A frequency band transfer method for high-frequency negative damping effect in flexible DC converter station

Abstract

The present invention discloses a method for transferring the frequency band of the high-frequency negative damping effect of a flexible direct current converter station, comprising the following steps: determining the negative damping effect interval of the converter station and then determining the negative damping effect transfer principle; selecting a suitable virtual series impedance controller structure; performing analytical calculation of the parameters of the virtual series impedance controller; and finally verifying the conductivity characteristics of the converter station and finally realizing the transfer of the frequency band of the negative damping effect of the flexible direct current converter station. The present invention provides the parameter analytical calculation steps for the virtual series impedance control transfer of the high-frequency negative damping effect, which can effectively transfer the high-frequency negative damping effect band, realize the reshaping of the high-frequency impedance characteristics of the flexible direct current converter station, and improve the high-frequency stability of the system.

Description

[0001] Technology Neighborhood

[0002] The present invention relates to the field of flexible direct current (HVDC), including application scenarios such as flexible direct current converter stations connected to alternating current power grids and new energy stations, and in particular to a method for transferring the high-frequency negative damping effect of a flexible direct current converter station. Background Art

[0003] Since my country's new energy and load are distributed inversely, high voltage direct current (HVDC) based on modular multilevel converter (MMC) is the preferred solution for realizing high-power long-distance transmission, large-scale transmission and consumption of new energy. It has been put into use in many domestic and foreign projects such as Zhangbei Flexible Direct Current Project and Rudong Offshore Wind Power Flexible Direct Current Project. However, with the widespread application of flexible direct current projects, the high-frequency oscillation problem induced by the interaction of power electronic devices in the system has gradually become prominent, such as the 1.6kHz oscillation on the AC side of the Faxi Project and the high-frequency oscillations of about 665Hz and about 1810Hz on the AC side of the Chongqing-Hubei Project. If the oscillation is not effectively suppressed, it will cause equipment damage, new energy disconnection and power outage accidents, seriously affecting the safe and efficient operation of the power system. Therefore, it is necessary to study relevant measures to suppress the high-frequency oscillation phenomenon of flexible direct current projects.

[0004] At present, the active damping control strategy has become one of the mainstream measures for suppressing high-frequency oscillations in flexible DC transmission systems, due to its advantage of being able to suppress high-frequency oscillations through control algorithms without increasing system power loss and external circuits. For example, related studies have proposed using virtual series impedance in the current inner loop of the flexible DC converter station to suppress high-frequency oscillations. However, the parameter design of the virtual series impedance in these studies is mainly achieved by drawing the impedance characteristic curve of the converter station under different parameters, optimizing and determining the parameters, or obtaining the parameter constraints of the virtual series impedance controller based on the impedance stability criterion. The former has limited design parameters and needs to redesign the parameters when the application scenario changes. The latter method cannot accurately determine the frequency range of the negative damping effect and does not reflect the essence of the frequency band transfer of the negative damping effect.

[0005] In view of the fact that virtual series impedance is essentially to achieve effective suppression of high-frequency oscillation by transferring the negative damping effect frequency band, the present invention patent proposes a method for transferring the high-frequency negative damping effect frequency band of a flexible DC converter station. This method utilizes the mechanism of virtual series impedance transferring the negative damping effect frequency band, determines the negative damping effect transfer principle, analyzes and calculates the parameter range of the virtual series impedance controller, reshapes the high-frequency impedance characteristics of the flexible DC converter station, and improves the high-frequency stability of the flexible DC transmission system. Summary of the invention

[0006] The purpose of the present invention is to determine the negative damping effect interval and transfer principle of the flexible DC converter station without adding additional hardware circuits, analyze and calculate the parameters of the virtual series impedance controller, transfer the negative damping effect frequency band, reshape the high-frequency impedance characteristics of the flexible DC converter station, and improve the high-frequency stability of the flexible DC transmission system.

[0007] A method for frequency band transfer of high-frequency negative damping effect of a flexible DC converter station provided by the present invention comprises the following steps:

[0008] S1. Determine the negative damping effect range of the converter station;

[0009] S2. Determine the negative damping effect transfer principle;

[0010] S3. Virtual series impedance controller structure selection

[0011] S4. Analytical calculation of virtual series impedance controller parameters;

[0012] S5. Verification of conductivity characteristics of converter station.

[0013] The determination of the negative damping effect interval of the converter station in step S1 is specifically as follows:

[0014] First, the theoretical calculation research work is carried out through the positive sequence impedance or admittance model, and then the positive and negative sequence verification is carried out. The positive sequence admittance model of the flexible DC converter station that can realize voltage feedforward and virtual series impedance decoupling analysis is adopted. Based on the positive sequence admittance model of the flexible DC converter station, the zero point of the converter station positive sequence conduction is calculated without considering the virtual series impedance. Thus, the negative damping effect range of the converter station is determined.

[0015] The principle for determining the negative damping effect transfer described in step S2 is as follows:

[0016] Based on the converter station admittance model, the virtual series impedance controller is equivalent to the real part and the imaginary part, and the converter station conductance is calculated when the virtual series impedance is considered. The principle of the virtual series impedance transfer negative damping effect is to minimize the Under the influence of the impedance characteristics of the converter station within the section, the zero-crossing point where the conductivity of the converter station changes from positive to negative moves to a higher frequency.

[0017] The virtual series impedance controller structure selection described in step S3 is as follows:

[0018] Based on the negative damping effect transfer principle determined in step S2, the conditions that the virtual series impedance controller must meet are determined, so as to select a virtual series impedance controller structure, whose types include but are not limited to low-pass filters, high-pass filters and band-pass filters, and whose orders include first-order, second-order and high-order, and which can be either a single controller or a superposition of multiple controllers.

[0019] The virtual series impedance controller parameter analytical calculation described in step S4 is specifically as follows:

[0020] Based on the virtual series impedance controller determined in step S3, the real and imaginary characteristics of the virtual series impedance controller are analyzed to determine the frequency point f at which the conductance of the converter station changes from positive to negative after adding the virtual series impedance. kx , and then according to the negative damping effect transfer principle, the inequality constraints that the virtual series impedance controller parameters need to satisfy are obtained to obtain the virtual series impedance controller parameter selection range. The negative damping interval range of a single virtual series impedance controller transfer is limited. According to actual needs, multiple virtual series impedance controllers can be superimposed to expand the negative damping effect transfer range. First, determine the single virtual series impedance controller parameters and the negative damping effect frequency range after transfer, and then determine the remaining controller parameters one by one. If necessary, the parameter selection range can be further optimized according to the converter station conductivity characteristic curve.

[0021] The verification of the conductivity characteristics of the converter station described in step S5 is specifically as follows:

[0022] According to the parameters of the virtual series impedance controller determined in step S4, the conductivity characteristics of the converter station are verified, the frequency range of the negative damping effect of the converter station before and after the virtual series impedance is added is analyzed, and the correctness of the negative damping effect transfer method and parameter design is verified.

[0023] Compared with the prior art, the present invention has the following beneficial effects:

[0024] The method for transferring the high-frequency negative damping effect frequency band of a flexible DC converter station proposed in the present invention has clear logic, can calculate the frequency range of the negative damping effect of the converter station, determine the frequency range of high-frequency oscillation risk, and provides the parameter analytical calculation steps for the virtual series impedance control transfer of the high-frequency negative damping effect, which can guide the analytical calculation of the parameters of the virtual series impedance controller, thereby effectively transferring the high-frequency negative damping effect frequency band, reshaping the high-frequency impedance characteristics of the flexible DC converter station, and improving the high-frequency stability of the system. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Figure 1 This is a schematic diagram of the main circuit of the flexible DC transmission system.

[0026] Figure 2 Schematic diagram of the control system implementation of virtual series impedance.

[0027] Figure 3 The real and imaginary characteristics of the second-order high-pass filter used in the specific implementation case provided for the patent of this invention can use other types of filters in addition to the second-order high-pass filter.

[0028] Figure 4 This is a step diagram for implementing the method for transferring the high-frequency negative damping effect of the flexible DC converter station in the patent of this invention. DETAILED DESCRIPTION

[0029] like Figure 4 The method for transferring the high-frequency negative damping effect of the flexible DC converter station provided by the present invention comprises the following steps:

[0030] S1. Determine the negative damping effect range of the converter station:

[0031] First, theoretical calculation research is carried out through the positive sequence impedance or admittance model. The positive sequence admittance model expression of the converter station that can realize voltage feedforward and virtual series impedance decoupling analysis is:

[0032]

[0033] Voltage feedforward includes step wave feedforward, direct feedforward and low-pass filter feedforward. This case uses direct feedforward as an example. Substituting the relevant parameters of this case, we can get f g z _ e m r m o c =796Hz, so the high-frequency negative damping effect interval of the converter station in this case is in the range of f>796Hz.

[0034] S2. Determine the transfer principle of negative damping effect:

[0035] Virtual series impedance control Figure 2 As shown, the transfer function of the virtual series impedance controller is equivalent in the form of real and imaginary parts. When the virtual series impedance is considered, the expression of the conductance of the converter station has the same sign as the following formula:

[0036] g n (ω)=(k pi +r vir )cos(ωT de )+(x vir +ωL eq )sin(ωT de )-(k pi +r vir ) (2)

[0037] The principle of virtual series impedance transfer negative damping effect is to minimize the impact on the impedance characteristics of the converter station in the range of f < 796Hz, so as to move the zero-crossing point where the converter station conductance changes from positive to negative to a higher frequency. Based on this principle, the virtual series impedance controller needs to satisfy the following inequality conditions as much as possible:

[0038] k pi +r vir <0 (3)

[0039]

[0040] The above formula is the principle of virtual series impedance transfer negative damping effect.

[0041] S3. Virtual series impedance controller structure selection:

[0042] According to the principle determined in step S2, a virtual series impedance controller structure can be selected, and its types include but are not limited to low-pass filters, high-pass filters and band-pass filters, and the orders include first-order, second-order and high-order. It can be a single controller or a superposition of multiple controllers. Since the second-order high-pass filter with a smaller damping ratio can make the real part have a larger negative value at the undamped oscillation frequency, it is easy to meet the first inequality condition, so this case only uses a second-order high-pass filter for illustration. In addition, other types of filters can also be used to transfer the frequency band of negative damping effect.

[0043] S4. Parameters analysis and calculation of virtual series impedance controller:

[0044] This case takes a single second-order high-pass filter as an example. The real and imaginary expressions of the second-order high-pass filter are:

[0045]

[0046] The real and imaginary characteristics of the second-order high-pass filter are determined by the above formula. vir When it is very small, it can not only reduce the impact of the low and medium frequency bands, but also effectively increase the extreme value of the real part, and the extreme value of the real part is consistent with the undamped oscillation frequency f vir It is irrelevant, which means that the conductivity characteristics in different frequency ranges can be affected by adjusting the undamped oscillation frequency. The real part extreme value of the second-order high-pass filter is about half of the imaginary part extreme value, and the imaginary part maximum value is about

[0047]

[0048] The real and imaginary characteristic curves of the second-order high-pass filter under a small damping ratio are as follows: Figure 3 As shown. Let k x Represents the imaginary part x vir To its maximum value The ratio of the imaginary part can be estimated to be 0.5k x k vir / ξ vir The frequency f kx In this case, k x = 0.2 as an example, it can be estimated that the frequency at this time is about

[0049] f kx ≈(1-2ξ vir )·f vir (7)

[0050] f kx As a damping controller, the negative damping effect of the converter is driven to a frequency point in a higher frequency range. That is, it is considered that the converter conductance changes from positive to negative at this point, and then the inequality constraints of the controller parameters are obtained, as shown in formula (8). Finally, the selection range of the virtual series impedance controller parameters is obtained.

[0051]

[0052] S5.Conductance characteristics verification of converter station:

[0053] According to the parameters of the virtual series impedance controller determined in step S4, by verifying the conductivity characteristics of the converter station, it is found that after the virtual series impedance is added, the negative damping effect range of the converter station is transferred to a higher frequency range, which verifies the correctness of the negative damping effect transfer method and parameter design.

[0054] At this point, the specific case of the patent of this invention has been implemented.

Claims

1. A method for frequency band transfer of high-frequency negative damping effect in a flexible DC converter station, characterized in that: The steps include: S1. Determine the negative damping effect range of the converter station; S2. Determine the negative damping effect transfer principle; Based on the converter station admittance model, the virtual series impedance controller is expressed in the form of real and imaginary parts, and the zero-crossing point of the positive sequence conductance of the converter station is calculated without considering the virtual series impedance. and the converter station conductance when considering virtual series impedance, where the principle of virtual series impedance transfer negative damping effect is to minimize the Under the influence of the impedance characteristics of the converter station in the section, the zero-crossing point where the conductance of the converter station changes from positive to negative moves to a higher frequency; S3. Virtual series impedance controller structure selection; S4. Analytical calculation of virtual series impedance controller parameters; S5. Verification of conductivity characteristics of converter station.

2. The method for transferring the high-frequency negative damping effect of a flexible DC converter station according to claim 1 is characterized in that Determining the negative damping effect interval of the converter station in step S1: First, the theoretical calculation research work is carried out through the positive sequence impedance or admittance model, and then the positive and negative sequence verification is carried out. The positive sequence admittance model of the flexible DC converter station that realizes the voltage feedforward and virtual series impedance decoupling analysis can be adopted. Based on the positive sequence admittance model of the flexible DC converter station, the zero crossing point of the positive sequence conductance of the converter station is calculated without considering the virtual series impedance. Thus, the negative damping effect range of the converter station is determined.

3. The method for transferring the high-frequency negative damping effect of a flexible DC converter station according to claim 1 is characterized in that The virtual series impedance controller structure selection described in step S3 is: Based on the negative damping effect transfer principle determined in step S2, the conditions that the virtual series impedance controller must meet are determined, so as to select the structure of the virtual series impedance controller; the virtual series impedance controller adopts a single filter or a superposition of multiple filters, and the types of filters adopted include low-pass filters, high-pass filters and band-pass filters, and the orders include first-order, second-order and high-order.

4. The method for transferring the high-frequency negative damping effect of a flexible DC converter station according to claim 1 is characterized in that The virtual series impedance controller parameter analytical calculation described in step S4 is: Based on the virtual series impedance controller determined in step S3, the real and imaginary characteristics of the virtual series impedance controller are analyzed to determine the frequency point f at which the conductance of the converter station changes from positive to negative after adding the virtual series impedance. kx Then, according to the negative damping effect transfer principle, the inequality constraints that the virtual series impedance controller parameters need to satisfy are obtained, and the selection range of the virtual series impedance controller parameters is obtained. The negative damping interval range of a single virtual series impedance controller is limited. According to actual needs, multiple virtual series impedance controllers can be superimposed to expand the negative damping effect transfer range. First, determine the parameters of a single virtual series impedance controller and the negative damping effect frequency range after transfer, and then determine the remaining controller parameters one by one. Finally, the parameter selection range can be further optimized according to the conductivity characteristic curve of the converter station.

5. The method for transferring the high-frequency negative damping effect of a flexible DC converter station according to claim 1 is characterized in that Verification of the conductivity characteristics of the converter station described in step S5: According to the parameters of the virtual series impedance controller determined in step S4, the conductivity characteristics of the converter station are verified, the frequency range of the negative damping effect of the converter station before and after the virtual series impedance is added is analyzed, and the correctness of the negative damping effect transfer method and parameter design is verified.

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