Author: Iko Järberg
This blog focuses on utility-scale solar power plants.
Foundation of Profitability: Grid Connection, Soil Conditions, and Solar Radiation
Every solar power project should begin with the same question: is this technically feasible in an economically viable way? This cannot be answered with a single spreadsheet—it requires a comprehensive feasibility study.
Key factors for feasibility include potential grid connection points and the conditions for accessing them. Ideally, the grid connection should be located on-site or within a few kilometers. The voltage level of the grid—whether 21 kV, 33 kV, 110 kV, or 400 kV—affects both costs and overall system design. Additionally, it must be assessed whether the connection can be implemented via underground cable or overhead line. The optimal connection method can be millions of euros cheaper than alternatives.
Beyond the grid connection, constructability forms the literal foundation of the project. Soil conditions determine how the panel mounting structures can be installed—whether piling is feasible, how deep the piles must go, or whether a floating foundation is required. Existing infrastructure, such as roads and access routes, also plays a significant role in reducing costs. On-site observations, soil drilling, and geotechnical surveys during the pre-design phase help identify potential risks and additional costs early on. Peat areas, former landfills, soft soil layers, or sites requiring extensive earthworks can significantly limit or increase the cost of construction. Solar power plants typically cover large areas and may impact nearby water bodies, making early-stage water management planning essential.
Once potential land areas are identified, the amount of solar radiation must be assessed. In Finland, annual global radiation ranges from approximately 1000 to 1200 kWh/m², typically higher along the coast due to lower cloud cover. With proper system optimization, solar radiation in Finland enables production of about 900–1000 kWh/kWp. Radiation data is available from sources such as NASA and Meteonorm, as well as more site-specific datasets. Once an initial layout is created, a production forecast can be developed, accounting for shading, electrical losses, and other losses. This forecast can then be translated into euros, providing a preliminary profitability estimate and a basis for more detailed planning.
"Every solar power project should start with the same question: is this technically feasible in a commercially viable way? The answer doesn’t lie in a single spreadsheet — it requires a comprehensive feasibility study."
The Puzzle Pieces
Designing a functional solar power plant is a systematic optimization process where each component affects the system’s overall efficiency and reliability. The puzzle pieces include components such as solar panels, inverters, and transformer stations.
Panel selection begins with their electrical and mechanical properties. Bifacial panels, typically rated between 550–650 Wp, are commonly used today. Technical specifications influence production forecasts and system sizing. It is also advisable to ensure multiple supplier options exist for the same power class to avoid availability issues.
Inverters play a central role by converting the DC electricity generated by the panels into grid-compatible AC electricity. In Finland, string inverters are preferred due to their scalability, maintainability, and reliability—largely because early projects used them and have proven successful. Larger projects may also use central inverters, which are installed in key locations. Inverter selection depends on factors such as power rating, maximum voltage and current, number of DC inputs, and MPPT channels. It is especially important that inverters meet the latest grid code requirements (VJV) and that PSSE and PSCAD models are available for grid compliance analysis. The power plant controller (PPC) must also meet these requirements.
Central inverters are comprehensive units that include the inverter, transformer, and switchgear. In addition to string inverters, transformer stations are needed to step up the low voltage from the inverters to medium voltage. Key considerations for transformer stations include the number of inverter outputs, transformer size, medium-voltage switchgear design, and suitability for Finnish conditions and regulations.
During the design phase, the number and optimal placement of these components are determined, along with sufficient access routes for emergency services and proper water management solutions.
System Design Based on Grid or Land Constraints
Electrical design of a solar power plant is an iterative process, often constrained by either the grid connection or the available land area. System design defines the components and their quantities.
The grid connection agreement specifies the maximum power that can be fed into the grid. Based on this, the number of transformer stations, inverters, panel strings, string lengths, mounting configurations, and panels are determined to maximize the use of the grid connection.
If the grid connection is not the limiting factor, the available land area becomes the constraint. In this case, the system is designed starting from the minimum row spacing and maximum number of panels, then proceeding to strings, inverters, and transformer stations. This approach aims to extract the maximum amount of solar energy from the land in a cost-effective manner.
System design is not mathematically straight forward — it must also consider the DC/AC ratio and reactive power capacity. The DC/AC ratio refers to the relationship between panel power (DC) and inverter power (AC), typically ranging from 1.2 to 1.5. This means that oversizing the panel capacity improves average performance. Reactive power capacity is regulated by the grid code and is usually met by limiting the inverter’s active power to about 85%. In some cases, reactive power can be provided by other means, such as capacitor banks at the substation. More on this topic is available in a previous blog post.
Read more about the reactive power capacity of solar power plants:
"In the end, the compatibility of all the pieces is verified through power system studies to ensure the solution meets grid requirements and functions as intended."."
Grid Connection Requirements
Solar power plants are typically connected to the grid via a dedicated substation. Smaller plants may connect via a feeder line to an existing substation or transformer station. Grid connection design must consider for example general connection terms (YLE2021), grid code requirements (VJV2024), emergency and restoration rules (NC ER), local grid operator requirements, and the plant owner’s needs. As a result, the connection may need to include specifications for the main transformer, a 110 kV pole operated circuit breaker, power plant controller, 24-hour battery system, disturbance recorder, and a dedicated room for communication equipment. Future expansions, such as energy storage, should also be considered—at least by reserving space.
A Functional and Profitable System
At first glance, a solar power plant appears simple, with no moving parts—unless you count the movement of electrons. However, designing a solar power plant requires not only an understanding of constructability but also of electrical requirements, which are often the stumbling blocks in otherwise promising projects. A well-designed system helps avoid surprises later in the project.
According to the renewable energy association, we are at the dawn of solar power in Finland. This reflects the fact that hundreds of projects are being developed and the first major investments are underway—but much more is yet to come. Ultimately, only the most profitable projects will be realized, which is why it pays to invest in proper system design from the very beginning.
Despro is a reliable partner throughout the entire lifecycle of solar power projects. We have extensive experience, having supported over 5000 MW in the development phase and over 300 MW in the construction phase. We offer all the services needed to design a functional and profitable system!
FAQ – Frequently Asked Questions
The planning process always starts with a feasibility assessment: is the project technically and economically viable? Key starting points include grid connection, soil suitability, and the site’s solar irradiance conditions.
The grid connection has a direct impact on both costs and technical implementation. The location of the connection point, voltage level, and method of connection (underground cable or overhead line) can shift the project’s cost by millions of euros.
Key components include solar panels, inverters, and transformers. Their selection impacts production capacity, maintainability, and overall system reliability. In Finland, string inverters are often the preferred choice.
Electrical design is an iterative process that takes into account constraints such as grid connection limitations and available land area. The goal is to optimize the system for maximum energy production in a cost-efficient way while meeting all technical requirements.
How can we help?
Despro is your reliable partner throughout the entire lifecycle of solar power projects. We have extensive experience with over 5,000 MW in project development and more than 300 MW in construction phases. We offer all the necessary services to design a functional and profitable system!
AUTHOR
Iko Järnberg works at Despro as Development Manager in renewable energy. He has over ten years of experience in wind, solar, and battery projects — with more than 50 design projects completed and main contracting responsibility for ten wind power plants.