purposes, record the value of that portion of the facility in a deferred debit account and shall treat this amount similar to the treatment of the allowance for funds used during construction. When that portion of the facility is returned to useful service, as provided in subdivision (c), the corporation may apply to the commission for the inclusion of its value and expenses related to its operation for purposes of the establishment of the corporation’s rates.

notification specified in subdivision (b), the commission shall institute an investigation to determine whether to reduce the rates of the corporation to reflect the portion of the electric, gas, heat, or water generation or production facility which is out of service. For purposes of this subdivision, out-of-service periods shall not include planned outages of predetermined duration scheduled in advance.

The commission’s order shall require that rates associated with that facility are subject to refund from the date the order instituting the investigation was issued. The commission shall consolidate the hearing on the investigation with the next general rate proceeding instituted for the corporation.

rates for being out of service for nine or more consecutive months pursuant to subdivision (a) or (b), has been restored to service and has achieved at least 100 continuous hours of operation, the commission may again consider that portion of the facility for purposes of establishing rates, and may adjust the corporation’s rates accordingly without a hearing, except that a hearing is required on whether to include, for purposes of establishing rates, any additional plant value added.

such a facility that the commission determines to be a major facility of the corporation, and does not include any facility determined by the commission to constitute a plant held for future use.