Column: Managing existing transformers in the grid transition

Modern transformers are designed for stationary loads and ‘clean’ sine waves, but they experience variable loads and higher harmonic content, which is not optimal.

byBrian Sparling


A brief history of line and equipment ratings

In the past eight or more decades, the electric power industry used a rating system based on conservative assumptions to determine the capacity available which lines and substations could use to safely carry the load. These are known as static lines ratings (SLR) systems, and substation capacity has had similar determinations made based on static or nameplate data of the equipment. It remains in use but is a very conservative method. Two newer methods evolved over time. One is the seasonally adjusted rating (SAR) system and the ambient adjusted rating (AAR), but they still rely on weather-related assumptions rather than real-time information.


In the past, the equipment was designed to operate under well-known conditions assuming a ‘clean’ sine wave of 50 or 60 Hz


It worked well in the past when the system was designed to generate energy in larger centrally located power plants and when they delivered that energy in one direction, from the source to the load centers. The equipment was designed to operate under well-known conditions assuming a ‘clean’ sine wave of 50 or 60 Hz.


The changing nature of the power system: Digital versus static

There has been a lot of technical changes in our industry, beginning with the shift from analog simulation systems, to first electronic and then quickly into the microprocessor-based devices that are common today.


The application of intelligent electronic devices (IEDs) on power transformers has been ongoing for the past 25 years. An example can be found in dissolved gas analysis (DGA) monitors, from basic key-gas units advancing to multi-gas monitors with inherent ability to alarm when gassing patterns begin to change and provide diagnostics about the nature, and sometimes, about the severity of a developing fault. The systems have evolved to today’s point, where virtually anything can be measured, computed locally, and communicated to centers where this actionable information is available for decision making.


Today, AC current is often ‘polluted’ with harmonics, as the inverters in many applications typically do not produce the clean sine wave that equipment, such as transformers, expect to see


With these “Smart Grid” IEDs, digital communication and data collecting/archiving systems throughout the grid, the opportunity to make use of the information and enable the move from the “Static” to a “Dynamic” understanding of the capacity available in the system is achievable.

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