A three-phase grid-connected inverter online insulation detection method and detection circuit

Abstract

The present invention provides an online insulation detection method for a three-phase grid-connected inverter, comprising: a step S1 of obtaining a voltage between the earth and a DC input, and obtaining an initial leakage current ix and an initial bus voltage; a step S2 of obtaining a current bus voltage according to a change in the bus voltage; a step S3 of configuring a voltage increment and a current leakage current iy according to the current bus voltage and the DC voltage; a step S4 of configuring a current insulation resistance according to the voltage increment, the initial leakage current and the current leakage current iy, by giving a voltage increment to a DC input terminal, obtaining a current increment through a leakage current sensor, and calculating the total equivalent impedance of the current DC input to the earth through the voltage increment and the current increment, so as to realize online detection of the insulation performance of the DC side when a non-isolated grid-connected inverter grounded on the AC side is connected to the grid, thereby being able to timely discover a decreasing trend of the insulation impedance of the DC input terminal.

Description

Technical Field

[0001] The invention belongs to the technical field of inverter insulation detection, and in particular relates to an online insulation detection method and a detection circuit for a three-phase grid-connected inverter. Background Art

[0002] On the one hand, when the output of a non-isolated photovoltaic grid-connected inverter is grounded, the input cannot be electrically connected to the earth to avoid the inverter from being damaged due to loss of control; on the other hand, after the photovoltaic array converges, the higher current and voltage will threaten the life safety of engineers. At the same time, the convergence of high-voltage currents is prone to static electricity, which also needs to be grounded to release it to avoid arc sparks and fire. However, in most industrial, commercial and civil power systems, the midpoint (neutral line) of the power transformer is connected to the earth, so when the grid-connected inverter is connected to the grid, the inverter is electrically connected to the earth. In this way, the insulation between the input end of the inverter and the earth is extremely important.

[0003] The usual practice is to perform insulation testing on the inverter before it is connected to the grid. This testing method is usually in offline mode. Obviously, offline testing must have its limitations. Considering the risk of electric shock caused by the degradation of the insulation performance of solar panels and the protection of photovoltaic inverters, one easy-to-think-of scenario is to set the inverter to be unable to connect to the grid when the insulation impedance of the motor to the ground of the input solar panel string of the inverter is low. The existing offline insulation detection method is difficult to perform real-time insulation detection on the input end when the inverter is connected to the grid. Since the inverter and the ground are electrically connected at this time, it can be imagined that the offline insulation detection method is difficult to respond to the decrease in the equivalent impedance of the inverter to the ground in the grid-connected state.

[0004] For example, Figure 1 A common insulation detection method using the bridge balance principle in the prior art is shown. Figure 1 , when relays RL1, RL2, and RL3 are disconnected, switches S1 and S2 are closed respectively, and the specific equivalent resistance values ​​of Rx and Ry can be calculated by measuring the voltage changes between the lines. That is, the offline insulation detection of the inverter input terminal is completed in the offline state. When relays RL1, RL2, and RL3 are closed, that is, when the inverter is connected to the grid, according to the three-phase voltage vector principle, under ideal conditions, the earth presents a fixed DC voltage to the DC input terminal of the inverter. At this time, if switches S1 and S2 are closed to connect R1 and R2, the voltage of the earth to the DC input terminal of the inverter cannot be changed. Therefore, the equivalent resistance of the DC input terminal of the inverter to the earth in the grid-connected mode cannot be obtained.

[0005] In view of this, the existing technology should be improved to solve the technical problem that insulation detection in the inverter grid-connected mode under the existing technology is difficult to achieve. Summary of the invention

[0006] The technical problem to be solved by the present invention is to overcome the deficiencies of the prior art and provide a three-phase grid-connected inverter online insulation detection method and detection circuit that can perform real-time online detection of the insulation impedance on the DC side when a non-isolated grid-connected inverter with the AC side grounded is in a grid-connected working state.

[0007] To solve the above technical problems, the present invention adopts an online insulation detection method for a three-phase grid-connected inverter, which includes the following steps: step S1 of obtaining the voltage between the earth and the DC input, and obtaining the initial leakage current ix and the initial bus voltage; step S2 of obtaining the current bus voltage according to the change of the bus voltage; step S3 of configuring to obtain the voltage increment and the current leakage current iy according to the current bus voltage and the DC voltage; step S4 of configuring to obtain the current insulation resistance according to the voltage increment, the initial leakage current and the current leakage current iy.

[0008] Preferably, in the step S1, it further includes a step S11 of averaging the voltage between the earth and the DC input and averaging the leakage current.

[0009] Preferably, in the step S2, the step of obtaining the current bus voltage according to the change of the bus voltage includes: a step S21 of configuring a boost chopper circuit and a duty cycle signal in the boost chopper circuit; and a step S22 of configuring to increase or decrease the duty cycle signal to change the initial bus voltage and obtain the current bus voltage.

[0010] Further preferably, in the step S2, the voltage between the earth and the DC input is set to be half of the bus voltage.

[0011] Still further preferably, in step S3, the voltage increment ΔV satisfies:

[0012] Voltage increment △V = 1 / 2 current bus voltage - 1 / 2 initial bus voltage;

[0013] The current increment △i satisfies:

[0014] Current increment △i = initial leakage current ix-current leakage current iy.

[0015] Still further preferably, in step S4, Rx is set as the equivalent resistance value between the DC input positive electrode and the earth, and Ry is set as the equivalent resistance value between the DC input negative electrode and the earth, then the insulation resistance satisfies: insulation resistance Rx / / Ry=voltage increment / current increment.

[0016] More preferably, in step S4, when Rx and Ry are both infinite, the leakage current is 0; when Rx is infinite and Ry becomes smaller, the leakage current increases in the positive direction; when Ry is infinite and Rx becomes smaller, the leakage current increases in the negative direction.

[0017] Correspondingly, the present invention also provides an insulation detection circuit of a three-phase grid-connected inverter based on the above-mentioned online insulation detection method of the three-phase grid-connected inverter, the detection circuit comprising: a DC bus, which is connected to the two poles of the DC input terminal; an inverter circuit composed of three bridge arms in parallel, each of which is composed of two switch tubes in series, and each of which is respectively connected to one phase of the grounded three-phase AC side; a first inductor, a second inductor and a relay are connected to the line connected to any phase of the three-phase AC side, and each of which is respectively connected to a compensation capacitor and then connected to the midpoint potential of the DC bus, wherein the DC input terminal and the DC bus include a first insulation resistor connecting the positive pole of the DC input terminal and the earth, a second insulation resistor connecting the negative pole of the DC input terminal and the earth, and a duty cycle signal switch, by controlling the timing and on-off of the duty cycle signal switch, and the leakage current sensor on the three-phase AC side to obtain the current voltage value and leakage current of the DC bus.

[0018] As a further preferred solution, the switch tube is a metal oxide semi-conductor field effect transistor or a turn-off thyristor.

[0019] Due to the adoption of the above technical solution, the present invention has the following beneficial technical effects compared with the prior art:

[0020] By increasing the duty cycle signal control at the DC input end, a voltage increment is given between the positive or negative pole of the DC input end and the earth, and a leakage current sensor is set on the AC side. The initial leakage current and the real-time leakage current are compared by the leakage current sensor to obtain the current increment, and finally the total equivalent impedance of the current DC input to the earth is calculated by the voltage increment and the current increment. In this way, when a non-isolated grid-connected inverter grounded on the AC side is connected to the grid and works, the insulation performance of the DC side is detected online, so that the trend of the insulation impedance decreasing at the DC input end can be discovered in time, and further failures caused by the continued operation of the inverter can be avoided. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Figure 1 Schematic diagram showing an equivalent circuit structure of insulation detection using the bridge balance principle in the prior art;

[0022] Figure 2 is a flow chart showing the process of the online insulation detection method of the three-phase grid-connected inverter described in a preferred embodiment of the present invention;

[0023] Figure 3 Schematic diagram showing the equivalent circuit structure of the online insulation detection circuit of the three-phase grid-connected inverter described in a preferred embodiment of the present invention. DETAILED DESCRIPTION

[0024] The following will describe an embodiment of an online insulation detection method and detection circuit for a three-phase grid-connected inverter according to the present invention with reference to the accompanying drawings. It will be appreciated by those skilled in the art that the described embodiments may be modified in various ways without departing from the spirit and scope of the present invention. Therefore, the drawings and descriptions are illustrative in nature and are not intended to limit the scope of protection of the claims. In addition, in this specification, the drawings are not drawn to scale and the same reference numerals represent the same parts.

[0025] It should be noted that the expressions “first” and “second” used in the embodiments of the present invention are intended to distinguish two non-identical entities with the same name or non-identical parameters. It can be seen that “first” and “second” are only for the convenience of expression and should not be understood as limitations on the embodiments of the invention. The subsequent embodiments will not explain this one by one.

[0026] The preferred embodiment of the present invention is to increase the voltage increment on the insulation impedance of the positive pole of the DC input terminal to the ground and the negative pole of the DC input terminal to the ground, so as to increase the leakage current on the AC side, and thus calculate the change trend of the equivalent insulation impedance of the positive pole or negative pole of the current input terminal to the ground according to the voltage increment and the leakage current increment. One way to increase the voltage increment on the insulation impedance is to add a boost chopper circuit and a duty cycle control signal between the DC input terminal and the DC bus, and change the voltage value of the DC bus by controlling the duty cycle signal. Then, according to the three-phase voltage vector principle, the change in the DC bus voltage value also causes the voltage value of the positive pole or negative pole of the DC input terminal to the ground to change accordingly, that is, it can be regarded as adding a voltage increment on the positive pole of the DC input terminal to the ground, or on the negative pole of the DC input terminal to the ground. Compared with the active way of increasing the voltage increment on the insulation impedance through the duty cycle control signal, another passive way of changing the voltage on the insulation impedance is, for example, the change of the maximum power point tracking solar controller (MPPT) caused by cloud cover, so that the voltage on the insulation impedance changes.

[0027] For the above two methods, although the generation method and reason of the voltage increment on the insulation impedance are different, the generated result and the processing method are consistent. In a preferred embodiment of the present invention, the active method of configuring the duty cycle control signal is taken as an example to illustrate the preferred embodiment of the present invention.

[0028] Figure 2FIG. 2 is a flow chart showing the process of the online insulation detection method of the three-phase grid-connected inverter described in a preferred embodiment of the present invention. Figure 2 As shown, the method for online insulation detection of a three-phase grid-connected inverter described in this preferred embodiment of the present invention includes the following steps: step S1 of obtaining the voltage between the earth and the DC input, and obtaining the initial leakage current ix and the initial bus voltage; step S2 of configuring a boost chopper circuit and a duty cycle signal in the boost chopper circuit; step S3 of configuring to increase or decrease the duty cycle signal to change the initial bus voltage and obtain the current bus voltage; step S4 of configuring to obtain a voltage increment and a current leakage current iy according to the current bus voltage and the DC voltage; step S5 of configuring to obtain a current insulation resistance according to the voltage increment, the initial leakage current and the current leakage current iy.

[0029] Specifically, Figure 3 FIG. 2 is a schematic diagram showing an equivalent circuit structure of an online insulation detection circuit of a three-phase grid-connected inverter according to a preferred embodiment of the present invention. Figure 3 The detection circuit structure includes a DC side and an AC side. The DC side includes a DC bus 11 connected to the two poles of the DC input terminal 10 and having a midpoint potential. A boost chopper circuit 12 and a duty cycle signal (PWM) are configured between the DC bus 11 and the DC input terminal 10.

[0030] A first insulation resistor Rx connecting the positive electrode of the DC input terminal 10 and the ground and a second insulation resistor Ry connecting the negative electrode of the DC input terminal 10 and the ground are included between the DC input terminal 10 and the DC bus 11 .

[0031] The inverter circuit on the DC side is composed of three bridge arms in parallel, and each bridge arm includes two switching tubes connected in series. The switching tube in the preferred embodiment of the present invention can be a metal oxide semi-field effect transistor, or a turn-off thyristor. The gate of the switching tube is turned on when a positive pulse is added, and turned off when a negative pulse is added. Correspondingly, the on and off of each switching tube can also be controlled by a duty cycle signal to achieve bidirectional energy transfer in different timing conditions. Each bridge arm is respectively connected to one phase of the grounded three-phase AC side. The line connecting the bridge arm to any phase of the three-phase AC side includes a first inductor 13, a second inductor 14 and relays (RL1, RL2, RL3). Continue to refer to Figure 3 Each bridge arm is also connected to a compensation capacitor (Ca, Cb, Cc) and then connected to the midpoint potential position of the DC bus 11. A leakage current sensor for detecting leakage current is also provided on the AC side.

[0032] In an intuitive judgment during actual grid-connected operation, it is assumed that if both Rx and Ry are infinite, the leakage current value on the AC side should be 0. If Rx is infinite and Ry suddenly decreases, the leakage current value should increase, and it should increase in the positive direction; on the contrary, if Ry is infinite and Rx suddenly decreases, the leakage current will also increase, but it should increase in the opposite direction. In other words, according to the positive or negative increase of the leakage current, the change trend of the insulation resistance between the positive or negative pole of the DC input terminal and the earth can be calculated. However, under actual working conditions, when Rx and Ry show a decreasing trend at the same time, it is impossible to derive the accurate change trend of the current insulation organization based on the positive or negative increase trend of the leakage current.

[0033] Therefore, in this preferred embodiment of the present invention, when the inverter is connected to the grid, the voltage between the earth and the DC input terminal 10 is first continuously obtained, and the leakage current on the AC side is continuously obtained through the leakage current sensor. Under actual working conditions, the voltage between the earth and the DC input terminal 10 is weighted and averaged, and the average value is used as the initial bus voltage. Correspondingly, the continuously obtained leakage current values ​​are weighted and averaged, and the average value is used as the initial leakage current value.

[0034] The voltage between the earth and the DC input terminal 10 is set to half the bus voltage. That is, according to the three-phase voltage vector principle, under ideal conditions, the earth presents a fixed DC voltage to the inverter DC input terminal 10. When the AC side is three-phase symmetrical, the voltage between the earth and the DC input terminal 10 should be half the bus voltage. For example, if the bus voltage is equal to the voltage of the DC input terminal 600V when the duty cycle is not increased, the voltage between the earth and the inverter DC input terminal 10 is 300V. The duty cycle signal (PWM) in the boost chopper circuit is controlled to increase or decrease the duty cycle to change the bus voltage value to obtain the current bus voltage. The difference between the current bus voltage and the initial bus voltage is the size of the voltage increment.

[0035] On the other hand, due to the change in bus voltage, the leakage current on the AC side also changes accordingly. The current leakage current value continuously obtained by the leakage current sensor is compared with the initial leakage current value to obtain the leakage current increment. Specifically, the voltage increment △V satisfies: voltage increment △V = 1 / 2 current bus voltage - 1 / 2 initial bus voltage; current increment △i satisfies: current increment △i = initial leakage current ix - current leakage current iy. As mentioned above, Rx is regarded as the equivalent resistance value between the positive pole of the DC input terminal and the earth, and Ry is regarded as the equivalent resistance value between the negative pole of the DC input terminal and the earth. The insulation resistance value should satisfy the insulation resistance Rx / / Ry = voltage increment / current increment. That is, the size and change trend of the current insulation organization can be calculated based on the voltage increment and leakage current increment.

[0036] Example

[0037] A resistor with a resistance of 10,000 ohms is connected in parallel between the positive pole of the DC input terminal and the ground, and between the negative pole and the ground, and the leakage current on the AC side is set to 0 in the grid-connected operation state. After the inverter is grid-connected, the current bus voltage is obtained to be 600V, then the voltage between the positive pole or negative pole of the DC input terminal and the ground should be 300V. At this moment, the duty cycle is adjusted from 0 in the initial state to 0.27 through the controller in the boost chopper circuit. Then the voltage between the positive pole of the current DC input terminal and the ground rises from 300V to 409V. According to the voltage increment △V=1 / 2 current bus voltage-1 / 2 initial bus voltage, the voltage increment △V should be 109V at this time. At the same time, the AC side leakage current sensor obtains the current leakage current value from 0 to 21.9mA in the negative direction. According to the current increment △i=initial leakage current ix-current leakage current iy, the current increment △i should be 0.0219mA at this time. Finally, according to the insulation resistance Rx / / Ry=voltage increment / current increment, the current insulation resistance Rx / / Ry should be 4.977K. Obviously, the current equivalent insulation impedance shows a downward trend.

[0038] Due to the adoption of the above technical solution, the present invention has the following beneficial technical effects compared with the prior art:

[0039] By increasing the duty cycle signal control at the DC input end, a voltage increment is given between the positive or negative pole of the DC input end and the earth, and a leakage current sensor is set on the AC side. The initial leakage current and the real-time leakage current are compared by the leakage current sensor to obtain the current increment, and finally the total equivalent impedance of the current DC input to the earth is calculated by the voltage increment and the current increment. In this way, when a non-isolated grid-connected inverter grounded on the AC side is connected to the grid and works, the insulation performance of the DC side is detected online, so that the trend of the insulation impedance decreasing at the DC input end can be discovered in time, and further failures caused by the continued operation of the inverter can be avoided.

[0040] The present invention has been described in detail above, and the description of the embodiments is only used to help understand the method and core idea of ​​the present invention, and its purpose is to enable people familiar with the technology in this field to understand the content of the present invention and implement it accordingly, and it cannot be used to limit the protection scope of the present invention. Any equivalent changes or modifications made according to the spirit of the present invention should be included in the protection scope of the present invention.

Claims

1. A three-phase grid-connected inverter online insulation detection method, characterized in that: The method comprises the following steps: Step S1 of obtaining the voltage between the earth and the DC input, and obtaining the initial leakage current ix and the initial bus voltage; The step S2 of acquiring the current bus voltage according to the change of the bus voltage includes the step S21 of configuring a boost chopper circuit and a duty cycle signal in the boost chopper circuit; the step S22 of configuring to change the initial bus voltage and acquire the current bus voltage by increasing or decreasing the duty cycle signal, wherein in the step S2, the voltage between the earth and the DC input is set to be half of the bus voltage; A step S3 of acquiring a current leakage current iy and acquiring a voltage increment according to the current bus voltage and the initial bus voltage is configured; Step S4 of obtaining the current insulation resistance value according to the voltage increment and the current increment is configured, and the voltage increment △V satisfies: Voltage increment △V = 1 / 2 current bus voltage - 1 / 2 initial bus voltage; The current increment △i satisfies: Current increment △i = initial leakage current ix-current leakage current iy; In step S4, Rx is set as the equivalent resistance value between the DC input positive electrode and the earth, and Ry is set as the equivalent resistance value between the DC input negative electrode and the earth. Then the insulation resistance satisfies: Insulation resistance Rx / / Ry = voltage increment / current increment.

2. The online insulation detection method for a three-phase grid-connected inverter according to claim 1, characterized in that: In the step S1, there is also included a step S11 of averaging the voltage between the ground and the DC input and averaging the leakage current.

3. The online insulation detection method for a three-phase grid-connected inverter according to claim 2, characterized in that: In the step S4, When Rx and Ry are both infinite, the leakage current is 0; When Rx is infinite and Ry becomes smaller, the leakage current increases in the positive direction; When Ry is infinite and Rx becomes smaller, the leakage current increases in the opposite direction.

4. A three-phase grid-connected inverter insulation detection circuit using the three-phase grid-connected inverter online insulation detection method according to any one of claims 1 to 3, characterized in that: The detection circuit comprises: A DC bus connected to two poles of the DC input terminal; An inverter circuit consisting of three bridge arms connected in parallel, each of which is composed of two switch tubes connected in series, and each of which is connected to one phase of the grounded three-phase AC side; The line connecting each bridge arm to any phase of the three-phase AC side is connected to a first inductor, a second inductor and a relay, and each bridge arm is connected to a compensation capacitor and then to the midpoint potential of the DC bus, wherein: The DC input terminal and the DC bus include a first insulation resistor connecting the positive electrode of the DC input terminal and the ground, a second insulation resistor connecting the negative electrode of the DC input terminal and the ground, and a duty cycle signal switch. The current voltage value and leakage current of the DC bus are obtained by controlling the timing and on-off of the duty cycle signal switch and the leakage current sensor on the three-phase AC side.

5. The three-phase grid-connected inverter insulation detection circuit according to claim 4, characterized in that: The switch tube is a metal oxide semi-conductor field effect transistor or a turn-off thyristor.

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