Static excitation system (SES) has been implemented in a specially designed synchronous machine installed in a testing laboratory. This is a large capacity single machine operated in dual mode (i.e., motor or generator) with the help of static sources. It is well known that bearings of the rotating machines are vulnerable to the effects of the shaft voltages caused by the static sources. Shaft voltage is the prime concern for this special machine too due to SES. To find out the exact cause of the shaft voltage, SES of this machine has been modelled with Power Systems software. Various waveforms drawn from the model are validated through computer simulations and actual laboratory tests. Sources of shaft voltages are also analysed thereafter with the FFT analysis of the rotor voltage and current waveforms. 1. Introduction There are various means of supplying dc power to the field winding of an electric machine, for example, dc generator, rotating exciter, and static converter. After the invention of semiconductor devices, static converter has become popular. It is also known as static excitation system (SES). SES came into existence in the beginning of the sixties. Till then the alternators were getting dc field power in the rotor from a dc generator, separately or coupled with the same alternator shaft. SES was experimented successfully on a steam turbine alternator in the year 1962 [1]. Later on it was implemented not only on the new alternators but also on the existing machines. Due to ease in retrofitting, the old rotating exciters were also replaced with the SES. Starting with steam power generator the SES was extended up to the captive power plant in paper and pulp industries, pump storage power plant, electric locomotive, and gas power plant [2–6]. SES has also been implemented in high power short circuit alternator [7]. Short circuit alternator is basically a synchronous generator in a high power test laboratory which supplies high value of short circuit currents during test on electrical power equipment. A strong dc field is essential to establish and maintain the alternator terminal voltage during the short circuit test. Old testing laboratories were equipped with a separately excited high power dc generator run by an induction motor. A permanent magnet dc generator feeds the field of high power dc generator. In cascading mode a high power dc is produced which is fed into alternator rotor through slip rings and brushes. Viewing the benefits of SES, short circuit alternators are also started equipped with high power thyristors and fast acting
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