In recent years, solar energy has become a major form of renewable energy due to its numerous environmental and economic benefits and proven reliability. Since the solar power system does not contain moving parts, the system does not actually require any maintenance once it is installed. This article describes the main application areas of solar photovoltaic technology, and focuses on the development status and application prospects of solar inverters.

Photovoltaic Technology Applications Photovoltaic technology is mainly used in the following areas:

Home grid-connected systems: This is the most popular type of home and enterprise-class solar photovoltaic power generation system in developed regions. The connection to the local power grid allows the surplus power generated by the PV system to be delivered to the grid and sold to the public agency. When there is no sun, electricity is output from the grid. The inverter is used to convert the direct current (DC) generated by the photovoltaic system into the alternating current (AC) required to run general electrical equipment.

Grid-connected power plants: At a site location, these grid-connected systems also generate large amounts of photovoltaic electricity, with capacities ranging from a few hundred kilowatts to several megawatts. Some of these power plants are located in large industrial buildings such as airports or railway stations. These types of power plants make use of the available space to compensate for some of the energy needed by users with high energy consumption.

Rural Electrified Off-grid Systems: Where electricity is not available, the PV system is connected to the battery via a charge controller. An inverter can be used to provide AC power for general electrical use. Typical off-grid applications are to power remote areas (such as mountainous houses and developing countries). Rural electrification refers to the following two types of applications: small-scale home solar systems that can meet the basic electricity needs of a family; or small solar grids that can provide enough electricity for a few homes.

Hybrid systems: Solar systems can be combined with other types of energy sources (such as biomass, wind or diesel) to ensure continuous and stable supply of electricity. Hybrid systems can be grid-connected, stand-alone or back-up supported by the grid.

Off-grid industrial applications: In the field of telecommunications, it is often necessary to use solar power for remote applications, especially where there is a need to connect remote rural areas with other parts of the country. There is also great potential for mobile phone relay station applications powered by photovoltaic or hybrid systems. Other applications include: traffic lights, HNA support systems, security phones, remote lighting, highway signs, and wastewater treatment plants. Because they can power areas where power is not transmitted, thus avoiding the high cost of laying wire mesh, these applications currently have cost advantages.

Solar PV Inverter A typical solar power system consists of a solar photovoltaic panel array and an inverter. Photovoltaic panels convert solar light directly into electrical energy in the form of a DC voltage; the inverter converts the DC voltage generated by the PV panel into an AC voltage that can be sent to the grid. Therefore, the inverter has become the core component of the grid-connected photovoltaic system.

In addition to having features such as high efficiency DC/AC conversion and maximum power point tracking (MPPT), the inverter should also meet the required quality—low total harmonic distortion (THD) current, high power factor ( Close to 1) and low level of electromagnetic interference, provide AC power and optimize the process of energy transfer from the PV array to the grid as much as possible. In addition, the inverter must also meet the safety requirements of users, equipment and the power grid itself.

There are several topologies that inverters can use. One of them is the use of an H-bridge driven linear transformer. This is the simplest and most reliable way to provide complete isolation between the power grid and the DC front end. It also circumvents this kind of situation that should be avoided when DC current enters the grid.

However, the severe power loss of linear transformers leads to low efficiency, which is the drawback of this topology; the large size and weight of linear transformers are also defects of this topology.

Replacing a bulky linear transformer with an output inductor is another topology. This method has the highest efficiency in all topologies; this inverter is much lighter and more cost-effective than the inverter with a linear transformer because of the small size of the output inductor. However, its disadvantage is that it does not provide any form of electrical isolation between the grid and the photovoltaic panels. Some countries with strict control regulations do not allow the use of such inverters.

Therefore, inverter manufacturers are eager to put high-frequency, high-efficiency, compact and lightweight transformers into front-end DC/DC converters. In this way, it not only provides electrical isolation between the grid and the photovoltaic panels, it also provides the inverter with a regulated and controlled DC bus voltage. Moreover, the realization of the MPPT function in the DC-DC converter section is another benefit of the above solution. The MPPT system in the inverter ensures that the inverter will always operate at the maximum power that the PV panel can output under all kinds of weather conditions and at any time during the day.

SunFarmer Solar Inverter Platform SunFarmer Solar Inverter Platform is the first inverter product developed by IMI's Singapore R&D team. The transformerless output topology is used at the output end, complemented by the front-end DC-DC converter. Use a small high-frequency high-efficiency transformer. The product is equipped with a phase-shifting DC-DC converter with zero voltage switching (ZVT) mechanism to reduce the switching losses of the semiconductor devices; at the output, two inductors are driven using a full-bridge topology.

At the heart of this IMI Solar Inverter is a 32-bit microprocessor with a fully digital control algorithm for grid-connected power management and MPPT algorithms developed in C language.

Tasks performed by the microprocessor include the MPPT tracking mechanism of the DC-DC section; detection of grid voltage zero crossings; synchronization of the grid using the PLL; Park and anti-Park transformation of the grid voltage and current; active power and reactive power calculations; and Other protection features.

What makes SunFarmer inverters unique is that they no longer require user intervention once they are connected to a photovoltaic cell. It will automatically detect the presence of grid voltage and control its output voltage to synchronize with grid voltage, frequency, and phase. The target life of this solar inverter is more than 8 years.

The safety features of the inverter are also important. When the grid connected to the photovoltaic system fails, it is necessary to observe the island conditions. When an islanding situation occurs, the photovoltaic power generation system must be disconnected from the main power grid immediately. If the PV output matches the load, the PV system can continue to supply only the local load. However, if the photovoltaic system is not disconnected from the main grid during an islanding situation, there will be transient over-current through the PV system inverter, which may damage the protection equipment such as circuit breakers.

Photovoltaic or any other distributed power generation system must be protected from the island phenomenon. The main reasons are as follows:

(1) The grid cannot control the island voltage and frequency, which may damage the client equipment if the grid cannot be controlled.

(2) The owners of utilities and distributed photovoltaic energy need to be responsible for the electrical damage to customer equipment connected to their grid.

(3) An island may cause harm to power system employees or the public because it will energize lines that are normally considered disconnected from all sources.

(4) After islanding is reconnected to the grid, the line may be tripped again due to phase shift closure, or the distributed energy generation equipment or other equipment connected may be damaged.

(5) An island may interfere with the operation of a public agency to manually or automatically restore the grid to restore normal service.

In addition to having the function of preventing island hazards, inverters also need to meet specific security regulations and norms proposed by specific regions.

Future inverter solar inverters need to have the following features.

Higher efficiency: At present, the highest efficiency of inverters in the U.S. market is up to 95%. In Europe, higher efficiency can be achieved thanks to transformerless design and innovative topology. For example: One product (SMASunnyMiniCentral8000TL) claims to achieve 98% efficiency.

Lower cost: The price of about 0.2-0.3 USD/W has been set as the price target of the inverter in 2020, which means that it is 50-75% lower than the current price. This goal is most likely to be achieved by increasing production and improving the learning curve.

Higher reliability: At present, the inverter MTBF (mean time between failures) is 5 to 10 years. Ideally, the life of the inverter should be as long as the other components in the PV system (25 years). However, many people doubt whether it is possible to achieve such a goal at a reasonable cost. In the medium to the near future, by improving quality control, better heat dissipation, and reduced complexity, the MTBF target of more than 10 years is likely to be achieved.

Advanced communication functions: Today, inverters can record and transmit information using manufacturer-specific protocols. Next-generation units should use a common communications standard to deliver more comprehensive system information to enable advanced diagnostic capabilities and communicate with public service agencies to support grid stability.