Unintentional Islanding Working Group Final Report

December 15, 2023

Claire Halbrook



In 2021, the California Public Utilities Commission (CPUC) established the Unintentional Islanding Working Group (UIWG) to review, discuss, evaluate, and recommend distribution system-level solutions to unintentional island formation arising from increased Distributed Energy Resource (DER) penetration. The Working Group met monthly for approximately one year. Gridworks served as the UIWG’s facilitator and lead author on the report

The Problem: Unintentional Islanding 

If an electric distribution circuit or segment disconnects from a larger electric grid, any connected DER must quickly de-energize or separate from the electric grid so that an unintentional “island” does not form (i.e., a portion of the area’s electric system remains energized). Unintentional islanding (UI) is defined as unplanned, unapproved energization of some portion of a power system by one or more DERs (following disconnection from any larger electric grid). UI can result in transient voltages and frequencies, damage to utility or customer equipment, or subsequent uncleared or delayed clearing faults. UI can technically occur on either the transmission or distribution system. However, no UI of distribution-level DERs within the service territories of California’s three investor-owned utilities (IOUs) has occurred. It is difficult, if not impossible, to determine whether this is the result of mitigations in place or whether no UI events would have occurred even in the absence of these mitigations.  

Approaches to Unintentional Islanding Mitigation

California’s IOUs currently take different approaches with respect to how they assess and manage the potential risks of UI formation, with Pacific Gas and Electric (PG&E) differing most significantly in its approach. PG&E requires additional screening of distribution-level DERs. If the DER does not pass the screens, the developer can either elect to conduct a Risk of Islanding (ROI) study with one of PG&E’s pre-approved third parties or move directly to installing mitigations. The ROI study is conducted at the developer’s expense. If the DER fails the ROI study, additional mitigations may need to be installed. The additional mitigations include direct transfer trip (DTT) and/or the addition of a Supervisory Control and Data Acquisition (SCADA) utility-operated recloser at the DER’s point of interconnection. DTT isolates the DER from the distribution and transmission system whenever an upstream breaker trips offline and is typically applied for transmission-level faults. The SCADA-operated recloser can be opened by the utility distribution operator during a UI event. Mitigations like DTT can introduce additional project costs for the developer of over $1 million and thus serve as a deterrent to further DER development. This is only a concern when a machine-based generator is present on the distribution circuit and is not an issue when only certified-inverter-based generation is present. 

Potential Alternatives

The UIWG found that other emerging UI mitigations could eventually prove viable alternatives to costly wired DTT. Some of these are ready for deployment or pilots and others still need additional work in laboratory settings. These include: 

  • Synchrophasors, also known as phasor measurement units 
  • Bulk system timing reference (BUSTR) 
  • Cellular Wireless DTT (rather than spread spectrum wireless)  
  • Power Line Carrier (PLC) Method 
  • Spread spectrum broadcast Generic Object-Oriented Substation Events (GOOSE) Protocol


In the near term, the UIWG supports PG&E’s efforts to pilot cellular wireless DTT and spread spectrum broadcast (GOOSE) as potential alternatives to the current wired DTT UI mitigation.  Due to limitations in the broadcast radius, PG&E questions whether synchrophasors are truly a cost-effective alternative to DTT.  

PLC is currently being deployed by a handful of utilities in the Northeast and Midwest. One of its main advantages is that it can cover all the DERs on a given circuit rather than needing to be relayed point to point. However, it currently faces propagation challenges. The signal cannot pass through transformers or underground distribution. Additionally, as higher frequencies are leveraged to achieve higher bandwidth, cross coupling with distribution lines not meant to communicate these signals unfortunately becomes possible. For these reasons, the UIWG is not recommending that PG&E prioritize further exploration of utilizing PLC as a UI mitigation at this time.  

In addition to the technologies recommended for piloting, the UIWG believes an additional UI mitigation warrants further testing. The BUSTR method should be explored and eventually be evaluated in the longer term for piloting. Sandia National Laboratories is currently assembling a coalition and pursuing funding opportunities to conduct demonstration projects and pilots at-scale. Sandia will report back on its progress and findings to the three California IOUs and other utilities over the coming years. The merits of different business models for BUSTR also need to be discussed.

The following table summarizes the known alternatives to wired DTT.

Post by Claire Halbrook