AUGUSTA, Maine — A cybersecurity firm sued the parent company of Central Maine Power Co. in federal court this week, alleging it engineered a bid-rigging scheme in the tens of millions of dollars aimed at increasing payouts from ratepayers. Avangrid, a U.S. subsidiary of the Spain-based Iberdrola, categorically denied the allegations contained in the bombshell lawsuit from Pennsylvania-based Security Limits Inc., calling its CEO “a disgruntled subcontractor” who was angry that he did not win more business. The lawsuit comes at a highly sensitive time for Avangrid and CMP.