Designing substations and transformers for bi-directional power flow

Interconnection of transformers in substations that need to accommodate the bi-directional flow of power.


In early November, on a Consulting-Specifying Engineer webcast, I presented concepts regarding the interconnection of transformers in substations that need to accommodate the bi-directional flow of power. This scenario is typical at large industrial facilities employing co-generation or solar or wind farms with the need to inject large amounts of power into the grid. 

The CSE webcast was titled “What’s New in Electrical Engineering: Smart Grid and Transformers.” (View the webcast here)

Though I’ve covered many of these topics in previous blogs, the webcast allowed me to weave them into a coherent whole, reflected in this two-part blog that provides links to more detail.

Smart Grid involves a lot of technology that’s been in play for many years. What’s different today is the communications element, the information sharing among devices and systems. Smart Grid really is a “system of systems,” which coordinates the flow of information going back and forth for better visualization, data management, simulation and control and cyber security. 

Designing a substation for a transformer that’s going to put power back into the grid is a different kettle of fish, however, in contrast to a traditional load transformer in which the utility feeds power in one direction.

I can recommend that the consulting specifying engineer become familiar with the IEEE C57 family of standards governing equipment aging, protection, design and configurations – all the elements needed for transformers in North America, largely as required by utilities running the grid, particularly in circumstances enabling a bi-directional flow of power.

Apparatus arrangements might include high-side breakers, low-side breakers or both, ground switches, disconnects and protection schemes. The protection scheme is very important. On utility protection and control requirements, the local utility will hand you its detailed needs for your design of the transformer interconnection. Utility crews need tag-out access authorization. Reclosing is an issue that will require resolution between you, your client and the utility.

The utility will also have a prescribed set of requirement for data visualization and control. Set points for volt/VAR support, anti-islanding commands and breaker control of the transformer itself may all be control points that the utility will require for their telemetry of the station. There is even the possibility that the utility will require synchronization control. Metering location figures in substation design, too, and is often dictated by the utility’s requirements. But your client will need access to the meter as well to verify utility bills.

As is often the case, standards provide guidance on many of these issues. The IEEE 1547 series of standards govern interconnections. (Slide 31 in the webcast deck illustrates examples of what’s covered by IEEE 1547.) IEEE 2030 provides a reference model for interoperability of the interconnection. For context here, if you start out with a conceptual reference model such as the ones put out by the National Institute for Science and Technology (NIST) or the International Electrotechnical Commission (IEC), those models speak at a high level. IEEE 2030 offers three different viewpoints of the conceptual reference model, using interoperability architecture perspectives (IAPs): power systems (PS), communications technology (CT) and information technology (IT).

You’ll sometimes find, in talking to a communications engineer, that they don’t really care what information is going back and forth, they just want to know how big a pipe they need to build. On the other hand, the power system engineer doesn’t really care how the data gets from Point A to Point B as long as the data gets transmitted. IEEE 2030 provides an interoperability approach for this, including the transformers and the transformer monitoring. That interoperability allows for a greater degree of success when interconnecting to an electric utility. You’ll find a lot of this documented in IAPs’ Interoperability Tables. Have a look, it’s always better to know these things ahead of time, rather than finding them out later and having to do costly rework.

Regulatory requirements may well come into play in this design scenario. Some states have regulations, such as ownership and control, on generation connected to distribution circuits. The consulting specifying engineer needs to be aware that these regulations change on a state-by-state basis. Fortunately, regulators typically have documents or application guides on these matters. But the consulting specifying engineer needs to look at these issues as they’re designing interconnections with distribution systems.

(For more detail, see my previous blog, “Political and Regulatory Patchwork Governs Interconnection Policies”)

Most of these standards have been adopted in North America and Canada. These are the guiding principles for interconnection and will also address anti-islanding, reclosing and protection schemes that need to be put in place.

Sam Sciacca is an active senior member in the IEEE and the International Electrotechnical Commission (IEC) in the area of utility automation. He has more than 25 years of experience in the domestic and international electrical utility industries. Sciacca serves as the chair of two IEEE working groups that focus on cyber security for electric utilities: the Substations Working Group C1 (P1686) and the Power System Relay Committee Working Group H13 (PC37.240). Sciacca also is president of SCS Consulting.

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