Ground fault detection and interruption scheme
As required by UL1741 CRD 2010, the TRIO inverter incorporates two separate methods for detecting a ground
fault in the ungrounded PV array, as described below:
Method 1: Pre-Start (Static RISO)
Any time conditions are suitable for the inverter to be connected to the grid, prior to connection internal circuitry
measures the insulation resistance (RISO) of the PV array conductors relative to ground. If the result of this static
insulation resistance test is less than the pre-programmed threshold value, the connection is aborted, the inverter
will show an error on the LCD screen and illuminate the red LED GF indicator on the inverter front panel. This test
is conducted prior to any attempt to connect to the grid; e.g., at daybreak and any other time during the day where
the inverter has been disconnected from the grid.
Method 2: Post Grid Connection (Dynamic Leakage Current)
Any time the inverter is connected to the grid, the inverter circuitry continuously checks for ground fault conditions
using a differential measurement of the three-phase AC lines, searching for any values that would indicate leakage
of current to ground. Measurement of the ground leakage current is carried out simultaneously by two independent
and redundant processors. If either processor detects an unacceptable value as dened below, the inverter will
immediately be disconnected from the grid, display an error on the LCD screen, and illuminate the red LED GF
indicator on the inverter front panel.
The inverter responds differently depending on the level and duration of leakage current detected. If any of the
following conditions is detected in measured values of differential current (IDIF) or a rapid change of IDIF over
time (ΔIDIF/Δt), the inverter will automatically disconnect from the grid and the front panel GF indicator will be il-
luminated:
• If IDIF > 300 mA for a period of 300 msec
• If ΔIDIF/Δt > 30 mA/sec for a duration of 300 msec
• If ΔIDIF/Δt > 60 mA/sec with duration of 150 msec
• If ΔIDIF/Δt > 150 mA/sec with duration of 40 msec
As a further safety precaution, in compliance with UL1741 CRD 2010, the inverter conducts an isolation monitor
interrupter self-test before connecting to the grid or every 24 hours, whichever is sooner. This test validates that the
circuitry needed to perform the isolation test operates normally and has not been damaged.
All errors generated from a detected ground fault are permitted to occur up to four times within a 24-hour period,
and ground fault errors require a manual reset of the inverter by a trained technician on the fth occurrence. This
is intended to ensure equipment with a ground fault is not connected to the grid.
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