PV is merely part of a system!
To generate solar power,
PV and the inverter must be inseparable
Jun, 2021

The Growth Trajectory of the ‘PV Inverter’ with PV Markets Speeding Up 

PV inverters and photovoltaic (PV) modules must be installed together to achieve solar power generation. In other words, if a 100GWp solar battery module is installed, a PV inverter with a capacity of 100 GW must be installed as well. Therefore, the PV inverter market grows in lock-step with the PV module market, the forecast for which can be considered an accurate barometer for the PV inverter market.

Based on the 2020 World Energy Outlook of the International Energy Agency (IEA), the Stated Policy Scenario (STEPS) forecasts that the share of renewable energy in 2025 will reach 34%, exceeding that of coal (32%), and that the share will increase to 38% in 2030. Over the next 10 years, the shares of solar PV power and wind power are predicted to grow at the fastest rate from 9% in 2020 to 18% in 2030[1]. So, 2030 will be the first year in which renewable energy exceeds the market share of coal energy.

<Share of renewable energy generation, Source: IEA>

In 2020, 100 GW of new PV installations were added to the market, with the market expected to grow to 151 GW in 2030 and 245 GW by 2040, meaning that in the next 20 years, the market is forecast to grow two-fold [2]. By region, the PV market in Europe may recede slightly, while the Japanese market may expand slightly. The PV market outlook in India is very bright, and China, the country with the largest PV market in the world, is expected to double in market scale in the near future. Of particular note, other than the traditional PV markets where PV generation has secured price competitiveness, regions including Southeast Asia, the Middle East and Africa, where PV has not been widely available, are expected to record significant growth.

<Forecast for solar PV and wind energy generation by country, Source: IEA>

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.

*REC (Renewable Energy Certificate): A certificate that proves the supplied electricity was generated from a renewable energy resource.
**RPS (Renewable Portfolio Standard): RPS is a regulatory mandate to increase the production of energy from renewable sources.

The definition and three functions of the PV inverter

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.

*Grid-connected: The act of delivering power by connecting a distribution-type power source, such as the PV inverter, to the electrical grid.

<Characteristics of output voltage-current based on solar exposure of a 485W silicon PV module>

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. 

*Isolation resistance: Refers to the resistance between conductors separated from each other by isolation materials.
**Fusing: Fuses or wires melted and shut off by excessive current

<Functions of a PV inverter >

There are five types of PV inverters 

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.

<5 types of PV power generation>

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.

[1] IEA. 2020b, “World Energy Outlook 2020”
[2] Energy Economy Newspaper (May 16, 2021): “KRW30 trillion in PV projects to be carried out by 2030… 8 public enterprises to complete KRW8 trillion in projects next year” 
[3] One-stop Renewable Energy Business Information Portal:
#Energy Insight
#Professional Column
#PV inverter
#grid-connected inverter
#Central inverter
#String inverter
#Multi-string inverter
#Bifacial PV Module
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