PHOTOVOLTAIC FUSE SIZING How to Calculate Fuse Sizes for Photovoltaic Installations TECHNICAL PAPER

PHOTOVOLTAIC FUSE SIZING

PHOTOVOLTAIC FUSE SIZING Every photovoltaic (PV) power application must use fuses that are properly sized to its system. When you use the incorrect fuse size (or a standard non-PV fuse), you put the system’s reliability and safety in jeopardy. Due to the explosive growth in PV system power, understanding how to properly size a fuse for a PV application is essential. direct current International Electrical Code National Electrical Code This paper provides insight into how fuse sizes affect PV applications, and how to calculate the correct fuse size for PV equipment. This paper will go over NEC’s...

PHOTOVOLTAIC FUSE SIZING As shown in Figure 1, PV fuses protect ■  Central inverters, Small inverters, Micro inverters, String inverters, Wiring harnesses, Array combiners. FIGURE 2. Reverse overcurrents occur when power flows back from the circuit to its source. Why PV Applications Specifically Require PV Fuses Due to the unique conditions of PV installations, PV fuses have three characteristics that are unnecessary in non-PV applications: ■  A higher dc voltage rating The ability to withstand harsh, fluctuating temperature changes FIGURE 3. Polysilicon panel failures include glass destruction,...

PHOTOVOLTAIC FUSE SIZING FIGURE 4. Thin-film panel failures result in heat damage to thermal gel and blocking diodes Exception for Crystalline Silicon Modules What Causes Failure in PV Panels For crystalline silicon modules, rather than using the ambient temperature correction factor formula, refer to NEC table 690.7(A) (see Table 690 below) to determine these values at different temperature ranges. Reverse overcurrents are one of the primary causes of PV panel failure. An overheated PV panel caused by a lack of fuse protection is subject to melting, arcing, fire, and heatdamaged equipment and...

PHOTOVOLTAIC FUSE SIZING Mono-Silicon PV Panel Failure In Figure 5, a sustained reverse current burned the back part of the panel. An overcurrent caused arcing across the panel cells, and the blocking diodes were destroyed from overheating. Fuse Sizing You can calculate the correct fuse size by using the following steps. This formula ensures the fuse is sized for the optimal protection and energy efficiency. FIGURE 5. Mono-silicon panel failures include arcing, burning and diode destruction. Step 1 The ambient temperature correction calculation compensates for potential variations that are caused...

PHOTOVOLTAIC FUSE SIZING The fuse voltage must be equal to or greater than the determined voltage using (2). You can determine the next When the irradiance is lower than the standard testing conditions, the short-circuit current will also be lower. highest standard fuse rating with NEC 240.4(B). PV Output Circuits NEC 690.8(A)(2) says that the maximum circuit current shall be the sum of a parallel source circuit as calculated in 690.8(A)(1). I = 1.25 × (I + … + I ) If we consider N strings, this implies: Determine the Maximum Circuit Current The formula to determine the maximum circuit current...

PHOTOVOLTAIC FUSE SIZING In = 1.25 × Im current rating. maximum circuit current. Use formula (6) to calculate the current ratings for the fuses located in the PV source circuit, and formula (7) for the fuses located in the PV output circuit. string short-circuit current. de-rate factor. current rating current rating. If the fuse is in a PV source circuit, use the formula de-rate factor. string short-circuit current. Determine the Fuse Size If the current rating is not a standard fuse ampere rating, choose the next highest standard fuse rating according to NEC 240.4(B). PV Output Circuit In =...

PHOTOVOLTAIC FUSE SIZING Based on (1), the voltage ambient temperature correction factor (Tatcf ) is 1.112. Now that we have the voltage ambient temperature correction factor, we can use it to determine the maximum system voltage (VSYS) using formula (2). where: 6 modules (2) FIGURE 8. Percent of uprating and downrating according to ambient temperature. Based on (2), the maximum system voltage is 1475.18 V. Since the current rating (In) factor does not equal the typical fuse ampere rating, we will use the next highest standard fuse rating according to NEC 240.4(B), which is 1500 V dc. Determine...

PHOTOVOLTAIC FUSE SIZING Step 5 foundation, tracker, and other components, as required to form a dc or ac power-producing unit. Central inverters: The core of a PV system, the central inverter converts the dc output from PV panels into ac electricity. A central inverter also controls the PV array system. The central inverter is usually stationed near the main electrical service switchboard away from potentially harmful environmental conditions. Determine the Fuse Size Based on (9), the fuse current rating (In) is 4.29 A. Because this is not a typical fuse rating, we will select the next highest...

PHOTOVOLTAIC FUSE SIZING Micro-inverters: Micro-inverters connect to an individual PV panel with the role of converting dc into ac for general power usage. Standard circuit protection devices: Standard circuit protection devices include non-PV fuses and circuit breakers. Subarray: An electrical subset of a PV array. Module short-circuit current (Isc): The amount of current generated when the positive and negative terminals of a module are shorted together. Thermal cycling: Thermal cycling describes the rise and drops in temperature over a specific period. Mono-silicon panel: A single-layer base...

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