Industrial Transformers: Dry Type vs Oil Type. Efficiency and Harmonic Distortions

1. Introduction: The challenge of reliability and functionality of transformers

Industrial transformers are critical components of electrical systems, providing voltage conversion to power production equipment. Their smooth operation is the basis of the reliability and efficiency of technological processes. The choice between dry-type and oil-type transformers, as well as consideration of their efficiency and the effect of harmonic distortion, has a direct impact on operating costs, safety and durability of the equipment. Incorrect selection or insufficient maintenance can lead to significant financial losses due to downtime, equipment damage and high power consumption. For Ukrainian industrial enterprises working in difficult operating conditions and striving for optimization, understanding these aspects is a top priority.

2. Fundamental Principles of Operation of Transformers

A transformer is a static electrical device that, using electromagnetic induction, converts electrical energy of one alternating current voltage into electrical energy of another alternating current voltage of the same frequency. The main components are a magnetic wire (core) and two or more windings, electrically isolated from each other, but magnetically connected. When an alternating voltage is applied to the primary winding, an alternating magnetic flux is created in the core, which induces an electromotive force (EMF) in the secondary winding, proportional to the number of turns. The transformation coefficient K = N1/N2 = U1/U2, where N is the number of turns, U is the voltage.

2.1. Dry Transformers

Dry transformers use air or a solid dielectric (such as epoxy resin) as an insulating and cooling medium. Windings are usually made of copper or aluminum and can be either completely cast resin (Cast Resin Dry-Type, CRT) or impregnated with varnish (Vacuum Pressure Impregnated, VPI). The absence of liquid reduces the risk of fire and leaks.

2.2. Oil Transformers

Oil transformers use mineral or synthetic insulating oil, which simultaneously serves as a dielectric and an effective cooling medium. The windings and core are immersed in oil in a sealed tank or a tank with an expansion tank. The oil removes heat from the windings and isolates them from the grounded parts of the tank.

3. Technical Characteristics and Standards

The choice of transformer is based on a comprehensive analysis of technical parameters regulated by international and national standards.

3.1. Dry Transformers

• Insulation: Insulation classes (F, H) determine the maximum permissible temperature of the windings. Class F (155°C), Class H (180°C). The overshoot temperature of the windings usually does not exceed 100-115°C for class F and 125-140°C for class H.
• Degree of protection: IP00 (without protective cover), IP21 (against drops of water and solid particles >12.5 mm), IP23, IP31. Protection against external influences according to IEC 60529 / DSTU EN 60529.
• Environmentally friendly: Low risk of contamination.
• Standards: IEC 60076-11 / DSTU EN 60076-11 (Power transformers. Part 11: Dry transformers).

3.2. Oil Transformers

• Insulating oil: Mineral oil (eg transformer oil from IEC 60296) or synthetic dielectric fluids (eg esters). The ignition temperature of mineral oil is usually higher than 140°C, the flash point is higher than 170°C.
• Cooling: ONAN (natural air cooling with natural oil circulation), ONAF (natural air cooling with forced air circulation and natural oil circulation).
• Expansion systems: With expansion tank or sealed.
• Standards: IEC 60076-1 / DSTU EN 60076-1 (Power transformers. Part 1: General provisions).

3.3. Efficiency

Transformer efficiency (η) is defined as the ratio of output power to input power, expressed as a percentage. η = (Pout / Pin) * 100%. Power losses consist of:

• No-load losses (P₀): Losses in the core (for remagnetization and eddy currents). Usually 0.1-0.3% of the rated power.
• Short-circuit losses (P_k): Losses in windings (copper) due to ohmic resistance and eddy currents. Depends on the load (proportional to the square of the load current). Usually 1.5-3.0% of the rated power at full load.

Modern transformers have an efficiency of 97-99.5%. EN 50588-1 / DSTU EN 50588-1 establishes energy efficiency classes such as A0, Ak for dry and N0, Nk for oil transformers, indicating the level of losses. This is critical to minimizing operating costs throughout the life cycle.

3.4. Harmonic Distortions

Harmonic distortions are caused by non-linear loads in the network (for example, frequency converters, uninterruptible power supplies, LED lighting, induction furnaces). These distortions create currents and voltages multiples of the main frequency (50 Hz in Ukraine). Harmonic currents flow through the transformer windings, increasing heat losses (especially eddy currents) and causing additional mechanical loads. This can lead to:

• Overheating of the windings and the core, which shortens the service life of the insulation.
• Reduction of transformer efficiency.
• Resonance phenomena in the network.
• False protections.

For systems with a high content of harmonics, transformers with a special design are used, for example, K-factor transformers (IEEE C57.110), which are designed to work with non-linear loads. K-factor from 1 to 50 indicates the ability of the transformer to withstand additional losses from harmonics.

4. Selection and Calculation Guide

Optimum selection of a transformer requires consideration of operating conditions, type of load and economic indicators.

4.1. Selection factors

• Operating environment: Temperature, humidity, pollution, altitude above sea level (change in dielectric strength of air). For aggressive environments or rooms with a high fire hazard (for example, woodworking shops, mines), dry transformers are preferred.
• Type of load: Standard transformers are suitable for linear loads. For non-linear loads with THD_I (total current harmonic distortion) greater than 5-10%, K-factor transformers should be considered.
• Safety requirements: Dry transformers are self-extinguishing and do not emit toxic gases, which makes them ideal for hospitals, shopping centers, offices, subways.
• Cost: The initial investment for dry-type transformers can be higher, but their running costs (due to less maintenance and no oil) are usually lower.
• Noise level: Dry transformers, as a rule, have a lower noise level (by 5-10 dB) compared to oil transformers, which is important for objects with increased acoustic requirements.

4.2. Power calculation

The nominal power of the transformer Snom should be greater than the maximum calculated power of the load Ssp, taking into account the reserve factor of 1.1-1.2.

For loads with harmonic distortions, additional losses must be taken into account. The equivalent load current, which produces the same heating as a non-sinusoidal current, is calculated as:

Iequ = √(I12 + Σh=2H (Ih2 * h2)), where I1 is the fundamental frequency current. Ih is a harmonic current of order h.

Or by the K factor: Sk = Snom / (1 + K * THDI2) (rough estimate).

5. Best Practices for Installation and Commissioning

Correct installation and commissioning is the key to long-term and safe operation of the transformer.

5.1. Preparation of the Maidanchik

• Foundation: Strong, even, able to withstand the weight of the transformer, taking into account dynamic loads.
• Ventilation: Adequate natural or forced ventilation for heat removal is critical for dry transformers. The temperature in the room should not exceed 40°C.
• Distances: Compliance with minimum distances to walls, other equipment and service passages according to PUE and manufacturer's instructions.
• Grounding: Reliable grounding system (protective and working) in accordance with DSTU EN 50522 (IEC 61936-1) to ensure the safety of personnel and equipment.

5.2. Electrical Connections

• Cables: Selection of cable cross-section according to nominal currents and permissible voltage losses. Application of flexible connections to compensate for thermal expansion.
• Connection: Phasing, observing the sequence of phases (for example, L1, L2, L3). Polarity check.
• Tightening: All contact connections must be tightened to the specified torque to avoid overheating.

5.3. Checks Before Energization

• Insulation resistance measurement: Using a megohmmeter (eg 2500V) between windings, windings and ground. The values ​​must comply with the manufacturer's norms and standards (for example, at least 100 MΩ for new transformers).
• Checking the transformation coefficient: Using special devices. The deviation should not exceed ±0.5% for the corresponding solders.
• Checking the winding connection group: For three-phase transformers.
• Protections: Checking the settings and functionality of relay protection and automation.
• Oil (for oily): Check the level, no air in the expander. Oil sampling for analysis (dielectric strength, moisture content).

6. Failure Modes and Root Cause Analysis

Understanding typical failures allows you to prevent and eliminate them in a timely manner.

6.1. Typical Failures of Dry Transformers

• Overheating of windings: The main reason is overload, insufficient ventilation, high ambient temperature or excessive content of harmonics. Visually - a change in the color of the insulation, the appearance of the smell of burnt insulation.
• Degradation of insulation: Prolonged exposure to elevated temperatures, mechanical damage, exposure to aggressive chemicals.
• Damage to windings: Short circuits, overvoltages (thunderstorm, commutation), mechanical vibrations.
• Cracking of the compound (for CRT): Under rapid temperature changes or cyclic loads, which creates a path for partial discharges.

6.2. Typical Failures of Oil Transformers

• Degradation of insulating oil: Decrease in dielectric strength due to the content of water, aging products, and mechanical impurities. Leads to overlap of insulation.
• Partial discharges and insulation breakdowns: Caused by aging insulation, local defects, high electric fields, overvoltages.
• Oil leaks: Due to leaky welds, seals. Low oil level leads to overheating and risk of breakdown.
• Input damage: Mechanical damage, surface contamination, aging of porcelain or polymer material. Visually - cracks, traces of the crown.
• Overheating of the windings/core: Overload, siltation of the cooling channels, malfunction of the cooling system (for ONAF), high harmonics.

6.3. Harmonic Refusals

Harmonic currents cause additional losses in the core and windings, especially in copper (eddy currents). This leads to local overheating, even if the total load current does not exceed the nominal one. The main reason is that the K-factor is not taken into account when choosing a transformer for non-linear loads.

7. Predictive Maintenance and Condition Monitoring

The implementation of predictive maintenance systems allows to identify the initial stages of defects and prevent emergency failures.

7.1. Monitoring of Dry Transformers

• Thermography: Regular thermal imaging control (for example, once every six months) using a thermal imager to detect overheating of contacts, windings, and tires. A temperature difference of up to 10-15°C from normal is permissible.
• Partial Discharge (PD) Monitoring: The PD measurement can indicate defects in the winding insulation or compound. Acoustic sensors or high-frequency current transformers are used.
• Measurement of winding temperature: Installation of PT100 or PT1000 sensors for constant monitoring of temperature and alarm activation when the set thresholds are exceeded.
• Visual inspection: Inspection for the presence of dust, dirt, cracking of the compound. Regular cleaning (for example, once a year).

7.2. Monitoring of Oil Transformers

• Dissolved gas chromatographic analysis (DGA): The most important diagnostic method. Analysis of gases (H₂, CH₄, C₂H₆, C₂H₄, C₂H₂, CO, CO₂) in oil allows to determine the type of defect (partial discharges, arc discharges, thermal damage to the windings or core). It is conducted annually or more often if suspected.
• Physico-chemical analysis of oil: Measurement of dielectric strength, acid number, water content, dielectric loss angle tangent (tgδ). Determines the degree of oil aging and contamination.
• Measuring the tangent of the dielectric loss angle (tgδ) of insulation: For windings and input. Detects moisture and insulation aging.
• Oil level and temperature monitoring: Constant control using sensors.
• Thermography: Similar to dry transformers, to detect overheating of contacts, input, radiators.

7.3. Harmonic monitoring

Use of power quality analyzers to measure the total harmonic distortion coefficient of voltage (THD_U) and current (THD_I). These data make it possible to estimate the level of harmonic pollution of the network and its potential impact on the transformer. Regular monitoring (for example, once a quarter) allows timely response to changes in the load.

8. Alternative Comparison Matrix

The choice of a particular type of transformer depends on the balance between requirements for safety, reliability, efficiency and cost.

9. Conclusion

Transformer selection is an engineering decision that requires a thorough understanding of operational requirements, environmental conditions, load characteristics, and long-term economic implications. Dry-type transformers offer high fire safety and low operating costs, while oil-type transformers offer high reliability for large capacities and outdoor installation. In today's industrial environment, where non-linear loads are increasingly common, consideration of harmonic distortion and selection of K-factor transformers is critical to ensure durability and efficiency. UNITEC-D GmbH is a reliable supplier of transformers, cable products and related equipment that meets the highest quality and safety standards, including CE and UkrSEPRO certificates.

For a more detailed introduction to the assortment and technical characteristics, we invite you to visit our electronic catalog UNITEC-D.

10. Links

1. IEC 60076-1. Power transformers – Part 1: General.
2. IEC 60076-11. Power transformers – Part 11: Dry-type transformers.
3. EN 50588-1. Medium power transformers 50 Hz, with the highest voltage for equipment not exceeding 36 kV – Part 1: General requirements.
4. IEEE C57.110. IEEE Recommended Practice for Establishing Liquid-Filled and Dry-Type Power and Distribution Transformer Capability When Supplying Non-Sinusoidal Load Currents.
5. DSTU EN 50522. Grounding of power supply installations with a nominal voltage of more than 1 kV alternating current.