Technical Application Papers No.10 Photovoltaic plants - ABB

6.1.2 Protection of the strings against reverse
current
Due to shading or fault a string becomes passive, absorbing
and dissipating the electric power generated by the other
strings connected in parallel to the same inverter through
a current which flows through the string under consideration
in a reverse direction with respect to that of standard
operation, with possible damages to the modules.
These are able to withstand a reverse current ranging
from 2.5 and 3 I (IEC TS 62257-7-1). Since with x strings
sc
in parallel connected to the same inverter the highest
reverse current is equal to I = (x-1) . 1.25 . I , it is not
inv sc
necessary to protect the strings if I ≤ 2.5 . I that is
inv sc
(x-1) . 1.25 ≤ 2.5 ⇒ x ≤ 33 .
6.1.3 Behaviour of the inverter
The contribution to the short-circuit on the DC side of the
inverter may come from the grid and from the discharge
of the capacitors inside the inverter.
The grid current is due to the recirculating diodes of the
bridge inverter which in this case act as a bridge rectifier.
Such current is limited by the impedances of the
transformer and of the inductors belonging to the output
circuit and by the protection fuses of the inverter on the
AC side chosen so that they can limit the thermal effects
of possible internal faults on the semiconductors. As a
consequence the I 2 t passing through will be normally
reduced. Indicatively a final current value (internal capacitors
completely discharged) of 10In can be an upper
limit value. This current is present in case of inverter with
galvanic insulation at 50Hz, while it is null in case of inverter
without transformer. In fact these inverters usually
have an input DC/DC converter so that the operation on
a wide voltage range of the PV generator is guaranteed;
this converter, due to its constructive typology, includes at
least one blocking diode which prevents the contribution
of the grid current to the short-circuit.
The discharge current of the capacitors is limited by the
cables between inverter and fault and exhausts itself with
exponential trend: the lowest the impedance of the cable
stretch, the highest the initial current, but the lowest the
time constant of the discharge. The energy which flows
is limited to that one initially stored in the capacitors.
Moreover, if a blocking diode or other similar device is in
3 The blocking diodes can be used, but they do not replace the protections against
overcurrent (IEC TS 62257-7-1), since it is taken into consideration the possibility that
the blocking diode does not work properly and is short-circuited. Moreover the diodes
introduce a loss of power due to the voltage drop on the junction, a loss which can be
reduced by using Schottky diodes with 0.4V drop instead of 0.7V of conventional diodes.
However the rated reverse voltage of the diodes shall be ≥ 2 U oc and the rated current ≥
1.25 I sc (CEI Guide 82-25).
series with one of the two poles, this contribution to the
short-circuit is null.
In each case, the short-circuit on the DC side causes
a drop of the direct voltage, the inverter certainly shuts
down and probably is disconnected from the grid. Normally
the shut down times of the inverter are of the order
of some milliseconds, while the disconnection times may
be of the order of some dozens of milliseconds. In the
interval between the shut down and the disconnection,
the grid might cause the above mentioned effect, while
the internal capacitors, if involved, participate up to their
complete discharge.
However, the influences of both the grid and the internal
capacitors on the short-circuit have only a transient nature
and they are usually not such as to affect the sizing
of the protection, switching and disconnection devices
positioned on the DC side.
6.1.4 Choice of the protective devices
As regards the protection against the short-circuits on the
DC side, the devices shall be obviously suitable for DC
use and have a rated service voltage Ue equal or higher
than the maximum voltage of the PV generator which is
4 equal to 1.2 U (IEC TS 62257-7-1).
oc
Moreover the protection devices shall be positioned at
the end of the circuit to be protected, proceeding from
the strings towards the inverter, that is in the various
subfield switchboards and inverter switchboards since
the short-circuit currents come from the other strings,
that is from the load side and not from the supply side
(IEC TS 62257-7-1).
In order to avoid unwanted tripping under standard operation
conditions, the protective devices positioned in the
subfield switchboards (device A in the Figure 6.1) shall
5 have a rated current I : n
I n ≥ 1.25 . I sc
[6.1]
These devices shall protect:
• every single string against the reverse current;
• the connection cable 6 string to subswitchboard (cable
1 of Figure 6.1) if the latter has a current carrying capacity
lower than the maximum short-circuit current of
the other x-1 strings connected to the same inverter
switchboard 7 , i.e. if:
I z < I sc2 = (x - 1) . 1.25 . I sc
4 Uoc is the no load voltage coming out of the strings (see Chapter 3).
[6.2]
5 For thermomagnetic circuit-breakers the [6.1] becomes I1 ≥ 1.25 . I sc , while for magnetic
only circuit-breakers I u ≥ 1.25 . I sc so that their overheating can be avoided.
6 Protection against short-circuit only because Iz ≥ 1.25 . I sc .
7 The short-circuit Isc1 = 1.25 . I sc (fig. 6.1) (Figure 6.1) is unimportant because the string
cable has a current carrying capacity not lower than 1.25 . I sc .
Photovoltaic plants
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6 Protection against overcurrents and overvoltages