Electrical Distribution Systems

After electricity is generated, it is first transmitted over long distances through transmission systems, then stepped down to lower voltage levels and transferred to distribution systems. Distribution systems are the general term for the infrastructure that enables electricity to reach end users. These systems are designed to suit different usage points such as cities, towns, industrial areas, and rural regions.

An electricity distribution system consists of transformer stations, medium and low voltage lines, switching devices, feeders, and busbars. All these components work together to ensure electricity reaches consumers safely, continuously, and with high quality. The network design is shaped according to the locations of load centers, consumption density, geographical conditions, and safety requirements.

The primary objectives of a distribution system include ensuring continuous energy supply, maintaining voltage and frequency stability, minimizing intervention time during faults, and reducing losses. Additionally, various transformers and switching systems are used to convert the high-voltage electricity received from transmission lines into voltage levels suitable for end users.

Distribution Levels According to Voltage Levels

Electricity distribution systems are classified into four main categories based on the voltage levels used: low voltage, medium voltage, high voltage, and extra-high voltage networks. This classification determines the level at which transmission or distribution occurs and the types of equipment used.

Low Voltage (LV) Networks:

These networks operate at voltage levels between 1 and 1000 volts and typically extend from distribution transformers to end users. LV systems are located at the closest stage to residential areas and individual consumers because their insulation and protection are relatively simple. However, they are inefficient for transmission due to high voltage drop and power losses.

Medium Voltage (MV) Networks:

Medium voltage networks operate at voltage levels between 1 kV and 35 kV and are commonly used to supply energy to industrial zones, small cities, and large facilities. They serve as a bridge between high voltage transmission networks and low voltage distribution networks. In Türkiye, common voltage values are 6.3 kV, 10.5 kV, 15 kV, and 30 kV. The appropriate voltage is selected based on line length; for example, 3–10 kV is preferred for distances up to 10 km, while 20–35 kV is chosen for distances between 30 and 70 km.

High Voltage (HV) Networks:

These networks operate at voltage levels between 35 kV and 154 kV. Energy generated at power plants is transmitted to major cities or regional centers at these voltage levels. High voltage networks are used for transmission purposes rather than direct distribution because power losses are lower at these levels. In Türkiye, 66 kV and 154 kV are standard values.

Extra-High Voltage (EHV) Networks:

These networks encompass voltage levels above 154 kV. In Türkiye, 380 kV is used, while in some countries even higher voltages such as 500 kV and 750 kV are employed. These networks are preferred for large-scale transmission such as intercity and international energy transport.

These four voltage levels form an integrated structure that ensures electricity reaches end users with minimal losses and with system security maintained.

Types of Electricity Distribution Networks

Electricity distribution systems are categorized into different network structures based on the method of delivering electricity to consumers. These structures are selected according to factors such as energy continuity, safety, maintenance ease, and cost. Four main network types are primarily used: radial, ring, mesh (lattice), and interconnected systems.

Radial (Feeder) Networks

The radial structure is the simplest form of distribution, supplying power from the source to consumers through a single line. The network resembles the structure of a tree trunk extending into branches. Multiple consumers can be connected to a single line, but in case of a fault, the entire line may lose power.

Advantages of the radial system include low initial investment cost, easy voltage control, and simple planning and analysis. However, it has low supply reliability and technical limitations such as significant voltage drop.

Ring (Closed) Networks

In ring networks, power is supplied by multiple transformers, all connected in parallel. When a fault occurs at one point, only that section is disconnected, minimizing power outages.

Although ring systems are more expensive than radial systems, they provide higher security and energy continuity. However, future expansions can be costly because entire lines may need to be simultaneously upgraded.

Mesh (Lattice) Networks

Mesh networks are an advanced version of ring systems. Lines are interconnected like a lattice, allowing transformer and consumer connections via multiple paths. This structure ensures that during a fault, only the affected section is isolated while other areas continue operating.

Mesh systems offer high reliability and voltage stability. However, installation and maintenance costs are higher, and the impact of short-circuit currents can be more significant.

Interconnected Systems

An interconnected system is a large-scale network that links production and consumption centers across different regions. These systems can operate at national or international levels. Türkiye has interconnected connections with Bulgaria, Greece, Georgia, Azerbaijan, Iran, and Iraq. Additionally, Türkiye is linked to the European electricity system via ENTSO-E.

Advantages of interconnected systems include high efficiency, energy supply security, and flexible system response during faults. However, high short-circuit currents and challenges in maintaining system stability require careful management.

Busbar Structures Used in Distribution Systems

A busbar is a conductive structure that collects and distributes electrical energy at the same voltage and frequency. In electricity distribution systems, busbars serve as central hubs directing energy from inputs to outputs. Different busbar systems are used depending on the facility’s structure and requirements. The selection of busbars considers factors such as energy continuity, maintenance ease, cost, and safety.

Single Busbar System

This is the simplest and most commonly used system, where inputs and outputs are connected to a single busbar. Investment cost is low, but any fault on the busbar or circuit breaker causes the entire system to lose power. Therefore, safety levels are low and it is generally used in areas where outages are tolerable.

Single Busbar System with Bypass Isolators

Bypass isolators are added to the single busbar system to allow temporary connections without interrupting power during maintenance. Power supply can be maintained during faults, but the bypass line lacks protection and must be used with caution.

Transfer Busbar System

This system includes a main busbar and an additional transfer busbar. When maintenance or a fault occurs on the main busbar, loads are transferred to the transfer busbar to maintain system operation. Switching operations between circuit breakers must be carefully planned. It provides higher safety than the single busbar system.

Double Busbar System

This system features two active busbars, making it easy to transfer loads from one busbar to another. It is preferred in locations where energy continuity is critical. It enables load sharing, facilitates maintenance, and offers flexible operation. However, initial investment cost is high.

Double Busbar / Single Breaker System

This system operates with two busbars and a single breaker, allowing power supply from either busbar. However, if the busbar breaker fails, all loads may be disconnected. It provides high flexibility but complex risk management.

Ring Busbar System

This system connects busbars in a ring configuration. Each output has its own circuit breaker, facilitating maintenance. It can operate without a main busbar. It offers low investment cost but complex operation and protection systems.

Breaker-and-a-Half System

This system operates on the principle of one and a half circuit breakers per two busbars. It provides high supply reliability and flexible operation. Faults are confined to a single breaker. However, it has a complex control structure and high investment cost.

Busbars are typically made of copper or aluminum. In indoor installations, lamellar-type busbars are preferred, while in outdoor installations, pipe-type or steel-cored aluminum (ST-Al) conductors are used. Busbars transfer energy to consumers via outgoing lines called feeders.

Power and Distribution Transformers

Transformers, one of the fundamental components of electricity distribution systems, are used to convert electrical energy to suitable voltage levels according to demand. This transformation enables energy to be transmitted and utilized safely and economically.

Transformers are generally divided into two categories:

• Power Transformers
• Distribution Transformers.

Power Transformers

Power transformers are used in power plants and transmission centers where high-power energy flows occur. These transformers typically step up voltage at power plants to reduce losses during transmission and step down voltage at switching stations to match the levels required by distribution systems.

These transformers have power ratings ranging from 2500 kVA to 1000 MVA and operate at voltage levels between 36 kV and 1500 kV. In Türkiye, transformers operating at 154/33 kV, 380/33 kV, and 380/154 kV are common. Automatic tap changers that allow voltage adjustment under load are typically located on the high-voltage side. For cooling, systems such as OFAF (Forced Oil and Air Forced) are preferred.

Distribution Transformers

Distribution transformers are located near end users and convert medium voltage to low voltage. Their power ratings range from 25 kVA to 2500 kVA. The standard secondary (output) voltage in Türkiye is 0.4 kV. Voltage adjustment is generally performed manually, and automatic systems are not used.

Types of Distribution Transformers

Distribution transformers are structurally divided into two types:

• Oil-Immersed Type
• Dry-Type

Oil-Immersed Distribution Transformers

In this type, windings are fully immersed in transformer oil. Cooling is achieved through oil circulation. Oil-immersed transformers are divided into two categories:

• Hermetic Type: Completely sealed from the atmosphere, requiring no maintenance. Since the oil does not contact oxygen, its lifespan is longer. Preferred in compact spaces.
• Expansion Tank Type: The oil is exposed to air, requiring regular maintenance and oil replacement. However, it is more economical than hermetic types.

Dry-Type Distribution Transformers

These transformers use epoxy resin insulation instead of oil. They are preferred in places with high human density such as schools, hospitals, and shopping malls, as well as in indoor installations. They offer advantages such as fire resistance, no maintenance requirements, and no emission of harmful gases. They operate with AN (Air Natural) or AF (Air Forced) cooling systems.

Dry-type transformers can also be safely used in 100% humidity environments due to their moisture-resistant construction. They are protected against overheating through temperature control systems. Additionally, their power capacity can be increased using fan-assisted cooling.

Current Status and Applications of Distribution Systems in Türkiye

In Türkiye, electricity distribution is carried out under the Electricity Market Law and related regulations. Private distribution companies licensed by the Energy Market Regulatory Authority (EPDK) are responsible for providing electricity distribution services in designated regions. Transmission systems are managed by Türkiye Elektrik İletim A.Ş. (TEİAŞ), while distribution systems operate under a public-private partnership model.

Legal and Technical Infrastructure

According to regulatory amendments published in the Official Gazette in 2022, distribution systems have been separated from transmission systems, and the concept of the distribution network has been more clearly defined. Under this framework, the distribution network is defined as the infrastructure operating at distribution voltage levels (33 kV and below) that delivers electrical energy to consumers.

In Türkiye, 380 kV and 154 kV are standardized for transmission lines, while 33 kV is the standard voltage level for distribution lines. Installation of systems outside this value is not permitted in new projects.

Network Structure and Applications

In Türkiye’s major cities, ring and mesh network systems are common, while radial (feeder) networks are preferred in rural areas due to cost advantages. In particular, in city centers, systems such as double busbar and transfer busbar are employed to ensure uninterrupted energy supply.

Busbars used in electricity distribution are generally made of copper or aluminum conductors, with lamellar-type busbars preferred in urban applications. Additionally, transformer stations have been configured across different regions of Türkiye, and specialized energy load maps have been developed for each region.

Türkiye’s Emphasis in Energy Summits and Policies

Türkiye actively participates in national and international summits to shape its energy policies. At events such as Power Summit, topics such as modernizing distribution infrastructure, promoting local production, and ensuring energy supply security have been highlighted.

Additionally, Türkiye has strengthened its interconnected infrastructure by connecting to the European electricity system via ENTSO-E. This has positioned Türkiye as a key player in both energy trade and regional energy supply security.

Future Approaches and Transformations

Technological advancements in the energy sector today are driving transformation in electricity distribution systems. Rising energy demand, environmental awareness, and digitalization trends have brought the need to restructure distribution infrastructure to the agenda. In Türkiye, parallel to this transformation, various steps are being taken and next-generation applications are being encouraged.

Smart Grid Approaches

Unlike traditional distribution systems, smart grids are equipped with features such as data collection, analysis, automatic control, and remote monitoring. These systems enable analysis of consumption patterns, implementation of demand response management, and dynamic adjustment of load balance on the network.

Smart grids are also critical for integrating renewable energy sources. Safe integration of variable generation sources such as solar and wind energy is only possible through real-time data analytics.

Energy Efficiency and Resource Management

New technologies offer solutions to improve energy efficiency. Applications such as reducing loss and theft rates, expanding energy monitoring systems, and optimizing reactive power management enable more efficient operation of distribution systems.

Additionally, the lifespan, performance, and maintenance needs of equipment used in transformer stations and distribution lines can now be monitored through sensors. This allows intervention before faults occur, reducing operational costs.

International Integration and Energy Trade

Türkiye’s connection to the European energy grid via ENTSO-E is considered a strategic step that could enhance future electricity trade. Strengthening the interconnected infrastructure ensures electricity supply security while enabling mutual energy support between countries.

Distributed Generation and Microgrids

In future distribution systems, distributed generation structures will be as prominent as centralized production. Elements such as solar panels installed on building rooftops, small wind turbines, and energy storage systems will be integrated into microgrid solutions that balance local production and consumption.

These developments transform the classic unidirectional energy flow into a bidirectional structure, turning users into producers. Thus, distribution systems are evolving from mere energy distributors into intelligent platforms that manage energy exchange.