Function and principle of generator excitation system?

The basic requirement for operation is that all generators integrated into the system operate in sync. In the operation of the power system, various disturbances will be encountered at any time, so the automatic excitation system is required to improve the three types of stable operation of the power system.

1. Static stability

If the no-load electromotive force Eq of the generator is constant, the active power Pf of the generator will change with the power Angle δ, usually such as △P/△t as the criterion of static stability of the power system, when such as △P/△t, the system is stable, and vice versa is unstable. For generators without automatic excitation regulation, the system is unstable at △δ<90°. So the stability limit Angle is 90 degrees.

If the generator can automatically adjust the excitation during operation, then Eq is the changing value. When the generator stator current caused by the increase reduces the generator terminal voltage, the excitation regulation device will adjust the excitation current to increase Eq, so that the generator voltage is stable at a certain level, thus achieving the function of static regulation.

Second, transient stability

It is a problem of transient stability research whether generators can keep running synchronously when they are subjected to large disturbances. Improving the forced excitation ability of the excitation system to improve the voltage multiplier and voltage rise speed is generally considered to be one of the most economical and effective means to improve the transient stability of the power system. The forced excitation turn-off with the excitation system mainly takes the voltage multiple of the excitation top value, the voltage response time of the excitation system and the voltage response ratio into consideration.

3. Dynamic stability

State stability is the study of the stability of the power system after being disturbed, which restores to the original equilibrium point or transitions to the new equilibrium point. It can be understood as the damping problem of electromechanical oscillation of power system. When the damping is positive, the dynamic is stable when the damping is negative, the dynamic is unstable and when the damping is zero, it is critical. For zero damping or small positive damping, are unsafe factors in power system operation, measures should be taken to improve the damping.

Automatic voltage regulation in excitation control system is one of the most important reasons that cause the damping of electromechanical oscillation of power system to become weak or even negative. In the normal application range, the negative damping effect of excitation voltage regulator will be strengthened with the increase of open-loop gain, and the requirement of improving voltage regulation accuracy and improving dynamic stability is contradictory. PPS (Electric Chest Stabilizer) is currently used to improve the dynamic stability of the power system.

Improve the accuracy of reinsurance

When a short circuit occurs in the power system, the short circuit current of the generator attenuates faster, which may make the generator backup protection unable to operate. At this time, the short circuit current of the generator even rises slightly due to the strong excitation of the automatic regulation system on the generator. In this way, the short circuit current of the power system is greatly increased, so that the sensitivity of the relay protection device is improved.

Rapid demagnetizing

When the generator or booster transformer adopts the internal fault of the unit connection, in order to reduce the damage caused by the fault, the generator is required to be able to quickly eliminate the magnetic field, so as to prevent the rotor overvoltage and ensure the safety of the generator in the event of an accident.

02 Obtaining excitation current

Excitation mode of DC generator power supply

This type of excitation generator has a special DC generator, the special DC generator is called DC exciter, the exciter is generally coaxial with the generator, the generator's excitation winding through the slip ring installed on the large shaft and fixed brush from the exciter to obtain DC current.

This excitation mode has the advantages of independent excitation current, reliable operation and reducing self-use electricity consumption, etc., is the main excitation mode of generators in the past decades, and has more mature operation experience. The disadvantage is that the excitation regulation speed is slow and the maintenance workload is large, so it is rarely used in units above 10MW.

Excitation mode of AC exciter power supply

Some modern high-capacity generators use AC exciter to provide excitation current. Ac exciter is also installed on the main shaft of the generator, and its output alternating current flows through the rectifier to supply the generator rotor excitation, at this time, the excitation mode of the generator belongs to the other excitation mode, and because of the static rectifier device, it is also known as the static excitation, AC secondary exciter provides excitation current.

The AC secondary exciter can be a permanent magnet machine or an alternator with a self-excited constant voltage device. In order to improve the excitation regulation speed, the AC exciter usually uses a 100-200Hz intermediate frequency generator, and the AC secondary exciter uses a 400-500Hz intermediate frequency generator.