System Design & String Sizing
MPPT windows, temperature-corrected voltage limits, DC/AC ratio, inverter loading, azimuth, tilt optimization, and shading analysis.
- Explain maximum power point tracking (MPPT) and its role in system optimization
- Calculate temperature-corrected string voltage for coldest conditions
- Define DC/AC ratio and explain why designers oversize arrays
- Describe optimal azimuth and tilt for Northern Hemisphere installations
- Estimate annual energy production using site-specific factors
Leçon 1
String Sizing & Inverter Voltage Windows
Maximum Power Point Tracking (MPPT)
Maximum power point tracking (MPPT) is an algorithm used by inverters and optimizers to continuously adjust the operating voltage and current to extract the maximum possible power from the PV array. As irradiance and temperature change throughout the day, the maximum power point on the IV curve shifts. The MPPT controller tracks this moving target in real time.
Exam Definition
MPPT is an algorithm that operates the PV array at the voltage and current combination that maximizes power output. It is not a fuse sizing method, a wiring type, or a temperature correction - it is an active control algorithm.
The MPPT Voltage Window
Every string inverter has an MPPT voltage window - the range of DC input voltages within which the inverter can perform maximum power point tracking. The array string voltage must stay within this window during normal operating conditions for the inverter to harvest maximum energy.
The inverter also has a maximum input voltage that must never be exceeded, or the inverter can be damaged. This maximum is an absolute limit, not an operating range.
Temperature-Corrected String Sizing
When sizing strings, the designer must account for the coldest expected site temperature to determine the maximum possible string VOC. Cold temperatures increase voltage (as covered in Module 1), so the worst-case maximum voltage occurs on a cold, clear morning.
String Sizing Tools
Inverter manufacturers provide string sizing tools that help designers determine the correct number of modules per string. These tools ensure that string VOC and VMP stay within the inverter's MPPT and maximum voltage windows across all expected temperature conditions.
Cold Morning Risk
VOC rises above STC value
Could exceed inverter max voltage
Must size for coldest expected temp
Hot Afternoon Risk
VMP drops below STC value
Could fall below MPPT minimum
Must verify minimum string length
When sizing strings, maximum open-circuit voltage is limited by the coldest expected site temperature - not STC, not average temperature. MPPT is an algorithm that operates the array at the voltage/current combination that maximizes power.