The influence of meter input impedance on the calibration of DC high voltage divider

Author： GOZ Electric Time：2024-07-01 09:12:25 Read：12

At present, DC high-voltage voltage dividers are usually calibrated by voltage ratio method or difference method. It is believed that the input impedance of the meter on the low-voltage side will have a certain impact on the measurement results, but there is a lack of quantitative analysis and theoretical basis. An experiment was conducted to test the influence of meter input impedance on the measurement results of the voltage divider ratio. By establishing a mathematical model, a calculation formula for the deviation of the meter input impedance on the measurement results was given, and a method for correcting the measurement results by the influencing factor was proposed. The experiment showed that the deviation between the corrected results and the actual measurement results did not exceed 0.01%.

Since the 1970s, foreign countries have successively established DC high-voltage standards of 100kV or higher voltage levels, and the measurement capabilities and levels of various high-voltage DC high-voltage dividers have gradually improved. Two calibration methods for DC voltage dividers were proposed in China, and comparative verification experiments were carried out. With the improvement of measurement level, the accuracy requirements of DC high-voltage voltage dividers are also higher. Relevant research work based on the characteristics of the resistor components of the voltage divider itself and the leakage current under high voltage has gradually been carried out. A method for measuring the temperature coefficient and voltage coefficient of precision resistors under high voltage is proposed, which provides a reference for the selection of resistor materials for DC high-voltage voltage dividers and improves the measurement uncertainty level of the voltage divider at the component level; a leakage current measurement device for DC high-voltage resistor voltage dividers is introduced, which can effectively overcome the influence of power supply ripple and drift on measurement accuracy, and optimize the design of the voltage divider through the measurement of leakage current, and further improve the measurement accuracy of the voltage divider under high voltage; methods for improving the measurement level of the voltage divider are proposed from the design levels of heat dissipation, radial discharge theory, etc.

At present, the influence of the deviation of the measurement system itself is relatively small in theory. The dual digital voltmeter method is simply discussed to accurately measure the voltage divider ratio of the voltage divider, mainly focusing on the synchronous design of the dual meter acquisition to avoid the influence of power supply ripple. In fact, for most measuring instruments, input impedance will be introduced, especially for the measurement of DC voltage exceeding 10V, the introduction of meter input impedance will directly affect the measurement results. This paper verifies the influence of meter input impedance on the voltage divider ratio measurement results through a verification test, gives the influence of meter input impedance on the measurement results, and proposes a method to correct the measurement results. By correcting the measurement results with the input impedance introduced, the measurement level is closer to the real measurement results.

The calibration of DC high-voltage dividers is based on the national metrological verification regulations JJG 1007-2005 "DC high-voltage dividers". The verification regulations give three verification methods for DC high-voltage dividers. The difference method and voltage ratio method are mainly used in laboratory and field environments.

Test experiment and results

At present, the national high-voltage metering station covers the full range of 10-1000 kV DC voltage measurement standards. Through the independently developed DC voltage test system, the DC input impedance state of the 344xx series digital multimeter can be set, and the meter count value can be sampled and read.

It is generally believed that the input impedance of the low-voltage arm measurement meter will have a great influence on the measurement of the voltage divider ratio of the DC high-voltage divider, but there is a lack of basis for the influence of input impedance on the two measurement methods and the judgment of the specific influence amount. This paper intuitively demonstrates the influence of input impedance on the two measurement methods through experiments, analyzes the causes and magnitude of the influence through the establishment of mathematical models, and corrects the experimental voltage divider ratio data by giving the influence factor, giving a more accurate measurement result.

Experimental conclusion

In the process of DC high-voltage voltage divider calibration, the experiment shows that the input impedance introduced by the meter has little effect on the differential measurement method, while the measurement results of the double meter method have affected the determination of the accuracy level of the test product. In the actual experimental process, on the one hand, the voltage difference between the test product and the low-voltage arm of the standard in the differential measurement method is very small (mV level), which has high requirements for the meter's measurement capability and surrounding environment, especially for field measurements under complex working conditions, data stability and reliability are difficult to guarantee; on the other hand, with the development of various ultra-high voltage and ultra-high voltage projects in China, the calibration of various DC high-voltage dividers based on ultra-high voltage and ultra-high voltage levels has gradually been rolled out, and the voltage measurement on the secondary side after voltage division is bound to be affected by the meter input impedance.

This paper analyzes the influence of meter input impedance in the process of voltage divider ratio measurement, and uses the correction method of the measurement result based on the influencing factor. It has certain guiding significance for the application of meters in the verification and calibration of DC high-voltage voltage dividers, and also provides a reference for improving the measurement accuracy of the double-meter method.