Replacement of the Generation Circuit Breaker at 23th Outage of Cofrentes NPP

Tlhe Generation Circuit Breaker has the function of isolating the output of the Main Generator from the Main Transformer and the Auxiliary Transformers, allowing the Plant to be fed from the external 400 kV network when the Generator is not working. The BBC (currently ABB) model DR 36 u 2500 D Circuit Breaker was installed at C.N. Cofrentes in 1997, being located in a building next to the Turbine building. Due to the declaration of obsolescence by the supplier of this Breaker model, its change was proposed to ensure the maximum reliability of the Plant’s Generation systems in the coming years. The new Breaker (model FKGA8 from GE Grid Solutions) maintains the electrical characteristics of the previous one, with a short-circuit capacity of 250 kA and the capability to operate with a rated nominal current up to 39 kA. Each pole of the Circuit Breaker is equipped with a mechanical drive for opening and closing and an SF6 chamber to extinguish the electric arc that is created during the opening and closing maneuvers of the Circuit Breaker. Due to the greater weight of the new Circuit Breaker, it has been necessary to verify the validity of the metallic structure of the building, modeling it completely and evaluating the stresses caused both by the weight of the Circuit Breaker and by the dynamic stresses that occur during the actuation maneuvers of the Circuit Breaker. The calculation concludes that the structure is valid and it´s not necessary any modification of the structure. The dimensional difference between the new Breaker and the existing one has made necessary to modify the design of the connection of the Breaker with the Isolated Phase Buss, being manufactured and installed new sections. The replacement of the Breaker have been carried out in a 23-day period at 23^th Outage of Cofrentes NPP (November and December 2021). The main phases of the assembly work have been:

1. Disconnection of the flexible connections between the Circuit Breaker and the Isolated Phase Bus and electrical and mechanical disconnection and removal of the Control Panel and Air and Water Auxiliary Systems.
2. Removal (with a 60 Tm crane) of the 3 Poles of the Breaker. For all the maneuvers it has been necessary the partial dismantling of the roof of the Building, being reinstalled at the end of each maneuver.
3. Cutting and removal of the 3 Baseplates on which the existing poles were mounted.
4. Cutting and removal of the connection sections of the Isolated Phase Bus with the Breaker.
5. Preparation of the floor and introduction with the crane of the new single Baseplate for the 3 new poles.
6. Introduction and positioning of the new sections of Isolated Phase Bars.
7. Introduction of the new poles with a 90 Tm crane.
8. Verification of the alignment Breaker-Isolated Phase Bus assembly and after that, complete welding of the new sections of Insulated Phase Busbars (conductor and enclosure).
9. Introduction of the new Control Panel.
10. Performance of both  SAT (Site Acceptance Tests) and functional (interface with the Plant) tests, after which the flexible connections between the Circuit Breaker and the Isolated Phase Bus were connected, leaving the Circuit Breaker ready to operate.

In the Design and Assembly phases, an exhaustive analysis of interferences, maneuvers and impacts has been carried in order to guarantee the successful execution of the works. The work and the involvement of all the areas of the Plant (Engineering, Maintenance and Operation), has made possible the success of the Project with the installation in Cofrentes NPP of a state-of-the-art Breaker, which will contribute to guarantee in the future the operation of the Plant with the maximum reliability and safety.