Technical Application Papers No.10 Photovoltaic plants - ABB

A metal separation between the PV plant and the public
network shall be guaranteed in order not to feed direct
currents into the grid. For plants with total generated
power not higher than 20kW such separation can be
replaced by a protection (generally inside the electronic
control and setting system of the inverter) which makes
the interface (or generator) device open in case of values
of total direct component exceeding 0.5% of the
r.m.s. value of the fundamental component of the total
maximum current coming out from the converters. For
plants with total generation power exceeding 20kW
and with inverters without metal separation between
the direct and alternating current parts, the insertion of
a LV/lv at industrial frequency is necessary (Guide CEI
82-25, II ed.).
Figure 4.2 shows a single-line diagram typical of a PV plant
connected to the LV grid in the presence of a user’s plant.
Figure 4.2
LV users not enabled
for “stand-alone” operation
Public LV network
Photovoltaic
generator (PV)
kWh
kvarh
Inverter
PV installations can deliver active energy with a power
factor (Guide CEI 82-25, II ed.) 3 :
• not lower than 0.8 delayed (absorption of reactive
power), when the supplied active power ranges from
20% to 100% of the installed total power;
• unitary;
• in advance, when they deliver a total reactive power
not exceeding the minimum value between 1kvar and
(0.05+P/20)kvar (where P is the installed total power
expressed in kW).
2 A high frequency transformer is not suitable since it has output direct current components
exceeding the allowed limits; moreover only a separation transformer is admitted
for more inverters.
3 Referred to the fundamental component.
Measure of the
energy taken from
and fed into the grid
Main
circuit-breaker
(DG)
Interface
device (DDI)
Circuit-breaker
of the generator
(DDG)
Distributor
User’s plant
kWh
27 - 59 - 81
Measurement of
produced energy
Interface
protection (PI)
Circuit-breaker
of the generator
(DDG)
Photovoltaic plants
35
4 Connection to the grid and measure of the energy