Brief Analysis on Overcurrent Protection of Cable DC High Voltage Generator

       Take the overcurrent control of a power supply line as an example. The current transformer detects the line current. When the current is too large, an alarm signal is issued and the power is cut off. The specific process is: the large current signal is converted into a small current through the current transformer, and converted into a voltage signal through a resistor, and then input into the inside of LOGO! through the analog input terminal of LOGO!. After the processing of LOGO!, after the delay time T1, the relay TM and the alarm signal light are controlled to cut off the power supply of the circuit and issue an alarm signal. And the circuit can be automatically connected after the cut-off. If the circuit current is normal at this time, the alarm light goes out. Moreover, within the specified time after the overcurrent, if the DC high-voltage generator overcurrent occurs again, there is no need to delay T1 time, only T2 time is needed to issue an alarm signal again. This makes LOGO! selective. When the current is too large for a moment due to motor startup, LOGO! will not start again, reducing the malfunction of the equipment. And after an alarm, if the current is too large again, it will alarm again in a shorter time, reducing the loss caused by the failure to eliminate the fault.

　　Debugging Problems

　　The current signal is converted into a voltage signal using a current-voltage DC high-voltage generator conversion device, and the output voltage can be adjusted to better meet actual requirements. In the frequency trigger module, the counting time period is set to 6S, so that it has a selection function, and no alarm signal will be issued in normal overcurrent conditions such as motor start-up. And when setting the number of pulses, the actual value is 80, not 300. This avoids the situation where the alarm device cannot be started when the power supply current is too small due to interference. At the same time, it reduces the situation where the delay time is too long when the overcurrent occurs within the timing time of B02. Similarly, when setting the number of pulses, B03 is only set to 70.

　　In order to obtain a higher output voltage value, the resistance R2 value and the voltage drop between the regulating end and the ground of the LM317 regulator are often large. A capacitor C3 of less than 10μF is connected in parallel at both ends of R2. The DC high-voltage generator can effectively suppress the ripple at the output end. When a short circuit occurs at the input or output end, the discharge of capacitor C3 will generate a surge voltage on R1, which will endanger the reference voltage circuit of the regulator. Therefore, a diode D3 is required to be connected in parallel at both ends of R1 to protect the regulator.

　　The output of the voltage regulator can work without adding capacitors. Since the voltage regulator works under 1:1 deep negative feedback, when the output load is a certain value of capacitance, the voltage regulator may self-excite. Therefore, a 0.1μF capacitor C1 is connected to the input of the voltage regulator, and a 1000μF electrolytic capacitor C5 is connected to the output to provide sufficient current supply. At the same time, it can prevent possible self-excited oscillation, reduce high-frequency noise and improve the transient response of the load. When the input is short-circuited, C5 discharges through the regulator's adjustment tube. If the C5 value is large, the impact current during discharge is very large, and the voltage will discharge through the output transistor inside the voltage regulator, which may cause the output transistor emitter junction to reverse breakdown. For this reason, a diode D2 is connected in parallel at both ends of the voltage regulator. When the input is short-circuited, C5 discharges through D2 to protect the voltage regulator.

　　The LOGO! model used in this device is 24RCLB11, its power supply is 24V DC, and the output is 220V AC, which is used to drive the relay and the alarm light. The relay TM of this device is normally closed and connected to the main circuit. The analog input terminals I1 and I2 of LOGO!. The outputs are Q1 and Q2. The current signal output by the transformer enters the I1 and I2 terminals of LOGO! through the current-voltage conversion device (I/V).

　　1. Design idea of ​​overcurrent protection

　　The programming of LOGO! is realized by using the functional modules in LOGO!. The logic diagram of the DC high voltage generator system software is shown in Figure 2

　　The working principle inside LOGO! is as follows: the analog quantity inputted by analog input terminals AI1 and AI2 is converted into digital quantity by analog triggers B08 and B12. The settings of modules B08 and B12 are the same. The voltage of analog switch on is 6V, and the voltage of off is 1V. When the peak value of the input signal is greater than 6V, the module outputs a digital pulse signal with a frequency of 50HZ. After passing B06, it is counted by frequency trigger B02. When 280 pulses are input within 6S, module B02 outputs a high level. At this time, output Q1 is turned on, connecting the TM trip controller and disconnecting the circuit. At the same time, output terminal Q2 is set to a high level and outputs an alarm signal. At this time, the circuit has been cut off and no analog signal is input. After 6s, the frequency trigger outputs a low level, that is, Q1 has no signal output, and the circuit is connected. If the circuit is not overcurrent at this time, the alarm light goes out. If the alarm light is not turned off, or there is still overcurrent in the time period after the alarm light is turned off (controlled by B09), then within 2S, as long as B09 detects 70 current pulses, the alarm device will start again to prevent the alarm device from processing normally when the fault is not eliminated. When Q2 is 0, B09 starts timing. If Q2 still has no alarm output after the self-determined time, B09 outputs a low level, which becomes a high level after being reversed by the NOT gate B10, and resets the RS trigger B05. At this time, the alarm device returns to its original state.

　　Considering that the alarm device will still be powered on after a delay of 6 seconds if the fault is not eliminated, B03 is added to avoid danger, and modules B04, B05, B09, and B10 assist in generating delayed signals. This makes the device more sensitive to overcurrent signals within a specified time after an overcurrent event, that is, when an overcurrent event occurs, the overcurrent signal only needs to last for 2 seconds before the alarm is sounded again, thus reducing danger.

　　The overcurrent protection of DC high voltage generator is widely used in cables, especially in the protection of cables in the power sector.