| Transmission line is one of the important component parts of power system. The power frequency parameters of transmission line generally include direct current resistance, positive sequence impedance, zero sequence impedance, positive sequence admittance, zero sequence admittance and zero sequence mutual inductive reactance between parallel lines. These parameters are necessary to set up mathematics model before calculating power flow and short circuit faults, relay setting and coordination, selecting condition of power system.Since the theoretical calculation is complicated and can not put all aspects into consideration, the power frequency parameters are required to be measured on site. Nowadays, the conventional measuring methods, that is, the way to use current, voltage and power meters, are still being used widely. However these traditional methods have many shortcomings: the measurement work is tedious and tiresome; the simultaneity of reading is worse; the result is easily distorted by interference of power frequency and harmonic frequency. To develop a more convenient and accurate measuring apparatus is of great significance.This paper analyses the source and characteristics of power frequency interference during the measurement deeply and then educes the approach to overcome such interference– the method of frequency shift. The principles of this method and data process algorithm are introduced. On the basis of this method, a measuring apparatus for power frequency parameters of transmission line based on PC104 is developed. The design of its hardware and software are described in detail. It is proved by practical test that the apparatus overcome the interference of power frequency and has many good features such as compact size, lightweight and simple in operation.
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