Connecticut DPUC Issues Draft Decision on Network Interconnection

On March 31, 2010, the Connecticut DPUC issued a draft decision to revise the small generator interconnection protocols of the state’s investor-owned utilities, the United Illuminating Company and the Connecticut Light and Power Company. This proceeding refers specifically to interconnections to area networks which are low voltage electrical systems served by multiple transformers, usually located in densely populated metropolitan areas to provide large numbers of customers with electrical service.  Originally, the DPUC did not require the EDCs to allow area network interconnection, but stated its belief that the utilities should work with generators to interconnect to area networks on a case-by-case basis, to determine if they can be safely connected.  In its Draft Decision the DPUC noted that, “Although area networks are only a very small fraction of the electric power system in Connecticut, it is important for energy policy reasons to allow small generators to safely interconnect to the maximum extent possible, particularly for renewable energy projects.“

After careful study the utilities have concluded that a limited amount of inverter-based generation may be interconnected to an area network.  The utilities filed a final report of their study, which included the following major findings:

  • Adding generation to an existing area network cannot enhance the operation of the system; however, it can negatively impact the operation of the system during normal loading and during fault conditions;
  • Synchronous and induction generation sources should not be allowed on an area network where time delay tripping is not used on network protectors; and
  • Small amounts of inverter based generation can be allowed to connect to an area network.  The amount of permissible inverter based generation should be limited to 3-4% of the installed network transformer capacity.


Based on the Final Report, the utilities propose revisions to the Interconnection Guidelines to introduce a process flow chart and screening criteria to determine whether proposed inverter based generation can safely interconnect with area networks, and to allow such interconnections where feasible.  The Final Report proposes the following requirements for inverter-based generators as amendments to the interconnection guidelines:

  • The unit is a certified inverter-based generator or it has inverter based utility grade relays exclusively used in its design;
  • The network primary feeders supplying the network to which the generation is connected are from the same electrical bus or from normally tied buses;
  • The maximum generator size will be limited to 50 kW at any location, to limit power flow through the cable limiters;
  • Total aggregate generation interconnected to an area network will be limited to 3% of the maximum network transformer connected kVA with the feeder supplying the largest number of network units out of service, or a maximum of 500 kW, whichever is less.


In the draft decision, the DPUC approves these changes.  IEEE is currently considering issuing a national standard regarding interconnection to networks titled IEEE 1547.6, Recommended Practice For Interconnecting Distributed Resources With Electric Power Systems Distribution Secondary Networks.  The DPUC additional required the utilities to follow the issuance of IEEE 1547.6 and report on how that standard differs from the revised Interconnection Guidelines. At that time they will revise the Guidelines again, in accordance with the IEEE standard, if necessary.


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