800kV DC voltage divider calibration analysis and application

Author： GOZ Electric Time：2024-05-14 09:40:46 Read：13

At present, China is the country with the largest number of DC transmission projects, the longest transmission lines, and the highest transmission voltage level in the world. And the DC transmission system already has a transmission capacity of 800 kV voltage level. The most common method to obtain high DC voltage is to use a DC voltage divider. It is an indispensable main equipment in the DC transmission system and plays important roles in energy measurement, power monitoring, and relay signal transmission. The accuracy of the data collected is also crucial to the safe operation of the power grid.

As the measured voltage gradually increases, the relative change of the DC voltage divider ratio with voltage becomes more and more prominent, which directly affects the accuracy of the measurement results. This change is called the voltage coefficient of the voltage divider ratio. Research results at home and abroad show that the characteristics of resistive elements, leakage current, external environment, corona current, operating overvoltage, etc. are all important factors that affect the voltage dividing ratio. As the voltage level increases, the influence of these factors increases, making it difficult to Through theoretical analysis, the voltage coefficient of the voltage division ratio within the full voltage range is obtained. To obtain the magnitude of the voltage division ratio at a higher voltage level, the key is to evaluate the voltage coefficient of the voltage division ratio.

A more effective method in the current power industry is to conduct test measurements under the actual working voltage of the voltage divider, and evaluate the voltage coefficient of the voltage divider ratio within the full voltage range by analyzing and calculating the measurement results. This article proposes a DC voltage divider calibration analysis and calculation model based on experimental measurements. It adopts a multi-layer comparative analysis thinking mode to simplify the data processing process and evaluate the voltage coefficient of the voltage divider ratio more accurately.

1. Working principle of DC voltage divider

The DC voltage divider ratio standard adopts the resistor dividing principle. As an important high-voltage measurement equipment, its main function is to convert the primary-side pole line DC high-voltage that needs to be measured into a secondary-side high voltage according to a certain ratio. Measure the low-voltage signals required by the system to ensure the safe, reliable and stable operation of the DC transmission system.

Long-term operating experience shows that leakage current and corona current will not only cause changes in the voltage dividing ratio of the DC voltage divider, but often cause DC voltage divider failure. In order to reduce corona leakage and reduce space conductance current, the DC voltage divider is A shielding resistance layer is added to the structural design. The 800 kV standard DC voltage divider is used on site. The shielding layer is located between the insulating shell and the measurement layer. The resistance is evenly distributed. The shielding layer and the measurement layer are connected in parallel wiring. The internal voltage of the DC voltage divider is evenly divided to form a local equipotential. environment, used to enhance the voltage divider's anti-interference ability to the external environment, while reducing the electric field intensity on the surface of the resistance element of the measurement layer, which greatly improves the stability of the DC voltage divider operation. Both the measurement layer resistance R and the shielding layer resistance r use electrical

It is a metal film resistor with small voltage coefficient and temperature coefficient, and Ig is the leakage current along the insulating shell.

2. Uncertainty of partial pressure ratio

Assuming that Rh and Rl in Figure 1 are ideal resistors with fixed resistance values, and do not consider the influence of factors such as leakage current and corona current, the voltage dividing ratio of the voltage divider is Rh+R R l l , which is a quantitative value. However, in reality, an ideal resistor does not exist at all. The resistance of the resistor changes as the voltage applied to both ends of the resistor changes, and the voltage dividing ratio becomes a variable. This phenomenon is called the uncertainty of the voltage dividing ratio. Among them, the voltage coefficient and temperature coefficient of the resistor are the main factors causing changes in the voltage dividing ratio of the DC voltage regulator. However, this instability of resistance is difficult to completely avoid. The rated working voltage of the measuring resistance element used in the high-voltage DC voltage ratio standard is generally about 1000V. At the same time, in order to reduce the influence of leakage current, the rated working current is generally not less than 0.1 mA, therefore, the rated working power of the resistor will be greater than 0.1W, which will inevitably cause the resistor conductor to heat up, and the resistor resistance will also change with its own temperature.

Conclusion

On the basis of studying the uncertainty of the voltage dividing ratio, a DC voltage divider calibration analysis and calculation model based on experimental measurements was constructed to conduct a comprehensive evaluation and calculation of the voltage coefficient of the voltage dividing ratio of the DC voltage divider based on 800kV. The test proved that the model The calibration level is as high as 10-5 to 10-6. Using this model to evaluate the voltage coefficient of the voltage dividing ratio not only reduces the workload of voltage divider calibration and testing, but also greatly improves the accuracy of DC voltage divider value measurement, ensuring the safe and stable operation of the UHV DC transmission system.