MMC converter station bridge arm current control method

Abstract

The invention provides an MMC converter station bridge arm current control method, and relates to the field of power systems. The method comprises the following steps: establishing a modular multilevel converter (MMC) mathematical model, and analyzing a bridge arm current reference value based on the MMC mathematical model; establishing a two-degree-of-freedom controller based on a quasi-proportional-integral resonance (PIR) controller and a decoupling extended state observer (DESO), wherein the two-degree-of-freedom controller is used for compensating MMC disturbance; performing disturbance compensation simulation verification and stability analysis; wherein the DESO is used for observing the working state of the MMC system in real time and feeding back MMC disturbance to the input end of the two-degree-of-freedom controller when the parameters and the running state of an MMC converter station are changed.

Description

Technical Field

[0001] The present application relates to the field of power systems, and in particular to a method for controlling arm current of an MMC converter station. Background Art

[0002] In high-voltage flexible direct current transmission projects, modular multilevel converters (MMC) have become increasingly popular due to their lower switching frequency and modular structure with better scalability. When MMC is used in direct current transmission systems, the AC side is generally connected to the grid, and grid imbalance often occurs, seriously affecting the reliable operation of the system.

[0003] MMC has an inherent circulating current problem. The circulating current will aggravate the distortion of the bridge arm current waveform, increase the bridge arm current peak and capacitor voltage fluctuation, increase the energy storage requirements of the capacitor, and reduce the safety margin of the power device, and increase the turn-on loss and turn-off loss of the device. Therefore, it is necessary to suppress the circulating current.

[0004] When the grid voltage is unbalanced, the generation of negative sequence voltage and negative sequence current will cause the double frequency circulating current to have three components: positive, negative and zero sequence. The MMC control system composed of AC current control, circulating current suppression and capacitor voltage balance control is relatively complex, and it is necessary to optimize the MMC control system. Summary of the invention

[0005] The present application aims to solve one of the technical problems in the related art at least to some extent.

[0006] The first aspect of the present application proposes a method for controlling the arm current of an MMC converter station, comprising: establishing a mathematical model of a modular multilevel converter MMC, and analyzing a reference value of the arm current based on the MMC mathematical model; establishing a two-degree-of-freedom controller based on a quasi-proportional integral resonant PIR controller and a decoupled extended state observer DESO, wherein the two-degree-of-freedom controller is used to compensate for MMC disturbances; performing disturbance compensation simulation verification and stability analysis; wherein the purpose of DESO is to observe the working status of the MMC in real time, and to feed back the MMC disturbance to the input end of the two-degree-of-freedom controller when the parameters and operating status of the MMC converter station change.

[0007] According to an embodiment of the present application, under the control target of suppressing the negative sequence double frequency current, the formula corresponding to the bridge arm current reference value is:

[0008]

[0009] In the above formula, is the reference value of the upper bridge arm current of phase k, is the reference value of the lower bridge arm current of phase k, is the reference value of the DC component of the bridge arm current, is the reference value of the AC component of the bridge arm current, where k = a, b, c.

[0010] According to one embodiment of the present application, when disturbance is considered, the dynamic characteristics of the bridge arm current of the MMC are:

[0011]

[0012] In the above formula, i k1 is the upper bridge arm current of phase k, i k2 is the k-phase lower bridge arm current, L r is the bridge arm inductance, R r is the equivalent resistance of the bridge arm, u k1 is the upper bridge arm voltage of phase k, u k2 is the voltage of the lower bridge arm of phase k, U dc is the DC side voltage, u sk is the k-phase voltage, d k1 is the unmodeled dynamics of the upper arm in the three-phase stationary coordinate system, d k2 is the unmodeled dynamics of the lower bridge arm in the three-phase stationary coordinate system.

[0013] According to one embodiment of the present application, the voltage drop on the equivalent resistance of the bridge arm is regarded as a disturbance, and the dynamic characteristic of the bridge arm current of the MMC is expressed as:

[0014]

[0015] in:

[0016] x=[i k1 i k2 ] T

[0017] B=diag(-1 / L r , =1 / L r , =1 / L r , =1 / L r , -1 / L r , =1 / L r )

[0018] u=[u k1 u k2 ] T

[0019]

[0020] In the above formula, i k1 is the upper bridge arm current of phase k, i k2 is the k-phase lower bridge arm current, L r is the bridge arm inductance, Rr is the equivalent resistance of the bridge arm, u k1 is the upper bridge arm voltage of phase k, u k2 is the voltage of the lower bridge arm of phase k, U dc is the DC side voltage, u sk is the k-phase voltage, d k1 is the unmodeled dynamics of the upper arm in the three-phase stationary coordinate system, d k2 is the unmodeled dynamics of the lower bridge arm in the three-phase stationary coordinate system, f k1 is the total observed disturbance corresponding to the upper arm, f k2 is the observed total disturbance corresponding to the lower bridge arm.

[0021] According to one embodiment of the present application, the transfer function of the PIR controller is:

[0022]

[0023] In the above formula, s is the Laplace operator, k i is the integral controller gain, k p is the proportional controller gain, k r is the gain of the resonant controller, ω 1 is the first resonant frequency, ω 1 =100π, the first resonant controller is used for tracking control of the base frequency AC current, ω 2 is the second resonant frequency, ω 2 =200π, the second resonant controller is used to suppress the double frequency harmonic circulating current, ω c is the resonance bandwidth.

[0024] According to one embodiment of the present application, when performing stability analysis, the open-loop transfer function used to determine stability is:

[0025] G op (s) = G k (s) / s

[0026] In the above formula, s is the Laplace operator, G k (s) is the transfer function of the PIR controller.

[0027] According to one embodiment of the present application, for the k-phase upper bridge arm, the control law of the PIR controller is:

[0028]

[0029] Where v k1 It is expressed as:

[0030]

[0031] In the above formula, is the reference value of the upper bridge arm current of phase k, is the estimated value of the upper bridge arm current of phase k obtained by the state observer, is the estimated value of the upper arm disturbance of phase k, is the rated value of the bridge arm inductance, where k = a, b, c;

[0032] According to one embodiment of the present application, for the k-phase lower bridge arm, the control law of the PIR controller is:

[0033]

[0034] Where v k2 It is expressed as:

[0035]

[0036] In the above formula, is the reference value of the lower bridge arm current of phase k, is the estimated value of the lower bridge arm current of phase k obtained by the state observer, is the estimated value of the disturbance of the lower bridge arm of phase k, is the rated value of the bridge arm inductance, where k = a, b, c.

[0037] According to one embodiment of the present application, DESO can be expressed as:

[0038] For the k-phase upper bridge arm:

[0039]

[0040] In the formula, is the rated value of the bridge arm inductance, β1 and β2 are the bandwidth of the state observer, is the estimated value of the upper arm disturbance of phase k, is the rate of change of the estimated value of the upper arm disturbance of phase k, is the rate of change of the estimated value of the upper bridge arm current of phase k obtained by the state observer, u k1 represents the control law of the PIR controller corresponding to the upper bridge arm of phase k, where k = a, b, c;

[0041] For the k-phase lower bridge arm:

[0042]

[0043] In the formula, is the rated value of the bridge arm inductance, β1 and β2 are the bandwidth of the state observer, is the estimated value of the disturbance of the lower bridge arm of phase k, is the rate of change of the estimated value of the disturbance of the lower bridge arm of phase k, is the rate of change of the estimated value of the lower bridge arm current of phase k obtained by the state observer, u k2 It represents the control law of the PIR controller corresponding to the lower bridge arm of phase k, where k = a, b, c.

[0044] The second aspect of the present application proposes an MMC converter station arm current control device, including: an analysis module, used to establish a modular multilevel converter MMC mathematical model, and analyze the bridge arm current reference value based on the MMC mathematical model; an establishment module, used to establish a two-degree-of-freedom controller based on a quasi-proportional integral resonant PIR controller and a decoupled extended state observer DESO, and the two-degree-of-freedom controller is used to compensate for MMC disturbances; a verification module, used to perform disturbance compensation simulation verification and stability analysis; wherein the purpose of the DESO is to observe the working status of the MMC in real time, and feed back the MMC disturbance to the input end of the two-degree-of-freedom controller when the parameters and operating status of the MMC converter station change.

[0045] The third aspect embodiment of the present application proposes an electronic device, comprising: at least one processor; and a memory communicatively connected to the at least one processor; wherein the memory stores instructions executable by the at least one processor, and the instructions are executed by the at least one processor to implement the MMC converter station bridge arm current control method as described in the first aspect embodiment of the present application.

[0046] The fourth aspect embodiment of the present application proposes a non-transitory computer-readable storage medium storing computer instructions, wherein the computer instructions are used to implement the MMC converter station arm current control method as described in the first aspect embodiment of the present application.

[0047] The fifth aspect embodiment of the present application proposes a computer program product, including a computer program, which, when executed by a processor, implements the MMC converter station bridge arm current control method as described in the first aspect embodiment of the present application.

[0048] The present application achieves at least the following beneficial effects: the present application establishes an MMC mathematical model based on the bridge arm current in a three-phase stationary coordinate system, analyzes the reference value of the bridge arm current under different control objectives, and secondly, for the unmodeled dynamics of the MMC device, establishes the state equation of each bridge arm current considering the disturbance, and proposes a two-degree-of-freedom controller whose tracking performance is completely decoupled from the anti-disturbance performance, builds a simulation model of the MMC rectifier side, and proves the advantages of this control method over the traditional quasi-proportional resonant control under balanced and unbalanced power grid conditions. BRIEF DESCRIPTION OF THE DRAWINGS

[0049] The above and / or additional aspects and advantages of the present application will become apparent and easily understood from the following description of the embodiments in conjunction with the accompanying drawings, in which:

[0050] Figure 1 It is an exemplary schematic diagram of a method for controlling the bridge arm current of an MMC converter station shown in the present application.

[0051] Figure 2 This is an MMC topology diagram and equivalent circuit diagram shown in this application.

[0052] Figure 3 It is a control block diagram of the upper bridge arm current of phase A based on the PIR+DESO controller shown in one embodiment of the present application.

[0053] Figure 4 This is a block diagram of the A-phase upper bridge arm current control according to an embodiment of the present application.

[0054] Figure 5(a) is k p Nyquist curve for different values.

[0055] Figure 5(b) is k r Nyquist curve for different values.

[0056] Figure 5(c) is k i Nyquist curve for different values.

[0057] Figure 5(d) is ω c Nyquist curve for different values.

[0058] FIG6( a ) shows the three-phase voltage drop simulation waveform under QPR control.

[0059] FIG6( b ) shows the three-phase voltage drop simulation waveform under PIR+DESO control.

[0060] FIG6( c ) shows the load dump simulation waveform under QPR control.

[0061] FIG6( d ) shows the load dump simulation waveform under PIR+DESO control.

[0062] FIG6( e ) shows the three-phase voltage recovery simulation waveform under QPR control.

[0063] FIG6( f ) shows the three-phase voltage recovery simulation waveform under PIR+DESO control.

[0064] FIG7( a ) shows the simulated waveform of the voltage drop of phase A under QPR control.

[0065] FIG7( b ) shows the simulated waveform of the voltage drop of phase A under PIR+DESO control.

[0066] FIG7( c ) shows the load dump simulation waveform under QPR control.

[0067] FIG7( d ) shows the load dump simulation waveform under PIR+DESO control.

[0068] FIG7( e ) shows the simulation waveform of the voltage recovery of phase A under QPR control.

[0069] FIG7( f ) shows the simulation waveform of phase A voltage recovery under PIR+DESO control. DETAILED DESCRIPTION

[0070] Embodiments of the present application are described in detail below, and examples of the embodiments are shown in the accompanying drawings, wherein the same or similar reference numerals throughout represent the same or similar elements or elements having the same or similar functions. The embodiments described below with reference to the accompanying drawings are exemplary and are intended to be used to explain the present application, and should not be construed as limiting the present application.

[0071] Figure 1 is an exemplary schematic diagram of a method for controlling the bridge arm current of an MMC converter station shown in the present application, such as Figure 1 As shown, the MMC converter station bridge arm current control method comprises the following steps:

[0072] S101, establishing a modular multilevel converter MMC mathematical model, and analyzing a bridge arm current reference value based on the MMC mathematical model.

[0073] Figure 2 This is a MMC topology diagram and equivalent circuit diagram shown in this application, such as Figure 2 As shown in the figure, according to Kirchhoff's law, the mathematical model of the modular multilevel converter MMC based on the bridge arm current in the three-phase stationary coordinate system is:

[0074]

[0075] In the above formula, i k1 is the upper bridge arm current of phase k, i k2 is the k-phase lower bridge arm current, L r is the bridge arm inductance, which can suppress the surge current and circulating current under DC side fault to a certain extent. r is the equivalent resistance of the bridge arm, u k1 is the upper bridge arm voltage of phase k, u k2 is the voltage of the lower bridge arm of phase k, U dc is the DC side voltage, u sk is the k-phase voltage, where k=a, b, c.

[0076] When considering the second harmonic circulating current, the upper and lower arm currents are:

[0077]

[0078] In the above formula, i k1 is the upper bridge arm current of phase k, i k2 is the lower bridge arm current of phase k, i dck is the DC component, I 2ωk is the secondary circulation amplitude, I 左 is the fundamental frequency current amplitude, ω is the fundamental frequency, θ k and φ are the power factor angle and the initial current phase angle respectively. Where, k = a, b, c.

[0079] Under normal working conditions, the circulating current suppression suppresses the double frequency circulating current to zero. Therefore, when the double frequency circulating current is suppressed to zero, as a feasible method, the formula corresponding to the upper and lower arm current reference values ​​is:

[0080]

[0081] In the above formula, is the reference value of the upper bridge arm current of phase k, is the reference value of the lower bridge arm current of phase k, is the reference value of the DC component of the bridge arm current, is the reference value of the AC component of the bridge arm current, where k = a, b, c.

[0082] In this application, X * Refers to the reference value of X, where X is the parameter mentioned in the formula in this application.

[0083] If the three-phase active power is the same, the DC component of the bridge arm current is the same, that is, In the formula, is the reference value of the DC component of the bridge arm current, I dc is the DC side current.

[0084] When the three-phase active power changes, the DC current component of the bridge arm also changes accordingly. When the loss caused by the resistance reactance of each phase of the MMC bridge arm is ignored, the instantaneous power on the AC and DC sides can be considered to be the same. The active power value on the AC side is measured, and the active DC component P is obtained using a low-pass filter. ack . Define the DC component for:

[0085]

[0086] In the above formula, is the reference value of the DC component of the bridge arm current, P ack is the active DC component, U dc is the DC side voltage.

[0087] Optionally, the bridge arm current reference value may be expressed in another form. For example, under unbalanced working conditions, an inner loop reference current expression in the αβ coordinate system is:

[0088]

[0089] In the above formula, u sa+ 、u sβ+ is the positive sequence voltage component, u sa- 、u sβ- is the negative sequence voltage component, u sαβ+ ,u sαβ- It is the voltage value in the stationary coordinate system obtained by equal power change on the AC side.

[0090] p 0 * ,q 0 * They are the reference values ​​of active and reactive power calculated by the voltage outer loop, and the current i with an asterisk is the reference value calculated by this formula.

[0091] Where K∈[-1 1], when K takes different values, the current reference value under different control objectives can be obtained.

[0092] Among them, different control objectives refer to suppressing negative sequence double frequency current, suppressing active power fluctuation and reactive power fluctuation.

[0093] Among them, the αβ coordinate system (also known as the stationary two-phase coordinate system) is a common coordinate transformation method used to simplify the analysis and control of three-phase motors and power electronic systems. It is mathematically equivalent to the three-phase coordinate system, but provides a more intuitive and simplified way to deal with these systems.

[0094] It is not difficult to understand that the balanced working condition is just a special case of the unbalanced working condition. The balanced working condition is a special unbalanced working condition, so the inner loop reference current expression in the αβ coordinate system is also applicable to the balanced working condition.

[0095] S102, a two-degree-of-freedom controller is established based on a quasi-proportional-integral-resonant (PIR) controller and a decoupled extended state observer (DESO), wherein the two-degree-of-freedom controller is used to compensate for the MMC disturbance, wherein the purpose of the DESO is to observe the working state of the MMC in real time, and to feed back the MMC disturbance to the input end of the two-degree-of-freedom controller when the parameters and operating state of the MMC converter station change.

[0096] The MMC bridge arm current contains DC components, AC components, and circulating current components. If traditional PI control is used in an unbalanced power grid, sequence control is required, which will make the MMC control system particularly complex. At the same time, since the coordinate change requires the detection of each sequence phase angle, the error and delay caused by the detection will also cause disturbances, reducing the tracking accuracy of the control system. The PIR controller can control the AC and DC components without sequence control, simplifying the MMC control structure.

[0097] The tracking performance and disturbance rejection performance of the linear active disturbance rejection controller of the decoupling extended state observer (DESO) are completely decoupled, which can improve the disturbance rejection performance of the system without affecting the tracking performance.

[0098] Aiming at the disadvantage of poor anti-interference ability of PIR bridge arm current control strategy, this section proposes a PIR+DESO two-degree-of-freedom controller, which adds DESO to observe and compensate for disturbances and is used for MMC bridge arm current control under unbalanced power grid.

[0099] When considering disturbances, the dynamic characteristics of the bridge arm current of the MMC are expressed as:

[0100]

[0101] In the above formula, i k1 is the upper bridge arm current of phase k, i k2 is the k-phase lower bridge arm current, L r is the bridge arm inductance, R r is the equivalent resistance of the bridge arm, u k1 is the upper bridge arm voltage of phase k, u k2 is the voltage of the lower bridge arm of phase k, U dc is the DC side voltage, u sk is the k-phase voltage, d k1 is the unmodeled dynamics of the upper arm in the three-phase stationary coordinate system, d k2 is the unmodeled dynamics of the lower bridge arm in the three-phase stationary coordinate system, where k = a, b, c.

[0102] Considering the voltage drop on the equivalent resistance of the bridge arm as a disturbance, the dynamic characteristics of the bridge arm current of the MMC can be expressed as:

[0103]

[0104] in:

[0105] x=[i k1 i k2 ] T

[0106] B=diag(=1 / L r, -1 / L r , -1 / L r , -1 / L r , -1 / L r , -1 / L r )

[0107] u=[u k1 u k2 ] T

[0108]

[0109] In the above formula, i k1 is the upper bridge arm current of phase k, i k2 is the k-phase lower bridge arm current, L r is the bridge arm inductance, R r is the equivalent resistance of the bridge arm, u k1 is the upper bridge arm voltage of phase k, u k2 is the voltage of the lower bridge arm of phase k, U dc is the DC side voltage, u sk is the k-phase voltage, d k1 is the unmodeled dynamics of the upper arm in the three-phase stationary coordinate system, d k2 is the unmodeled dynamics of the lower bridge arm in the three-phase stationary coordinate system, f k1 is the total observed disturbance corresponding to the upper arm, f k2 is the observed total disturbance corresponding to the lower bridge arm, where k = a, b, c.

[0110] Taking the upper bridge arm of phase A as an example, the bridge arm current control based on the PIR+DESO controller is designed. The transfer function of the PIR controller is:

[0111]

[0112] In the above formula, s is the Laplace operator, k i is the integral controller gain, k p is the proportional controller gain, k r is the gain of the resonant controller, ω 1 is the first resonant frequency, ω 1 =100π, the first resonant controller is used for tracking control of the base frequency AC current, ω 2 is the second resonant frequency, ω 2 =200π, the second resonant controller is used to suppress the double frequency harmonic circulating current, ω c is the resonance bandwidth.

[0113] Next, we take the upper bridge arm of phase a as an example to analyze and deduce the disturbance f a1 Expanding it into state variables, we get the expanded state form of the bridge arm current:

[0114]

[0115] Where h a1 f a1 The differential of is an unknown bounded quantity. According to the above formula, the extended state observer is designed as:

[0116]

[0117] Where e is the error, is the rated value of the bridge arm inductance. 1 , β 2 is the bandwidth of the state observer, is the estimated value of the upper arm disturbance of phase a, is the rate of change of the estimated value of the upper arm disturbance of phase a, is the rate of change of the estimated value of the upper bridge arm current of phase a obtained by the state observer, u a1 It represents the control law of the PIR controller corresponding to the upper bridge arm of phase a.

[0118] in Represents the estimated value of variable x, where x is the parameter mentioned in the formula in this application.

[0119] Based on the above state observer expression formula, in this application, it can be obtained That is to say, the estimated value of the upper bridge arm current of phase a is obtained.

[0120] The control law obtained in this application is:

[0121]

[0122] Where v a1 It is expressed as:

[0123]

[0124] In the above formula, is the reference value of the upper bridge arm current of phase a, is the estimated value of the upper bridge arm current of phase a obtained by the state observer, is the estimated value of the upper arm disturbance of phase a, is the rated value of the bridge arm inductance.

[0125] The control block diagram of the upper bridge arm current of phase A based on the PIR+DESO controller can be obtained as follows Figure 3 shown. Figure 3 is the overall control diagram of the inner loop current controller, where β1 and β2 are the bandwidths of the state observer, G k (s) is the transfer function of the PIR controller mentioned above, and the MMC part refers to Figure 2 The MMC topology in is a mathematical modeling of MMC. Figure 3 In the control block diagram shown, LESF is the system control law, which is the PIR controller and the disturbance estimate The first-order link shown in the MMC part is the MMC mathematical model obtained according to the switch average state modeling method. The DESO part is an extended state observer that decouples disturbance resistance and observability. It replaces the traditional feedback link to obtain the inner loop feedback, that is, to obtain the estimated value.

[0126] The system obtains the reference value of the inner loop current according to the outer loop controller This reference value is then compared with the estimated value obtained by the state observer Instead of making a difference in the system quantity in traditional control and entering the inner loop controller, this is done because the system parameters such as resistance and inductance may change during system operation, and due to the modeling method of power electronic topology, there are unmodeled errors. The observer can timely observe the changes of these disturbances, not only passively tracking, but also actively resisting and eliminating the impact of disturbances.

[0127] The above is based on the upper bridge arm of phase a. It is not difficult to understand that the principles of the lower bridge arm of phase a, the upper and lower bridge arms of phase b, and the upper and lower bridge arms of phase c are the same.

[0128] That is, if a comprehensive description is given, the introduction of the PIR controller designed in this application is as follows:

[0129] For the k-phase upper bridge arm, the control law of the PIR controller is:

[0130]

[0131] Where v k1 It is expressed as:

[0132]

[0133] In the above formula, is the reference value of the upper bridge arm current of phase k, is the estimated value of the upper bridge arm current of phase k obtained by the state observer, is the estimated value of the upper arm disturbance of phase k, is the rated value of the bridge arm inductance, where k = a, b, c;

[0134] For the k-phase lower bridge arm, the control law of the PIR controller is:

[0135]

[0136] Where v k2It is expressed as:

[0137]

[0138] In the above formula, is the reference value of the lower bridge arm current of phase k, is the estimated value of the lower bridge arm current of phase k obtained by the state observer, is the estimated value of the disturbance of the lower bridge arm of phase k, is the rated value of the bridge arm inductance, where k = a, b, c.

[0139] The DESO formula designed in this application can be expressed as:

[0140] For the k-phase upper bridge arm:

[0141]

[0142] In the formula, is the rated value of the bridge arm inductance, β 1 , β 2 is the bandwidth of the state observer, is the estimated value of the upper arm disturbance of phase k, is the rate of change of the estimated value of the upper arm disturbance of phase k, is the rate of change of the estimated value of the upper bridge arm current of phase k obtained by the state observer, u k1 represents the control law of the PIR controller corresponding to the upper bridge arm of phase k, where k = a, b, c;

[0143] For the k-phase lower bridge arm:

[0144]

[0145] In the formula, is the rated value of the bridge arm inductance, β 1 , β 2 is the bandwidth of the state observer, is the estimated value of the disturbance of the lower bridge arm of phase k, is the rate of change of the estimated value of the disturbance of the lower bridge arm of phase k, is the rate of change of the estimated value of the lower bridge arm current of phase k obtained by the state observer, u k2 It represents the control law of the PIR controller corresponding to the lower bridge arm of phase k, where k = a, b, c.

[0146] S103, performing disturbance compensation simulation verification and stability analysis.

[0147] Define the state observer estimation error e 1 for e 2According to the above formula, the observed state error state equation of DESO is:

[0148]

[0149] In the above formula, h is f a The differential of a is the disturbance of phase a.

[0150] From the above formula, we can get the system characteristic equation:

[0151] s 2 +β 1 s+β 2 =0

[0152] According to Lyapunov theory, if both solutions of the above equation have negative real roots in the left half plane of the s domain, then DESO is asymptotically stable. 1 >0, DESO is convergent and its estimated value can track the actual value. Then we can get:

[0153]

[0154] From the above, we have deduced that the state observer can observe and track the state of the system in real time. Then the state observer can be represented by a line in the entire control block diagram, that is, we get the control block diagram of the inner loop current. For example, the control block diagram of the upper bridge arm current of phase A is as follows Figure 4 shown.

[0155] It can be obtained that the open-loop transfer function used to judge the stability is:

[0156] G op (s) = G k (s) / s

[0157] In the above formula, s is the Laplace operator, G k (s) is the transfer function of the PIR controller.

[0158] Through the stability analysis of the open loop, it can be seen that when the control parameters of the controller change, the control system is stable.

[0159] Figure 5(a) is k p Nyquist curve for different values.

[0160] Figure 5(b) is k r Nyquist curve for different values.

[0161] Figure 5(c) is k i Nyquist curve for different values.

[0162] Figure 5(d) is ω c Nyquist curve for different values.

[0163] It can be seen from Figure 5(a), Figure 5(b), Figure 5(c), and Figure 5(d) that when different parameters take different values, the Nyquist curve does not surround the point (-1, j0), and the number of poles of the open-loop transfer function in the right half is 0, so the system is stable.

[0164] In order to verify the bridge arm current control strategy based on the PIR+DESO controller, this section builds an MMC simulation model in Matlab based on the simulation parameters in Table 1. The effectiveness of the proposed control strategy is verified in balanced and unbalanced power grids.

[0165] Table 1 Simulation parameters

[0166]

[0167] In order to verify the effectiveness of the control method proposed in this paper on the bridge arm current and circulating current, the simulation results of the PIR+DESO bridge arm current control method are compared with the traditional quasi-proportional resonant control method for inner loop current and circulating current suppression.

[0168] This section sets the changes in voltage and load conditions.

[0169] (1) Under balanced grid conditions, the three-phase voltages drop by 10% at 0.3s, the load drops suddenly at 0.7s, and the three-phase voltages recover at 1.1s.

[0170] (2) Under unbalanced grid conditions, the A-phase voltage drops by 90% at 0.3s, the load suddenly drops at 0.7s, and the A-phase voltage recovers at 1.1s.

[0171] Next, we will introduce the simulation waveforms of the inner loop current and circulating current under the balanced power grid, where the inner loop current and circulating current are controlled by QPR, and the bridge arm current is controlled by PIR+DESO. Specifically:

[0172] FIG6( a ) shows the three-phase voltage drop simulation waveform under QPR control.

[0173] FIG6( b ) shows the three-phase voltage drop simulation waveform under PIR+DESO control.

[0174] FIG6( c ) shows the load dump simulation waveform under QPR control.

[0175] FIG6( d ) shows the load dump simulation waveform under PIR+DESO control.

[0176] FIG6( e ) shows the three-phase voltage recovery simulation waveform under QPR control.

[0177] FIG6( f ) shows the three-phase voltage recovery simulation waveform under PIR+DESO control.

[0178] As shown in Figure 6(b) and Figure 6(f), when the three-phase voltage drops and recovers, the three-phase current is overstable, the circulating current suppression effect is obvious, and the steady-state error is 0.5A. Compared with the inner loop current and circulating current in Figure 6(a), the AC current overshoot is smaller, and the circulating current and AC current recovery time is faster. As shown in Figure 6(d), when a load dump occurs, the three-phase current remains symmetrical, and the circulating current value produces an overshoot. Compared with Figure 6(c), the circulating current overshoot is smaller, and the response speed of restoring the steady state is faster.

[0179] Next, the current and circulating current simulation waveforms under the bridge arm electrical control with the goal of eliminating negative sequence current are introduced under the unbalanced power grid, where the inner loop current and circulating current adopt QPR control.

[0180] FIG7( a ) shows the simulated waveform of the voltage drop of phase A under QPR control.

[0181] FIG7( b ) shows the simulated waveform of the voltage drop of phase A under PIR+DESO control.

[0182] FIG7( c ) shows the load dump simulation waveform under QPR control.

[0183] FIG7( d ) shows the load dump simulation waveform under PIR+DESO control.

[0184] FIG7( e ) shows the simulation waveform of the voltage recovery of phase A under QPR control.

[0185] FIG7( f ) shows the simulation waveform of phase A voltage recovery under PIR+DESO control.

[0186] As shown in Figure 7(b) and Figure 7(f), when the A phase voltage drops and recovers, compared with the traditional QPR control shown in Figure 7(a) and Figure 7(e), the three-phase current has a smaller overshoot and a faster response speed to restore the steady state. In Figure 7(b) and Figure 7(f), the circulating current has a small fluctuation and returns to the steady state after 0.1s. The steady-state amplitude of the circulating current is ±0.8A, which is faster than the circulating current control strategy using QPR and has a smaller steady-state amplitude. As shown in Figure 7(c) and Figure 7(d), under load mutation conditions, compared with the circulating current suppression strategy of QPR, the circulating current overshoot under the PIR+DESO bridge arm current control is smaller and the response speed to restore stability is faster.

[0187] This application establishes an MMC mathematical model based on the bridge arm current in a three-phase stationary coordinate system, and analyzes and derives the bridge arm current reference values ​​for different control targets under an unbalanced power grid. Secondly, in view of the shortcomings of PIR's poor anti-disturbance ability, a two-degree-of-freedom PIR+DESO control strategy is proposed. This strategy can improve the anti-disturbance performance of the control system without changing the tracking performance, while simplifying the MMC current control structure. The simulation results show that under balanced and unbalanced conditions, when the grid voltage suddenly changes and the load changes, the PIR+DESO control strategy has better dynamic performance than the traditional quasi-proportional resonant control strategy that separates the inner loop current and the circulating current, and the steady-state value of the circulating current is lower.

[0188] The present application also proposes a bridge arm current control device for an MMC converter station, comprising:

[0189] The analysis module is used to establish a modular multi-level converter MMC mathematical model and analyze the bridge arm current reference value based on the MMC mathematical model.

[0190] A module is established, which is used to establish a two-degree-of-freedom controller based on a quasi-proportional integral resonant PIR controller and a decoupled extended state observer DESO, wherein the two-degree-of-freedom controller is used to compensate for MMC disturbances.

[0191] Verification module, used for disturbance compensation simulation verification and stability analysis.

[0192] The purpose of the DESO is to observe the working status of the MMC in real time and to feed back the MMC disturbance to the input end of the two-degree-of-freedom controller when the parameters and operating status of the MMC converter station change.

[0193] The present application also proposes an electronic device, comprising: at least one processor; and a memory communicatively connected to the at least one processor; wherein the memory stores instructions executable by the at least one processor, and the instructions are executed by the at least one processor to implement the MMC converter station bridge arm current control method as described in the first aspect embodiment of the present application.

[0194] The present application also proposes a non-transitory computer-readable storage medium storing computer instructions, wherein the computer instructions are used to implement the MMC converter station arm current control method as described in the first aspect embodiment of the present application.

[0195] The present application also proposes a computer program product, including a computer program, which, when executed by a processor, implements the MMC converter station bridge arm current control method as described in the first aspect embodiment of the present application.

[0196] In the description of this specification, the description with reference to the terms "one embodiment", "some embodiments", "example", "specific example", or "some examples" etc. means that the specific features, structures, materials or characteristics described in conjunction with the embodiment or example are included in at least one embodiment or example of the present application. In this specification, the schematic representations of the above terms do not necessarily refer to the same embodiment or example. Moreover, the specific features, structures, materials or characteristics described may be combined in any one or more embodiments or examples in a suitable manner. In addition, those skilled in the art may combine and combine the different embodiments or examples described in this specification and the features of the different embodiments or examples, without contradiction.

[0197] Although the embodiments of the present application have been shown and described above, it can be understood that the above embodiments are exemplary and cannot be understood as limitations on the present application. Ordinary technicians in the field can change, modify, replace and modify the above embodiments within the scope of the present application.

Claims

1. A method for controlling the bridge arm current of an MMC converter station, characterized in that: include: Establishing a modular multilevel converter MMC mathematical model, and analyzing a bridge arm current reference value based on the MMC mathematical model; A two-degree-of-freedom controller is established based on a quasi-proportional integral resonant PIR controller and a decoupled extended state observer DESO, wherein the two-degree-of-freedom controller is used to compensate for MMC disturbances; Conduct disturbance compensation simulation verification and stability analysis; The purpose of the DESO is to observe the working status of the MMC in real time and to feed back the MMC disturbance to the input end of the two-degree-of-freedom controller when the parameters and operating status of the MMC converter station change.

2. The method according to claim 1, characterized in that Under the control target of suppressing the negative sequence double frequency current, the formula corresponding to the bridge arm current reference value is: In the above formula, is the reference value of the upper bridge arm current of phase k, is the reference value of the lower bridge arm current of phase k, is the reference value of the DC component of the bridge arm current, is the reference value of the AC component of the bridge arm current, where k = a, b, c.

3. The method according to claim 2, characterized in that When considering disturbances, the dynamic characteristics of the bridge arm current of the MMC are expressed as: In the above formula, i k1 is the upper bridge arm current of phase k, i k2 is the k-phase lower bridge arm current, L r is the bridge arm inductance, R r is the equivalent resistance of the bridge arm, u k1 is the upper bridge arm voltage of phase k, u k2 is the voltage of the lower bridge arm of phase k, U dc is the DC side voltage, u sk is the k-phase voltage, d k1 is the unmodeled dynamics of the upper arm in the three-phase stationary coordinate system, d k2 is the unmodeled dynamics of the lower bridge arm in the three-phase stationary coordinate system, where k = a, b, c.

4. The method according to claim 3, characterized in that Considering the voltage drop on the equivalent resistance of the bridge arm as a disturbance, the dynamic characteristics of the bridge arm current of the MMC can be expressed as: in: x=[i k1 i k2 ] T B=diag(-1 / L r ,-1 / L r ,-1 / L r ,-1 / L r ,-1 / L r ,-1 / L r ) in=[in k1 in k2 ] T In the above formula, i k1 is the upper bridge arm current of phase k, i k2 is the k-phase lower bridge arm current, L r is the bridge arm inductance, R r is the equivalent resistance of the bridge arm, u k1 is the upper bridge arm voltage of phase k, u k2 is the voltage of the lower bridge arm of phase k, U dc is the DC side voltage, u sk is the k-phase voltage, d k1 is the unmodeled dynamics of the upper arm in the three-phase stationary coordinate system, d k2 is the unmodeled dynamics of the lower bridge arm in the three-phase stationary coordinate system, f k1 is the total observed disturbance corresponding to the upper arm, f k2 is the observed total disturbance corresponding to the lower bridge arm, where k = a, b, C.

5. The method according to claim 4, characterized in that The transfer function of the PIR controller is: In the above formula, s is the Laplace operator, k i is the integral controller gain, k p is the proportional controller gain, k r is the gain of the resonant controller, ω1 is the first resonant frequency, ω1=100π, the first resonant controller is used for tracking control of the fundamental frequency AC current, ω2 is the second resonant frequency, ω2=200π, the second resonant controller is used for suppression of the double harmonic circulating current, ω c is the resonance bandwidth.

6. The method according to claim 5, characterized in that When performing stability analysis, the open-loop transfer function used to determine stability is: G op (s)=G k (s) / s In the above formula, s is the Laplace operator, G k (s) is the transfer function of the PIR controller.

7. The method according to claim 6, characterized in that The method further comprises: For the k-phase upper bridge arm, the control law of the PIR controller is: Where v k1 It is expressed as: In the above formula, is the reference value of the upper bridge arm current of phase k, is the estimated value of the upper bridge arm current of phase k obtained by the state observer, is the estimated value of the upper arm disturbance of phase k, is the rated value of the bridge arm inductance, where k = a, b, c; For the k-phase lower bridge arm, the control law of the PIR controller is: Where v k2 It is expressed as: In the above formula, is the reference value of the lower bridge arm current of phase k, is the estimated value of the lower bridge arm current of phase k obtained by the state observer, is the estimated value of the disturbance of the lower bridge arm of phase k, is the rated value of the bridge arm inductance, where k = a, b, c.

8. The method according to claim 7, characterized in that DESO can be expressed as: For the k-phase upper bridge arm: In the formula, is the rated value of the bridge arm inductance, β1 and β2 are the bandwidth of the state observer, is the estimated value of the upper arm disturbance of phase k, is the rate of change of the estimated value of the upper arm disturbance of phase k, is the rate of change of the estimated value of the upper bridge arm current of phase k obtained by the state observer, u k1 represents the control law of the PIR controller corresponding to the upper bridge arm of phase k, where k = a, b, c; For the k-phase lower bridge arm: In the formula, is the rated value of the bridge arm inductance, β1 and β2 are the bandwidth of the state observer, is the estimated value of the disturbance of the lower bridge arm of phase k, is the rate of change of the estimated value of the disturbance of the lower bridge arm of phase k, is the rate of change of the estimated value of the lower bridge arm current of phase k obtained by the state observer, u k2 It represents the control law of the PIR controller corresponding to the lower bridge arm of phase k, where k = a, b, c.

9. A bridge arm current control device for an MMC converter station, characterized in that: include: An analysis module, used for establishing a modular multi-level converter MMC mathematical model, and analyzing a bridge arm current reference value based on the MMC mathematical model; Establishing a module, used for establishing a two-degree-of-freedom controller based on a quasi-proportional integral resonant PIR controller and a decoupled extended state observer DESO, wherein the two-degree-of-freedom controller is used for compensating for MMC disturbances; Verification module, used for disturbance compensation simulation verification and stability analysis; The purpose of the DESO is to observe the working status of the MMC in real time and to feed back the MMC disturbance to the input end of the two-degree-of-freedom controller when the parameters and operating status of the MMC converter station change.

10. An electronic device, comprising: at least one processor; as well as, a memory communicatively connected to the at least one processor; wherein, The memory stores instructions that can be executed by the at least one processor, and the instructions are executed by the at least one processor to enable the at least one processor to perform the method according to any one of claims 1 to 8.

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