A system solution for testing leakage protector

introduction

According to incomplete statistics, electric shock accidents and fires caused by leakage in my country cause thousands of deaths and billions of economic losses every year, so higher requirements are put forward for the performance of leakage protectors that can prevent leakage fires and personal electric shock protection. The automatic test system for the action characteristics of leakage protectors introduced in this article can measure the leakage action current value, breaking time and leakage non-action current value of the leakage protector, and provide the main technical parameters for improving the reliability of the leakage protector. The detection process has a high level of automation and can detect leakage protectors that are running online and offline.

System hardware design

There are many parameters that characterize the leakage protector, among which the leakage action performance is the most closely related to the personal safety of electricity users. The main parameters describing the leakage action performance are the rated leakage action current (I△n) and the leakage action time. The rated leakage action current is the leakage action current value that the leakage protector must operate under specified conditions as specified by the manufacturer, which reflects the leakage action sensitivity of the leakage protector. The leakage action time refers to the time from the sudden application of the leakage action current to the cutting off of the protection circuit. In order to prevent the leakage protector from misoperation, the national standard stipulates the leakage non-operation current (I△n0), which refers to the leakage current value that the leakage protector must not operate under specified conditions (the preferred value is 0.5I△n), which is a necessary technical parameter for operation on the power grid.

Figure 1 Block diagram of leakage protection device test system

The system is based on LPC2132 and has an extended test current generation and adjustment module, action execution unit, current detection circuit, keyboard and other peripherals. LPC2132 is a microcontroller that supports real-time simulation and tracking of 32-bit ARM7TDMI-S core, 1 10-bit 8-channel A/D converter, 2 32-bit timers/counters, 6-channel PWM unit output, 2 hardware I2C interfaces and 47 GPIOs, 2 16C550 industrial standard UARTs, and up to 9 edge or level triggered external interrupts. 16kB on-chip static RAM and 64kB on-chip Flash program memory avoid LPC2132 external memory expansion, simplify the circuit and improve the running speed. The block diagram of the automatic test system for the action characteristics of the leakage protector is shown in Figure 1.

1 Test current generation and regulation module

The test current generation and regulation module is shown in Figure 2. The test current is generated by passing 50Hz, 220V sinusoidal AC through a 220:12 step-down transformer and an electric voltage regulator to output 0~12V sinusoidal AC, and then passing through the loop resistor to generate the required test current. The test current generation is divided into 3 levels to meet different measurement ranges. When relay J1 is energized, a test current of 0~1000mA can be generated; when relay J2 is energized, a test current of 0~500mA can be generated; when relays J1 and J2 are not energized, a test current of 0~100mA can be generated. The regulation of each level of test current is achieved by controlling the electric voltage regulator through LPC2132. In order to make the test current change evenly, the electric voltage regulator adopts AC servo control. During the test, LPC2132 compares the test current value collected in the real-time loop with the set value, and calculates the control amount to control the rotation of the servo motor, driving the brush of the electric voltage regulator to slide steadily on the secondary side, causing the secondary side voltage to change, thereby changing the current in the loop. The P0.2 pin of LPC2132 outputs a pulse signal to control the movement speed of the servo motor, and the P0.3 pin outputs a high or low level signal to control the direction of rotation of the servo motor.

2 Current Detection Circuit

The current detection circuit is shown in Figure 3. The test current is sampled through the current transformer, and the secondary output signal of the current transformer is converted into a unipolar voltage signal (0~5V) that can be collected by the A/D module and sent to the LPC2132 through filtering, amplification, voltage boost and other circuits.

Figure 3 Current detection circuit

When detecting the magnitude of the current, sampling is performed at 40 points per cycle according to the cycle (power frequency) of the test current. After sampling one cycle, the actual effective value of the current is calculated through a software algorithm based on the attenuation multiple of the current transformer and the value of the boost voltage. The circuit should meet the following conditions: when the instantaneous value of the AC current reaches the positive peak value, the amplifier outputs 5V; when the instantaneous value of the AC current reaches the negative peak value, the amplifier outputs 0V. 3 A/D conversion and control circuit [page]

The output signal VOUT of the current detection circuit is sent to the built-in 8-channel 10-bit high-speed A/D conversion input terminal of LPC2132 to detect the size of the leakage current.

Since the A/D conversion is 10 bits, when the input voltage is 5V, the output data value is 1024 (4FFH), so the maximum resolution is 0.0049V (5V/1024). If the loop resistance that generates the test current is 12Ω, the resolution of the leakage current is 0.4mA (0.0049V/12Ω), which fully meets the test needs. The start signal generated by the leakage current of the leakage protector and the disconnection signal of the moving and static contacts are respectively sent to the external interrupt input terminal of LPC2132, and the disconnection time of the moving and static contacts of the leakage protector is detected by interruption. The P0.5 and P0.6 pins control the closing and disconnection of relays J1 and J2 respectively, and select three test currents with different measurement ranges. Serial communication is used between LPC2132 and the host computer. Since the system is a 3.3V system, SP3232E is used for RS-232 level conversion. SP3232E is an RS-232 conversion chip with a 3V working power supply. The A/D conversion and control circuit is shown in Figure 4.

software design

The design of the LPC2132 software part is based on embedded C language and adopts a modular program structure. It includes the main program, system initialization subroutine, human-machine interface control function subroutine, current sampling subroutine, leakage action current detection subroutine, leakage action time subroutine, communication subroutine with the host computer, and host PC monitoring program.

The main program is the core program of the leakage action characteristic detection test. After the test system starts working, the program keeps running in the main program in a loop, calling other functional subroutines according to different needs. After the calling is completed, the program returns to the main program to continue the loop. The main program flow chart is shown in Figure 5.

Figure 5 Main program flow chart

The system initialization subroutine mainly completes the system initialization work, including pin configuration initialization, A/D conversion initialization, timer initialization, interrupt initialization, system parameter initialization, setting detection items and parameters, etc. The human-machine interface control function subroutine controls the communication between the system and the human, and mainly realizes the scanning of key functions. The current sampling subroutine performs A/D conversion on the test current signal sent to LPC2132 (which becomes a unipolar voltage signal of 0~5V after filtering, amplification, and voltage boost), and restores the obtained digital quantity to the current value in the actual circuit through calculation. The leakage action current detection subroutine is used to detect the leakage current value (I△) at the moment when the leakage protector is disconnected. The leakage action time subroutine completes the detection of the leakage action time of the leakage protector. The communication subroutine with the host computer mainly completes the communication between LPC2132 and the host PC. The host PC monitoring program mainly realizes the control of LPC2132 by the host PC and displays the measurement results.

The host PC monitoring program of the test system is developed based on the LabVIEW8.6 platform. Through the graphical programming environment of LabVIEW, the function functions of serial communication operations in the serial port sub-panel of LabVIEW8.6 are used. Through the configuration of the serial port function, it is relatively easy to compile a what-you-see-is-what-you-get program interface, which simplifies the serial communication programming of Windows and realizes the reception and transmission of data.

Figure 6 Test system interface

The control interface is shown in Figure 6, which displays a measurement data.

After the system starts testing, the test equipment is automatically initialized according to the set test parameters. The software formulates the control port control word to select the appropriate hardware circuit loop and sampling resistor. The test current starts from less than 0.2 I△n and increases steadily to I△n within 30s. The leakage current value I△ at the moment when the leakage protector is disconnected is measured. If I△n0 is satisfied,

in conclusion

This test system overcomes the drawbacks of the traditional manual test method. It has a simple operation interface. You only need to enter the test conditions and parameters to start the test. The test results are clear at a glance, and the measurement is automated and intelligent. It can detect both non-online leakage protectors and online leakage protectors. It improves the test level of leakage protectors and provides an effective means for performance research, quality inspection and production of leakage protectors.