Case Study: Repowering a High School Array
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 failed, cutting production in half. The array wasn’t making money with 15+ years remaining on the power purchase agreement (PPA). It was time for some serious upgrades.
Mayfield Renewables 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.
Project Goals
The repower project began in 2022 when one of the two central inverters failed. Mayfield worked with Cleanleaf to acquire as-built documentation and outline the changes needed to re-permit the updated system.
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?
Answers to these questions painted a fuzzy picture. The existing equipment, layout, and code compliance introduced many engineering challenges. Through collaboration with the equipment manufacturers, the California Division of State Architects (DSA), Clean Leaf Energy, and the system owner, the picture slowly came into focus.
Project Hurdles
Mayfield encountered many hurdles during project development, which were overcome by proper due diligence and thoughtful value engineering:
Interconnection and Power Purchase Agreements
The array had to retain its AC capacity to satisfy the 1 MW PPA and interconnection agreements. The system owner requested to keep one of the existing central inverters which was still operational, as well as all of the PV modules. So, Mayfield set out to redesign the remaining 500 kW of AC capacity.
Mayfield 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, a system configuration that was a relic of its time and introduced engineering complexity.
Mayfield 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
Mayfield’s 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 had to be addressed prior to redesigning the system.
After Cleanleaf confirmed the conduits installed, Mayfield adjusted the wire schedule to repurpose existing DC conduits for AC circuits. Mayfield also upsized conductors for voltage drop while ensuring conduit fill capacities were met.
Results
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. The updated array was also compliant with the current codes and standards.
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.
Mayfield Renewables is an engineering consultancy specializing in commercial and industrial PV and microgrid engineering. Contact us today for a consultation.
Contact Clean Leaf Energy to learn more about this project: info@cleanleafenergy.com
