When to Consider Repowering Your PV Plant: Engineering Insights from the Field

by Rolandas Sliautaris, Lead Project Engineer at Detra Solar.

Many solar PV assets commissioned in the early 2010s are approaching a critical juncture. As components age and technology advances, asset owners are increasingly weighing the benefits of repowering—especially where inverter obsolescence, declining performance, or safety concerns are present.

Our engineering team recently completed an optioneering study for a ~13.5 MWp UK solar farm built in 2014. The site was originally equipped with 27 central inverters and 1000V DC modules, and although performance wasn’t critically low, aging equipment, compliance risks, and inverter obsolescence prompted the client to explore repowering options.

Here’s what we learned—and what you should consider—before embarking on a repowering project.

First: Assess the Condition, Not Just the Age

Before jumping into replacement strategies, we conducted a detailed engineering review, covering:

• Electrical design integrity – The DC string configuration and AC cable routes were mostly within industry norms, with DC losses averaging 1.15% (max 2.95%) and AC cable losses around 0.1%. However, certain components—like oversized MCCBs and busbar-sharing in inverter buildings—were flagged for correction to meet updated protection standards.
• Earthing compliance – This was a critical issue. Many rows of mounting tables were not bonded according to IEC standard, and module piles were not connected to the main earthing matrix—both direct violations of IEC 62548. Without a proper equipotential earth, the risk of inverter malfunction, undetected ground faults, and arc flash was significantly higher.
• Monitoring system risks – CAT5e data cables stretched up to 314 meters—more than triple the recommended maximum of 100m. While functional, this setup posed real risks for communication reliability, prompting recommendations to upgrade to RS-485 or PLC systems.

These findings reinforced a key point: even systems that appear operational may have underlying issues that compromise both safety and long-term performance.

Inverter Technology: Legacy Constraints Meet Modern Choices

The original inverters were SolarMax 360TS-SV units—no longer supported, and designed around a 1000V DC architecture. While many modern inverters now operate at 1500V, we needed to find replacements that could align with the existing system voltage, MPPT range, and string configurations.

We evaluated two main replacement strategies:

• String inverters – Brands like SMA, Huawei, and Sungrow offered viable 1000V-compatible options (e.g., SMA STP110-60, Huawei SUN2000-150K-MG0). These would require 74 to 101 new units, depending on the chosen DC/AC ratio. Benefits included:
  • Higher system resilience in case of inverter failure
  • Easier O&M with lower replacement complexity
  • Flexibility for phased implementation
• However, this came with trade-offs: extensive LV AC trenching, removal of combiner boxes, and transformer and switchgear redesign.
• Central inverters – Fewer units (5 to 7), such as SMA Sunny Central 2200, could potentially use existing DC cable infrastructure and reduce trenching. But:
  • Spare parts and serviceability are more complex
  • Maintenance typically requires expert technicians
  • A single fault can impact larger sections of the plant

PV String Configuration

Both options required significant transformer and LV switchgear updates due to output voltage differences (385–400V vs the original 320V).

Energy Yield: Small Changes Add Up

We modelled eight repowering scenarios using PVSyst, comparing both inverter types and DC/AC configurations.

• The Performance Ratio (PR) ranged from 80.94% (current system) up to 82.86% (repowered)
• Specific energy production improved from ~979 to ~1002 kWh/kWp/year
• This translates to 100–300 MWh/year of additional output—enough to affect ROI over the lifetime of the asset

Optimizing the inverter configuration and carefully balancing the DC/AC ratio proved essential to avoid undervoltage issues at low temperatures while maximizing annual production.

Don’t Ignore the Earthing System

One of the most concerning findings was the earthing configuration. The lack of table-to-table bonding and disconnected metal piles posed:

• Safety hazards (electric shock, arc flash)
• Fault detection failures in inverters
• Elevated electromagnetic interference (EMI), risking comms reliability

We proposed strengthening the site’s earthing system either by upgrading to a ring-type equipotential grid or, at minimum, by correcting the existing setup—this includes bonding adjacent tables and ensuring each table row is connected to the main earthing via the mounting structure. Either approach significantly improves safety, reduces the risk of floating potentials, and supports the correct functioning of inverter protection and RS-485/PLC communication systems.

Earthing layout (lack of connection to tables)

Final Thoughts: Full or Partial Repowering?

Given the current system was still operating but with high risk of component failure and rising O&M costs, we recommended partial repowering across three of the site’s seven substations.

This approach:

• Lowers upfront CAPEX
• Allows re-use of removed inverters for spares
• Enables live testing of new systems with minimal disruption

It’s a cost-effective way to modernize without committing to a full plant retrofit in one step.

Key Takeaways for Asset Owners

• Don’t wait for failure – Inverter obsolescence, ageing cabling, and non-compliant earthing may already be eroding your ROI and increasing your risk exposure.
• Start with a deep engineering study – Visual checks and SCADA data rarely tell the full story.
• Compare all angles – Cost, yield, safety, O&M—repowering is more than a financial decision.
• Consider phased repowering – It balances risk and return, and builds resilience over time.

If your site is approaching 10+ years in operation, it may be time to ask:  What’s the cost of doing nothing?

Considering a repowering project?

At Detra Solar, we help asset owners make informed, engineering-led decisions—whether it’s through optioneering studies, design audits, or full repowering strategies.

If you’re looking to maximise the performance and lifespan of your PV asset, get in touch with our team to explore how we can support your next steps.