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.

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.

The DC excitation winding and three-phase AC winding of this generator are wound in the stator slot, the rotor only teeth and slots without winding, like a gear, therefore, it has no brush, slip ring and other rotating contact parts, with reliable work, simple structure, convenient manufacturing process and other advantages. The disadvantage is that the noise is larger, and the harmonic component of the AC potential is also larger.

Excitation mode without exciter

No special exciter is set up in the excitation mode, and the excitation power is obtained from the generator itself, and then the generator itself is excited after rectification, which is called self-excited static excitation. Self-excited static excitation can be divided into two ways: self-shunt excitation and self-compound excitation. Self-shunt excitation mode it obtains excitation current through the rectifier transformer connected to the generator outlet, and supplies the generator excitation after rectification. This excitation mode has the advantages of simple junction, less equipment, less investment and less maintenance work.

The self-compound excitation mode not only has no rectifier transformer, but also has a high-power current transformer in series in the generator stator loop. The function of the transformer is to provide a larger excitation current to the generator when a short circuit occurs, so as to make up for the deficiency of the output of the rectifier transformer. This exciting mode has two exciting power sources, the voltage power source obtained by the rectifier transformer and the current source obtained by the series transformer.

03 Method of automatically adjusting excitation current

In changing the excitation current of the generator, it is generally not carried out directly in its rotor loop, because the current in the loop is very large, it is not easy to directly adjust, and the method usually used is to change the excitation current of the exciter to achieve the purpose of adjusting the rotor current of the generator. The common methods include changing the resistance of the excitation circuit of the exciter, changing the additional excitation current of the exciter, changing the on-angle of the thyristor, etc.

Here mainly talk about the method of changing the thyristor on-angle, it is according to the generator voltage, current or power factor changes, and correspondingly change the on-angle of the thyristor rectifier, so the generator excitation current will change. This device is generally composed of transistors, thyristor electronic components, with sensitive, fast, no failure zone, large output power, small size and light weight and other advantages.

In the case of accident, the generator overvoltage can be effectively suppressed and the magnetic field can be quickly eliminated. The automatic regulating excitation device is usually composed of measuring unit, synchronizing unit, amplifying unit, adjusting unit, stabilizing unit, limiting unit and some auxiliary units. The measured signal (such as voltage, current, etc.) is compared with the given value after being transformed by the measurement unit, and then the comparison result (deviation) is amplified by the pre-amplification unit and the power amplification unit, and is used to control the on-angle of the thyristor to achieve the purpose of adjusting the generator excitation current.

The function of the synchronization unit is to synchronize the trigger pulse output of the phase shift part with the AC excitation power supply of the thyristor rectifier to ensure the correct triggering of the controlled silicon. The function of the differential adjustment unit is to make the generators running in parallel to distribute the reactive load stably and reasonably.

Stability unit is a unit introduced to improve the stability of power system. The excitation system stabilization unit is used to improve the stability of the excitation system. The limiting unit is designed to prevent the generator from operating under overexcited or underexcited conditions. It must be pointed out that not every kind of automatic regulating excitation device has the above-mentioned units, and the units of a regulator device are related to the specific tasks they undertake.

04 Automatic adjustment of excitation components and auxiliary equipment

Components of automatic excitation regulation organic end voltage transformer, machine end current transformer, excitation transformer; The excitation device needs to provide the following current, factory AC380v, factory DC220v control power supply. Factory DC220v closing power supply; Need to provide the following empty contact, automatic boot. Automatic shutdown. Grid-connected (one normally open, one normally closed) increase, decrease; The following analog signals need to be provided, the generator terminal voltage is 100V, the generator terminal current is 5A, the bus voltage is 100V, and the excitation device outputs the following relay contact signals; Excitation variation overcurrent, loss of excitation, excitation device abnormality, etc.

Excitation control, protection and signal circuit are composed of magnetic switch, magnetic circuit, fan, magnetic switch steal, excitation change overcurrent, regulator fault, generator abnormal condition, power transmitter, etc. In addition to the internal failure of the synchronous generator must be disassembled, but also must be out of the magnetic field, the rotor magnetic field as soon as possible to weaken to the minimum degree, to ensure that the rotor is not the case, so that the magnetic time as short as possible, is the main function of the magnetic device. According to the rated excitation voltage, it can be divided into linear resistance and nonlinear resistance.

In the past ten years, due to the emergence and use of new technologies, new processes and new devices, the excitation mode of generators has been continuously developed and improved. In the field of automatic regulating excitation devices, many new regulating devices have been developed and popularized. Because of the obvious advantages of the automatic regulation excitation device realized by microcomputer software, many countries are developing and testing the digital automatic regulation excitation device composed of microcomputer computer with corresponding external equipment, which can realize the adaptive optimal regulation.