Computer-Based Control System Of Power Frequency Test Equipments

AC Withstand Voltage test is the most important test item to AC electric equipments, is a valid method of trying out the ability of the insulation withstanding AC voltage of tested object. It can effectively discover the defect in insulation and has an important value on guaranteeing the safe operation of equipments. Power frequency withstand voltage test commonly acquires high voltage by testing transformer. National standard GB/T 16927.1-1997 has a definitive prescript on the waveform and the tolerance of testing voltage, requesting the voltage on the tested object follows up the anticipative voltage-time curve and that the measured value on testing voltage must maintain between 99% and 101% to the prescriptive value. At the same time the standard also has a definitive prescript on the testing process. In actual projects it is difficult for hand control to follow up the voltage-time curve and control the speed of step-up and step-down for voltage regulator. So it is necessary for us to design a computer-based control system of power frequency testing transformer.The system chooses PLC as the main control element, realizing the control of PLC through the friendly and powerful human-computer interface based on PC, further controlling the withstand voltage process of system. At the same time the testing voltage and the testing current .etc the main parameter is measured and is displayed on PC's screen. Someone also designed a similar system ago, but there is a main problem that the refreshing rate of system is so slow, refreshing about 2 to 3 seconds every time. This is because the communication speed between PLC and PC is so slow. So the article mainly has completed the next jobs: software realization of communication between PLC and PC directly through PC's serial port; programming for soft elements in PLC and design for ladders to control the withstand voltage process of system; design for images displaying the global state of system and withstanding voltage state of system; detailed programming and design for the interface of users'operation. Because considering that power frequency test circuit in test may occur series resonance, the article further proves the non-linear feature of variable voltage transformer and gives an available model for simulation according to the reference.In succession the article design a scheme for testing the refreshing rate of system and has done the experiment. The outcome proves that it has an good effect on the system and reaches the refreshing rate of 2 times per second. In addition, the author has established a simulative testing circuit in lab, in order to verify if the system can control the withstand voltage test process or not. It proves that the system can plot voltage-time curve in the opinion of the requirements of experimentalist, and makes the voltage on the tested object follow up the voltage-time curve, at the same time displaying the instantaneous value of current voltage. The voltage relay and the current relay installed in circuit and the quick-speed protection equipment can cut off the circuit at an abnormal time to guarantee the security of equipments. The human-computer interface can display all the key parameters of system, realizing the designed function. At last, the article makes a declaration of the localization of this system and puts the direction of research forward, in the hope of the further development in this system. The success in test proves that the system has reached the requirements of project design and it can be applied in actual projects.