Bipolar PV Systems and the NEC

This article is designed to help designers, installers and inspectors of PV systems understand the unique NEC requirements related to bipolar PV systems. I start with a brief history of bipolar PV systems and an overview of some fundamental definitions. I conclude with an overview of design and installation best practices for building bipolar systems that are safe and efficient.

History and Definitions

Though they are atypical, bipolar PV arrays have been in use for nearly two decades. Omnion Power Engineering, a pioneering solar inverter manufacturer that was based in East Troy, Wisconsin, introduced bipolar utility-interactive inverters in the mid-1990s. The Sacramento Municipal Utility District (SMUD) used bipolar Omnion inverters in its residential PV Pioneer program as early as 1994. Failure rates were high, however (≈15%), and by the time the NEC was revised in the early 2000s to address specific safety concerns associated with bipolar PV arrays, Omnion was no longer active in the market.

While bipolar PV systems are still the exception rather than the rule, both AE Solar Energy (AESE) and Ideal Power Converters (IPC) currently offer utility-interactive inverters intended for use with bipolar PV arrays. AESE offers three models in its NX inverter series (formerly the Solaron series)—rated at 250 kW, 333 kW and 500 kW—that use bipolar PV arrays. The IPV-30kW-480 inverter from IPC is rated at 30 kW and accommodates bipolar array configurations.

You know that dc power sources, like PV systems, have one positive and one negative pole. So what exactly is a bipolar system? According to the definition added to NEC Section 690.2 in 2002, a bipolar PV array is defined as “a photovoltaic array that has two outputs, each having opposite polarity to a common reference point or center tap.”

Historically, bipolar PV arrays—like those used with Omnion inverters—were configured like the 3-wire system shown in Figure 1. In a 3-wire bipolar system, there are two ungrounded current-carrying conductors, both of which are switched and fused, and one grounded current-carrying conductor, which is center-tapped between the positive and negative poles. This schematic may look familiar, since it is analogous to the split-phase 120/240 Vac distribution system used to serve dwellings in the US. Since the grounded conductor of a bipolar PV array is connected to the midpoint of a 3-wire dc system, it even meets the technical definition of a neutral conductor as found in NEC Article 100. Note that the PV output circuit in Figure 1 includes all three of the current-carrying conductors, and all wires are run together in the same raceways, trenches, disconnects and so forth.

In response to new system topologies and their configuration requirements, the code-making panel (CMP) responsible for NEC Article 690 introduced new content related to bipolar PV arrays as part of the 2011 cycle of revisions. It added the following definition for a monopole subarray to Section 690.2: “A PV subarray that has two conductors in the output circuit, one positive (+) and one negative (-). Two monopole PV subarrays are used to form a bipolar PV array.”

This new definition clarifies that a negatively grounded monopole sub-array and a positively grounded monopole sub-array can be electrically connected together into a single PV power source, as shown in Figure 2. This 4-wire system schematic is representative of the bipolar PV arrays used today. Note that the conductors and connected equipment dedicated to each of the monopole sub-arrays in Figure 2 may need to be physically separated in accordance with NEC Section 690.4(G).

While I refer to the configuration in Figure 2 as a 4-wire bipolar configuration, inspectors and plan checkers are used to applying the term 4-wire to a 3-phase ac system with a center-tapped neutral. In your conversations with AHJs, therefore, you may wish to refer to this as “a bipolar array made up of two separate monopole arrays that comply with NEC Section 690.7(E).” While this is a mouthful, it points plan checkers and inspectors to the specific Code reference that allows 4-wire bipolar systems, which should expedite design review and system acceptance.

Design and Deployment

Conceptually, a 4-wire bipolar PV system is straightforward. As described in the “Installation and Field Service Instructions” for IPC’s IPV-30kW-480: “The inverter operates from two separate photovoltaic arrays with opposite polarity. The positive array uses positive voltages and negative grounding, while the negative array uses negative voltages and positive grounding.” This is a unique array configuration, one used only with inverters that are designed and listed for use in bipolar PV systems.

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