A method and device for suppressing grid voltage imbalance

Abstract

The present application provides a method and device for suppressing grid voltage imbalance, which obtains a grid voltage with a 90° lag through a differential tracker, calculates instantaneous power using the grid voltage with a 90° lag, and then calculates a power converter voltage based on the instantaneous power, thereby achieving suppression of grid voltage imbalance. The function of the differential tracker is to extract differential information from a noise-contaminated signal. Because the differential of a trigonometric function is proportional to its 90° lag, the grid voltage with a 90° lag can be calculated using a differential tracker. Since the use of an array is eliminated, the method is insensitive to changes in grid frequency, and its control performance is not affected when the grid frequency fluctuates, and a large amount of memory resources are saved.

Description

Technical Field

[0001] The present application relates to the field of power electronics technology, and in particular to a method and device for suppressing power grid voltage imbalance. Background Art

[0002] To ensure normal operation of a three-phase active front-end rectifier (AFER) under an unbalanced power grid, achieving adjustable bus voltage, unity power factor, and a total harmonic distortion (THD) of less than 5% (reflecting the quality of the grid current; lower THD indicates higher current quality), existing technologies use proportional-integral controllers, repetitive controllers, and resonant controllers to obtain the output voltage acting on the AFER.

[0003] Among them, the proportional-integral controller (PI) is used to track the fundamental current in the current and obtain the first component of the output voltage; the repetitive controller is used to track the sixth frequency component in the current to suppress the 6n±1 harmonics and obtain the second component of the output voltage; the resonant controller is used to track the second frequency component in the current to suppress the third harmonic and obtain the third component of the output voltage; the output voltage of the rectifier is the sum of the first component of the output voltage, the second component of the output voltage and the third component of the output voltage.

[0004] This achieves the purpose of simultaneously suppressing the 6n±1 harmonics (such as the fifth and seventh) caused by the grid voltage and dead zone effect and the third harmonic caused by the grid imbalance.

[0005] Existing technology uses a repetitive controller to track the 6-fold frequency component. This method is extremely sensitive to the frequency of the power grid. Once the power grid frequency fluctuates, the control performance will be greatly deteriorated. In addition, the repetitive control also requires the development of a pair of 400 floating-point arrays (corresponding to a 50Hz power grid and a 20kHz control frequency), which consumes a lot of memory. Summary of the Invention

[0006] The purpose of the embodiments of the present application is to provide a method and device for suppressing grid voltage imbalance, so as to solve the problem that the control method in the prior art is sensitive to grid frequency, resulting in deterioration of control performance when the grid frequency fluctuates.

[0007] An embodiment of the present application provides a method for suppressing grid voltage imbalance, which is applied to a PWM rectifier and includes:

[0008] Obtain grid voltage and grid current;

[0009] According to the grid voltage, the grid voltage with a 90° lag is calculated by the differential tracker;

[0010] The instantaneous power is calculated based on the grid voltage and grid current that lag 90°;

[0011] The power converter voltage is obtained based on the instantaneous power calculation;

[0012] According to the power converter voltage, a corresponding pulse sequence is obtained to suppress the grid voltage imbalance.

[0013] The above technical solution obtains the grid voltage with a 90° lag through a differential tracker, calculates the instantaneous power using the grid voltage with a 90° lag, and then calculates the power converter voltage based on the instantaneous power, thereby suppressing the grid voltage imbalance. The role of the differential tracker is to extract differential information from the noise-contaminated signal. Because the differential of the trigonometric function is proportional to its 90° lag, the grid voltage with a 90° lag can be calculated using a differential tracker. Since the use of the array is eliminated, it is insensitive to changes in the grid frequency, and its control performance is not affected when the grid frequency fluctuates, and a large amount of memory resources are saved.

[0014] In some optional implementations, obtaining the grid voltage and the grid current includes:

[0015] Obtain three-phase AC voltage and three-phase AC current;

[0016] Perform Clarke transformation on the three-phase AC voltage and three-phase AC current to obtain the grid voltage and grid current in the orthogonal stationary coordinate system.

[0017] In some optional implementations, the instantaneous power includes instantaneous active power and instantaneous reactive power.

[0018] In the above technical solution, an improved instantaneous power theory is adopted. The instantaneous active power and instantaneous reactive power are calculated using the grid voltage and grid current with a 90° lag. The instantaneous active power and instantaneous reactive power are tracked under an unbalanced grid, which can more accurately reflect the actual grid power situation, strengthen the control of instantaneous power, and thus have a higher ability to suppress grid imbalance.

[0019] In some optional implementations, calculating the power converter voltage based on the instantaneous power includes:

[0020] Perform Park transformation on the grid voltage in the orthogonal stationary coordinate system to obtain the d-axis grid voltage and the q-axis grid voltage in the orthogonal rotating coordinate system;

[0021] The instantaneous active power is subtracted from the active power reference, and the offset obtained by the proportional-integral controller and the resonant controller is added to the d-axis grid voltage to obtain the d-axis power converter voltage.

[0022] The instantaneous reactive power is subtracted from the reactive power reference, and the offset obtained by the proportional-integral controller and the resonant controller is added to the q-axis grid voltage to obtain the q-axis power converter voltage.

[0023] An inverse Park transform is performed on the d-axis power converter voltage and the q-axis power converter voltage to obtain an α-axis power converter voltage and a β-axis power converter voltage.

[0024] In the above technical solution, the difference between the instantaneous reactive power and the reactive power reference is taken and then enters the proportional-integral controller and the resonant controller. The proportional-integral controller is used to eliminate the static error of the control, and the resonant controller is introduced to reconstruct the distorted grid component to eliminate the current harmonics caused by the grid voltage harmonics when the grid is unbalanced.

[0025] In some optional implementations, obtaining a corresponding pulse sequence based on the power converter voltage to suppress grid voltage imbalance includes:

[0026] According to the α-axis power converter voltage and the β-axis power converter voltage, a corresponding pulse sequence is obtained by using a modulation strategy;

[0027] The pulse train is input into the power converter to suppress the grid voltage imbalance.

[0028] In some optional implementations, the modulation strategy adopts pulse width modulation, space vector pulse width modulation or sinusoidal pulse width modulation.

[0029] An embodiment of the present application provides a device for suppressing grid voltage imbalance, comprising:

[0030] An acquisition module, used for acquiring grid voltage and grid current;

[0031] A differential tracker is used to calculate the grid voltage with a 90° lag based on the grid voltage through the differential tracker;

[0032] The instantaneous power calculation module is used to calculate the instantaneous power based on the grid voltage and grid current that lag 90 degrees;

[0033] A power converter voltage calculation module, configured to calculate the power converter voltage based on the instantaneous power;

[0034] The pulse sequence acquisition module is used to obtain the corresponding pulse sequence according to the power converter voltage to suppress the grid voltage imbalance.

[0035] The grid voltage imbalance suppression device of the above technical solution includes an acquisition module, a differential tracker, an instantaneous power calculation module, a power converter voltage calculation module and a pulse sequence acquisition module. The function of the differential tracker is to extract differential information from the noise-contaminated signal. Because the differential of the trigonometric function is proportional to its 90° lag, the grid voltage with a 90° lag can be calculated using the differential tracker. There is no need to open a pair of 100 floating-point arrays in the microcontroller. Therefore, it is insensitive to changes in grid frequency, and its control performance is not affected when the grid frequency fluctuates, and a large amount of memory resources are saved.

[0036] In some optional implementations, the acquisition module is further configured to acquire a three-phase AC voltage and a three-phase AC current; perform a Park transform on the three-phase AC voltage and the three-phase AC current to obtain a grid voltage and a grid current in an orthogonal rotating coordinate system;

[0037] The instantaneous power calculation module is also used to calculate the instantaneous active power and instantaneous reactive power based on the grid voltage and grid current that lag 90 degrees.

[0038] The instantaneous power calculation module of the above technical solution uses the grid voltage and grid current with a 90° lag to calculate the instantaneous active power and instantaneous reactive power. It tracks the instantaneous active power and instantaneous reactive power under an unbalanced grid, can more accurately reflect the actual grid power situation, strengthen the control of instantaneous power, and thus have a higher ability to suppress grid imbalance.

[0039] In some optional implementations, the power converter voltage calculation module is further configured to:

[0040] A Park transform is performed on the grid voltage in the orthogonal stationary coordinate system to obtain the d-axis grid voltage and the q-axis grid voltage in the orthogonal rotating coordinate system; the instantaneous active power is subtracted from the active power reference, and the bias obtained by entering the proportional-integral controller and the resonant controller is added to the d-axis grid voltage to obtain the d-axis power converter voltage; the instantaneous reactive power is subtracted from the reactive power reference, and the bias obtained by entering the proportional-integral controller and the resonant controller is added to the q-axis grid voltage to obtain the q-axis power converter voltage; the d-axis power converter voltage and the q-axis power converter voltage are subjected to an inverse Park transform to obtain the α-axis power converter voltage and the β-axis power converter voltage.

[0041] The power converter voltage calculation module in the above technical solution takes the difference between the instantaneous reactive power and the reactive power reference and enters the proportional-integral controller and the resonant controller. The proportional-integral controller is used to eliminate the static error of the control, and the resonant controller is introduced to reconstruct the distorted grid component to eliminate the current harmonics caused by the grid voltage harmonics when the grid is unbalanced.

[0042] In some optional implementations, the pulse sequence acquisition module is further configured to:

[0043] According to the α-axis power converter voltage and the β-axis power converter voltage, a corresponding pulse sequence is obtained by using a modulation strategy; the pulse sequence is input into the power converter to suppress the grid voltage imbalance.

[0044] An embodiment of the present application provides a computer-readable storage medium, which stores a computer program. When the computer program is executed by a processor, any of the above methods is executed.

[0045] The above description is only an overview of the technical solution of the present application. In order to more clearly understand the technical means of the present application, it can be implemented in accordance with the contents of the specification. In order to make the above and other purposes, features and advantages of the present application more obvious and easy to understand, the specific implementation methods of the present application are listed below. BRIEF DESCRIPTION OF THE DRAWINGS

[0046] In order to more clearly illustrate the technical solutions of the embodiments of the present application, the following briefly introduces the drawings required for use in the embodiments of the present application. It should be understood that the following drawings only illustrate certain embodiments of the present application and should not be considered as limiting the scope. A person of ordinary skill in the art can also derive other relevant drawings based on these drawings without inventive effort. In the drawings:

[0047] Figure 1 A flowchart of a method for suppressing grid voltage imbalance provided in an embodiment of the present application;

[0048] Figure 2 A flowchart of the specific steps for obtaining grid voltage and grid current provided in an embodiment of the present application;

[0049] Figure 3 A flowchart of the steps for suppressing grid voltage imbalance by using power converter voltage according to an embodiment of the present application;

[0050] Figure 4 A functional module diagram of a PWM rectifier provided in an embodiment of the present application;

[0051] Figure 5 A flowchart of a method for suppressing grid voltage imbalance provided in an embodiment of the present application;

[0052] Figure 6 This is a functional module diagram of a grid voltage imbalance suppression device provided in an embodiment of the present application.

[0053] Icons: 1-acquisition module, 2-differential tracker, 3-instantaneous power calculation module, 4-power converter voltage calculation module, 5-pulse sequence acquisition module. DETAILED DESCRIPTION

[0054] The following embodiments of the technical solution of the present application will be described in detail with reference to the accompanying drawings. The following embodiments are only used to more clearly illustrate the technical solution of the present application and are therefore only examples and are not intended to limit the scope of protection of the present application.

[0055] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by those skilled in the art to which this application belongs; the terms used herein are only for the purpose of describing specific embodiments and are not intended to limit this application; the terms "including" and "having" and any variations thereof in the specification and claims of this application and the above-mentioned figure descriptions are intended to cover non-exclusive inclusions.

[0056] References herein to "embodiments" mean that a particular feature, structure, or characteristic described in connection with the embodiments may be included in at least one embodiment of the present application. The appearance of this phrase in various places in the specification does not necessarily refer to the same embodiment, nor does it constitute an independent or alternative embodiment that is mutually exclusive of other embodiments. It is understood, both explicitly and implicitly, by those skilled in the art that the embodiments described herein may be combined with other embodiments.

[0057] AC charging piles often operate in areas with unbalanced power grids, but industry standards require them to operate normally within the range of 50Hz+-5Hz.

[0058] The applicant noted that the existing technology is sensitive to grid frequency fluctuations, which can easily cause charging piles to fail and even affect the stable operation of the grid in the area.

[0059] In order to avoid the failure of charging piles due to sensitivity to grid frequency fluctuations, the applicant discovered that the instantaneous active power and instantaneous reactive power can be calculated in real time using the instantaneous power theory, and the conventional current inner loop can be improved to a power inner loop to directly track the instantaneous active power and instantaneous reactive power. However, under an unbalanced power grid, the conventional instantaneous power theory cannot accurately calculate the reactive power, and the control of the instantaneous power is also greatly weakened. Therefore, the applicant adopted an improved instantaneous power theory, which can achieve good tracking of reactive power under an unbalanced power grid. However, since the improved instantaneous power calculation requires a voltage value that lags 90°, it is still necessary to open up a pair of 100 floating-point arrays, which is still a considerable challenge for the microcontroller.

[0060] Based on the above considerations, the applicant introduced a differential tracker in the active front-end rectifier control algorithm to obtain the 90° lagging voltage by utilizing the characteristic that the differential of the trigonometric function is proportional to its 90° lag, thereby avoiding the need to open a pair of 100 floating-point arrays for calculation, saving a large amount of memory consumption. In addition, this calculation method becomes less sensitive to changes in grid frequency, ensuring the normal operation of the active front-end rectifier under conditions of grid imbalance and grid frequency changes.

[0061] For example, assuming the grid frequency changes from the original power frequency of 50Hz to 45Hz, the differential value calculated using a differential tracker will also follow the frequency change, thereby accurately calculating the active and reactive power. If the array method is used, 50Hz corresponds to a 400-point array (at a control frequency of 50us), 55Hz corresponds to 363 (363.63) array elements, and 45Hz corresponds to 444 (444.44) array elements. Without the use of a dynamic array, it is difficult to calculate relatively accurate power when the grid frequency changes. Furthermore, due to the high requirements for embedded security, the use of dynamic arrays carries the risk of memory overflow, so dynamic arrays cannot be used.

[0062] It should be clear that the following embodiments of the present application can be applied in situations where industry standards require the grid frequency to be within the range of 50Hz+-5Hz, but are not limited to this situation and can also be applied in other possible grid frequency fluctuation situations.

[0063] Please refer to Figure 1 , Figure 1 A flowchart of a method for suppressing grid voltage imbalance provided in an embodiment of the present application, which is applied to a PWM rectifier, includes:

[0064] Step 100: Obtain grid voltage and grid current;

[0065] The grid voltage and grid current in step 100 are the grid voltage and grid current in the orthogonal stationary coordinate system. The grid voltage and grid current in the orthogonal stationary coordinate system can be directly obtained, or the three-phase AC voltage and three-phase AC current can be obtained and then Clarke transform is performed on them to obtain the grid voltage and grid current in the orthogonal stationary coordinate system.

[0066] Step 200: Calculate the grid voltage with a 90° lag by using a differential tracker according to the grid voltage;

[0067] The function of the differential tracker in step 200 is to extract differential information from the noise-contaminated signal, and to track the grid voltage based on the proportional relationship between the trigonometric function differential and its 90° lag, thereby obtaining the grid voltage with a 90° lag.

[0068] Step 300: Calculate the instantaneous power based on the grid voltage and grid current that are lagged by 90°.

[0069] When calculating instantaneous power in step 300, it can be further distinguished into instantaneous active power and instantaneous reactive power, or it can be not distinguished. There is no limitation here. As long as the grid voltage and grid current with a 90° lag can calculate the instantaneous power, the bus voltage is directly related to the power on the bus. Therefore, the power inner loop can obtain a faster response speed than the current inner loop.

[0070] Step 400: Calculate the power converter voltage based on the instantaneous power;

[0071] Step 500: Obtain a corresponding pulse sequence according to the power converter voltage to suppress grid voltage imbalance.

[0072] Among them, PWM (Pulse Width Modulation) rectifiers are indispensable power electronic devices in industrial applications and are widely used in the fields of electricity, electronics, communications, transportation, medical care, etc.; the differential tracker adopts Active Disturbance Rejection Control Technique (ADRC); the power converter voltage is the voltage after the power converter converts the grid voltage; the instantaneous power is equal to the product of the instantaneous values ​​of instantaneous voltage and current; the instantaneous active power is related to the energy of the load, and the instantaneous reactive power controls the unity power factor.

[0073] The above technical solution uses a differential tracker to obtain the grid voltage with a 90° lag, uses this 90° lag to calculate the instantaneous power, and then calculates the power converter voltage based on the instantaneous power, thereby suppressing grid voltage imbalance. Because the differential tracker replaces a pair of 100 floating-point data arrays containing 90° lag information on the three-phase AC grid voltage, eliminating the need for arrays, the system is insensitive to grid frequency changes, maintaining control performance even when grid frequency fluctuates. It also saves a significant amount of memory resources.

[0074] In some optional implementations, please refer to Figure 2 , Figure 2 The specific steps for obtaining the grid voltage and grid current are shown in the flowchart, that is, step 100 further includes:

[0075] Step 101: Obtain three-phase AC voltage and three-phase AC current;

[0076] Step 102: Perform Clarke transformation on the three-phase AC voltage and the three-phase AC current to obtain the grid voltage and the grid current in an orthogonal stationary coordinate system.

[0077] In the embodiment of the present application, the three-phase AC voltage and the three-phase AC current are directly collected from the busbar. Therefore, the method of this embodiment can directly process the collected three-phase AC voltage and three-phase AC current.

[0078] In some optional implementations, the instantaneous power is further divided into instantaneous active power and instantaneous reactive power; then, step 300 includes: calculating the instantaneous active power and instantaneous reactive power based on the grid voltage and grid current that lag 90°.

[0079] The method for suppressing grid voltage imbalance in the embodiment of the present application adopts an improved instantaneous power theory, and uses the grid voltage and grid current with a 90° lag to calculate the instantaneous active power and instantaneous reactive power. The instantaneous active power and instantaneous reactive power are tracked under an unbalanced grid, which can more accurately reflect the actual grid power situation, strengthen the control of instantaneous power, and thus have a higher ability to suppress grid imbalance.

[0080] In some optional implementations, in step 400, the calculation process of obtaining the power converter voltage based on the instantaneous power calculation specifically includes:

[0081] A Park transform is performed on the grid voltage in the orthogonal stationary coordinate system to obtain the d-axis and q-axis grid voltages in the orthogonal rotating coordinate system. The instantaneous active power is subtracted from the active power reference, and the resulting bias is fed into the proportional-integral controller and the resonant controller. This bias is then added to the d-axis grid voltage to obtain the d-axis power converter voltage. The instantaneous reactive power is subtracted from the reactive power reference, and the resulting bias is then fed into the proportional-integral controller and the resonant controller to obtain the q-axis grid voltage. The d-axis and q-axis power converter voltages are then inversely Park transformed to obtain the α-axis and β-axis power converter voltages. The reactive power reference is the set value of reactive power, and the active power reference is the set value of active power. In a proportional-integral controller, the integral action makes the controller output proportional to the integral of the deviation, resulting in no residual error at the end of the transition process. However, adding the integral action reduces stability. Increasing the proportionality with the integral action maintains stability, but increases overshoot and oscillation period, and the transition time. This controller is the most widely used and is suitable for systems with small regulation channel lag, small load variations, and process requirements that require no residual error. A resonant controller is a circuit composed of inductors and capacitors that can resonate at one or more frequencies, collectively referred to as a resonant circuit. When the reactive power reference is 0, its interpolated value is the opposite of the instantaneous reactive power, that is, the negative instantaneous reactive power. Grid-connected inverters generally operate in unity power factor mode, in which the reactive power is zero, the active power transferred to the grid is maximized, and the losses are minimized for the same active power. Depending on the situation, the reactive power may not be zero. For example, if the grid-connected inverter is required to provide low voltage ride-through function, a certain amount of reactive power needs to be injected into the grid to maintain grid voltage stability.

[0082] In an embodiment of the present application, the instantaneous reactive power is subtracted from the given reactive power and then fed into a proportional-integral controller and a resonant controller. The proportional-integral controller is used to eliminate the static error of the control, and the resonant controller is then introduced to reconstruct the distorted grid components to eliminate the current harmonics caused by the grid voltage harmonics when the grid is unbalanced. Moreover, the improved instantaneous power under unbalanced conditions can better reflect the actual grid conditions. Therefore, the reconstructed distorted grid components reconstructed using the improved instantaneous power are more accurate than those reconstructed using the d-axis current and the q-axis current. For example, in an unbalanced three-phase grid with a grid frequency of 46 Hz, because the arrays of 400 floating-point elements each for the α-axis current and the β-axis current are predetermined, the solution of using the array to obtain a voltage that lags 90° will not be able to adapt to changes in the grid frequency, and its performance will be greatly deteriorated. For example: Assume that the grid frequency changes from the original power frequency of 50Hz to 45Hz. If the array method is used, 50Hz corresponds to a 400-point array (under a control frequency of 50us), 55Hz corresponds to 363 (363.63) array elements, and 45Hz corresponds to 444 (444.44) array elements. That is, if a 400-point array is used, a complete cycle of data (greater than 400) cannot be stored at 45Hz. The access algorithm at 55Hz is quite complicated, and the existence of decimals leads to certain errors.

[0083] This solution uses a differential tracker to obtain a grid voltage that lags 90° to calculate the improved instantaneous power theory. In this process, the calculation of the differential controller is independent of the frequency of the grid and does not require a large number of arrays. The calculation of instantaneous power requires the grid voltage that lags 90°. The conventional method is to obtain it by an array, which is also affected by the grid frequency. This solution uses a differential controller to obtain it. Therefore, this solution can still operate stably under a frequency fluctuating grid and effectively suppress grid voltage harmonics.

[0084] In some optional implementations, please refer to Figure 3 , Figure 3 The flowchart of the steps for suppressing grid voltage imbalance by using power converter voltage, namely step 500 specifically includes:

[0085] Step 501: Obtain a corresponding pulse sequence using a modulation strategy according to the α-axis power converter voltage and the β-axis power converter voltage;

[0086] Step 502: Input the pulse sequence into the power converter to suppress the grid voltage imbalance.

[0087] In the embodiment of the present application, a modulation strategy is used to obtain a corresponding pulse sequence for the obtained power converter voltage, so that the power converter can suppress the grid voltage imbalance according to the pulse sequence.

[0088] In some optional implementations, the modulation strategy adopted in step 501 includes pulse width modulation (PWM), sinusoidal pulse width modulation (SPWM), space vector pulse width modulation (SVPWM), etc.

[0089] According to some embodiments of the present application, referring to Figure 4 , Figure 4 This is the functional module diagram of the PWM rectifier. Figure 4 In the figure, Ea, Eb, and Ec are respectively the sampling of the three-phase grid voltages a, b, and c, and the sampling point is at the end of the inductor close to the grid. Ia, Ib, and Ic are respectively the sampling of the three-phase currents a, b, and c, and the sampling is the current flowing through the three-phase inductor. Vdc is the bus voltage sampling, and the sampling is the voltage on the output capacitor. Q1~Q6 are switching tubes, and the pulse sequence obtained by the algorithm acts on the switching tubes to control their on and off. The Phase Lock Loop (PLL) calculates the frequency and phase of the grid, and the output w is the grid angular frequency. The coordinate transformation is the Park transformation and the Clark transformation, which converts the three-phase stationary coordinate system into a two-phase stationary coordinate system and a two-phase rotating coordinate system. The power calculation uses the grid voltage and real-time grid current that lag 90°. In this application, the grid voltage that lags 90° is obtained by the differential tracker. The results calculated by this module are instantaneous active power and instantaneous reactive power.

[0090] In traditional vector control, the outer loop is generally the output voltage loop, which controls the output voltage stability, and the inner loop is the current loop or power loop, which controls the inductor current or the overall power stability. The embodiment of the present application is based on the improvement of the power loop. The instantaneous active power is related to the energy of the load, and the instantaneous reactive power controls the unit power factor. The coordinate inverse transformation is the inverse Park transformation and the inverse Clarke transformation. The PWM module can be SVPWM or SPWM or other PWM modulation methods. PWM1~PWM6 are the outputs of the PWM module, which control the on and off of the Q1~Q6 switches respectively.

[0091] Please refer to Figure 5 , Figure 5 The following is a working flow chart of a method for suppressing grid voltage imbalance, wherein the specific steps include:

[0092] Collect bus voltage Vdc, three-phase AC voltages Ea, Eb, Ec and three-phase AC currents Ia, Ib, Ic.

[0093] Clarke transformation is performed on the three-phase AC voltages Ea, Eb, and Ec, and the three-phase AC currents Ia, Ib, and Ic to obtain the grid voltages Eα and Eβ (i.e., the α-axis grid voltage Eα and the β-axis grid voltage Eβ) and the grid currents Iα and Iβ (i.e., the α-axis grid current Iα and the β-axis grid current Iβ) in the orthogonal stationary coordinate system, as shown in the following equations:

[0094]

[0095]

[0096] The grid voltages Eα and Eβ in the orthogonal stationary coordinate system are transformed by Park transformation to obtain the grid voltages Ed and Eq in the orthogonal rotating coordinate system, namely the d-axis grid voltage Ed and the q-axis grid voltage Eq, as shown in the following formula:

[0097]

[0098] Applying the differential tracker to the grid voltages Eα and Eβ in the orthogonal stationary coordinate system yields the grid voltages Eα' and Eβ' that lag 90° in the orthogonal stationary coordinate system. The differential tracker calculation is shown in the following equation:

[0099] xα1(k)=xα1(k-1)+h·xα2(k-1)

[0100] xα2(k)=xα2(k-1)-h·r^2(xα1(k-1)-Eα(k-1)+2 / r·xα2(k-1))

[0101] xβ1(k)=xβ1(k-1)+h·xβ2(k-1)

[0102] xβ2(k)=xβ2(k-1)-h·r^2(xβ1(k-1)-Eβ(k-1)+2 / r·xβ2(k-1))

[0103] Eα lags behind the voltage by 90°, Eα'=(Xα2(k)) / w

[0104] Eβ lags behind the voltage by 90°, Eβ'=(Xβ2(k)) / w

[0105] Among them, r is the speed tracking factor, the larger the value of r, the faster the tracking speed, h is the filter factor, the larger the value of h, the smoother the waveform, xα1(k) is the tracking of Eα at the current time k, xα2(k) is the tracking of the differential of Eα, xβ1(k) is the tracking of Eβ, xβ2(k) is the tracking of the differential of Eβ, xα1(k-1) is the tracking result of Eα at the k-1 moment, xα2(k-1) is the tracking result of the differential of Eα at the k-1 moment, xβ1(k-1) is the tracking result of Eα at the k-1 moment, xβ2(k-1) is the tracking result of the differential of Eα at the k-1 moment; w is the grid angular frequency, which is obtained by the phase lock loop (PLL).

[0106] The improved instantaneous power P and Q are calculated using the grid voltage with a 90° lag in the orthogonal stationary coordinate system and the actual grid current in the orthogonal stationary coordinate system, as shown in the following formula:

[0107] Instantaneous active power P = Eα'·Iβ-Eβ'·Iα

[0108] Instantaneous reactive power Q = Eα'·Iα+Eβ'·Iβ

[0109] The controller obtains the active power set value P* according to the bus voltage set value Vdc* and the sampling value Vdc through a proportional-integral controller.

[0110] The difference between the given active power P* and the instantaneous active power P enters the proportional integral controller ( Figure 4 PI in) and resonant controller ( Figure 4 The d-axis power converter voltage Vd is obtained by subtracting the output of the proportional-integral controller from the output of the resonant controller and adding the grid voltage Ed in the orthogonal rotating coordinate system.

[0111] The difference between the reactive power given by 0 and the instantaneous reactive power Q enters the proportional-integral controller and the resonant controller. The offset obtained by subtracting the output of the proportional-integral controller from the output of the resonant controller is added to the grid voltage Eq in the orthogonal rotating coordinate system to obtain the q-axis power converter voltage Vq.

[0112] The transfer function of the proportional-integral controller is:

[0113]

[0114] The transfer function of the resonant controller is:

[0115]

[0116] Where Kp, Ki, Kr, ωc and s are proportional gain, integral gain, resonant gain, cutoff frequency and differential operator respectively.

[0117] The input of the outer loop proportional-integral controller is the difference between the given voltage and the actual voltage, Vdc*-Vdc, and the output is the active power given, P*. The input of the active power inner loop proportional-integral controller and the resonant controller is the difference between the given active power and the calculated active power, P*-P, and the output is Vd'. The input of the reactive power inner loop proportional-integral controller and the resonant controller is the difference between the given reactive power and the calculated reactive power, Q*-Q, and the output is Vq'. Then, the converter voltages Vd and Vq are shown as follows:

[0118] Vd=Ed-Vd'

[0119] Vq=Eq-Vq'

[0120] The d-axis and q-axis power converter voltages Vd and Vq are converted into the α-axis and β-axis power converter voltages Vα and Vβ through inverse Park transformation, as shown in the following equation:

[0121]

[0122] Finally, the α-axis and β-axis power converter voltages Vα and Vβ are converted into corresponding pulse sequences through SVPWM.

[0123] In the above embodiment, a proportional-integral controller is used to eliminate the static error of the control; and a resonant controller is introduced to reconstruct the distorted grid components to eliminate the current harmonics caused by the grid voltage harmonics under unbalanced grid conditions; the improved instantaneous power under unbalanced conditions can better reflect the actual grid conditions, and the reconstructed distorted grid components reconstructed using the improved instantaneous power are more accurate than the reconstructed distorted grid components reconstructed using the d-axis current and the q-axis current; in order to eliminate the need to use a pair of arrays of 100 floating-point elements in calculating the improved instantaneous power, a differential tracker is introduced to obtain a grid voltage with a 90° lag.

[0124] In summary, the advantages of one or more embodiments of the present application are: 1. It is insensitive to changes in grid frequency and can operate normally under a 50Hz+-5Hz grid; 2. It has low memory consumption and does not require a pair of arrays of 400 floating-point elements to record the repeated controller output of one power frequency cycle; 3. It has a better suppression effect on grid imbalance.

[0125] Please refer to Figure 6 , Figure 6This is a functional block diagram of a grid voltage imbalance suppression device provided in an embodiment of the present application, comprising an acquisition module 1, a differential tracker 2, an instantaneous power calculation module 3, a power converter voltage calculation module 4, and a pulse sequence acquisition module 5. The acquisition module 1 is used to acquire the grid voltage and grid current; the differential tracker 2 is used to calculate a grid voltage with a 90° lag based on the grid voltage through a differential tracker; the instantaneous power calculation module 3 is used to calculate the instantaneous power based on the grid voltage and grid current with a 90° lag; the power converter voltage calculation module 4 is used to calculate the power converter voltage based on the instantaneous power; and the pulse sequence acquisition module 5 is used to obtain a corresponding pulse sequence based on the power converter voltage to suppress grid voltage imbalance.

[0126] In the grid voltage imbalance suppression device of the embodiment of the present application, the function of the differential tracker 2 is to extract differential information from the noise-contaminated signal. Since the differential of the trigonometric function is proportional to its 90° lag, the grid voltage with a 90° lag can be calculated using the differential tracker. There is no need to open a pair of 100 floating-point arrays in the microcontroller. The device is insensitive to grid frequency changes, and its control performance is not affected when the grid frequency fluctuates, and a large amount of memory resources is saved.

[0127] In some optional embodiments, the acquisition module 1 is also used to obtain the three-phase AC voltage and the three-phase AC current; perform Park transformation on the three-phase AC voltage and the three-phase AC current to obtain the grid voltage and the grid current in the orthogonal rotating coordinate system; the instantaneous power calculation module 3 is also used to calculate the instantaneous active power and the instantaneous reactive power based on the grid voltage and the grid current that are lagged by 90°.

[0128] The instantaneous power calculation module 3 of the embodiment of the present application uses the grid voltage and grid current that are lagged by 90° to calculate the instantaneous active power and instantaneous reactive power, and tracks the instantaneous active power and instantaneous reactive power under an unbalanced grid, which can more accurately reflect the actual grid power situation, strengthen the control of instantaneous power, and thus have a higher ability to suppress grid imbalance.

[0129] In some optional embodiments, the power converter voltage calculation module 4 is also used to: perform Park transform on the grid voltage in the orthogonal stationary coordinate system to obtain the d-axis grid voltage and the q-axis grid voltage in the orthogonal rotating coordinate system; add the bias obtained by subtracting the instantaneous active power from the active power setting and entering the proportional-integral controller and the resonant controller to obtain the d-axis power converter voltage; add the bias obtained by subtracting the instantaneous reactive power from the reactive power setting and entering the proportional-integral controller and the resonant controller to obtain the q-axis power converter voltage; and perform inverse Park transform on the d-axis power converter voltage and the q-axis power converter voltage to obtain the α-axis power converter voltage and the β-axis power converter voltage.

[0130] In some optional embodiments, the pulse sequence acquisition module 5 is further used to: obtain a corresponding pulse sequence using a modulation strategy based on the α-axis power converter voltage and the β-axis power converter voltage; and input the pulse sequence into the power converter to suppress grid voltage imbalance.

[0131] The present application also provides a computer-readable storage medium having computer program instructions stored thereon. When the computer program instructions are read and executed by a computer processor, the method provided by the present application is executed. For example, the computer-readable storage medium can implement the following: obtaining a grid voltage and a grid current; calculating a grid voltage that lags 90° based on the grid voltage using a differential tracker; calculating instantaneous power based on the grid voltage and grid current that lags 90°; calculating a power converter voltage based on the instantaneous power; and obtaining a corresponding pulse sequence based on the power converter voltage to suppress grid voltage imbalance.

[0132] In the embodiments provided in this application, it should be understood that the disclosed devices and methods can be implemented in other ways. The device embodiments described above are merely schematic. For example, the division of the units is only a logical function division. There may be other division methods in actual implementation. For example, multiple units or components can be combined or integrated into another system, or some features can be ignored or not executed. Another point is that the mutual coupling or direct coupling or communication connection shown or discussed can be through some communication interface, indirect coupling or communication connection of devices or units, which can be electrical, mechanical or other forms.

[0133] In addition, the units described as separate components may or may not be physically separate, and the components shown as units may or may not be physical units, that is, they may be located in one place or distributed across multiple network units. Some or all of the units may be selected according to actual needs to achieve the purpose of the solution of this embodiment.

[0134] Furthermore, the functional modules in each embodiment of the present application can be integrated together to form an independent part, or each module can exist independently, or two or more modules can be integrated to form an independent part.

[0135] In this document, relational terms such as first and second, etc. are used merely to distinguish one entity or operation from another entity or operation, but do not necessarily require or imply any actual relationship or order between these entities or operations.

[0136] The above description is merely an embodiment of the present application and is not intended to limit the scope of protection of the present application. For those skilled in the art, various modifications and variations of the present application are possible. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the present application shall be included in the scope of protection of the present application.

Claims

1. A method for suppressing grid voltage imbalance, characterized in that: Applied to PWM rectifiers, including: Obtain grid voltage and grid current; According to the grid voltage, a grid voltage that lags behind by 90° is calculated by a differential tracker; Calculating instantaneous power based on the grid voltage and the grid current that are lagged by 90°; Calculating a power converter voltage based on the instantaneous power; and According to the power converter voltage, a corresponding pulse sequence is obtained to suppress the grid voltage imbalance; The obtaining of the grid voltage and grid current includes: Obtain three-phase AC voltage and three-phase AC current; Performing Clarke transformation on the three-phase AC voltage and the three-phase AC current to obtain the grid voltage and the grid current in an orthogonal stationary coordinate system; Wherein, the instantaneous power includes instantaneous active power and instantaneous reactive power; The step of calculating the power converter voltage according to the instantaneous power comprises: Performing a Park transform on the grid voltage in the orthogonal stationary coordinate system to obtain a d-axis grid voltage and a q-axis grid voltage in the orthogonal rotating coordinate system; The instantaneous active power is subtracted from the set active power and fed into the proportional-integral controller and the resonant controller to obtain an offset value, which is then added to the d-axis grid voltage to obtain the d-axis power converter voltage; The instantaneous reactive power is subtracted from the reactive power reference, and the offset obtained by the proportional-integral controller and the resonant controller is added to the q-axis grid voltage to obtain the q-axis power converter voltage; An inverse Park transform is performed on the d-axis power converter voltage and the q-axis power converter voltage to obtain an α-axis power converter voltage and a β-axis power converter voltage.

2. The method according to claim 1, wherein Obtaining a corresponding pulse sequence according to the power converter voltage to suppress grid voltage imbalance includes: Obtaining a corresponding pulse sequence using a modulation strategy according to the α-axis power converter voltage and the β-axis power converter voltage; The pulse sequence is input into a power converter to suppress grid voltage imbalance.

3. The method according to claim 2, wherein The modulation strategy adopts pulse width modulation, space vector pulse width modulation or sinusoidal pulse width modulation.

4. A device for suppressing voltage imbalance in a power grid, characterized in that: include: An acquisition module, used for acquiring grid voltage and grid current; A differential tracker is used to calculate a grid voltage that lags behind by 90° based on the grid voltage through the differential tracker; An instantaneous power calculation module, configured to calculate the instantaneous power based on the grid voltage and the grid current that are lagged by 90°; A power converter voltage calculation module, configured to calculate the power converter voltage based on the instantaneous power; A pulse sequence acquisition module, configured to obtain a corresponding pulse sequence according to the power converter voltage to suppress grid voltage imbalance; The acquisition module is further configured to acquire a three-phase AC voltage and a three-phase AC current; perform a Park transformation on the three-phase AC voltage and the three-phase AC current to obtain the grid voltage and the grid current in an orthogonal rotating coordinate system; The instantaneous power calculation module is further configured to calculate the instantaneous active power and the instantaneous reactive power based on the grid voltage and the grid current that are lagged by 90°; Wherein, the power converter voltage calculation module is further used for: Performing a Park transform on the grid voltage in the orthogonal stationary coordinate system to obtain a d-axis grid voltage and a q-axis grid voltage in the orthogonal rotating coordinate system; The instantaneous active power is subtracted from the set active power and fed into the proportional-integral controller and the resonant controller to obtain an offset value, which is then added to the d-axis grid voltage to obtain the d-axis power converter voltage; The instantaneous reactive power is subtracted from the reactive power reference, and the offset obtained by the proportional-integral controller and the resonant controller is added to the q-axis grid voltage to obtain the q-axis power converter voltage; An inverse Park transform is performed on the d-axis power converter voltage and the q-axis power converter voltage to obtain an α-axis power converter voltage and a β-axis power converter voltage.

5. The device according to claim 4, characterized in that The pulse sequence acquisition module is further used for: Obtaining a corresponding pulse sequence using a modulation strategy according to the α-axis power converter voltage and the β-axis power converter voltage; The pulse sequence is input into a power converter to suppress grid voltage imbalance.

6. A computer-readable storage medium, characterized in that The storage medium stores a computer program, which, when executed by a processor, executes the method according to any one of claims 1 to 3.

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