ISOLATED PHASE BUS BAR CIRCULATORY CURRENTS
The currents flowing in the conductors produce large magnetic fields around them. These fields induce currents in the enclosures and generate forces between all the con- ductors and the enclosures. The extreme ends of each phase enclosure are bonded together to reduce these forces. This allows a balanced current flow in the enclosures of the three phases (Fig. 14.2). This arrangement is called electrically continuous IPBB with short-circuit.
The enclosures are supported by insulated foot mountings to insulate them from the grounds. They are also grounded at one point. The circulating currents in the enclosures are controlled by isolating them from all the systems in the plant that are connected to the conductors. Rubber bellows are used to connect the enclosures to the systems in the plant. These bellows provide electrical isolation of the enclosures from the plant systems as well as physical protection of the conductors. The large magnetic field produced by the conductor currents can induce circulating currents and heat in the adjacent steel- work. This problem should be considered during the design phase because excessive heat is an unnecessary loss and can cause expansion that can be hazardous to personnel. All generating units having a rating lower than 660 MW use the naturally cooled main connections system (Fig. 14.3). Units having higher rating rely normally on forced- cooling designs.
