- Electrical Power Distribution System Definition: An electrical power distribution system is defined as a network that delivers power to individual consumer premises at a lower voltage level.
- Components of Distribution Networks: Distribution networks consist of distribution substations, primary distribution feeders, distribution transformers, distributors, and service mains.
- Radial Power Distribution System: This system has feeders radiating from a substation but can cause power interruptions if a feeder fails.
- Ring Main Power Distribution System: A ring main distribution system uses a ring network of distributors fed by multiple feeders, providing continuous power supply even if one feeder fails.
- Section Isolators: These devices in ring main systems isolate parts of the network for maintenance or faults, maintaining power supply to other parts.
What is an Electrical Power Distribution System?
An electrical power distribution system delivers power from substations to customer premises. It carries electric power at medium and low voltage levels, below those normally used to transfer bulk power over long transmission lines.
A distribution network commonly includes the following parts:
- Distribution substation
- Primary distribution feeder
- Distribution transformers
- Distributors
- Service mains
At a distribution substation, transformers reduce transmission or subtransmission voltage to the primary distribution level.
Primary distribution feeders carry this power from the substation to each distribution transformer or other distribution point. A feeder may use overhead lines, underground cables or both.
Overhead conductors may be bare, covered or formed into aerial cable systems. Their material, support and insulation depend on the voltage, environment, utility standard and required capacity. Underground cables are also used where routing, safety, reliability or planning requirements favour them.
Distribution transformers may be single-phase or three-phase and may be mounted on poles, pads or inside buildings and underground structures. Their secondary circuits supply distributors or local service connections, depending on the network design.
Service mains connect customers to a distributor or another local distribution circuit. Some systems use sub-distributors to supply groups of service mains. The exact terms and boundaries vary by utility and jurisdiction.
The service connection point depends on the network layout, customer demand, protection design and utility connection rules.
In the terminology used here, a feeder carries power from a source to distribution points, while a distributor has multiple taps that supply customers. Utilities may use these terms differently.
A feeder in this simplified definition has no intermediate customer taps. Equipment along a real feeder may allow voltage to be stepped down or stepped up, while branches, shunt equipment, distributed resources and losses can change the current along the circuit. Sending-end and receiving-end current should not be assumed equal.
A distributor supplies loads at several points, so its current normally changes along the circuit as power is delivered or injected.
Radial Electrical Power Distribution System
A radial feeder has one normal source and one normal power path from the substation towards its loads.
A radial electrical power distribution system is simple to protect and operate, but a fault on the only available path interrupts customers downstream until protection, repair or switching restores a healthy section.
A failed transformer or feeder section can interrupt the customers it supplies. Protective devices isolate the fault. If the network has switches and a tie to another source, operators may restore some healthy sections before the failed equipment is repaired.
Ring Main Electrical Power Distribution System
A radial electrical power distribution system can gain more restoration options when it is arranged as a ring main electrical power distribution system.
In this network topology, the circuit forms a loop with switches that divide it into sections. A ring may have one or more supply points and may operate with a normally open point. After a fault or planned outage, operators can isolate the affected section and may resupply healthy sections from the other direction. Restoration depends on switch locations, protection coordination and the spare capacity of the remaining path, so continuous supply is not guaranteed.
Ring main systems use sectionalising switches at selected points. Protection first clears a fault, then suitable switches can isolate the affected section before healthy sections are re-energised where the design permits it.
This arrangement can reduce the number of customers affected by a section outage. The number and rating of supply points connected to the ring main electrical power distribution system depend on factors such as the following.
- Maximum Demand of the System: The supply points and circuits must carry normal and permitted contingency loads without exceeding their ratings.
- Total Length of the Ring Main Distributors: Circuit length and conductor impedance affect losses and voltage drop, which can influence conductor size, voltage level and supply-point placement.
- Required Voltage Regulation: The design must keep customer voltage within the applicable limits during normal operation and specified contingencies.
Distribution transformers, sub-distributors and service connections are placed at suitable points according to the loads, route, voltage level and protection scheme.
A sub-distributor may supply a group of service mains where a direct connection to the main circuit is unsuitable or not permitted.
