A DC component detection circuit for inverter output current

Abstract

The present application provides an inverter output current DC component detection circuit, which relates to the technical field of inverters. The inverter output current DC component detection circuit provided by the embodiment of the present application directly detects the voltage DC component between the midpoints of the inverter bridge arms to determine whether the inverter output current DC component exists and the size of the DC component, thereby eliminating the influence of the zero drift and temperature drift of the Hall current sensor. The present application designs a first differential amplifier circuit with a low-pass filter function to filter the sinusoidal pulse width modulation voltage between the midpoints of the inverter bridge arms into a sine wave, and realizes the function of filtering out the DC component in the sine wave through a coupling capacitor, realizes the impedance matching of the front and rear stages through a first voltage follower and a second voltage follower, and designs a second differential amplifier circuit with a low-pass filter function to make a difference between the original sinusoidal wave signal and the sinusoidal wave signal with the DC component filtered out, thereby eliminating the AC component and retaining only the DC component, thereby improving the accuracy of detection.

Description

Technical Field

[0001] The present application relates to the technical field of inverters, and in particular to a detection circuit for a DC component of an inverter output current. Background Art

[0002] In the field of renewable energy grid-connected power generation technology, inverter circuit topology is commonly used to convert electrical energy from DC to AC, and transmit the inverted AC to the power grid. Since the inverter circuit is controlled by PWM (pulse width modulation), there are inevitably differences between the inverter's drive wave and the switch device, which may cause the inverter's switch tube wave to be asymmetric, resulting in an asymmetric output voltage of the inverter, and then causing the inverter's output current to be asymmetric, with a DC component.

[0003] The injection of DC components into the power grid will cause great harm to power equipment. In the field of new energy grid-connected power generation technology, in order to prevent the DC component of the inverter output current from damaging power equipment, the inverter needs to detect and control the DC component of its output current. After detecting that the DC component of the inverter output current exceeds the standard, the inverter needs to eliminate the DC component of the inverter output current by adjusting the PWM wave. Therefore, it is very important to accurately detect the DC component of the inverter output current.

[0004] At present, in the grid-connected inverter system, the commonly used DC component detection circuit first detects the inverter's AC output current through the current Hall, and then processes the AC current signal output by the Hall to remove the power frequency component to obtain the DC component detection signal of the inverter output current. In actual use, since it is impossible to achieve infinite attenuation of the 50Hz power frequency signal, the actual filter output signal still has a weak 50Hz fluctuation, which affects the actual sampling accuracy. At the same time, since the current Hall sensor itself will also have zero drift and temperature drift, it is easy to cause DC component detection deviation. Utility Model Content

[0005] The purpose of the present application is to provide an inverter output current DC component detection circuit to address the problems in the above-mentioned prior art. The DC component of the inverter output current is reflected by directly detecting the DC component of the voltage between the inverter bridge arms. The solution has the advantages of low cost and high precision, and can completely eliminate the 50Hz power frequency fluctuation in the detection signal. In addition, since the signal output by the Hall current sensor is avoided, the influence of the zero drift and temperature drift of the Hall current sensor is eliminated.

[0006] To achieve the above purpose, the technical solution adopted in the embodiment of the present application is as follows:

[0007] The embodiment of the present application provides a DC component detection circuit for an inverter output current, comprising: a first differential amplifier circuit, a second differential amplifier circuit, an intermediate coupling capacitor circuit, a first voltage follower, and a second voltage follower; the first differential amplifier circuit is arranged between two bridge arms of the inverter, and takes the voltage signals of the two bridge arms of the inverter as input signals respectively, and is used to condition the SVPWM voltage signal between the two bridge arms of the inverter into a sinusoidal voltage signal; the intermediate coupling capacitor circuit is connected in series to the output end of the first differential amplifier circuit and the input end of the first voltage follower, and is used to isolate the DC and pass the AC; the first voltage follower is connected in series between the intermediate coupling capacitor circuit and the in-phase input end of the second differential amplifier circuit, and the second voltage follower is connected in series between the output end of the first differential amplifier circuit signal and the inverting input end of the second differential amplifier circuit, and the first and second voltage followers have the characteristics of high input impedance and low output impedance, and are used to achieve impedance matching of the front and rear stage circuits. The signal input end of the second differential amplifier circuit is connected to the signal output ends of the first and second voltage followers, and is used to eliminate the AC component in the positive standard sinusoidal voltage signal, and only retain the DC component, so as to achieve the detection of the DC component of the inverter current.

[0008] Optionally, the first differential amplifier circuit includes a first resistor, a second resistor, a third resistor, a fourth resistor, a first capacitor, a second capacitor, a first operational amplifier, a first adjustable resistor group and a second adjustable resistor group; the fourth resistor and the second capacitor are connected in parallel to form a first parallel circuit; the third resistor and the first capacitor are connected in parallel to form a second parallel circuit; the inverting input terminal of the first operational amplifier is respectively connected to the second resistor and one end of the first parallel circuit, the other end of the first parallel circuit is connected to the signal output terminal of the first operational amplifier, the non-inverting input terminal of the first operational amplifier is respectively connected to the first resistor and one end of the second parallel circuit, and the other end of the second parallel circuit is grounded; the other end of the second resistor is connected to the midpoint of the first bridge arm of the inverter by connecting the first adjustable resistor group in series; the other end of the first resistor is connected to the midpoint of the second bridge arm of the inverter by connecting the second adjustable resistor group in series.

[0009] Optionally, the first adjustable resistor group and the second adjustable resistor group can be independently selected from a resistor group formed by a single resistor or a plurality of resistors connected in series.

[0010] Optionally, the second differential amplifier circuit includes a fifth resistor, a sixth resistor, a seventh resistor, an eighth resistor, a fourth capacitor, a fifth capacitor and a fourth operational amplifier; the seventh resistor and the fourth capacitor are connected in parallel to form a third parallel circuit; the eighth resistor and the fifth capacitor are connected in parallel to form a fourth parallel circuit; the inverting input terminal of the fourth operational amplifier is respectively connected to the sixth resistor and one end of the fourth parallel circuit, the other end of the fourth parallel circuit is connected to the signal output terminal of the fourth operational amplifier, and the signal output terminal of the fourth operational amplifier outputs a DC signal with the AC component eliminated; the non-inverting input terminal of the fourth operational amplifier is respectively connected to the fifth resistor and one end of the third parallel circuit, the other end of the third parallel circuit is connected to the reference voltage; the other ends of the fifth resistor and the sixth resistor are respectively connected to the output terminals of the first voltage follower and the second voltage follower.

[0011] Optionally, the intermediate-stage coupling capacitor circuit is a third capacitor.

[0012] Optionally, the first voltage follower is the second operational amplifier, and the second voltage follower is the third operational amplifier; the non-inverting input terminal of the second operational amplifier is connected to the intermediate-stage coupling capacitor circuit, the inverting input terminal of the second operational amplifier is connected to the output terminal of the second operational amplifier, and the output terminal of the second operational amplifier is connected to the fifth resistor; the non-inverting input terminal of the third operational amplifier is connected to the output terminal of the first differential amplifier circuit signal, the inverting input terminal of the third operational amplifier is connected to the output terminal of the third operational amplifier, and the output terminal of the third operational amplifier is connected to the sixth resistor.

[0013] 1) The beneficial effects of the present application include: the present application provides an inverter output current DC component detection circuit, which directly detects the voltage DC component between the midpoints of the inverter bridge arm to determine whether the inverter output current DC component exists and the size of the DC component, and can completely eliminate the 50Hz power frequency fluctuation in the detection signal, and because the signal output by the Hall current sensor is avoided, the influence of the zero drift and temperature drift of the Hall current sensor is eliminated;

[0014] 2) The present application filters the sinusoidal pulse width modulation voltage between the midpoints of the inverter bridge arms into a sine wave by designing a first differential amplifier circuit with a low-pass filter function; and eliminates the AC component by designing a second differential amplifier circuit with a low-pass filter function, leaving only the DC component, thereby improving the accuracy of detection;

[0015] 3) The present application can isolate the DC component of a signal input to the second differential amplifier circuit by adding an intermediate-stage coupling capacitor circuit between the two-stage differential amplifier circuits of the detection circuit, thereby differentiating the input signal of the second differential amplifier circuit, which is beneficial to eliminate the AC component in the operation of the second differential circuit and accurately obtain the DC component; at the same time, it ensures that the sine wave input to the second differential amplifier circuit is not attenuated or phase shifted. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] In order to more clearly illustrate the technical solutions of the embodiments of the present application, the drawings required for use in the embodiments will be briefly introduced below. It should be understood that the following drawings only show certain embodiments of the present application and therefore should not be regarded as limiting the scope. For ordinary technicians in this field, other related drawings can be obtained based on these drawings without paying creative work.

[0017] Figure 1 A circuit diagram of a DC component detection circuit for an inverter output current provided in an embodiment of the present application;

[0018] Figure 2 One of the waveform diagrams of a DC component detection circuit of an inverter output current provided in an embodiment of the present application;

[0019] Figure 3 A second waveform diagram of a DC component detection circuit for an inverter output current provided in an embodiment of the present application;

[0020] Figure 4 The third waveform diagram of a DC component detection circuit of an inverter output current provided in an embodiment of the present application;

[0021] Figure 5 A fourth waveform diagram of a DC component detection circuit for an inverter output current provided in an embodiment of the present application;

[0022] Figure 6 A fifth waveform diagram of a DC component detection circuit for an inverter output current provided in an embodiment of the present application;

[0023] Figure 7 This is a sixth waveform diagram of an inverter output current DC component detection circuit provided in an embodiment of the present application.

[0024] Icon: Rx-first adjustable resistor group; Ry-second adjustable resistor group; R1-first resistor; R2-second resistor; R3-third resistor; R4-fourth resistor; R5-fifth resistor; R6-sixth resistor; R7-seventh resistor; R8-eighth resistor; C1-first capacitor; C2-second capacitor; C3-third capacitor; C4-fourth capacitor; C5-fifth capacitor; U1-first operational amplifier; U2-second operational amplifier; U3-third operational amplifier; U4-fourth operational amplifier; Vref-reference voltage; A-midpoint of the first bridge arm; B-midpoint of the second bridge arm; Q1-first switch tube; Q2-second switch tube; Q3-third switch tube; Q4-fourth switch tube. DETAILED DESCRIPTION

[0025] In order to make the purpose, technical solution and advantages of the embodiments of the present application clearer, the technical solution in the embodiments of the present application will be clearly and completely described below in conjunction with the drawings in the embodiments of the present application. Obviously, the described embodiments are part of the embodiments of the present application, rather than all the embodiments. The components of the embodiments of the present application described and shown in the drawings here can be arranged and designed in various different configurations.

[0026] Therefore, the following detailed description of the embodiments of the present application provided in the accompanying drawings is not intended to limit the scope of the present application for protection, but merely represents selected embodiments of the present application. It should be noted that, in the absence of conflict, the various features in the embodiments of the present application can be combined with each other, and the combined embodiments are still within the scope of protection of the present application.

[0027] In the field of renewable energy grid-connected power generation technology, inverter circuit topology is commonly used to convert electrical energy from DC to AC, and transmit the inverted AC to the power grid. Since the inverter circuit is controlled by PWM (pulse width modulation), there are inevitably differences between the inverter's drive wave and the switch device, which may cause the inverter's switch tube wave to be asymmetric, resulting in an asymmetric output voltage of the inverter, and then causing the inverter's output current to be asymmetric, with a DC component.

[0028] The injection of DC components into the power grid will cause great harm to power equipment, mainly manifested in the serious impact on power transformers. Power transformers are key power equipment in the power system. Whether the transformer operates normally will affect power transmission and the quality of power consumption of users. The DC component of the inverter output injected into the power grid will cause DC bias magnetization of the power transformer. The harm of DC bias magnetization is that it causes serious magnetic saturation of the transformer core every half cycle, resulting in serious current distortion, high harmonic content, and increased transformer loss, which in turn causes the power system to fail to work properly.

[0029] In the field of new energy grid-connected power generation technology, in order to prevent the DC component of the inverter output current from damaging power equipment, the inverter needs to detect and control the DC component of its output current. After detecting that the DC component of the inverter output current exceeds the standard, the inverter needs to eliminate the DC component of the inverter output current by adjusting the PWM wave. Therefore, it is very important to accurately detect the DC component of the inverter output current.

[0030] At present, in the grid-connected inverter system, the commonly used DC component detection circuits first detect the inverter's AC output current through the current Hall, and then process the AC current signal output by the Hall to remove the power frequency component to obtain the DC component detection signal of the inverter output current. There are two main types of signal processing circuits: one is to use a low-pass filter to forcibly filter out the 50Hz power frequency component in the detection signal and only retain the DC component; the other is to use a notch filter to perform band-stop filtering on the 50Hz component in the detection signal. Since the inverter output defaults to 50Hz AC, the output signal after filtering by the notch filter is ideally only the DC component. In actual use, since these two types of filters cannot achieve infinite attenuation for the 50Hz power frequency signal, the actual filter output signal still has a weak 50Hz fluctuation, which affects the actual sampling accuracy. At the same time, since the current Hall sensor itself also has zero drift and temperature drift, it is easy to cause DC component detection deviation.

[0031] Aiming at the influence of the zero drift and temperature drift of the Hall current sensor itself on the sampling accuracy of the DC component of the inverter output current, and the low-pass filter and notch filter in the output signal processing circuit of the Hall current sensor cannot infinitely attenuate the 50Hz power frequency component in the detection signal, the influence of the 50Hz power frequency fluctuation in the DC component detection signal on the sampling accuracy of the DC component. The present application proposes a method of directly detecting the DC component of the voltage between the inverter bridge arms to reflect the DC component of the inverter output current. This solution has the advantages of low cost and high accuracy, and can completely eliminate the 50Hz power frequency fluctuation in the detection signal. In addition, by avoiding the use of the signal output by the Hall current sensor, the influence of the zero drift and temperature drift of the Hall current sensor is eliminated.

[0032] In order to solve the above technical problems, the technical solution adopted in this application is: an inverter output current DC component detection circuit, such as Figure 1As shown, it includes: a first differential amplifier circuit, a second differential amplifier circuit, an intermediate coupling capacitor circuit, a first voltage follower and a second voltage follower; the first differential amplifier circuit is arranged between the two bridge arms of the inverter, and takes the voltage signals of the two bridge arms of the inverter as input signals respectively, and is used to condition the SVPWM voltage signal between the two bridge arms of the inverter into a positive standard sinusoidal voltage signal; the intermediate coupling capacitor circuit is connected in series between the output end of the first differential amplifier circuit and the in-phase input end of the first voltage follower, and is used to isolate DC and pass AC; the first voltage follower is connected in series between the intermediate coupling capacitor circuit and the in-phase input end of the second differential amplifier circuit, and the second voltage follower is connected in series between the output end of the first differential amplifier circuit signal and the inverting input end of the second differential amplifier circuit, and is used to achieve impedance matching of the front and rear stages; the signal input end of the second differential amplifier circuit is connected to the signal output ends of the first and second voltage followers, and is used to eliminate the AC component in the positive standard sinusoidal voltage signal, and only retain the DC component, so as to achieve DC component detection.

[0033] Specifically, the first differential amplifier circuit includes a first resistor R1, a second resistor R2, a third resistor R3, a fourth resistor R4, a first capacitor C1, a second capacitor C2, a first operational amplifier U1, a first adjustable resistor group and a second adjustable resistor group; the fourth resistor R4 and the second capacitor C2 are connected in parallel to form a first parallel circuit; the third resistor R3 and the first capacitor C1 are connected in parallel to form a second parallel circuit; the inverting input terminal of the first operational amplifier U1 is respectively connected to the second resistor R2 and one end of the first parallel circuit, the other end of the first parallel circuit is connected to the signal output terminal of the first operational amplifier U1, the non-inverting input terminal of the first operational amplifier C1 is respectively connected to the first resistor R1 and one end of the second parallel circuit, and the other end of the second parallel circuit is grounded; the other end of the second resistor R2 is connected to the midpoint A of the first bridge arm of the inverter by being connected in series with the first adjustable resistor group Rx; the other end of the first resistor R1 is connected to the midpoint B of the second bridge arm of the inverter by being connected in series with the second adjustable resistor group Ry.

[0034] Specifically, the first adjustable resistor group Rx and the second adjustable resistor group Ry can be independently selected from a resistor group formed by a single resistor or a plurality of resistors connected in series.

[0035] Specifically, the second differential amplifier circuit includes a fifth resistor R5, a sixth resistor R6, a seventh resistor R7, an eighth resistor R8, a fourth capacitor C4, a fifth capacitor C5 and a fourth operational amplifier U4; the seventh resistor R7 and the fourth capacitor C4 are connected in parallel to form a third parallel circuit; the eighth resistor R8 and the fifth capacitor C5 are connected in parallel to form a fourth parallel circuit; the inverting input terminal of the fourth operational amplifier U4 is respectively connected to the sixth resistor R6 and one end of the fourth parallel circuit, and the other end of the fourth parallel circuit is connected to the signal output terminal of the fourth operational amplifier U4, and the signal output terminal of the fourth operational amplifier U4 outputs a DC signal with the AC component eliminated; the non-inverting input terminal of the fourth operational amplifier U4 is respectively connected to the fifth resistor R5 and one end of the third parallel circuit, and the other end of the third parallel circuit is connected to the reference voltage Vref; the other ends of the fifth resistor R5 and the sixth resistor R6 are respectively connected to the output ends of the first voltage follower and the second voltage follower.

[0036] Specifically, the intermediate-stage coupling capacitor circuit is a third capacitor C3.

[0037] Specifically, the first voltage follower is the second operational amplifier U2, and the second voltage follower is the third operational amplifier U3; the non-inverting input terminal of the second operational amplifier U2 is connected to the intermediate-stage coupling capacitor circuit, the inverting input terminal of the second operational amplifier U2 is connected to the output terminal of the second operational amplifier U2, and the output terminal of the second operational amplifier U2 is connected to the fifth resistor R5; the non-inverting input terminal of the third operational amplifier U3 is connected to the output terminal of the first differential amplifier circuit signal, the inverting input terminal of the third operational amplifier U3 is connected to the output terminal of the third operational amplifier U3, and the output terminal of the third operational amplifier U3 is connected to the sixth resistor R6.

[0038] Working principle:

[0039] As a key device in the new energy power generation system, the inverter mainly completes the task of converting the direct current generated by the new energy system into alternating current and delivering it to the power grid. The first switch tube Q1, the second switch tube Q2, the third switch tube Q3, and the fourth switch tube Q4 in the inverter are all controlled by SVPWM (space vector pulse width modulation). The voltage between the midpoints of the two bridge arms of the inverter: the midpoint A of the first bridge arm and the midpoint B of the second bridge arm is a SVPWM (sinusoidal pulse width modulation) signal, which is a voltage in both positive and negative directions, and its amplitude is basically equal to the bus voltage output by the photovoltaic panel. Since the voltages at points A and B are PWM waves modulated by sine waves, the signal can be detected and low-pass filtered to be conditioned into a standard sine wave. This part of the operation is implemented by a first differential amplifier with a low-pass filter function composed of a first operational amplifier U1, a first resistor R1, a second resistor R2, a third resistor R3, a fourth resistor R4, a first capacitor C1, and a second capacitor C2. Its input voltage is Vsin (sinusoidal voltage source), and the signal waveform is as follows: Figure 2 shown.

[0040] The third capacitor C3 is connected in series with the output end of the first operational amplifier U1 and the in-phase input end of the second operational amplifier U2. The function of the third capacitor C3 is to isolate the DC component of the AC signal output by the first operational amplifier U1 and only retain the AC component in the signal. The third capacitor C3 is directly connected to the in-phase input end of the second operational amplifier U2. The in-phase input end of the second operational amplifier U2 cannot be connected to any device to avoid phase shift between the in-phase input end signal of the second operational amplifier U2 and the output end signal of the first operational amplifier U1. Since the in-phase input end of the second operational amplifier U2 itself is a megohm impedance, it will not cause phase shift to the signal. The output voltage of the first operational amplifier U1 and the voltage waveform of the in-phase input end of the second operational amplifier U2 are shown in Figure 3.

[0041] The second operational amplifier U2 and the third operational amplifier U3 are used as voltage followers, and their main function is impedance matching. They have the advantages of high input impedance and low output impedance. When connected in series in the signal loop, they can avoid the influence between the previous and next stage circuits and provide detection accuracy.

[0042] The second differential amplifier circuit with low-pass filter function, which is composed of the fourth operational amplifier U4, the fifth resistor R5, the sixth resistor R6, the seventh resistor R7, the eighth resistor R8, the fourth capacitor C4 and the fifth capacitor C5, realizes the function of amplifying and filtering the weak DC component, thereby improving the DC component detection accuracy, and the output voltage is Vdc (DC voltage). The output voltage waveform is as follows: Figure 4 shown. Specific implementation method:

[0044] The embodiment of the present application takes a single-phase photovoltaic grid-connected inverter as an example. The grid voltage is a standard AC 220V, the photovoltaic panel output voltage is a DC 400V, the first adjustable resistor group Rx is 3 510k1206 packaged precision resistors connected in series, and the second adjustable resistor group Ry is 3 510k 1206 packaged precision resistors connected in series. The first resistor R1, the second resistor R2, the third resistor R3, and the fourth resistor R4 are all 30k precision resistors. The first capacitor C1 and the second capacitor C2 are 0.1uF capacitors. The first operational amplifier U1, the second operational amplifier U2, the third operational amplifier U3, and the fourth operational amplifier U4 are all precision operational amplifiers OPA2188. The third capacitor C3 is a 0.1uF capacitor. The fifth resistor R5 and the sixth resistor R6 are 1k precision resistors, the seventh resistor R7 and the eighth resistor R8 are 10k precision resistors, and the fourth capacitor C4 and the fifth capacitor C5 are 1uF capacitors.

[0045] There are three working scenarios in this application embodiment:

[0046] Scenario 1: The inverter output voltage has no DC component. At this time, the wave between the two bridge arms of the inverter is completely symmetrical. After a sine wave cycle, the average voltage between the midpoint A of the first bridge arm and the midpoint B of the second bridge arm of the inverter is zero, and there is no DC component. At this time, the voltage SVPWM signal between the midpoint A of the first bridge arm and the midpoint B of the second bridge arm of the inverter is symmetrical. After attenuation and filtering by the first differential circuit composed of the first adjustable resistor group Rx, the second adjustable resistor group Ry, the first resistor R1, the second resistor R2, the third resistor R3, the fourth resistor R4, the first capacitor C1, the second capacitor C2, and the first operational amplifier U1, as shown in FIG. Figure 5 As shown, the voltage signal output by the first operational amplifier U1 is a standard sine wave signal with a peak value of -6V to 6V. The positive and negative half-waves of the signal are completely symmetrical, and there is no DC component. The sine signal output by the first operational amplifier U1 is input to the in-phase input terminal of the second operational amplifier U2 without attenuation or phase shift after being isolated by the third capacitor C3. The output terminal voltages of the second operational amplifiers U2 and U3 are completely consistent. After passing through the differential amplifier circuit composed of the fifth resistor R5, the sixth resistor R6, the seventh resistor R7, the eighth resistor R8, the fourth capacitor C4, the fifth capacitor C5, and the fourth operational amplifier U4, the output voltage of the fourth operational amplifier U4 is equal to Vref. Since the inverter output current is determined by the inverter output voltage, in this scenario, because the inverter output voltage is completely symmetrical, the inverter output current is also symmetrical, and there is no DC component.

[0047] Scenario 2: The inverter output voltage has a positive DC component. At this time, the wave generation between the two bridge arms of the inverter is asymmetric. After a sine wave cycle, the voltage average value between the midpoints of the two bridge arms of the inverter is positive, and there is a DC component. At this time, the voltage SVPWM signal between the midpoints A and B of the inverter bridge arms is an asymmetric signal. After attenuation and filtering by the differential circuit composed of the first adjustable resistor group Rx, the second adjustable resistor group Ry, the first resistor R1, the second resistor R2, the third resistor R3, the fourth resistor R4, the first capacitor C1, the second capacitor C2, and the first operational amplifier U1, it is assumed that the voltage signal output by the first operational amplifier U1 is a standard sine wave signal with a peak value of -5.9V to 6.1V. The positive and negative half-waves of the signal are asymmetric and there is a DC component. The sine signal with a DC component output by the first operational amplifier U1 is isolated by the third capacitor C3 to generate a pure sine signal without attenuation, phase shift, and DC component, which is input to the non-inverting input terminal of the second operational amplifier U2. The amplitude of the signal is -6V to 6V. The pure sinusoidal signal after the third capacitor C3 isolates the DC and the output signal of the first operational amplifier U1, after passing through the voltage followers formed by the second operational amplifier U2 and the third operational amplifier U3, respectively, are output without phase shift and attenuation. The differential amplifier circuit with a low-pass filter function, which is composed of the fifth resistor R5, the sixth resistor R6, the seventh resistor R7, the eighth resistor R8, the fourth capacitor C4, the fifth capacitor C5, and the fourth operational amplifier U4, differentiates and amplifies the output signals of the second operational amplifier U2 and the third operational amplifier U3, and finally outputs a DC voltage of Vref-1V. The existence of this -1V DC voltage means that there is a DC component in the output voltage of the inverter, which will cause a DC component in the output current of the inverter. The inverter control software needs to appropriately reduce the PWM wave duty cycle of the positive half-cycle bridge arm, so that the positive and negative half-cycle waveforms of the inverter's output voltage are symmetrical and the DC component is eliminated. The waveform is as follows Figure 6 shown.

[0048] Scenario 3: The inverter output voltage has a negative DC component. At this time, the wave generation between the two bridge arms of the inverter is asymmetric. After a sine wave cycle, the voltage average value between the midpoints of the two bridge arms of the inverter is negative, and there is a DC component. At this time, the voltage SVPWM signal between the midpoints A and B of the inverter bridge arms is an asymmetric signal. After attenuation and filtering by the differential circuit composed of the first adjustable resistor group Rx, the second adjustable resistor group Ry, the first resistor R1, the second resistor R2, the third resistor R3, the fourth resistor R4, the first capacitor C1, the second capacitor C2, and the first operational amplifier U1, it is assumed that the voltage signal output by the first operational amplifier U1 is a standard sine wave signal with a peak value of -6.1V to 5.9V. The positive and negative half-waves of the signal are asymmetric and there is a DC component. The sine signal with a DC component output by the first operational amplifier U1 is isolated by the third capacitor C3 to generate a pure sine signal without attenuation, phase shift, and DC component, which is input to the non-inverting input terminal of the second operational amplifier U2. The amplitude of the signal is -6V to 6V. The pure sinusoidal signal after the third capacitor C3 isolates the DC and the output signal of the first operational amplifier U1, after passing through the voltage followers formed by the second operational amplifier U2 and the third operational amplifier U3, respectively, are output without phase shift and attenuation. The differential amplifier circuit with a low-pass filter function formed by the fifth resistor R5, the sixth resistor R6, the seventh resistor R7, the eighth resistor R8, the fourth capacitor C4, the fifth capacitor C5, and the fourth operational amplifier U4, differentiates and amplifies the output signals of the second operational amplifier U2 and the third operational amplifier U3, and finally outputs a DC voltage of Vref+1V. The existence of the +1V DC voltage means that there is a DC component in the output voltage of the inverter, which will cause a DC component in the output current of the inverter. The inverter control software needs to appropriately reduce the PWM wave duty cycle of the negative half-cycle bridge arm, so that the positive and negative half-cycle waveforms of the inverter's output voltage are symmetrical and the DC component is eliminated. The waveform is as follows Figure 7 shown.

[0049] An inverter output current DC component detection circuit provided in an embodiment of the present application determines whether the inverter output current DC component exists and the size of the DC component by detecting the DC component of the voltage between the midpoints of the inverter bridge arms; isolates the DC component by using a coupling third capacitor C3 in the signal conditioning circuit, and ensures that the sine wave has no attenuation and no phase shift; and detects the DC component of the inverter output current by detecting the DC component of the voltage between the midpoints of the inverter bridge arms. The differential amplifier circuit with low-pass filter function formed by capacitor C2 filters the sinusoidal pulse width modulation voltage between the midpoints of the inverter bridge arm into a sine wave; the differential amplifier circuit with low-pass filter function formed by the fourth operational amplifier U4, the fifth resistor R5, the sixth resistor R6, the seventh resistor R7, the eighth resistor R8, the fourth capacitor C4, and the fifth capacitor C5 makes a difference between the sine wave without DC component output by the second operational amplifier U2 and the sine wave with DC component output by the third operational amplifier U3, thereby eliminating the AC component and retaining only the DC component. This application is applicable to all new energy grid-connected power generation equipment, and has the advantages of low cost and high precision. At the same time, this solution can avoid the influence of zero drift and temperature drift of the Hall current sensor in the traditional sampling method, and can completely eliminate the 50Hz power frequency component that is difficult to eliminate in the traditional signal conditioning circuit, avoiding the problem of low detection accuracy of the DC component of the inverter output current in the traditional detection method, and can play a key role in reducing and eliminating the DC component of the inverter output current.

[0050] The above description is only the preferred embodiment of the present application and is not intended to limit the present application. For those skilled in the art, the present application may have various modifications and variations. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present application shall be included in the protection scope of the present application.

Claims

1. An inverter output current DC component detection circuit, characterized in that: include: A first differential amplifier circuit, a second differential amplifier circuit, an intermediate coupling capacitor circuit, a first voltage follower, and a second voltage follower; The first differential amplifier circuit is arranged between the two bridge arms of the inverter, and takes the voltage signals of the two bridge arms of the inverter as input signals respectively, and is used to condition the SVPWM voltage signal between the two bridge arms of the inverter into a sinusoidal voltage signal; The intermediate coupling capacitor circuit is connected in series between the output end of the first differential amplifier circuit and the input end of the first voltage follower, and is used to block direct current and pass alternating current; The first voltage follower is connected in series between the intermediate-stage coupling capacitor circuit and the in-phase input terminal of the second differential amplifier circuit, the second voltage follower is connected in series between the output terminal of the first differential amplifier circuit signal and the inverting input terminal of the second differential amplifier circuit, and the signal input terminal of the second differential amplifier circuit is connected to the signal output terminals of the first and second voltage followers, so as to eliminate the AC component in the sinusoidal voltage signal and retain only the DC component.

2. The inverter output current DC component detection circuit according to claim 1, characterized in that: The first differential amplifier circuit includes a first resistor, a second resistor, a third resistor, a fourth resistor, a first capacitor, a second capacitor, a first operational amplifier, a first adjustable resistor group, and a second adjustable resistor group; The fourth resistor and the second capacitor are connected in parallel to form a first parallel circuit; The third resistor and the first capacitor are connected in parallel to form a second parallel circuit; The inverting input terminal of the first operational amplifier is connected to the second resistor and one end of the first parallel circuit respectively, the other end of the first parallel circuit is connected to the signal output terminal of the first operational amplifier, the non-inverting input terminal of the first operational amplifier is connected to the first resistor and one end of the second parallel circuit respectively, and the other end of the second parallel circuit is grounded; The other end of the second resistor is connected to the midpoint of the first bridge arm of the inverter by being connected in series with the first adjustable resistor group; the other end of the first resistor is connected to the midpoint of the second bridge arm of the inverter by being connected in series with the second adjustable resistor group.

3. The inverter output current DC component detection circuit according to claim 2, characterized in that: The first adjustable resistor group and the second adjustable resistor group can be independently selected from a resistor group formed by a single resistor or a plurality of resistors connected in series.

4. The inverter output current DC component detection circuit according to claim 1, characterized in that: The second differential amplifier circuit includes a fifth resistor, a sixth resistor, a seventh resistor, an eighth resistor, a fourth capacitor, a fifth capacitor and a fourth operational amplifier; The seventh resistor and the fourth capacitor are connected in parallel to form a third parallel circuit; The eighth resistor and the fifth capacitor are connected in parallel to form a fourth parallel circuit; The inverting input terminal of the fourth operational amplifier is connected to the sixth resistor and one end of the fourth parallel circuit respectively, the other end of the fourth parallel circuit is connected to the signal output terminal of the fourth operational amplifier, and the signal output terminal of the fourth operational amplifier outputs a DC signal with the AC component eliminated; the non-inverting input terminal of the fourth operational amplifier is connected to the fifth resistor and one end of the third parallel circuit respectively, and the other end of the third parallel circuit is connected to a reference voltage; The other ends of the fifth resistor and the sixth resistor are connected to the output ends of the first voltage follower and the second voltage follower, respectively.

5. The inverter output current DC component detection circuit according to claim 1, characterized in that: The intermediate-stage coupling capacitor circuit is a third capacitor.

6. The inverter output current DC component detection circuit according to claim 4, characterized in that: The first voltage follower is a second operational amplifier, and the second voltage follower is a third operational amplifier; The non-inverting input terminal of the second operational amplifier is connected to the intermediate coupling capacitor circuit, the inverting input terminal of the second operational amplifier is connected to the output terminal of the second operational amplifier, and the output terminal of the second operational amplifier is connected to the fifth resistor; The non-inverting input terminal of the third operational amplifier is connected to the output terminal of the first differential amplifier circuit signal, the inverting input terminal of the third operational amplifier is connected to the output terminal of the third operational amplifier, and the output terminal of the third operational amplifier is connected to the sixth resistor.

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