The Importance of Partial Discharge Testing for Instrument Transformers

Why Partial Discharge Testing Is Critical for Instrument Transformer Reliability

When internal voids, cracks, or cavities exist within an instrument transformer, repeated electrical discharges occur once the equipment is energized. These discharges are typically not intense enough to cause immediate burnout, which is why they are classified as partial discharges (PD) . Over months or even years of continuous operation, each discharge event gradually erodes and decomposes a portion of the insulating material, generating black carbon powder that progressively forms tree-like carbonization channels. When a carbonization channel eventually bridges the high-voltage and low-voltage potentials, the main insulation experiences complete breakdown. The resulting high-temperature arc can destroy the winding and cause catastrophic rupture of the transformer housing.

The insidious nature of partial discharge makes it particularly dangerous—it is nearly impossible to observe during early stages and is typically discovered only after breakdown and failure have already occurred. This is precisely why partial discharge testing is mandatory during factory acceptance testing of instrument transformers before shipment.

Partial discharge testing is a critical quality control measure in the manufacturing of voltage transformers and current transformers. It detects insulation defects that other routine tests—such as power frequency withstand test or insulation resistance test—may miss. Diagnostic PD measurements accurately detect and localize insulation defects, enabling reliable assessment of insulation condition and risk of equipment failure.

CNYIHU implements comprehensive PD testing protocols across our full range of instrument transformers, including the JDZW-10R Voltage Transformer, JLSZV-10 Combined Transformer, and LZZBJ9-10 Current Transformer. Every unit undergoes rigorous partial discharge testing before leaving our production facility. Our voltage transformer manufacturing process article details the complete 6-step production flow, while our current transformer manufacturing process article covers the 7-step process, both emphasizing PD testing as a final quality gate.

The Role of Partial Discharge Testing in Quality Assurance

Our epoxy resin APG (Automatic Pressure Gelation) process incorporates vacuum degassing to eliminate internal voids before casting, significantly reducing the risk of partial discharge initiation. For more details on our APG equipment and manufacturing capabilities, contact our technical team.