Why to use fuse in PV system ?
Fuse play a very important role in solar power projects. There are various locations where the fuses are used in the solar PV systems, these locations includes the string combiner boxes, inverters, on the DC side of the system. Fuses are also used in the AC side of the system. The AC fuses are different as compared to the DC fuses. But here we are only considering the DC fuse design.
In case of large number of strings connected in parallel it is necessary to ensure protection of PV panels against reverse currents, and overcurrent protection of cables of PV array. Fuses are primarily used to protect the system against short circuit and fire hazards. Fuses in PV installation are subjected to extreme condition of environment like exposure to the sunlight causing abnormal temperature of the fuse which affects the performance of the fuse and thus sizing and selection of the cable. Moreover the PV modules also produce continuous current drawing more concern towards the proper fuse sizing. In this article we will discuss the step by step sizing of the fuse for the case of PV array to inverter.
Generally fusing is done for three or more parallel connected strings as the fault in one string will subject the faulted string to a maximum circuit current of all other connected strings with each string delivering 1.25 x Isc under worst-case conditions(Isc =Short circuit current of the module).The combined fault currents will be larger than the withstand rating of the installed wiring sized at 1.56 x Isc as well as the series fuse rating of the PV modules and in such a condition both PV modules and conductors are subjected to damage.Below shown are the SIBA 25 A fuses used in the Arrayguard combiner box.
Figure 1 PV fuse Location on DC side of PV plant
Key Steps to Fuse Sizing for Solar Photovoltaics
The following steps given below should be used for proper sizing of fuse of a string as per article 690.8 of National Electrical Code 2011.
But before jumping into calculations, a few NEC definitions will be helpful, since the rules for correction factors and overcurrent requirements can change based on the specific circuit. Working from the array to the inverter, we have
- PV source circuits :These are conductors between the modules, and from modules to a common connection point in DC system, typically a combiner box. Sometimes these are often called the “home runs” from the individual strings(see figure below).
- PV output circuits : These are conductors between the PV source circuits and the inverter or DC utilization equipment. These are the circuit conductors after a combiner box to the inverter or charge controller(see figure below).
- Inverter input circuits: In a battery-based system, these are the conductors between the inverter and the battery bank. In a grid-tied system, they are the conductors between the inverter and PV output circuits. Typically, these are the conductors between the inverter’s integrated DC disconnect and the inverter’s DC input connection.
- Inverter output circuits: These are the AC conductors from the inverter to the ultimate connection to the AC distribution system for either stand-alone or utility-interactive systems.
Figure 3 Combiner box to inverter connection with fuses in CB.
Step 1 : Calculating the maximum circuit current.
For PV source circuits, NEC Section 690.8(A)(1) states that the maximum circuit current (Imax), or continuous current, is defined as 1.25 multiplied by the PV module rated short-circuit current (Isc) or the sum of parallel PV module rated short-circuit currents. Therefore, for n strings, the equation for determining the maximum circuit current (Imax) is:
Imax= (ISC1+ ISC2+ ISC3+ … + ISCn) x 1.25
Where ISC = Short circuit current of an individual PV module.
Step 2 : Calculating the nominal fuse ampere rating.
As per NEC section 690.8(B)(1) PV system currents are considered to be continuous, the maximum currents as calculated above must be multiplied by 125% to calculate the minimum conductor size. This calculation ensures that the conductors do not carry more than 80% of the continuous current value (0.8 is the inverse of 1.25). In the PV industry, the result of this calculation is commonly referred to as the “156% factor.” When this rule is applied, the module’s rated Isc has been multiplied by 156% (125% × 125% = 156%). Therefore equation for determining the nominal fuse ampere rating (In) is:
In= Imax x 1.25
Step 3 : De-rating of the fuse nominal rating due to abnormal temperature condition.
Fuse nameplate ampere ratings are calculated by the fuse manufacturer under Standard Test Conditions, where ambient temperatures are 25°C.But when operating temperatures are greater than 40°C ,the manufacturer’s temperature correction factors shall be applied. To apply temperature correction factor divide the the nominal fuse ampere rating (In) calculated in Step 2 by the de-rating coefficient to determine the de-rated ampere rating of the fuse (Irated).
Irated = In / Kf
(In) = Nominal fuse ampere rating
Kf = De-rating correction factor
Step 4 : Calculating the nameplate ampere rating of Fuse.
If the de-rated ampere rating of the fuse (Irated) is not a readily available fuse ampere rating, then choose the next higher rating fuse but this rating should not exceed the conductor ampacity. For example, if the fuse ampere rating is calculated to be 8.5A, a 10A fuse should be used as it is the next highest available ampere rating.
Step 5 : Verifying the fuse rating w.r.t Conductor rating.
The fuse nameplate rating, after any corrections for conditions of use (Steps 3 and 4), must be less than or equal to the Ampacity of the conductor selected. If not, then select the conductor of higher in order to ensure safety.
To illustrate the procedure explained above we have taken an sample module with the following specifications given below. Here we are doing the calculation for 3 string with 24 module each as shown in the figure above.
Open circuit voltage(STC) – 37.4 V
Short Circuit Current(STC) -8.83 A
Correction factor(Kf) at 50 °C (we have assumed the ambient temperature to be 50 °C
Combining and applying All the steps explained above.
Irated = (Imax * 1.56)/ Kf
Irated = ((8.88+18.104.22.168) * 1.56)/ 0.90
Irated = 45.916 A
If the fuse of this rating is not available then select the fuse of next higher rating i.e 46 or 47 which ever is available.
Conclusion – In a nutshell it can be said that small PV system of single or double string size does not requires fuse but when number of strings goes higher than 3 then fusing your PV modules becomes essential for the protection of modules as well as the conductor cable. The sizing of the fuse for your plant should be done according to the NEC codes as done above.