Bus Duct

Do you mean "between generator and step-up transformer"? From the step-up transformer to High voltage Switchgear or overhead line we use cables, indeed.

Usually from generator to step-up transformer -or to and from generator breaker-you could use cables and terminations but many cables in parallel. Let's say for a 300-700 MVA generator at 20 kV the rated current is 8-20 kA. Ampacity of a 1000 MCM copper cable in a cable tray-10 ft. wide- is 580-600 A. Then you need 15 up to 35 parallel cables per phase [up to 100 single-core cables] provided with 200 terminations and terminals and so on.

Because "simple cable" is too simple for something as complex as the interconnection of a generator to the grid. First there's the reliability question, whenever there are multiple terminations per phase of paralleled cables there's the problem of a poorly made connection causing an unbalanced current flow through the other connections/cables.

This is a degenerative situation which can lead to cascading failures as others now have to pick up excess current, which in turn leads to overheating/failure of the next poorest joint, things can degrade very rapidly from there and lead to a unit outage.

Another not so simple consideration is how to balance the current flow through all those cables given the physical spacing required to arrange and keep cool all those conductors.

But the real reason is revealed when a professional performs a FMEA (Failure Mode and Effects Analysis) and considers what happens when there is a fault, particularly a short circuit on the circuit between the generator terminals and the GSU (Generator Step-Up) transformer. The mechanical forces on those conductors and their supports are tremendous, leading to mechanical stresses that can permanently distort the conductors and/or break them free from their supports.

Parallel conductor systems are particularly vulnerable to conductor "galloping" whereby the magnetic forces due to a fault cause the unrestrained cables to break free and whip around ferociously for the duration of the fault. A properly designed bus duct system is mechanically robust to withstand those forces and keep the phases from contacting each other or ground and causing additional faults, it also has provisions for normal cooling and containing any arc products from causing any further damage to other equipment and/or personnel.

Which would you rather sign-off on, a pretested assembly with 6 bolted connections, or one with say 6 cables per phase for a total of 36 crimps and 36 bolted connections?