The Research On Data Acquisition System Based On Electronic Transformer And Transmission Of Sampled Value

The active Electronic current transformer (ECT) with air core coil as transducer, compared with conventional transformer with iron core coil as transducer, has excellent transforming characteristics, high precision and wide bandwidth. As analog signal is transformed to digital signal in high voltage side, ECT transfers sampled value with optic fiber or twisted-pair network, instead of lots of electric cable between process level and bay level.The conventional substation automation system realized commutation by network between bay level and substation level. The digital output makes communication between process level and bay level use network as also so as the whole substation use unified communication networks to realize exchange of various data. All of this lay the foundation for achieving complete digital of substation.Firstly, this paper analyzes the characteristics of digital substation and introduces IEC61850 specifications——substation communication network and system, which employ abstract functional modeling and analyses method with information model as core, which virtualizes actual automation function as information service. Information model consist of attribute and service, whose attribute standardize content and meaning of exchanged information while service standardize object and method of information exchange. Modeling lays the theoretical foundation for realizing interoperability and seamless communication among intelligent electronic devices.Digital output of ECT/EVT is presupposition of digital transform of information of process level. This paper analyze detailed data acquisition system for output of ECT with Rogowski coil as transducer, including AD convert control at high voltage side, data receiveing and digital signal process at low voltage side and so on. The whole system makes use of CPLD chips to get a good timing control and DSP chip for mathematic algorithms to realize digital integrate and filtering.There are all kinds of information exchange and IEC61850 defines information exchange as information service, which has unified abstract interface and specific mapping according to reliability. The digital output of ECT/EVT is sampled value, which is foundation of correct working of relay protection and measuring devices. For sampled service has a strict command of real-time performance, it has a simple mapping method. IEC61850 define two specific sampled value (SV) transmitting methods: serial point to point link and transmitting based on IEC8802.3, also as Ethernet protocol. The difference of the two methods is analyzed in this paper. MU is interface between ECT/EVT and secondary devices and publisher of SV. The function of MU and how the synchronous sampling signal is produced and interface between receiving card and Merging Unit is analyzed in this paper.This paper constructs a simple data acquisition and process system which uses relay-protection tester as a signal source, and serial point to point link is adopted as the path between MU to protection device. The experiment result shows real-time performance and presition of this system reach the base requirements of relay protection.