Solar Generator and Array Junction Boxes

Many of the components discussed in the previous section are accommodated in a special junction box near the solar generator and referred to as the generator junction box. The string wiring for large multi­array PV generators is initially realized in array junction boxes that exhibit the same structure as generator junction boxes. The array wires that lead out of the array junction boxes are then wired, in the superordinate generator junction boxes, to the PV generator as a whole (see Section 4.5.6 for more information concerning large PV generators). Figure 4.80 displays the basic structure of a generator junction box, which should be readily accessible to allow a technician to check the fuses and surge diverters from time to time. These boxes protect the solar generator/array components from the elements and for outdoor installations should exhibit a UV-resistant housing that meets protection class IP54 and preferably IP65 requirements. The housing should be constructed of a non-combustible or barely combustible self-extinguishing material.

The various solar generator strings, which are wired in parallel into the generator junction boxes, contain the elements necessary for such wiring, such as string connection terminals, string diodes, string fuses and disconnect terminals, as well as (in many cases) DC circuit isolating switches and varistors for power surge protection. Each generator junction box should also be rendered grounding fault – and short – circuit-proof via double-insulated terminals and positive and negative conductors, which should exhibit adequate reciprocal clearance and may also need to be insulated from each other. If desired, the positive and negative conductors can be accommodated in separate generator junction boxes, although this costs

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Figure 4.80 Layout of a generator junction box for a PV system for the wiring of nSP = n parallel strings, each of which can be measured separately if desired, and disconnected from the other solar generator devices via a disconnect terminal and fuse. The box also accommodates: (a) a DC circuit isolating switch that allows the system to be powered down before a fuse is changed or a junction box is opened; and (b) the surge diverters for the main positive and negative conductors. For systems with low nSP values such as nSP < 3, disconnect terminals can be used in lieu of fuses

more and precludes optimal lightning current protection. Metal housings are non-combustible, which is a major advantage in the event of a terminal fire inside the housing. But as these housings are usually grounded forreasons of safety, realization of grounding fault-proof wiring inside the housing is somewhat more difficult.

Once the strings have been wired, each string is in turn wired to the string connection terminals (with the isolating switch open, the disconnect terminals open and all fuses removed) and the open-circuit voltage at each string is then measured.

Asa rule, faulty strings are immediately detectable by virtue of their open-circuit voltage reading being considerably lower than the mean value. Fuses should be changed or disconnect terminals closed (followed by closing of the isolating switch) only if an anomaly is detected in one or more strings.

As noted in Section 4.3.3.3, it is absolutely essential to avoid erroneously activating a string whose polarity is reversed. In other words, both string voltage and polarity must be verified.

Faulty fuses can be readily detected by simply measuring their voltage while the installation is running normally, i. e. while a wired inverter is operating at the MPP or if a stand-alone system is drawing current from the solar generator (i. e. when the battery bank is not fully charged). In such a case, a string with a faulty fuse will be in an open-circuit state and will exhibit considerably higher voltage than that measured at the DC isolating switch. Prior to replacement of a faulty fuse, however, the isolating switch must be opened so that the fuse can be replaced in a currentless state. Many of today’s generator junction boxes for larger PV plants integrate a continuous monitoring mechanism for all string currents.

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Figure 4.81 Older metal generator junction box for wiring in parallel up to 20 strings (only 10 are integrated here) with a high operating voltage ( Voca up to around 800 V). The positive side of the various strings integrates large DC – compatible fuses and string diodes (the bottom fuse is shown with the dedicated fuse removal and replacement tool). The disconnect terminals for the negative wires for disconnection of all poles in the various strings, which is a desirable feature, is missing here. The two varistors can be seen at the lower left and to their right the four-pole system isolating switch. In the centre is the DC isolating switch with a special shielded 2 ■ 10 mm2 cable and 10 mm2 common shielding. The wires are connected via spring-loaded terminals (Photo: Tritec)

Figure 4.81 provides a view inside a generator junction box used for parallel connection of up to 20 strings with high operating voltage (normally around 500 V). The strings integrate string diodes and large DC-compatible string fuses, although the installation around 15 kWp shown here has only 10 strings. In accordance with Swiss regulations that came into effect in the mid 1990s, the isolating switch integrates a 10 mm2 common shielding that provides good lightning protection (see Section 6.9.4).

Figure 4.82 displays an SMA array junction box with an integrated string monitoring system for larger PV installations with central inverters. This box can accommodate up to 8 or 16 strings, depending on configuration. For each input, the positive and negative conductor integrates a DC-compatible fuse (not shown here for reasons of clarity). The current in the eight connecting leads (each with one or two strings) is monitored using converters from LEM.

The string output signals of the monitoring device (upper right) are received by an RS-485 interface (no connection terminals in the right centre) and are routed to a central monitoring element that immediately detects a string failure. In order for the surge diverters to work, a ground cable (as short as possible but with an adequately large gauge) must be attached to the large grounding terminal at the centre (see Chapter 6 for more on PV system lightning protection). The outbound DC wire cannot be switched, and should instead be wired into the superordinate solar generator junction boxes (preferably to a circuit breaker).

Surge arresters Measurement of string currents with LEM sensors

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Figure 4.82 SMA arrayjunction box for a maximum of 8/16 strings (for direct wiring in parallel of two strings with Impp < 5.6A) (Photo: based on an SMA document)

Figure4.83 displays a Sputnik array junction box for up to 16 strings, each of which integrates the following elements: a fuse and ammeter on the positive terminal side for each string; a disconnect terminal on the negative terminal side for each string; varistors for surge protection; and a common load break switch. A remote string monitoring device can also be integrated into this box if desired.

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Figure 4.83 Sputnik MaxConnect Array junction box for up to 16 strings: a, surge diverter (varistors); b, string module with ammeters and string fuses (for 16 strings); c, 6 mm2 string, positive-pole connection terminals (without string monitoring electronics); d, terminal for potential free message output; e, RJ-45 jacks for connection to the MaxComm monitoring network; f, string monitoring electronics (optional feature); g, strain-relief rail with quick wire installation element; h, 6 mm2 positive-pole string connection terminals (with string monitoring electronics); i, 35 mm2 grounding terminal; j, 6 mm2 negative-pole string connection terminals and disconnect terminal; k, DC load break switch; l, connection terminal for array connection wire (50 or 150 mm2); m, 6 mm2 terminal for centre-point transmission (Photo: Sputnik Engineering AG)

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Figure 4.84 Plastic generator junction box for a small 3.3 kWp system with a high operating voltage (Voca-stc around 650 V, installation as in Figure 4.47). As the system comprises only two strings, string diodes and fuses can be dispensed with. The two strings are connected (in the centre) solely via two disconnect terminals each. To the right are the thermally monitored surge diverters; the three-pole system isolating switch is on the left

Figure 4.84 displays a generator junction box for a 3.3 kWp system comprising only two strings whose nominal operating voltage is around 500 V. No string fuses are needed here, thus considerably simplifying the installation. The two strings are wired to the positive and negative conductor solely via a disconnect terminal. Here too, the isolating switch integrates a 10 mm2 common shielding.

In the arrangement shown in Figure 4.84, the string wires are attached to the module frames by conventional white cable ties, which constitutes an egregious defect since such ties become brittle over the years, causing the wires to dangle from the box. The cable ties in the installation shown here will have to be replaced within a few years. Black cable ties are usually more durable. UVand weather-resistant cable ties should always be used to secure wires.

Updated: August 7, 2015 — 12:35 pm