Insulation withstand voltage problem in LED street light anti-surge interference design

Commonly used devices for preventing surge or transient immunity include gas discharge tubes, metal oxide varistors, silicon transient voltage absorbing diodes, and solid discharge tubes, and combinations thereof. The LED street light lightning protection circuit and its device are generally integrated with the LED control device, and a combination of a gas discharge tube and a varistor is commonly used.

First, the suppression circuit principle composed of a combination of a gas discharge tube and a varistor

Since the varistor (VDR) has a large parasitic capacitance, it is used in an AC power system, and a considerable leakage current is generated. After using the varistor with a poor performance for a period of time, the leakage current may become hot and may self-explosion. To solve this problem, a gas discharge tube is inserted between the varistor. In Figure 1, the varistor is placed in series with the gas discharge tube. Since the parasitic capacitance of the gas discharge tube is small, the total capacitance of the series branch can be reduced to a few pF. In this branch, the gas discharge tube will act as a switch. When there is no transient voltage, it can separate the varistor from the system, leaving the varistor with almost no leakage current. However, this has the disadvantage that the reaction time is the sum of the reaction times of the devices. For example, the reaction time of the varistor is 25 ns, and the reaction time of the gas discharge tube is 100 ns. The reaction time of R2, G, and R3 in Fig. 2 is 150 ns. To improve the reaction time, the R1 varistor is added, so that the reaction time is 25ns.

The voltage-current characteristics of the metal oxide varistor (MOV) are shown in Figure 3. The metal oxide varistor (MOV) characteristics are shown in Table 1. The voltage-current characteristics of the gas discharge tube (GDT) are shown in Figure 4. The characteristics of the gas discharge tube (GDT) are shown in Table 2.

Due to the surge interference, once the voltage applied across the gas discharge tube exceeds the spark discharge voltage (u1 in Fig. 4), the gas inside the discharge tube is ionized and the discharge tube begins to discharge. The voltage drop across the discharge tube drops rapidly to the glow discharge voltage (u2 in Figure 4) (u2 is 140V or 180V in Table 2, depending on the characteristics of the tube itself), and the current in the tube begins to rise. As the discharge current is further increased, the discharge tube enters an arc discharge state. In this state, the voltage across the tube (arc voltage) drops very low (u3 in Figure 4) (u3 is 15V or 20V in Table 2, depending on the characteristics of the tube itself), and the arc voltage is quite wide The current variation range (from the i1 → i2 process in Figure 4) remains stable. Therefore, the external high-voltage surge interference is resolved into a protected condition of low voltage and large current (u3 and i2) due to the discharge of the gas discharge tube, and this current (from i2 → i3 in Fig. 4) The gas discharge tube itself flows back into the interference source, eliminating the possibility of interference to the luminaire. As the surge overvoltage subsides, the current flowing through the gas discharge tube drops below the minimum required to maintain the arc discharge state (approximately 10 mA to 100 mA, which is related to the characteristics of the tube itself), and the arc discharge stops and again After the glow discharge state, the entire discharge state (arc stop) is ended.

Second, the insulation withstand voltage problem common to LED street lamps with anti-surge interference function

1. The status quo of the lamp withstand voltage problem

In the LED street lamp using the combination of the above gas discharge tube and varistor to prevent surge interference, the insulation withstand voltage problem is that the contact between the live parts of the lamp and the metal part cannot withstand 2U+1000(V). The voltage of the insulation is usually breakdown at around 600V. The root cause of the insulation withstand voltage problem is the unreasonable selection of the withstand voltage parameters of the gas discharge tube. It is not so much the insulation withstand voltage problem of LED street lamps, but rather the insulation withstand voltage problem of LED control devices. Because the surge protection circuit is usually located in the LED control unit. The LED control device with anti-surge interference function shall comply with GB 19510.14-2009 "Lighting control device Part 14: Special requirements for DC or AC electronic control devices for LED modules" and GB19510.1-2000 "Light control devices" Part 1: General requirements and safety requirements.

2. Problems and answers to the problem of withstand voltage

Manufacturers often question the test method for insulation withstand voltage, and believe that the series circuit of the gas discharge tube and the varistor should be disconnected during the insulation withstand voltage test.

The doubts mainly come from the following two reasons: Reason for question 1: GB 7000.1-2007 "Lamps - Part 1: General requirements and tests", "10.2 Insulation resistance and electrical strength" stipulates: "When performing these tests, the following parts It should be disconnected so that the test voltage is applied to the insulation of the component, not to the capacitive or inductive functional components of these components: a) bypass connected capacitors; b) capacitors between live parts and the lamp housing; c ) Chokes and transformers connected between live parts.) Should L-R2-G-PE or N-R3-G-PE in Figure 2 be disconnected?

For the above parts of GB7000.1-2007, 10.2, the parts to be disconnected during the test refer to the additional capacitors or components in the lamp lighting circuit, and do not include the components in the LED control unit (component). The electrical strength test of the LED control device is based on the requirements of GB 19510.14-2009 and GB19510.1-2000. When the luminaire is tested for electrical strength, the capacitors or components in the control unit should not be disconnected.

Capacitors a) and b) of 10.2 in GB 7000.1-2007 are disconnected during the withstand voltage test of the luminaire. There is no pressure requirement. Actually, there are clear requirements in the luminaire structure, such as 4.10.1 double insulation and GB 7000.1-2007. Reinforced insulation provisions: "Inhibition of interference capacitors shall comply with the provisions of GB/T 14472, and the connection method shall comply with the requirements of 8.6 of IEC 60065:2001." GB/T 14472-1998 "Fixed capacitors for electronic equipment - Part 14: Sub-standards for the suppression of power supply electromagnetic interference with fixed capacitors. Table 1B specifies: Y1 capacitors are used across the double insulation or reinforced insulation, rated voltage ≤ 250V, peak pulse voltage 8.0kV; Y2 capacitors are used to jump across the basic insulation Or supplementary insulation, rated voltage: 150V ≤ U ≤ 250V, peak pulse voltage 5.0kV; Y3 capacitor is used to bridge between basic insulation or supplementary insulation, rated voltage: 150V ≤ U ≤ 250V; Y4 capacitor is applied across Connected between basic insulation or supplementary insulation, rated voltage <150V, peak pulse voltage 2.5kV. According to the type of jumper insulation, etc., the capacitance of the corresponding classification level is determined. Actually, in the structure of the lamp, the requirements of the capacitor withstand voltage corresponding to the corresponding insulation are proposed.

The power supply unit in Figure 2 - R2-G - ground loop or power supply - another live component - R3-G - ground loop is clearly not the object that should be disconnected during insulation withstand voltage or electrical strength test.

Reason 2: IEC 60950-1:2005 "Safety of information technology equipment Part 1: General requirements" "5.2.2 Test procedures" in "5.2 Electrical strength" Note 4: "Provided in parallel with the insulation to be tested The components of the DC path (such as the discharge resistor of the filter capacitor, the voltage limiting device or the surge absorber) should be disconnected. Should the L-R2-G-PE or N-R3-G-PE in Figure 2 be disconnected?

The disconnection voltage limiting device or surge absorber of the above components refers only to a varistor (VDR) (for example, R1 in Fig. 2), and does not involve a series circuit of a gas discharge tube and a varistor (the power supply in Fig. 2 is charged). Component - R2-G - Ground loop or another live component of the power supply - R3-G - Ground loop). The reason is that “1.5.9.1 General Requirements” in “1.5.9 Surge Absorbers” of IEC 60950-1:2005 states: “Secondary circuits allow the use of any type of surge absorber, including varistor (VDR). The surge absorber used for the primary line shall be a VDR (varistor) and the VDR complies with the standard Appendix Q. VDR is sometimes referred to as a Varistor or a metal oxide varistor (MOV). For example, a gas discharge tube Devices such as carbon blocks and non-linear voltage/current characteristics of semiconductor devices are not considered to be VDRs in this standard."

For what insulation can be used VDR, what insulation can not use VDR, IEC60950-1:2005 1.5.9.3, 1.5.9.4 and 1.5.9.5 are as follows:

(1) It is allowed to be insulated with a VDR jumper function.
(2) It is allowed to use a VDR with one side grounded and a basic insulation.

VDR bridging basic insulation This equipment should be one of the following:
- Type B pluggable device; or - Permanently connected device; or - Device with permanently attached protective earth conductor assembly and instructions for installation of the conductor.

(3) It is not allowed to use a VDR to bridge additional insulation, double insulation or reinforced insulation. To prevent transient voltages above the maximum continuous voltage, thermal overload due to leakage currents in the VDR, and VDR combustion and explosion in the event of a short-circuit fault, 1.5.9.2 of IEC 60950-1:2005 specifies a series connection with the VDR. A circuit breaker with sufficient fuse capability. The surge absorber is used as a differential mode protection for the device, or as a common mode protection case, which is different for safety. Differential mode protection refers to anti-interference protection provided between one pole and the other pole of the power supply. The protection object is equipment and does not involve personal safety protection. Common mode protection refers to the anti-interference protection provided between any pole of the power supply and the grounding of the power supply. In addition to protecting the equipment, it is more important to refer to the safety when the person touches the equipment. In this sense, security should be ensured first, and then the requirements of the EMS should be considered.

3, LED control device standard test method for withstand voltage

As the LED control device, it should meet the requirements of GB 19510.14-2009 "Lighting control device Part 14: Special requirements for DC or AC electronic control devices for LED modules", whether it is GB19510.14-2009 or its reference standard GB19510 The "dielectric strength" of .1-2000 "Control devices for lamps - Part 1: General requirements and safety requirements" is not required to be disconnected.

4, OSM / EE resolution on insulation withstand voltage

EN 60950 "Safety of Information Technology Equipment" and EN 60950-1 "Safety of Information Technology Equipment Part 1: General Requirements" 1.5.1 related OSM/EE No. 98/2 (European Electrotechnical Standardization Committee electrical equipment operation The staff meeting) stated in the resolution of the use of the varistor on the power supply: "The combination of the varistor (the varistor does not have to be separately certified) in series with the arrester or gas discharge tube between the power supply and the protective ground shall comply with the basic insulation (eg Electrical strength and external creepage distance to arrester or gas discharge tube) and protection against short circuit. 1. Type A pluggable device: accepted by all countries. 2. Type B pluggable device and permanently connected device : Accepted by all countries." The above OSM/EE resolution on the use of varistor on the power supply clearly demonstrates that the L-R2 in Figure 2 of the LED street lamp is subjected to a withstand voltage test between live parts and metal parts. -G-PE or N-R3-G-PE should not be disconnected.

5, the solution of insulation withstand voltage problem

In order to make the LED street lamp meet the safety requirements, the pressure selection of the gas discharge tube is very important. The gas discharge tube with sufficient pressure should be selected to be matched with the varistor. The series circuit of the varistor and the gas discharge tube (Fig. 1) should be Capable of withstanding the withstand voltage of basic insulation. The relationship between the DC spark discharge voltage and the AC breakdown voltage of several gas discharge tubes is shown in Table 3. The DC spark discharge voltage of the gas discharge tube should generally be no less than 2500V.

Third, the conclusion

1. The EMS design of LED street lamps should be based on meeting safety requirements and should not be at the expense of safety in exchange for meeting EMS requirements. The safety requirements are not up to the point, I am afraid I don’t know.

2. LED street light design input must be sufficient.

(1) The safety of LED street lamps must comply with GB 7000.5-2005 "Safety requirements for roads and street lighting" and GB 7000.1-2007 "Lighting lamps Part 1: General requirements and tests";

(2) The performance of LED street lamps can be based on the standard GB/T 24827-2009 "Performance requirements for road and street lighting fixtures" and GB/T 9468-2008 "General requirements for luminaire distribution photometric measurement";

(3) EMI of LED street lamps is required to comply with GB 17743-2007 "Limitations and Measurement Methods for Radio Disturbance Characteristics of Electrical Lighting and Similar Equipment", GB 17625.1-2003 "Electromagnetic Compatibility Limits Harmonic Current Emission Limits (Equipment Per Phase Input current ≤16A)" and GB 17625.2-2007 "Electromagnetic compatibility limit for each phase of rated current ≤ 16A and unconditional access to equipment in the public low-voltage power supply system voltage changes, voltage fluctuations and flicker restrictions";

(4) The EMS of LED street lamps needs to meet the requirements of GB/T 18595-2001 "Electromagnetic Compatibility Immunity Requirements for General Lighting Equipment". 

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references

[1] "Transient Interference Absorption Device" Qian Zhenyu

[2] GB/T 18802.311-2007/IEC 61643-311:2001 Low Voltage Surge Protection Components Part 311: Gas Discharge Tube (GDT) Specification

[3] GB/T 18802.331-2007/IEC 61643-331:2003 Low-voltage surge protective components - Part 331: Metal oxide varistor (MOV) specification

[4] IEC 60950-1:2005 Information technology equipment –Safety –Part1:General equirements

[5] IEC 61347-2-13:2006 Lamp controlgear –Part 2-13: Particularrequirements for dc or ac supplied electronic controlgear for LEDmodules

[6] IEC 60384-14:2005 Fixed capacitors for use in electronic equipment–Part 14:
Sectional specification: Fixed capacitors for electromagnetic interference suppression and connection to the supply mains

[7] OSM/EE DECISION SHEET, Sheet no. 98/2, Subject: Use of Varistors on the mains

[8] GB 7000.1-2007 lamps - Part 1: General requirements and tests

[9] GB / T 14472-1998 electronic equipment part 14 is fixed capacitors: electromagnetic interference suppression specification Fixed capacitors