A DC active filter device based on square wave modulation

Abstract

This invention provides a DC active filter device based on square wave modulation. The device includes: at least one independent DC active filter unit; each DC active filter unit is connected in parallel across the two ends of a DC bus; each DC active filter unit includes: a measurement circuit that measures the ripple current of the DC bus; a control circuit that calculates the frequency components of the ripple current and then obtains a switching signal through square wave modulation, the frequency of which is equal to the target frequency of the ripple current to be compensated; and a drive circuit that sends the switching signal to the main power circuit, causing the switching devices in the main power circuit to operate and generate a compensation current to compensate for the ripple current of the target frequency component of the DC bus. This invention solves the problem of poor ripple suppression performance of high-power DC active filters at low switching frequencies. It enables the application of DC active filters at lower switching frequencies up to MW-level power applications, meeting the DC bus ripple suppression requirements for long-term operation of high-power power supplies.

Description

Technical Field

[0001] This invention belongs to the field of DC active filtering technology, and more specifically, relates to a DC active filtering device based on square wave modulation. Background Technology

[0002] The Accelerating Electrode Power Supply (AGPS) of the CFETR Neutral Beam Injector (NBI) prototype is a high-voltage power supply with a rated voltage of -200kV / 25A. To reduce neutral beam divergence and improve the heating efficiency of the NBI system, the AGPS output voltage ripple needs to be strictly limited. The DC bus voltage ripple of the AGPS is transmitted to the output voltage through the circuit structure, increasing the output voltage ripple. Therefore, a low-ripple DC bus voltage is essential.

[0003] To reduce ripple on the DC bus voltage of the AGPS power supply, a large number of capacitors are connected in parallel on the DC bus. These capacitors occupy most of the power supply's volume, making it bulkier. Furthermore, capacitors are among the most prone to failure components in the entire power supply, and the extensive use of capacitors reduces the power supply's reliability.

[0004] Active DC filters are a new type of ripple suppression device. By switching on and off, they can compensate for ripple current in the power supply, thereby reducing ripple on the DC bus voltage. They have advantages such as small size and good ripple suppression effect. However, in high-power power supply applications, the operating frequency of the switching devices is limited by power capacity (typically tens to hundreds of kilohertz). The ripple suppression effect of the active DC filter will significantly deteriorate due to the reduction in switching frequency, which limits its application in high-power applications. In addition, most current control strategies or improved circuit schemes for active DC filters incorporate sinusoidal grid voltage, which is unsuitable for all high-power power supplies with DC bus structures. Summary of the Invention

[0005] To address the shortcomings and improvement needs of existing technologies, this invention proposes a DC active filter based on square wave modulation, aiming to solve the problem of poor ripple suppression performance in high-power DC active filters at low switching frequencies. By determining key electrical parameters, the DC active filter can be applied to MW-level power applications with existing switching device power capacities, greatly satisfying the DC bus ripple suppression requirements for long-term operation of high-power power supplies, and demonstrating good market application prospects and value.

[0006] To achieve the above objectives, the present invention provides a DC active filter device based on square wave modulation, wherein the DC active filter device is connected to a DC bus and is used to compensate for the ripple current of the DC bus.

[0007] The DC active filter device includes: a DC active filter, a measurement circuit, a control circuit, and a drive circuit; the DC active filter includes at least one independent DC active filter unit, and the DC active filter units are all connected in parallel to both ends of the DC bus;

[0008] The measuring circuit is used to measure the ripple current of the DC bus;

[0009] The control circuit calculates each frequency component of the ripple current and then obtains a switching signal through square wave modulation. The frequency of the switching signal is equal to the target frequency of the ripple current to be compensated.

[0010] The driving circuit sends the switching signal to the DC active filter, and the switching device in the DC active filter unit operates to generate a compensation current to compensate for the ripple current of the target frequency component of the DC bus.

[0011] Optionally, the number of the DC active filter units is equal to the number of frequency components of the ripple current;

[0012] Each of the aforementioned DC active filter units is used to compensate for ripple current of one frequency component.

[0013] Optionally, the DC active filter unit consists of a power half-bridge, an inductor, and a capacitor; the two ends of the capacitor are respectively connected to the upper and lower ends of the power half-bridge, the first end of the inductor is connected to the positive terminal of the DC bus, the second end of the inductor is connected to the midpoint of the power half-bridge, and the lower end of the power half-bridge is connected to the negative terminal of the DC bus.

[0014] Optionally, the power half-bridge consists of two switching devices that are complementary in being turned on or off.

[0015] When the sinusoidal phase of the target frequency component of the ripple current is in arrive During this period, the upper switching device of the power half-bridge is turned off and the lower switching device is turned on, and the output voltage of the power half-bridge is 0.

[0016] When the sinusoidal phase of the target frequency component of the ripple current is in arrive During this period, the upper switching device of the power half-bridge is turned on and the lower switching device is turned off, and the output voltage of the power half-bridge is twice the DC bus voltage.

[0017] Optionally, the compensation current is a triangular wave. By setting the target electrical parameters of the inductor and capacitor, the phase of the compensation current is made to be the same as the phase of the sinusoidal ripple current of the target frequency component, and the amplitude I of the compensation current is... tri The amplitude I of the sinusoidal ripple current with respect to the target frequency componentCrf Satisfying relation: I tri =1.1384I Crf .

[0018] Optionally, the inductor L f The calculation formula is:

[0019]

[0020] Among them, T rf It is the period of the ripple current with frequency f, I Crf It is the amplitude of the ripple current, V dc It is the voltage of the DC bus;

[0021] The capacitor C f The calculation formula is:

[0022]

[0023] Among them, V Cf C is the capacitor voltage. rf % represents the voltage ripple rate of the capacitor.

[0024] Optionally, the ripple current is the output current i of the rectifier on the DC bus. rec With the inverter's input current i inv The difference;

[0025] The rectifier output current i rec The input current i of the inverter is obtained by measuring the current of the filter inductor of the DC bus. inv It is obtained from the following formula:

[0026]

[0027] Among them, V a V b V c i is the three-phase output voltage of the inverter connected to the DC bus. a i b i c This refers to the three-phase output current of the inverter connected to the DC bus.

[0028] In summary, the above-described technical solutions conceived in this invention can achieve the following beneficial effects:

[0029] (1) This invention reduces the switching frequency of the DC active filter device from at least 10 times the ripple frequency to 1 times the ripple frequency, solving the problem of the difficulty in designing and developing DC active filters for high power supplies, making it possible to apply DC active filters in MW-level high power supplies, and has high practicality.

[0030] (2) Each DAPF unit of the present invention can compensate for a ripple current component individually, making the DAPF unit circuit simple, easy to design, highly reliable, and capable of compensating for ripple current with rich harmonic content, and highly versatile.

[0031] (3) Each DAPF unit of the present invention is independent of each other and does not affect each other. It can be used in combination according to the ripple suppression requirements, which has high flexibility. Furthermore, the method of generating the switching signal is simple and does not rely on closed-loop control, thus solving the problem that a large number of switching devices lead to a very complex control strategy.

[0032] (4) This invention can accurately obtain the ripple current of the DC bus, which solves the problem of difficulty in measuring the ripple current of high power power supplies due to the DC bus structure. It provides an accurate and reliable measurement solution for inventors and scholars in the industry to continue to study the DC bus ripple suppression of high power power supplies. Attached Figure Description

[0033] Figure 1 A system block diagram of a DC active filter device provided in an embodiment of the present invention;

[0034] Figure 2 This is a schematic diagram of the main power circuit of the DC active filter unit provided in an embodiment of the present invention;

[0035] Figure 3 This is a schematic diagram of the DC bus circuit related to the accelerating electrode power supply provided in an embodiment of the present invention;

[0036] Figure 4 A schematic diagram of the primary circuit of the accelerating electrode power inverter and the isolation transformer provided in an embodiment of the present invention;

[0037] Figure 5 The simulation results of the DC active filter unit provided in the embodiment of the present invention. Detailed Implementation

[0038] 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.

[0039] Figure 1 This is a schematic diagram of a DC active filter device based on square wave modulation, provided as an embodiment of the present invention.

[0040] like Figure 1As shown, a DC active filter 1 based on square wave modulation is connected to a DC bus 2 to compensate for the ripple current i of the DC bus 2. Cr ;

[0041] The DC active filter device 1 includes: at least one independent DC active filter unit 11; the DC active filter unit 11 includes: a main power circuit, a measurement circuit, a control circuit and a drive circuit; the DC active filter units 11 are all connected in parallel to both ends of the DC bus 2;

[0042] The measurement circuit is used to measure the ripple current i of DC bus 2. Cr ;

[0043] The control circuit calculates the ripple current i. Cr The frequency components are then modulated by a square wave to obtain a switching signal. The frequency of the switching signal is related to the ripple current i to be compensated. Cr The target frequencies are equal;

[0044] The drive circuit sends a switching signal to the main power circuit, and the switching devices in the main power circuit operate, generating a compensation current i. Lf The ripple current i used to compensate for the target frequency component of DC bus 1 Cr .

[0045] The device includes multiple DC active filter (DAPF) units 11, all of which are connected in parallel to the DC bus 2 of the power supply. Each unit can compensate for a sinusoidal ripple current of a specific frequency, based on the ripple current i. Cr The number of frequency components requiring compensation is determined, the number of DC active filter units 11 is determined, and the number of DC active filter units 11 and the ripple current i are set. Cr The number of frequency components is equal, allowing for the simultaneous processing of ripple current i across multiple frequency components. Cr Compensation will be provided.

[0046] The active power filter unit 11 includes: a main power circuit, a measurement circuit, a control circuit, and a drive circuit. The main power circuit reference... Figure 2 As shown. The measurement circuit measures the ripple current i of DC bus 2. CrA typical active power filter (APS) consists of a voltage or current sensor and a signal conditioning circuit. Its input is connected to the voltage or current to be measured in the circuit, and its output is connected to the analog-to-digital converter (ADC) pin of the controller in the control circuit. The controller in the control circuit can be a programmable processor such as a microcontroller, a digital signal processor, or a single-chip microcomputer. It receives signals from the measurement circuit. The controller generates a switching signal for the DC active filter unit based on Fast Fourier Transform (FFT) analysis and a square wave modulation strategy. The frequency of the switching signal is equal to the target frequency of the ripple current to be compensated. The drive circuit receives the switching signal from the controller and generates a drive signal suitable for the switching device, driving the switch in the main power circuit to operate and generate a compensation current. The generated compensation current is a triangular wave with the same frequency as the ripple current, thus providing a good compensation effect.

[0047] Based on the above embodiments, optionally, the main power circuit consists of a power half-bridge H and an inductor L. f and capacitor C f Composition; Capacitor C f The two ends of the inductor L are connected to the upper end a and the lower end b of the power half-bridge H, respectively. f The first terminal d1 is connected to the positive terminal of the DC bus, and the inductor L f The second end d2 is connected to the midpoint e of the power half-bridge H, and the lower end b of the power half-bridge H is connected to the negative terminal of the DC bus.

[0048] By selecting an inductor L with special parameters f (The subscript "f" indicates the ripple current component with compensation frequency f in the DAPF unit) and a specific switching signal, causing the inductor current i in each DC active filter unit to... Lf It is a symmetrical triangular wave with positive and negative amplitudes, I. tri The amplitude I of the ripple current component of the sinusoidal waveform Crf Satisfying a specific relationship allows i to... Lf with i Crf The waveforms approximately overlap. Therefore, the capacitor C originally flowing into the DC bus... dc ripple current i Cr The main current flowing into the DC active filter will be i in the time domain of a certain ripple current frequency component of the DC bus capacitor. Lf with i Crf The difference will be greatly reduced in magnitude compared to the original value, thereby achieving ripple suppression of DC bus voltage.

[0049] Optionally, the power half-bridge H consists of two switching devices (T... n1 and T n2 Composed of switching devices that complement each other to turn on or off;

[0050] When the sinusoidal phase of the target frequency component of the ripple current is in arrive During this period, the upper switching device T of the power half-bridge H... n1 Turn off, switch device T n2 When the power half-bridge H is turned on, the output voltage V SWf =0;

[0051] When the sinusoidal phase of the target frequency component of the ripple current is in arrive During this period, the upper switching device T of the power half-bridge H... n1 On and off switching devices T n2 When turned off, the output voltage V of the power half-bridge H is... SWf DC bus voltage V dc Twice as much.

[0052] Furthermore, i Lf The change can be represented as:

[0053]

[0054] Among them, V dc t is the DC bus voltage of the power supply. on and t off These are the opening times of the upper and lower pipes, respectively. From the above formula, we can see that i... Lf The triangular wave's switching signal duty cycle will affect the duration of the falling phase, and thus influence i. Lf The waveform. Analysis shows that when the rise and fall times of the triangular wave are half a cycle of the sinusoidal ripple current, and i Lf and i Crf When kept in phase, the triangular wave and the sine wave will approximately coincide.

[0055] The voltage at the midpoint e of the power half-bridge H is the input voltage of the power half-bridge H, and the voltage across the upper switching device T is... n1 Turn off, switch device T n2 When the circuit is turned on, the voltage V at the midpoint of the power half-bridge H is... cf The output voltage V of the power half-bridge SWf To ensure that the rising and falling speeds of the triangular wave are consistent, V Cf It should satisfy the formula: V Cf =2V dc By setting different phase sine waves to correspond to different switching signals, the phase of the triangular wave current and the ripple current of the sine wave can be made the same. Furthermore, by setting the amplitude of the triangular wave, the compensated DC bus ripple current can be minimized.

[0056] Optionally, by setting the inductor L f and capacitor C f The target electrical parameters make the compensation current iLf The sinusoidal ripple current i with phase and target frequency component Cr The phases are the same, and the compensation current i Lf Amplitude I tri sinusoidal ripple current i with target frequency component Cr Amplitude I Crf Satisfying relation: I tri =1.1384I Crf .

[0057] Furthermore, to ensure the amplitude of the triangular wave meets the above requirements, the required inductance value is calculated based on the inductor's volt-ampere characteristics. The calculation formula is as follows:

[0058]

[0059] Among them, T rf It is the period of the ripple current with frequency f, I Crf It is the amplitude of the ripple current, V dc It is the voltage of the DC bus;

[0060] The capacitor C f The calculation formula is:

[0061]

[0062] Among them, V Cf C is the capacitor voltage. rf % represents the voltage ripple rate of the capacitor.

[0063] Only when the upper transistor T of the power half-bridge... n1 When turned on, the capacitor C of the DC active filter unit f There is current only when it is above, and at this time the current is related to i. Lf They are equal. At this point, i... Lf Located in the descending portion of the triangular wave. Since the DC component of the ripple is 0 in steady state, i Lf In T off The value reaches 0 at time / 2 and continues to decrease until the shutdown time T. off End. If the design of C... f The maximum allowable voltage ripple rate is C rf % indicates that, based on the capacitor's volt-ampere characteristics and the current waveform during that period, C f The parameter range can be determined as follows:

[0064]

[0065] Through the above process, the switching signals and key electrical parameters of the main power circuit can be determined.

[0066] The above process relies on the controller knowing the amplitude and phase of each frequency component of the ripple current. This requires the ability to measure the DC bus capacitance C. dc Ripple current i Cr .

[0067] In high-power power supplies, large-size laminated busbars are commonly used to reduce leakage inductance in the circuit. However, current sensors require a conductor carrying the measured current to pass through their sampling coil for accurate current measurement, but this coil is often difficult to wind successfully around the laminated busbar. To accurately and reliably obtain the real-time waveform of the DC bus ripple current, the following measurement method is employed:

[0068] Depend on Figure 3 As can be seen, the DC bus in AGPS is located between the rectifier and the inverter. The smoothing reactor on the output side of the rectifier is connected to the DC bus via copper wire. The current sensor can measure the lead current of the smoothing reactor, which is the output current i of the rectifier. rec If the ripple current i induced by the inverter on the DC bus capacitor can be measured... inv i Cr It can then be calculated using the following formula:

[0069] i Cr =i rec -i inv

[0070] AGPS's inverters employ a neutral-point clamped (NPC) three-level inverter topology. The inverter output is connected to an isolation transformer with a D-type winding, such as... Figure 4 As shown, at this moment, the voltage of phase A is high, while the voltages of the other two phases are low. The dashed line in the figure represents the path of current flow. At this time, the capacitive current of the positive DC bus is the output current i of phase A. a The capacitor current of the negative DC bus is the output current i of phase B. b and C-phase output current i c The sum. Based on the connection method of the D-type winding, the following formula applies:

[0071] i a +i b +i c =0

[0072] Depend on Figure 3 It can be seen that the DC bus of the accelerating electrode power supply consists of positive and negative parts, corresponding to i inv There are also two in total, represented by i. inv+ and i inv- This indicates that the sum of the output currents of phases B and C is the negative of the output current of phase A. Therefore, i inv+ and i inv-They are also opposites of each other, and both can be represented by i. a To express.

[0073] Because the DC bus and the three-phase inverter are symmetrical and have positive and negative polarity, the capacitance corresponding to the positive DC bus is i. inv+ The time-domain waveform can be represented as:

[0074]

[0075] By acquiring the three-phase output voltage and current of the inverter, the ripple current of the DC bus can be obtained; by using the FFT algorithm of the measurement circuit and controller, i can be obtained. Cr Each frequency component is calculated; then each controller generates a corresponding switching signal based on the real-time phase of the frequency component ripple current compensated by its unit.

[0076] Simulation results for compensating 900Hz ripple current with a single DC active filter unit are shown below. Figure 5 At this point, the variable's subscript "f" is replaced by "900". (See diagram u.) dcr900 and u dcA900 i represents the DC bus voltage ripple at a frequency of 900Hz before and after the DC active filter unit is connected. Cr900 This represents the DC bus ripple current. To better compare the inductor current and i in the DAPF... Cr900 The waveform differences between them are represented by the inverse number -i for the inductor current of the DAPF in the figure. L900 This indicates that -i L900 with i Cr900 The near-overlapping values ​​indicate that most of the ripple current is compensated by the current of the DAPF unit, significantly reducing the actual ripple current of the DC bus. Therefore, the DC bus voltage ripple is reduced from the original 8.15V to 1.40V, a reduction of 82.8%. The working principle and implementation effect of DC active filter units at other ripple frequencies are almost the same, only the parameters are changed accordingly, so the corresponding simulation results are not listed.

[0077] This invention enables the application of active DC filters in high-power power supplies. Simulation results show that the device can reduce ripple current by approximately 80% for each frequency component, significantly reducing DC bus voltage ripple. This technique also reduces DC bus capacitance significantly while maintaining the same DC bus ripple voltage. Therefore, this invention improves the power density and reliability of high-power power supplies and can also be used in other high-power applications requiring low DC ripple voltage.

[0078] 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 DC active filter based on square wave modulation, characterized in that, The DC active filter is connected to the DC bus and is used to compensate for the ripple current of the DC bus. The DC active filter device includes: at least one independent DC active filter unit; the DC active filter unit includes: a main power circuit, a measurement circuit, a control circuit, and a drive circuit; the DC active filter units are all connected in parallel to both ends of the DC bus; The measuring circuit is used to measure the ripple current of the DC bus; The control circuit calculates each frequency component of the ripple current and then obtains a switching signal through square wave modulation. The frequency of the switching signal is equal to the target frequency of the ripple current to be compensated. The drive circuit sends the switching signal to the main power circuit. The switching devices in the main power circuit activate, generating a compensation current to compensate for the ripple current of the target frequency component of the DC bus. This compensation current is a triangular wave, and by setting the target electrical parameters of the inductor and capacitor, the phase of the compensation current is made to be the same as the phase of the sinusoidal ripple current of the target frequency component. The amplitude of the compensation current... I tri The amplitude of the sinusoidal ripple current with respect to the target frequency component I Crf Satisfying Relationship: .

2. The DC active filter device according to claim 1, characterized in that, The number of DC active filter units is equal to the number of frequency components of the ripple current; Each of the aforementioned DC active filter units is used to compensate for ripple current of one frequency component.

3. The DC active filter device according to claim 2, characterized in that, The main power circuit consists of a power half-bridge, an inductor, and a capacitor. The two ends of the capacitor are connected to the upper and lower ends of the power half-bridge, respectively. The first end of the inductor is connected to the positive terminal of the DC bus, the second end of the inductor is connected to the midpoint of the power half-bridge, and the lower end of the power half-bridge is connected to the negative terminal of the DC bus.

4. The DC active filter device according to claim 3, characterized in that, The power half-bridge consists of two switching devices that are complementary in being turned on or off. When the sinusoidal phase of the target frequency component of the ripple current is in arrive During this period, the upper switching device of the power half-bridge is turned off and the lower switching device is turned on, and the output voltage of the power half-bridge is 0. When the sinusoidal phase of the target frequency component of the ripple current is in arrive During this period, the upper switching device of the power half-bridge is turned on and the lower switching device is turned off, and the output voltage of the power half-bridge is twice the DC bus voltage.

5. The DC active filter device according to claim 1, characterized in that, The inductor L f The calculation formula is: in, T rf It is the period of the ripple current with frequency f. I Crf It is the amplitude of the ripple current. V dc It is the voltage of the DC bus; The capacitor C f The calculation formula is: in, V Cf This is the capacitor voltage. C rf The percentage represents the voltage ripple rate of the capacitor.

6. The DC active filter device according to claim 1, characterized in that, The ripple current is the output current of the rectifier on the DC bus. i rec With the input current of the inverter i inv The difference; The rectifier output current i rec The input current of the inverter is obtained by measuring the current of the filter inductor of the DC bus. i inv+ It is obtained from the following formula: in, V a , V b , V c This refers to the three-phase output voltage of the inverter connected to the DC bus. i a , i b , i c This refers to the three-phase output current of the inverter connected to the DC bus.

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