Transformers are essential electrical devices that transfer energy between circuits through electromagnetic induction. Their primary function is to step up or step down AC voltage, enabling efficient long-distance power transmission and ensuring electrical safety. Additionally, transformers provide electrical isolation, protecting equipment from surges and improving system security.
Core Functions
Voltage Conversion: Adjusts voltage levels to suit different grid systems or device requirements.
Electrical Isolation: Prevents fault propagation between primary and secondary circuits, enhancing safety.
Transmission Efficiency: High-voltage transmission reduces line current and energy loss, improving overall efficiency.
Classification by Voltage Level
1. Power Transformers
Step-up Transformers
Definition: Increase low voltage to high voltage.
Work Principle： Uses a turns ratio (N₂>N₁) between primary and secondary windings. Electromagnetic induction increases AC voltage proportionally to the turns ratio, with power conserved (losses excluded).
Applications: Power plants, HVDC transmission systems.
Advantages: Reduces long-distance transmission loss, improves efficiency.
Disadvantages: Requires high insulation; relatively expensive.
Definition: Reduce high voltage to lower levels.
Work Principle： Reversed turns ratio (N₂
Applications: Distribution networks, industrial power systems.
Advantages: Simple structure, low maintenance cost.
Disadvantages: Efficiency fluctuates with load; energy waste under light load.
Classification by Purpose and Function
Definition: Used in power networks to step voltage up or down (typically above 33kV); high capacity and designed for continuous operation.
Applications: Power plants, substations, inter-province transmission lines, large industrial zones.
Advantages: High efficiency (up to 99%), supports high current and power, long service life.
Disadvantages: Bulky, expensive, complex cooling systems.
Definition: Step down medium voltage (10–35kV) to low voltage (400/230V) for end users; typically <2000kVA.
Applications: Residential communities, office buildings, malls, schools, hospitals.
Advantages: Cost-effective, easy to install and maintain; suitable for outdoor or pole-mounted use.
Disadvantages: Lower full-load efficiency; energy loss under light load; limited voltage/capacity range.
3. Autotransformers
Definition: Primary and secondary share part of the winding; voltage adjusted via taps.
Applications: Motor starting, voltage regulation, power testing systems.
Advantages: Compact, low cost, high efficiency.
Disadvantages: No isolation; lower safety, greater fault risk.
4. Instrument Transformers
Voltage Transformers (VTs)
Definition: Scale down voltage for measurement/protection.
Applications: Voltage meters, protection relays, energy metering.
Advantages: High accuracy, electrical isolation from high-voltage systems.
Disadvantages: Secondary must not be short-circuited; cost-sensitive.
Current Transformers (CTs)
Definition: Scale down current for safe measurement or protection.
Applications: Current meters, fault current detection, protection systems.
Advantages: Accurate measurement, isolates high-voltage from low-voltage equipment.
Disadvantages: Secondary must not be open-circuited; prone to residual magnetism.
General Instrument Transformers
Definition: Convert high-voltage/current signals into safe, low-level signals.
Applications: Substations, metering, relay protection.
Advantages: Safe measurement, high accuracy, standardization.
Disadvantages: Sensitive to impedance and saturation; requires calibration and correct grounding.
5. Isolation Transformers
Definition: Complete isolation between primary and secondary; often 1:1 ratio.
Applications: Medical devices, data centers, laboratories, precision instruments.
Advantages: Enhances safety, reduces common-mode interference, and eliminates ground loops.
Disadvantages: Usually does not change voltage; relatively high cost; large footprint.
Classification by Capacity
In IEC 60076-6, transformers can be classified by capacity into small, middle, and large transformers. Small mainly refers to transformers without additional radiators/coolers/pipes/corrugated oil tanks. Medium transformers refer to transformers with three-phase capacity ≤100 MVA or single-phase capacity ≤33.3 MVA. Large transformers refer to transformers with three-phase capacity >100 MVA or single-phase capacity >33.3 MVA.
Classification by Cooling Medium
According to the cooling medium, transformers can be divided into oil-immersed transformers and dry-type transformers. Then dry-type transformers can be divided into resin cast type transformers and vacuum pressure impregnated transformers. Vacuum pressure-impregnated transformers are usually called VPI transformers.
Definition: Uses circulating insulating oil for heat dissipation; common in high-capacity outdoor systems.
Applications: Substations, industrial power hubs, high-voltage transmission networks.
Advantages: Excellent cooling, supports large loads, stable operation.
Disadvantages: Risk of fire, leaks, and pollution; requires regular oil maintenance; limited in eco-sensitive areas.
Dry-Type Transformers (Cast Resin / VPI)
Definition: Uses air or forced cooling; windings sealed with epoxy resin or fiberglass.
Applications: Commercial buildings, hospitals, subways, factory control rooms, densely populated areas.
Advantages: Safe, eco-friendly; no oil leakage; easy installation and low maintenance.
Disadvantages: Lower cooling capacity; capacity limited (generally <35kV); sensitive to humidity.
Comparison between dry type and oil immersed transformer
| Features | Dry-Type Transformer | Oil-Immersed Transformer |
| Cooling medium | Air or other gases | Transformer oil |
| Safety | High, no risk of fire and explosion | Low, there is a risk of oil combustion and explosion |
| Maintenance | Simple, no need to regularly replace the cooling medium | Requires regular oil replacement and maintenance |
| Environmental protection | High, no pollution to the environment | Low, there is a risk of oil leakage and environmental pollution |
| Application areas | High-rise buildings, subways, hospitals, etc | Outdoor substations, industrial parks, etc. |
Classification by Phase
1. Single-Phase Transformer
Definition: A transformer that operates with a single-phase AC input and output.
Applications: Household appliances (air conditioners, EV chargers), rural power grids (single-phase distribution), power supply for small electronic devices.
Advantages: Simple structure, low cost, ideal for low-capacity applications.
Disadvantages: Limited capacity (typically < 100 kVA); efficiency drops when phase imbalance occurs.
2. Three-Phase Transformer
Definition: A transformer that operates with three-phase AC input and output, typically composed of three separate windings or a three-limb core.
Applications: Industrial power systems (motors, production lines), urban power distribution networks, data centers.
Advantages: Efficient for high-power transmission, balanced load across phases; saves ~20% in materials and space compared to using three single-phase transformers.
Disadvantages: Complex structure, larger failure impact area, requires precise phase synchronization, and higher maintenance costs.
Classification by Core Material and Design
1. By core Material
Iron Core Transformer
Definition: Uses laminated silicon steel sheets as the magnetic core to guide magnetic flux. Core design often includes mitred joints or step-lap laminations to reduce reluctance. The thickness of the silicon steel sheet is inversely proportional to the operating frequency (e.g., 0.3 mm for 50 Hz, 0.1 mm for 400 Hz).
Applications: Power transmission (50/60 Hz systems), line-frequency power supplies, large motor control-ideal for high-power, cost-sensitive electrical systems.
Advantages: High efficiency (95–99%), large power capacity (up to GVA level), low cost; laminated design and optimized magnetic circuits improve energy conversion efficiency.
Disadvantages: Bulky due to laminated sheets; significant losses at high frequency (eddy current and hysteresis); prone to vibration and noise. Not suitable for high-frequency operation due to increased losses.
Ferrite Core Transformer
Definition: Utilizes ferrite (ceramic magnetic material) as the magnetic core, suitable for high-frequency applications. Mn-Zn ferrite is optimal below 1 MHz, while Ni-Zn ferrite suits frequencies above 1 MHz. The Curie temperature (80–300°C) determines the maximum operating temperature.
Applications: Switching power supplies (e.g., phone chargers), high-frequency inverters, RF circuits, electronic ballasts-suitable for compact, low-loss, high-frequency devices.
Advantages: Extremely low high-frequency losses (above 1 MHz), compact size, strong anti-saturation capability; materials tailored for specific frequency bands ensure high transmission efficiency.
Disadvantages: Limited power capacity (<10 kW), magnetic permeability varies with temperature, fragile and prone to cracking; performance degrades in high-temperature environments.
Air-Core Transformer
Definition: Lacks a magnetic core, relying entirely on air or non-magnetic media to transmit magnetic flux. Effective in microwave frequencies (GHz range), such as RFID applications, using multilayer or honeycomb winding structures to improve coupling.
Applications: RF communication (antenna tuning), Tesla coils, high-frequency measurement instruments, superconducting equipment-ideal for high-frequency or high-linearity environments.
Advantages: No hysteresis or eddy current loss, no magnetic saturation, high linearity; coreless design eliminates magnetic loss, offering stable performance at high frequencies.
Disadvantages: Low efficiency due to poor magnetic coupling, large size, limited to high-frequency applications (>100 kHz); not suitable for low-frequency or high-power scenarios.
2. By Core Design
Solenoidal Core Transformer
Definition: Windings are wrapped around a central limb of the core, which is usually E-type or U-I type, commonly used in core-type transformer structures where the magnetic flux loops through a closed magnetic path.
Applications: Distribution transformers, power transformers, and general industrial/electrical equipment.
Advantages: Mature manufacturing process, suitable for standardized mass production; ample insulation space allows high-voltage operation; favorable for oil or air cooling systems.
Disadvantages: Longer magnetic circuit leads to higher leakage flux, slightly higher vibration and noise; relatively larger footprint.
Toroidal Core Transformer
Definition: Uses a closed-ring magnetic core with windings uniformly wound around it, allowing a fully enclosed magnetic flux path.
Applications: High-end audio equipment, medical devices, precision instruments, lab equipment, power adapters, compact power supplies.
Advantages: Extremely low magnetic leakage and electromagnetic interference; high efficiency, quiet operation; compact and lightweight, flexible installation.
Disadvantages: Complex winding process, higher manufacturing cost; unsuitable for high-voltage applications; difficult to maintain or replace.
3. By Core Structure
Core-Type Transformer
Definition: Windings surround the core limbs, with magnetic flux forming a rectangular (loop-like) path. Common in large power transformers.
Applications: Power transmission and distribution systems, power station transformers, high and ultra-high voltage (110 kV and above).
Advantages: Simple structure, easy to manufacture; good insulation and cooling performance; minimal air gap and relatively continuous magnetic circuit.
Disadvantages: Slightly higher leakage flux than shell-type; weaker short-circuit withstand capability; may require more installation space.
Shell-Type Transformer
Definition: Windings are enclosed by the magnetic core, forming a rectangular “box” shape for magnetic flux. Often used in special-purpose or precision control transformers.
Applications: Railway traction transformers, furnace transformers, audio transformers, and small electronic devices.
Advantages: Low leakage flux, strong short-circuit withstand capacity; excellent heat dissipation and high efficiency; low EMI, high operational stability.
Disadvantages: Complex and heavy structure; higher manufacturing cost; harder to inspect or maintain; occupies more space.
Special Transformers
1. Rectifier Transformers
Definition: Supplies specific voltages to rectifier units; multi-winding designs reduce harmonics.
Applications: Aluminum smelting, DC transmission, traction power, electroplating.
Advantages: Handles harmonics well; stable output; suited for high-power rectification.
Disadvantages: High heat due to harmonics; expensive cooling systems.
2. Furnace Transformers
Definition: Supplies low voltage (10–100V) and high current (up to tens of kA) for industrial furnaces.
Applications: Steelmaking, metal smelting, thermal processing.
Advantages: High, adjustable current output; supports frequent short-circuits.
Disadvantages: Lower efficiency; high energy consumption; requires cooling.
3. Testing Transformers
Definition: Produces high voltage (up to several hundred kV) for short-term insulation testing.
Applications: Cable testing, insulation testing, and factory acceptance testing.
Advantages: High adjustable output; strong short-time overload capacity.
Disadvantages: Large size; limited operating time; complex maintenance.
4. Welding Transformers
Definition: Provides low-voltage, high-current power for arc welding; uses magnetic shunt or leakage reactance to shape output.
Applications: Manual arc welding, spot welding, and construction sites.
Advantages: Stable output, suited for frequent arcing; high safety.
Disadvantages: Low power factor; complex control; requires compensation.
This section outlines the classification of electrical transformers through multiple dimensions, including the transformer’s voltage level, purpose and function, phases, core material, core design, core structure, and cooling medium. A comparative analysis of these categories is provided to guide optimal transformer selection based on specific operational demands and environmental constraints.
