Distribution Systems

Table of Contents
1. Introduction
2. High-voltage (Primary) Distribution
3. Low-voltage (Secondary) Distribution
4. Main Elements of a Distribution System
5. Common Methods of Distribution
6. Conductor Types and Arrangements
7. Corona: Causes, Effects and Mitigation
8. Voltage Levels and Phase Relationships
9. Performance Parameters and Practical Considerations
10. Practical Applications and Design Choices
11. Summary
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Introduction

The conductor system by means of which electrical energy is conveyed from bulk power sources (generating stations or major substations supplied over transmission lines) to consumers is divided into two broad parts: high-voltage (primary) distribution and low-voltage (secondary) distribution. The choice of voltages, conductor arrangement and system configuration depends on the amount of power to be conveyed, the distance, the nature of loads and the need to limit losses, voltage drop and electromagnetic interference.

High-voltage (Primary) Distribution

Power from generating stations is normally transmitted over extra high tension transmission lines at voltages typically in the range 33 kV to 765 kV. At receiving substations the transmission voltage is stepped down by transformers to distribution voltages such as 11 kV, 6.6 kV or 3.3 kV (11 kV being the most common). Power at these voltages is carried to distribution substations and large bulk consumers. This part of the network is referred to as the primary distribution system. The selected primary distribution voltage depends on power levels and the distances between substations and load centres.

Functions of Primary Distribution

• Carry bulk power from transmission substations to distribution substations and large industrial consumers.
• Provide points for voltage transformation and network sectionalisation.
• Limit transmission losses by operating at higher voltages.
• Facilitate protective relaying and system stability by zonal separation.

Low-voltage (Secondary) Distribution

At distribution substations the voltage is stepped down for final delivery to consumers. A common supply level used historically is 415 volts between phases and 240 volts between phase and neutral in a three-phase star-connected secondary network. From these substations low-voltage distributors radiate out and feed individual consumers; this network is known as the secondary distribution system.

Typical Secondary Equipment and Supplies

• Distribution transformers that step down primary distribution voltages (e.g., 11 kV) to low-voltage levels for consumer supply.
• Low-voltage distributors (overhead or underground) that carry 3-phase supplies.
• Service mains and service connections that deliver power from the distributor to the consumer's meter and installation.
• Neutral and earthing systems to ensure safety and correct operation of protective devices.

Main Elements of a Distribution System

• Substation - contains transformers, switchgear and protection to step down and distribute power.
• Feeders - conductors that connect substations to distribution areas or major loads; usually radial or ring type.
• Distributors (or distribution mains) - lines which supply groups of consumers; they carry reduced-voltage power from the feeder/substation.
• Service connections - final conductors from the distributor to the consumer installation.
• Switchgear and protection - circuit breakers, fuses, relays and isolators to protect equipment and ensure selectivity.

Common Methods of Distribution

• Radial system - simple, low cost; each consumer or distributor is fed from one direction. Faults cause interruption to downstream supply.
• Ring main system - a looped feeder that supplies distributors from two directions improving reliability and allowing maintenance without interruption.
• Interconnected (network) system - many interconnections between feeders and substations; high reliability and good voltage regulation for dense urban areas.
• Double-main and main-and-tap systems - variants for industrial/railway applications where higher reliability or segregated supplies are required.

Conductor Types and Arrangements

Conductors used in distribution and transmission lines may be single solid conductors, stranded conductors, composite conductors or bundle conductors. Selection depends on mechanical strength, electrical performance, corona behaviour and economic considerations.

Bundle Conductors

Definition: A bundle conductor consists of two or more subconductors per phase kept separated from each other by a nearly constant distance for the full length of the line using spacers. Typical spacing between subconductors lies in the range 0.2 m to 0.6 m depending on the line voltage and environmental conditions.

Difference from a composite conductor: The subconductors of a bundle conductor are purposely separated from each other by spacers; a composite conductor has its wires in contact with one another.

Advantages of Bundle Conductors

• Reduction of corona loss and radio interference: Increased effective conductor diameter reduces electric field intensity at the surface, lowering corona inception and associated audible noise and RFI.
• Reduced reactance (inductance) per phase: The external magnetic field distribution changes so that line inductance per phase is reduced.
• Increased capacitance to neutral: Spreading the phase charge over several subconductors increases the line capacitance, giving higher charging currents.
• Improved transmission efficiency: Lower reactance reduces voltage drop and line losses for the same delivered power.
• Higher maximum power transfer capability: Lower surge impedance and reduced reactance allow higher transfer of active power for a given voltage.
• Improved power factor (under certain conditions): Higher charging currents associated with increased capacitance may help to offset inductive loads and improve system power factor locally.

Remarks on Surge Impedance and Line Parameters

The characteristic or surge impedance of a transmission line is given by the relation Zs = √(L/C), where L is the per-phase inductance and C is the per-phase capacitance to neutral. For bundle conductors the effective L decreases and C increases, therefore Zs reduces, which helps increase the maximum power transfer capability and changes the voltage profile along the line.

Corona: Causes, Effects and Mitigation

• Cause: High electric field near conductor surface when the local field exceeds the dielectric strength of air; intensified by small conductor radius or sharp points.
• Effects: Audible noise, radio interference, power loss (corona loss), ozone generation and possible surface damage to insulators under discharges.
• Mitigation: Use of larger effective conductor diameter (bundle conductors), smooth conductors, corona rings on terminals and optimal conductor spacing.

Voltage Levels and Phase Relationships

In a three-phase star-connected secondary system the line-to-line voltage VLL and line-to-neutral voltage VLN are related by the square root of three:

VLN = VLL / √3.

Using the commonly cited example:

VLL = 415 V.

VLN = 415 / √3 ≈ 240 V.

Performance Parameters and Practical Considerations

• Voltage regulation: The change of voltage between the receiving and sending ends under load; improved by lower line reactance and suitable reactive compensation.
• Losses: Technical losses (I2R copper losses, magnetic losses in transformers, corona, dielectric losses) and commercial losses (metering error, theft, pilferage).
• Power factor control: Capacitor banks are commonly used near distribution substations or large consumers to improve the power factor and reduce line currents.
• Protection and selectivity: Coordination of fuses, circuit breakers and relays is required so that only the faulty section is isolated while supply to other consumers continues.
• Earthing and neutral arrangements: Correct neutral earthing at substations and distribution transformers is essential for safety, fault current return path and protection operation.

Practical Applications and Design Choices

• For long distances and large bulk power transfer use higher primary distribution voltages and, where required, bundle conductors to reduce corona and losses.
• For dense urban networks prefer ring or interconnected systems to improve supply reliability and simplify maintenance switching.
• For rural or lightly loaded feeders radial systems are economical and simpler to operate but have lower reliability.
• Select conductor type, spacing and insulation to balance initial cost, transmission efficiency, corona performance and mechanical constraints (wind, ice, sag).

Summary

A distribution system links transmission and consumers through a hierarchy of substations, feeders, distributors and service connections. The primary distribution operates at higher voltages (commonly 3.3 kV, 6.6 kV, 11 kV or higher) while the secondary distribution provides the final low-voltage supply to consumers (commonly 415 V line-to-line and 240 V line-to-neutral in the cited practice). Bundle conductors are used on extra-high-voltage lines to reduce corona, lower inductance, increase capacitance, improve transmission efficiency and raise the maximum power transfer capability. Proper arrangement of network topology, conductor selection, protection and reactive compensation is essential for efficient, reliable and safe distribution of electrical energy.