Refer to figures, Figure 1 is a split bus system, Figure 2 is parallel bus system.
Learning Objective 021.09.04.03.02: Describe the following distribution consequences: power transfer between different power supplies; power transfer in the event of a supply failure; loss of all normal AC supplies.

The two primary methods of distributing AC power in an aircraft are via the split bus and parallel bus systems.

Split bus systems have each generator feeding its own load bus, which are not usually connected at all. They run independently of each other, and usually have a TRU (Transformer Rectifier Unit) each to provide DC electrical power.

Parallel bus systems are naturally "paralleled" when possible, and have a synchronising bus (or sometimes 2) which means that many generators are all putting power into all the load busses. This system is most commonly used on aircraft with more than 2 engines, for simplicity.

In the event of a generator failure in a split bus system, the generator is cut off from its load bus and the BTB (Bus Tie Breaker) is closed to allow the working generator to feed both load busses. The APU can then be used to take over the failed generator and return the system back to normal if it is available. As this happens after generator cut-off, there is no need to parallel the supplies in normal service.

In a parallel bus system, a generator failure means that the failed generator will be cut off from the rest of the system, and the load busses will all be fed from the remaining generators that are already connected to it, so essentially the bus tie breaker is already closed, and the generator just needs to be disconnected via the GCB (Generator Circuit Breaker).

The batteries provide DC power, which can be put through an inverter to feed the essential AC bus, but this would only occur in the event of a multiple generator failure, where the batteries are the only remaining power source.