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Uncovering the Unexpected Impacts of PV Inverters on Local Voltage Sags
This white paper examines how photovoltaic (PV) systems—though theoretically expected to reduce voltage sags by lowering system impedance—can sometimes worsen them. Field data shows that inverter design, current limits, and control algorithms may fail to support the voltage during fast disturbances and can even deepen a sag or trigger a disconnect. The paper explains why the simple “add a source in parallel” model breaks down in practice: the inverter’s switching architecture, finite crest-factor and current-limit behavior, and control-loop priorities restrict available support during the few-cycle events that define most sags. Using day-versus-night recordings from a commercial service, it shows how solar production can reduce measured current yet still coincide with a deeper voltage dip when the inverter hits its limit or merely rides through the event.
Readers get a practical framework for interpreting sag traces: what to watch in one-cycle RMS minimums and coincident current peaks, how to correlate events with irradiance and load steps, and how to distinguish inverter behavior from upstream impedance or transformer effects. The discussion is vendor-neutral and diagnostic-focused, setting realistic expectations for mitigation without revealing proprietary settings.
Key topics include:
- Simple Distributed Generation Model
- PV Inverter Limitations
- Real-World Example
Why utilities should care:
As PV penetration grows, planning depends on sound assumptions about inverter behavior. In practice, current limiting, PLL latency, and protection logic complicate voltage complaints, motor-start analysis, and feeder coordination. Momentary disconnections during sags can stack load back onto the utility. The paper outlines practical monitoring—PQ recorders upstream/downstream of the PV point of common coupling, RMS and event logs—and decision points for settings review and ride-through policy. With clearer expectations, utilities can improve root-cause analysis, prioritize mitigation, and avoid over-promising that “PV will fix sags.”
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