Recent efforts in California and other states have focused on expanding the acceptable methods of export control to permit the use of certified Power Control Systems for both non- and limited-export functions. These can be especially useful for smaller systems where a relay is impractical,((R. Brent Alderfer,, Monika M. Eldridge, and Thomas J Starrs, Making Connections: Case Studies of Interconnection Barriers and Their Impact on Distributed Power Projects, United States Department Of Energy Distributed Power Program Office of Energy Efficiency and Renewable Energy, Office of Power Technologies (July 2000), though DERs of any size might employ them.

Power Control Systems are composed of a controller, sensors, and inverters, any of which may or may not be contained in separate devices. PCS have been used to limit export to the distribution system where no export is allowed, or to limit the maximum export to a value less than the Nameplate Rating of the DER. One possible configuration of a PCS is shown in Figure 1. Here, separate PV and storage inverters are controlled by signals derived from a discrete PCS controller. As connected, the current transformer monitors the entire load, while the PCS uses the sensor information to create power setpoints for the inverter(s). In this configuration, either or both of the inverters could be controlled to an export limit, and import limiting to the storage inverter could be implemented. Other configurations with alternative connections or setups could be used to achieve different control strategies (e.g., see Appendix B).

Figure 1. Local Power Control System Supporting Export Limiting (EPRI)

Storage may include PCS export or import controls in order to maintain export or import limits within distribution system constraints. Storage could also use PCS to enable energy storage to comply with Net Energy Metering requirements, typically when set for export only to ensure that a battery is charged entirely from solar or import only to ensure that a battery does not export for NEM credit.

Since PCS are control devices, as opposed to a signaling device which trips a circuit breaker at a definite time delay (like a relay does), their response times are characterized in terms of open loop response time (OLRT), which reflects the time for the output to reach 90% of the reduction toward the final value. PCS can introduce inadvertent export as a result of changes to load, similar to other systems, but they do not “trip” at any definite time. Though some PCS are able to respond in timeframes similar to the typical settings for reverse power relays, others are slower—while still generally being fast enough to avoid distribution system impacts such as interactions with voltage regulators.

Arizona, Colorado, Nevada, Maryland, Minnesota, and Hawaii have included provisions in interconnection rules for these types of systems, including a maximum 30second response time,((AZ Administrative Code § R14-2-2603(E)(4) (inadvertent export duration limited to 30 seconds); Section 4 Code of Colorado Regulations § 723-3, 3853(c)(I); HI Pub. Util. Comm., Rule 22, at Sheet 44B-1 to Sheet 44B-2 (Appendix II) (same); MN TIIR § 11.3, at p. 33 (same); NV Pub. Util. Comm., Dkt 17-06014, NV Power Co. Rule 15 § I.4(b) (same); Code MD Regs., Sec. but those rules largely predated any certification test protocol. The UL Certification Requirement Decision (CRD)((CRDs are the preliminary documents developed through UL’s deliberative process to inform revisions to UL’s existing or future listings. They are a primary vehicle for addressing hardware or control requirements in standards. The CRD for PCS contains tests to assess a set of PCS functionalities not previously addressed in UL 1741.)) for PCS (issued for UL 1741((UL 1741 is a product safety standard that stipulates the manufacturing and product testing requirements for the design and operation of inverters, converters, controllers, and other interconnection equipment intended for DER. Solar and storage inverters, as well as other products, are listed to the safety standard UL 1741, which requires grid-interactive equipment to pass the tests in IEEE 1547.1.)) on March 8, 2019) now defines conformance tests that allow PCS to be certified. While not yet part of the UL 1741 standard, the CRD document is required to be utilized for UL product certification programs. The tests are planned to be incorporated into the UL 1741 standard such that the CRD will no longer be needed.

The test protocol can be used to demonstrate that a PCS supports: (1) export limiting from all sources, (2) export limiting from ESS, and (3) import limiting to ESS. Additionally, unrestricted, export only, import only, and no exchange operating modes may optionally be supported by the PCS. More detail on the CRD test procedures is given in Appendix B.


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