In order to improve the efficiency of photovoltaic grid-connected systems and reduce costs as much as possible, under the condition that no mandatory electrical isolation is required (the relevant standards of some countries require mandatory electrical isolation for photovoltaic grid-connected systems), a non-isolated transformerless type can be used Topology scheme. The non-isolated photovoltaic grid-connected inverter eliminates the need for bulky power frequency transformers, and has many advantages such as small size, light weight, high efficiency, and low cost, and has a good development prospect.
Generally speaking, non-isolated photovoltaic grid-connected inverters can be divided into single-stage and multi-stage according to their structure. Below we will talk about single-stage non-isolated photovoltaic grid-connected inverters.
The structure of the single-stage non-isolated photovoltaic grid-connected inverter is shown in Figure 1. The single-stage photovoltaic grid-connected inverter can complete the DC/AC grid-connected inverter function with only one level of energy conversion. It has simple circuits and elements. There are many advantages such as fewer devices, high reliability, high efficiency, and low power consumption.
In fact, when the output voltage of the photovoltaic array meets the grid-connected inverter requirements and does not require isolation, the isolation transformers in the various topologies of the power frequency isolated photovoltaic grid-connected inverter can be omitted, thereby evolving into a single-stage non-isolation Various topologies of photovoltaic grid-connected inverters, such as full-bridge, half-bridge, three-level, etc.
Although the single-stage non-isolated photovoltaic grid-connected inverter eliminates the power frequency transformer, the conventional structure of the single-stage non-isolated photovoltaic grid-connected inverter has filter inductors on the grid side. The filter inductor flows through the power frequency current, so it also has a certain volume and quality; in addition, the conventional single-stage non-isolated photovoltaic grid-connected inverter requires the photovoltaic components to have sufficient voltage to ensure grid-connected power generation. In order to solve the above shortcomings, two single-stage non-isolated photovoltaic grid-connected inverters are introduced below.
(1) Single-stage non-isolated type based on Buck-Boost circuit
In order to overcome the shortcomings of the conventional single-stage non-isolated photovoltaic grid-connected inverter, and further reduce the weight and volume of the photovoltaic grid-connected inverter, a single-stage non-isolated photovoltaic grid-connected inverter based on BuckBoost circuit has appeared. The topology of the device is shown in Figure 2.
This single-stage non-isolated photovoltaic grid-connected inverter topology based on Buck-Boost circuit consists of two sets of photovoltaic arrays and Buck-Boost choppers. Thanks to the Buck-Boost type chopper, a wider photovoltaic array voltage can be adapted to meet the grid-connected power generation requirements without a transformer. The two Buck-Boost choppers work in a DCM (discontinuous current mode) with a fixed switching frequency, and control the two sets of photovoltaic arrays to work alternately during the positive and negative half cycles of the power frequency grid. Due to the presence of the intermediate energy storage inductor, the output AC end of this non-isolated photovoltaic grid-connected inverter does not need to be connected to the inductor that flows through the power frequency current, so the volume and quality of the inverter are greatly reduced. In addition, compared with a multi-level non-isolated photovoltaic grid-connected inverter with DC voltage adaptability, the number of switching devices used in this inverter system is relatively small.
Generally, the output power of a single-stage non-isolated photovoltaic grid-connected inverter based on Buck-Boost circuit is less than 1kW, which is mainly used for household photovoltaic grid-connected systems. In this non-isolated system, theoretically, there is no large leakage current, and the main circuit of the system is relatively simple. However, since each group of photovoltaic arrays can only work within half a week of the power frequency grid, the efficiency is relatively low.
(2) Single-stage non-isolated type based on Z-source network
The conventional voltage source single-stage non-isolated topology has the following problems:
1) It can only be applied to occasions where the DC voltage is higher than the grid voltage amplitude. In order to achieve grid connection, the photovoltaic input voltage must be higher than the grid voltage.
2) Two tubes on the same bridge wall should be connected with dead time to prevent short circuit of the DC side capacitor caused by the direct connection.
3) The support capacitor value on the DC side should be designed to be large enough to suppress the DC voltage ripple.
In view of the shortcomings of the above-mentioned conventional topologies, some documents have proposed a single-stage non-isolated inverter based on a Z-source network. Compared with the inverter of the traditional structure, it can achieve the DC side boost through a unique through state. The purpose of the inverter is to achieve any voltage output requirements of the inverter. This new type of Z-source photovoltaic grid-connected inverter has the following characteristics:
1) In theory, the input voltage of any size photovoltaic array can be connected to the grid through a Z-source inverter.
2) There is no need to set the dead time, so the grid-connected current has better waveform quality.
Figure 3 shows the general topology of a single-stage non-isolated inverter based on the Z-source network. It consists of photovoltaic arrays, diode VD, Z-source symmetrical network (L1=L2, C1=C2), and full-bridge inverters (S1~S4). ) And five parts of the output filtering link.
In a traditional voltage-type inverter, simultaneous conduction (through state) of the upper and lower switch tubes of the same bridge arm is prohibited, because in this case, the input DC capacitor will cause a sudden current increase due to the instantaneous through state. Damage to the switching device. However, the introduction of the Z-source network makes the pass-through state possible in the inverter, and it is through this pass-through state that the entire Z-source inverter provides the inverter with unique boost characteristics.