intensity of the receiving grid considering hybrid DC infeed.

[0050] S1 includes calculating the power bifurcation Jacobian matrix equation suitable for power stability analysis of hybrid DC feed-in systems based on the quasi-steady-state model and power flow calculation equations of the AC / DC hybrid system.

[0051] The power bifurcation Jacobian matrix equation is specifically expressed as follows:

[0052] ;

[0053] in:

[0054] ;

[0055] ;

[0056] ;

[0057] ;

[0058] In the above expression, the active power of the DC side of conventional DC LCC-HVDC is expressed as Indicates voltage Indicates current Indicates the shut-off angle γ Indicates and commutation reactance Χ express, φ Indicates the power factor of conventional DC. Conventional DC AC voltage is expressed as U 1 ∠δ 1 express, k is the ratio of the voltage reference value of the DC side to the AC side of the conventional DC; the active power of the DC side of the flexible DC VSC-HVDC is Indicates reactive power Indicates conventional DC / AC voltage U 2 ∠δ 2 express, Represents the admittance between nodes m and n, J PdId etc. are all partial derivatives between the DC side / AC measured variables corresponding to the subscripts.

[0059] The power interaction factor is calculated by the power bifurcation Jacobian matrix equation. It is specifically expressed as follows:

[0060] .

[0061] S2 includes the use of saddle-node bifurcation analysis methods to calculate the equations of power interaction characteristics through matrix transformation. The equations characterizing the power interaction of the hybrid DC feed-in system are expressed as follows:

[0062] ;

[0063] ;

[0064] ;

[0065] ω represents the angular frequency of the system, Bc represents the admittance of the filter capacitor, and the active power of the DC side of the conventional DC LCC-HVDC is expressed as The apparent power injected into the AC side by the flexible direct current VSC-HVDC is represented by S2, and the active power is represented by Indicates reactive power Denotes, det(B) represents the determinant of the system admittance matrix, Indicates the admittance between nodes m and n.

[0066] S2 also includes defining the short-circuit ratio of the hybrid DC feed system, converting the power interaction characteristic equation into an explicit function of the short-circuit ratio, and solving it to obtain the critical short-circuit ratio of the hybrid DC feed system. The SCR and CSCR are specifically expressed as follows:

[0067] ;

[0068] ;

[0069] .

[0070] S3 includes qualitative analysis using sensitivity analysis. The specific expression of the sensitivity analysis method is as follows:

[0071] ;

[0072] ;

[0073] According to the above analysis, we can get that The greater the active power injected into the AC side by the flexible DC, the greater the coefficient a affecting the critical short-circuit ratio CSCR, the greater the critical short-circuit ratio, the smaller the difference between the short-circuit ratio and the critical short-circuit ratio, the closer the system is to the power bifurcation point, and the smaller the stability margin; The greater the reactive power injected by the flexible DC into the AC side, the smaller the coefficient a affecting the critical short-circuit ratio CSCR, the smaller the critical short-circuit ratio, the greater the difference between the short-circuit ratio and the critical short-circuit ratio, the farther the system is from the power bifurcation point, and the greater the power stability margin.

[0074] The judgment method in S2 includes determining the system stability margin, which is the difference between the short-circuit ratio and the critical short-circuit ratio. When the hybrid DC-fed system short-circuit ratio (SCR) is greater than the critical short-circuit ratio (CSCR), the system maintains power controllability. When the short-circuit ratio (SCR) is less than the critical short-circuit ratio (CSCR), the system loses power controllability. The difference between the SCR and CSCR reflects the system stability margin. The larger the difference between the SCR and CSCR, the further the system operating point is from the power bifurcation point, indicating better power stability.

[0075] like Figure 1As shown, the method of the present invention is used for processing. Typical parameters of DC equipment are used in combination with specific grid information to calculate the short-circuit ratio and critical short-circuit ratio of the hybrid DC feed-in system. The power stability of the system is judged, and the critical short-circuit ratio obtained by the simulation model built with PSCAD / EMTDC is compared with the numerical result calculated by the present invention.

[0076] In PSCAD / EMTDC software, Figure 2 The hybrid DC feed system is shown in Figure 2. The specific parameters of the DC system are as follows: Figure 3 The flexible DC in the system adopts constant power control mode.

[0077] in, 、 Indicates the active power of conventional DC and flexible transmission; 、 is the reactive power absorbed by the two DC circuits respectively; Z 11 ∠θ 1 、Z 22 ∠θ 2 、Z 12 ∠θ 3 is the Thevenin equivalent impedance of the AC system; U 1 ∠δ 1 、 U 2 ∠δ 2 Respectively represent the AC bus voltage amplitude and phase angle of the converter station; 、 is the equivalent admittance of the reactive compensation devices on bus 1 and bus 2; 、 Represents the equivalent electromotive force of each AC system.

[0078] The control parameter T of the conventional DC system is set to 1.5. The system network parameters considering hybrid DC access are shown in Table 1, and the system benchmark values ​​are shown in Table 2.

[0079] Table 1 System network parameters

[0080] Line impedance number Impedance per unit value Line impedance number Impedance per unit value Line impedance number Impedance per unit value Z1 0.6 Z2 0.8 Z12 0.05

[0081] Table 2 System per unit base value

[0082] system Voltage / kV Power / MW Inverter side AC system 230 990 Conventional DC 525 1 000 Flexible DC 115

[0083] Figure 4The black dot in the middle table represents the short-circuit ratio calculated when the system reaches the power instability point in the PSCAD simulation model. The corresponding curve corresponds to the critical point of the system reaching power bifurcation obtained using the numerical calculation method proposed in this invention. It can be seen that the curve of the critical condition for the system to reach static voltage stability in the electromagnetic transient simulation is very close to the curve of the system reaching the critical generalized short-circuit ratio (SCR=CgSCR=2), and the maximum relative error is 1.04%, which proves the accuracy of the short-circuit ratio indicator and power stability margin calculation method proposed in this paper. In addition, the larger the system's short-circuit ratio, the stronger the system's electric power stability is, the farther it is from the critical stability curve, and the larger the stability margin is, which can be consistent with the characteristics of the DC single-input short-circuit ratio.

[0084] Although the present invention has been described in detail above using general descriptions and specific embodiments, it will be apparent to those skilled in the art that modifications and improvements may be made thereto. Therefore, such modifications and improvements, without departing from the spirit of the present invention, are intended to be within the scope of protection claimed herein.

Claims

1. A method for analyzing power interaction of an AC / DC system considering hybrid DC feed, characterized in that: The steps include: S1. Based on the quasi-steady-state model and power flow calculation equation of the AC / DC hybrid system, the power bifurcation Jacobian matrix equation is calculated. The power interaction factor of the hybrid DC feed-in system is calculated through the power bifurcation Jacobian matrix equation. S2. An analytical method using a saddle-node bifurcation is used to calculate an equation for power interaction characteristics by transforming a power interaction factor matrix, converting the power interaction characteristic equation into an explicit function of the short-circuit ratio, solving the equation to obtain the critical short-circuit ratio of the hybrid DC feed-in system, and determining the power bifurcation characteristics of the hybrid DC feed-in system based on the difference between the short-circuit ratio and the critical short-circuit ratio; S3. Analyze the impact of active and reactive power injected into the AC side of the hybrid feed system by flexible DC on the critical short-circuit ratio index and power stability.

2. The method for analyzing power interaction of an AC / DC system considering hybrid DC feed-in according to claim 1, characterized in that: The S1 includes calculating a power bifurcation Jacobian matrix equation suitable for power stability analysis of a hybrid DC feed-in system based on a quasi-steady-state model and a power flow calculation equation of the AC / DC hybrid system.

3. The method for analyzing power interaction of an AC / DC system considering hybrid DC feed-in according to claim 2, characterized in that: The power interaction factors are calculated using the power bifurcation Jacobian matrix equation.

4. A method for analyzing power interaction of an AC / DC system considering hybrid DC feed-in according to claim 1 or 3, characterized in that: The S2 calculates the determinant of the power interaction factor, expands and simplifies it to obtain an equation characterizing the power interaction of the hybrid DC feed system.

5. The method for analyzing power interaction of an AC / DC system considering hybrid DC feed-in according to claim 4, characterized in that: The S2 also includes defining the short-circuit ratio of the hybrid DC feeding system, converting the power interaction characteristic equation into an explicit function of the short-circuit ratio, and solving it to obtain the critical short-circuit ratio of the hybrid DC feeding system.

6. A method for analyzing power interaction of an AC / DC system considering hybrid DC feed-in according to claim 1 or 5, characterized in that: The S3 involves performing a qualitative analysis using sensitivity analysis.

7. The method for analyzing power interaction of an AC / DC system considering hybrid DC feed-in according to claim 1, characterized in that: The determination method in S2 includes determining the system stability margin, where the system stability margin is the difference between the short circuit ratio and the critical short circuit ratio.

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