CN114499259B - Single-phase five-level photovoltaic grid-connected inverter and control method thereof - Google Patents

Abstract

The present invention provides a single-phase five-level photovoltaic grid-connected inverter with active power decoupling and boosting functions and a control method thereof. The five-level inverter is composed of a photovoltaic cell, a bus capacitor, a first half-bridge commutation circuit, a second half-bridge boosting circuit, a third half-bridge T-type three-level circuit and a filter circuit. The upper and lower ends of the first half-bridge commutation circuit are connected to the positive and negative electrodes of the bus capacitor, and the midpoint of the bridge arm is connected to the negative end of the power grid. The positive electrode of the photovoltaic cell is connected to the midpoint of the second half-bridge circuit through a DC filter inductor, and together they form the second half-bridge boosting circuit. The input end of the third half-bridge T-type three-level circuit is connected to the photovoltaic cell, the upper and lower ends are connected to the positive and negative electrodes of the bus capacitor, the midpoint is connected to one end of the filter circuit, and the other end of the filter circuit is connected to the positive electrode of the power grid. The power decoupling of the single-phase five-level inverter system can be automatically realized by controlling the photovoltaic cell voltage to a constant value and the grid-connected current to a sinusoidal value in real time through a closed loop. The proposed inverter topology and control strategy have boosting and active power decoupling functions while realizing multi-level output, and can effectively improve the maximum power point tracking efficiency and reliability of the photovoltaic inverter system.

Description

A single-phase five-level photovoltaic grid-connected inverter and control method thereof

Technical Field

The present invention relates to the field of power electronics technology and photovoltaic new energy power generation, and in particular to a single-phase photovoltaic five-level grid-connected inverter with active power decoupling and boosting functions and a control strategy thereof.

Background technique

With the increasing exhaustion of traditional fossil energy and the increasingly serious environmental pollution problem, clean energy such as photovoltaic power generation has become a hot topic in the world. As an important part of the photovoltaic power generation system, the photovoltaic inverter plays a vital role in the safe and reliable operation of the system and the efficient use of energy.

With the development of power electronics technology and the "silicon in, copper out" strategy of the photovoltaic industry, photovoltaic inverters are developing towards high efficiency, high power density and medium and high voltage. Compared with traditional three-level inverters, multi-level inverters have the advantages of low voltage stress, low grid-connected current harmonics and small filter inductance, so they are widely used in medium and high voltage applications. However, traditional multi-level inverters require a large number of components and have a single function, so the device utilization rate is low.

In addition, in a single-phase inverter system, the inherent double frequency power pulsation of the system will affect the grid current power quality and reduce the system maximum power point tracking efficiency. In order to buffer the double frequency power pulsation, there are usually two methods: 1) Incorporating a larger electrolytic capacitor or LC series resonant circuit on the DC side; 2) Adding an additional active decoupling circuit. Method 1 is simple and direct, but the bulky electrolytic capacitor or LC resonant circuit reduces the power density and reliability of the system; while method 2 requires additional hardware circuits, which increases the cost of the system.

Summary of the invention

The present invention provides a single-phase five-level photovoltaic grid-connected inverter and a control strategy thereof, the purpose of which is to achieve functions such as multi-level output, voltage boost and power decoupling of a single-phase photovoltaic power generation system while reducing the cost of the photovoltaic power generation system.

In order to achieve the above objectives, an embodiment of the present invention provides a single-phase five-level photovoltaic grid-connected inverter with active power decoupling and boosting functions and a control strategy thereof. The specific technical solution is as follows:

In a first aspect, an embodiment of the present invention provides a single-phase five-level photovoltaic grid-connected inverter topology, including a photovoltaic cell, a bus capacitor, a first half-bridge commutation circuit, a second half-bridge boost circuit, a third half-bridge T-type three-level circuit and a filter circuit; wherein,

The first half-bridge commutation circuit is composed of upper and lower bridge arm switch tubes _S1 and _S2 ; the second half-bridge boost circuit is composed of upper and lower bridge arm switch tubes _S3 , _S4 and a DC filter inductor; the third half-bridge T-type three-level circuit is composed of upper and lower bridge arm switch tubes _S6 , _S7 and a midpoint switch tube _S5 ; the filter circuit is composed of an L-type filter or an LC or LCL-type filter.

The collector of the switch tube _S1 of the first half-bridge commutation circuit is connected to the positive electrode of the bus capacitor, and the emitter of the switch tube _S1 is connected to the electrode of the switch tube _S2 , and the connection point is the midpoint of the first half-bridge commutation circuit, and the midpoint is connected to the negative electrode of the power grid; the emitter of the switch tube _S2 is connected to the negative electrode of the bus capacitor;

The collector of the switch tube _S3 of the second half-bridge boost circuit is connected to the positive electrode of the bus capacitor, and the emitter of the switch tube _S3 is connected to the electrode of the switch tube _S4 , and the connection point is the midpoint of the second half-bridge boost circuit, and the midpoint is connected to one end of the DC filter inductor _L1 ; the other end of the DC filter inductor _L1 is connected to the positive electrode of the photovoltaic cell; the emitter of the switch tube _S4 is connected to the negative electrode of the bus capacitor;

The collector of the switch tube _S6 of the third half-bridge T-type three-level circuit is connected to the positive electrode of the bus capacitor, and the emitter of the switch tube _S6 is connected to the electrode of the switch tube _S7 . The connection point is the midpoint of the T-type three-level circuit, the midpoint is connected to one end of the switch tube _S5 , and the other end is connected to one end of the AC filter; the other end of the switch tube _S5 is connected to the positive electrode of the photovoltaic cell, and the emitter of the switch tube _S7 is connected to the negative electrode of the bus capacitor; the other end of the AC filter circuit is connected to the positive electrode of the power grid.

The single-phase five-level inverter topology can output five levels in one power frequency cycle, specifically: when the switch tubes _S2 and _S7 are turned on or the switch tubes _S1 and _S6 are turned on, and the other switch tubes are turned off, the converter output voltage _Vab is 0; when the switch tubes _S2 and _S5 are turned on, and the other switch tubes are turned off, the converter output voltage _Vab is _Vpv ; when the switch tubes _S2 and _S6 are turned on, and the other switch tubes are turned off, the converter output voltage _Vab is _Vdc ; when the switch tubes _S1 and _S5 are turned on, and the other switch tubes are turned off, the converter output voltage _Vab is _Vpv - _Vdc ; when the switch tubes _S1 and _S7 are turned on, and the other switch tubes are turned off, the converter output voltage _Vab is _-Vdc .

In a second aspect, an embodiment of the present invention provides a single-phase five-level photovoltaic grid-connected inverter control strategy, which is applied to the single-phase five-level photovoltaic grid-connected inverter described in the first aspect, and the control method is:

The inverter function is completed by the first half-bridge commutation circuit and the third half-bridge T-type three-level circuit; the boost function, active power decoupling function and photovoltaic maximum power point tracking function are completed by the second half-bridge boost circuit.

Among them, the control implementation steps of the inverter part are as follows:

Step 11: Set the bus capacitor voltage reference constant to U _dcref , make a difference between the bus capacitor reference voltage and the feedback voltage processed by the double frequency notch filter, input the difference value into the voltage outer loop controller, and the voltage loop controller outputs the amplitude of the current inner loop reference;

Step 12: The current loop reference is obtained by multiplying the current reference amplitude with the cosine value of the phase-locked loop lock-out angle. The current loop reference is subtracted from the grid-connected feedback current and then sent to the current loop controller.

Step 13: The current loop controller outputs a modulated wave, which is compared with a phase-shifted carrier or a stacked carrier to generate a PWM wave to drive the switch tubes S ₁ , S ₂ , S ₅ , S ₆ , and S ₇ .

The control steps for the boost and power decoupling parts are as follows:

Step 21: Obtain the photovoltaic cell reference voltage U _pvref through the maximum power point tracking algorithm, make a difference between the reference voltage and the feedback voltage and send it to the voltage loop controller, and the output of the voltage loop controller is the current loop reference;

Step 22: Subtract the current reference from the feedback current and send it to the current loop controller, and the output of the current loop controller is a modulated wave;

Step 23: Compare the modulation wave with the carrier wave to generate a PWM wave to drive the switch tubes _S3 and _S4 .

Furthermore, in step 21, the voltage loop controller needs to adopt a multi-resonance controller or a repetitive controller with the ability to suppress odd harmonics such as the 3rd, 5th, and 7th harmonics to achieve power decoupling between the photovoltaic input side and the inverter output side.

The present invention proposes a single-phase five-level photovoltaic grid-connected inverter and a control strategy thereof, which has the following advantages over the prior art:

1. Fewer switching devices and lower cost. The proposed inverter topology only needs seven switches to achieve boost and five-level output functions, which reduces system cost while realizing multi-functions of photovoltaic inverters;

2. High reliability and high power density. Through the proposed control strategy, the system power can be naturally decoupled without additional hardware circuits, and small film capacitors can be used to replace bulky electrolytic capacitors, which improves the system reliability and power density.

Therefore, the solution proposed in the present invention is very suitable for a single-phase photovoltaic power generation system.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG1 is a topological structure diagram of a single-phase five-level photovoltaic grid-connected inverter according to an embodiment of the present invention;

FIG2-3 is a modulation diagram and a schematic diagram of the working principle of the single-phase five-level photovoltaic inverter shown in FIG1;

FIG4 is a schematic diagram of a control strategy for the single-phase five-level inverter shown in FIG1 ;

FIG. 5 is a diagram of dynamic and steady-state simulation waveforms of an embodiment of the present invention.

[Description of Reference Numerals]

1-first half-bridge commutation circuit; 2-second half-bridge boost circuit; 3-third half-bridge T-type three-level circuit; 4-filter circuit;

Detailed ways

In order to make the technical problems, technical solutions and advantages to be solved by the present invention more clear, a detailed description will be given below with reference to the accompanying drawings and specific embodiments.

As shown in FIG1 , a single-phase five-level photovoltaic grid-connected inverter with active power decoupling and boosting functions includes a photovoltaic cell PV, a bus capacitor C _dc , a first half-bridge commutation circuit, a second half-bridge boosting circuit, a third half-bridge T-type three-level circuit and a filter circuit; wherein,

The first half-bridge commutation circuit is composed of upper and lower bridge arm switch tubes _S1 and _S2 ; the second half-bridge boost circuit is composed of upper and lower bridge arm switch tubes _S3 , _S4 and a DC filter inductor; the third half-bridge T-type three-level circuit is composed of upper and lower bridge arm switch tubes _S6 , _S7 and a midpoint switch tube _S5 ; the filter circuit is composed of an L-type filter or an LC or LCL-type filter.

The collector of the switch tube _S1 of the first half-bridge commutation circuit is connected to the positive electrode of the bus capacitor, and the emitter of the switch tube _S1 is connected to the electrode of the switch tube _S2 , and the connection point is the midpoint of the first half-bridge commutation circuit, and the midpoint is connected to the negative electrode of the power grid; the emitter of the switch tube _S2 is connected to the negative electrode of the bus capacitor;

The collector of the switch tube _S3 of the second half-bridge boost circuit is connected to the positive electrode of the bus capacitor, and the emitter of the switch tube _S3 is connected to the electrode of the switch tube _S4 , and the connection point is the midpoint of the second half-bridge boost circuit, and the midpoint is connected to one end of the DC filter inductor _L1 ; the other end of the DC filter inductor _L1 is connected to the positive electrode of the photovoltaic cell; the emitter of the switch tube _S4 is connected to the negative electrode of the bus capacitor;

The collector of the switch tube _S6 of the third half-bridge T-type three-level circuit is connected to the positive electrode of the bus capacitor, and the emitter of the switch tube _S6 is connected to the electrode of the switch tube _S7 . The connection point is the midpoint of the T-type three-level circuit, the midpoint is connected to one end of the switch tube _S5 , and the other end is connected to one end of the AC filter; the other end of the switch tube _S5 is connected to the positive electrode of the photovoltaic cell, and the emitter of the switch tube _S7 is connected to the negative electrode of the bus capacitor; the other end of the AC filter circuit is connected to the positive electrode of the power grid.

It should be noted that the switch tube _S5 is composed of two conventional unidirectional switch tubes connected in reverse series; the other switch tubes are all composed of switch tubes with anti-parallel diodes.

FIG2 and FIG3 are respectively a modulation waveform diagram and a working principle diagram based on the single-phase five-level inverter shown in FIG1 .

As shown in Fig. 3(a) and 3(b), when the switch tubes _S2 and _S7 are turned on or the switch tubes _S1 and _S6 are turned on, and the other switch tubes are turned off, the output voltage of the converter is 0 level;

As shown in FIG3(c), when the switch tubes _S2 and _S5 are turned on and the other switch tubes are turned off, the converter output voltage is _Vpv ;

As shown in FIG3(d), when the switch tubes _S2 and _S6 are turned on and the other switch tubes are turned off, the converter output voltage is V _dc ;

As shown in FIG3(e), when the switch tubes _S1 and _S5 are turned on and the other switch tubes are turned off, the converter output voltage is V _pv -V _dc ;

As shown in FIG3(f), when the switch tubes _S1 and _S7 are turned on and the other switch tubes are turned off, the converter output voltage is -V _dc .

Based on the single-phase five-level inverter shown in Figure 1, an embodiment of the present invention provides a control strategy for this circuit, and its control structure diagram is shown in Figure 4. The control of the system includes an inverter part and a boost and power decoupling part.

The control steps of the inverter part are as follows:

Step 11: Set the bus capacitor voltage reference constant to U _dcref , make a difference between the bus capacitor reference voltage and the feedback voltage processed by the double frequency notch filter G _noth (s), input the difference value into the voltage outer loop controller G _iv (s), and the voltage loop controller outputs the amplitude I _m of the current inner loop reference; wherein the transfer function of the double frequency notch filter is:

Where ξ is the bandwidth coefficient, ω _n = 4πf ₀ , and f ₀ is the fundamental frequency.

Step 12: The current reference amplitude is multiplied by the sine value of the phase-locked loop lock-out angle to obtain the current loop reference I _gref , and the current loop reference is subtracted from the grid-connected feedback current and sent to the current loop controller G _ii (s);

Step 13: The current loop controller outputs a modulation wave v _m1 , which is compared with the phase-shifted carrier or stacked carrier v _cr to generate a PWM wave to drive the switch tubes S ₁ , S ₂ , S ₅ , S ₆ , S ₇ .

The control steps for the boost and power decoupling parts are as follows:

Step 21: Obtain the photovoltaic cell reference voltage U _pvref through the maximum power point tracking algorithm, and send the difference between the reference voltage and the feedback voltage to the voltage loop controller G _bv (s), and the output of the voltage loop controller is the current loop reference I _lref ;

Step 22: Subtract the current reference from the feedback current and send it to the current loop controller G _bi (s), and the output of the current loop controller is the modulation wave v _m2 ;

Step 23: Compare the modulation wave with the carrier wave v _tr to generate a PWM wave to drive the switch tubes S ₃ and S ₄ .

It should be emphasized that the voltage loop controller G _bv (s) in step 21 needs to adopt a multi-resonant controller or a repetitive controller with the ability to suppress odd harmonics such as the 3rd, 5th, and 7th harmonics to prevent the pulsating power from penetrating into the photovoltaic cell side, thereby achieving natural power decoupling between the photovoltaic input side and the inverter output side.

In order to verify the superior performance of the proposed topology and control strategy, a single-phase five-level photovoltaic grid-connected inverter simulation platform shown in Figure 1 was built, and the system parameters are shown in Table 1. Figure 5 (a) is a simulation waveform diagram under steady state, and Figure 5 (b) is a dynamic simulation waveform diagram of the system when the illumination changes suddenly. It can be seen from the above figures that the system proposed in the present invention has a boost and five-level output while achieving good power decoupling, so it is very suitable for a single-phase photovoltaic power generation system.

Table 1 System parameters

The above is a preferred embodiment of the present invention. It should be pointed out that for ordinary technicians in this technical field, several improvements and modifications can be made without departing from the principles of the present invention. These improvements and modifications should also be regarded as the scope of protection of the present invention.

Claims (7)

1. The single-phase five-level photovoltaic grid-connected inverter with the active power decoupling and boosting functions is characterized by comprising a photovoltaic cell, a bus capacitor, a first half-bridge reversing circuit, a second half-bridge boosting circuit, a third half-bridge T-shaped three-level circuit and a filter circuit;

The first half-bridge reversing circuit consists of an upper bridge arm switch tube S ₁ and a lower bridge arm switch tube S ₂; the second half-bridge boost circuit consists of an upper bridge arm switch tube S ₃,S₄, a lower bridge arm switch tube S ₃,S₄ and a direct current filter inductor, the third half-bridge T-shaped three-level circuit consists of an upper bridge arm switch tube S ₆、S₇, a lower bridge arm switch tube S ₆、S₇ and a midpoint switch tube S ₅, and the filter circuit consists of an L-shaped filter or an LC or LCL-shaped filter;

The collector of the switching tube S ₁ of the first half-bridge reversing circuit is connected with the positive electrode of the bus capacitor, the emitter of the switching tube S ₁ is connected with the electrode of the switching tube S ₂, the connecting point is the midpoint of the first half-bridge reversing circuit, the midpoint is connected with the negative electrode of the power grid, and the emitter of the switching tube S ₂ is connected with the negative electrode of the bus capacitor;

The collector of the switching tube S ₃ of the second half-bridge boosting circuit is connected with the positive electrode of the bus capacitor, the emitter of the switching tube S ₃ is connected with the electrode of the switching tube S ₄, the connecting point is the midpoint of the second half-bridge boosting circuit, the midpoint is connected with one end of the direct current filter inductor L ₁, the other end of the direct current filter inductor L ₁ is connected with the positive electrode of the photovoltaic cell, and the emitter of the switching tube S ₄ is connected with the negative electrode of the bus capacitor;

The collector of the third half-bridge T-shaped three-level circuit switching tube S ₆ is connected with the positive electrode of the bus capacitor, the emitter of the switching tube S ₆ is connected with the electrode of the switching tube S ₇, the connecting point is a midpoint of the T-shaped three-level circuit, the midpoint is connected with one end of the switching tube S ₅ and one end of the alternating current filter, the other end of the switching tube S ₅ is connected with the positive electrode of the photovoltaic cell, the emitter of the switching tube S ₇ is connected with the negative electrode of the bus capacitor, and the other end of the alternating current filter circuit is connected with the positive electrode of the power grid.

2. The single-phase five-level photovoltaic grid-connected inverter according to claim 1, wherein the bidirectional switching tube S ₅ in the circuit is composed of two conventional unidirectional switching tubes connected in series in opposite directions, and the switching tubes S ₁,S₂,S₃,S₄,S₆,S₇ are composed of switching tubes with antiparallel diodes.

3. The single-phase five-level photovoltaic grid-connected inverter according to claim 1, wherein five levels can be output in one power frequency period, respectively:

The switching tube S ₂,S₇ or the switching tube S ₁,S₆ is turned on, other switching tubes are turned off, and the output voltage V _ab of the converter is 0;

Switching on the switching tubes S ₂ and S ₅, switching off the other switching tubes, and outputting voltage V _ab of the converter to be V _pv;

Switching on the switching tubes S ₂ and S ₆, switching off the other switching tubes, and outputting voltage V _ab of the converter to be V _dc;

switching on the switching tubes S ₁ and S ₅, switching off the other switching tubes, and outputting voltage V _ab of the converter to be V _pv-V_dc;

Switching on the switching tubes S ₁ and S ₇, switching off the other switching tubes, and enabling the output voltage V _ab of the converter to be-V _dc;

wherein V _pv is photovoltaic voltage, and V _dc is direct current bus voltage.

4. The control method of the single-phase five-level photovoltaic grid-connected inverter according to any one of claims 1 to 3 comprises the following steps:

The inversion function is completed by the first half-bridge reversing circuit and the third half-bridge T-shaped three-level circuit; the boosting function, the active power decoupling function and the photovoltaic maximum power point tracking function are completed by the second half-bridge boosting circuit.

5. The control method of the single-phase five-level photovoltaic grid-connected inverter according to claim 4, wherein the control implementation step of the inversion section is as follows:

Step 11: setting the voltage reference of the bus capacitor as U _dcref, making a difference between the reference voltage of the bus capacitor and the feedback voltage processed by the frequency doubling trap, inputting the value obtained after the difference into a voltage outer ring controller, and outputting the voltage outer ring controller as the amplitude value of the current inner ring reference;

Step 12: multiplying the current reference amplitude value by the cosine value of the locking angle of the phase-locked loop to obtain a current loop reference, and sending the current loop reference and the grid-connected feedback current into a current loop controller after making a difference;

Step 13: the current loop controller outputs a modulated wave, compares the modulated wave with a phase-shifted carrier or a laminated carrier, and generates a PWM wave to drive the switching tube S ₁,S₂,S₅,S₆,S₇.

6. The control method of the single-phase five-level photovoltaic grid-connected inverter according to claim 4, wherein the control implementation steps of the step-up and power decoupling portion are as follows:

step 21: obtaining a photovoltaic cell reference voltage U _pvref through a maximum power point tracking algorithm, and sending the difference between the reference voltage and the feedback voltage into a voltage loop controller, wherein the voltage loop controller outputs the reference voltage as a current loop reference;

Step 22: the current reference and the feedback current are fed into a current loop controller after being differenced, and the current loop controller outputs modulated waves;

Step 23: the modulated wave is compared with the carrier wave to generate a PWM wave to drive the switching transistors S ₃ and S ₄.

7. The method according to claim 6, wherein the voltage loop controller in step 21 is a multi-resonant controller or a repetitive controller with 3 times, 5 times, 7 times, etc. odd harmonic suppression capability to achieve power decoupling between the photovoltaic input side and the inverter output side.