Connecting renewable energy systems

Electrical engineers have several items to consider when interconnecting renewable energy production systems.

02/08/2016


Learning objectives:

  • Explain the basics of connecting a renewable energy production system (REPS) to a building's electrical system.
  • Help system designers, integrators, and owners consider the implications of different interconnection options and requirements for utility interactive REPS.

Figure 1: The 8,000-panel (2 MW) photovoltaic energy generating system at FedExField, home of the Washington Redskins, also includes 800 covered parking spaces. This array generates enough power to serve all of the facility's electrical needs on nongame dWhether designing a renewable energy production system (REPS) to connect into an existing building's electrical system, or designing a solar-ready facility where only a means to interconnect a future REPS is provided, the details of the interconnection can have a major impact on the design, cost, and schedule of a REPS project.

The design of an electrical utility grid-connected REPS, whether it be photovoltaic (PV), wind, or alternative technology, can be broken into two parts:

  • The energy system itself, including production equipment, conversion equipment, and any other balance of system components
  • The electrical interconnection to the grid.

The design of the former is usually straightforward, and the rough cost and design is easily predicted as early as the proposal process. The interconnection design, which is often overlooked until later in the design process, can have drastic implications on the system size, project schedule, and project costs—both the upfront cost and lifecycle cost of the system. Considering the electrical interconnection early will improve the likelihood of eliminating project pitfalls associated with these items.

The major factors influencing the interconnection design are code requirements, utility requirements, owner requirements, and existing equipment conditions. These factors are discussed in detail in regards to their impact on the overall REPS design. For the sake of this article, all REPS are assumed to use utility interactive inverters, thus fall under the requirements for those types of inverters.

Code requirements

NFPA 70: National Electric Code (NEC) Article 705 provides requirements for interconnecting REPS. Article 710.12 addresses the point of interconnection and offers three possibilities. Each of these possibilities presents different benefits and restrictions, and each can affect the maximum system size allowed, the cost of the interconnection, and other system parameters. The interconnection possibilities are presented in the order in which they are likely assessed during design.

Figure 2: This shows a typical load-side tap in which 120% of 1,200 amp is 1,440 amp, so a renewable energy production system is limited to 225 amp. Point of interconnection opposite of main overcurrent protection. Courtesy: DLR GroupNEC 705.12(D)(2)—load-side interconnection: The most common type of interconnection for smaller-scale REPS is a load-side interconnection, commonly known as a load-side tap. The general rule for this type is that the interconnection overcurrent protection (OCP) must be placed at the opposite end of the busbar from the main OCP, and that the sum of the main OCP and the interconnection OCP cannot exceed 120% of the bus rating (see Figure 2). For smaller REPS, this option is often chosen because it is relatively inexpensive, uses spare capacity within existing equipment, and is well-understood by the authority having jurisdiction (AHJ) and electric utilities. For larger REPS, however, this option may present a challenge as the 120% rule limits the OCP for the REPS and, therefore, the size of the REPS.

Figure 3: Reducing main overcurrent protection—the main circuit breaker, in this case—allows for an increased size of the renewable energy production system. Courtesy: DLR GroupTo increase the potential size of the REPS with a load-side tap, either the busbar size must be increased or the main OCP must be decreased. For existing buildings, increasing the busbar size is unlikely to be a cost-effective option. It is also uncommon for the main OCP to have a lower rating than the busbar. However, it is likely that the peak demand on the existing equipment will be substantially less than the OCP rating. If metering the main OCP shows that this is the case, the main OCP may be reduced (either replaced or, if an electronic trip unit is present, dialed down) to allow for a larger REPS to be connected on the load side (see Figure 3). If a main OCP reduction is desired, the implications of the resulting reduced system capacity should be discussed with the owner. If a main OCP reduction is not possible, an alternate method must be considered.

NEC 705.12(A)—supply-side interconnection: This article allows the REPS to be interconnected ahead of the main disconnecting means for the site. This option, usually referred to as a supply-side tap or a line-side tap, allows for output amperage of the REPS to be as large as the busbar rating.

Figure 4: Providing an overcurrent protection (OCP) device for interconnection ahead of the main OCP is easily done with new construction. The OCP can be rated as high as the busbar. Courtesy: DLR GroupThe big question that usually arises for this interconnection is, "Where exactly do we make the tap?" For a new system, the answer is easy. A tap location ahead of the main OCP can be provided in the gear in a similar fashion as a fire pump would be connected (see Figure 4). The REPS OCP could be an externally fused disconnect or a breaker/disconnect integrated into the switchgear.

Figure 5: A retrofit tap ahead of the main overcurrent protection and downstream of the meter may be feasible. Confirm with the manufacturer and/or field-listing agency. Courtesy: DLR GroupFor existing installations, this option can be tricky. Adding a breaker or tap section to an existing piece of gear will likely be costly, as it would require significant modification of the equipment. This leaves tapping the busbar somewhere ahead of the main OCP but after the utility meter (see Figure 5). With the many different gear configurations out there, this may or may not be possible. To find out, a few parties will need to be involved.

To evaluate a line-side tap, it is recommended to first get the gear identification number, usually located on a sticker on the front of the gear. With this, you can contact the manufacturer and find out, based on the shop drawings, whether a tap can be made ahead of the main without voiding the UL listing of the gear. The manufacturer may affirm it is possible, and even be able to provide you with a part number for a tap kit. If the board contains multiple metering sections, the manufacturer may say it is possible within one of the metering sections, in which case you would need to contact the utility and get permission to tap in one of their unused sections.


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