Inasmuch as solar modules are actually super solar cells that exhibit high voltages and currents, the observations in Sections 4.2.3 and 4.3.2 also apply to directly parallel-connected strings of the same type. Even in cases where numerous strings of the same type are wired to each other in parallel, shading of
Figure 4.34 Reverse current Ir induced by a string malfunction in a very large solar generator comprising nTG arrays with nSP-TG parallel-connected strings each. Inasmuch as nTG is usually very high and much greater than 1, the cable array (TGK) needs to be protected, via fuses or preferably circuit breakers, against reverse current from the solar generator as a whole. Since (a) nSP ^ 1, reverse current in a string can be extremely high in the event of a malfunction; and (b) in such cases the reverse current comes not only from the (nSP. TG — 1) intact strings of the native arrays, but also from the (nTG — 1) intact arrays. Hence in such cases as well, string fuses should be used, including for low nSP-TG values. Integrating cooled diodes into each array that is sized for 1.25 ■ nSP. TG ■ Isc-stc is also a viable solution
individual intact strings does not engender hazardous reverse current, even if the solar generator is at open circuit. That said, prior to direct parallel connection of strings, it is essential to verify that the strings are wired correctly and present identical open-circuit voltage. Hence it is indispensable to mount disconnect terminals or special PV plugs for each string.
It may be advisable to dispense completely with anti-reverse current elements if particularly well – insulated protection class II solar modules and dual-insulation cables that are resistant to grounding faults and short circuits (as recommended in [4.2]) are used. In addition, [4.19] reports that aR fuses (which do not protect an entire area) have caused fires. However, just because use of the wrong type of fuse has caused fires does not mean that fuses should be eschewed – providing of course that the right ones (namely, type gPV or gR) are used. But as the solar module and laminate cover glass as well as the relatively thin back foils used in these devices are extremely fragile and easily damaged by impacts, PV plants comprising numerous parallel-connected strings are prone to damage that can induce grounding faults, or short circuits in individual modules or module groups (see Section 184.108.40.206). Wiring errors can also occur from time to time. Electrical engineering students are always told (in accordance with what is undeniably the state of the art for electricity distribution) to design systems that can withstand the worst-case scenario,
as well as the damage induced by such situations. Hence it would be nothing short of negligent to reduce system costs marginally by dispensing with elements that avoid reverse current and protect string conductors, which usually exhibit a very low gauge.
The elevated voltages engendered by short circuits or grounding faults in directly parallel-connected strings result in appreciably higher output than is the case with directly parallel-connected solar cells or modules. As solar generator short-circuit current is subject to a fixed upper limit, two strings can always be directly parallel connected since at nSP = 2 string fuses can never shut down a reverse current.
In most cases, three strings can also be parallel connected without anti-reverse-current elements, since effective protection against reverse current from two adjacent strings is a virtual impossibility using commercial fuses. However, in such settings disconnect terminals orpreferably special PV plugs should be usedonbothsides. However, inordertodothisthestringwiregaugesmustbelargeenoughandprecautions must be taken to obviate reverse current from connected appliances such as battery banks and inverters.
Four strings can often also be directly parallel connected, although this is somewhat risky in the absence of explicit reverse current specifications from the vendor, in the event of damage. Diligent planners that wish to rule out all possible risks only opt for nSP = 2 direct parallel-connected string solutions.
However, if nSP is greater than 3 or 4, or if a PV generator is subdivided into a series of arrays, string fuses should be used, unless the vendor datasheet specifies a maximum reverse current IR-mod for the relevant module, such that the maximum number nSP of allowable direct parallel-connected strings can be determined using Equation 4.8 or 4.11. However, transient current overload events induced by cloud enhancements or snow can result, causing upwards of 1.25 ■ ISC-STC to flow into the modules. Hence to avoid damage to string conductors, they need to be rated for at least 1.25 ■ (nSP — 1) ■ ISC-STC.
Incorrect string polarity in a solar generator with numerous nSP should be avoided at all costs, as fuses can do nothing against this problem (see Figure 4.35).
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Bypass diodes may overheat and blow! IF “ (nSP"2)’ISC I—КЗ—I—И—I—W—I—і
———— E I—E I—E *—E H——————– =-
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— main switch of PV array (two-pole or four-pole)
Figure 4.35 Solar generator with parallel-connected series strings, one of which is incorrectly polarized. Bypass diodes can be damaged beyond repair in such a case in a system with a high nSP value
If a solar generator with nSP parallel-connected strings is incorrectly polarized, the generator will in effect be short-circuited via the bypass diodes of the incorrectly polarized string, and (as shown in Figure 4.35) a current of around (nSP — 1) ■ ISC will flow through the string. As the modules of such a string are usually as insolated as the other modules, an ISC current will flow into them as well, and an (nSP — 2) ■ ISC current will flow through the bypass diodes. In such a case, inasmuch as bypass diode capacity is very low, at nSP > 4 all such diodes in the incorrectly polarized string could easily be damaged beyond repair before the string fuses are tripped.