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PV grid-connected inverters

2022-02-09 14:31:42

As the core of grid-connected PV system energy control and conversion, the grid-connected PV inverter converts the DC power output from solar modules into AC power that meets the grid-connected requirements and connects it to the public grid. The specific circuit topologies are numerous and can be divided into voltage-source inverters and current-source inverters according to the requirements of the nature of the power supply on the input side. The DC side of the voltage source inverter is a voltage source, or a large capacitor is connected in parallel with the voltage source, and the DC side voltage is basically pulsation-free; while the DC side of the current source inverter is connected in series with a large inductor, which is equivalent to the current source, and the DC side current is basically pulsation-free, but this large inductor will lead to poor dynamic response of the system, so most of the mainstream grid-connected inverters in the world are voltage source inverters. Here we also discuss the voltage source inverter.

According to the presence or absence of isolation transformers, grid-connected PV inverters can be classified into isolated and non-isolated types, as follows

Grid-connected photovoltaic inverter
Isolated grid-connected PV inverter Non-isolated PV grid-connected inverter
Industrial Frequency Isolated High Frequency Isolated Single-stage non-isolated Multi-stage non-isolated

The following discussion focuses on the basic working methods of different organizations in this category.

Isolated PV grid-connected inverter

In the isolated PV grid-connected inverter, it is divided into two types: industrial frequency isolated type and high frequency isolated type according to the operating frequency of the isolation transformer.

PV grid-connected inverters 1

Grid-connected Inverter Structure

The IFA isolation type is the most commonly used structure in grid-connected PV inverters, and is the earliest developed and most widely used PV inverter structure in the market. This circuit structure converts the DC power output from the PV array into 50Hz AC power through the IFC or HF inverter, and then feeds it into the grid through the IFC transformer and the input and output filters. The circuit structure is simple, the DC input voltage of the PV array has a wide range of matching, and has bidirectional power flow, single-phase power conversion (DC-LFAC), high conversion efficiency and large size.

High conversion efficiency and large volume, mass and audio noise characteristics. Due to the isolation of the transformer, on the one hand, it can ensure that no DC-side component will be injected into the grid, effectively preventing the saturation of the distribution voltager and pollution of the public grid; on the other hand, it can effectively prevent the public grid from causing harm to people through the electric circuit bridge arm when they come into contact with the PV-side circuit, improving the safety of the system.

IFE isolated grid-connected inverter can be realized by square wave, step wave into, pulse width modulation and other inverters, and its topology family includes push-pull, push-pull forward, half bridge, full bridge and other circuits.

With the development of grid-connected inverter technology, high-frequency grid-connected inverters have been developed to solve the problems of large size and mass and noise on the basis of retaining the frequency inverters.

High-frequency isolated grid-connected inverter structure

The high frequency PV grid-connected inverter circuit uses a high frequency transformer with small size and mass and low noise, which overcomes the main disadvantages of the industrial frequency transformer. The solar array output DC power is transformed into high frequency voltage by high frequency transformer, isolated, converted and voltage ratio adjusted by high frequency transformer.

And then through the high-frequency AC to low-frequency AC conversion, the transmission of low-frequency current electricity to the grid. The conversion from high-frequency AC to low-frequency AC can be a cascade of high-frequency rectifier and polarity reversing inverter bridge, or a circumferential converter.

The topological families of high-frequency PV grid-connected inverters include push-pull, push-pull forward, half-bridge and full-bridge single-tube forward, parallel interleaved single-tube forward, double-tube forward, and parallel interleaved double-tube forward, etc.

PV grid-connected inverters 2

Non-isolated PV grid-connected inverter

Non-isolated PV grid-connected inverter eliminates the need for bulky frequency transformers. This method has advantages in terms of cost, size, weight and efficiency, making this inverter structure a promising one. Generally speaking, non-isolated PV grid-connected inverters are divided into single-stage and multi-stage types. Compared with the isolated inverter, the non-isolated inverter has the advantages of small size, low cost and high efficiency, but because there is no isolation between the output and the transmitter, the PV module has a large parasitic capacitance to ground, which leads to a large leakage current to ground, and this leakage current can seriously affect the working mode of the inverter and may also cause safety accidents.

Single-stage non-isolated grid-connected inverters

Single-stage non-grid inverters can be divided into the following three structures according to the relationship between the input voltage and output voltage: Buck inverter, Boost inverter and Buck-Boost inverter. Among them, Buck-Boost inverters are widely used in the market.

This Buck-Boost based channel inverter circuit is a four-switch non-isolated half-bridge inverter, consisting of two PV arrays and a Buck Boost chopper, which can adapt to a wide range of PV array output voltages and meet grid requirements without installing a transformer because of the chopper. It divides the PV power supply at the input side into two parts to supply the two Buck Boost circuits, and the two Buck Boost circuits work alternately, each working for half a grid voltage cycle. It eliminates the disadvantage of working asymmetrically in the positive and negative half-cycles of the grid. In addition, there are only two switching tubes operating at high frequency in each half-cycle, which has the advantages of low switching losses, weak electromagnetic interference and high reliability. However, the topology suffers from low utilization of the PV module and the increase in size caused by the DC filter capacitor.

PV grid-connected inverters 3

Multi-stage non-isolated grid-connected inverter

For the traditional non-isolated PV grid-connected system, the output voltage of the PV array should be greater than the peak grid voltage at all times, so series connection of solar modules is needed to increase the output voltage of the array. However, the output voltage of the PV array drops severely due to cloud cover and other factors, and it is impossible to guarantee that the output voltage of the array is greater than the peak voltage on the public grid side at any time, and it is difficult to achieve both the maximum power tracking and grid-connected inverter functions by only -level conversion. The above Buck Boost inverter circuit solves this problem well, but the two PV modules work alternately, so a multi-stage non-isolated grid-connected PV inverter can be used to overcome this deficiency.

For DC-DC converter circuits, Buck and Boost have the highest conversion efficiency. As the Buck chopper circuit is a buck converter circuit, it cannot be boosted, so to realize the output voltage of the array after boosting and then connected to the grid, it is more likely to use the Boost circuit with boost conversion, so as to meet the PV array working in a wide voltage range, thus making the adaptation of the PV modules on the DC side more flexible; and through a suitable control method, the voltage on the input side of the Boost converter circuit can be made The Boost circuit structure is grounded together with the lower bridge arm of the inverter, and the circuit is quite simple to drive.

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