A Recording-Trigger Method for DC-Side Insulation Anomalies of Photovoltaic Inverters Under Wide-Range Voltage Fluctuations

An ungrounded bipolar photovoltaic (PV) DC link has no stable voltage reference: the bus swings 500–1500 V under normal maximum power point tracking (MPPT), irradiance and temperature changes, so a fixed threshold false-starts or misses low-voltage faults. We combine two physically different observables in a layered, non-redundant trigger: a millisecond voltage-imbalance arm and a persistent insulation-resistance confirmation before latching, with a resistance-only backup and a slow-aging branch, each judged against the episode’s own baseline, not an ohmic cutoff. Sampling (400 Hz anti-alias, 10 kHz, 1 kHz) is Nyquist-consistent; the nominal 4.7 MΩ spans 2.1–5.6 MΩ across baselines. On 18,000 Monte Carlo cases the detector reaches an overall Pd=0.9223, detects 233/234 faults at the empirical 2 MΩ 95% limit with 0/6000 false records (bound 5.0 × 10−4), a 329 ms median confirmation latency and a 263 ms median arm lead. With the voltage channel unusable the resistance backup retains coverage (McNemar p=1); independent C and Python detectors agree sample by sample. Stress tests give Pd=1.00 under simultaneous bipolar breakdown, zero false records under 80 V ringing or strong common mode, and only a bounded delay during an estimator gap.