H Utilisation Of Generated Power - Viessmann Vitovolt 200 Technical Manual

Hot−spot effects
B
+
−
Utilisation of generated power
Inverter function
Photovoltaic panels supply DC power.
However, AC power is required to be fed
into the grid. Therefore, one of the main
tasks of the inverter is to change the DC
power into AC power suitable for the grid
with the highest possible level of
efficiency.
In addition the inverter constantly
regulates the optimum operating point of
the system (MPP maximum power
point) and so matches the system to the
dynamic weather and radiation
conditions. The ENS function is another
task of the inverter.
ENS means a circuit which safely
separates the photovoltaic system from
the grid during mains failure or work on
the grid (ENS = Equipment for Mains
monitoring with associated Control
components in series [abbreviation
translated from German]).
VITOVOLT
A Solar cell
B Bypass diode
The inverter must separate the
photovoltaic system from the mains, if
the power supply system is switched OFF ,
e.g. for maintenance purposes. Otherwise
power from the photovoltaic system will
be fed into the grid, causing a risk to the
maintenance personnel. Maintenance of
the inverter switching criteria are checked
in accordance with VDEW guidelines and
ENS regulations.
1.3 Photovoltaic system
When all cells of the photovoltaic panel
are wired in series, and any cell is
shaded, that cell will act as an electrical
resistance, i.e. as consumer. This causes
the so−called "Hot−spot effect". The entire
array can only supply as much current as
can flow through the shaded cell, which
will become hotter.
The "hot spot effect" should always be
avoided.
H It reduces the system output.
H It causes damage to the shaded cells
through overheating.
Viessmann photovoltaic panels are
equipped with bypass diodes. If a row of
cells is shaded, the bypass diode opens
and the current bypasses that row of
cells.
In addition the power fed into the mains
by the inverter must not exceed the
harmonic content set by EN 60555.
The inverter must meet the legal
requirements regarding the
electro−magnetic compatibility of
equipment.
The inverter used by Viessmann meets
the ENS function, as well as the EN 60555
requirements and those regarding EMC.
7