Technical Application Papers No.10 Photovoltaic plants - ABB

5 Earthing and protection against indirect contact
5.1 Earthing
The concept of earthing applied to a photovoltaic (PV)
system may involve both the exposed conductive parts
(e.g. metal frame of the panels) as well as the generation
power system (live parts of the PV system e.g. the
cells).
A PV system can be earthed only if it is galvanically separated
(e.g. by means of a transformer) from the electrical
network by means of a transformer. A PV insulated system
could seem apparently safer for the people touching
a live part; as a matter of fact, the insulation resistance
to earth of the live parts is not infinite and then a person
may be passed through by a current returning through
such resistance. This current rises when the voltage to
earth of the plant and the plant size increase since the
insulation resistance to earth decreases. Besides, the
physiological decay of the insulators, due to the passage
of time and the presence of humidity, reduces the
insulation resistance itself. Consequently, in very big
plants, the current passing through a person in touch
with the live part may cause electrocution and therefore
the advantage over the earthed systems is present only
in case of small plants.
5.2 Plants with transformer
In the plants with transformer, in addition to the analysis
of the PV system either insulated or earthed, for the
protection against indirect contacts it is necessary to
make a difference between the exposed conductive parts
upstream and downstream the transformer 1 .
5.2.1 Exposed conductive parts on the load side
of the transformer
5.2.1.1 Plant with IT system
In this type of plant the live parts result insulated from
earth, whereas the exposed conductive parts are earthed 2
(Figure 5.1).
Figure 5.1
+ - + - + -
B
Id
Load
Re
A
In this case the earthing resistance R e of the exposed
conductive parts shall meet the condition (CEI 64-8):
Figure 5.2
1 In this case upstream and downstream are referred to the direction of the electric power
produced by the PV plant.
R e
120
2 For safety reasons the earthing system of the PV plant results to be in common with the
consumer’s one. However, to make the insulation controller of the inverter operate properly
and monitor the PV generator it is necessary that the frames and/or the supporting
structures of the panels (even if of class II) are earthed.
I d
[5.1]
where I d is the current of first fault to earth, which is not
known in advance, but which is generally very low in
small-sized plants. As a consequence, the earthing resistance
R e of the consumer plant, which is defined for a
fault in the network, usually satisfies only the relation [5.1].
In case of a double earth fault, since the PV generator
is a current generator, the voltage of the interconnected
exposed conductive parts shall be lower than:
I sc . R eqp ≤ 120V [5.2]
where I sc is the short-circuit current of the cells involved,
whereas R eqp is the resistance of the conductor interconnecting
the exposed conductive parts affected by
fault. For instance, if R eqp = 1Ω (value approximated by
excess), the relation [5.2] is fulfilled for I sc not exceeding
120A, which is usual in small-sized plants; therefore the
effective touch voltage in case of a second earth fault
does not result hazardous. On the contrary, in large-sized
plants it is necessary to reduce to acceptable limits the
chance that a second earth fault occurs by eliminating
the first earth fault detected by the insulation controller
(either inside the inverter or external).
5.2.1.2 Plant with TN system
In this type of plant the live parts and the exposed conductive
parts are connected to the same earthing system
(earthing system of the consumer’s plant). Thus a TN
system on the DC side is obtained (Figure 5.2).
+ - + - + -
B
Id
Load
Re
A
Photovoltaic plants
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5 Earthing and protection against indirect contact