The domestic PV market is being led by the Korea Renewable Energy 3020 Plan, the Green New Deal and Carbon Neutrality Policy. Four power companies (Korea Southern Power, Korea Energy, Korea Western Power and Korea Midland Power) began operating 288 MW PV plants in 2020, and six power companies (Korea Hydro Nuclear Power, Korea Energy, Korea Southern Power, Korea East-West Power, Korea Western Power and Korea Midland Power) as well as two public organizations (Korea Water Resources Corporation and Korean Rural Corporation) are carrying out large-scale, 1.6 GW PV projects for completion in 2023[2].

For the government’s policies to be successfully implemented, initiatives driven by private entities are essential and, thus, government policies should be supplemented accordingly. To support the initiatives, appropriate *REC prices need to be maintained at the current level, which is about one-fourth of the price in 2017[3]. Power companies and public organizations that need to purchase REC must be engaged in large-scale power businesses and the adjustment of the REC price system and the price itself is needed to accommodate the **RPS volume, which has increased sharply since 2018. Now is the time for the private entities to take active steps in line with the government authorities’ efforts.

An inverter refers to a device that converts direct current (DC) electricity produced by a module into alternating current (AC). The official name of the PV inverter is a “grid-connected inverter”. This can be simply understood as the addition of a ‘PV’ function and ‘*grid’ function to a power converter, the inverter. The modifier ‘PV’ means that the output characteristics of the PV module are not linear. 

For example, to be linear, the amount of voltage has to increase proportionally to that of the current. However, as demonstrated in the graph below, the PV module has a power current of ‘0’ when the voltage reaches its highest level, and the voltage is ‘0’ when the power current is at its highest. Therefore, a function that keeps track of the maximum power is required. ‘Grid-connected’ means that it meets the grid code of all countries that use the AC grid system. Therefore, a PV inverter can be defined as a power converter that converts the maximum direct power created by PV modules into alternating power and transmits it to the grid.

The functions of a PV inverter can be classified into ‘PV module (input)’, ‘grid function (output)’, and ‘safety.’ The output of the PV module changes according to the solar exposure level; therefore, a MMPT (maximum power point tracking) function must be performed to generate the maximum amount of power at any time and deliver it to the electrical grid. 

The electrical grid maintains voltage and frequency within a normal range, but, if the voltage and frequency exceed that range (sudden increase of load, sudden decrease of power supply, etc.) and a blackout or an accident occurs, the PV inverter must be switched off within a predetermined time. If islanding (a condition where power is supplied by a distributed generator in a blackout) occurs, it must be detected and stopped. To increase the stability level of the electrical grid, a function creating reactive power must be performed at the request of a power company. In addition, while operating connected to a power grid, the level of current harmonics has to be maintained at less than 5%. 

Safety-related controls must derate or stop the power output if the internal temperature of the PV inverter exceeds the safety range. Also, the power output must be stopped if there is excessive input or output current, the *isolation resistance of the input (PV array) is critically low, the vector sum of output power is not ‘0,’ or the direct current in the power output exceeds the regulated limit. In addition, the requirements of Electromagnetic Compatibility (EMC) must be met. These functions are required to deliver the maximum amount of direct current power generated from a PV module safely to the electrical grid. 

Finally, PV inverters have the function to communicate the status of PV power generation to consumers that can be monitored any time. Monitoring may appear to have no direct relevance to the power generation volume, but it is necessary for users to respond to the suspension of PV operations caused by certain factors, such as snow piled up on a PV module, the system breaker being activated, fusing problems, or other factors. If a suspension is identified, users can request service from the PV inverter manufacturer to have service personnel visit the site and get the PV inverter up and running again. The availability of AS support is an important element when choosing a PV inverter. 

Compared to PV cell modules, PV inverters are classified into a greater number of product types. PV inverters are classified based on several characteristics, including the type (single-phase, three-phase, low voltage, very-high voltage) of grid, volume of output (low, medium, high), internal structure (central, string, multi-string, module level, etc.) of the PV inverter, use of transformers (transformer, transformerless) and the type (low frequency, high frequency) of the transformer used. PV inverters that are widely available today can be classified into five types based on the MPPT. 

A string inverter and a central inverter use the MPPT technique in string and array units, respectively. Module level power electronics use MPPT per module. There also are micro inverters and optimizers, which is a combination of micro converters (DC modules) and grid-connected inverters.

Central inverters (centralized), which have been used for the longest period among inverters, have one MPPT function. Its PV inverter structure is relatively simple and is used in MW-scale power generation. A string inverter is a smaller version of a central inverter. For example, ten 100 kW inverters are used in a system where only one 1 MW central inverter is used. In this case, there is an increase in the number of MPPT based on the number of string inverters employed. Generally, the power efficiency rises as the number of MPPT increases. A multi-string inverter is one that integrates more MPPTs into a string inverter. Most of the PV inverters widely used for systems of less than 500 kWP in capacity are multi-string inverters. The criteria for distinguishing a string inverter from a multi-string inverter is the number of MPPTs. A multi-string inverter has more than 2 MPPTs. Also, the number of serial and parallel connections of a PV module has to be the same for a central inverter, but this rule does not apply to a multi-string inverter.

PV inverters that use MPPT(s) per module are called micro inverters and power optimizers with grid-connected inverters. 

Due to the price sensitive nature of the PV system, the most appropriate system, based on the current technology and the price level in 2020, is a multi-spring inverter. For installation areas with a lot of shade, the module level method (power optimizer+grid-connected inverter or micro inverter) is the best way to increase power generation efficiency.

The bifacial PV module is drawing attention in the PV market today. To respond to the popularity of bifacial solar modules, the industry is changing PV inverter specifications to increase the current input capacity. However, it is a pity that many modules have been changed from traditional monofacial to bifacial type without considering which PV inverter is the most appropriate for bifacial applications. 

When installing bifacial solar modules, a sufficient distance between modules has to be maintained to increase the amount of light that reaches the floor surface. When treating a floor surface, the albedo effect (reflectance) must be considered. Foil is often installed on a floor surface, but this may violate the Aviation Act. Lastly, which type of PV inverter is to be used should be considered. If there is not much shade in an installation location, multi-string inverters would be most appropriate. The number of strings per MPPT should not exceed two to ensure maximum efficiency. 

PV power generation can be maintained even if a problem occurs in one or two PV modules, but not if there is any error in the PV inverter. Therefore, it can be said that PV inverters are essential for PV power generation. This is why the role and development of PV inverters has become important for the sustainable growth of the PV market, which is gaining momentum.