Firstly, we need to limit the scope of the discussion to avoid making it too imprecise. The generator discussed here refers to a brushless, three-phase AC synchronous generator, hereinafter referred to only as the “generator”.
This type of generator consists of at least three main parts, which will be mentioned in the following discussion:
Main generator, divided into main stator and main rotor; The main rotor provides a magnetic field, and the main stator generates electricity to supply the load; Exciter, divided into exciter stator and rotor; The exciter stator provides a magnetic field, the rotor generates electricity, and after rectification by a rotating commutator, it supplies power to the main rotor; Automatic Voltage Regulator (AVR) detects the output voltage of the main generator, controls the current of the exciter stator coil, and achieves the goal of stabilizing the output voltage of the main stator.
Description of AVR voltage stabilization work
The operational goal of AVR is to maintain a stable generator output voltage, commonly known as a “voltage stabilizer”.
Its operation is to increase the stator current of the exciter when the output voltage of the generator is lower than the set value, which is equivalent to increasing the excitation current of the main rotor, causing the main generator voltage to rise to the set value; On the contrary, reduce the excitation current and allow the voltage to decrease; If the output voltage of the generator is equal to the set value, the AVR maintains the existing output without adjustment.
Furthermore, according to the phase relationship between current and voltage, AC loads can be classified into three categories:
Resistive load, where the current is in phase with the voltage applied to it; Inductive load, the phase of the current lags behind the voltage; Capacitive load, the phase of the current is ahead of the voltage. A comparison of the three load characteristics helps us better understand capacitive loads.
For resistive loads, the larger the load, the greater the excitation current required for the main rotor (in order to stabilize the output voltage of the generator).
In the subsequent discussion, we will use the excitation current required for resistive loads as a reference standard, which means that larger ones are referred to as larger; We call it smaller than it.
When the load of the generator is inductive, the main rotor will require a greater excitation current in order for the generator to maintain a stable output voltage.
Capacitive load
When the generator encounters a capacitive load, the excitation current required by the main rotor is smaller, which means that the excitation current must be reduced in order to stabilize the output voltage of the generator.
Why did this happen?
We should still remember that the current on the capacitive load is ahead of the voltage, and these leading currents (flowing through the main stator) will generate induced current on the main rotor, which happens to be positively superimposed with the excitation current, enhancing the magnetic field of the main rotor. So the current from the exciter must be reduced in order to maintain a stable output voltage of the generator.
The larger the capacitive load, the smaller the output of the exciter; When the capacitive load increases to a certain extent, the output of the exciter must be reduced to zero. The output of the exciter is zero, which is the limit of the generator; At this point, the output voltage of the generator will not be self stable, and this type of power supply is not qualified. This limitation is also known as’ under excitation limitation ‘.
The generator can only accept limited load capacity; (Of course, for a specified generator, there are also limitations on the size of resistive or inductive loads.)
If a project is troubled by capacitive loads, it is possible to choose to use IT power sources with smaller capacitance per kilowatt, or use inductors for compensation. Do not let the generator set operate near the “under excitation limit” area.
Post time: Sep-07-2023
