Harmonics are the unwanted positive frequency multiples of the fundamental frequency of the output AC power of the solar inverter and electronics. These introduce noise to the system, which changes and distorts the wave shape of the AC power. Thus, a pure AC waveform is free of harmonics and shows as a perfect sine wave. Harmonics also cause certain phenomena in the system that adversely affect its performance, e.g., Resonance, heating, increased skin effect in coil devices, malfunctions of grid-side protection equipment, additional grid losses, production of zero sequence currents, etc., are all caused due to such harmonics.
Figure 1: How a 3rd Harmonic Distorts the Fundamental Waveform. THD = 30%:
Where do Harmonics Come from?
The impact of the nonlinear load in modern industrial power electric arc furnace, converter, data center server and household LED lights, distorts the pure sine wave current work. Even if these devices are connected to an ideal sinusoidal voltage, their current waveforms will still be distorted, injecting high-frequency harmonic currents into the power grid. For instance, when a medium-frequency melting furnace is in operation, the current is rectified and then inverted, generating a large number of high-order harmonics such as the 11th, 13th, and 23rd harmonics, causing the total voltage distortion rate to surge to 17.7%, far exceeding the national standard limit of 3%.
The saturation effect of the transformer's own core
The magnetization curve of the transformer core is essentially nonlinear. When the operating voltage rises or the designed magnetic flux density approaches the saturation zone, the excitation current will be severely distorted, mainly generating the 3rd, 5th, and 7th odd harmonics. Actual measurements show that when a 66kV dry-type transformer operates no-load at 110% of its rated voltage, the content of the 5th harmonic voltage can reach 7.4%, and for every 3% increase in voltage, the harmonic increase exceeds 20%. Light-load high-voltage operating conditions (such as at night) are typical scenarios for such harmonic bursts.
The Destructive Power of Harmonics: From Electricity meters to Motors
The harmonic frequency can reach dozens of times that of the fundamental wave (50Hz) (such as 2500Hz), and its physical effects far exceed imagination:
Sharp increase in loss: High-frequency current triggers the "skin effect", causing a significant increase in copper loss in transformers. At the same time, the core's iron loss rises due to the dramatic change in magnetic flux. When 10 medium-frequency furnaces in a certain foundry were not treated, the temperature rise of the 7000kVA transformer was abnormal and the noise was shrill. It was measured that the temperature rise was reduced by 10℃ only by a set of filtering devices.
Protection against misoperation: The 5th harmonic can easily penetrate the negative sequence voltage filter. When the 5th harmonic content of the 66kV busbar reaches 11%, it can trigger the relay with a set value of 9V to malfunction, causing an unexplained power outage.
Inaccurate measurement: Mechanical energy meters slow down due to the resistance of high-frequency eddy currents, while electronic meters may mistakenly count harmonic energy as power generation, resulting in "mysterious power loss".
Phase-shifting transformer group: It adopts a combination of Δ-Zigzag and Δ-Y dual transformers, and through a 30° phase shift, the two groups of harmonic currents cancel each other out. For instance, for single-phase nonlinear loads, the secondary harmonics of the two transformers are superimposed out of phase, significantly reducing the total harmonic distortion (THD).
Phase Shifting and Harmonics
The best way to eliminate harmonics is to use a technique known as “phase shifting.” The concept of phase shifting involves separating the electrical supply into several outputs; each output being phase shifted with the other outputs with an appropriate angle for the harmonics to be mitigated. The idea is to displace the harmonic currents in order to bring them to a 180° phase shift so that they cancel each other out.
Hence, an angular displacement of
Phase-Shifting Transformers Designed for Non-Linear Loads
The level of harmonic currents may be reduced using phase-shifting transformers. Impedance plays a crucial role in reducing voltage distortion.
Phase-shifting transformers allow the treatment of harmonic currents through cancellation. Moreover, these transformers are designed to withstand the additional overheating caused by harmonic currents. The quality and reliability of the electrical system can be improved considerably by using harmonic mitigating transformers.
Electrified railway: The 31st, 35th and 41st harmonics on the 380V side of the traction substation exceeded the standard (with a content of 17%). After parallel high-pass filters, the distortion rate was reduced from 19% to 3.98%. The principle is that it presents a high impedance (nearly open circuit) at power frequency, while it shows a low impedance path for high-frequency harmonics.
Minimizing harmonics improves the system performance, reduces power system losses, and improves the system power factor. Therefore, it is important to identify and reduce harmonics as much as possible, especially when the non-linear load on the system is increased.
