Performance Analysis of Resistor-Capacitor Voltage Divider Type Voltage Transformer

Author： GOZ Electric Time：2024-11-19 09:37:01 Read：16

1 Application Status of Electronic Voltage Transformer

At present, with the rapid development of smart grid, active electronic voltage transformers are being used more and more widely in the construction and operation of smart substations above 110kV. Commonly used are two types of AIS electronic voltage transformers and GIS electronic voltage transformers. Among them, AIS electronic voltage transformers are often used in open substations, and their capacitor voltage dividers mainly use the method of high-voltage capacitors and low-voltage capacitors in series; GIS electronic voltage transformers are often used in combination with gas-insulated fully enclosed electrical equipment, and coaxial capacitor voltage dividers are used. At present, many discussions on electronic voltage transformers focus on operating stability and measurement accuracy, and the impact of transient processes, ambient temperature and other factors on operating stability and accuracy has not been given enough attention. Although the mainstream research direction in the new generation of voltage transformer research is capacitor voltage divider type electronic voltage transformers, there are still some problems to be solved in the existing capacitor voltage divider electronic transformers. First, a phase compensation circuit needs to be applied. Usually, a sampling resistor is connected in parallel to the low-voltage capacitor, which further increases the phase error of the entire system. In order to ensure the measurement accuracy, a phase compensation circuit needs to be added.

Second, because the integral circuit restoration is applied to the measured voltage signal, the error link is increased. In order to restore the original signal during actual operation, the integral link should be added to the signal processing stage. Both the integral resistor and the integral capacitor are easily affected by temperature. When encountering a large temperature change, their stability will be significantly affected. It is difficult to ensure the stability of the integral time constant temperature characteristics, resulting in a large error drift in the actual measurement process.

The third is the risk of transient retained charge. If the electronic voltage transformer used in the high voltage sensor is a pure capacitive voltage divider, then its retained charge phenomenon is likely to cause transient problems. This is mainly because once the power line is cut off, it is difficult to accurately disconnect when the voltage passes through zero. A certain amount of voltage will remain in the line, which will cause the capacitor to generate induced charge. The amount of induced charge is determined by the voltage phase at the moment of cutting off. If there is no discharge circuit in the system, the induced charge will be retained on the capacitor for a long time until it is connected to the line again. The residual charge will decrease according to the time constant determined by the parallel load resistance and superimposed on the sine wave signal. This process is likely to cause large

errors.

Fourth, the accuracy is unstable. The low-voltage capacitor is connected in parallel with a relatively small resistance value, which leads to the fact that the voltage divider ratio of the capacitor voltage divider is not fully utilized. In addition, the capacitor and the resistor are not in the same space, so it is difficult to maintain the temperature characteristics relatively stable, so the measurement accuracy is difficult to ensure.

2 Performance analysis of RC voltage divider type electronic voltage transformer

Whether it is a capacitor voltage divider or a RC voltage divider type electronic voltage transformer, there are strict requirements for the voltage divider. At present, only a very small number of companies with a high level of technology can achieve this product performance.

2.1 Principle of RC voltage divider type electronic voltage transformer

From the above analysis, it can be seen that with the rapid development of smart substations, electronic voltage transformers have become the main components of substations. In high-voltage sensors, the method of connecting capacitors and resistors in parallel can be reasonably applied, which can achieve the advantage of reducing the loss of capacitors during operation, and after applying parallel resistors, the transient charge can be discharged, which significantly improves the measurement accuracy of the transient process of the electronic voltage transformer. The principle of the RC voltage divider type electronic voltage transformer is shown in Figure 1.

2.2 High precision

The voltage sensor uses a RC parallel voltage divider. When designing, the resistor shunt only accounts for 0.03% of the capacitor current. In addition, the discharge time constant is controlled to meet the discharge requirements. Therefore, the capacitor plays a decisive role in the voltage divider ratio, that is, K= (C1+C2)/C1. C1 and C2 have the same temperature characteristics, so the deviation range of K in operation is not large.

The capacitor uses a metallized film, and its electrode is evaporated on the dielectric to eliminate the harm of the gas gap between the material and the plate, while also improving the reliability of the capacitor temperature. In order to prevent the volume damage caused by the self-healing of the metallized high-voltage capacitor C1 and the low-voltage capacitor C2 during operation, the magnetic field strength of C1 should be set to (1/3~1/2) or a lower field strength film rated voltage to ensure the self-healing condition during operation, and a low line current strength design below 0.003A/mm2 should be used, and a series of anti-oxidation measures should be adopted to effectively prevent capacitor wear and extend the service life, and ensure the long-term operation precision of the voltage divider.

3 No transient retained charge hidden danger

The national standard GB/T20840.7-2007 shows that in this transient process, the use of a resistor-capacitor hybrid voltage divider to transmit the correct signal is currently the best way to deal with it. Because each capacitor unit is connected in parallel with a discharge resistor, the transient deviation is eliminated.