Pay Attention To The Harmonic Problem Of LED Lamp

- Oct 28, 2018-

LED lighting is the development direction of future lighting, the trend is very obvious, and has been verified by reality.

However, some problems are not well recognized and need further research and attention, for example, the harmonic of LED lamp must be paid attention.

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

Two of the three brands are international famous brands and one is domestic famous brand, representing the high level of LED lights.

The harmonic of these samples can be seen more intuitively by using the curve.

The red curve is the requirement of China's standard value, and it can be seen from the above figure that half of the samples are over harmonic. Therefore, attention should be paid to it.

In order to reduce the harmonic effect of the power supply and obtain the test data accurately, the power supply of this test adopts UPS uninterruptible power supply device to obtain the power supply with stable voltage and frequency.

In addition to testing LED lights, energy-saving lamps and straight fluorescent lamps were also tested.

To match the theme of this article, only the LED data is listed.

The nominal power of LED lamp and actual measured power have different degrees of deviation, both positive and negative errors, the maximum deviation is nearly 25%, very outrageous.

Under normal circumstances, active power P, reactive power Q and apparent power S satisfy the hook string law.

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

In fact, P, Q, D and S satisfy the cuboid law, that is, P, Q and D are the length, width and height of cuboid respectively, and S is the diagonal line of cuboid (see the lower left corner of the figure above).

The power factor of LED lamp is PF and cos, which respectively represent the total power factor and the base wave power factor. The two definitions are different, and the algorithm is also different. See the figure above.

PF is directly related to harmonics.

As shown in the figure above, the red curve is PF and the green curve is THDi.

According to the figure, the smaller PF value is, the larger THDi is, and the more serious the harmonic is.

Conclusion: at present, the quality of LED lamps is uneven, and the problem of harmonic is common. Therefore, the problem of harmonic must be paid attention to, so as not to affect lighting distribution, protection, wiring, control, etc.

Harmonic hazard

The harm of harmonics is briefly introduced below.

1. The harmonic wave will cause overload of equipment and conductor and heat

The fundamental current is the normal current, and the harmonic current is the extra current, belonging to the abnormal current, therefore, the harmonic current is easy to cause the conductor overload, overheating, leading to the conductor insulation damage and burnt.

The conductor has skin effect on the high frequency harmonic current, which reduces the rated load flow.

2. The harmonic will increase the loss of the transformer

The harmonic current will also increase the copper loss and magnetic leakage loss of the transformer.

The harmonic voltage will increase the transformer's iron loss.

At the same time, power harmonics can also increase the machine noise of the transformer and increase the extra temperature rise. The higher the harmonic frequency is, the more obvious the noise and temperature rise characteristics are.

3. The harmonic will increase the loss of the rotating motor

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

Increase the temperature of the motor.

4. Harmonics will affect the quality of communication system

Because the harmonic current or the harmonic voltage in the power line will produce the induced electromagnetic field, which affects the communication quality of the adjacent communication line.

5. Possible interference protection relay works normally

Different types of protective relays have different sensitivity to harmonics, and different harmonic phase angles of the same magnitude of harmonics have different effects on the protection relays.

According to relevant literatures, when the harmonic content is less than 5%, most protection relays will not be affected. When the harmonic content is more than 10%, the protection relay may not work normally. Therefore, it should be evaluated to analyze whether the protection relay can work normally.

6. The table will not work properly

The meter and measuring instrument may affect the normal operation due to harmonic pollution, especially when the harmonic is amplified due to resonance, the meter and measuring instrument may cause large error.

Generally speaking, when the total harmonic distortion is greater than 20%, the meter will have obvious error.

However, the harmonic interference can be effectively avoided when the meter and measuring instrument adopt the instrument special transformer.

7. It may cause adverse effects on switching appliances

Switching equipment will cause additional temperature rise and loss due to harmonics. As a result, the rated capacity of switching equipment will be reduced and its insulation and life will be affected.

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

Therefore, it is possible that the harmonic may cause the switch to fail to protect properly.

8. Overload of neutral conductor

The zero sequence harmonic will cause the distortion of current and voltage. The zero sequence harmonic current in each phase line will be superimposed on the neutral line due to the same phase, thus increasing the possibility of the neutral line overload, and the neutral line safety is threatened.