CURRENT OVERSIZING OF FRONIUS INVERTER OR MPP-TRACKER
(IMPP > IDC,MAX)
The DC inputs of inverter or MPP-tracker (MPPT) have a lot of parameters for classification and limitation.
(Udc,max; Umpp,max; Udc,r; Umpp,min; Idc,max; Ipv,max; Pdc,max; Ppv,max; ….)
This paper should clarify the key characteristics of the electrical current.
In practice the maximum MPP current of the PV array can exceed the maximum input current of the inverter /
MPPT. Such a case is called Current Oversizing (Impp,stc of the PV array > Idc,max of the inverter / MPPT).
The Fronius SnapINverter generation (Galvo, Symo, Symo-Hybrid, Primo, Primo-Hybrid, ECO) show a high
robustness concerning high current (and high power also)!
The current oversizing may account for 50% in any case, for some inverters even more (lo datasheets: max.
array short circuit current = Ipv,max). That means, the maximum short circuit current of the connected PV array
(Isc,stc) may exceed the inverters maximum input current (Idc,max) to at last 50%, without voiding our warranty
and without damaging the inverter.
Such a configuration might only cause a slight reduction of the annual energy yield.
Maximum yield in spite of current limiting
Due to the characteristic of a PV module and the frequency (rareness) of high irradiations, current limiting leads
to a negligible reduction of the energy yield, which ensures profitability of such a configuration.
(Please find a more detailed explanation later in this article.)
/ Example 1: Fronius Symo Hybrid 5.0-3-S (Idc,max = 16A) with 2 module strings, each Impp,stc = 8.8A
The PV arrays current (2 x 8.8Astc = 17.6A) is 10% higher than the current the inverter will maximally take
(Idc,max = 16A).
With this configuration the annually energy yield may get reduced by less than 0.25% caused by the
current limiting. (fig. 1)
Fig. 1: Energy yield per irradiation class for a year (Munich / South oriented roof)
Impp,stc - 10% = Idc,max,inv
1/4
/ Example 2: Fronius Symo Hybrid 5.0-3-S (Idc,max = 16A) with 2 module strings, each Impp,stc = 9,5A
The PV arrays current (2 x 9.5Astc = 19A) is 18.75% higher than the current the inverter will maximally take
(Idc,max = 16A).
With this configuration the annually energy yield may get reduced by less than 0.75% caused by the
current limiting. (fig. 2)
Fig. 2: Energy yield per irradiation class for a year (Munich / South oriented roof)
Impp,stc – 18.75% = Idc,max,inv
2/4
WHY IS ONLY SO LITTLE LOSS?
(TECHNICAL EXPLANATION)
Maximum Operating Current (input current) Idc,max
With Fronius inverters this is not a safety limit.
In fact, this value may be exceeded.
Similarly to the power oversizing the current consumption is limited by shifting the operating point towards the
open circuit voltage. As power is the product of current and voltage, power drops much less than the current.
Pmpp
6000
18
Impp
Idc,max
16
14
220
Fig. 3a:
Umpp
275 Udc [V] 330
current reduction caused by
shifting the operating point
Umpp
385
5000
220
Fig. 3b:
275 Udc [V] 330
385
power reduction caused by
shifting the operating point
(characteristic curves by PV-Syst)
These charts make it clearly visible. The limitation of the current of (e.g.) 17.6A at 16A is achieved by increasing
the voltage of 6.5% (operating point shift of the inverter; see fig. 3a). Due to this voltage increase, at this
moment the power gets reduced of 3.2% (fig. 3b).
The smaller the current limiting (e.g. 16.5A to 16A) the much smaller is the power reduction. (non-linear)
As in a year, these moments of high irradiation and so far high current (current is linearly depending on
irradiation) only happen seldom (in this example for only 2.5% of the operating time it is above 950W/m²), the
annual energy yield reduces of negligible less than 0.25%.
Fig. 4:
percentage distribution of the irradiation in one year (Munich / South oriented roof)
Impp,stc - 10% = Idc,max,inv
3/4
Maximum Short Circuit Current Ipv,max
a safety-relevant limit for the current
The so called Ipv,max is the maximum short circuit current (Isc at STC), that must not be exceeded by the PV
array connected to the inverter (MPPT).
(lo datasheets: max. array short circuit current = Ipv,max)
Equal to the Udc,max, applying as the absolute limit of the maximally arising voltage (theoretically maximum
open circuit voltage at average irradiation and lowest temperatures), there is also a maximum permissible
current which must not get exceeded.
Different from the open circuit voltage this current only occurs in the case of a short circuit.
All components must be capable of carrying a potentially arising short circuit current.
A DC-disconnector, as it is integrated in an inverter, must be able to safely disconnect this short circuit current.
Fronius International GmbH
Solar Energy
Wels, 20. August 2016
4/4