# Harmonic Control Scheme of LED Display Screen

Due to the large number of LED display screens installed, a large number of harmonics are generated in the display screen, which leads to zero line overheating, tripping and other faults. So what is the harmonic control scheme and method of LED display screen?Harmonic problem of LED display screen - excessive zero line current

In fact, not only the LED display screen will have this problem, but also some advertising light boxes using electronic ballasts will have similar problems. The reason for this problem is that the power supply of LED display screen is switching power supply. The input of switching power supply is rectifier circuit, so it is a typical harmonic source. The harmonic current of single-phase bridge rectifier circuit has 3 times, 5 times, 7 times and 9 times. The 3rd harmonic current is arithmetically superimposed on the zero line, and the effective value is 1.73 times of the phase line current. Since the cross-sectional area of the zero line is the same as that of the phase line during wiring, when the phase line reaches the rated current, the current on the zero line will exceed the rated current, resulting in overheating of the zero line.

It should be noted that when AC current flows on a conductor, there is a physical phenomenon called skin effect, or skin effect. The skin effect means that the AC current is not evenly distributed in the conductor, but concentrated on the conductor surface. The higher the frequency of AC, the more significant this effect is. The current concentration near the surface means that the actual cross-sectional area of the conductor is reduced. Therefore, when the third harmonic current flows through the zero line, the zero line presents a greater resistance to the third harmonic current.The AC resistance is directly proportional to the square root of the frequency. Therefore, the resistance of a conductor to the third harmonic current is 1.7 times that of the fundamental current. According to the calculation of power P = I2R, the calorific value of the third harmonic current with the same amplitude is 1.7 times that of the fundamental current. This exacerbates the overheating of the zero line.

We know that when the current flows through the conductor, it will cause the conductor to heat. The heat is directly proportional to the resistance of the conductor and the square of the current flowing through the conductor. Therefore, if the cross-sectional area of the zero line is the same as that of the phase line, when the zero line current reaches twice the phase line current, the calorific value of the zero line is 4 times that of the phase line. If the cross-sectional area of the zero line is 1 / 3 of the phase line and the resistance of the zero line is 3 times that of the phase line, when the zero line current reaches 2 times that of the phase line, the heating capacity of the zero line is 12 times that of the phase line!

Through the above analysis, we know that when the current on the zero line exceeds the phase line current, it will inevitably lead to overheating of the zero line, which may lead to fire. Excessive zero line current is a serious fire hazard, because there is no overcurrent protection device on the zero line, unlike safety devices on the phase line. When the current is too large, cut off the current. When the current is too large, it can only allow its temperature to rise, accelerate cable aging, and even cause fire.

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