What Is Harmonic Distortion?

Harmonics are distortions of normal current waveforms that are typically emitted by non-linear loads. Harmonic distortion (HD) refers to the ratio of the root-mean-square (RMS) value of the target harmonic (second-order, third-order), etc. to the electrical root mean square value of the signal ^[1] . Harmonic distortion is caused by the system not being completely linear. In audio applications, it is usually expressed as a percentage, and in communication applications, it is usually expressed as dB. The measurement method is to apply a pure sine wave to an amplifier and observe the output of the amplifier with a spectrum analyzer.

Periodic signal analysis

A periodic signal can pass

Because the current method of measuring distortion is a single sine wave, it does not reflect the full picture of the amplifier. The actual music signal is a complex wave with various rates, including rate conversion,

Power Industry

VFD (variable frequency drive) has many advantages in speed regulation and optimized power consumption of key motors, but it also easily causes harmonic distortion of adjacent power distribution systems. The power distribution system can absorb some distortion, but when the VFD is directly connected to the generator-driven circuit, the interference may also affect the reliability of the operation.

Water treatment plants are usually equipped with VFDs, ozone generators, and other loads that can cause harmonic distortion. Most factories are also equipped with emergency backup generators to provide power to important equipment in the event of an external power outage or abnormality. However, plant operators have been worried that generators may fail in the event of extended emergency operation time. In order to determine the degree of failure, the operator edits the harmonic measurement results, compares the degree of distortion during normal use and the use of a backup generator, and evaluates the harmonic suppression technology through engineering analysis based on the measured data.

How much distortion is acceptable

The IEEE519-1992 standard "Recommended Practices and Requirements for Harmonic Control of Power Systems" provides some guidelines on "how much harmonic distortion is acceptable". Initially, the standard was used as a reference for the recommended power supply unit and its customers; the standard was popularized and used by many factories and enterprises as a guideline for measuring harmonic currents of existing equipment.

Tests conducted on normal power usage of the desalination plant and backup generator power supply have shown that the main power system parameters, including voltage calibration and imbalance, and current imbalance, are within acceptable limits. Although the degree of harmonic distortion is not serious enough to have a significant impact on the normal production operation of the plant, further reduction of harmonics is still a work that cannot be ignored. Operators are concerned about the long-term impact of harmonics, and since the harmonics when using a backup generator often exceed the harmonic range specified in the IEEE519-1992 standard, they are more worried about whether the generator can be stable and reliable under the extended emergency operation time continue working. In addition, harmonic reduction technology can also extend the life of equipment and enhance system reliability.

The harmonic limit table shows test results compared with the IEEE519-1992 standard harmonic limits commonly used in power generation equipment. As demonstrated, the data measured online exceeded this limit. In addition, Schneider Electric also carried out computer simulations of different wave suppression technologies.

Harmonic simulation can estimate the reduction of harmonic current under different conditions. As mentioned earlier, the 5th and 7th harmonic current harmonic elimination effects have been obtained when 250hp equipment and 60hp equipment are working simultaneously. The system requires the current of the 5th harmonic to be reduced by 27% and the current of the 7th harmonic to be reduced by 16%, although the total RMS current is increased by 19%. The worst case is the highest harmonic peak, which occurs when only 250hp equipment is operating. In general, there are 4 solutions:

Bypass a Y Insulated Transformer-Each 250hp device has a Y Insulated Transformer. Bypassing one of the Y insulation transformers can achieve good results, which reduces the amount of harmonic current distortion. Coupled with the 250hp and 60hp equipment mentioned above, the 5th and 7th harmonic currents are reduced. However, the 5th and 7th harmonic currents of the bypass circuit remain unchanged, and once one or two 250hp devices are operated, it will have an additional elimination effect. However, this technique is only suitable for temporary retrofits without a more effective solution.

Replace a Y Insulated Transformer-A more effective harmonic attenuation technique is to replace one of the Y insulated transformers with a Y interleaved winding transformer instead of bypassing it. Because the harmonic current does not move through the phase of the Y interleaved winding transformer, this improvement also enhances the cancellation effect of the 5th and 7th currents. This method also retains the positive effects of harmonic attenuation.

Passive harmonic filter-A 5th-order passive harmonic filter can be installed in the 480V main circuit, but this method is not practical because the passive harmonic filter also increases the basic power factor. Because the power factor of the equipment is already high (up to 94% at full load), the system cannot tolerate more loads without the state-of-the-art power factor in the factory.

Active Harmonic Filter-The factory's best solution is to reduce the harmonic current by installing an active filter on the 480V main circuit. Active filters can measure the amount of harmonic current required by the load and cause a 180 ° phase shift of the current. This method can greatly reduce the degree of harmonic distortion, and is usually used in situations where strict compliance with harmonic limits is required. In addition, Schneider Electric recommends installing on-site power monitoring equipment to track equipment performance, voltage quality, interference, and cost in harmonic distortion.

Transitional and permanent solutions

By temporarily setting the bypass to bypass the insulation transformer, thereby increasing harmonic elimination, the factory reduces the degree of distortion of harmonics. This unconventional approach helped the plant weather its peak summer operations. After that, staff installed an active harmonic filter on the main switchboard. The device ultimately eliminates the effects of harmonic distortion. The measurement results after installing the active filter show that the current distortion is less than 8% and the voltage distortion is less than 2%.