Case Study
Repowering a High School Array
Cleanleaf Energy
Project Highlights
Location
Lancaster, CA
System Specs
1 MW / 200 V AC
Equipment
PV modules: Trina Solar TSM-230PA05 String inverters: SolarEdge SE 17.3K & SE50K PV optimizers: SolarEdge P1100 2:1
Challenge
In September 2022, Cleanleaf Energy (formerly Borrego) approached Mayfield Renewables with a repower project at Antelope Valley High School in Southern California. Originally installed in 2011, one of the system’s two 500 kW central inverters had failed, cutting production in half. With 15+ years remaining on the power purchase agreement (PPA), the array wasn’t making any money. It was time for some serious upgrades.
Approach
As with any repower project, we began by asking a few questions:
- How was the original system built?
- What equipment can we keep?
- What codes and standards must the repowered array comply with?
- What other parties will be involved?
- Are there any strict system DC voltage, AC voltage, and/or AC power requirements?
The answers painted a fuzzy picture. The existing equipment, layout, and code compliance introduced many engineering challenges. By collaborating with the equipment manufacturers, the California Division of State Architects (DSA), Cleanleaf Energy, and the system owner, we gradually brought the picture into focus.
Project Hurdles
Interconnection & power purchase agreements
The array had to retain its AC capacity to satisfy the 1 MW PPA and interconnection agreements. The system owner wanted to keep one of the existing central inverters, which was still operational, as well as all of the PV modules. So, we set out to redesign the remaining 500 kW of AC capacity.
We overcame this hurdle by replacing one 500 kW central inverter with a mix of distributed string inverters that summed to 500 kW. Some inverters were derated to meet the desired system capacity.
Abnormal AC Voltage
The old central inverters had a 200 Vac output connected to a 4160/200 V step-up transformer. This system configuration was a relic of its time and introduced engineering complexity.
We determined that SolarEdge SE17.3K and SE50K string inverters paired with SolarEdge P1100 optimizers were the best fit given the AC voltage limitations. This enabled the new array circuits to tie into the existing step-up transformer at 200 Vac with the least re-work. Optimizers also helped to boost the efficiency of the aging array.
Constructability
Our approach to system design emphasizes constructability. For this repower, the site owner wanted to avoid any new trenching and conduit fill, so the existing underground DC conduit had to be repurposed for AC circuits. The as-built drawings also had discrepancies that we had to address prior to redesigning the system.
After Cleanleaf Energy confirmed the conduits installed, we adjusted the wire schedule to repurpose existing DC conduits for AC circuits. We also upsized conductors for voltage drop while ensuring conduit fill capacities were met.
Result
The resulting plan set included installation details to replace one of the existing central inverters while retaining 1 MWac total system capacity and 200 Vac system voltage.
We assisted in product selection, delivered a full permit plan set with demolition plans, and brought the system up to NEC 2020 safety standards—including rapid shutdown—while retaining as much existing equipment as possible.
After a lengthy DSA review process, the plans were approved and the system was re-energized in July 2024. It is operating at full capacity today, with its original PPA and interconnection agreements intact.
