Study Of Key Technologies For Multi-Digital Source Fast Differential Protection Of Large Generator

In recent years, ultra-high voltage power transmission has been developed rapidly. New technologies in telecommunication networks, and non-conventional instrument transformer technologies have been applied in power grid and power station automation systems, which solved some problems faced by traditional protection, and at the same time, brought some new challenges on large generator protection. Large generators are important units in power system; they have higher requirements for protection. The protection system should deal with new problems in new application environment as to meet the higher requirements in term of reliability. This thesis proposed the concept of multi-digital source environment of protection. In multi-digital source environment, protection IEDs use process layer sampled value from network. From the standpoint of a protection, sampling and sampled data are not controlled by protection any more, and the sampled data is not use by a particular protection algorithm but shared by all bay layer IEDs, this brings more error sources to a particular protection algorithm than traditional way. This thesis focus on enhancement of protection reliability and seamless connection between protection algorithm and sampled data, relating specifically to sampling synchronization, transmission delay of sampled data, differences in instrument transformer principles, restraining performance of protection algorithm and seamless connection method between protection algorithm and sampled data.In multi-digital source environment, IEEE 1588 based sampling synchronization scheme is an optimal solution with high accuracy. But the IEEE1588 scheme is based on the hypothetical proposition that the transmission of synchronization message is symmetric between master and slave clock, which in real application environment is not satisfied. Because of various error factors in multi-digital source environment, reducing the synchronization error is of significance. This thesis proposed an enhanced IEEE 1588 synchronization scheme based on multi-interaction algorithm, which can choose the best synchronization message time label for synchronization. By reducing the asymmetric possibility of synchronization message, the proposed scheme can enhance the synchronization precision.In multi-digital source environment, transmission of sampled data takes more time than traditional way, and the transmission time difference influences the quality of sampled data transmission. The thesis uses network calculus theory to calculate the upper bound for the transmission of sampled data. Different parameters such as message priority、background traffic、largest size of lower priority message、the ratio of other messages and network bandwidth are set to analysis the factors that influence the transmission upper bound. The calculation results give certain guideline for designing data buffer of protection and network planning.The unbalance current of differential protection is the important influence of protection reliability. The thesis analyzed the differences of differential protection two sides’signals in multi-digital source environment and built the equivalent filter models of difference sources and signal processing units. This thesis analyzed the influence of operating performance and calculating error of fundamental frequency phasor by different synchronization error、transforming error、signal processing error and sampled data transmitting error. Also, this thesis analyzed the error tolerance of percentage restraint differential protection and scalar product restraint theory by using amplitude and phase angle plane method.Starting from the protection algorithm to enhance protection reliability, this thesis proposes a generator differential algorithm based on transient fitting restraint characteristic. The algorithm fits the slope of restraint curve with the change of transient unbalanced current with time when CT saturate. Besides, by using modified longest common substring algorithm to comparing the correlation of two sides current, the theses proposes a new restraining algorithm by the characteristics of the change of correlation degree in different fault condition. This new restraining algorithm can not only tolerate the amplitude and phase error caused by CT saturation but also tolerate the particular phase error cause by phase shift CT saturation and synchronization error.From the point of connection between protection algorithm and sampled data, the thesis proposes a resampling algorithm based on spline interpolation and time scale transformation. The propose algorithm realizes sampling rate conversion from the fixed sampling rate of instrument transformers to the sampling rate which protection algorithm requires. Sampled data transmission out of time problem and frequency leakage problem caused by non-integer period sampling can also be solved using this algorithm, which realizes "seamless connection" between sampled data and protection algorithms.According to the transforming and signal processing differences of both sides signal, the thesis proposes the equivalence filter concept. A compensating filter is designed using least square algorithm to compensate the difference of the equivalence filter of both sides. The compensating filter can compensate current signal’s amplitude and phase at certain frequency which realized signals of both sides processed in unified framework so that smaller amplitude and phase error can be obtained.