A low switching frequency modular dc active filter device and control method

Abstract

The application provides a low-switching-frequency modular DC active filter device and a control method. The device comprises an inductor, a plurality of DC active filter units, a measurement circuit and a controller. The measurement circuit measures the ripple current of the DC bus. The controller controls the working state of the power half-bridge of the DC active filter unit according to the switching signal to discharge on the inductor to generate a compensation current. The DC active filter units are connected in series and then in parallel to the two ends of the DC bus. The number of DC active filter units is the same as the number of frequency components of the ripple current to be compensated, and each DC active filter unit compensates for the ripple current of one frequency component. The DC active filter unit comprises a power half-bridge and a capacitor. The parameters of the capacitor and the switching signal are determined according to the ripple current to be compensated corresponding to the DC active filter unit. The problems of complex circuit and control and low reliability of the modular multilevel converter in the high-power scene are solved.

Description

Technical Field

[0001] This invention belongs to the field of DC active filtering technology, and more specifically, relates to a modular DC active filtering device and control method with low switching frequency. Background Technology

[0002] High-power power supplies have low switching frequencies and large ripple currents, so a large number of capacitors are often used on the DC bus. These capacitors occupy a lot of space and reduce the power density of the power supply. In addition, capacitors are the most prone to failure components in a power supply. Using a large number of capacitors will reduce the reliability of the power supply. During long-term operation, capacitors often need to be replaced due to damage, which increases the operation and maintenance costs of the power supply and reduces the continuous operating time of the power supply.

[0003] Active power filters are a new type of filtering device that uses power electronics technology and control methods to output current of arbitrary waveforms. When this current cancels out ripple or harmonic currents in the circuit, the ripple or harmonic content can be reduced. Currently, active power filters are widely used in AC power grids, playing a crucial role in reducing the harmonic current content of the grid.

[0004] However, most existing active filters can only be applied to AC power grids, with limited research on their application in DC. Current DC active filters are mainly used to reduce the output voltage ripple of diode rectifiers or PWM rectifiers. They add a bridge arm to the rectifier and connect it to the AC power grid on the input side. While this achieves a filtering effect, it still only works in applications with sinusoidal AC voltage. Without sinusoidal AC voltage, these active filters will no longer function properly.

[0005] To achieve filtering effects, the switching frequency of active power filters is typically more than 10 times the ripple frequency. This ensures that the current variation caused by a single switching frequency is sufficiently small, allowing the overall current of the active power filter to match the ripple current waveform. However, in high-power applications, a single power semiconductor device generally cannot meet the power capacity requirements. Therefore, power supplies typically employ a modular multilevel converter topology. By increasing the number of individual modules, the power capacity of the power supply is increased. However, this approach results in each module having a high switching frequency and relatively low capacity, leading to a large number of modules, highly complex circuitry and control, high development costs, and decreased reliability. Summary of the Invention

[0006] In response to the shortcomings of existing technologies and the need for improvement, this invention proposes a modular DC active filter device and control method with low switching frequency, aiming to solve the problems of numerous high-power active filter modules, complex structure, and high cost.

[0007] To achieve the above objectives, in a first aspect, the present invention provides a modular DC active filter with a low switching frequency, wherein the DC active filter is connected in parallel across a DC bus to compensate for the ripple voltage of the DC bus.

[0008] The DC active filter device includes: an inductor, multiple independent DC active filter units, a measurement circuit, and a controller; the DC active filter unit includes a power half-bridge and a capacitor;

[0009] The measuring circuit is used to measure the ripple current of the DC bus;

[0010] The controller is used to control the working state of the power half-bridge of the DC active filter unit according to the switching signal, and to generate a compensation current by discharging on the inductor. The voltage fluctuation generated by the compensation current on the DC bus is opposite to the direction of the ripple voltage generated by the ripple current to be compensated.

[0011] The DC active filter units are connected in series and then in parallel across the DC bus. The number of DC active filter units is the same as the number of frequency components of the ripple current to be compensated, and each DC active filter unit is used to compensate for one frequency component of the ripple current. The parameters of the capacitor and the switching signal are determined according to the ripple current to be compensated corresponding to the DC active filter unit.

[0012] Optionally, the capacitor of each of the DC active filter units is connected in parallel at the upper and lower ends of the power half-bridge;

[0013] The midpoint of the power half-bridge of the first DC active filter unit is connected to one end of an inductor, and the other end of the inductor is connected to the positive terminal of the DC bus. The lower end of the power half-bridge of the first DC active filter unit is connected to the midpoint of the power half-bridge of the second DC active filter unit. The lower end of the power half-bridge of the last DC active filter unit is connected to the negative terminal of the DC bus.

[0014] Each of the DC active filter units consists of a power half-bridge composed of two power switching devices that are complementary in being turned on or off.

[0015] Optionally, the frequency of the switching signal is four times the frequency of the ripple current to be compensated.

[0016] Secondly, the present invention also provides a control method for a low-switching-frequency modular DC active filter, applied to a low-switching-frequency modular DC active filter as described in any one of the first aspects, comprising:

[0017] The measuring circuit measures the ripple current of the DC bus;

[0018] The frequency components of the ripple current are calculated by the controller's fast Fourier transform algorithm to obtain a preset number of ripple current frequency components; based on the frequency components of the ripple current to be compensated and the unit compensation current, the parameters of the switching signal and capacitor are determined by the optimal parameter algorithm.

[0019] Each DC active filter unit operates according to the corresponding switching signal, generating a unit compensation current on the inductor. The voltage fluctuation generated by the compensation current on the DC bus is opposite in direction to the ripple voltage generated by the ripple current to be compensated, corresponding to the compensation of a frequency component of the ripple current.

[0020] Optionally, the step of determining the parameters of the switching signal and capacitor using an optimal parameter algorithm based on the frequency component of the ripple current to be compensated and the unit compensation current includes:

[0021] By iterating through the preset switching signal parameters and preset capacitor voltage parameters of each DC active filter unit, the unit compensation current and unit compensation voltage are calculated.

[0022] Calculate the difference between the compensation voltage and the ripple voltage of the corresponding frequency component, i.e., the compensated ripple voltage; based on the voltage amplitude constraint of the compensated ripple voltage and the constraint of continuous operation of the DC active filter, select the target switching signal parameters and the target capacitor voltage parameters.

[0023] Optionally, before calculating the unit compensation current and unit compensation voltage by iterating through the preset switching signal parameters and preset capacitor voltage parameters of each DC active filter unit, the method further includes:

[0024] S1. Confirm the type and range of the parameters to be solved. The parameters of the switching signal include the duty cycle d of the switching period and the proportion of the initial phase in the switching period. and the voltage V of the capacitor in the DC active filter unit Cf The parameter range is 0 ≤ d ≤ 1. V d ≤V Cf ≤4V d The switching frequency of each DC active filter is four times the ripple to be compensated. Therefore, d in each switching cycle can be specifically represented as d1, d2, d3, d4. It can be specifically expressed as

[0025] S2, divide the above parameters into discrete parameter sequences, and determine the size of the discrete interval according to the preset calculation precision, calculation resources and time;

[0026] S3, from d, and V CfOne parameter is taken out in sequence from the parameter sequence to form a set of preset parameters to be solved.

[0027] Optionally, the step of traversing the preset switching signals and preset capacitor voltage parameters of each DC active filter unit to calculate the unit compensation current and unit compensation voltage includes:

[0028] S4. According to formulas (1) and (2), calculate the inductor current (unit compensation current) i generated by the active filter unit corresponding to the selected preset parameter set in one switching cycle. Lf and unit compensation voltage u dcAf :

[0029]

[0030]

[0031] Among them, C dc L is the DC bus capacitor, C is the inductor in the DC active filter, and i is the capacitor in the DC active filter unit. Lf (0) and u dcAf (0) represents the initial values ​​of the compensation current and compensation voltage in the switching cycle, respectively; the subscript "f" represents the frequency of the current ripple current component.

[0032] Optionally, the difference between the compensated voltage and the ripple voltage of the corresponding frequency component is calculated, i.e., the compensated ripple voltage; based on the voltage amplitude constraint of the compensated ripple voltage and the constraint of continuous operation of the DC active filter, the target switching signal parameters and capacitor voltage parameters are selected, including:

[0033] S5, within the first switching cycle, u dcAf1 With ripple voltage u dcrf1 The difference is the ripple voltage of the DC bus after compensation using the active filter unit;

[0034] Assume the maximum absolute value of the compensated ripple voltage during this switching cycle is u. dcmf1 , if u dcmf1 The amplitude is less than the ripple voltage u dcrf1 30% of the amplitude, and the inductor current i at the end of the first switching cycle. Lf1 (T) and the difference between the current and the ripple current of the DC bus at this frequency are less than 3%, and the voltage fluctuation u at the end of the first switching cycle dcAf1 If the difference between (T) and the ripple voltage at this frequency is less than 10%, proceed to S6; otherwise, return to S4.

[0035] S6 will compensate the voltage u at the end of the first switching cycle. dcAf1 (T) and compensation current i Lf1The value of (T) is used as the initial value of the compensation voltage u for the next switching cycle. dcAf2 (0) and the initial value of the compensation current i Lf2 (0), keep V Cf Without changing S4 and S5, iterate through d in the second switching cycle. The voltage fluctuation u in the second switching cycle is calculated using the formula. dcAf2 and inductor current i Lf2 ;

[0036] S7, Calculate the DC bus voltage ripple u after using the active filter unit during the second switching cycle. dcmf2 , if u dcmf2 The amplitude is less than the ripple voltage u dcrf1 30% of the amplitude, and the inductor current i at the end of the second switching cycle. Lf2 (T) The difference between the current and the ripple current of the DC bus at this frequency is less than 3%, and the voltage fluctuation u at the end of the second switching cycle dcAf2 If the difference between (T) and the ripple voltage at that frequency is less than 10%, then record this set of parameters (d1, d2、 and V Cf );

[0037] S8. Determine whether all parameter sequences have been traversed. If they have been traversed, proceed to S9; otherwise, return to S3.

[0038] S9, repeat S3-S8, calculate the set of all parameters (d3, ...) that satisfy the constraints for the last two switching cycles. d4、 and V Cf );

[0039] S10, in the parameter set that satisfies the constraints in the first two switching cycles and the last two switching cycles, find the parameter with the same V. Cf The parameter set forms the full-cycle parameter set (i.e., d1, ...). d2、 d3、 d4、 and V Cf If there are no identical V Cf Then V is selected from the parameter sets of the first two switching cycles and the parameter sets of the last two switching cycles. Cf The most similar parameter set forms the full-cycle parameter set; if the full-cycle parameter set contains multiple sets, then V is chosen. Cf The smallest set of parameters is taken as the target set of parameters.

[0040] In summary, the above-described technical solutions conceived in this invention can achieve the following beneficial effects:

[0041] (1) The modular DC active filter device with low switching frequency provided by the present invention includes multiple DC active filter units. The number of DC active filter units is determined according to the ripple suppression requirements. Each DC active filter unit can compensate for one ripple frequency component. The DC active filter unit can meet the filtering requirements of high power supply, expand the application scenarios of active filter. Compared with directly adding capacitors, the present invention improves the reliability of power supply, reduces space and site occupation, and saves the cost and effort of subsequent operation and maintenance.

[0042] (2) Compared with the modular multilevel technology in the prior art, the modular multilevel active filter of the present invention reduces the switching frequency, greatly increases the power capacity of a single module, avoids the use of a large number of modules, simplifies the modular multilevel circuit, reduces the development cost of the modular multilevel circuit and reduces the number of modules used.

[0043] (3) The control method provided by the present invention determines the switching signal and capacitor voltage parameters according to the optimal parameter design method, so that a single module can independently compensate for the ripple current of a frequency component, reducing the coupling degree between modules and simplifying the modular multilevel control strategy; and it has strong versatility and strong practicality in high power applications. Attached Figure Description

[0044] Figure 1 The circuit structure diagram of the modular multilevel DC active filter provided in the embodiment of the present invention is shown.

[0045] Figure 2 The equivalent circuit structure diagram of the modular multilevel DC active filter provided in the embodiment of the present invention is shown.

[0046] Figure 3 The diagram shows the equivalent circuit structure of a single module provided in an embodiment of the present invention.

[0047] Figure 4 The simulation results of the single-module equivalent circuit provided in the embodiments of the present invention when compensating for ripple current are shown. Detailed Implementation

[0048] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the invention. Furthermore, the technical features involved in the various embodiments of this invention described below can be combined with each other as long as they do not conflict with each other.

[0049] Example 1

[0050] like Figure 1As shown, this embodiment of the invention provides a low-switching-frequency modular DC active filter for suppressing DC bus voltage ripple in a power supply, wherein C dc and V dc These are the DC bus capacitance and DC bus voltage, i Cr This is the ripple current of the DC bus. The DC active filter device includes n DC active filter units (APFs), an inductor L, measurement circuitry, and a controller; each DC active filter unit (APF) includes a power half-bridge H. f and a capacitor C f ,

[0051] The measurement circuit is used to measure the ripple current i of the DC bus. Cr ;

[0052] The controller is used to control the power half-bridge H of the DC active filter unit (APF) according to the switching signal. f In its operating state, a compensation current i is generated by discharging through inductor L. Lf Compensation current i Lf Voltage fluctuations generated on the DC bus u dcAf With a frequency of ripple current i to be compensated Crf The generated ripple voltage u dcrf The opposite direction;

[0053] The active DC filter units (APFs) are connected in series and then in parallel across the DC bus; the number of APFs is related to the ripple current i to be compensated. Cr The number of frequency components is the same, and each DC active filter unit (APF) is used to compensate for the ripple current i of one frequency component. Crf Capacitor C f The parameters and switching signals are based on the ripple current i to be compensated corresponding to the DC active filter unit APF. Crf Sure.

[0054] Optionally, the frequency of the switching signal is four times the frequency of the ripple current to be compensated.

[0055] Among them, the ripple current i Cr The ripple current can be decomposed into multiple frequency components using the Fast Fourier Transform algorithm. The n largest ripple currents are then represented by n AC current sources, i Cr It can be approximated as a parallel connection of these n current sources, such as... Figure 2 As shown, the subscript "f" represents the frequency of the current ripple current component. The DC active filter device corresponds to n DC active filter units (APFs) that compensate for the ripple current i of n frequency components. Crf The DC active filter unit is an active power filter (APF).

[0056] The output voltage of each DC active filter unit (APF) can be expressed as a square wave voltage V. SWn This means that the DC bus voltage can be considered to be V. dc The DC voltage source can be divided into n voltages of size V. d A DC voltage source, where V d For V dc / n. According to the superposition theorem, the DC active filter unit and the DC bus can be regarded as the superposition of n equivalent circuits.

[0057] The DC active filter device provided in this embodiment uses a switching signal frequency that is four times the frequency of the ripple current to be compensated. Under the premise of satisfying ripple suppression, the switching frequency is greatly reduced, which can improve the power capacity of a single modular DC active filter unit, avoid the use of a large number of modules, and simplify the modular multilevel circuit.

[0058] refer to Figure 1 and Figure 3 The DC active filter device is divided into n equivalent circuits based on the number of DC active filter units. Each equivalent circuit includes one DC active filter unit (APF). f An inductor L f and a DC bus sub-capacitor C dcf DC bus sub-capacitor C dcf voltage V d V is the voltage across the DC bus capacitor. dc 1 / n; the inductance value, DC bus capacitance value, and DC active filter unit capacitance value are consistent with the corresponding parameter values ​​of the DC active filter device.

[0059] Optionally, the capacitor C of each DC active filter unit (APF) f Parallel connection in power half-bridge H f The upper and lower ends;

[0060] The midpoint of the power half-bridge of the first DC active filter unit APF1 is connected to one end of inductor L, and the other end of inductor L is connected to the positive terminal of the DC bus. The lower end of the power half-bridge of the first DC active filter unit APF1 is connected to the midpoint of the power half-bridge of the second DC active filter unit APF2; the last DC active filter unit APF... n The lower end of the power half-bridge is connected to the negative terminal of the DC bus;

[0061] The power half-bridge in each of the aforementioned DC active filter units (APF) consists of two power switching devices (T). n1 and T n2 The power switching devices (T) are connected in series. n1 and Tn2 Complementary conduction or shutdown.

[0062] By selecting an inductor C with special parameters f And a specific switching signal, causing the inductor current i of each DC active filter unit (APF) to... Lf i Lf The voltage fluctuation caused on the DC bus is the compensation voltage u. dcA When the compensation voltage u dcA and DC bus ripple current i Crf The generated ripple voltage u dcrf When they can cancel each other out, the ripple voltage of the DC bus will be significantly reduced, thereby achieving ripple suppression of the DC bus voltage.

[0063] Example 2

[0064] A control method for a low-switching-frequency modular DC active filter includes:

[0065] The measuring circuit measures the ripple current of the DC bus;

[0066] The frequency components of the ripple current are calculated by the controller's fast Fourier transform algorithm to obtain a preset number of ripple current frequency components; based on the frequency components of the ripple current to be compensated and the unit compensation current, the parameters of the switching signal and capacitor are determined by the optimal parameter algorithm.

[0067] Each DC active filter unit operates according to the corresponding switching signal, generating a unit compensation current on the inductor. The voltage fluctuation generated by the compensation current on the DC bus is opposite in direction to the ripple voltage generated by the ripple current to be compensated, corresponding to the compensation of a frequency component of the ripple current.

[0068] The compensation current is the sum of the unit compensation currents generated by each DC active filter unit, and the unit compensation current generated by each DC active filter unit is used to compensate for the ripple current of the corresponding frequency component.

[0069] According to the superposition theorem, a ripple current source and a V dc A combination of a DC voltage source and an APF module. Figure 2 It can be split into n parts Figure 3 .therefore Figure 1 The circuit can be decomposed and equivalently represented as Figure 3 To conduct analysis, Figure 3 The subscript "f" for each variable indicates the frequency of the ripple current compensated by that module. For example... Figure 3 As shown, the assumed ripple voltage u of the APF module dcrf It is a cosine waveform with an amplitude of U. dcrf ripple current i Cr The amplitude is ICr , then u dcrf and i Cr The expression within the ripple period is:

[0070]

[0071] Assume the capacitance and capacitor voltage of the APF module are C and C respectively. f and V Cf Then, the module output voltage V within one switching cycle T SW for:

[0072]

[0073] Where t is time, and d is the duty cycle of the upper pipe of the bridge arm. It is the ratio of the initial phase of the square wave (the rising edge of the square wave) to T. From the above equation, it is easy to see that d and The scope is as follows:

[0074]

[0075] Furthermore, the voltage U of inductor L Lf for:

[0076]

[0077] The current in inductor L is called the unit compensation current i. Lf Specifically:

[0078]

[0079] Unit compensation current i Lf The voltage fluctuation caused on the DC bus is the unit compensation voltage, denoted by u. dcAf It means, u dcAf It is t, d, and V Cf Functions:

[0080]

[0081] The above formula assumes i Lf and u dcAf The initial value is 0, but since the inductor current and capacitor voltage cannot change abruptly, consider i Lf initial value i Lf (0) and u dcAf initial value u dcAf (0) of u dcAf for:

[0082]

[0083] From formula (8), we can see that udcAf The time-domain waveform during one switching cycle is a combination of three parabolas, when V Cf At a certain time, in a suitable d, Under the parameters, u dcAf The waveform is basically the same as the waveform of the four stages of a cosine wave.

[0084] Under these specific parameters, u for 4 switching cycles dcAf The waveform that forms an approximate cosine wave, if u dcAf With ripple voltage u dcr If the magnitudes are equal but the directions are opposite, the ripple on the DC bus will decrease. The reduced ripple voltage amplitude u dcmf For |u dcAf -u dcrf The maximum value within one ripple cycle (4 switching cycles). Therefore, the switching frequency of each DC active filter unit is 4 times the ripple to be compensated.

[0085] Next, we will introduce d. and V Cf The optimal parameter calculation method. Specifically, this set of parameters includes d1, d2、 d3、 d4、 (Subscript numbers indicate the time sequence of the switching cycles) and V Cf There are a total of 9 parameters.

[0086] The following constraints must be satisfied during the solution process:

[0087] 1. Minimum ripple constraint: The above parameters should ensure that the active filter has a sufficiently good filtering effect. In this example, u dcmf It should meet the following requirements:

[0088] u dcmf <30%U dcrf (10)

[0089] 2. Continuity constraint: Inductor current and capacitor voltage cannot change abruptly, so they must be equal at the end of the previous switching cycle and at the beginning of the next switching cycle, i.e.:

[0090]

[0091] Where m is one of 1, 2, or 3, representing the m-th switching cycle.

[0092] 3. Periodic constraint: Since the active filter compensates for ripple voltage under steady-state conditions, its operation should be continuous, i.e., u dcrfThe ripple cycle should repeat periodically, and considering that the inductor current and capacitor voltage cannot change abruptly, the final values ​​of the inductor current and capacitor voltage in the current ripple cycle should be used as the initial values ​​for the next ripple cycle.

[0093]

[0094] In addition to the constraints described above, some additional assumptions are required for this example to facilitate calculations.

[0095] 1. Initial value assumptions: It is assumed that at the initial moment of the first switching cycle, the inductor current and ripple current of the active filter are equal, and the capacitor voltage and ripple voltage are equal, that is:

[0096]

[0097] 2. Median Assumption: It is assumed that at the end of the second switching cycle, the inductor current and ripple current of the active filter are equal, and the capacitor voltage and ripple voltage are equal, i.e.:

[0098]

[0099] The process of calculating the optimal parameters is as follows:

[0100] S1: Defines the range of parameters, including the range of capacitor voltage: V d ≤V Cf ≤4V d d、 The range is shown in formula (5).

[0101] S2: Divide the above parameters into discrete parameter sequences. Determine the size of the discrete intervals based on the needs of computational accuracy, computational resources, and time. In this implementation, the parameter sequence is divided into 100 intervals.

[0102] S3: From d1, and V Cf One parameter is taken out in sequence from the parameter sequence to form a set of parameters to be solved.

[0103] S4: Calculate the unit compensation current i generated by the active filter unit corresponding to this parameter set in the first switching cycle based on the initial value of formula (13) and formulas (7) to (9). Lf1 and unit compensation voltage u dcA1 .

[0104] S5: During the first switching cycle, u... dcAf1 With ripple voltage u dcrf1 The difference is the DC bus voltage ripple after using the active filter unit.

[0105] Assume its maximum absolute value during this switching cycle is u. dcmf1 Record the inductor current i at the end of the first switching cycle. Lf1 (T) and voltage fluctuation u dcAf1 (T), if u dcmf1 Satisfying the minimum ripple constraint (i.e., Equation (10)), and i Lf1 (T) and the ripple current i at this moment Crf1 The error of (T) is within 3% (i.e., |i Lf1 (T)-(-I Crf )|<0.03I Crf ), and u dcAf1 (T) and ripple voltage u dcrf1 The error of (T) is within 10% of the ripple voltage amplitude (i.e., |u dcAf1 (T)-0|<0.1U dcrf If the condition is met, proceed to S6; otherwise, return to S3.

[0106] S6: Compensate voltage u at the end of the first switching cycle dcAf1 (T) and compensation current i Lf1 The value of (T) is used as the initial value of the compensation voltage u for the next switching cycle. dcAf2 (0) and the initial value of the compensation current i Lf2 (0), keep V Cf V selected with the first switching cycle Cf Same; from d2 and One parameter is taken sequentially from the parameter sequence to form a set of solution parameters. The solution parameter set of the second switching cycle is traversed, and the calculation result of S5 (i) is used to calculate the solution parameter set. Lf1 (T) and u dcrf1 (T) and the continuity constraint formula (11) are used to determine the initial value of the second calculation cycle, and then the unit compensation current i is calculated according to formula (9). Lf2 and unit compensation voltage u dcA2 .

[0107] S7: During the second switching cycle, u... dcAf2 With ripple voltage u dcrf2 The difference is the DC bus voltage ripple after using the active filter unit.

[0108] Assume its maximum absolute value during this switching cycle is u. dcmf2 Record the inductor current i at the end of the second switching cycle. Lf2 (T) and voltage fluctuation u dcAf2 (T). If u dcmf2 Satisfying the minimum ripple constraint (i.e., Equation (10)), and i Lf2 (T) and the ripple current i at this momentCrf2 The error of (T) is within the ripple current amplitude I Crf Within 3% (i.e., |i Lf2 (T)-0|<0.03I Crf ), and u dcAf2 (T) and ripple voltage u dcrf2 The error of (T) is within 10% |u dcAf2 (T)-(-U dcrf )|<0.1U dcrf Then record this set of parameters (d1, ... d2、 and V Cf ).

[0109] S8: Determine whether all parameter sequences have been traversed. If they have, proceed to S9; otherwise, return to S3.

[0110] S9: Repeat S3-S8 to calculate the set of all parameters (d3, ...) that satisfy the constraints for the last two switching cycles. d4、 and V Cf ).

[0111] S10: In the parameter sets that satisfy the first two and last two constraints, find the parameter with the same V. Cf The parameter set, which constitutes the parameter set of the entire period (i.e., d1, ... d2、 d3、 d4、 and V Cf If there are no two identical V's. Cf Then choose V. Cf The most similar parameter sets form the full-cycle parameter set; if the full-cycle parameter set contains multiple sets, then V is selected. Cf The smallest set of parameters is taken as the target set of parameters.

[0112] S11: Determine the capacitor of the DC active filter unit based on the capacitor voltage parameters in the target parameter set, and control each DC active filter unit according to the switching signals in the target parameter set.

[0113] Figure 4 The simulation results show a single APF cell designed using this parameter method, with a ripple frequency of 300Hz. The upper part of the figure shows the ripple voltage u before the APF cell is used. dcr300 (Amplitude is U) dcr300 ) and the ripple voltage u after use dcm300 The middle part is the ripple current i. Cr300 and inductor current i L300 The waveform is shown below; the half-bridge output voltage V of the DC active filter unit is shown below.SW300 This unit compensates for the DC bus voltage V. d For a voltage of 1250V, the optimal parameter set for the APF obtained by the parameter design method is (d1 = 0.38, ...).

[0114] d2 = 0.94 d3 = 0.88 d4 = 0.42 and V C300 =1925V).

[0115] As shown in the figure, the switching frequency of the APF unit is four times the ripple current frequency, and it can reduce the ripple voltage from 20.9V to 3.09V, a reduction of 85.2%. This achieves excellent ripple suppression while reducing the switching frequency.

[0116] The technical solution of this invention provides a modular DC active filter device with low switching frequency. The number of DC active filter modules is determined according to ripple suppression requirements, making it flexible and convenient to use. Each DC active filter unit can compensate for one ripple frequency component, and the switching frequency of each DC active filter unit is only four times the ripple frequency, achieving a ripple suppression effect of approximately 85%. This invention solves the technical problems of high difficulty in developing high-power active filters and low reliability, enriching the technical solutions for active filters in the field of DC ripple suppression.

[0117] Those skilled in the art will readily understand that the above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A modular DC active filter with low switching frequency, characterized in that, The DC active filter is connected in parallel across the two ends of the DC bus to compensate for the ripple voltage of the DC bus. The DC active filter device includes: an inductor, multiple independent DC active filter units, a measurement circuit, and a controller; the DC active filter unit includes a power half-bridge and a capacitor; The measuring circuit is used to measure the ripple current of the DC bus; The controller is used to control the working state of the power half-bridge of the DC active filter unit according to the switching signal, and to generate a compensation current by discharging on the inductor. The voltage fluctuation generated by the compensation current on the DC bus is opposite to the direction of the ripple voltage generated by the ripple current to be compensated. The DC active filter units are connected in series and then in parallel across the DC bus. The number of DC active filter units is the same as the number of frequency components of the ripple current to be compensated, and each DC active filter unit is used to compensate for one frequency component of the ripple current. The parameters of the capacitor and the switching signal are determined according to the ripple current to be compensated corresponding to the DC active filter unit. Among them, the optimal parameter algorithm is used to select the target switching signal parameters and target capacitor voltage parameters based on the voltage amplitude constraints of the compensated ripple voltage and the constraints of continuous operation of the DC active filter; the frequency of the switching signal is 4 times the frequency of the ripple current to be compensated.

2. The DC active filter device according to claim 1, characterized in that, The capacitor of each of the DC active filter units is connected in parallel at the upper and lower ends of the power half-bridge; The midpoint of the power half-bridge of the first DC active filter unit is connected to one end of an inductor, and the other end of the inductor is connected to the positive terminal of the DC bus. The lower end of the power half-bridge of the first DC active filter unit is connected to the midpoint of the power half-bridge of the second DC active filter unit. The lower end of the power half-bridge of the last DC active filter unit is connected to the negative terminal of the DC bus. Each of the DC active filter units consists of a power half-bridge composed of two power switching devices that are complementary in being turned on or off.

3. A control method for a low-switching-frequency modular DC active filter, applied to the low-switching-frequency modular DC active filter as described in any one of claims 1-2, characterized in that, include: The measuring circuit measures the ripple current of the DC bus; The frequency components of the ripple current are calculated by the controller's fast Fourier transform algorithm to obtain a preset number of ripple current frequency components. Based on the frequency component of the ripple current to be compensated and the unit compensation current, the parameters of the switching signal and capacitor are determined using an optimal parameter algorithm. Each DC active filter unit operates according to the corresponding switching signal, generating a unit compensation current on the inductor. The voltage fluctuation generated by the compensation current on the DC bus is opposite in direction to the ripple voltage generated by the ripple current to be compensated, corresponding to the compensation of a frequency component of the ripple current.

4. The control method according to claim 3, characterized in that, The step of determining the parameters of the switching signal and capacitor using an optimal parameter algorithm based on the frequency component of the ripple current to be compensated and the unit compensation current includes: By iterating through the preset switching signal parameters and preset capacitor voltage parameters of each DC active filter unit, the unit compensation current and unit compensation voltage are calculated. Calculate the difference between the compensation voltage and the ripple voltage of the corresponding frequency component, i.e., the compensated ripple voltage; based on the voltage amplitude constraint of the compensated ripple voltage and the constraint of continuous operation of the DC active filter, select the target switching signal parameters and the target capacitor voltage parameters.

5. The control method according to claim 4, characterized in that, Before calculating the unit compensation current and unit compensation voltage by iterating through the preset switching signal parameters and preset capacitor voltage parameters of each DC active filter unit, the following steps are also included: S1, confirm the type and range of the parameters to be solved. The parameters of the switching signal include the duty cycle of the switching period. d The proportion of the initial phase in the switching cycle φ and the voltage of the capacitor in the DC active filter unit. V Cf The parameter range is 0≤ d ≤1, 0≤ φ ≤1- d , V d ≤ V Cf ≤4 V d The switching frequency of each of the aforementioned DC active filter switches is four times the ripple to be compensated, therefore, in each switching cycle... d It can be specifically expressed as d 1, d 2, d 3, d 4. In each switching cycle φ It can be specifically expressed as φ 1, φ 2, φ 3, φ 4; S2, divide the above parameters into discrete parameter sequences, and determine the size of the discrete interval according to the preset calculation precision, calculation resources and time; S3, from d , φ and V Cf One parameter is taken out in sequence from the parameter sequence to form a set of preset parameters to be solved.

6. The control method according to claim 5, characterized in that, The process of iterating through the preset switching signals and preset capacitor voltage parameters of each DC active filter unit to calculate the unit compensation current and unit compensation voltage includes: S4. According to formulas (1) and (2), calculate the inductor current generated by the active filter unit corresponding to the selected preset parameter set in one switching cycle. i Lf and unit compensation voltage u dcAf : （1） （2） in, C dc It is a DC bus capacitor. L It is the inductor in a DC active filter device. C It is a capacitor in a DC active filter unit. i Lf (0) and u dcAf (0) represents the initial values ​​of the compensation current and compensation voltage in the switching cycle, respectively; the subscript f is the frequency of the current ripple current component.

7. The control method according to claim 6, characterized in that, Calculate the difference between the compensated voltage and the ripple voltage of the corresponding frequency component, i.e., the compensated ripple voltage; based on the voltage amplitude constraint of the compensated ripple voltage and the constraint of continuous operation of the DC active filter, select the target switching signal parameters and capacitor voltage parameters, including: S5 will be used in the first switching cycle. u dcAf1 With ripple voltage u dcrf1 The difference is the ripple voltage of the DC bus after compensation using the active filter unit; Assuming the maximum absolute value of the compensated ripple voltage during this switching cycle is u dcmf1 ,like u dcmf1 The amplitude is less than the ripple voltage. u dcrf1 30% of the amplitude, and the inductor current at the end of the first switching cycle. i Lf1 ( T The difference between the ripple current at this frequency and the current at the DC bus is less than 3%, and the voltage fluctuation at the end of the first switching cycle is... u dcAf1 ( T If the difference between the voltage and the ripple voltage at that frequency is less than 10%, proceed to S6; otherwise, return to S4. S6, at the end of the first switching cycle, compensates the voltage. u dcAf1 ( T and compensation current i Lf1 ( T The value of ) is used as the initial value of the compensation voltage for the next switching cycle. u dcAf2 (0) and the initial value of the compensation current i Lf2 (0), keep V Cf With S4 and S5 unchanged, iterate through the second switching cycle. d , φ Calculate the voltage fluctuation of the second switching cycle according to the formula. u dcAf2 and inductor current i Lf2 ; S7, Calculate the DC bus voltage ripple after using the active filter unit during the second switching cycle. u dcmf2 ,like u dcmf2 The amplitude is less than the ripple voltage. u dcrf1 30% of the amplitude, and the inductor current at the end of the second switching cycle. i Lf2 ( T The difference between the ripple current at this frequency and the DC bus ripple current at this time is less than 3%, and the voltage fluctuation at the end of the second switching cycle... u dcAf2 ( T If the difference between the current ripple voltage and the ripple voltage at that frequency is less than 10%, then record this set of parameters {( d 1, φ 1, d 2, φ 2, V Cf )}; S8. Determine whether all parameter sequences have been traversed. If they have been traversed, proceed to S9; otherwise, return to S3. S9, repeat S3-S8, calculate the set of all parameters that satisfy the constraints for the last two switching cycles {( d 3, φ 3, d 4, φ 4, V Cf )}; S10, in the parameter set that satisfies the constraints in the first two switching cycles and the last two switching cycles, find parameters with the same... V Cf The parameter set, forming the full-cycle parameter set {( d 1, φ 1, d 2, φ 2, d 3, φ 3, d 4, φ 4, V Cf If there are no identical ones V Cf Then, select from the parameter sets of the first two switching cycles and the parameter sets of the last two switching cycles. V Cf The most similar parameter set forms the full-cycle parameter set; if the full-cycle parameter set contains multiple sets, then choose... V Cf The smallest set of parameters is taken as the target set of parameters.

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