Primary and secondary power distribution systems are essential components of the electrical grid, each serving a specific function in the delivery of electricity from generation plants to end users.
Primary Power Distribution Systems:
These systems consist of feeders that deliver power from distribution substations to distribution transformers.
A feeder typically starts with a feeder breaker at the substation and may be routed underground or overhead to cover the service territory.
The primary system operates at medium voltages, usually ranging from 600 V to 35 kV2.
It can be configured as a radial system, which is common in low-density rural areas, or as interconnected networks for greater reliability.
Secondary Power Distribution Systems:
These systems take over from the primary distribution transformers and step down the voltage to levels suitable for end-user consumption.
Secondary distribution operates at lower voltages, typically 120 to 480 V, and supplies power directly to customers’ meters.
The layout is often radial, with each customer connected to a single source, ensuring a safe and usable form of electricity is delivered.
For an in-depth understanding of the layouts and components of these systems, one may consult technical articles and resources that offer thorough perspectives on the design and operation of these distribution systems. Comprehending these systems is vital for guaranteeing efficient and dependable electricity supply to residential, commercial, and industrial consumers.
Table of Contents:
1. Primary Distribution Systems
Primary distribution systems are composed of feeders that transport power from distribution substations to distribution transformers. Typically, a feeder starts with a feeder breaker located at the distribution substation. Numerous feeders exit the substation through concrete ducts and proceed to an adjacent pole. Here, the underground cable changes to an overhead three-phase main trunk.
The primary trunk is routed throughout the feeder service territory and can be linked to other feeders via normally open tie points. While underground main trunks are feasible and even typical in urban settings, they are significantly more expensive than overhead construction.
Lateral taps branching from the main trunk serve the majority of a feeder's service area. Typically, these taps are single-phase, although they can also be two-phase or three-phase. While laterals can be connected directly to main trunks, they are usually safeguarded by protective devices like fuses, reclosers, or automatic sectionalizers.
Overhead laterals employ pole-mounted distribution transformers to serve customers, while underground laterals utilize pad-mounted transformers. Feeder routes are designed to pass close to every customer. To achieve this, each substation operates multiple feeders to encompass the designated service territory.
Figure 1 displays an illustrative feeder, highlighting various types of laterals and devices.
The most basic primary distribution system is made up of independent feeders, with each customer linked to a single feeder. Due to the lack of interconnections between feeders, any fault will disrupt service to all customers downstream until the fault is fixed.
This setup is known as a radial system, frequently used in low-density rural regions where the implementation of more intricate systems is not cost-effective.
A somewhat more prevalent configuration involves connecting two feeders at their endpoints using a normally open tie switch. This primary loop configuration enhances reliability, as it enables customers downstream of a fault to regain power by opening a switch upstream and closing the tie switch. The only customers who remain without power are those located in the switchable section where the fault has occurred.
Numerous distribution systems feature several tie switches among multiple feeders, offering reliability advantages akin to a primary loop but with enhanced switching flexibility.
These primary distribution systems, known for their high interconnectivity, are termed radially operated networks.
Some customer categories demand a level of reliability that exceeds what a single feeder can offer.
Primary selective service links each customer to a preferred and an alternate feeder. Should the preferred feeder lose power, a transfer switch will disconnect it and connect the alternate feeder instead.
Secondary selective service yields comparable outcomes by utilizing switches on secondary voltages instead of primary voltages. For optimal reliability advantages, each distribution transformer in a secondary selective service must be capable of supplying the full load.
Spot Networks cater to customers demanding the utmost reliability. This setup links two or more transformers, powered by at least two separate feeders, in parallel to activate the secondary bus. To inhibit reverse power flow through the transformers, specialized network protectors equipped with sensitive reverse power relays are employed. Spot networks enable several component failures to transpire without any discernible effect on the customers.
Primary distribution systems are prevalent in central business districts and areas with high density. They are increasingly used in peripheral regions for substantial commercial services where the availability of multiple supply feeders is possible. Typical configurations of primary distribution systems are depicted in Figure 2.
2. Secondary Distribution Systems
A low-voltage network, also known as a secondary network, is a segment of electrical power distribution that transports electric energy from distribution transformers to the electricity meters of end users.
These secondary networks function at a low voltage level, usually corresponding to the main voltage of electrical appliances. The majority of contemporary secondary networks operate with an AC-rated voltage ranging from 100–120 or 230–240 volts and a frequency of either 50 or 60 hertz.
The operating voltage, the number of required phases (either three-phase or single-phase), and the necessary reliability determine the network's topology and configuration.
Electric power distribution systems aim to provide customers with dependable and high-quality electricity. The typical distribution system is composed of straightforward radial circuits, known as feeders, which may be situated overhead, underground, or as a mix of both.
Feeders extend from the distribution substation to deliver power to end customers, creating the medium-voltage or primary network. This network operates at a medium voltage level, usually between 5–35 kV. The length of feeders varies, spanning from a few to several tens of kilometres. They are designed to serve all customers within a specific distribution area, which means they frequently wind and branch out across the designated corridors.
Distribution transformers, also known as secondary transformers, are positioned along feeders and serve to step down the voltage from a medium level to a low level, which is appropriate for direct use by end consumers (mains voltage).
Typically, a rural primary feeder can supply up to 50 distribution transformers, which are distributed across a broad area, although this number can vary greatly based on the configuration. These transformers are located on pole tops, in cellars, or on designated small plots.
From these transformers, a low-voltage or secondary network extends to customer connections at their premises, which are outfitted with electricity meters.
Customers are linked to distribution systems through service drops. Those situated near a distribution transformer can have service drops directly attached to the transformer's secondary connections. Customers further away are served by extending a secondary main to facilitate service drop connections.
The two varieties of service connections are depicted in Figure 3 above.
Systems that use secondary mains are typically characterized by having fewer large distribution transformers instead of a multitude of smaller ones.
This approach may be cost-effective for areas with low load density and large lot sizes, yet it leads to increased ohmic losses and higher voltage drops. While more exposed lines may decrease reliability, having fewer transformers can enhance it.
Numerous underground systems link service drops straight to distribution transformers, bypassing the need for secondary mains. This necessitates placing distribution transformers within a few hundred feet of each customer, thereby removing the reliability issues tied to the T-splices needed for connecting underground service drops to secondary mains.
2.1 Configuration of Secondary Distribution Systems
The configuration of secondary distribution systems can vary, but here are some common layouts:
Simple Radial Systems: A single path from the transformer to the customer. It’s cost-effective but less reliable since any fault can cause an outage for downstream customers.
Expanded Radial System: Similar to a simple radial system but with additional branches to serve more customers from a single transformer.
Primary Selective System: Offers more reliability by connecting the system to two or more transformers. If one fails, the system can switch to another without interrupting supply.
Primary Loop System: Forms a loop of the feeders, allowing for an alternative path for electricity if one part of the loop fails.
Secondary Selective System: Uses Main–Tie–Main (MTM) circuit breakers to allow for automatic switching between sources in case of a fault.
Ring Bus System: A variation of the loop system where each customer is connected to two transformers, enhancing reliability.
Spot Network: Used in high-density areas like city centres, where multiple transformers are connected to a single point, providing high reliability and quality of power.
Every configuration offers its own benefits and is selected according to criteria such as reliability, efficiency, cost, and operational complexity. For comprehensive diagrams and in-depth explanations, consulting technical resources or engineering textbooks on electric power distribution is advisable.
2.1.1 Radial Networks
Radial operation represents the most common and cost-effective design for both medium voltage (MV) and low voltage (LV) networks. It ensures an adequate level of reliability and service continuity for the majority of consumers.
In American 120V systems, customers typically receive power directly from distribution transformers through relatively short service drop lines arranged in a star-like topology. In contrast, 240V systems serve customers through multiple low-voltage feeders, which may be a combination of overhead lines, aerial cables, or underground cables.
In overhead networks, service drops extend from the tops of poles to roof connections. In cable networks, essential connections and protective devices are usually housed in pad-mounted cabinets or sometimes in manholes, as buried T-joint connections tend to be failure-prone.
Designing and implementing power-system protection in radial networks is straightforward because short-circuit currents are limited to a single path that requires interruption.
Fuses are typically employed for protection against short-circuits and overloads, whereas low-voltage circuit breakers are utilized under specific conditions.
2.1.2 Spot Networks
Spot networks are utilized to enhance the reliability of supply for critical customers. The low-voltage network receives power from two or more distribution transformers located at a single site, with each transformer connected to a distinct medium-voltage (MV) feeder that may come from the same or different substations.
Transformers are interconnected on the secondary side using a bus or cable, known as a paralleling bus or collector bus. Typically, the paralleling bus lacks connecting cables to other network units, defining such configurations as isolating spot networks. Conversely, when connecting cables are present, these configurations are identified as spot networks with reach.
In certain situations, fast-acting secondary bus tie breakers can be utilized between bus sections to isolate faults within the secondary switchgear, thereby minimizing service disruption.
Spot systems are frequently utilized in areas with high load density, including business districts, major hospitals, small industries, and critical facilities like water supply systems.
During regular operations, the energy supply is delivered in parallel by both primary feeders. If one primary feeder experiences an outage, the network protector at the corresponding spot transformer's secondary side will automatically disengage. The other transformers will then persist in supplying energy via their respective primary feeders.
Customers will only experience a service interruption if the short circuit occurs at the paralleling bus or there is a complete loss of the primary supply. Faults within the low-voltage network are managed by fuses or local circuit breakers, which isolate the issue, limiting the service disruption to the loads directly affected.
2.1.3 Grid Networks
Grid networks consist of an interconnected grid of circuits, powered by multiple primary feeders through distribution transformers at various locations. These networks are commonly found in the downtown areas of large cities, where connecting cables are installed in underground conduits along the streets.
A multitude of cables provides various pathways for current to flow from each transformer to every load in the grid.
Similar to spot networks, network protectors serve to guard against faults in primary feeders and prevent the propagation of fault current from the grid to the primary feeder.
Individual cable sections can be safeguarded by cable limiters at both ends, which are special fuses designed for rapid short-circuit protection. The built-in redundancy of the system typically ensures that customers do not experience outages.
Sources:
Electric Distribution Systems by Abdelhay A. Sallam and Om P. Malik
Electric Power Distribution Handbook by Short, T.A.
