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Use high-magnetic-conductivity silicon steel core to ensure a stable and clean power supply waveform.
2026-07-06
Select high-magnetic-conductivity silicon steel core to reduce grid harmonics and ensure stable and clean power supply waveform
In modern industrial power grids, frequency converters, servo drives, switching power supplies, and silicon-controlled rectifier power control devices have been widely adopted. The grid voltage is no longer a standard, pure sine wave, and the harmonic components are increasing. Third, fifth, and seventh high-frequency harmonics are superimposed on the 50Hz power frequency. This not only causes voltage waveform distortion but also leads to a sharp increase in no-load current, overheating of the core, increased noise, and more spikes in the output voltage in ordinary power frequency transformers. Many industrial control equipment power supplies frequently experience baseline jitter and sampling data drift. Repeatedly adding multiple-level filtering circuits has little effect. The root cause is that ordinary low-grade silicon steel sheets are highly sensitive to harmonics, and the magnetic flux is prone to distortion.
The core is the core of energy conversion in transformers. The grain orientation, magnetic permeability, and iron loss indicators of silicon steel sheets directly determine the transformer's ability to resist harmonic distortion. High-magnetic-damping silicon steel sheets have low hysteresis loss and a wide linear range. When faced with harmonic disturbances, the magnetic flux is less likely to saturate, which can effectively filter out some high-frequency noise and make the secondary output voltage closer to the standard sine wave.
- How Grid Harmonics Affect the Normal Operation of Transformers
1. Harmonic Currents Intensify Core Magnetic Saturation
The ideal power frequency is only the fundamental wave at 50Hz. The transformer core operates within the linear magnetization range. When the grid mixes in a large amount of high-frequency harmonics, the instantaneous voltage peak will be continuously raised, and the superimposed instantaneous flux will far exceed the design value. Ordinary silicon steel sheets have a relatively low inflection point in their magnetization curve, and are easily prone to entering the magnetic saturation zone. Once the core is saturated, the excitation current will sharply increase, and the current waveform will be severely peaked. The originally smooth sinusoidal excitation current becomes a sharp pulse, further transmitting high-frequency noise to the secondary circuit, causing distortion in the output voltage waveform. Eventually, this manifests as severe overheating of the transformer when no load is applied, increased buzzing noise, a large number of spikes in the downstream power supply, and continuous jittering in the analog signal acquisition.
The ideal power frequency is only the fundamental wave at 50Hz. The transformer core operates within the linear magnetization range. When the grid mixes in a large amount of high-frequency harmonics, the instantaneous voltage peak will be continuously raised, and the superimposed instantaneous flux will far exceed the design value. Ordinary silicon steel sheets have a relatively low inflection point in their magnetization curve, and are easily prone to entering the magnetic saturation zone. Once the core is saturated, the excitation current will sharply increase, and the current waveform will be severely peaked. The originally smooth sinusoidal excitation current becomes a sharp pulse, further transmitting high-frequency noise to the secondary circuit, causing distortion in the output voltage waveform. Eventually, this manifests as severe overheating of the transformer when no load is applied, increased buzzing noise, a large number of spikes in the downstream power supply, and continuous jittering in the analog signal acquisition.
2. High-frequency harmonics significantly increase core eddy current losses
Harmonics contain a large number of high-frequency components. The higher the frequency, the more intense the eddy current effect within the silicon steel sheet. Ordinary hot-rolled silicon steel and low-grade non-oriented silicon steel sheets have low resistivity and, when faced with high-frequency harmonics, the eddy current losses increase exponentially. Even if the transformer is not under load, the core will continue to abnormally heat up. The temperature rise further compresses the magnetic flux linear range, forming a vicious cycle of "harmonics → increased losses → temperature rise → more prone to saturation". Operating in a harmonic power grid for a long time, the insulation of ordinary core transformers will age at a rate that is more than twice as fast.
3. Harmonic disturbances cause primary-secondary voltage coupling distortion
High-frequency noise will pass through the parasitic capacitance of the winding and directly enter the low-voltage output circuit through the transformer. The low-magnetic-permeability core has unstable magnetic field, and the magnetic flux oscillates repeatedly, which will amplify the harmonic coupling effect. At this time, simply relying on the rear-end LC filter can only handle a part of the conducted interference, and the waveform distortion caused by the source magnetic flux distortion is difficult to be completely eliminated. Many industrial control equipment power supplies have high background noise and unstable voltage baseline. The essence is that the core's ability to resist harmonics is insufficient.
- Core Performance Advantages of High Magnetic Conductivity Oriented Silicon Steel Sheets
High magnetic conductivity oriented silicon steel (GO silicon steel), after undergoing high-temperature rolling and grain orientation treatment, the magnetic domains are neatly arranged along the rolling direction, forming a distinct generation gap from ordinary non-oriented silicon steel (NGO), and is also the most critical hardware upgrade for resisting grid harmonics.
1. Initial magnetic permeability is extremely high, and the linear working range is wider.
High-grade oriented silicon steel can achieve a very high flux density even under low magnetic fields. The magnetization curve is gentle, and the linear segment is longer. In the face of instantaneous voltage surges caused by harmonics, the core is less likely to enter saturation prematurely. Under the same input distorted waveform, the high-magnetic-permeability core can still stabilize the flux within the linear region, and the excitation current will not form a sharp pulse. It suppresses current distortion from the source and keeps the excitation waveform smooth. In simple terms: Ordinary cores saturate when encountering harmonics, and the current distortion becomes increasingly severe; high-magnetic-permeability cores have strong shock resistance, the fundamental flux is stable, and high-frequency harmonics are difficult to trigger sharp current pulses.
2. The iron loss is extremely low, significantly reducing the temperature rise caused by harmonics.
The losses caused by harmonics mainly consist of eddy current loss and hysteresis loss. High-dielectric orientation silicon steel sheets not only have a narrow hysteresis loop, low hysteresis loss, and higher sheet resistivity, but also effectively suppress the eddy current at high frequencies. In a non-sinusoidal grid containing harmonics, for the same-sized iron core, the total iron loss of high-grade silicon steel is 30% to 50% lower than that of ordinary silicon steel. Even when subjected to voltage distortion for a long time, the no-load temperature rise remains very low, and it will not be further compressed due to heating. The transformer temperature is stable, the magnetization characteristics remain consistent over the long term, and the power supply waveform does not gradually deteriorate with the operating time.
3. The magnetic flux is uniform and stable, reducing the outward transmission of noise.
The silicon steel grains are arranged in a regular pattern, and the magnetic flux distribution of the entire core is uniform. There will be no phenomenon of local magnetic flux congestion or local magnetic saturation. The high-frequency noise in the power grid is difficult to cause intense oscillations in the magnetic field, and the electromagnetic conversion between the primary and secondary is more stable. High-frequency harmonics are difficult to be transmitted to the secondary winding through magnetic coupling. Compared with recycled silicon steel sheets and hot-rolled iron sheets, the brand-new high-conductivity oriented silicon steel can effectively smooth the voltage waveform, filter out the distorted peaks to a certain extent, make the secondary output waveform closer to the standard sine wave, and significantly reduce the noise level of the downstream power supply.
4. The magnetic expansion is smaller, reducing the vibration noise caused by harmonics.
Harmonics cause the magnetic field to fluctuate violently repeatedly, and the ordinary iron core vibrates at a high frequency, generating chaotic whistling and buzzing sounds. The high-conductivity-oriented silicon steel has an extremely low magnetic expansion coefficient. Under alternating magnetic fields, its deformation is small. Even if there is voltage distortion, the vibration amplitude of the iron core can be controlled at a very low level, which not only reduces noise but also avoids the displacement of the winding due to vibration, maintaining the stability of electrical parameters.
- Classification standards for silicon steel material selection in harmonic environments
Grade 1: Ordinary civilian weak harmonic power grid
Ordinary indoor lighting, single-phase stabilized power supply. The grid has low harmonic content, and conventional non-oriented cold-rolled silicon steel sheets can be selected. The cost is low, and it can meet the requirements of smooth sinusoidal operation.
Ordinary indoor lighting, single-phase stabilized power supply. The grid has low harmonic content, and conventional non-oriented cold-rolled silicon steel sheets can be selected. The cost is low, and it can meet the requirements of smooth sinusoidal operation.
Grade 2: Industrial control workshop, with a moderate harmonic environment in the frequency converter area
PLC power supply, control cabinet isolation transformer, instrument and meter power supply. The power grid contains a large amount of 5th and 7th harmonic waves. It is necessary to select oriented silicon steel sheets of the 35WW300 and above grades. Preferentially use brand-new genuine grain-oriented materials, avoid second-hand recycled materials, ensure wide-range linear magnetization characteristics, resist instantaneous peak voltages, and suppress excitation current spikes.
Grade 3: Variable frequency equipment workshop, and the surrounding area of medium-frequency equipment with strong harmonic environment
The harsh power grid containing a large amount of high-frequency distorted voltage. It is recommended to select high-grade high-magnetic-conductivity silicon steel, such as 23QG085, 27QG100 ultra-thin oriented silicon steel. The ultra-thin sheet further reduces eddy current loss, and can still keep iron loss at a low level in the face of high-frequency harmonics. The magnetic flux linear reserve is sufficient, even if the voltage distortion is severe, it will not easily cause magnetic saturation, ensuring the stability of the power supply waveform.
Strictly prohibited use of inferior materials:
Reclaimed dismantled silicon steel sheets, hot-rolled black iron sheets, mixed material cores. These materials have uneven magnetization curves, and local magnetic intensity is highly prone to exceeding the limit. When encountering grid harmonics, local saturation occurs immediately, and current peaks, overheating, waveform distortion will all burst simultaneously. They are completely unable to cope with industrial distorted power grids. Many low-priced transformers use recycled silicon steel. They function normally under clean mains power, but once placed in the frequency converter workshop, they overheat and the noise increases sharply. The root cause lies in the core material.
- Optimization of supporting processes to maximize the anti-harmonic capability
Just replacing the silicon steel material without matching the process still fails to fully leverage the advantages of the high-magnetic-conductivity core. Therefore, three process optimizations must be carried out in conjunction.
1. Reasonably reduce the working magnetic flux density and reserve the anti-harmonic margin
When designing the transformer, the magnetic flux density should not be pushed to the maximum. Under a pure sine grid, the magnetic flux density of an ordinary core can be set at 1.3 to 1.4 T; in industrial sites with more harmonics, even if high-conductivity oriented silicon steel is used, the working magnetic flux density should be reduced to 1.1 to 1.25 T. Reserve sufficient flux margin to avoid the instantaneous magnetic flux exceeding the linear range after the harmonic voltage is superimposed. When the magnetic flux density is lowered, the core always operates in a gentle linear section. No matter how strong the harmonics are, they cannot trigger a saturation peak, and the excitation current remains smooth, effectively suppressing the output voltage distortion.
2. EI type laminations are staggered and interlocked to reduce air gaps and stabilize the overall magnetic circuit.
Highly oriented silicon steel sheets must be processed using a full staggered lamination method. E-type laminations and I-type laminations are alternately interlocked. This reduces the gap between adjacent laminations. The larger the air gap, the higher the magnetic resistance, and the more likely the magnetic flux is to become unbalanced, making the device more sensitive to voltage fluctuations and harmonics. The laminations of the iron core are compressed and compacted to reduce the gap between the laminations, ensuring uniform magnetic resistance of the entire iron core and preventing the magnetic flux from concentrating locally. A more stable magnetic circuit means a stronger ability to resist harmonic disturbances.
3. The winding is designed with a shielding structure to block the harmonic coupling channel.
The high-conductivity iron core is responsible for stabilizing the fundamental magnetic flux and suppressing the magnetic saturation spikes; a copper foil electrostatic shielding layer is added between the primary and secondary windings to cut off the high-frequency harmonics that pass through the parasitic capacitance of the winding into the low-voltage circuit. The combination of material and shielding effectively suppresses the magnetic flux distortion on the magnetic path and the common-mode conducted interference on the electrical path, thereby doubly weakening the impact of grid harmonics on the power supply quality.
At the same time, vacuum impregnation is carried out to cure the iron core and the coil, avoiding the loosening and vibration of the iron core caused by the alternating electromagnetic force of harmonics, and preventing the continuous disturbance of the magnetic field from introducing voltage noise.
- V. Practical Application Results: The Improved Visual Changes
After replacing the high-conductivity oriented silicon steel core transformer in a variable frequency converter workshop with severe harmonics, four significant improvements are typically observed:
First, the peak excitation current during no-load operation becomes significantly smoother, the pulse peaks disappear, and the current waveform approaches a sine wave;
Second, the amplitude of the no-load temperature rise is significantly reduced, and there is no longer abnormal overheating during long-term power supply, avoiding the compression of residual flux due to temperature rise; Third, the spikes in the secondary output voltage decrease, the waveform baseline becomes more flat, and the number of jumps in analog quantity collection at the rear end and signal drift problems are significantly alleviated;
Fourth, the noise of the transformer changes from a chaotic whistling back to a stable low-frequency humming, the magnetic field oscillation is suppressed, and the spatial radiation interference is simultaneously reduced.
Many projects only need to upgrade the core brand without the need to add additional multi-stage filtering circuits, and can smoothly pass the EMC electromagnetic compatibility test, significantly simplifying the workload of subsequent power system rectification.
The voltage distortion caused by grid harmonics will increase the peak value of the instantaneous magnetic flux, forcing ordinary silicon steel cores to enter magnetic saturation prematurely, resulting in excitation current spikes, overheating of the core, and distortion of the output waveform. Simply relying on post-processing filtering can only handle the stray waves that are conducted out, but it cannot solve the problem of the original magnetic flux distortion. Selecting high-magnetic-conductivity grain-oriented silicon steel sheets, which can broaden the magnetization linear range, reduce iron loss, and stabilize the magnetic circuit distribution, is the fundamental solution to weaken the harmonic impact from the source.
The complete rectification plan can be summarized into three points:
1. In industrial power grids with high harmonic content, instead of using ordinary non-oriented silicon steel and recycled iron cores, upgrade to brand-new grain-oriented high-conductivity silicon steel, expand the linear magnetization range, resist instantaneous voltage peaks, and avoid current spikes caused by magnetic saturation;
2. Moderately reduce the designed magnetic flux density, reserve sufficient margin for anti-distortion, and at the same time adopt the staggered laminated pressing process to ensure uniform magnetic resistance of the entire iron core and no local congestion of magnetic flux;
3. In combination with primary and secondary electrostatic shielding, cut off the capacitive coupling channel of high-frequency harmonics, achieving dual noise reduction of stable magnetic circuit + circuit isolation.
By using high-quality silicon steel cores to stabilize the magnetic flux and smoothing out the distorted voltage peaks, the transformer can output clean and stable sine voltage. This can significantly reduce the interference of grid harmonics on industrial control power supplies, ensuring the long-term stable operation of sensors, PLCs, and communication modules, and greatly reducing the cost of later circuit rectification.
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