seegongsik
PW-C5 · Transmission and per-unit

Distribution systems: buying reliability with wire

Even when power is divided among the same customers, how the network is laid out greatly changes how far a single fault cuts off supply. Switch with the tabs from radial lines that branch one way, to loops joined into a ring, to mesh-like networks, and learn the trade of buying reliability at the cost of wires and switchgear.

Switch the grid and watch the outage area

With one and the same fault point (the red mark), the distribution network is switched. Under radial, everything downstream of the fault goes dark; under the loop, power is back-fed from the other side so only one faulted section drops; under the network, automatic rerouting means no one loses power. The lit dots are customers receiving power.

The tabs switch the distribution scheme. Out of 8 customers, 4 lose power under radial, 1 under the loop and 0 under the network.
Customers interrupted by a single fault
4 / 8
Branching one way only, everything downstream of the fault loses power. Cheapest, but least reliable.
Wide outage

Radial · simple and cheap

Power branches out in one direction only from the substation to reach customers. It uses the least wire, so it is cheap and protection coordination is simple, but a fault at any one point cuts off every customer connected below it. It is widely used in rural areas with low load density or where reliability demands are modest.

Loop · opening a second path

Joining the two ends of a radial line into a ring gives every customer two paths, left and right. Normally one point is kept open; when a fault occurs, only the faulted section is cut out from both sides and the open point is closed to feed power from the other direction, so the outage is confined to the faulted section and the other customers are restored quickly. It needs more wire and switchgear but greatly raises reliability, and it is common in urban distribution.

Observeradial: out = Ndown
Under radial, everything downstream of the fault loses power.
Chooseloop: out = ?
Under the loop, back-feeding cuts only the faulted section.
Fill inmesh: out = ?
Under the network, automatic rerouting means no outage.
On your ownpaths ↑ → cost ?
Raising reliability raises equipment cost.

Network · a mesh, no outage

Customers are tied together like a mesh by several paths from several substations. Even if one path fails, the remaining paths take up the load at once, so outages are rare, but wiring, protection and operation are the most complex and expensive, so it is used for city centers and critical loads that can hardly tolerate outages, such as hospitals and data centers. Meanwhile the service to customers divides into single-phase 2-wire (household lighting and outlets), three-phase 3-wire (motor power) and three-phase 4-wire (buildings that use single-phase and three-phase together); Korea's three-phase 4-wire 380/220 V supplies 380 V three-phase between lines and 220 V single-phase between a line and the neutral at the same time.

Back to the first screen

The fault was at one and the same point, yet under radial half of the downstream went dark, under the loop only one section, and under the network no one. What changed was not the fault but whether power had another path to take. Each extra path shrinks the harm of a single fault but raises the cost of wire and switchgear, so distribution design is in the end deciding how much reliability an area needs and at what cost to buy it.

Distribution systems: the network layout sets how far a single fault cuts off supply. Radial (one direction) is cheap but everything downstream of the fault loses power, the loop (ring) cuts only the faulted section by back-feeding, and the network (mesh) has no outage thanks to automatic rerouting but is the most expensive. More paths are a trade of reliability↑ for cost↑, and customer service divides into single-phase 2-wire, three-phase 3-wire and three-phase 4-wire.
The next step

So far we have looked at transmission and distribution, the paths that send and divide electricity. The next section D covers how power actually flows along those paths and how they are protected when something goes wrong. Its first unit (PW-D1) is power flow: once the voltage magnitude and phase of each bus are set, the active and reactive power flowing in each line follow, and we trace how that flow is pushed along by phase differences.