Common Misapplications of SPDs
SPDs have been applied to power circuits for many years. Most contemporary power circuits are alternating current systems. As such, most surge protection equipment has been designed for use in ac systems. The relatively recent introduction of large commercial and utility-scale PV systems and the increasing number of systems deployed has, unfortunately, led to the misapplication to the dc side of SPDs designed for ac systems. In these cases, the SPDs perform improperly, especially during their failure mode, due to the characteristics of dc PV systems.
MOVs provide excellent characteristics for serving as SPDs. If they are rated properly and applied correctly, they perform in a quality manner for that function. However, like all electrical products, they may fail. Failure can be caused by ambient heating, discharging currents that are greater than the device is designed to handle, discharging too many times or being exposed to continuous over-voltage conditions.
Therefore, SPDs are designed with a thermally operated disconnecting switch that separates them from the parallel connection to the energized dc circuit should that become necessary. Since some current flows through as the SPD enters failure mode, a slight arc appears as the thermal disconnect switch operates. When applied on an ac circuit, the first zero crossing of the generator-supplied current extinguishes that arc, and the SPD is safely removed from the circuit. If that same ac SPD is applied to the dc side of a PV system, especially high voltages, there is no zero crossing of the current in a dc waveform. The normal thermally operated switch cannot extinguish the arc current, and the device fails.
Placing a parallel fused bypass circuit around the MOV is one method to overcome the extinguishing of the dc fault arc. Should the thermal disconnect operate, an arc still appears across its opening contacts; but that arc current is redirected to a parallel path containing a fuse where the arc is extinguished, and the fuse interrupts the fault current.
Upstream fusing ahead of the SPD, as may be applied on ac systems, is not appropriate on dc systems. The short-circuit available current to operate the fuse (as in an overcurrent protection device) may not be sufficient when the generator is at reduced power output. As a consequence, some SPD manufacturers have taken this into consideration in their design. UL has modified its earlier standard by its supplement to the latest surge protection standard—UL 1449. This third edition is specifically applicable to PV systems.