Current transformers and voltage transformers

Current transformers (CTs) and voltage transformers (VTs/PTs), collectively instrument transformers, are core components in modern power systems, acting as the "sensing bridge" between high-voltage circuits and low-voltage measurement/protection devices.

Their operation is based on Faraday's Law of Electromagnetic Induction, similar to power transformers but optimized for precise signal conversion rather than energy transmission.

1. Core Classification & Working Characteristics

1.1 Current Transformers (CTs)

The primary winding is series-connected to the high-current circuit, with few turns and robust conductors for heavy loads. The secondary winding, with more turns, outputs standardized small current (typically 5A or 1A for industrial use).

1.2 Voltage Transformers (VTs/PTs)

The primary winding is parallel-connected to the high-voltage circuit, with many turns matching system voltage. The secondary winding outputs standardized low voltage (usually 100V phase-to-phase, 100/√3 V phase-to-neutral).

2. Core Functions

1. Parameter Conversion: Converts high voltage (kV) and large current (kA) to standardized low voltage/current, matching meters, energy meters, and protection devices.

2. Electrical Isolation: Separates high-voltage primary and low-voltage secondary circuits, protecting operators and low-voltage equipment from high-voltage hazards.

3. Standardization: Unifies secondary device input specs, enabling mass production and interchangeability, reducing system costs.

3. Application Scenarios & Standards Compliance

Instrument transformers are widely used in power plants, substations, industrial distribution systems, and large electrical equipment (e.g., transformers, switchgear).

Products must comply with strict international/regional standards:

• IEC 61869 Series: International standard for instrument transformers

• ANSI/IEEE C57.13: American standard for voltage transformers

• GB 1208/GB 1207: Chinese national standard for CTs/VTs

4. Material & Structural Features

High-quality instrument transformers use cold-rolled silicon steel or amorphous alloy cores to reduce iron loss and improve accuracy. Winding insulation adopts heat/moisture-resistant materials (epoxy resin casting, oil-immersed), ensuring stable performance in harsh environments (high temp, humidity, corrosion).