Figure 4.33 illustrates the scenario involving a severe module string malfunction induced by module short circuits, mechanical damage resulting from an accident, or a catastrophic string wiring error. Such events occur now and again (a) in building-integrated PV plants or infrastructure installations such as highway or railway line sound barriers, or (b) in the event of an earth fault in a system whose poles are grounded.
In such a case, in the absence of string diodes or fuses (i. e. nSP strings wired to each other in parallel), the following reverse current IR flows into the intact solar generators:
where: ISC is the short-circuit current in a solar module or string; and nSP is the number of parallel – connected strings.
The actual current in a short circuit is ISC higher, i. e. the following holds true for short-circuit current IK for strings that are wired to each other in parallel:
Figure 4.33 Reverse current IR in a diode-free string resulting from an event such as a string short circuit involving a number of modules, caused by a severe malfunction such as the mechanical effects of an accident, or a bypass diode short circuit induced by lightning. This same scenario can also occur if too few modules are available owing to a major string wiring error
The following scenarios can arise, depending on solar generator configuration:
• In the presence of functioning string diodes, the short-circuit current IK that arises is limited to the short – circuit current ISC of a string, i. e. IK — ISC and IR — 0.
• In the absence of string diodes or if they are short-circuited, but in the presence of string fuses with kSN as in Equation 4.6 and kf as in Equation 4.7, Ik initially equates to aggregate solar generator short-circuit current nSP ■ ISC. Thus, depending on the scope of nSP, the transient reverse current IR induces a greater or lesser temporary overload in the module through which IR flows. In the longer term, however, the maximum reverse current from the intact array is around IR ~ kf ■ kSN ■ ISC-STC, since otherwise the fuse would be tripped, i. e. IK < ISC + kf ■ kSN ■ ISC-sTC applies. IR — 0 for decoupled fuses; the sole remaining continuous current is the short-circuit current ISC of a string, i. e. IK — ISC.
• In the case of directly parallel-connected strings, the short-circuit current remains at around the same level as the short-circuit current IK — nSP ■ ISC of the solar generator as a whole, such that for higher nSP values string wiring can incur severe damage, as can the string modules through which reverse current IR ~ (nSP — 1) ■ ISC flows.
The situation is somewhat more complex for extremely large-scale solar generators comprising nTG parallel-connected arrays (TG) with nSP_TG parallel strings each (i. e. for which nSP — nTG ■ nSP-TG) (see Figure 4.34).
The cable array (TGK) used for large-scale solar generators should be protected against reverse current from the solar generator as a whole, via fuses, or preferably circuit breakers. I > ISN may also occur briefly until these devices are activated.
As the array wiring current is higher than string current, in accordance with Equation 4.6 the following approximate guide value applies to nominal current ISN-TG for such array fuses:
kSN TG — the ratio between nominal fuse current ISN-TG and nominal current in the array wiring (approximate value as in Equation 4.6: 1.4 to 2)
ISC STC — short-circuit current in a solar module or string at STC power output nSP-TG — number of parallel-connected strings in an array
In the event of a malfunction in a large solar generator, a kf ■ ISN-TG current can flow back through the array conductor for a lengthy period in the presence of intact array fuses. Hence the possible string reverse current IR in a large solar generator comprising a series of arrays is as follows:
Even with low nSP-TG values, appreciable reverse current can occur, e. g. with nSP-TG — 3, kSN-TG — 2and kf — 1.45 at STC power output IR reaches 10.7 ■ ISC-STC. Hence in large multi-array solar generators, fuses should be installed not only for the individual array conductors, but also for the strings of the various arrays.