What Are Combined Transformer Standards? Key Specs & Tests

Combined Instrument Transformers: Technical Requirements and Testing Standards Explained with Data

A combined instrument transformer integrates a voltage transformer (VT) and a current transformer (CT) into a single unit. Its design and performance are governed by comprehensive standards covering technical specifications, testing procedures, and operational reliability.

1. Technical Requirements

Rated Voltage:
The primary rated voltages include 3kV, 6kV, 10kV, and 35kV, among others. The secondary voltage is typically standardized at 100V or 100/√3 V. For example, in a 10kV system, the primary rated voltage of the combined transformer is 10kV, while the secondary output is 100V—meeting both measurement and protection requirements.

Rated Current Ratio:
The CT section offers various rated current ratios such as 50/5, 100/5, and 200/5. These ratios can be selected based on actual system current levels to accurately transform primary current to the secondary side (typically 5A), ensuring precise monitoring and relay protection operation.

2. Testing Standards

Insulation Tests:
These verify the dielectric strength of the transformer under normal and transient overvoltage conditions.

• Power Frequency Withstand Test:
  For a 10kV combined transformer, the test voltage is typically 42kV RMS, applied for 1 minute. This ensures the insulation can withstand sustained power-frequency overvoltages during service.

• Impulse Withstand Test:
  The peak impulse voltage is generally 75kV, simulating lightning surge conditions. This test evaluates the transformer’s ability to endure transient overvoltages without breakdown.

Accuracy (Error) Tests:
Strict error limits are defined based on accuracy classes.

• Voltage Transformer (0.2 Class):
  At rated voltage, the ratio error must not exceed ±0.2%, and the phase angle error must be within ±10 minutes (′).

• Current Transformer (0.2S Class):
  Over a wide range from 1% to 120% of rated current, the ratio error remains within approximately ±0.2%, with tightly controlled phase angle error. This high accuracy is essential for metering applications, especially under low-load conditions.

Temperature Rise Test:
This test ensures safe long-term operation under full load.

• Conducted at rated load and specified ambient temperature (usually 40°C), the average winding temperature rise must not exceed 65K. This limit prevents insulation degradation and ensures reliable performance over the transformer’s service life.

Summary

Combined instrument transformers are engineered to meet rigorous international standards (such as IEC 61869 series and GB/T 20840). Their technical parameters—like 10kV primary voltage, 100V secondary output, and 100/5 current ratio—are selected based on system requirements. Compliance with tests including 42kV power frequency, 75kV impulse withstand, ±0.2% accuracy, and 65K temperature rise guarantees safety, precision, and durability in power systems.