Planning the electrical installation 81
Implementing ground fault monitoring in IT (ungrounded)
systems
The internal ground fault monitoring of the inverter is based on total sum of the phase
current measurements. The monitoring will detect severe ground faults in IT (ungrounded)
systems. However, often the ground fault leakage current does not exceed the trip level
and the inverter remains is operation. The inverter can be equipped with insulation
monitoring device option +Q954, +Q976 or +Q981, or the IT system must be monitored
otherwise with a monitoring device suitable for use with inverters. Because of the leakage
currents of inverters, many ground fault monitoring devices do not work properly with
them.
Insulation monitoring device (options +Q954, +Q976 and +Q981)
Options +Q954, +Q976 and +Q981 include an insulation monitoring device with a coupling
device that enables measurement of the insulation resistance. According to the IEC
62109-2 standard, the measurement is needed before the inverter can be started.
The insulation monitoring device measures insulation resistance between the DC busbars
and protective earth (PE). When the inverter is operating, the insulation resistance of the
AC busbars against the protective earth is also measured indirectly. The monitoring device
reacts to all ground faults in IT systems which are galvanically connected to each other.
If the insulation resistance between the conductors and the ground falls below the set
response values, the state of the alarm relay in the insulation monitoring device is
changed, and in R7i and R8i the indication LED on the cabinet door lights up. The inverter
is tripped or an alarm is generated depending on the parameter settings. The measured
insulation resistance value can be read from the inverter parameters in 3 × R8i frames
equipped with option +Q954.
When auxiliary power to the insulation monitoring device is switched off, its alarm relays
are switched to the fault position.
With option +Q954, the response values and parameters of the insulation monitoring
device can be set with its function keys.
Note: The insulation monitoring device measures the insulation resistance of the solar
generator correctly according to the settings when the inverter is not operating. For
disabling the monitoring device during the inverter operation, contact ABB.
Safety information
The insulation monitoring device is constructed according to state-of-the-art and
recognized technical safety rules. Nevertheless, when the device is used, hazards may
occur to the life and limb of the user or of third parties, or there may be adverse effects on
the monitoring device or on other valuable property. The monitoring device must only be
used
• for the purpose for which it is intended
• when it is in perfect technical condition as far as safety is concerned.
Only one insulation monitoring device may be used in each interconnected IT system.
When insulation or voltage test is to be carried out, the device is to be isolated from the
system for the test period.
The ground fault monitoring function (+Q954, +Q976 and +Q981) is not a personnel safety
or fire protection feature.
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