Network-configuration type flexible direct current station control method and device based on coupled virtual impedance

Abstract

The application discloses a network-constructing type flexible direct current station control method and device based on coupled virtual impedance, and the method comprises the following steps: obtaining three-phase currents and three-phase voltages of an alternating current side of a flexible direct current station; obtaining a control voltage of the flexible direct current station according to the three-phase voltages; filtering dq-axis fundamental frequency components of the three-phase currents according to a preselected filter to obtain dq-axis oscillation components; obtaining a dq-axis direct compensation voltage according to the dq-axis oscillation components and a first control coefficient; performing decoupling control on the dq-axis oscillation components, a second control coefficient and a third control coefficient, and then performing integral control to obtain a dq-axis decoupling compensation voltage; adding the dq-axis direct compensation voltage and the dq-axis decoupling compensation voltage to obtain a dq-axis damping remodeling compensation voltage; and superimposing the control voltage of the flexible direct current station and the dq-axis damping remodeling compensation voltage to perform modulation and control.

Description

Technical Field

[0001] This invention relates to the field of power electronics, and in particular to a control method and device for a grid-type flexible DC converter station based on coupled virtual impedance. Background Technology

[0002] In recent years, the power system has been developing a trend towards a high proportion of new energy sources and high proportion of power electronic equipment. The converters commonly used in engineering are typically grid-connected control frameworks. With the large-scale integration of grid-connected converters into the grid, local grid weakening occurs, and the interaction between the grid-connected converter and the weak grid increases the probability of broadband oscillations. Grid-connected control converters, because they do not require traditional phase-locked loops (PLLs) for synchronization, eliminate the interaction between the weak grid and the PLL, making them suitable for weak grid and islanded grid applications. However, they still face similar oscillation problems as grid-connected converters.

[0003] The inner-loop control of network-based control technology mainly includes two structures: voltage-current dual closed-loop and voltage single closed-loop. The voltage-current dual closed-loop structure, due to the effective damping provided by the proportional element of its current loop PI controller, has its oscillation risk concentrated in the mid-to-high frequency range (above 200Hz), while exhibiting better impedance characteristics in the low-frequency range. In contrast, the voltage single closed-loop structure, on the one hand, controls a relatively slowly changing AC voltage, resulting in a narrower controller bandwidth, weaker damping, and a susceptibility to near-power frequency oscillations with low correlation to the control strategy. On the other hand, due to the dynamic switching of multiple sub-modules in each phase arm, the internal dynamics and circulating current suppression of the MMC (modular multilevel converter) make it more complex than the VSC (voltage source converter), and high-frequency oscillations are unavoidable due to the time delay.

[0004] Therefore, a new technical solution is urgently needed to address the technical problem of how to improve the impedance characteristics of grid-type MMC flexible DC converter stations based on voltage single closed-loop control over a wide frequency range. Summary of the Invention

[0005] This invention provides a control method and apparatus for a grid-type flexible DC converter station based on coupled virtual impedance, which solves the technical problem of how to improve the impedance characteristics of a grid-type MMC flexible DC converter station based on voltage single closed-loop control over a wide frequency range.

[0006] To achieve the above objectives, this invention provides a control method for a grid-type flexible DC converter station based on coupled virtual impedance, comprising:

[0007] Obtain the three-phase current and three-phase voltage on the AC side of the converter station; obtain the converter station control voltage based on the three-phase voltage; filter out the dq-axis fundamental frequency component of the three-phase current using a pre-selected filter to obtain the dq-axis oscillation component;

[0008] The direct compensation voltage of the dq axis is obtained based on the dq axis oscillation component and the first control coefficient; after decoupling control based on the dq axis oscillation component, the second control coefficient, and the third control coefficient, integral control is performed to obtain the decoupling compensation voltage of the dq axis.

[0009] The dq-axis direct compensation voltage and the dq-axis decoupling compensation voltage are summed to obtain the dq-axis damping reshaping compensation voltage; the converter station control voltage and the dq-axis damping reshaping compensation voltage are superimposed and then modulated and controlled.

[0010] Preferably, the converter station control voltage is obtained based on the three-phase voltage, including:

[0011] The dq-axis control quantity is obtained by subtracting the reference value from the dq-axis component of the three-phase voltage and then passing it through an integral controller.

[0012] The converter station control voltage of the dq axis is obtained by summing the dq axis control quantity with the setpoint feedforward. and :

[0013] ;

[0014] in, For the Laplace operator; The coefficients of the integral element; and These are the d-axis components of the three-phase voltage. and the q-axis component of the three-phase voltage of Domain value; and They are respectively and of Domain reference value; This is the rated voltage value.

[0015] Preferably, obtaining the dq-axis direct compensation voltage based on the dq-axis oscillation component and the first control coefficient includes:

[0016] dq axis oscillation component and With the first control coefficient K Multiplying by 1 yields the direct compensation voltage for the dq axis. and First control coefficient K 1 is the preset direct control coefficient; and Represented as:

[0017] .

[0018] Preferably, after decoupling control based on the dq-axis oscillation component, the second control coefficient, and the third control coefficient, integral control is performed to obtain the dq-axis decoupling compensation voltage, which includes:

[0019] Based on the dq-axis oscillation component and the second control coefficient K 2 and the third control coefficient K 3. Perform decoupling control, and then obtain the dq-axis decoupling compensation voltage through integral control. and Second control coefficient K 2 and the third control coefficient K 3 are all preset decoupling control coefficients; and Represented as:

[0020] .

[0021] Preferably, the dq-axis damping reshaping compensation voltage is obtained by summing the dq-axis direct compensation voltage and the dq-axis decoupling compensation voltage, including:

[0022] The dq-axis direct compensation voltage and the dq-axis decoupled compensation voltage are summed to obtain the dq-axis damping reshaping compensation voltage. and , is represented as:

[0023] .

[0024] Preferably, the modulation and control of the converter station control voltage and the dq-axis damping reshaping compensation voltage by superimposing them includes:

[0025] By superimposing the converter station control voltage and the dq-axis damping reshaping compensation voltage, the virtual potential of the flexible DC converter station is obtained:

[0026] ;

[0027] The control of the grid-type flexible DC converter station is achieved by modulating the virtual potential.

[0028] The present invention also provides a control device for a grid-type flexible DC converter station based on coupled virtual impedance, the device being used to implement the method of the present invention.

[0029] The present invention has the following beneficial effects:

[0030] This invention presents a control method for grid-type flexible DC converter stations based on coupled virtual impedance. It eliminates the need for a voltage loop proportional element, employs a constant-value feedforward approach to improve response speed, and enhances the impedance characteristics of grid-type MMC flexible DC converter stations based on single-voltage closed-loop control over a wide frequency range through the coupling of d-axis and q-axis currents. This method requires no additional external hardware circuitry; it only needs to obtain the compensation voltage based on the d-axis and q-axis currents of the flexible DC converter station, eliminating complex algorithmic operations and offering ease of implementation. This method requires no trigger signal and does not alter the performance of the original control system under steady-state conditions, allowing for continuous operation alongside the control system. This method effectively improves the damping characteristics of grid-type MMC based on single-voltage closed-loop control, enabling stable operation under varying grid strength conditions. This method is applicable not only to providing voltage support to passive renewable energy bases from grid-type flexible DC converter stations but also to providing support to weak AC grids. The method of this invention is simple in algorithm, easy to implement, does not affect dynamic and transient operating characteristics, does not change the original control and protection system parameters, and does not add hardware equipment, effectively reducing the risk of instability over a wide frequency range. This method can effectively suppress oscillations after a system experiences wide-frequency oscillations; continued operation of this method can effectively reduce the risk of wide-frequency oscillations and improve system stability.

[0031] The control device for a grid-type flexible DC converter station based on coupled virtual impedance of the present invention, when used in the method of the present invention, has the same beneficial effects as the method of the present invention.

[0032] In addition to the objectives, features, and advantages described above, the present invention has other objectives, features, and advantages. The invention will now be described in further detail with reference to the accompanying drawings. Attached Figure Description

[0033] The accompanying drawings, which form part of this invention, are used to provide a further understanding of the invention. The illustrative embodiments of the invention and their descriptions are used to explain the invention and do not constitute an undue limitation of the invention. In the drawings:

[0034] Figure 1 This is a schematic diagram of the main circuit of a grid-type MMC flexible DC converter station according to a preferred embodiment of the present invention.

[0035] Figure 2 This is a schematic diagram of the method flow of a preferred embodiment of the present invention.

[0036] Figure 3 This is a schematic diagram of a single-loop control structure for coupled virtual impedance voltage according to a preferred embodiment of the present invention.

[0037] Figure 4 This is a schematic diagram of a coupling virtual impedance controller according to a preferred embodiment of the present invention.

[0038] Figure 5 This is a diagram showing the low-frequency positive sequence impedance characteristics of a converter station after adopting the method of the present invention in a preferred embodiment of the present invention.

[0039] Figure 6 This is a diagram showing the low-frequency negative sequence impedance characteristics of a converter station after adopting the method of the present invention in a preferred embodiment of the present invention.

[0040] Figure 7 This is a diagram showing the full-band positive sequence impedance characteristics of the converter station after adopting the method of the present invention in a preferred embodiment of the present invention.

[0041] Figure 8 This is a diagram showing the full-band negative sequence impedance characteristics of the converter station after adopting the method of the present invention in a preferred embodiment of the present invention. Detailed Implementation

[0042] The embodiments of the present invention will be described in detail below with reference to the accompanying drawings, but the present invention can be implemented in many different ways as defined and covered by the claims.

[0043] The main circuit structure of the grid-type MMC flexible DC converter station of the preferred embodiment of the present invention is shown below. Figure 1 ;in, The phase voltage amplitude at the PCC point (point of common coupling, grid connection point); This represents the phase difference between the voltage at point PCC and the equivalent power source of the grid. and The equivalent resistance and inductance of the converter transformer; The current flowing into the converter station at point PCC; and Inject the active and reactive power of the converter station into the PCC point; and These are DC voltage and DC current, respectively.

[0044] See Figure 2 In a preferred embodiment of the present invention, a control method for a grid-type flexible DC converter station based on coupled virtual impedance is provided, comprising:

[0045] Q1. Obtain the three-phase current and three-phase voltage on the AC side of the converter station; obtain the converter station control voltage based on the three-phase voltage; and obtain the dq-axis oscillation component by filtering out the dq-axis fundamental frequency component of the three-phase current using a pre-selected filter.

[0046] In a preferred embodiment of the present invention, obtaining the three-phase current and three-phase voltage on the AC side of the converter station includes:

[0047] By monitoring the instantaneous three-phase components on the AC side of the converter station using AC current and voltage measuring devices, the three-phase current and three-phase voltage on the AC side of the converter station can be obtained.

[0048] When the system is running, if broadband oscillations occur, the voltage, current, and power on the AC side of the flexible DC converter station will all oscillate. To suppress broadband oscillations, current control is performed through coupled virtual impedance. The current control of the flexible DC converter station has two degrees of freedom, implemented on the d-axis and q-axis respectively. The corresponding dq components can be obtained by performing a Park transformation on the three-phase current and three-phase voltage.

[0049] In a preferred embodiment of the present invention, obtaining the converter station control voltage based on the three-phase voltage includes:

[0050] See Figure 3 The dq-axis control quantity is obtained by subtracting the reference value from the dq-axis component of the three-phase voltage and then passing it through an integral controller. The dq-axis control quantity is then summed with the setpoint feedforward to obtain the converter station control voltage on the dq-axis. and :

[0051] ;

[0052] in, For the Laplace operator; The coefficients of the integral element; and These are the d-axis components of the three-phase voltage. and the q-axis component of the three-phase voltage of Domain value; and They are respectively and of Domain reference value; This is the rated voltage value.

[0053] In a preferred embodiment of the present invention, the dq-axis oscillation component is obtained by filtering out the fundamental frequency component of the three-phase current using a pre-selected filter, including:

[0054] The DC component of the three-phase current dq component is the fundamental frequency signal in the original three-phase current. In order to obtain the dq axis oscillation component, the fundamental frequency component of the dq axis should be filtered out.

[0055] The filter is selected according to actual needs. Commonly used options include first-order high-pass filters, higher-order high-pass filters, second-order and higher-order band-pass filters, etc., and only one needs to be selected. In the preferred embodiment of the present invention, a first-order high-pass filter is selected. Then, the dq-axis fundamental frequency component of the three-phase current is filtered out by the selected first-order high-pass filter, and the dq-axis oscillation component is obtained. and exist In the domain, it is represented as:

[0056] ;

[0057] in, This is the bandwidth of a first-order high-pass filter; and These represent the dq components of the three-phase current, respectively. Domain value.

[0058] Q2. Obtain the direct compensation voltage of the dq axis based on the dq axis oscillation component and the first control coefficient; after decoupling control based on the dq axis oscillation component, the second control coefficient, and the third control coefficient, perform integral control to obtain the decoupling compensation voltage of the dq axis.

[0059] To suppress oscillating components, by means of... Figure 4 The coupled virtual impedance controller shown is used for processing. The parameters can be... K 1 is considered the proportional parameter of the PI controller. K 2 and K 3 can be considered as the integral parameter of a PI controller. Unlike a PI controller, however, it needs to pass through [a parameter] before entering the integral stage. K 2 and K 3. Interaction, eliminating the coupling effect between d-axis current and q-axis.

[0060] In a preferred embodiment of the present invention, obtaining the dq-axis direct compensation voltage based on the dq-axis oscillation component and the first control coefficient includes:

[0061] dq axis oscillation component and With the first control coefficient K Multiplying by 1 yields the direct compensation voltage for the dq axis. and First control coefficient K 1 is the preset direct control coefficient; and Represented as:

[0062] .

[0063] In a preferred embodiment of the present invention, after decoupling control based on the dq-axis oscillation component, the second control coefficient, and the third control coefficient, integral control is performed to obtain the dq-axis decoupling compensation voltage, including:

[0064] Based on the dq-axis oscillation component and the second control coefficient K 2 and the third control coefficient K 3. Perform decoupling control, and then obtain the dq-axis decoupling compensation voltage through integral control. and Second control coefficient K 2 and the third control coefficient K3 are all preset decoupling control coefficients; and Represented as:

[0065] .

[0066] Q3. The direct compensation voltage and the decoupled compensation voltage of the dq axis are summed to obtain the damped reshaping compensation voltage of the dq axis; the converter station control voltage and the damped reshaping compensation voltage of the dq axis are superimposed and then modulated and controlled.

[0067] In a preferred embodiment of the present invention, the dq-axis direct compensation voltage and the dq-axis decoupling compensation voltage are summed to obtain the dq-axis damping reshaping compensation voltage, which includes:

[0068] The dq-axis direct compensation voltage and the dq-axis decoupled compensation voltage are summed to obtain the dq-axis damping reshaping compensation voltage. and , represented as:

[0069] .

[0070] In a preferred embodiment of the present invention, the modulation and control of the converter station control voltage and the dq-axis damping reshaping compensation voltage by superimposing them includes:

[0071] By superimposing the converter station control voltage and the dq-axis damping reshaping compensation voltage, the virtual potential of the flexible DC converter station is obtained:

[0072] .

[0073] Expanding on this further, it can be represented as:

[0074] ;

[0075] The control of the grid-type flexible DC converter station is achieved by modulating the virtual potential.

[0076] After the system experiences broadband oscillations, the power fluctuates across multiple frequency bands. By altering the power absorbed or released by the flexible DC converter station, the broadband oscillations can be suppressed. The superposition of the dq-axis damping reshaping compensation voltage and the control voltage generated by the single-voltage closed loop changes the MMC virtual potential value, ultimately altering the instantaneously absorbed power of the flexible DC converter station and achieving the goal of suppressing low-frequency oscillations.

[0077] This invention presents a control method for grid-type flexible DC converter stations based on coupled virtual impedance. It eliminates the need for a voltage loop proportional element, employs a constant-value feedforward approach to improve response speed, and enhances the impedance characteristics of grid-type MMC flexible DC converter stations based on single-voltage closed-loop control over a wide frequency range through the coupling of d-axis and q-axis currents. This method requires no additional external hardware circuitry; it only needs to obtain the compensation voltage based on the d-axis and q-axis currents of the flexible DC converter station, eliminating complex algorithmic operations and offering ease of implementation. This method requires no trigger signal and does not alter the performance of the original control system under steady-state conditions, allowing for continuous operation alongside the control system. This method effectively improves the damping characteristics of grid-type MMC based on single-voltage closed-loop control, enabling stable operation under varying grid strength conditions. This method is applicable not only to providing voltage support to passive renewable energy bases from grid-type flexible DC converter stations but also to providing support to weak AC grids. The method of this invention is simple in algorithm, easy to implement, does not affect dynamic and transient operating characteristics, does not change the original control and protection system parameters, and does not add hardware equipment, effectively reducing the risk of instability over a wide frequency range. This method can effectively suppress oscillations after a system experiences wide-frequency oscillations; continued operation of this method can effectively reduce the risk of wide-frequency oscillations and improve system stability.

[0078] In a preferred embodiment of the present invention, a control device for a grid-type flexible DC converter station based on coupled virtual impedance is also provided, the device being used to implement the method of the present invention.

[0079] The control device for a grid-type flexible DC converter station based on coupled virtual impedance of the present invention, when used in the method of the present invention, has the same beneficial effects as the method of the present invention.

[0080] Verification section:

[0081] In a preferred embodiment of the present invention, the impedance characteristic analysis of the converter station obtained after using the method of the present invention is described in [reference needed]. Figures 5 to 8 According to the Nyquist impedance criterion, the impedance angle outside ±90° is in the negative damping region, posing a significant risk of broadband oscillation when interacting with AC systems. After adopting the method of this invention, Figure 5 The positive sequence impedance in the low-frequency band of the medium-frequency converter station is effectively reduced in the negative damping region below 50Hz. Figure 6 The negative sequence impedance in the low-to-mid frequency range has no negative damping region, effectively reducing the risk of oscillation below 100Hz; after adopting the method of this invention... Figures 7 to 8 The impedance angles of the positive and negative sequence impedances are all within ±90° in the frequency band above 100Hz, effectively reducing the negative damping region above 100Hz and lowering the risk of mid-to-high frequency oscillations. In summary, compared with traditional typical control methods, the method of this invention can greatly improve the impedance characteristics across the entire frequency band and reduce the risk of broadband oscillations.

[0082] The above description is merely a preferred embodiment of the present invention and is not intended to limit the invention. Various modifications and variations can be made to the present invention by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A control method for a grid-type flexible DC converter station based on coupled virtual impedance, characterized in that, include: Obtain the three-phase current and three-phase voltage on the AC side of the converter station; obtain the converter station control voltage based on the three-phase voltage; The dq-axis fundamental frequency component of the three-phase current is filtered out by a pre-selected filter to obtain the dq-axis oscillation component; The dq-axis direct compensation voltage is obtained based on the dq-axis oscillation component and the first control coefficient; after decoupling control based on the dq-axis oscillation component, the second control coefficient, and the third control coefficient, integral control is performed to obtain the dq-axis decoupling compensation voltage. The dq-axis direct compensation voltage and the dq-axis decoupling compensation voltage are summed to obtain the dq-axis damping reshaping compensation voltage. The converter station control voltage and the dq-axis damping reshaping compensation voltage are superimposed and then modulated and controlled. The converter station control voltage is obtained based on the three-phase voltage, including: The dq-axis control quantity is obtained by subtracting the reference value from the dq-axis component of the three-phase voltage and then passing it through an integral controller. The converter station control voltage of the dq axis is obtained by summing the dq axis control quantity with the setpoint feedforward. and : ； in, For the Laplace operator; The coefficients of the integral element; and These are the d-axis components of the three-phase voltage. and the q-axis component of the three-phase voltage of Domain value; and They are respectively and of Domain reference value; This is the rated voltage value; The dq-axis direct compensation voltage and the dq-axis decoupling compensation voltage are summed to obtain the dq-axis damping remodeling compensation voltage, which includes: dq-axis damping remodeling compensation voltage. and Represented as: ； in, K 1 is the first control factor; K 2 is the second control coefficient; K 3 is the third control coefficient; and This refers to the dq-axis oscillation component; The modulation and control of the superimposed converter station control voltage and the dq-axis damping reshaping compensation voltage includes: The virtual potential of the flexible DC converter station is obtained by superimposing the control voltage of the converter station and the dq-axis damping reshaping compensation voltage: ； The virtual potential is modulated to achieve control of the grid-type flexible DC converter station.

2. The control method for a grid-type flexible DC converter station based on coupled virtual impedance according to claim 1, characterized in that, The direct compensation voltage for the dq axis is obtained based on the dq axis oscillation component and the first control coefficient, including: The dq axis oscillation component and With the first control coefficient K Multiplying by 1 yields the direct compensation voltage for the dq axis. and The first control coefficient K 1 is the preset direct control coefficient; and Represented as: 。 3. The control method for a grid-type flexible DC converter station based on coupled virtual impedance according to claim 2, characterized in that, After decoupling control based on the dq-axis oscillation component, the second control coefficient, and the third control coefficient, integral control is performed to obtain the dq-axis decoupling compensation voltage, which includes: Based on the dq-axis oscillation component and the second control coefficient K 2 and the third control coefficient K 3. Perform decoupling control, and then obtain the dq-axis decoupling compensation voltage through integral control. and The second control coefficient K 2 and the third control coefficient K 3 are all preset decoupling control coefficients; and Represented as: 。 4. A control device for a grid-type flexible DC converter station based on coupled virtual impedance, characterized in that, The apparatus is used to implement the method according to any one of claims 1 to 3.

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