Pay Attention To The Harmonic Problem Of LED Lamp

- Aug 01, 2018-

LED lighting is the future direction of lighting development, the trend is very obvious, and has been verified in reality. (LED Flood Light)

However, some problems are not well recognized and need further study and attention. For example, the harmonics of LED lamps must be paid attention to. (LED Flood Light)




The author tested seven LED lamps of three brands. See the above table for relevant data.

Two of the three brands are international famous brands. One is a domestic famous brand, which represents the high level of LED lights. (LED Flood Light)




The harmonics of these samples can be seen more directly by using the curves.

Among them, the red curve is the requirement of China's standard value. As can be seen from the figure above, half of the samples have excessive harmonics, so it should be paid attention to. (LED Flood Light)




In order to reduce the influence of power harmonics and get the test data accurately, the power supply in this test adopts the UPS uninterrupted power supply device to obtain the power supply with stable voltage and stable frequency.


In addition to testing LED lights, this test also tested energy-saving lamps, dc fluorescent lamps. (LED Flood Light)

To match the theme of this article, this article only lists the data of LED lights. (LED Flood Light)




The nominal power of LED lamp has different degrees of deviation from the actual measured power, both positive and negative errors, with a maximum deviation of nearly 25%. (LED Flood Light)




Under normal circumstances, the active power P, the reactive power Q and the apparent power S satisfy the hook string law. (LED Flood Light)

However, it can be seen from the last column in the figure above that P, Q and S do not meet this rule, and the extra part indicates that there is a third power D in LED lights. (LED Flood Light)

In fact, P, Q, D and S meet the cube rule, that is, P, Q and D are respectively the length, width and height of the cube, and S are the diagonal lines of the cube (see the lower left corner of the figure above). (LED Flood Light)




The power factors of LED lamps are PF and cos, which respectively represent the total power factor and the fundamental wave power factor. The two definitions are different and the algorithm is different, as shown in the figure above. (LED Flood Light)




PF is directly related to harmonics. (LED Flood Light)

As shown in the figure above, the red curve is PF and the green curve is THDi. (LED Flood Light)

It can be seen from the figure that the smaller PF value is, the larger THDi is, the more serious the harmonics are.


Conclusion: at present, the quality of LED lights is not uniform in the market, and the problem of harmonics is common. (LED Flood Light)



Harm of harmonics


The harm of harmonics is described briefly below. (LED Flood Light)




1. The harmonics will overload equipment and conductors and generate heat


The base wave current is normal current, and the harmonic current is extra current, which belongs to abnormal current. Therefore, the harmonic current is easy to cause the conductor overload and overheat, leading to the conductor insulation damage and burning. (LED Flood Light)

The conductor has a skin effect on high frequency harmonic current, which reduces the rated load flow. (LED Flood Light)


2. Harmonics will increase the loss of transformer


The harmonic current will also increase the loss of transformer copper and leakage magnetic loss. (LED Flood Light)

The harmonic voltage will increase the transformer iron loss. (LED Flood Light)

At the same time, the power harmonics also increase the working mechanical noise of transformer and increase the extra temperature rise of transformer. The higher the harmonic frequency is, the more obvious the noise and temperature rise characteristics are. (LED Flood Light)


3. Harmonics will increase the loss of the rotating motor


Similar to transformers, harmonic voltage and harmonic current will cause additional iron and copper damage to the rotating motor, which will affect the mechanical efficiency and stable torque of the rotating motor.

The temperature rise of the motor is also increased.


Harmonics affect the quality of communication systems


Because the harmonic current or harmonic voltage in the power line will generate induced electromagnetic field, which affects the communication quality of adjacent communication lines. (LED Flood Light)


5. It may interfere with the normal operation of the protective relay


There are many kinds of protective relays and their sensitivity to harmonics is also different. Different harmonic phase angles of the same size of harmonics will have different effects on protective relays. (LED Flood Light)

According to relevant literature records, when the harmonic content is less than 5%, it will not affect most protective relays. When the harmonic content is more than 10%, the protective relays may not work normally. Therefore, it is necessary to evaluate whether the protective relays can work normally. (LED Flood Light)


6. The meter will not work properly


The meter and measuring instrument may affect their normal operation due to harmonic pollution, especially when harmonics are amplified due to resonance, the meter and measuring instrument may produce large error. (LED Flood Light)

Generally speaking, when the total harmonic distortion is greater than 20%, there will be obvious errors in the meter. (LED Flood Light)

However, when measuring meter and measuring instrument adopt special transformer of instrument, it can avoid harmonic interference effectively. (LED Flood Light)


7. It may cause adverse effects on switching appliances


Additional temperature rise and loss will be caused by harmonics in switching equipment. As a result, the rated capacity of switching equipment will be reduced and its insulation and life will be affected. (LED Flood Light)

The higher the content of harmonics, the greater the influence of harmonics on switching appliances.

Therefore, it is possible that harmonics will cause the switch to fail to protect normally. (LED Flood Light)


8. Overload of neutral conductor


The zero-order harmonics will cause distortion of current and voltage, and the zero-order harmonic current in each phase line will be superimposed on the neutral line due to the same phase, thus increasing the possibility of overload of the neutral line, and the safety of the neutral line is threatened. (LED Flood Light)