Evaluation of Measurement Uncertainty of 0.5-level DC High Voltage Divider

Author： GOZ Electric Time：2024-07-07 09:29:19 Read：14

Based on the relevant specifications for the verification of DC high-voltage voltage dividers, the "difference method" was used to calibrate the voltage ratio of the DC high-voltage voltage divider, the measurement uncertainty of the voltage ratio calibration result was evaluated, and the "transfer comparison method" was used to verify the calibration result. Through the calculation of the relative standard uncertainty, the corresponding technical analysis suggestions were put forward, which is helpful to improve the accuracy of the measurement results of the 0.5-level DC high-voltage voltage divider.

In order to ensure the accuracy and reliability of the measurement standard value of the laboratory DC high-voltage voltage divider and improve the level of metrological verification, the laboratory calibration personnel used the "difference method" to calibrate the voltage ratio of the DC high-voltage voltage divider according to the verification procedures of JJG1007-2005 "DC High-voltage Voltage Divider", and evaluated the measurement uncertainty of the voltage ratio calibration result.

1. Overview and calibration basis

High-voltage DC voltage dividers are equipment mainly used for voltage parameter measurement in high-voltage electrical systems, and are of great value for maintaining the stable operation of power systems. The development and progress of science and technology have promoted the wide application of high-voltage DC voltage dividers in high-voltage electrical equipment manufacturers, substations, power plants and high-voltage test laboratories. Therefore, the current industry standards and specifications have higher requirements for the accuracy of 0.5-level DC high-voltage dividers. A DC high-voltage divider is a measuring device used to measure DC high voltage. It can convert the measured DC high voltage into a DC voltage that can be directly measured by a low-voltage DC voltmeter at a certain ratio. It consists of a high-voltage arm and a low-voltage arm. The high-voltage arm is usually composed of multiple resistors in series.

1.1 Calibration basis

The high-voltage divider is one of the standard equipment frequently used in DC high-voltage measurement. Its biggest features are high accuracy, light weight, small size, and easy use. According to different uses, different types of high-voltage dividers are selected, and then equipped with low-voltage measuring instruments with appropriate functions, which can almost complete all the relevant contents of DC high-voltage measurement. According to the verification regulations of JJG1007-2005 "DC High-voltage Divider", the accuracy determination of 0.5-level DC high-voltage dividers is generally carried out by the "difference method".

1.2 Sample equipment name: standard DC divider; accuracy level: 0.5 level.

1.3 Measurement method The DC high voltage divider is a special instrument for field measurement. The whole set of equipment consists of a voltage divider and a measuring instrument. It is mainly used for field DC high voltage measurement and is the best DC high voltage measurement equipment to replace the electrostatic voltmeter. The voltage divider adopts a balanced equipotential shielding structure and uses high-quality electronic components inside a completely sealed insulating tube, so that the entire device has the advantages of accurate testing, good linearity and stable performance. According to Article 6.3.4.2 "Difference method" in the verification regulations of JJG1007-2005 "DC High Voltage Divider".

2. Relative standard uncertainty

2.1 Standard uncertainty component u1 introduced by repeated measurements At present, most of the calibration systems for traditional high voltage dividers are composed of digital multimeters, standard high voltage dividers and high voltage test power supplies. During the test, the high voltage sides of the standard voltage divider and the calibrated voltage divider are connected to the high voltage power supply of the test system, and the low voltage sides are connected to the digital multimeters of their respective circuits. The high voltage dividers must be reliably grounded. Under repeatability conditions, the uncertainty component u1 caused by the measurement repeatability of the measured DC high-voltage divider and the standard DC high-voltage divider is evaluated by Class A method. Under repeatability conditions, 10 independent measurements are performed, and the U0 and ΔU measurement data are measured respectively, and the measured voltage divider ratio of each measurement is calculated.

2.2 Standard uncertainty component u2 introduced by the accuracy of the standard DC voltage divider This uncertainty component mainly comes from the standard DC high-voltage divider. The standard DC high-voltage divider has been qualified by the statutory metrology agency, and the maximum allowable indication error of (0~200) kV is ±0.1%, which can be considered to be uniformly distributed within the interval.

2.3 Standard uncertainty u3 introduced by the accuracy of the digital multimeter This uncertainty component mainly comes from the uncertainty items introduced by the digital multimeter, and the uncertainty of the digital multimeter is given by the traceability calibration certificate. The traceability certificate of the digital multimeter states that the relative expansion uncertainty is 0.0013% when the range is 10V and the measurement point is 2V.

2.4 Uncertainty component u4 introduced by the stability of the DC high-voltage power supply According to the provisions of JJG1007-2005 Section 6.1.2.1, the error caused by the stability of the DC high-voltage power supply should be less than 1/10 of the allowable error of the DC high-voltage voltage divider under test, and it should be uniformly distributed in this range.

2.5 Other influencing factors Other uncertainty components include the corona effect during high voltage operation, the thermal temperature coefficient of the voltage divider ratio of the DC high-voltage standard voltage divider, the thermoelectric potential effect of the digital multimeter wiring, and the rounding of the measurement readings. Since these uncertainty components have little effect on the accuracy of the measurement results, they can be ignored in the uncertainty assessment of the measurement results.

3. Technical analysis and suggestions

The calibration method used this time is the direct measurement method. The technical analysis and suggestions are as follows:

1) Try to use the difference method to measure during calibration, which can reduce the impact of the DC voltage source voltage instability.

2) The standard high-voltage divider, the calibrated voltage divider and the DC high-voltage generator should be single-point grounded to better protect the safety of personnel and equipment. In low-voltage circuits, the voltage is not very high, especially for signal sources used as standards, the rated output current is generally small, so safety issues are not too prominent during work. However, the situation is much more serious in high-voltage measurements. If safety is not taken seriously enough, accidents may occur to personnel and equipment.

3) The connection line between the high-voltage end of the calibrated DC high-voltage divider and the high-voltage end of the standard DC divider should be at a 90° angle to the DC high-voltage generator body to reduce the impact of creepage.

4) The input impedance of the secondary instrument and the digital multimeter should be properly matched. Improper matching will cause large deviations in the measurement results.