Research on Online Monitoring System for Micro-moisture of Transformer Insulation pressboard

Moisture in the insulating structure of the power transformer will reduce the dielectric strength and mechanical strength, which will bring hidden dangers to the normal operation of the transformer. Therefore, it is very important to monitor the moisture content in the insulating pressboard during the manufacturing and working process of the transformer to keep it within a reasonable range.

Starting from the feasibility of using dielectric measurement methods to measure the physical properties of materials, the measurement of trace moisture in insulating paperboard using dielectric measurement methods was studied, and an online monitoring system for micro moisture of insulating paperboard was developed. One of the keys to the research on the on-line measurement system of micro moisture in insulating paperboard is the fringe field capacitive sensor. The technical indicators and design requirements of the sensor are introduced. The fringe field capacitive sensor is designed according to the requirements of the measured object structure size, electrodes and substrate, number of channels, penetration depth, signal strength and ease of installation.

The two kinds of micro-moisture measurement sensors listed in this article have their own characteristics, which represent the trend of small sensors and sensors with large measurement depth. The measurement system conditioning circuit directly affects the measurement results. In view of the fact that the induced signal output by the fringe field capacitive sensor under the action of a low-frequency sinusoidal drive signal is very weak, the conditioning circuit adopts a driving shielding method to buffer the measurement signal, uses a band stop filter to improve the signal-to-noise ratio, and uses the corresponding conversion circuit to find the capacitance. The phase angle and amplitude gain of the system is to obtain the complex dielectric constant of the insulating cardboard. The conditioning circuit has reliable performance and high accuracy, and can meet the requirements for measuring the moisture of insulating paperboard. The data acquisition system uses the 51 series new single-chip AT89S51 as the core component, supported by the software program compiled in assembly language, which can process and calculate the three voltage signals of amplitude, phase and temperature, and real-time control of the heating power supply through the relay.

After processing the output amplitude signal and phase signal by the conditioning circuit, the corresponding humidity value at this time is calculated according to the quality of the insulating cardboard, and a table corresponding to the voltage signal and the humidity value is made. Load the table into the ROM of the single-chip microcomputer, and use the table look-up method and interpolation method to get the humidity value.

The experimental results show that the overall design of the system is reasonable and the performance is reliable, and the expected design goals are achieved. Capacitive sensors and conditioning circuits can be used to accurately measure the micro moisture in pressboard. The above research can provide benefit for the measurement of micro-moisture in transformer insulation board.