An AC current sampling system for inverter

Abstract

The present invention discloses an AC current sampling system for an inverter, wherein the AC current sampling system includes an AC current sampling circuit, which includes a current sampling unit, a modulation unit, a filtering unit, and a calculation unit. The current sampling unit is connected to the middle and lower switching tubes of the inverter bridge arm and is used to sample the inverter bridge arm current signal; the modulation unit is connected to the output end of the current sampling unit and is used to perform high-speed modulation on the inverter bridge arm current signal to obtain the inverter bridge arm high-speed current data stream; the filtering unit is connected to the output end of the modulation unit and is used to perform digital extraction and filtering on the inverter bridge arm high-speed current data stream to obtain the average value of the inverter bridge arm current within the switching cycle; the calculation unit is connected to the output end of the filtering unit and is used to divide the average value of the inverter bridge arm current by the duty cycle of the corresponding middle and lower switching tubes of the bridge arm to obtain the sampling value of the inverter bridge arm AC current.

Description

Technical Field

[0001] The present invention relates to the technical field of inverters, and in particular to an AC current sampling system and method for an inverter, and an inverter. Background Art

[0002] An existing three-phase inverter consists of a capacitor C, switches Q1-Q6, and an inductor Lf. Switches Q1-Q2, Q3-Q4, and Q5-Q6 form three bridge arms, respectively. The left side of the three-phase inverter inputs DC and the right side outputs AC. Controlling a three-phase inverter generally requires sampling the output AC current. Existing AC current sampling methods for three-phase inverters typically use isolated Hall effect sensors. However, Hall effect sensors are bulky, costly, and have a narrow bandwidth.

[0003] The above problems can be avoided by using a series resistor to sample the AC current. However, the signal obtained by the sampling resistor needs to be electrically isolated before it can be used by the controller, which also increases the cost.

[0004] Another AC sampling method uses a sampling resistor to sample the bridge arm current of the three-phase inverter and infer the AC current. The reference ground of the sampled signal is the DC negative terminal, which can be shared with the controller without the need for additional electrical isolation.

[0005] During normal operation, the upper and lower switches in one arm of a three-phase inverter alternately turn on. The lower arm current is identical to the AC current only when the lower switch is on. Therefore, sampling the arm current during this period is necessary to obtain the actual AC current. However, during normal operation, the lower switch's on-time varies. In some periods, the lower switch's on-time is extremely short, leaving little time for sampling. Furthermore, these periods are subject to significant interference, severely impacting current sampling accuracy. To improve sampling accuracy, the sampling speed can be increased, but chips with high sampling speeds are expensive. Summary of the Invention

[0006] In view of this, the present invention proposes an AC current sampling system, method and inverter for an inverter. First, the bridge arm current data stream within the switching cycle is sampled and obtained, and the average bridge arm current within the switching cycle is obtained through digital extraction and filtering. The average bridge arm current is divided by the duty cycle of the switching tube to obtain the AC current. No controller is required, the cost is low, and the sampling accuracy is high.

[0007] A first aspect of the present invention provides an AC current sampling system for an inverter, the system comprising an AC current sampling circuit, the AC current sampling circuit comprising:

[0008] A current sampling unit, connected to the lower switch tube in the bridge arm of the inverter, and used to sample the current signal of the bridge arm of the inverter;

[0009] a modulation unit connected to the output end of the current sampling unit, configured to receive the bridge arm current signal of the inverter sampled by the current sampling unit, and perform high-speed modulation on the bridge arm current signal of the inverter to obtain a high-speed current data stream of the bridge arm of the inverter;

[0010] a filtering unit connected to the output end of the modulation unit, and configured to perform digital extraction and filtering on the high-speed current data stream of the bridge arm of the inverter output by the modulation unit to obtain an average value of the bridge arm current of the inverter within a switching cycle;

[0011] The calculation unit is connected to the output end of the filtering unit and is used to divide the average value of the bridge arm current of the inverter by the duty cycle of the lower switch tube in the corresponding bridge arm to obtain a sampling value of the bridge arm AC current of the inverter.

[0012] Furthermore, the current sampling unit includes a sampling resistor and an amplifier. The sampling resistor is connected to the lower switch tube in the bridge arm of the inverter and is used to convert the bridge arm current signal of the inverter into a voltage signal; the positive input end of the amplifier is connected to the sampling resistor, and the output end of the amplifier is connected to the modulation unit, which is used to obtain the voltage signal at both ends of the sampling resistor and amplify it to obtain the bridge arm current sampling signal of the inverter and transmit it to the modulation unit.

[0013] Furthermore, the modulation unit is a sigma-delta modulator.

[0014] Furthermore, the filtering unit is a digital decimation filter.

[0015] Furthermore, the inverter is a three-phase inverter, and the number of the AC current sampling circuits is three.

[0016] A second aspect of the present invention provides an AC current sampling method for an inverter, the method comprising: sampling the current signal of each phase bridge arm of the inverter; receiving the sampled current signal of each phase bridge arm of the inverter, performing high-speed modulation on the current signal of each phase bridge arm of the inverter to obtain a high-speed current data stream of each phase bridge arm of the inverter; performing digital extraction and filtering on the high-speed current data stream of each phase bridge arm of the inverter to obtain an average current value of each phase bridge arm of the inverter during a switching cycle; and calculating a sampled value of the AC current of each phase bridge arm of the inverter based on the average current value of each phase bridge arm of the inverter and the duty cycle of the lower switch tube in the corresponding bridge arm.

[0017] Furthermore, the method for digitally extracting and filtering the high-speed current data stream of each phase bridge arm of the inverter includes: setting the digital extraction frequency to the switching frequency; based on the switching frequency, digitally extracting and filtering the high-speed current data stream of each phase bridge arm of the inverter to obtain the average current value of each phase bridge arm of the inverter during the switching cycle.

[0018] Furthermore, the calculation method of the sampling value of the AC current of each phase bridge arm of the inverter is: dividing the average value of the current of each phase bridge arm of the inverter by the duty cycle of the lower switch tube in the corresponding bridge arm to obtain the sampling value of the AC current of each phase bridge arm of the inverter.

[0019] A third aspect of the present invention provides an inverter, which includes a three-phase inverter and three AC current sampling circuits as described above, respectively connected to each phase bridge arm of the three-phase inverter, each of the AC current sampling circuits being used to collect the AC current of each phase bridge arm of the three-phase inverter.

[0020] The above-mentioned inverter AC current sampling system and method first obtains the bridge arm current data stream within the switching cycle through sampling by the current sampling unit and sigma-delta modulation by the modulation unit, and then digitally extracts and filters the bridge arm current data stream through the filtering unit to obtain the average value of the bridge arm current; finally, the calculation unit divides the average value of the bridge arm current by the duty cycle of the lower switch tube in the bridge arm to obtain the sampling value of the inverter AC current, which has high sampling efficiency. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] For purposes of illustration and not limitation, the present invention will now be described with reference to preferred embodiments thereof, particularly with reference to the accompanying drawings, in which:

[0022] Figure 1 It is the circuit structure diagram of the three-phase inverter;

[0023] Figure 2 This is the circuit structure diagram of the bridge arm current sampling method;

[0024] Figure 3 is a circuit diagram of an AC current sampling circuit provided by an embodiment of the present invention;

[0025] Figure 4 It is a schematic diagram of the relationship between the bridge arm current and the AC current;

[0026] Figure 5 It is a schematic diagram of a 1-bit high-speed digital data stream obtained by Sigma-Delta modulation;

[0027] Figure 6 It is the waveform of AC current and the waveform obtained by sampling;

[0028] Figure 7is a flow chart of an AC current sampling method for an inverter provided by another embodiment of the present invention;

[0029] Figure 8 It is a structural diagram of an inverter provided by another embodiment of the present invention. DETAILED DESCRIPTION

[0030] In order to more clearly understand the above-mentioned objects, features and advantages of the present invention, the present invention is described in detail below with reference to the accompanying drawings and specific embodiments. It should be noted that the embodiments of the present invention and the features therein can be combined with each other without conflict.

[0031] The following description sets forth numerous specific details to facilitate a thorough understanding of the present invention. The embodiments described are merely some, not all, of the present invention. All other embodiments derived by persons of ordinary skill in the art based on the embodiments of the present invention without inventive effort are intended to fall within the scope of protection of the present invention.

[0032] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as those commonly understood by those skilled in the art of the present invention. The terms used in this specification of the present invention are only for the purpose of describing specific embodiments and are not intended to limit the present invention.

[0033] Figure 1 This is a schematic diagram of the structure of an existing three-phase inverter. Figure 1 As shown, the three-phase inverter includes a capacitor C, switch tubes Q1-Q6, and three inductors Lf. Among them, the switch tubes Q1-Q2 form a first bridge arm, whose middle node is connected to the grid side through an inductor Lf, the switch tubes Q3-Q4 form a second bridge arm, whose middle node is connected to the grid side through another inductor Lf, and the switch tubes Q5-Q6 form a third bridge arm, whose middle node is connected to the grid side through another inductor Lf.

[0034] Figure 2 This is a schematic diagram of AC current sampling of an existing inverter. Figure 2As shown, three sampling resistors are used to sample the currents in the first, second, and third bridge arms, respectively, and infer the AC current. The reference ground for the sampled signal is the negative DC terminal, which can be shared with the controller without the need for additional electrical isolation. However, during normal operation, the upper and lower switches in any bridge arm are turned on alternately. The lower arm current is the same as the AC current only when the lower switch is on. Therefore, the bridge arm current must be sampled when the lower switch is on to obtain the actual AC current. However, during normal operation, the on-time of the lower switch varies continuously. In some periods, the on-time of the lower switch is very short, leaving little time for sampling. Furthermore, interference during these periods is relatively high, severely impacting the accuracy of current sampling. To improve sampling accuracy, the sampling speed can be increased, but chips with high sampling speeds are expensive.

[0035] In order to address the deficiencies of the above-mentioned prior art, an embodiment of the present invention provides an AC current sampling system for an inverter. The system first obtains a bridge arm current data stream within a switching cycle through sampling by a current sampling unit and sigma-delta modulation by a modulation unit. The bridge arm current data stream is then digitally extracted and filtered by a filtering unit to obtain an average bridge arm current value. Finally, a calculation unit divides the average bridge arm current value by the duty cycle of the lower switch tube in the bridge arm to obtain the AC current of the inverter.

[0036] Taking the inverter as a three-phase inverter as an example, Figure 3 FIG. 1 is a schematic diagram of the structure of an AC current sampling system for an inverter provided by an embodiment of the present invention. Figure 3As shown, the AC current sampling system for the inverter includes three AC current sampling circuits 10, each connected to the three-phase bridge arm of the inverter. Each AC current sampling circuit 10 includes a current sampling unit 11, a modulation unit 12, a filtering unit 13, and a calculation unit 14. The sampling terminal of the current sampling unit 11 is connected to the lower switching transistor in a single-phase bridge arm of the inverter, and the signal output terminal of the current sampling unit 11 is connected to the input terminal of the modulation unit 12. The current sampling unit 11 is used to sample the single-phase bridge arm current signal il of the inverter and transmit it to the modulation unit 12. The input terminal of the modulation unit 12 is connected to the signal output terminal of the current sampling unit 11, and the output terminal of the modulation unit 12 is connected to the input terminal of the filtering unit 13. The modulation unit 12 is used to perform high-speed modulation on the sampled single-phase bridge arm current signal il of the inverter to obtain a high-speed current data stream is1 of the single-phase bridge arm of the inverter, and transmit it to the filtering unit. The input terminal of the filtering unit 13 is connected to the output terminal of the modulation unit 12, and the output terminal of the filtering unit 13 is connected to the calculation unit 14. Filtering unit 13 is configured to digitally extract and filter the high-speed current data stream is1 of the inverter's single-phase bridge arm based on the set switching frequency to obtain an average value is2 of the inverter's single-phase bridge arm current during the switching cycle. Calculation unit 14 is configured to divide the average value is2 of the inverter's single-phase bridge arm current by the duty cycle of the lower switching transistor in that bridge arm to obtain a sampled value is of the inverter's single-phase bridge arm AC current iac.

[0037] In this embodiment, the current sampling unit 11 includes a sampling resistor R and an amplifier 15. One end of the sampling resistor R is connected to the lower switching transistor (Q2 / Q4 / Q6) in a single-phase bridge arm of the inverter, and the other end of the sampling resistor R is grounded. The positive input of the amplifier 15 is connected to one end of the sampling resistor R, the negative input of the amplifier 15 is grounded, and the output of the amplifier 15 is connected to the input of the modulation unit 12. The sampling resistor R is used to convert the single-phase bridge arm current signal of the inverter into a voltage signal. The amplifier 15 is used to obtain and amplify the voltage signal across the sampling resistor R to obtain the single-phase bridge arm current sampling signal i1 of the inverter, and transmit it to the modulation unit 12.

[0038] In this embodiment, the modulation unit 12 may adopt a sigma-delta modulator to perform sigma-delta high-speed modulation on the sampled single-phase bridge arm current signal il of the inverter through the sigma-delta modulator to obtain the single-phase bridge arm high-speed current data stream is1 of the inverter.

[0039] In this embodiment, the filtering unit 13 adopts a digital extraction filter, sets the extraction frequency of the digital extraction filter to the switching frequency, and digitally extracts and filters the high-speed current data stream is1 of the single-phase bridge arm of the inverter through the digital extraction filter to obtain the average value is2 of the single-phase bridge arm current of the inverter during the switching cycle.

[0040] The AC current sampling system of the inverter described above first samples the bridge arm current signal through a current sampling unit, performs sigma-delta modulation on the bridge arm current signal through a filtering unit to obtain a bridge arm current data stream within a switching cycle, and then digitally extracts and filters the bridge arm current data stream through the filtering unit to obtain an average value of the bridge arm current; finally, the calculation unit divides the average value of the bridge arm current by the duty cycle of the lower and middle switching tubes in the bridge arm to obtain a sampling value of the inverter's bridge arm AC current, thereby obtaining the inverter's AC current. This system does not require a controller, has low cost, and has high sampling accuracy.

[0041] right Figure 3 The circuit shown is simulated and analyzed using a sine wave modulation. Figure 4 The figure shows the relationship between the bridge arm current and the AC current. The blue color is the AC current waveform, and the red color is the current waveform of the bridge arm resistance. The two waveforms can overlap for a period of time in each cycle, and the overlapping time is exactly the conduction time of the lower switch tube (Q2).

[0042] Figure 5 Shown is the 1-bit data stream generated by sigma-delta modulation. Figure 6 It is the waveform of AC current and the waveform obtained by sampling.

[0043] Figure 7 FIG is a flow chart of an AC current sampling method for an inverter provided by another embodiment of the present invention. Figure 7 , the AC current sampling method of the inverter includes the following steps:

[0044] S100, sampling the current signal il of each phase bridge arm of the inverter.

[0045] In this embodiment, a sampling resistor R is connected in series with the lower switching transistor in the inverter bridge arm to convert the inverter bridge arm current signal into a voltage signal. The voltage signal across the sampling resistor R is obtained and amplified by an amplifier to obtain the inverter bridge arm current sampling signal i1.

[0046] S200 , performing high-speed modulation on the sampled current signal il of each phase bridge arm of the inverter to obtain a high-speed current data stream is1 of each phase bridge arm of the inverter.

[0047] In this embodiment, a sigma-delta high-speed modulation is performed on the sampled bridge arm current signal il of the inverter by a sigma-delta modulator to obtain a high-speed current data stream is1 of the bridge arm of the inverter.

[0048] S300 , setting an extraction frequency, performing digital extraction and filtering on the high-speed current data stream is1 of each phase bridge arm of the inverter, and obtaining an average current is2 of each phase bridge arm of the inverter during a switching cycle.

[0049] In this embodiment, a digital decimation filter is used, and the decimation frequency of the digital decimation filter is set to the switching frequency. The digital decimation filter is used to digitally extract and filter the high-speed current data stream is1 of the inverter bridge arm to obtain the average value is2 of the inverter bridge arm current during the switching cycle.

[0050] S400 , dividing the average current value is2 of each phase bridge arm of the inverter by the duty cycle of the lower switch tube in the bridge arm to obtain a sampling value is of the AC current iac of each phase bridge arm of the inverter.

[0051] The AC current sampling method for the inverter described above first samples the current signal of each phase bridge arm, then performs sigma-delta modulation on each bridge arm current signal to obtain a current data stream of each phase bridge arm within a switching cycle, and then performs digital extraction and filtering on each phase bridge arm current data stream to obtain an average bridge arm current value; finally, the average bridge arm current value is divided by the duty cycle of the lower and middle switching tubes in the bridge arm to obtain a sampling value of the AC current of each phase bridge arm of the inverter, thereby obtaining the AC current of the inverter.

[0052] Figure 8 FIG1 is a schematic diagram of a three-phase inverter provided by another embodiment of the present invention. In the three-phase inverter, an AC current sampling circuit 10 is connected to each bridge arm of the three-phase transformer. The AC current sampling circuit 10 collects the AC current of each bridge arm of the three-phase transformer, thereby obtaining the AC current of the three-phase inverter.

[0053] The above specific embodiments do not limit the scope of protection of the present invention. Those skilled in the art will appreciate that various modifications, combinations, sub-combinations, and substitutions may occur depending on design requirements and other factors. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention are intended to be included within the scope of protection of the present invention.

Claims

1. An AC current sampling system for an inverter, characterized in that: An AC current sampling circuit is included, and the AC current sampling circuit includes: A current sampling unit, connected to the lower switch tube in the bridge arm of the inverter, and used to sample the current signal of the bridge arm of the inverter; a modulation unit connected to the output end of the current sampling unit, configured to receive the bridge arm current signal of the inverter sampled by the current sampling unit, and perform high-speed modulation on the bridge arm current signal of the inverter to obtain a high-speed current data stream of the bridge arm of the inverter; a filtering unit connected to the output end of the modulation unit, and configured to perform digital extraction and filtering on the high-speed current data stream of the bridge arm of the inverter output by the modulation unit to obtain an average value of the bridge arm current of the inverter within a switching cycle; The calculation unit is connected to the output end of the filtering unit and is used to divide the average value of the bridge arm current of the inverter by the duty cycle of the lower switch tube in the corresponding bridge arm to obtain a sampling value of the bridge arm AC current of the inverter.

2. The AC current sampling system for an inverter according to claim 1, characterized in that: The current sampling unit includes a sampling resistor and an amplifier. The sampling resistor is connected to the lower switch tube in the bridge arm of the inverter and is used to convert the bridge arm current signal of the inverter into a voltage signal. The positive input end of the amplifier is connected to the sampling resistor, and the output end of the amplifier is connected to the modulation unit. The voltage signal across the sampling resistor is amplified to obtain the bridge arm current sampling signal of the inverter and transmit it to the modulation unit.

3. The AC current sampling system for an inverter according to claim 1, characterized in that: The modulation unit is a sigma-delta modulator.

4. The AC current sampling system for an inverter according to claim 1, characterized in that: The filtering unit is a digital decimation filter.

5. The AC current sampling system for an inverter according to claim 1, characterized in that: The inverter is a three-phase inverter, and the number of the AC current sampling circuits is three.

6. A method for sampling AC current of an inverter, characterized in that: include: Sampling a current signal of each phase bridge arm of the inverter; receiving a sampled current signal of each phase bridge arm of the inverter, performing high-speed modulation on the current signal of each phase bridge arm of the inverter, and obtaining a high-speed current data stream of each phase bridge arm of the inverter; Performing digital extraction and filtering on the high-speed current data stream of each phase bridge arm of the inverter to obtain an average current value of each phase bridge arm of the inverter during a switching cycle; Calculating a sampling value of the alternating current of each phase bridge arm of the inverter according to the average current value of each phase bridge arm of the inverter and the duty cycle of the lower switch tube in the corresponding bridge arm; The calculation method of the sampling value of the AC current of each phase bridge arm of the inverter is: The average current value of each phase bridge arm of the inverter is divided by the duty cycle of the lower switch tube in the corresponding bridge arm to obtain a sampling value of the AC current of each phase bridge arm of the inverter.

7. The AC current sampling method for an inverter according to claim 6, characterized in that: The method for digitally extracting and filtering the high-speed current data stream of each phase bridge arm of the inverter includes: Set the digital decimation frequency to the switching frequency; Based on the switching frequency, digital extraction and filtering are performed on the high-speed current data stream of each phase bridge arm of the inverter to obtain an average current value of each phase bridge arm of the inverter during the switching cycle.

8. An inverter, characterized in that: The invention comprises a three-phase inverter and three AC current sampling circuits according to any one of claims 1 to 5 respectively connected to each phase bridge arm of the three-phase inverter, wherein each AC current sampling circuit is used to collect the AC current of each phase bridge arm of the three-phase inverter.

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