A power grid harmonic state estimation adaptive correction strategy considering transmission line parameter errors

Abstract

The rapid development of power electronic technology represented by DC power transmission has injected a large amount of harmonics into the power grid, resulting in the characteristics of wide frequency and multi-source of the grid harmonics. In order to effectively monitor the complex grid harmonics and take into account the economy of the measurement system, the current mainstream measures are to install wide frequency synchronous phasor measurement units (bPMU) at some key nodes and to use harmonic state estimation technology to calculate the harmonic state of the remaining nodes. However, under this technical framework, the accuracy of harmonic state estimation depends on the accuracy of the grid impedance parameters. Since the line parameters are affected by the harmonic frequency, system operation mode and many environmental factors, it is difficult to realize real-time and accurate acquisition of the grid parameters in practical application, which leads to the decrease of the accuracy of harmonic state estimation. In view of this, the present invention discloses a grid wide frequency harmonic state estimation adaptive correction method for grid impedance parameter error, which can correct the grid impedance parameter error of a specific frequency band based on the wide frequency measurement data of the specific frequency band and correct the harmonic state estimation result. The core idea is to use the harmonic state estimation result to correct the grid impedance / admittance parameters and to re-estimate the grid harmonic state based on the corrected parameters. This cycle is repeated until the calculation result converges. The example results show that the strategy proposed in the present invention can effectively improve the accuracy of the harmonic state estimation result under the condition that there is an error in the grid impedance parameters.

Description

Technical Field

[0001] This invention relates to the field of power system harmonic measurement technology, and in particular to an adaptive correction strategy for power grid harmonic state estimation that takes into account transmission line parameter errors. Background Technology

[0002] With the continuous development of DC transmission technology, the power grid is showing a trend of AC / DC hybrid development. Simultaneously, the widespread application of new energy power generation technologies and power electronics technologies, while improving system controllability and flexibility, also injects a large number of harmonics into the system. This complicates the harmonic situation, leading to power quality deterioration and increased power loss, accelerated equipment aging, and other problems. Due to the nonlinear characteristics of power electronic devices, harmonic problems in power systems with a high proportion of power electronic equipment exhibit broadband and multi-source characteristics. Accurate measurement and source tracing of harmonics, and correctly obtaining the harmonic situation within the power grid, are prerequisites for carrying out power quality management and ensuring the safe, reliable, and stable operation of the power grid.

[0003] To address the challenge of broadband harmonics in power grids, high-performance measurement devices such as broadband synchronous phasor measurement units (bPMUs) have emerged and been put into application, enabling accurate synchronous measurement of harmonics from the 1st to the 50th order and interharmonics. However, due to the high hardware and installation costs of bPMUs, in practical applications, bPMUs are typically configured only at certain nodes. Harmonic state estimation methods are then used to estimate the harmonic state of the remaining nodes using limited broadband harmonic measurement data, combined with the impedance parameters of the power grid structure.

[0004] However, within this technical framework, the accuracy of harmonic state estimation depends on the accuracy of the power grid impedance parameters. However, due to the following reasons, it is difficult to achieve real-time and accurate acquisition of these parameters in practical applications. First, line parameters exhibit frequency-varying characteristics, while the impedance parameters provided by power grid operators are often measured based on 50Hz power frequency conditions, which are not applicable to mid-to-high frequency bands far from the fundamental frequency. Second, the power grid's operating mode is constantly changing, and the grid structure and parameters also vary with different operating modes, making it difficult to address the variable operating conditions of the power grid using impedance parameters specific to a particular operating mode. Finally, grid parameters are also affected by various random environmental factors, such as changes in air temperature / humidity and conductor swaying in the wind, all of which can cause changes in line parameters. Currently, broadband harmonic measurement devices have achieved measurement accuracy of ±0.2% amplitude and ±5° phase angle, providing high-precision data support for broadband harmonic state estimation of the power grid. Therefore, improving the tolerance of the harmonic state estimation process to errors in grid parameters has become a key issue in power grid harmonic monitoring.

[0005] In response, this invention discloses an adaptive correction method for broadband harmonic state estimation of power grids, which is oriented towards errors in power grid parameters. It can correct errors in grid parameters of different frequency bands based on broadband measurement data and correct the harmonic state estimation results. Summary of the Invention

[0006] 1. Π-type equivalent model of transmission lines

[0007] The π-type equivalent model is the most commonly used transmission line model, such as... Figure 1 As shown, it consists of series branches and ground-connected branches on both sides. The series branches represent the transmission impedance, which is expressed as follows under the h-th harmonic:

[0008] Z xy h =R xy h +j·h·ω·L xy h =(R xy，0 h +j·h·ω·L xy，0 h )l (1)

[0009] In the formula, Z xy h The xy series impedance of the line is represented by h, the harmonic order is represented by h, the fundamental angular frequency of the system is represented by ω, and the unit imaginary number is represented by j. R xy h and L xy h R represents the line resistance and inductance corresponding to the h-th harmonic, respectively; xy，0 h and L xy，0 h Here, l represents the resistance and inductance per unit length of the line, and l represents the line length.

[0010] The ground-to-ground branches on both sides represent the line's ground admittance. Generally, the ground conductance is small and can be approximately ignored, so only the capacitance term is retained, i.e.:

[0011]

[0012] In the formula, Y xy h C represents the ground branch admittance of line xy. xy h C represents the line capacitance corresponding to the h-th harmonic, respectively. xy，0 h The capacitance per unit length of the line.

[0013] according to Figure 1 Given the positive direction of the electrical quantities shown, the current equation for the branch at terminal x can be established as follows:

[0014]

[0015] In the formula, This represents the h-th harmonic current of branch xy measured at point x. and These represent the h-th harmonic voltages at nodes x and y, respectively.

[0016] If a broadband measurement device is installed at node x, the h-th harmonic voltage and h-th harmonic current at node x can be considered as measured quantities, and the voltage at node y can be used as the quantity to be estimated, that is:

[0017]

[0018] The h-th harmonic current at node y can be estimated as:

[0019]

[0020] In the formula, This represents the h-th harmonic current of branch xy measured at point y.

[0021] On the other hand, according to equations (3) and (5), if the voltage and current at both ends are known quantities, the impedance / admittance parameters of the transmission line can be calculated as follows:

[0022]

[0023] 2. Adaptive Correction Method for Line Impedance Parameter Errors and Harmonic State Estimation Results

[0024] This invention discloses an adaptive method for correcting line impedance parameter errors and harmonic state estimation results. To achieve error correction, the harmonic state of each node to be estimated must be estimated based on data from two or more measurement nodes. In other words, each node to be estimated must have two or more adjacent nodes equipped with bPMU devices. The core idea of ​​this method is as follows:

[0025] S1: Based on the pre-set or updated line impedance / admittance data from the previous iteration, establish a harmonic state estimation model for the power grid;

[0026] S2: For any branch in the power grid, using the harmonic measurement data of the external nodes of the branch, based on the harmonic state estimation model established in S1, the harmonic state at both ends of the branch is estimated, and then the impedance / admittance parameters of the branch are corrected.

[0027] S3: Repeat S2 until the impedance / admittance corrections for all branches in the example power grid are completed;

[0028] S4: Repeat S1 to S3 until the difference between the harmonic state estimates of the two iterations meets the set error threshold, then exit the iterative calculation;

[0029] S5: Output the final harmonic state estimation results. Attached Figure Description

[0030] Figure 1 This is a schematic diagram of the Π-type equivalent model of the transmission line mentioned in this invention;

[0031] Figure 2 It is the 3-node minimum system mentioned in this invention that satisfies the application conditions of the proposed method;

[0032] Figure 3 This is a flowchart illustrating the implementation of the method proposed in this invention. Detailed Implementation

[0033] This invention is achieved through Figure 2 The illustrated 3-node minimum system that meets the conditions specifically illustrates the implementation of the present invention. The example includes three nodes x, y, and z, and two branches xy and yz, wherein nodes x and z are equipped with bPMU devices. For the illustrated minimum system, the adaptive implementation method for correcting line impedance parameter errors and harmonic state estimation results is as follows:

[0034] S1: Read bPMU measurement data, including the h-th node harmonic voltage phasor at x. and branch harmonic current phasor and the h-th node harmonic voltage phasor at z and branch harmonic current phasors

[0035] S2: Line impedance data Z obtained from pre-given values ​​or calculations in the previous iteration. xy h (k-1), Y xy h (k-1), Z yz h (k-1), Y yz h (k-1), using equation (4), from and Calculate the h-th harmonic voltage phasor at node y respectively. And record the result as and The number k in parentheses represents the iteration round. For the first iteration where k=1, the branch parameters are pre-defined as Z. xy h (0), Y xy h (0), Z yzh (0), Y yz h (0).

[0036] S3: Using equation (5), according to calculate Record the result as At the same time, according to calculate Record the result as

[0037] S4: Using equation (6), according to Estimate the impedance / admittance of the line in the xy direction, denoted as Z. xy h (1) Y xy h (1); According to Estimate the impedance / admittance of the line in the xy direction, denoted as Z. yz h (1) Y yz h (1).

[0038] S5: Let k = k+1 to update the iteration round, use the latest estimated line impedance / admittance parameters to correct the model, repeat S2 to S4 until the difference in harmonic state estimation between two adjacent iterations meets the set threshold.

[0039] S6: Complete the iterative calculation and output the harmonic state estimation results for the final iteration.

[0040] This invention further illustrates the practical application effects of the proposed method through specific examples. The example structure is the same as... Figure 2 The specific parameters are as follows: the rated voltage of the line is 500kV; the line is an overhead line; the conductor type is GJ-630 / 35; under power frequency conditions, the line resistance, inductance, and capacitance are 12.5mΩ / km, 0.8934mH / km, and 6.78nF / km, respectively; the lengths of the xy and yz segments are 45km and 55km, respectively. Nodes x and z are equipped with bPMU devices, while node y has a 7th voltage harmonic source with an amplitude of 2.5kV.

[0041] To verify the correctness and effectiveness of the proposed method, it is assumed that the pre-measured impedance / admittance values ​​of segments xy and yz in the initial model for harmonic state estimation have errors of +5% and -5%, respectively. Table 1 shows the fifth harmonic impedance / admittance and harmonic voltage at node y before and after correction using the proposed method. It can be seen that the proposed method can automatically correct the line impedance / admittance parameter errors, reducing the line parameter errors from 5% to approximately 1%, and significantly improving the accuracy of the final harmonic state estimation result.

[0042] Table 1 shows the computational application effects of the method proposed in this invention.

[0043]

Claims

1. An adaptive correction strategy for power grid harmonic state estimation that takes into account transmission line parameter errors, wherein the strategy is applied to a power grid in which each node to be estimated must have two or more adjacent nodes equipped with broadband synchronous phasor measurement units (bPMUs), characterized in that... It mainly includes the following steps: S1: Based on the pre-set or updated line impedance / admittance data from the previous iteration, establish a harmonic state estimation model for the power grid; S2: For any branch in the power grid, use the harmonic measurement data collected by the bPMU at the external node of the branch, and estimate the harmonic state at both ends of the branch based on the harmonic state estimation model established in S1, and then correct the impedance / admittance parameters of the branch. The impedance and admittance parameters of the line can be calculated based on the h-th harmonic voltage and current phasors at the xy-ends of the line, and have the following mathematical form: ； In the formula, h is the harmonic order; and These represent the h-th harmonic voltage phasors at the beginning and end of the xy line, respectively, with the ground as the reference potential; and These represent the h-th harmonic current phasors of line x and y at the beginning and end of the line, respectively, with the positive direction being the flow from outside the line into the line; Z xy h Y represents the h-th harmonic series impedance of the line xy; xy h This represents the h-th harmonic admittance of the line xy to the ground branch; S3: Repeat S2 until the impedance / admittance corrections for all branches in the example power grid are completed; S4: Repeat S1~S3 until the difference between the harmonic state estimates of the two iterations meets the set error threshold, then exit the iterative calculation; S5: Output the final harmonic state estimation results.

2. The adaptive correction strategy for power grid harmonic state estimation considering transmission line parameter errors according to claim 1, characterized in that, Based on the known voltage and current phasors at the beginning of the transmission line, as well as the impedance and admittance parameters of the line, the voltage and current phasors at the end of the transmission line can be calculated, which has the following mathematical form: ； ； In the formula, h is the harmonic order; and These represent the h-th harmonic voltage phasors at the beginning and end of the xy line, respectively, with the ground as the reference potential; and These represent the h-th harmonic current phasors of line x and y at the beginning and end of the line, respectively, with the positive direction being the flow from outside the line into the line; Z xy h Y represents the h-th harmonic series impedance of the line xy; xy h This represents the h-th harmonic admittance of the line xy to the ground branch.

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