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Automatic wear bushing machine CT polarity measurement report

Today Dongguan Hui Yue Technology Engineering test automatic machine transformer bushing wear through the core CT polarity measurement, electrical test, it is often necessary to have assembled retest its transformer bushing current transformer polarity, the conventional method is between the outlet and the transformer neutral point plus 3V DC, power instantly, in the current transformer secondary side file pointer multimeter measured in millivolts "DC induction method." But when larger capacity transformer bushing installed later, the transformer inductance coil has a great use of "DC induction method" can not measure the polarity of current transformer bushing (for more capacity 240MVA transformers that use 24V DC voltage measurement does not come out, if it continues to raise the voltage transformer regulations do not allow). In our work, the use of the transformer high side A, B, C three-phase short circuit transformer winding up the elimination of the flux synthesis method, in which the neutral point plus 1.5V DC voltage to determine the polarity of the current transformer. This "action to eliminate inductance measurements" have proved to be feasible and accurate measurements, to work to bring a great convenience.

2 Principle of three columns with three hearts transformer (analysis of five three hearts and three hearts column transformer transformer is consistent with three columns) as an example for analysis.

2.1 A-phase coil is powered on when a phase is powered, when the momentary switch K is closed, there is an increasing current flows through the transformer coil, the flux linkage change produced ψ, an induced electromotive force eL. According to the law of electromagnetic induction and Lenz eL = -dψ / dt, the transformer iron hearts larger air gap, and working in the non-saturation can be considered linear inductance. This gave eL = - (dI / dt). The actual direction of the self-induced emf is always an attempt to stop the current changes. Because of the role of transformer cores, inductors greatly enhanced, ie the flux generated by the current surge in the chain units, so that eL higher. From the physical sense, a high voltage must be applied to overcome fi, but the real test in the field is not allowed. Therefore, the current flowing through the coil is small, sensitive to the current transformer secondary current is smaller and can not be seen from the measuring meter pointer deflection direction, unable to correctly judge the transformer polarity.

2.2 Three-phase short circuit powering the coil, after a three-phase plus neutral point coil short circuit, three-phase short circuit power on the principle of measuring current transformer polarity wiring diagram, you can make the magnetic flux generated by each phase chain offset each other (drain flux is negligible), thereby eliminating the self-induced emf of the coil, to achieve the purpose of increasing the loop current. Assuming the transformer core section equal everywhere, every phase of zero leakage flux. Thus, when the closing moments K, A, B, C three-phase current through the coil is equal to the magnetic flux generated by the current chain is equal. Direction of the flux linkage shown, the ultimate performance in each phase flux linkage is 0, there is self-induced emf is zero. Thus, to overcome the impact of the applied voltage without self-induced emf, the entire circuit can be understood as a purely resistive circuit, I = V / R, R and the shunt resistor and the source resistance is the sum of three coils. Depends only on the size of the current loop resistance power potential and size, much larger than the current value of single-phase coil is powered. Due to increased primary current, EMF current transformer secondary will also increase, therefore, from the measurement of the meter can accurately determine the polarity of the transformer.

Hui Li Gong Yue Technology summarized using the method "to eliminate the role of measuring inductance polarity" to solve the transformer bushing and transformer body when assembled, measuring transformer bushing polarity problems. This method is accurate and reliable, simple, and "DC induction method," there is no difference from the procedure. Applicable to the neutral point leads to various high-frequency transformer.

Source：HUITUE transformer bushing auto through machine Manufacturers