Current status and application prospects of solar inverters

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 solar power generation systems do not contain moving parts, once the system is installed, it does not require virtually any maintenance. This article introduces the main application areas of solar photovoltaic technology and focuses on the development status and application prospects of solar inverters.

Photovoltaic technology application areas

Photovoltaic technology is mainly used in the following fields:

Home grid-connected system: This is the most popular type of home and business solar photovoltaic power generation system application in developed regions. The connection to the local power grid allows the excess electricity generated by the photovoltaic system to be transmitted to the grid and sold to the public agency. When the sun is not shining, electricity is exported from the grid. Inverters are 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: These systems, which are also connected to the grid, generate large amounts of photovoltaic power at a site, with capacities ranging from a few hundred kilowatts to several megawatts. Some of these plants are located in large industrial buildings, such as airports or railway stations, making use of the already available space and compensating part of the power required by high-energy users.

Rural electrification off-grid systems: In places without electricity, the PV system is connected to the battery through a charge controller. An inverter can be used to provide AC power for common appliances. Typical off-grid applications are to provide electricity to remote areas such as mountain homes and areas in developing countries. Rural electrification refers to two types of applications: small home solar systems that can meet the basic electricity needs of a family; or slightly larger small solar grids that can provide enough power for several families.

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

Off-grid industrial applications: In the telecommunications sector, solar power generation is often needed for remote applications, especially when remote rural areas need to be connected to other parts of the country. Mobile phone relay stations powered by photovoltaic or hybrid systems also have great potential. Other applications include: traffic lights, marine support systems, security phones, remote lighting, highway signs and wastewater treatment plants. Because they can provide power to areas where electricity cannot be transmitted, thus avoiding the high cost of laying wire networks, these applications currently have cost advantages.

Solar Photovoltaic Inverter

A typical solar power generation system consists of a solar photovoltaic panel array and an inverter. Photovoltaic panels directly convert sunlight into electrical energy in the form of DC voltage; inverters convert the DC voltage generated by photovoltaic panels into AC voltage that can be sent to the power grid. Therefore, inverters have become the core components of grid-connected photovoltaic systems.

In addition to 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 electromagnetic interference level, provide AC power, and optimize the process of energy transmission from the photovoltaic array to the grid as much as possible. In addition, the inverter must also meet the safety requirements of users, equipment and the grid itself.

There are several topologies that can be used for the inverter. One of them is to use a linear transformer driven by an H-bridge. This is the simplest and most reliable way, and it provides complete isolation between the grid and the DC front end. It also avoids the situation where DC current enters the grid, which should be avoided. However, the high power loss of the linear transformer leads to low efficiency, which is a disadvantage of this topology; the large size and weight of the linear transformer are also disadvantages of this topology.

Another topology is to use an output inductor instead of a bulky linear transformer. This method has the highest efficiency of all topologies and is much lighter and more cost-effective than inverters using linear transformers due to 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 PV panels. Some countries with strict regulations do not allow the use of such inverters.

Therefore, inverter manufacturers try their best to make high-frequency, high-efficiency, compact and lightweight transformers into the front-end DC/DC converter. In this way, it not only provides electrical isolation between the grid and the photovoltaic panels, but also provides a regulated and controlled DC bus voltage for the inverter. Moreover, the ability to implement the MPPT function in the DC-DC converter part is another benefit of the above solution. The MPPT system in the inverter ensures that the inverter always works at the maximum power that the photovoltaic panel can output under all weather conditions and at any time of the day.

Figure 1. A solar photovoltaic inverter connected to the grid.

Sun Farmer Solar Inverter Platform

The Sun Farmer solar inverter platform is the first inverter product developed by IMI Singapore's R&D team. Its output end adopts a transformer-free topology, supplemented by a small high-frequency and high-efficiency transformer in the front-end DC-DC converter. The product has a phase-shifted DC-DC converter with a zero voltage conversion (ZVT) mechanism to reduce the switching loss of semiconductor devices; a full-bridge topology is used to drive two inductors at the output end.

The core of this IMI solar inverter is a 32-bit microprocessor with fully digital control algorithms for grid-connected operation power management and MPPT algorithms. The firmware is developed in C language.

The tasks performed by the microprocessor include the MPPT tracking mechanism of the DC-DC section; grid voltage zero-crossing detection; grid phase synchronization using PLL; Park and anti-Park transformation of grid voltage and current; active and reactive power calculation; and other protection functions.

The special thing about the Sun Farmer inverter is that once connected to the photovoltaic cells, no user intervention is required. It automatically detects the presence of grid voltage and controls its output voltage to synchronize with the 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 a fault occurs in the grid connected to the PV system, it is necessary to observe the islanding situation. When an islanding situation occurs, the PV power generation system must be disconnected from the main grid immediately. If the PV output matches the load, the PV system can continue to supply power to the local load only. However, if the PV system is not disconnected from the main grid during an islanding situation, there will be a transient overcurrent passing through the PV system inverter, which may damage protection devices such as circuit breakers.

Photovoltaic or any other distributed generation system must be protected from islanding for the following reasons:

(1) The grid cannot control the voltage and frequency of the island, which may damage the client equipment when the grid cannot control it.

(2) Utilities and owners of distributed photovoltaic energy are responsible for electrical damage to customer equipment connected to their grids.

(3) Islanding can pose a hazard to power system employees or the public because it can energize lines that are normally considered to be disconnected from all excitation sources.

(4) After the island is reconnected to the grid, the line may trip again or damage the distributed energy generation equipment or other connected equipment due to phase shift closure.

(5) Islanding may interfere with the utility’s efforts to manually or automatically restore the grid to normal service.

In addition to having the function of preventing islanding hazards, inverters also need to meet the specific safety regulations and specifications put forward in specific regions.