1. Nonlinear characteristics

The diodes, thyristors and other power electronic devices in rectifier equipment have nonlinear voltammetry characteristics. Take the diode as an example, when the input AC voltage is in positive half circle, the diode is positive and the current can pass through; At the negative half cycle, the diode is reversed off and the current is almost zero. This characteristic does not change linearly with the input voltage, so that the output current and voltage after rectification are no longer standard sine waves, resulting in harmonics.

2. Commutation process

In a rectifier circuit, the commutation process occurs when the phase sequence of the AC power supply changes or when devices such as thyristors trigger on-off and off-off. At the moment of commutation, the current is transferred from one branch to another branch. Due to the existence of inductors, capacitors and other components in the circuit, the current cannot suddenly change, and a transient process will be generated, resulting in distortion of the current waveform and harmonics. For example, in a three-phase bridge rectifier circuit, the commutation process of the thyristor will introduce harmonics of a specific frequency into the output current.

3. Circuit structure

The circuit structure of the rectifier equipment will also affect the generation of harmonics. For example, half wave rectifier circuit only uses half cycle of AC power supply, the discontinuity of output waveform is more obvious, and the harmonic content is relatively high. Although the full wave rectifier circuit and bridge rectifier circuit use the whole cycle of AC power supply, due to the characteristics of their circuit structure, harmonics will inevitably be generated in the rectification process.

4. Load characteristic

When the rectifier equipment has a nonlinear load, such as electric arc furnace, frequency converter, etc., the nonlinear characteristics of the load itself will distort the input current waveform of the rectifier equipment, resulting in harmonics. Taking the inverter as an example, the power electronics inside it will chop and invert the power supply according to the control signal when working, so that the input current is no longer a sine wave, but contains a large number of harmonic components.