194 / 2018-09-07 11:34:34

Influence of Low-temperature Characteristics on Accuracy of Full- Fiber ECT

low temperature,full- fiber ECT,accuracy

Owing to the advance of optical fiber and electronic technology, electronic transformers based on various sensing principles are rapidly developed in recent years. With the large-scale construction and development of UHV AC and DC projects and various new-generation smart substations of the State Grid Corporation of China, higher requirements for the performance of the electronic transformers with optical properties used in substations are brought forward. As some substations of UHV and smart grids are operating in low-temperature areas in China, with the average temperature of below -20°C for a long time, the low-temperature environment causes certain problems for the stability of operation of electronic transformers which are the important equipment in substations.The traditional transformer transmits voltage and current signals based on the electromagnetic induction theorem. Low temperature environment has little influence on its materials and key component parts. The electronic transformer is a new type of transformer which works to convert current and voltage signals into optical information that is transmitted to the secondary equipment through the optical fiber. Although its insulation structure is greatly simplified, a large number of optical devices are required. Since most optical devices are very sensitive to changes in temperature, the relative performance of electronic transformers under low- temperature characteristics is not guaranteed. Full-fiber ECT is at the highest level of development of optical current transformers at present. It has been widely used in UHV DC projects and various kinds of smart substations. A large number of optical sensing devices are used in full-fiber ECT. The low-temperature environment has a great influence on the primary sensor (sensing ring) and transmission fiber components, especially causing the instability of their accuracy. Therefore, it is of great significance to research the accuracy characteristics of electronic transformers in low- temperature environment in order to guarantee the reliability of UHV engineering and smart grid equipment.Full-fiber ECT works based on the Faraday effect. Due to the Faraday effect, the magnetic field generated by the current can cause magnetically induced circular birefringence in the sensing fiber. And the residual linear birefringence of the sensing fiber itself can bring about an inseparable detection error for the detection of circular birefringence. Especially under small current conditions, the circular birefringence produced by the Faraday effect is of small magnitude, and the influence of circular birefringence on the output light intensity signal is almost annihilated by linear birefringence. The influence of the perturbation of linear birefringence on the light intensity can cause the fluctuation of the current measurement value. Therefore, how to overcome the influence of the linear birefringence factor becomes the key of the full-fiber current sensor. Fiber current transformers work all year round in harsh environments such as substations. Alternating from cold to hot, day to night, the intense fluctuation in temperature can cause the instability of linear birefringence. In addition, it is also the main factor influencing the stability of the system measurement. Another aspect of the influence of temperature on the stability of measurement of system is the influence on the verdet constant. The verdet constant is a function of temperature and wavelength of light, which in reality changes with temperature. Studies have shown that when the temperature changes from -40°C to +80 °C, the fluctuation in the verdet constant alone can cause the scale factor to fluctuate by 0.9%, failing to meet the requirements of the IES 0.2S standard. Therefore, temperature compensation measures must be taken. In addition, the parameters of the electronic components and photoelectric components of the circuit part can also be drifted under the influence of temperature. Therefore, the influence of temperature can not be ignored although it is not the most important one on the stability of system.The main factors influencing the accuracy of full-fiber ECT are as follows: 1. Primary sensors, and 2. Acquisition and conversion electronic devices. Therefore, an accuracy test of full-fiber ECT at low temperature needs to be carried out. The accuracy test is carried out by placing the primary sensor and the acquisition and conversion device into a low-temperature test chamber, and applying the primary current, in accordance with the requirements of GB 20840.In this paper, the factor influencing the low- temperature characteristics of full-fiber ECT based on the principle of full-fiber ECT is analyzed, and a conclusion is obtained that it is the temperature characteristics of the primary sensor and the acquisition and processing unit. In addition, a low-temperature accuracy test of the full-fiber ECT is focused to research the accuracy characteristics under the conditions of 0°C to -60°C, thus, improved measures for reliable operation of full-fiber ECT at low temperatures are obtained.