4.3.1. Synchronous generator
Figure 41 sets out the interrelationship between the generated voltage U0, the terminal voltage U and the ohmic load current.
In order to retain the required, constant terminal voltage, the generated voltage must be supplied.
According to the generator equation
U0 = O • Φ • n
one can determine the induced voltage in the synchronous generator.
As the frequency is speed-dependent according to the equation
and, generally speaking, a constant frequency is required by energy consumers, it is only possible to control the voltage U0 by means of the exciting current Ie:
Ie → Φ → U0
The exciter current is set by means of field voltage plate resp. carbon regulator, tyrill regulator or electronic facility.
Figure 42 shows a synchronous generator with a self-exciting direct current generator. The circuit diagram indicates that the current can be set from zero to maximum. A short-circuit terminal q has also been provided for the exciter circuit of the synchronous generator. Switching off excitation cannot ensue through immediate interruption of the exciting circuit as induction leads to voltage peaks. Moreover, even where the exciting circuit has been short-circuited by means of a terminal q, voltage peaks corresponding to normal operating value arise in the operational winding through residual magnetism.
L.1; L.2; L.3 conductors
The following should be heeded when setting the voltage in synchronous generators:
Synchronous generators are sometimes run in isolated operation. A number of important installations, for instance transmitting units of the postal and telecommunication services, must continue operations in the event of a power failure. Consequently, for this reason, standby generator sets have been installed in many works and institutions. These standby units ensure power supplies if the national grid fails. The drive machines of these standby units are mainly diesel motors which drive a synchronous generator along with the accompanying, self-exciting generator. We differentiate between three load categories. The synchronous generator can be loaded either with active current, inductive or capacitive reactive current. Figure 43 indicates the terminal voltage yielded according to the load.
1 Voltage, 2 Current, 3 Rated current, upper curve U = f (I) given capacitive load and lower curve U = f (I) given inductive load
Thus, one can deduce that the exciting current must be continuously set given a required, constant terminal voltage.
Rigid network operation
Where a synchronous generator feeds power to a network whose voltage also remains constant in the face of load differences, then one refers to a rigid network machine. Generally speaking several generators operate within such a network, for instance, as is customary for energy generation in power stations. One also refers to parallel or compound operation. Where several or merely one generator works within the network, then the following must be heeded when switching on the second resp. subsequent generator:
Special measuring devices resp. circuits have been devised to ensure that these conditions are adhered to or, as one says, that the generator to be switched on is “synchronized”.
The most frequently employed measuring devices are the double frequency measuring unit, the double voltage measuring device and the synchronoscope. The double frequency measuring unit contains two vibration measuring devices independent of one another, which indicate network frequency as well as the frequency of the generator to be switched on. Both frequencies can be read off simultaneously. The correct frequency is attained by setting the speed of the drive machine of the generator to be synchronized. Switching on must always ensue at the same frequency as, otherwise, the generator and/or the drive machine may be damaged. The double voltage measuring unit has two iron moving instruments independent of one another which indicate the voltage of the network and the generator. Voltage setting of the generator ensues through the exciting current. In order to control the phase position and phase sequence one can utilise light, dark or mixed circuits; the latter is also called light-dark circuit. The three circuits feature in Figure 44. The lamps are generally arranged in circular formation for better observation.
(1) Bright connection, (2) Dark connection, (3) Mixed connection
The light circuit shall be switched on when all three lamps light up brightly at the same time. In this conjunction note that the lamps are brightest given a 60 degree phase shift between generator and network. The circular arrangement of the lamps yields a rotating light reflex where the frequencies of the network and the generator do not concur. The speed of the generator drive machine is controlled in line with the rotation of the rotating light reflex. The rotational speed of the light reflex is a measure for the difference between network and generator frequencies. The rotational speed of the light reflex is a measure for the difference between the network and generator frequencies. The direction indicates whether the generator is running too quickly or too slowly.
The generator to be switched on has been positioned in the correct phase position in the network - given synchronizing dark connection - if all three lamps light up and extinguish simultaneously. If this is not the case, then two connections should be interchanged. Generator switching on ensues when all three lamps extinguish.
Where the lamps have been arranged in circular position within a mixed circuit, they light up one after the other and the light reflex wanders in accordance with the either too low or too great generator frequency. Switching on occurs when the lamp H1 extinguishes and lamps H2 and H3 light up brightly.
As synchronizing with these cited circuits is a troublesome business, power stations utilize a so-called synchronoscope. The synchronoscope indication ensues through an indicator which is controlled by two systems operating in accordance with the induction principle. The indicator is furthermore illuminated by a phase lamp in synchronizing bright connection. By this yields an optical impression of the rotational movement of the indicator is achieved according to which the speed controls of the prime mover ensue in line with the indicator movement. Figure 45 shows a customary circuit for synchronization.
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