C. Traditional Export Control Methods
Where DER systems require export limiting in order to interconnect, control has been achieved over the years in multiple ways with existing equipment, mostly only for larger systems. This is often achieved using protective relays implementing a reverse power limiting function (known as Reverse Power Protection) or minimum import function (known as Minimum Power Protection). Relays are sensing and computational devices which can signal a circuit breaker to trip based on measured quantities of voltage and current, dependent on the function(s) implemented. For a non-export system, the relay would be set to trip the circuit breaker if reverse power is sensed for longer than a short delay time or, alternatively, if import power falls below a minimum amount. A similar concept can be used for limited-export systems to trip the breaker when reverse power exceeds a certain level (known as Directional Power Protection).
DER systems which employ this type of protection to control export may have an additional control system acting internally to ensure export power does not reach the level which would cause the relay to trip. Alternatively, the systems could be designed based on an analysis of the load and generation at the site, such that export power is very unlikely to ever exceed the limit. In this case, inadvertent export (previously described in Chapter I.A.3 and Chapter III) could be introduced where some export beyond the limit occurs, but is not of sufficient duration to cause a trip. Inadvertent export would usually occur due to a fast drop in load, such as a large air conditioning unit or other large load turning off. DERs with control systems in place can recognize this violation of the export limit and respond quickly to reduce generation so export no longer exceeds the limit. Chapter V will discuss inadvertent export in more detail.
Another way to control export is by reducing the export capability of the DER via an internal setting to a value below its Nameplate Rating. Inverters typically have an ability to limit maximum output power via a settable parameter or via a firmware change, the latter typically requiring the intervention of the manufacturer. IEEE 1547-2018 has formalized this concept by allowing the changing of nameplate parameter values via configuration (known as Configured Power Rating). This optional feature can be tested with the IEEE 1547.1-2020 test procedures.((IEEE Standard Conformance Test Procedures for Equipment Interconnecting Distributed Energy Resource with Electric Power Systems and Associated Interfaces, IEEE Std 1547.1-2020, https://standards.ieee.org/standard/1547_1-2020.html.)) While limiting power via configuration settings does limit export power, it would also generally limit the ability to serve any onsite load when this limit affects the power at the inverter terminals, as is typically done today.
Another option is to use probabilistic methods to ensure export power does not exceed a limit, without the need for additional protection functions or relays. This is typically only done for non-export systems, by analyzing the load in comparison to the generation in order to have a high degree of certainty that load will always be higher than generation, usually by a wide margin (known as Relative Distributed Energy Resource Rating).
The above practices have been used in many areas of the country and around the world, but in the U.S. have thus far only been formalized in a few interconnection rules. California, Nevada, and Hawaii have for some years included a list of recognized non-export methods in interconnection rules which include relay and probabilistic methods.((California Rule 21 G.1.i; Nevada Rule 15 I.4.b; and Hawaiian Electric Rule 22 Appendix II.))