# Corrosion Inhibitor Addition in Transformer Insulating Oil: Dosage and Method
## Abstract
Transformer insulating oil is crucial for electrical insulation and heat dissipation in power transformers. However, the presence of corrosive sulfur compounds in the oil can lead to the formation of copper sulfide deposits on winding insulation, causing insulation degradation and transformer failures. This paper discusses the optimal dosage and addition methods of corrosion inhibitors in transformer insulating oil, focusing on benzotriazole (BTA) and its derivatives, to mitigate sulfur corrosion risks effectively.
## Introduction
Transformer insulating oil, primarily composed of mineral oils, synthetic esters, or natural esters, serves as both an electrical insulator and a coolant. Over time, sulfur-containing compounds in the oil, such as dibenzyl disulfide (DBDS), can react with copper windings to form copper sulfide (Cu₂S) deposits. These deposits reduce the dielectric strength of insulation paper, increase dielectric loss, and potentially trigger partial discharges, leading to catastrophic transformer failures. To address this issue, corrosion inhibitors are added to the oil to form protective films on metal surfaces, thereby suppressing sulfur corrosion.
## Mechanism of Corrosion Inhibitors
Corrosion inhibitors function by adsorbing onto metal surfaces, forming a thin, protective film that isolates the metal from corrosive species. Based on their mode of action, inhibitors are classified as cathodic, anodic, or mixed-type. In transformer oils, mixed-type inhibitors like BTA are preferred due to their ability to simultaneously retard both anodic and cathodic reactions. BTA forms a stable, insoluble complex with copper ions, preventing their interaction with sulfur compounds and thus inhibiting Cu₂S formation.
## Optimal Dosage of Corrosion Inhibitors
The optimal dosage of corrosion inhibitors depends on several factors, including the type and concentration of corrosive sulfur compounds, oil temperature, and the specific inhibitor used. Studies have shown that BTA concentrations ranging from 50 to 200 ppm are effective in suppressing sulfur corrosion in transformer oils. However, excessive inhibitor concentrations can lead to adverse effects, such as increased oil viscosity or reduced dielectric properties.
### Dosage Determination Methods
1. **Laboratory Accelerated Aging Tests**:
Laboratory experiments involve adding known concentrations of corrosive sulfur compounds (e.g., DBDS) and BTA to fresh transformer oil, followed by accelerated thermal aging at elevated temperatures (e.g., 130°C). The corrosion rate is evaluated by measuring the deposition of Cu₂S on copper windings and insulation paper using techniques like scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDX). The optimal BTA dosage is determined based on the minimum concentration required to significantly reduce Cu₂S deposition.
2. **Electrochemical Analysis**:
Electrochemical methods, such as linear polarization resistance (LPR) and electrochemical impedance spectroscopy (EIS), are used to assess the corrosion inhibition efficiency of BTA. These techniques measure the polarization resistance of copper electrodes immersed in inhibitor-treated oil, providing quantitative data on the inhibitor's effectiveness. The optimal dosage corresponds to the concentration at which the polarization resistance reaches a maximum value, indicating maximal inhibition of the corrosion reaction.
3. **Field Trials**:
Field trials involve adding BTA to transformer oils in operational units and monitoring the corrosion rate over extended periods. Oil samples are periodically analyzed for Cu₂S content, dielectric properties, and inhibitor concentration. The optimal dosage is refined based on long-term performance data, ensuring compatibility with real-world operating conditions.
## Addition Methods of Corrosion Inhibitors
The addition of corrosion inhibitors to transformer oil must be carefully controlled to ensure uniform distribution and avoid contamination. The following methods are commonly employed:
1. **Offline Addition**:
In this method, the transformer is taken out of service, and the oil is drained into a clean, dry container. The required amount of BTA is dissolved in a small volume of oil and then mixed thoroughly with the bulk oil using a high-shear mixer or recirculation pump. The treated oil is then reintroduced into the transformer, and the unit is re-energized after ensuring proper oil circulation and air removal.
2. **Online Addition**:
For large transformers where offline treatment is impractical, online addition is preferred. A metering pump is used to inject a pre-calculated volume of BTA solution into the oil stream during normal operation. The inhibitor is gradually dispersed throughout the oil system via natural convection or forced circulation. Online addition requires precise control of the injection rate to avoid overdosing or underdosing.
3. **Inhibitor-Impregnated Materials**:
Some manufacturers offer inhibitor-impregnated materials, such as cellulose-based adsorbents, that can be placed inside the transformer tank. These materials slowly release the inhibitor into the oil over time, providing continuous protection against sulfur corrosion. This method is particularly useful for retrofitting existing transformers without the need for oil replacement.
## Conclusion
The addition of corrosion inhibitors like BTA to transformer insulating oil is an effective strategy for mitigating sulfur corrosion risks. The optimal dosage, typically ranging from 50 to 200 ppm, should be determined through laboratory tests, electrochemical analysis, and field trials. Addition methods, including offline and online treatment, as well as inhibitor-impregnated materials, offer flexibility in implementing corrosion protection measures. By adopting these best practices, power utilities can enhance the reliability and longevity of their transformer fleets, ensuring safe and efficient electricity transmission.
