Transmission Upgrade Deferral
Transmission upgrade deferral involves delaying – and in some cases avoiding entirely – utility investments in transmission system upgrades, by using relatively small amounts of storage. Consider a transmission system with peak electric loading that is approaching the system’s load- carrying capacity (design rating). In some cases, installing a small amount of energy storage downstream from the nearly overloaded transmission node could defer the need for the upgrade for a few years.
The key consideration is that a small amount of storage can be used to provide enough incremental capacity to defer the need for a large lump investment in transmission equipment. Doing so reduces overall cost to ratepayers, improves utility asset utilization, allows use of the capital for other projects, and reduces the financial risk associated with lump investments.
Notably, for most nodes within a transmission system, the highest loads occur on just a few days per year, for just a few hours per year. Often, the highest annual load occurs on one specific day with a peak somewhat higher than any other day. One important implication is that storage used for this application can provide significant benefits with limited or no need to discharge. Given that most modular storage has a high variable operating cost, this may be especially attractive in such instances.
Although the emphasis for this application is on transmission upgrade deferral, a similar rationale applies to transmission equipment life extension. That is, if storage use reduces loading on existing equipment that is nearing its expected life, the result could be to extend the life of the existing equipment. This may be especially compelling for transmission equipment that includes aging transformers and underground power cables.
Technical Considerations
Storage System Size Range: 10 – 100 MW
Target Discharge Duration Range: 2 – 8 hours
Minimum Cycles/Year: 10 – 50
Energy storage must serve sufficient load, for as long as needed, to keep loading on the transmission equipment below a specified maximum.
Figure 11 illustrates the use of storage for transmission deferral. The lower plot shows storage being discharged on Wednesday afternoon to compensate for the high load on the substation transformer, as shown in the upper plot. The storage is recharged when the feeder load reduces in the late evening. Alternatively, the storage can be recharged during the late night as long as it is available to serve the peak load that the transformer is likely to see the following day(s).
