Inertia evaluation method of power system considering influence of direct current FLC

Abstract

The application discloses a kind of power system inertia evaluation method considering the influence of DC FLC, first according to the frequency data of the bus at the grid-connected place of generator unit before and after power disturbance, to calculate the center frequency of sending end system, then carry out standardization processing;Secondly, according to the active power data collected on the AC tie line of sending end system, carry out mean removal, standardization processing to obtain the active power variation standardization data of sending end system tie line;According to the frequency data collected on the AC side of rectifier station, the DC power output after the action of DC FLC controller is obtained, and it is standardized;Thirdly, according to the center frequency fluctuation data of sending end system, the time of disturbance occurrence is identified;Finally, select the preset time period after the time of disturbance occurrence as the time period to be solved;In the time period to be solved, the first equivalent inertia time constant of asynchronous sending end power system is calculated by using the method of sliding data window, and on this basis, the final equivalent inertia time constant of asynchronous sending end power system is obtained by averaging all the calculated first equivalent inertia time constants.The application can effectively solve the inertia evaluation problem of asynchronous sending end power system under the action of DC FLC.

Description

Technical Field

[0001] The present invention relates to a method for evaluating inertia of a power system taking into account the influence of direct current (DC) FLC, and belongs to the field of power system frequency stability analysis. Background Art

[0002] Asynchronous operation of large power grids is an important measure to ensure the safe and stable operation of large interconnected power systems. Asynchronous operation of power grids can effectively solve the power angle stability problem of AC interconnected power systems, but it will also increase the risk of frequency stability operation, especially for asynchronous sending-end power systems. In addition, with the development of new power systems, the proportion of renewable energy in the system is increasing. Since the mechanical inertia of wind power and photovoltaic power is very small, even close to zero, the overall inertia level of the system has decreased, further exacerbating the frequency stability problem of the power system. Under the same power disturbance conditions, the lower the system inertia level, the larger the range of system frequency fluctuations and the higher the risk of frequency instability. Therefore, the inertia assessment of the power system has become an important link in ensuring the reliability of power supply.

[0003] To improve the frequency stability of asynchronous sending-end power systems, they are typically equipped with a DC FLC controller. This controller serves as an auxiliary control link for the DC power control on the AC side of the rectifier station. It provides rapid power support for forward and reverse power shortfalls in the sending-end power system, thereby improving the frequency stability of the sending-end system. However, when the system experiences large power disturbances, the frequency response of the DC FLC is strongly coupled with the inertia response of the synchronous generators and the primary frequency regulation response, jointly participating in the system frequency regulation. This makes it difficult for existing inertia assessment methods to effectively assess the system inertia level. Therefore, it is necessary to develop an inertia assessment method that addresses the DC FLC interference conditions for asynchronous sending-end power systems. This method is crucial for ensuring the safe and stable operation of asynchronous sending-end power systems involving DC FLC. Due to the unique nature of inertia assessment in sending-end power systems, few existing inertia assessment methods address this research context. Summary of the Invention

[0004] The present invention provides a power system inertia evaluation method taking into account the influence of DC FLC, so as to evaluate the inertia of an asynchronous sending-end power system involving DC FLC.

[0005] The technical solution of the present invention is:

[0006] According to a first aspect of the present invention, a method for estimating inertia of a power system taking into account the influence of DC FLC is provided, comprising the following steps:

[0007] Step 1: Under a large disturbance in the asynchronous sending-end power system, frequency data of the buses at the grid-connected locations of all generator sets in the asynchronous sending-end power system are collected, and active power data on the AC tie lines of the sending-end system of the asynchronous sending-end power system are synchronously collected;

[0008] Step 2: Perform per-unit processing on the frequency data of the busbar at the grid-connected generator set to obtain per-unit frequency data, and calculate the per-unit data of the center frequency of the sending-end system based on the obtained per-unit frequency data; perform de-averaging and per-unit processing on the active power data on the AC tie line of the sending-end system to obtain the per-unit data of the active power change of the sending-end system;

[0009] Step 3: Collect the AC side frequency data of the rectifier station, perform de-averaging on the collected AC side frequency data of the rectifier station, and obtain the frequency variation data of the AC side of the rectifier station; calculate the output DC power after the DC FLC controller is actuated based on the frequency dead zone parameter, proportional link gain coefficient, integral link gain coefficient, positive / negative power output limit value, and the AC side frequency variation data of the rectifier station; and obtain the active power of the DC FLC controller acting on the asynchronous sending-end power system based on the conversion efficiency and the output DC power after the DC FLC controller is actuated, and perform per-unit normalization processing on the active power;

[0010] Step 4: Identify the instantaneous moment when the disturbance occurs based on the normalized data of the center frequency of the sending-end system;

[0011] Step 5. Take the instantaneous moment of the disturbance determined in step 4 as the demarcation point; select a preset time period after the demarcation point as the time period to be calculated, fix the data window length in the time period, and use the sliding data window method to traverse the time period to be calculated with the same step length; based on the frequency and power data in the time period to be calculated obtained in steps 1 to 3, calculate the first equivalent inertia time constant of the asynchronous sending-end power system in each data window within the time period to be calculated; average the first equivalent inertia time constants of the asynchronous sending-end power system calculated in all data windows to obtain the final equivalent inertia time constant of the asynchronous sending-end power system.

[0012] Furthermore, assuming that the frequency dead zone of the DC FLC controller is [Δf - ,Δf + ], the active power ΔP acting on the asynchronous sending-end power system by the DC FLC controller FLC As shown below:

[0013]

[0014] ΔP FLC =ηΔP DC ;

[0015] Where ΔPDC is the output DC power after the DC FLC controller is activated; ΔP up ΔP is the DC boost power value; down is the DC buck power; ΔP FLC is the active power of the DC FLC controller acting on the asynchronous sending-end power system; η is the conversion efficiency; Δf is the frequency variation data of the AC side of the rectifier station.

[0016] Furthermore, the step 4 is specifically as follows:

[0017] The EWMA algorithm is designed for the center frequency normalized data of the sending system to detect the data disturbance point through the following discriminant. If the discriminant is satisfied, the moment is considered to be the disturbance point. The first disturbance point is the instant when the disturbance occurs:

[0018] res t >alpha·sig

[0019] Where: res t is the residual of the data at time t; alpha is the significance level; sig represents the standard deviation of the residual sequence.

[0020] Furthermore, with i as the order, the calculation expression for the first equivalent inertia time constant is:

[0021]

[0022] Among them, H aream-i is the calculated value of the first equivalent inertia time constant of the asynchronous sending-end power system; t i,0 Indicates the starting time of the calculation of the first equivalent inertia time constant of the i-th element, t i,1 represents the end point of the calculation of the i-th first equivalent inertia time constant; P s is the per-unit value of the disturbance power; ΔP m ΔP' is the normalized data of the active power variation of the sending end system; FLC represents the per-unit active power value of the DC FLC controller acting on the asynchronous sending-end power system; f COI It is the normalized data of the center frequency of the sending system.

[0023] According to a second aspect of the present invention, there is provided an asynchronous sending-end power system inertia assessment system taking into account DC FLC, comprising: one or more processors; and a memory for storing operable instructions, wherein the instructions, when executed by the one or more processors, cause the one or more processors to perform operations, the operations comprising the steps of any one of the methods described above.

[0024] The beneficial effects of the present invention are as follows: for the problem of inertia assessment of the asynchronous sending-end power system involving DC FLC, when the system is subjected to a large disturbance, the previous inertia assessment method cannot assess the inertia of the asynchronous sending-end power system involving DC FLC. However, the present invention can effectively assess the inertia level of the asynchronous sending-end system under the influence of the DC FLC action based on the obtained per-unit data of the active power change of the asynchronous sending-end system, the per-unit data of the active power output of the DC power acting on the asynchronous sending-end power system after the DC FLC controller is activated, and the center frequency of the sending-end system. Verification has shown that the inertia level of the asynchronous sending-end system is smaller than the error of the real inertia time constant. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Figure 1 is a flow chart of the present invention;

[0026] Figure 2 This is the four-machine, two-area DC interconnected distribution system with DC FLC used in the evaluation example of the present invention;

[0027] Figure 3 is the DC FLC controller model used in the evaluation example of the present invention;

[0028] Figure 4 is the center frequency fluctuation data of the sending-end system obtained by calculation in the present invention;

[0029] Figure 5 It is the normalized data of the active power variation of the asynchronous sending end system after preprocessing;

[0030] Figure 6 The DC power output by the DC FLC controller after operation is obtained by calculation in the present invention;

[0031] Figure 7 This is a schematic diagram of the EWMA algorithm designed by the present invention to detect the moment when disturbance occurs. DETAILED DESCRIPTION

[0032] To make the purpose, technical solutions and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative work are within the scope of protection of the present invention. It should be noted that, in the absence of conflict, the embodiments in this application and the features in the embodiments can be combined with each other in any way.

[0033] Example 1: Figure 1-7 As shown, according to a first aspect of an embodiment of the present invention, a method for estimating inertia of a power system taking into account the influence of DC FLC is provided, comprising the following steps:

[0034] Step 1: Under a large disturbance in the asynchronous sending-end power system, collect frequency data of the buses at the grid-connected locations of all generator sets in the asynchronous sending-end power system, and simultaneously collect active power data on the AC interconnection lines of the asynchronous sending-end power system. Generally, a large disturbance in the asynchronous sending-end power system occurs when the large disturbance causes active power fluctuations on the DC interconnection, resulting in a maximum frequency fluctuation value of the asynchronous sending-end power system exceeding the dead zone value of the DC FLC.

[0035] Step 2: Perform per-unit processing on the frequency data of the busbar at the grid-connected generator set to obtain per-unit frequency data, and calculate the per-unit data of the center frequency of the sending-end system based on the obtained per-unit frequency data; perform de-averaging and per-unit processing on the active power data on the AC tie line of the sending-end system to obtain the per-unit data of the active power change of the asynchronous sending-end system;

[0036] For each sampling point, the following formula is used to calculate the per-unit center frequency data of the sending system:

[0037]

[0038] Where, f COI Indicates the per-unit value of the center frequency of the sending end system; f k is the normalized frequency data of the kth generator set; H k is the inertia time constant of the kth generator set; S k is the rated capacity of the kth generator set.

[0039] Step 3. Obtain the topology and control parameters of the DC FLC controller on the AC side of the rectifier station; simultaneously, synchronously collect the frequency data of the AC side of the rectifier station, perform de-averaging on the collected frequency data of the AC side of the rectifier station, and obtain the frequency variation data of the AC side of the rectifier station; calculate the DC power output after the DC FLC controller is actuated based on the control parameters of the DC FLC controller and the frequency variation data of the AC side of the rectifier station; and obtain the active power of the DC FLC controller acting on the asynchronous sending-end power system based on the conversion efficiency and the DC power output after the DC FLC controller is actuated, and perform per-unit normalization processing on the active power;

[0040] The details are as follows:

[0041] Assume that the frequency dead zone of the DC FLC controller is [Δf - ,Δf + ], the active power ΔP acting on the asynchronous sending-end power system by the DC FLC controller FLC As shown below:

[0042]

[0043] ΔP FLC =ηΔPDC ;

[0044]

[0045] Where ΔP DC The DC power output after the DC FLC controller is activated; ΔP up ΔP is the DC boost power value; down is the DC buck power; ΔP FLC is the active power of the DC FLC controller acting on the asynchronous sending-end power system; ΔP' FLC Indicates ΔP FLC Per unit value, P base It represents the active power reference value; η is the conversion efficiency, which depends on the conversion rate of DC power into active power, and the reference value range is 0.9 to 1; Δf is the frequency change data on the AC side of the rectifier station.

[0046] The DC boost power value and DC drop power value are expressed as follows:

[0047]

[0048] Where K p , K I are the proportional link gain coefficient and the integral link gain coefficient respectively;

[0049] And the constraints must be met:

[0050]

[0051] Where ΔP up.max Indicates the forward power output limit value; ΔP down.min Indicates the negative power output limit value;

[0052] For example, the DC FLC controller adopts a reverse frequency difference reset type FLC controller, and the topology and parameters of the reverse frequency difference reset type FLC controller are as follows: Figure 3 As shown, the specific parameters include frequency dead zone parameters, proportional link gain coefficient, integral link gain coefficient, and positive / negative power output power limit values.

[0053] Step 4: Use the EWMA algorithm to detect the fluctuation of the center frequency of the sending end system and identify the instant when the disturbance occurs;

[0054] The step 4 is specifically as follows:

[0055] The EWMA algorithm is designed for the center frequency fluctuation data of the sending system. The expression is as follows:

[0056] v t =β·x t-1+(1-β)·θ t

[0057] Where: The exponentially weighted moving average v at time t is denoted as v t ;x t-1 is the center frequency of the sending end system at time t-1; θ t is the value of the variable v at time t-1; β is the attenuation weight, which is 0.2;

[0058] Recursively deducing the above formula, we can obtain:

[0059] θ t =v t-1 =β·x t-2 +(1-β)·θ t-1 ;

[0060] Combining the above two formulas, we can get:

[0061] v t =β·x t-1 +(1-β)·(β·x t-2 +(1-β)·θ t-1 );

[0062] Calculate the residual sequence and calculate the standard deviation of the residual sequence:

[0063] res t =v t -x t ;

[0064]

[0065] The data disturbance point can be detected by the following discriminant:

[0066] res t >alpha·sig;

[0067] Where: res t is the residual of the data at time t; alpha is the significance level, which is 0.02; sig represents the standard deviation of the residual sequence; if the discriminant is satisfied, the moment can be considered as the moment when the disturbance occurs, that is, the disturbance point; the first disturbance point is the instant when the disturbance occurs.

[0068] Step 5: Use the instantaneous moment of the disturbance determined in Step 4 as the demarcation point; select a preset time period after the demarcation point as the time period to be determined, and fix the data window length within this time period. Use the sliding data window method to traverse the time period to be determined with the same step length; based on the frequency and power data for the time period to be determined obtained in Steps 1-3, calculate the first equivalent inertia time constant of the asynchronous sending power system in each data window within the time period to be determined; average the first equivalent inertia time constants of the asynchronous sending power system calculated in all data windows to obtain the final equivalent inertia time constant of the asynchronous sending power system. For example, assume that the instantaneous moment of the disturbance identified in Step 4 is 15 seconds, and use [15.1,16] as the time period to be determined; within this time period, calculate the first equivalent inertia time constant every 0.1 seconds, and take the average of the nine first equivalent inertia time constants calculated after 15.1 seconds as the final equivalent inertia time constant of the asynchronous sending power system. It should be noted that the starting and ending times of each time period to be determined are sampling points.

[0069] Taking i as the order, the calculation expression of the first equivalent inertia time constant is given as follows:

[0070]

[0071] Among them, H aream-i is the calculated value of the first equivalent inertia time constant of the asynchronous sending-end power system; t i,0 Indicates the starting time of the calculation of the first equivalent inertia time constant of the i-th element, t i,1 represents the end point of the calculation of the i-th first equivalent inertia time constant; P s is the per-unit value of the disturbance power; ΔP m ΔP' is the normalized data of the active power variation of the sending end system; FLC represents the per-unit active power value of the DC FLC controller acting on the asynchronous sending-end power system; f COI is the center frequency normalized data of the sending end system, f COI (t i,0 ) represents t i,0 The center frequency of the sending end system at the time of normalization data, f COI (t i,1 ) represents t i,1 The center frequency normalized data of the sending end system at time t.

[0072] According to a second aspect of the present invention, there is provided a power system inertia assessment system that takes into account the influence of DC FLC, comprising: one or more processors; and a memory for storing operable instructions, wherein when the instructions are executed by the one or more processors, the one or more processors perform operations, the operations comprising the steps of any one of the methods described above.

[0073] Obviously, those skilled in the art will appreciate that the various steps of the present invention described above can be implemented using a general-purpose computing device, can be centralized on a single computing device, or can be distributed across a network of multiple computing devices. They can be implemented using program code executable by the computing device, and thus, can be stored in a storage device and executed by the computing device. In some cases, the steps shown or described herein can be performed in a different order than that described herein, or can be implemented as separate integrated circuit modules, or multiple modules or steps can be implemented as a single integrated circuit module. Thus, the present invention is not limited to any particular combination of hardware and software.

[0074] Example 2: Combination Figure 1-7 As shown, the present invention provides an optional method for estimating the inertia of an asynchronous power system sending end taking into account the DC FLC, which is specifically described as follows: a DC FLC controller is added to the rectifier station of a four-machine two-area system of high-voltage direct current transmission (LCC-HVDC), such as Figure 2 As shown, the DC FLC controller topology and parameters are as follows Figure 3 As shown. Figure 2 For reference, a power disturbance event was set at bus 7 to remove a 120MW load, with a load shedding duration of 1s. The sampling rate was set to 100Hz, and the PMU device collected frequency data from the bus at the generator grid connection point, active power data on the regional sending-end system AC tie line, and frequency data on the AC side of the rectifier station before and after the large power disturbance.

[0075] Step 1: Under the condition of large disturbance in the asynchronous power sending system, collect the frequency data of the busbars where all generators in the asynchronous power sending system are connected to the grid through the PMU device, and simultaneously collect the active power data on the AC tie line of the asynchronous power sending system; generally, a frequency disturbance exceeding 0.15Hz is regarded as a large disturbance. Figure 7 In the example shown, a given disturbance causes the system frequency to change by approximately 0.3 Hz.

[0076] Step 2: The collected frequency data of the busbar at the grid-connected generator set are sequentially normalized to obtain normalized frequency data, and the normalized data of the center frequency of the sending-end system is calculated based on the obtained normalized frequency data; the collected active power data on the AC tie line of the sending-end system is de-averaged and normalized to obtain the normalized data of the active power variation of the asynchronous sending-end system; for example, Figure 4 The display shows the center frequency change curve of the sending end system under the sampling time of 1-40s; Figure 5 The figure shows the normalized data of the active power variation of the asynchronous sending-end system under the sampling time of 1-40s.

[0077] Step 3, obtain the topology and control parameters of the DC FLC controller; synchronously collect the AC side frequency data of the rectifier station, remove the mean value of the collected AC side frequency data of the rectifier station, and obtain the AC side frequency variation data of the rectifier station; calculate the output DC power of the DC FLC controller after the action based on the control parameters of the DC FLC controller and the AC side frequency variation data of the rectifier station; and obtain the active power of the DC FLC controller on the asynchronous sending end power system based on the conversion efficiency and the output DC power after the DC FLC controller is acted, and perform per-unit standardization processing on it; such as Figure 6 The figure shows the DC power output ΔP after the DC FLC controller is activated. DC .

[0078] S4, using the EWMA algorithm to detect the fluctuation of the center frequency of the sending end system and identify the instantaneous moment when the disturbance occurs; Figure 7 As shown, the instantaneous moment of the identified disturbance is 15.01s;

[0079] S5. Based on the instantaneous moment of the disturbance identified in step 4, 15.01s is used. [15.1, 16] is used as the time period to be calculated. The sliding data window length is 0.1s, and the sliding step is 0.1s. In this time period, the first equivalent inertia time constant is calculated every 0.1s. The average of the nine first equivalent inertia time constants calculated after 15.1s is taken as the final equivalent inertia time constant of the asynchronous sending-end power system. The calculated first equivalent inertia time constant of the asynchronous sending-end system is shown in Table 1:

[0080] Table 1 Calculated values ​​of equivalent inertia time constant of asynchronous sending end system at different time periods

[0081]

[0082]

[0083] From Table 1, we can see that the final equivalent inertia time constant H is calculated. aream The total inertia time constant is 4.61s. All other variables in the power system model remain unchanged, and only the DC FLC controller is changed. The DC FLC controller is deactivated. The perturbation method is used to obtain the final equivalent inertia time constant of the asynchronous sending-end power system with the DC FLC controller deactivated. This is used as the system's true inertia time constant, which is 4.67s. The proposed method deviates from the true inertia time constant of the sending-end system by only -1.28%. This demonstrates that the proposed method can accurately perform inertia assessment in an asynchronous sending-end power system that takes the DC FLC into account.

[0084] The specific embodiments of the present invention are described in detail above with reference to the accompanying drawings. However, the present invention is not limited to the above embodiments. Various changes can be made within the knowledge of ordinary technicians in this field without departing from the scope of the present invention.

Claims

1. A method for evaluating power system inertia taking into account the influence of DC FLC, characterized in that: The following steps are involved: Step 1: Under a large disturbance in the asynchronous sending-end power system, frequency data of the buses at the grid-connected locations of all generator sets in the asynchronous sending-end power system are collected, and active power data on the AC tie lines of the sending-end system of the asynchronous sending-end power system are synchronously collected; Step 2: Perform per-unit processing on the frequency data of the busbar at the grid-connected generator set to obtain per-unit frequency data, and calculate the per-unit data of the center frequency of the sending-end system based on the obtained per-unit frequency data; perform de-averaging and per-unit processing on the active power data on the AC tie line of the sending-end system to obtain the per-unit data of the active power change of the sending-end system; Step 3: Collect the AC side frequency data of the rectifier station, perform de-averaging on the collected AC side frequency data of the rectifier station, and obtain the frequency variation data of the AC side of the rectifier station; calculate the output DC power after the DC FLC controller is actuated based on the frequency dead zone parameter, proportional link gain coefficient, integral link gain coefficient, positive / negative power output limit value, and the AC side frequency variation data of the rectifier station; and obtain the active power of the DC FLC controller acting on the asynchronous sending-end power system based on the conversion efficiency and the output DC power after the DC FLC controller is actuated, and perform per-unit normalization processing on the active power; Step 4: Identify the instantaneous moment when the disturbance occurs based on the normalized data of the center frequency of the sending-end system; Step 5. Take the instantaneous moment of the disturbance determined in step 4 as the demarcation point; select a preset time period after the demarcation point as the time period to be calculated, fix the data window length in the time period, and use the sliding data window method to traverse the time period to be calculated with the same step length; based on the frequency and power data in the time period to be calculated obtained in steps 1 to 3, calculate the first equivalent inertia time constant of the asynchronous sending-end power system in each data window within the time period to be calculated; average the first equivalent inertia time constants of the asynchronous sending-end power system calculated in all data windows to obtain the final equivalent inertia time constant of the asynchronous sending-end power system.

2. The method for estimating power system inertia taking into account the influence of DC FLC according to claim 1, characterized in that: Assume that the frequency dead zone of the DC FLC controller is [Δf - ,Δf + ], the active power ΔP acting on the asynchronous sending-end power system by the DC FLC controller FLC As shown below: ΔP FLC =ηΔP DC ; Where ΔP DC is the output DC power after the DC FLC controller is activated; ΔP up ΔP is the DC boost power value; down is the DC buck power; ΔP FLC is the active power of the DC FLC controller acting on the asynchronous sending-end power system; η is the conversion efficiency; Δf is the frequency variation data of the AC side of the rectifier station.

3. The method for estimating power system inertia taking into account the influence of DC FLC according to claim 1, characterized in that: The step 4 is specifically as follows: The EWMA algorithm is designed for the center frequency normalized data of the sending system to detect the data disturbance point through the following discriminant. If the discriminant is satisfied, the moment is considered to be the disturbance point. The first disturbance point is the instant when the disturbance occurs: travel t >alpha·say Where: res t is the residual of the data at time t; alpha is the significance level; sig represents the standard deviation of the residual sequence.

4. The method for estimating power system inertia taking into account the influence of DC FLC according to claim 1, characterized in that: Taking i as the order, the calculation expression for the first equivalent inertia time constant is: Among them, H aream-i is the calculated value of the first equivalent inertia time constant of the asynchronous sending-end power system; t i,0 Indicates the starting time of the calculation of the first equivalent inertia time constant of the i-th element, t i,1 represents the end point of the calculation of the i-th first equivalent inertia time constant; P s is the per-unit value of the disturbance power; ΔP m ΔP' is the normalized data of the active power variation of the sending end system; FLC represents the per-unit active power value of the DC FLC controller acting on the asynchronous sending-end power system; f COI It is the normalized data of the center frequency of the sending system.

5. A method and system for evaluating power system inertia taking into account the influence of DC FLC, characterized in that: include: one or more processors; A memory for storing operable instructions, wherein when the instructions are executed by the one or more processors, the one or more processors are caused to perform operations, wherein the operations include the steps of the method according to any one of claims 1 to 4.

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