Development On Test Device For Restraining DC Magnetic Bias Of Conveter Transformer

The unbalanced distribution of our country’s energy and load determines that the mass transportation over a long distance of our country’s energy is a inevitable trend. Along with the general policy "West-east electricity transmission, North-south power exchange and Nation-wide interconnection", the HVDC is the main selection of long distance transmission. However, when the HVDC transmission system operating with ground return may cause DC magnetic bias in conveter transformer and make a sharp rise in vibration and noise so that endanger the safety of system operation. In order to solve this problem, this paper researched and developmented a small-scale automatic suppression device based on reverse current injection method. The research work is carried out as follow:The causes for the DC bias of the converter transformer were understanded by reading a large number of literatures. The solutions of the problem at home and abroad were analyzed such as series current-limiting resistance, series capacitance at the neutral points of transformers and injecting reverse DC current at neutral points of transformers. The design scheme of a small-scale automatic suppression device based on reverse current injection method was put forward. On this basis, a simulation model for software simulating was established to determine the parameters of the system according to the actual transmission line parameters, so that laid a theoretical foundation for the design and development of suppression device which adopts the reverse current injection method.In order to meet the requirements that the amplitude and direction of reverse injection direct current can be automatically adjusted to the appropriate, the numerical control automatic AC voltage regulation circuits have been designed, it divided into two parts, the data acquisition board and the control board. Including master control module with single-chip microcomputer as the core, measuring module, LCD module, serial port communication module, etc. Data acquisition board can collect the amplitude and direction of the DC current in the circuit and send the data to the control board through a serial port communication. According to the data of measured, control board can implement AC voltage regulation and make the relay produces corresponding action to control the direction of the injection current. And then provide controllable voltage to the rectifier circuit at a lower level to output reverse direct injection into the test circuit.Software programming in the hardware circuit of data acquisition board and control board have been done and then debugged all the modules. The results show that the data acquisition board and control board have effective communication, the trigger pulse waveform come from control panel is accurate, LCD screen display correctly. According to the data of the simulation model, the power resistance and air-core reactor used in the simulation test have been designed and produced, the test platform have been built. Test platform mainly includes the data acquisition board and control board hardware circuit and the power board, voltage regulator, transformer, AC contactors, power resistance, air-core reactor, DC voltage source. Debugging results show that the device can output the required reverse direct current, the offset effect is good and the design achieved expected requirements and met the application.