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Surge (Shock) Immunity Test and Calibration Method Discussion
Release time:
2022-05-27 00:00
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Surge (Shock) Immunity Test and Calibration Method Discussion
1 Introduction
In order to truly simulate the voltage or current surges caused by lightning strikes or switching transients that the equipment may encounter in actual operation, and to evaluate the immunity level of electrical and electronic equipment to lightning strikes, the standard requires that the equipment under test (EUT) be in Under the specified live (DC or AC power supply) working condition, use a combined wave signal generator capable of generating 1.2/50us open-circuit voltage wave and 8/20us short-circuit current wave to conduct lightning surge immunity test to test the equipment under test Anti-interference ability in the actual operation process. Based on the actual situation of the laboratory, this paper introduces the main components and working principle of the lightning surge test equipment, builds a lightning surge test platform and analyzes the differences between different coupling methods.
2 Definition of surge waveform
GB 17626.5-2019 provides a more operable waveform and parameter definition in actual test work, as shown in Figure 1 and Figure 2.
Figure 1 The open-circuit voltage waveform (1.2/50us) of the output terminal of the generator not connected to the CDN
Among them, Tw is the duration, which is defined as the time elapsed between the rising edge and falling edge of the voltage waveform at 50% of the peak voltage, and T is the wave front time, defined as the rising edge of the voltage waveform at 30% of the peak voltage and 90% of the peak voltage Elapsed time between:
Wave front time: Tf=1.67×T=1.2×(1±30%)µs, 1.67 is the reciprocal of the difference between 0.9 and 0.3 thresholds.
Duration: Td=Tw=50×(1±20%)µs
Figure 2 Short-circuit current waveform (8/20us) at the output of the generator not connected to CDN
The waveform parameters of the short-circuit current are still wave front time and duration, and the definition is the same as that of the open-circuit voltage, so no more details are given. The standard definition is:
Wave front time: Tf=1.25×Tr=8×(1±20%)µs, 1.25 is the reciprocal of the difference between 0.9 and 0.1 thresholds.
Duration: Td=1.18×Tw=20×(1±20%)µs, 1.18 is the empirical value.
As for the (10/700)µs–(5/320)µs waveform, since this waveform is only used for synchronous interconnection cables directly connected to the outdoor communication network, the scope of application is small. Therefore, the standard puts the definition of this waveform into Appendix A, as shown in Figure 3 and Figure 4.
Figure 3 Open circuit voltage waveform (10/700us)
Among them, Tw is the duration, which is defined as the time elapsed between the rising edge and falling edge of the current waveform at 50% of the peak current, and T is the wave front time, defined as the rising edge of the current waveform at 30% of the peak voltage and 90% of the peak current Elapsed time between: the time elapsed by 50%~50% of the peak value of the waveform, T is the time elapsed by 30%~90% of the peak value of the wave front pulse. Standard definition:
Wave front time: Tf=1.67×T=10×(1±30%)µs
Duration: Td=Tw=700×(1±20%)µs
Figure 4 Short circuit current waveform (5/320us)
The waveform parameters of the short-circuit current are still wave front time and duration, and the definition is the same as that of the open-circuit voltage, so no more details are given. The standard definition is:
Wave front time: Tf=1,25×Tr=5×(1±20%)µs
Duration: Td=Tw=320×(1±20%)µs
3 Surge (shock) test system
The surge (shock) test system is mainly composed of two parts: a surge signal generator and a coupling/decoupling network. In order to generate a lightning surge signal waveform that conforms to the definition of the standard, the circuit structure and performance indicators of the combined wave generator are specified in the standard GB/T 17626.5-2019.
3.1 Surge signal generator
In the entire surge test system, the surge combined wave generator is the most critical component, and its main function is to generate surge waveforms. In the standard GB/T 17626.5-2019, there is a circuit structure of a surge combined wave generator. Figure 5 is a circuit schematic diagram of a (1.2/50)µs-(8/20)µs waveform generator:
Figure 5 Circuit schematic diagram of combined wave generator (1.2/50μs-8/20μs)
As shown in the circuit schematic diagram of the (1.2/50)µs-(8/20)µs waveform generator in Figure 5, the circuit of the surge generator is composed of two parts, namely the charging circuit and the pulse forming circuit. The circuit of charging circuit is composed of high voltage generator U, charging resistor Rc and energy storage capacitor Cc, and the circuit of pulse forming circuit is composed of pulse duration adjusting resistors RS1, RS2, impedance matching resistor Rm and inductance Lr formed by adjusting rise time.
The working principle is: when the switch is off, the circuit will charge the energy storage capacitor Cc through the RC and CC charging circuits. After the energy storage capacitor Cc is fully stored with the required voltage and electricity, the switch of the main circuit is closed to form a discharge circuit. During the initial period when the switch is closed, the energy storage capacitor Cc discharges to Rm, Lr and RS2. At this time, the waveform of the lightning surge voltage measured at both ends of RS2 is in the rising state of the waveform. During this period, the inductor Lr is in the pulse circuit It also plays the role of energy storage. As the power of the energy storage capacitor Cc gradually decreases, the inductor Lr and capacitor Cc discharge the resistors RS1, Rm and RS2 at the same time. At this time, the lightning surge voltage waveform measured at both ends of RS2 changes from the peak value begin descending. The resistor Rm ensures that the internal resistance of the entire generator is 2Ω, and finally the required (1.2/50)µs-(8/20)µs lightning surge waveform can be obtained.
Similarly, Figure 6 is a circuit schematic diagram of a (10/700)µs-(5/320)µs waveform generator:
Figure 6 Circuit schematic diagram of combined wave generator (10/700μs-5/320μs)
As shown in the circuit schematic diagram of the (10/700) µs-(5/320) µs waveform generator in Figure 6, the circuit of the surge generator is also divided into a charging circuit and a pulse forming circuit. The circuit of the charging circuit is composed of a high-voltage generator U, a charging resistor Rc and an energy storage capacitor Cc. The circuit of the pulse forming circuit is composed of pulse duration adjustment resistors RS, Rm1, Rm2 and a capacitor Cs for adjusting the rise time. When the switch S1 is closed, Use external matching impedance.
The working principle of the charging circuit is the same as that of the (1.2/50)µs-(8/20)µs waveform generator, and the working principle of the pulse forming circuit is slightly different, due to the rise time and duration of the (10/700)µs waveform The time is much longer than the (1.2/50)µs waveform, so the capacitor Cs is used instead of the inductor Lr. At the same time, the built-in impedance Rm1 is changed to 15Ω according to the standard requirements, and the built-in impedance Rm2 is 25Ω. At this time, the internal resistance of the generator is 40Ω. It should be noted that when the switch S1 is closed, the resistor Rm2 is short-circuited, and an external matching resistor can be used at this time. . Finally, the required (10/700)µs-(5/320)µs surge waveform can be obtained.
3.2 Coupling/Decoupling Network
The lightning surge coupling and decoupling network can be divided into a coupling network part and a decoupling network part according to its working principle. The role of the coupling network is to transmit the surge signal of the combined wave generator to the device under test, limiting the current flowing into the combined wave generator from the power line of the device under test to cause damage to the generator body, and reducing the impact on the surge waveform. Influence. The role of the decoupling network is to provide sufficient decoupling impedance for the surge signal, so as to prevent the surge signal from entering the grid and adversely affecting the non-test equipment powered by the same power supply. In addition, other equipment connected to the same power supply may contain lightning protection devices. In the absence of a decoupling network, the lightning protection devices on the non-test equipment will prevent the application of surges on the EUT and affect the surge test results.
The coupling and decoupling network for lightning surge can be divided into: power line coupling/decoupling network and interconnection line coupling/decoupling network according to the scope of application, where the power line coupling/decoupling network includes single-phase AC or DC coupling/decoupling network , Three-phase AC coupling/decoupling network; interconnection line coupling/decoupling network includes unshielded asymmetric coupling/decoupling network, unshielded symmetric coupling/decoupling network.
Generally, there are capacitive coupling and gas discharge tube coupling for the surge signal to be coupled to the EUT. The latter has a more obvious impact on the output waveform of the combined wave generator, so capacitive coupling is often used. If a small coupling capacitance value is selected, the residual surge voltage on the power supply side is low, but the efficiency of generating inrush current is low; if a large coupling capacitance value is selected, the coupling efficiency to the EUT is high, but the residual voltage is high. In order to take into account both output efficiency and residual voltage, the national standard stipulates that the line-to-line coupling (differential mode) adopts a capacitor of 18uF, and the line-to-ground coupling (common mode) adopts a capacitor of 9uF. The design of the power line coupling and decoupling network should meet the waveform parameter requirements of the network port in the standard. The decoupling network part provides a higher impedance than the lightning surge wave, and at the same time, this impedance cannot affect the normal operation of the equipment under test. powered by. The coupling components of the coupling network use high-voltage capacitors. Its function is just the opposite of that of the decoupling network. It needs to make the lightning surge waveform pass through completely. Similarly, the design of the signal line coupling/decoupling network should also meet the waveform parameter requirements of the network port in the standard, and its coupling elements can be capacitors, clamps or arresters.
When testing, the corresponding coupling and decoupling network should be selected according to different products. The selection method is shown in Figure 7:
Figure 7 Selection of coupling/decoupling network
The circuit topology diagram of the coupling and decoupling network is given in the standard GB/T 17626.5-2019. Figure 8 to Figure 11 are single-phase/DC power supply coupling and decoupling network line-to-line coupling, single-phase/DC power supply coupling and decoupling respectively Network line-to-ground coupling, three-phase power supply coupling and decoupling network line-to-line coupling, and three-phase power supply coupling-decoupling network line-to-ground coupling.
Figure 8 for line-line coupling on AC/DC lines
Figure 9 for line-to-ground coupling on AC/DC lines
Figure 10 For line-line coupling on three-phase AC lines
Figure 11 for line-ground coupling on three-phase AC lines
It can be seen from Fig. 8 to Fig. 11 that no matter it is single-phase or three-phase power coupling/decoupling network, the topological structure of the decoupling network part is consistent. Its structure is an LC low-pass filter composed of a decoupling capacitor C between lines and a decoupling inductance L on each line.
The selection of the decoupling inductance L should not be too large, otherwise it will not only cause a large voltage drop on the coupling/decoupling network, but also cause a large inductance, which will bring inconvenience to manufacturing and installation. Generally speaking, in order to achieve the standard: "The voltage drop generated on the coupling/decoupling network shall not exceed 10% of the input voltage of the coupling/decoupling network under rated current conditions". Assuming that the rated current of each phase of the tested product is ≤200A, the L value should be ≤1.5mH. When the rated current is >200A, the L value is shown in Figure 12.
Figure 12 Inductive reactance value of decoupling line in coupling/decoupling network with rated current greater than 200A
For the coupling network, because the actual situation simulated by the coupling method is different, it is divided into two cases: line-line and line-ground. Since the source impedance of the low-voltage power grid to the ground is 12Ω, for a combined wave generator with a virtual impedance (defined as the ratio of the peak value of the open circuit voltage to the peak value of the short circuit current) of 2Ω, when performing line-to-ground coupling, it is necessary to connect another 10Ω in series Additional resistors to increase the effective source impedance.
All power line coupling/decoupling networks are only suitable for (1.2/50)µs-(8/20)µs lightning surge waveforms.
Figures 13 to 15 are unshielded asymmetric interconnection line coupling/decoupling network, unshielded symmetrical interconnection line coupling/decoupling network and unshielded outdoor symmetrical communication line coupling/decoupling network respectively.
Figure 13 Coupling/decoupling network for unshielded asymmetrical interconnection lines
Figure 14 Coupling/decoupling network for unshielded symmetrical interconnection lines
Figure 15 Coupling/decoupling network of unshielded outdoor symmetrical communication lines
In GB/T 17626.5-2019, it is divided into:
1) In the unshielded asymmetric interconnection coupling/decoupling network (as shown in Figure 13), R=40Ω, CD is a 0.5uF capacitor and a gas discharge tube;
2) When using a (1.2/50)µs combined wave generator, in the unshielded symmetrical interconnection coupling/decoupling network (as shown in Figure 14), Rc=n×40Ω, n is the number of interconnection wires, CD can be a capacitor, a gas discharge tube, a clamp, an avalanche device or anything that allows the EUT to transmit data normally while meeting the specified surge waveform parameters;
3) When using a (10/700)µs combined wave generator, in the unshielded outdoor symmetrical communication line coupling/decoupling network (as shown in Figure 15), Rc=25Ω, CD is a 0.5µF capacitor and a gas discharge tube.
4 Surge test system test configuration and test process requirements
4.1 Test configuration
In the standard, there are clear requirements for matters needing attention, test layout and test procedures when performing surge tests. The test configuration of the lightning surge test system mainly includes the following equipment:
ü Device under test
ü Auxiliary equipment (when needed)
ü Cable (specified type and length)
ü Coupling/decoupling network
ü Combined wave generator
ü Reference ground plane
Figure 16 to Figure 18 are the surge test configuration diagrams of the power port, the unshielded interconnection port and the shielded interconnection respectively.
Figure 16 Power port surge test configuration diagram
Figure 17 Unshielded interconnection port surge test configuration diagram
Figure 18 Surge test configuration diagram for shielded interconnection wires
4.2 Test process requirements
After the test arrangement is made according to the product standard or product category standard or general standard corresponding to the tested equipment, the test configuration is also carried out according to the requirements, including the following requirements.
4.2.1 Confirm the test level according to the corresponding standard, as shown in Figure 19.
Figure 19 Test level
4.2.2 Number of surges
Unless specified in the relevant product standards, for each coupling path, the number of surge pulses applied to the DC power supply terminal and the interconnection line should be 5 times in positive and negative polarity respectively; , 180º, and 270º phases apply positive and negative surge pulses 5 times respectively.
4.2.3 Inter-pulse time
The interval between continuous surge pulses cannot be greater than 1 minute, and the surge test with an interval of less than 1 minute has a higher severity. Exceed the actual test voltage level.
4.2.4 The equipment under test should be in typical working condition
The equipment under test should be tested under typical working conditions to simulate the actual situation.
4.2.5 Ports to which surges are applied
The power port input, output and signal ports of the device under test need to be tested, but the specific situation should refer to the relevant product standards. In cases where there are several identical lines, it may be sufficient to select only a certain number of lines for measurement. As for the surge test on the output port, it is only recommended to conduct the surge test on the output port where the surge may enter the EUT through this port (for example, the switching of a large power consumption load).
When carrying out the common mode test, each wire shall be tested one by one, unless the corresponding other requirements are given by the product standard.
Since the equipment under test may be equipped with surge protection devices such as piezoresistors or air discharge tubes, or the voltage-current characteristics of the equipment under test are in a non-linear state, the test level should be gradually increased from low to high until To achieve the required test level, the equipment under test shall be able to meet the requirements specified in the test plan or standard in all levels of tests.
5 Surge Calibration Parameters and Calibration Configuration Diagram
For surge test equipment, the general practice is to measure it periodically (one year), and a third-party measurement agency will issue a calibration report. As long as the parameters in the calibration report meet the standard requirements, it can be used normally.
5.1 Calibration parameters of combined wave generator
GB/T 17626.5-2019 has the following requirements for the output parameters of the combined wave generator, see Table 1 and Table 2.
|
Open circuit voltage parameter range |
Short circuit current parameter range |
||
|
1.2/50us KV |
10/700us KV |
1.2/50us THE |
10/700us A |
|
0.45~0.55 |
0.45~0.55 |
0.225~0.275 |
11.25~13.75 |
|
0.9~1.1 |
0.9~1.1 |
0.45~0.55 |
22.5~27.5 |
|
1.8~2.2 |
1.8~2.2 |
0.9~1.1 |
45~55 |
|
3.6~4.4 |
3.6~4.4 |
1.8~2.2 |
90~110 |
Table 1 Lightning surge waveform at the output of combined wave generator - tolerance range of open circuit voltage and short circuit current amplitude parameters
|
|
1.2/50us |
10/700us |
||
|
parameter |
Front time |
duration |
Front time |
duration |
|
open circuit voltage |
0.84µs~1.56µs |
40µs~60µs |
7µs~13µs |
560µs~840µs |
|
short circuit current |
6.4µs~9.6µs |
16µs~24µs |
4µs~6µs |
256µs~384µs |
Table 2 Lightning surge waveform at the output of the combined wave generator - tolerance range of time parameters
5.2 Calibration parameters of power port coupling/decoupling network
In order to compare the impact of different coupling/decoupling networks on the test results, the coupling/decoupling networks should be calibrated periodically. The waveform parameters measured at the EUT side of the coupling/decoupling network are generator-dependent and are only valid for the specific combination of generator and coupling/decoupling network being tested. Figure 20~Figure 22 are applicable to the calibration of CDN of AC/DC power ports with rated current ≤ 200A per line.
Figure 20 The relationship between the peak value of the open circuit voltage and the peak value of the short circuit current of the EUT port of the coupling/decoupling network
Figure 21 Voltage waveform requirements of the EUT port of the coupling/decoupling network
Figure 22 Current waveform requirements of the EUT port of the coupling/decoupling network
5.3 Calibration parameters for interconnect port coupling/decoupling networks
5.3.1 Coupling/decoupling network calibration for unshielded asymmetric interconnection lines, suitable for surge waveform (1.2/50us combined wave).
According to Figure 23, the peak value, front time and pulse duration of the EUT output port of the coupling/decoupling network shall be measured at the rated pulse voltage and current of the coupling/decoupling network. Figure 24 shows the surge waveform requirements at the EUT port of the coupling/decoupling network for unshielded asymmetric interconnection lines.
Figure 23 Calibration of coupling/decoupling networks for unshielded asymmetrical interconnection lines
Figure 24 Surge waveform requirements at the EUT port of the coupling/decoupling network of unshielded asymmetric interconnection lines
5.3.2 Coupling/decoupling network calibration for unshielded symmetrical interconnection lines, suitable for surge waveform (1.2/50us combined wave).
According to Figure 25, the peak value, front time and pulse duration of the EUT output port of the coupling/decoupling network shall be measured at the rated pulse voltage and current of the coupling/decoupling network. Figure 26 shows the surge waveform requirements at the EUT port of the coupling/decoupling network of unshielded symmetrical interconnection lines.
Figure 25 Calibration of coupling/decoupling networks for unshielded symmetrical interconnection lines
Figure 26 Surge waveform requirements at the EUT port of the coupling/decoupling network of unshielded symmetrical interconnection lines
5.3.3 Coupling/decoupling network calibration for unshielded outdoor symmetrical communication lines, suitable for surge waveforms (10/700us combined waves).
It is necessary to measure the peak value, wave front time and pulse duration of the pulse voltage in the case of open circuit and the pulse current in the case of short circuit, and the pulse should be applied to the coupled pair line at one time during the measurement. At the same time, in order to measure the pulse voltage and pulse current of the EUT output port, the input of the AE side of the decoupling network should be shorted to PE. Figure 27 shows the calibration process requirements for the coupling/decoupling network of unshielded outdoor symmetrical communication lines. Figure 28 Surge waveform requirements at the EUT port of the coupling/decoupling network for unshielded outdoor symmetrical communication lines.
Figure 27 Calibration process of CDN for unshielded outdoor symmetrical communication lines
Figure 28 Surge waveform requirements of CDN EUT port for unshielded outdoor symmetrical communication lines
5.4 Calibration Configuration Diagram
Based on the requirements for parameters and waveforms in the above standards, the calibration of surge instruments must have the function of measuring voltage, current and time parameters. Since the rising edge time of the surge waveform needs to reach an accuracy of 0.1µs or even 0.01µs, the bandwidth of the digital oscilloscope must be greater than 100MHz; at the same time, the voltage level of the surge test is mostly kV level, so it is necessary to use a high voltage probe or a differential probe to convert the high voltage It is low voltage for oscilloscope detection; see Figure 29 and Figure 30 for specific calibration diagrams.
Figure 29 Schematic diagram of open circuit voltage calibration
Figure 30 Schematic diagram of short-circuit current calibration