Transient stability evaluation method and device for new energy generator set grid-following converter grid-connected system, terminal equipment and computer readable storage medium

Abstract

The invention discloses a transient stability evaluation method and device for a new energy generator set grid-following converter grid-connected system, terminal equipment and a computer readable storage medium, and the method comprises the steps: obtaining system parameters after a fault occurs, and calculating an energy value of each time step after the fault occurs through a dynamic equation and an approximate energy function; whether the energy value of each time step is smaller than a preset key energy value or not is judged, if yes, it is judged that the corresponding time step is in a stable state, and if not, it is judged that the corresponding time step is in an unstable state; taking the maximum time step corresponding to the stable state as the target maximum fault clearing time after the fault occurs, and performing transient stability evaluation according to the target maximum fault clearing time; wherein the dynamic equation and the approximate energy function are determined by the following steps of: constructing the dynamic equation and the energy function according to the structure of the grid-connected system of the new energy generator set and the grid-connected converter; and performing approximate processing on a damping term in the energy function to obtain an approximate energy function.

Description

Technical Field

[0001] The present invention relates to the field of power system stability analysis, and in particular to a transient stability assessment method, device, terminal equipment and computer-readable storage medium for a grid-connected system of a new energy generator set and a grid converter. Background Art

[0002] The high penetration of renewable energy has become a prominent feature of modern power systems. The grid-connected converter is the main interface for connecting renewable energy generation to the grid. The multi-point synchronization between the grid-connected converter and the power grid is achieved through a phase-locked loop (PLL). Compared with traditional devices such as synchronous generators, the phase-locked loop has significantly different transient dynamic characteristics. Its transient stability characteristics under fault conditions need to be further analyzed.

[0003] Existing research shows that the transient equation of the grid-connected converter of the phase-locked loop controlled renewable energy generator is similar to the synchronous machine swing equation in form, both of which are composed of mechanical power terms, electromagnetic power terms and damping terms. However, unlike the constant damping coefficient of the synchronous machine, the damping coefficient of the grid-connected converter varies with the phase angle, and is negative when the phase angle is large; this makes the traditional transient analysis method inapplicable. The equal area method and the Lyapunov method are currently the most widely used direct transient stability analysis methods, among which the energy function of the Lyapunov method is constructed from the perspective of energy conservation. In both methods, the damping term of the system is simply ignored, and it has been proven that this does not introduce a larger stability domain or a radical estimate of the critical cut-off time, which is acceptable. However, when the damping can be negative, conservative estimates cannot be guaranteed. Therefore, the traditional direct method cannot be directly used for transient analysis of the grid-connected converter. The current method to solve the negative damping problem mainly adds a damping term to the energy function, which can more accurately reflect the actual operating status of the system. However, the exact value of the damping energy consumption / accumulation is highly correlated with the transient trajectory and is a complex nonlinear function. The calculation requires a lot of computing resources and time, and real-time online evaluation may not be possible. Summary of the invention

[0004] The embodiments of the present invention provide a transient stability assessment method, apparatus, terminal device and computer-readable storage medium for a grid-connected system of a new energy generator set and a grid-connected converter, which can reduce the amount of damping calculation and improve the calculation efficiency of transient stability analysis, thereby realizing real-time online assessment.

[0005] An embodiment of the present invention provides a transient stability assessment method for a grid-connected system of a new energy generator set and a grid converter, comprising:

[0006] After a fault occurs in the grid-connected system of the new energy generator set and the grid-connected converter, the system parameters of the grid-connected system of the new energy generator set and the grid-connected converter are obtained; the system parameters include: system voltage, impedance load, fault resistance, grid side impedance and converter side impedance;

[0007] According to the system parameters, the energy value of each time step after the fault occurs is calculated through the dynamic equation of the grid-connected system of the new energy generator set and the grid converter and the approximate energy function;

[0008] Determine whether the energy value of each time step is less than the preset critical energy value. If so, it is determined that the corresponding time step of the grid-connected system of the new energy generator set and the grid converter is in a stable state. If not, it is determined that the corresponding time step of the grid-connected system of the new energy generator set and the grid converter is in an unstable state.

[0009] The maximum time step corresponding to the stable state is taken as the target maximum fault removal time after the fault occurs, and the transient stability of the grid-connected system of the new energy generator set and the grid converter is evaluated according to the target maximum fault removal time;

[0010] The dynamic equation and the approximate energy function are determined as follows:

[0011] According to the structure of the grid-connected system of the new energy generator set and the grid-converter, the dynamic equation of the grid-connected system of the new energy generator set and the grid-converter and the energy function of the grid-connected system of the new energy generator set and the grid-converter are constructed;

[0012] The damping term in the energy function is approximated to obtain an approximate energy function.

[0013] Furthermore, the dynamic equation is:

[0014]

[0015]

[0016] P e,c =K i Z eq1 U g ;

[0017] D c ′=D c cos(δ c -θ1)=K p Z eq1 U g cos(δ c -θ1);

[0018] Among them, U cq represents the system voltage of the grid-connected system of the qth renewable energy generator set and the grid converter, Z eq1 and Z eq2 represents the imaginary impedance, θ1 and θ2 represent the phase angle of the imaginary impedance, U g represents the grid side voltage, δ cRepresents the output angle of the phase-locked loop, I c Indicates the grid-connected current, Indicates the grid-connected current I c Phase angle difference with the phase-locked loop d-axis, Z l ′ represents the virtual load impedance. In the pre-fault and post-fault stages, Z l ′=Z l , in the fault-on phase, Z l ′=Z l / / R f , Z l Represents a constant impedance load, R f represents the fault resistance, / / represents the parallel relationship, Z g represents the virtual grid impedance, Z c represents the virtual converter impedance, and the dot on it indicates that the variable is a vector, ω c represents the angular velocity of the transformer side, ω0 represents the angular velocity reference value, P m,c represents the imaginary mechanical power of the dynamic equation, P e,c represents the imaginary electromagnetic power of the dynamic equation, D c ′ represents the virtual damping coefficient, K i Indicates the integral coefficient of the phase-locked loop, K p Represents the proportionality factor of the phase-locked loop.

[0019] Furthermore, the energy function is specifically:

[0020]

[0021] Among them, V tr represents the energy function of the grid-connected system of the new energy generator set and the grid converter, δ c,se Denotes the phase angle of the stable equilibrium point, D c Represents the damping coefficient.

[0022] Furthermore, the damping term in the energy function is approximated to obtain an approximate energy function, including:

[0023] The damping term in the energy function is approximated by the trapezoidal rule to obtain the following first approximate energy function, and the first approximate energy function is used as the approximate energy function:

[0024]

[0025] D c,i+1 =D c cos(δ c,i -θ1);

[0026] Among them, V tr1represents the first approximate energy function, N represents the number of steps of approximate processing, Δt represents the time interval of each step of approximate processing, and D c,i represents the damping coefficient of the i-th step approximation, δ c,i represents the phase angle of the i-th step approximation, ω c,i represents the angular velocity of the approximation process in the i-th step.

[0027] Furthermore, the damping term in the energy function is approximated to obtain an approximate energy function, including:

[0028] The damping term in the energy function is approximated by ray approximation to obtain the following second approximate energy function, which is used as the approximate energy function:

[0029] V tr2 =0.5(ω c -ω0) 2 -P e,c (δ c -δ c,se )+P e,c (cos(δ c -θ1)-cos(δ c,se -

[0030] θ1))+D c ω c sin(δ c -θ1)-D c ω0sin(δ c,se -θ1)+D c Δω c / Δδ c [cos(δ c -θ1)-cos(δ c,se -θ1)];

[0031] Among them, V tr2 represents the second approximate energy function, Δδ c Represents the phase angle difference from the current state to the stable equilibrium point, Δω c Represents the angular velocity difference from the current state to the stable equilibrium point.

[0032] Furthermore, the preset critical energy value is determined by:

[0033] Conduct several electromagnetic transient simulations on the target faults of the grid-connected system of the new energy generator set and the grid converter to obtain the maximum fault removal time without system instability under the target fault;

[0034] According to the maximum fault removal time under which the system does not lose stability under the target fault, the transient stability critical point of the target fault is determined;

[0035] Through the approximate energy function, the energy value required for the grid-connected system of the new energy generator set and the grid converter to go from the stable equilibrium point through the transient stability critical point to the unstable equilibrium point is calculated to obtain the preset key energy value.

[0036] Furthermore, based on the target maximum fault removal time, the transient stability of the grid-connected system of the new energy generator set and the grid converter is evaluated, including:

[0037] Obtain the actual fault clearing time after the fault occurs;

[0038] Compare the actual fault clearing time with the target maximum fault clearing time;

[0039] If the actual fault removal time does not exceed the target maximum fault removal time, it is determined that the grid-connected system of the new energy generator set and the grid converter remains stable after the fault is removed;

[0040] If the actual fault clearing time exceeds the target maximum fault clearing time, it is determined that there is a risk of instability after the new energy generator set and the grid-connected converter grid-connected system clear the fault.

[0041] Based on the above method embodiment, the present invention provides a corresponding device embodiment, including: a system parameter acquisition module, an energy value calculation module, a single time step state determination module and a system transient stability assessment module;

[0042] After a fault occurs in the grid-connected system of the new energy generator set and the grid-connected converter, the system parameters of the grid-connected system of the new energy generator set and the grid-connected converter are obtained; the system parameters include: system voltage, impedance load, fault resistance, grid side impedance and converter side impedance;

[0043] According to the system parameters, the energy value of each time step after the fault occurs is calculated through the dynamic equation of the grid-connected system of the new energy generator set and the grid-converter and the approximate energy function; wherein the dynamic equation and the approximate energy function are determined in the following manner: according to the structure of the grid-connected system of the new energy generator set and the grid-converter, the dynamic equation of the grid-connected system of the new energy generator set and the grid-converter and the energy function of the grid-connected system of the new energy generator set and the grid-converter are constructed; the damping term in the energy function is approximated to obtain the approximate energy function;

[0044] Determine whether the energy value of each time step is less than the preset critical energy value. If so, it is determined that the corresponding time step of the grid-connected system of the new energy generator set and the grid converter is in a stable state. If not, it is determined that the corresponding time step of the grid-connected system of the new energy generator set and the grid converter is in an unstable state.

[0045] The maximum time step corresponding to the stable state is taken as the target maximum fault clearing time after the fault occurs, and the transient stability of the grid-connected system of the new energy generator set and the grid converter is evaluated based on the target maximum fault clearing time.

[0046] Based on the above-mentioned method embodiment, the present invention provides a corresponding terminal device embodiment, including: a processor, a memory, and a computer program stored in the memory and configured to be executed by the processor. When the processor executes the computer program, the steps of the transient stability assessment method of the grid-connected system of the new energy generator set and the grid converter as described in the present invention are implemented.

[0047] Based on the above-mentioned method embodiment, the present invention provides a corresponding computer-readable storage medium embodiment, including: a stored computer program, when the computer program is running, controlling the device where the computer-readable storage medium is located to execute the steps of the transient stability assessment method of the grid-connected system of the new energy generator set and the grid converter as described in the present invention.

[0048] Compared with the prior art, the beneficial effects of the embodiment of this solution are:

[0049] After a fault occurs in the grid-connected system of the new energy generator set and the grid-connected converter, the present invention obtains system parameters of the grid-connected system, including system voltage, impedance load, fault resistance, grid-side impedance and converter-side impedance. Then, for each time step after the fault occurs, according to the system parameters, the energy value of each time step after the fault occurs is calculated through the dynamic equation of the grid-connected system of the new energy generator set and the grid-connected converter and the approximate energy function. The energy value can reflect the energy change of the grid-connected system after the fault occurs. The dynamic equation and the approximate energy function are determined in the following ways: First, according to the structure of the grid-connected system of the new energy generator set and the grid-connected converter, the dynamic equation of the grid-connected system of the new energy generator set and the grid-connected converter and the energy function of the grid-connected system of the new energy generator set and the grid-connected converter are constructed. When constructing the energy function, since the damping term is a complex nonlinear function, direct calculation takes a long time, and even the calculation takes too long. The time exceeds the time for transient stability of the fault. Therefore, the present invention approximates the damping term in the energy function to obtain an approximate energy function, which takes damping into account, thereby more accurately reflecting the actual operating state of the system, while reducing the amount of calculation and improving the calculation efficiency; then, it is determined whether the energy value of each time step is less than the preset key energy value. If so, it is determined that the corresponding time step of the grid-connected system of the new energy generator set and the grid converter is in a stable state. If not, it is determined that the corresponding time step of the grid-connected system of the new energy generator set and the grid converter is in an unstable state, so as to determine the system state of each time step. Finally, the maximum time step corresponding to the stable state is used as the target maximum fault removal time after the occurrence of the fault, and according to the target maximum fault removal time, the transient stability of the grid-connected system of the new energy generator set and the grid converter is evaluated, thereby quickly and accurately reflecting the transient stability of the grid-connected system after the fault occurs. BRIEF DESCRIPTION OF THE DRAWINGS

[0050] Figure 1 It is a flow chart of a transient stability assessment method for a grid-connected system of a new energy generator set and a grid converter provided by an embodiment of the present invention;

[0051] Figure 2 It is a structural schematic diagram of a grid-connected system of a new energy generator set and a grid converter provided in one embodiment of the present invention;

[0052] Figure 3 It is a transient trajectory diagram of a grid-connected system of a new energy generator set and a grid converter provided by an embodiment of the present invention;

[0053] Figure 4 It is a structural schematic diagram of a transient stability assessment device for a grid-connected system of a new energy generator set and a grid converter provided in one embodiment of the present invention. DETAILED DESCRIPTION

[0054] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0055] In the description of the present invention, it should be understood that the terms "first" and "second" are only used for descriptive purposes and cannot be understood as indicating or implying relative importance or implicitly indicating the number of the indicated technical features.

[0056] like Figure 1 As shown, an embodiment of the present invention provides a transient stability assessment method for a grid-connected system of a new energy generator set and a grid converter, the method comprising at least the following steps:

[0057] Step S1: after a fault occurs in the grid-connected system of the new energy generator set and the grid-connected converter, system parameters of the grid-connected system of the new energy generator set and the grid-connected converter are obtained; the system parameters include: system voltage, impedance load, fault resistance, grid side impedance and converter side impedance;

[0058] For step S1, the system block diagram of the grid-connected system of the new energy generator set and the grid-connected converter of the present invention is shown as X, including the grid-connected converter, the ideal voltage source and various impedances, wherein the grid is regarded as an ideal voltage source, and its voltage is represented by U g ∠0° represents: the grid-following converter is regarded as a fixed current amplitude I c A controllable current source is used to connect and convert the electric energy generated by the new energy generator set to the power grid. Its phase angle δ c Determined by the phase-locked loop (PLL); when the system is operating normally, the load is a constant impedance load Z l , when a fault occurs, the fault resistance R f It is connected in parallel with the load impedance, thereby changing the current and voltage characteristics of the system. It should be noted that in the actual power grid transient process, the frequency fluctuation is often relatively small, so in the present invention, the effect of frequency fluctuation on the reactance is ignored, and the reactance in the system is assumed to be constant, which helps to simplify the analysis process while still maintaining sufficient accuracy to evaluate the transient stability of the system.

[0059] In this embodiment, after a fault occurs in the grid-connected system of the new energy generator set and the grid-connected converter, it is necessary to immediately obtain the system parameters of the grid-connected system of the new energy generator set and the grid-connected converter. These system parameters include system voltage, impedance load, fault resistance, grid-side impedance and converter-side impedance. The specific parameter values ​​are shown in Table 1 below, where parameter I c , The corresponding active output of the current transformer is 300MW and the reactive output is 50MVar respectively.

[0060] Table 1 System parameters and their corresponding values

[0061]

[0062] Step S2: according to the system parameters, the energy value of each time step after the fault occurs is calculated through the dynamic equation of the grid-connected system of the new energy generator set and the grid converter and the approximate energy function;

[0063] The dynamic equation and the approximate energy function are determined as follows:

[0064] According to the structure of the grid-connected system of the new energy generator set and the grid-converter, the dynamic equation of the grid-connected system of the new energy generator set and the grid-converter and the energy function of the grid-connected system of the new energy generator set and the grid-converter are constructed;

[0065] The damping term in the energy function is approximated to obtain an approximate energy function.

[0066] For step S2, according to Figure 2 The structure of the grid-connected system of new energy generator sets and grid converters is constructed by combining power system theory, control theory and dynamic system theory. The dynamic equation describes the law of time-varying state of the grid-connected system of new energy generator sets and grid converters, and can reflect the dynamic behavior of the system under normal and fault conditions.

[0067] Preferably, the dynamic equation is specifically:

[0068]

[0069] P e,c =K i Z eq1 U g ;

[0070] D c ′=D c cos(δ c -θ1)=K p Z eq1 U g cos(δ c -θ1);

[0071] Among them, U cq represents the system voltage of the grid-connected system of the qth renewable energy generator set and the grid converter, Z eq1 and Z eq2 represents the imaginary impedance, θ1 and θ2 represent the phase angle of the imaginary impedance, U g Indicates the grid side voltage, δc Represents the output angle of the phase-locked loop, I c Indicates the grid-connected current, Indicates the grid-connected current I c Phase angle difference with the phase-locked loop d-axis, Z l ′ represents the virtual load impedance. In the pre-fault and post-fault stages, Z l ′=Z l , in the fault-on phase, Z l ′=Z l / / R f , Z l Represents a constant impedance load, R f represents the fault resistance, / / represents the parallel relationship, Z g represents the virtual grid impedance, Z c represents the virtual converter impedance, and the dot on it indicates that the variable is a vector, ω c represents the angular velocity of the transformer side, ω0 represents the angular velocity reference value, P m,c represents the imaginary mechanical power of the dynamic equation, P e,c represents the imaginary electromagnetic power of the dynamic equation, D c ′ represents the virtual damping coefficient, K i Indicates the integral coefficient of the phase-locked loop, K p Represents the proportionality factor of the phase-locked loop.

[0072] Specifically, the phase-locked loop is synchronized by detecting the q-axis voltage at the grid connection point. Under normal operation, the phase-locked loop can accurately track the frequency and phase of the grid to ensure a stable connection between the generator set and the grid. When a fault occurs in the system or the fault is cleared, the q-axis grid connection point voltage U cq A sudden change may occur, which in turn causes ω to c mutation.

[0073] Next, based on the physical characteristics and dynamic equations of the system, the energy function of the grid-connected system of the new energy generator set and the grid converter is constructed to quantify the total energy of the system at a certain moment, so as to further analyze the stability and dynamic behavior of the system.

[0074] Preferably, the energy function is specifically:

[0075]

[0076] Among them, V tr represents the energy function of the grid-connected system of the new energy generator set and the grid converter, δ c,se Denotes the phase angle of the stable equilibrium point, D c Represents the damping coefficient.

[0077] Specifically, it can be seen from the mathematical representation of the energy function that the first three terms of the energy function are only related to the current state and the stable equilibrium point (SEP), but the last term, that is, the time integral of the damping term, is related to the system state motion trajectory in the transient process, which enables the energy function to more comprehensively describe the dynamic behavior of the system. However, the time integral of this term also increases the complexity of the energy function. When faced with such a complex energy function, the traditional unstable equilibrium point (UEP) method is difficult to quickly determine the key energy value V cr . The critical energy value is the boundary between the system being able to maintain stability and instability, and is crucial for evaluating the stability of the system. In the power system, the fault transient time may be very short, for example, only 0.2 to 0.3 seconds, but the traditional calculation method may take a longer time, such as 0.5 seconds or even longer, to get the result, which obviously cannot meet the real-time requirements. In order to solve this problem, the present invention simplifies the calculation process of the energy function through approximate calculation by trapezoidal rule approximation or ray approximation, especially the time integral part of the damping term, thereby eliminating the correlation of the state motion trajectory to the calculation result and improving the calculation efficiency.

[0078] In a preferred embodiment, the damping term in the energy function is approximated to obtain an approximate energy function, including:

[0079] The damping term in the energy function is approximated by the trapezoidal rule to obtain the following first approximate energy function, and the first approximate energy function is used as the approximate energy function:

[0080]

[0081] D c,i+1 =D c cos(δ c,i -θ1);

[0082] Among them, V tr1 represents the first approximate energy function, N represents the number of steps of approximate processing, Δt represents the time interval of each step of approximate processing, and D c,i represents the damping coefficient of the i-th step approximation, δ c,i represents the phase angle of the i-th step approximation, ω c,i represents the angular velocity of the approximation process in the i-th step.

[0083] In one embodiment of the present invention, the trapezoidal rule approximation is based on the Newton-Cotter formula, which approximates the original integrated area of ​​each step to a total area of ​​a trapezoid. Through the trapezoidal approximation, the damping term E d,r , which is the last term in the energy function can be rewritten as:

[0084]

[0085] D c,i+1 =D c cos(δ c,i -θ1);

[0086] The damping term E d,r Substituting into the energy function, we can get the approximate energy function V based on the trapezoidal rule approximation: tr1 :

[0087]

[0088] D c,i+1 =D c cos(δ c,i -θ1);

[0089] Where N is the number of steps of approximate processing, Δt is the time interval of each step of approximate processing, and D c,i represents the damping coefficient of the i-th step approximation, δ c,i represents the phase angle of the i-th step approximation, ω c,i represents the angular velocity of the approximation process in the i-th step.

[0090] In a preferred embodiment, the damping term in the energy function is approximated to obtain an approximate energy function, including:

[0091] The damping term in the energy function is approximated by ray approximation to obtain the following second approximate energy function, which is used as the approximate energy function:

[0092] V tr2 =0.5(ω c -ω0) 2 -P e,c (δ c -δ c,se )+P e,c (cos(δ c -θ1)-cos(δ c,se -

[0093] θ1))+D c ω c sin(δ c -θ1)-D c ω0sin(δ c,se -θ1)+D c Δω c / Δδ c [cos(δ c -θ1)-cos(δ c,se -θ1)];

[0094] Among them, Vtr2 represents the second approximate energy function, Δδ c Represents the phase angle difference from the current state to the stable equilibrium point, Δω c Represents the angular velocity difference from the current state to the stable equilibrium point.

[0095] In one embodiment of the present invention, when only the original state and the final state are considered, the variables in the damping term are approximately:

[0096] δ c =δ c,se +λΔδ c ;

[0097] ω c =ω0+λΔω c ;

[0098] Where λ is a construction variable in the range [0,1], Δδ c Represents the phase angle difference from the current state to the stable equilibrium point, Δω c represents the angular velocity difference from the current state to the stable equilibrium point. c =δ c,se +λΔδ c and ω c =ω0+λΔω c Substituting into the energy function, the damping term has only one variable λ, which can be solved analytically. By ray approximation, the damping term E d,r , which is the last term in the energy function can be rewritten as:

[0099] E d,r =D c ω c sin(δ c -θ1)-D c ω0sin(δ c,se -θ1)+D c Δω c / Δδ c [cos(δ c -

[0100] θ1)-cos(δ c,se -θ1)];

[0101] The damping term E d,r Substituting into the energy function, we can get the approximate energy function V based on ray approximation tr2 :

[0102] V tr2 =0.5(ω c -ω0) 2 -P e,c (δ c-δ c,se )+P e,c (cos(δ c -θ1)-cos(δ c,se -

[0103] θ1))+D c ω c sin(δ c -θ1)-D c ω0sin(δ c,se -θ1)+D c Δω c / Δδ c [cos(δ c -θ1)-cos(δ c,se -θ1)];

[0104] Among them, V tr2 represents the second approximate energy function, Δδ c Represents the phase angle difference from the current state to the stable equilibrium point, Δω c Represents the angular velocity difference from the current state to the stable equilibrium point.

[0105] At each time step, the dynamic equation of the grid-connected system of the new energy generator set and the grid converter is used to calculate the system state at each time step. Next, the system status Substitute the approximate energy function to calculate the energy value of the system. This energy value should be able to reflect the energy level of the system at the corresponding time step.

[0106] Step S3: Determine whether the energy value of each time step is less than a preset critical energy value. If so, determine that the corresponding time step of the grid-connected system of the new energy generator set and the grid converter is in a stable state. If not, determine that the corresponding time step of the grid-connected system of the new energy generator set and the grid converter is in an unstable state.

[0107] For step S3, the energy value of each time step calculated in step S2 is compared with the preset critical energy value. If the calculated energy value is less than the preset critical energy value, then it can be determined that the grid-connected system of the new energy generator set and the grid converter is in a stable state at the current time step, which means that the operating state of the system at the corresponding time step after the fault occurs is safe and meets the stability requirements; if the calculated energy value is not less than the preset critical energy value, then the system is determined to be in an unstable state at the current time step, which means that there are potential risks or instability factors in the system at the corresponding time step after the fault occurs, and further analysis and intervention measures are required.

[0108] Preferably, the preset critical energy value is determined by:

[0109] Conduct several electromagnetic transient simulations on the target faults of the grid-connected system of the new energy generator set and the grid converter to obtain the maximum fault removal time without system instability under the target fault;

[0110] According to the maximum fault removal time under which the system does not lose stability under the target fault, the transient stability critical point of the target fault is determined;

[0111] Through the approximate energy function, the energy value required for the grid-connected system of the new energy generator set and the grid converter to go from the stable equilibrium point through the transient stability critical point to the unstable equilibrium point is calculated to obtain the preset key energy value.

[0112] Specifically, first, multiple electromagnetic transient simulations are performed on the target faults of the grid-connected system of the new energy generator set and the grid converter to simulate the dynamic response of the system when a specific fault occurs. By continuously adjusting the fault removal time, the maximum fault removal time that ensures that the system does not lose stability is found. After the fault is removed at this time point, the mutation point of the system state is the transient stability critical point. At the maximum fault removal time, the transient trajectory of the system is as follows Figure 3 As shown. It should be noted that for different fault types, the dynamic response and stability of the system may be different. The maximum fault removal time may only apply to specific fault types and cannot fully reflect the system stability under other fault types, while the critical energy value can be used to evaluate the system's recovery capability after a fault. Therefore, in order to estimate the critical energy value of the unstable equilibrium point (UEP), the total energy change required for the grid-connected system of the new energy generator set and the grid converter to go from the stable equilibrium point (SEP) through the transient stability critical point to the unstable equilibrium point (UEP) is calculated by approximating the energy function, as the critical energy value V cr In this embodiment, the critical energy value V cr It is 67.407. This energy change reflects the energy barrier that the system needs to overcome during the failure process, so it can be used as a key indicator to evaluate the stability of the system.

[0113] Step S4: taking the maximum time step corresponding to the stable state as the target maximum fault clearing time after the fault occurs, and evaluating the transient stability of the grid-connected system of the new energy generator set and the grid converter according to the target maximum fault clearing time;

[0114] In a preferred embodiment, the transient stability of the grid-connected system of the new energy generator set and the grid converter is evaluated according to the target maximum fault removal time, including:

[0115] Obtain the actual fault clearing time after the fault occurs;

[0116] Compare the actual fault clearing time with the target maximum fault clearing time;

[0117] If the actual fault removal time does not exceed the target maximum fault removal time, it is determined that the grid-connected system of the new energy generator set and the grid converter remains stable after the fault is removed;

[0118] If the actual fault clearing time exceeds the target maximum fault clearing time, it is determined that there is a risk of instability after the new energy generator set and the grid-connected converter grid-connected system clear the fault.

[0119] For step S4, the maximum time step corresponding to the stable state calculated in step S3 is used as the target maximum fault removal time after the fault occurs. This time step represents the maximum fault removal time allowed after the system fails, so as to ensure that the system can remain stable. Preferably, in this embodiment, transient stability assessment is performed through the following steps:

[0120] First, obtain the actual fault removal time, which is the time taken from the occurrence of a fault to the time the system removes the fault. Then, compare the actual fault removal time with the target maximum fault removal time. If the actual fault removal time does not exceed the target maximum fault removal time, it means that the fault is removed in time and the system can maintain stable operation after the fault is removed. If the actual fault removal time exceeds the target maximum fault removal time, it means that the fault removal time is too long and the system may not be able to recover stability in time, and there is a risk of system instability.

[0121] In this embodiment, in order to verify the validity and accuracy of the target maximum fault removal time, the fault resistance R f To simulate various fault conditions that may be encountered in the actual operation of the grid-connected system of the new energy generator set and the grid converter, as shown in Table 2, the fault resistance R is calculated by the transient stability evaluation method of the grid-connected system of the new energy generator set and the grid converter of the present invention. f The target maximum fault removal time is calculated under the conditions of 3Ω, 0.5Ω and 0.1Ω respectively. Then, the grid-connected system of the new energy generator set and the grid converter is experimentally tested under the conditions of fault resistance R f The maximum fault removal time that can maintain stable operation when the resistance is 3Ω, 0.5Ω and 0.1Ω is the real maximum fault removal time. The results show that the estimation error is within ±7%, indicating that the target maximum fault removal time can accurately determine whether the system can maintain stable operation after the fault occurs and the fault is removed, which meets the actual application requirements.

[0122] Table 1 Different fault resistance R f Transient analysis results

[0123] <![CDATA[R f ]]> 3Ω 0.5Ω 0.1Ω Target maximum fault clearing time 0.2462s 0.2266s 0.2254s True maximum fault clearing time 0.2635s 0.2214s 0.2158s error 6.57％ -2.35％ -4.45％

[0124] like Figure 4As shown, based on the above method embodiment, a corresponding device embodiment is provided;

[0125] An embodiment of the present invention provides a transient stability assessment device for a grid-connected system of a new energy generator set and a grid converter, comprising: a system parameter acquisition module, an energy value calculation module, a single time step state determination module, and a system transient stability assessment module;

[0126] After a fault occurs in the grid-connected system of the new energy generator set and the grid-connected converter, the system parameters of the grid-connected system of the new energy generator set and the grid-connected converter are obtained; the system parameters include: system voltage, impedance load, fault resistance, grid side impedance and converter side impedance;

[0127] According to the system parameters, the energy value of each time step after the fault occurs is calculated through the dynamic equation of the grid-connected system of the new energy generator set and the grid-converter and the approximate energy function; wherein the dynamic equation and the approximate energy function are determined in the following manner: according to the structure of the grid-connected system of the new energy generator set and the grid-converter, the dynamic equation of the grid-connected system of the new energy generator set and the grid-converter and the energy function of the grid-connected system of the new energy generator set and the grid-converter are constructed; the damping term in the energy function is approximated to obtain the approximate energy function;

[0128] Determine whether the energy value of each time step is less than the preset critical energy value. If so, it is determined that the corresponding time step of the grid-connected system of the new energy generator set and the grid converter is in a stable state. If not, it is determined that the corresponding time step of the grid-connected system of the new energy generator set and the grid converter is in an unstable state.

[0129] The maximum time step corresponding to the stable state is taken as the target maximum fault clearing time after the fault occurs, and the transient stability of the grid-connected system of the new energy generator set and the grid converter is evaluated based on the target maximum fault clearing time.

[0130] It can be understood that the above-mentioned device item embodiments correspond to the method item embodiments of the present invention, which can implement the transient stability assessment method of the grid-connected system of the new energy generator set and the grid converter provided by any one of the above-mentioned method item embodiments of the present invention.

[0131] It should be noted that the device embodiments described above are merely illustrative, and some or all of the modules may be selected according to actual needs to achieve the purpose of the present embodiment. In addition, in the accompanying drawings of the device embodiments provided by the present invention, the connection relationship between the modules indicates that there is a communication connection between them, which may be specifically implemented as one or more communication buses or signal lines. Those of ordinary skill in the art may understand and implement the present invention without creative work.

[0132] Based on the above-mentioned embodiment of the transient stability assessment method for a grid-connected system of a new energy generator set and a grid-connected converter, another embodiment of the present invention provides a terminal device, which includes a processor, a memory, and a computer program stored in the memory and configured to be executed by the processor. When the processor executes the computer program, the transient stability assessment method for a grid-connected system of a new energy generator set and a grid-connected converter according to any embodiment of the present invention is implemented.

[0133] Exemplarily, in this embodiment, the computer program may be divided into one or more modules, which are stored in the memory and executed by the processor to implement the present invention. The one or more module elements may be a series of computer program instruction segments capable of implementing specific functions, which are used to describe the execution process of the computer program in the terminal device.

[0134] The terminal device may be a computing device such as a desktop computer, a notebook, a PDA, a cloud server, etc. The terminal device may include, but is not limited to, a processor and a memory.

[0135] The processor may be a central processing unit (CPU), other general-purpose processors, digital signal processors (DSP), application-specific integrated circuits (ASIC), field-programmable gate arrays (FPGA) or other programmable logic devices, discrete gate or transistor logic devices, discrete hardware components, etc. The general-purpose processor may be a microprocessor or any conventional processor, etc. The processor is the control center of the terminal device, and uses various interfaces and lines to connect various parts of the entire terminal device.

[0136] Based on the above method embodiment, another embodiment is provided: another embodiment of the present invention provides a computer-readable storage medium, including a stored computer program, wherein when the computer program is running, the device where the computer-readable storage medium is located is controlled to execute the transient stability assessment method of the grid-connected system of the new energy generator set and the grid-connected converter described in any one of the above method embodiments of the present invention.

[0137] Wherein, the module / unit integrated with the transient stability assessment device / terminal device of the grid-connected system of the new energy generator set and the grid-connected converter can be stored in a computer-readable storage medium if it is implemented in the form of a software functional unit and sold or used as an independent product. Based on such an understanding, the present invention implements all or part of the processes in the above-mentioned embodiment method, and can also be completed by instructing the relevant hardware through a computer program. The computer program can be stored in a computer-readable storage medium, and the computer program can implement the steps of the above-mentioned various method embodiments when executed by the processor. Wherein, the computer program includes computer program code, and the computer program code can be in source code form, object code form, executable file or some intermediate form, etc. The computer-readable medium may include: any entity or device capable of carrying the computer program code, recording medium, U disk, mobile hard disk, magnetic disk, optical disk, computer memory, read-only memory (ROM, Read-Only Memory), random access memory (RAM, Random Access Memory), electric carrier signal, telecommunication signal and software distribution medium, etc.

[0138] The above is a preferred embodiment of the present invention. It should be pointed out that a person skilled in the art can make several improvements and modifications without departing from the principle of the present invention. These improvements and modifications are also considered to be within the scope of protection of the present invention.

Claims

1. A transient stability assessment method for a grid-connected system of a new energy generator set and a grid converter, characterized in that: include: After a fault occurs in the grid-connected system of the new energy generator set and the grid converter, system parameters of the grid-connected system of the new energy generator set and the grid converter are obtained; The system parameters include: system voltage, impedance load, fault resistance, grid side impedance and converter side impedance; According to the system parameters, the energy value of each time step after the fault occurs is calculated through the dynamic equation of the grid-connected system of the new energy generator set and the grid converter and the approximate energy function; Determine whether the energy value of each time step is less than the preset critical energy value. If so, it is determined that the corresponding time step of the grid-connected system of the new energy generator set and the grid converter is in a stable state. If not, it is determined that the corresponding time step of the grid-connected system of the new energy generator set and the grid converter is in an unstable state. The maximum time step corresponding to the stable state is used as the target maximum fault removal time after the fault occurs, and the transient stability of the grid-connected system of the new energy generator set and the grid converter is evaluated according to the target maximum fault removal time; The dynamic equation and the approximate energy function are determined in the following way: According to the structure of the grid-connected system of the new energy generator set and the grid-converter, the dynamic equation of the grid-connected system of the new energy generator set and the grid-converter and the energy function of the grid-connected system of the new energy generator set and the grid-converter are constructed; The damping term in the energy function is approximated to obtain an approximate energy function.

2. The transient stability assessment method of the grid-connected system of the new energy generator set and the grid converter according to claim 1 is characterized in that: The dynamic equation is specifically: P e，c =K i Z eq1 U g ; D′ c =D c cos(δ c -θ1)=K p Z eq1 U g cos(δ c -θ1); Among them, U cq represents the system voltage of the grid-connected system of the qth renewable energy generator set and the grid converter, Z eq1 and Z eq2 represents the imaginary impedance, θ1 and θ2 represent the phase angle of the imaginary impedance, U g Indicates the grid side voltage, δ c Represents the output angle of the phase-locked loop, I c Indicates the grid-connected current, Indicates the grid-connected current I c Phase angle difference with the d-axis of the phase-locked loop, Z′ l represents the virtual load impedance, before and after the fault, Z′ l =Z l , in the fault-on stage, Z′ l =Z l / / R f , Z l Represents a constant impedance load, R f represents the fault resistance, / / represents the parallel relationship, Z g represents the virtual grid impedance, Z c represents the virtual converter impedance, and the dot on it indicates that the variable is a vector. c represents the angular velocity of the transformer side, ω0 represents the angular velocity reference value, P m，c represents the imaginary mechanical power of the dynamic equation, P e，c represents the imaginary electromagnetic power of the dynamic equation, D′ c represents the imaginary damping coefficient, K i Indicates the integral coefficient of the phase-locked loop, K p Represents the proportionality factor of the phase-locked loop.

3. The transient stability assessment method of the grid-connected system of the new energy generator set and the grid converter according to claim 2 is characterized in that: The energy function is specifically: Among them, V tr represents the energy function of the grid-connected system of the new energy generator set and the grid converter, δ c，se Denotes the phase angle of the stable equilibrium point, D c Represents the damping coefficient.

4. The transient stability assessment method of the grid-connected system of the new energy generator set and the grid converter according to claim 3 is characterized in that: The damping term in the energy function is approximated to obtain an approximate energy function, including: The damping term in the energy function is approximated by the trapezoidal rule to obtain the following first approximate energy function, and the first approximate energy function is used as the approximate energy function: D c，i+1 =D c cos(δ c，i -θ1); Among them, V tr1 represents the first approximate energy function, N represents the number of steps of approximate processing, Δt represents the time interval of each step of approximate processing, and D c，i represents the damping coefficient of the i-th step approximation, δ c，i represents the phase angle of the i-th step approximation, ω c，i represents the angular velocity of the approximation process in the i-th step.

5. The transient stability assessment method of the grid-connected system of the new energy generator set and the grid converter according to claim 4 is characterized in that: The damping term in the energy function is approximated to obtain an approximate energy function, including: The damping term in the energy function is approximated by ray approximation to obtain the following second approximate energy function, and the second approximate energy function is used as the approximate energy function: V tr2 =0.5(ω c -ω0) 2 -P e，c (d c -d c，se )+P e，c (cos(δ c -θ1)-cos(δ c，se -θ1))+D c oh c sin(δ c -θ1)-D c ω0sin(δ c，se -θ1)+D c Give c / DD c [cos(δ c -θ1)-cos(δ c，se -θ1)]; Among them, V tr2 represents the second approximate energy function, Δδ c Represents the phase angle difference from the current state to the stable equilibrium point, Δω c Represents the angular velocity difference from the current state to the stable equilibrium point.

6. The transient stability assessment method of the grid-connected system of the new energy generator set and the grid converter according to claim 1 is characterized in that: The preset critical energy value is determined by: Conduct several electromagnetic transient simulations on the target faults of the grid-connected system of the new energy generator set and the grid converter to obtain the maximum fault removal time without system instability under the target fault; According to the maximum fault removal time under which the system does not lose stability under the target fault, the transient stability critical point of the target fault is determined; By approximating the energy function, the energy value required for the grid-connected system of the new energy generator set and the grid converter to go from the stable balance point to the transient stability critical point to the unstable balance point is calculated to obtain the preset key energy value.

7. The transient stability assessment method of the grid-connected system of the new energy generator set and the grid converter according to claim 1 is characterized in that: According to the target maximum fault removal time, the transient stability of the grid-connected system of the new energy generator set and the grid converter is evaluated, including: Obtain the actual fault clearing time after the fault occurs; Compare the actual fault clearing time with the target maximum fault clearing time; If the actual fault removal time does not exceed the target maximum fault removal time, it is determined that the grid-connected system of the new energy generator set and the grid converter remains stable after the fault is removed; If the actual fault clearing time exceeds the target maximum fault clearing time, it is determined that there is a risk of instability after the new energy generator set and the grid-connected converter grid-connected system clear the fault.

8. A transient stability assessment device for a grid-connected system of a new energy generator set and a grid converter, characterized in that: include: System parameter acquisition module, energy value calculation module, single time step state judgment module and system transient stability assessment module; After a fault occurs in the grid-connected system of the new energy generator set and the grid converter, system parameters of the grid-connected system of the new energy generator set and the grid converter are obtained; The system parameters include: system voltage, impedance load, fault resistance, grid side impedance and converter side impedance; According to the system parameters, the energy value of each time step after the fault occurs is calculated through the dynamic equation of the grid-connected system of the new energy generator set and the grid-converter and the approximate energy function; wherein the dynamic equation and the approximate energy function are determined in the following manner: according to the structure of the grid-connected system of the new energy generator set and the grid-converter, the dynamic equation of the grid-connected system of the new energy generator set and the grid-converter and the energy function of the grid-connected system of the new energy generator set and the grid-converter are constructed; the damping term in the energy function is approximated to obtain the approximate energy function; Determine whether the energy value of each time step is less than the preset critical energy value. If so, it is determined that the corresponding time step of the grid-connected system of the new energy generator set and the grid converter is in a stable state. If not, it is determined that the corresponding time step of the grid-connected system of the new energy generator set and the grid converter is in an unstable state. The maximum time step corresponding to the stable state is taken as the target maximum fault clearing time after the fault occurs, and the transient stability of the grid-connected system of the new energy generator set and the grid converter is evaluated based on the target maximum fault clearing time.

9. A terminal device, characterized in that: It includes a processor, a memory, and a computer program stored in the memory and configured to be executed by the processor. When the processor executes the computer program, it implements the transient stability assessment method of the grid-connected system of the new energy generator set and the grid converter as described in any one of claims 1 to 7.

10. A computer-readable storage medium, characterized in that: include: A stored computer program, wherein when the computer program is running, the device where the computer-readable storage medium is located is controlled to execute the transient stability assessment method of the grid-connected system of the new energy generator set and the grid converter as described in any one of claims 1 to 7.

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