What are harmonics in electrical systems, and what causes them?

In an electrical system, harmonics refer to the components with frequencies that are integer multiples greater than the fundamental frequency obtained by decomposing a periodic non-sinusoidal alternating quantity through Fourier series, and they are usually called higher harmonics.

The main causes of harmonics are as follows:

• Existence of Nonlinear Loads: This is the primary cause of harmonics generation. For example, power electronic devices, including rectifiers, inverters, and frequency converters, the semiconductor devices inside will distort the current and voltage waveforms during operation, generating harmonics. Take a rectifier as an example. It converts alternating current into direct current. During the conversion process, the input current will show a non-sinusoidal waveform and contain a large number of harmonic components. In addition, equipment such as arc furnaces and fluorescent lamps are also common nonlinear loads. In the process of steelmaking in an arc furnace, the instability of the arc will lead to current fluctuations and generate harmonics. Due to the function of the ballast in a fluorescent lamp, the current waveform will also be distorted, generating harmonics.

• Excitation Current of Transformers: When a transformer is in operation, magnetic saturation will occur in its iron core, resulting in the excitation current no longer being a sinusoidal wave, thus generating harmonics. Especially when the transformer is switched on without load or operating under a light load, the harmonic components in the excitation current will be more obvious.

• Unbalance of the Power Supply System: When the loads of each phase in a three-phase power supply system are unbalanced, it will lead to the asymmetry of the current and voltage, thus generating harmonics. For example, in some industrial sites, due to the different equipment connected to each phase, the situation of three-phase load unbalance may occur, causing harmonics to appear in the power supply system.

Harmonics can cause increased heating and greater losses in electrical equipment, affecting the normal operation and service life of the equipment. They may also interfere with communication systems and trigger resonances in the power system and other issues. Therefore, corresponding measures need to be taken for treatment.