Method 2: Post Grid Connection (Dynamic Leakage Current)
While the inverter is connected to the grid, the inverter circuitry continuously
checks for ground fault conditions using a differential measurement of the
AC line, 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, and it will lluminate 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 red front panel GF LED will illuminate:
• 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
conrms that the circuitry needed to perform the isolation test is operating
normally, and has not been damaged.
Ground fault errors are permitted to occur up to four times within a
24-hour period, after which a fth ground fault error within a 24 hour
period requires a manual reset. The system must be given a thorough
examination before the reset, the cause of the ground fault located and
corrected. This is intended to ensure equipment with a ground fault is not
connected to the grid.
Other safeguards
The inverter also has:
- Constant monitoring of the grid voltage to ensure the voltage and
frequency remain within operating limits;
- Internal temperature control to automatically limit the power (if necessary)
to prevent overheating.
The numerous control systems and redundancy ensure safe operation.
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