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Best Practices for Utility-Scale Solar Inverter Design



Utility-scale solar projects rely heavily on the type of inverter used, as it directly impacts installation, efficiency, cost, and maintenance. There are three primary types of utility-scale solar inverters: multiple MPPT string inverters, single MPPT inverters with DC combiners, and central inverters. Each has unique features, advantages, and drawbacks, which we'll explore in detail below. This article will also provide key takeaways for inverter design on your next project. 


1. Multiple MPPT String Inverters

Multiple MPPT (Maximum Power Point Tracking) string inverters connect directly to the PV strings and are installed near the racking system. Each string can be monitored individually, allowing for detailed performance tracking.


Pros:

  • Cost-Effective Installation: These inverters are the cheapest in terms of installation, labor, and materials.

  • String-Level Monitoring: They allow for detailed monitoring of each string's current and voltage, facilitating maintenance and early fault detection.

  • Flexibility: Easy to connect and install directly next to the racking system.


Cons:

  • Distributed Layout: The inverters are spread across the field, leading to longer AC wire runs to the transformer or AC switchboard.

  • Voltage Drop: Significant AC voltage drop occurs between the inverter output and the AC switchboard input, leading to irreversible losses.

  • Lower Efficiency: These systems are less efficient operationally due to the aforementioned voltage drops.

  • Communication Costs: Long communication wires are needed due to the distributed layout, adding to the overall cost.

  • Brand Reliability: Often manufactured by overseas brands, which might be less preferred due to quality concerns.


Best Use Cases:

  • Ideal for projects up to 5 MW AC where initial installation cost savings are prioritized.

  • Suitable for scenarios where detailed string-level monitoring is crucial for maintenance.


2. Single MPPT Inverters with DC Combiner

Single MPPT inverters with DC combiners feature a single DC input, requiring multiple strings to be combined before connecting to the inverter. DC combiners are spread across the field and connected to the inverter, which is placed centrally.


Pros:

  • High Efficiency: These systems offer the highest production efficiency due to minimized AC voltage drop.

  • Reduced Communication Costs: Shorter communication wires are needed, as inverters are centrally located.

  • Centralized Layout: Easier installation and maintenance due to the centralized location.


Cons:

  • Higher Initial Costs: Installation involves additional costs for DC combiners and wiring.

  • Limited Monitoring: Only allows monitoring at the DC combiner level, not individual string level.

  • Complex Maintenance: Requires experienced maintenance teams to detect and diagnose issues at the DC combiner level.


Best Use Cases:

  • Optimal for projects where maximizing production efficiency is critical.

  • Suitable for larger systems where detailed string-level monitoring is less critical, and maintenance teams are experienced.


3. Central Inverters

Central inverters are large, high-capacity units typically housed in containers with integrated medium voltage (MV) equipment. These systems consolidate multiple DC inputs into a single AC output.


Pros:

  • Cost-Effective Per Watt: Generally offers the lowest specific price per watt compared to decentralized solutions.

  • Reliability: High reliability with industrial-scale components, leading to less frequent failures.

  • Integrated Solutions: Often provided as containerized solutions with pre-connected MV transformers and switchgear, reducing installation time and labor costs.

  • Simplified Maintenance: Easier to maintain as fewer units are involved.


Cons:

  • Risk of Significant Production Loss: A single inverter failure can result in substantial production loss.

  • Limited Monitoring: Monitoring is typically limited to the DC input level, which may not provide the granularity some operators desire.

  • Long Lead Times: High demand can result in long lead times for certain brands.


Best Use Cases:

  • Ideal for large-scale projects over 5 MW AC where cost-efficiency and reliability are priorities.

  • Suitable for scenarios where quick installation and reduced maintenance complexity are essential.


High-Level Takeaways

  1. Multiple MPPT String Inverters: Best for smaller projects up to 5 MW AC, where installation cost savings and detailed string-level monitoring are valued.


  2. Single MPPT Inverters with DC Combiner: Optimal for maximizing production efficiency in mid to large-scale projects, balancing higher initial costs with operational efficiency.


  3. Central Inverters: Preferred for large-scale projects over 5 MW AC, offering the lowest cost per watt, high reliability, and simplified maintenance, despite potential risks of significant production loss and longer lead times.


Selecting the right inverter type is crucial for optimizing the performance and cost-effectiveness of utility-scale solar projects. Each inverter type presents unique benefits and challenges, requiring careful consideration based on project size, budget, and specific operational requirements. Get in touch with a technical expert at Castillo Engineering for a consultation on your next project.

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