Mode of operation
Referring to Fig. 2, the alternator (1) is excited by a main exciter (3), which is subsequently excited by an auxiliary exciter (4), following the rectification on the output of the auxiliary exciter by a static rectifier (5). The auxiliary exciter also produces, through a separate a. c. winding, the requisite power supply for the voltage regulator thyristor trigger equipment (6).
the output of the main exciter is rectified by a rotary rectifier bridge (2) consisting of 3 thyristors and 3 nonregulated diodes, a small amount of the rectifier power, being used for exciting the auxiliary exciter. The exciter continuously supplies a voltage which is normally above the maximum requisite voltage, and the excitation current to the alternator is regulated to the correct value by the voltage regulator controlling the thyristors. During a voltage drop in the alternator caused by a momentary loading the regulator generates a signal to effect a full phase advance of the thyristor, the high excitation voltage being momentarily applied to the alternator field winding. By virtue of this arrangement, very rapid regulation of the alternator voltage can be obtained, which is fully comparable to the regulation that is effected by a compounding rectifier exciter systems.
The recovery time for a voltage drop of 15% is approximately 0,1 seconds.
The control pulses from the regulator which is a transistorized regulator, are transmitted to the thyristors via a rotary transformer (7) with a static primary and a rotary secondary winding^
The excitation circuit is dimensioned so that the alternator itself is excited without help from any external means. The alternator retains the excitation even during external short-circuits. This allQws the alternator short-circuit current to be maintained so that selective discrimination can be achieved.
In order to set the voltage level within the range ±5%, a potentiometer is supplied which is preferably fitted to the main switchboard. For applying a coarse setting to the voltage the regulator also comprises a potentiometer. The voltage accuracy is better than ±1% within entire load range for frequency deviations of ±10%. In addition the voltage regulator also allows alternators to be run in parallel, without any special measures^being necessary. This may be effected by a compensating impendance in the regulator, the voltage regulation becoming thereby current dependent.
S
i
52
S3
A current transformer (8), which is situated in the alternator terminal box, supplies a current proportional to the alternator current to the regulator. When an increase of reactive power occurs, the voltage falls approx. 5 per cent, this being necessary to prevent reactive circulation current between the alternators and to effect stable distribution of the reactive power. Under normal loading, i. e. at the normal power factor, this voltage reduction is arrested by the compensating impendance increasing the voltage when active power increases within the entire loading range.
In order to protect the generator and exciter from over-heating when running at speeds lower than normal a frequency discriminator is incorporated in the voltage regulator, which reduces the generator voltage to a value proportional to frequency at speeds below 95% of rated value.