Electrical Distribution System Arrangements

It is necessary to ensure that the distribution of electric power from centralized generation stations to individual households and companies is carried out in a manner that is both profitable and efficient. 

The electric power distribution system, that is an essential component of the overall infrastructure of the electric grid, is the individual responsible for carrying out this duty. 

For this purpose, there are several types of distribution systems that are deployed, including the radial distribution system representing one of the layouts that is applied the most frequently.

What is an Electric Power Distribution System?

An electrical power distribution system is the infrastructure that delivers electricity from transmission system to individual users. 

Transmission lines feed power into distribution networks, which then transport it to 

• Industrial, 
• Commercial, & 
• Residential consumers. 

The basic function of an electric power distribution system in an organization or structure is to collect power from one or more supply points and distribute it to lighting, motors, chillers, elevators, and other electrical loads.

Several frequently utilized electrical distribution schemes are described below

There are many types of distribution systems that depend on connection configurations: 

1. Radial Distribution System
2. Expanded Radial System
3. Radial System with Primary Selectivity
4. Primary and Secondary Simple Radial System
5. Primary Loop System
6. Secondary Selective System
7. Primary Selective System
8. Sparing Transformer System
9. Secondary Spot Network
10. Composite Systems
11. Ring Main Distribution and
12. Interconnected Systems.

Radial Distribution System

A radial distribution system is a type of electricity distribution network in which each circuit originates from a single source or substation & branches out radially to serve individual loads and customers. 

Power flows in one way from the source to the loads, with no other supply path available. 

Radial systems are popular because of their simple, easy design and cheaper starting costs compared to networked arrangements.

The radial system is the most basic electrical distribution design & is not particularly expensive in terms of equipment startup cost. It is also the least stable configuration because it only has one utility source.

The traditional simple radial system gets power at the utilities supply voltage from a single substation & steps it down to the utilization level. A loss of service will occur if the utility source, transformer, (or) service or distribution equipment fails.

Furthermore, loads must be turned off in order to do system maintenance. This configuration is most typically utilized when the necessity for low initial cost, simplicity, & space efficiency outweighs the need for increased reliability.

This system architecture typically includes a single-unit substation with a fused primary switch, a transformer large enough to power the loads, & a low-voltage switchboard.

Expanded Radial System

One of the key advantages of simple radial system is that it may be simply expanded by adding additional transformers. As the number of substations expands, reliability improves since the loss of one transformer does not result in the interruption of service for all loads.

To reduce the voltage drop, those extra transformers might be placed at the middle of each set of loads. If the failure of a transformer (or) feeder does not result in the interruption of service to a portion of the facility, a more dependable system architecture is necessary.

Radial System with Primary Selectivity

When 2 utility sources are accessible, radial systems with primary selectivity offer almost the same economic benefits as simple radial systems with improved dependability, as the failure of a single utility source does not result in a complete loss of service.

Although the utility sources are paralleled, there will be a temporary outage between the loss of the primary and the changeover to the alternate. 

The transformer failure (or) distribution equipment remains to result in a loss of service.

An automatic transfer method between the 2 primary sources can switch a failing utility source to an accessible one. 

For primary system maintenance, all loads must be turned off.

Primary and Secondary Simple Radial System

An improved version of the standard basic radial system distributes power at a primary voltage. In certain load zones, the voltage is reduced to the utilization level using secondary unit substation transformers.

Each secondary unit substation is a fully assembled unit that includes 

• A transformer, 
• An interconnected primary fused switch, & 
• Low-voltage switchgear (or) switchboard. 

• Circuit breakers or 
• Fused switches 

connect circuits to each load.

A fault in a primary feeder circuit (or) in a single transformer will only affect the secondary loads handled by that feeder or transformer. 

In the event of a major primary bus fault (or) a utility service outage, service is suspended to all loads until the problem is resolved.

Because electricity is supplied to the load areas at a primary voltage, losses are decreased, voltage control is improved, and, in various cases, the load circuit breakers’ interrupting duty is reduced.

Primary Loop System

This distribution design is made up of one or more “Primary Loops” that are connected to two or more transformers. 

This technique works effectively when the utility provides two services.

The fundamental advantage of the loop system over radial designs is that if one transformer or feeder cable fails, no part of the facility will lose service, & one feeder cable will be maintained without resulting in a loss of service.

To avoid concurrent functioning of the sources, each primary loop is configured so that one of the loop sectionalizing switches remains open. When a feeder cable fails, a portion of the system will experience an outage until the loop is switched to compensate the cable’s loss.

By operating the necessary sectionalizing switches, any part of the loop conductors can be disconnected from the remainder of the system. To prevent the loop’s sectionalizing devices from being closed, a key interlocking scheme is typically used.

Secondary Selective System

The secondary selective system is another way to keep a distribution system operational once one component fails. In these system, each transformer secondary is coupled in the standard double-ended unit substation configuration.

To avoid parallel operation, each unit substation has two secondary main circuit breakers & a secondary tie breaker that are mechanically or electrically interconnected. If the secondary source voltage is lost on one side, the loads can be transferred to the opposite side, restoring power to all secondary loads. This can be accomplished manually (or) automatically.

Primary Selective System

If a single primary source is employed in the secondary selective system, any voltage drop at that source would cause the system to fail completely. To increase dependability, duplicate sources from the power supply point should be used, along with two primary main circuit breakers & a primary tie breaker.

A primary selective system consists of two primary main breakers & a primary tie breaker that are mechanically or electrically linked to avoid parallel operation. If one side’s primary voltage supply fails, manual or automatic transfer can be employed to restore power to all primary loads.

Metal-clad switchgear is most typically utilized with this type of setup due to the restrictions of metal-enclosed load interrupter switches. Secondary radial (or) selective systems may be coupled with the primary selective arrangement to form a composite system.

Sparing Transformer System

The transformer sparing scheme is a larger-scale implementation of the secondary selection system. It effectively substitutes double-ended substations with the single-ended substations plus one or more “sparing” transformer substations, which are all connected via a common secondary bus.

This particular type of electrical distribution system provides a high degree of switching flexibility. If a substation transformer fails (or) is turned off for maintenance, the sparing transformer provides power to one load bus.

All major breakers, including the spare main breaker, are generally closed, whereas the tie breakers are (NO) normally open. A transformer is disconnected from the circuit by opening the secondary main breaker & closing the tie breaker, allowing the spare transformer to serve its loads.

When connecting many transformers in parallel, exercise caution because the fault current increases with each paralleled transformer, and directional relaying on the secondary main circuit breakers is required to selectively isolate a failed transformer.

An electrical (or) key interlock method is essential to ensure the proper working modes of this sort of system, especially as switching occurs across multiple pieces of equipment that may be in various places. An automatic transfer technique can be used to transition between a failed & an available transformer.

Secondary Spot Network System

Secondary network systems are commonly utilized in high-density locations where huge loads must be supplied while maintaining a high level of reliability. In this configuration, many electrical services are paralleled at low voltage, resulting in a highly dependable system.

The key advantage of secondary network system is the continuity of service. No one fault on the principal system will disrupt service to any of the system’s loads.

Network protectors are specially designed circuit breakers that are installed on the transformer secondary to isolate transformer faults that are fed back into the low voltage system. Most issues will be resolved without disrupting service to any load.

The ordinary secondary bus is frequently referred to as the “collector bus.” Secondary Spot Network systems are widely utilized in hospitals, high-rise office buildings, & institutional buildings that demand a high level of service reliability from utility sources.

Composite Systems

The system arrangements outlined above are the fundamental building blocks of power distribution system topologies, however they are rarely employed alone in a given system. To improve system reliability, it is frequently required to combine two or more layouts.

As reliability rises, so do complexity and cost. Economic concerns typically govern how complicated a system arrangement may be used, which has a significant impact on the system’s reliability.

Ring Main Distribution System

A ring main distribution system consists of feeders connected in a closed loop (or) ring form, with distributors tapped at various points along the ring. The distinctive characteristic is the development of a continuous loop network with no ending end in the feeders. 

Distribution transformers scale down the power and supply loads through distributors tapped from rings. The ring main configuration enables each distributor to get its supply from two routes, ensuring redundancy in the event of a feeder failure.

Ring Circuit Breakers form an outside ring, and Ring Feeders are connected radially to it. 

To provide consumer loads, distribution transformers & distributors are tapped off of ring feeders. Sectional Ring Main Units on the feeders enable the isolation of problematic sections.

Interconnected Distribution System 

The distribution feeder network of an integrated distribution system receives power from two or more substations (or) generation units.

Some important points concerning interconnected distribution systems:

• It enables redundant power delivery from various sources, ensuring uninterrupted power supply even if a single source fails due to an outage or maintenance.
• Substations often activate closed loops or rings of the distribution network from two distinct places. This looped structure creates backup power paths.
• Sectionalizing switches allow sections of distribution ring to be separated quickly in the event of a malfunction, without affecting supply to other areas.
• Load balancing can be achieved across sources by dynamically managing power flows from each location. This increases the usage of generation assets.
• Voltage regulation is more efficient since it may be coordinated across several injection points in an interconnected network.
• The power supply is substantially more reliable than typical radial (or) ring main networks, which rely on a single source.
• Interconnected distribution is often utilized for essential loads that cannot withstand power outages, such as process industries, data centers, and urban regions.
• Advanced monitoring and automation allow to optimize benefits while overcoming complexity in operation & maintenance.