The Role of Reverse and Short-Circuit Current in Solar Generator Malfunctions

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:

Подпись: Maximum reverse current IR for directly parallel-connected strings : IR ~ (nSP — 1) ■ ISC (4.8)

where: ISC is the short-circuit current in a solar module or string; and nSP is the number of parallel – connected strings.

Подпись: Short-circuit current IK = ISC + IR « ISC + (nSp — 1) ■ ISC ~ nSp ■ ISC (4.9)

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:

image408

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.

image409 Подпись: (4.10)

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:

where:

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

Подпись: Possible return current IR for arrays : IR ~ kf ■ ISN-TG + (nSP-TG — 1) ■ ISC (4.11)

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.

Updated: August 5, 2015 — 2:12 pm