Constant Pressure Device for Diesel Generator Set

In the past, the excitation system of AC synchronous generators used DC exciters. However, due to the relatively troublesome maintenance and management, it is rarely used nowadays. Later, a self-excited and constant-voltage excitation system of a phase compound exciter without an exciter was developed. In modern times, synchronous generators use an excitation system of a separately excited synchronous generator with an AC exciter and a rotating rectifier, which is called the brushless synchronous generator excitation system. For small synchronous generators, a reactance phase-shifting type phase compound excitation self-excited and constant-voltage device is generally used. It is an uncontrollable phase compound excitation method. To understand its basic principle, let's first discuss the self-excitation voltage build-up and the current superposition principle.

Working Principle of the Compound Excitation Self-Excited and Constant-Voltage Device of Diesel Generator Sets

In the magnetic synthesis self-excited and constant-voltage circuit, DK is a three-phase linear reactor, also known as a phase-shifting reactor; BF is a three-phase three-winding transformer. W1 is the voltage winding of BF, W2 is the output winding, and W3 is its current winding. ZL is a three-phase bridge rectifier, R is the regulating rheostat, and LQ is the excitation winding of the generator. In the diesel generator set, the volume shrinks and the temperature rises rapidly, reaching the ignition point of diesel. The diesel is ignited, and the mixed gas burns violently. The volume expands rapidly, pushing the piston downward, which is called "power generation".

It can be seen that the current I2 in the W2 winding of the compound excitation transformer is the excitation current IL of the generator after rectification. For a three-phase bridge rectifier circuit, IL = 1.23I2. Therefore, discussing the variation law of I2 is also discussing the variation law of the generator excitation current. The magnitude of I2 is obviously related to the currents in the W1 and W2 windings. According to the magnetic potential balance condition of the transformer (pressure variable), it can be seen that the excitation current of the generator is composed of two parts superimposed: one part is Iu = K12I1, which is basically provided by the generator terminal voltage through the phase-shifting reactor and is called the self-excitation component or voltage component of the excitation current; the other part is Ii = K32I3, which is provided by the generator load current and is called the current component or compound excitation component of the excitation.

Since Iu flows through a phase-shifting reactor with a very large inductive reactance, it lags behind the generator terminal voltage by approximately 90° in phase, while I1 is in the same phase as the generator load current. Taking these two points into account, it is easy to draw the vector of I2. It also shows the variation law of the excitation current I2 when the power factor angle φ changes.

Now, let's compare the vector of the excitation current I2 with the vector of the generator potential. Under the condition that the generator voltage remains unchanged, it is the simplified vector of the generator potential at different load currents and power factors. By comparing the two, it can be seen that the variation law of the generator excitation current I2 is completely consistent with that of the generator potential. This shows that due to the vector superposition of the compound excitation component and the voltage component of the excitation, the excitation current can theoretically meet the requirements of the generator potential change and keep the generator voltage constant. This is also the origin of the name of the phase compound excitation self-excited and constant-voltage.

On the one hand, the self-excited and constant-voltage device has the advantages of reliability, simplicity, and rapid response, so it has been widely used. On the other hand, when factors such as temperature rise, magnetic circuit non-linearity, and speed fluctuation cause generator voltage deviation, this type of circuit does not have the ability to eliminate this deviation. Therefore, it is difficult to maintain a high voltage regulation accuracy. In general, its static voltage variation rate is ±1%, and in some cases, it can even reach ±5%. Since this type of circuit is usually designed according to the non-differential voltage regulation characteristic, there are also certain difficulties in the balanced distribution of reactive loads during parallel operation, especially for the parallel operation of generator sets of different models.

Dingbo Power was founded in 1974 and is one of the earliest manufacturers of generators and diesel generator sets in China. If you want get more information, please feel free to send email to [email protected] we will pay highly attention on your question.