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Interconnection Standards

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The Washington Utilities and Transportation Commission (UTC) adopted interconnection standards for distributed generation (DG) systems up to 20 megawatts (MW) in capacity in 2007, and revised these standards in July 2013. The revised standards provide for three separate levels of interconnection based on system capacity and other requirements. The first level, Tier 1 systems, applies generally to systems up to 25 kilowatts (kW) using inverter-based interconnection equipment. The second tier applies generally to systems sized between 26 kW and 500 kW, and Tier 3 applies to all systems up to 20 MW that do not qualify for Tier 1 or Tier 2 standards. The UTC's standards apply to the state's investor-owned electric utilities, but not to municipal utilities or electric cooperatives.

Tier 1 systems must include an inverter certified to meet the Underwriters Laboratories (UL) 1741 standard. 

Tier 2 systems must be capable of separating the generating facilities from the electric system as required by the Institute of Electrical and Electronics Engineers (IEEE) 1547 specification. Inverter-based systems must comply with UL 1741 and IEEE 1547. A utility may offer to modify  its electrical system to accommodate the generating facility, but the customer must agree to pay for the minor modifications. Systems larger than 50 kW may be required to use a three-phase connection. A visible, locking AC disconnect switch is generally required for Tier 2 systems, though a utility may waive the requirement if the customer agrees that service may be disconnected entirely if the system must be physically disconnected for any reason, and the customer retains documentation of operating and maintaining the inverter in accordance with the manufacturer’s guidelines.

Tier 3 systems must also isolate the generating facility from the electric system as specified by IEEE 1547, and inverter based systems must meet IEEE 1547 and UL 1741 specifications. The system must be designed to prevent a single point of failure from causing a loss of protective functions, and must provide ground fault protections and breaker failure detection. Utilities may require a feasibility or system impact study.

All interconnections of systems must conform to all applicable codes and standards for safe and reliable operation, including the National Electric Code (NEC); National Electric Safety Code (NESC); the standards of the Institute of Electrical and Electronics Engineers (IEEE); the standards of the North American Electric Reliability Corporation (NERC); the standards of the Western Electricity Coordinating Council (WECC); American National Standards Institute (ANSI); Underwriters Laboratories (UL) standards; local, state and federal building codes, and any electrical company's written electric service requirement approved by the UTC.

Utilities must have a designated point of contact for customers seeking to interconnect systems, and must file with the UTC a standard form application and a model interconnection agreement. Application fees are limited to $100 for systems up to 25 kW, $500 for systems greater than 25 kW but not greater than 500 kW, and $1,000 for systems greater than 500 kW but not greater than 20 MW. The UTC's rules include provisions for dispute resolution.