Power Supply Lesson 1 Understanding Harmonics

1 Basic knowledge of harmonics

(1) What is the fundamental wave?

The frequency of the periodically changing voltage or current in the power network is the fundamental wave (also called the primary wave). The frequency of my country's power grid is 50 Hz, so the fundamental wave is 50 Hz.

(2)What are harmonics?

In the power network, except for the fundamental wave (50 Hz), any periodic voltage or current signal with a frequency higher than the fundamental wave (50 Hz) is called a harmonic.

In a power grid or circuit, the harmonics generated by voltage are called voltage harmonics;

The harmonics generated by current are called current harmonics.

(3)How many types of harmonics are there?

Integer harmonics: refers to harmonics whose frequencies are integer (1) times the fundamental frequency, that is, 2nd, 3rd, 4th, 5th, 6th, 7th, 8th, 9th, 10th and other harmonics.

Even harmonics: refers to harmonics whose frequencies are even multiples of the fundamental frequency, such as 6, 8, 10, etc.

Odd harmonics: refers to harmonics whose frequencies are 3, 5, 7, 9, 11 and other odd multiples of the fundamental frequency.

Positive sequence harmonics: The harmonic order is 3k+1 (k is a positive integer), i.e. 4th, 7th, 10th, etc.

Negative sequence harmonics: The harmonic order is 3k-1 (k is a positive integer), i.e. 2nd, 5th, 8th etc. harmonics.

Zero-sequence harmonics: refers to harmonics whose frequencies are integer multiples of the fundamental frequency of 3, such as the 3rd, 6th, 9th, 12th, and 15th harmonics.

High-frequency harmonics: refers to harmonics with a frequency of kHz.

(4)How to calculate harmonic frequency?

The harmonic frequency is equal to the harmonic order × the fundamental wave frequency. For example: the 1st harmonic frequency is 100 Hz × 50 Hz, the 2nd harmonic frequency is 7 × 50 Hz × 50 Hz, and so on.

(5) Which devices or circuits are likely to generate harmonics?

1) Non-linear load, such as diode rectifier circuit (AC/DC).

2) Three-phase voltage or current asymmetric load.

3) Inverter circuit (DC/AC).

4) UPS power supply (for PC), EPS power supply (for high-power power), that is, uninterruptible power supply.

5) Thyristor voltage regulator or speed regulation circuit.

6) Electroplating equipment.

7) Electric arc furnace, ore-fired furnace, manganese ore furnace, phosphate ore furnace, calcium carbide furnace, ferrosilicon furnace.

8) Electrolytic cell.

9) Electric welding machine (arc welding, seam welding, spot welding, butt welding, etc.).

10) Battery charger.

11) Frequency converter (low voltage or high voltage frequency converter).

12) Pulse amplitude modulation (PAM) voltage regulation circuit or pulse width modulation (PWM) frequency modulation circuit.

13) The harmonic order is related to the number of phases of the rectifier circuit, for example, three-phase, six-phase, twelve-phase, eighteen-phase, twenty-four-phase. When the number of phases increases, the harmonic order and harmonic components can be reduced by phase shifting. For example, the unit using input transformer phase shifting technology is connected in series with the main circuit of the high-voltage frequency converter.

14) Switching power supply.

15) Chopper circuit, chopper speed regulation.

16) Industrial frequency electric furnace.

17) Medium frequency electric furnace.

18) Overhead crane and lifting machinery.

19) Gas discharge lighting fixtures, such as energy-saving lamps, fluorescent lamps (T5, T8), metal halide lamps, sodium lamps, mercury lamps, krypton lamps, tritium lamps, etc., all generate certain harmonics when in use.

20) Soft start device (using SCR voltage regulation).

(6) What is the direction of harmonic current?

Harmonic current flows from the load to the power supply transformer and then to the power supply network, that is, from the terminal load to the previous stage.

(7)What are the effects of harmonics?

1) Impact on transformer Harmonic current increases copper loss and magnetic leakage; harmonic voltage increases iron loss; harmonic power increases noise and temperature rise.

2) Impact on power lines Harmonic currents are prone to overload, leading to overheating, insulation damage, and increased skin effect.

3) Impact on the motor Harmonic current increases copper loss, harmonic voltage increases iron loss, and the power of harmonics causes a decrease in mechanical efficiency, a drop in power factor, and a decrease in effective torque.

4) Impact on the control system Voltage zero point drift, line voltage imbalance, inaccurate indication of instruments and equipment, resulting in control judgment errors and even loss of control system.

5) Impact on communication equipment Harmonics will generate induced electromagnetic fields, affecting communication quality.

6) Impact on capacitors Generally, large motors or low-voltage power grids have reactive power compensation or power factor compensation, which are basically achieved using capacitors. Due to the generation of harmonics, capacitors are prone to overcurrent and overvoltage.

Voltage and over-temperature cause breakdown damage. The reason is that harmonic current is prone to parallel resonance, generating large current, and series resonance generates overvoltage. Generally, the operating current of capacitors should be 1.3 times the rated current, otherwise they will be quickly damaged. The harmonic currents are mainly 5th, 7th and 11th.