Investigations On The Key Technology And Relevant Theory Of Electronic Transformers

Electronic transformers represent an attractive solution for alternating current and voltage measurement at high voltage grades power system, since they feature a wide bandwidth, high degree of insulation, and low cost compared with traditional inductive instrument transformers. Being encouraged by recent progress in information technology and requirements of the competitive power market, considerable interest has been paid to the electronic transducers, and new technologies are constantly being introduced to electronic transformer with the aim of improving reliability, reducing total life cycle costs and minimizing environmental impact. This dissertation is devoted to correlative theory and test research on the key technology in the practical application of the electronic transformers, including new type sensors, data fusion algorithm of double-sensors, design of primary sensors, digit interface research, combined power supply, low power technology and supervisory functions.At present the open-looped structure with single sensor is applied to the majority of electronic current transformers (ECTs). And the accuracy and reliability requirements of ' every link are high in the open-looped structure, it severely limit the improvement of the overall performance of ECT and the practicality of ECTs. This dissertation introduces the principle of two new sensors: low power current transformer (LPCT) with the structure of iron core coil and the printed circuit board (PCB) air core coil with digital integrator. On the base of those above, a primary current sensor is designed with combined structure of LPCT and PCB air core coil, and it integrates both advantages of these two kinds of sensors and possesses the features of parallel connection. The data fusion algorithm is used to achieve high precision measurement and improve system reliability. A new method to distinguish the saturation of LPCT is proposed. The experiment and simulation results show that the primary current sensor can cover a wide current range and achieve the accuracy requirements in IEC 60044-8 standard about the amplitude measurement, compound error of protective relaying and peak value of transient response, and also it can be used as a multi-purpose device.The principle, structure and output signal of the electronic voltage transformer, which based on a resistive voltage divider, are very different from the traditional voltage transformer. A resistor divider voltage sensor for 10kV system has been designed that can replace traditional voltage transformer. Through the experiments and calculations, the conclusion can be drawn that the performance of a resistor voltage transformer could be affected by the resistors and stray capacitance. Also some solutions are suggested to reduce the error .The accuracy test results have showed that the resistor divider could satisfy the requirement of IEC 60044-7 and the measurement accuracy could reach class 0.2.The intermal digital design and standard interface is the key technology of electronic transformers, this paper designs an applied digital system by putting forward a 10kV combined electronic transformer as an example. The resistor divider acts as the voltage sensor of the combined primary sensor, and the current sensor is a combined structure which base on the data fusion algorithm. With regard to standard interface, it solved two problems: the synchronization of transformers and the internal synchronization between different sensors. Also, the realization of 100M Ethernet interface is accomplished in accordance with IEC 61850-9-1 standard.Power supply for those active electronic elements at the high-voltage part is a key technique in high voltage electronic current transformer system. Two methods for power supply are proposed in this paper: self-supplying current transformer (CT) power source and optical power supply. In the self-supplying CT power source, energy is obtained directly from the power line by some tailor-made CT. By setting an inductance in series with the CT and the rectifier bridge, the voltage on the rectifier is reduced to the acceptable level and the output current from the CT is also limited to very small. Thus, the inductance helps steady the voltage and protect the follow circuit. The optical power supply is based on state-of-the-art optical-to-electrical power converters, solid-state diode lasers and optical fibers. Command data signals can be transmitted via the same up-fiber which is used to transmit power from secondary power supply to primary converter. The data transmission is finished during the brief interruptions of optical power supply and at the same time it does not affect the steady electrical power-supply. The command data can be picked through a simple comparator circuit added to the power-conversion circuit in the primary converter. Finally, the paper proposes a power supply which is combined of the self-supplying power source and the optical, and provides the switch technique between the two supplies. The dead area is got rid of in the self-supplying CT power source and the lifetime of the optical power supply is improved.As a general application, adopting the low power consumption technology, a combined supply powered ECT based on LPCT used in high voltage power system has been developed. The primary converter includes two primary sensor data channels with active temperature compensation and self-calibration at the high voltage part. It ensures even higher accuracy when it works in an extended temperature range and a wide current range. The secondary converter at the low voltage side with analog and digital output interfaces can receive data from primary converter and sends the signal to control the primary converter. The data signals of synchronization and calibration can be transmitted via the same optical fiber which is used to transmit power to primary converter. Some online supervisory functions have also been successfully accomplished in this system, and this form of preventive maintenance or built-in test will enhance the reliability of the system by giving advance warning and indicating necessary maintenance. The experimental results show that the primary converter which includes the improved circuit driving fiber transmitter can work reliably consuming averagely no more than 40mW power; data signal can be transmitted at baud rate as fast as 200kb/s from the second to the primary converter, and 2Mb/s at the opposite direction; the ECT can meet the accuracy requirement of IEC 60044-8 standard 0.2S class for measuring current transformer and 5TPE class for protective current transformer.