Frequently asked questions in the photovoltaic field
1) How to select rated voltage and rated current of fuse-links for d.c. protection of PV sources?
Parameters shall be selected with regard to resulting series-parallel connection of PV panels and their characteristics.
Un ≥ 1.2 × VOC_STC × M
M...number of panels in series
VOC_STC ...open circuit voltage of the PV panel
The factor 1.2 makes provision for voltage increase at low ambient temperatures, manufacturing tolerances of PV panels, etc.
For selection of rated current of the fuselink the following applies:
1.4 × ISC ≤ In ≤ 0.85 IMOD_REVERSE
for fuse-links with characteristic gR; gS; gG
In ≥ 10 A
1.4 × ISC ≤ In ≤ 0.7 IMOD_REVERSE
for fuse-links with characteristic gR; gS; gG
In < 10 A
The factor 1.4 makes provision for the use at ambient temperature of 60°C, radiation intensity of 1000 W/m2, and influence of cyclic load. If the manufacturer of the panel prescribes a maximum protection value, this value shall be accepted.
Note: For collective protection of series-parallel connection of PV panels, the resulting current is proportional to the number of parallel branches.
2) Under what conditions is overcurrent protection necessary on d.c. side of PV applications?
Overcurrent protection need not be implemented for PV conductors of strings and PV arrays, if conductor rating capacity is equal to or higher than 1.25 × ISC STC in any places. Overcurrent protection need not be implemented for main PV conductors, if conductor rating capacity is equal to or higher than 1.25 × ISC STC of PV source.
3) Is it possible to use series connection of fuse-links to achieve higher rated voltage of the fuse group?
In no case. For series combination of fuselinks it is not possible to guarantee uniform distribution of cut-off processes in case of failure. One fuse-link always takes over higher part of cut-off processes, and for this reason it cannot be overloaded above its design characteristic.
4) When and why to use overvoltage protection on d.c. side close to both the inverter and PV panels?
In case of application, where PV panels are at a distance from the inverter (more than 10 m), it is recommended to use the overvoltage protection both upstream the inverter and at the photovoltaic panels. Due to the fact that voltage in long lines can rise significantly thanks to induction in the line.
5) Why are surge voltage arresters with gap preferred in PV applications?
In low-voltage networks of 230/400 V varistor-based overvoltage protective devices are commonly used.
The varistor itself shows a leakage current at operating voltage. In common applications with one type of varistor-based overvoltage protection the leakage current is negligible.
This is not, however, true for the photovoltaic applications, where tens of surge voltage arresters are used. In this case the total leakage current is not negligible, because it decreases the resulting output.
6) What fuse-link type and characteristic to select for protection of d.c. side?
The decisive parameter is rated d.c. voltage of the fuse. For protection of PV applications it is required to use fuse-links with full tripping range, i.e. type „g". Fuse-links with tripping range „a" cannot be used, because their breaking capacity is limited in the range of small overloads.
7) What protection to use at current values higher than 20 A?
Fuse-links PF10 are used advantageously for rated currents up to 20A. For currents higher than 20 A it is possible to use fuselinks for protection of semiconductors (char. gR, gS), e.g. PT22, PV514, P51R06, P51U06 with regard to required values of operating d.c. voltage. These types of fuselinks with characteristic gR are up to rated current of 80 A.
8) Is it possible to use calculation program Sichr for design of main electrical system of a.c. part of a photovoltaic power plant delivering electrical energy to distribution network?
Yes, it is. In this case the photovoltaic power plant is considered a load. The source will be the connection point (transformer or general source). Consequently, we will enter the output of the photovoltaic power plant in kW in the outlet at the end of the line or on the line. Then we will select simultaneity and power factor (cos φ) equal to 1.
Reverse direction of current in the line has no effect on the line rating. This configuration is even necessary for the design of overcurrent protection of the line and ensuring protection by self-disconnection from the source of the a.c. side of the photovoltaic power plant. The advantage of program Sichr is its possibility of performing economic optimization of cross-section of the proposed lines and thus reduction of total acquisition and operating costs.
9) You recommend using disconnector 5TE2 515-1 (63 A, 1000 V d.c.) as a disconnector on d.c. side of PV sources. At the same time you note that for this purpose it is possible to use also circuit breakers of LPN-DC and LST-DC series.
The circuit breakers LPN-DC and LST-DC are able to fulfil also the function of d.c. circuit disconnector. That is why this possibility is mentioned.