With the widespread and vigorous promotion of electronic technology in various industries in recent years, especially frequency conversion, soft starter, uninterrupted power supply, rectification, and thyristor switching devices, which are widely used in drag, electric heating, water pumps, and fans, it has brought great convenience to production, especially the excellent performance of frequency converters, which are constantly favored by people in industrial control, and the energy-saving effect is even more remarkable.
However, the resulting harmonic problem is gradually exposed in the power supply system. Why does the metering instrument shake and lose its accuracy, which is easy to be damaged, the circuit breaker is easy to burn contacts, the capacitor bulge, PT insulation breakdown, the neutral line of low-voltage power cable is easy to heat and breakdown, which causes interphase short circuit, local heat and breakdown short circuit of motor winding and life reduction, which has caused immeasurable losses to equipment in production, Due to the significant harm of high order harmonics to the normal operation of electrical equipment, problems such as heating, breakdown, short circuit, and reduced lifespan of local windings of motors often occur in this power system.
In terms of the power system, sinusoidal voltage is applied to linear passive resistors, inductors, and capacitors in a proportional integral and differential relationship, respectively. Presenting a sine wave of the same frequency, when a sine voltage is applied to a nonlinear circuit, the current becomes a non sine wave. Abnormal current generates a voltage drop on the impedance of the power grid, and its waveform also becomes a non sine wave. From the Fourier series expansion, we can see that the harmonic is the sine component of the integral multiple fundamental frequency with a frequency greater than 1 in a periodic electrical quantity. As the integer of harmonic frequency is higher than the fundamental frequency, it is self-evident that harmonics will inevitably cause harm to electrical equipment at the fundamental frequency. In addition, the greatest feature of the Fourier series expansion formula is that it decomposes the electromotive force into the sum of the DC component and each harmonic component. Moreover, the series converges step by step, and the higher the harmonic order, the smaller the amplitude. We can clearly see that in common waveforms:
1. Although the waveforms are different, the harmonics of different orders in the Fourier Series expansion have the same amplitude Fm for consideration.
2.Different waveforms have different harmonic structures, appearing odd or even.
3. As the number of harmonics increases, the harmonic content will sharply decrease.
4. With different waveform expansions, it is much easier for us to calculate harmonics of different orders in the circuit.
In the three-phase four wire low-voltage power supply system, a large number of electronic components that have been confirmed as harmonic sources, such as pad mounted transformer, converter equipment, rectifier, inverter, frequency converter, soft starter, transistor switch circuit, various uninterruptible power supplies, energy-saving appliances, have been added to the power system, so the low-voltage power system has become an important source of harmonic generation, and some harmonics are injected into the upper power supply system, The other part of harmonics can harm electrical equipment on-site.
Under the skin effect, harmonic high-frequency currents are squeezed through the surface of the conductor, thereby reducing the effective area of the conductor and increasing its resistance. Secondly, when zero sequence current flows through the neutral line, the current density on the surface of the conductor is also influenced by its alternating current in the phase line, which is the famous proximity effect theory. The proximity effect also increases the local current density on the surface of the neutral wire, further reducing the total conductive area of the neutral wire, increasing the effective resistance, and making it possible for the neutral wire conductor to generate local heat. This is the reason why the neutral wire is prone to heating under the action of harmonics. If the neutral wire has a joint, the probability of heating and damage to the neutral wire will increase several times. So in specific engineering, it is necessary to fully consider the calculation load situation. If there is a harmonic source, the neutral line cross-section should be appropriately increased, and corresponding measures can be taken to reduce harmonics. If necessary, filtering devices should be installed.
The structure of a soft starter is composed of three-phase reverse parallel thyristors connected in series to the controlled motor on the power supply. By utilizing the switching characteristics of the thyristors, the output voltage is adjusted by adjusting the conduction angle of the thyristors for voltage regulation, ultimately achieving the control of the motor’s starting process. Due to the fact that the load motor carried by the soft starter is a star connected voltage regulating circuit, the phase voltage of the load can be decomposed using Fourier transform. The content of each harmonic in the triangle connected input current controlled by the circuit is smaller than that in the star connected input current. As the number of harmonics increases, the content of harmonics decreases sharply. Generally speaking, in symmetrical three-phase loads, soft starters produce odd harmonics, and the slightly higher content is harmonics below 9th order.
The superposition of these harmonics and fundamental waves causes waveform distortion, and the waveform is no longer sinusoidal in shape. The noise and vibration of the motor significantly increase after input, and the operating temperature of the cable has also increased significantly due to skin effect.
Measures to eliminate harmonics
1) Which method is more convenient and economical to eliminate the harm of harmonics to equipment? Undoubtedly, the electronic device that generates harmonics will be decommissioned as soon as possible after completing the task, completely eliminating the harmonic source. To this end, we have made corresponding modifications to the working circuit of the soft starter by adding an AC contactor to the soft starter. By using the main contact of the contactor, the starter is short circuited after completing the starting task to exit operation. This effectively reduces the generation of harmonics, needless to say, this effect is quite good. The most direct effect is that the electromagnetic noise, vibration, and heat generation of the motor are significantly reduced. The working temperature of the repaired power cable has decreased by about 15 º C, and it operates normally.
2) Because the zero sequence harmonics of the neutral line are diverted from the phase line. Most of the integral multiple harmonics of 3 will be naturally eliminated in the three-phase circuit. Due to the influence of asymmetric load, shunt will be generated. Therefore, the best way to keep the integral multiple harmonics of 3, namely zero sequence harmonics, in the three-phase circuit and reduce the shunt to the neutral line, so that the heating problem of the neutral line due to harmonics will be reduced.
The method of limiting or diverting zero sequence current on the cable centerline is to string a reactor at the end of the cable centerline to increase the impedance of the centerline and limit harmonic inflow.
3. Add a harmonic filter into an electrical switchboard.
Conclusion, we know that there are two basic methods to solve the problem of harmonic pollution. One is to install a harmonic compensation device to compensate for harmonics, which is applicable to all types of harmonics; The other is to modify the power electronic device itself to prevent harmonic generation, which is still a difficult problem that requires a long time to solve. The pragmatic approach is to adapt to local conditions and solve harmonic problems based on the actual research of the power system.
Please contact our expert engineers to solve your harmonic problems.