WHAT IS LED STREET LIGHT?

- May 07, 2020-

Definition

Road lighting is an important part of urban lighting. Traditional street lamps are often used. High-pressure sodium lamps emit light at 360 degrees. The disadvantage of large light loss causes a huge waste of energy. At present, the global environment is deteriorating, and all countries are developing clean energy. With the rapid growth of the national economy, the contradiction between energy supply and demand in China is becoming increasingly prominent, and there is a serious shortage of electricity supply. Energy conservation is an urgent problem to be solved. Therefore, the development of new high-efficiency, energy-saving, long life, high color rendering index, and environmentally friendly LED street lamps is of great significance to the energy saving of urban lighting.

Road lighting is closely related to people's production and life. With the acceleration of China's urbanization process, LED street lights gradually take advantage of directional light emission, low power consumption, good driving characteristics, fast response, high earthquake resistance, long service life, and green environmental protection. Entering people's field of vision and becoming the world's newest energy-saving light source with the advantages of replacing traditional light sources, therefore, LED street lights will become the best choice for energy-saving road lighting transformation.


Features

The difference between LED street lamps and conventional street lamps is that the LED light source uses low-voltage DC power supply, high-efficiency white light synthesized by GaN-based power blue LEDs and yellow. Unique advantages can be widely used in roads. The outer cover can be made with high temperature resistance up to 135 degrees and low temperature resistance up to -45 degrees.

advantage

1. Its own characteristics-the unidirectional light, no diffusion of light, to ensure lighting efficiency.

2. The LED street lamp has a unique secondary optical design, which illuminates the light of the LED street lamp to the area required for illumination, and further improves the lighting efficiency to achieve the purpose of energy saving.

3. The LED has reached 110-130lm / W, and there is still a lot of room for development, with a theoretical value of 360lm / W. The luminous efficiency of high-pressure sodium lamps increases only with the increase of power. Therefore, the overall light efficiency of LED street lamps is stronger than that of high-pressure sodium lamps; ).

4. The color rendering of LED street lamps is much higher than that of high-pressure sodium lamps. The color rendering index of high-pressure sodium lamps is only about 23, and the color rendering index of LED street lamps reaches more than 75. From the perspective of visual psychology, to achieve the same brightness, the average illumination of LED street lamps can be More than 20% lower than high-pressure sodium lamps.

5. The light decay is small, less than 3% in one year, and it still meets the road requirements after 10 years of use, and the high-pressure sodium lamp has decayed, which has dropped by more than 30% in about one year. Can be lower than high pressure sodium lamps.

6. The LED street lamp has an automatic control energy-saving device, which can achieve the maximum possible power reduction and energy saving when meeting the lighting requirements of different periods. It can realize humanized functions such as computer dimming, time-division control, light control, temperature control, and automatic inspection.

7. Long lifespan: it can be used for more than 50,000 hours and provide a three-year quality guarantee. The disadvantage is that the life of the power supply cannot be guaranteed.

8. High luminous efficiency: the use of chips with ≥100LM or more can save energy by more than 75% compared with traditional high-pressure sodium lamps.

9. Easy installation: no need to bury cables or rectifiers, etc., directly install the lamp pole or nest the light source in the original lamp housing.

10. Excellent heat dissipation control: the temperature in summer is controlled below 45 degrees, and passive heat dissipation is adopted, and the heat dissipation guarantee in summer is insufficient.

11. Reliable quality: The circuit power supply uses high-quality components, and each LED has a separate overcurrent protection without worrying about damage.

12. Uniform light color: no lens is added, uniform light color is not sacrificed to improve brightness, so as to ensure uniform light color without aperture.

13. LED does not contain harmful metal mercury, which will not cause harm to the environment when scrapped.

The energy saving effect based on the above principle is remarkable, and it can save more than 60% of electricity by replacing the high-pressure sodium lamp.

Low maintenance cost: Compared with traditional street lamps, the maintenance cost of LED street lamps is extremely low. After comparison, the total input cost can be recovered in less than 6 years.


Disadvantages

1. Low single LED power. In order to obtain high power, multiple parallel use is required.

2. Low color rendering. The color displayed under LED illumination is not as real as an incandescent lamp. This is a technical issue to analyze from the spectral distribution.

3. Light spot. Due to the defects in the manufacturing process of the white LED itself and the matching error with the reflective cup or lens, it is easy to cause the "yellow circle" problem.

4. Uniformity of LED irradiation. If the secondary optical design is not carried out, the illumination of the LED is relatively concentrated, so the secondary optical design must be carried out so that the light intensity distribution diagram is bat-shaped.

5. The light decay of LED. Compared with low-power LEDs, the light attenuation of high-power LED street lamps will be much better. But the heat dissipation of low-power LED is very small. High-power LEDs have the problem that there is no solution to heat dissipation, and the brightness will be significantly reduced after heating, so the power cannot be increased. The largest Spark LED street lamp on the market is 360W.


Design requirements


  1. The biggest feature of LEDs for lighting is the function of directional light emission, because almost all power LEDs are equipped with reflectors, and the efficiency of such reflectors is significantly higher than that of lamps. In addition, the efficiency of the self-reflector has been included in the detection of LED light efficiency. Road lamps using LEDs should make full use of the characteristics of the directional emitted light of the LEDs, so that each LED in the road lamps directs the light directly to each area of the illuminated road surface, and then uses the auxiliary light distribution of the lamp reflector to achieve Very reasonable comprehensive light distribution of road lamps. It should be said that road lamps must truly meet the illuminance and illuminance uniformity requirements of CJJ45-2006 and CIE31 and CIE115 standards. The luminaire should include the function of three times light distribution to achieve better. However, LEDs with reflectors and reasonable beam output angles themselves have a good primary light distribution function. In the lamp, the installation position of each LED and the direction of the emitted light can be designed according to the height of the road lamp and the width of the road surface to achieve a good secondary light distribution function. The reflector in this type of lamp is only used as an auxiliary third light distribution method to ensure a better uniformity of road illumination.

  2. In the design of actual road lighting fixtures, each LED can be fixed to the fixture with a ball gimbal under the premise of basically setting the direction of each LED. When the fixture is used at different heights and irradiation widths At this time, the spherical gimbal can be adjusted to achieve a satisfactory result in the irradiation direction of each LED. When determining the power and beam output angle of each LED, according to E (lx) = I (cd) / D (m) 2 (the law of inverse square ratio of light intensity and illuminance distance), calculate the basic selection of each LED separately The power that should be available at the beam output angle, and the light output of each LED can reach the expected value by adjusting the power of each LED and the different power output from the LED drive circuit to each LED. These adjustment methods are unique to road lamps that use LED light sources. Taking full advantage of these features can achieve the purpose of reducing the lighting power density and meeting the goal of saving energy while satisfying the illuminance and uniformity of road surfaces.The power supply system of LED street lamps is also different from traditional light sources. The constant current drive power required by LEDs is a cornerstone to ensure their normal operation. Simple switching power supply solutions often cause damage to LED devices. How to make a group of LEDs tightly packed together safe is also an indicator for investigating LED street lights. The requirement of the LED for the driving circuit is to ensure the characteristics of constant current output. Because the relative change area of the junction voltage when the LED is working in the forward direction is small, the constant of the LED drive current is basically guaranteed to ensure the constant of the LED output power. For the current situation of unstable power supply in China, it is very necessary for the driving circuit of the road lamp LED to have a constant current output characteristic, which can ensure a constant light output and prevent the LED from overpowering.


To make the LED drive circuit exhibit constant current characteristics, the output internal impedance must be high from the output end of the drive circuit. During operation, the load current also passes through this output internal impedance. If the drive circuit is composed of a DC constant current source circuit after buck, rectification and filtering, or a general switching power supply plus a resistance circuit, it must also consume a lot of active power on it. Power, so the efficiency of these two types of drive circuits can not be high under the premise of basically meeting the constant current output. The correct design scheme is to use active electronic switching circuits or high-frequency currents to drive LEDs. Using the above two schemes can make the drive circuit still have high conversion efficiency while maintaining good constant current output characteristics

The road lamps in China basically adopt the mode of HID light source plus trigger and inductive ballast. Although this mode has the problems of low energy efficiency and strobe. An important aspect that threatens the plasticity of LED lamps that use electronic drive circuits when used in outdoor lighting is the lightning induction problem.


As we all know, the lightning in the air emits a broad spectrum of radio waves, and the overhead road lamp power supply line is well-accepted for wireless. The radio waves emitted by the same lightning received by the two power lines are common-mode interference signals to the drive circuit. This common-mode interference can reach hundreds of volts to thousands of volts to the ground, and it is easy to break through the drive circuit. EMC grounding capacitors or small electrical clearances to the ground (to the housing) cause damage to the drive circuit.

In addition, because China's power supply line is a three-phase four-wire neutral line grounded polar power supply, at the instant when a radio wave of lightning is induced at each section of the two overhead power supply lines, due to the two power supply lines to the ground The difference in instantaneous impedance causes a differential mode interference voltage between the two power supply lines. This instantaneous differential mode interference voltage can also reach hundreds of volts to more than 3000 volts. This voltage often breaks the power supply rectifier diode and printed circuit of the drive circuit. The LED controller will also damage the drive circuit due to the electrical gap between the electrodes of different polarities on the circuit board.


To solve this problem, it is necessary to connect a fast-responsive varistor at the input end of the LED drive circuit to ensure the release of differential mode interference. Since the induced interference of lightning is repeated many times, when the interference voltage is high, the varistor may conduct a large amount of current. The varistor should not only have a fast response capability, but also have an instantaneous conduction. Pass tens of amps of discharge capacity without damage. In addition to the use of varistor, the input end of the LED drive circuit should also be combined with the protection of conducted interference (EMI), and a composite LC network is designed, so that these LC networks can not only hinder the internal EMI leakage to the power grid, but also The interference signal of lightning has obvious suppression effect.

In addition, the electrical clearance of each point of the LED drive circuit to the ground should be maintained at 7mm or more. The ground capacitance of the EMI protection and the insulation strength of the drive circuit to the ground should meet the requirements of reinforced insulation (4V + 2750V). The drive circuit has good resistance to differential mode and common mode lightning induction.


Purchase points

Luminous efficiency

The technology is becoming more and more mature, high-power LED light source can meet the needs of general street lamps. The general high-pressure sodium lamp has a light efficiency of 100LM / W, and the commonly used high-power LED is 50-60LM / W. Using foreign LED chips can achieve 80LM / W. The higher the luminous efficiency, the better the energy saving effect, which is also the choice One of the most important indicators of LED street lights. However, the LED standard does not clearly specify this, so you must carefully confirm when purchasing LED street lights.

Light decay

In order to reduce costs, some businesses use hundreds of 0.5W low-power LEDs. However, the light attenuation of this low-power LED is very serious, and its light attenuation to 80% of the life is only 1000 hours. Therefore, as a street lamp that needs to be used for a long time, it is absolutely not allowed to use this low-power LED. If you choose a high-power LED (generally refers to more than 30W), its light attenuation will be much better.

Weight problem

Due to the high technical content and complex composition of LED street lamps, some high-power lamp heads will weigh far more than ordinary high-pressure sodium lamps, so the requirements for the corresponding supporting materials are higher. However, some LED street lamp manufacturers have been trying to reduce the weight of the LED lamp head as much as possible, from the original single lamp head of about 30 kg to more than 10 kg, and the weight reduction has a further downward trend.


Heat dissipation

Because LED is a semiconductor component, its wafer is reduced to 30% of the initial luminous flux due to temperature, it will lose the significance of lighting, that is, the end of life. Theoretically, the life of high-power LED street lamps is 3-50000 hours, The important premise is good heat dissipation. It is worth mentioning that domestic manufacturers have adopted the world's leading needle cooling technology.


Cost comparison


Suppose that a city has three sections of roads with lengths of 3 km, 5 km, and 10 km, and the street lights are laid every 30 meters, and one on each side. , 668, traditional street lamps are based on high-pressure sodium lamps with a power of 250W, and LED street lamps are compared with a power of 50W.

The unit price of traditional street lamps is 1,000 yuan per lamp, and the unit price of LED street lamps is 3,000 yuan. * 1000 = 668,000 yuan, the cost of light sources required to lay the roads with three lengths of LED street lights are: 202 * 3000 = 606,000 yuan, 334 * 3000 = 1.002 million yuan, 668 * 3000 = 2.004 million yuan

Let's analyze the cost of laying cables with two light sources on three sections of the road. The load flow of the main line of the traditional street lamp on the 3 km road is I = P / U = 202 * 250/220 = 230A. For cable, the unit price is 120 yuan / meter, the cable cost is 120 * 3000 = 360,000 yuan, the load flow of the LED street light trunk cable l = P / U = 202 * 50/220 = 44A, it needs a cross-sectional area of 1Omm2 Copper core cable, it needs a copper core cable with a cross-section of 12mm2, the unit price is 12 yuan / meter, the cable cost is 12 * 300 = 36,000 yuan, and the load flow of the 5km road traditional street lamp trunk line cable is I = P / U = 333 * 250/220 = 377A, it needs a copper core cable with a cross section of 90mm2, the unit price is 180 yuan / m, the cable cost is 980 * 5000 = 900,000 yuan, the load flow of the LED street lamp main line cable I = P / U = 333 * 50/220 = 73A, it needs a copper core cable with a cross-section of 20mm2, the unit price is 25 yuan / cost is 25 * 5000 = 12.5 million, and the cable flow of the main line of the traditional street lamp on 10 kilometers of road I = P / U = 666 * 250/220 = 680A, it needs a copper core cable with a cross section of 400m2, the unit price is 360 yuan / meter, the cable cost is 360 * 10000 = 3.6 million yuan, the load flow of the LED street lamp trunk line cable is I = P / U = 666 * 50220 = 1 46A, it needs a copper core cable with a cross section of 42mm2, the unit price is 60 yuan / m, and the cost is 600,000 yuan.