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Calibration Method of Pulsed Magnetic Field Immunity System

Release time:

2023-10-16 15:52

Source:

1. Preface

At present, the domestic pulse magnetic field immunity calibration specification is only applicable to the independent calibration of the combined wave generator host and the induction coil, and the lack of pulse magnetic field test system calibration method, and the deformation of the induction coil caused by improper use during use will cause The performance index of the whole system does not meet the requirements of the test standard.

Therefore, in order to solve the above problems, this paper proposes a calibration method of the test system, and verifies the correctness of this calibration method, which can also provide guidance for the relevant testers to carry out the test system during the verification. In order to increase the repeatability and stability of the pulse magnetic field immunity test results, it is necessary to calibrate the test system regularly.

2. Overview of Pulsed Magnetic Field Immunity System

The pulsed magnetic field immunity test system is used to simulate the equipment subjected to lightning strikes on buildings and other metal structures and by the initial fault transient in the power system, or to simulate the pulse magnetic field interference generated by the circuit breaker switching high-voltage bus and high-voltage line in the high-voltage substation.

The pulse magnetic field immunity test system is composed of a combined wave generator (including or externally connected with a 1 8μF capacitor) and an induction coil. The combined wave generator is used as a test power supply to generate a pulse current with a certain intensity that meets the standard requirements. The induction coil is used as an energy converter to convert the current signal flowing through the coil itself into a space magnetic field interference signal to check whether the tested equipment can work normally. The schematic diagram of the test system is shown in Figure 1.

Fig.1 Schematic diagram of pulse magnetic field immunity system

2.1 combined wave generator

The composition of the combined wave generator is shown in fig. 2. it consists of two circuits. the left half u is used as a voltage source, Rc is used as a charging resistor, and Cc is used as an energy storage element to form a charging circuit. The right half Rs1 and Rs2 are used to adjust the duration of the waveform, Lr inductance is used to control the rise time of the waveform, Rm is used as a matching impedance resistor, and forms a discharge loop with Rm, the switch is used to control the charging and discharging process of the circuit.

Figure 2: Schematic diagram of combined wave generator

The combined wave generator is used as the current source of the test system. The short-circuit current measured without connecting the induction coil is 8/20μs waveform, I .e. the wave front time of the short-circuit current is 8 μs, and the duration of the short-circuit current is 20 μs. The waveform parameters are shown in Table 1, and the waveform diagram is shown in Figure 3.

Table 1 Short-circuit current waveform parameters

Figure 3: Combined Wave Generator 8/20 μs Waveform

2.2 induction coil

The IEC61000-4-9 specifies two standard single-turn induction coils with dimensions of 1m × 1m and 1m × 2.6m respectively, which are made of copper, aluminum or other non-magnetic materials. The coils can generate the magnetic field required by the standard. The magnetic field uniformity requirement is limited to a single direction perpendicular to the coil plane. During the test, the magnetic fields in different directions are obtained by continuously rotating the induction coil.

3. Characteristics and requirements of pulse magnetic field system calibration

Characteristic index for calibration of 3.1 test system

The essence of the calibration of the test system is to calibrate the parameters of the pulse current signal output from the combined wave generator to the induction coil. The characteristic indicators are the wave front time, duration and pulse peak current of the pulse current waveform. The specific indicators are shown in Table 2 and Table 3. X in the table is used as an open level, and higher current level tests can be carried out according to the needs of users. Coefficients 0.9 and 0.66 are the calculated coil factors of standard induction coils.

Table 2: Test System Waveform Parameters

Table 3 Peak Current Parameters of Test System

Calibration Requirements for 3.2 Test Systems

The calibration environment is required to be carried out under free space conditions without the equipment under test, and there is no electromagnetic interference and mechanical vibration that affect the normal calibration work. When using a current probe to measure short-circuit current, a suitable probe should be selected to avoid magnetic flux saturation, and the low frequency (-3dB) point of the probe should be less than 100Hz. The current probe and oscilloscope or other similar equivalent instruments used for calibration shall have a bandwidth of not less than 1MHz.

4. Calibration method

Do these steps to calibrate the test system:

A. connect the pulse magnetic field immunity test system according to fig. 4, the current probe is sleeved on the output connection line of the combined wave generator, the output end of the current probe is connected with the input end of the digital oscilloscope, and the ground end is reliably connected with the ground port of the oscilloscope. If the connection line between the output end of the test generator and the standard induction coil is not shielded coaxial cable, it shall be connected with twisted pair (length less than 3m) to eliminate the differential mode signal caused by magnetic field induction on the cable;

Fig. 4 Calibration connection diagram of test system

B. After the connection is completed and checked according to Fig. 4, the oscilloscope and the combined wave generator are turned on and preheated for 30min;

c. According to the process of pulse magnetic field immunity test, according to the requirements of Table 3, set the corresponding test level and other parameters on the combined wave generator, then set the input impedance of the oscilloscope to 1MΩ, and adjust the appropriate voltage scale, time scale, trigger mode, trigger level and sampling rate;

D. turn on the output switch of the combined wave generator, make the oscilloscope capture the pulse current waveform, then adjust the scanning time and trigger level of the oscilloscope, make the center of the oscilloscope screen display the complete waveform, use the oscilloscope cursor to manually measure the amplitude y-axis and the time parameter x-axis, and read and record the data.

e. According to the above steps, the combined wave generator outputs 5 pulses of positive polarity and 5 pulses of negative polarity respectively at each test level, the time interval of each pulse is not less than 10s, and the calibration results are recorded in turn.

5. Calibration Verification

According to the above calibration method and calibration requirements, the existing pulse magnetic field immunity measurement system in the laboratory is calibrated to verify the feasibility and correctness of the calibration method. The coil is a standard coil with a size of 1m × 1m and a coil factor of 0.9.

5.1 calibration equipment

① The oscilloscope uses R & S's RTO 2024 oscilloscope with a bandwidth of 2GHz (see Figure 5).

Figure 5:RTO 2024 Oscilloscope

② The current probe uses Pearson company's model 411 probe with a bandwidth of 20MHz and a current measurement range of 5000A (see Figure 6).

Figure 6:411 Current Probe

5.2 calibration arrangements and results

The calibration arrangement is shown in Figure 7, and the calibration is carried out according to the method in Section 4. The result after calibration is shown in Figure 8.

Figure 7: Real map of calibration of pulsed magnetic field system in Century Huize Laboratory

Fig. 8 is a waveform diagram of the calibration results of the combined wave generator with a magnetic field setting of 100A/m. the diagram shows that the peak current is 104.4A, the wave front time is 1.25*8.14 μs ≈ 10.18 μs, and the duration is 23.5 μs. Meet the requirements of the test standards in Table 2 and Table 3 for equipment performance indicators.

Figure 8:100A/m calibration waveform

　　

 

It is hoped that readers can master the calibration method of pulsed magnetic field immunity system through this article, and can also provide guidance for relevant testers to carry out the period verification of the test system.

Century Wisdom (Suzhou) Testing Technology Co., Ltd

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