Excitation systems for synchronous machines

from Wikipedia, the free encyclopedia
Turbine hall of the Schwarze Pump power plant : In the front right in the cuboid yellow housing power plant exciter machines of a brushless power plant exciter device. Behind the turbo generator, yellow cylindrical unit in the middle. Then the multi-stage, blue painted steam turbine.

Excitation systems for synchronous machines are used to provide the magnetic excitation field required to operate single-phase and three-phase synchronous machines . This is obtained when the on the flywheel housed excitation coil with a direct current I e fed or permanent magnets used.

Basics

In order to be able to control the reactive power behavior of the machine, this current must be adjustable. A ramp-function generator is often used so that the current does not suddenly change when the load changes . With large generators, the excitation current can have a value of over 10  kA . The excitation power for small synchronous machines (a few 100 kVA ) assumes  3 to 5% of the rated apparent power S N and for larger synchronous machines (a few 100 M VA) about 0.5%. With a two-pole turbo generator with 100 kVA, around 3 kW of excitation power would have to be  generated and around 4000 kW for a 1000 MVA generator. Different excitation systems have been developed depending on the type of application and the power range of the synchronous machine (generator, motor, phase shifter , island or network operation).

Types of pathogens

Coupled DC exciter

There is also an exciter and an auxiliary exciter on the shaft of the three-phase synchronous machine. The auxiliary excitation machine is self-excited, ie it draws its excitation power from its stator circle. It feeds the excitation circuit of the excitation machine with auxiliary excitation power. This auxiliary excitation power is regulated depending on the stator voltage of the three-phase synchronous machine.

This excitation system was previously used for turbo generators up to 150 MVA. This exciter machine was out of the question for larger generators, as the load limit for the mechanical and electrical stress on the armature had to be taken into account. Another major disadvantage is the large time constant of the excitation winding of the excitation machine, which does not allow rapid changes in the excitation power on the generator.

Separately installed excitation converter

This system, which can also be used for larger generator outputs, consists of a three-phase asynchronous motor that drives a self-excited direct current generator. The asynchronous motor is a low voltage - auxiliary system fed. In the excitation circuit there is an actuator with which the level of the excitation current can be regulated. In practice, this system is rarely used.

Converter exciter

Modern excitation systems work with components of the power electronics and can be used up to the highest power.

Typical representatives of converter excitation are:

Permanent magnet excitation of synchronous machines

Mainly synchronous motors ( servos ) of small to medium power are permanently excited by means of permanent magnets . This type of excitation is now also used in generators in some wind turbines . Hard magnetic ferrites and rare earth magnet materials such as neodymium-iron-boron or samarium-cobalt come into consideration as the material for the permanent magnets . The use of permanent magnets would be uneconomical for machines with higher capacities because of the high price. The use of permanent magnets results in a constant excitation field. It is therefore not possible to influence the reactive power behavior of the machine. A great advantage of permanent magnet excitation is that there is no need for an excitation device. In addition, no excitation power needs to be applied.

Degree of excitation

The degree of excitation of a synchronous machine is the degree of excitation:

with the pole wheel voltage and the mains voltage

In addition, the voltages are phase-shifted by the angle and different in size by the degree of excitation, i.e.

The degree of excitation can be displayed graphically in the current locus .

Applicable standards

  • IEC 60034-16-1 - Rotating electrical machines - Part 16-1: Excitation systems for synchronous machines - Terms

literature

  • Rolf Fischer: Electrical machines. 14th, updated and expanded edition. Carl Hanser Verlag, Munich 2007, ISBN 978-3-446-41754-0 .
  • Helmut Lindner, Harry Brauer, Constans Lehmann: Taschenbuch der Elektrotechnik und Elektronik 7th, completely revised edition. Fachbuchverlag Leipzig in Carl Hanser Verlag, Munich 1999, ISBN 978-3-446-21056-1 .