De-excitation circuit
If in the single-phase and three-phase synchronous machines used in electrical power engineering , such. B. the turbo generator , internal faults in the form of a short-circuit or a short-circuit occur, it is not sufficient to disconnect the machine from the mains, but the magnetic flux of the excitation current must also be reduced. This means that a rapid dissipation of the magnetic energy that is stored in the field winding must be brought about. This process is called de-energizing , the necessary to electrical circuit is as Entregungsschaltung referred. This procedure applies not only to synchronous machines, but to inductive loads of all kinds.
De-excitation circuits are used in larger machines with outputs of over 1 MW .
Basics
If a machine is not de-energized, the field of the excitation current continues to induce voltages in the short-circuit circuits caused by faults, the currents of which extend the damage that occurs, which leads to the mechanical destruction of the synchronous machine. In order to keep the extent of the damage as small as possible, it is necessary to de-excite as quickly as possible. With such a rapid de-excitation , it should be noted that magnetic energy is stored in the magnetic field of the excitation current that cannot be transported out of the machine in any short time. From the point of view of electromagnetic processes, the excitation winding represents an inductance , the current of which can only change according to the time constants of the excitation circuit.
Methods
The simplest type of de-excitation is to short-circuit the field winding. The short circuit can occur by reducing the voltage of the excitation voltage source (exciter or controlled converter) as quickly as possible to zero. De-excitation can be accelerated if the excitation winding of the synchronous machine is short-circuited not directly, but via an external resistor, the so-called de-excitation resistor. This resistance reduces the time constant. However, it must not be selected too large in view of the voltage that the exciter current, which continues to flow in its original magnitude, causes across it at the first moment. If the dimensioning is too large, the voltage of the excitation current would endanger the isolation of the excitation winding. The same thing happens when you try to break down the excitation field by interrupting the excitation current.
A further acceleration of the field reduction is achieved in that the excitation winding, in order to initiate de-excitation, is not short-circuited via the de-excitation resistor, but is connected to a negative DC voltage. This process is achieved with excitation with an excitation machine by reversing the polarity of the winding (s) at the same time as switching on the de-excitation resistor. The excitation current of the synchronous machine can drop to values below zero (negative values).
When the converter is excited, negative DC voltage can be applied by switching the converter to inverter operation. The thyristors each switch the negative half-wave of the applied alternating voltage. In this case, however, the excitation current cannot be less than zero, as it can only flow in one direction. If you want to force negative excitation currents, a second converter arrangement must be provided in anti-parallel connection. In addition, the polarity of the field winding can be reversed using a switch when the field current has dropped to zero. Negative excitation currents are important because additional short-circuit circuits, such as those represented by the damper cage or the solid rotor body of the machine, try to maintain the original field. A finite air gap field still exists when the excitation current zero is reached. In order to reduce this field as quickly as possible, a short-term counter-excitation from the field winding must take place with the help of negative excitation currents. This counter-excitation must be switched off at the moment when the air gap field has disappeared; otherwise a field would build up again. However, countercurrent de-excitation is rarely used nowadays. On the one hand, it forms an additional source of interference, on the other hand, it only brings about an insignificant acceleration of the de-excitation, because reactive power is practically always used, which means a cross-field in the machine that cannot be influenced by the counter-current de-excitation . An impressive reduction in the de-excitation time is only achieved with no-load de-excitation (which, however, has no meaning in this context), in which only one longitudinal field exists. Therefore, there are almost no systems with countercurrent de-excitation anymore.
literature
- H. Koettnitz, G. Winkler and K. Weßnigk: Fundamentals of electrical operating processes in electrical energy systems . VEB German publishing house for basic industry, Leipzig 1986, ISBN 3342000872 .