Generator Excitation Method and Working Principle

Generator excitation

The generator excitation system is the collective name for the power supply and its ancillary equipment that supplies the excitation current of the synchronous generator. It generally consists of two main parts: excitation power unit and excitation regulator.

Generator excitation system includes DC exciter, non-exciter, AC exciter, etc. In the past ten years, due to the emergence and use of new technologies, new processes and new devices, the excitation methods of generators have been continuously developed and improved. In terms of automatic adjustment of excitation devices, many new adjustment devices have been continuously developed and promoted.

System composition

The excitation power unit provides excitation current to the synchronous generator rotor; and the excitation regulator controls the output of the excitation power unit according to the input signal and the given regulation criterion. The automatic excitation regulator of the excitation system plays a considerable role in improving the stability of parallel units in the power system. In particular, the development of modern power systems has led to a reduction in the stability limit of units, which has also prompted the continuous development of excitation technology. The excitation system of the synchronous generator is mainly composed of two parts: the power unit and the regulator (device). as the picture shows:

The excitation power unit refers to the excitation power supply part that provides DC excitation current to the rotor winding of the synchronous generator, while the excitation regulator is a device that controls the output of the excitation power unit according to the input signal required by the control and the given adjustment criteria. The entire system consisting of the excitation regulator, the excitation power unit and the generator itself is called the excitation system control system. The excitation system is an important part of the generator. It has a great impact on the safe and stable operation of the power system and the generator itself.

  main effect

The main functions of the excitation system are:

1) Adjust the excitation current accordingly according to changes in generator load to maintain the machine terminal voltage at a given value;

2) Control the reactive power distribution between generators running in parallel;

3) Improve the static stability of generators operating in parallel;

4) Improve the transient stability of generators operating in parallel;

5) When a fault occurs inside the generator, demagnetize it to reduce the degree of fault loss;

6) Implement excitation limits and excitation limits on the generator according to operating requirements.

1. Several ways for a generator to obtain excitation current

1. Excitation mode of DC generator power supply: The generator of this excitation mode has a dedicated DC generator. This dedicated DC generator is called a DC exciter. The exciter is generally coaxial with the generator. The excitation of the generator The windings obtain DC current from the exciter through slip rings and fixed brushes mounted on the large shaft. This excitation method has the advantages of independent excitation current, relatively reliable operation and reduced self-consumption of electricity. It has been the main excitation method of generators in the past few decades and has relatively mature operating experience. The disadvantage is that the excitation adjustment speed is slow and the maintenance workload is large, so it is rarely used in units above 10MW.

2. Excitation method powered by AC exciter. Some modern large-capacity generators use AC exciters to provide excitation current. The AC exciter is also installed on the main shaft of the generator. The AC current it outputs is rectified and supplied to the generator rotor for excitation. At this time, the excitation mode of the generator is a separate excitation mode. Since it uses a static rectifier device, it is also called For separately excited static excitation, the AC auxiliary exciter provides excitation current. The AC auxiliary exciter can be a permanent magnet machine or an alternator with a self-excited constant voltage device. In order to increase the excitation adjustment speed, the AC exciter usually uses a 100-200HZ intermediate frequency generator, while the AC auxiliary exciter uses a 400-500HZ intermediate frequency generator. The DC excitation winding and three-phase AC winding of this kind of generator are wound in the stator slots. The rotor has only teeth and slots without windings, like a gear. Therefore, it has no brushes, slip rings and other rotating contact parts, making it reliable in operation. , 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.

3. Excitation method without exciter:

In the excitation mode, there is no special exciter, but the excitation power is obtained from the generator itself, and then supplied to the generator itself for excitation after rectification, which is called self-excited static excitation. Self-excited static excitation can be divided into two methods: self-shunt excitation and self-re-excitation. The self-shunt excitation method obtains the excitation current through the rectifier transformer connected to the generator outlet, and supplies the generator excitation after rectification. This excitation method has the advantages of simple structure, less equipment, low investment and low maintenance workload. In addition to not having a rectifier transformer, the self-re-excitation method also has a high-power current transformer connected in series with the stator circuit of the generator. The function of this transformer is to provide a larger excitation current to the generator to make up for the lack of output of the rectifier transformer when a short circuit occurs. This excitation method has two excitation power sources, a voltage source obtained through a rectifier transformer and a current source obtained through a series transformer.

2. Characteristics related to generator and excitation current

1. Voltage adjustment

The automatic adjustment excitation system can be regarded as a negative feedback control system with voltage as the adjusted variable. The reactive load current is the main cause of the voltage drop at the generator terminal. When the excitation current remains unchanged, the terminal voltage of the generator will decrease as the reactive current increases. However, in order to meet the user's requirements for power quality, the terminal voltage of the generator should remain basically unchanged. The way to achieve this requirement is to adjust the excitation current of the generator as the reactive current changes.

2. Adjustment of reactive power:

When the generator is running in parallel with the system, it can be considered to be running with the busbar of an infinite large-capacity power supply. If the generator excitation current is changed, the induced potential and stator current will also change. At this time, the reactive current of the generator will also change. When a generator is running in parallel with an infinitely large capacity system, in order to change the reactive power of the generator, the excitation current of the generator must be adjusted. The changed generator excitation current at this time is not what is commonly called "voltage regulation", but only changes the reactive power fed into the system.

3. Distribution of reactive load:

Generators moving in parallel distribute reactive current in proportion according to their respective rated capacities. Large-capacity generators should bear more reactive load, while smaller-capacity generators should provide less reactive load. In order to achieve automatic distribution of reactive loads, the excitation device of the automatic high-voltage adjustment can be used to change the excitation current of the generator to maintain its terminal voltage unchanged. The inclination of the voltage regulation characteristics of the generator can also be adjusted to realize parallel operation of the generators. Reasonable distribution of reactive power loads.

3. Method of automatically adjusting the excitation current

When changing the excitation current of the generator, it is generally not done directly in its rotor loop, because the current in this loop is very large and it is inconvenient to directly adjust. The commonly used method is to change the excitation current of the exciter to adjust the generator. purpose of the rotor current. Commonly used 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 silicon-controlled rectifier. It is to change the conduction angle of the silicon-controlled rectifier accordingly according to the change of the generator voltage, current or power factor, so the excitation current of the generator will change accordingly. This device is generally composed of transistors and thyristor electronic components. It 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.

Automatically adjusting excitation devices usually consist of a measurement unit, a synchronization unit, an amplification unit, a difference adjustment unit, a stabilization unit, a 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 measuring unit, and then the comparison result (deviation) is amplified by the preamplifier unit and power amplification unit, and used to control the conduction of the thyristor angle to achieve the purpose of adjusting the generator excitation current. The function of the synchronization unit is to synchronize the trigger pulse output by the phase-shifting part with the AC excitation power of the silicon-controlled rectifier to ensure the correct triggering of the silicon-controlled rectifier.

The function of the differential control unit is to enable 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. 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 over- or under-excitation conditions. It must be pointed out that not every automatic regulating excitation device has the above-mentioned various units. The units of a regulator device are related to the specific tasks it undertakes. 4. Components and auxiliary equipment for automatic adjustment of excitation

The components of automatic adjustment of excitation include organic terminal voltage transformer, machine terminal current transformer and excitation transformer; the excitation device needs to provide the following current, factory AC380v and factory DC220v control power supply. Factory DC220v closing power supply; the following empty contacts need to be provided to automatically start up. Automatic shutdown. Grid-connected (one normally open, one normally closed) increases and decreases; 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, the excitation device outputs the following relay contact signal; the excitation changes current, loss of excitation, abnormality of excitation device, etc.

The excitation control, protection and signal circuit are composed of excitation switch, excitation circuit, fan, excitation switch tripping, excitation transformer overcurrent, regulator failure, abnormal generator working condition, power transmitter, etc. When an internal fault occurs in a synchronous generator, in addition to being demagnetized, it must also be demagnetized to weaken the rotor magnetic field as quickly as possible to ensure that the rotor fails and shorten the demagnetization time as much as possible. This is the main function of the demagnetization device. . According to the rated excitation voltage, it can be divided into linear resistor demagnetization and nonlinear resistor demagnetization.

In the past ten years, due to the emergence and use of new technologies, new processes and new devices, the excitation methods of generators have been continuously developed and improved. In terms of automatic adjustment of excitation devices, many new adjustment devices have been continuously developed and promoted. Since the use of microcomputer software to realize automatic adjustment of excitation devices has significant advantages, many countries are currently developing and testing digital automatic adjustment of excitation devices composed of microcomputers and corresponding external equipment. This kind of adjustment device will be able to realize automatic adjustment. Adapt to adjustment. The method of obtaining the excitation current is called the excitation method. The currently used excitation methods are divided into two categories: one is a DC exciter excitation system that uses a DC generator as the excitation power supply; the other is a rectifier excitation system that uses a silicon rectifier device to convert AC into DC and then supply excitation. The description is as follows:

  1 . DC exciter excitation DC exciter is usually coaxial with the synchronous generator and adopts shunt excitation or separate excitation connection method. When the separately excited connection method is adopted, the excitation current of the exciter is supplied by another coaxial DC generator called the auxiliary exciter.

  2 . Static rectifier excitation There are three alternators on the same axis, namely the main generator, the AC main exciter and the AC auxiliary exciter. The excitation current of the auxiliary exciter is initially provided by an external DC power supply, and then turns to self-excitation after the voltage is established (permanent magnet generators are sometimes used). The output current of the auxiliary exciter is rectified by a static thyristor rectifier and then supplied to the main exciter, while the AC output current of the main exciter is rectified by a static three-phase bridge silicon rectifier and then supplied to the excitation winding of the main generator.

3. Rotating rectifier excitation The DC output of the static rectifier must pass through the brushes and collector rings before it can be delivered to the rotating excitation winding. For large-capacity synchronous generators, the excitation current reaches thousands of amperes, causing the collector rings to seriously overheat. Therefore, in large-capacity synchronous generators, rotating rectifier excitation systems that do not require brushes and slip rings are often used. The main exciter is a rotating armature three-phase synchronous generator. The alternating current of the rotating armature is rectified by a silicon rectifier that rotates with the main shaft, and then directly sent to the rotor excitation winding of the main generator. The excitation current of the AC main exciter is supplied by the coaxial AC auxiliary exciter after being rectified by a static thyristor rectifier. Since this excitation system eliminates the collector ring and brush device, it is also called a brushless excitation system.

HZ9110 Generator Rotor AC Impedance Tester