In Chapter 1 of Power Systems Analysis (Grainger/Stevenson), the authors present a sequence of representations of the same power system (Fig 1.25 --> Fig 1.26 --> Fig 1.27). I am confused by the last of these three.
Why is the neutral line in the third figure labelled "neutral bus" (shown in a red circle)? A bus is quite different from a line, right? Surely 200-mile-long busbars don't exist, do they?
Schematic diagrams do not reflect physical layout. I think all that is meant by calling it a "bus" is that for purposes of analysis we are going to assume that the voltage is the same everywhere on the line (always the case in schematic diagrams).
I guess what they are trying to convey is that all the impedances that the authors wish to consider have been "rolled up" into the upper conductor, so now the neutral can be treated like a bus.
I don't think you should get hung up on this point for the moment.
It's a silly figure of speech. Unnecessarily confusing, but of course it's not a busbar. It's a bus implemented with a cable. And then calling it a bus is a bit weird indeed.
The idea is, though, that since all the impedances are in series, you can have an equivalent circuit with zero impedance in the neutral line. You just shift them around :)
You see where transformers "step up" the voltage for the long 200-mile transmission line.
You know in 3-phase there is "wye" (4-wire, neutral in the middle) and "delta" (3 wires, corners only, no neutral, ground in the middle). Delta can be derived from wye simply by not hooking up the neutral.
In the real world, that 200-mile transmission line will be wired as "delta". Why bring a 4th wire when you don't need to?
Also, high-tension line insulation stacks are expensive. They want to keep the phase-ground voltage as low as possible. So they don't want the 3 phases floating at random voltages to ground. They want to equipotential bond the center of the delta triangle to earth ground. Easiest way to do that is use "wye" transformers, which is what Figure 1.25 shows in fact.
So even though they aren't transmitting neutral, an imaginary "neutral" can be inferred from the connection, and correlates roughly with earth ground potential. Figure 1.27 is asking you to "think about" only 1 of the phases. As such, its reference is the "imaginary neutral", which being imaginary, has no voltage drop.
And that is accurate. In practical home wiring, when you wire a 240V appliance or panelboard with neutral for 120V loads... when calculating voltage drop on the feeder/circuit, you consider the outer phase conductors only. Neutral current is presumably minimal if the panel has been competently balanced.
The diagram is talking about a single phase, and in that context the return current flows through the neutral.
In real life, it's a three-phase transmission line and there isn't any continuous neutral. The return current flows through the other two phases instead.
A bus is just a conductor (or set of conductors) that connects more than two other things.
From Power Systems Analysis (Grainger/Stevenson) with reference to Figure 1-27:
The per-phase impedance and reactance diagrams discussed here are sometimes called the per-phase positive-sequence diagrams since they show impedances to balanced currents in one phase of a symmetrical three-phase system.
Figure 1-25 is a single line diagram of a three-phase system.
Figure 1-26 is an extracted per-phase (single phase of three-phase) for analysis.
Figure 1-27 is the circuit simplified to a single-phase circuit only concentrating on key components.
Source: SSR and falstad.
A 4-wire balanced three-phase circuit can be replaced by three equivalent single-phase circuits for analysis.
One being: Leave source to C, to c, through load to n, to N and back to source. Follow the blue line.
But in a three-phase distribution system, there is no neutral wire, but that does not stop the authors from extracting a single-phase (per-phase) circuit for easier analysis.
They cannot call it a neutral wire, so they call it a neutral bus, where they use the earth as the neutral, but no current actually flows through the neutral bus, since the load is balanced. So it is an illusion to allow analysis. Determine current magnitude and phase angle and then extrapolate it to the three-phase world.
