Transmission Congestion Relief
Transmission congestion occurs when available, least-cost energy cannot be delivered to all or some loads because transmission facilities are not adequate to deliver that energy. When transmission capacity additions do not keep pace with the growth in peak electric demand, the transmission systems become congested. Thus during periods of peak demand, the need and cost for more transmission capacity increases along with transmission access charges. Transmission congestion may also lead to increased congestion costs or locational marginal pricing (LMP) for wholesale electricity at certain transmission nodes.
Electricity storage can be used to avoid congestion-related costs and charges, especially if the costs become onerous due to significant transmission system congestion. In this service, storage systems would be installed at locations that are electrically downstream from the congested portion of the transmission system. Energy would be stored when there is no transmission congestion, and it would be discharged (during peak demand periods) to reduce peak transmission capacity requirements.
Technical Considerations
Storage System Size Range: 1 – 100 MW
Target Discharge Duration Range: 1 – 4 hours
Minimum Cycles/Year: 50 – 100
The discharge duration needed for transmission congestion relief cannot be generalized easily, given all the possible options. As with the Transmission upgrade deferral service, it may require only a few hours of support during the year when congestion relief is required. Generally, congestion charges apply for just a few occurrences during a year when there are several consecutive hours of transmission congestion.
Figure 12 illustrates the storage response in transmission congestion relief service. The upper plot shows four instances in which load exceeds the capacity of the transmission line. The lower plot shows storage discharge during those four events and a recharge during the late night when the system load is lower and the transmission line is lightly loaded.
