Wels, September 15th 2015
INFORMATION ON TESTING PROGRAM FOR CROATIA
Fronius Information on the application of the standard testing program for parallel operation of solar power
plants (“PROGRAM ISPITIVANJA PARALELNOG POGONA SUNČANE ELEKTRANE PRIKLJUČNE SNAGE
do 30 kW S MREŽOM”).
The test document explains several requirements and tests for interconnecting a PV plant.
Fronius International highly recommends not to do any tests with disconnected neutral or where neutral gets
disconnected during the test. (Test 15. and 16.) In that case defects are possible and will result in losing the
guarantee.
All other evaluations and tests can be done with Fronius inverters.
Explanation:
Fronius inverters are internally protected against overvoltages. Not only with having trip limits, that measure the
voltage and disconnect in case the voltage gets outside of the limits, but also protection with varistors to protect
against voltage transients (surge).
Varistors are located between phase and neutral. They are designed to protect against overvoltages based on
phase-to-neutral voltages.
In case neutral is not connected the potential of the neutral conductor is not controlled (Fronius Symo and Eco
inverters are not supplying the neutral). The potential of the neutral is floating somewhere inside the voltage
triangle spanned by the phase-to-phase voltages. With even small asymmetries (like single phase loads, e.g.
fan) the neutral potential can move near a phase potential and cause phase-to-phase voltage on the other
phases to neutral (See figure 2).
Figure 1: System with neutral connected Figure 2: Possible voltages with
neutral disconnected
02/2011
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The varistors between phase and neutral inside the inverters can get damaged in such a situation.
Fronius International GmbH
Solar Energy Division
Froniusplatz 1
A-4600 Wels
DI Thomas Mühlberger
Head of Solution Management
02/2011
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