Method for Automatically Adjusting Excitation Current

When changing the excitation current of a generator, it is usually not done directly in its rotor circuit, because the current in the circuit is very large and it is not convenient to adjust it directly. The method usually adopted is to change the excitation current of the exciter to achieve the purpose of adjusting the rotor current of the generator. Common methods include changing the resistance of the excitation circuit of the exciter, changing the additional excitation current of the exciter, changing the conduction angle of the thyristor, etc.

Here we mainly talk about the method of changing the conduction angle of the thyristor. It is based on the change of the voltage, current or power factor of the generator, and the conduction angle of the thyristor rectifier is changed accordingly, so the excitation current of the generator changes accordingly. This device is generally composed of transistors and thyristor electronic components, and has the advantages of sensitivity, speed, no failure zone, high output power, small size and light weight. In the event of an accident, it can effectively suppress the overvoltage of the generator and achieve rapid demagnetization.

The automatic excitation regulating device is usually composed of a measuring unit, a synchronizing unit, an amplifying unit, a differential regulating unit, a stabilizing unit, a limiting unit and some auxiliary units. The measured signal (such as voltage, current, etc.) is transformed by the measuring unit and compared with the given value, and then the comparison result (deviation) is amplified by the preamplifying unit and the power amplifying unit, and used to control the conduction angle of the thyristor to achieve the purpose of regulating the excitation current of the generator. The function of the synchronizing unit is to synchronize the trigger pulse output by the phase shifting part with the AC excitation power supply of the thyristor rectifier to ensure the correct triggering of the thyristor.

The function of the differential unit is to make the generators running in parallel to distribute the reactive load stably and reasonably. The stabilization unit is a unit introduced to improve the stability of the power system. The excitation system stabilization unit is used to improve the stability of the excitation system. The limiting unit is set to prevent the generator from running under over-excitation or under-excitation conditions. It must be pointed out that not every automatic excitation device has the above-mentioned units. The units of a regulator device are related to the specific tasks it undertakes. 4. Components and auxiliary equipment of automatic excitation

The components of automatic excitation control include machine-end voltage transformer, machine-end current transformer, and excitation transformer; the excitation device needs to provide the following current, AC380v for factory use, DC220v for factory use control power supply. DC220v for factory use closing power supply; the following open contacts need to be provided, automatic start-up. Automatic shutdown. Grid connection (one normally open, one normally closed) increase and decrease; the following analog signals need to be provided, generator end voltage 100V, generator end current 5A, bus voltage 100V, the excitation device outputs the following relay contact signals; excitation transformer overcurrent, demagnetization, excitation device abnormality, etc.

The excitation control, protection and signal circuits are composed of demagnetization switch, auxiliary magnetic circuit, fan, demagnetization switch tripping, excitation transformer overcurrent, regulator failure, generator operating abnormality, power transmitter, etc. In the event of an internal fault in the synchronous generator, in addition to being decoupled, demagnetization is also required to reduce the rotor magnetic field to a certain extent as quickly as possible to ensure that the rotor does not pass through and shorten the demagnetization time as much as possible. This is the main function of the demagnetization device. According to the size of the rated excitation voltage, it can be divided into linear resistance demagnetization and nonlinear resistance demagnetization.

There are several methods for automatically adjusting the excitation current:

1. Open-loop control method: that is, manually adjust the excitation current according to the changes in load to keep the output voltage of the motor or generator stable.

2. Closed-loop control method: Automatically adjust the excitation current through feedback control to ensure the stability of the output voltage.

3. PI or PID control algorithm: Based on the difference between the reference value and the feedback value, the control quantity is calculated and the excitation current is further adjusted to make the output voltage reach the expected value.

4. Inverter control method: The output voltage of the inverter is controlled by the excitation current. The purpose of regulating the excitation current is achieved by adjusting the PWM waveform and frequency of the inverter.

The above methods can all achieve the purpose of automatically adjusting the excitation current, but in specific applications it is necessary to select a suitable control method based on actual conditions.