The synchronous generator excitation system is designed to produce the rotor magnetic field that can be varied to control the voltage and reactive power of the ship generator.
In modern ship high-power machines, the synchronous reactance Xs is around 1.5 × base impedance of the machine. With such a high reactance, Ef or the rotor field current required at the rated load at 0.9 lagging power factor can be more than twice that at no load with the same terminal voltage.
A typical excitation system has the corresponding current and voltage ratings, with the capability of varying the voltage Ef over a wide range of 1 to 3, or even more, without undue saturation in the magnetic circuit. Most excitation systems operate at 200 to 1,000 Vdc.
The excitation power to overcome the rotor winding I2R loss ranges from ½% to 1% of the generator rating.
All types of excitation on ship generators
For a large ship utility generator, four types of excitation system – dc, ac, static, and brushless.
DC exciter on ship generator
DC exciter: A suitably designed dc generator supplies the main field winding excitation through conventional slip rings and brushes. Due to low reliability and a high maintenance requirement, the conventional dc exciter is seldom used in modern ac generators of large ratings.
AC exciter on ship generator
AC exciter: It consists of a permanent magnet pilot exciter that excites the main exciter. The ac output of the pilot exciter is converted into dc by a floor-standing rectifier and supplied to the main exciter through slip rings. The main exciter’s ac output is converted into dc by means of a phase-controlled rectifier whose firing angle is changed in response to the terminal voltage variations.
After filtering the ripples, the dc is fed to the main generator field winding.
Static excitier on ship generator
Static exciter: It has no moving parts, as opposed to the rotating exciters
described. In the static exciter scheme, the controlled dc voltage is obtained from a suitable stationary ac source rectified and filtered. The dc voltage
is then fed to the main field winding through slip rings. This excitation
scheme has a fast dynamic response and is more reliable because it has no
rotating exciter with mechanical inertia.
Brushless exciter on ship generator system
Brushless exciter: Most modern synchronous generators of large ratings use
the brushless scheme of excitation to eliminate the need for slip rings and
brushes. The brushless exciter is placed on the same shaft as the main generator. The ac voltage induced in the exciter is rectified by rotating diodes on the rotor and filtered into pure dc. The dc is then fed directly into the rotor field coil.
The excitation control system modeling for analytical studies must be carefully done as it forms multiple feedback control loops that can become unstable. The IEEE has developed an industry standard for modeling the excitation systems. The model must account for any nonlinearity due to magnetic saturation that may be present in practical designs. The control system stability can be improved by supplementing the main control signal by auxiliary signals, such as speed and power, as required by the feedback control system stability.