Conn. Gen. Stat. § 12-268d

(b) If any company or municipal utility has not made its return within one month after the time specified in section 12-249 , 12-256 , 12-264 or 16-331cc , the commissioner may make such return at any time thereafter, according to the best information obtainable and according to the form prescribed. To the tax imposed upon the basis of such return, there shall be added an amount equal to ten per cent of such tax, or fifty dollars, whichever is greater. The tax shall bear interest at the rate of one per cent per month or fraction thereof, from the due date of such tax until the date of payment. No taxpayer shall be subject to a penalty under both subsections (a) and (b) of this section in relation to the same tax period.

(c) Subject to the provisions of section 12-3a , the commissioner may waive all or part of the penalties provided under this chapter when it is proven to his satisfaction that the failure to pay any tax was due to reasonable cause and was not intentional or due to neglect.