The Method of Shortening Transformer DC Resistance Test Time

Abstract

Transformer DC resistance test is one of the routine test in the transformer core project, but the test efficiency is generally low.The paper ,through the analysis of transient process of large DC resistance measurement of transformer coil, core hysteresis phenomenon and the influence of residual magnetism on the measurement process.Improve the working principle, a method of fast measurement, large transformer DC resistance does not increase ,in other devices,characteristics of combination of YNd11 connection of three-phase transformer testing instrument and physical characteristics of the proposes. Field test proves that this method effectively utilizes the remanence, can get the main transformer winding DC resistance test results quickly and accurately. Introduction Transformer is the core equipment of electric power system in the substation, in order to ensure the safe and reliable operation, related test in the transformer factory, after installation,in operation, before and after overhaul should.At the same time, the measured data of vertical and horizontal analysis, in order to make the correct judgment on the health status of equipment. Winding DC resistance test can check out the contact wire and the casing state, he location of each contact state of the tap switch.Whether the actual position match to the position of the tap switch.the quality of welding joint of the winding wire, welding quality wire and winding, winding used wire specification compliance with design requirements, multi strand and around whether strand broken, whether there is inter turn short cake, lead wire fault and other fault conditions[1-3]. Winding DC resistance test and compared with other tests is more time-consuming, such as the 220kV main transformer routine tests of DC resistance test time ,accounted for the total test time of 1/3. With the progress of science and technology, the improvement of the instrument, measuring the operation more simple, measured by time shortening. But some problems also appeared in the use of the process. For example, the limited conditions , relay can not pull in the power supply low voltage , liquid crystal LCD of very low temperatures is not displayed or garbled, occasionally,the buzzer ringing of instrument discharge time is no stopping etc. It is the cause of the instrument itself, or external conditions. There are also some situation is reason of user aspects, such as sometimes in the trial process, long charging time, digital disorder etc.. In order to solve this problem, this paper analyzes some problems in practical use[4, 5]. Analysis of transient process of transformer DC resistance test Transformer winding in the pass into the DC current, its equivalent circuit is shown in Figure 1.It is a first order circuit. A power supply voltage of US, inductance L, winding resistance is R, because the initial conditions for the io-=0, the circuit response to a first-order zero state response. Differential equation of electric circuit: US=Ldi/dt+iR Solution i=US/R∙(1-e), τ=L/R Charge current speed which achieve the steady state I = US / R depends on the time constant τ. The general circuit is switched on after 5 τ ,current has reached steady state value of 99.3%