其次综述国内对这三部文学作品的研究成果及存在问题，并提出论文的写作目的及意义。
In the next place, it summarizes research result and existent problems of the three literary works in our country, and comes up with purpose and significance of writing thesis.
前言介绍了选题的原因、目的、意义及论文的大体框架。
The foreword introduced the selected topic reason, the goal, the significance and the thesis roughly the frame.
本论文首先对电力系统的各种扰动进行了概述，分析了国内外励磁控制的研究方法，提出本论文的研究目的及意义。
At first, the paper introduces the various disturbances in the power system, analyzes international exciter control research methods, presents purpose and meaning of this paper.
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