Wind power is ubiquitous. Massive wind turbines can be seen from miles away all across the United States. While the inner workings of wind turbines are complicated, designing power systems to support a wind power plant are not. Article 694 in the 2020 NEC describes the requirements for designing a wind power plant system. The block diagram below describes the major components of a typical wind turbine electrical system.
High Level Wind Turbine Block Diagram
The alternator converts rotational energy from the wind turbine blades into AC power. The rectifier takes that variable AC electrical output and converts it to steady DC power. The inverter then takes that DC and converts it to a steady AC output for interconnection with local loads, a collection system, or the electrical grid. In short, AC => DC => AC.
Many times, wind turbine manufacturers will already have pre-installed or provided cabling between the alternator, rectifier, and inverter. If there's a step-up transformer after the inverter, this might be included as well. If this cabling is not designed by the manufacturer, NEC 694 provides some guidance: size to 125% of the maximum continuous output current of the upstream device. In other words, consider all loads from wind turbines as if they are continuous.
To some, this might seem strange: Wind power is intermittent. Why should I size it as if it is continuous? The answer to this comes back to the definition of a continuous load from NEC Article 100: "A load where the maximum current is expected to continue for 3 hours or more". Even though a wind turbine is intermittent, the load current can absolutely continue for 3 hours at its maximum value. In fact, the load current could continue for days at a time at its maximum value. Considering the output of a wind turbine as anything other than continuous would be a mistake.
All conductors must be protected at sources of potential overcurrent. Modern inverters generally function as current-limited devices, meaning that they can't contribute dangerous, sustained short circuit currents to a circuit. If your conductors from the inverter output onward are sized appropriately and there are no other sources of overcurrent, then an overcurrent device may not be required at all (see NEC 694.15). However, if the output conductors of the inverter are connected to other inverters and/or to the grid, overcurrent protection may be necessary to prevent unsafe currents. As with any power system, designing for the worst-case scenario is what keeps things safe and operating when you need them the most.
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