B. Types of System Modifications That an ESS Could Implement to Mitigate Impacts
Due to the flexibility that ESS provides, both to the customer and as a resource to the grid, it is important to recognize the manner in which system parameters may be changed to mitigate impacts identified during the interconnection process. The below paragraphs discuss the various modifications that may be utilized by an ESS project to mitigate or avoid impacts during the review process.
An ESS project may offer one or more use cases, such as self-supply and peak shaving. The ESS may employ operating schedules, potentially through the use of a Power Control System (PCS) or other export limiting equipment (see Chapter III for a discussion of the methods for controlling export and Chapter IX for further discussion of how the use of schedules can be relied upon and communicated to the utility). Also note that the same storage system may offer grid support functions (such as volt-var or fixed power factor) though this is not explored further herein since it applies to all inverter-based DERs.
PCS can be utilized by interconnection customers to limit export to the distribution system to a value less than the Nameplate Rating of the DER. Customers with storage may include PCS in their DER design, either in the original application or as a design change to address an identified impact (such as maintaining export limits within distribution system constraints). Where a PCS was included in the original DER design, the utility will have evaluated the system’s proposed Export Capacity in its analysis and screens, per Chapter IV. To address certain impacts, it may be possible for the customer to revise the Export Capacity to a new limit. On the other hand, where a PCS was not included in the original DER design, the utility will have evaluated the system’s full nameplate capacity in its analysis. It is possible for the customer to add PCS equipment that would change the Export Capacity to a new limit. Customers may wish to operate ESS in a manner that mitigates impacts during periods with grid constraints. As an example, during days (or hours) where the grid is restricted, the storage system could be scheduled to charge or discharge following a local operating schedule or one based on control signals. Where an ESS operating schedule is verifiable and can maximize hosting capacity and mitigate impacts during grid constraint periods, a customer could be allowed to modify the ESS operating schedule such that Export Capacity does not increase beyond a predetermined value. Alternatively, where utility control systems (such as a distributed energy resource management system, or DERMS) are deployed, signaling may be used to change export limits dynamically in response to real-time grid constraints.
Customers may consider adding storage to a DER design (that did not originally contain ESS) in order to address identified upgrades or screen failures. For example, an exporting PV system could charge an ESS which could then discharge at a later time (“time-shifting”) and implement a reduced Export Capacity. This concept could be extended by applying a schedule or dynamic signal to avoid grid constraints at certain hours. Note that adding AC-coupled energy storage increases the Nameplate Rating of the DER as well as the rated fault current. Where a PCS maintains or decreases Export Capacity, adding AC-coupled storage can be acceptable, but the utility may need to reassess the fault current impacts.
In the initial application, the interconnection customer will identify the proposed ESS operating profile and the utility will evaluate such characteristics in the applicable screening and/or study process. The following sections will provide recommendations on how information can be provided during the interconnection review process to: (1) identify where modifications may be feasible to mitigate impacts, and then (2) provide defined opportunities for any of the above storage characteristics to be modified, so long as they are designed to mitigate the grid impacts identified in the screen or study results.