Inrush Current
Inrush Current
Inrush current — the large, brief surge of current drawn when a system is first energised — is often overlooked in PV commissioning but can cause nuisance tripping, breaker damage, and connector stress if not properly understood and managed. Inverter DC bus capacitor charging, transformer magnetisation, and coupled storage systems all contribute to inrush events of varying magnitude and duration. This section covers measurement equipment, data capture techniques, and safe energisation sequencing to characterise and manage inrush events on PV systems.
The primary source is inverter DC bus capacitor charging. When DC voltage is first applied to an inverter, the large capacitors on its DC input bus are uncharged and draw a surge of current to charge to operating voltage — the magnitude depends on capacitance, source impedance, and how fast voltage is applied. Transformer magnetisation inrush is a secondary source in transformer-coupled systems. Neither is a fault; both are normal system behaviour.
Yes. Repeated high-magnitude inrush events stress MC4 connectors and string wiring beyond their rated continuous current. A single connection event may not cause visible damage, but frequent energisation cycles — particularly in systems where strings are switched individually — accelerate connector wear and can initiate the arc fault conditions that degraded connectors are prone to. This risk is amplified with high-efficiency and bifacial modules, which produce higher operating currents than conventional PERC.
Inrush current protection on instruments like the Emazys Z300 HE limits the current surge that flows through the meter when it is connected to an energised string for Voc or Isc measurement. Without protection, the instantaneous current at connection can spike well above the instrument's rated continuous current. It is most important when testing TOPCon, HJT, and bifacial module strings, which operate at higher currents than conventional PERC modules.
Pre-charge the inverter DC bus at reduced voltage before applying full string voltage — most string inverters and central inverters have a soft-start function for this purpose. Bring strings online sequentially rather than simultaneously. Avoid connecting multiple strings in parallel at the combiner before the inverter is energised. If your inverter does not have configurable soft-start behaviour, consult the manufacturer's commissioning guidance for string sequencing recommendations.