Smart Cities and the Smart T&D Electric Grid

Much of the discussion around smart cities centers around smart buildings and the proliferation of smart meters (i.e., advanced metering infrastructure). Also discussed is the growing importance of distributed energy resources (DER) and the multitude of smart devices that make up the Internet of Things (IoT). However, the criticality of the electric transmission and distribution (T&D) grid that powers the smart city or smart community is rarely or only casually mentioned. Regardless, many T&D technologies and features will likely be critical to the smart city of the future.

Generation: The shift from local coal or nuclear generators supplying urban population centers to remote utility-scale wind and solar generation resources is rapidly occurring and can be seen across North America and Europe. Large-scale wind and solar generation farms are becoming an increasing portion of the generation mix. Electric power must be transferred from these remote sites to urban populations over hundreds (if not thousands) of miles of new high-voltage transmission lines using high-voltage direct current (HVDC) and extra high-voltage alternating current (HVAC) transformers and converter stations.

Transmission grid technologies: In turn, these lines require new approaches to monitoring and control necessary to maintain voltage levels and synchronize the three-phase power delivery at each substation along the way. Relatively new phasor measurement units (PMUs) and digital protective relays collect voltage, current, and power factor information up to 60 times per second, time stamping it for comparison purposes. Synchrophasor analytics make real-time comparisons of status at each end of the transmission lines, warn operators, and automatically correct voltage or power factor when readings diverge from optimal operating conditions. These high-speed incidents go largely unnoticed with traditional SCADA monitoring and control and can sometimes create major reliability incidents.

Distribution substations: The digital substation will also be a critical part of the new smart city. As every device in the substation is upgraded to have digital communications and control, substations will be ringed with high-speed fiber optic networks. These networks connect the various devices, including transformers, switchgear, protective relays, and other intelligent electronic devices. This sets the stage for the virtual substation, where every piece of equipment is modeled, operating data is shared, and system operations are monitored, controlled, and automated at the local and centralized operations centers.

Distribution feeders and low-voltage (LV) distribution transformers: Distribution feeders connect the substation with customers in both urban and rural locations. Urban distribution feeder systems are complex meshed networks, with fleets of disconnect switches, reclosers, and other devices that allow the network to be reconfigured and continually operated when isolated system faults occur. These intelligent electronic devices increasingly include local and autonomous decision-making and control capabilities. They communicate with adjacent devices and reconfigure the network or managing voltage and power factor without control by the substation or central operations center.

There are also millions of LV distribution transformers that operate at the edge of the grid, stepping down voltage for delivery to the customer. These transformers have traditionally been mechanical/electrical devices with no monitoring capabilities, but are now being gradually replaced with smart transformers that measure and report critical operating condition information. Sophisticated transformers may provide control and automation capabilities, which are becoming increasingly critical for managing the distribution grid as DER penetration increases. Retrofit monitoring and control devices are also now available and can be installed close to problematic or overloaded transformers.