Excitation control on ship generator is required to
- maintain the normal operating voltage
- vary kVAR generation to match with the load
- increase the steady-state and dynamic stability
The manual control with field rheostat can be adequate for small generators, but the automatic voltage regulator (AVR) is common for large ship machines.
The complex power S˜ delivered by the generator must match the complex power drawn by the load. In S˜ = P + jQ the real power P is balanced by the prime mover fuel flow rate that is controlled by the prime mover governor. The reactive power Q, on the other hand, is balanced by the field current
controlled by the AVR.
Governor and AVR on ship generator
The governor and the AVR are two independent controllers in an ac generator. The AVR is a part of the excitation system, which works as follows in brushless machines.
The AVR senses the voltage in the ship main generator winding and controls the excitation to maintain the generator output voltage within the specified limits, compensating for the load, speed, temperature, and power factor of the generator.
Three-phase root mean square (rms) sensing is employed for finer voltage regulation. The excitation current is derived from a dedicated three-phase permanent magnet generator to isolate the AVR control circuits from the effects of nonlinear loads and to reduce radio frequency interference on the generator terminals. Protection of the exciter against sustained generator short circuit current is another feature of the permanent magnet rotor used in AVR.
Additional features found in ship AVRs
Underspeed protection on ship generator
A frequency-measuring circuit continually monitors the shaft speed of the
generator and provides underspeed protection of the excitation system by
102 Shipboard Electrical Power Systems reducing the generator output voltage proportionally with speed below a presettable threshold.
Maximum excitation protection on ship generator
The maximum excitation is limited to a safe value by internal shutdown of
the AVR output device. This condition remains latched until the generator
has stopped.
Remote voltage trimmer on ship AVR
Provision is made for the connection of a remote voltage trimmer, allowing the user to finely control the generator output. The AVR has the facility to allow parallel running with other similarly equipped generators.
Speed response of AVR on ship generator
Typical transient response times are: AVR itself in 10 ms, field current
to 90% in 80 ms, and machine voltage to 97% in 300 ms. The AVR also
includes a stability or damping circuit to provide good steady-state and
transient performance of the generator.
Soft start on ship generator
AVR includes a soft start or voltage ramp-up circuit to control the rate of
voltage buildup when the generator runs up to speed. This is normally preset and sealed to give a voltage ramp-up time of approximately 3 sec. If required, this can be adjusted between the limits defined in the AVR specifications.
