SMA SC 500CP XT - Manual (Page 89)

10.2.2.4 Passive Islanding Detection
The inverter is equipped with passive islanding detection. This function can be activated if required. The islanding
detection function detects the formation of stand-alone grids and disconnects the inverter from the utility grid.
Islanding can occur when at the time of utility grid failure, the load in the shut-down sub-grid is roughly equivalent to
the current feed-in power of the PV power plant.
Unlike active islanding detection, with passive islanding detection the utility grid is not actively influenced, but simply
passively monitored. This involves monitoring the speed of the frequency change.
If the power frequency changes by a certain amount in a certain time, a stand-alone grid is detected and the inverter
disconnects from the utility grid. The magnitude of the frequency change and the time in which this change must take
place can be configured via parameters on the grid monitoring relay.
10.2.3 Grounding and Insulation Monitoring
10.2.3.1 Mode of Operation
In grounded PV arrays
The ground-fault monitoring is implemented by means of a residual-current monitoring device. If a ground fault occurs,
the residual currents are detected and interrupted.
• Ground fault on the ungrounded terminal
If a ground fault occurs on the ungrounded terminal of the PV array, the normally ungrounded terminal of the PV
array is grounded non-specifically by the ground fault and a residual current flows to the grounded terminal. This
residual current flows through the ground-fault monitoring device, e.g. the GFDI, and triggers it.
• Ground fault on the grounded terminal
The GFDI is bypassed when a ground fault occurs on the grounded terminal of the PV array. A ground fault on
the grounded terminal cannot be reliably detected. If an undetected ground fault occurs on the grounded
terminal, this will pose a safety risk. A further ground fault occurring on the ungrounded terminal will lead to high
residual currents that cannot be interrupted by the ground-fault monitoring unit.
Residual current monitoring in grounded systems
In order to ensure the residual current monitoring function in grounded systems, the PV array insulation must be
checked at regular intervals. It is therefore advisable to use an additional insulation monitoring device in
grounded systems. This will enable the insulation to be checked at regular intervals.
In ungrounded PV arrays
An insulation monitoring device constantly determines the insulation resistance using an active measurement
procedure. As soon as the insulation resistance falls below the warning threshold specified in the insulation monitoring
device, an insulation warning will appear on the touch display. As a result, preventative measures can be taken before
errors such as personal injury due to leakage currents or system failure occur. If the insulation resistance falls below the
configured warning threshold, the PV power plant switch off. Use the parameter IsoErrIgn to activate or deactivate the
disconnection process under fault conditions.
10.2.3.2 GFDI
Depending on the order option, ground-fault monitoring in the inverter may be carried out via ground fault detection
and interruption (GFDI). This grounds one terminal of the PV array. GFDI is performed via a high-performance K-type
circuit breaker with adjustable operating current. The GFDI is integrated in the inverter and connected between an
input busbar and the grounding busbar.
10 Function Description SMA Solar Technology AG
User Manual 89 SCCPXT-BA-E4-en-54