Technical Application Papers No.10 Photovoltaic plants - ABB

If a voltage is applied from the outside to the PV cell in
reverse direction with respect to standard operation,
the produced current remains constant and the power
is absorbed by the cell. When a certain value of inverse
voltage (“breakdown” voltage) is exceeded, the junction
P-N is perforated, as it occurs in a diode, and the current
reaches a high value thus damaging the cell. In absence
of light, the generated current is null for reverse voltage
up to the “breakdown” voltage, then there is a discharge
current similarly to the lightening conditions (Figure 2.3
– left quadrant).
Figure 2.3
Current [A]
2.3 Grid connection scheme
A PV plant connected to the grid and supplying a consumer
plant can be represented in a simplified way by
the scheme of Figure 2.4.
The supply network (assumed to be at infinite short-circuit
power) is schematized by means of an ideal voltage
generator the value of which is independent of the load
conditions of the consumer plant. On the contrary, the
PV generator is represented by an ideal current generator
(with constant current and equal insolation) whereas the
consumer plant by a resistance R u .
Figure 2.4
PV
generator
V inv
Current [A]
Ig
0
N
Iu
RU
Ir
U
V oc
Voltage [V]
Network
The currents I g and I r , which come from the PV generator
and the network respectively, converge in the node N of
Figure 2.4 and the current I u absorbed by the consumer
plant comes out from the node:
I u = I g + I r
[2.4]
Since the current on the load is also the ratio between
the network voltage U and the load resistance R u :
I u = U
R u
the relation among the currents becomes:
I r = U
R u
I r = U
R u
I g = U
R u
- I g
[2.5]
[2.6]
If in the [2.6] we put I g = 0, as it occurs during the night
hours, the current absorbed from the grid results:
[2.7]
On the contrary, if all the current generated by the PV
plant is absorbed by the consumer plant, the current
supplied by the grid shall be null and consequently the
formula [2.6] becomes:
[2.8]
When the insolation increases, if the generated current
I g becomes higher then that required by the load I u , the
current I r becomes negative, that is no more drawn from
the grid but put into it.
Multiplying the terms of the [2.4] by the network voltage
U, the previous considerations can be made also for the
powers, assuming as:
• P u = U . I u = U2
R u
the power absorbed by the user plant;
• P g = U . I g the power generated by the PV plant;
• P r = U . I r the power delivered by the grid.
Photovoltaic plants
19
2 Energy production