Generator excitation
The generator excitation system is a collective term for the power supply that provides the excitation current to the synchronous generator and its auxiliary equipment. It generally consists of two main parts: the excitation power unit and the excitation regulator.
Generator excitation systems include DC exciters, no-excitation exciters, and AC exciters. Over the past decade or so, the emergence and use of new technologies, processes, and devices have led to continuous development and improvement in generator excitation methods. In the area of automatic excitation control devices, many new types of control devices have also been continuously researched, developed, and widely adopted.
System Composition
The excitation power unit provides excitation current to the synchronous generator rotor; the excitation regulator controls the output of the excitation power unit according to the input signal and given regulation criteria. The automatic excitation regulator of the excitation system plays a significant role in improving the stability of parallel generating units in the power system. In particular, the development of modern power systems, leading to a trend of decreasing unit stability limits, has also spurred the continuous development of excitation technology. The excitation system of a synchronous generator mainly consists of two parts: the power unit and the regulator (device). As shown in the figure:
The excitation power unit refers to the excitation power supply that provides DC excitation current to the rotor windings of the synchronous generator, while the excitation regulator is the device that controls the output of the excitation power unit according to the input signal of the control requirements and the given regulation 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 component of the generator, and it has a significant impact on the safe and stable operation of the power system and the generator itself.
Main function
The main functions of the excitation system are:
1) Adjust the excitation current according to the changes in generator load to maintain the terminal voltage at the given value;
2) Control the reactive power distribution among generators operating 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 extent of the damage.
6) Implement maximum and minimum excitation limits for the generator according to operational requirements.
I. Several ways for a generator to obtain excitation current
1. Excitation method powered by DC generator: This type of generator has a dedicated DC generator, called a DC exciter. The exciter is generally coaxial with the generator, and the generator's excitation winding obtains DC current from the exciter through slip rings and fixed brushes mounted on the main shaft. This excitation method has advantages such as independent excitation current, relatively reliable operation, and reduced self-consumption of electricity. It has been the main excitation method for generators for the past few decades and has mature operating experience. The disadvantages are slow excitation regulation speed and high maintenance workload, so it is rarely used in units above 10MW.
2. Excitation Method Powered by AC Exciter: Some large-capacity generators use AC exciters to provide the excitation current. The AC exciter is also mounted on the generator shaft. Its output AC current is rectified and supplied to the generator rotor for excitation. In this case, the generator's excitation method is separately excited, and because a static rectifier is used, it is also called separately excited static excitation. An AC auxiliary exciter provides the excitation current. The AC auxiliary exciter can be a permanent magnet generator or an AC generator with a self-excited constant voltage device. To improve the excitation regulation speed, the AC exciter usually uses a 100-200Hz medium-frequency generator, while the AC auxiliary exciter uses a 400-500Hz medium-frequency generator. In this type of generator, both the DC excitation winding and the three-phase AC winding are wound in the stator slots. The rotor only has teeth and slots without windings, resembling a gear. Therefore, it has no brushes, slip rings, or other rotating contact parts, offering advantages such as reliable operation, simple structure, and convenient manufacturing. The disadvantages are relatively high noise and a large harmonic component of the AC potential.
3. Excitation method without an exciter:
In a self-excited static excitation system, no dedicated exciter is used; instead, the excitation power is obtained from the generator itself, rectified, and then supplied to the generator for excitation. Self-excited static excitation can be divided into two types: self-shunt excitation and self-compound excitation. The self-shunt excitation method obtains the excitation current through a rectifier transformer connected to the generator outlet, which is then rectified and supplied to the generator for excitation. This method has advantages such as simple structure, fewer equipment, lower investment, and less maintenance. The self-compound excitation method, in addition to lacking a rectifier transformer, also includes 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 in the event of a short circuit, compensating for the insufficient output of the rectifier transformer. This excitation method has two excitation power sources: a voltage source obtained through the rectifier transformer and a current source obtained through the series transformer.
II. Relevant Characteristics of Generators and Excitation Current
1. Voltage regulation
An automatic excitation control system can be viewed as a negative feedback control system with voltage as the controlled variable. Reactive load current is the main cause of generator terminal voltage drop. When the excitation current remains constant, the generator terminal voltage will decrease as the reactive current increases. However, to meet users' requirements for power quality, the generator terminal voltage should remain essentially constant. This is achieved by adjusting the generator's excitation current according to changes in reactive current.
2. Reactive power adjustment:
When a generator operates in parallel with a system, it can be considered to be operating on the bus of an infinitely large power source. Changing the generator's excitation current will cause changes in the induced electromotive force and stator current, thus altering the generator's reactive current. When a generator operates in parallel with an infinitely large capacity system, adjusting the generator's excitation current is necessary to change its reactive power. However, this change in the generator's excitation current is not the same as the commonly understood "voltage regulation," but rather a change in the reactive power supplied to the system.
3. Reactive load distribution:
In parallel operation, generators distribute reactive current proportionally according to their rated capacity. Larger-capacity generators should handle more reactive load, while smaller-capacity generators should provide less. To achieve automatic reactive load distribution, an automatic high-voltage regulating excitation device can be used to change the generator's excitation current to maintain a constant terminal voltage. The slope of the generator's voltage regulation characteristics can also be adjusted to achieve a reasonable distribution of reactive load among parallel-operating generators.
III. Methods for Automatically Adjusting Excitation Current
In changing the excitation current of a generator, it is generally not done directly in its rotor circuit because the current in that circuit is very large and not convenient for direct adjustment. The usual method is to change the excitation current of the exciter to achieve the purpose of regulating the generator rotor current. Common methods include changing the resistance of the exciter's excitation circuit, changing the additional excitation current of the exciter, and changing the conduction angle of the thyristor.
This section primarily discusses methods for changing the conduction angle of a thyristor. This involves adjusting the conduction angle of the thyristor rectifier based on changes in generator voltage, current, or power factor, thereby altering the generator's excitation current. This system typically consists of transistors and thyristor electronic components, offering advantages such as sensitivity, speed, no failure zone, high output power, small size, and light weight. In emergency situations, it can effectively suppress generator overvoltage and achieve rapid demagnetization.
Automatic excitation control devices typically consist of a measurement unit, a synchronization unit, an amplification unit, a differential adjustment unit, a stabilization unit, a limiting unit, and some auxiliary units. The measured signal (such as voltage or current) is transformed by the measurement unit and compared with a given value. The comparison result (deviation) is then amplified by the preamplifier and power amplifier units and used to control the conduction angle of the thyristor, thereby regulating the generator excitation current. The synchronization unit synchronizes the trigger pulse output from the phase-shifting section with the AC excitation power supply of the thyristor rectifier, ensuring correct thyristor triggering.
The function of the drooping unit is to ensure the stable and rational distribution of reactive load among generators operating in parallel. The stabilization unit is introduced to improve the stability of the power system. The excitation system stabilization unit is used to improve the stability of the excitation system. The limiting unit is set up to prevent the generator from operating under over-excitation or under-excitation conditions. It must be noted that not every automatic excitation regulating device has all of the above-mentioned units; the units a regulator device has are related to its specific task. IV. Components and Auxiliary Equipment of Automatic Excitation Regulation
The components of the automatic excitation regulating device include an organic terminal voltage transformer, a generator terminal current transformer, and an excitation transformer. The excitation device requires the following currents: AC380V and DC220V control power supply for the plant; DC220V closing power supply for the plant; and the following open contacts for automatic start-up, automatic shutdown, and grid connection (one normally open, one normally closed) increase/decrease. It also requires the following analog signals: generator terminal voltage 100V, generator terminal current 5A, bus voltage 100V. The excitation device outputs the following relay contact signals: excitation transformer overcurrent, loss of excitation, and excitation device malfunction.
The excitation control, protection, and signal circuit consists of a demagnetizing switch, an auxiliary excitation circuit, a fan, a demagnetizing switch tripping mechanism, an excitation transformer overcurrent protection mechanism, a regulator fault protection mechanism, a generator operating abnormality protection mechanism, and a power transmitter. When an internal fault occurs in a synchronous generator, in addition to disconnecting the generator, demagnetization is also necessary. The main function of the demagnetizing device is to reduce the rotor magnetic field to a minimum as quickly as possible, ensuring the rotor does not fail to pass through the grid, and to minimize the demagnetization time. Based on the rated excitation voltage, demagnetization can be divided into linear resistance demagnetization and nonlinear resistance demagnetization.
Over the past decade or so, the emergence and use of new technologies, processes, and devices have led to continuous development and improvement in generator excitation methods. In the area of automatic excitation control devices, many new types of control devices have been continuously researched, developed, and widely adopted. Due to the significant advantages of automatic excitation control devices implemented using microcomputer software, many countries are currently researching and testing digital automatic excitation control devices composed of microcomputers and corresponding external equipment. Such control devices can achieve adaptive optimal regulation. The method of obtaining excitation current is called the excitation method. Currently, excitation methods are divided into two main categories: one is the DC exciter excitation system using a DC generator as the excitation power source; the other is the rectifier excitation system using a silicon rectifier to convert AC to DC before supplying the excitation current. These are explained below:
1. DC Exciter Excitation: DC exciters are typically coaxial with synchronous generators and are connected in either shunt or separately excited configurations. In the separately excited configuration, the excitation current of the exciter is supplied by another coaxial DC generator, known as the auxiliary exciter.
2. The static rectifier excitation system has three AC generators on the same shaft: the main generator, the AC main exciter, and the AC auxiliary exciter. The auxiliary exciter's excitation current is initially supplied by an external DC power source, and then switches to self-excitation (sometimes using a permanent magnet generator) once the voltage is established. The auxiliary exciter's output current is rectified by a static thyristor rectifier and supplied to the main exciter, while the main exciter's AC output current is rectified by a static three-phase bridge silicon rectifier and supplied to the main generator's excitation winding.
3. Rotating Rectifier Excitation: The DC output of the static rectifier must pass through brushes and slip rings before being delivered to the rotating excitation winding. For large-capacity synchronous generators, the excitation current reaches thousands of amperes, causing severe overheating of the slip rings. Therefore, large-capacity synchronous generators often employ a rotating rectifier excitation system that does not require brushes and slip rings. The main exciter is a rotating armature type three-phase synchronous generator. The AC current of the rotating armature is rectified by a silicon rectifier rotating 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 a coaxial AC auxiliary exciter after rectification by a static thyristor rectifier. Because this excitation system eliminates the slip rings and brushes, it is also called a brushless excitation system.
