The power source that supplies the excitation current to the synchronous generator and its associated equipment is collectively referred to as the excitation system. It generally consists of two main components: the excitation power unit and the excitation regulator. The excitation power unit supplies the excitation current to the rotor of the synchronous generator; while the excitation regulator controls the output of the excitation power unit based on the input signal and the given regulation criteria. The automatic excitation regulator of the excitation system plays a significant role in improving the stability of the parallel units in the power system. Especially with the development of modern power systems, the stability limit of the units has decreased, which also prompts the continuous development of excitation technology.
System Introduction
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Excitation
Generally, we refer to the process of generating a rotating magnetic field in the rotor of a generator based on the principle of electromagnetic induction as excitation. Additionally, providing a working magnetic field for “electrical equipment that operates based on the principle of electromagnetic induction” such as generators is also called excitation. Sometimes, the device that supplies the rotor power to the generator is also called excitation.
With the development of power construction, the power system industry in China has entered the stage of large networks, high voltages, and large units. The stability of large-capacity units during operation is crucial for the stability and safety of the entire power grid. However, the most significant factor affecting the stability of the generator is the excitation system of the power grid. The excitation system plays a very important role in ensuring the stable operation of the units and is also a lever for reactive power and voltage regulation in the entire power grid. [1]
Main Functions
1. Maintain the generator terminal voltage at a given value. When the generator load changes, it regulates the strength of the magnetic field to keep the generator terminal voltage constant.
According to the supply mode of the AC power source for generator excitation
1. AC excitation (external excitation) system
Powered by an AC excitation machine coaxial with the generator. The system can be divided into four types:
1) AC excitation machine (magnetic field rotation) plus static silicon rectifier (brushed).
2) AC excitation machine (magnetic field rotation) plus static thyristor rectifier (brushed).
3) AC excitation machine (armature rotation) plus silicon rectifier (brushless).
4) AC excitation machine (armature rotation) plus thyristor rectifier (brushless).
2. Full static excitation (self-excitation) system
Powered by a transformer. When the excitation transformer is connected to the generator’s terminal or to the plant power bus of a unitized generator set, it is called self-excitation excitation mode. The connection method of the generator terminal excitation transformer in series with the excitation converter that supplies power to the generator rotor is called self-redundant excitation excitation mode. This combination method also has four types:
1) DC side parallel
2) DC side series
3) AC side parallel
4) AC side series Function
1. Adjust the excitation current accordingly in response to changes in the generator load, to maintain the terminal voltage at the specified value;
2. Control the distribution of reactive power among the generators operating in parallel;
3. Enhance the static stability of generator parallel operation;
4. Improve the transient stability of generator parallel operation;
5. In the event of a fault within the generator, perform excitation disconnection to reduce the extent of fault losses;
6. According to operational requirements, implement maximum excitation limitation and minimum excitation limitation for the generators. Method
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Direct Current Generator
This type of excitation method for generators features a dedicated direct current generator. This dedicated direct current generator is called a direct current exciter. The exciter is usually coaxial with the generator. The excitation winding of the generator obtains direct current from the exciter through slip rings installed on the large shaft and fixed brushes. This excitation method has the advantages of independent excitation current, reliable operation, and reduced self-power consumption. It was the main excitation method for generators in the past few decades and has mature operational experience. However, its disadvantage is that the excitation regulation speed is relatively slow and the maintenance workload is large. Therefore, it is rarely used in units with a capacity of 10MW and above. [2]
Alternating Current Exciter
Some modern large-capacity generators use alternating current exciter machines to provide excitation current. The alternating current exciter is also installed on the large shaft of the generator. The alternating current it outputs is rectified and then supplied to the rotor excitation of the generator. At this time, the excitation method of the generator belongs to the self-excitation method. Also, due to the use of a stationary rectification device, it is also called self-excitation static excitation. The alternating secondary exciter provides excitation current. The alternating secondary exciter can be a permanent magnet machine or an alternating generator with a self-excitation constant voltage device. To improve the excitation regulation speed, the alternating current exciter usually uses a 100-200Hz medium-frequency generator, while the alternating secondary exciter uses a 400-500Hz medium-frequency generator. This type of generator has its DC excitation winding and three-phase alternating winding wound in the stator slots. The rotor only has teeth and slots without windings, like a gear, so it has no brush, slip ring, etc. for rotational contact components. It has the advantages of reliable operation, simple structure, and convenient manufacturing process. However, its disadvantage is that it has higher noise and larger harmonic components of the alternating electromotive force. [2]
Unexcited machine
In the excitation method, no dedicated excitation machine is set up. Instead, the excitation power is obtained from the generator itself, rectified, and then supplied to the generator for excitation. This is called self-excited static excitation. Self-excited static excitation can be divided into two types: self-series excitation and self-recovery excitation. The self-series excitation method obtains the excitation current by connecting a rectifier transformer at the generator outlet, rectifies it, and then supplies it to the generator for excitation. This excitation method has the advantages of simple structure, fewer equipment, lower investment, and less maintenance workload. The self-recovery excitation method, in addition to having a rectifier transformer, also has a high-power current transformer connected in series in the generator stator circuit. The function of this transformer is to provide a larger excitation current to the generator when a short circuit occurs, to make up for the deficiency of the output of the rectifier transformer. This excitation method has two excitation power sources, namely the voltage source obtained through the rectifier transformer and the current source obtained through the series transformer. [2]
About the features
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Voltage regulation
The automatic excitation system can be regarded as a negative feedback control system with voltage as the controlled variable. Reactive load current is the main cause of the decrease in generator terminal voltage. When the excitation current remains constant, the generator’s terminal voltage will decrease as the reactive current increases. However, in order to meet the users’ requirements for power quality, the generator’s terminal voltage should remain basically unchanged. The way to achieve this requirement is to adjust the generator’s excitation current according to the changes in reactive current. [2]
Reactive power
When a generator operates in parallel with the system, it can be considered to be operating on the bus of an infinitely large-capacity power source. To change the generator’s excitation current, the induced electromotive force and the stator current also change accordingly, and at this time, the generator’s reactive current also changes. When the generator operates in parallel with an infinitely large-capacity system, in order to change the generator’s reactive power, the generator’s excitation current must be adjusted. At this time, the adjusted generator excitation current is not what is commonly referred to as “voltage regulation”, but merely changes the reactive power sent into the system. [2]
Reactive load
The generators operating in parallel distribute the reactive current proportionally according to their rated capacities. The larger-capacity generators should bear a greater reactive load, while the smaller-capacity ones should provide a smaller reactive load. To achieve automatic distribution of the reactive load, an automatic high-voltage regulation excitation device can be used to change the excitation current of the generators to maintain their terminal voltages unchanged. The slope of the generator voltage regulation characteristics can also be adjusted to achieve a reasonable distribution of the reactive load among the parallel-running generators. [2]
Automatic adjustment
Announcement Method
When changing the excitation current of the generator, it is generally not done directly in the rotor circuit because the current in this circuit is very large and it is not convenient for direct adjustment. The common method is to change the excitation current of the exciter to achieve the purpose of regulating the rotor current of the generator. [2]
Common methods include: changing the resistance of the excitation circuit of the exciter, altering the additional excitation current of the exciter, and modifying the conduction angle of the thyristor, etc.
Here, we mainly discuss the method of changing the conduction angle of the thyristor. It is based on the changes in the generator’s voltage, current or power factor, and accordingly changes the conduction angle of the thyristor rectifier. Thus, the excitation current of the generator changes accordingly. This device is generally composed of transistors and thyristor electronic components, featuring sensitivity, rapidity, no failure zone, large output power, small size and light weight. In emergency situations, it can effectively suppress the overvoltage of the generator and achieve rapid excitation extinction.
Component units
The automatic excitation regulation device is usually composed of a measurement unit, a synchronization unit, an amplification unit, a regulation unit, a stabilization unit, a limiting unit and some auxiliary units. [2]
1. Measurement Unit
The measured signal (such as voltage, current, etc.) is transformed by the measurement unit and compared with the given value. Then, the comparison result (deviation) is amplified by the preamplifier unit and the power amplifier unit, and is used to control the conduction angle of the thyristor to achieve the purpose of regulating the generator excitation current.
2. Synchronization Unit
The function of the synchronization unit is to make the trigger pulse output by the phase-shifting part synchronous with the AC excitation power supply of the thyristor rectifier, so as to ensure the correct triggering of the thyristor.
3. Compensation Unit
The function of the compensation unit is to enable the parallel-operated generators to stably and reasonably distribute the reactive load.
4. Stability Unit
The stability unit is introduced to improve the stability of the power system. The excitation system stability unit is used to improve the stability of the excitation system.
5. Limiting Unit
The limiting unit is set up to prevent the generator from operating under over-excitation or under-excitation conditions.
It should be noted that not every automatic excitation regulation device has all these units. The units possessed by a regulator device are related to the specific task it undertakes.
Component Composition
The components of the automatic excitation regulation include the generator terminal voltage transformer, the generator terminal current transformer, and the excitation transformer; the excitation device needs to provide the following currents: factory AC 380v, factory DC 220v control power supply, factory DC 220v closing power supply; it also needs to provide the following contact points: automatic startup, automatic shutdown, grid connection (one normally open, one normally closed), increase, decrease; it needs to provide the following analog signals: generator terminal voltage 100V, generator terminal current 5A, bus voltage 100V, excitation device output relay contact signals; excitation transformer overcurrent, loss of excitation, excitation device abnormality, etc.
Excitation control, protection and signal return are composed of the excitation cut-off switch, the booster circuit, the fan, the excitation cut-off switch trip, excitation transformer overcurrent, regulator failure, generator operating condition abnormality, voltage transmitter, etc. In the event of an internal fault in the synchronous generator, in addition to disconnection, it is also necessary to excite the machine, to quickly reduce the rotor magnetic field to the minimum extent, ensuring that the rotor does not overheat, and minimizing the excitation time as much as possible is the main function of the excitation device. According to the rated excitation voltage, it can be divided into linear resistance excitation cut-off and non-linear resistance excitation cut-off.
Digital Automatic Excitation Regulation Device
In the past decade, due to the emergence and use of new technologies, new processes and new devices, the excitation method of the generator has been continuously developed and improved. In the aspect of automatic excitation regulation devices, many new regulating devices have been developed and promoted for use. Due to the significant advantages of the automatic excitation regulation devices realized by the microcomputer computer using software, many countries are developing and testing digital automatic excitation regulation devices composed of microcomputer computers and corresponding external equipment. This regulating device will be able to achieve adaptive regulation.