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IEC TR 61620: Insulating Liquids — Estimation of Acidity by Automatic Titration
IEC TR 61620 is a technical report that provides guidance on the estimation of acidity in insulating liquids using automatic potentiometric titration. The acidity of insulating oil — quantified as the neutralization number (NN) or acid number — is one of the most important indicators of oil degradation in electrical equipment such as power transformers, load tap changers, and circuit breakers. As insulating oil ages under the combined effects of heat, oxygen, moisture, and electrical stress, hydrocarbon molecules undergo oxidation reactions that produce acidic byproducts. These acids accelerate further degradation, increase the oil’s conductivity, corrode metallic components, and impair the paper insulation’s mechanical and dielectric properties.
Critical: An increasing acid number is often the first detectable sign of transformer oil degradation — preceding visible changes such as discoloration, sludge formation, or significant changes in dielectric breakdown voltage. Regular acidity monitoring per IEC TR 61620 enables early intervention before irreversible damage occurs to the paper insulation system.
1. Principles of Acidity Determination
IEC TR 61620 describes the estimation of both strong acid number (SAN) and total acid number (TAN) in insulating liquids. The strong acid number measures the content of mineral acids (such as sulfuric, hydrochloric, and nitric acids) that may be introduced through external contamination or severe oxidation. The total acid number measures all acidic constituents, including both strong acids and weak organic acids (naphthenic acids, carboxylic acids, and phenolic compounds) formed during oil oxidation.
The method employs automatic potentiometric titration using a glass indicating electrode and a calomel or silver/silver chloride reference electrode. The oil sample (typically 10–20 g) is dissolved in a titration solvent consisting of toluene, isopropyl alcohol, and a small amount of water. The solution is titrated with standardized alcoholic potassium hydroxide (KOH) solution (0.02 N or 0.1 N, depending on the expected acidity range) while the potential is continuously monitored. The endpoint is determined from the inflection point of the potential-versus-volume titration curve.
The standard specifies two titration procedures. Procedure A determines the total acid number by titrating from the initial potential to the inflection point corresponding to neutralization of all acidic species. Procedure B separately determines the strong acid number by first titrating to an endpoint at pH 3–4 (where strong acids are neutralized) and then continuing to the inflection point at pH 10–12 (where weak acids are neutralized).
Tip: The titration solvent composition is critical for reliable results. A ratio of 500 mL toluene : 495 mL isopropyl alcohol : 5 mL water provides optimum solubility for both fresh (low-acidity) oils and aged (high-acidity) oils while maintaining adequate conductivity for stable potentiometric measurements.
2. Automatic Titration Equipment and Procedures
IEC TR 61620 specifies the requirements for automatic titration equipment. The titrator must be capable of delivering titrant in increments of 0.01 mL or smaller, with a potential measurement resolution of 0.1 mV. The titration rate should not exceed 0.1 mL/min near the endpoint to ensure accurate detection of the inflection point. Modern automated titrators satisfy these requirements and offer the additional advantage of programmable methods, automatic sample changers, and data archiving.
The procedure requires careful electrode maintenance. The glass electrode must be stored in distilled water when not in use, and the reference electrode must be filled with the appropriate electrolyte (saturated KCl for calomel electrodes, 3 M KCl for Ag/AgCl electrodes). Before each series of measurements, the electrode system must be calibrated using standard buffer solutions at pH 4.0, 7.0, and 10.0. The standard also recommends daily verification of the KOH titrant concentration against a primary standard (typically potassium hydrogen phthalate, KHP).
A blank titration (solvent only, without oil sample) must be performed with each batch of samples. The blank correction is subtracted from the sample titration volume to account for acidity contributed by the solvent system. The standard specifies that the blank titration volume should not exceed 0.1 mL of 0.02 N KOH; higher values indicate contamination of the solvent system and require investigation.
| Parameter | Procedure A (Total Acid Number) | Procedure B (Strong Acid Number) |
|---|---|---|
| Sample size | 10–20 g | 10–20 g |
| Titration solvent | Toluene/IPA/water (500:495:5) | Toluene/IPA/water (500:495:5) |
| Titrant | 0.02 N KOH (TAN < 0.1) 0.1 N KOH (TAN ≥ 0.1) |
0.02 N KOH |
| Titration endpoint | Inflection point (full curve) | pH 3–4 for SAN; inflection for TAN |
| Detection limit | Approximately 0.01 mg KOH/g | Approximately 0.005 mg KOH/g |
| Repeatability | 0.01 mg KOH/g (TAN < 0.1) | 0.005 mg KOH/g |
| Typical analysis time | 10–15 min per sample | 15–20 min per sample |
Warning: Carbon dioxide (CO₂) from the atmosphere dissolves in the titration solvent, forming carbonic acid that contributes a positive bias to the acid number. The titration vessel must be purged with nitrogen gas (99.995% purity) at a flow rate of 50–100 mL/min throughout the titration to exclude CO₂. This is especially important for low-acidity oils (TAN < 0.05 mg KOH/g) where atmospheric CO₂ can introduce errors exceeding 50% of the measured value.
3. Interpretation of Acid Number Results and Engineering Actions
IEC TR 61620 provides guidance on interpreting acidity results for transformer condition assessment. The acid number is not a standalone indicator — it must be evaluated alongside other oil quality parameters such as dielectric breakdown voltage, interfacial tension, water content, and dissolved gas analysis (DGA). However, the acid number trend over time is one of the most reliable indicators of the oil oxidation rate.
The following guidelines are generally accepted for mineral insulating oils in power transformers: a TAN below 0.05 mg KOH/g indicates oil in good condition; 0.05–0.10 mg KOH/g indicates moderate degradation requiring increased monitoring frequency; 0.10–0.20 mg KOH/g indicates significant degradation where oil reclamation (clay treatment or vacuum treatment) should be considered; above 0.20 mg KOH/g indicates severe degradation requiring immediate action, which may include oil replacement or regeneration.
For the paper insulation system, the acid number is correlated with the degree of polymerization (DP) of cellulose. Acids catalyze the hydrolysis of cellulose, accelerating the depolymerization process that reduces the mechanical strength of paper insulation. A TAN exceeding 0.15 mg KOH/g is often associated with a significant acceleration of cellulose degradation, particularly in transformers with high moisture content. This correlation makes acidity monitoring a valuable tool for estimating remaining paper life.
Design Insight: The acid number of transformer oil typically follows an exponential growth pattern over the service life. A sudden change in the rate of acidity increase (an inflection in the TAN vs. time curve) often signals a change in the degradation mechanism — such as the onset of catalytic oxidation by copper ions, depletion of the oxidation inhibitor (DBPC), or increased moisture ingress. Identifying these inflection points enables targeted maintenance actions, such as inhibitor replenishment or online drying.
| TAN Range (mg KOH/g) | Oil Condition | Recommended Action | Monitoring Frequency |
|---|---|---|---|
| < 0.05 | Good | None required | Annual |
| 0.05 – 0.10 | Moderate | Increase monitoring; check inhibitor level | 6 months |
| 0.10 – 0.20 | Degraded | Plan oil reclamation; check DGA | 3 months |
| 0.20 – 0.40 | Severe | Oil reclamation or replacement | Monthly |
| > 0.40 | Critical | Immediate oil replacement; assess paper condition | Continuous |
FAQs
Q1: What is the difference between acid number and neutralization number?
A: Both terms are used interchangeably in the context of insulating oil analysis and both represent the quantity of base (KOH) required to neutralize the acidic constituents in 1 gram of oil. The units are mg KOH/g. The term “neutralization number” is more common in North America (ASTM D974), while “acid number” is more common in Europe (IEC standards). IEC TR 61620 uses “estimation of acidity” but the reported value is functionally equivalent.
A: Both terms are used interchangeably in the context of insulating oil analysis and both represent the quantity of base (KOH) required to neutralize the acidic constituents in 1 gram of oil. The units are mg KOH/g. The term “neutralization number” is more common in North America (ASTM D974), while “acid number” is more common in Europe (IEC standards). IEC TR 61620 uses “estimation of acidity” but the reported value is functionally equivalent.
Q2: Can acidity measurement be performed on in-service transformers without taking an oil sample?
A: While online sensors for moisture and dissolved gases are well established, reliable online acidity sensors remain an active area of research. The electrochemical sensors developed to date suffer from drift, interference from other oil constituents, and limited long-term stability under transformer operating conditions. For now, periodic sampling and laboratory analysis per IEC TR 61620 remains the standard approach for acidity monitoring.
A: While online sensors for moisture and dissolved gases are well established, reliable online acidity sensors remain an active area of research. The electrochemical sensors developed to date suffer from drift, interference from other oil constituents, and limited long-term stability under transformer operating conditions. For now, periodic sampling and laboratory analysis per IEC TR 61620 remains the standard approach for acidity monitoring.
Q3: How does the presence of oxidation inhibitor (DBPC) affect the acidity measurement?
A: DBPC (2,6-di-tert-butyl-p-cresol) and similar phenolic antioxidants do not directly interfere with the potentiometric titration because they are not acidic at the titration endpoint pH. However, the oxidation byproducts of DBPC (such as stilbenequinone and hydroperoxycyclohexadienones) do contribute to the acid number. A sudden increase in TAN accompanied by a decrease in DBPC concentration indicates that the inhibitor has been depleted and the oil is entering rapid oxidation.
A: DBPC (2,6-di-tert-butyl-p-cresol) and similar phenolic antioxidants do not directly interfere with the potentiometric titration because they are not acidic at the titration endpoint pH. However, the oxidation byproducts of DBPC (such as stilbenequinone and hydroperoxycyclohexadienones) do contribute to the acid number. A sudden increase in TAN accompanied by a decrease in DBPC concentration indicates that the inhibitor has been depleted and the oil is entering rapid oxidation.
Q4: Why is the sample size important for accurate acidity measurement?
A: The sample size determines the titration endpoint volume. For optimal accuracy, the endpoint volume should be between 2 mL and 8 mL of titrant. A sample that is too large may require more titrant than the burette capacity (typically 10–20 mL), while a sample that is too small results in a small endpoint volume where the relative error from the blank correction and endpoint detection becomes significant. IEC TR 61620 recommends adjusting the sample size based on the expected acidity range.
A: The sample size determines the titration endpoint volume. For optimal accuracy, the endpoint volume should be between 2 mL and 8 mL of titrant. A sample that is too large may require more titrant than the burette capacity (typically 10–20 mL), while a sample that is too small results in a small endpoint volume where the relative error from the blank correction and endpoint detection becomes significant. IEC TR 61620 recommends adjusting the sample size based on the expected acidity range.
