Bulgaria's Electricity System Operator (ESO) said that it has signed eight framework agreements for the overhaul of power substation equipment and infrastructure, worth up to 10 million levs ($5.6 million/5.1 million euro) each.The contracts were landed by four local tie-ups, each of which inked two agreements, ESO said in a notice on Tuesday.Four of the deals, concerning 110 kV power substations, were signed on September 9, while the other four were inked on August 30.One of the winning tie-ups - Rudin-Start, comprises local companies Rudin and Start Engineering. Another winning bidder - Ruesta tie-up, includes local firms El-Test, Etra-El and Elia. Elektroluks Tabakov i sinove and Streza form the third winning tie-up, while the last successful bidder comprises T&D Engineering, El Control and Emu.