Harmonic knowledge of power equipment hazards

(1) Hazards to Power Capacitors When there is a harmonic in the power grid, the terminal voltage increases after the capacitor is inserted, and the current through the capacitor increases more, which increases the power loss of the capacitor. For film-membrane composite dielectric capacitors, although the power loss when harmonics are allowed is 1.38 times the power loss without harmonics, the power loss when harmonics are allowed for all-film capacitors is 1.43 times that without harmonics, but if Higher harmonic content, beyond the capacitor allowable conditions, will make the capacitor over-current and overload, the power loss exceeds the above value, so that the capacitor abnormal heat, the insulation medium under the effect of the electric field and temperature will accelerate aging. Especially when the capacitor is put into the power grid where the voltage has been distorted, the harmonic of the power grid may also be exacerbated, that is, the phenomenon of harmonic expansion occurs. In addition, the presence of harmonics tends to make the voltage appear sharp. The steep-peak voltage wave easily induces partial discharge in the medium. Due to the large voltage change rate and partial discharge intensity, it can accelerate the aging of the insulating medium, thus shortening it. The service life of the capacitor. In general, for every 10% increase in voltage, the life of the capacitor is reduced by about 1/2. In addition, in the case of severe harmonics, the capacitor will bulge, breakdown or explosion.

(2) Harm to Power Cables As the frequency of harmonics increases, the skin effect becomes more significant as the cross-sectional area of ​​the cable conductor increases, resulting in an increase in the AC resistance of the conductor and a decrease in the allowable passing current of the cable. In addition, the resistance of the cable, the system bus side and the line inductance are connected in series with the system, and the capacitance and resistance of the capacitor and line used to increase the power factor are connected in parallel with the system, and resonance may occur under a certain value of inductance and capacitance.

(3) Hazardous harmonics to the power transformer increase the copper loss of the transformer, including resistive losses, eddy current losses in the conductor, and stray losses due to leakage flux outside the conductor. Harmonics also increase the iron loss of the transformer, which is mainly manifested in the increase of the hysteresis loss in the core, and the worse the waveform of the voltage caused by the harmonics, the greater the hysteresis loss. At the same time, because of the increased loss in the above two aspects, it is necessary to reduce the actual capacity of the transformer, or to consider the harmonic content of the grid when selecting the rated capacity of the transformer. In addition, harmonics also cause the transformer noise to increase. The vibration and noise of the transformer is mainly caused by the magnetostriction of the iron core. With the increase of the number of harmonics, the components with the vibration frequency around 1KHZ increase the mixed noise. Also emits metal sounds.

(4) Hazards to Electric Motors Harm to Electric Motors The influence of harmonics on asynchronous motors is mainly to increase the additional loss of the electric motor, reduce the efficiency, and cause the electric motor to overheat when it is severe. In particular, negative-sequence harmonics generate a negative-sequence rotating magnetic field in the motor, forming a torque opposite to that of the motor, and acting as a brake, thereby reducing the output of the motor. In addition, the harmonic current in the motor will cause the motor to generate mechanical vibration when the frequency is close to the natural frequency of a certain part, which will cause great noise.

(5) Hazards to low-voltage switchgear For circuit breakers used in power distribution, all-magnetic-type circuit breakers are susceptible to harmonic currents, which increase iron loss and generate heat. At the same time, they are affected by the influence of electromagnets and eddy currents. The tripping is difficult, and the higher the number of harmonics is, the greater the influence is; the thermal magnetic type circuit breaker, due to the collector skin times of the conductor and the increased iron consumption, causes the heat generation, which causes the rated current to decrease and the tripping current to decrease; the electronic type circuit breaker The harmonic current also has to reduce its rated current, especially the electronic circuit breaker that detects the peak, and the rated current is reduced more. It can be seen that the three types of power distribution circuit breakers described above may be generated due to harmonics.

For leakage circuit breakers, due to the effect of harmonic leakage currents, the circuit breakers may be abnormally heated and cause malfunction or failure. For electromagnetic adapters, the harmonic currents increase the temperature rise of the magnet components, affecting the contacts, and the coil temperature rises to lower the rated current. For thermal relays, the rated current is also reduced due to harmonic currents. They can all cause malfunctions at work.

(6) Interference to low-voltage system equipment For weak equipment such as computer networks, communications, cable television, and alarm and building automation, the harmonics in the power system are coupled to these systems through electromagnetic induction, electrostatic induction, and conduction to generate interference. The coupling strength between the induction and the electrostatic induction is proportional to the interference frequency. The conduction is coupled through the common ground. A large amount of unbalanced current flows into the grounding pole, thus interfering with the weak current system.

(7) Impact on the accuracy of power measurement The currently used power measurement instruments are magneto-electric and inductive. They are greatly affected by harmonics. Especially the electric energy meter (adopts the induction type more), when the harmonic wave is bigger, will produce the measuring confusion, the measurement is not accurate.

(8) Harmonic effects on human body From the aspect of human physiology, when human cells are stimulated and excited, they will rapidly fluctuate or reversibly flip on the basis of the resting potential of the cell membrane. If the frequency is close to the harmonic frequency, the grid Harmonic electromagnetic radiation will directly affect the human brain magnetic field and the magnetic field.

When the harmonic pollution degree of the power grid is less than the national standard, it usually does not affect the system. As the degree of pollution increases, the influence of harmonics gradually emerges. In the case of severely exceeded harmonics, serious harmonic consequences will occur if harmonics are not controlled. The characteristics of harmonic sources are very complex, because the generation of harmonics depends not only on the load itself that generates the harmonics, but also on the short-circuit capacity of the grid, the composition of the grid, and the nature of other loads in the grid.