Deep Analysis of Partial Discharge Control and Online Monitoring Technology for Dry-type Transformers

1. Introduction: Partial Discharge - The "Invisible Killer" of Dry-type Insulation

For dry-type transformers, especially those using Cast Resin (CR) technology, Partial Discharge (PD) is a core indicator determining their service life. Unlike oil-immersed transformers, the insulation of dry-type units suffers irreversible damage once PD occurs (such as treeing discharge tracks), which cannot be restored through oil filtration or replacement. Controlling PD below 10pC (or even 5pC) is not only a standard requirement but also a foundation for ensuring 30 years of safe operation.

2. Microscopic Mechanisms and Physical Defects of PD

Through 29 years of technical observation, PD in dry-type transformers typically originates from three types of physical defects:

3. Electric Field Optimization and Shielding Technology

The primary means of reducing PD is to improve electric field distribution and eliminate electric field peaks:

• Electrostatic Shielding Design: Install highly rounded voltage-equalizing shielding rings at the ends of high- and low-voltage coils and at tap leads.
• Rounding and Filleting Optimization: All metal supports and busbar connections must strictly follow R-angle processing standards. Even tiny burrs can become continuous PD sources under 10kV or 35kV voltages.
• Redundancy in Insulation Distances: In high-altitude or dusty environments, the air surface flashover distance must be calculated through software modeling, with a safety margin of 15%-20% reserved.

4. Core Control Parameters in the Casting Process

High-quality dry-type transformers are "manufactured," not just "designed."

5. Comparison and Evolution of PD Monitoring Technologies

With the requirements of grid digitalization, PD monitoring has shifted from laboratory-based to online monitoring:

• Pulse Current Method (IEC 60270): The standard method, offering high sensitivity but being extremely susceptible to onsite electromagnetic noise interference.
• Ultra-High Frequency (UHF) Method: Monitoring frequencies between $300\text{MHz}\sim 1500\text{MHz}$, offering strong anti-interference capabilities. It effectively identifies internal discharge pulses and is particularly suitable for urban substations with complex electromagnetic environments.
• Ultrasonic Detection: Used as a supplementary tool to locate specific physical discharge points (e.g., loose bushings or external surface tracking).

6. Expert Insights: Trend Analysis Over Absolute Values

In practical maintenance, while the absolute value of a single PD test is important, the Trend Analysis of PD volume over time holds greater predictive value.

• If the PD volume rises exponentially within six months, it indicates deep physical deterioration of the insulation system, even if the current value is within the qualified range.
• Using PRPD (Phase-Resolved Partial Discharge) patterns, experienced engineers can distinguish between "internal void discharge" and "surface tracking," allowing for the formulation of targeted maintenance plans.

7. Conclusion

PD control in dry-type transformers is a systematic project covering material science, electric field theory, and manufacturing processes. Through refined process control and advanced UHF online monitoring, we can elevate the reliability of dry-type units to a level equivalent to or even better than oil-immersed units, providing long-term power security for critical loads.