CN206658046U - A kind of switching overvoltage protection device of multilevel photovoltaic grid-connected inverter - Google Patents

Abstract

An operating overvoltage protection device for a multi-level photovoltaic grid-connected inverter, the device includes a DC input unit, a multi-level inverter unit, a control unit, a voltage detection unit, and a current detection unit; the output terminal of the DC input unit is connected to the The DC input terminals of the multi-level inverter unit are connected, the AC output terminals of the multi-level inverter unit are connected to the power grid through the grid-side switch K, but not limited to the power grid, and the input terminals of the voltage detection unit are connected to the grid side. Connection, the output terminal of the voltage detection unit is connected with the input terminal of the control unit, the input terminal of the current detection unit is connected with the AC output terminal of the multi-level inverter unit, the output terminal of the current detection unit is connected with the other input terminal of the control unit The output terminal of the control unit is connected with the driving terminal of the switching tube in the multi-level inverter unit; the utility model can effectively reduce the output voltage of the inverter unit to within the threshold range, and timely protect the AC output side electronics The safety of the load; and the system loss is small.

Description

An operating overvoltage protection device for a multi-level photovoltaic grid-connected inverter

technical field

The utility model relates to the technical field of photovoltaic power supply. The utility model relates to an operation overvoltage protection device, in particular to an operation overvoltage protection device applied to a multi-level photovoltaic grid-connected inverter.

Background technique

The AC output terminal of the photovoltaic grid-connected inverter is directly connected to the grid through a common connection point. At the same time, other load devices may be connected to the port. After connecting to the grid, the port voltage is clamped by the grid, and the port voltage amplitude is determined by the grid voltage. When grid islanding occurs, the grid-side switch is disconnected, the inverter is disconnected from the grid, and the voltage at the grid-side public connection point is no longer clamped by the grid. Continue to provide reactive power to the grid, causing the voltage at the common connection point on the AC side of the inverter to rise. In order to prevent the electronic load at the AC output end of the inverter from being impacted by higher voltage, the transient voltage at the AC output end of the inverter should not exceed the requirements in "NB/T 32004-2013 Photovoltaic Power Grid-connected Inverter Technical Specifications" limit.

For the three-phase grid-connected inverter, aiming at the above problems in grid-connected, the existing technology establishes an orthogonal dq rotating coordinate system according to the three-phase grid voltage, and calculates the inverter voltage d axis of the three-phase grid-connected inverter component or the effective value of the grid voltage to judge whether an operating overvoltage has occurred at the AC output of the inverter. When it is judged that the operation overvoltage has occurred, the voltage on the grid side has often risen. Even if the grid connection is stopped at this time, the voltage at both ends of the safety capacitor Cx (generally X capacitor) at the AC output end of the inverter is already high. Discharge occurs through the internal resistance in the inverter. Obviously, the smaller the capacitance of the safety capacitor Cx, the faster the discharge, but it will affect the electromagnetic compatibility (EMC) characteristics of the inverter; the smaller the resistance of the internal resistance of the inverter, the faster the discharge, but it will bring the inverter Additional losses increase the cost of the inverter.

Contents of the invention

In order to solve the above technical problems, the utility model provides an operation overvoltage protection device and method applied to multi-level photovoltaic grid-connected inverters. The device mainly detects the grid voltage through the detection unit, and detects whether the grid voltage In the isolated island, the control unit controls the inverter to switch from the grid-connected working state to the operating overvoltage protection working state, until the AC output voltage of the inverter drops to a given voltage, the entire operation overvoltage protection process ends, and the inverter In the standby state, waiting for re-connection to the grid; the device and protection method adopt a simple circuit structure, combined with a control method, when the grid is islanded, first disconnect the inverter from the grid quickly, and then switch the inverter to AC The output voltage is reduced to the range required by the standard, which effectively protects the electronic equipment connected to the AC side of the inverter.

In order to achieve the above object, the utility model adopts the following technical solutions:

An operating overvoltage protection device for a multi-level photovoltaic grid-connected inverter, the device includes a DC input unit, a multi-level inverter unit, a control unit, a voltage detection unit, and a current detection unit; the output of the DC input unit terminal is connected to the DC input terminal of the multi-level inverter unit, the AC output terminal of the multi-level inverter unit is connected to the power grid through the network side switch K but not limited to being connected to the power grid, and the voltage detection unit’s The input end is connected to the grid side for detecting the grid voltage, the output end of the voltage detection unit is connected to the input end of the control unit, the input end of the current detection unit is connected to the AC output end of the multi-level inverter unit The output end of the current detection unit is connected with the other input end of the control unit, and the output end of the control unit is connected with the driving end of the switching tube in the multi-level inverter unit, which is used to control the multi-voltage The working state of the switching tube of the flat inverter unit.

The multi-level inverter unit is single-phase or three-phase; the multi-level inverter unit is composed of a DC bus module, an inverter bridge arm, and an AC filter module connected in series in sequence, and the DC bus module is n-1 busbars Capacitors are connected in series, n is the number of levels; the multi-level inverter bridge arm includes a "zero" level switch group, a "positive" level switch group and a "negative" level switch group, and the "zero" level switch group The flat switch group is connected in series between the midpoint of the bus capacitor and the "positive" level switch group and the "negative" level switch group. The AC filter module is composed of a filter inductor L and a filter capacitor C _inv . The filter inductor L One end of is connected with the output of the inverter bridge arm, the other end is the positive output end of the multi-level inverter unit, one end of the filter capacitor C _inv is connected with the positive output end of the multi-level inverter unit, and the filter capacitor The other end of C _inv is connected to the midpoint of the bus capacitor as the negative output end of the multi-level inverter unit.

Compared with the prior art, the utility model has the following advantages:

1. The utility model device can effectively reduce the output voltage of the inverter unit to within the threshold range when the output voltage of the inverter unit exceeds the threshold, so as to timely protect the safety of the electronic load on the AC output side.

2. The protection device and method feed back the excess electric energy of the AC output to the DC bus instead of consuming it through the internal resistance of the system, so the system loss is small.

Description of drawings

Figure 1 Block diagram of the operating overvoltage protection device.

Fig. 2 block diagram of multi-level inverter unit circuit.

Fig. 3 is a control flow chart of the operation overvoltage protection method.

Figure 4 is a single-phase T-type three-level inverter.

Figure 5 shows the four operating modes of the single-phase T-type three-level inverter after operating overvoltage, where (a) in Figure 5 is the transfer of the filter capacitor to the filter inductor L during the [0,D×Ts] period (b) in Figure 5 is the circuit working state in which the energy of the filter inductor is fed to the DC bus module during the [D×Ts,Ts] period, and (c) in Figure 5 is [0,D×Ts] The energy of the filter capacitor is transferred to the circuit working state in the filter inductor L during the time period. (d) in Figure 5 is the working state of the circuit in which the energy of the filter inductor is fed to the DC bus module during the [D×Ts, Ts] period.

Figure 6 is a three-phase T-type three-level inverter.

Figure 7 is a single-phase I-type three-level inverter.

Figure 8 shows the four working modes of the single-phase I-type three-level inverter after the operation overvoltage occurs, in which (a) in Figure 8 is the transfer of the filter capacitor to the filter inductor L during the [0, D×Ts] period (b) in Figure 8 is the circuit working state in which the energy of the filter inductor is fed to the DC bus module in the [D×Ts,Ts] period, and (c) in Figure 8 is [0,D×Ts] The energy of the filter capacitor is transferred to the circuit working state in the filter inductor L during the time period, and (d) in Figure 8 is the circuit working state in which the energy of the filter inductor is fed to the DC bus module during the [D×Ts, Ts] period.

Figure 9 is a three-phase I-type three-level inverter.

Figure 10 is a three-phase I-type five-level inverter.

detailed description

The utility model will be further described below in conjunction with the embodiment shown in the accompanying drawings.

Figure 1 is a block diagram of an operating overvoltage protection device, as shown in the figure, an operating overvoltage protection device and method applied to a multi-level photovoltaic grid-connected inverter, the device includes a DC input unit and a multi-level inverter unit , a control unit, a voltage detection unit and a current detection unit. The output end of the DC input unit is connected to the DC input end of the multi-level inverter unit, the AC output end of the multi-level inverter unit is connected to the power grid through the network side switch K, and the AC output of the inverter unit is The terminal can also be connected with other electronic loads, the input terminal of the voltage detection unit is connected with the grid side for detecting the grid voltage, the output terminal of the voltage detection unit is connected with the input terminal of the control unit, the current detection unit The input end of the unit is connected to the AC output end of the multi-level inverter unit, the output end of the current detection unit is connected to the other input end of the control unit, and the output end of the control unit is connected to the multi-level inverter unit. The drive terminals of the switching tubes in the unit are connected to control the working state of the switching tubes of the multi-level inverter unit.

FIG. 2 is a circuit block diagram of a multi-level inverter unit. As shown in the figure, the multi-level inverter unit is single-phase or three-phase. The multi-level inverter unit is composed of a DC bus module, an inverter bridge arm, and an AC filter module connected in series in sequence. The DC bus module is composed of (n-1) bus capacitors connected in series, and n is the number of levels; the multi-level inverter bridge arm includes a "zero" level switch group, a "positive" level switch group and a "positive" level switch group. Negative level switch group, the "zero" level switch group is connected in series between the midpoint of the bus capacitor and the "positive" level switch group and the "negative" level switch group, the AC filter module is composed of a filter inductor L and filter capacitor C, one end of the filter inductor L is connected to the output of the multi-level inverter bridge arm, the other end is the positive output end of the inverter unit, one end of the filter capacitor C _inv is connected to the multi-level The output ends of the inverter unit are connected, and the other end of the filter capacitor C _inv is connected to the midpoint of the bus capacitor to be the negative output end of the multi-level inverter unit.

Fig. 3 is a control flow chart of the operation overvoltage protection method, the control method of the operation overvoltage protection device:

Step 1: The voltage detection unit detects the grid voltage V in real time, and sends the detected voltage to the control unit for data processing;

Step 2: The control unit processes the sent voltage data. When it detects that the effective value of the grid voltage V _ave is less than the threshold value V _G given by the system, the control unit controls the multi-level inverter unit to continue to be in the grid-connected working state; when When it is detected that the effective value of the grid voltage is greater than the threshold value V _G given by the system, the control unit sends a control signal to disconnect all the switches of the multi-level inverter unit, and at the same time, the multi-level inverter unit immediately switches from the grid-connected state to the operation Overvoltage protection state, waiting for the control unit to re-send the control signal;

Step 3: Operate in the overvoltage protection state, that is, the control unit sends a control signal to the zero-level switch group again to make the zero-level switch group conduct and work, and a current loop is formed inside the multi-level inverter unit to make the multi-level inverter The voltage at the AC output end of the transformation unit is fed back to the DC bus module at the DC input end of the inverter unit.

Step 4: When the voltage value V of the AC output terminal of the multilevel inverter unit is greater than the safe voltage value V _safe (the safe voltage value V _safe is the safety value required by the inverter when it is operating overvoltage), continue to execute Step 3 is to perform operation overvoltage protection; when the voltage V of the AC output terminal of the multi-level inverter unit is less than the safe voltage value _Vsafe , the control unit will send a control signal to turn off the zero-level switch group, and the operation overvoltage protection state will stop. The grid-side switch K is turned off, the multi-level inverter unit is in a standby state, and the operation overvoltage protection process ends.

Figure 4 is a single-phase T-type three-level inverter. As shown in the figure, the DC bus capacitor is connected in series by the first capacitor C1 and the second capacitor C2, and the zero-level switch is connected between the first capacitor C1 and the second capacitor C2 Between the midpoint of the inverter bridge arm and the midpoint of the inverter bridge arm, the zero-level switch is connected in series by the first zero-level switch tube S2 and the second zero-level switch tube S3, and the first zero-level switch tube S2 The collector is connected to the midpoint of the first capacitor C1 and the second capacitor C2, the collector of the second zero-level switch tube S3 is connected to the midpoint of the inverter bridge arm, and the first zero-level switch The emitter of the tube S2 is connected to the emitter of the second zero-level switching tube S3. The collector of the positive level switch S1 is connected to the positive pole of the first capacitor C1, the emitter of the positive level switch S1 is connected to the collector of the negative level switch S4, and the emitter of the negative level switch S4 is connected to the second capacitor C1. The negative pole of C2 is connected. One end of the inductance L is connected to the midpoint of the inverter bridge arm, the other end of the inductance L is connected to one end of the filter capacitor C _inv , the other end of the filter capacitor C _inv is connected to the first capacitor C1 and the second capacitor The midpoints of C2 are connected.

Figure 5 shows the four working modes of the single-phase T-type three-level inverter after the operation overvoltage occurs. As shown in the figure, the "zero" level switching tube includes the first zero level switching tube S2 and the second zero level switching tube S2 Level switch tube S3. When the digital controller detects that an operation overvoltage has occurred, the digital controller sends a driving signal with a duty cycle D (0<D<1) to the "zero" level switch tube, and the switching period is Ts, and at the same time prohibits the rest of the switch tubes work. At this time, when the "zero" level switching tube is turned on or off, the inverter has four working modes:

Mode 1: The equivalent circuit is shown in (a) in Figure 5. At this time, the voltage across the filter capacitor C _inv is greater than zero. During the [0,D×Ts] period of the switching cycle, the "zero" level switch The first zero-level switch tube S2 and the second zero-level switch tube S3 are turned on, and the other power switch tubes are turned off. The energy of the filter capacitor C _inv is transferred to the filter inductor L;

Mode 2: The equivalent circuit is shown in (b) in Figure 5. At this time, the voltage across the filter capacitor C _inv is greater than zero, and all switches are turned off during the [D×Ts, Ts] period of the switching cycle. The current of the filter inductor L freewheels through the body diode _of the positive level switch S1, and the energy of the filter inductor is fed to the DC bus module;

Mode 3: The equivalent circuit is shown in (c) in Figure 5. At this time, the voltage across the filter capacitor C _inv is less than zero. During the [0,D×Ts] period of the switching cycle, the "zero" level switch The first zero-level switch tube S2 and the second zero-level switch tube S3 are turned on, and the other power switch tubes are turned off. The energy of the filter capacitor C _inv is transferred to the filter inductor L;

Mode 4: The equivalent circuit is shown in (d) in Figure 5. At this time, the voltage across the filter capacitor C _inv is less than zero, and all the switches are turned off during the [D×Ts, Ts] period of the switching cycle. _The filter inductor current freewheels through the body diode of the negative level switch S4, and the energy of the filter inductor L is fed to the DC bus module;

Please follow my modification of the description in Figure 5, and modify the names of the components in the yellow font below, and note that the same letters use the same names as the description in Figure 5!

Figure 6 is a three-phase T-type three-level inverter. As shown in the figure, the DC bus capacitor is connected in series by the first capacitor C1 and the second capacitor C2, and the zero-level switch group of phase a is connected between the midpoint of the first capacitor C1 and the second capacitor C2 and the midpoint of the inverter bridge arm Between points, the zero-level switch is respectively connected in series by the first zero-level switch tube S2a and the second zero-level switch tube S3a, and the collector of the first zero-level switch tube S2a is connected to the first capacitor C1 and the first capacitor C1. The midpoints of the two capacitors C2 are connected, the collector of the second zero-level switching tube S3a is connected to the midpoint of the inverter bridge arm, and the emitter of the first zero-level switching tube S2a is connected to the second zero-level switching tube S2a. The emitters of the level switch tube S3a are connected. The collector of the positive level switch S1a is connected to the positive pole of the first capacitor C1, the emitter of the positive level switch S1a is connected to the collector of the negative level switch S4a, and the emitter of the negative level switch S4a is connected to the first capacitor C1. The negative poles of the two capacitors C2a are connected. The output terminal of the phase a is connected to an AC filter module. The circuits of phase b and phase c are the same as those of phase a, and will not be repeated here. At the same time, the operating mode of the overvoltage protection of the circuit is the same as that of the single-phase T-type three-level inverter.

Figure 7 is a single-phase I-type three-level inverter. As shown in the figure, the single-phase I-type three-level circuit is composed of four switching tubes with anti-parallel diodes connected in series, and the drain of the positive level switching tube S1 is connected to the positive pole of the first capacitor _C1 , the positive voltage The source of the flat switch S1 is connected in series with the drain of the _first zero-level switch S2, and the source of the first zero _- level switch S2 is connected in series with the drain of the _second zero _- level switch S3 _The source of the second zero-level switching transistor S3 is connected in series with the drain of the negative _- level switching transistor S4, and the source of the negative _- level switching transistor S4 is connected to the negative electrode of the second capacitor C2. A first capacitor C1 and a second capacitor C2 are connected in series between the positive and negative sides of the power supply, and the positive level switch S1 and the _first zero level switch S2 are connected in series with the first capacitor C1 and the _second capacitor C1. The first zero-level diode D _{1 is connected between the series points N of the capacitor C2, the anode of the zero-level first diode D 1 is} connected to point N, and the zero-level first diode D _{1 The} cathode is connected to the series connection point of the positive level switching tube S1 and the zero level _first switching tube S2. A zero-level second diode D ₂ is connected between the series connection point N of the zero-level second switching tube S ₃ and the negative-level switching tube S ₄ and the series connection point N of the first capacitor C1 and the second capacitor C2. The cathode of the level _second diode D2 is connected to point N, and the anode of the zero-level _second diode D2 is in phase with the series connection point of the zero _- level second switch tube S3 and the negative _- level switch tube S4. connection, the series point of the zero _- level first switching tube S2 and the zero-level _second switching tube S3 is connected to a filter inductor L, and the other end of the filter inductor L is connected to the first capacitor C1 and the second capacitor C2 A filter capacitor C _inv is connected between the midpoints N, and a load resistor R is connected in parallel with both ends of the filter capacitor C _inv .

Figure 8 shows the four operating modes of the single-phase I-type three-level inverter after the operation overvoltage occurs. As shown in the figure, the "zero" level switch tube includes the power switch tube zero _- level first switch S2 and Zero level second switch S ₃ . When the digital controller detects that an operation overvoltage has occurred, the digital controller sends a driving signal with a duty cycle D (0<D<1) to the "zero" level switch tube, and the switching period is Ts, and at the same time prohibits the rest of the switch tubes work. At this time, when the "zero" level switching tube is turned on or off, the inverter has four working modes:

Mode 1: The equivalent circuit is shown in (a) in Figure 8. At this time, the voltage across the filter capacitor C _inv is greater than zero. During the [0,D×Ts] period of the switching cycle, the "zero" level switch The first zero-level switch S2 and the second zero _- level switch S3 are turned on, and the other power switches are turned off. The energy of the filter capacitor C _inv is transferred to the filter inductor L;

Mode 2: The equivalent circuit is shown in (b) in Figure 8. At this time, the voltage across the filter capacitor C _inv is greater than zero, and all the switches are turned off during the [D×Ts, Ts] period of the switching cycle. _The current of the filter inductor L freewheels through the body diode of the positive level switch S1 and the body diode _of the zero level switch S2, and the energy of the filter inductor L is fed to the DC bus module;

Mode 3: The equivalent circuit is shown in (c) in Figure 8. At this time, the voltage across the filter capacitor C _inv is less than zero. During the [0,D×Ts] period of the switching cycle, the "zero" level switch The first zero-level switch S2 and the _second zero-level switch S3 are turned on, and the other power switches are turned off. The energy of the filter capacitor Cin is transferred to the filter inductor L;

Mode 4: The equivalent circuit is shown in (d) in Figure 8. At this time, the voltage across the filter capacitor C _inv is less than zero, and all the switches are turned off during the [D×Ts, Ts] period of the switching cycle. The voltage at both ends of the filter capacitor C _inv is less than zero, the inverter inductive current continues to flow through the body diode of the second zero-level switch S3 _{and the} body diode of the negative _- level switch S4, and the energy of the filter inductor L is fed to the DC bus;

Figure 9 is a three-phase I-type three-level inverter. As shown in the figure, the circuit is composed of three single-phase I-type three-level circuits. The working mode is similar to that of the single-phase I-type three-level inverter in the embodiment, and will not be repeated here.

Figure 10 is a three-phase I-type five-level inverter. As shown in the figure, the circuit is composed of three single-phase I-type five-level circuits. Transformers are similar and will not be repeated here.

It can be seen from the above-mentioned embodiments that the multi-level inverter circuit applicable to this scheme can have various modifications. When an operating overvoltage occurs, only the switching on and off of the "zero" level switching tube is controlled, and the remaining power switches If the tube remains turned off, the protection time requirement for operating overvoltage can be met. Under the premise of not adding additional hardware overhead, this solution can meet the needs of operation overvoltage protection through a simple control method.

The above-mentioned embodiments only illustrate the technical concept and characteristics of the present utility model, and its purpose is to allow those familiar with this technology to understand the content of the present utility model and implement it accordingly, and cannot limit the protection scope of the present utility model. All equivalent changes or modifications made according to the spirit of the utility model shall fall within the protection scope of the utility model.

Claims (2)

1. An operating overvoltage protection device for a multi-level photovoltaic grid-connected inverter, characterized in that: it includes a DC input unit, a multi-level inverter unit, a control unit, a voltage detection unit and a current detection unit; the DC The output end of the input unit is connected to the DC input end of the multi-level inverter unit, and the AC output end of the multi-level inverter unit is connected to the power grid through the grid-side switch K, but not limited to being connected to the power grid. The input end of the voltage detection unit is connected to the grid side for detecting the grid voltage, the output end of the voltage detection unit is connected to the input end of the control unit, the input end of the current detection unit is connected to the multi-level inverter unit The AC output terminal of the current detection unit is connected with the other input terminal of the control unit, and the output terminal of the control unit is connected with the driving terminal of the switching tube in the multi-level inverter unit, and the It is used to control the working state of the switching tube of the multi-level inverter unit. 2. An operating overvoltage protection device for a multi-level photovoltaic grid-connected inverter according to claim 1, characterized in that: the multi-level inverter unit is single-phase or three-phase; The variable unit is composed of a DC bus module, an inverter bridge arm, and an AC filter module connected in series in sequence. The DC bus module is composed of n-1 bus capacitors connected in series, and n is the number of levels; the multi-level inverter bridge arm Including "zero" level switch group, "positive" level switch group and "negative" level switch group, the "zero" level switch group is connected in series with the "positive" level switch group and Between the "negative" level switch groups, the AC filtering module is composed of a filter inductor L and a filter capacitor C _inv , one end of the filter inductor L is connected to the output of the inverter bridge arm, and the other end is a multi-level inverter The positive output end of the variable unit, one end of the filter capacitor C _inv is connected to the positive output end of the multi-level inverter unit, and the other end of the filter capacitor C _inv is connected to the midpoint of the bus capacitor as a multi-level inverter unit negative output terminal.