What is a photovoltaic inverter?

The photovoltaic inverter, also called frequency converter, is the heart of every photovoltaic system. Its quality impacts not only the efficiency of electricity conversion, but also the safety of home installation. What should you know about inverters before installing your own PV system?

How does an inverter work?

The task of the inverter in a photovoltaic system is to process and convert the energy produced by the PV panels. The photons incident on the photovoltaic modules produce direct current (DC), while alternating current (AC), which is available in sockets, is needed to power the home electrical grid. The inverter converts and regulates frequency, voltage and current in such a way as to produce usable alternating current of 230V/400V 50 Hz. The better the inverter, the lower the power losses and the more stable the sinusoidal current.

However, the role of modern inverters does not end with voltage conversion. They are also responsible for synchronisation of the photovoltaic system with the electricity grid, which is necessary in the most popular on-grid installations. The inverters are also responsible for recording operating data, monitoring electricity production results and communicating with an external management system, such as a smartphone application. Thanks to this, the user can remotely manage the photovoltaic software, control current production and check archived yield data.

Structure of the inverter

An inverter is composed of a rectifier, an intermediate stage (consisting of capacitors), a final stage (consisting of transistors), and a control and protection system. Appropriate switches and safety switches are essential, especially on the more dangerous, DC side of the installation. A high-class inverter cooperating with a photovoltaic system is equipped, among others, with overvoltage protection, an integrated DC disconnector, short-circuit and overvoltage protection of AC and DC type II, as well as monitoring of parameters, overvoltage and ground fault.

Maximum Power Point Tracking (MPPT) can be found in the input circuit of modern inverters, responsible for tracking the maximum power point, depending on insolation and temperature. MPPT charge controllers increase the efficiency of a photovoltaic system by up to 20% in relation to old inverters. Thanks to them, the installation works efficiently both on sunny and cloudy days. The output system consists of anti-interference filters called chokes. Their task is to limit alternating current without power loss, prevent current peaks and protect cables and devices connected to the home electric system from damage.

Types of photovoltaic inverters

Different types of inverters can be used in domestic photovoltaic micro-systems, depending on the type of PV installation. The basic division of inverters distinguishes:

• On-grid (grid) inverters – the most popular type of inverters, adapted to cooperate with the electric grid. In such a system, surplus energy is returned to the grid, which in the discount system acts as “energy storage”. This allows the user to use 80% or 70% (installations over 10 kWp) of the energy produced at a later time.
• Off-grid (island) inverters – designed for a closed system, using energy storage. Surplus energy is not fed into the grid, but is stored in a battery bank. Some of the energy generated can also be stored as heat.
• Hybrid inverters – the most advanced inverters, combining on-grid and off-grid functions. They can use external or built-in energy storage. Once the storage is fully charged, the rest of the energy goes to the grid. This is the most efficient and safest, but also the most expensive solution

How does a hybrid photovoltaic system with energy storage work? Diagram

1. electricity generation from the sun
2. self-consumption of the energy produced
3. storage of surplus energy
4. receipt of surplus energy from the grid
5. use of stored electrical energy

Remote monitoring of the plant operation and the amount of produced energy from the sun

Photovoltaic inverters are also divided according to other criteria. A distinction can be made between single phase inverters, which are suitable for installations with a capacity of a few kWp, and three phase inverters, which also supply electricity via a power outlet (400V). Another division of inverters takes into account the presence or absence of a transformer. Transformerless inverters are characterised by a lighter design, higher efficiency over a wide load range, but have a more complex structure. Transformer models, on the other hand, have fewer failures, are safer to use (they have galvanic isolation of DC and AC) and provide higher quality of voltage. Microinverters, serving only one PV panel, are also gaining popularity. Their advantage is that they make it possible to produce electricity even with little sunlight.

How to choose the inverter power?

Selecting the inverter power for a photovoltaic system may seem unintuitive at first. The recommended inverter power should be around 80-95% of the total PV installation power. This is dictated by maximising the efficiency of the inverter, which most of the time will be operating at a lower power generated by the panels anyway. The efficiency of inverters for PV plants is highest when they are loaded with the maximum energy from the sun.  However, a small loss during a few sunny days will be made up for by running the PV longer on the other, less sunny days. Choosing a slightly lower inverter output will ensure that it is operating in the optimum power range most of the time. Maximum energy production in PV panels is rare anyway, and a slight overload of the inverter does not cause any problems. In the case of east-west PV systems, it is advisable to oversize the PV panel capacity even more in relation to the inverter, even in the range of 160%. The detailed selection of the inverter power should, however, be preceded by a detailed assessment of the conditions on site.

Where to install the inverter?

The photovoltaic inverter serving the photovoltaic system should be located in a place that is safe, shaded and inaccessible to children and animals. Although most models have IP65 protection, the inverter should be sheltered from rain and snow. At the same time, the inverter should be mounted as close to the photovoltaic panels as possible to keep the high voltage DC cables as short as possible. At the same time, the device must have adequate cooling, as it generates a considerable amount of heat during operation.

Inverters are divided into internal and external ones. The former are mainly used in the most popular photovoltaic systems mounted on the roof of a building. The most frequently chosen place to install the inverter is a utility room, garage or boiler room. The cooling system of an inverter can be noisy, so it is not recommended to install the device on walls adjacent to bedrooms. A laundry room (high humidity), a boiler room with a coal boiler (high dust) or a corridor is also a bad choice. Due to high temperature, the inverter must not be hidden in cupboards, wardrobes or built-ins. In the case of ground-mounted photovoltaic systems, the inverter is usually mounted just below the panels. The panels provide shade and protection from rain and snow. However, you have to make sure that the inverter has IP65 protection and that the rack under the device is stable. The inverter itself should be mounted vertically. The maximum deviation is approx. 15%.

How do I connect the photovoltaic inverter to the grid?

After completing a photovoltaic system with the on-grid inverter, it is necessary to connect it to the grid. This requires work on both the AC and DC side. If the inverter is not equipped with an automatic DC disconnector, an external one must be installed. The installation must also have a residual current device and surge protectors. Before connecting the installation to the grid, it must still be earthed.

To connect the micro-system to the grid, a connection application must be submitted. Necessary documentation is required, which includes: wiring diagram, owner’s data, technical parameters of the PV installation, and in the case of businesses – an excerpt from the CEIDG Central Register and Information on Economic Activity or KRS National Court Register. Each electricity supplier has its own application template, in which all the necessary information must be included. The operator has 30 days to verify the application. After the documents have been accepted, the operator connects the micro-system to the meter. If there is a one-way meter in the building, it will be replaced by a two-way meter. From then on, the photovoltaic system can start operating and any surplus energy will be returned to the grid. The user becomes a prosumer. The conditions for the recovery of the energy produced are defined in the contract with the operator.

Want to know more about photovoltaic inverters? Check the inverter ranking prepared by our experts at the Solmix photovoltaic wholesaler. Read more: 2021/2022 ranking of photovoltaic inverters. Read more: 2021/2022 ranking of photovoltaic inverters

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