Solar rooftop system sizing: Over current protection
OVER CURRENT PROTECTION FOR SOLAR PV PLANT:
It is important for any solar pv system to keep fuses to protect the pv system through over current and effective solar pv installation should have fuses at module level, at the junction box level, at the inverter level in D.C system. Similarly the fuses are also installed in A.C side the pv system for effective electrical protection of the solar pv system. On D.C side typically about 20-24 solar panel are connected in series and operating voltage of around 600-700V D.C. The string currents are typically in the range of 7.5-8Amp.
As per national electrical code the maximum circuit current should be about 125% of the pv module current (Isc).Hence the conductors and over current protection devices are sized at 125% of maximum or (1.56x Isc).
Additionally international standards such as BS EN 7671 SEC 712 for solar photovoltaic power supply system’s specify that conductors current carrying ability must be equal to (or) greater than 1.25 x Isc STC at any location ,Isc is published by the module manufactures in data sheets. The Isc is typically only 110-115% of the maximum power current ( Impp) of the pv module.
This means that unlike typical grid connected A.C system; the available of short circuit current is limited and over protective devices will need to operate effectively on low levels of fault current.
Depending on the desired capacity of the pv system, there may be a several strings are connected in parallel to achieve high currents and subsequently more power. Pv system that have three or more strings are connected in parallel need to have each string protected (system that have less than three strings will not generate enough fault current to damage conductors/equipment therefore do not present a safety hazard as long as conductor was sized properly based on the local code requirements)
Where three or more strings are connected in parallel a fuse on each string will protect the conductors from damage and help minimize any safety hazards. It will also isolate the faulting string so that the rest of the pv system can generate electricity continuously.
Similarly system should have circuit breakers in A.C side to protect from over current .In the solar pv system two types of breakers available, they are RCCB and MCCB. In the A.C side MCB acting like fuse and it has 32Amp auxiliary power supply to ground.
EXAMPLE CALCULATION FOR FUSE RATING:
[ezcol_1half]Let us calculate fuse rating for spv plant which has 4 strings and 24 solar panels are connected in series. Isc = 8.83A , Impp = 8.31A Maximum system voltage: 900V D.C Conductor size formula: 1.56 x Isc. = 8.83 x 1.56 = 13.77. Conduct size: cu 4mm2 = 18A @(80C). Array max Isc = (N-1) X Isc. Where N is no. of parallel strings Array max Isc = (4-1) x 8.83 = 26.49A Array max Isc is greater than conductor withstand, therefore string fuse’s are required.In =1.56 x Isc = 13.77A .