| As an important part of the digital substation, the stability and reliability of electronic transformers (ETs) is the guarantee of safe and stable operation of power systems. Compared with traditional transformer, the ET has many advantages, such as wide dynamic range, excellent transient performance, and digital output. It is developed rapidly for it adapt to the development direction of smart grid. However, as a new technology, the ET’s experience in design and operation is insufficient, and some problems have been exposed in the field operation. According to the statistics of State Grid Corporation of China, the failure rate of electronic current transformers (ECTs) and electronic voltage transformers (EVTs) are4.91and11.37times per hundred machines in2010, which is much higher than that of traditional transformers, the failure rate of traditional transformers are0.0027and0.0040times per hundred machines in2010, respectively.Because of the perfect performance evaluation technology, the failure rate of traditional transformer is low. The existing problems of transformer can be timely found and improved by using performance evaluation, and then its operation reliability can be guaranteed. The electromagnetic interference of running ET is more serious than traditional transformer for its sensing principle is various and it contains many electronic components, so the performance evaluation technology for ET is more complex than traditional transformer. Although ET’s performance evaluation technology is constantly developing in recent years, there are some shortcomings for the whole performance evaluation system, which are reflected in that the performance evaluation system is not perfect, and the existing performance evaluation tests cannot reflect the actual operation condition. As a result, the ET that passed all performance evaluation tests will still encounter some problems in the field operation, and the failure rate is high. Therefore, in order to examine the performance of ET more reasonable, the research of performance evaluation system specifically for ET is in demand, and then the defects and deficiencies of ET can be found timely and its operation reliability can be improved.Considering the shortcomings of performance evaluation technology for ET and referring to the performance evaluation system for traditional transformer, this paper studies the performance evaluation system for ET preliminarily. The performance evaluation can be divided into three evaluation stages, which are type test, delivery test and field test. Studies are performed according to the shortcomings of performance evaluation technology in each testing stage. Firstly, a transient performance calibration system for ECT is given for performance evaluation in the field testing stage, which is based on self-calibration of mutual inductance coefficient of air-core coil. The air-core coil has small volume, wide bandwidth, and wide dynamic range, and is suitable for measuring transient current, however, its accuracy is sensitive to the position of the conductor. The iorn-core coil has high accuracy, and is insensitive to the position of the conductor, however, the volume is large and the weight is heavy when measuring transient current, and is unsuitable for field use. In this paper, the iorn-core coil is used to correct the error of mutual inductance coefficient of the air-core coil at small currents, which may be caused by position of the conductor, and then the calibrated air-core coil can be used as the standard to measure transient current. Besides, an improved Al-Alaoui digital integrator based on the direct current negative feedback principle is used to restore the outputs of the air-core coil, which avoids the temperature drift and zero drift of analog integrator. Accuracy test has been performed in China Electric Power Research Institute of Wuhan and transient performance test has been performed in Dynamic Model Laboratory. The results show that the ratio error is less than0.1%and the phase error is less than3’in the range of100A to40kA, respectively. The total weight of air-core coil and iron-core coil is2.5kg, so it is suitable for field application.The calibration mode of running ET in field test is generally the off-line calibration, which has long calibration period and complex power-off operation. This paper proposes two online calibration systems for ET, which can calibrate the running ET when the power line is energized. The online calibration system for ECT is based on the combined clamp-shape coil, which is consisted of a clamp-shape iron-core coil and a clamp-shape air-core coil. The iron-core coil has high accuracy, but its output amplitude and phase is sensitive to the air gap, while the output phase of the air-core coil is almost not changing with the air gap. By using the characteristics of the two coils, the output phase of the air-core coil can be used as the criteria to determine whether the iron-core coil is closed tightly or not, and the accuracy of the combined clamp-shape coil can be guaranteed. The online calibration of EVT is realized by using a SF6insulation voltage transformer and an automatic lifting device. It has high accuracy and good reliability in complex electromagnetic environment of substation. The two calibration systems have been calibrated in China Electric Power Research Institute of Wuhan, and the results show that the calibration systems can meet0.05accuracy class. The calibration systems also have been successfully applied in Guizhou electric power company, and the test results show that these calibration systems are easy to operate.In addition, the electromagnetic compatibility of ET is studied. Based on the "Testing of switching small capacity current with disconnector" in the performance test for ET in2011, a simulation model is established to study the influence of ET in this test. The simulation results can provide reasonable suggestions to improve the electromagnetic compatibility of ET, and also is helpful to promote the development and application of ET. |
PDF Full Text Request