To ensure the generator’s stable operation, it is important that there are no major voltage fluctuations. For this purpose, an excitation current is created to provide power to the AVR (Automatic Voltage Regulator), a device that maintains the generator's nominal voltage at a preset value, even when there are sudden load variations or in the case of special operations, such as:
parallel operation between generators
parallel operation between the generator and the grid
start-up of more demanding equipment
In regular situations, the AVR is powered by the energy generated in the alternator itself, what we call the SHUNT system.
In installations with more demanding loads, which lead to voltage drops and transient regimes, the AVR's work suffers because the voltage it receives is reduced. This may cause it to be unable to return the voltage to the preset value.
Therefore, for more demanding cases, such as those mentioned above, there are other excitation systems - the PMG and the AREP -, which ensure that the generator can withstand non-linear loads, with minimum voltage deviation.
PMG
With a PMG (Permanent Magnetic Generator) system, the energy to power the AVR is generated by rotating magnets external to the alternator. This system constitutes an almost constant source of energy, allowing the AVR to quickly balance the generator’s voltage, according to the pre-established voltage.
Because it is independent of the alternator, a PMG can be installed later and its repair does not entail great logistics.
In higher power alternators, this equipment is incorporated as standard by most manufacturers.
AREP
In the AREP system, there are two independent auxiliary windings. The first works as the SHUNT system and the second ensures that the AVR's power is sufficient to guarantee up to 300% charge for 10 seconds, which is enough to prevent power quality problems from shutting down the system.
As it is a system applied inside the alternator, its maintenance and repair imply its disassembly.