DC Combiners Revisited
Inside this Article
Since the publication of “Pulling It All Together: Strategies for Making Common Connections in PV Power Circuits” (April/May, 2009, SolarPro magazine), many advances have been made on dc combining methodologies and devices. In addition to product advancements, there are changes in the 2011 edition of the National Electrical Code that have implications regarding how combiners are deployed in the field and what features these devices include.
Here I consider these recent developments and address more basic questions, such as: What is a dc combiner? Where is it used? Why is it used? How is it used properly? I also discuss product selection criteria, and the tables (CLICK HERE) include specifications for 148 combiner and recombiner boxes from 11 original equipment manufacturers.
Understanding PV Circuits
Before discussing dc combiners in particular, it is necessary to briefly review PV circuits in general. As defined in the NEC, the photovoltaic source circuit is located “between modules and from modules to the common connection point(s) of the dc system.” This means that all conductors from the PV module junction box to the point of common connection are PV source circuits. This includes the pigtail conductors that come attached to the PV module’s junction box. The photovoltaic output circuit, however, by definition includes all “conductors between the photovoltaic source circuit(s) and the inverter or dc utilization equipment.” This circuit typically includes all conductors between the common connection point in the PV source circuit and the inverter. The NEC defines the inverter input circuit as “the conductors between the inverter and the photovoltaic output circuits.”
Larger grid-interactive PV systems with complex dc distribution networks have inverter input circuits in addition to PV output circuits and PV source circuits. It is also possible to have a PV system that uses only PV source circuits and does not have any PV output circuits. For example, in a PV system that uses string inverters with an integrated fused disconnect, the first common connection point is typically at the inverter input. In this case, the current-carrying conductors terminating at the inverter are the PV source circuits.
The basic definition of a common connection point is anywhere two or more PV circuits come together in parallel. Common connection points can occur at a number of places in a PV system: in source circuits, in PV output circuits or in inverter input circuits. Wherever these common connection points occur, it is necessary to analyze the circuit to determine the available fault current.
Fault Analysis for Combined PV Circuits
Where are all the sources of current in a PV system? It is a rare PV system where a fault on a conductor does not draw current from sources on both ends of the conductor. Therefore, just as your parents taught you to look both ways before crossing the road, you must look for possible sources of current from both directions on a conductor: the normal current and the reverse current. Reverse current, also called backfeed current, is the current that can flow through a conductor to a fault in the opposite direction of the normal current flow; this is usually the current that the conductor needs to be protected against.
Single source circuit. In a PV system consisting of a single source circuit, where the conductor is connected to the PV string on one end and to the inverter on the other, a fault on the conductor has two possible sources of current, as shown in Diagram 1. One source is the maximum PV source-circuit current, which NEC Article 690.8(A)(1) defines as the shortcircuit current multiplied by 125%. Therefore the fault current (IFAULT) at Point A in Diagram 1 is calculated as follows:
IFAULT = ISC x 1.25
= 8.19 x 1.25
The conductor at Point A is already sized to carry the maximum circuit current. The other potential source is the inverter backfeed current, or I BACKFEED at Point B. Most transformer-based inverters have zero backfeed current, which is assumed in Diagrams 1–3.