| As an important part of smart grid, the occurrence and development of digital substation has largely promoted the progress of smart grid. And as the primary sensor elements in digital substations, electronic transformers (ETs) are data sources of all relay protection devices and electric energy metering equipments. Comparing with traditional transformers, ETs are used widely because there are outstanding features including simple insulation, digital output, wide dynamic range and the frequency band. But ETs are newborn things which need further running experience including reliability, working life and EMC. On-line calibration of ETs periodically or real-time can greatly improved reliability during running time. At present, calibration of both traditional transformers and ETs stayed on the off-line calibration lack of efficient measures for on-line monitoring performance of ETs. Because of this, there is no related calibration specification which limited the performance monitoring and improving of ETs and the application in digital substation. In the traction of China Southern Power Grid and real project, the paper conducted more thorough research to the on-line calibration technology of ETs with digital output focus on the portability of on-line operations, standard current sensor design and data processing algorithm in non-linear load. The main researches and achievements are as follows.A new calibration method for electronic current transformers (ECTs) was proposed. The proposed system can reach the precision class up to0.05and the system can calibrate the ETs for0.2S in the range of600A-1500A at rated current which the synchronization method can be PPS or IRIG-B. Actual onsite calibration shows that the system has high-accuracy, and is easy to operate with satisfactory stability.Earth potential working was used in the system and a clamp operating mechanism based on insulated operating rod was designed. The performance differences were systemically analyzed between a clamp-shape iron core sensor and a clamp-shape air core sensor and a high-accuracy method based on combined clamp-shape current sensors was proposed originally according with the performance differences in the paper. Clamp-shape current sensor when open and close, may be the cause of opening cannot be closed completely, even if there is air gap only micron, will also cause a significant reduction in the standard current accuracy. In online calibration under the condition of high voltage, air gap exists or not, the operator can’t be judged by conventional means of low pressure.Principle deduction and simulation were carried out and experimental analysis details were made for the air-gap influence on clamp-shape sensors. The actual tests results show that the novelty method can make judgment on air-gap of amp-shape iron core sensors accurately. A high-accuracy algorithm is one of the most important key technologies and a new method based on data preprocessing was proposed after the analysis of waveform components and algorithms before, In complicated non-linear load calibration environments, the algorithm has high accuracy in inhibition of errors resulted from spectral leakage, frequency fluctuation, inter-harmonics and mutual interference.A detailed design of on-line calibration system was given in the paper. Digital sampling of analog output signal of standard channel is the factor influencing accuracy of calibration system so the paper introduced the influences on calibration accuracy in different parameters from two aspects:high resolution and wide dynamic range. Meanwhile, considering stability and reliability, EMC design was made from bandwidth design, ground and shielding. Four improvements were given according to the design of data frame program. PC software mainly realizes the modular sensor head characteristic judgment function, standard channel data acquisition, merging unit data reception and decoding, the fundamental wave extraction algorithm and error calculation, data display, record, storage, and other functions.At the end of this paper, a series of performances testing verification and application of the method and technology of the whole paper were made for the combined electronic transformers (1500A/5A) in Bai Cheng110kV substation in Gui Zhou province. The experimental results prove that the on-line calibration system can realize the real-time calibration of ETs which provide the data guarantee for the running of substation and the running stability and quality were improved. Besides that, on-site calibration norm and on-line monitoring specification of ETs and electric energy metering equipments were established which can provide calibration norm for the on-line calibration of ETs. |
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