Electrical safety of large photovoltaic installations
Electrical safety of large photovoltaic plants: insulation monitoring and protection
Requirements of standards
According to IEC 60364-7-712:2017 -
Low-voltage electrical installations - Part
7-712: Requirements for special installati-
ons or locations - Solar photovoltaic (PV)
power supply systems, PV installations
must be equipped with insulation monito-
ring equipment. This insulation monito-
ring equipment must meet the require-
ments of ČSN EN 61557-8 - Electrical
safety in low-voltage distribution systems
up to 1000 V a.c. and 1500 V d.c. - equ-
ipment for testing, measuring, or monito-
ring of protective measures - Part 8: Insu-
lation monitoring devices for IT systems.
Annex C to the standard stipulates that in-
sulation monitoring devices for PV instal-
lations must also meet dynamic reference
characteristics. A monitor integrated into
each inverter monitors these characteris-
tics continuously on the basis of the opti-
mum operating point. The insulation mo-
nitoring device must use a measurement
method that operates reliably even if the
DC voltage of the PV power source fluc-
tuates, especially during start-up in the
morning and shutdown in the evening.
Such devices are easy to tell as they bear
the PV symbol on them.
Environment
as an influencing factor
Changes in the insulation value of photo-
voltaic installations are caused by factors
such as the interaction between tempera-
ture and humidity. After a sub-zero night,
the morning humidity can cause a tenfold
increase in the stray capacitance (Ce) of a
photovoltaic installation, while reducing
the insulation resistance (Rf) by as much
as over 30%.
In addition, this phenomenon also
occurs in the event of rain or snowfall as
well as in areas with increased humidity,
for example, close to bodies of water.
Conventional insulation measurement in-
struments are unable to account for these
changes intelligently and therefore give
incorrect readings. These can, in turn, ha-
ve a negative impact on the protection of
people and equipment.
Age as a risk factor
Degradation is another major problem fa-
ced by PV operators. More and more in-
stallations are reaching beyond their best
lifetime and becoming damaged, for in-
stance, as the foil in PV panels becomes
loose or porous and begins to let moistu-
re through. As a result, the insulation value
deteriorates. In obsolete semiconductor
designs, these problems are accompanied
by the so-called PID (potential induced de-
gradation) effect. In such cases, leakage
current and voltage potential cause nega-
tive ions to leave the semiconductor. At
the same time, positive ions become rele-
ased from the surface of the glass, the ca-
sing, and the surrounding environment
and move towards the semiconductor.
This polarisation effect can be reserved by
the so-called PID shift. During the process,
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Electrical Safety of Large
Photovoltaic Installations
The electrical safety of large photovoltaic installations is a challenging task. In addition
to the effects of natural ageing, weather conditions and other factors must also be taken
into consideration. The ISOMETERS
®
adaptive measuring instruments offer a safe and simple
solution compliant with the requirements of the applicable standards to manage problems
throughout the lifetime of photovoltaic installations. Read more about how to operate
a photovoltaic system with electrical safety and economy in mind.
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