Dynamic and Transmission Error Characteristics of Resistor-Capacitor Voltage Divider Type Voltage Transformer

Author： GOZ Electric Time：2024-11-21 09:41:11 Read：10

1 Dynamic characteristics of RC voltage transformer

In the operation of the power system, in order to improve the stability of the system operation, a transformer with good transient characteristics should be selected during the transition of its fault state. Taking the 220kV voltage sensor as an example, the main capacitor C1 and the sub-capacitor C2 are selected as 0.005mF and 0.043mF respectively. The mathematical model of the transformer under specific parameters is simulated and calculated with the help of MATLAB simulation tools. After the relevant parameters are input, the corresponding spectrum domain characteristic curve can be obtained, as shown in Figure 2.

Its error for the measurement result is not higher than 0.01%, which shows that the magnetic saturation phenomenon caused by the core material will not have a great impact on the output result.

2 Transmission error characteristics of RC voltage transformer

In the actual design process, although the construction of the standard RC voltage transformer model can be realized by using precise design parameters, the parameters of each component of the transformer will be affected by many factors during the manufacturing and application of the RC voltage transformer. Under the influence of these factors, there will be deviations between the original design values of the relevant participants, resulting in transmission errors of the sensor. Therefore, when analyzing the relevant performance of the RC voltage transformer, the analysis of the transmission error is very necessary, and electromagnetic compatibility and other aspects need to be taken into account. In the simulation calculation of the transmission deviation, the design is carried out by using the function model H in which a certain parameter changes within a certain range and other parameters are fixed. When the other parameters are fixed and only one parameter changes, the output results of different parameter values of the variable can be observed to see the degree of influence. The measurement of its output results can be carried out from the two aspects of ratio difference and phase.

The simulation results show that when the variation range of parameters such as C2, Lm, and RL is controlled within the range of 20%, the impact on the sensor output is very small, and the maximum contrast difference does not exceed 0.001%. Therefore, after determining these parameters, changes in the external environment or temperature have little effect on the sensor output, and can even be ignored. During the transient process of the power system, the change of the transformer core excitation impedance will be in a wide range. Its effect on the measurement result is also not higher than 0.01%, which shows that the magnetic saturation phenomenon caused by the core material will not have a great impact on the output result.

3 Design of RC voltage transformer

3.1 Design of RC parallel voltage divider

In the RC parallel voltage transformer, the high voltage part is basically borne by the capacitor voltage divider, and the withstand voltage strength of the capacitor medium will directly affect the measurement accuracy and the reliability of the equipment operation. Therefore, in order to ensure the measurement accuracy and insulation reliability, the quality of the RC voltage divider should be continuously improved, and the relevant design should be continuously improved. In actual operation, whether it is a low-voltage capacitor unit or a high-voltage capacitor unit, its temperature stability will directly affect the stability of the transformer. The primary signal processing unit and high-voltage sensing part of the electronic voltage transformer are located outdoors in the substation. Their temperature is greatly affected by the outdoor temperature, resulting in a very large temperature change range. In order to adapt well to such a large range of temperature changes, a self-healing metallized capacitor structure is usually selected during the design process. For high-voltage sensors, the main body is the capacitor, and it has the function of equalizing the series resistance. The main function of the resistor is to eliminate the transient error. In the process of carrying out related designs, in order to ensure the consistency of temperature characteristics and reduce the influence of distributed inductance, non-inductive structures are selected for both high-voltage and low-voltage resistors, and the selected materials are the same, so that their outer diameters are also consistent. For metallized polypropylene film, its capacitance temperature coefficient is a negative temperature coefficient, so a positive temperature coefficient is usually selected for low-voltage resistors during the design process, so that the resistance value in its operating temperature range meets the resistance shunt error requirements and meets the corresponding time constant requirements.

The transient performance of the RC voltage divider type electronic voltage transformer is good, but the stray capacitance will directly affect the measurement accuracy. In addition, the main capacity is small, the anti-interference level is low, and it is greatly affected by the distributed parameters. Therefore, in the process of sensor design, a very critical parameter is the RC voltage divider. Usually, the capacitance is controlled at about 0.005mF. After determining the capacitance, the appropriate resistance value should be selected. The following points should be noted when determining the resistance value: First, the power consumption is small, and the internal temperature rise caused should not be higher than 5℃. Second, the temperature characteristic of the resistance is poor. In the process of design, the error caused by the shunt effect should be reduced to avoid the influence on the accuracy of the capacitor voltage divider ratio. Usually, the parallel resistor shunt should be controlled within 0.03% of the capacitive current.

Third, the time constant must meet the actual requirements of charge discharge. The rated operation cycle of the circuit breaker is usually O-0.3s-CO-180s-CO. If the discharge time constant is determined by the first reclosing time of 0.3s, the selected resistance value will be very small, and the advantage of capacitor voltage division is difficult to achieve. Therefore, the discharge time constant is usually determined based on the second reclosing time of 180s.

3.2 Error temperature characteristics of resistor-capacitor voltage division type voltage transformer

The measurement of voltage transformer is a dynamic test method, and the measurement error cannot be completely avoided during the test process. The measurement error generated is also a dynamic generation process. The deviation of the digital converter is usually small and can be kept relatively stable. In the actual design process, the circuit structure can be improved, high-quality components can be applied as much as possible, and software correction measures can be used to eliminate the error. The voltage error of the RC voltage divider type electronic voltage transformer is mainly the voltage divider ratio error of the RC parallel voltage divider. In the actual operation process, the input impedance of the voltage follower is not in the same space, and there are obvious differences in properties. Once the temperature coefficient changes, the time constant will also change accordingly, and temperature compensation is usually required for correction.

3.3 Anti-interference design of RC voltage divider type voltage transformer

The electromagnetic environment in which the power system is located during operation is relatively harsh, and it faces very complex influencing factors during operation, which will affect the anti-interference ability of the transformer. Therefore, in the process of carrying out relevant designs, the anti-interference ability of the RC voltage divider type voltage transformer should be improved as much as possible to improve the safety and stability of the power system operation.

Conclusion

Among various active electronic voltage transformers, it is an ideal way to use a RC voltage divider with a relatively large capacitor as a high-voltage sensor. It can not only eliminate the harm of transient retained charge, but also does not need to restore the integral circuit, thereby improving the long-term operation stability of the high-voltage sensor. This paper briefly analyzes the performance of the RC voltage transformer and puts forward some suggestions on the design of the RC voltage transformer in terms of capacitance, error temperature characteristics and anti-interference ability.