Prejudgment method for maximum acceptable unit off-network power of power grid

Abstract

The invention provides a power grid maximum acceptable unit off-network power pre-judgment method, which comprises the following steps: constructing a power grid equivalent rotor motion equation, linearizing a primary frequency modulation process, and obtaining power vacancy caused by a unit off-network fault; the boundary inertia under the power grid fault condition is represented by using the time of the power vacancy and the frequency extreme value; the inertia loss caused by the fault is homogenized, and the minimum inertia demand of the system under the off-network fault of the unit is obtained; setting a frequency safety boundary condition, and representing the power shortage of the power grid by using the minimum inertia demand of the system; calculating the maximum acceptable unit off-network power of the power grid, and setting a fault according to the maximum acceptable unit off-network power; according to the method, the maximum acceptable unit off-network power meeting the frequency safety boundary requirement is calculated based on the frequency lowest point safety boundary constraint, and guidance is provided for a power grid dispatching department to pre-judge, prevent and control frequency instability.

Description

Technical Field

[0001] The present invention relates to the technical field of power system frequency security and stability analysis, and in particular to a method for predicting the maximum acceptable unit tripping power of a power grid. Background Art

[0002] Due to the continuous increase in the new energy penetration rate of the power system, the frequency anti-disturbance ability decreases, and the probability of power deficit increases. Therefore, it has become urgent to introduce a method for predicting the maximum acceptable unit tripping power of the power grid. When studying the dynamic frequency characteristics of a power system containing new energy, the primary problem to be faced is that the power deficit and inertia loss are unknown. At present, to determine whether the power system frequency is stable after a power grid fault, it is judged by whether the current equivalent inertia of the system is greater than the equivalent critical inertia of the system. However, since the operating state and mode of the power system are constantly changing, the calculated value of the equivalent inertia at the previous moment is not necessarily greater than the critical inertia of the system at the next moment, and the accuracy cannot be guaranteed. Therefore, there is an urgent need for a method for predicting the maximum acceptable unit tripping of the power grid. With the continuous progress of the construction process of the new power system and the continuous increase in the new energy grid connection penetration rate, the equivalent inertia of the power grid is weakened, and the frequency anti-disturbance ability of the power system is significantly reduced. Referring to an online evaluation method for the tripping risk of a wind farm in a source-network-load precise control system with the Chinese patent publication number CN112488434A, the steps are as follows: collecting information of each wind turbine in the wind farm in the normal working state to form an operating section before the accident; during the accident, collecting the terminal voltage of each unit and the corresponding unit body protection action information, and calculating the tripping risk coefficient of each unit; through the operating conditions of each unit collected during the fault, combined with the operating conditions of the unit before the accident, comprehensively calculating the expected tripping power and risk index coefficient of each unit and the wind farm; the source-network-load control master station makes a comprehensive decision based on the tripping risk indicators calculated for each wind farm. This method is used for cutting from the new energy grid connection side, and preferentially cuts off the wind farms and units with high tripping risks. However, it does not consider the widespread grid connection of large-capacity synchronous generator sets and power plants (the installed capacity can be close to 10,000 megawatts). When a large-capacity unit / power plant trips due to a fault, it often causes a sudden drop in the power system frequency and touches the frequency safety operation boundary, resulting in serious consequences of frequency instability. Summary of the Invention

[0003] The present invention solves the problem that the prior art lacks the prediction of large-capacity unit / power plant tripping due to faults, and proposes a method for predicting the maximum acceptable unit tripping power of a power grid. Based on the safety boundary constraint of the lowest frequency point, calculating the maximum acceptable unit tripping power that meets the frequency safety boundary requirements provides guidance for the power grid dispatching department to predict and prevent the occurrence of frequency instability.

[0004] To achieve the above object, the following technical solutions are proposed: A method for predicting the maximum acceptable unit tripping power of a power grid includes the following steps: S1. Construct the equivalent rotor motion equation of the power grid, linearize the primary frequency regulation process, and obtain the power deficit caused by the unit tripping fault. S2. Use the time of the power deficit and the frequency extreme value to characterize the boundary inertia under the power grid fault condition. S3. Homogenize the inertia loss caused by the fault and obtain the minimum system inertia demand under the unit tripping fault. S4. Set the frequency safety boundary condition and use the minimum system inertia demand to characterize the power deficit of the power grid. S5. Calculate the maximum acceptable unit tripping power of the power grid, and then set the fault with the maximum acceptable unit tripping power.

[0005] The present invention calculates the maximum acceptable unit tripping power that meets the frequency safety boundary requirements by establishing the equivalent rotor motion equation of the power grid and based on the frequency lowest point safety boundary constraint. The calculation result can provide guidance for the power grid dispatching department to predict and prevent the occurrence of frequency instability, and is particularly significant for the frequency safety operation of the low-inertia receiving-end power system. Essentially, the maximum acceptable unit tripping power prediction method is obtained through the current system operation state and some approximate processing methods, and can well judge whether the system frequency meets the frequency constraint after the system fails.

[0006] Preferably, the S1 includes the following steps: Equivalent all the power sources in the power grid to a single aggregated synchronous generator set and construct the rotor motion equation of the synchronous generator set. Linearize the active-frequency response of the system after the unit tripping fault occurs to obtain the active power-time response curve and the frequency-time response curve. Extract the time of the power deficit and the system frequency reaching the extreme value during the linearization process of the primary frequency regulation.

[0007] The step 1 of the present invention specifically includes the following steps: Equivalent all the power sources in the power grid to a single aggregated synchronous generator set. The rotor motion equation of the synchronous generator set is: (1) In formula (1), H equ is the inertia of the equivalent generator set, that is, the equivalent inertia of the power grid. f(t) is t the system frequency at time P m ( t ) and P e ( t ) are the instantaneous values of the mechanical power and the electromagnetic power of the equivalent generator set respectively. D reflects the grid damping effect coefficient in the frequency response process (approximately taken as 0.06).∆f(t) is the instantaneous value of the system frequency deviation. If a generator tripping fault occurs at time 0 in the system, when the frequency starts to deviate from the rated frequency, the synchronous generator undergoes t r a time delay and then starts primary frequency regulation, and at t m the frequency drops to the lowest point. Integrating Equation (1) from 0 to t m gives: (2) According to Figure 2 as shown, a linearized simulation is used to represent the active power - frequency response of the system after a generator tripping fault occurs. The active power - frequency in this figure consists of two parts: the active power - time response curve (upper figure) and the frequency - time response curve (lower figure). In the figure, f r and f m represent t r the system frequency at time R s and the lowest frequency point (corresponding to the frequency safety boundary, 49 Hz); P = R s (t m - t r ) m t m = t r +∆ P / R s .

[0008] Preferably, the left - hand side of the rotor motion equation of the synchronous generator set is the product of twice the inertia of the equivalent generator set and the derivative of the system frequency with respect to time, and the right - hand side of the rotor motion equation of the synchronous generator set is the instantaneous value of the mechanical power of the equivalent generator set minus the instantaneous value of the electromagnetic power of the equivalent generator set minus the product of the grid damping effect coefficient and the instantaneous value of the system frequency deviation.

[0009] Preferably, the step S2 includes the following steps: integrating and solving the rotor motion equation of the synchronous generator set from the moment of 0 to the moment when the system frequency reaches the extreme value to obtain the boundary inertia, where the boundary inertia is equal to the sum of the integral value of the overall unbalanced active power of the system from the moment of 0 to the current moment and the integral value of the overall unbalanced active power of the system from the current moment to the moment when the system frequency reaches the extreme value, divided by the difference between twice the system frequency safety boundary constraint value and the system frequency at the current moment, and then subtracting one-fourth of the product of the moment when the system frequency reaches the extreme value and the grid damping effect coefficient.

[0010] The step 2 of the present invention specifically includes the following steps: integrating the rotor motion equation in formula (2) from 0 to t m and performing integral solution to obtain: (3) In formula (3), H bor is the boundary inertia, and its magnitude corresponds to the frequency safety boundary f m requirement; W 1 and W 2 are respectively the integral values of the overall unbalanced active power of the system in the time periods [0, t r and t r , t m ; f m is the system frequency safety boundary constraint, which is 49 Hz; f N is t r the system frequency at the moment of

[0011] Preferably, the step S3 includes the following steps: calculating that the system minimum inertia is equal to the boundary inertia minus the inertia loss when a unit tripping fault occurs in the grid; wherein, homogenizing the inertia in the unit power in the grid, and obtaining that the inertia loss when a unit tripping fault occurs in the grid is equal to the product of the power deficit, the inertia of the equivalent generating unit, and the total number of units in the system, divided by the sum of the rated capacities of all units in the system.

[0012] The step 3 of the present invention specifically includes the following steps: when a unit tripping fault occurs in the system, the system minimum inertia H min = H bor + H loss , where H minis the minimum inertia of the system, H loss is the inertia loss when a generator trips in the power grid. The inertia in unit power in the power grid is homogenized. Therefore, the inertia loss caused by generator tripping is related to the power deficit ∆ P as follows: (4) In formula (4), m is the total number of generators in the system; S Gb is the b th generator's rated capacity in the system. The above inertia loss is the ideal situation after homogenization. However, in an actual power grid, the inertia loss caused by generator tripping may be higher than, equal to, or lower than this homogenized inertia loss H loss . To simulate the actual tripping situation, three different situations of the inertia loss of the tripped generator need to be considered.

[0013] Preferably, S4 includes the following steps: According to the frequency safety boundary conditions, an inequality is constructed using the minimum inertia requirement of the system to characterize the power deficit of the power grid. The inequality is that the minimum inertia requirement of the system is less than or equal to the inertia of the equivalent generating units.

[0014] Step 4 of the present invention specifically includes the following steps: When a generator tripping fault occurs in the system, the equivalent inertia of the system will decrease. If the equivalent inertia of the system is less than the minimum inertia requirement, a serious frequency instability accident will occur in the system. Therefore, it is necessary to ensure that the equivalent inertia of the system in each case is greater than the minimum inertia requirement, that is, to satisfy H equ ≥ H min . When a generator tripping fault occurs in the system, considering the operating conditions of each generator in the system and the impact caused by the tripped generator, from the above inequality relationship, we can obtain: (5) In formula (5), H Gb respectively represent the inertia of the b th synchronous machine in the system; x t ∈[0, 1], 0 represents the generator shutdown state, and 1 represents the generator startup state.

[0015] Preferably, S5 includes the following steps: When the most serious generator tripping fault occurs in the system, the maximum power deficit caused by the generator tripping fault is obtained on the condition that the inertia of the equivalent generating units is greater than or equal to the minimum inertia of the system, and the fault is set with the maximum power deficit caused by the generator tripping fault.

[0016] Step 5 of the present invention specifically includes the following steps: When the most serious unit tripping fault occurs in the system, the constraint of formula (5) still needs to be satisfied. At this time, the maximum power deficit caused by the unit tripping fault is: (6) Preferably, the present invention further includes a simulation example verification step. Set that a unit tripping fault occurs in the system at a certain moment, calculate its power deficit and inertia loss, simulate the frequency response curve under the unit tripping fault, and judge whether the steady-state frequency requirement is satisfied according to the frequency response curve under the unit tripping fault. If so, set the fault with the current maximum acceptable unit tripping power; if not, re-determine the minimum inertia of the system and return to S3.

[0017] Preferably, the condition for judging whether the steady-state frequency requirement is satisfied is to compare whether the lowest frequency point in the frequency response curve is equal to the frequency safety boundary.

[0018] Preferably, when the present invention is verified, three different situations of inertia loss caused by tripped units are considered, which are 75% of the homogenized inertia loss, 100% of the homogenized inertia loss, and 125% of the homogenized inertia loss respectively.

[0019] The beneficial effects of the present invention are as follows: By establishing the equivalent rotor motion equation of the power grid and based on the constraint of the lowest frequency point safety boundary, the present invention calculates the maximum acceptable unit tripping power that meets the frequency safety boundary requirement. The calculation results can provide guidance for the power grid dispatching department to predict and prevent the occurrence of frequency instability, and are particularly important for the frequency safety operation of low-inertia receiving-end power systems. Essentially, the maximum acceptable unit tripping power prediction method is obtained through the current operating state of the system and some approximate processing methods, and can well judge whether the system frequency meets the frequency constraint after a system fault occurs. Brief Description of the Drawings

[0020] Figure 1 It is a flowchart of an embodiment of the present invention.

[0021] Figure 2 It is a schematic diagram of the linearization of the primary frequency regulation response of a synchronous generator set.

[0022] Figure 3 It is a simulation system diagram of an embodiment of the present invention.

[0023] Figure 4 It is a frequency response curve when the unit tripping causes 75% of the homogenized inertia loss.

[0024] Figure 5 It is a frequency response curve when the unit tripping causes 100% of the homogenized inertia loss.

[0025] Figure 6Frequency response curve when the unit is disconnected from the grid, resulting in a 125% loss of uniform inertia. Detailed implementation mode

[0026] Example 1: This example proposes a method for predicting the maximum acceptable power of a unit disconnected from the grid, referring to Figure 1 , and specifically includes the following steps: Step 1: By establishing the equivalent rotor motion equation of the power grid and linearizing the primary frequency regulation process, determine the power deficit caused by the unit disconnection fault; The specific steps of step 1 of the present invention include the following steps: Equivalent all the power sources of the power grid to an aggregated synchronous generator set, and the rotor motion equation of the synchronous generator set is: (1); In formula (1), H equ is the inertia of the equivalent generator set, that is, the equivalent inertia of the power grid; f(t) is t the system frequency at time P m ( t ) and P e ( t ) are respectively the instantaneous values of the mechanical power and electromagnetic power of the equivalent generator set; D reflects the power grid damping effect coefficient in the frequency response process (approximately taken as 0.06); ∆f(t) is the instantaneous value of the system frequency deviation. If a unit disconnection fault occurs at time 0 of the system, when the frequency starts to deviate from the rated frequency, the synchronous generator starts primary frequency regulation after t r delay, and the frequency drops to the lowest point at t m time. Integrate formula (1) from 0 to t m to get: (2); According to Figure 2 shown, use linearization to simulate the active power-frequency response of the system after the unit disconnection fault occurs. The active power-frequency in this figure includes two parts: the active power-time response curve (upper figure) and the frequency-time response curve (lower figure). In the figure, f r and f m respectively represent t r the system frequency at time R sIndicates the equivalent primary frequency regulation droop coefficient of the system (approximately taken as 0.05). Therefore, during the linearization process of primary frequency regulation, the power deficit ∆ P = R s (t m - t r ) , where the time when the system frequency reaches the extreme value t m = t r +∆ P / R s .

[0027] Step 2: According to the equivalent rotor motion equation of the power grid, use the power deficit and the time of frequency extreme value to characterize the boundary inertia under the condition of power grid fault; The step 2 of the present invention specifically includes the following steps: Integrate and solve the rotor motion equation in formula (2) from 0 to t m to obtain: (3); In formula (3), H bor is the boundary inertia, and its magnitude corresponds to the frequency safety boundary f m requirement; W 1 and W 2 are respectively the integral values of the overall unbalanced active power of the system in the time periods [0, t r and t r , t m ; f m is the system frequency safety boundary constraint, which is 49 Hz; f N is t r the system frequency at time

[0028] Step 3: Calculate the minimum inertia requirement of the system under the condition of generator tripping fault, and analyze the inertia loss caused by the fault; The step 3 of the present invention specifically includes the following steps: When a generator tripping fault occurs in the system, the minimum inertia of the system H min = H bor + H loss , where H min is the minimum inertia of the system,H loss It is the inertia loss when a generator trips in the power grid.

[0029] The inertia in unit power in the power grid is homogenized. Therefore, the relationship between the inertia loss caused by generator tripping and the power deficit ∆ P is as follows: (4); In formula (4), m is the total number of generators in the system; S Gb is the b th generator's rated capacity in the system. The above inertia loss is the ideal situation after homogenization. However, in an actual power grid, the inertia loss caused by generator tripping may be higher than, equal to, or lower than this homogenized inertia loss. H loss , To simulate the actual tripping situation, three different situations of the inertia loss of the tripped generator need to be considered.

[0030] Step 4: According to the frequency safety boundary conditions, use the minimum inertia demand of the system to characterize the power deficit of the power grid. The specific steps of Step 4 of the present invention are as follows: When a generator tripping fault occurs in the system, it will cause the equivalent inertia of the system to decrease. If the equivalent inertia of the system is less than the minimum inertia demand, a serious frequency instability accident will occur in the system. Therefore, it is necessary to ensure that the equivalent inertia of the system is greater than the minimum inertia demand in each case, that is, to satisfy H equ ≥ H min . When a generator tripping fault occurs in the system, considering the operating conditions of each generator in the system and the impact caused by the tripped generator, from the above inequality relationship, we can get: (5); In formula (5), H Gb respectively represent the inertia of the b th synchronous machine in the system; x t ∈[0, 1], 0 represents the shutdown state of the generator, and 1 represents the operating state of the generator.

[0031] Step 5: Integrate the above Steps 2, 3, and 4 to calculate the maximum acceptable generator tripping power of the power grid, and then set the fault with the maximum acceptable generator tripping power.

[0032] The specific steps of Step 5 of the present invention are as follows: When the most serious generator tripping fault occurs in the system, the constraint of formula (5) still needs to be satisfied. At this time, the maximum power deficit caused by the generator tripping fault is: (6).

[0033] Embodiment 2: Based on Embodiment 1, this embodiment adds a verification step and proposes a method for predicting the maximum acceptable power of generator sets tripping off the grid in a power grid. Refer to Figure 1 , which specifically includes the following steps: Step 1: By establishing an equivalent rotor motion equation for the power grid and linearizing the primary frequency regulation process, determine the power deficit caused by the generator set tripping fault. The specific steps of Step 1 of the present invention are as follows: Equivalent all power sources in the power grid to an aggregated synchronous generator set. The rotor motion equation of the synchronous generator set is: (1); In formula (1), H equ is the inertia of the equivalent generator set, that is, the equivalent inertia of the power grid; f(t) is t the system frequency at P m ( t ) and P e ( t ) are the instantaneous values of the mechanical power and electromagnetic power of the equivalent generator set respectively; D reflects the grid damping effect coefficient in the frequency response process (approximately taken as 0.06); ∆f(t) is the instantaneous value of the system frequency deviation. If a generator set tripping fault occurs at time 0 in the system, when the frequency begins to deviate from the rated frequency, the synchronous generator starts primary frequency regulation after t r delay, and the frequency drops to the lowest point at t m . Integrate formula (1) from 0 to t m to get: (2); According to Figure 2 shown, use linearization to simulate the active power-frequency response of the system after the generator set tripping fault occurs. The active power-frequency in this figure includes two parts: the active power-time response curve (upper figure) and the frequency-time response curve (lower figure). In the figure, f r and f m respectively represent t r the system frequency at R s and the lowest frequency point (corresponding to the frequency safety boundary, 49 Hz); P = Rs (t m - t r ) , where the time when the system frequency reaches the extreme value t m = t r + ∆ P / R s .

[0034] Step 2: According to the equivalent rotor motion equation of the power grid, use the power deficit and the time of the frequency extreme value to characterize the boundary inertia under the power grid fault; The said Step 2 of the present invention specifically includes the following steps: Integrate and solve the rotor motion equation of formula (2) from 0 to t m to obtain: (3); In formula (3), H bor is the boundary inertia, and its magnitude corresponds to the frequency safety boundary f m requirement; W 1 and W 2 are respectively the integral values of the overall unbalanced active power of the system in the time periods [0, t r and t r , t m ; f m is the system frequency safety boundary constraint, which is 49 Hz; f N is t r the system frequency at time

[0035] Step 3: Calculate the minimum inertia requirement of the system under the generator tripping fault, and analyze the inertia loss caused by the fault; The said Step 3 of the present invention specifically includes the following steps: When a generator tripping fault occurs in the system, the minimum inertia of the system H min = H bor + H loss , where H min is the minimum inertia of the system, H loss is the inertia loss when a generator tripping fault occurs in the power grid.

[0036] Homogenize the inertia in unit power of the power grid, so the inertia loss and power deficit ∆ caused by the generator tripping P The relationship is: (4); In formula (4), m is the total number of generators in the system; S Gb is the b th generator's rated capacity in the system. The above inertia loss is the ideal situation after homogenization, but in the actual power grid, the inertia loss caused by generator tripping may be higher than, equal to, or lower than this homogenized inertia loss H loss . To simulate the actual tripping situation, three different situations of the inertia loss of the tripped generator need to be considered.

[0037] Step 4: According to the frequency safety boundary conditions, use the minimum inertia demand of the system to characterize the power deficit of the power grid; The step 4 of the present invention specifically includes the following steps: When a generator tripping fault occurs in the system, it will cause the equivalent inertia of the system to decrease. If the equivalent inertia of the system is less than the minimum inertia demand, a serious frequency instability accident will occur in the system. Therefore, it is necessary to ensure that the equivalent inertia of the system is greater than the minimum inertia demand in each case, that is, to satisfy H equ ≥ H min . When a generator tripping fault occurs in the system, considering the operating conditions of each generator in the system and the influence caused by the tripped generator, from the above inequality relationship, it can be obtained that: (5); In formula (5), H Gb respectively represent the inertia of the b th synchronous machine in the system; x t ∈[0, 1], 0 represents the generator shutdown state, and 1 represents the generator startup state.

[0038] Step 5: Integrate the above steps 2, 3, and 4, calculate the maximum acceptable generator tripping power of the power grid, and then set the fault with the maximum acceptable generator tripping power; The step 5 of the present invention specifically includes the following steps: When the most serious generator tripping fault occurs in the system, the constraint of formula (5) still needs to be satisfied. At this time, the maximum power deficit caused by the generator tripping fault is: (6).

[0039] Step 6: Compare the lowest value of the frequency response with the frequency safety boundary to verify the accuracy of the calculation result. Set t At the moment when a generator tripping fault occurs in the system, calculatet At a certain moment, the power deficit and inertia loss are obtained through simulation to get the frequency response curve under the generator tripping fault. The lowest frequency point is obtained based on the frequency response curve under the generator tripping fault, and it is judged whether the steady-state frequency requirement is met. If not, the minimum inertia of the system needs to be re-determined, and return to S3; if so, set the fault reference value with the current maximum acceptable generator tripping power as the guidance.

[0040] Embodiment 3: Based on Embodiment 2, this embodiment optimizes the verification steps and proposes a method for predicting the maximum acceptable generator tripping power of the power grid, referring to Figure 1 , specifically including the following steps: Step 1: By establishing the equivalent rotor motion equation of the power grid and linearizing the primary frequency regulation process, determine the power deficit caused by the generator tripping fault. The specific steps of Step 1 of the present invention include the following steps: Equivalent all power sources of the power grid to a single aggregated synchronous generator set, and the rotor motion equation of the synchronous generator set is: (1); In Equation (1), H equ is the inertia of the equivalent generator set, that is, the equivalent inertia of the power grid; f(t) is t the system frequency at the moment of P m ( t ) and P e ( t ) are the instantaneous values of the mechanical power and electromagnetic power of the equivalent generator set respectively; D reflects the power grid damping effect coefficient in the frequency response process (approximately taken as 0.06); ∆f(t) is the instantaneous value of the system frequency deviation. If a generator tripping fault occurs at time 0 of the system, when the frequency begins to deviate from the rated frequency, the synchronous generator starts primary frequency regulation after t r delay, and the frequency drops to the lowest point at t m moment. Integrate Equation (1) from 0 to t m to get: (2); According to Figure 2 shown, linearize to simulate the active power-frequency response of the system after the generator tripping fault occurs. The active power-frequency in this figure includes two parts: the active power-time response curve (upper figure) and the frequency-time response curve (lower figure). In the figure, f r and fm respectively represent t r the system frequency at a certain moment and the lowest frequency point (corresponding to the frequency safety boundary, 49 Hz); R s represents the equivalent primary frequency regulation droop coefficient of the system (approximately taken as 0.05). Therefore, the power deficit ∆ during the linearization process of primary frequency regulation P = R s (t m - t r ) , where the time when the system frequency reaches the extreme value t m = t r +∆ P / R s .

[0041] Step 2: According to the equivalent rotor motion equation of the power grid, use the power deficit and the time of frequency extreme value to characterize the boundary inertia under the condition of power grid fault; The step 2 of the present invention specifically includes the following steps: Integrate and solve the rotor motion equation of formula (2) from 0 to t m to obtain: (3); In formula (3), H bor is the boundary inertia, and its magnitude corresponds to the frequency safety boundary f m requirement; W 1 and W 2 are respectively the integral values of the overall unbalanced active power of the system in the time periods [0, t r and t r , t m ; f m is the system frequency safety boundary constraint, which is 49 Hz; f N is t r the system frequency at a certain moment.

[0042] Step 3: Calculate the minimum inertia requirement of the system under the condition of generator tripping fault, and analyze the inertia loss caused by the fault; The step 3 of the present invention specifically includes the following steps: When a generator tripping fault occurs in the system, the minimum inertia of the system H min =H bor + H loss , where H min is the minimum inertia of the system, H loss is the inertia loss when the generator trips from the power grid due to a fault.

[0043] Homogenize the inertia per unit power in the power grid. Therefore, the relationship between the inertia loss caused by generator tripping and the power deficit ∆ P is as follows: (4); In Equation (4), m is the total number of generators in the system; S Gb is the b th generator's rated capacity in the system. The above inertia loss is the ideal situation after homogenization. However, in an actual power grid, the inertia loss caused by generator tripping may be higher than, equal to, or lower than this homogenized inertia loss H loss . To simulate the actual tripping situation, three different cases of the inertia loss of the tripped generator need to be considered.

[0044] Step 4: According to the frequency security boundary condition, use the minimum inertia requirement of the system to characterize the power deficit of the power grid. The specific steps of Step 4 of the present invention are as follows: When a generator tripping fault occurs in the system, the equivalent inertia of the system will decrease. If the equivalent inertia of the system is less than the minimum inertia requirement, a serious frequency instability accident will occur in the system. Therefore, it is necessary to ensure that the equivalent inertia of the system is greater than the minimum inertia requirement in each case, that is, to satisfy H equ ≥ H min . When a generator tripping fault occurs in the system, considering the operating conditions of each generator in the system and the impact caused by the tripped generator, from the above inequality relationship, we can obtain: (5); In Equation (5), H Gb respectively represent the inertia of the b th synchronous machine in the system; x t ∈[0, 1], 0 represents the shutdown state of the generator, and 1 represents the startup state of the generator.

[0045] Step 5: Integrate the above Steps 2, 3, and 4 to calculate the maximum acceptable generator tripping power of the power grid, and then set the fault with the maximum acceptable generator tripping power; Step 5 of the present invention specifically includes the following steps: When the most severe unit tripping fault occurs in the system, the constraint of formula (5) still needs to be satisfied. At this time, the maximum power deficit caused by the unit tripping fault is: (6).

[0046] Step 6: Compare the lowest value of the frequency response with the frequency safety boundary to verify the accuracy of the calculation results. Set t When a unit tripping fault occurs in the system at time, calculate t the power deficit and inertia loss at time, simulate the frequency response curve under the unit tripping fault, obtain the lowest point of the frequency according to the frequency response curve under the unit tripping fault, and determine whether the steady-state frequency requirement is met. If not, the minimum inertia of the system needs to be re-determined and return to S3; if so, set the fault reference value with the current maximum acceptable unit tripping power as the guide.

[0047] Integrating the above steps 2, 3, and 4, calculate the inertia losses in three different cases. Then, according to formula (3), formula (4), and formula (6), calculate the maximum acceptable power deficit Δ P max .

[0048] Verify its accuracy through simulation. In the MATLAB / Simulink environment, establish a 10-machine 39-node simulation system, as Figure 3 shown. The parameter settings of the simulation system are shown in Table 1.

[0049] Table 1 Parameter values of the example system Generator Type <![CDATA S N / MW]]> / s G1 Thermal power 1000 50 G2 Thermal power 1000 3.03 G3 Thermal power 1000 3.58 G4 Thermal power 1000 2.86 G5 Thermal power 1000 2.60 G6 Thermal power 1000 3.48 G7 Thermal power 1000 2.64 G8 Thermal power 1000 2.43 G9 Photovoltaic 1000 3.45 G10 Wind power 1000 4.2 Set a unit tripping fault in the simulation operation, and there are both power deficit and inertia loss in the unit tripping fault.

[0050] Under the above fault, execute according to the Figure 1 control flow chart. Integrating steps 2, 3, and 4, according to formula (3), formula (4), and formula (6), calculate the maximum acceptable power deficit and the homogenized inertia loss of the system, which are Δ P max = 655.22MW, H loss= 0.513 s. Considering the three different situations of inertia loss caused by the tripped units described in Step 3, when the inertia loss is 75% of the calculated value, the maximum power deficit acceptable to the system is 823.94 MW according to Equation (6); when the inertia loss is equal to 100% of the calculated homogenized inertia loss, the maximum power deficit acceptable to the system is 655.22 MW; when the inertia loss is 125% of the calculated homogenized inertia loss, the maximum power deficit acceptable to the system is 445.61 MW according to Equation (6).

[0051] Example 1: The specific implementation steps of the simulation are as follows: Set t = 40 s, a unit tripping fault occurs in the system. The power deficit caused by the unit tripping is 823.94 MW, and the inertia loss is 0.385 s.

[0052] To verify the accuracy of the maximum power deficit acceptable to the grid during unit tripping faults, the frequency response curve under unit tripping faults is obtained through simulation, as Figure 4 shown. The lowest point of the frequency in the frequency response curve is exactly equal to the frequency safety boundary (49 Hz), indicating the accuracy of the calculation of the maximum acceptable unit tripping power. The steady-state frequency value is around 49.6 Hz, meeting the steady-state frequency requirements. The above shows that this power deficit is the maximum power deficit acceptable to the grid under this inertia loss.

[0053] Example 2: The specific implementation steps of the simulation are as follows: Set t = 40 s, a unit tripping fault occurs in the system. The power deficit caused by the unit tripping is 655.22 MW, and the inertia loss is 0.513 s.

[0054] To verify the accuracy of the maximum power deficit acceptable to the grid during unit tripping faults, the frequency response curve under unit tripping faults is obtained through simulation, as Figure 5 shown. The lowest point of the frequency in the frequency response curve is exactly equal to the frequency safety boundary (49 Hz), indicating the accuracy of the calculation of the maximum acceptable unit tripping power. The steady-state frequency value is around 49.65 Hz, meeting the steady-state frequency requirements. The above shows that this power deficit is the maximum power deficit acceptable to the grid under this inertia loss.

[0055] Example 3: The specific implementation steps of the simulation are as follows: Set t = 40 s, a unit tripping fault occurs in the system. The power deficit caused by the unit tripping is 445.61 MW, and the inertia loss is 0.641 s.

[0056] To verify the accuracy of the maximum power deficit acceptable to the grid during unit tripping faults, the frequency response curve under unit tripping faults is obtained through simulation, asFigure 6 As shown. The lowest frequency point in the frequency response curve is exactly equal to the frequency safety boundary (49 Hz), indicating the accuracy of the calculation of the maximum acceptable unit tripping power. The steady-state frequency value is around 49.67 Hz, meeting the steady-state frequency requirements. The above shows that this power deficit is the maximum acceptable power deficit of the power grid under this inertia loss.

Claims

1. A method for predicting the maximum acceptable off-grid power of a power grid unit, characterized in that: The following steps are involved: S1, construct the grid equivalent rotor motion equation, linearize the primary frequency regulation process, and obtain the power shortage caused by the unit off-grid failure; S2, using the time of power shortage and frequency extreme value to characterize the boundary inertia under power grid fault conditions; S3, inertia loss caused by equalization fault, to obtain the minimum inertia requirement of the system under the unit off-grid fault; S4, setting frequency safety boundary conditions and using the system minimum inertia requirement to characterize the power shortage of the power grid; S5, calculate the maximum acceptable off-grid power of the unit in the power grid, and then set the fault according to the maximum acceptable off-grid power of the unit.

2. A method for predicting the maximum acceptable off-grid power of a power grid unit according to claim 1, characterized in that: The S1 comprises the following steps: Equivalently equate all power sources in the power grid to one aggregated synchronous generator set and construct the rotor motion equation of the synchronous generator set; Linearize the active power-frequency response of the system after the unit is disconnected from the grid to obtain the active power-time response curve and frequency-time response curve; Extract the time when the power deficit and system frequency reach the extreme value during the primary frequency modulation linearization process.

3. A method for predicting the maximum acceptable off-grid power of a power grid unit according to claim 2, characterized in that: The left side of the equal sign of the rotor motion equation of the synchronous generator set is the product of twice the inertia of the equivalent generator set and the derivative of the system frequency and time, and the right side of the equal sign of the rotor motion equation of the synchronous generator set is the product of the instantaneous value of the mechanical power of the equivalent generator set minus the instantaneous value of the electromagnetic power of the equivalent generator set minus the instantaneous value of the grid damping effect coefficient and the instantaneous value of the system frequency deviation.

4. A method for predicting the maximum acceptable off-grid power of a power grid unit according to claim 3, characterized in that: The S2 comprises the following steps: integrating the rotor motion equation of the synchronous generator set from time 0 to the time when the system frequency reaches an extreme value to obtain a boundary inertia, wherein the boundary inertia is equal to the sum of the integral value of the system's overall unbalanced active power from time 0 to the current time and the integral value of the system's overall unbalanced active power from the current time to the time when the system frequency reaches an extreme value, divided by twice the difference between the system frequency safety boundary constraint value and the system frequency at the current time, minus one-fourth of the time when the system frequency reaches an extreme value multiplied by the grid damping effect coefficient.

5. The method for predicting the maximum acceptable off-grid power of a power grid unit according to claim 3 is characterized in that: The S3 comprises the following steps: The minimum inertia of the calculated system is equal to the boundary inertia minus the inertia loss when the unit is disconnected from the grid; The inertia per unit power in the grid is evenly processed, and the inertia loss when a unit fails to connect to the grid is equal to the product of the power shortage, the inertia of the equivalent generator set, and the total number of units in the system, divided by the sum of the rated capacities of all units in the system.

6. A method for predicting the maximum acceptable off-grid power of a power grid unit according to claim 5, characterized in that: S4 includes the following steps: according to the frequency safety boundary condition, an inequality is constructed using the system minimum inertia requirement to characterize the power shortage of the power grid, and the inequality is that the system minimum inertia requirement is less than or equal to the inertia of the equivalent generator set.

7. A method for predicting the maximum acceptable off-grid power of a power grid unit according to claim 5, characterized in that: S5 comprises the following steps: when the most serious unit off-grid fault occurs in the system, the maximum power shortage caused by the unit off-grid fault is obtained based on the condition that the inertia of the equivalent generator set is greater than or equal to the minimum inertia of the system, and the fault is set based on the maximum power shortage caused by the unit off-grid fault.

8. A method for predicting the maximum acceptable off-grid power of a power grid unit according to any one of claims 1 to 7, characterized in that: The method includes a simulation example verification step, wherein a unit off-grid fault is set to occur in the system at a certain moment, the power shortage and inertia loss are calculated, and the frequency response curve under the unit off-grid fault is obtained by simulation. According to the frequency response curve under the unit off-grid fault, it is determined whether the steady-state frequency requirement is met. If so, the fault is set with the current maximum acceptable unit off-grid power; if not, the minimum inertia of the system is re-determined and the system is returned to S3.

9. A method for predicting the maximum acceptable off-grid power of a power grid unit according to claim 8, characterized in that: The condition for judging whether the steady-state frequency requirement is met is to compare whether the lowest frequency point in the frequency response curve is equal to the frequency safety margin.

10. A method for predicting the maximum acceptable off-grid power of a power grid unit according to claim 8, characterized in that: During the calculation, three different conditions of inertia loss caused by the off-grid unit are considered, namely 75% of the uniform inertia loss, 100% of the uniform inertia loss, and 125% of the uniform inertia loss.

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