Test Plan for Dielectric Loss Factor of Power Transformers

Dielectric loss factor tan δ A New Method of Measurement - Digital Measurement

1) The digital automatic balance bridge still adopts the principle of zero value comparison, but uses a transformer proportional arm instead of a common impedance proportional arm, and is controlled and processed by a computer; Thus achieving automatic balancing and measurement.

2) Real and imaginary part measurement method: It is a fully automatic measurement bridge developed based on the separation method of real and imaginary parts. It is connected to a vector voltmeter in a standard capacitor and a sample channel to convert the flowing current into voltage, and then the obtained voltage is divided into real and imaginary parts. A microcomputer is used to analyze and calculate these data to obtain the capacitance Cx and tan δ。

3) Digital sampling waveform analysis system: It is an intelligent system that includes two main parts. One is the sampling system, which is responsible for converting continuous signals of voltage and current on the test object into discrete digital quantities; The second is a data processing system, mainly composed of computers, which performs Fast Fourier Transform (FFT) on the sampled voltage and current signals, calculates the amplitude and phase of the fundamental component of the signal, and finally obtains tan δ Value and sample capacitance value. 

4) Portable measuring instrument (phase difference method): It uses the phase method (also known as square wave shaping method) to determine tan from the angle between the voltage applied to the test object and the current flowing through the test object δ。 The voltage and current signals obtained from the test sample are respectively filtered, amplified by limiting amplitude, and compared with zero crossing circuits, which are called square wave signals. Finally, they are transformed into behavioral pulses through XOR gates. This phase pulse is filled with time scale pulses after passing through AND gates, and finally sent to the microcontroller for counting, processing, and display. The system also takes measures in signal extraction, zero drift, and quantization error to make measurements more accurate.

Test Plan for Dielectric Loss Factor of Power Transformers

Test conditions

This test should be conducted in good weather, with the temperature of the sample and environment not less than+5 ℃ and the relative humidity of the air not exceeding 80%.

Test preparation

Before testing, the insulation resistance between the electrodes of the sample should be measured first. If necessary, clean or dry the surface of the test object (such as the external porcelain sleeve or capacitor sleeve voltage divider small porcelain sleeve, secondary terminal board, etc.). Understand the electrical and chemical properties of insulating oil for oil-filled power equipment, including the tan of the oil δ) Recent experimental results.

Due to the direct grounding of the transformer casing, the on-site measurement is carried out using the AC bridge reverse connection method (or other instruments such as M-type dielectric testers). To avoid measurement errors caused by winding inductance and excitation losses, it is necessary to short-circuit each phase of the measuring winding and ground each phase of the non measuring winding during the test (using an M-type dielectric tester with shielding).

Analysis and judgment of test results

Tan of transformer δ During major repairs and handover, when compared at the same temperature, the value should not be greater than 1.3 times the factory test value. Compared with preventive tests over the years, there should be no significant changes in the value. The results of major repairs and preventive tests should be comprehensively judged in accordance with DL/T596 regulations.
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