Common sense of current harmonics
Time:2022-12-22
Views:1441
Introduce the common sense about harmonics, hoping to be helpful to readers
What are harmonics? We know that the load connected to the power grid can be divided into linear load and nonlinear load, as shown in Figure 1:
Figure 1
The dotted line represents the voltage waveform. Our current waveform is normally a sinusoidal alternating current with amplitude of 220V and frequency of 50HZ. When the load current waveform is also a sinusoidal waveform of 50HZ, we call it a linear load. When the phase of the current waveform is consistent with the voltage, we call it a resistive load, as shown in the yellow line in Figure 1, such as common white flag lamps, circuits, and electric water heaters. When the current phase is ahead of the voltage, we call this kind of load capacitive load. As shown in the black line in Figure 1, there are few pure capacitive loads. For example, the reactive compensation cabinet commonly used in general distribution rooms is pure capacitive load. When the load current phase lags behind the voltage waveform, this kind of load is called inductive load, as shown in the blue line in Figure 1. Common inductive loads such as motors, transformers, and inductive ballast fluorescent lamps.
The characteristic of the above linear load is that no matter its current phase is ahead of or behind the voltage waveform, its current waveform is always a sine wave without distortion, which is the key of the linear load.
Nonlinear load means that its current waveform is not 50HZ sine wave, or it is understood that there is greater distortion compared with 50HZ sine wave. For example, the current common loads: computers, color TVs, induction cookers, industrial power frequency UPS, frequency converters, electroplating, etc. The load current waveform is not 509HZ sine wave. Figure 2 is the current waveform of computers, color TVs, etc. in the most common single-phase power supply:
Figure 2
It can be seen that the difference between the current waveform and the sinusoidal waveform of the voltage is large. Figure 3 is the common input current waveform of the inverter for general three-phase power supply:
Figure 3
It can also be seen from Figure 3 that the current waveform is greatly distorted compared with the voltage waveform. This load with non sinusoidal waveform is called nonlinear load. Its current waveform is not 50HZ sine wave, which can be decomposed into many sine waves with different frequencies in electrotechnics. For example, the third harmonic is 150HZ, and the fifth harmonic is 250HZ. What are the hazards of these harmonics?
In daily life, sometimes when a high-power induction cooker is turned on, many interference fringes will appear on the color TV screen. This is what harmonic waves are doing, because the induction cooker is a nonlinear load. The equipment in our power grid is designed according to 50HZ, such as motors and transformers. Generally, if the harmonic wave is too large, the coils will flow 150HZ, 250HZ and other currents with frequencies much higher than 50HZ, which will inevitably lead to abnormal heating of motors and transformers.
Other equipment in the power grid is designed according to 50HZ power supply. Once there is a load with large harmonic in the same power grid, it will cause great interference to other electrical equipment, because their filtering is generally designed according to 50HZ, and the high-frequency component in the harmonic often makes other equipment unable to work normally.
When the harmonics are large, it is inevitable that the power factor is not high. This is because the power grid also needs to transmit additional reactive power when transmitting general active power, resulting in a waste of transmission line capacity and a decrease in the capacity of transmission transformer equipment.
It is precisely because of the many hazards of harmonics that all countries are paying attention to this point at present, and have successively issued relevant laws and regulations to strictly regulate the input power factor and current harmonic weight of equipment entering the power grid.
In the UPS industry, we call the models with high input power factor and low input current harmonics green power supply. For example, our KR series, the input power factor can reach 0.99, and the input current harmonics can reach below 5%, which is a real green power supply. For high-power power frequency UPS, the input current harmonics can also be reduced to less than 10% through the adoption of very mature 12 pulse rectification technology. While ensuring high reliability, Also called green power.
In conclusion, reactive power can be understood as the phase difference between the load current waveform and the voltage waveform. The greater the phase difference, the greater the reactive power. Harmonic can be understood as the distortion of load current waveform and 50HZ voltage waveform. The greater the distortion, the greater the harmonic.
For the treatment of reactive power and harmonic, the current technology is more and more mature, and there are many means and methods. In the future, we will introduce them in a popular way.
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