Research on a new type of withstand voltage test system

Research on a New Type of Withstanding Voltage Testing System

YANG Shengbing, LI Dong, WANG Xiaofei, WANG Yanlin

(Department of Electronic Engineering, Beijing Institude of Machinery,
Beijing 100085, China)

Key words: withstanding voltage; test; IPC

The withstand voltage of electrical devices, insulating materials and insulating structures reflects the safety performance of the equipment under actual working conditions and is an electrical parameter that has a direct impact on human safety. Therefore, withstand voltage testing is one of the important technical indicators for testing safety performance.

The basic theory of withstand voltage test is: if a product can maintain normal condition in a very harsh environment, it can be determined that it can also maintain normal condition in a normal environment. The withstand voltage test mainly achieves the following purposes: 1. Check the ability of insulation to withstand working voltage or overvoltage; 2. Check the insulation manufacturing or maintenance quality of electrical equipment; 3. Eliminate the factors affecting insulation due to raw materials, processing or transportation; 4. Check the insulation electrical clearance and creepage distance. When conducting
a withstand voltage test (as shown in Figure 1), a high voltage generator generates a set high voltage. By controlling the opening and closing of switch K, a voltage higher than normal operation is applied to the device under test for testing. The applied voltage value is measured by the voltmeter at both ends of the high voltage generator. This voltage must last for a specified period of time. The withstand voltage capacity of the tested device is determined by observing the leakage current value of the ammeter in the loop. If the leakage current of a device or component under test is also kept within the specified range within the specified time, it can be determined that the device under test can operate safely under normal conditions.
Different products have different technical specifications. For general equipment, the withstand voltage test is to test the leakage current value between the live wire and the housing. The basic rule is: take twice the working voltage of the object under test plus 1000V as the standard test voltage. The test voltage of some products may be higher than twice the working voltage plus 1000V. IEC61010 stipulates that within 5s, the test voltage gradually rises to the required test voltage value (such as 5kV, etc.), and then maintains it for a specified time (5s). At the same time, the leakage current value of the circuit is measured to determine whether the withstand voltage test meets the test standard. Then, within the specified time (such as 5s), the test voltage is gradually reduced to zero. [1]?

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The test system consists of power supply, signal acquisition, conditioning, A/D conversion, SPWM generator and computer. When conducting a withstand voltage test, the computer outputs appropriate digital quantities to obtain the corresponding SPWM wave, thereby generating a specified high voltage on the high-voltage side of the transformer, which is applied to the test piece. The computer collects the voltage at both ends of the test piece and the current parameters of the test loop through voltage and current sensors. The voltage specified for the test can be obtained through closed-loop control technology, and the withstand voltage capability of the test piece can be determined through software. The new withstand voltage test system introduced in this article complies with the latest IEC61010 standard and adopts industrial computer control technology; it monitors the test process in real time, has a friendly software interface, is easy to operate, and can be conveniently combined with several other test parameters for comprehensive testing. It should be noted that the withstand voltage test system is an indicator of the comprehensive test system project for instrument safety performance. The main indicators are: withstand voltage output 0~5kV, automatic voltage boost (AC); leakage current 0.1~20mA, arbitrarily set (AC); accuracy (1~3)%; test time 5s (IEC61010 stipulates 5s, but can be set arbitrarily as needed). The withstand voltage test system diagram is shown in Figure 2.

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Voltage sampling is achieved by a sensor with the same winding as the high-voltage transformer. The high-voltage side of the voltage sensor varies between 0 and 5000V, while the low-voltage output side of the sampling varies between 0 and 5V. There is a good linear relationship between them. Optical isolation measures are used before the sampling voltage enters the signal conditioning part. Current sampling is completed by a current transformer connected in series in the test loop. Since the maximum setting value of the leakage current is 20mA, the current sensor uses a current transformer with a primary current of 40mA and a secondary current of 17mA. After the conditioned signal is collected by the multi-channel analog switch and A/D in the acquisition board and collected to the computer, the root mean square value is calculated; finally, after the collected signal is calibrated through the experiment, the corresponding function relationship is saved in the program, and then the scale transformation is performed through the software according to the test needs to calculate the actual test voltage value, current value, test time, etc. The scale transformation includes the steps of amplification link processing, resistance value conversion, transmission ratio conversion and system comprehensive calibration.
2.2 Computer interface circuit and control logic, control level interface
The data acquisition circuit uses a self-designed ISA board, including address decoding circuit, bus interface, A/D conversion, analog switch and other parts; it can complete analog input/output and digital input/output.
In the system, the opening and closing of the AC contactor is controlled by the high and low of the digital output signal, including the start, protection and stop of the withstand voltage tester, and the generation of SPWM signals in the power module, the start and blockage of the drive signal and other logical controls are realized through the control of the digital output signal. In addition, the operation of the system is realized by computer programs, and the digital I/O signal logic level of the data acquisition card of the computer bus cannot directly drive the contactor. Therefore, a related control level conversion circuit is designed in the hardware circuit, such as: driving the contactor through the solid-state relay SSR.
2.3 Power supply
The power supply part adopts computer closed-loop control technology. The DC voltage U?2 generated by the AC voltage U?1 (50Hz, 220V) after rectification and filtering varies within the range of (0.9～1.414)［2］ times the AC power supply voltage value; the single-phase inverter is realized by the H-bridge composed of IGBT, and the required AC voltage is obtained by controlling the gate trigger pulse sequence of the H-bridge arm of the IGBT. The pulse sequence designed in this system is controlled by SPWM wave, and the computer outputs appropriate digital signals, which form a closed loop through the voltage sampling link to output basically continuously adjustable AC voltage; a first-level filtering link is added on the AC output side to obtain a relatively ideal sine wave. The control of SPWM wave is achieved by controlling the modulation index m (the ratio of the sine wave amplitude to the triangle wave amplitude); changing the amplitude of the sine wave changes the modulation index m (the modulation index m has 256 variable values), thereby achieving the purpose of controlling the test voltage of the withstand voltage tester. The AC voltage U3 output by the inverter is the product of 0.5 times the DC side voltage and the modulation index m［2］. Since the withstand voltage test requires a higher voltage, a first-level step-up transformer link is designed for this purpose. Through the voltage sampling link and the use of computer closed-loop control technology, the set test high voltage U4 is applied to the device under test for testing (Figure 3).

The software part mainly completes the tasks of data collection, storage, analysis, output, and display. Of course, the software implementation process must reflect the test standards in IEC61010. The program is used to accurately control the test power supply. When the test voltage reaches the test requirement, the timer starts to start the test. After the test is completed, the corresponding test indicators are displayed on the software interface, and it is also displayed whether the product is qualified and an alarm is generated (see Figure 4 for the main test flow chart). Whether in the process of boosting or constant voltage, when the test sample breaks down, the test device will rapidly reduce the voltage to zero and send out an audible and visual alarm signal, showing the actual breakdown voltage value and actual breakdown current of the sample. This software can also realize self-calibration and self-diagnosis before testing, automatically eliminate possible error factors or alarm for faults, etc. [3], [4]. ?

The test software is used to control the step-up transformer to generate high voltage, which is applied to the large load resistor, and the leakage current is collected through the current transformer; under the IEC61010 standard test requirements, multiple tests are carried out with different high voltages, and the measurement data has high repetition accuracy; by changing different large load resistors, the measurement data has high linearity; and the comparison test with the actual withstand voltage test instrument shows good data consistency. The measured leakage current is shown in Table 1. The withstand voltage test system we designed realizes the integration of measurement, control and display through a computer, and has functions such as over-standard alarm; the software interface is friendly, the operation is simple, and it is easy to upgrade.

References?

［2］Chen Guocheng. PWM variable frequency speed regulation and soft switching power conversion technology［M］. Beijing: Machinery Industry Press, 2001.
［3］Lin Weixing. Application of 16-bit single-chip microcomputer in voltage withstand tester［J］. Industrial Control Computer, 2002, 15(6).
［4］Lai Shouhong. Microcomputer control technology［M］. Beijing: Machinery Industry Press, 2002.