Series and parallel hybrid four-leg voltage regulating device

Abstract

Large-scale renewable energy power generation is connected to a power distribution network in a distributed mode, random faults of a power system are overlaid, and the three-phase voltage unbalance problem of the power distribution network is increasingly prominent. In order to solve the problem, the invention discloses a series and parallel hybrid four-leg voltage regulating device, and formulates control strategies of a parallel part and a series part and a modulation strategy corresponding to a special topological structure, thereby realizing a compensation function for various types of three-phase voltage problems in a three-phase three-wire system power distribution system. And reference significance is provided for voltage treatment of the high-proportion new energy permeability power distribution network.

Description

Technical Field

[0001] The present invention belongs to the technical field of flexible voltage regulation in distribution networks, and in particular to a method for realizing a low-cost voltage regulation device for multiplexing multiple bridge arms. Background Art

[0002] Large-scale distributed access to renewable energy generation has brought serious voltage quality problems to the distribution network. Due to the random, intermittent and volatile characteristics of renewable energy output, the power at the grid connection point fluctuates, which in turn causes voltage fluctuations near the grid connection point; secondly, since the peak load period of the power grid does not coincide with the peak power period of new energy sources such as photovoltaics and wind power, the power flow of the distribution line flows in both directions in different time periods, causing overvoltage and undervoltage problems to coexist at some nodes; in addition, the disorderly access of a large number of single-phase distributed grid-connected equipment and the random start and stop of high-power single-phase loads cause asymmetric three-phase voltages in the distribution network. Voltage quality issues are related to the actual interests of power users. A temporary voltage drop alone will cause a series of serious consequences, including abnormal operation of equipment, reduced product quality, damage to precision products, scrapping of raw materials, disordered production lines, and disordered subsequent startup.

[0003] Traditional voltage regulating equipment in distribution networks includes a series of compensation devices such as on-load voltage regulating transformers and switching capacitors. However, in the face of complex working conditions where voltage quality problems in new energy distribution networks change rapidly, overvoltage and undervoltage problems coexist, and three-phase asymmetry problems are serious, traditional voltage regulating equipment is no longer applicable. In recent years, many flexible voltage regulating devices such as dynamic voltage restorers (DVRs) and unified power quality controllers (UPQCs) have been widely studied. These devices are connected in series with distribution lines and actively adjust the compensation voltage of the series ports to keep the load voltage rated after compensation. However, these devices are expensive, especially high-power semiconductor devices, which account for a large part of the overall cost. Therefore, in order to effectively solve the voltage quality problem of new energy distribution networks, it is urgent to reduce the manufacturing cost of flexible voltage regulating devices.

[0004] The multiplexing bridge arm technology is an important means to reduce the manufacturing cost of flexible voltage regulation devices. Some bridge arms of the H bridge with multiple ports are merged to achieve the purpose of reducing the number of bridge arms used. In order to avoid mutual interference between the multiplexing bridge arms in the use of different ports, it is necessary to design a special method for decoupling control in the modulation strategy. Common multiplexing bridge arm topologies include a single-phase three-bridge arm back-to-back structure. This structure combines two bridge arms in the traditional single-phase four-bridge arm topology into one bridge arm for multiplexing, thereby reducing the use of a pair of power electronic bridge arms. In terms of modulation, a special space vector modulation method is used to decouple the multiplexing bridge arm, and the output voltage references of the two ports are regarded as the x-axis and y-axis respectively, and the overall reference voltage vector is synthesized, and then modulated using the traditional space vector modulation method. The single-phase three-bridge arm back-to-back structure can be applied in the field of single-phase voltage regulation. In comparison, the multiplexing bridge arm technology in the field of three-phase voltage regulation has not been widely used. Summary of the invention

[0005] The purpose of the present invention is to overcome the deficiencies of the prior art and propose a series-parallel hybrid four-bridge-arm voltage regulating device. The parallel port is connected in parallel with the A and B phases of the grid voltage, and the two series ports are connected in series with the B and C phases of the distribution line through an isolation transformer. The device has four power electronic bridge arms (bridge arm a, bridge arm b, bridge arm c, bridge arm n), the parallel port is composed of bridge arm a and bridge arm n connected to an LCL filter, the B-phase parallel port is composed of bridge arm b and bridge arm n connected to an LC filter, and the C-phase parallel port is composed of bridge arm c and bridge arm n connected to an LC filter. The device uses a voltage sensor to collect real-time data of the grid voltage, and after analysis and calculation by a controller, controls the series compensation port to output a compensation voltage to ensure the rated three-phase voltage after compensation.

[0006] The present invention solves the technical problem by adopting the following technical solutions: A series-parallel hybrid four-bridge-arm voltage regulating device comprises the following steps: Step 1, connect the DC side of the four-arm converter (arm a, arm b, arm c, arm n) to a capacitor, connect the midpoint output ports of arm a and arm n to the port on the side with large inductance of the LCL filter, and then connect the other side port of the LCL filter to the A phase and B phase of the distribution line in parallel; connect the midpoint output ports of arm b and arm n to the inductance side port of the LC filter, and then connect the other side port of the LC filter to the primary side of the isolation transformer, and the complex side of the isolation transformer is connected in series to the B phase of the distribution line; connect the midpoint output ports of arm c and arm n to the inductance side port of the LC filter, and then connect the other side port of the LC filter to the primary side of the isolation transformer, and the complex side of the isolation transformer is connected in series to the C phase of the distribution line; Step 2, the voltage sensor measures the DC bus voltage and sends the obtained data to the controller, the controller makes a difference between the DC bus set voltage and the measured DC bus voltage, and sends the result to the proportional integral controller for calculation, and then multiplies the result with the sine value of the real-time voltage phase angle between the AB phases obtained by the phase-locked loop to obtain the parallel port input current reference, makes a difference between the current reference and the input current of the bridge arm a, and sends the result to the proportional resonant controller for calculation to obtain the voltage reference between the bridge arm a and the bridge arm n; Step 3, the voltage sensor measures the three-phase grid voltage to obtain real-time data of the three-phase grid voltage, and sends the data to the controller. The controller first shifts the three-phase grid voltage backward by a quarter cycle, and uses the coordinate transformation of the three-phase grid voltage before and after the phase shift from the abc coordinate system to the αβ coordinate system. Then, the α component of the grid voltage before the phase shift is subtracted from the β component of the grid voltage after the phase shift and divided by 2 to obtain the α component of the positive-sequence grid voltage, the α component of the grid voltage after the phase shift is summed with the β component of the grid voltage before the phase shift and divided by 2 to obtain the β component of the positive-sequence grid voltage, the α component of the grid voltage before the phase shift is summed with the β component of the grid voltage after the phase shift and divided by 2 to obtain the α component of the negative-sequence grid voltage, and the β component of the grid voltage before the phase shift is subtracted from the α component of the grid voltage after the phase shift and divided by 2 to obtain the β component of the negative-sequence grid voltage. The obtained positive sequence voltage component is transformed into the abc coordinate system, and then the three-phase positive sequence component is phase-locked. The sine value of the obtained real-time phase angle is multiplied by the rated grid voltage as the reference value of the three-phase voltage after compensation. Then the reference value is subtracted from the actual grid voltage, where the difference between the line voltage AB and its rated voltage is used as the B-phase compensation voltage reference, and the inverse of the difference between the line voltage CA and its rated voltage is used as the C-phase compensation voltage reference; Step 4, the voltage sensor measures the voltage of the compensation port of phase B and phase C, and sends the result to the controller. The controller makes a difference between the output reference voltage of the series port of phase B and the actual output voltage, and sends the result to the proportional resonant controller for calculation to obtain the output current reference of bridge arm b, and makes a difference between the reference and the actual output current of bridge arm b, and then sends the result to the proportional resonant controller for calculation to obtain the voltage reference between bridge arm b and bridge arm n. The controller makes a difference between the output reference voltage of the series port of phase C and the actual output voltage, and sends the result to the proportional resonant controller for calculation to obtain the output current reference of bridge arm c, and makes a difference between the reference and the actual output current of bridge arm c, and then sends the result to the proportional resonant controller for calculation to obtain the voltage reference between bridge arm c and bridge arm n. Step 5. Arrange the voltage reference between bridge arms a and n, the voltage reference between bridge arms b and n, and the voltage reference between bridge arms c and n in order of size to obtain the maximum voltage reference and the minimum voltage reference, set the output voltage reference of bridge arm n to the inverse of the average value of the maximum voltage reference and the minimum voltage reference, set the output voltage reference of bridge arm a to the sum of the voltage reference of bridge arm n and the voltage reference between bridge arms a and n, set the output voltage reference of bridge arm b to the sum of the voltage reference of bridge arm n and the voltage reference between bridge arms b and n, and set the output voltage reference of bridge arm c to the sum of the voltage reference of bridge arm n and the voltage reference between bridge arms c and n, and then obtain the turn-on time and turn-off time of the upper and lower switches of each bridge arm according to the volt-second balance principle.

[0007] The advantages and positive effects of the present invention are: The present invention is reasonably designed and reduces the manufacturing cost of the three-phase series flexible voltage regulating device through the bridge arm multiplexing technology. In addition, the topological structure of the traditional three-phase voltage regulating equipment is simplified, and the functional structure irrelevant to the voltage regulating function is removed. Compared with the traditional series flexible distribution network voltage regulating equipment, the voltage regulating equipment proposed by the present invention reduces the manufacturing cost by 30% compared with the traditional equipment without affecting the voltage compensation effect. BRIEF DESCRIPTION OF THE DRAWINGS

[0008] Figure 1 This is a topological diagram of a proposed series-parallel hybrid four-bridge-arm voltage regulator device; Figure 2 This is the parallel port control block diagram of the proposed series-parallel hybrid four-bridge-arm voltage regulator device; Figure 3 This is the control block diagram of the series port of the proposed series-parallel hybrid four-bridge-arm voltage regulator; Figure 4 It is a modulation strategy diagram of a proposed series and parallel hybrid four-bridge-arm voltage regulator; Figure 5 This is a compensation effect diagram of the proposed series-parallel hybrid four-bridge-arm voltage regulator facing a three-phase symmetrical voltage drop scenario; Figure 6 This is a diagram showing the compensation effect of the proposed series-parallel hybrid four-arm voltage regulator facing a three-phase asymmetric voltage drop scenario. DETAILED DESCRIPTION

[0009] The implementation steps of the present invention are further described in detail below with reference to the accompanying drawings and specific examples.

[0010] A series and parallel hybrid four-bridge arm voltage regulating device is designed according to the capacity of 60kVA per phase. The specific implementation method includes the following steps: Step 1: Build the main circuit of the voltage regulator Firstly, three IGBT bridge arm modules with rated current of 120A and breakdown voltage of 1200V and one IGBT bridge arm module with rated current of 150A and breakdown voltage of 1200V are selected to form a four-bridge arm converter. The three bridge arm modules with rated current of 120A are used as bridge arm a, bridge arm b and bridge arm c respectively, and the bridge arm module with rated current of 150A is used as bridge arm n. The DC bus of the converter is connected to a group of split capacitors. The upper and lower capacitor parameters of the split capacitors are both rated voltage of 450V and capacity of 5000μF. An LCL filter is connected between bridge arms a and n, wherein the inductor is connected to the side of bridge arm a, the inductor of the LCL filter converter side is 0.4mH, the capacitor is 60μF, and the inductor of the distribution line side is 0.1mH. The output of the LCL filter is connected in parallel to the A and B phases of the distribution line, wherein the inductor side is connected to the A phase and the other side is connected to the B phase; an LC filter is connected between bridge arms b and n, wherein the inductor is 0.4mH and the capacitor is 40μF, the filter inductor is connected to the side of bridge arm b, the output of the LC filter is connected to the primary side of the isolation transformer, and the secondary side of the transformer is connected in series in the B phase distribution line, wherein the primary-to-secondary turns ratio is 4:1; an LC filter is connected between bridge arms c and n, wherein the inductor is connected to the side of bridge arm c, the inductor is 0.4mH and the capacitor is 40μF, the output of the LC filter is connected to the primary side of the isolation transformer, and the secondary side of the transformer is connected in series in the C phase distribution line, wherein the primary-to-secondary turns ratio is 4:1. The overall topology of the device is as follows Figure 1 In addition, voltage sensors are installed at both ends of the three-phase power grid, the two isolation transformer secondary sides, and the DC busbars, and current sensors are installed at the outgoing lines of the converter bridge arm a, bridge arm b, and bridge arm c, and the sensor data are transmitted to the controller.

[0011] Step 2: Parallel port control.

[0012] The DC bus reference voltage The DC bus voltage collected by the voltage sensor The result is sent to the proportional integral controller for operation to obtain the DC bus input current reference , which can be expressed as: ; in, k p Take it as 5, k i Take it as 2. Then perform phase-locked loop operation on the line voltage between phases A and B, perform sine operation on the result, and then multiply it by the DC bus current reference to obtain the parallel port current reference ,Will The output current of bridge arm a collected by the current sensor The difference is calculated and the result is sent to the proportional resonant controller to obtain the reference voltage between bridge arm a and bridge arm n. , expressed as: ; in, k p Take it as 20, k r Take it as 400, ω c Take it as 4, ω 0 is taken as 100π. The control block diagram of the parallel part is as follows Figure 2 shown.

[0013] Step 3: Grid voltage phase sequence decomposition Change the collected grid voltage to the αβ coordinate system to obtain the α-axis component of the grid voltage and the β-axis component , the transformation method is as follows: ; After that, the α-axis component and β-axis component of the grid voltage are respectively input into the transfer function F ( s ) performs a quarter-cycle phase shift operation to obtain the α-axis component of the grid voltage after phase shifting and the β-axis component ,in, F ( s ) is: ; After that, the grid voltage α component before phase shifting V g,α The β component of the grid voltage after phase shift Take the difference and divide by 2 to get the α component of the positive sequence grid voltage , the grid voltage α component after phase shift The β component of the grid voltage before phase shift V g,β Sum and divide by 2 to get the positive sequence grid voltage β component , the grid voltage α component before phase shift V g,α The β component of the grid voltage after phase shift Sum and divide by 2 to get the negative sequence grid voltage α component , the β component of the grid voltage before phase shift V g,β The α component of the grid voltage after phase shift Take the difference and divide by 2 to get the β component of the negative sequence grid voltage , the specific transformation formula is: ; Thereby, the α-axis component and the β-axis component of the positive sequence component and the α-axis component and the β-axis component of the negative sequence component are respectively.

[0014] Step 4: Serial port control The positive sequence component of the grid voltage is transformed back to the abc coordinate system, and then the phase-locked loop is used to obtain the real-time phase of the positive sequence voltage component. , , , the sine value of the three-phase voltage phase and the amplitude of the rated grid voltage V N,max Multiply them to get the compensated three-phase load voltage reference , , , then, the line voltage between phases AB is referenced Real-time line voltage between phases AB V AB Subtract and get the B phase compensation voltage reference , the line voltage between the CA phases is referenced Real-time line voltage between AC phases V CA Subtract and take the opposite number to get the C phase compensation voltage reference , expressed as: ; The B phase compensation voltage reference The actual compensation voltage of phase B V B,sc The result is sent to the proportional resonant controller to calculate the output current reference of the B phase series port. , and then compare the reference with the actual input current of bridge arm b I b,sc The difference is calculated and the result is sent to the proportional resonant controller for operation to obtain the reference voltage between bridge arm b and bridge arm n. The C phase compensation voltage reference The actual compensation voltage of phase B V B,sc The result is sent to the proportional resonant controller to calculate the output current reference of the C phase series port. , and then compare the reference with the actual input current of bridge arm c I c,sc The difference is calculated and the result is sent to the proportional resonant controller for operation to obtain the reference voltage between bridge arm c and bridge arm n. The specific control block diagram is as follows: Figure 3 shown.

[0015] Step 5: Compensation device modulation strategy The reference voltage between bridge arms a and n is obtained , the reference voltage between bridge arms b and n , the reference voltage between bridge arms c and n Arrange them in order of size and get the maximum voltage of the three V max and minimum voltage V min After that, set the output voltage reference of bridge arm n It is the inverse of the average value of the maximum voltage and the minimum voltage, and is expressed by the formula: ; The output voltage reference of bridge arm a is the output voltage reference of bridge arm n. The reference voltage between bridge arms a and n The output voltage reference of bridge arm b is the output voltage reference of bridge arm n. The reference voltage between bridge arms b and n The output voltage reference of bridge arm c is the output voltage reference of bridge arm n. The reference voltage between bridge arms c and n The sum of; expressed by formula: ; After that, within one cycle, the volt-second balance principle is used to obtain the on and off times of the upper and lower tubes of the bridge arm, and then the PWM signal driven by each IGBT is formed. The specific control block diagram of the modulation strategy is shown in the figure. Figure 4 shown.

[0016] Figure 5 and Figure 6 This is the experimental waveform diagram of the present invention. It can be seen that the proposed series-parallel hybrid four-arm voltage regulating device can cope with different types of voltage quality problems such as three-phase symmetrical voltage drop and three-phase asymmetric voltage drop, and the voltage compensation has a fast response speed and high compensation accuracy. It can be compensated to a steady state after a quarter of a cycle after the grid voltage drops, and the steady-state voltage waveform has no obvious distortion, and the effective value is the rated grid voltage value. The experimental waveform diagram further verifies the effectiveness of the proposed method.

[0017] It should be emphasized that the embodiments described in the present invention are illustrative rather than restrictive, and therefore the present invention is not limited to the embodiments described in the specific implementation manners. Any other implementation manners derived by those skilled in the art based on the technical solutions of the present invention also fall within the scope of protection of the present invention.

Claims

1. A series-parallel hybrid four-bridge-arm voltage regulating device, consisting of a parallel port and two series ports, characterized in that The parallel port and the series port are composed of four power electronic bridge arms (bridge arm a, bridge arm b, bridge arm c, bridge arm n), an LCL filter, and two LC filters. The parallel port is connected in parallel with the A phase and the B phase of the three-phase power grid, and the two series compensation ports are respectively connected in series in the B phase and the C phase lines through isolation transformers.

2. A series-parallel hybrid four-bridge-arm voltage regulating device according to claim 1, characterized in that The parallel port part is composed of an H bridge consisting of bridge arm a and bridge arm n, which is connected to an LCL type filter. The B phase series port part is composed of an H bridge consisting of bridge arm b and bridge arm n, which is connected to an LC type filter. The C phase series port part is composed of an H bridge consisting of bridge arm c and bridge arm n, which is connected to an LC type filter.

3. A series-parallel hybrid four-bridge-arm voltage regulating device according to claim 1, characterized in that The high-voltage side port of the isolation transformer is connected to the series output port of the device, and the low-voltage side port is connected in series with the power grid line. In addition, the same-name end of the high-voltage side is connected to the bridge arm n, and the same-name end of the low-voltage side is connected to the load side line.

4. A series-parallel hybrid four-bridge-arm voltage regulating device according to claim 1, characterized in that The calculation method of the series port output voltage reference is as follows: first, the grid voltage is decomposed into phase sequence, and secondly, the obtained positive sequence voltage component is subjected to phase-locked loop calculation, and then the obtained real-time phase angle of the grid positive sequence component voltage is multiplied by the rated amplitude of the voltage to obtain the rated reference of the three-phase grid voltage, and then the rated reference of the B phase voltage is subtracted from the actual voltage of the B phase to obtain the output voltage reference of the B phase series port, and the rated reference of the C phase voltage is subtracted from the actual voltage of the C phase and the inverse is taken to obtain the output voltage reference of the C phase series port.

5. A series-parallel hybrid four-bridge-arm voltage regulating device according to claim 4, characterized in that The phase sequence decomposition method has the following specific steps: first, the grid voltage is phase-shifted by a quarter cycle; second, in the αβ coordinate system, the α component of the actual grid voltage is subtracted from the β component of the grid voltage after the phase shift and divided by 2 to obtain the α component of the positive-sequence grid voltage; the α component of the grid voltage after the phase shift and the β component of the actual grid voltage are summed and divided by 2 to obtain the β component of the positive-sequence grid voltage; the α component of the actual grid voltage and the β component of the grid voltage after the phase shift are summed and divided by 2 to obtain the α component of the negative-sequence grid voltage; the β component of the actual grid voltage is subtracted from the α component of the grid voltage after the phase shift and divided by 2 to obtain the β component of the negative-sequence grid voltage.

6. A series-parallel hybrid four-bridge-arm voltage regulating device according to claim 2, characterized in that The closed-loop control strategy of the series port is as follows: the reference output voltage of the B-phase compensation port is subtracted from the actual output voltage of the B-phase compensation port as the B-phase output voltage error, the error is sent to the proportional resonant controller for calculation, and the result is subtracted from the output current of the bridge arm b, the difference is sent to the proportional resonant controller for calculation, and the output voltage reference of the bridge arm b and the bridge arm n is obtained; The difference between the reference output voltage of the C-phase compensation port and the actual output voltage of the C-phase compensation port is used as the C-phase output voltage error. The error is sent to the proportional resonant controller for calculation, and the result of the bridge arm c output current is subtracted and sent to the proportional resonant controller for calculation to obtain the output voltage reference of bridge arm c and bridge arm n.

7. A series-parallel hybrid four-bridge-arm voltage regulating device according to claim 2, characterized in that The control strategy of the parallel port is as follows: the DC bus reference voltage is subtracted from the actual DC bus voltage, and the difference is sent to the proportional-integral controller for calculation to obtain the parallel port output current reference, and the reference is subtracted from the actual output current of bridge arm a, and the result is sent to the proportional-integral controller to obtain the output voltage reference of bridge arm a and bridge arm n.

8. A series-parallel hybrid four-bridge-arm voltage regulating device according to claims 6 and 7, characterized in that The modulation method of the proposed device is as follows: first, the output voltage references of bridge arm a and bridge arm n, the output voltage references of bridge arm b and bridge arm n, and the output voltage references of bridge arm c and bridge arm n are selected according to their sizes, and the maximum and minimum values ​​are selected; secondly, the output voltage reference of bridge arm n is set to the inverse of the average value of the maximum voltage and the minimum voltage, the reference voltage of bridge arm a is set to the sum of the output voltage references of bridge arm a and bridge arm n and the output voltage reference of bridge arm n, the reference voltage of bridge arm b is set to the sum of the output voltage references of bridge arm b and bridge arm n and the output voltage reference of bridge arm n, and the reference voltage of bridge arm c is set to the sum of the output voltage references of bridge arm c and bridge arm n and the output voltage reference of bridge arm n, and then the action time of the up and down switches of each switch bridge arm is determined according to the volt-second balance rule.

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