Study On The Influence Factors On The Error Characteristics Of The Capacitor Voltage Transformer

The capacitor voltage transformer(CVT) has the advantages of convenient adjustment, high insulation reliability and low cost, and is widely used in the high-voltage power networks of 110 kV and above. Compared to the PT, due to the existence of the capacitive divider and the compensation reactor inside the CVT, the accuracy of the CVT is easily affected by the insulation state, secondary load, ambient temperature, contamination, external electric field, ambient humidity and other factors. Accuracy, one of the most fundamental indicators of the power transformers, affects power measurement and protection action. Therefore, it is of great theoretical and practical significance to study the influence of various factors on the performance of the CVT. This dissertation includes the following contents:The physical structure and working principle of the CVT were analyzed. The influence mechanisms of both the internal electrical parameters(such as insulation state and secondary load) and the external working environment(such as ambient temperature, external electric field, contamination and ambient humidity) that would affect the accuracy of the CVTs were also investigated. The simplified CVT equivalent model was set up and it is found out that various factors affect the amplitude and phase of the secondary output voltage by changing the equivalent electrical parameters of the CVT model, leading to the deviation of its accuracy. From the perspective of the circuit, the relationship between the amplitude and phase of the secondary output voltage and the electrical parameters of the equipment was analyzed.Based on the traditional CVT mathematical model, the dissertation focuses on the influence of self-electrical parameters on the measurement error, combines the factors such as insulation state, secondary load, ambient temperature, external electric field, contamination and ambient humidity, and builds the simulation model of the influence of CVT equivalent electrical parameters on the error characteristics. By establishing the connection between the different influence factors and the equivalent electrical parameters, the dielectric loss, capacitance and secondary load of the capacitive divider were chosen to for simulation of the influence on its accuracy.The low-voltage level CVT model test platform and the 220 kV CVT error characteristic test platform was set up. The tests of the influence of various capacitance and dielectric loss on the error characteristic of CVT was carried out based on the low-voltage CVT model test platform. The results show that the dielectric loss of capacitive divider had great influence on its phrase difference. Field tests show that it should be paid attention when the dielectric loss of the high / low voltage capacitor exceed 0.12%~0.15% because of the risk of exceeding error, and the detailed accuracy tests should be carried out. The capacitance of capacitive divider had great influence on its ratio difference. Field tests show that it should be paid attention when the capacitance of the high / low voltage capacitor exceed 0.08%~0.1%.The tests of the influence of various secondary loads and ambient temperature on the error characteristic of CVT was carried out based on the 220 kV CVT error characteristic test platform. The results show that the various loads had certain influence on the CVT accuracy. The load in rated range would not cause the CVT to exceed error. However, it should be paid attention to the conditions of the light-load, no-load and when the power factor was below 0.8, which had a risk of exceeding error. Therefore, the load type and capacity should be based on the theoretical design in the installation and operation of on-site CVTs. Ambient temperature had a limited influence on capacitance, dielectric loss and accuracy of the CVT. In the operating temperature range, the temperature would not cause the CVT to exceed error. However, in the poor working surroundings, the areas with large temperature difference between day and night, and the conditions when other factors affected the CVT cooperatively, there are risks of exceeding error as well. Other corresponding compensation and measures should be taken to ensure the ratio and phase difference of the CVT within the prescribed range.