Types of Potential Transformers

The stable operation, precise metering, and reliable protection of modern power systems all depend on the accurate feedback from core sensing equipment. The Potential Transformer (PT), also widely known as the Voltage Transformer (VT), is the starting point of this feedback chain. It safely and accurately converts the high voltage signals of the power grid into standard low voltages that can be processed by secondary equipment.

With the global power grid evolving towards ultra-high voltage, digitalization, and intelligence, and with increasing demands for insulation safety, compactness, and resistance to environmental factors, every reliable potential transformer manufacturer must diversify the technical routes and structural designs of these instrument transformer products. From traditional oil-immersed electromagnetic technology to new electronic sensing technology, each type of potential transformer is designed for specific voltage levels, application scenarios, and environmental conditions.

What is a Potential Transformer (PT)

The Potential Transformer (PT/VT) serves as a bridge connecting the high-voltage primary circuit and the low-voltage secondary system. Understanding the distinction between a current transformer vs voltage transformer is crucial here: while a current transformer measures current, the PT/VT’s role in the power system is irreplaceable: it precisely scales down voltages, often tens or even hundreds of thousands of volts, to safe, standardized, and operable low voltages.

Core Functions: The Dual Mission of Metering and Protection

These transformers must possess dual functionality to meet the core needs of grid operation:

1. Precise Metering (Measurement): Provides high-accuracy voltage signals for energy meters, monitoring instruments, etc., ensuring the accuracy of energy transaction and operational monitoring data. Metering PTs require extremely high accuracy within the normal operating voltage range, typically corresponding to accuracy classes such as 0.2 or 0.5 (M class), which represent minimal error.
2. Relay Protection (Protection): Accurately and quickly transmits fault voltage signals to relay protection devices when the grid experiences faults like short circuits or ground faults, allowing for timely isolation of the fault point to prevent accident escalation. Protection VTs focus on transient response and reliability, avoiding saturation even under high overvoltages, and typically corresponding to accuracy classes such as 3P or 5P (P class), representing reliability even under fault conditions.

Basic Principle and Key Parameters

Potential Transformers operate based on the principle of electromagnetic induction, achieving voltage reduction and isolation through high turns ratios. Their design must strictly comply with the following standards:

1. Voltage Reduction and Isolation: Utilizes magnetic coupling between the primary and secondary windings to achieve accurate step-down of the high-voltage signal, while ensuring complete electrical isolation between the secondary system and the primary high voltage, guaranteeing operational safety.
2. Rated Voltage Ratio K_e: This is the most fundamental parameter of a PT, defined as the ratio of the rated primary voltage U_1N to the rated secondary voltage U_2N (K_e = U_1N/U_2N). All performance indicators of the voltage transformer are designed around this fixed ratio.
3. Standardized Output: The standard secondary side output voltage for traditional electromagnetic transformers is typically 100V, 110V, or 120V. For new Electronic Voltage Transformers (EVT), the output is either extremely low voltage or a digital signal conforming to international standards.

I. Potential Transformer Types by Technical Principle

Classification by operating principle and internal structure is the most fundamental and decisive dimension in potential transformer (PT/VT) technology. The chosen technical principle directly determines the fundamental performance differences in high-voltage isolation capability, transient response speed, core anti-saturation, and the final signal output mode. Understanding this classification helps to accurately grasp the core technical routes and performance characteristics adopted by different voltage transformers when dealing with various voltage levels.

1. Electromagnetic Potential Transformer

Electromagnetic Voltage Transformers are traditional devices based on the principle of electromagnetic induction, and are the most widely used and technically mature type in medium and low-voltage power systems today. Their core structure is similar to a step-down power transformer. In scenarios where accuracy requirements are relatively stable and there are no special high-frequency signal demands, electromagnetic VTs have unparalleled cost and maintenance advantages. These products cover the full range of electromagnetic applications from 3kV to 35kV.

• Features: Composed of primary winding, secondary winding, and iron core, similar to a small step-down transformer.
• Advantages: Simple structure, mature technology, high operational reliability, and relatively low cost.
• Liyond Typical Product Series (3~10kV Indoor): JDZ(J)-3, 6, 10Q Type Epoxy Resin Cast Single-Phase Voltage Transformer, JDZX10-3, 6, 10 Type Indoor Epoxy Resin Cast Voltage Transformer
• Application Scenarios: Widely used in metering and protection for medium and low-voltage systems (such as 35kV and below).

2. Capacitive Potential Transformer (CVT)

Capacitive Voltage Transformers are the preferred solution for high-voltage and ultra-high-voltage power systems (110kV and above). Their operating principle is twofold: first, they perform capacitive voltage division, reducing the extremely high voltage to a medium voltage level; then, a small electromagnetic unit performs secondary step-down and electrical isolation. This structure offers better economics and volume advantages at high voltage levels compared to pure electromagnetic PTs.

• Principle: Performs voltage division through a series capacitor bank, followed by secondary step-down and isolation via an electromagnetic unit.
• Advantages:
  • Cost-Effectiveness: At high voltage levels, CVTs are smaller and less expensive than electromagnetic VTs.
  • Carrier Compatibility: The capacitor bank can serve as the coupling capacitor for Power Line Carrier communication (PLC).
• Application Scenarios: High-voltage and ultra-high-voltage transmission lines, metering and protection in substations, and power line carrier communication.

3. Electronic Potential Transformer (EVT / EPT)

Electronic Potential Transformers (Electronic Instrument Transformer, EIT) are modern technologies that have emerged with the development of smart grids. They completely break away from the traditional electromagnetic induction principle, utilizing photoelectric or electronic techniques. The core advantage of this electronic VT (EVT) lies in its coreless design, which eliminates ferroresonance and saturation issues, offering an extremely wide frequency response. It directly outputs digital signals compliant with the IEC 61850 standard.

• Principle: Utilizes photoelectric or electronic technology to directly output low-voltage digital signals.
• Advantages:
  • High Precision: High accuracy, good linearity, and no saturation phenomena.
  • Lightweight: Extremely small in size and weight.
  • Digitalization: Output complies with the IEC 61850 standard, facilitating access to smart substations.
• Application Scenarios: Smart grids, digital substations, and scenarios demanding extremely high measurement accuracy and response speed.

II. Potential Transformer Types by Insulation Medium

The insulation medium is the core factor determining the long-term operational reliability, maintenance cost, and environmental adaptability of potential transformers. In high-voltage electric fields, the selection of insulation medium is crucial, as it must balance the need to withstand huge electrical stress, effectively conduct operational heat, and comply with fire safety and environmental regulations.

1. Cast-Resin Potential Transformer (Dry Type)

Cast-resin transformers use epoxy resin as the solid insulation medium. Through vacuum casting, the windings and iron core are completely encapsulated in a solid insulation block, forming an oil-free, void-free dry structure. Due to their excellent fire resistance and maintenance-free advantages, cast-resin PTs have become the preferred choice for indoor medium-voltage switchgear.

• Insulation Medium: Epoxy resin.
• Features: The windings and iron core are entirely cast in epoxy resin to form a solid insulation block (dry-type VT).
• Advantages: Maintenance-free, good fire performance (non-flammable), small size, no risk of oil leakage, and environmentally friendly operation.
• Liyond Typical Product Series (3~35kV): JDZX10-6/10Q Epoxy Resin Cast Voltage Transformer, JDZXW-35(G) Type Outdoor Composite Insulated Dry-Type Voltage Transformer
• Application Scenarios: Widely used in indoor medium-voltage (such as 35kV and below) switchgear, distribution rooms, and places with strict fire safety requirements.

2. Oil-Immersed Potential Transformer

Oil-immersed VTs use transformer oil as the main insulation and cooling medium. The oil not only provides high dielectric strength but also efficiently dissipates heat generated by the windings. This structure is very effective in suppressing partial discharge under high voltage, thus retaining an important position in high-voltage applications.

• Insulation Medium: Transformer oil.
• Features: Windings and iron core are immersed in transformer oil, which serves both as an insulating medium and for cooling.
• Advantages: Superior insulation performance, high cooling efficiency, suitable for high voltage levels (above 35kV).
• Liyond Typical Product Series (3~35kV): JDJ-3, 6, 10 Type Outdoor Oil-Immersed Voltage Transformer (High Reliability Type)
• Application Scenarios: Outdoor substations and transmission lines at high voltage levels.

3. Gas-Insulated Potential Transformer

Gas-insulated transformers are mainly used in gas insulated switchgear solutions (GIS), utilizing SF6 gas as the insulation medium. The transformer components are encapsulated in a sealed metal enclosure filled with SF6 gas, completely isolated from the external environment. This design achieves miniaturization and high reliability of the PT. In the compact GIS structure, the integration of both the voltage transformer and the current transformer is often employed.

• Insulation Medium: SF6 (Sulfur Hexafluoride) gas.
• Features: The transformer is encapsulated in a sealed metal enclosure filled with SF6 gas, providing extremely high insulation strength and a compact structure.
• Advantages: High insulation strength, unaffected by environmental pollution, compact size, and minimal maintenance workload.
• Application Scenarios: GIS and compact substations or underground substations.

III. Potential Transformer Types by Specialized Function

To address complex grid operating environments and the demand for multi-function integration, potential transformers have evolved into specialized products with unique functions. These specialized PTs are designed to solve specific problems (such as the risk of resonance) or meet space optimization goals.

1. Three-Phase Common Tank Potential Transformer

Three-phase common tank VTs integrate three-phase voltage measurement units into a single insulating enclosure, mainly used for metering or protection in three-phase systems. Compared to a combination of three single-phase transformers, the common tank structure offers advantages such as smaller footprint and simplified installation wiring.

• Features: Integrates three single-phase voltage transformers into one insulating enclosure.
• Liyond Typical Product Series: JSZVR-3, 6, 10W Type Indoor Cast Three-Phase Common Tank Voltage Transformer
• Application Scenarios: Overall voltage monitoring, metering, and relay protection for three-phase distribution networks.

2. Anti-resonance Potential Transformer

Anti-resonance transformers are specifically optimized to address the problem of ferroresonance, which can easily occur in certain medium-voltage grids. They typically connect a damping resistor in parallel with the secondary winding to quickly dissipate the energy generated by resonance, thereby effectively suppressing ferroresonance overvoltage and ensuring equipment safety.

• Features: Specifically designed to solve overvoltage problems caused by ferroresonance, with integrated damping components or special winding structures.
• Liyond Typical Product Series: JSZJK-3, 6, 10Q Type Indoor Cast Anti-Ferroresonance Three-Phase Voltage Transformer
• Application Scenarios: Medium-voltage grids with large capacitive earth currents and a high propensity for ferroresonance.

3. Discharge Coil and Auxiliary Power Potential Transformer

These Potential Transformers typically have multiple secondary windings. In addition to providing standard low-voltage measurement and protection windings, they may also include an open delta winding for ground fault protection, and an auxiliary power winding specifically to provide stable low-voltage operating power for downstream control devices.

• Features: Used for measurement, providing operating power, or cooperating with arc suppression coils, usually featuring multiple independent secondary windings.
• Liyond Typical Product Series: JDZF(X)-10Q/10W Type Transformer with Auxiliary Power/Discharge Coil Function
• Application Scenarios: Compensating arc suppression coil circuits, providing reliable auxiliary power for relay protection devices, and situations requiring ground protection.

IV. Potential Transformer Types by Installation Environment

Potential Transformers are clearly divided into two major types based on their final installation location. This is a critical classification dimension, directly determining the equipment’s enclosure protection capability, insulation material choice, and resistance to climate and pollution.

1. Indoor Potential Transformer

Indoor VTs are designed to be installed in indoor environments with roofs, protected from direct rain and snow, such as switchgear or control rooms. They usually use cast-resin (dry-type) insulation, emphasizing compact size and high integration.

• Features: Typically uses dry-type/cast-resin insulation, featuring a smaller size, compact structure, and focusing on insulation performance and dust prevention.
• Liyond Typical Product Series: JDZ(X) Series Indoor Epoxy Resin Voltage Transformer
• Application Scenarios: Medium and low-voltage switchgear, distribution boxes, control rooms.

2. Outdoor Potential Transformer

Outdoor PTs must withstand harsh natural environments, including sun exposure, rain and snow erosion, and environmental contamination. Therefore, they typically use oil-immersed or post-type structures with robust composite insulation. The insulating external surface is designed with a large creepage distance to ensure reliable operation even under heavy pollution conditions.

• Features: Must have a robust enclosure and good resistance to moisture and contamination flashover, featuring stable structure and strong weather resistance.
• Liyond Typical Product Series: JDZW-3, 6, 10(G) Type Outdoor Composite Insulated Voltage Transformer, JDJ Series Outdoor Oil-Immersed Voltage Transformer
• Application Scenarios: Outdoor substations, transmission line towers, and applications with high demands for environmental adaptability.

Summary and Future Trends

Potential Transformers (PTs/VTs) are the cornerstone of accuracy and safety in modern power systems. They not only perform the fundamental task of precise conversion from high voltage to low voltage but also carry the core value of providing reliable data support for energy metering and relay protection.

In the context of the power system’s continued push towards intelligence, the trend of transformer technology iteration is increasingly clear. For any leading instrument transformer manufacturer, the market demands higher standards for miniaturization, high precision, maintenance-free operation, and specialized functions like anti-resonance. Particularly, Electronic Voltage Transformers (EVT) with digital output capabilities are gradually replacing traditional electromagnetic products, becoming key sensors for digital substations. Ultimately, every technical advancement in potential transformers directly enhances the operating efficiency and risk resilience of the entire power grid.

Liyond is dedicated to providing a full range of high-performance potential transformer solutions, covering various operating conditions and technical routes, to meet your strict requirements for different voltage levels, installation environments, and functional needs. Please specify your detailed project requirements, and we will provide you with a customized VT selection plan that aligns with both technical specifications and economic benefits.