LED street light power supply environment and impact analysis

Nowadays, LED energy-saving products have been recognized and favored by the government and users in the renovation of street lamps, and have greatly promoted the development of LED street lamps. However, in the actual use of LED street lights, it can be found that the actual application status of many street lamp power sources is not optimistic. “Power” is an important part of LED street lights. Environmental factors and installation grounding methods have a great impact on the service life of the power supply. According to the comprehensive analysis of the field survey of LED power failures, the failure products mostly occur in minefields, waters, open areas, etc. Therefore, LED streetlight products must have certain characteristics of anti-induction lightning, and improve grounding technology. These two points require the joint efforts of LED street lamp manufacturers and LED power supply manufacturers to extend the service life of LED street lamp power supplies.

The following analysis of the failures encountered in the use of LED street light power for many years, and put forward some personal improvement technical advice, hoping to contribute to the development of LED street lights.

Main power supply failure mechanism and analysis

Problem Analysis 1:
High grid voltage: not isolated from the industrial grid, causing the voltage to rise in the middle of the night, exceeding the maximum input voltage of the power supply specification.
The effect of grid voltage fluctuations on the use of LED power supplies.

Example: In the maintenance of LED street light power supply, the monitoring and monitoring of the grid voltage is too high in different areas:

The following methods are proposed for the above problems for reference:

Add a leakage protection switch between the power supply and the power supply.
When input overvoltage and continuous strong lightning strike occur, you can switch the power supply, protect it, and then manually close the switch during maintenance, without damaging the device.
Need to pay attention to the choice of leakage current to prevent frequent movements.

Problem Analysis 2: Mainly damaged by lightning strikes

The grounding effect of the lamp post installation is not good, resulting in poor lightning protection effect, affecting the normal operation of the power supply:
Soil environment analysis The pole grounding condition of the power supply grounding pole is poor, resulting in poor lightning protection.
The power supply protection effect of the power grid is not good, resulting in poor lightning protection.

Lightning in the air emits a spectrum of radio waves, overhead road lights and power lines that are good to receive antennas. This instantaneous differential mode interference voltage can also reach hundreds of volts to more than 3,000 volts. This voltage tends to break through the electrical gaps in the power supply circuit of the electronic device and can also damage the drive circuit. Therefore, the power supply needs to be designed for lightning protection. The lightning protection design is a system engineering, and lightning protection design is required from the power supply grid, system grounding, and power equipment.

The following is a detailed analysis of lightning protection design and current status:

First, the lightning failure example (see the figure below)

During the analysis, such as MOV rupture, it is caused by lightning damage.

Second, four types of lightning strike mode and power supply lightning failure mechanism

1. Direct lightning strike: Contains great energy, lightning current with peak voltage up to 5000kv into the ground, with great destructive power. There are three effects that can be caused:
1) The huge current is flowing down in a few microseconds, causing the ground potential to rise rapidly, causing a counterattack accident and endangering the safety of people and equipment.
2) Lightning current generates powerful electromagnetic waves, which induce extremely high pulse voltages on power lines and signal lines.
3) Lightning current flows through electrical equipment to generate extremely high heat, causing fire or explosion accidents.

2. Conducted lightning: Distant lightning strikes the line or the extremely high voltage generated by electromagnetic induction, which is transmitted from the outdoor power line and communication line to the electrical equipment inside the building.
3. Inductive lightning: Frequent discharge between clouds produces powerful electromagnetic waves that cause common mode and differential mode interference, affecting the operation of electrical equipment.
4. Switch overvoltage: Inductive and capacitive load in the power supply system is turned on or off, short circuit of the ground, short circuit of the power supply line, etc., can generate high voltage pulse on the power line, the pulse voltage can reach 3 to 5 of the normal voltage. Double, can seriously damage the device. The damage effect is similar to lightning strikes.

National Military Standard "GJB 6784-2009 General Requirements for Lightning Protection of Military Ground Electronic Facilities", as shown in the following figure:

Third, lightning protection system design:

The TN-S structure refers to the transformer coming out, using three-phase four-wire transmission, and the neutral line is connected to the protection ground. This structure facilitates the rapid release of inrush current. The degree of protection at this time is Class IV (4KV); after transmission to the building, the degree of protection is Class III (2.5KV); after the special machine room, the degree of protection is Class II (1.5KV); finally, on the equipment, the degree of protection is lowered. It is class I (1.0KV).

Lightning protection is a system engineering. On the one hand, it is necessary to absorb the energy step by step, and the principle of classification of the protection interval, it is necessary to do multi-level protection. On the other hand, it is necessary to carry out protection design from three aspects: power supply system, equipment grounding, and products.

The lightning protection design of the power supply grid can be based on the requirements of Chapter 6 of the latest standard "CJJ45-2006 Urban Road Lighting Design Standards": "6.1.9 The grounding form of the road lighting distribution system should adopt TN-S system or TT system, metal lamp The exposed conductive parts of poles and components, lamp housings, power distribution and control box screens shall be protected and grounded and shall comply with the relevant national standards."

According to the on-site maintenance feedback information, there are three main power supply lines: FireWire, Neutral, and Protective Ground (PE). It should be a TN-S system with an independent grounding wire, which provides a way for the discharge of lightning surge current.

The lightning protection grounding design of the light pole shall be based on GB50057-2000 "Lightning Protection Design Code for Buildings", Section 3.2.2 "3. The lightning protection grounding device shall be shared with the electrical equipment grounding device, and its power frequency grounding resistance shall not be Should be greater than 10 Ω.” There is also the latest standard “CJJ89-2001 Urban Road Lighting Project Construction and Acceptance Regulations” Chapter 6 requirements:

6.2.7 When the metal power equipment such as the light pole and distribution box is grounded, the grounding resistance should not exceed 4Ω.

6.3.2 The buried depth of the grounding body shall comply with the design regulations. When the design is not specified, the buried depth shall not be less than 0.6m.

In summary, the grounding of the lamppost should follow the protective grounding of no more than 4Ω and the lightning protection grounding of no more than 10Ω.

The soil has an influence on the grounding resistance, and the lightning protection grounding design of the lamp post is to adopt an effective resistance reduction method. When the soil at the grounding point (such as rock, sandy soil and long-term frozen soil) has a high electrical resistivity, in order to meet the requirements of grounding resistance, measures must be taken to reduce the resistivity of the soil. These measures include:

1. Change soil: Replace the soil with higher resistivity with black soil, clay and sandy clay with lower resistivity. Generally, the soil with the length of 1/3 of the upper part of the grounding body and 0.5 m or less around it is replaced.
2. Deep burial: If the deep soil resistivity of the grounding point is low, the buried depth of the grounding body can be appropriately increased. Deep burial can also ignore the effects of increased resistivity caused by soil freezing and drying.
3. External grounding: The grounding body is buried in the place where the soil resistivity is low by the metal lead.
4. Chemical treatment: Mixing slag, charcoal powder, salt and other chemical substances in the soil at the grounding point, and using a special chemical resistance reducing agent can effectively reduce the soil resistivity.
5. Soil conservation: Take measures to keep the soil at the grounding point moist for a long time.
6. Treat the frozen soil: add peat to the soil at the grounding point in winter to prevent the soil from freezing, or bury the grounding body under the building.

Fourth, the lightning protection design of the power supply:

1. LED power supply requirements

Refer to GBT 17626.5-2008 Electromagnetic Compatibility Test and Measurement Technology Surge (Impact) Immunity Test" is divided into 1\2\3\4\X level. The first-level parameters of the power line differential mode test are not given, and the remaining levels are 0.5KV\1KV\2KV and are to be determined. The parameters of the power line common mode test are 0.5KV\1KV\2KV\4KV. The severity of the test depends on the environment (the environment subject to the possibility of a surge) and the installation conditions. The general classification is:

Level 1: A well-protected environment, such as the control room of a factory or power station;
Level 2: A certain protected environment, such as a factory without strong interference;
Level 3: Ordinary electromagnetic disturbance environment: no special installation requirements are specified for the equipment, such as cable network installed in general, industrial workplace and substation;
Level 4: Environments subject to severe harassment, such as civil overhead lines, unprotected high-voltage substations;
Class X: Special level, determined by the user and the manufacturer after negotiation.
From the perspective of the use environment, the LED driver power supply needs to consider the requirements of level 3 or higher.

2. Field application analysis of power supply

There are three main power supply lines for on-site maintenance feedback: FireWire, Neutral, and PGND (PE). It should be a TN-S system with an independent grounding wire, which provides a way for the discharge of lightning surge current.

During installation, the PE is not directly connected to the PE line corresponding to the input line of the power supply, but directly connected to the exposed metal part at the bottom of the lamp post: it is desired to pass the lamp post to the outer casing of the lamp module, and then from the mounting screw to the power supply case. (The outer casing of the power supply is connected to the ground of the internal wiring). More prominent in some old lamp renovation.

Power grounding is a problem that cannot be ignored. The fault is often caused by the installation project. The power protection ground is connected to the outer casing (lightning protection ground) and crosstalks each other; the related joints are easily corroded, resulting in many unreliable problems. For example, after the power supply case is anti-oxidation treatment, it is non-conductive, making the grounding unreliable and causing poor lightning protection. Even after passing the lamp, the pole to the ground connection to the PE line, especially the PE line connection, because there is no anti-oxidation treatment, after six months, rust will occur, resulting in the contact resistance not meeting the requirements, so that the power supply Lightning protection is reduced.

The power protection ground is connected to the lamp pole casing (lightning protection ground), and the surge voltage is cross-talked. This complicates the lightning protection engineering. The surge voltage generated by the induction lightning received by the power grid acts on different poles. The effect (because the grounding resistance of the lamp pole is different), the surge voltages of the multiple minefields are staggered, and the varistor breakdown failure of the enhanced characteristics is invalid.

After analysis, mainly focused on field applications, the following methods are proposed for reference:

The power supply grid must use the TN-S system - live wire, neutral wire, ground wire;
The grid protection ground (PE line) and the lamp post lightning protection ground (cylinder) should be separated, do not connect to prevent crosstalk;
Connect the power PE line to the PE line of the power grid reliably;
Grounding resistance is less than 0.1 ohm;
The connection joint is waterproofed to ensure long-term connection reliability;
The grounding of the lamp post is greatly affected by the weather such as the rainy season and the dry season, but it is necessary to keep the grounding resistance less than 10 ohms at all times;
The main factors that consider the grounding resistance are the soil resistivity, the size, shape and depth of the grounding body, the connection of the grounding wire to the grounding body, etc.
The joint needs to be treated with anti-oxidation and anti-corrosion treatment;
In the project of the bridge and the expressway, it is necessary to weld the metal connecting piece of the lamp post base with the lightning protection system of the bridge to form an overall lightning protection;
Suburbs need to pay more attention to lightning protection than urban routes.

Fifth, pay attention to the impact of the installation method:

The problem of actual use of LED power supply can not be ignored. In the early stage of retrofitting conventional lighting streetlights, it is recommended to cooperate with the power supply provider to take into account the actual use of the ground to reduce unnecessary losses during the installation process. "Guangdong Zhaoxin Lighting Co., Ltd." cooperated closely with "Shenzhen Maoshuo Technology Co., Ltd." in the process of retrofitting LED street lamps for 103 ordinary roads in the east of Guiyang, Nanhai District, Foshan City, and dispatched technicians to inspect the soil and waters of the project site. , lightning strikes, voltage and other conditions, scientific design of lighting and power supply technical parameters, making more than 8,000 street lamp renovation projects successfully implemented in a short period of time, won the praise of the owners, added a new local energy-saving renovation project Beautiful scenery!

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