In a non-isolated photovoltaic grid-connected system with traditional topology, the output voltage of solar cell modules must be greater than the peak voltage of the grid at any time, so photovoltaic modules need to be connected in series to increase the input voltage level of the photovoltaic system. However, the series connection of multiple photovoltaic modules may often cause some solar panels to be shielded by external factors such as clouds, resulting in a serious loss of output energy of photovoltaic modules and a drop in output voltage. There is no guarantee that the output voltage will be greater than the peak voltage of the grid at any time, making the entire photovoltaic grid-connected system It cannot work normally, and it is often difficult to achieve the two functions of maximum power tracking and grid-connected inverter at the same time by only one-level energy conversion. Although the above-mentioned single-stage non-isolated photovoltaic grid-connected inverter based on BuckBoost circuit can overcome this shortcoming, it requires two sets of photovoltaic arrays to be connected and work alternately. For this, a multi-stage conversion non-isolated photovoltaic grid-connected inverter can be used. Converter to solve this problem.
Usually the topology of a multi-stage non-isolated photovoltaic grid-connected inverter consists of two parts, namely the front-stage DC/DC converter and the latter-stage DC/AC converter, as shown in Figure 1.
The key to the design of a multi-level non-isolated photovoltaic grid-connected inverter lies in the selection of the circuit topology of the DC/DC converter. From the perspective of the efficiency of the DC/DC converter, Buck and Boost converters have the highest efficiency. Because Buck converter is a buck converter and cannot boost voltage, if it is to be connected to the grid to generate electricity, the voltage requirement of the photovoltaic array must be matched to a higher level, which will bring many problems to the photovoltaic system, so there are few Buck converters Used in photovoltaic grid-connected power generation system. The Boost converter is a boost converter, which can make the photovoltaic array work in a wide voltage range, so the voltage configuration of the DC side photovoltaic components is more flexible; because the input voltage of the Boost converter can be fluctuated through an appropriate control strategy It is very small, which improves the accuracy of maximum power point tracking. At the same time, the Boost circuit structure shares the same ground with the power tube of the lower arm of the grid-side inverter, and the drive is relatively simple. It can be seen that the Boost converter is an ideal choice in the topology design of multi-level non-isolated photovoltaic grid-connected inverters.
(1) Basic Boost multi-stage non-isolated type
The main circuit topology diagram of Boost multi-stage non-isolated photovoltaic grid-connected inverter is shown in Figure 2. This circuit is a two-stage power conversion circuit. The front stage uses a Boost converter to complete the DC-side photovoltaic array output voltage boost function and the maximum power point tracking (MPPT) of the system, and the rear stage DC/AC part generally uses a classic full-bridge inverter circuit to complete the grid-connected inverter of the system Features.
The unipolar frequency doubling modulation method can make the pulsating frequency of the output SPWM wave twice that of the conventional unipolar modulation method under the condition that the switching frequency is unchanged. In this way, the unipolar frequency doubling modulation method can double the equivalent switching frequency of the circuit output under the condition of constant switching loss. Compared with bipolar modulation, unipolar frequency doubling modulation obviously has smaller harmonic components. Therefore, for the single-phase bridge voltage inverter circuit, the performance of the unipolar frequency multiplication modulation method is better than that of the conventional single and bipolar modulation.
(2) Dual-mode Boost multi-stage non-isolated type
In the basic Boost multi-stage non-isolated photovoltaic grid-connected inverter shown in Figure 2, both the front-stage Boost converter and the back-stage full-bridge converter work at high frequencies, and the switching loss is relatively large. To this end, some scholars have proposed a novel dual-mode (dual-mode) Boost multi-level non-isolated photovoltaic grid-connected inverter, which has the advantages of small size, long life, low loss, and high efficiency. The main circuit is shown in Figure 3. Different from the basic Boost multi-level non-isolated inverter shown in Figure 2, the dual-mode Boost multi-level non-isolated photovoltaic grid-connected inverter circuit adds a bypass diode VDb.
When the input voltage Uin is less than the absolute value of the given sinusoidal output voltage Uout, the switch Vc of the Boost circuit runs at high frequency, and the previous stage works in the Boost circuit mode to produce a quasi-sinusoidal voltage waveform on the intermediate DC capacitor. At the same time, the full-bridge circuit works in a power frequency modulation mode to synchronize the output voltage with the grid polarity. For example, when the output is a positive half-wave, only V1 and V4 are turned on. When the output is a negative half-wave, only V2 and V3 are turned on. This working mode is called PWM boost mode.
When the input voltage Uin is greater than or equal to the absolute value of the given sinusoidal output voltage Uout, the switch Vc is turned off. The full bridge circuit works in SPWM modulation mode. At this time, the input current does not pass through the Boost inductor Lb and the diode VD, but passes through the bypass diode in a continuous manner. This working mode is called full-bridge inverter mode.
In summary, no matter what mode the dual-mode Boost multi-level non-isolated photovoltaic grid-connected inverter circuit works in, only the first-level circuit works in high-frequency mode at the same time, which is different from the traditional basic Boost multi-level non-isolation. Compared with the type inverter, the total number of switching is reduced. In addition, the system works in the full-bridge inverter mode, and the input current passes through the bypass diode VDb in a continuous manner instead of passing through the inductor Lb and the diode VDc, reducing system losses. Finally, due to the unique working mode of this dual-mode Boost multi-level non-isolated photovoltaic grid-connected inverter circuit, there is no need to maintain a constant voltage in the intermediate DC link, so the large electrolytic capacitor commonly used in the intermediate link of the circuit can use a small capacity The thin film capacitor is replaced, thereby effectively reducing the volume, weight and loss of the system, and increasing the life, efficiency and reliability of the system.
(3) Double Boost type
As the power level of the system becomes larger and larger, in order to reduce the harmonic content and speed up the dynamic response, the contradiction between the inverter power processing capability and the switching frequency becomes more and more serious. In the multi-level non-isolated photovoltaic grid-connected inverter, it is necessary to consider the combination of multiplexing, multi-level and power frequency modulation technology to solve this contradiction. The following introduces a current-type photovoltaic grid-connected inverter based on dual Boost converters.
The main circuit topology of the current-type photovoltaic grid-connected inverter based on dual Boost converters is shown in Figure 4. The main design idea is to use current multiplexing design in the input stage. In order to take advantage of this multiple current design, a current source inverter topology is selected in the output stage, and power frequency modulation is used to convert the multiple current of the input stage into a multi-level current waveform output by the inverter, thereby effectively The volume and system loss of the grid-side filter are reduced. The specific analysis is as follows:
1) Multiple design: This topology consists of S1, VD1, Li and S2, VD2. and Lb to form two sets of Boost converters in parallel. The angular wave is used as the carrier to perform PWM modulation to obtain the drive signals of S1 and S2, thereby providing the drive signal for obtaining the required output current waveform. Since the current source inverter is adopted in the latter stage, in order to make the current source inverter work in the power frequency modulation mode, the input current of the current source inverter must be a steamed wave. The specific working process of the dual Boost circuit is: if the current ib on the control inductor Lb is equal to 0.5i, when S1 and S2 are both disconnected, the input current flows out with currents iVD1 and iVD2 respectively, and the sum of the two is io, and iVD1 And iVD2 are equal to 0.5i, io is the input current of the subsequent current source inverter; when S1 is on and S2 is off, iVD1=0, iVD2=ib=0.5i, and the current flowing through S1 is also 0.5 ii. At this time, the inductor Li is used as an energy storage element to store the energy emitted by the photovoltaic array; when S1 is disconnected and S2 is turned on, the Boost converter composed of Li and VD1 transfers the stored energy of Li to the current source inverter of the subsequent stage , Where iVD1=0.5i;, iVD2=0, the other part of the electric energy is stored through Lb; when S1 and S2 are both turned on, both L1 and Lb are used as energy storage elements to receive photovoltaic energy from the photovoltaic array, at this time iVD1=0 , IVD2=0.
2) Multi-level current and power frequency modulation: The half-sine wave modulation of the previous dual Boost converter makes the output current waveform of the dual Boost converter a multi-level waveform of superimposed half-sine modulation. The multi-level waveform mainly contains half-sine information corresponding to the power frequency. Therefore, the switching tube of the subsequent grid inverter can adopt power frequency modulation, which effectively reduces the switching loss of the current source inverter; in addition, Through the power frequency modulation of the current source inverter, the output current waveform of the inverter is a multi-level waveform of superimposed sinusoidal modulation of each group, which effectively reduces the current harmonics, reduces the size of the grid-side filter, and improves Improve the grid-connected current quality.
In fact, the current-type photovoltaic grid-connected inverter structure of the above-mentioned dual Boost converter can be extended to the current-type photovoltaic grid-connected inverter structure of multiple Boost converters, and the distribution of the main current among the multiple Boost converters makes The inverter topology can be used in high-power applications (MW level). In short, the advantage of this inverter topology is that the current of the power device due to multiple, multi-level current modulation and balanced distribution, reduces the rate of rise of the current of the power device, reduces the EMI interference of the system and the size of the filter. At the same time, the power frequency modulation of the output inverter also reduces the switching loss. These advantages will be beneficial to the design of photovoltaic grid-connected systems. The main problem of the current-type photovoltaic grid-connected inverter structure of this multiple Boost converter is: when the system is running at a higher voltage level, the working efficiency of the circuit is reduced, which is mainly due to the use of high-power inductors in the topology. Therefore, low-loss inductors should be selected as much as possible.