PV fuse selection guide
PV systems with arrays >4 module strings the fault current can easily exceed the operating current and reach a level that can cause overheating and damage of wire insulation.
To ensure the best possible protection for the system and people working on the equipment, each string of solar panels MUST be protected with one fuse on each + and – pole .
These fuses will also isolate the faulted string so that the rest of the PV system can continue to generate electricity.
In operation, fuse links, like thermal devices, are influenced by ambient temperature. The current capability of the PV fuse links shall therefore be derated according to their corresponding Temperature Derating Curve.
When a fault occurs in the DC circuit of the PV installation, the absence of natural zero voltage makes the interruption of DC faults more difficult than the interruption of AC faults as only the fuse arc will force the current to decrease to zero.
The correct interruption depends on three parameters:
The value of the DC voltage
The value of the ratio L/R (time constant) of the fault path
The value of the fault current
Due to the unique requirement in PV systems of having to clear a very low level fault, it is important that a fuse with full range capability is used. This means that the fuse is designed for clearing overloads as well as short circuit faults and requires the use of a fuse with a gPV characteristic.
In order to calculate the best fuse for a general recommendation, the following information is required:
The array open circuit voltage (with the temperature coefficient)
The lowest application temperature
The ambient temperature
The maximum power (MPP) current
The short circuit current (with temperature coefficient of the short circuit current) of the string
The number of parallel strings
The max irradiance
Checklist:
(1)Taking the fact that the polarities + and – are never connected to the earth (neither one nor the other in photovoltaic equipment) into account, each chain of modules has to be fitted with two fuses: one fuse on the positive output and also one fuse on the negative output.
(2)The first rule has to be applied when the number of chains in parallel (N) is equal to or higher than 4. (for 1, 2, 3 parallel chains, the fusing is not necessary).
(3)The maximum DC operating voltage of the fuse must be higher than or equal to 1.20 x M x (Voc STC).
Calculation method to choose the correct fuse for a PV power system:
Example: 20kW PV installation
Check the datasheet of the module (STC-values):
Voltage at PMAX (Maximum Point of Power) VMPP = 29.2V
Open Circuit Voltage VOC = 36.4V
MPP-Current IMPP = 7.9A
Short Circuit Current ISC = 8.7A
Check the datasheet of the generator (STC-values):
Number of strings N N = 4
Number of modules per string M = 22
MPP Array Voltage = UMPP x M VARRAY = 642V
Array-Open Circuit Voltage = UOC MOD x M VOC ARRAY = 800V
Temperature inside combiner box 60 °C
Evaluation of the fuse rated voltage:
1. Calculate the smallest fuse link voltage:
VP MIN –25 °C (with temperature coefficient of UCO = 0.36%/°C)
VP MIN >= VOC ARRAY x (1+ (25+25) x 0.0036) =945V
taken: VN = 900 V (VP = 1000 V)
At an IMPP – Current of 7.9A and an open circuit voltage of 945V for the string-protection should be using a gPV type fuse such as ADLER A73 range.
2. Evaluation of the fuse rated current:
Determine the fuse deratings:
Ambient temperature of 60 °C 0.84
Cycling factor for full-range fuse-links 0.9
De-rating of multiple encapsulated fuse-holders should also be considered
Calculate:
Ambient Smallest fuse rating IN MIN = I MPP /
De-rating = 7.9 A / 0.84 / 0.9 10.5A
Next highest available fuse rating IN chosen: 12A
Narrow in on the selection:
Reduced fuse rating in the junction box 12A x 0.84 x 0.9 = 9.1A
Check, if the reduced fuse rating is > ISC MOD 9.1 A > 8.7 A ()
String short circuit current ISC STRING = ISC x (N-1) ISC STRING'' = 26.1A
ISC STRING at 70 °C (with temp. coefficient of ISC with 0.065%/°C)
ISC STRING' = ISC STRING'' x (1+ (70-25) x 0.00065)) ISC STRING' = 27A
Consideration of max. irradiance
ISC STRING' at 1200 W/m² ISC STRING =27x1.2 = 32.4A
Melting time ts of the fuse rating 12A at ISC STRING 30 s
3. Results:
In this example, ADLER’s A7332120700 fuse is the best choice.
It is a gPV type fuse, has dimensions 10mm x 38mm, is rated at 1000VDC with a rated current of 12A.
This will carry the MPP current of 7.9A and will interrupt a string short circuit current of 32.4A in a time of about 30s.
Fuse ratings above 12A are possible, but have to be calculated on the above scheme.
Source: ADLER Elektrotechnik
