Basics of Medium-Voltage Wiring
Inside this Article
Medium-voltage wiring is typically used for ac collection systems in utility-scale PV power plants. Depending upon project capacity and site complexity, it can also be a useful design option in commercial-scale PV applications.
The ac collection system in a utility-interactive PV system includes all of the wiring and components from the inverter output circuit(s) to the interconnection point with the utility. Commercial-scale PV plants are generally interconnected to the utility via building wiring systems at common building utilization voltages, such as 208 Vac or 480 Vac. However, as the rated capacity of these interactive systems increases, so does the physical footprint of the PV power plant and the distances between wiring points. Based on the site layout, the desired PV generating capacity and other project-specific variables, system designers working on commercial-scale projects may find a practical need to use medium-voltage (MV) wiring for ac collection systems.
In this article, I provide an introduction to some common components used in MV circuits in PV systems and discuss basic design considerations for their application. I cover MV components for use in ac collection systems, including distribution transformers, overhead and underground feeders, pad-mounted switchgear, and metal-enclosed and metalclad switchgear. I also provide example single-line diagrams (see below) showing three representative uses of MV wiring methods and components in commercial- and utility-scale PV systems.
SYSTEM VOLTAGE CLASSES AND WORKER SAFETY
I use the term MV here to describe electrical system components rated between 5 kV and 38 kV. This definition corresponds to common US utility-distribution voltages.
In practice, voltage class definitions are somewhat tricky to pin down. They vary from industry to industry and from one set of codes or product standards to another. On one hand, The Authoritative Dictionary of IEEE Standards Terms (IEEE 100) defines medium voltage as “a class of nominal system voltages greater than 1,000 V but less than 100,000 V.” On the other, Article 490 of the National Electrical Code defines high voltage as “more than 600 volts, nominal.” While the NEC does not include a stand-alone definition of medium voltage, Article 328 details the Code requirements related to medium-voltage or Type MV cable. As described in NEC Sections 328.2 and 328.10, MV cable is rated at “2001 volts or higher” and is “permitted for use on power systems rated up to and including 35,000 volts, nominal.”
While the exact definition of MV may vary by context, the implications for worker safety are indisputable. The voltage and potential fault energy levels of MV wiring systems pose significant safety hazards. In addition to electric shock and arc-flash burn hazards, blunt force and projectile injuries are also possible due to arc-blast hazard. While it is beyond the scope of this article to go into the details of MV electrical safety, personnel must always be trained to recognize the particular hazards associated with the specific class of system voltage they will be working with and understand how to mitigate those hazards through safe work practices. Workers must have the proper tools and PPE, and be trained in their use.
In addition to applicable city, state and county requirements, work in PV power plants may fall under one or both of the following OSHA standards:
- CFR 1910.269—Electrical Power Generation, Transmission and Distribution
- CFR 1910 Subpart S—Electrical
GENERAL DESIGN CONSIDERATIONS
The design goals for a MV ac collection system are essentially the same as those for other power distribution systems—or any electrical system, for that matter. According to Eaton Corporation’s Consulting Application Guide (see Resources), “The best distribution system is one that will, cost-effectively and safely, supply adequate electric service to both present and future probable loads.” The only distinction is that the MV wiring in a utility-interactive PV application does not supply loads, but rather is designed to supply adequate service to interconnected electric power production sources.
The Consulting Application Guide goes on to list seven electrical distribution design goals, the most important of which is a safe installation that does not present any electrical hazards to people or equipment. The other six design goals can be summarized as follows: