A Comprehensive Analysis of Voltage Level Systems: The Power "Ladder" From Ultra-High Voltage To Low Voltage

If a modern power system is likened to the human circulatory system, then voltage levels represent the "network of blood vessels" connecting various regions. Once generated, electrical energy is not delivered directly to end-users; instead, it undergoes step-up transformation, long-distance transmission, and multi-stage step-down transformation before finally being converted to voltage levels suitable for industrial equipment, commercial buildings, and residential use.

From ultra-high-voltage transmission lines spanning vast distances and underground urban cables connecting industrial parks to the 220V power supply in residential buildings, the entire power system relies on the coordination of various voltage levels to ensure safe, efficient, and stable operation.

In 2025, the scale of transmission lines rated at 220kV and above continued to expand, while China’s ±1100kV ultra-high-voltage direct current (UHVDC) transmission technology maintained its global leading position. Behind these massive power infrastructure systems lies a mature and well-established voltage level hierarchy.

For electrical engineers, power equipment procurement professionals, and transformer industry practitioners, understanding voltage levels is not only essential for comprehending the structure of the power grid, but also directly influences transformer selection, equipment configuration, and project design.

This article will systematically analyze the five major voltage levels — ultra-high voltage, extra-high voltage, high voltage, medium voltage, and low voltage — from the complete process of power generation, transmission, distribution, and end-user consumption, while explaining the key equipment and practical applications associated with each voltage level.

I. Overview of Voltage Levels: The Five-Tier Hierarchy of the Power System

Based on function, they are primarily categorized into:

ultra-high voltage (UHV), extra-high voltage (EHV), high voltage, medium voltage, and low voltage.

Different voltage levels handle distinct power transmission and distribution tasks.

The design of voltage levels follows a fundamental principle:

The longer the transmission distance, the higher the required voltage; the shorter the distance, the lower the voltage level used.

The reason is that:

During the transmission of electrical energy, line losses are primarily caused by the electric current.

According to Joule's law:

P = I⊃2;R

When the transmitted power remains constant, increasing the voltage allows for a reduction in transmission current, thereby minimizing line losses.

therefore:

· Generation side → Step-up                             · Long-distance transmission → UHV/EHV

· Urban areas → High/Medium voltage         · User end → Low voltage

This is the "tiered voltage structure" of modern power systems.

II. Ultra-High Voltage (UHV): The "Highways" of Power Transmission

1.Voltage levels:

AC: 1,000 kV and above; DC: ±800 kV and above.

Ultra-high voltage (UHV) is currently one of the world's highest-level power transmission technologies, primarily used to bridge the gap between energy resources and load centers.

my country's energy resources are concentrated in the western regions, while economic load centers are concentrated in the eastern coastal areas; therefore, UHV technology is essential to achieve high-capacity, long-distance, and low-loss power transmission.

2.Characteristics of UHV Power Transmission

① High transmission capacity

A single circuit of an UHVDC (Ultra-High Voltage Direct Current) transmission project can transmit millions of kilowatts of power.

For example:

The ±1100 kV Changji–Guquan UHVDC project: transmission distance exceeds 3,000 kilometers; transmission capacity reaches 12 million kilowatts.

It stands as one of the world's UHVDC projects with the longest transmission distance and the highest capacity.

② Low power transmission losses

Compared to conventional power transmission methods, UHV transmission can significantly reduce line losses and improve energy utilization efficiency.

③ Suitable for large-scale integration of new energy

As the scale of new energy bases—such as wind and solar power—expands, UHV (Ultra-High Voltage) technology has become vital infrastructure for the cross-regional consumption of new energy.

3.Core UHV Equipment: Converter Transformer

Converter transformers are core components of UHVDC (Ultra-High Voltage Direct Current) transmission systems.

Their primary functions include stepping up AC voltage, facilitating AC/DC conversion in conjunction with converter valves, and supporting the stable operation of DC power transmission.

Due to their extremely high operating voltages, UHV converter transformers are characterized by:

Ultra-high insulation requirements; high-capacity design; complex cooling system; high reliability requirements.

A single unit can weigh hundreds of tons, representing a significant example within the power equipment manufacturing sector.

III. Ultra-High Voltage: The "Main Arteries" of Regional Power Grids

Voltage ratings: AC 330 kV–750 kV; DC ±500 kV–±660 kV.

Extra-high voltage (EHV) systems primarily handle power exchange between regional power grids and serve as vital corridors connecting large-scale energy bases with urban load centers.

Typical applications: Power transmission from Northwest energy bases; interconnection of regional power grids; power supply to large-scale industrial bases.

Typical Equipment: Large Power Transformers

In extra-high voltage (EHV) substations—such as those operating at 750 kV or 500 kV—power transformers perform the function of voltage transformation.

For instance, after a 750 kV transmission line enters a regional substation, the voltage must be stepped down by a large main transformer before the power is fed into the 220 kV or 110 kV grid.

EHV transformers typically feature ultra-high capacity, high insulation levels, robust short-circuit withstand capability, and a long service life; their capacity often reaches several hundred MVA.

IV. High Voltage: The "Main Arteries" of Urban Power Supply

Voltage levels: 10 kV–220 kV

High-voltage levels are a crucial component of urban power grids and industrial power supply systems.

Different voltage levels serve different functions:

High-Voltage Equipment Application Cases

Taking an industrial park as an example:

The power grid supplies 110 kV power, which is stepped down to 10 kV via a 110 kV substation before entering the plant's power distribution system.

Key equipment includes: oil-immersed power transformers, GIS (Gas-Insulated Switchgear) assemblies, and high-voltage switchgear cabinets.

Applications of Oil-Immersed Power Transformers

Oil-immersed transformers are most widely used in 110 kV and 220 kV substations.

Typical parameters:

Oil-immersed transformers utilize insulating oil for cooling and insulation, offering strong heat dissipation capabilities, a wide capacity range, and stable operation.

They are widely used in industrial facilities, power grid substations, and new energy projects.

V. Medium Voltage: The "Connection Layer" of the Distribution Network

1.Voltage ratings: 1 kV–10 kV

Medium voltage represents the large-scale distribution network closest to the end-user. Among the power distribution systems in my country's urban and rural areas, 10 kV lines are the most widely used.

Typical workflow:

110kV substation

↓

10kV distribution line

↓

10kV/0.4kV distribution transformer

↓

End user

2.Core Equipment: Distribution Transformers and Box-type Substations

Distribution Transformers

Common Capacities: 200 kVA; 400 kVA; 800 kVA; 1,000 kVA; 1,600 kVA

Primary Applications: Residential power supply; commercial buildings; industrial plants

Box-type Substation Application

3.A box-type substation integrates:

· high-voltage switchgear · transformers · low-voltage distribution cabinets

into a single enclosure.

4.Advantages:

① Rapid installation

Factory-prefabricated for direct on-site installation.

② Small footprint.

Suitable for areas with limited urban space.

③ Strong protective capability.

④ Capable of operating in complex environments such as high temperature, high humidity, salt spray, and sand/dust.

VI. Low Voltage: The "Last Mile" at the User End

Voltage levels: 220V and 380V represent the low-voltage levels for end-user connections.

Key applications include:

220V

Used for: household lighting and home appliances.

380V

Used for: electric motor equipment, industrial machinery, and commercial equipment.

Major components of low-voltage systems: low-voltage distribution cabinets, distribution boxes, electricity meters, and protective devices.

For specialized environments such as hospitals, data centers, and laboratories, additional equipment is used—such as isolation transformers and UPS systems—to ensure power supply reliability.

VII. Functions of Transformers at Different Voltage Levels

Transformers are integral to the entire power system and serve as the core equipment for voltage transformation.

VIII. Principles for Equipment Selection Based on Voltage Levels

In actual engineering projects, once the voltage level has been determined, it is necessary to select suitable equipment.

Key considerations:

1. Capacity

Determine transformer capacity based on load calculations, including: peak load, future expansion requirements, and starting inrush current.

2. Environmental conditions

Different environments require specific designs:

Standard environments: Standard oil-immersed transformers

High-temperature regions: Enhanced heat dissipation; derated design

High-humidity environments: Anti-corrosion coatings; sealed construction

Desert regions: High IP rating; dust-proof design

3. Equipment type

Common options:

IX. Summary: Understanding voltage levels means understanding the entire power system.

From ultra-high-voltage transmission lines to urban distribution networks and finally to residential electricity consumption, voltage levels form a complete energy transmission chain.

Different voltage levels perform different functions:

Voltage levels are not simply numerical classifications; they represent a comprehensive power system architecture developed through decades of engineering practice.

Understanding the voltage level system not only helps electrical engineers select equipment correctly but also enables enterprises to achieve more accurate transformer configuration and system design in overseas power projects, grid construction, and industrial supporting facilities.