In the isolated photovoltaic grid-connected inverter, according to the working frequency of the isolation transformer, it can be divided into two types: power frequency isolation type and high frequency isolation type.
1. Power frequency isolated photovoltaic grid-connected inverter structure
The power frequency isolation type is the most commonly used structure of photovoltaic grid-connected inverters, and it is also the most commonly used type of photovoltaic inverter on the market. Its structure is shown in Figure 1. The direct current generated by the photovoltaic array is converted into 50Hz alternating current through an inverter, and then input into the grid through a power frequency transformer, which performs voltage matching and isolation functions at the same time. Since the power frequency isolation inverter uses a power frequency transformer to isolate the input and output ends, the main circuit and control circuit are relatively simple, and the DC input voltage of the photovoltaic array has a large matching range. The isolation of the transformer, on the one hand, can effectively prevent people from contacting the positive or negative pole of the DC side, reduce the possibility that the grid current will form a loop through the bridge arm to cause harm to people, and improve the safety of the system; on the other hand, it also guarantees In this way, the system will not inject DC components into the grid, effectively preventing the saturation of the distribution transformer.
However, the power frequency transformer has the disadvantages of large volume, heavy weight, high noise, and low efficiency. It accounts for about 50% of the total weight of the inverter, making it difficult to reduce the size of the inverter. In addition, the existence of power frequency transformers Increase the loss and cost of the system, and increase the difficulty of transportation and installation. Figure 2 shows the impact of the isolated power frequency transformer on the system efficiency in a high-efficiency megawatt-level photovoltaic grid-connected system.
The conventional topological forms of power frequency isolated photovoltaic grid-connected inverters include single-phase structure, three-phase structure, and three-phase multiple structure.
(1) Power frequency isolation system-single-phase structure
The single-phase structure of the power frequency isolated photovoltaic grid-connected inverter is shown in Figure 3. Generally, a full-bridge or half-bridge structure can be used. This type of single-phase structure is often used in photovoltaic grid-connected systems with power levels below several kilowatts, where the DC working voltage is generally less than 600V, and the working efficiency is less than 96%.
(2) Power frequency isolation type-three-phase structure
The three-phase structure of the power frequency isolated photovoltaic grid-connected inverter is shown in Figure 4. Generally, a full-bridge or three-level half-bridge structure can be used. This type of three-phase structure is often used in photovoltaic grid-connected systems with a power level of tens or even hundreds of kilowatts. Among them, the DC working voltage of the three-phase full-bridge structure is generally 450~820V, and the working efficiency can reach 97%; while the DC working voltage of the three-level half-bridge structure is generally 600~1000V, and the working efficiency can reach 98%. In addition, the three-level half-bridge structure can achieve better waveform quality.
(3) Power frequency isolation system 1-three-phase multiple structure
The three-phase multi-structure power frequency isolated photovoltaic grid-connected inverter generally adopts a three-phase full bridge structure. It is worth mentioning that this three-phase multiple structure can switch between the photovoltaic array and the inverter connection combination according to the change of solar irradiance to improve the operating efficiency of the inverter. For example, when the solar irradiance is low, all The array is connected to one inverter, and when the solar irradiance is large enough, the two inverters are put into operation. In addition, this three-phase multiple structure can also use transformer secondary winding d or y to connect when two inverters are working at the same time to eliminate low-order harmonic currents, or use phase-shift multiple technology to increase the equivalent switching frequency. Reduce the switching loss of each inverter.
Power frequency isolated photovoltaic grid-connected inverter is the earliest development and application of the main circuit form of photovoltaic grid-connected inverter. With the development of inverter technology, on the basis of retaining the advantages of isolated photovoltaic grid-connected inverter , In order to reduce the volume and quality of the inverter, a high-frequency isolation photovoltaic grid-connected inverter structure came into being.
2. High frequency isolation type
The difference between the high frequency isolation type and the power frequency isolation type photovoltaic grid-connected inverter is the use of a high frequency transformer. Compared with the power frequency transformer, the high frequency transformer is small in size and light in weight, which overcomes the main shortcomings of the power frequency isolation inverter. With the improvement of devices and control technology, the efficiency of high-frequency isolation photovoltaic grid-connected inverters can also be made very high. Classified by circuit topology, there are two main types of high-frequency isolation photovoltaic grid-connected inverters: DC/DC conversion type and cycle conversion type, as shown in Figure 5. High-frequency isolation inverters mainly use high-frequency chain inverter technology.
The concept of high frequency link inverter technology was proposed by Espelage and B.K.Bose in 1977. The high-frequency chain inverter technology replaces the power-frequency transformer in the low-frequency inverter technology with a high-frequency transformer to achieve electrical isolation between input and output, reducing the size and quality of the transformer, and significantly improving the characteristics of the inverter.
In photovoltaic power generation systems, a variety of high-frequency photovoltaic grid-connected inverters based on high-frequency link technology have been developed. Generally speaking, it mainly includes DC/DC conversion type DC/HFAC (high frequency AC)/DC/LFAC (low frequency AC, low frequency AC) and cycle conversion type (DC/HFAC/LFAC).
(1) DC/DC conversion type high frequency chain
1) Circuit composition and working mode
The DC/DC conversion type high frequency chain photovoltaic grid-connected inverter has the advantages of electrical isolation, light weight, and small size. The capacity of a single unit is generally within a few kilowatts, and the system efficiency is above 93%. Its structure is shown in Figure 6. In this kind of DC-DC conversion inverter, the electrical energy output by the photovoltaic array is converted into the power grid through DC/HFAC/DC/LFAC, and the DC/AC/HFT/AC/DC link constitutes the DC/DC converter. In addition, in the DC/DC conversion type high-frequency photovoltaic grid-connected inverter circuit structure, two DC/AC links are designed on the input and output sides: DC/AC used on the input side converts the DC output of the photovoltaic array High-frequency alternating current is used to transform and isolate high-frequency transformers, and then high-frequency rectification to obtain the required voltage level of direct current; while the DC/AC used on the output side inverts the intermediate-level direct current to low-frequency sinusoidal alternating voltage , And connect to the grid.
The DC/DC conversion type high-frequency chain photovoltaic grid-connected inverter mainly has two working modes: the first working mode is shown in Figure 7, the DC power output by the photovoltaic array is transformed into an equal duty cycle by the front-stage high-frequency inverter (50%) of the high-frequency square wave voltage is isolated by a high-frequency transformer, rectified by a rectifier circuit into direct current, and then passed through a post-stage PWM (pulse width modulation) inverter and an LC filter to filter the electrical energy Feed into the power frequency grid; the second mode of operation is shown in Figure 8. The DC power output by the photovoltaic array is inverted into SPWPM (sinusoidal pulse width position modulation) by the front-stage high-frequency inverter. After passing through a high-frequency isolation transformer, it is rectified and filtered into a half-sine wave (steamed bun wave), and finally fed into the power-frequency power grid by a power-frequency inverter in the subsequent stage.
2) Full-bridge DC/DC conversion type high-frequency chain
In the specific circuit structure, the high frequency inverter part of the front stage of the DC/DC conversion type high-frequency chain photovoltaic grid-connected inverter can adopt the form of push-pull, half-bridge, and full-bridge conversion circuits, and the latter The inverter part of the inverter can be in the form of half-bridge and full-bridge conversion circuits. Generally speaking, push-pull circuits are suitable for low-voltage input conversion occasions, and half-bridge and full-bridge circuits are suitable for high-voltage input occasions. In practical applications, the appropriate circuit topology can be determined according to the final output voltage level and power. The following takes the full-bridge topology as an example to expand the specific discussion.
The circuit topology of the full-bridge DC/DC conversion type high-frequency chain photovoltaic grid-connected inverter is shown in Figure 9. The full-bridge DC/DC conversion type high-frequency chain photovoltaic grid-connected inverter consists of a high-frequency voltage full-bridge inverter, a high-frequency transformer, an uncontrollable bridge diode full-wave rectifier, a DC filter inductor and a polarity Reverse inverter bridge composition. Among them: the high-frequency voltage full-bridge inverter adopts the SPWPM modulation method to invert the DC voltage emitted by the photovoltaic array into a bipolar three-level SPWPM high-frequency pulse signal. The high-frequency transformer boosts the signal and transmits it to the subsequent uncontrollable bridge diode full-wave rectifier circuit; the SPWPM pulse signal is rectified here, filtered by the DC filter inductor, and transformed into a half-sine waveform (steamed bun wave); The linear reversal inverter bridge reverses the half sine wave into a full sine wave at the power frequency, and feeds electrical energy into the power frequency grid. It can be seen that the full-bridge DC/DC conversion high-frequency chain photovoltaic grid-connected inverter adopts the aforementioned second working mode, and its high-frequency side adopts the SPWPM modulation method.
The circuit adopts high-frequency electrical isolation, mature technology, and the transformer can increase the output voltage of the previous inverter, reduce the system current, and facilitate the selection of power devices; the control of the front and rear stages is independent of each other, and the control is simple; the voltage stress of the back stage circuit is low , And can realize ZVZCS (Zero voltage zero current switching, zero voltage open zero current shut off), the front-end circuit can also realize ZVS (Zero voltage switching, zero voltage open).
(2) Frequency conversion type high frequency chain
1) Circuit composition and working mode
The DC/DC conversion type high frequency chain photovoltaic grid-connected inverter circuit structure uses two-level power conversion (DC/HFAC/DC/LFAC), which has more conversion links and increases power loss. In order to improve the efficiency of high-frequency photovoltaic grid-connected inverter circuits, it is hoped that high-frequency transformers can be directly used to complete the tasks of voltage transformation, isolation, and SPWM (sinusoidal pulse width modulation) inverters at the same time. Therefore, some scholars have proposed High frequency chain inverter technology based on cycle transform. The circuit structure of the cycle-transformed high-frequency chain photovoltaic grid-connected inverter is shown in Figure 10. The topology of this type of photovoltaic grid-connected inverter is composed of three parts: a high-frequency inverter, a high-frequency transformer, and a cycle converter, forming a DC/HFAC/LFAC two-stage circuit topology. The power conversion link of this structure has only two stages, which improves the efficiency of the system. Because there is no intermediate rectification link, it can even realize the two-way transmission of power; because the first-level power inverter is less used, the purpose of simplifying the structure, reducing the volume and quality, and improving the efficiency is achieved. This is for the realization of grid-connected inverters. The high frequency, high efficiency and high power density have created the conditions.
Similar to the DC/DC conversion type high frequency chain photovoltaic grid-connected inverter, the cycle conversion type high frequency chain photovoltaic grid-connected inverter also has two main working modes: the first working mode is shown in Figure 11, the DC power output by the photovoltaic array First, it is inverted into a high-frequency square wave voltage of equal duty cycle (50%) by a high-frequency PWM inverter. After passing through a high-frequency isolation transformer, it is controlled by a cycle converter to directly output power frequency alternating current; the second working mode is such as As shown in Figure 12, the DC power output by the photovoltaic array is first converted into a high-frequency SPWPM wave through a high-frequency SPWPM inverter, and after passing through a high-frequency isolation transformer, it is controlled by a cycle converter to directly output power frequency AC power.
2) Full-bridge frequency conversion type high-frequency chain
The full-bridge circuit power switch has low voltage stress, a large number of power switches, and a high utilization rate of transformer windings, which is suitable for high-voltage output occasions. The topology of the full-bridge cycle conversion type high-frequency chain photovoltaic grid-connected inverter is shown in Figure 13.
When using the traditional PWM technology, the commutation of the power device of the cycle converter will shut off the continuous current in the leakage inductance and cause the inevitable voltage overshoot. Therefore, this type of high-frequency isolated photovoltaic grid-connected inverter often adopts the second type. Operating mode. It can better solve the technical problems of voltage overshoot and the conversion flow of the cycloconverter without increasing the complexity of the circuit topology. This working mode is to use a high-frequency switching inverter to invert the input DC voltage into SPWPM waves. After passing through the high-frequency isolation transformer, it is transmitted to the secondary side of the transformer, and then the SPWPM waves are converted by the synchronously working cycle converter. Into SPWM wave. Since the electric energy conversion does not go through the rectification link and adopts a two-way switch, the circuit topology can realize two-way power flow, which is very necessary for a system equipped with an energy storage link.