Impulse Voltage Testing of Instrument Transformers Using HV Lightning Impulse Voltage Generator Systems

Instrument transformers, including current transformers (CTs) and voltage transformers (VTs), are essential components of protection and metering systems in power networks. Although physically smaller than power transformers, they are exposed to the same transient overvoltages and must maintain accuracy and insulation integrity under impulse stress. Lightning impulse voltage testing using HV lightning impulse voltage generator systems is therefore a required type test for these devices. This article explains the standards, test configurations, and practical considerations for impulse testing of instrument transformers.

Purpose of Impulse Testing on Instrument Transformers

Impulse testing verifies the dielectric strength of instrument transformer insulation and confirms that the device can withstand lightning overvoltages without failure. Specific objectives include:

• Confirming the insulation design of primary and secondary windings.
• Detecting manufacturing defects such as weak insulation, voids, or incorrect clearances.
• Verifying the integrity of the primary insulation to ground and between sections.
• Ensuring that protective and metering functions remain accurate after impulse exposure.
• Providing quality assurance for critical substation equipment.

Applicable Standards

Impulse testing of instrument transformers is governed by the following standards:

• IEC 61869-1: General requirements for instrument transformers.
• IEC 61869-2: Additional requirements for current transformers.
• IEC 61869-3: Additional requirements for inductive voltage transformers.
• IEC 60060-1: Defines standard lightning impulse waveforms and tolerances.
• IEEE C57.13: Standard requirements for instrument transformers.

These standards specify test voltage levels based on the rated insulation level, waveform parameters, and acceptance criteria.

Characteristics of Instrument Transformers as Test Objects

Instrument transformers present specific characteristics that influence impulse testing:

• Relatively Low Capacitance: Compared with power transformers, instrument transformers have lower winding capacitance, which generally simplifies waveform shaping.
• Compact Insulation: Small clearances make them sensitive to defects and contamination.
• Multiple Insulation Paths: Primary-to-secondary, primary-to-ground, and secondary-to-ground insulation must all be considered.
• Winding Inductance: The winding inductance affects the impulse response and may cause oscillations.
• Test Terminal Configuration: CTs and VTs have different terminal arrangements requiring adapted test setups.

Test Configuration for Current Transformers

Current transformer impulse testing typically involves:

• Primary Terminal Impulse: The impulse is applied to the primary terminal with the secondary winding short-circuited and grounded according to the test plan.
• Grounding Configuration: The tank or base is grounded at a single point.
• Waveform Shaping: Front and tail resistors are adjusted to compensate for the relatively low capacitance of the CT.
• Measurement: The impulse voltage divider is connected directly to the primary terminal.

Test Configuration for Voltage Transformers

Voltage transformer impulse testing typically involves:

• Primary Winding Impulse: The impulse is applied to the primary winding with the secondary winding connected according to the test plan.
• Grounding: The tank and secondary windings are grounded as specified.
• Waveform Shaping: Resistor values are selected based on the higher capacitance of the VT winding.
• Measurement: The divider is connected directly to the primary terminal, with attention to the effect of winding inductance.

Test Procedure

A typical instrument transformer impulse test follows these steps:

• Preparation: Inspect the transformer, clean insulators, and verify that oil or gas conditions meet specifications.
• Circuit Verification: Apply a reduced-level impulse to verify waveform and measurement.
• Full-Level Impulses: Apply the specified test voltage, typically three impulses per terminal and polarity.
• Waveform Recording: Record all waveforms and compare them for consistency.
• Post-Test Checks: Perform insulation resistance and ratio tests to confirm that no damage occurred.

Waveform Interpretation and Fault Detection

Waveform analysis is essential for detecting insulation defects in instrument transformers:

• Consistency Across Impulses: Stable waveforms indicate healthy insulation.
• Sudden Changes: Alterations in waveform shape may indicate partial discharge or breakdown.
• Oscillation Patterns: Changes in oscillation frequency or amplitude can reveal winding or insulation problems.
• Voltage Collapse: A sharp drop in voltage during the impulse indicates flashover or breakdown.
• Polarity Effects: Differences between positive and negative impulses may reveal asymmetric defects.

Digital recording systems with high sampling rates and appropriate analysis software assist in detecting these indicators.

Common Challenges

Impulse testing of instrument transformers presents several practical challenges:

• Small Clearances: Compact insulation requires careful control of test voltage and waveform to avoid flashover.
• Winding Resonance: Internal resonance can distort the impulse waveform and complicate interpretation.
• Multiple Test Configurations: Different terminals and windings require repeated circuit reconfiguration.
• Measurement Accuracy: Fast transients require high bandwidth dividers and recorders.
• Safety: High voltage and stored energy demand strict safety procedures.

Post-Test Evaluation

After impulse testing, the instrument transformer should be inspected for visible damage, and functional tests should be performed to confirm accuracy and insulation integrity. Any indication of failure requires further investigation, including repeat testing at reduced levels or internal inspection. A successful impulse test provides confidence that the device will perform reliably under transient overvoltages in service.

Conclusion

Impulse voltage testing of instrument transformers using HV lightning impulse voltage generator systems is essential for verifying insulation integrity and ensuring accurate performance in protection and metering systems. By following international standards, preparing the test object carefully, and interpreting waveforms rigorously, manufacturers and utilities can ensure that instrument transformers will withstand lightning overvoltages throughout their service life.