Analysis of current situation and characteristics of resistance-capacitance voltage-dividing electronic voltage transformer

Author： GOZ Electric Time：2024-05-18 09:42:48 Read：18

    With the development of smart grids and the large-scale promotion and application of GIS, the application scope of electronic voltage transformers and electronic voltage/current combined transformers has rapidly expanded. However, the new generation of smart substations has put forward new and higher requirements for primary equipment such as electronic transformers. However, the current electronic transformer technology is still immature. Among them, voltage transformers are mainly characterized by unstable temperature characteristics and poor measurement accuracy. Therefore, it is very necessary to research and design high-performance electronic voltage transformers.

     In smart substations of 110 kV and above, there are two main types of active electronic voltage transformers currently used: GIS electronic voltage transformer. The voltage uses a coaxial capacitor voltage divider and is used in conjunction with gas-insulated fully enclosed combined electrical appliances. ; AIS electronic voltage transformer, the voltage uses high and low voltage capacitors in series with capacitor divider, used in open substations.

     In view of the fact that the current research on electronic voltage transformers mainly focuses on aspects such as measurement accuracy and operational stability, ambient temperature and transient processes are the main factors causing accuracy and operational stability problems. Therefore, we focus on analyzing the technical status of AIS capacitive voltage-dividing electronic voltage transformers, and conduct in-depth research on the existing problems such as temperature characteristics and transient retained charges. The key components of the high-voltage sensor, the high-voltage and low-voltage capacitors, are designed with a metallized polypropylene film structure and precision resistors with positive temperature coefficients. A resistance-capacitance parallel voltage dividing type electronic voltage transformer is developed, which solves the long-standing problem of temperature characteristics and Problems such as transient errors.

1 Analysis of the current status and characteristics of electronic voltage transformers

Capacitive voltage divider electronic voltage transformer has become the leading research direction of the new generation of voltage transformer. However, due to various problem factors in the system structure design, this electronic voltage transformer has some defects that are difficult to overcome.

1.1 The accuracy is not stable enough

1) The low-voltage capacitor is connected in parallel with a resistor R with a smaller resistance. From the relationship between the secondary output voltage and the primary input voltage, it can be seen that the capacitive voltage divider has lost the role of the voltage dividing ratio [(C1+C2)/C1], u2 ( The accuracy of t) is determined by the temperature characteristics of the resistor and capacitor product RC1. Since the resistor R and the capacitor C1 are not in the same space, the temperature characteristics are difficult to stabilize, resulting in poor measurement accuracy.

2) The gap between the aluminum foil and the dielectric film of high and low voltage capacitor plates changes irregularly with temperature, resulting in discrete changes in capacitance.

1.2 There is a hidden danger of transient retained charge

     For electronic voltage transformers that use purely capacitive voltage dividers as high-voltage sensors, the most important transient problem is caused by the phenomenon of trapped charge. Because when the power line is cut off, it is difficult to disconnect it exactly when the voltage crosses zero. The residual voltage of the line will cause the capacitor to generate induced charges. The amount of the induced charges depends on the voltage phase at the moment of cut-off. If there is no leakage circuit, these charges will remain on the capacitor for a long time. When the line is reconnected, the residual charge decays exponentially according to the time constant determined by the parallel load resistor, and is superimposed on the sine wave signal, thus causing a large error.

1.3 The measured voltage signal needs to be restored by an integrating circuit, which increases the error factor.

1.4 Phase compensation circuit is required. This is because the low-voltage capacitor is connected in parallel with the sampling resistor, which increases the phase error.