An Improved Switch-Coupled Inductor Quasi-Z-Source Inverter

Abstract

The present invention belongs to the technical field of DC-AC conversion equipment and relates to an improved switch-coupled inductor quasi-Z-source inverter. The main structure includes a DC power supply, a first inductor, a first capacitor, a first diode, a second diode, a third diode, a second capacitor, a third capacitor, a first winding, a second winding, a third winding, a fourth winding and six power switch tubes, wherein the second diode, the third diode, the third capacitor, the first winding, the second winding, the third winding and the fourth winding constitute a boost unit; the first winding, the second winding, the third winding and the fourth winding are coupled in pairs and are all coupled in the same direction, and the unique connection method between the same-name ends of the four coupled inductors can effectively reduce the current stress of the winding, reduce the loss, and can reduce the resonance problem of the converter circuit, and the output efficiency is high.

Description

Technical field:

[0001] The invention belongs to the technical field of DC-AC conversion equipment and relates to an improved switch coupled inductor quasi-Z source inverter (MSCL qZSI). Background technology:

[0002] Since the 21st century, as energy crisis and environmental pollution problems have become increasingly severe, the rapid development of renewable clean energy has become the focus of energy development in countries around the world. Renewable clean energy such as solar energy, wind energy, hydropower, and nuclear energy have been widely used. Photovoltaic systems based on solar energy have attracted widespread attention from countries around the world due to their advantages of being pollution-free, noise-free, and rich in development resources. They have been identified as the most promising new energy technology in the future and are now widely used in industrial production and various fields of the national economy. Among them, voltage source inverters, as the most mature type of converter, have been widely used in uninterruptible power supplies, AC motor speed regulation, and new energy power generation. As a power conditioning system (PCS), inverters play a vital role in solar power generation systems. Traditional single-stage photovoltaic power generation systems use DC / AC inverters to directly transfer the energy output by photovoltaic cells to the power grid. Although this method can achieve high conversion efficiency, it also has great defects: first, it requires a higher input voltage, and multiple photovoltaic modules must be connected in series, which will greatly increase the system cost and failure rate and lead to serious power loss under non-ideal conditions; second, in actual operation, in order to track the maximum power point voltage of the photovoltaic cell, the DC bus voltage of the inverter needs to fluctuate within a large range, which leads to an additional increase in the power capacity of the inverter when designing the inverter. On the basis of a single-stage power generation system, the two-stage control structure of the cascaded DC-DC circuit can be used to solve the above problems. A DC / DC circuit with maximum power point tracking function is added between the photovoltaic module and the inverter. The DC circuit can be used to boost the voltage, thereby reducing the number of photovoltaic modules in series and obtaining a constant DC link voltage, realizing independent control of the inverter when it is connected to the grid and tracking the maximum power point of the photovoltaic cell. However, there are also some disadvantages: first, the addition of the DC-DC converter leads to a decrease in the efficiency of the entire system; second, the number of hardware circuits in the system is increased, the reliability of the system operation is reduced, and its maintenance cost is increased. In order to solve the problem of low gain and efficiency, Professor FZPeng proposed a new type of single-stage buck-boost inverter, the Z-Source Inverter (ZSI) in 2002. The Z-source inverter (ZSI) can boost and reduce the voltage of a single-stage inverter, and has certain advantages when used in photovoltaic power generation grid-connected systems. The development of single-chip photovoltaic module capacity has promoted the rapid development of micro-inverter grid-connected systems. High-voltage gain ZSI is very suitable for such application scenarios. However, experimental studies have found that traditional ZSI also has some shortcomings: First, due to its own topological structure, its voltage gain is low; second, its input side current is intermittent; third, the voltage across the energy storage capacitor is large; fourth, there are problems such as startup impact current and common mode noise.In order to solve these problems, many scholars have made a series of improvements to the traditional ZSI, among which the most classic improved circuit is the quasi-Z-source inverter (qZSI) circuit. The quasi-Z-source inverter (qZSI) is used in photovoltaic power generation grid-connected systems, which can realize the maximum power point tracking (MPPT) of photovoltaic modules and facilitate the grid connection of the inverter. The continuous input current of qZSI is conducive to the MPPT of photovoltaic modules, and the "low" voltage overshoot of the DC bus is conducive to the type selection of switches and the improvement of the electromagnetic environment (EMI) of the inverter. However, the low DC link boost capability of qZSI requires a large number of photovoltaic modules to be connected in series to achieve the grid-connected voltage level, which leads to high cost and failure rate of photovoltaic module systems. In the qZSI framework, embedding a specific boost unit will lead to a new high-voltage gain qZSI. Such an inverter can achieve the desired grid-connected voltage level by adjusting the turns ratio n and the through duty cycle D of the coupled inductor. It can maintain continuous input current and low DC bus voltage overshoot, and increase the boost capability of the inverter, but the current stress of the components in the boost unit is high, which increases the difficulty and cost of component selection. Therefore, it is emphasized to embed an improved boost unit in the qZSI. Hafiz Furqan Ahmed, Honnyong Cha et al. proposed the SCL qZSI, in which the bootstrap capacitor and the symmetrical parallel structure are used to improve the boost capability at a smaller shoot-through duty cycle D. The symmetrical parallel structure can reduce the current stress of the components. In addition, the third winding N23 added to the coupled inductor improves the boost capability of the inverter. However, the copper loss in the winding in the non-shoot-through state reduces the efficiency. The mSSCL qZSI proposed by Saeed Sharifi and Mohammad Monfared achieves a high voltage gain at a small D by using a bootstrap capacitor and a switched coupled inductor unit. However, during the shoot-through and non-shoot-through states, the third winding N23 of the switched coupled inductor has a high current stress. The high current stress leads to high copper loss of the coupled inductor, which in turn reduces the inverter efficiency. Therefore, seeking an inverter circuit with a simple structure, high conversion efficiency, low current stress, and a winding that is suitable for high boost occasions has become a challenging work topic in photovoltaic power generation systems. Summary of the invention:

[0003] The object of the present invention is to overcome the shortcomings of the prior art and to provide an improved switched coupled inductor quasi-Z-source inverter (MSCL qZSI) to provide continuous input current and low DC bus voltage spikes.

[0004] To achieve the above object, the main structure of the improved switched-coupled inductor quasi-Z-source inverter of the present invention includes a DC power supply, a first inductor, a first capacitor, a first diode, a second diode, a third diode, a second capacitor, a third capacitor, a first winding, a second winding, a third winding, a fourth winding, and six power switches. Among them, the second diode, the third diode, the third capacitor, the first winding, the second winding, the third winding, and the fourth winding form a boost unit; both ends of the first inductor are respectively connected to the positive pole of the DC power supply, the anode of the first diode, and the cathode of the second capacitor. The cathode of the first diode is respectively connected to the anode of the first capacitor, the same-named end of the first winding, and the anode of the third diode. The cathode of the third diode is connected to the different-named end of the fourth winding. The different-named end of the first winding is respectively connected to the different-named end of the third winding and the cathode of the third capacitor. The same-named end of the third winding is connected to the anode of the second diode. The anode of the third capacitor is respectively connected to the same-named end of the second winding and the same-named end of the fourth winding. The cathode of the second diode is respectively connected to the anode of the second capacitor and the different-named end of the second winding. The drains of the upper-bridge-arm power switches are respectively connected to the anode of the second capacitor, the cathode of the second diode, and the different-named end of the second winding. The sources of the lower-bridge-arm power switches are respectively connected to the negative pole of the DC power supply and the cathode of the first capacitor; the first winding, the second winding, the third winding, and the fourth winding are pairwise coupled, and their corresponding turns ratios are N 23 / N 21 = N 24 / N 22 = n, 0 < n < 1, and all are coupled in the same direction.

[0005] The present invention switches the circuit operating state by controlling the conduction or cut-off of the power switches, thereby controlling whether the DC power supply provides the energy required for the circuit operation to the coupled inductor. By changing the duty cycle and the turns ratio of the windings of the coupled inductor, the change of the input-output voltage gain is realized, so as to realize the step-up and step-down control of the output voltage with respect to the DC power supply.

[0006] Compared with the prior art, in actual operation, due to the conduction and cut-off of the switches, the process of continuous charging and discharging of the coupled inductor unit is realized, thereby achieving the purpose of high boost gain; moreover, the unique connection method between the same-named ends of the four coupled inductors can effectively reduce the current stress of the windings, reduce losses, and can reduce the resonance problems occurring in the converter circuit, and the output efficiency is high. BRIEF DESCRIPTION OF THE DRAWINGS:

[0007] Figure 1 It is a schematic diagram of the circuit principle of the main structure of the present invention.

[0008] Figure 2 It is a schematic diagram of the working state of the circuit when the power switches S1 - S6 (labeled as S eq ) of the present invention are conducting.

[0009] Figure 3 This is a schematic diagram of the working state of the circuit when the power switch tubes S1-S6 of the present invention are turned off.

[0010] Figure 4 4 is a relationship diagram between the boost factor B and the through duty cycle D of the three inverters described in the embodiment of the present invention.

[0011] Figure 5 Graph showing the relationship between winding current and voltage gain G of the three inverters described in the embodiment of the present invention.

[0012] Figure 6 Graph showing the functional relationship between the efficiency and output power of the three inverters described in the embodiment of the present invention. Specific implementation method:

[0013] The present invention will be further described below in conjunction with the accompanying drawings and specific implementations.

[0014] Example:

[0015] The main structure of the MSCL qZSI described in this embodiment is as follows Figure 1 As shown, the boost unit containing the coupled winding is used to replace the single independent energy storage inductor in the traditional boost topology. By utilizing the characteristics of the coupled inductor being charged and discharged simultaneously, the original inverter only has the duty cycle D as an adjustment factor, and the turns ratio as an adjustable free factor is added. By changing the turns ratio of the coupled winding, the high voltage conversion capability is achieved, specifically including the DC power supply V g , a first inductor L1, a first capacitor C1, a first diode D1, a second diode D 21 , the third diode D 22 , the second capacitor C2, the third capacitor C 21 、The first winding N 21 , the second winding N 22 、The third winding N 23 、The fourth winding N 24 and six power switch tubes S1-S6, among which the second diode D 21 , the third diode D 22 , the third capacitor C 21 、The first winding N 21 , the second winding N 22 、The third winding N 23 and the fourth winding N 24 The two ends of the first inductor L1 are connected to the DC power supply V g The positive electrode of the first diode D1 is connected to the anode of the first capacitor C1, the cathode of the first winding N 21The same-name terminal and the third diode D 22 The anode of the third diode D 22 The cathode of the fourth winding N 24 The opposite end of the first winding N 21 The opposite end of the third winding N 23 The opposite end and the third capacitor C 21 The cathode of the third winding N 23 The same-name terminal of the second diode D 21 The anode of the third capacitor C 21 The anode of the second winding N 22 The same-name end and the fourth winding N 24 The second diode D 21 The cathode of the second capacitor C2 is connected to the anode of the second winding N 22 The drains of the upper bridge wall power switch tubes S1-S3 are connected to the anode of the second capacitor C2 and the second diode D 21 The cathode and the second winding N 22 The opposite ends are connected, and the sources of the lower bridge arm power switches S4-S6 are connected to the DC power supply V g The negative electrode of the first winding N is connected to the cathode of the first capacitor C1; 21 , the second winding N 22 、The third winding N 23 and the fourth winding N 24 Two-to-two coupling, the corresponding turns ratio is N 23 / N 21 =N 24 / N 22 =n, and both are coupled in the same direction.

[0016] This embodiment adopts a unipolar SPWM mode to control the on or off of the power switch tube to complete the switching of different working modes, thereby reducing the switching loss in the overall circuit structure and improving the overall working efficiency of the circuit. The different working states of the circuit when the switch tube is turned off and on are respectively as follows: Figure 2 and Figure 3 As shown:

[0017] In the forward state, the first diode D1 is reverse biased, while the second diode D 21 and the third diode D 22 The first inductor L1 is connected by the second capacitor C2 and the input power supply V g Charging, four windings N 21 、N 22 、N 23 and N 24 All are charged by the first capacitor C1, and the third capacitor C 21The energy from the first capacitor C1 is stored. The specific current loop is as follows: Figure 2 As shown. At this time, the circuit has the following voltage and current relationship:

[0018]

[0019]

[0020] In the non-through state, the second diode D 21 and the third diode D 22 is reverse biased, the first diode D1 is turned on, and the current stored in the first winding N 21 , the second winding N 22 The energy in the first inductor L1 is equal to the DC power supply V g Combined to supply power to the load, the first capacitor C1 and the second capacitor C2 are charged in this state. The specific current loop is as follows: Figure 3 As shown, at this time, the circuit structure has the following voltage and current relationship:

[0021]

[0022] Among them, V L1-ON , V N-ON , V L1-OFF , V N-OFF are the voltages across the magnetic components (inductors and windings) in the through state and the non-through state, respectively. C is the capacitor voltage, V PN is the peak DC link voltage,

[0023] Applying the volt-second balance theory to L1 and N 21 ,get:

[0024]

[0025] Applying the ampere-second balance theory to C2, we obtain:

[0026]

[0027] From the current relationship in the non-through state, we can get:

[0028]

[0029] Where V N23 =V N24 =-nV N21 =-nV N22

[0030]

[0031] VL1-ON =(1-D)V PN

[0032]

[0033] This gives the peak DC link boost factor B of the MSCL qZSI:

[0034]

[0035] Where B is the peak DC link boost factor of the inverter and D is the shoot-through duty cycle (0 <D<1),n=N 23 / N 21 =N 24 / N 22 is the turns ratio, 0 <n<1,N 21 =N 22 ; The peak DC link boost factor of SCL qZSI and mSSCL qZSI in the prior art is as follows:

[0036]

[0037] The three inverters mentioned above have the same type of boost factor, namely B = k N / 1-kD, k>1, k N >1 is a coefficient associated with the structure of the inverter. Although the proposed inverter has four windings, in order to make the comparison more valuable, in this embodiment, the comparison is based on the same total turns ratio of the coupled inductors in different inverters. Therefore, the total turns ratio N in MSCL qZSI is 23 / N 21 +N 24 / N 22 =0.15+0.15=0.3 and the turns ratio in SCL qZSI and mSSCL qZSI is n=N 23 / N 21 = 0.3, in the relationship diagram between the boost factor B and the direct duty cycle D (such as Figure 4 As shown in Figure 2, the boost factor of MSCL qZSI is between the boost factors of mSSCL qZSI and SCL qZSI;

[0038] Using a simple boost control method, the relationship between D and M is: D = 1-M

[0039]

[0040]

[0041] In the relationship diagram between winding current and voltage gain G (such as Figure 5As shown in the figure, the winding N of MSCL qZSI 23 With N 24 The current of winding N is obviously lower than that of mSSCL qZSI and SCL qZSI. 21 With N 22 The current mSSCL qZSI and SCL qZSI are similar; the functional relationship between the efficiency and output power of the three inverters is shown in the figure Figure 6 As shown, the MSCL qZSI has a higher efficiency than the other two inverters due to the lower current stress on windings N23 and N24. Although the mSSCL qZSI has a high boost capability, its efficiency is the lowest among the three inverters due to the high current stress in windings N23 and N24. The power dissipation of the coupled inductor in the MSCLqZSI is much smaller than that of the other two candidates.

[0042] In this embodiment, the MSCL qZSI is tested under the test conditions of input voltage 100V, output phase voltage effective value 120V and output power 1kW, and the maximum efficiency reaches 92.8%, which basically meets the design requirements. The above analysis and experimental results show that the MSCL qZSI of this embodiment has the advantages of high voltage gain and high efficiency. It provides continuous input current and low DC bus voltage spike, and can achieve maximum power point tracking (MPPT) of photovoltaic modules; due to the unique design of coupled inductor, the winding current stress in the proposed MSCL qZSI is lower than that of SCL qZSI and mSSCL qZSI, which reduces the power loss of the winding and improves the efficiency of the proposed inverter.

[0043] Although the above describes the specific implementation mode of the present invention in conjunction with the accompanying drawings, it is not intended to limit the scope of protection of the present invention. Those skilled in the art should understand that various modifications or variations that can be made by those skilled in the art on the basis of the technical solution of the present invention without creative work are still within the scope of protection of the present invention.

Claims

1. An improved switch-coupled inductor quasi-Z-source inverter, characterized in that The main structure includes a DC power supply, a first inductor, a first capacitor, a first diode, a second diode, a third diode, a second capacitor, a third capacitor, a first winding, a second winding, a third winding, a fourth winding, and six power switching tubes. Among them, the second diode, the third diode, the third capacitor, the first winding, the second winding, the third winding, and the fourth winding form a boost unit; both ends of the first inductor are respectively connected to the positive electrode of the DC power supply, the anode of the first diode, and the cathode of the second capacitor. The cathode of the first diode is respectively connected to the anode of the first capacitor, the same-named end of the first winding, and the anode of the third diode. The cathode of the third diode is connected to the different-named end of the fourth winding. The different-named end of the first winding is respectively connected to the different-named end of the third winding and the cathode of the third capacitor. The same-named end of the third winding is connected to the anode of the second diode. The anode of the third capacitor is respectively connected to the same-named end of the second winding and the same-named end of the fourth winding. The cathode of the second diode is respectively connected to the anode of the second capacitor and the different-named end of the second winding. The drains of the upper-bridge-arm power switching tubes are respectively connected to the anode of the second capacitor, the cathode of the second diode, and the different-named end of the second winding. The sources of the lower-bridge-arm power switching tubes are respectively connected to the negative electrode of the DC power supply and the cathode of the first capacitor; the first winding and the third winding are coupled, and the second winding and the fourth winding are coupled, and their corresponding turns ratio is N 23 / N 21 = N 24 / N 22 =n, 0 < n < 1, and they are all coupled in the same direction.

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