Harm of Harmonics to Electric Power and Electrical Equipment (II)

3. Harm to power transformers

Harmonics increase the copper loss of the transformer, including resistance 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 by an increase in the hysteresis loss in the core, and the worse the waveform of the voltage becomes, the greater the hysteresis loss. At the same time, due to the increase of the above two aspects of loss, it is necessary to reduce the actual use capacity of the transformer, or to consider the harmonic content in the power grid when selecting the rated capacity of the transformer. In addition, harmonics also lead to increased transformer noise. The vibration noise of the transformer is mainly caused by the magnetostriction of the core. As the number of harmonics increases, the component with a vibration frequency of about 1 kHz increases the noise. Also emits a metallic sound.

4. Hazards to electrical equipment

Harm to the motor

The influence of harmonics on the asynchronous motor is mainly to increase the additional loss of the motor, reduce the efficiency, and cause the motor to overheat in severe cases. In particular, the negative-sequence harmonics generate a negative-sequence rotating magnetic field in the motor, forming a torque opposite to the direction of rotation of the motor, and acting as a brake, thereby reducing the output of the motor. In addition, the harmonic current in the motor causes the motor to vibrate mechanically when the frequency is close to the natural frequency of a part, causing a large noise.

5. Hazards to low voltage switchgear

For the circuit breakers for power distribution, the all-electromagnetic type circuit breaker is susceptible to the influence of harmonic currents to increase the iron loss and generate heat. At the same time, the tripping is difficult due to the influence of the electromagnet and the influence of the eddy current, and the harmonics are more The higher the high impact; the thermomagnetic type circuit breaker, because the collector of the conductor should be heated and the iron consumption is increased, so that the rated current is reduced and the trip current is reduced; the electronic type circuit breaker, the harmonics must also be rated The current is reduced, especially for electronic circuit breakers that detect peaks, and the rated current is reduced even more. It can be seen that all three types of power distribution circuit breakers may be generated by harmonics.

For the leakage circuit breaker, due to the harmonic leakage current, the circuit breaker may be abnormally heated, and malfunction or malfunction may occur. For the electromagnetic angler, the harmonic current increases the temperature rise of the magnet component, affecting the contact point, and the coil temperature rises to lower the rated current. For thermal relays, the rated current is also reduced due to the influence of harmonic currents. They can cause malfunctions at work.