The capacitive load problem often encountered by diesel generator sets in data center

First of all, we need to limit the scope of discussion to avoid being too imprecise. The generator discussed here refers to a brushless, three-phase AC synchronous generator, which is referred to as a “generator” below.

This generator contains at least the following three main parts, which will be mentioned in the following discussion:

The main generator is divided into a main stator and a main rotor; the main rotor provides a magnetic field, and the main stator generates electricity to supply the load; the exciter is divided into an exciter stator and a rotor; the exciter stator provides a magnetic field, and the rotor generates electricity, which is rectified by a rotating rectifier and then supplied to the main rotor; the automatic voltage regulator (AVR) detects the output voltage of the main generator and controls the current of the stator coil of the exciter to achieve the purpose of stabilizing the output voltage of the main stator.

Description of the voltage stabilization work of AVR

The operating goal of AVR is to stabilize the output voltage of the generator, and it is also commonly called a “voltage stabilizer”.

Its operation is: when the generator output voltage is lower than the set value, increase the stator current of the exciter, which is equivalent to increasing the excitation current of the main rotor, so that the voltage of the main generator rises to the set value; otherwise, reduce the excitation current to reduce the voltage; if the generator output voltage is equal to the set value, the AVR maintains the existing output without adjustment.

Let’s talk about the load. According to the phase relationship between current and voltage, AC loads can be divided into three categories:

Resistive load, the current is in phase with the voltage applied to it; inductive load, the current phase lags behind the voltage; capacitive load, the current phase leads the voltage. Comparing the characteristics of the three loads helps us better understand capacitive loads.

For resistive loads, the larger the load, the greater the excitation current required by the main rotor (in order to stabilize the output voltage of the generator).

In the following discussion, we take the excitation current required by the resistive load as a reference standard, that is, we call it larger than it; we call it smaller than it.

When the load of the generator is inductive, the main rotor will require a larger excitation current to maintain a stable output voltage.

Capacitive load

When the generator encounters a capacitive load, the main rotor requires a smaller excitation current, that is, the excitation current must be reduced to stabilize the output voltage of the generator.

Why does this happen?

We should remember that the current on the capacitive load is ahead of the voltage. These leading currents (flowing through the main stator) will generate induced currents on the main rotor, which are positively superimposed with the excitation current, enhancing the magnetic field of the main rotor. Therefore, the current from the exciter must be reduced to keep the generator output voltage stable.

The larger the capacitive load, the smaller the output of the exciter must be; when the capacitive load increases to the maximum 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 time, the output voltage of the generator will not be able to stabilize itself, and this power supply is unqualified. This limitation is also called “underexcitation limitation”.

Generators can only accept limited load capacitance; (of course, for a given generator, there are also limits on the size of resistive or inductive loads)

If a project is troubled by capacitive loads, you can choose to use IT power with less capacitance per kilowatt of power, or use inductance for compensation, and never let the generator set operate near the “underexcitation limit”.