1. Synchronous generator excitation system

Generally speaking, a generator set consists of an engine (providing kinetic energy), a generator (generating current), and a control system. Wind turbines rely on wind power to drive the generator to rotate and generate current; hydroelectric generators use the drop of water flow to generate power to drive the generator to generate electricity. Fuel generators rely on the combustion of diesel or gasoline to generate power to drive the generator set. Although the power sources of various power generation systems may be different, they all have one important part in common, and that is the generator. The electrical energy we use in production and life comes from power plants, and they are inseparable from motors. The rotor can be driven by a water turbine, steam turbine, internal combustion engine, etc. The main components are shown in the figure below.

Currently, 99% of the electrical energy used by humans is generated by synchronous generators. In order to achieve energy conversion, a synchronous generator needs a DC magnetic field. The DC current that generates this magnetic field is called the excitation current of the generator. According to the supply method of the excitation current, any generator that obtains the excitation current from other power sources is called a separately excited generator, and a generator that obtains the excitation power from the generator itself is called a self-excited generator.

The excitation system of a synchronous generator generally consists of two parts. One part is used to provide DC current to the field winding of the generator to establish a DC magnetic field, which is usually called the excitation power output part (or power unit). The other part is used to adjust the excitation current during normal operation or in the event of an accident to meet operational needs. This part includes excitation regulator, forced excitation, forced demagnetization and automatic demagnetization, etc., and is generally called the excitation control part (or control unit).

2. Synchronous generator voltage regulation

Maintaining the terminal voltage of the generator equal to a given value is the main means of voltage regulation in the power system. So how to ensure that the terminal voltage of the synchronous generator is a given value?

We say that when the load changes, the excitation must be adjusted. Let's analyze its specific principle using the simplified phasor diagram of the generator.

From the above figure we can get:

In the formula: Eq——the no-load potential of the generator; Uf——the terminal voltage of the generator; If——the load current ratio of the generator.

The above formula shows that when the generator no-load potential Eq is constant, the generator terminal voltage Uf will decrease with the increase of the load current If. In order to ensure that the generator terminal voltage Uf is constant, it must increase with the increase of the generator load current If. Increase (or decrease), increase (or decrease) the no-load potential Eq of the generator, and Eq is a function of the generator excitation current Ifq (if saturation is not considered, Eq and Ifq are proportional), so when the generator is running As the generator load current changes, the excitation current must be adjusted to keep the generator terminal voltage constant.