Photovoltaic arrays output direct current, while the typical power grid is an alternating current grid, and most electrical equipment uses alternating current power. In order to ensure that the power generated by the photovoltaic array is fed to the grid, an inverter is required to convert the direct current output by the photovoltaic array into alternating current.
The circuit design of the inverter makes this conversion function possible: the inverter forms alternating current through a switching mechanism, that is, quickly turn on and turn off the circuit. Then, a transformer is used to increase the voltage to the voltage level required by the grid.
There are two main types of inverters: battery inverters that use batteries as power sources and grid-interactive inverters for grid-connected photovoltaic systems.
① Battery inverter
Many people are not unfamiliar with the use of inverters. They are used when using DC power sources such as car batteries or larger batteries to drive standard AC appliances. The inverter takes the DC power from the battery and converts it into AC power for use by the AC circuit and load. These inverters are often referred to as battery inverters or power inverters. Battery inverters are widely used and are used in independent photovoltaic systems. The types of inverters in grid-connected systems are very different (see Chapter 1). Generally, the continuous output power range of these inverters is 1 ~ 5kW.
There are inverter-charger products in independent systems and backup systems. These products can be connected to the grid and have a variety of different working methods:
●Charge the battery with photovoltaic power;
●Power can be transmitted to AC load or grid through inverter;
●The battery can provide AC power through the inverter; etc.
The working range of various inverter-charger products is very different, so it is necessary to carefully check the product specifications to determine the applicability of the product.
②Grid interactive inverter
This kind of inverter is also called grid-connected inverter, which is used in grid-connected system. There are many different types of products. The DC input of this inverter comes from the photovoltaic array and matches the AC output required by the grid. The inverter can work normally only when the grid is running, and work within a specific voltage and frequency range. Whether an inverter-charger can deliver power to the grid depends on its own operating specifications. For example, an inverter charger manufacturer in the United States has a product that can only be used as a power source or a DC input power source, and cannot be output to the grid: There is also a product with a set of switching software that allows inverter AC power to be output to the grid.
All grid-interactive inverters can implement the following basic functions:
●Convert the DC power of the photovoltaic array into AC power. The AC power can be used on-site or sent to the grid through an electric meter. If there is no grid-interactive inverter, it is impossible to output the power of the photovoltaic system to the grid.
●Ensure that the power sent to the grid has an appropriate frequency and voltage. If the inverter converts DC power but cannot match the frequency and voltage of the grid, it will not send power to the grid.
●Use the “Maximum Power Point Tracking” (MPPT) function to ensure that the inverter finds the maximum power of the photovoltaic array and converts it into alternating current.
●The inverter integrates active and passive safety protection functions. When the grid is not running within the acceptable voltage and frequency range, the inverter automatically shuts down. This section will discuss the inverter protection system.
Grid interactive inverters may vary greatly in the following aspects:
●Whether the inverter has a transformer.
●The switching frequency of the transformer.
●The interface between the photovoltaic array and the inverter.
●Inverter rated capacity.
●The inverter has a single string of photovoltaic power input or multiple strings of photovoltaic power input.
●The inverter is used for single-phase power supply or multi-phase power supply.
A transformer is a device that uses a magnetic field to increase or decrease the power supply voltage. Traditionally, grid-interactive inverters must be used in conjunction with transformers, such as high-frequency transformers or low-frequency transformers. Due to the rapid development of electronic devices, there are now many transformerless inverter products. Although transformerless inverters are not yet widely used in the United States, they are becoming more common in Europe and Australia. The advantages of transformerless inverters are lighter weight and higher efficiency than traditional products. However, transformers provide electrical isolation because the circuits are connected through a magnetic field (often called electrical isolation), rather than through physical wiring like a transformerless inverter. The disadvantage of transformerless inverters is that due to lack of electrical isolation, a small amount of direct current may be injected into the grid. Sometimes a small isolation transformer is used to prevent the DC component from being injected.
The transformer used in the inverter can be low-frequency or high-frequency. High-frequency transformers are more efficient and lighter than low-frequency transformers, but they are more complicated to manufacture.