1 Introduction

Synchronous generators are key elements of a power system. Therefore, it is necessary for the optimal operation of a power system that we understand their behaviour. Since it is the airgap flux of a generator that dominates the operation, this is the characteristic that we should come to understand.

However, generator characteristics to date have been related to terminal voltages and currents. From these quantities, we cannot know the actual behaviour of airgap flux (time and space harmonics) and its effect on the generator operation. Until now, there has been no effective and practical means of detecting airgap flux of an on-load synchronous generator directly.

We have analyzed a synchronous generator from the point of view of its airgap flux [1], and we have devised a method for measuring airgap flux distribution by the Fourier expansion of induced voltages of search coils which are placed at armature tooth tips.

Until now, we have analyzed the induced voltages of search coils using a large scale computer after having recorded them onto an analog data recorder. We need, however, to study the behaviour of airgap flux distribution in real time at the point where a synchronous generator actually supplies electric power to various loads. We have been therefore developing a microprocessor-based instrument for the measurement of airgap flux distribution of on-load synchronous generators.

This paper describes how the instrument has been developed. First, the principle and method af measuring airgap flux distribution are described. Next, the outline of the form of the measuring instrument is explained. Finally, examples of measurements of airgap flux distribution in some loading conditions are reported, and these results are examined.

This instrument converts the induced voltages of three search coils into digital values using three analogue-to-digital convertors (ADC) and calculates airgap flux distribution by expanding the set of values into Fourier series using two microprocessors. The airgap flux distribution is extracted through digital-to-analogue convertors (DAC).

This measuring instrument is small, of low-cost, and can be made easily. By using it, we will be able to look directly at airgap flux distribution of the on-load synchronous generator provided that we install search coils and the d-axis pickup in the generator. Noting that no use of airgap flux is made in control systems for synchronous generators at present, we are developing a stabilizing controller for a generator as the application of this instrument. When information on airgap flux is added to the control system and the new control system is perfected, we can significantly improve the stability of generators.
[1] UENOSONO, C., UEDA, Y., NANAHARA, T., and KANEKO, K.: 'Air-gap flux of a synchronous generator at steady-state and flux based analysis of generator characteristics', Trans. Inst. Electr. Eng. Jpn. Part B (Japan), May 1982, 102, (5), pp. 281-288, English translation, Scripta, 1982, 102, (3), pp. 42-49
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