In modern power systems, the widespread use of non-linear loads (such as variable frequency drives, UPS systems, LED lighting, and EV chargers) has led to increasingly severe harmonic pollution in electrical grids. Dry-type transformers, thanks to their excellent fire resistance and low maintenance requirements, are widely used in data centers, commercial buildings, industrial facilities, and renewable energy applications. However, harmonics can cause additional heating, accelerated insulation aging, increased noise, and even premature failure of dry-type transformers. Therefore, effectively mitigating the negative impact of harmonics on dry-type transformers is critical for ensuring safe and reliable power system operation. This article first discusses the hazards of harmonics and then systematically presents key technical measures to prevent harmonic effects on dry-type transformers, along with Xinhong Electrical's professional solutions in this field.
Before understanding “how to avoid,” it is necessary to understand the specific problems caused by harmonics:
Increased Eddy Current and Stray Losses
Harmonic currents generate high-frequency magnetic flux in the transformer windings and core, significantly increasing eddy current losses. Odd-order harmonics (especially 3rd, 5th, and 7th) intensify the skin effect and proximity effect in windings, making local overheating a major concern.
Increased Core Magnetostriction
Harmonics distort the core flux waveform, causing non-linear changes in magnetostriction, which in turn generates noticeable mechanical noise. Dry-type transformers rely on air cooling, making noise issues particularly prominent in quiet environments.
Additional Thermal Stress on Insulation System
The additional losses caused by harmonics cannot be fully dissipated by conventional cooling, so hot-spot temperatures may exceed the insulation class limits (e.g., Class F or H). Long-term operation under such conditions accelerates aging of epoxy resin and other insulating materials, reducing service life.
Neutral Point Overload Risk
In a star-connected secondary side, 3rd-order harmonics and their multiples accumulate in the neutral conductor, causing abnormally high neutral current – sometimes even exceeding phase current – leading to overheating or burnout of neutral connection components.
Voltage Distortion and System Interference
Harmonic currents flowing through transformer impedance produce harmonic voltage drops, distorting the output voltage waveform and affecting the normal operation of downstream sensitive loads.
K-factor is an important indicator of a transformer’s ability to withstand harmonic currents. Standard dry-type transformers have a K-factor of 1 and are suitable only for pure sinusoidal loads. When non-linear loads exceed 20% of the total load, a harmonic-mitigating transformer with K-4, K-9, K-13, or even K-20 rating should be selected.
Design features include:
Multi-segmented or interleaved winding structures to reduce eddy current losses;
Increased conductor cross-section and use of multi-strand or foil winding to suppress skin effect;
Larger core cross-section and flux density margin to avoid saturation;
Enhanced cooling channels to ensure hot-spot temperature rise is kept under control.
Xinhong Electrical offers a full range of K-factor dry-type transformers (K-4 to K-20). By optimizing winding arrangement and using high-permeability, low-loss silicon steel, these transformers reduce additional temperature rise by more than 30% in harmonic environments while maintaining low noise operation.
Addressing harmonics at the system level is more effective than forcing the transformer to endure them alone. Common solutions include:
Passive harmonic filters: Composed of reactors and capacitors, they provide a low-impedance path for specific frequencies (e.g., 5th, 7th) to absorb harmonic currents. Low cost, suitable for applications with relatively stable loads.
Active harmonic filters (AHF): They detect harmonic currents in real time and inject compensating currents, achieving dynamic harmonic mitigation. Total harmonic distortion (THD) can be reduced to below 5%. For demanding applications such as data centers and precision manufacturing, AHF is the best choice.
It is recommended to install an active filter on the secondary side (low-voltage bus) of the dry-type transformer and coordinate it with the transformer’s operation. Xinhong Electrical provides an integrated “dry-type transformer + AHF” solution, with parameters matched and commissioning completed before delivery, reducing on-site engineering time.
The correct connection method can block or attenuate specific harmonic orders:
Dyn11 connection (delta primary, star secondary with neutral) is the most recommended scheme. The delta connection provides a closed path for zero-sequence currents and 3rd harmonics, preventing 3rd harmonics from propagating to the primary side while reducing neutral current.
Yyn0 connection is not recommended for applications with a high proportion of non-linear loads, as it cannot suppress 3rd harmonics and the neutral point is prone to overload.
All industrial-grade dry-type transformers from Xinhong Electrical default to Dyn11 connection, and other special connections (such as Dd0, Yd11, etc.) can be customized according to user requirements, structurally reducing harmonic impact.
When harmonics exist in the system but are not fully mitigated, the simplest and most reliable method is to derate the transformer. According to IEEE Std C57.110, the derating factor for a dry-type transformer can be calculated based on the measured harmonic current spectrum and K-factor. For example:
When the total harmonic distortion of load current (THDi) = 30%, mainly consisting of 5th and 7th harmonics, it is recommended to derate to 80% of rated capacity;
When THDi exceeds 50%, derating to 60% or even lower may be necessary.
Derating sacrifices equipment utilization but prevents overheating and burnout. For new projects, Xinhong Electrical provides free harmonic compatibility calculation services to help users accurately select the right transformer, avoiding wasted investment due to excessive derating.
For three-phase four-wire systems with a large number of single-phase non-linear loads (such as computers, LED power supplies), neutral current can reach more than 1.5 times the phase current. To address this:
Specify that the neutral terminal and internal neutral conductor of the transformer have a cross-section not less than 1.5 times that of the phase conductor;
Or add a third-harmonic filter winding inside the transformer, using electromagnetic cancellation to confine 3rd harmonic energy within the transformer without flowing back to the grid.
Xinhong Electrical’s special dry-type transformers can integrate a zero-sequence harmonic cancellation winding, reducing neutral 3rd harmonics by approximately 70% without external filters – ideal for office buildings, hospitals, data centers, and similar applications.
In harmonic environments, transformer losses are higher, so cooling must be enhanced:
Ensure the dry-type transformer is installed in a well-ventilated indoor space with ambient temperature not exceeding 40°C;
Add forced air cooling (fans) with a thermostat – when winding temperature exceeds the set point, fans start automatically;
Regularly clean dust from air ducts and the core surface to avoid cooling efficiency degradation.
Xinhong Electrical dry-type transformers come standard with an intelligent temperature monitoring system that measures hot-spot temperatures of all three phases in real time, and provides fan control and alarm dry contacts for easy integration into user monitoring systems.
A large data center originally used two 1600 kVA dry-type transformers to power IT equipment. Due to a large number of switching power supplies, measured 5th harmonic current was 28% and 7th harmonic 15%. This caused transformer operating temperatures to reach 145°C (Class F insulation maximum allowable is 155°C), noise exceeded 65 dBA, and the neutral connection bar showed frequent discoloration due to overheating.
Xinhong Electrical provided the following solution:
Replaced the old transformers with K-13 rated harmonic-mitigating dry-type transformers using foil windings and an interleaved core structure;
Installed two 300 A active harmonic filters (AHF) in the low-voltage main incoming switchgear, operating in redundant mode;
Changed transformer connection from Yyn0 to Dyn11 and increased neutral conductor cross-section;
Added independent forced-air cooling and a temperature control unit.
After the upgrade, the hot-spot temperature of the transformers under full load dropped to 118°C, noise decreased to 52 dBA, system THD was reduced from 35% to 3.8%, and neutral current dropped from 280 A to 45 A. The user reports that the equipment has been operating stably for more than two years without any overheating or insulation failures.
Avoiding harmonic effects on dry-type transformers is not a single measure but requires a comprehensive approach covering equipment selection, system-level mitigation, and operation & maintenance. The key points are summarized below:
| Measure | Application Scenario | Effect |
|---|---|---|
| Select K-factor transformer | Known harmonics but cannot be fully eliminated | Directly improves transformer’s harmonic withstand capability |
| Install active harmonic filter | High power quality requirements, variable loads | Dynamic harmonic elimination, THD < 5% |
| Dyn11 connection | All three-phase four-wire systems | Blocks 3rd harmonics, reduces neutral current |
| Derating | Existing transformers coping with harmonics temporarily | Simple and effective but reduces utilization |
| Enhance cooling and monitoring | All harmonic-prone applications | Prevents heat accumulation, extends life |
For new projects, it is recommended to commission harmonic simulation assessment from a professional manufacturer during the design stage. Xinhong Electrical has over 15 years of experience in harmonic mitigation for dry-type transformers, offering a full suite of services from harmonic measurement, K-factor selection, filter configuration, to intelligent monitoring. Our products comply with IEC 60076, GB 1094, ANSI/IEEE C57.12, and other international standards, and can be customized as harmonic-tolerant, ultra-low noise, or compact dry-type transformers according to customer requirements.
To learn more about selecting the most economical harmonic mitigation solution based on your load characteristics, please contact the Xinhong Electrical technical team for free consultation and selection guidance.
This article was written by the Xinhong Electrical Technical Center to share engineering best practices for harmonic protection of dry-type transformers. Reproduction without permission is prohibited.
