Long-term voltage calculation method for power systems involving discrete actions

Abstract

The present invention provides a method for calculating medium- and long-term voltage of an electric power system including discrete actions, comprising: establishing a medium- and long-term quasi-steady-state model of the electric power system; using a collocation method to approximate a model curve affected by a variable parameter in the model to obtain an approximated voltage trajectory; selecting any adjacent first discrete action and second discrete action, obtaining a first voltage and a first voltage derivative corresponding to the first discrete action in the medium- and long-term quasi-steady-state model at a moment before the first discrete action, and obtaining a second voltage and a second voltage derivative corresponding to the second discrete action in the medium- and long-term quasi-steady-state model at a moment after the second discrete action; obtaining a voltage polynomial function according to the first voltage, the second voltage, the first voltage derivative and the second voltage derivative as the voltage trajectory between the first discrete action and the second discrete action; dividing the approximated voltage trajectory into two sections, a front section and a back section, according to the earliest time when the discrete action occurs, and replacing the rear section of the approximated voltage trajectory with the voltage trajectory between the first discrete action and the second discrete action.

Description

Technical Field

[0001] The present invention relates to the technical field of power systems, and in particular to a method for calculating long-term voltage in a power system including discrete actions. Background Art

[0002] In recent years, the transmission power of power grid has been increasing, and the power system has been operating near the limit point more and more frequently. The voltage instability problem has become an increasingly serious threat to the safe operation of the power grid. Voltage instability is divided into transient voltage instability and medium- and long-term voltage instability at the minute level according to its time frame.

[0003] In the power system, changes in certain parameters can affect the operating state of the system. Such parameters are called variable parameters, such as generator output, reactive compensation, load interruption, etc. With the increase in the operating pressure of the power system and the increase in variable parameters in the power system, the medium- and long-term voltage instability problem of the power system has become increasingly prominent. In the calculation process of the medium- and long-term voltage trajectory, due to the generation of discrete action events, the polynomial approximation method directly approximates the medium- and long-term dynamic process, resulting in a decrease in the approximation accuracy of the medium- and long-term voltage trajectory, and a large approximation error of the voltage trajectory, which brings great challenges to the analysis of the medium- and long-term voltage stability problem. Summary of the invention

[0004] The purpose of the present invention is to provide a method for calculating medium- and long-term voltage in an electric power system including discrete actions, which can obtain medium- and long-term voltage trajectories with higher accuracy, reduce the approximation error of the voltage trajectory, and reduce the difficulty of analyzing medium- and long-term voltage stability problems.

[0005] In order to achieve the above object, the present invention provides a method for calculating medium- and long-term voltage in a power system including discrete actions, comprising:

[0006] A medium- and long-term quasi-steady-state model of the power system is established based on the state variables of fast dynamic elements, algebraic variables of the power system, state variables of slow dynamic elements, state variables of discrete actions, and variable parameters;

[0007] From the medium- and long-term quasi-steady-state model, a collocation method is used to approximate a model curve affected by variable parameters to obtain an approximate voltage trajectory;

[0008] Select any adjacent first discrete action and second discrete action, obtain a first voltage corresponding to the first discrete action in the medium- and long-term quasi-steady-state model at a moment before the first discrete action and a first voltage derivative of the first voltage with respect to time, and obtain a second voltage corresponding to the second discrete action in the medium- and long-term quasi-steady-state model at a moment after the second discrete action and a second voltage derivative of the second voltage with respect to time;

[0009] Obtaining a voltage polynomial function according to the first voltage, the second voltage, the first voltage derivative and the second voltage derivative to represent a voltage trajectory between a first discrete action and a second discrete action; and

[0010] The approximate voltage trajectory is divided into two sections according to the earliest time when the discrete action occurs, and the voltage trajectory between the first discrete action and the second discrete action is used to replace the latter section of the approximate voltage trajectory. The front section of the approximate voltage trajectory and the voltage trajectory between the first discrete action and the second discrete action constitute a medium- and long-term voltage trajectory.

[0011] Optionally, in the method for calculating medium- and long-term voltage in the power system, the medium- and long-term quasi-steady-state model is as follows:

[0012]

[0013] Among them, x is the state variable of the fast dynamic element in the power system, y is the algebraic variable in the power system, and z c is the state variable of the slow dynamic element in the power system, z d is the state variable of the discrete action in the power system, p is the medium- and long-term variable parameter of the power system, f represents the equilibrium form of the differential equation describing the fast dynamic element; g represents the algebraic equation based on the power system network equation; h c represents the differential equation describing the slow dynamic element; h d represents the discrete equation that describes the process of discrete actions; x - is the state variable of the fast dynamic element in the power system at the moment before the discrete action occurs; - is the algebraic variable in the power system at the previous moment when the discrete action occurs; is the state variable of the slow dynamic element in the power system at the moment before the discrete action occurs; It is the state variable of the discrete action in the power system at the moment before the discrete action occurs.

[0014] Optionally, in the method for calculating the medium- and long-term voltage in the power system, the state variables of the fast dynamic element include: generator speed, excitation winding flux and power angle.

[0015] Optionally, in the method for calculating the medium- and long-term voltage of the power system, the algebraic variables of the power system include: node voltages of the power system, node currents of the power system, input power of the generator set, and output power of the generator set.

[0016] Optionally, in the method for calculating the medium- and long-term voltage in the power system, the state variables of the slow dynamic element include: the recovery amount of the self-recovery load and the reactive power limit amount of the overexcitation limiter.

[0017] Optionally, in the method for calculating the medium- and long-term voltage in the power system, the state variables of the discrete actions include: on-load tap-changing transformer action and overexcitation limiter action.

[0018] Optionally, in the method for calculating the long-term voltage in the power system, the variable parameters include: load shedding time and load shedding amount.

[0019] Optionally, in the method for calculating the long-term voltage in the power system, the method for using the collocation method to approximate the model curve affected by the variable parameters to obtain the approximate voltage trajectory includes:

[0020]

[0021] Where: v(t; p) represents the simulated voltage trajectory in compact form of variables x and y; v(t; p) and To approximate the voltage trajectory.

[0022] Optionally, in the method for calculating the medium and long term voltage in the power system, the method for obtaining the first voltage derivative includes:

[0023]

[0024] in: is the first voltage, is the first voltage derivative, is the time point t at which the medium- to long-term discrete action event occurs i,j (p) The voltage at the next moment Δt, where Δt is the step size of the numerical simulation.

[0025] Optionally, in the method for calculating the medium and long term voltage in the power system, the method for obtaining the second voltage derivative includes:

[0026]

[0027] in: is the second voltage, is the second voltage derivative, is the time point t at which the medium- to long-term discrete action event occurs i,j (p) The voltage at the previous time Δt, where Δt is the step size of the numerical simulation.

[0028] Optionally, in the method for calculating the medium- and long-term voltage in the power system, a method for obtaining a voltage polynomial function according to the first voltage, the second voltage, the first voltage derivative and the second voltage derivative includes:

[0029]

[0030] Where: G θ(t; p) is the voltage trajectory polynomial between the first discrete action and the second discrete action; θ0(p), θ1(p), θ2(p), θ3(p) are the coefficients of the polynomial.

[0031] In the method for calculating medium- and long-term voltage in an electric power system including discrete actions provided by the present invention, discrete actions are taken into account when calculating medium- and long-term voltages, thereby reducing the approximation error of the voltage trajectory and obtaining a more accurate medium- and long-term voltage trajectory. BRIEF DESCRIPTION OF THE DRAWINGS

[0032] Figure 1 is a flow chart of a method for calculating long-term voltage in a power system including discrete actions according to an embodiment of the present invention;

[0033] Figure 2 Schematic diagram of the voltage and the first-order derivative of the voltage with respect to time at the time point when the medium- and long-term discrete action occurs in an embodiment of the present invention;

[0034] Figure 3 It is a schematic diagram comparing the third-order approximation voltage trajectory and the medium- and long-term actual voltage curve according to an embodiment of the present invention. DETAILED DESCRIPTION

[0035] The specific implementation of the present invention will be described in more detail below in conjunction with the schematic diagram. The advantages and features of the present invention will become clearer based on the following description. It should be noted that the drawings are all in a very simplified form and are not in exact proportions, and are only used to facilitate and clearly assist in explaining the purpose of the embodiments of the present invention.

[0036] Hereinafter, the terms "first", "second", etc. are used to distinguish between similar elements and are not necessarily used to describe a particular order or chronological sequence. It is to be understood that these terms used in this manner are interchangeable where appropriate. Similarly, if the method described herein includes a series of steps, the order of these steps presented herein is not necessarily the only order in which these steps can be performed, and some of the steps described may be omitted and / or some other steps not described herein may be added to the method.

[0037] Please refer to Figure 1 The present invention provides a method for calculating medium- and long-term voltage in a power system including discrete actions, comprising:

[0038] S11: Establish a medium- and long-term quasi-steady-state model of the power system based on the state variables of fast dynamic elements, algebraic variables of the power system, state variables of slow dynamic elements, state variables of discrete actions and variable parameters;

[0039] S12: From the medium- and long-term quasi-steady-state model, the model curve affected by the variable parameters is approximated by the collocation method to obtain the approximate voltage trajectory;

[0040] S13: Select any adjacent first discrete action and second discrete action, obtain a first voltage corresponding to the first discrete action in the medium- and long-term quasi-steady-state model at a moment before the first discrete action and a first voltage derivative of the first voltage with respect to time, and obtain a second voltage corresponding to the second discrete action in the medium- and long-term quasi-steady-state model at a moment after the second discrete action and a second voltage derivative of the second voltage with respect to time;

[0041] S14: acquiring a voltage polynomial function according to the first voltage, the second voltage, the first voltage derivative, and the second voltage derivative, to represent a voltage trajectory between the first discrete action and the second discrete action; and

[0042] S15: Divide the approximate voltage trajectory into two sections according to the earliest time when the discrete action occurs, and use the voltage trajectory between the first discrete action and the second discrete action to replace the latter section of the approximate voltage trajectory. The front section of the approximate voltage trajectory and the voltage trajectory between the first discrete action and the second discrete action constitute the medium- and long-term voltage trajectory.

[0043] It is known that the medium- and long-term quasi-steady-state model of the power system consists of a set of differential-algebraic-discrete equations. Combined with the influence of variable parameters on the medium- and long-term processes of the power system, the medium- and long-term quasi-steady-state model can be expressed as follows:

[0044]

[0045] Among them, x is the state variable of the fast dynamic element in the power system, y is the algebraic variable in the power system, and z c is the state variable of the slow dynamic element in the power system, z d is the state variable of the discrete action in the power system, p is the medium- and long-term variable parameter of the power system, f represents the equilibrium form of the differential equation describing the fast dynamic element; g represents the algebraic equation based on the power system network equation; h c represents the differential equation describing the slow dynamic element; h d represents the discrete equation that describes the process of discrete actions; x - For discrete actions

[0046] - is the state variable of the fast dynamic element in the power system at the previous moment; y is the algebraic variable in the power system at the previous moment when the discrete action occurs; is the state variable of the slow dynamic element in the power system at the moment before the discrete action occurs; It is the state variable of the discrete action in the power system at the moment before the discrete action occurs.

[0047] In an embodiment of the present invention, the state variables of the fast dynamic element include: the state variables of the fast dynamic element include: generator speed, excitation winding flux and power angle. The algebraic variables of the power system include: node voltage of the power system, node current of the power system, input power of the generator set and output power of the generator set. The state variables of the slow dynamic element include: the recovery amount of the self-recovering load and the reactive power limit amount of the overexcitation limiter. The state variables of the discrete action include: on-load tap-changing transformer action and overexcitation limiter action. The variable parameters include: load shedding time and load shedding amount.

[0048] In an embodiment of the present invention, a method for approximating a model curve affected by a variable parameter using a collocation method to obtain an approximate voltage trajectory includes:

[0049]

[0050] Where: v(t; p) represents the simulated voltage trajectory in compact form of variables x and y; v(t; p) and To approximate the voltage trajectory, φ k (p) is the basis function of polynomial approximation, which consists of a set of orthogonal polynomials that satisfy:

[0051]

[0052] Where k is the number of basis functions, k1 and k2 represent any value within the range of k, and <·,·> represents the inner product calculation; k represents the count of the number of basis functions approximated by the polynomial; are the coefficients of the corresponding basis functions; N b represents the number of basis functions of the polynomial approximation; using the collocation method to find c k (t), time domain simulation is required at M parameter matching points to obtain M voltage trajectories (Note: some matching points will be counted multiple times). The calculation method of M is as follows:

[0053]

[0054] Where d represents the number of variable variables; l represents the approximation order; M is the total number of collocation points; Indicates the calculation of the number of combinations; then, the coefficients of the basis functions are calculated according to the following formula:

[0055]

[0056] Where m represents the number of matching points; p m represents the mth parameter matching point; q m For p m The corresponding integral coefficient; E[·] is the expected value calculation.

[0057] Preferably, the method of the first voltage derivative comprises:

[0058]

[0059] in: is the first voltage, is the first voltage derivative, is the time point t at which the medium- to long-term discrete action event occurs i,j (p) The voltage at the next moment Δt, where Δt is the step size of the numerical simulation.

[0060] Preferably, the method for obtaining the second voltage derivative includes:

[0061]

[0062] in: is the second voltage, is the second voltage derivative, is the time point t at which the medium- to long-term discrete action event occurs i,j (p) The voltage at the previous time Δt, where Δt is the step size of the numerical simulation.

[0063] Preferably, the method of obtaining a voltage polynomial function according to the first voltage, the second voltage, the first voltage derivative and the second voltage derivative by using the Hermite interpolation method includes:

[0064]

[0065] Where: G θ (t; p) is the voltage trajectory polynomial between the first discrete action and the second discrete action; θ0(p), θ1(p), θ2(p), θ3(p) are the coefficients of the polynomial.

[0066] In step S15, the approximate voltage trajectory is known and the calculated voltage trajectory G θ (t; p), available G θ (t;p) Replacement The latter part of the , combined into a new medium- and long-term voltage trajectory F affected by variable parameters U (t; p), as the medium- and long-term voltage of the power system obtained by the embodiment of the present invention. This method can indirectly obtain the medium- and long-term voltage trajectory after the discrete action occurs, and the approximate trajectory F under this method U (t; p) has a high degree of overlap with the transient simulation trajectory v(t; p), which improves the approximation accuracy of the voltage trajectory.

[0067] The functional relationship is shown in the following formula:

[0068]

[0069] Among them, t i,j (p) is the time when the discrete action occurs, The voltage trajectory obtained by directly approximating the medium- and long-term process using the collocation method is i,j (p) can be divided into two sections, namely 0≤t<t i,j (p) and t i,j (p)≤t≤t i,j+1 (p) two paragraphs; G θ (t; p) is the voltage trajectory calculated based on the voltage at the time of two adjacent medium- and long-term discrete actions and the corresponding voltage derivative; F U (t; p) is G θ (t;p) Replacement The medium- and long-term voltage trajectory obtained after the discrete action event occurs is G θ (t; p) replace interval t i,j (p)≤t≤t i,j+1 (p) α0(t), α1(t), α2(t), and α3(t) are coefficients in the polynomial approximating the voltage trajectory and can be calculated by the collocation method.

[0070] Example

[0071] In the power system in the embodiment of the present invention, the delay time of the on-load tap-changing transformer is 30s, and the mechanical action time is 6s; except for the generator G1, all generator sets are equipped with overexcitation limiters; the loads all adopt static ZIP loads, and the ZIP parameters of the loads are listed in Table 1, which is a static load ZIP coefficient table.

[0072] Table 1

[0073] Constant impedance coefficient Constant current coefficient Constant power factor Load L1 0.0 0.0 1.0 Load L2 1.0 0.0 0.0

[0074] The fault is a three-phase short circuit in a high-voltage transmission line between nodes 4 and 5, which is then cut off. The discrete action event is the low-voltage load shedding action. When the voltage at node 10 is lower than 0.9pu, the first round of load shedding is triggered after a delay of 6s; when the voltage at node 10 continues to drop and is lower than 0.87pu, the second round of load shedding is triggered after a delay of 2s. The load L2 at node 10 is selected as the interrupted load; the variable parameter is the removal amount of load L2.

[0075] First, a long-term quasi-steady-state model of the power system considering variable parameters is established:

[0076]

[0077] Where x(t) is the state variable of the fast dynamic element in the power system, y(t) is the algebraic variable in the power system, and z c (t) is the state variable of the slow dynamic element in the power system, z d (t + ) is the state variable of the discrete action in the power system, p is the medium- and long-term variable parameter of the power system, f represents the equilibrium form of the differential equation describing the fast dynamic element; g represents the algebraic equation based on the power system network equation; h c represents the differential equation describing the slow dynamic element; h d represents the discrete equation describing the process of discrete action; x(t - ) is the state variable of the fast dynamic element in the power system at the moment before the discrete action occurs; y(t - ) is the algebraic variable in the power system at the moment before the discrete action occurs; z c (t - ) is the state variable of the slow dynamic element in the power system at the moment before the discrete action occurs; z d (t - ) is the state variable of the discrete action in the power system before the discrete action occurs, and p is the amount of load L2 removed during the two rounds of low-voltage load shedding, denoted as p1 and p2 respectively. The range of their variation is p1∈[0,5]%, p2∈[0,5]%.

[0078] Next, the voltage trajectory of node 10 is approximated based on the collocation method:

[0079] In order to quantify the influence of the variable parameter on the voltage trajectory of the node 10, a polynomial relationship between the variable parameter and the voltage of the node 10 is established to obtain the approximate voltage trajectory of the node 10. It can be expressed as:

[0080]

[0081] The approximation order is 3rd order, v(t; p) represents the simulated voltage trajectory in the compact form of variables x and y; v(t; p) and To approximate the voltage trajectory, φ k (p) is the basis function of the polynomial approximation.

[0082] Next, the collocation method is used to approximate the voltage at the time point of the long-term discrete action in the system and the time derivative of the voltage at that time point:

[0083] like Figure 2 As shown in the figure, the collocation method is used to approximate the voltage at the time point of the long-term discrete action in the system and the first-order derivative of the voltage at this time point with respect to time. i,j(p) and the voltage at the moment before and after the discrete action occurs and as well as and First-order voltage derivative with respect to time and The two voltage derivatives obtained are as follows:

[0084]

[0085] In the formula, is the first voltage, is the first voltage derivative, is the time point t at which the medium- to long-term discrete action event occurs i,j (p) The voltage at the next moment Δt, where Δt is the step size of the numerical simulation; is the second voltage, is the second voltage derivative, is the time point t at which the medium- to long-term discrete action event occurs i,j (p) The voltage at the previous time Δt, where Δt is the step size of the numerical simulation. Figure 2 middle, and and and and Represent the voltage at point 0, point 1, point 1', and point 2 and the first-order derivative of the voltage with respect to time respectively; point 0 and point 1 are the voltages at the discrete action time points in the first round of low-voltage load shedding, point 1 is the voltage before the mutation in the first round of low-voltage load shedding, and point 1 is the voltage after the mutation; point 1' and point 2 are the voltages at the discrete action time points in the second round of low-voltage load shedding, point 1' is the voltage before the mutation, and point 2 is the voltage after the mutation.

[0086] Next, based on the voltage and the corresponding derivative at the time points of two adjacent medium- and long-term discrete actions, a voltage polynomial function is calculated:

[0087] According to voltage and And the voltage derivative and A cubic polynomial function can be calculated as follows:

[0088]

[0089] G θ(t; p) is the voltage trajectory polynomial between the first discrete action and the second discrete action; θ0(p), θ1(p), θ2(p), θ3(p) are the coefficients of the polynomial. Next, the voltage trajectory between two adjacent mid- and long-term discrete action occurrence points is used to replace the partial trajectory after the discrete action event in the approximate voltage trajectory:

[0090] Get the calculated voltage trajectory G between two rounds of low voltage load shedding θ (t; p) after, replace the approximate voltage trajectory Middle and G θ The part with the same time interval (t; p) is replaced by the part of the track after the discrete action event occurs in the approximate voltage track, and combined into a new long-term voltage F of the power system affected by the variable parameters. U (t; p). The functional relationship is as follows:

[0091]

[0092] G θ (t;p)=θ0(p)+θ1(p)t+θ2(p)t 2 +θ3(p)t 3 ,(t i,j (p)≤t≤t i,j+1 (p))

[0093]

[0094] Finally, t i,j (p) is the time when the discrete action occurs, The voltage trajectory obtained by directly approximating the medium- and long-term process by the collocation method is i,j (p) can be divided into two sections, namely 0≤t<t i,j (p) and t i,j (p)≤t≤t i,j+1 (p) two paragraphs; G θ (t; p) is the voltage trajectory calculated based on the voltage at the time of two adjacent medium- and long-term discrete actions and the corresponding voltage derivative; F U (t; p) is G θ (t;p) Replacement The medium- and long-term voltage trajectory obtained after the discrete action event occurs is G θ (t; p) replace interval t i,j (p)≤t≤t i,j+1 (p) α0(t), α1(t), α2(t), and α3(t) are the coefficients of the polynomial in the approximation voltage trajectory. Select a set of parameter values ​​and plot the approximation voltage trajectory v(t; p) at node 10 and the long-term voltage F U (t; p) Figure 3 As shown, point 0 and point 1 are the voltages at discrete action time points in the first round of low-voltage load shedding, point 1 is the voltage before the mutation in the first round of low-voltage load shedding, and point 2 is the voltage after the mutation; point 1' and point 2 are the voltages at discrete action time points in the second round of low-voltage load shedding, and similarly, point 1' is the voltage before the mutation, and point 2 is the voltage after the mutation.

[0095] In summary, in the method for calculating medium- and long-term voltage in a power system including discrete actions provided in an embodiment of the present invention, discrete actions are taken into account when calculating medium- and long-term voltages, which reduces the approximation error of the voltage trajectory and obtains a more accurate medium- and long-term voltage trajectory.

[0096] The above is only a preferred embodiment of the present invention and does not limit the present invention in any way. Any technician in the relevant technical field, without departing from the scope of the technical solution of the present invention, makes any form of equivalent replacement or modification to the technical solution and technical content disclosed in the present invention, which does not depart from the content of the technical solution of the present invention and still falls within the protection scope of the present invention.

Claims

1. A method for calculating medium- and long-term voltage in an electric power system including discrete actions, characterized in that: include: A medium- and long-term quasi-steady-state model of the power system is established based on the state variables of fast dynamic elements, algebraic variables of the power system, state variables of slow dynamic elements, state variables of discrete actions, and variable parameters; From the medium- and long-term quasi-steady-state model, a collocation method is used to approximate a model curve affected by variable parameters to obtain an approximate voltage trajectory; Select any adjacent first discrete action and second discrete action, obtain a first voltage corresponding to the first discrete action in the medium- and long-term quasi-steady-state model at a moment before the first discrete action and a first voltage derivative of the first voltage with respect to time, and obtain a second voltage corresponding to the second discrete action in the medium- and long-term quasi-steady-state model at a moment after the second discrete action and a second voltage derivative of the second voltage with respect to time; Obtaining a voltage polynomial function according to the first voltage, the second voltage, the first voltage derivative and the second voltage derivative to represent a voltage trajectory between a first discrete action and a second discrete action; as well as The approximate voltage trajectory is divided into two sections according to the earliest time when the discrete action occurs, and the voltage trajectory between the first discrete action and the second discrete action is used to replace the latter section of the approximate voltage trajectory. The front section of the approximate voltage trajectory and the voltage trajectory between the first discrete action and the second discrete action constitute a medium- and long-term voltage trajectory.

2. The method for calculating medium and long term voltage in an electric power system according to claim 1, characterized in that: The medium- and long-term quasi-steady-state model is as follows: Among them, x is the state variable of the fast dynamic element in the power system, y is the algebraic variable in the power system, and z c is the state variable of the slow dynamic element in the power system, z d is the state variable of the discrete action in the power system, p is the medium- and long-term variable parameter of the power system, f represents the equilibrium form of the differential equation describing the fast dynamic element; g represents the algebraic equation based on the power system network equation; h c represents the differential equation describing the slow dynamic element; h d represents the discrete equation that describes the process of discrete actions; x - is the state variable of the fast dynamic element in the power system at the moment before the discrete action occurs; - is the algebraic variable in the power system at the previous moment when the discrete action occurs; is the state variable of the slow dynamic element in the power system at the moment before the discrete action occurs; It is the state variable of the discrete action in the power system at the moment before the discrete action occurs.

3. The method for calculating medium and long term voltage in an electric power system according to claim 1, characterized in that: The state variables of the fast dynamic element include: generator speed, excitation winding flux and power angle.

4. The method for calculating medium and long term voltage in an electric power system according to claim 1, characterized in that: The algebraic variables of the power system include: node voltages of the power system, node currents of the power system, input power of the generator sets, and output power of the generator sets.

5. The method for calculating medium and long term voltage in an electric power system according to claim 1, characterized in that: The state variables of the slow dynamic element include: the recovery amount of the self-recovery load and the reactive power limit amount of the overexcitation limiter.

6. The method for calculating medium and long term voltage in an electric power system according to claim 1, characterized in that: The state variables of the discrete actions include: on-load tap-changing transformer action and overexcitation limiter action.

7. The method for calculating medium and long term voltage in an electric power system according to claim 1, characterized in that: The variable parameters include: load shedding time and load shedding amount.

8. The method for calculating medium and long term voltage in an electric power system according to claim 2, characterized in that: Methods for approximating a model curve affected by variable parameters using a collocation method to obtain an approximate voltage trajectory include: Where: v(t; p) represents the simulated voltage trajectory in compact form of variables x and y; To approximate the voltage trajectory, φ k (p) is the basis function of polynomial approximation, N b is the number of basis functions of the polynomial approximation, are the coefficients of the basis function and t is the time.

9. The method for calculating medium and long term voltage in an electric power system according to claim 2, characterized in that: The method of obtaining the first voltage derivative includes: in: is the first voltage, is the first voltage derivative, The time point at which medium- to long-term discrete action events occur The voltage at the next moment Δt, where Δt is the step size of the numerical simulation.

10. The method for calculating medium- and long-term voltage in an electric power system according to claim 9, characterized in that: The method of obtaining the second voltage derivative includes: in: is the second voltage, is the second voltage derivative, The time point at which a medium- to long-term discrete action event occurs The voltage at the previous time Δt, where Δt is the step size of the numerical simulation.

11. The method for calculating medium- and long-term voltage in an electric power system according to claim 10, characterized in that: The method for obtaining a voltage polynomial function according to the first voltage, the second voltage, the first voltage derivative and the second voltage derivative comprises: Where: G θ (t; p) is the voltage trajectory polynomial between the first discrete action and the second discrete action; θ0(p), θ1(p), θ2(p) and θ3(p) are the coefficients of the polynomial.

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