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Improved measurement of loop resistance tester

time:2021/2/11   source:华天电力  reading:519 time

From the perspective of the development of loop resistance testing, conventional double-arm DC bridges were generally used to measure the DC resistance of transformer coils and the contact resistance of high-voltage circuit breakers. The test current of this type of bridge is only mA level, and it is difficult to find that the transformer coils are conductive. The defect that the cross-sectional area of the loop conductor is reduced. When measuring the contact resistance of the conductive loop of a high-voltage switch, due to the influence of the oxide layer between the contact surface and the moving and static contacts, the measurement deviation is large, which conceals the true contact resistance value.

The principle of the loop resistance tester currently used in the field test is to use a typical four-wire measurement method. The high-frequency switching power supply generates a DC test current greater than 100A, which is added to the two ends of the test object, and the measured current flows through the test object. The sampling circuit samples the voltage input terminal and the internal current shunt voltage at the same time. The obtained signal is amplified by the amplifier, and the analog signal is converted into a digital signal by the A/D converter, and then the data is processed by the microprocessor. Filtering, calculation, processing, the DC resistance value of the measured body is obtained by calculating the ratio of the voltage value and the current value, and finally the display shows the current and resistance value measured. And when the current test loop is disconnected or poorly connected, the instrument will judge the current loop to be poorly connected or open according to the voltage on the current shunt. Problems with existing measurement methods. The loop resistance tester designed in accordance with the above conventional design principles found that there is a common problem: when the vacuum oil filter tester’s voltage wiring circuit has poor contact or open circuit, the tester still A value will be displayed, and the following situations will occur:

Micro Ohmmeter.png

1) The voltage loop is open and there is no strong electric field interference at the test site: In this case, since the differential mode voltage input by the amplifier is basically 0, the test value displayed by the instrument is close to 0. If the tester has sufficient field test experience, you can It is judged that the instrument voltage loop test line is abnormal. After the instrument voltage loop test line abnormality is eliminated, the final correct test result can be obtained; if the tester does not have sufficient field test experience, it may misjudge that the tester has a problem and interrupt the test , Replace or repair the instrument, delay the power outage time, and bring unnecessary trouble to the test work.

2) Poor contact of the voltage circuit. In most cases, the wiring terminals of the circuit breaker will have an oxide film or oil film on the outer surface of the terminal block after long-term operation. When the voltage test clamp of the circuit resistance meter is clamped to such a terminal block, contact may occur. Not good, both the voltage test line clamp itself also generates a certain contact resistance. When the contact resistance reaches the same value as the internal resistance of the voltage sampling loop, it will have a serious impact on the test results. Suppose the resistance of the test product is Rx, the input internal resistance of the voltage sampling loop is Ri, the voltage test line clamp clamp resistance is R1, and the instrument displays a false value of Riˊ (assuming current sampling is not affected). At this time, the measured false false value Riˊ=Rx×Ri/(R1+Ri) displayed by the instrument, it can be seen that the false value Riˊ<the actual resistance value Rx of the circuit breaker is displayed by the instrument. It can be seen that due to the existence of R1, the input internal resistance Ri of the voltage sampling loop and the clamp clamp resistance R1 have become a voltage division relationship. The more R1 is divided, the less Ri is divided, and the measured value displayed by the instrument The smaller. If the actual resistance value of the circuit breaker has exceeded the upper limit of the limit, and the value displayed by the tester is within the qualified range, no matter whether the tester has on-site testing experience or not, it will lead to a misjudgment. The safe operation of equipment is a serious threat.

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