Function and principle of generator excitation system?

The excitation system of synchronous generator is an important component

01 Main tasks of the excitation system

1. Maintain the generator terminal voltage at a given level;

2. Control reactive power distribution;

3. Improve the stability of parallel operation of synchronous generator;

4. Improve the power system relay protection device operation accuracy;

5. Quickly disable the magnetic field.

Maintain the terminal voltage

Under the normal operation of the generator, the excitation regulation device shall maintain the generator terminal voltage or the high voltage side of the main transformer at a given level. When the generator load changes, the excitation must be adjusted to ensure that the generator terminal voltage is a given value. The simplified phasor diagram from the generator is shown below:

From the generator simplified phasor diagram:

Eq=Uf+jLXd

Among them, EQ-generator no-load potential;

Uf- generator terminal voltage;

If- generator stator current;

Xd- generator synchronous reactance.

As can be seen from the above formula, when the generator no-load potential Eq is constant, the generator terminal voltage Uf will decrease with the increase of the generator stator current If, and increase with the decrease of the generator stator current If. To ensure that the generator terminal voltage Uf is constant, the no-load potential Eq of the generator must be increased or decreased with the increase or decrease of the generator load current. Eq is a function of the generator excitation current IL, and if saturation is not considered, the no-load potential Eq is proportional to the excitation current IL. Therefore, in the operation of the generator, as the generator load current changes, the generator terminal voltage will also change, in order to maintain the generator terminal voltage at a given level, it is necessary to automatically increase or reduce the excitation current through the regulation of the excitation device.

Reactive power distribution

When the generator is running side by side with the power system, its output active power depends on the prime mover input mechanical power, and the output reactive power is related to the generator excitation current, in actual operation, the generator bus running side by side will not be infinite bus, then change the generator excitation will make the generator terminal voltage and reactive power are changed, but the end voltage change is small, and reactive power There will be large variations in power. Controlling reasonable reactive power distribution between parallel generators is an important function of the excitation regulation device. How to distribute reactive power reasonably between generators running in parallel is related to the difference rate of generator terminal voltage. The adjustment rate of generator terminal voltage is defined as the input of excitation device adjustment function, the given voltage of the generator is unchanged, and the power factor of the generator is zero, when the reactive load of the generator increases from zero to rated, the terminal voltage change rate K expressed by the percentage of rated voltage of the generator, that is, formula 2:

K(%)=(UF0-UFR)/UFE

Where, K- generator error adjustment rate;

Ufo-generator no-load terminal voltage;

UFE- rated voltage of generator;

Uer-generator terminal voltage when the reactive current is the rated current of the stator.

The difference adjustment rate of generator terminal voltage reflects the change rate of generator terminal voltage UFR with reactive power output under the action of excitation regulation device. The generator terminal voltage UF may decrease with the increase of the output reactive current IR of the generator, that is, UF<UFO, then the generator has a positive voltage adjustment or the generator terminal voltage UF may increase with the output reactive current IR of the generator, that is, UF>UFO, then the generator has a negative voltage adjustment; If the generator terminal voltage UF does not change with the generator output reactive current IR change, that is, UF=UFO said that the generator has no voltage adjustment, no difference regulation. There are three modulation characteristics shown in the figure below.

Generator adjustment characteristic diagram

When multiple generator ends work directly in parallel, in order to have a stable reactive power distribution between the parallel sets, these generators must have a positive voltage adjustment, and the adjustment rate K=3%~5%. If the generator is a unit connection, that is, it is connected in parallel on the high-voltage bus through the booster transformer, the generator is required to have a negative adjustment, and the role of the negative adjustment is to partially compensate the voltage drop formed by the reactive current on the booster transformer, so that the high-voltage bus of the power plant is more stable.

Improve operational stability

The stability problems of power system can be divided into three types: static stability, transient stability and dynamic stability. The so-called static stability refers to the stability of the power system after small interference, that is, the ability to restore the original equilibrium state after small interference, while the transient stability refers to the stability of the power system after large interference, which is mainly the short-circuit effect, that is, whether the system can work stably in the new equilibrium state after large interference, and the dynamic stability refers to the power system after small interference and large interference Interference, taking into account the role of various automatic control devices, the stability of the long process.