Generator sets can be divided into permanent magnet generators and excitation generators. The biggest difference between permanent magnet generators and excitation generators is that their excitation magnetic field is generated by permanent magnets. Permanent magnets are both a magnetic source and an integral part of the magnetic circuit in the motor. The excitation power unit provides excitation current to the synchronous generator rotor; while the excitation regulator controls the output of the excitation power unit of the permanent magnet generator according to the input signal and the given regulation criterion.
The excitation power unit provides an excitation current to the synchronous generator rotor; while 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).
The excitation power unit refers to the part of the excitation power supply 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.
The main functions of the excitation system are:
1) Adjust the excitation current according to changes in generator load to maintain the machine terminal voltage at a given value;
2) Control the reactive power distribution between 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 degree of fault loss;
6) Implement maximum excitation limits and minimum excitation limits on the generator according to operating requirements.
1. Excitation method of DC generator power supply
This type of excitation generator has a dedicated DC generator. This dedicated DC generator is called a DC exciter. The exciter is generally coaxial with the generator. The excitation winding of the generator passes through a slip ring mounted on the large shaft. and fixed brushes obtain DC from the exciter. 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 of AC exciter power supply
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 it outputs is rectified and supplied to the generator rotor for excitation. At this time, the excitation mode of the generator is separate. 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. 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, or 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.
4. Permanent magnet generator
The biggest difference between a permanent magnet generator and an excitation generator is that its excitation magnetic field is generated by permanent magnets. Permanent magnets are both a magnetic source and an integral part of the magnetic circuit in the motor. The magnetic properties of permanent magnets are not only related to the manufacturing process of the manufacturer but also to the shape and size of the permanent magnet, the capacity of the magnetizer, and the magnetization method. The specific performance data are highly discrete. Moreover, the magnetic flux and magnetomotive force that permanent magnets can provide in the motor also vary with the material properties and size of the rest of the magnetic circuit and the operating status of the motor. In addition, the magnetic circuit structures of permanent magnet generators are diverse, the magnetic leakage circuit is very complex and the leakage magnetic flux accounts for a large proportion, the ferromagnetic material part is relatively easy to saturate, and the magnetic conductance is nonlinear. These increase the complexity of the electromagnetic calculation of the permanent magnet generator, making the calculation results less accurate than that of the electric excitation generator. Therefore, new design concepts must be established, and the magnetic circuit structure and control system must be reanalyzed and improved; modern design methods must be applied and new analysis and calculation methods must be studied to improve the accuracy of design calculations; the use of advanced testing methods and manufacturing Craftsmanship must be studied.
1. Control issues
After the permanent magnet generator is made, it can maintain its magnetic field without external energy, but it also makes it extremely difficult to adjust and control its magnetic field from the outside. These limit the application range of permanent magnet generators. However, with the rapid development of control technology of power electronic devices such as mosFET and IGBTT, permanent magnet generators do not require magnetic field control but only motor output control in applications. The design requires the combination of three new technologies: NdFeB materials, power electronic devices, and microcomputer control so that the permanent magnet generator can operate under new working conditions.
2. Irreversible demagnetization problem
If designed and used improperly, the permanent magnet generator will operate under the armature reaction caused by the impact current or severe mechanical vibration when the temperature is too high (neodymium iron boron permanent magnet) or too low (ferrite permanent magnet). Irreversible demagnetization, or loss of excitation, may sometimes occur, causing the motor's performance to degrade or even become unusable. Therefore, it is necessary to not only research and develop methods and devices for checking the thermal stability of permanent magnet materials suitable for use in motor manufacturers, but also to analyze the anti-demagnetization capabilities of various structural forms, so that corresponding measures can be taken during design and manufacturing to ensure permanent magnet generators do not lose magnetism.
3. Cost issue
Since the current price of rare earth permanent magnet materials is relatively expensive, the cost of rare earth permanent magnet generators is generally higher than that of electric excitation generators, but this cost will be better compensated by the high performance and operation of the motor. In future designs, performance and price comparisons will be made based on specific use occasions and requirements, and structural innovation and design optimization will be carried out to reduce manufacturing costs.
The output differences between the excitation generator and the permanent magnet generator are as follows:
1. The initial electromotive force is provided in different ways
When the excitation generator is started, there must be an initial electromotive force for the excitation coil to generate a magnetic field. It relies on the small electromotive force generated by the generator set to provide electromotive force from an external power supply or a permanent magnet. When it works normally, it relies on its output voltage to work. The permanent magnet generator set relies on permanent magnets to provide the initial electromotive force.
2. Different magnetic field strengths
The excitation generator can change the excitation magnetic field by changing the current of the excitation coil. The magnetic field intensity can be large and controllable. The permanent magnet generator set is prone to magnetic field containment.