Photovoltaic ground fault and blind spot electrical simulations.

Abstract

Ground faults in photovoltaic (PV) systems pose a fire and shock hazard. To mitigate these risks, AC-isolated, DC grounded PV systems in the United States use Ground Fault Protection Devices (GFPDs), e.g., fuses, to de-energize the PV system when there is a ground fault. Recently the effectiveness of these protection devices has come under question because multiple fires have started when ground faults went undetected. In order to understand the limitations of fuse-based ground fault protection in PV systems, analytical and numerical simulations of different ground faults were performed. The numerical simulations were conducted with Simulation Program with Integrated Circuit Emphasis (SPICE) using a circuit model of the PV system which included the modules, wiring, switchgear, grounded or ungrounded components, and the inverter. The derivation of the SPICE model and the results of parametric fault current studies are provided with varying array topologies, fuse sizes, and fault impedances. Closed-form analytical approximations for GFPD currents from faults to the grounded current carrying conductor-known as "blind spot" ground faults-are derived to provide greater understanding of the influence of array impedances on fault currents. The behavior of the array during various ground faults is studied for a range of ground fault fuse sizes to determine if reducing the size of the fuse improves ground fault detection sensitivity. The results of the simulations show that reducing the amperage rating of the protective fuse does increase fault current detection sensitivity without increasing the likelihood of nuisance trips to a degree. Unfortunately, this benefit reaches a limit as fuses become smaller and their internal resistance increases to the point of becoming a major element in the fault current circuit.4

PV Systems Reliability Final Technical Report.

“…Electrical simulations of series and parallel arc-faults showed that it was possible to detect the fault type through proper instrumentation [87]. But, ultimately, the requirement for parallel arc-fault detection has not been included in the NEC because of the low occurrence rate and difficult mitigation method: either shorting the array or module-level electronics [88].…”

Photovoltaic ground fault detection recommendations for array safety and operation

“…GFPD resistance vs. rating for a variety of 10x38 mm ("midget") fuses by various PV fuse manufacturers. In general, the more sensitive the fuse, the higher the intrinsic resistance [10][11][12][13][14][15][16][17].…”

Electrical simulations of series and parallel PV arc-faults

“…The construction of a single module is accomplished using a standard one-diode model [6]. This model consists of an ideal current source with a value equal to the module short-circuit current (I sc ) in parallel with a diode and shunt resistance (R sh ) all in series with a series resistance (R s ).…”

“…In order to validate the SPICE model and expand on the work in [6], a series of arc-fault tests were performed on real PV arrays at DETL composed of two parallel strings of seven 200 W monocrystalline Si modules connected in series. In each experiment, the fault was installed between modules using MC4 T-branch connectors.…”

“…To better understand the different fault types and determine if there is a surefire method of differentiating the faults, Sandia National Laboratories (SNL) has developed a PV array model [6] using a simulation program with integrated circuit emphasis (SPICE) [7]-a free node-based electrical modeling software-to better understand the electrical dynamics of the series and parallel arc-faults. The SPICE models of different arc-faults have been studied in hopes of creating a quick, accurate differentiation scheme between different types of parallel and series arc-faults using current/voltage information that is already being collected by the inverter during PV array operation.…”