A critical short-circuit ratio index calculation method based on power angle stability and steady-state voltage safety

Abstract

This invention discloses a method for calculating the critical short-circuit ratio based on power angle stability and steady-state voltage safety, specifically relating to the field of power system stability and operational safety. This invention obtains the critical short-circuit ratio under power angle stability constraints; obtains the critical short-circuit ratio under steady-state voltage safety constraints; and selects the maximum value of the critical short-circuit ratio under power angle stability constraints and the critical short-circuit ratio under steady-state voltage safety constraints as the critical short-circuit ratio. This invention enables the assessment of the stable operation capability of new energy power systems.

Description

Technical Field

[0001] This invention relates to the field of power system stability and operational safety, specifically to a method for calculating the critical short-circuit ratio of a new energy power system considering steady-state voltage safety and power angle stability constraints. Background Technology

[0002] Due to the reverse distribution characteristics of renewable energy sources in my country, there is a phenomenon of large-scale renewable energy being centrally fed into local areas at the end of the power grid and then transmitted over long distances to weak AC systems. The stability of such renewable energy multi-feeding systems is a key issue that urgently needs to be addressed in the construction of new power systems.

[0003] Because my country's renewable energy resources and electricity load are distributed inversely, large-scale renewable energy power plants, such as wind power in deserts and Gobi and photovoltaic power on plateaus, mostly adopt a method of centralized feeding from multiple renewable energy power plants, which are then collected at a collection station before being transmitted over long distances. These multi-feedback renewable energy systems are often built at the edge of the power grid, and in some areas, the voltage level of the sending-end AC system is low, the grid architecture is weak, and the voltage support capacity is insufficient. This makes it difficult to meet the national standard for short-circuit ratio, which greatly limits the grid integration capacity of renewable energy and also poses potential system stability risks.

[0004] The short-circuit ratio is commonly used as an indicator to evaluate the voltage support strength of grid-connected systems, reflecting the grid's ability to maintain voltage stability after the integration of new energy power plants. The critical short-circuit ratio, on the other hand, is the short-circuit ratio corresponding to the critical stability of a power plant, reflecting its tolerance to a weak grid. By comparing the relative magnitudes of the short-circuit ratio and the critical short-circuit ratio, the stable operation capability of new energy power plants can be assessed.

[0005] Assessing the stable operation capability of renewable energy power plants also requires calculating the critical short-circuit ratio. Previously, the calculation of the critical short-circuit ratio largely relied on empirical formulas, with values ​​ranging from 2.0 to 2.5. This resulted in an inability to accurately assess the system status. Therefore, proposing a method for calculating the critical short-circuit ratio is of great significance for assessing the stable operation capability of renewable energy power plants under weak power grid conditions and improving the safety and stability of renewable energy grid-connected systems. Summary of the Invention

[0006] In view of the aforementioned deficiencies in the prior art, this invention provides a critical short-circuit ratio calculation system and method to assess the stable operation capability of new energy power systems. This helps improve the safety and stability of new energy grid-connected systems and provides a basis for calculating the maximum grid-connected capacity of new energy.

[0007] This invention provides a method for calculating the critical short-circuit ratio based on power angle stability and steady-state voltage safety, comprising the following steps:

[0008] Obtain the critical short-circuit ratio index under the power angle stability constraint ;

[0009] Obtain the critical short-circuit ratio index under steady-state voltage safety constraints ;

[0010] Select the critical short-circuit ratio index under the power angle stability constraint. Critical short-circuit ratio index under steady-state voltage safety constraints The maximum value is used as the critical short-circuit ratio index.

[0011] Furthermore, the critical short-circuit ratio index under the power angle stability constraint... The methods for obtaining the information include:

[0012] The constraint equations for new energy power plants under power angle stability are obtained by solving the state-space equations of the grid synchronization link at the equilibrium point.

[0013] Based on the power flow equations of the grid-connected system and the constraints under power angle stability, the critical active power output of the grid-connected power station under power angle stability constraints is determined. The critical active power Substituting these values ​​into the short-circuit ratio calculation formula, we obtain the critical short-circuit ratio index under the power angle stability constraint. .

[0014] Furthermore, the critical short-circuit ratio index under the steady-state voltage safety constraint... The methods for obtaining it include:

[0015] Constructing a mathematical model of the fault steady-state voltage of a multi-infeed system:

[0016] ;

[0017] in, For grid connection point For grid connection points The active influence factor;

[0018] The solution yields the output active power limit under steady-state voltage constraints. The output active power limit value under the voltage constraint. Substituting these values ​​into the short-circuit ratio calculation formula, we obtain the critical short-circuit ratio index for new energy power plants under steady-state voltage safety constraints. .

[0019] Furthermore, grid connection points For grid connection points Active Influence Factor for:

[0020] .

[0021] Furthermore, the critical short-circuit ratio index under the power angle stability constraint... for:

[0022] ;

[0023] in, The limit value of output power of grid-type power stations under power angle stability constraints; This is the voltage value of the equivalent AC power supply.

[0024] Furthermore, the critical active power output of the grid-type power station under the power angle stability constraint. for:

[0025] ;

[0026] in, and These are the proportional and integral coefficients of the phase-locked loop, respectively. This is the equivalent inductance for connection to the power grid.

[0027] Furthermore, the critical short-circuit ratio index under the steady-state voltage safety constraint... for:

[0028] ;

[0029] in, This represents the maximum output active power under steady-state voltage constraints.

[0030] Furthermore, the output active power limit value under the steady-state voltage constraint for:

[0031] .

[0032] Compared with the prior art, the present invention has the following beneficial effects:

[0033] This invention differs from traditional methods that rely on empirical formulas to derive the critical short-circuit ratio. It reveals the physical meaning of the critical short-circuit ratio index from a mechanistic analysis perspective. Furthermore, compared to considering only voltage stability constraints, this invention comprehensively considers power angle stability constraints and steady-state voltage safety constraints. The proposed critical short-circuit ratio index is more accurate and effective in assessing the stable operation capability of new energy power systems, and helps to improve the safety and stability of new energy grid-connected systems.

[0034] The following will further explain the concept, specific structure, and technical effects of the present invention in conjunction with the accompanying drawings, so as to fully understand the purpose, features, and effects of the present invention. Attached Figure Description

[0035] Figure 1This is a flowchart illustrating a specific embodiment of the critical short-circuit ratio calculation method of the present invention;

[0036] Figure 2 This is a simulation structure diagram of a multi-feed system according to an embodiment of the present invention;

[0037] Figure 3 This is the curve showing the change of the short-circuit ratio index of the new energy power station in this embodiment of the invention;

[0038] Figure 4 These are the power angle variation curves of the new energy power station under operating states A and B in this embodiment of the invention; wherein, Figure 4 'a' represents the curve showing the change in the field station's power angle under state A. Figure 4 b represents the station power angle variation curve under state B;

[0039] Figure 5 These are the output active power variation curves of the new energy power station under operating states A and B in this embodiment of the invention, wherein... Figure 5 'a' represents the curve showing the change in active power output from the power station under state A. Figure 5 b represents the curve of the station's output active power change under state B;

[0040] Figure 6 These are the voltage change curves at the grid connection point of the new energy power station under operating states A and B in this embodiment of the invention. Figure 6 'a' represents the voltage variation curve at the grid connection point of the substation under state A. Figure 6 b is the voltage change curve at the grid connection point of the substation under state B. Detailed Implementation

[0041] The following specific examples illustrate the implementation of the present invention. Those skilled in the art can easily understand other advantages and effects of the present invention from the content disclosed in this specification. The present invention can also be implemented or applied through other different specific embodiments, and various details in this specification can also be modified or changed based on different viewpoints and applications without departing from the spirit of the present invention. It should be noted that, unless otherwise specified, the following embodiments and features described therein can be combined with each other.

[0042] like Figure 1 As shown, in one specific embodiment, a method for calculating the critical short-circuit ratio based on power angle stability and steady-state voltage safety is provided, including the following steps:

[0043] S1. Obtain the critical short-circuit ratio index under the power angle stability constraint. ;

[0044] Specifically, it includes:

[0045] S11. Establish the state-space equations for the new energy power station at the equilibrium point;

[0046] The operation of renewable energy power plants requires the stability of the power angle of the synchronization element to ensure synchronization with the AC power grid. The state-space equation of the grid-synchronized element at the equilibrium point is:

[0047] ;

[0048] in, It is a coefficient matrix; , for The equilibrium point; .

[0049] S12. Based on the state-space equations, solve the constraint equations of the new energy power station under power angle stability using the Lyapunov indirect method.

[0050] If we consider the grid-connected power station as a controlled current source, then the second-order state-space model of the grid-connected power station based on PLL is as follows:

[0051] ;

[0052] In the formula: and These are the phase and frequency deviations of the grid-type field stations, respectively. , ; and These are the phase angle and angular frequency of the phase-locked loop, respectively. and These are the phase angle and angular frequency of the power grid, respectively. and These are the proportional and integral coefficients of the phase-locked loop, respectively. To match the output current of the grid-type power station, For the equivalent inductance connected to the power grid, This is the voltage value of the equivalent AC power supply.

[0053] Based on the Hamiltonian energy function, when When the converter exhibits synchronous generator characteristics, it supplies power to the grid-connected system; when... At this time, the converter exhibits synchronous motor characteristics and needs to draw power from the grid-connected system. Therefore, the basic condition for grid-connected power plants to transmit active power to the power system is... ,Right now .

[0054] According to the Lyapunov indirect method, if the coefficient matrix All eigenvalues ​​have negative real parts, i.e. ( All eigenvalues ​​in the identity matrix If all coefficients are less than 0, the system is asymptotically stable at the equilibrium point. This is determined by the coefficient matrix based on the synchronization element. It can be seen that, satisfying and At this time, the power angle of the grid-connected renewable energy power station is stable. Therefore, under stable power angle conditions, the constraints that the renewable energy power station needs to satisfy can be expressed as:

[0055] ;

[0056] Step S13: Calculate the active power output limit under the power angle stability constraint based on the solution results of step S12. And define the critical short-circuit ratio index under the power angle stability constraint based on the short-circuit ratio;

[0057] Based on the power flow equations of the grid-connected system and the constraints under power angle stability, the critical active power output of the grid-connected power station under power angle stability constraints is determined. for:

[0058] ;

[0059] The critical short-circuit ratio index is defined as the short-circuit ratio of a power plant under given reactive power conditions and active power limit output. Therefore, under the power angle stability constraint, the critical short-circuit ratio index of a new energy power plant is... for:

[0060] .

[0061] S2. Obtain the critical short-circuit ratio index under steady-state voltage safety constraints. ;

[0062] Specifically, the steps include the following:

[0063] S21. Establish a mathematical analysis model for steady-state voltage after a fault in a new energy power station.

[0064] The voltage phasor method is used to analyze the steady-state voltage security problem after a fault in a multi-infeed system. The magnitude of the voltage drop between the grid connection point voltage and the system potential is approximately equal to the sum of the transverse components of each voltage level:

[0065] ;

[0066] in, For grid connection point The transverse voltage component between the point of convergence and the point of convergence; This is the transverse component of the voltage between the point of convergence and the system potential. For grid connection point The current injected into the system, total A side road.

[0067] The short-circuit capacity of the reactance between the grid connection point and the collection point is:

[0068] ;

[0069] Taking into account that the current injected into the system from the collection point is the sum of the currents injected from each grid connection point, the analytical model for the relationship between the voltage of the grid connection point and the scale of new energy sources and the short-circuit capacity of the system in the fault steady-state voltage security problem of multi-infeed systems is as follows:

[0070] ;

[0071] S22. Based on the aforementioned analysis model, construct the relationship between steady-state voltage and active power after a fault by considering the influencing factors among various grid connection points.

[0072] Further analysis was conducted on the relationship between voltage and power at each grid connection point. , The ratio of the voltage transverse component between the convergence points is:

[0073] ;

[0074] Define and describe the grid connection point With grid connection point Active power influencing factor of the power relationship between them for:

[0075] ;

[0076] Will Substitute the values ​​and use the power at other points as the grid connection point. Representing the power, the steady-state voltage security analysis model for the multi-infeed system is obtained as follows:

[0077] ;

[0078] Rewriting it yields:

[0079] .

[0080] S23. The relationship model between the steady-state voltage and active power is used to solve the output active power limit value under the steady-state voltage constraint, and the critical short-circuit ratio index under the steady-state voltage safety constraint is defined according to the short-circuit ratio.

[0081] Analyzing the relationship between steady-state voltage and active power, the calculation expression for the scale of new energy grid connection is obtained as follows:

[0082] ;

[0083] Analyzing the monotonicity between active power and grid connection point voltage, the derivative of the renewable energy scale with respect to the grid connection point voltage is obtained as follows:

[0084] ;

[0085] When the grid connection point voltage When the derivative is less than zero, for It is a monotonically decreasing function. To ensure the safe and stable operation of the system, based on the low voltage ride-through requirements, the grid connection point voltage... Corresponding to the maximum grid-connected capacity of new energy sources under the condition of satisfying the steady-state voltage safety constraint after a fault, the maximum grid-connected capacity of new energy sources is:

[0086] ;

[0087] The critical short-circuit ratio index is defined as the short-circuit ratio of a power plant under given reactive power conditions and active power limit output. Therefore, the critical short-circuit ratio index of a renewable energy power plant under steady-state voltage safety constraints can be derived. for:

[0088] ;

[0089] S3. Select the critical short-circuit ratio index under the power angle stability constraint. Critical short-circuit ratio index under steady-state voltage safety constraints The maximum value of is used as the critical short-circuit ratio index. That is, the critical short-circuit ratio index based on power angle stability and steady-state voltage safety is defined as:

[0090] .

[0091] To further illustrate the effects of the above embodiments, the following embodiments are used for verification and explanation.

[0092] Build Figure 2 The simulation structure of the multi-infeed system shown is as follows. The multi-infeed system consists of three new energy power stations (each power station is composed of a doubly fed wind turbine) that are connected to the AC system through a collection point. The base capacity is 1000MVA. The structural parameters of the new energy power stations and the system are shown in Table 1 below.

[0093] Table 1 Structural Parameters of New Energy Power Stations and Systems

[0094]

[0095] In this example, we take renewable energy power station 1 as the research object (the same applies to other power stations), and set renewable energy power station 1 to operate in two working states: A: Power station active power output is 1.2 pu; B: Power station active power output is 1.5 pu.

[0096] According to the expressions for the critical short-circuit ratio index under the power angle stability constraint and the critical short-circuit ratio index under the steady-state voltage safety constraint proposed in this invention, the critical short-circuit ratio variation curves under the two constraints are obtained by calculation, as shown below. Figure 3 As shown. The solid line... The critical short-circuit ratio curve under the power angle constraint is shown by the dashed line. This is the critical short-circuit ratio curve under steady-state voltage safety constraints.

[0097] Depend on Figure 3 It can be seen that the short-circuit ratio of station 1 in state A is 2.51, the short-circuit ratio of station 1 in state B is 2.18, the critical short-circuit ratio under power angle constraint is 2.29, and the critical short-circuit ratio under voltage safety constraint is 1.86.

[0098] To verify the operational stability of the power station under operating conditions A and B, a three-phase short circuit occurred on the busbar at 1 second, causing the voltage to drop to 0.2 pu. The fault lasted for 2 seconds, and was cleared at 3 seconds, restoring normal operation. The power angle variation curves of the renewable energy power station under operating conditions A and B are obtained as follows: Figure 4 As shown; the output active power variation curves of the new energy power stations under operating conditions A and B are as follows. Figure 5 As shown; the voltage variation curves at the grid connection point of the new energy power station under operating conditions A and B are as follows. Figure 6 As shown.

[0099] according to Figures 4 to 6 The operating results of station 1 under operating states A and B are shown in Table 2 below.

[0100] Table 2. Operational results of Station 1 under operating conditions A and B.

[0101]

[0102] Simulation curves show that under operating condition A, the power angle of station 1 remained at 2.15 pu during the fault, which did not exceed the safety threshold. After the fault was cleared, the steady-state voltage recovered to 0.91 pu, meeting the voltage safety constraint, and the station operated stably. Under condition B, the steady-state voltage of station 1 recovered to 0.93 pu after the fault was cleared, meeting the voltage safety constraint, but the power angle oscillated and became unstable during the fault. Therefore, station 1 was unstable under operating condition B.

[0103] In summary, according to the critical short-circuit ratio calculation method based on power angle stability and steady-state voltage safety proposed in this invention, the short-circuit ratio value of the new energy power station in operating state A is 2.51, which is greater than 2.29 and 1.86, indicating that the power station 1 is operating stably in operating state A. In operating state B, the short-circuit ratio value is 2.18, which is greater than 1.86 but less than 2.29, indicating that the power station 1 is unstable in operating state B, consistent with the simulation results. However, the traditional method of calculating the critical short-circuit ratio only considering voltage safety results in a short-circuit ratio greater than 1.86 in both operating states A and B for the new energy power station, thus requiring the power station 1 to be assessed as operating stably in both states, which contradicts the simulation results. Therefore, the critical short-circuit ratio calculation method based on power angle stability and steady-state voltage safety proposed in this invention is more accurate and effective in assessing the operational stability of new energy power stations.

[0104] The above embodiments are merely illustrative of the principles and effects of the present invention and are not intended to limit the invention. Any person skilled in the art can modify or alter the above embodiments without departing from the spirit and scope of the present invention. Therefore, all equivalent modifications or alterations made by those skilled in the art without departing from the spirit and technical concept disclosed in the present invention should still be covered by the claims of the present invention.

Claims

1. A method for calculating the critical short-circuit ratio based on power angle stability and steady-state voltage safety, characterized in that, Includes the following steps: Obtain the critical short-circuit ratio index under the power angle stability constraint ; Obtain the critical short-circuit ratio index under steady-state voltage safety constraints ; Select the critical short-circuit ratio index under the power angle stability constraint. Critical short-circuit ratio index under steady-state voltage safety constraints The maximum value is used as the critical short-circuit ratio index.

2. The critical short-circuit ratio calculation method based on power angle stability and steady-state voltage safety according to claim 1, characterized in that, The critical short-circuit ratio index under the power angle stability constraint The methods for obtaining the information include: The constraint equations for new energy power plants under power angle stability are obtained by solving the state-space equations of the grid synchronization link at the equilibrium point. Based on the power flow equations of the grid-connected system and the constraints under power angle stability, the critical active power output of the grid-connected power station under power angle stability constraints is determined. The critical active power Substituting these values ​​into the short-circuit ratio calculation formula, we obtain the critical short-circuit ratio index under the power angle stability constraint. .

3. The critical short-circuit ratio calculation method based on power angle stability and steady-state voltage safety according to claim 1, characterized in that, The critical short-circuit ratio index under steady-state voltage safety constraints The methods for obtaining it include: Constructing a mathematical model of the fault steady-state voltage of a multi-infeed system: ； in, For grid connection point For grid connection points The active influence factor; The solution yields the output active power limit under steady-state voltage constraints. The output active power limit value under the voltage constraint. Substituting these values ​​into the short-circuit ratio calculation formula, we obtain the critical short-circuit ratio index for new energy power plants under steady-state voltage safety constraints. .

4. The critical short-circuit ratio calculation method based on power angle stability and steady-state voltage safety according to claim 3, characterized in that, Grid connection point For grid connection points Active Influence Factor for: 。 5. The critical short-circuit ratio calculation method based on power angle stability and steady-state voltage safety according to claim 1, characterized in that, The critical short-circuit ratio index under the power angle stability constraint for: ； in, The limit value of output power of grid-type power stations under power angle stability constraints; This is the voltage value of the equivalent AC power supply.

6. The critical short-circuit ratio calculation method based on power angle stability and steady-state voltage safety according to claim 5, characterized in that, Critical active power output of grid-type power plants under power angle stability constraints for: ； in, and These are the proportional and integral coefficients of the phase-locked loop, respectively. This is the equivalent inductance for connection to the power grid.

7. The critical short-circuit ratio calculation method based on power angle stability and steady-state voltage safety according to claim 1, characterized in that, The critical short-circuit ratio index under steady-state voltage safety constraints for: ； in, This represents the maximum output active power under steady-state voltage constraints.

8. The critical short-circuit ratio calculation method based on power angle stability and steady-state voltage safety according to claim 7, characterized in that, The output active power limit value under steady-state voltage constraint for: 。

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