Different focus points make the classification of photovoltaic grid-connected power generation systems slightly different. The following three aspects are classified and explained.
- According to the connection method of photovoltaic cell modules and power electronic conversion circuits
From the perspective of the connection method between photovoltaic cell modules and power electronic conversion circuits, photovoltaic grid-connected power generation systems are mainly divided into centralized, multi-branch, single-branch, DC modular, AC modular and other forms. Figure 1.
1) Centralized
The same photovoltaic modules are connected in series and parallel to form a photovoltaic matrix (usually called photovoltaic array), which is connected to the grid by a high-power power electronic conversion circuit, as shown in Figure 1(a). It is usually used in photovoltaic power plants, and the power is generally between 100kW and 1MW for three-phase systems. The advantage is that the circuit cost is low, and the disadvantage is that the MPPT function is implemented in a centralized manner. Since the photovoltaic array area is very large, it is easy to cause some shadow problems caused by dust, clouds, snow, etc. blocking the sun. In addition, when one or more photovoltaic modules fail, the power generation and conversion efficiency of the entire photovoltaic power generation system will be affected.
2) Branch type
There are two types of single-branch and multi-branch. The single-branch type is that the same PV modules are connected in series to form a PV module branch, which is connected to the grid by a small and medium power power electronic conversion circuit [99], as shown in Figure 1(b). The multi-branch type is an extension of the single-branch type. It consists of multiple photovoltaic module branches. Each photovoltaic module branch is connected to an independent DC/DC conversion circuit with MPPT function, which is realized by a public power electronic conversion circuit. Grid connection, as shown in Figure 1(c), can be used for single-phase or three-phase systems. The branch type is usually used for building integration, and the power is generally between 1 and 100kW. Their advantage is that the photovoltaic modules can be organically combined with the building surface, and at the same time, the MPPT function can be well realized, and the hardware cost is slightly higher. For the multi-branch type, the same branch uses the same photovoltaic modules, but different branches can use photovoltaic modules of different power and quantity, which is conducive to building integration and maintenance. The disadvantage is that the power loss and shading problems are partially improved, but there is still the problem of PV module series failure.
3) AC Modular
Each photovoltaic module is connected to a low-power power electronic conversion circuit (referred to as micro-inverter or micro-inverter), and it is directly connected to the grid [100.101], as shown in Figure 1(d). The power is generally between 50 and 300W, which is determined by the power of a single photovoltaic module, and is generally used in single-phase systems. Since each PV module has an independent MPPT function, the shadow problem is completely solved. The disadvantages are high cost, low circuit efficiency and poor power quality. If there is a filter electrolytic capacitor, the circuit life will be greatly shortened.
4) DC module type
Similar to the multi-branch circuit structure, the difference is that each photovoltaic module is connected to a DC/DC conversion circuit with MPPT function. This structure is called a DC module, which is divided into parallel type and series type. The parallel type means that multiple DC modules are connected in parallel to the common DC bus, and the grid connection is realized through the common power electronic conversion circuit [102], as shown in Fig. 1(e). The series type means that multiple DC modules are connected in series to the input terminal of the power electronic conversion circuit [103], as shown in Fig. 1(f). Also generally used in single-phase systems. The advantage is that the shadow problem is completely solved, and the circuit efficiency is high and the power quality is high. The disadvantage is that the cost is higher, but much lower than the AC modular type. It is better to use organic film capacitors or long-life electrolytic capacitors instead of ordinary electrolytic capacitors, which is beneficial to prolong the life of the circuit.
- According to the characteristics of the power electronic conversion circuit itself
From the characteristics of the power electronic conversion circuit, the photovoltaic grid-connected power generation system is mainly divided into single-stage circuit topology, two (multi-)-level circuit topology and circuit topology based on common DC bus, as shown in Figure 2.
1) Single-stage circuit topology
The photovoltaic array is directly connected to the grid-connected inverter circuit. In the case of isolation requirements, there is generally no other circuit for transmitting energy except the power frequency transformer, as shown in Figure 2(a). The single-stage circuit topology should not only realize the maximum power point tracking, but also realize the grid-connected control function. Without power frequency isolation, it has the advantages of small size, high efficiency, and low cost. The disadvantage is that the input voltage range is limited, the energy transmission is also limited, the control is complicated, and there are problems such as common mode leakage current and DC current injection. . In the case of power frequency isolation, it has the advantages of large input voltage range and high reliability, but the disadvantage is that it is bulky and bulky, and the transformer bias and temperature rise cannot be ignored.
2) Two (multi) stage circuit topology
For the two (multiple) stage circuit topology, the front stage is the DC/DC conversion circuit that realizes the maximum power point tracking, and the latter stage is the inverter circuit that realizes the grid-connected function, as shown in Figure 2(b), which is usually used for small and medium-sized In power applications, it is mostly used in single-phase systems. It is further divided into two-level circuit topology and multi-level circuit topology. Most of the two-level circuit topologies belong to non-isolated circuit topologies, and most of the multi-level circuit topologies belong to high-frequency isolation circuit topologies. The advantage is that MPPT control and grid-connected control are decoupled through software and hardware circuits, the control is simple and clear, and at the same time, it is small in size, light in weight, low in noise and high in efficiency. The disadvantage is that the more circuit stages, the lower the efficiency and the lower the reliability.
3) Circuit topology based on common DC bus
It is a photovoltaic grid-connected power generation system based on modular design ideas. Based on the common DC bus, its front-end part is similar to the multi-branch type or DC module type. Multiple DC/DC conversion circuits convert the low-voltage DC power output from the corresponding photovoltaic branches or components into high-voltage DC power, and operate in parallel. , to achieve their respective MPPT functions; its back end is a plurality of inverter grid-connected modules running in parallel, feeding the electric energy on the DC bus to the AC grid [104-106], as shown in Figure 2(c). Typically used in building integration applications and three-phase systems. Modular structure improves system redundancy and reliability. In this photovoltaic grid-connected power generation system, each module is centrally controlled by the host computer or the main module through communication to realize group control of multiple units. According to the external environmental conditions, a reasonable number of grid-connected inverter modules are started and closed to avoid multiple The modules operate at light loads, thereby increasing system efficiency. The disadvantage is high cost and complicated control.
- Classification according to the nature of circuit isolation
According to whether the circuit is isolated, photovoltaic grid-connected power generation systems can also be divided into non-isolated and isolated types. Isolation type is divided into power frequency isolation type and high frequency isolation type.
1) Non-isolated type
For non-isolated circuits, it is divided into single-stage circuit topology and two-stage circuit topology. As shown in Figure 3(a), the single-stage circuit topology requires the output voltage of the PV array to be higher than the maximum AC grid voltage peak, so it is more restricted. For the two-stage circuit topology, the front stage is a DC/DC conversion circuit with MPPT function, generally chopper circuits such as BUCK and BOOST, and the rear stage is a DC/AC inverter circuit with grid-connected control function, as shown in Figure 3 ( b) shown. Most are used in single-phase or three-phase systems of small and medium power. The advantages are small size, light weight and high conversion efficiency. The disadvantage is that there are electromagnetic compatibility, grid-connected current asymmetry and ground current suppression problems, which do not meet individual photovoltaic grid-connected power generation standards in countries or regions.
2) Power frequency isolation type
The power frequency isolation type is basically a single-stage circuit topology type, as shown in Figure 4(a). Usually suitable for medium and high power three-phase systems, single-phase systems are also used. The advantage is that the circuit structure is simple, the safety is high, and the efficiency is high. The disadvantage is that it is bulky, bulky and noisy. For the centralized high-power photovoltaic grid-connected circuit topology, two high-power grid-connected inverters with the same capacity are directly connected to the medium-voltage grid through the Y/△/Y three-phase multiple power frequency medium-voltage transformer, which can improve the Equipment efficiency and cost reduction, as shown in Figure 4(b).
3) High frequency isolation type
Most of the high-frequency isolation types belong to multi-level circuit topology types, which are generally divided into voltage source type, current source type and cyclic converter type. The voltage source type is shown in Figure 5(a). Its typical circuit is in the form of DC-HFAC-HFDC-LFAC. The front stage is a forward, flyback, push-pull, half-bridge or full-bridge circuit with a high-frequency transformer. The latter stage is a high-frequency rectifier and a DC/AC inverter circuit based on a high-frequency SPWM wave. The current source type is shown in Figure 5(b), and its typical circuit is in the form of DC-sine half-wave HFAC-sine half-wave HFDC-power frequency polarity inversion converter, which is mainly used in photovoltaic micro-inverters, and its front-end Usually, it is a forward excitation, flyback, push-pull circuit, etc., which is modulated by a sine half-wave. After high-frequency rectification, a sine half-wave current is generated, which is synchronized with the grid voltage through the power frequency polarity reversal control. The typical circuit of the cyclic converter type is in the form of DC-HFAC-LFAC, as shown in Figure 5(c). There is no rectified DC link. The advantages of these three types are small size, light weight, high safety and low noise.
The main disadvantage of the voltage source type is low efficiency. The current source type can only be used in the occasion of unity power factor control. The main disadvantage of the cyclic converter type is that the bidirectional power switch needs to overlap the commutation process, and the control is relatively complicated.
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