City Research Online

Abstract

The requirement of digital differential protection has become more and more severe with the increasing capacity of Generator-Transformer unit and the development of power industry. An advanced principle, its application and perspective of differential protection are developed in this thesis.

Firstly, it is demonstrated in principle why the scalar product differential protection is more sensitive than the percentage differential protection. Then a best solution for the choice of two quantities (I max and I Σ-1) of the product is achieved.

The possible potential problems of the scalar product differential protection due to its sensitivity to phase angle are also put forward. A novel protection principle on ANN techniques is then proposed which breaks through the former ANN application bottleneck in relay.

Secondly, a new method to distinguish inrush state using multi-condition restraint criterion is presented. Incorporated with the introduction of breaker switching state and transformer voltage state, the transformer working state such as inrush, short circuit, fault-recovery inrush, sympathetic inrush, switching on unenergized fault transformer, and fault right after switching on unenergized transformer and so on can be differentiated by the multi-characteristics of inrush current. Moreover, there is no direct relation between the inrush restraint method and tripping time during switching on unenergized fault transformer. It is suggested that special over-excitation restraint function in the transformer differential protection is not necessary.

Thirdly, anti-CT saturation principles based on speed control, trap setting and deceleration are presented for preventing mis-tripping of differential protection. All these methods possess good performance in actual practice. The percentage restraint differential protection performance is analyzed both theoretically and practically when CT is saturated. Conventionally, mis-tripping is blocked during CT saturation. But in fact when external fault occurs, the calculated data are very easy to enter the action zone but will not stay too long because of continuous transient status of CT transient saturation. Accordingly, the transient process of digital algorithm and time staying in action area are also crucial to avoid the mis-tripping in addition to the percentage restraint characteristic.

At last, the sensitivity of the differential protection is discussed. It is pointed out clearly in this thesis that the differential protection sensitivity should be understood in a comprehensive way. Sensitivity is closely related to the principle, setting and realization of protection, which contribute to the direction to improve sensitivity. Object-Oriented software design method for protection brings forward a platform for digital protection research. Additionally, due to the protection configuration it is intended to emphasize main protection and simplify backup protection for generator and transformer unit.

Publication Type:	Thesis (Doctoral)
Subjects:	T Technology > TA Engineering (General). Civil engineering (General) T Technology > TK Electrical engineering. Electronics Nuclear engineering
Departments:	School of Science & Technology > Engineering > Electrical & Electronic Engineering School of Science & Technology > School of Science & Technology Doctoral Theses Doctoral Theses

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