Impedance matching of RC voltage divider DC voltage transformer

Author： GOZ Electric Time：2024-11-27 09:49:17 Read：10

1.1 Analysis of impedance matching principle

When the impedance at the end of the transmission cable is not matched, the measured signal will be reflected at the end of the cable. Under normal circumstances, the characteristic impedance of the coaxial cable is 50Ω or 75Ω, while the input resistance of the merging unit generally reaches the kilo-ohm level, which is much larger than the characteristic impedance of the cable. The reflection coefficient at the end of the transmission cable is large, and the reflected wave at the end is also large. The incident signal wave superimposed on the reflected wave will oscillate, causing signal distortion, which seriously affects the accuracy of the transformer measurement. Therefore, it is necessary to study the impedance matching of the resistance-capacitance voltage divider DC voltage transformer.

From the transmission line theory, it can be seen that the voltage and current at any position on the transmission line are the superposition of the incident wave and the reflected wave at that point, and the voltage reflection coefficient at that position is the ratio of the reflected wave to the incident wave. The basic principle of impedance matching is to make the load impedance equal to the characteristic impedance of the transmission line, so that the reflection coefficient is 0. There is only an incident wave traveling from the head end to the end on the transmission line without a reflected wave, the transmission signal will not be distorted, and the electromagnetic energy is absorbed by the load to the greatest extent.

For the convenience of analysis, it is assumed that the loss of the transmission line can be ignored, that is, the transmission line is considered to be a uniform and lossless transmission line. Figure 2 is a schematic diagram of a typical voltage divider measurement system.

In actual engineering, in order to meet the impedance matching requirements of DC voltage transformers, it is usually necessary to select a coaxial cable with double shielding layers (insulation is sandwiched between layers). At the same time, the ends of the cable should be welded to a special plug, and the grounding jacket of the plug should be tightly connected to the outer metal shell of the low-voltage arm of the voltage divider or the end secondary voltage divider in a standardized screw connection method or conductive glue. The connection at both ends must be standardized, and it is forbidden to use long wires instead of plugs, otherwise it will cause abnormal oscillation in waveform measurement. In addition, it is necessary to install a certain impedance matching device at one or both ends of the transmission cable.

1.2 Impedance matching simulation study

In order to study the impedance matching inside the RC voltage divider DC voltage transformer, this paper uses MATLAB software to establish a simulation model of the RC voltage divider DC voltage transformer. The DC voltage transformer simulation model is mainly divided into three parts: resistor-capacitor voltage divider, coaxial cable, and merging unit module. Among them, the coaxial cable model uses the PISectionLine transmission line element in the Simulink simulation component library for simulation; the merging unit model is established based on the actual structure of the transformer and is composed of various modules in the Simulink simulation component library. The remote module digital-to-analog converter in the simulation model is implemented using a 16-bit conversion module, and the dual sampling comparison technology is used to ensure the full-scale measurement accuracy; the photoelectric conversion and electro-optical conversion are simulated using a photoelectric coupler. The parameters of each part of the simulation model are established based on the actual XRC200 resistor-capacitor voltage divider DC voltage transformer parameters.

1.3 Impedance matching experimental test

The experiment uses a square wave voltage generator with a rated voltage level of 200kV to output a step signal wave with an amplitude of 20kV as the input measurement signal, and the standard capacitor voltage divider and the XRC200 resistor-capacitor voltage divider DC voltage transformer are connected in parallel to the experimental circuit. The rated voltage level of the standard capacitor voltage divider and the RC DC voltage transformer are both 200kV, and the voltage division ratio is 2000:1. Two step response experiments were carried out as a control to study the impedance matching of the transformer merging unit.

3 Impedance matching measures

Impedance mismatch will cause distortion and distortion of the measurement signal, resulting in inaccurate voltage measurement. The current economical and effective impedance matching method is to use filters to suppress overvoltage at the transmission line end. The national standard GB/T14549 1993 stipulates that in the relay protection and metering of the power system, it is generally required to retain harmonics below the 19th order. Therefore, using a low-pass, anti-aliasing filter as the filter of the voltage transformer can meet the requirements. This paper adopts two methods, RC passive filter and active filter, to eliminate oscillation and suppress overvoltage.

Conclusion

RC DC voltage transformers have been widely used with the large-scale construction of DC transmission systems, but there is still little research on the impedance matching of the cable transmission link inside the DC voltage transformer. Through theoretical analysis, experimental testing, and MATLAB modeling and simulation research, this paper draws the following conclusions:

1) Through simulation analysis and experimental measurement, it is found that the internal impedance matching of the transformer is not ideal. The merging unit module has a certain impedance matching effect, but it cannot completely eliminate the oscillation. Further impedance matching measures need to be taken;

2) A low-pass filter is used as an effective measure for the internal impedance matching of the transformer. Through simulation comparison and analysis, it is found that compared with the RC passive filter, the second-order Butterworth filter has better filtering characteristics and is a more reliable and effective impedance matching measure.

This paper studies the impedance matching of the cable transmission link inside the RC voltage transformer, and proposes solutions to the existing problems, which has certain guiding significance for the design and application of DC voltage transformers.