The principle and measurement influencing factors of high-precision resistor-capacitor voltage divider

Author： GOZ Electric Time：2024-12-03 09:55:44 Read：11

1 Technical principle of high-precision RC voltage divider

The main circuit of the voltage divider measurement includes two parts: the high-voltage arm and the low-voltage arm, which are respectively composed of several resistors and capacitors stacked in series and parallel. The circuit principle of the traditional RC voltage divider and the high-precision RC voltage divider is shown in Figure 1.

When measuring DC voltage, the capacitor branch is equivalent to an open circuit, and the voltage division function is realized by the resistor branch. The RC voltage divider can be equivalent to a resistor voltage divider; when measuring AC voltage, the RC voltage divider topology can be equivalent to a capacitor voltage divider.

2 Solution method for additional error of stray capacitance of RC voltage divider

The high-voltage arm measurement circuit of the high-precision RC voltage divider is composed of 4 RC units in series, and each RC unit is composed of several capacitors and resistors stacked in series and parallel. The outer layer of the measurement circuit is arranged with a shielding circuit, which is also composed of 4 RC units in series, and there is no electrical connection between the measurement circuit and the shielding circuit. When modeling, it is believed that the shielding loop plays a good shielding role for the measurement loop. Therefore, there is no direct coupling relationship between the measurement loop and the external conductor, and there is only stray capacitance between the shielding loop and the external conductor. The modeling is carried out with the resistor-capacitor unit as the research object, and the equivalent mathematical model of the stray capacitance of the resistor-capacitor voltage divider is finally obtained.

3 Analysis of the influence of stray capacitance on the resistor-capacitor voltage divider; In the Ansys environment, according to the design requirements of the prototype developed in this project, a three-dimensional simulation model of a 500kV traditional resistor-capacitor voltage divider and a high-precision resistor-capacitor voltage divider is established, and the five types of stray capacitance mentioned in the previous section are extracted and a partial capacitance matrix is constructed. In a wide range, the high-voltage arm measurement capacitance of the traditional resistor-capacitor voltage divider, the high-voltage arm measurement capacitance of the high-precision resistor-capacitor voltage divider, and the shielding capacitance are assigned, and the additional errors caused by the stray parameters of the above two resistor-capacitor voltage dividers in the AC environment are solved.

At present, foreign DC projects clearly require that the AC measurement error of the RC voltage divider in the frequency band of 0~20kHz should not exceed 0.2%, that is, the RC voltage divider needs to have both high measurement accuracy and excellent transient response performance in a wide frequency range. By selecting a larger rated capacitance value, the influence of stray capacitance on the measurement accuracy of the voltage transformer can be effectively reduced. For example, the primary rated capacitance value of the capacitive voltage transformer is usually more than several thousand pF. However, the increase in the rated capacitance value will increase the time constant of the RC voltage divider, thereby significantly reducing its transient response performance. The rated capacitance values of the AC RC voltage divider products of Trench, ABB and other companies are all around 1000pF, while the rated capacitance values of the DC RC voltage divider products are only a few hundred pF. For the fast protection of the DC power grid, the measuring device needs to have a bandwidth of more than 200kHz, and the measured capacitance value needs to be further reduced to ensure the excellent transient performance of the product.

The rated capacitance value of the traditional RC voltage divider needs to be at least 2000 or 3000pF to make the additional error caused by stray capacitance reach 0.2% and 0.1%. The additional error of the high-precision RC voltage divider is about 0.2% when the measured capacitance and shielding capacitance are both 400pF. When the measured capacitance and shielding capacitance are 400 and 1200pF respectively, the additional error is only 0.076%. The calculation results show that the high-precision RC voltage divider has a good shielding effect, which can ensure that the measurement accuracy of the product will not be offset before leaving the factory and after arriving at the project site, and has the ability to avoid calibration. At the same time, the capacitance value measured by the high-precision RC voltage divider is significantly smaller than that of the traditional RC voltage divider, which ensures the excellent transient performance of the product. During the design process, the measured capacitance value should be reduced as much as possible, and the shielding capacitance value should be selected in conjunction.