Differential mode frequency measurement method based on power measurement

Abstract

The invention is suitable for analyzing the time-space distribution characteristics of the frequency of a power system, and provides a differential mode frequency measurement method based on power measurement, which comprises the following steps: acquiring the power flowing through a node and the admittance from a disturbance point to the node, and determining the frequency of the node according to the power flowing through the node and the admittance from the disturbance point to the node; on the basis of the obtained frequencies of the different nodes, performing difference on every two nodes to obtain difference mode frequencies between the different nodes; based on the established simulation system model, obtaining a formula capable of obtaining the difference mode frequency between different nodes of the system by using the Kirchhoff voltage law; after disturbance, the differential mode frequency between the nodes can be obtained only by measuring the transmission power of each branch. Compared with PLL (Phase Locked Loop) measurement and PMU (Phase Measurement Unit) measurement, the differential mode frequency of the system can be more accurately obtained, spatial and temporal distribution characteristic analysis is carried out on the differential mode frequency obtained by a measurement method, and a reference basis can be provided for evaluation and improvement of frequency stability after a new energy unit is accessed.

Description

Technical Field

[0001] The present application is applicable to analyzing the temporal and spatial distribution characteristics of the frequency of an electric power system, and provides a differential mode frequency measurement method based on power measurement. Background Art

[0002] With the massive access to new energy sources such as wind power and photovoltaics, the system equivalent inertia time constant continues to decline, and the degree of frequency change under disturbance events increases. In 2021, the power flow caused by the tripping of bus and line circuit breakers in the European power grid shifted significantly to adjacent lines, which eventually led to large frequency fluctuations; among them, the "7.24" accident, when Spain and Portugal were disconnected from the European continental power grid, the rate of change of frequency (RoCoF) of different buses in their power grids showed significant differences in temporal and spatial distribution. The above accidents are closely related to the system inertia support and frequency regulation capabilities when the fault occurs. Therefore, studying the temporal and spatial distribution characteristics of the frequency of the power system is of great significance to the stability of the power system.

[0003] In addition, with the large-scale replacement of synchronous machines by new energy sources, the power system gradually presents the characteristics of reduced system inertia and uneven distribution of frequency modulation resources. Since the node frequency response is closely related to the dynamics, distribution and network topology of all devices in the system, its analysis is more complicated than the global frequency. In terms of differential mode frequency analysis, most existing studies focus on establishing indicators and quantifying frequency characteristics based on the measured frequency trajectory. The method of describing the spatiotemporal distribution characteristics of frequency response based on the observation space has errors caused by uneven distribution of observation points and incomplete geographic information; the small signal model is used to obtain the differential mode frequency by calculation. Under small disturbances, the small signal model usually approximates the system parameters, which is convenient for theoretical analysis, but there will be deviations in the case of large disturbances, affecting the accuracy of the calculation, so the small signal model is only suitable for differential mode frequency analysis under small disturbances. In the above theory, the frequency response is mostly obtained by PLL (phase-locked loop) or PMU (synchronous phasor measurement unit) in the simulation system, and there are certain errors. Therefore, it is necessary to propose a more accurate power measurement method to obtain the frequency change during disturbance through the power change of the system.

[0004] However, existing analysis of the spatiotemporal distribution characteristics of power system frequency often focuses more on the frequency change rate and the maximum frequency offset, which can accurately reflect the overall interference situation and frequency support capacity of the power system. At present, the global response analysis of power system frequency usually uses the common mode frequency of the system, and rarely uses the differential mode frequency to describe the spatiotemporal distribution characteristics of the system frequency after a fault. Summary of the invention

[0005] In order to overcome the errors existing in related measurement methods, the present application proposes a differential mode frequency measurement method based on power measurement to reduce the error when measuring the differential mode frequency of the system.

[0006] This application is implemented through the following technical solutions, including:

[0007] Based on the P / ω "admittance" circuit modeling method, the active power-frequency response small signal circuit model of the simulation system is obtained; the power flowing through the node and the admittance from the disturbance point to the node are obtained, and the frequency of the node is determined according to the power flowing through the node and the admittance from the disturbance point to the node; based on the different node frequencies obtained by the power measurement method, the differential mode frequencies between different nodes are obtained by taking the difference between them; based on the constructed simulation system model, Kirchhoff's voltage law is used to obtain the formula for obtaining the differential mode frequencies between different nodes of the system.

[0008] The P / ω “admittance” circuit modeling method is used to obtain the active power-frequency response small signal circuit model, and the transmission power calculation formula between different nodes i and j is obtained, including:

[0009] The line transmission power mentioned above refers to the power flowing between different nodes after the power system is disturbed. The calculation formula is:

[0010]

[0011] Where: P ij is the power flowing through the line; U i , U j is the voltage across the line, X lineij That is, the reactance component of the line, δ ij The phase angle difference of the voltage at both ends of the line.

[0012] The linearized differential mode frequency small signal calculation formula obtained based on the P / ω “admittance” circuit modeling includes:

[0013] The differential mode frequency refers to the frequency difference between different nodes after a disturbance occurs in the power system. The small signal calculation formula is:

[0014]

[0015] Where: Δω ij is the frequency difference mode component between different nodes i and j, ΔP ij is the change in active power flowing through nodes i and j before and after the disturbance.

[0016] The calculation method is based on a small signal model obtained by P / ω "admittance" circuit modeling. By selecting any node as a reference node, the differential mode frequency of any two nodes in the system can be obtained. The calculation formula includes:

[0017] The node admittance matrix, taking a node in the circuit model as the reference node, the node admittance matrix expression of the system circuit model is:

[0018]

[0019] Where: Y lineij is the “admittance” of each section of the transmission line, Y Ti is the "admittance" of the transformer.

[0020] Select a reference node, take node 4 as an example, and according to Kirchhoff's voltage law, the "potential" expression of each node in the system can be obtained:

[0021]

[0022] Where: is the potential of node i with respect to node 4 as the reference.

[0023] Finally, substitute the above two equations into the difference formula, and the expression is:

[0024] Δω ij =Δω i4 -Δω j4 (i≠j)

[0025] Where: Δω ij is the "potential difference" Δω between any two nodes ij

[0026] In summary, the present invention is directed to a method for calculating differential mode frequency. Considering that the current frequency response is mostly obtained by PLL (phase-locked loop) or PMU (synchronous phasor measurement unit) in the simulation system, there are certain errors; various differential mode frequency acquisition methods are highly targeted and not universal, so a differential mode frequency measurement method based on power measurement is proposed. After example simulation and experimental verification, the power measurement method can better reflect the frequency-time-space distribution characteristics during a fault compared to the differential mode frequency measured by PLL and PMU, and the power measurement method has better accuracy and adaptability. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] Figure 1 This is a structural diagram of a simulation system built for a differential mode frequency measurement method based on power measurement in this application.

[0028] Figure 2 This is a comparison diagram of differential mode frequencies between generator sets under load surge conditions obtained by a differential mode frequency measurement method based on power measurement in this application.

[0029] Figure 3 This is a short-circuit fault condition obtained by a differential-mode frequency measurement method based on power measurement in this application, and a differential-mode frequency comparison diagram between generator sets. DETAILED DESCRIPTION

[0030] In order to make the purpose, features and advantages of the present invention more obvious and easy to understand, the present invention will be described clearly and completely below in conjunction with the drawings in the simulation and experiment of the present invention. Obviously, the simulation and experiment described below are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0031] A differential mode frequency measurement method based on power measurement of the present invention is described in detail below in conjunction with the accompanying drawings and simulation schemes.

[0032] Figure 1 This is a simulation structure diagram of a 7-machine 29-node example system built by a differential mode frequency measurement method based on power measurement in this application.

[0033] The system structure of the 7-machine 29-node example is as follows Figure 1 The system has four units in the northwest area and two units in the northeast area. Unit G5 is connected to the southwest side. Under different working conditions, three groups of data are taken for comparison, namely, group 1 takes two units in the northwest area, group 2 takes two units in the northeast area, and group 3 takes one unit each in the northwest area and the northeast area.

[0034] Verify the validity of the differential mode frequency obtained by the power measurement method in this system. Take the differential mode component of the rotor frequency of two synchronous machines in each group and compare it with the differential mode obtained by the power measurement method.

[0035] When the system is disturbed, the differential mode frequency component obtained by the system power measurement method is basically consistent with the simulated differential mode component obtained by the generator set angular frequency. However, at the moment of disturbance, there is a deviation in the measurement of differential mode frequency due to power jump. The result is as follows Figure 2 , Figure 3 shown.

[0036] In order to further verify the effectiveness of the power measurement method in measuring differential mode frequency, experimental tests were carried out on a three-machine parallel system, focusing on analyzing the differential mode frequency response of the multi-VSC (voltage source inverter) system after disturbance under different control schemes.

[0037] Two schemes are used for comparison. The control strategy of scheme A is the grid-building type. In scheme B, VSC3 adopts the grid-following control mode, VSC1 is controlled as VSG (virtual synchronous control), and the inertia parameter K J1 =1.6kg / m2; VSC2 is to improve VSG control, inertia parameter K J2 =1.6kg / m2; VSC3 adopts VSG control or virtual inertia control, and the inertia parameters are K J3=0.8kg / m2.

[0038] The specific control strategies of VSC1-3 are shown in the table:

[0039]

[0040] Three operating conditions are set: 1) VSC1 power command increases ΔP ref1 =±1.0kW.

[0041] 2) VSC2 power command increases by ΔPref2 = ±0.7 kW.

[0042] 3) Switching on and off a load of ΔP = 1.45 kW.

[0043] The experimental waveforms and calculated results of the differential mode frequency responses of the two schemes are depicted.

[0044] The experimental results are shown in blue, where the experimental differential mode frequency is obtained from the system's internal angular frequency. The power measurement calculation results are shown in red, which are derived from the mechanical admittance model.

[0045] In scenario A, the multi-VSC system is in ΔP ref1 and ΔP disturbances, but VSC3 does not participate in the oscillation process. ref2 Under disturbance, there is no oscillation and active power overshoot in the dynamic process. Regardless of the above control strategy, the measurement method can effectively track the differential mode frequency response after disturbance.

[0046] In scheme B, at ΔP ref2 The dynamic process under disturbance is relatively stable, but in ΔP ref1 The active power response of VSC1 under the disturbance of ΔP appears overshoot. VSC3 in Scheme B adopts virtual inertia control, which has no internal potential and internal angular frequency, so Scheme B has only one set of differential mode frequencies.

[0047] Under the three disturbances, the differential mode frequency obtained by the measurement method is basically consistent with the experimental differential mode frequency. Therefore, using the measurement method to obtain the differential mode frequency can effectively verify the frequency-time-space distribution characteristics after the system is disturbed.

[0048] In order to more clearly reflect the effectiveness of the measurement method in analyzing the spatial and temporal distribution characteristics of the system frequency, the maximum amplitude of the measured differential mode frequency in the experiment is compared with the experimental differential mode frequency.

[0049] The maximum amplitude errors of the differential mode frequency obtained by the power measurement method and the experimental method are both within an acceptable range and can accurately characterize the frequency and spatial distribution of the system after the disturbance.

[0050] As mentioned above, the VSC3 in solution B uses virtual inertia control, which has no internal potential and internal angular frequency. Therefore, the frequency of its port is measured by the PLL of the VSC3 controller, i.e., f bus .

[0051] The differential mode frequency (f i -f bus ) is compared with the differential mode frequency of power measurement. The differential mode frequency obtained by the phase-locked loop has a larger error in the maximum amplitude compared to the power measurement method, and the time when the maximum amplitude occurs is also different.

[0052] This error is quantitatively analyzed with the power measurement value as the benchmark.

[0053] The application provides a differential mode frequency measurement method based on power measurement, and obtains the active power-frequency response small signal circuit model of the simulation system based on the P / ω "admittance" circuit modeling method; obtains the power flowing through the node and the admittance from the disturbance point to the node, and determines the frequency of the node based on the power flowing through the node and the admittance from the disturbance point to the node; based on the different node frequencies obtained by the power measurement method, the differential mode frequencies between different nodes are obtained by taking the difference between them; based on the constructed simulation system model, Kirchhoff's voltage law is used to obtain a formula that can obtain the differential mode frequencies between different nodes of the system.

[0054] The embodiments described above are only used to illustrate the technical solutions of the present application, rather than to limit them. Although the present application has been described in detail with reference to the aforementioned embodiments, a person skilled in the art should understand that the technical solutions described in the aforementioned embodiments may still be modified, or some of the technical features may be replaced by equivalents. Such modifications or replacements do not deviate the essence of the corresponding technical solutions from the spirit and scope of the technical solutions of the embodiments of the present application, and should all be included in the protection scope of the present application.

Claims

1. A differential mode frequency measurement method based on power measurement, characterized in that: include: Based on the P / ω "admittance" circuit modeling method, the active power-frequency response small signal circuit model of the simulation system is obtained; The power flowing through the node and the admittance from the disturbance point to the node are obtained, and the frequency of the node is determined according to the power flowing through the node and the admittance from the disturbance point to the node; Based on the different node frequencies obtained by the power measurement method, the differential mode frequencies between different nodes are obtained by subtracting them from each other. Based on the constructed simulation system model, Kirchhoff's voltage law is used to obtain the formula for obtaining the differential mode frequency between different nodes of the system.

2. The differential mode frequency calculation method based on power measurement according to claim 1, characterized in that: When a disturbance occurs in the system, the differential mode frequency between nodes can be obtained by measuring the power transmitted by each branch and the admittance from the disturbance point to the node.

3. The differential mode frequency calculation method based on power measurement according to claim 1, characterized in that: The P / ω "admittance" circuit modeling method is used to obtain the active power-frequency response small signal circuit model, and the transmission power calculation formula between different nodes i and j is obtained, including: The line transmission power mentioned above refers to the power flowing between different nodes after a disturbance occurs in the power system. The calculation formula is: Where: P ij is the power flowing through the line; U i , U j is the voltage across the line, X lineij That is, the reactance component of the line, δ ij The phase angle difference of the voltage at both ends of the line; The line transmission power can be determined by calculating the voltage at both ends of the line, the reactance component of the line and the phase angle difference of the voltage at both ends of the line.

4. The calculation method according to claim 1, characterized in that: The linearized differential mode frequency small signal calculation formula obtained based on the P / ω "admittance" circuit modeling includes: The differential mode frequency refers to the frequency difference between different nodes after a disturbance occurs in the power system. The small signal calculation formula is: Where: Δω ij is the frequency difference mode component Δω between different nodes i and j ij , ΔP ij is the change in active power flowing through nodes i and j before and after the disturbance.

5. The calculation method according to claim 1, characterized in that: The calculation method is based on a small signal model obtained by P / ω "admittance" circuit modeling. By selecting any node as a reference node, the differential mode frequency of any two nodes in the system can be obtained. The calculation formula includes: The node admittance matrix, taking a node in the circuit model as the reference node, the node admittance matrix expression of the system circuit model is: Where: Y lineij is the "admittance" of each section of the transmission line, Y Ti is the "admittance" of the transformer; Select a reference node, take node 4 as an example, and according to Kirchhoff's voltage law, the "potential" expression of each node in the system can be obtained: Where: Δω i4 is the potential of node i with respect to node 4; Finally, substitute the above two equations into the difference formula, and the expression is: Give ij =Here i4 -See j4 (i≠j) Where: Δω ij is the "potential difference" Δω between any two nodes ij .

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