Research On AC Equivalent Resistance Automatic Measurement System Used For Cable Conductor

Cables are widely used in many industrial fields as an important carrier for transmitting electrical energy,such as energy,transportation,information and communication,construction,railways and automobile industry.The DC resistance of the conductor in cable is equal to the ratio of its heating power to the square of the current.When alternating current flows through the conductor,the AC resistance is greater than DC resistance due to the skin effect and proximity effect.Therefore,in order to characterize the heating effect of the cable conductor accurately,AC equivalent resistance is introduced.At present,there are two ways to obtain the AC equivalent resistance,they are computational method and experimental method.Computational method is complex and not an ideal method because it’s difficult to model.Many experimental methods have been tried by a large number of researchers,but they are unable to meet the rapid development of measurement need.Until now,there has been no commercial device and mature experimental methods.So it is necessary to study the method of AC equivalent resistance measurement.After having analyzed the existed measurement methods for AC equivalent impedance,this paper presented a new automatic method based on FPGA and voltage controlled current source.This method can improve the accuracy of the measurement by comparison with a current comparator which is the key in this method.The process of the measurement as follows.The signals of voltage on the sample and the unbalanced current of CC are sent to the control system.According to the signals,calculating the compensation current which will be converted by VCCS and sent into compensation coil of CC.The judgement of the compensation effect depends on the signal in detecting coil of CC.To realize the balance of the CC by modifying the compensation current repeatedly.Finally calculating the parameters of the sample according to the Ampere-turn equilibrium principle.The necessary hardware and software testing platform for the measurement method proposed in this paper,The hardware consist of a power module,a voltage measurement module,a current comparison measurement module and a control module.The output current of the power module is 0~10A.The voltage measurement module mainly contains a program-controlled amplifiers,and the gain magnitudes are 1,10,100,and 1000.The essence of the current measurement part is a current comparator The current comparator and a sensitivity adjustment resistor are in parallel to increase its sensitivity.NI SingleBoard RIO was selected as the control module,which includes FPGA,RT,appropriate AD and DA conversion modules,and digital output modules.For the software,the program is designed from the measurement process first,and then assign FPGA,RT and host computer different functions according to their respective characteristics.Finally the Lab VIEW software is adopted to realize the the whole measurement program.The verification of the whole system was performed under the laboratory conditions.The conductor with resistance of 0.3m? taken as sample was measured in power frequency firstly,The results show that the relative error is 16%,and the source of error was analyzed.Then the low inductive resistance of 0.1? and 0.2? were measured under different magnitude and frequency currents.The results show that the system measurement accuracy is better than 0.5% and verify the feasibility of the principle for the system in this paper.Which lays the foundation for further research.