Research On The Accurate Continuous Calibration System For Current Transformer

The current transformer, as one of the basic electrical system components, by its accuracy and stability, ensures the metering, protection and analysis of the power system. The current transformers are wildly used since 1884, when the electromagnetic transforms were invented. Recently, the capacity of electrical power is greatly enhanced, and the intelligent electric grid is built. To meet the requirement of intelligent electric grid, the electronic current transformer is substituting for the conventional current transformer because of it is without a ferromagnetic core and it has the wide dynamic range and the frequency band.The improvement of current transformer definitely expedites the growth of its calibration system. The diversity of conventional current transformer and electronic current transformer brings the new problems of calibration system. The existed maintenance system of conventional current transformer is prevented calibration system, which means calibrating and updating conventional current transformer with a regular period like five or ten years. However, this maintenance system is not suitable for the electronic current transformers. A lot of electronic devices using in electronic current transformer cause the stability problem of its application. As a result, a new kind of calibration system which is more reasonable and reliable should be proposed.A kind of continuous calibration system which can monitor the current transformer without deenergization in the operation is proposed in this paper. It means that this system can avoid a long calibrating period in the power system and avert the loss of power metering expense. Firstly, the sensor of this system is based on a kind of electronic instrument current transformer, which is a clamp-shape air core coil. Secondly, this system uses an optical fiber transmission system for its signal transmission and power supply. Finally the digital integrator and fourth-order convolution window algorithm as error calculation method are realized by the virtual instrument with a personal computer.The continuous calibration system has two innovation points. One is the design of high precision standard current transformer with a clamp-shape which can be fixed without deenergization and the other one is the high accuracy error calculation method which can reduce the errors caused by asynchronous sampling effectively. This paper demonstrates the two innovation points respectively. First, a standard sensor made by a clamp-shape air coil is introduced in this paper. It has a wide dynamic range and non-magnetic saturation, which especially fits into the high voltage and high current power line. This paper gives the deduction of its design and principle, and also gives the simulation experimental researches, such as the open air gasp test, the position influence tests which are the off-center test, the off-center rotational test, the edge-vertical test, the edge-parallel test and the temperature test.Second, this paper introduced a new error calculation method, which is the fourth-order convolution window and interpolation algorithm. Compared with Fast Fourier Transform, which is recommended by standard IEC-60044-8 (Electronic current transformers), it has higher accuracy. The relative measuring errors caused by asynchronous sampling could be reduced effectively without any special hardware technique adopted.At the end of this paper, a series of the performance testing researches are presented, including the standard transformer verification test, the calibration system accuracy experiment in Henan province Electric Power research Institute and the on-site operation test in an 110kv substation. The experimental results prove that this continuous calibration system can reach the 0.05 accuracy class and it is easy to operate on site.Besides that, this paper also demonstrates the implements of this continuous calibration, including the hardware design and the software design. The hardware consists of distant sampling circle, distant power supply circle, the optical fiber transmission module and the local received module. The software involves the serial communication between PC and local module, the realization of digital integrator and the fourth-order convolution window and interpolation algorithm as the error calculation.