Performance evaluation of new resistor-capacitor voltage transformer

Author： GOZ Electric Time：2024-05-30 09:27:09 Read：13

    In my country's power grid, capacitive voltage transformer (CVT) has been widely used as a voltage measurement device. Compared with traditional electromagnetic transformers, CVT has the advantages of simple insulation structure, high cost performance and easy maintenance. However, due to the limitations of the measurement principle of CVT itself, its transient response is unsatisfactory, which affects the real-time performance and reliability of the measurement and relay protection systems. Although the passive optical voltage sensor in the electronic voltage transformer has high accuracy and is suitable for microcomputer protection of power systems, the instability of the optical system it faces is the main obstacle to its use. In addition, almost all optical voltage transformers currently do not solve the problem of compatibility with traditional power equipment. Therefore, the study of new active electronic voltage transformers with non-optical sensor heads has become a current research hotspot.

    Improving the capacitive voltage transformer model is the key to solving its poor transient response. By connecting a small resistor in parallel with the equivalent ground capacitance to form a new voltage measurement circuit, the transient process time during a fault can be effectively shortened, the fundamental voltage amplitude error can be reduced, and it is conducive to the correct action of the relay protection system. The author established a resistor-capacitor voltage transformer model with a rated voltage of 220kV, and verified the linearity and stability of the model through simulation calculations, which can meet the requirements of 0.5 level accuracy.

    Resistor-capacitive voltage transformers have the advantages of accurately tracking primary voltage changes, accurate and reliable measurement, fast transient response, good linearity and no saturation. Compared with active optical voltage transformers, resistive-capacitive voltage transformers adopt a small PT structure and do not require an external power supply. Therefore, they become an ideal choice for the design of electronic voltage sensors to meet the voltage signal sampling requirements of the power system.

    The application of resistance-capacitance voltage transformer in the measurement, control and protection of high-voltage and ultra-high-voltage systems is expected to achieve widespread development and become the research and development direction of the new generation of high-voltage transformers in power systems.

    For the situation where the motor is approximately a power point, the equivalent reactance can be calculated based on its feedback capacity, thereby calculating the feedback current after the motor passes through the two low-voltage windings of the three-winding transformer. For large-capacity units with two voltage levels of 10kV and 3kV in high-voltage plant power systems, three-winding transformers are usually used instead of split transformers, but the selection of their reactance values is similar. Analysis and calculation results show that for two sections of high-voltage factory buses powered by the same three-winding transformer, when a short circuit occurs on one section of the bus, the feedback current provided by the motor on the other section of the bus is very small and can be ignored.