A circuit and method for detecting insulation impedance of a grid-connected inverter to ground

Abstract

The present invention discloses a circuit and method for detecting the insulation impedance to ground of a grid-connected inverter. The insulation impedance to ground detection circuit includes a disturbance resistor and a voltage sampling device. The bridge arm of the inverter bridge closest to the bus capacitor includes a first switch and a second switch. One end of the first switch is connected to the positive DC bus, and one end of the second switch is connected to the negative DC bus. The first end of the disturbance resistor is grounded, and the second end of the disturbance resistor is connected to the middle point of the first switch and the second switch. The voltage sampling device is connected between the disturbance resistor and the negative DC bus. The detection method includes: driving all switches of the inverter bridge to be disconnected, and recording the sampling voltage Vpe1 in this state; driving the first switch to be closed, keeping other switches disconnected, and recording the sampling voltage Vpe2 in this state; calculating the insulation impedance to ground, where R is the resistance value of the disturbance resistor. The present invention can quickly solve the insulation impedance size, and the hardware cost is low.

Description

Technical Field

[0001] The invention belongs to the field of ground insulation impedance detection of photovoltaic grid-connected inverters, and relates to a ground insulation impedance detection circuit and method of a grid-connected inverter. Background Art

[0002] In the photovoltaic inverter system, the protection of the insulation impedance of the photovoltaic grid-connected inverter to the ground is very important. Each region or country has made very clear regulations on it. The insulation impedance of the input end to the earth (casing) must be less than the specified threshold standard before it can be used. There are many ways to detect the insulation impedance, and most of them are based on Kirchhoff's law to solve it. These methods also involve equivalent infinite solutions, which are relatively complicated. In addition, the circuit design uses the signal quantity on the panel input side, which is also relatively high in hardware cost.

[0003] Chinese patent CN105548719B discloses a circuit and method for detecting insulation resistance to ground. Figure 1 The insulation resistance detection circuit to ground shown detects the insulation impedance of the photovoltaic inverter. Among them, resistors R01 and R02 are equivalent resistances of the insulation impedance to ground of the device to be detected, and resistors R30, R40, R50, R60 and switch Q10 constitute the topological structure of the detection circuit. The detection circuit introduces resistance disturbances and solves the resistance values ​​of resistors R01 and R02 according to Kirchhoff's law. However, in this insulation impedance detection circuit and method, the insulation impedance to ground is obtained by solving an equivalent calculation formula. At least one switch is required to assist the hardware. The equivalent formula is solved by the opening and closing of the switch and the input side voltage. Switches and other devices are added to the original circuit of the photovoltaic inverter, the hardware cost is high, the algorithm is complex, and the calculation amount is large. Summary of the invention

[0004] In view of the above technical problems, the present invention provides an improved circuit and method for detecting the insulation impedance to ground of a grid-connected inverter.

[0005] On one hand, the present invention provides a ground insulation impedance detection circuit of a grid-connected inverter, comprising a positive DC bus, a negative DC bus, a bus capacitor electrically connected between the positive DC bus and the negative DC bus, and an inverter bridge, wherein the inverter bridge at least comprises a first bridge arm closest to the bus capacitor, the first bridge arm comprises a first switch and a second switch connected in series with each other, one end of the first switch is electrically connected to the positive DC bus, and one end of the second switch is electrically connected to the negative DC bus, the ground insulation impedance detection circuit also comprises a disturbance resistor and a voltage sampling device, the first end of the disturbance resistor is grounded, the second end of the disturbance resistor is electrically connected to the middle point of the first switch and the second switch, and the voltage sampling device is electrically connected between the disturbance resistor and the negative DC bus; the ground insulation impedance detection circuit also comprises a controller, the controller is used to drive all switches of the inverter bridge to be disconnected in a first state and to drive the first switch to be closed while other switches remain disconnected in a second state.

[0006] Preferably, the controller is used to calculate the insulation impedance Xiso of the grid-connected inverter according to the following formula:

[0007]

[0008] Wherein, R is the resistance value of the disturbance resistor, Vpe1 and Vpe2 are the sampled voltages in the first state and the second state respectively, and Vbus is the DC bus voltage.

[0009] Preferably, the controller is used to receive the sampled voltage acquired by the voltage sampling device in real time or at a fixed time.

[0010] Optionally, the inverter bridge includes but is not limited to: an H4 inverter bridge and an H6 inverter bridge, and the first inverter bridge is a bridge arm of the H4 inverter bridge or the H6 inverter bridge that is closest to the input ends of the positive DC bus and the negative DC bus.

[0011] In a specific embodiment, the inverter bridge also includes a second bridge arm, the second bridge arm is connected in parallel with the first bridge arm, the second bridge arm includes a third switch and a fourth switch connected in series, one end of the third switch is electrically connected to the positive DC bus, one end of the fourth switch is electrically connected to the negative DC bus, and the second switch, the third switch and the fourth switch are all kept disconnected in the first state and the second state.

[0012] Furthermore, the output of the first bridge arm is electrically connected to the positive electrode of the AC output terminal through a connecting line provided with a first inductor, and the output of the second bridge arm is electrically connected to the negative electrode of the AC output terminal through a connecting line provided with a second inductor.

[0013] Preferably, the controller and the voltage sampling device are electrically connected.

[0014] Preferably, the controller is electrically connected to each switch of the inverter bridge.

[0015] Another aspect of the present invention provides a method for detecting the ground insulation impedance of a grid-connected inverter, which uses the ground insulation impedance detection circuit as described above. The ground insulation impedance detection method comprises the following steps:

[0016] A. driving all switches of the inverter bridge to be turned off, and recording the sampled voltage Vpe1 obtained by the voltage sampling device in this state;

[0017] B. driving the first switch to close, keeping other switches open, and recording the sampled voltage Vpe2 obtained by the voltage sampling device in this state;

[0018] C. Calculate the insulation impedance Xiso of the grid-connected inverter according to the following formula:

[0019]

[0020] Wherein, R is the resistance value of the disturbance resistor, and Vbus is the DC bus voltage.

[0021] Preferably, the method for detecting insulation impedance to ground further comprises the following steps: sending the sampled voltage acquired by the voltage sampling device to the controller, and the controller periodically updates the sampled voltage.

[0022] Preferably, the ground insulation impedance detection method further includes the following steps: when the ground insulation impedance Xiso is less than a set threshold, the grid-connected inverter enters the next state detection; when the ground insulation impedance Xiso is greater than or equal to the set threshold, the grid-connected inverter re-self-checks.

[0023] The present invention adopts the above solution, which has the following advantages compared with the prior art:

[0024] The ground insulation impedance detection circuit and ground insulation impedance detection method of the grid-connected inverter of the present invention are based on the existing hardware inverter topology structure, do not need to add other switches such as relays, and can quickly solve the insulation impedance by reusing the BUS voltage, and the hardware cost is low. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] In order to more clearly illustrate the technical solution of the present invention, the accompanying drawings required for use in the description of the embodiments will be briefly introduced below. Obviously, the accompanying drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other accompanying drawings can be obtained based on these accompanying drawings without paying creative work.

[0026] Figure 1 The present invention is a hardware topology of a grid-connected inverter ground insulation impedance detection circuit in the prior art;

[0027] Figure 2 A hardware topology of a ground insulation impedance detection circuit according to an embodiment of the present invention;

[0028] Figure 3 The figure is a flow chart of a method for detecting insulation impedance to ground according to an embodiment of the present invention. DETAILED DESCRIPTION

[0029] The preferred embodiments of the present invention are described in detail below in conjunction with the accompanying drawings so that the advantages and features of the present invention can be more easily understood by those skilled in the art. It should be noted that the description of these embodiments is used to help understand the present invention, but does not constitute a limitation of the present invention. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without making creative work are within the scope of protection of the present invention.

[0030] by Figure 2 Taking the H4 inverter shown as an example, a ground insulation impedance detection circuit and a ground insulation impedance detection method of a grid-connected inverter according to an embodiment of the present invention are described in detail. The hardware topology of the grid-connected inverter can be expanded to any form, such as an H6 inverter bridge.

[0031] Reference Figure 2, the ground insulation impedance detection circuit of the grid-connected inverter includes: a positive DC bus Vbus+, a negative DC bus Vbus-, a bus capacitor C and an inverter bridge. Among them, the positive DC bus Vbus+ and the negative DC bus Vbus- respectively have input terminals located on the DC side of the grid-connected inverter, which are used to electrically connect the photovoltaic panel. The bus capacitor C is also located on the DC side of the grid-connected inverter, and the first end of the bus capacitor C is electrically connected to the positive DC bus Vbus+, and the second end is electrically connected to the negative DC bus Vbus-. The inverter bridge includes a first bridge arm 1 and a second bridge arm 2 connected in parallel to each other, and the first bridge arm 1 is closest to the bus capacitor C, that is, closest to the input terminals of the positive DC bus Vbus+ and the negative DC bus Vbus-. The first bridge arm 1 includes a first switch K1 and a second switch K3 connected in series to each other, one end of the first switch K1 is electrically connected to the positive DC bus Vbus+, and one end of the second switch K3 is electrically connected to the negative DC bus Vbus-. The second bridge arm 2 includes a third switch K4 and a fourth switch K2 connected in series, one end of the third switch K4 is electrically connected to the positive DC bus Vbus+, and one end of the fourth switch K2 is electrically connected to the negative DC bus Vbus-. The outputs of the first bridge arm 1 and the second bridge arm 2 are electrically connected to the positive pole of the AC output terminal and the negative pole of the AC output terminal of the grid-connected inverter, respectively. The positive pole of the AC output terminal and the negative pole of the AC output terminal can be electrically connected to the power grid to feed the AC power converted by the inverter bridge into the power grid AC. Specifically, the middle point of the first switch K1 and the second switch K3 is electrically connected to the positive pole of the AC output terminal of the grid-connected inverter through a connecting line provided with a first inductor L1; the middle point of the third switch K4 and the fourth switch K2 is electrically connected to the negative pole of the AC output terminal through a connecting line provided with a second inductor L2. The first switch K1, the second switch K3, the third switch K4 and the fourth switch K2 are respectively connected in parallel with a rectifier diode.

[0032] Furthermore, the ground insulation impedance detection circuit also includes a disturbance resistor R and a voltage sampling device Vpe. The first end of the disturbance resistor R is connected to the ground PE, and the second end of the disturbance resistor R is electrically connected to the middle point of the first switch K1 and the second switch K3. The voltage sampling device Vpe is electrically connected between the disturbance resistor R and the negative DC bus Vbus-, and the voltage sampling device Vpe is used to collect the voltage of the negative DC bus Vbus- to the ground PE. The voltage sampling device Vpe can be a voltage sampling circuit composed of an operational amplifier, etc., which samples the voltage analog signal of the positive DC bus Vbus+ to the ground and converts it into a digital signal that can be recognized by the MCU. The disturbance resistor R and the voltage sampling device Vpe are both located between the bus capacitor C and the first bridge arm 1 of the inverter bridge.

[0033] Furthermore, the ground insulation impedance detection circuit further includes a controller, which is used to drive all switches of the inverter bridge to be disconnected in the first state and to drive the first switch K1 to be closed while other switches remain disconnected in the second state. The controller may specifically include an MCU.

[0034] The controller is electrically connected to the voltage sampling device Vpe, and is used to receive the sampled voltage acquired by the voltage sampling device Vpe in real time or at a fixed time.

[0035] The controller and each switch of the inverter (including the first switch K1, the second switch K3, the third switch K4 and the fourth switch K2) are electrically connected. When the controller outputs a low-level driving signal to the switch, the switch is opened. When the controller outputs a high-level driving signal to the switch, the switch is closed.

[0036] The detection principle of the ground insulation impedance detection circuit is as follows:

[0037] First, the controller controls the switches K1-K4 of all the inverter bridge arms of the inverter bridge to be disconnected, so that the grid-connected inverter is in the first state. The value of the sampling voltage of the DC bus voltage to the ground in this state can be expressed as formula (1):

[0038]

[0039] Optimize formula (1) to formula (2)

[0040] Vpe1*X1+Vpe1*X2=X2*Vbus (2)

[0041] Further optimized to formula (3)

[0042]

[0043] Then, the controller controls the first switch K1 to close, so that the grid-connected inverter is in the second state. The value of the sampling voltage of the DC bus voltage to the ground in this state can be expressed as formula (4):

[0044]

[0045] Substituting equation (3) into equation (4) yields equation (5)

[0046]

[0047] Further optimization gives formula (6):

[0048] Vpe2*(Vpe1*X2+R*Vbus)=Vpe1*R*Vbus+Vpe1*X2*Vbus (6)

[0049] We can get formula (7)

[0050]

[0051] From the hardware principle, we can get the total insulation resistance Xiso = X1 / / X2, and combined with formula (3), we can get formula (8)

[0052]

[0053] Then through Finally, we can get formula (9):

[0054]

[0055] In the above derivation process, X1 is the equivalent insulation impedance of the positive DC bus Vbus+ to the ground PE, X2 is the equivalent insulation impedance of the negative DC bus Vbus- to the ground PE, and R is the disturbance resistance.

[0056] Therefore, the controller can calculate the insulation impedance Xiso of the grid-connected inverter according to the following formula:

[0057]

[0058] Wherein, R is the resistance value of the disturbance resistor, Vpe1 and Vpe2 are the sampled voltages obtained by the voltage sampling device in the first state and the second state respectively, and Vbus is the DC bus voltage.

[0059] Reference Figure 3 The method for detecting the insulation impedance of the grid-connected inverter of this embodiment comprises the following steps:

[0060] S1. Send the sampled voltage Vpe obtained by the voltage sampling device to the MCU, and the MCU updates the sampled voltage every 20ms;

[0061] S2, the inverter enters the wait self-test mode, the MCU controls the driving of switches K1~K4, waits for T1 delay, pulls the driving signal low, and switches K1~K4 are all disconnected. At this time, the sampling voltage Vpe1 obtained by the voltage sampling device in this state is recorded;

[0062] S3, wait for T2 delay, set the driving signal of the first switch K1 to high, at this time the first switch K1 is closed, and the other three switches are open, at this time, record the sampled voltage Vpe2 obtained by the voltage sampling device in this state;

[0063] S4, wait for T3 delay, according to the formula Finally, the insulation impedance Xiso of the grid-connected inverter to ground is calculated;

[0064] S5, judgment condition: when Xiso is less than the software preset threshold value Xset, the machine enters the next state function detection; when Xiso is greater than or equal to the software preset threshold value Xset, the inverter reports an insulation impedance error to ground, and the inverter re-self-checks.

[0065] The above embodiment is only for illustrating the technical concept and features of the present invention, and is a preferred embodiment. Its purpose is to enable people familiar with the technology 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 transformation or modification made according to the spirit of the present invention should be included in the protection scope of the present invention.

Claims

1. A ground insulation impedance detection circuit for a grid-connected inverter, comprising a positive DC bus, a negative DC bus, a bus capacitor electrically connected between the positive DC bus and the negative DC bus, and an inverter bridge, wherein the inverter bridge at least comprises a first bridge arm closest to the bus capacitor, the first bridge arm comprises a first switch and a second switch connected in series, one end of the first switch is electrically connected to the positive DC bus, and one end of the second switch is electrically connected to the negative DC bus, characterized in that: The ground insulation impedance detection circuit also includes a disturbance resistor and a voltage sampling device, the first end of the disturbance resistor is grounded, the second end of the disturbance resistor is electrically connected to the middle point of the first switch and the second switch, and the voltage sampling device is electrically connected between the disturbance resistor and the negative DC bus; the ground insulation impedance detection circuit also includes a controller, which is used to drive all switches of the inverter bridge to open in a first state and drive the first switch to close while other switches remain open in a second state.

2. The ground insulation impedance detection circuit according to claim 1, characterized in that: The controller is used to calculate the insulation impedance Xiso of the grid-connected inverter according to the following formula Wherein, R is the resistance value of the disturbance resistor, Vpe1 and Vpe2 are the sampled voltages in the first state and the second state respectively, and Vbus is the DC bus voltage.

3. The ground insulation impedance detection circuit according to claim 1 or 2, characterized in that: The controller is used to receive the sampled voltage acquired by the voltage sampling device in real time or at a fixed time.

4. The ground insulation impedance detection circuit according to claim 1, characterized in that: The inverter bridge also includes a second bridge arm, which is connected in parallel with the first bridge arm. The second bridge arm includes a third switch and a fourth switch connected in series, one end of the third switch is electrically connected to the positive DC bus, and one end of the fourth switch is electrically connected to the negative DC bus. The second switch, the third switch and the fourth switch remain disconnected in the first state and the second state.

5. The ground insulation impedance detection circuit according to claim 4, characterized in that: The output of the first bridge arm is electrically connected to the positive electrode of the AC output terminal through a connecting line provided with a first inductor, and the output of the second bridge arm is electrically connected to the negative electrode of the AC output terminal through a connecting line provided with a second inductor.

6. The ground insulation impedance detection circuit according to claim 1, characterized in that: The controller is electrically connected to the voltage sampling device.

7. The ground insulation impedance detection circuit according to claim 1, characterized in that: The controller is electrically connected to each switch of the inverter bridge.

8. A method for detecting insulation impedance of a grid-connected inverter, characterized in that: Using the ground insulation impedance detection circuit according to any one of claims 1 to 7, the ground insulation impedance detection method comprises the following steps: A. driving all switches of the inverter bridge to be turned off, and recording the sampled voltage Vpe1 obtained by the voltage sampling device in this state; B. driving the first switch to close, keeping other switches open, and recording the sampled voltage Vpe2 obtained by the voltage sampling device in this state; C. Calculate the insulation impedance Xiso of the grid-connected inverter according to the following formula: Wherein, R is the resistance value of the disturbance resistor, and Vbus is the DC bus voltage.

9. The method for detecting insulation impedance to ground according to claim 8, characterized in that: The method for detecting insulation impedance to ground further comprises the following steps: sending the sampled voltage acquired by the voltage sampling device to the controller, and the controller periodically updating the sampled voltage.

10. The method for detecting insulation impedance to ground according to claim 8, characterized in that: The ground insulation impedance detection method also includes the following steps: when the ground insulation impedance Xiso is less than a set threshold, the grid-connected inverter enters the next state detection; when the ground insulation impedance Xiso is greater than or equal to the set threshold, the grid-connected inverter re-self-checks.

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