Generator Excitation Methods: DC, AC, and Self-Excitation

 
In generator systems, the choice of excitation method is crucial for the performance and efficiency of the generator. Excitation methods provide the magnetic field necessary for the generator's rotor to effectively generate electromotive force (EMF). In modern power systems, different excitation methods have their own advantages and disadvantages, making them suitable for various applications. This article introduces the two main excitation methods: direct current (DC) excitation and alternating current (AC) excitation, as well as the application of self-excitation stop excitation.

DC Excitation Method

 
The DC excitation method is a traditional way of exciting, mainly by using a dedicated DC generator (called a DC exciter) to provide excitation current for the generator. This exciter is typically mounted coaxially with the generator, and DC current is delivered to the generator's excitation windings through slip rings and fixed brushes on the generator's main shaft.

1. DC Excitation Advantages

 
Independent Excitation Current: The DC exciter provides an independent excitation current to the generator, unaffected by the generator's output, ensuring system stability and reliability.
Operational Reliability: Due to the independence of the excitation current, this method demonstrates high reliability during operation and is less susceptible to external grid fluctuations.
Low Self-Consumption: The design of the DC excitation system results in lower self-consumption of power during generator operation, enhancing overall efficiency.

2. DC Excitation Disadvantages

 
Slower Excitation Adjustment Speed: The DC exciter has a slower adjustment speed, limiting its ability to respond quickly to load changes and making it difficult to adapt to grid fluctuations.
High Maintenance: The use of slip rings and brushes requires regular maintenance, leading to higher operational costs and increased workload.
 
Due to these disadvantages, the DC excitation method is less commonly used in modern large-capacity generators (above 10 MW) and is more often applied in smaller or specialized power generation equipment.

AC Excitation Method

 
With the advancement of power technology, the AC excitation method has gradually become the mainstream, especially widely used in large-capacity generator sets. The AC excitation method uses an AC exciter to provide excitation current, which is also mounted on the generator's main shaft. The alternating current output from this exciter is rectified and then supplied to the generator's rotor excitation.

1. AC Excitation Characteristics

 
Separate Stop Excitation: This method is categorized as a separate excitation method because it uses external rectification equipment to convert the AC exciter's output current into DC, which is then supplied to the generator's rotor.
Multiple Excitation Sources: In an AC excitation system, auxiliary excitation current can be provided using an auxiliary AC exciter, which can be a permanent magnet machine or an AC generator with self-regulating constant voltage equipment, offering redundancy and reliability to the system.
High-Frequency Generators: To improve excitation adjustment speed, AC exciters typically use medium-frequency generators operating at 100-200 Hz, while auxiliary AC exciters use medium-frequency generators at 400-500 Hz. These high-frequency generators can respond faster to load changes, meeting the demands of modern power grids.

2. AC Excitation Advantages

 
High Reliability: The absence of rotating contact components like brushes and slip rings reduces wear and potential failure points, enhancing equipment reliability.
Simple Structure: Compared to DC exciters, AC exciters have a simpler structure and are easier to manufacture, facilitating large-scale production and application.

3. AC Excitation Disadvantages

 
High Noise Levels: Due to the operation of high-frequency generators, the equipment produces significant noise at high frequencies, requiring noise reduction measures.
High Harmonic Content in AC Potential: The AC potential generated contains substantial harmonic components, which may affect system stability and the normal operation of other equipment, necessitating suppression through filtering and other means.

Self-Excitation Stop Excitation

 
Self-excitation stop excitation is an excitation method that does not require an independent exciter; instead, it directly draws excitation power from the generator itself, which is rectified and then supplied back to the generator's excitation.

1. Self-Shunt Method

 
This method obtains excitation current through a rectifier transformer connected to the generator's output, which, after rectification, is supplied to the generator's excitation. It utilizes the generator's output voltage and converts it through a rectifier into DC excitation current.
 
Advantages: Simple structure, fewer devices, low investment, and reduced maintenance workload, making it suitable for small to medium-sized generator sets.

2. Self-Series Method

 
In addition to the absence of a rectifier transformer, this method also includes a high-power current transformer connected in series with the generator stator circuit to provide large excitation current during short circuits, compensating for the rectifier transformer's output deficiency.
 
Characteristics: This method offers two excitation sources: voltage sourced from the rectifier transformer and current sourced from the series transformer, allowing better response to short circuits and load variations.
 
The exciting method of the generator directly affects its operating characteristics and system stability. Although the DC excitation method has high reliability, it is complex to maintain and has a slow adjustment speed, and has gradually been replaced by the AC excitation method. The AC excitation method has become the mainstream choice for modern large-capacity generators due to its simple structure, convenient maintenance, and fast adjustment. At the same time, self-excited shutoff excitation, as a flexible exciting method, has also been widely used in medium and small generators. By choosing the appropriate exciting method and reasonably adjusting the excitation current, the performance of the generator can be effectively improved to ensure the safety and stable operation of the power system.