Safety Devices; Anti-Islanding; Photovoltaic Panel Ground Fault; Arc Fault Detection (Afd) - ABB UNO-DM-TL-PLUS-US Product Manual

S
afety devices
A
nti-Islanding
P
hotovoltaic panel ground fault
A
rc Fault Detection (AFD)
G
round fault detection and interruption scheme
In the event of a local utility company grid outage, or when the equipment is
switched off for maintenance, the inverter must be physically disconnected
to ensure protection of people working on the grid, all in accordance with
the relevant national standards and laws. To prevent possible islanding
(the condition in which a distributed generator continues to power a
location even though electrical grid power from the electric utility is no
longer present), the inverter is equipped with an automatic disconnection
system called an Anti-islanding system.
This inverter must be used with photovoltaic modules connected with
"floating" connections -- that is with positive and negative terminals that are
not grounded. An advanced ground fault protection circuit continuously
monitors the ground connection and disconnects the inverter when a
ground fault is detected. The ground fault condition is indicated by a red
LED on the front panel.
This safety function allows the inverter to recognize series electrical
arcing on DC cables. If arcing is detected, the inverter turns itself OFF,
disconnects from the grid, and remains disconnected until manually reset.
An AFD fault is reset by pressing the button on the left side of the DC
wiring box, but a thorough check of hte DC cables and panels must be
completed first, to locate the source of the arcs.
The AFD circuitry runs a self-test at each power up.
As required by UL1741 CRD 2010, the inverter incorporates two separate
methods for detecting a ground fault in the ungrounded PV array. These
are described below:
Method 1: Pre-Start (Static RISO)
Any time conditions are suitable for the inverter to be connected to the
grid, internal circuitry measures the insulation resistance (RISO) of the PV
array conductors to ground. If the result of this static insulation resistance
test is less than the pre-programmed threshold, the connection is aborted,
the inverter will show an error, and illuminate the red LED GF indicator
on the inverter's 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.
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2 - Characteristics